958136143-MIT

Enhancing quantum-dot luminescence
in visible and infrared light emitting devices
by
Geoffrey James Sasajima Supran
B.A., Trinity College, University of Cambridge (2009)
Submitted to the Department of Materials Science and Engineering
in partial fulfillment of the requirements for the degree of
Doctor of Philosophy in Materials Science and Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 2016
c Massachusetts Institute of Technology 2016. All rights reserved.
○
Author . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Department of Materials Science and Engineering
April 29, 2016
Certified by . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Vladimir Bulović
Professor of Electrical Engineering and Computer Science
Thesis Supervisor
Accepted by . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Professor Donald R. Sadoway
Chair, Departmental Committee on Graduate Students
2
Enhancing quantum-dot luminescence
in visible and infrared light emitting devices
by
Geoffrey James Sasajima Supran
Submitted to the Department of Materials Science and Engineering
on April 29, 2016, in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy in Materials Science and Engineering
Abstract
We investigate how the external quantum efficiency (EQE) of colloidal quantum-dot
light emitting devices (QD-LEDs) can be enhanced by addressing in situ QD photoluminescence (PL) quenching mechanisms occurring with and without applied bias.
QD-LEDs promise efficient, high colour-quality solid-state lighting and displays, and
our cost analysis of industrial-scale QD synthesis suggests they can be cost competitive. Efficiency ‘roll-off’ at high biases is among the most enduring challenges facing
all LED technologies today. It stands in the way of high efficiencies at high brightness,
yet it has not previously been studied in QD-LEDs. Simultaneous measurements of
QD electroluminescence (EL) and PL in an operating device allow us to show for
the first time that EQE roll-off in QD-LEDs derives from the QD layer itself, and
that it is entirely due to a bias-driven reduction in QD PL quantum yield. Using the
quantum confined Stark Effect as a signature of local electric fields in our devices,
the bias-dependence of EQE is predicted and found to be in excellent agreement
with the roll-off observed. We therefore conclude that electric field-induced QD PL
quenching fully accounts for roll-off in our QD-LEDs. To investigate zero-bias PL
quenching, we fabricate a novel near-infrared (NIR)-emitting device based on coreshell PbS-CdS QDs synthesised via cation exchange. QDs boast high PL quantum
yield at wavelengths beyond 1 𝜇m, making them uniquely suited to NIR applications
such as optical telecommunications and computing, bio-medical imaging, and on-chip
bio(sensing) and spectroscopy. Core-shell PbS-CdS QDs enhance the peak EQE of
core-only PbS control devices by 50- to 100-fold, up to 4.3 %. This is more than double
the efficiency of previous NIR QD-LEDs, making it the most efficient thin-film NIR
light source reported. PL measurements reveal that the efficiency enhancement is due
to passivation of the PbS core by the CdS shell against a non-radiative recombination
pathway caused by a neighboring conductive layer within the device architecture.
Thesis Supervisor: Vladimir Bulović
Title: Professor of Electrical Engineering and Computer Science
3
4
Acknowledgments
"What’s cookin’ good lookin’ ?!" No words better sum up Prof. Vladimir Bulović’s
indefatigable curiosity, passion, and good humour, than his own. It has been an
honour - and more fun than I could have imagined - to work under the supervision
of such a smart, energetic, and kind mentor, and a pioneer of our field. When the
President of the United States paid our lab a visit during my first year, I knew I was in
for an adventure in ONELab (Organic and Nanostructured Electronics Laboratory),
and throughout the years, Vladimir has continually nurtured an incredible playground
of talented colleagues, amazing equipment, and world-class ideas. I am particularly
grateful to Vladimir for his patience, interest, and support as I have explored my
passion for climate change work during this defining period of my career. He has
stood unwaveringly behind me during the hardest of times, and been there to cut
cake and celebrate with me during the best of them. Thank you, Vladimir, for these
unforgettable and formative years!
I am also deeply indebted to Prof. Moungi Bawendi and Prof. Marc Baldo, whose
doors have always been open. It has been a privilege to draw on their unparalleled
expertise during much of my work, and a pleasure to strike up fruitful collaborations
with their students and postdocs. I also sincerely thank Prof. Silvia Gradeçak and
Prof. Geoff Beach, who have kindly served on my thesis committee and provided
valuable guidance over the years.
My work was made possible thanks to the financial support of the DOE Excitonics
Center (an Energy Frontiers Research Center funded by the U.S. Department of
Energy) and an MIT Energy Initiative Fellowship. I also gratefully acknowledge QD
Vision, Inc., who have provided materials and technical support.
I have been so lucky that in my colleagues I have found such good friends. From
day one of grad school, and for my first several years in ONELab, Yasu Shirasaki
was my QD-LED partner-in-crime. I am very grateful to him for taking the time
to show me the ropes. Together we made our first QD-LED, inevitably reinvented
some wheels, and in the process enjoyed a tremendous collaboration on the roll-off
5
project. I look back fondly on our years sitting side-by-side in 13-3150. A little while
later, we were joined in the office by Katherine Song, whose many extreme injuries
had me convinced there was a curse on our office! Thank you to Katherine for being
such a great friend and labmate, without whom the IR QD-LED project would have
been neither as successful nor as fun. That project was also made possible only with
the synthetic skills of Gyuweon Hwang in the Bawendi group, to whom I am very
grateful. Most recently, it has been my pleasure to collaborate with Sihan (Jonas)
Xie: QD-LEDs are in your hands now, Jonas!
Beyond specific project collaborators, there are several others I must thank. No
one could have asked for nicer, smarter lab mates than Ni Zhao and Tim Osedach,
who not only mentored us newbies in the early years, but brought such levity to
days and nights in lab. I’ll never forget the time Ni took us to listen to Chinese
opera...argh! Patrick Brown and I joined ONELab together, and ours continues to
be amazingly eclectic friendship, working on everything from transfer line repairs
to climate activism. Thanks immensely for introducing me to fossil fuel divestment,
Patrick - as you know, it changed everything for me! Joel Jean has also become a great
friend and climate comrade, and is by far the most happy, least injured marathon
runner I’ve ever had the fortune to meet. Thank you also to Apoorva Murarka for
all the fun office chats, and cheers to Gleb Akselrod for getting my British sense of
humour better than most.
My heartfelt thanks, as well, to Prof. Will Tisdale, Wendi Chang, Trisha Andrew,
Farnaz Niroui, Deniz Bozyigit, Annie Wang, Sam Stranks, Andrea Maurano, Dong
Kyun Ko, Alexi Arango, and LeeAnn Kim. Indeed, to all collaborators and all past
and present members of ONELab with whom I’ve had the chance to work: it’s been
a privilege and a pleasure, thanks for all the help, and for all the memories.
I once read that Richard Feynman had a habit of turning down invitations to
receive prestigious awards, only to say "yes" to the local science club. It is in this
spirit that I will be forever grateful to those professors who said "yes" to letting a
precocious high schooler and undergrad tinker in their labs. Without their generosity
I may never have discovered my love of science. Thank you to Profs. Jeremy Baum6
berg (Southampton University, now at Cambridge), Sir Richard Friend (Cambridge),
Paul Alivisatos (UC Berkeley), Alan Heeger (UC Santa Barbara); Ray Goldstein
(Cambridge); and the late, great David MacKay (Cambridge).
Beyond the lab, climate advocacy and activism has become a huge part of my
life during grad school, and I am so thankful for the passionate, supportive, and
selfless friends I’ve had the pleasure to make. Helping to lead Fossil Free MIT in
our years of campaigning to divest MIT’s endowment from fossil fuel companies has
been one of the most fulfilling, transformative, and stressful(!) experiences of my
life. Thank you to all the brave students, staff, faculty, and alumni who continue to
speak truth to power, including but in no way limited to Ploy Achakulwisut, Ioana
Aron, Patrick Brown, Jane Conner, Ben Franta, Jason Jay, Daniel Mascoop, Jeremy
Poindexter, Ben Scandella, and Profs. Ian Condry, Kerry Emanuel, Charlie Harvey,
Naomi Oreskes, Jim Recht, Kieran Setiya, and John Sterman. I will never forget our
116 day #ScientistsSitIn! A big thank you, as well, to SustainUS and 350.org, and
to Bill McKibben in particular.
Last but not least, thank you to those who are always beside me. To Ploy: your
love, support, patience, and bravery - through all the ups and downs of grad school,
climate activism, and life - are all I could ever ask for. To my brother Jon: thanks for
keeping me grounded by literally being a rock star back in London, and for providing
the soundtrack to my Ph.D. And to Miko and David Supran, a.k.a. Mum and Dad:
There aren’t enough words to recount all the love, support, and opportunities you
have given me, which have carried me here and carry me on. Thanks, simply, for
being Mum and Dad - for being there "always and always, no matter what happens".
If you can fill the unforgiving minute
With sixty seconds’ worth of distance run,
Yours is the Earth and everything that’s in it,
And—which is more—you’ll be a Man, my son!
From "If—", by Rudyard Kipling.
7
Dedicated to my family - Mum, Dad, and Jon and to a Fossil Free MIT.
8
Contents
1 Introduction
19
1.1
Thesis Organisation . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20
1.2
Energy efficiency in lighting and displays . . . . . . . . . . . . . . . .
22
2 Quantum-Dots as Luminophores
2.1
2.2
31
Benefits for solid state lighting and displays . . . . . . . . . . . . . .
31
2.1.1
Tunable and pure colours
. . . . . . . . . . . . . . . . . . . .
31
2.1.2
Bright emission . . . . . . . . . . . . . . . . . . . . . . . . . .
34
2.1.3
Solution processable . . . . . . . . . . . . . . . . . . . . . . .
37
2.1.4
Stable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
37
2.1.5
Cost competitive . . . . . . . . . . . . . . . . . . . . . . . . .
38
Quantum-dot light emitting devices . . . . . . . . . . . . . . . . . . .
44
2.2.1
Evolution of QD-LEDs . . . . . . . . . . . . . . . . . . . . . .
44
2.2.2
Beyond Cd-based QDs: infrared and non-toxic QD-LEDs . . .
50
3 Colloidal Quantum-Dots
55
3.1
Colloidal versus epitaxial QDs . . . . . . . . . . . . . . . . . . . . . .
55
3.2
Colloidal QD synthesis . . . . . . . . . . . . . . . . . . . . . . . . . .
56
3.3
Optical properties of QDs . . . . . . . . . . . . . . . . . . . . . . . .
58
4 Origin of efficiency roll-off in QD-LEDs
63
4.1
Role of QD luminescence in QD-LED efficiency . . . . . . . . . . . .
63
4.2
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64
9
Contents
10
4.3
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64
4.4
Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
67
4.5
Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . .
68
5 Bias-driven QD luminescence quenching in QD-LEDs
73
5.1
Bias-dependent QD quenching mechanisms . . . . . . . . . . . . . . .
73
5.2
Electric field-induced quenching in QD-LEDs . . . . . . . . . . . . . .
78
5.2.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
78
5.2.2
Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
78
5.2.3
Results and discussion . . . . . . . . . . . . . . . . . . . . . .
80
5.2.4
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
87
6 Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
91
6.1
Bias-independent QD quenching mechanisms . . . . . . . . . . . . . .
91
6.2
Efficient infrared QD-LEDs using core-shell QDs . . . . . . . . . . . .
94
6.2.1
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
94
6.2.2
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
95
6.2.3
Methods summary . . . . . . . . . . . . . . . . . . . . . . . .
97
6.2.4
Results and discussion . . . . . . . . . . . . . . . . . . . . . .
99
6.2.5
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
6.2.6
Detailed Experimental Methods . . . . . . . . . . . . . . . . . 106
7 Conclusion
111
7.1
Thesis summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
7.2
Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
Appendix A Supplementary information to Chapter 6
121
A.1 Calculation and error estimate of EQE and radiance . . . . . . . . . . 132
A.1.1 EQE calculation
. . . . . . . . . . . . . . . . . . . . . . . . . 132
A.1.2 Radiance calculation . . . . . . . . . . . . . . . . . . . . . . . 133
A.1.3 EQE measurement error . . . . . . . . . . . . . . . . . . . . . 134
A.1.4 Device-to-device EQE variation . . . . . . . . . . . . . . . . . 134
Contents
11
A.2 QD film thickness calibration . . . . . . . . . . . . . . . . . . . . . . 134
12
List of Figures
13
List of Figures
1-1 Growth dynamics of carbon dioxide emissions. . . . . . . . . . . . . .
24
1-2 Joint influence of carbon intensity and energy intensity improvements
for limiting global warming. . . . . . . . . . . . . . . . . . . . . . . .
25
1-3 Evolution, approaches, and challenges of nitride-based LEDs. . . . . .
28
2-1 Tunable and pure colour light emission from colloidal quantum dots. .
33
2-2 Optical advantages of colloidal QDs for display and solid-state lighting
applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
2-3 Cost analysis for large-scale QD synthesis, example flowchart for synthesis of core-shell PbS-CdS QDs. . . . . . . . . . . . . . . . . . . . .
40
2-4 Progression of orange/red-emitting QD-LED performance over time in
terms of peak EQE and peak brightness. . . . . . . . . . . . . . . . .
45
2-5 QD Excitation mechanisms. . . . . . . . . . . . . . . . . . . . . . . .
47
2-6 Modern QD-LED architecture and operation. . . . . . . . . . . . . .
50
2-7 State-of-the-art QD-LEDs. . . . . . . . . . . . . . . . . . . . . . . . .
52
3-1 Schematic of colloidal QD synthesis. . . . . . . . . . . . . . . . . . . .
56
3-2 Photoluminescence spectra and quantum yield during PbS-CdS QD
cation exchange reaction. . . . . . . . . . . . . . . . . . . . . . . . . .
59
3-3 Absorption spectra of PbSe QDs as a function of nanocrystal diameter. 61
4-1 Efficiency roll-off in different LED technologies. . . . . . . . . . . . .
66
4-2 Simultaneous EL-PL measurement setup. . . . . . . . . . . . . . . . .
69
4-3 Absorption spectra of QD-LED components. . . . . . . . . . . . . . .
70
List of Figures
4-4 EQE roll-off in a QD-LED and correlation with QD PL. . . . . . . .
14
71
4-5 Bias-induced QD PL quenching, normalised by optical excitation intensity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
72
5-1 Heat-induced QD PL quenching. . . . . . . . . . . . . . . . . . . . . .
74
5-2 Zero-bias and bias-driven QD PL quenching mechanisms. . . . . . . .
76
5-3 Thermal image of QD-LED under constant current bias. . . . . . . .
79
5-4 QD-LED EL spectra. . . . . . . . . . . . . . . . . . . . . . . . . . . .
79
5-5 QD PL electric field-dependence measurement technique. . . . . . . .
81
5-6 QD-LED EL measurement setup. . . . . . . . . . . . . . . . . . . . .
82
5-7 Comparison of QD PL spectra and QD EL spectra at corresponding
peak emission energies, for three different biases. . . . . . . . . . . . .
84
5-8 Electric field-dependence of QD-LED PL and EL. . . . . . . . . . . .
85
5-9 Measured EQE and predicted EQE as a function of voltage. . . . . .
86
5-10 Transient PL of QDs under electric fields in a QD-LED. . . . . . . . .
88
5-11 Influence of QD electronic confinement on bias-driven PL quenching
mechanisms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
90
6-1 Influence of QD surface passivation and electronic confinement on zerobias and bias-driven PL quenching mechanisms. . . . . . . . . . . . .
93
6-2 Infrared QD PL, as seen by eye and with an infrared camera. . . . . .
96
6-3 QD synthesis and QD-LED design. . . . . . . . . . . . . . . . . . . .
98
6-4 QD-LED performance using core-only versus core-shell QDs. . . . . . 101
6-5 Progression in efficiencies over time of visible- and near-infrared-emitting
QD-LEDs, and comparison with this work and with commercial nearinfrared LEDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
6-6 Correlation of QD-LED EQE with QD PL quantum yields. . . . . . . 104
6-7 Transient spectroscopy of shell-dependent QD PL quenching. . . . . . 105
7-1 Progression of blue-emitting LED EQEs over time. . . . . . . . . . . 114
7-2 QD-LED operating lifetimes. . . . . . . . . . . . . . . . . . . . . . . . 120
List of Figures
15
A-1 Raw EQE-current density data with measurement errors and deviceto-device variations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
A-2 Current density-voltage behavior. . . . . . . . . . . . . . . . . . . . . 124
A-3 Test-to-test variations in peak EQE.
. . . . . . . . . . . . . . . . . . 125
A-4 Day-to-day variations in raw EQE-current density data. . . . . . . . . 126
A-5 Radiance-current density characteristics. . . . . . . . . . . . . . . . . 127
A-6 Enhancement in radiant intensity efficiency. . . . . . . . . . . . . . . 128
A-7 EQE-current density characteristics of anomalous ‘champion’ QD-LEDs.129
A-8 Dependence of QD-LED EQE-shell thickness correlation on current
density.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
A-9 Transient photoluminescence of QDs on glass. . . . . . . . . . . . . . 131
A-10 QD film thickness-concentration calibration curves on zinc oxide. . . . 136
16
List of Tables
17
List of Tables
2.1
Calculated large-scale chemical synthesis costs for red- and near-infraredemitting QDs and for two archetypal organic dyes. . . . . . . . . . . .
43
A.1 Summary of reported NIR QD-LED peak external quantum efficiencies
and power efficiencies. . . . . . . . . . . . . . . . . . . . . . . . . . . 123
A.2 PL lifetimes of spin-cast films of core-only (PbS) and core-shell (PbSCdS) QDs on ZnO, calculated from Fig. 6-7. . . . . . . . . . . . . . . 131
18
Chapter 1
Introduction
The generational challenges of climate change and global sustainable development
compel technological innovations in both renewable energy and energy efficiency. High
performance lighting is a key target for energy efficiency improvements and solid state
light emitting diodes are already revolutionising this field. In the quest for low cost,
large area, high colour-quality lighting, thin-film light emitting devices (LEDs) based
on nanostructured materials have emerged, and have already established themselves
as multi-billion dollar display markets. This thesis explores the use of visible and
infrared colloidal nanocrystal "quantum-dots" (QDs) as bright, wavelength-tunable
luminophores in optically- and electrically-driven LEDs.
Near the onset of this work, we conducted two comprehensive literature reviews
of the quantum-dot light emitting device (QD-LED) field [1, 2]. Our conclusion was
that from a device efficiency perspective, the three key challenges facing QD-LEDs
today are QD photoluminescence (PL) quenching, poor photon outcoupling, and a
limited understanding of the fundamental operating mechanisms contributing to the
recombination of electrons and holes, yielding light, in QD-LEDs. We also found
that the operational lifetime and cost of QD-LEDs must be addressed if they are to
become a commercial reality.
Improving device shelf life and reducing the cost of materials and manufacture are
surely the greatest hurdles to the commercialisation of existing QD-LEDs. Enhancing
the efficiency with which generated light is outcoupled from QD-LEDs is also a critical
19
Chapter 1. Introduction
20
step, yet it is expected that this may largely be a matter of extending to QD-LEDs
some of the many structural modifications that have been successfully implemented
in organic LEDs (OLEDs) [3, 4] and nitride-based LEDs [5]. Relatively speaking, all
three present predominantly manufacturing-based challenges on the R&D chain.
In contrast, better understanding carrier recombination and QD PL quenching in
QD-LEDs compels relatively fundamental research. As our overview of past QD-LED
research in Chapter 2 shows, substantial attention has been devoted to improving the
efficiency of carrier recombination - bringing electrons and holes together onto QD
emitters - through the engineering of four distinct generations of device architectures.
In this thesis, we instead focus on the last step of the light generation process, which
to date has received less dedicated attention: once an exciton has formed on a QD,
what processes dictate its probability of recombining to emit a photon? In particular,
we seek to characterise and, where possible, mitigate the influence of zero-bias and
bias-driven QD PL quenching on the performance of QD-LEDs. Indeed, the external
quantum efficiency (EQE) of QD-LEDs is directly proportional to QD PL quantum
yield (𝜂PL = number of photons emitted per photon absorbed), and in some devices it
is now the limiting factor in efficient light generation [6]. As we shall see, controlling
𝜂PL presents opportunities for engineering tailored materials solutions, allowing us to
make progress towards QD-LEDs with higher efficiency and brightness.
1.1
Thesis Organisation
Chapter 1 outlines the need for more energy efficient lighting technologies to help
decarbonise the global energy system and describes the challenges faced by white
LEDs and OLEDs. Chapter 2 introduces colloidal QDs and the benefits they bring to
lighting and display markets, and traces the evolution of QD-LEDs, from invention
through to state-of-the-art. It includes a quantitative cost analysis of large-scale
QD synthesis, which suggests QDs are cost-competitive with the U.S. Department of
Energy (DOE)’s solid state lighting (SSL) cost targets for OLEDs. Chapter 3 presents
a rudimentary background on the chemical approaches to QD synthesis relevant to this
Chapter 1. Introduction
21
thesis and the quantum mechanical origins of QDs’ fundamental optical properties.
Experimental methods are summarised at the start of each relevant chapter, and
where necessary, further details are presented at their end. We note that, expect
where explicitly described, the fabrication and characterisation tools employed in this
thesis are standard in the field, and the interested reader is directed to ref. [7–10],
which provide a basic contemporary overview of all of these techniques as used in our
laboratory.
Chapter 4 begins with an overview of the different processes that may contribute
to quenching of QD PL in QD-LEDs. This leads us to broadly categorise PL quenching mechanisms as either ‘zero-bias’ (occurring even without bias) or ‘bias-driven’
(dependent on applied bias). While these quenching pathways have previously been
identified for isolated QDs, here, in situ device measurements allow us for the first
time to directly observe the detrimental impact of these mechanisms in operating
QD-LEDs, and to engineer solutions to mitigate these effects.
In the second part of Chapter 4, we focus on bias-driven PL quenching in QDLEDs. By simultaneously measuring the PL and EL of operating red-emitting QDLEDs, we demonstrate for the first time that the so-called ‘roll-off’ in efficiency typically observed at high applied biases in QD-LEDs can be wholly explained by a
simultaneous, bias-driven, reversible reduction in QD 𝜂PL . This understanding is
crucial to designing devices with high efficiencies at high brightness.
This leads us in Chapter 5 to explore different possible mechanisms for this biasdependent QD PL quenching. By measuring the electric field dependence of QD PL,
we are able to quantitatively predict the EQE roll-off in our devices and therefore to
deduce that it is almost entirely governed by electric field-induced QD PL quenching,
and not carrier leakage or QD charging, as is often the conventional wisdom. In situ
transient PL measurements of our QD-LEDs suggest that the cause of quenching
is either a decrease in radiative exciton recombination rate (for example, due to a
decrease in the overlap of electron and hole wavefunctions) [11] or a decrease in the
efficiency of thermalized-exciton formation [12, 13].
Chapter 6 turns to zero-bias PL quenching in QD-LEDs. Using near-infrared
Chapter 1. Introduction
22
(NIR)-emitting QD-LEDs as a testbed of relatively unfulfilled technological potential,
we show that quenching of QD PL due to the QDs’ local device environment makes
𝜂PL a limiting factor in EQE. To address this, we fabricate a novel NIR QD-LED
architecture based on surface-passivated NIR-emitting QDs synthesised by overcoating core-only QDs with a wide band-gap shell. We show that this shell passivation
enhances in situ 𝜂PL by almost two orders of magnitude by mitigating quenching
caused by a neighbouring metal oxide charge transport layer. The correlated increase
in EQE, to more than double the previous record value, yields the most efficient
thin-film NIR light emitting device ever reported.
Finally, Chapter 7 presents a summary of this work, as well as an outlook for
QD-based light emitting technologies. Note that Chapter 2 draws substantially from
our work published in ref. [1, 2]. Chapters 4 and 5 appear in ref. [14]. Chapter 6 is
based on our work published in ref. [15] (for patent application see ref. [16]).
1.2
Energy efficiency in lighting and displays
A few months before the submission of this thesis, at the end of 2015, the world’s
nations adopted the Paris Agreement [17]. As part of the United Nations Framework Convention on Climate Change, this treaty commits countries to "Holding the
increase in the global average temperature to well below 2 ∘ C above pre-industrial
levels and to pursue efforts to limit the temperature increase to 1.5 ∘ C above preindustrial levels, recognizing that this would significantly reduce the risks and impacts
of climate change". Just a few months earlier, the United Nations General Assembly
also adopted the global Sustainable Development Goals, which include "Ensur[ing]
access to affordable, reliable, sustainable and clean energy for all" and "Tak[ing] urgent action to combat climate change and its impacts" [18]. As it happened, 2015 was
also the United Nations’ International Year of Light and Light-Based Technologies,
just months after the 2014 Nobel Prize in Physics was awarded "for the invention of
efficient blue light-emitting diodes which has enabled bright and energy-saving white
light sources" [19]. The prospect of designing and deploying high efficiency lighting
Chapter 1. Introduction
23
technologies to improve energy efficiency motivates the - necessarily fundamental research of novel QD-based light-emitting devices described in these pages.
Climate policy targets intending to limit the global mean temperature increase to
2 ∘ C will likely require that greenhouse gas emissions peak within the next decade,
and then fall rapidly to zero (or below) well before the end of this century. While
much focus rightly falls on supply-side strategies for reducing the carbon intensity
of energy production technologies, the Kaya Identity (which relates greenhouse gas
emissions to the complex, self-reinforcing interactions between carbon intensity, energy intensity, population, and affluence) makes clear that demand-side energy efficiency also has a vital role to play [20]. Fig. 1-1 shows how the exponential rise in
global greenhouse gas emissions since the Industrial Revolution has been driven by
an even faster exponential growth in energy use, only partially offset by a gradual
decline in carbon intensity [21]. Indeed, it is an inescapable theme of decarbonization scenarios that three fundamental energy system transformations are required:
fuel-switching (electrification); decarbonization of electricity; and energy efficiency
and conservation [20, 22–27]. As Fig. 1-2 shows, ensuring a reasonable probability of
holding global warming below 2 ∘ C requires on the order of a doubling in the rate
of energy intensity improvement in tandem with improvements in carbon intensity.
Energy efficiency improvements play an even more vital role in lower (1.5 ∘ C) stabilisation scenarios [28]. The Intergovernmental Panel on Climate Change projects
that efficiency improvements will be especially important in the near term, prior to
roughly 2030, compared to supply-side measures [27]. Buildings, in particular, have
the largest low-cost potential for improved energy efficiency [27]. Lighting, which is
the second largest user of building energy, and which accounts for roughly 19 % of
global electricity consumption (conservatively expected to grow by 80 % by 2030) [29]
and 6.5 % of the world’s greenhouse gas emissions [30], has therefore been identified
as a key area for early energy intensity reductions [26]. The U.S. DOE, for example,
is coordinating a major lighting research effort, and is directed by the Energy Policy
Act of 2005 to systematically accelerate SSL development [31]. Pertaining to the
U.N.’s Sustainable Development Goals, another key driver for lighting innovation is
Chapter 1. Introduction
24
Figure 1-1: Growth dynamics of carbon dioxide emissions. Global anthropogenic carbon dioxide (CO2 ) emissions, U (1015 g C yr−1 ) from fossil fuel and
land-use change, with exponential trend ln𝑈 = 𝛼(𝑡 − 𝑡1 ); (𝛼 = 0.0179 ± 0.0006
yr−1 , 𝑡1 = 1883 ± 1.7). Global primary energy use, E (1018 J yr−1 ) with trend
ln𝐸 = 𝜂(𝑡 − 𝑡1 ); (𝜂 = 0.0238 ± 0.0008 yr−1 , 𝑡1 = 1775 ± 3.5). Carbon intensity of
global primary energy, 𝑐 = 𝑈/𝐸 (g C 106 J−1 ) with trend ln𝑐 = (𝛼 − 𝜂)(𝑡 − 𝑡1 ).
Adapted from ref. [21].
the fact that 1.6 billion people today lack access to reliable lighting. Nearly all of
them instead rely on kerosene lamps, which account for 1 % of global lighting yet
20 % of lighting-based CO2 emissions [32], and which are responsible for 1.5 million
deaths per year [33].
To this end, almost all OECD (Organisation for Economic Co-operation and Development) governments have implemented policies aimed at phasing out incandescent lighting [35], which converts electrical energy to light with an efficiency of just 5
%, and with an efficacy of roughly 15 lm/W [32]. Fluorescent tubes (efficiency ∼25
%, efficacy ∼60-100 lm/W), and increasingly compact fluorescent lamps (efficiency
∼20 %, efficacy ∼35-80 lm/W), have come to provide the bulk of global lighting,
but owing to their mercury content and inferior efficacy relative to LEDs (discussed
below), they are perceived by many as a stop-gap measure to replace incandescent
lamps and their derivatives (halogen and high-intensity discharge lamps) until more
efficient, non-toxic white light sources become available and cost effective [29, 30].
Chapter 1. Introduction
25
Figure 1-2: Joint influence of carbon intensity and energy intensity improvements for limiting global warming. The plot, adapted from ref. [34], shows the
joint influence of these two factors on limiting warming to below 2 ∘ C during the
twenty-first century, in a large illustrative set of scenarios (𝑛 > 100). Average global
rates are for the period 2010 to 2050. The yellow star indicates the historically observed rate between 1971 and 2005, and six-pointed stars indicate the values found
in the SRES (Special Report on Emissions Scenarios - see ref. [34]) scenarios. SRES
marker scenarios are highlighted in red. The arrow illustrates the direction of increasing climate protection. Other symbols are colour-coded as a function of their
probability of limiting warming to below 2 ∘ C, and the shape of the symbols reflects
the base level of future energy demand assumed in the scenarios (diamond: high;
circle: intermediate; square: low). Note that whereas all scenarios in ref. [34] assume a consistent evolution of climate mitigation over the course of the full century,
the SRES scenarios represent baseline scenarios without climate mitigation. The
long-term development in the SRES scenarios might thus differ from their short-term
trends.
Chapter 1. Introduction
26
SSL based on inorganic LEDs continues to emerge as the most likely contender.
Akin to Moore’s Law for transistors, SSL development has progressed exponentially
for more than 40 years according to Haitz’ Law; each decade exhibiting a ten-fold
drop in cost-per-lumen and a twenty-fold rise in light generated per LED package
(for a given wavelength of emission) [30]. Sales of high-brightness LEDs have had a
compound annual growth rate of over 46 % since 1995 [29]. Whereas CFL efficacy
has lately remained flat at ∼35-80 lm/W, in just the last part of 2014, the average efficacy of LEDs increased from ∼70 lm/W up to almost 100 lm/W [36]. And
unlike incumbent technologies, LEDs are not limited by fundamental physics-based
limitations that prevent more than incremental improvements (for example, incandescents are limited by the laws governing a blackbody radiator) [30]. For SSL to
realise decisive energy efficiency benefits, however, even higher performance levels
must be achieved. The U.S. DOE’s Multi-Year Program Plan for SSL aims to reduce lighting-based energy use by 40-60 % by 2030 (2,216-3,900 TWh), equivalent to
roughly $220-380 billion in energy savings [37]. To do so, a stretch goal of 200 lm/W
by 2030 has been set, which envisions LED lighting virtually eliminating the use of
high intensity discharge sources (e.g. sodium lamps) and reducing the installed base
of linear fluorescent lamps to one-third of its current share. LED lighting sales (in
lumen-hours) would need to increase in the U.S., for example, from less than 4 % in
2014 to 88 % in 2030 [31].
Yet in most regions of the world right now, even with government policy support, fewer than 10 % of existing lighting installations (installed base) use SSL products [31]. A major research challenge of reaching the above goals is that white LEDs
(WLEDs) based on SSL must achieve such high efficacies while simultaneously delivering exceptional colour quality at low cost. Fig. 1-3b highlights three approaches
that are being explored to achieve these aims. The single-chip phosphor-conversion
approach based on a blue LED (InGaN, for example) backlight and a yellow or green
(and sometimes red, too; Fig. 1-3b, left) down-converting phosphor [38] are presently
the commercial technology of choice, but suffer from a tradeoff between efficacy and
colour quality (quantified by colour rendering index, CRI, explained in Section 2.1.1)
Chapter 1. Introduction
27
(Fig. 2-2b). This is due to the absorption and emission losses of the phosphor(s) [39],
as well as a 20-25 % energy loss because of the Stokes shift from a blue pump to lower
energy phosphor(s) [30]. The optimisation of phosphors, especially narrow-band redemitting ones with less thermal instability, is a key step to achieving efficacies truly
surpassing those of fluorescent lamps while maintaining high CRI [39,40]. Ultimately,
however, the Stokes shift may preclude the realisation of very high efficacy (∼200
lm/W) WLEDs [29]. An alternative avenue for achieving higher CRI, illustrated in
Fig. 1-3b, right, is the use of multi-chip red, green, blue (and sometimes yellow, too:
RYGB) WLEDs. But this approach is simultaneously made complicated and costly
by various aspects of assembly, colour mixing, and feedback drive circuitry required to
continuously balance the different colour emitters that age at different rates [38, 39].
Critically, RYGB WLEDs also face the decade-long challenge of improving the low
efficiencies of LEDs in the ‘green-yellow gap’ wavelengths [40,41]. To attain long-term
SSL performance targets, high LED internal quantum efficiencies of ⩾ 90 % will need
to be achived at particular green and yellow wavelengths [39]. Another LED challenge, relevant to both phosphor conversion and multi-chip approaches, is efficiency
‘droop’, which is the fall in efficiency of (nearly all GaN-based) LEDs of even 70 %
or more at desired high operating currents. This phenomenon is discussed in detail
in Chapter 4.
Meanwhile, OLEDs have emerged as a potential complimentary white light technology owing to their prospects as flexible, thin, lightweight, large-area, and diffuse
light sources. For example, ultrathin, large OLED panels can provide the same light
output as a compact GaN-based LED while reducing the brightness per unit area, resulting in a glare-free light source [42]. However, despite research progress, in practice,
the development of OLEDs for lighting has lagged behind that of nitride-based LEDs.
White OLEDs for lighting are only just nearing commercial availability because it has
not been possible so far to simultaneously achieve high brightness at high efficiency
with long lifetime [29]. For example, the relatively broad line-width of red OLED
emission makes it difficult to achieve excellent colour quality and high efficacy at the
same time [31]. Another well-known roadblock to the commercialisation of OLEDs is
Chapter 1. Introduction
28
Figure 1-3: Evolution, approaches, and challenges of nitride-based LEDs.
a, Haitz’ Law evolution of LEDs: (filled) Exponential rise in the (highest) flux-perlumen of commercially available red and cool-white LEDs between 1968 and 2010;
(unfilled) exponential fall in the (lowest) original equipment manufacturers (OEM)
cost-per-lumen of commercially available red and cool-white LEDs between 1973 and
2010. Adapted from ref. [30]. b, Approaches for generating white light from LEDs
and their corresponding power spectra: (left) single-chip blue LED pumps red and
green phosphors; (right) multi-chip RYGB WLED. Adapted from ref. [39]. c, EQEs
of various state-of-the-art LEDs based on two material systems, demonstrating the
‘green-yellow’ efficiency gap. InGaN: (1) Thin-Film Flip-Chip LEDs; (2) InGaN
vertical thin-film LED; and (3) conventional chip LED. AlGaInP: (4) Truncatedinverted-pyramid LEDs. 𝑉 (𝜆) is the luminous response curve of the human eye.
Reproduced from ref. [39].
Chapter 1. Introduction
29
the particular challenge of achieving deep blue electrophosphorescence with combined
high efficiency, brightness, and long-term operational stability [43]. Deep-blue emission is crucial to realizing a wide color gamut for higher colour quality SSL. In terms
of efficiency, while fluorescent organic dyes can certainly achieve deep blue light, they
show comparatively much lower efficiencies than their phosphorescent counterparts.
Conversely, phosphorescent dyes enable far higher efficiencies, but with only sky-blue
or bluish-green emission [44, 45]. Likewise, achieving high brightness in deep blue
phosphorescent OLEDs leads to a commensurate roll-off in efficiency [43]. And unlike
red- and green-emitting OLEDs, achieving industry-standard device lifetimes in blue
devices currently requires the use of only inefficient fluorescent emitters [42].
Overall, there has been a gradual market penetration of inorganic LEDs from
mobile applications to displays to general lighting [46]. This is because many of the
challenges faced by WLED lighting are similar to those in developing display technologies - both requiring high brightness, multi-colour emitters. As Haitz et al. have
observed in mapping the evolution of SSL, applications like displays provide critical
"stepping stone" markets that help drive the innovation and cost reductions necessary
for ultimately penetrating the general illumination sector [30]. OLED displays, too,
are already an established multi-billion-dollar market reality [42]. This is in part because of more aggressive, profit-driven research funding in the display market [31], but
also because of their low energy consumption, wide viewing angles and high contrast
values (in comparison to back-lit liquid crystal displays (LCDs)), and compatibility
with high-resolution patterning.
Colloidal QDs offer a possible solution to some of the challenges currently facing the design and manufacture of LEDs for lighting and display applications, as
we describe in the next chapter. These solution-processed semiconducting nanocrystals boast unique size-dependent optical properties that have motivated increasingly
active research aimed at applying them in the next generation of optoelectronic and
bio(medical) technologies. Since the first directed QD synthesis three decades ago, use
of QD thin-films has been demonstrated in a range of optoelectronic devices including LEDs [47–50], photovoltaics (PVs) [51], photodiodes [52], photoconductors [53],
Chapter 1. Introduction
30
and field-effect transistors [54], while QD solutions have been used in a myriad of
in vivo and in vitro imaging, sensing, and labelling techniques [55]. In passing,
we note that the research of QD-LEDs therefore not only extends the frontiers of
lighting and display technologies, but can offer fundamental insights into the photophysical properties of QDs, which can help inform sister technologies like QD PVs.
The infrared-emitting QD-LED architecture described in this thesis, for example, is
extremely similar to that used by some of today’s most efficient QD PVs [56].
Chapter 2
Quantum-Dots as Luminophores
2.1
Benefits for solid state lighting and displays
2.1.1
Tunable and pure colours
Colloidal quantum-dots (QDs) are solution-processed nanoscale crystals of semiconducting materials. They comprise a small inorganic semiconductor core (1-10 nm
in diameter), often a wider-bandgap inorganic semiconductor shell, and a coating
of organic passivating ligands (Fig. 2-1b, insets). They emit bright, pure and tunable colours of light, making them excellent candidates for colour centers in nextgeneration display and SSL technologies. The greatest asset of QDs for light-emitting
applications is their tunable bandgap, governed by the quantum size effect. Confinement of electron-hole pairs (excitons) on the order of the bulk semiconductor’s Bohr
exciton radius (5.6 nm for CdSe, a common QD material) leads to quantisation of
bulk energy levels, resulting in atomic emission-like spectra. Another result of this
confinement is that as its size decreases, the QD’s bandgap increases, leading to a
blue shift in emission wavelengths [57]. This is shown in Figure 2-1a, which also illustrates how this spectral tunability can be extended through changes in QD chemical
compositions and stoichiometries [58, 59]. Such systematic and precise spectral tunability of efficient emission, even in the near-infrared (NIR) region, is a distinguishing
and significant technological advantage of QDs over organic dyes, which we exploit
31
Chapter 2. Quantum-Dots as Luminophores
32
in engineering the NIR QD-LEDs described in this thesis. The quantum mechanical
origins of this colour tunability are discussed further in Chapter 3. CdSe-based coreshell QDs are currently the material of choice in visible QD-LEDs [58,60–62] and lead
chalcogenide QDs dominate NIR devices [63,64]. In the visible, the spectrally-narrow
emission of QDs (see Figure 2-1b; full width half maximum (FWHM) ∼30 nm for
CdSe) [65] compared with those of inorganic phosphors (FWHM ∼50-100 nm) [66]
identify QDs as outstanding luminescent sources of saturated emission colour.
This high colour quality can be quantified using the Commission International
de l’Eclairage (CIE) chromaticity diagram (Figure 2-2a), which maps colours visible
to the human eye in terms of hue and saturation. By combining the emission of
three light sources, such as red, green, and blue (RGB) emissive display pixels, a set
of apparent colours can be generated corresponding to the colours enclosed by the
triangle on the CIE diagram defined by the coordinates of the three pixels. Figure
2-2a shows that, with the highly saturated colours of QD emission, it is possible
to select RGB QD-LED sources whose subtended colour gamut is larger than that
required by high-definition television (HDTV) standards (dashed line) [67].
Broad spectral tunability also allows a more controlled combination of colours,
such that higher quality white light, with a precisely tailored spectrum, can be generated. The white light’s quality can be measured in terms of correlated colour temperature (CCT) and colour rendering index (CRI), which compare LED emission with
that from the Sun (the ‘ideal’ white light source, with a CRI of 100). Conventional
white LEDs, comprising a blue inorganic LED backlight coated with a yellow phosphor optical down-converter (see Fig. 1-3), typically exhibit a cool bluish emission,
characteristic of high CCTs (>5000 K) and low CRIs (mostly between 80 and 85), as
shown in Figure 2-2b. For lower CCT lights (for example, 2700 K) it is particularly
hard to simultaneously maintain high luminous efficacy and high colour quality because the required red luminophores must have relatively narrow emission spectra so
as to avoid photon loss as infrared emission. The emission spectra of conventional red
phosphors is unfortunately too broad (>60 nm FWHM) to avoid this loss. In contrast,
the narrow spectral emission (∼30 nm FWHM) of QDs in QD Vision Inc.’s Quan-
Chapter 2. Quantum-Dots as Luminophores
33
Figure 2-1: Tunable and pure colour light emission from colloidal quantum
dots. a, Solutions of colloidal QDs of varying size and composition, exhibiting PL
under optical (ultraviolet) excitation [58]. b, PL spectra of CdSe-ZnS and PbS-CdS
core-shell colloidal QDs [67, 68]. The upper inset shows a schematic of a typical
core-shell colloidal QD. The lower inset is a high-resolution transmission electron microscope image of a CdSe QD (scale bar, 1.5 nm). a demonstrates the size- and
composition-dependent tunability of QD emission colour, whereas b shows the extension of this narrowband emission into the near-infrared. QD-LED electroluminescence
typically closely matches corresponding PL spectra.
Chapter 2. Quantum-Dots as Luminophores
34
tum LightTM optic offer supplementary and more selective optical down-conversion
of some of the backlight’s bluer emission (generated by Nexxus Lighting, Inc. LED
lighbulbs) into redder light, leading to a CRI >90 % and a superior CCT of 2700 K,
while maintaining a very high 65 lm/W efficacy [67] (see Figure 2-2b for a comparison
with other LED light sources). QDs thus enable higher colour quality and, accordingly, lower power consumption in SSL sources. Through analogous approaches, QDs
can be utilised as backlights in high-colour-quality liquid crystal displays (LCDs). In
fact, Sony’s 2013 line of Triluminos LCD televisions use edge-mounted red and green
QDs from QD Vision, Inc. to optically downconvert some of the television’s blue LED
backlight (absorbing some of the blue light and re-emitting it as red and green light),
optimising its colour balance so that it fulfills >100 % of the NTSC colour television
standard gamut, compared with ∼70 % for conventional LCD screens [69, 70]. The
result is a television picture with colour quality comparable to that of organic LED
(OLED) screens, but achieved at the cost of an LCD display [2].
2.1.2
Bright emission
Overcoating QDs with wider bandgap inorganic semiconductor shell(s) (Figure 21b, inset) has been shown to dramatically enhance their PL quantum yield (𝜂PL )
and photostabilities. Overcoating passivates surface non-radiative recombination sites
more effectively than organic ligands alone, and, simultaneously, shifts the electron
wavefunction, confining excitons to the QD core, away from surface trap states [71–
73]. For example, solutions of CdSe-ZnS core-shell QDs can be routinely synthesised
with 𝜂PL of between 30 % and 95-plus %, almost one order of magnitude greater than
those of native CdSe cores [74]. As noted above, the extendibility of QD emission
into the NIR is of significant technological interest. Whereas organic molecules have
negligible optical activity beyond wavelengths of 𝜆=1 𝜇m (𝜂PL is <5 % at these
wavelengths) and exhibit poor chemical and photostability, QDs are relatively stable
and retain 𝜂PL >50 % throughout the NIR [74–78]. As with visible-emitting QDs, this
high NIR brightness is owing in large part to improvements in 𝜂PL with overcoating
[75].
Chapter 2. Quantum-Dots as Luminophores
35
Figure 2-2: Optical advantages of colloidal QDs for display and solid-state
lighting applications. a, CIE chromaticity diagram showing that the spectral
purity of QDs enables a colour gamut (dotted line) larger than the high-definition
television standard (dashed line). b, Plot showing the luminous efficacy and colour
rendering index (CRI) of various lighting solutions. The first commercial QD-based
solid-state light source, developed by QD Vision and Nexxus Lighting, consists of
sheets of red QDs backlit by a blue LED with a yellow phosphor coating, resulting
in a high CRI without compromising high luminescence efficacy. Recently, Philips’
A-Style LED, which employs remote phosphors, has demonstrated even more energyefficient lighting. There is evidently an emerging market for high-quality optical
downconverters, such as QDs.
Chapter 2. Quantum-Dots as Luminophores
36
Since QD-LEDs often comprise neat films of QDs, it is their 𝜂PL in this closepacked form that then dictates maximum device efficiencies. For core-only QDs in
solution, 𝜂PL is typically reduced by one to two orders of magnitude when deposited
as thin films [79]. Evidence suggests that this ‘self-quenching’ results from efficient
non-radiative Förster Resonant Energy Transfer (FRET) of excitons within the inhomogeneous size distribution of QDs to non-luminescent sites [6, 80–82], where they
recombine non-radiatively [83]. It follows from the very strong inter-dot spacing dependence of FRET efficiency (it decreases as spacing increases) that QD ligand length
and shell thickness can profoundly impact the degree of QD ‘self-quenching’. Thin
films of core-shell CdSe-ZnS QDs with long oleic acid ligands, for example, typically
retain 𝜂PL of 10-20 %, which directly affects the external quantum efficiency (EQE)
of QD-LEDs containing those QD films. The EQE of a QD-LED is defined as the
ratio of the number of photons emitted by the LED in the viewing direction to the
number of electrons injected. It may be expressed as:
EQE = 𝜂r 𝜒𝜂PL 𝜂oc
(2.1)
where 𝜂r is the fraction of injected charges that form excitons in QDs, 𝜒 is the fraction
of these excitons whose states have spin-allowed optical transitions, 𝜂PL is the QD
PL quantum yield resulting from these transitions, and 𝜂oc is the fraction of the
emitted photons which are coupled out of the device. The internal quantum efficiency
(IQE) is the efficiency of the charge recombination process, independent of 𝜂oc (i.e.
IQE = EQE/𝜂oc ). A full description of how EQE is defined and measured in our
experiments can be found in Appendix A, Section A.1.
It is also technologically significant that for CdSe QDs, for example, 𝜒 ≈ 1,
identical to that of the most efficient organic phosphors, which are used in highefficiency organic LEDs (OLEDs) [84]. In CdSe QDs, the high 𝜒 is a result of the
small energetic separation (<25 meV) of the ‘bright’ and ‘dark’ band-edge excitonic
states [85], which have spin-allowed and spin-forbidden transitions to the ground
state, respectively. Thermal mixing at room temperature enables efficient crossing of
Chapter 2. Quantum-Dots as Luminophores
37
excitons from dark states to higher energy bright states, leading to a high effective 𝜒.
With 𝜒 ≈ 1, IQE is then limited by 𝜂r 𝜂PL . The greatest hurdle to achieving high
efficiency QD-LEDs is the simultaneous maximisation of 𝜂r and 𝜂PL . QD-LEDs have
been reported that appear to demonstrate PL-normalised IQEs (that is, IQE/𝜂PL )
approaching (at least within a factor of two) 100 %, and that are therefore limited
only by 𝜂PL [6,58,63,86,87]. This motivates the focus of this thesis on understanding
and controlling the quenching of QD PL in QD-LEDs.
2.1.3
Solution processable
QD surface ligands confer solubility in a variety of organic solvents. This enables the
use of low cost QD deposition techniques such as spin-coating [88], mist coating [89],
inkjet printing [90,91], and microcontact printing [92,93]. Ligands can also be chosen
[50] (or cross-linked post-deposition [94, 95]) to enable the deposition of subsequent
materials in orthogonal solvents. These methods have led to, for example: organicQD hybrid structures [96]; molecular length-scale control of dot-to-dot separation
[97]; QDs deposited on curved surfaces [98]; QD monolayers [88]; QD multilayer
superstructures [99]; and one-dimensional chains [100].
2.1.4
Stable
It is commonly attested that the photostability of QDs exceeds that of organic chromophores, and that this advantages their application in LEDs. Yet oxidation of QDs
has been seen to cause spectral diffusion (blue-shifting) and PL quenching in both
single QDs [101, 102] and ensembles of QDs [103]. Exposure to light generally exacerbates these effects through photo-oxidation and photo-bleaching [102], though substantial photo-brightening (increased 𝜂PL following exposure to light) has also been
observed [104–106]. Beyond the presence of oxygen, these phenomena have been
found to be critically dependent on an array of factors including humidity [107, 108],
QD film geometry [109], and the duration [103, 106], intensity, and wavelength [109]
of illumination.
Chapter 2. Quantum-Dots as Luminophores
38
Nevertheless, QD shells markedly improve photostability [102] by passivating surface traps, by confining excitons to QD cores, and by hindering the diffusion of oxygen,
for example, into QD cores. Moreover, thick inorganic multishells [110, 111], surfacepassivating ligands [112], and radially graded alloyed shells [113] have been shown to
heavily attenuate, and even entirely suppress the PL intermittency phenomenon of
CdSe QDs known as ‘blinking’. Reductions in blinking are relevant to QD-LEDs because they translate to higher ensemble 𝜂PL [114]. Talapin et al. recently synthesised
QDs with inorganic molecular metal chalcogenide ligands [115] and with metal-free
ionic ligands [116], relieving QDs of instabilities associated with photo-damage of organic ligands [110]. We caution that many of the above studies, however, are based
on single-QD spectroscopy at cryogenic temperatures.
Overall, at least as bioanalytical labels, QDs are proving to be more photostable
than organic dyes [117]. Whether this comparison could hold good in LEDs, however,
is yet unclear. Tremendous opportunities exist to better understand and improve the
longevity of QDs in QD-LEDs by investigating the chemistry and photo-physics of
films of QDs under operating conditions [118].
2.1.5
Cost competitive
From a manufacturing standpoint, QD-LEDs may be approximated as QD-enhanced
OLEDs. The manufacturing cost of QD-LEDs can be broadly divided into the cost of
raw materials and the fabrication costs of processing these materials. The similarity
of the constituent materials of QD-LEDs and OLEDs means that they are fabricated using a similar toolbox of thin-film processing techniques, so that QD-LED
commercialisation would benefit from the manufacturing infrastructure and expertise
developed for OLED production. Aside from the QDs themselves, the materials typically employed in QD-LEDs (metals, metal oxides and organic small molecules) are
also very similar to those found in OLEDs. Their materials costs should therefore be
commensurate with those that are enabling the growth of OLED markets, and would
benefit from their economies of scale.
To estimate the materials costs of QDs typical in light-emitting applications when
Chapter 2. Quantum-Dots as Luminophores
39
produced in large quantities, we perform a quantitative analysis based on published
small-scale synthetic procedures, which colleagues have previously used to asses the
commercial viability of organic materials for photovoltaics [119]. We consider a few of
the most common and promising QD-LED materials and synthetic preparations: redemitting ‘legacy’ CdSe QDs [65], ‘modern’ CdSe QDs [120], ‘legacy’ CdSe-ZnS coreshell QDs [72], and ‘modern’ CdSe-CdS core-shell QDs [120] (all with trioctylphosphine oxide (TOPO) ligands); NIR-emitting PbS-CdS core-shell QDs (with oleic acid
ligands); and, lastly, PbS QDs (with oleic acid ligands), which are not only commonly
used in NIR QD-LEDs but have also garnered tremendous interest as an active material in QD-based solar cells. The ‘legacy’ and ‘modern’ labels refer to the synthetic
recipe evaluated, as discussed below. As detailed in ref. [119], our cost analysis takes
into account all of the material inputs to these procedures in order to estimate the
total material costs for each type of QD (based on our assembled database of quotations from major chemical suppliers for each of the input materials). As an example,
our model for the synthesis of PbS-CdS is represented graphically as a flowchart in
Figure 2-3. The first box in the flowchart represents the starting material, lead (II)
oxide. Red arrows indicate reagents, green arrows indicate solvents, and blue arrows indicate additional materials required for workup and purification (‘crash out’).
The indicated quantities of input materials and waste are calculated to produce one
kilogram of product.
The materials cost results from our models for each synthetic procedure are summarised in Table 2.1. We note that the materials costs that we consider represent
only one component of the overall cost to produce these materials. In the case of
pharmaceutical drugs, for example, materials only account for 20-45 % of the cost
of drug synthesis. The balance includes contributions for labor, capital, utilities,
maintenance, waste treatment, taxes, insurance, and various overhead charges [119].
We find that the materials costs of visible-emitting ‘legacy’ CdSe QDs ($569-660
g-1 ; lower value is without workup, upper value is with workup) exceed those of ‘modern’ CdSe QDs ($58-59 g-1 ) by an order-of-magnitude as a consequence of similarly
sizeable differences in the costs of reagents, solvents, and workup materials. This
Chapter 2. Quantum-Dots as Luminophores
40
Figure 2-3: Cost analysis for large-scale QD synthesis, example flowchart
for synthesis of core-shell PbS-CdS QDs. The flowchart describes the synthesis
of 1 kg of PbS-CdS core-shell QDs. The requisite quantities of (red arrow) reagent,
(green arrow) solvents, and (blue arrow) workup (‘crash out’) materials are indicated
for this single-step process. Note that a quantitative yield is assumed, as discussed
in the caption of Table 2.1.
reflects the 20-year evolution in synthetic procedures that has led to the use of significantly smaller quantities of more economical input materials, notably a significantly
cheaper and air-stable source of Cd (cadmium oxide replaces dimethyl cadmium).
These advances carry forward to the ‘modern’ CdSe-CdS core-shell QDs ($61-65 g-1 ),
which cost only fractionally more than their core-only equivalents, again owing to
the use of an economical source of Cd for the shell. In contrast, the ‘legacy’ CdSeZnS core-shell QDs ($1884-1996 g-1 ) inherit the ten-fold higher costs of their starting
CdSe QDs and require the use of an expensive source of zinc for their shell (replacing
dimethyl zinc is key to lower costs). It is possible that reports of QD costs of up
to $10,000 g-1 (ref. [69]) may result from evaluation of antiquated ‘legacy’ syntheses
rather than more economical state-of-the-art approaches. The materials costs of NIRemitting QDs (PbS, Method 1: $18-29 g-1 ; PbS, Method 2: $45-68 g-1 ; and PbS-CdS:
Chapter 2. Quantum-Dots as Luminophores
41
$68-97 g-1 ) are roughly commensurate with those of the ‘modern’ visible-emitting
QDs.
Current target prices for QDs synthesised via scaled-up continuous processes are
∼$10 g-1 (ref. [121]). As a guide, the materials cost of Alq3 - an archetypal organic
dye used in OLEDs since the 1980s, and therefore subject to considerable economies
of scale - is ∼$4 g-1 (ref. [119]). However, most heavy-metal-based phosphors found in
high-performance OLEDs are considerably more expensive. A representative example
is Ir(ppy)2 (acac) (ref. [122]), for which we calculate a materials cost of $658-1297 g-1 .
Nevertheless, direct comparison of the materials costs of QDs with those of organic
dyes is complicated by the specifics of a given application, which determine how much
material is consumed. Considerations include whether it is used at a neat film or as a
dopant dispersed in a host matrix, the thickness of such a film, and the wastefulness of
the deposition technique employed. As is to be expected, the first QD-based products
address optical down-conversion by using a compact edge-mounted geometry [70] that
requires relatively small amounts of QDs (often dispersed to maximize 𝜂PL ).
One way to try to assess our results is to translate them into approximate materials
costs-per-unit-area (Table 2.1). The per-area costs are based on the assumption that
a typical QD-LED might comprise a 25 nm film of hexagonally close packed QDs
separated by ∼0.5 nm due to their surrounding organic ligands. As expected, we
obtain similar values for ‘modern’ CdSe ($3 m-2 ) and CdSe-CdS ($4 m-2 ) QDs as
for PbS (Method 1: $1-2 m-2 ; Method 2: $3-5 m-2 ) and PbS-CdS ($4-6 m-2 ) QDs.
‘Legacy’ CdSe and CdSe-ZnS QDs are significantly more expensive ($35-41 m-2 and
$90-95 m-2 , respectively). Especially given the likelihood that QD syntheses will
be subject to some economies-of-scale [69], this is competitive, for example, with
the DOE’s 2020 SSL target cost of organic materials of $10-15 m-2 for OLEDs [123].
Moreover this is just one possible metric, which does not necessarily reflect the higher
materials costs that luxury items such as displays might be able to shoulder.
One significant assumption that has so far been made, however, is that we can
deposit QDs with 100 % efficiency. In reality, the spin-casting technique (and therefore
microcontact printing, which in published studies involves a spin-casting step) that
Chapter 2. Quantum-Dots as Luminophores
42
has so far dominated laboratory demonstrations of QD-LEDs wastes ∼95 % of the
starting solution [124]. Unless the QD waste is recyclable, the associated 20-fold
increase in QD materials costs could render some applications economically unviable.
This points to the importance of developing low-waste QD-deposition techniques.
For example, as mentioned earlier, inkjet printing of multi-coloured pixel arrays of
QDs for both down-conversion [125] and RGB QD-LED [124] technologies has been
demonstrated, but further refinements in film quality and device performance are
required.
[120]
[72]
[120]
‘Modern’ CdSe (TOPO) QD
‘Legacy’ CdSe-ZnS (TOPO) QD
‘Modern’ CdSe-CdS (TOPO) QD
[75]
[75]
PbS (OA) QD, Method 2
PbS-CdS (OA) QD*
[128]
Ir(ppy)2 (acac)
1
1
1
1
1
1
1
1
1
Steps
621.66
0.44
49.62
36.00
14.66
45.23
1,613.13
42.41
35.86
0.00
18.39
8.99
3.25
16.12
271.14
15.30
271.14
($ g-1 )
($ g-1 )
298.22
Solvent
Reagents
639.26
3.90
29.26
22.51
11.08
3.24
111.48
1.62
90.82
($ g-1 )
Workup
657.52
0.44
68.01
44.99
17.90
61.34
1,884.27
57.71
569.36
($ g-1 )
-
-
4.27
3.24
1.29
4.11
105.37
3.35
35.35
($ m-2 )
Total (w/o workup)
1,296.78
4.34
97.27
67.50
29.98
64.59
1,995.76
59.34
660.18
($ g-1 )
-
-
6.11
4.86
2.09
4.33
111.60
3.44
40.99
($ m-2 )
Total (w/ workup)
Cost-per-gram for ‘legacy’ CdSe QDs (with trioctylphosphine oxide (TOPO) ligands); ‘modern’ CdSe QDs (TOPO ligands; wurtzite-CdSe synthesis); ‘legacy’ CdSe-ZnS core-shell QDs (TOPO ligands); ‘modern’ CdSe-CdS core-shell QDs (TOPO ligands; wurtzite-CdSe synthesis); PbS (via
two methods) and PbS-CdS core-shell QDs (oleic acid ligands); fluorescent dye, tris(8-hydroxyquinolinato) aluminium (Alq3 ); and phosphorescent
dye, acetylacetonatobis(2-phenylpyridine) iridium (Ir(ppy)2 (acac)). The cost analysis accounts for all material inputs (reagents, solvents, and workup
[‘crash out’]), yielding total costs-per-gram both without and with workup. For the QDs, these have been converted into costs-per-area assuming a QD
film of 25 nm thickness, as detailed in the text. *In the absence of literature yields, we assume quantitative yields; although this is clearly a slight overestimate, it is a reasonable approximation given the synthetic refinements and waste recycling that will surely accompany scale-ups in QD synthesis.
[127]
Alq3
Organics
[126]
PbS (OA) QD, Method 1
IR QDs
[65]
Ref.
‘Legacy’ CdSe (TOPO) QD
Visible QDs
Compound
Table 2.1: Calculated large-scale chemical synthesis costs for red- and near-infrared-emitting QDs and for two
archetypal organic dyes.
Chapter 2. Quantum-Dots as Luminophores
43
Chapter 2. Quantum-Dots as Luminophores
2.2
44
Quantum-dot light emitting devices
A typical electrically driven colloidal QD-LED comprises two electrodes, which inject
charge into a series of active layers sandwiched between them (see Fig. 3a, for example). Since their invention in 1994 [48], their performance has improved dramatically.
Fig. 2-4 summarizes this progress for the case of orange/red-emitting (almost always
CdSe-based) QD-LEDs in terms of two metrics: (a) peak EQE; and (b) peak brightness. EQE is directly proportional to power conversion efficiency, a key metric for
SSL and displays, and brightness values of 103 − 104 cd m-2 and 102 − 103 cd m-2 are
required for SSL and display applications, respectively.
These achievements have in part been a result of evolutions in device architecture,
which in two recent reviews of the field, we classified into the four device ‘types’ depicted in the inset of Fig. 2-4a [1,2]. It can be seen that these four types have evolved
roughly chronologically. Despite the scattered data, we observe steady increases in
both EQE and brightness, with values approaching those of (phosphorescent) OLEDs.
In the following sections, we outline the distinguishing features of each type of QDLED.
2.2.1
Evolution of QD-LEDs
Type-I: QD-LEDs with polymer charge transport layers
These earliest QD-LEDs, pioneered in the early 1990s, were a natural progression
from polymer LEDs (PLEDs). Devices comprised a CdSe core-only QD-polymer
bilayer or blend [130], sandwiched between two electrodes. QD electroluminescence
(EL) was achieved but EQEs were extremely low (<0.01 % EQE, ∼100 cd m-2 ), in
part due to the low 𝜂PL of QDs without shells (10 % in solution). The low brightness
was a consequence of the very low current densities that could be reached while using
insulating QDs as both charge transport and emissive materials. Core-shell CdSe QDs
were later employed in type-I structures to take advantage of their higher 𝜂PL [131],
and EQEs of up to 0.22 % (max. 600 cd m-2 ) were reported using CdS shells [132].
However, these devices still exhibited significant parasitic polymer EL, indicative of
Chapter 2. Quantum-Dots as Luminophores
45
Figure 2-4: Progression of orange/red-emitting QD-LED performance over
time in terms of peak EQE and peak brightness. a, Peak EQE. b, Peak
brightness. QD-LEDs (a substantial but non-exhaustive selection from the literature)
are classified into one of four ‘types’, as described in the text, and are compared
with selected orange/red-emitting (phosphorescent) OLEDs. Solid lines connect new
record values. Data for a are taken from references: (type-I) [129–132]; (type-II)
[47, 58, 59, 87, 88, 133–136]; (type-III) [49, 137, 138]; (type-IV) [60, 61, 86, 94, 139–142];
and (OLEDs) [84, 93, 118, 143–146]. Data for b are taken from references: (typeI) [48, 89, 129, 131, 132]; (type-II) [47, 62, 88, 135, 136]; (type-III) [137, 147]; (typeIV) [6, 60, 61, 86, 93, 94, 139–142]; and (OLEDs) [145, 148–150].
Chapter 2. Quantum-Dots as Luminophores
46
inefficient exciton formation in QDs.
In these initial QD-LEDs, QD EL has been speculated to be driven by direct
charge injection [49] (Fig. 2-5b), FRET (Fig. 2-5c), or both, with the relative contribution of these mechanisms remaining unclear [61, 129, 151, 152]. In the case of direct
charge injection, an electron and a hole are injected from charge transport layers
(CTLs) into a QD, forming an exciton, which subsequently recombines via emission
of a photon. FRET is also a viable mechanism, unique to devices having luminescent
species, such as emissive polymers [153], small molecule organics [154], or inorganic
semiconductors [155,156], in close proximity to the QDs. In this scheme, an exciton is
first formed on a luminescent CTL. Thereafter, the exciton energy is non-radiatively
transferred to a QD via dipole-dipole coupling (Fig. 2-5f, blue arrows). The relative
contribution of these mechanisms, in all four types of QD-LEDs in fact, remains unclear and a better understanding of their roles, for example as a function of QD-LED
architecture, will be essential in designing more efficient and brighter devices.
Type-II: QD-LEDs with organic small molecule charge transport layers
Type-II QD-LEDs were first introduced by Coe et al. in 2002 and instead comprise
a monolayer of QDs at the interface of a bilayer OLED [47] (Fig. 2-6). These devices demonstrated record EQEs of 0.5 %; an efficiency that has been augmented by
an additional factor-of-ten through optimisations. The enhanced performance was
attributed to the use of a single monolayer of QDs (enabled by the development of
spin-coating [47] and microcontact printing [92, 157, 158] techniques), which decouples the luminescence process in the QDs from charge transport through the organic
layers [47, 49, 133, 154, 155, 158].
Using the microcontact printing method, Anikeeva et al. showed a series of QDLEDs with emission tunable across the entire visible spectrum by varying the composition of QDs sandwiched between two organic CTLs (Fig. 2-6c) [58]. A maximum
EQE of 2.7 % was achieved for orange emission. The spectral purity and tunability of
the QD-LEDs reported in this work clearly demonstrates the potential that QD-LEDs
hold for EL displays. It has also been demonstrated that white-emitting QD-LEDs
Chapter 2. Quantum-Dots as Luminophores
47
Figure 2-5: QD Excitation mechanisms. There are four routes by which to generate excitons in QDs that have been used in QD-LEDs. a, Optical excitation: an
exciton is formed in a QD by absorbing a high-energy photon. b, Charge injection:
an exciton is formed by injection of an electron and a hole from neighboring charge
transport layers. c, Energy transfer: an exciton is transferred to a QD via FRET
from a nearby donor molecule. d, Ionisation: a large electric field ionises an electron
from one QD to another, thereby generating a hole. When these ionisation events
occur throughout a QD film, generated electrons and holes can meet on the same QD,
forming excitons. e, Energy band diagram of a typical type-II QD-LED that outlines
the two suspected QD excitation mechanisms: charge injection and energy transfer.
can be fabricated by simply mixing different compositions [134] or sizes [159] of QDs.
A CRI of 86 was achieved by mixing red, green, and blue QDs. As shown in Fig.
2-2a, even higher CRIs should be achievable through the mixing of a greater variety
of colours of QDs, suggesting the realisability of white QD-LEDs as SSL sources. Furthermore, compatibility of type-II QD-LEDs with flexible substrates is exemplified
by the flexible white-light-emitting QD-LED shown in Fig. 2-7c.
Studies in our group have indicated that, at least in certain type-II QD-LED
geometries, FRET is the dominant QD excitation mechanism [133]. Yet, for example,
the achievement of EQEs >2 % in QD monolayer-based devices comprising organic
donor materials with very low 𝜂PL [135] challenges the universality of the FRET
model.
Combining an OLED architecture with a monolayer of QDs was, nevertheless, a
significant step forward in demonstrating efficient QD-LEDs. These devices boast
all of the advantages of OLEDs, with the added benefits of enhanced spectral purity
Chapter 2. Quantum-Dots as Luminophores
48
and tunability, as illustrated by the tunable EL shown in Fig. 2-6b, c. However, the
use of organic layers introduces device instabilities upon air exposure [160, 161]. As
with OLEDs, commercialised QD-LEDs would then require protective encapsulation,
which adds to manufacturing costs and hinders certain applications, such as flexible
technologies. Furthermore, the relatively insulating nature of organic semiconductors
can limit the current densities achievable in QD-LEDs prior to device failure, therefore limiting their brightness.
Type-III: QD-LEDs with inorganic charge transport layers
Replacing the organic CTLs of type-II QD-LEDs with inorganic CTLs could lead to
greater device stability in air [160,161], and could enable the passage of higher current
densities and therefore brighter emission.
One such all-inorganic QD-LED (apart from the organic ligands) was made by
Mueller et al. by sandwiching a monolayer of QDs between epitaxially grown nand p-type GaN (ref. [49]). QD EL was observed, though at very limited efficiencies
(EQE <0.01 %). The epitaxial growth of GaN, however, diminishes the advantage
of using colloidal QDs to inexpensively fabricate large area devices. This necessitates
alternative approaches to developing QD-LEDs with inorganic CTLs.
One such alternative is to use sputtered metal oxides as CTLs. Like organic materials, metal oxide and chalcogenide thin films can be deposited at room temperature by
sputtering. The broad variety of metal oxide and chalcogenide compositions enables
fine-tuning of their energy bands, as needed for the optimal operation of QD-LEDs.
In addition, metal oxides can be more conductive than their organic counterparts,
with the conductivity of metal oxides tunable by controlling oxygen partial pressure
during thin film growth. Caruge et al. applied this technique in QD-LEDs comprising
zinc tin oxide and NiO as n- and p-type CTLs, respectively [137]. As expected, these
devices were able to pass higher current densities (up to 4 A cm-2 ) but the EQE was
less than 0.1 %.
This inefficiency was attributed to the damage of QDs during sputtering of the
overlying oxide layer, to carrier imbalance (due to a large hole injection barrier be-
Chapter 2. Quantum-Dots as Luminophores
49
tween the p-type metal oxide and the QDs), and to quenching of QD PL by the
surrounding conductive metal oxide [162]. To our knowledge, there has not been a
report of a type-III QD-LED with efficiencies comparable to those of type-II devices.
Aside from these DC-driven QD-LEDs, another sort of all-inorganic QD-LED that
operates by an altogether different excitation mechanism has emerged over the past
few years. These are capacitive structures consisting of two contacts sandwiching
a film of dielectric material, with a layer of QDs at its centre [162, 163]. High AC
voltages drive these devices, resulting in operation by field-assisted ionisation of QDs
to generate free carriers (Fig. 2-5d) [164]. This architecture eliminates the need for
CTLs and energy band alignment between different semiconductors.
Type-IV: QD-LEDs with hybrid organic-inorganic charge transport layers
The hybrid structure of type-IV QD-LEDs offers a compromise between type-II and
type-III QD-LEDs, often comprising an inorganic metal oxide electron transport layer
(notably solution processed ZnO [60, 61]) and an organic small molecule hole transport layer [50,60,61,139,142]. A typical device structure and its corresponding EL are
shown in Fig. 2-6a, b. Significant efficiency gains have resulted, with recent visibleemitting QD-LEDs reaching EQEs as high as 20 % [140, 142] (Fig. 2-4a and 2-6a);
approaching those of commercially mature phosphorescent OLEDs [118] as well as the
outcoupling-limited ceiling of ∼20-25 % [165]. The brightness of type-IV QD-LEDs
has also reached record levels of 218,800 cd m-2 (ref. [60]).
Type-IV devices can, if desired, be solution processed using colloidal metal oxide nanoparticles as the electron transport layer [60, 61]. In particular, Qian et al.
have demonstrated red, green, and blue solution-processed (excluding electrodes) QDLEDs. The EQEs of these devices were 1.7 %, 1.8 %, and 0.22 %, with maximum
brightness values of 31,000 cd m-2 , 68,000 cd m-2 , and 4,200 cd m-2 for red, green, and
blue devices, respectively. These brightness values are within range of the highest
reported [61].
High-resolution microcontact printing of QD films (>1000 pixels per inch, 25 𝜇m
Chapter 2. Quantum-Dots as Luminophores
50
Figure 2-6: Modern QD-LED architecture and operation. a, Inverted hybrid
organic-QD-inorganic type-IV QD-LED structure that has risen to prominence owing
to its record efficiencies and brightness. It typically comprises a few monolayers
of QDs sandwiched between an inorganic metal oxide electron transport layer and
an organic hole transport layer. Photographs of tunable electroluminescence (EL)
colours from: b, type-IV [2]; and c, type-II QD-LEDs (together with respective EL
spectra) [58].
features) [92] (Fig. 2-7b) has already enabled the fabrication of full-colour 4-inch
QD-LED displays [93] and, by mixing different compositions [134] or sizes [159] of
QDs, white-emitting QD-LEDs that are highly amenable to SSL have been demonstrated with excellent colour quality (CRI ∼90). White-emitting QD-LEDs on flexible
substrates (Fig. 2-7c) have also been realized at QD Vision, Inc. [1, 70].
The energy transfer scheme that is suspected to dominate QD excitation in typeII QD-LEDs requires migration of one carrier type through the close-packed QDs of
a monolayer film, so as to form excitons in an adjacent donor material (Figure 25e) [133]. Since type-III and type-IV QD-LEDs, in contrast with type-II QD-LEDs,
employ QD films thicker than one monolayer (up to ∼50 nm), the working mechanism
of type-IV QD-LEDs is more compatible with a charge injection model.
2.2.2
Beyond Cd-based QDs: infrared and non-toxic QD-LEDs
The paucity of high-𝜂PL NIR molecular and polymeric dyes provides a compelling
impetus to extend the EL of QD-LEDs from the visible into the NIR range (780-
Chapter 2. Quantum-Dots as Luminophores
51
2,500 nm). As EQEs of OLED and polymer LEDs emitting at 𝜆 > 1 𝜇m remain less
than 0.3 % [166], NIR-emitting QD-LEDs boast unique potential. For example, one
can envision solution-processable (potentially Si-compatibile) sources of EL in the 1.31.55 𝜇m telecommunications band, which can be deposited on any substrate and at
lower cost than existing (usually epitaxially grown) IR-emitters, finding application
in optical telecommunications and computing [63, 77, 167–170]. Utilising biological
transparency windows between 800 nm and 1700 nm (ref. [171]), one can also imagine
deep tissue biomedical imaging and optical diagnostic [77,171] applications, as well as
on-chip bio(sensing) and spectroscopy usages [55, 77, 172] (for example, low cost NIR
QD-LEDs in microfluidic point-of-care devices [173]). Night-vision-readable displays
[174], night-time surveillance, and other security applications also present themselves.
Extension of the wet chemical methods previously discussed has readily enabled
the synthesis of a variety of efficient NIR-emitting colloidal QDs, which in the telecommunications band include PbE (E=S, Se, Te), InAs, and HgTe, as well as core-shell
QDs such as PbE-CdS and InAs-ZnSe. In many cases, high 𝜂PL have been achieved
(⩾50 %). There are a number of in-depth reviews [74, 76, 77, 177] for the interested
reader.
Most NIR QD-LEDs have been based largely on type-I architectures and coreonly NIR (lead chalcogenide) QDs [64, 151, 168, 169, 178, 179], with EQEs of up to
∼2 % reported [64] (Fig. 2b). At QD Vision, Inc., NIR QD-LEDs with active areas
of up to 4 cm2 (Fig. 2b, inset) and radiances of up to 18.3 W sr-1 m-2 have been
achieved; comparable to commercial IR LEDs and sufficient to serve as large-area IR
illuminators (Fig. 2-7d). In this thesis, we describe the realisation of devices with
efficiencies exceeding 4 % [15] - more than double the previous record - by transitioning
to a type-IV device structure and exploiting the enhanced passivation of core-shell
NIR QDs. Just as with visible-emitting QD-LEDs, it has been argued that electrical
excitation of QDs in these NIR devices occurs either by FRET [63, 170] or by direct
charge injection [168,169,180–182]; a balance between the two is likely in most cases.
Recently, Cheng et al. described the first Si QD-based LEDs, with very high
EQEs of 0.6 % (type-I) [180] and 8.6 % (type-II) [183], though with rather blue NIR
Chapter 2. Quantum-Dots as Luminophores
52
Figure 2-7: State-of-the-art QD-LEDs. a, EQE versus applied voltage for very
high performance (peak EQE of 18 %) red-emitting QD-LEDs (photograph in inset)
[175]. b, The first demonstration of red-green-blue electroluminescence from (typeII) QD-LED pixels, patterned using microcontact printing [92]. c, Flexible whiteemitting type-II QD-LED [176]. d, Large-area infrared illumination by an infraredemitting QD-LED, as seen through an infrared camera [2].
Chapter 2. Quantum-Dots as Luminophores
53
EL of ∼850 nm. Although emission at wavelengths beyond 1 𝜇m may be difficult
to achieve with Si QDs (efficient blue and green emitters have also not yet been
demonstrated), this new breed of heavy-metal-free QD-LEDs nevertheless addresses
growing concerns regarding the risks that cations such as cadmium, lead, and mercury
pose to our health and to the environment [1]. The European Union’s Restriction of
Hazardous Substances Directive, for example, severely limits the use of these materials
in consumer electronics. Therefore, the likelihood of commercial success of QD-LEDs
will be greatly increased if these devices can be fabricated using heavy-metal-free
QDs.
As an aside though, we ask, how much cadmium (Cd) is actually in a QD-LED?
We can estimate this by assuming that a typical QD-LED might comprise a 5 cm × 5
cm film, 25 nm thick, of hexagonally close-packed CdSe-CdS core-shell QDs separated
by ∼0.5 nm due to their surrounding organic ligands. Then, the effective density of
CdSe-CdS QDs with a diameter of 4 nm and a shell thickness of 2 nm is 3.5 g cm-3 ,
which translates to a mass of 0.16 mg of CdSe-ZnS and therefore 63 𝜇g of Cd. This
is comparable to one’s daily Cd intake: the age-weighted mean Cd intake for males
in the United States is 0.35 𝜇g kg-1 day-1 , or 24.5 𝜇g day-1 for a 70 kg male [184].
All the same, the commercial demand for novel down-converters in single-chip white
LEDs has specifically led to QDs being identified as likely candidate ‘phosphors’,
yet concerns over Cd compliance issues threaten to inhibit their use [40]. There are
therefore strong incentives to develop Cd-free QDs competitive with their traditional
counterparts.
Chapter 2. Quantum-Dots as Luminophores
54
Chapter 3
Colloidal Quantum-Dots
3.1
Colloidal versus epitaxial QDs
Quantum-dots (QDs) may be categorised by their synthetic route as either ‘colloidal’
or ‘epitaxial’ (or ‘self-assembled’). Whereas the latter derive from relatively highenergy-input ‘dry’ methods of epitaxial growth from the vapour phase [185], colloidal
QDs are synthesised by ‘wet’ chemical approaches [74] and are the focus of this
thesis. The precise size and shape control, as well as the high monodispersity, spectral
purity and 𝜂PL afforded by the chemical synthesis of QDs, are unmatched by epitaxial
techniques. Colloidal QDs are freestanding and therefore amenable to a plethora
of chemical post-processing and thin-film assembly steps, in contrast to epitaxial
QDs, which are substrate-bound [99]. Additionally, the relatively inexpensive, facile,
and scalable solution-based conditions necessary for the synthesis of nearly defectfree colloidal QDs have an impurity tolerance far exceeding that of the ultra-high
vacuum environments required for epitaxial growth. Moreover, only weak quantumconfinement effects are observed in epitaxial QDs [186] owing to their relatively large
lateral dimensions (typically >10 nm) and difficulties in size control. This is in stark
contrast to the size-tunable emission of colloidal QDs, which are therefore favourable
as luminophores in LEDs [187].
55
Chapter 3. Colloidal Quantum-Dots
56
Figure 3-1: Schematic of colloidal QD synthesis. Adapted from ref. [9]
3.2
Colloidal QD synthesis
The benchmark preparation of colloidal quantum-dots (QDs, throughout), yielding
high quality and monodisperse (size variation <4 % r.m.s.) nanoparticles, involves pyrolysis of organometallic precursors (such as dimethyl cadmium and trioctylphosphine
selenium for CdSe and, in this thesis, lead (II) oleate and hexamethyldisilathiane for
PbS) injected into a hot organic coordinating solvent (at temperatures of 120 to 360
∘
C) [65, 74, 188]. The experimental procedure is shown schematically in Fig. 3-1.
Thermally activated nucleation and growth of small crystallites from the precursors
ensues until arrested by cooling. Fine control of QD size (for example, from 1.5 nm
to 12 nm for CdSe QDs [65]) and size dispersion can thus be achieved through control
of reaction time and temperature, as well as precursor and surfactant concentrations.
QD size is monitored by measuring absorption spectra of aliquots extracted from
the growth solution during synthesis. Overcoating CdSe core-only QDs with ZnS
shells has been shown to enhance QD stability and brightness [71,72]. This is usually
achieved by injecting organometallic precursors to the shell material into a flask containing the cores. The resulting QDs are dressed with organic ligands, which confer
solubility in a diversity of common non-polar solvents. Commercial red-emitting QDs
used in this thesis were obtained from QD Vision, Inc.
Post-synthesis size-selective precipitation can further increase monodispersity in
Chapter 3. Colloidal Quantum-Dots
57
colloidal QD solutions. Typically, prior to use in devices, QDs are precipitated and
redissolved at least twice in order to eliminate excess ligands, then recast in a suitable
solvent for solution-based deposition, such as spin-casting. For QDs with oleic acid
capping ligands, as the case in this thesis, we precipitate our QD starting solution
using butanol and then methanol (roughly 1 part butanol and 8 parts methanol to
1 part QD solution, by volume), redisperse is hexane, precipitate a second time, and
finally redisperse the QDs in chloroform. Each round of precipitation is accelerated
by centrifuging the solution at 3,500 rpm for 5 minutes.
As shall be the focus of Chapter 6, NIR-emitting QDs such PbSe and PbS are
relatively unstable and prone to oxidation and PL quenching, and accordingly, display
relatively low 𝜂PL . Pietryga et al. have demonstrated a partial cation exchange
method to controllably overcoat lead chalcogenide cores to form more stable coreshell QDs such as PbSe-CdSe, PbSe-CdSe-ZnS, and PbS-CdS [75]. As compared
to the conventional shell growth approach mentioned above, cation exchange relies
on the lability of Pb ions, which are susceptible to exchange with Cd ions when
immersed in a highly concentrated solution of cadmium oleate. Gyuweon Hwang in
Moungi Bawendi’s group used this technique to synthesise stable PbS-CdS QDs with
𝜂PL in solution of up to 53 % and emission peaks from 1.1 to 1.4 𝜇m. Synthesis
conditions were tailored to allow slower shell growth than normal by lowering the
reaction temperature to 80-100 ∘ C and reducing the Cd-precursor concentration [189].
Fig. 3-2a shows PL spectra at various times throughout the cation exchange reaction
of two batches of starting PbS QDs. The gradual blue-shifting of PL peaks during
the reaction reflects the in-growth of the CdS shell during Pb-to-Cd cation exchange,
effectively shrinking the confining PbS core. Fig. 3-2b plots 𝜂PL as a function of
shell thickness for the second batch. The 72 % rise in 𝜂PL upon addition of a shell
attests to the passivation of surface traps on PbS cores. However, this initial increase
(corresponding to a shell thickness of ∼2.5 monolayers) is followed by a gradual
decline in 𝜂PL as shell thickness increases further, attributable to a rise in defect
density resulting from lattice mismatch-induced stress [189].
Scaling up these synthesis techniques so as to reduce the cost of QDs is a pre-
Chapter 3. Colloidal Quantum-Dots
58
requisite for the commercialisation of QD technologies, and increases in yield, from
milligrams to kilograms per week, have been reported [121, 190].
3.3
Optical properties of QDs
The size-tunable colour of QD PL and absorption spectra is arguably their most
unique and important property, and is governed by the quantum size effect. See, for
example, ref. [191–193] for in-depth discussions. The intrinsic material constituting
the QD confers a bulk crystal band-gap, but in a QD, electrons and holes are subject
to quantum confinement effects that quantise these bulk energy levels, resulting in
atomic absorption- and emission-like spectra [192]. When the radius of a QD is less
than the material’s Bohr radius (𝑎B = ~2 𝜅/𝜇e2 , where 𝜅 is the dielectric constant of
the semiconductor and 𝜇 is the effective mass of the electron-hole pair) the energy
levels of a QD can be analytically approximated by modeling the QD as a particle-ina-sphere with infinite potential walls. This applies, for example, to PbS QDs, whose
exciton Bohr radius is approximately 18 nm [9], versus typical QD radii diameters of
2-5 nm. This gives a confinement energy of,
𝑒,ℎ
𝐸𝑙,𝑛
=
~2 𝜑2𝑙,𝑛
2𝑚𝑒,ℎ 𝑎2
(3.1)
where 𝜑𝑙,𝑛 is the nth root of the spherical Bessel function of order l, 𝑗𝑙 (𝜑𝑙,𝑛 ) = 0,
and 𝑚𝑒,ℎ is the effective mass of the electron or hole. This can be viewed as the
kinetic energy of a free particle, but with quantised wave vectors, 𝑘𝑙,𝑛 . Since QDs
are typically larger than the lattice constant of their constituent material, one can
apply the effective mass approximation, modeling the conduction and valence bands
as parabolic and describing particle wavefunctions as linear combinations of Bloch
functions. It is then possible to derive the energy of the electron-hole pair states,
𝐸 = 𝐸𝑔 + 𝐸 ℎ (𝑎) + 𝐸 𝑒 (𝑎) − 1.8
𝑒2
𝜅𝑎
(3.2)
The first term on the right-hand side of the equation is the fundamental band-gap of
Chapter 3. Colloidal Quantum-Dots
59
Figure 3-2: Photoluminescence spectra and quantum yield during PbS-CdS
QD cation exchange reaction. a, PL spectra of aliquots collected at representative
times with increasing CdS shell thickness (right to left) throughout two separate
cation exchange reactions. First reaction batch (dashed) shell thicknesses and reaction
times: 0 nm (i.e. PbS cores); 0.57 nm (5 min); 0.75 nm (30 min); and 0.89 nm (2
h). Second reaction batch (solid) shell thicknesses and reaction times: 0nm (i.e.
PbS cores); 0.18 nm (5 min, 80 ∘ C); 0.21 nm (5 min, 100 ∘ C); 0.69 nm (2 h); 0.8
nm; and 0.9 nm. The red-side of the red-most spectrum is clipped because of the
spectral responsivity limit of the InGaAs array detector used to collect the data. b,
PL quantum yield as a function of CdS shell thickness for the second batch (which
we make use of in Chapter 6) of QDs in chloroform solution.
Chapter 3. Colloidal Quantum-Dots
60
the QD material, 𝐸𝑔 . The two middle terms are the quantum confinement energies
of holes and electrons, respectively, defined above, which explain the size-dependent
band-gap that we observe in QDs: as a QD is reduced in size, its confinement increases, the energy of electron or hole wavefunctions increases, and electron-hole recombination will emit a higher energy (bluer) photon. The final term is a first order
Coulombic correction that accounts for the attraction between electrons and holes in
the QD.
The quantum confinement effect derived from the simple parabolic band approximation is elegantly manifested in spectral absorption measurements of QDs, wherein
the absorption band edges move to higher energies as QDs are reduced in size (Fig.33). It also accounts for the correlation between QD size and PL energy. However,
given our focus in this thesis on QD PL, it is worth noting that a more detailed
multi-band effective mass approximation is in fact required to explain some of the intricacies of QD PL spectra. These include their red Stokes shift with respect to their
first absorption peak and their unusually long radiative lifetimes (roughly an order
of magnitude longer than in the bulk [194]). These are explained by the fact that,
as mentioned in Section 2.1.2, the lowest energy exciton state of a QD is in fact an
optically inactive "Dark Exciton" state, which sits at lower energy than an optically
active "Bright Exciton" state [194]. Their splitting energy is on the order of 𝑘B T and
is therefore overcome at room temperature, effectively rendering QDs ‘phosphors’ at
this temperature.
Chapter 3. Colloidal Quantum-Dots
61
Figure 3-3: Absorption spectra of PbSe QDs as a function of nanocrystal
diameter. Reproduced from ref. [9]
Chapter 3. Colloidal Quantum-Dots
62
Chapter 4
Origin of efficiency roll-off in
QD-LEDs
4.1
Role of QD luminescence in QD-LED efficiency
Much of the focus of QD-LED research over the past two decades has been on understanding and improving the efficiency of free carrier recombination in these devices,
𝜂r . This is reflected in the engineering of the multiple generations of QD-LED architectures described in Chapter 2, and is of course motivated by the direct dependence
of EQE on 𝜂r by way of Eq. 2.1. This expression also shows that EQE is directly proportional to the photoluminescence (PL) quantum yield (𝜂PL ) of our QDs, yet this
factor has received less attention. In particular, while there have been substantial
efforts to elucidate the mechanisms of PL quenching in experimentally isolated QDs
(for example, single QDs or QD ensembles) and QDs under very specific experimental
conditions (for example, in electrochemical cells), much less scrutiny has been paid to
QD PL quenching in operational QD-LEDs; accounting for their unique in situ environment, which includes neighbouring conductive layers, large electric fields, charge
current flow, and the associated device heating. We therefore begin by looking to
the earlier literature of isolated QDs to identify the different processes that may be
suspected to dictate the probability of QD PL in QD-LEDs. In this chapter and the
next, we limit our scope to the bias dependence of QD-LED efficiency and investigate
63
Chapter 4. Origin of efficiency roll-off in QD-LEDs
64
the contribution of QD PL quenching on this behaviour. Zero-bias (occurring even
at zero bias) contributions to PL quenching will be the focus of Chapter 6.
4.2
Overview
Even casual inspection of the typical EQE-versus-current density (or voltage) plot of
a QD-LED reveals a pronounced decline in EQE (Fig. 4-1c): from its maximum value
at relatively low currents (∼ 0.01 to 0.1 A cm-2 at ∼ 2 V) to roughly half this value as
current increases to ∼ 1 A cm-2 at ∼ 8 V. This so-called efficiency ‘roll-off’ or ‘droop’
in fact plagues all LED technologies to date, yet until now, it has not been studied in
QD-LEDs. Understanding its cause is essential to developing high-brightness, highefficiency devices. Does a bias-driven reduction in QD 𝜂PL account for this efficiency
roll-off in QD-LEDs? If so, what mechanism(s) are responsible for this quenching?
These are the questions we set out to address in this chapter and the next.
4.3
Introduction
Nitride-based LEDs find wide and growing commercial application, and OLEDs and
QD-LEDs have the potential to capitalise on the vast solid state lighting (SSL) and
display markets. For all lighting technologies, however, efficiency roll-off presents a
considerable challenge. Indeed, roll-off is among the most significant and enduring
hurdles facing GaN-based LEDs and SSL, severely limiting the efficiencies achievable
with high-brightness devices. Today’s commercial blue GaInN LEDs, for example,
often suffer more than a 40 % loss in EQE at desired high current densities (∼35
A cm-2 ) compared to their peak EQEs at <10 A cm-2 [195] (Fig. 4-1a). Roll-off
raises SSL costs by requiring the use of multiple LEDs in a single lamp to keep efficiencies high. It adds constraints on optics and design, and it increases thermal
loads [196]. All major LED companies reportedly have active ongoing research into
roll-off effects [197]. According to the U.S. DOE’s latest assessment, addressing rolloff is among the highest priorities for LED-based lighting R&D [31]. Roll-off has also
Chapter 4. Origin of efficiency roll-off in QD-LEDs
65
been the topic of substantial investigation in efficient phosphor-based OLEDs over
the past two decades. Like QD-LEDs, OLED efficiencies fall rapidly beyond ∼ 0.01
to 0.1 A cm-2 (Fig. 4-1b). In a comprehensive review of white OLEDs, Reineke et
al. recently proposed that the largest improvement factor needed for enhancing the
efficiencies of this technology may come from reducing roll-off [198]. Importantly,
the exciton annihilation processes that have been found to be responsible for roll-off
in OLEDs (outlined briefly below) may also be a source of material degradation, because the associated high excitation energies may break the chemical bonds conferring
functionality to some organic materials [199].
Despite all of this, confirming the origins of efficiency roll-off in GaN-based LEDs
continues to be an active area of experiment and theory. Various mechanisms have
been proposed, most prominently thermionic electron leakage from the light-emitting
active region and Auger recombination (described in the next chapter) within this region, but also carrier delocalisation and poor hole injection [31, 195, 202]. Meanwhile,
efficiency roll-off in OLEDs has been attributed to mechanisms including triplettriplet annihilation, triplet-polaron quenching, and electric field-induced exciton dissociation [198, 203, 204]. Among these three, the first two (especially the former) are
considered to play larger roles [201].
Surprisingly, efficiency roll-off and its origins have never previously been investigated closely in QD-LEDs. To do so, we begin by reexamining the expression for
EQE in Eq. 2.1. Based on our physical understanding of each term in the equation, as well as inspection of the commonly cited mechanisms in the GaN and OLED
literature, discussed above, we expect only two of the terms, 𝜂PL and 𝜂r , to have
a strong bias-dependence (we neglect outcoupling efficiency, which may strictly also
have some bias dependence, for example due to dipole reorientation under applied
electric fields). That is, EQE (V) = 𝜂r (V)𝜒𝜂PL (V)𝜂oc ∝ 𝜂r 𝜂PL . Efficiency roll-off can
therefore be approximated as attributable to the bias dependence of 𝜂PL , of 𝜂r , or of
some combination of the two. Falling 𝜂PL would signify changes to the QD emitter
itself, and the numerous potential mechanisms responsible for bias-driven QD PL
quenching will be discussed in Chapter 5. In contrast, a reduction in 𝜂r would re-
Chapter 4. Origin of efficiency roll-off in QD-LEDs
66
Figure 4-1: Efficiency roll-off in different LED technologies. External quantum
efficiency (EQE) is seen to fall dramatically at high current densities in: a, GaNbased LEDs [200]; b, OLEDs [201]; and c, QD-LEDs (a typical red-emitting device
fabricated in our laboratory and investigated in this thesis). Note that a exhibits
roll-off due to both current density and thermal stress, a distinction discussed briefly
in Chapter 5.
Chapter 4. Origin of efficiency roll-off in QD-LEDs
67
sult from one or more forms of carrier leakage, such as those for GaN-based LEDs
above, whereby electrons and/or holes are not confined to the QD region and leak
through their respective blocking layers in a QD-LED. Unsurprisingly, distinguishing
between 𝜂PL and 𝜂r as the origin of EQE bias-dependence in GaN-based LEDs and
OLEDs has been key to uncovering the mechanisms discussed above. For example, in
GaInN/GaN multi-quantum well (MQW) LEDs, Kim et al. measured EQE and PL
as a function of excitation power and found no correlation, concluding that roll-off
must be due to recombination outside of the MQWs [205]. They deduced that electron
leakage was instead the dominant cause of roll-off. In contrast, Reineke et al. [204]
and Kalinowski et al. [206] both observed good correlations between EL and PL in
OLEDs, leading to the advancement of models for roll-off based on PL quenching via
triplet-triplet annihilation. In this chapter, we take a similar approach to quantifying
the contribution of PL quenching (versus carrier leakage) to roll-off in QD-LEDs.
4.4
Methods
The device structure we investigate is a type-IV QD-LED (see Section 2.2) with
organic-inorganic hybrid charge transport layers that has recently attracted attention
owing to its record high EQEs and brightness [2]. This red-emitting QD-LED is also
more conveniently compatible with the visible-wavelength spectroscopic tools at our
disposal, which we make use of in this chapter and the next. The device was fabricated
on a glass substrate coated with indium tin oxide (ITO) and has the structure: ITO
(150 nm) / zinc oxide (ZnO) (50 nm) / QDs (50 nm) / 4, 4-bis(carbazole-9-yl) biphenyl
(CBP) (100 nm) / molybdenum oxide (MoO3 ) (10 nm) / aluminium (Al) (100 nm).
ZnO was radio-frequency sputtered, QDs were spin-cast out of chloroform, and CBP,
MoO3 , and Al were thermally evaporated. We used CdSe-ZnCdS core-shell QDs with
a peak PL wavelength of 610 nm, provided by QD Vision, Inc. The energy band
diagram of the device is shown in the inset of Fig. 4-4 and is based on literature
values [58, 93, 207, 208].
Based on our discussion in the previous section, the decrease in EQE at high
Chapter 4. Origin of efficiency roll-off in QD-LEDs
68
biases may be the result of either charge carriers leaking out of the QD layer or a
reduction in QD luminescence efficiency. To identify which of these two mechanisms
dominates, we perform a simultaneous EL-PL experiment and monitor the relative
PL efficiency of the QDs as the device bias is swept. The experimental setup is shown
in Fig. 4-2. To isolate the PL contribution from total luminescence, we modulate
the PL excitation source (𝜆 = 530 nm LED) at 1 kHz and send the combined ELPL signal (collected using a Si photodiode and a current preamplifier) to a lock-in
amplifier. The PL intensity is intentionally kept low (PL/EL <0.001 % at 13 V) to
avoid significantly increasing the charge density within the QD layer. An excitation
wavelength of 530 nm ensures that the QD layer is excited without exciting the
surrounding wider band gap charge transport layers, as absorption spectra of the
QD-LEDs constituent materials confirm (Fig. 4-3).
4.5
Results and discussion
Current density and normalised EQE measured for a typical device as described above
are shown in Fig. 4-4a. The EQE peaks at 2 % for 4 V applied bias and rolls off by 50
% by 8 V, quite typical of a modern QD-LED. The results of the simultaneous EL-PL
experiment are shown in Fig. 4-4b, with EQE and QD PL intensity normalised at 4
V applied bias. Below 4 V, the PL intensity remains constant (voltage independent).
In contrast, above 4 V, the PL intensity decreases monotonically with increasing
bias, tracking the decrease in EQE of the QD-LED. The correspondence between
the decreasing PL intensities and EQE with applied bias identifies the change in the
QD 𝜂PL to be sufficient to explain the QD-LED roll-off behavior. Importantly, this
is the first time it has been demonstrated that efficiency roll-off in QD-LEDs can
be ascribed to the QD emitters themselves, allowing us to discount extrinsic charge
leakage effects as the cause, and to focus our investigation specifically on bias-driven
QD PL quenching mechanisms.
To verify that the optically generated excitons used to probe our QD-LED do
not interfere with the device’s intrinsic exciton dynamics, we conduct the same PL
Chapter 4. Origin of efficiency roll-off in QD-LEDs
69
Figure 4-2: Simultaneous EL-PL measurement setup. Relative EL intensity is
collected by a photodetector mounted in front of the biased QD-LED. Simultaneously,
QD PL is induced by excitation of the QD-LED with 530 nm light from an LED
pulsed at 1 kHz passed through a microscope objective. The resulting combined EL
and PL signal is collected by another photodetector and fed into a lock-in amplifier
that separates the EL (d.c.) and PL (a.c.) components, enabling relative PL intensity
to be isolated.
Chapter 4. Origin of efficiency roll-off in QD-LEDs
70
Figure 4-3: Absorption spectra of QD-LED components. Absorption spectra
of CdSe-ZnCdS QDs (red), CBP (dark blue), and ZnO (light blue) confirm that at
the excitation wavelength of 530 nm, only the QDs absorb significantly.
measurements at multiple excitation intensities, from 15.6 𝜇W to 44.5 𝜇W (Fig. 45). Normalising each PL signal by its excitation intensity, we confirm that the PL
quenching trend is independent of this variable.
In the context of the discussion in Section 4.3, unlike some GaN-based LEDs
but similarly to some OLEDs, we therefore conclude that we can neglect charge
leakage effects in accounting for efficiency roll-off in QD-LEDs. We note that another
possibility - that roll-off could be ascribed to permanent or semi-permanent thermal
degradation of our device - can be neglected given the reproducibility of the above
experiment when repeated on a one second timescale. In the next chapter, we probe
the mechanistic origins of the bias-driven QD PL quenching deduced here.
Chapter 4. Origin of efficiency roll-off in QD-LEDs
71
Figure 4-4: EQE roll-off in a QD-LED and correlation with QD PL. a, Current
density-voltage and EQE-voltage characteristics of the QD-LED under investigation.
Inset: Energy band diagram of the device, with indicated energy values referenced
to the vacuum level. b, Simultaneous EQE and QD PL intensity of the QD-LED
(normalised at 4 V, when the peak EQE is 2 %) as a function of voltage. Roll-off of
the EQE above 4 V is seen to reflect reduced QD PL efficiency at high biases.
Chapter 4. Origin of efficiency roll-off in QD-LEDs
72
Figure 4-5: Bias-induced QD PL quenching, normalised by optical excitation
intensity. Relative QD PL intensities under 530 nm excitation at multiple intensities
confirms the excitation probe does not disturb the device’s intrinsic exciton dynamics:
(black) 15.6 𝜇W; (dark grey) 33.4 𝜇W; (light grey) 44.5 𝜇W.
Chapter 5
Bias-driven QD luminescence
quenching in QD-LEDs
Our measurements of visible-emitting QD-LEDs in Chapter 4 have identified biasdriven QD photoluminescence (PL) quenching to be responsible for the roll-off in
efficiency of our QD-LEDs at high current densities. In this chapter, we conduct
in situ measurements of QD-LED electroluminescence (EL) and PL spectra, and by
comparing the two, reveal that strong electric fields are responsible for the reduced QD
PL quantum yield (𝜂PL ). The quantum confined Stark effect is found to consistently
explain the observed phenomena.
5.1
Bias-dependent QD quenching mechanisms
Potential mechanisms responsible for bias-dependent QD PL quenching are encapsulated by Eq. 5.1, which expresses QD 𝜂PL (and therefore external quantum efficiency,
EQE) in terms of radiative (𝑘r ) and non-radiative (𝑘nr ) recombination rates:
EQE(V) ∝ 𝜂PL (V) =
𝑘r (V)
𝑘r (V) + 𝑘nr (V)
(5.1)
We have explicitly reflected the potential bias-dependence (V) of each term. First
consider 𝑘nr . As we shall see in Chapter 6, one contribution to the non-radiative
73
Chapter 5. Bias-driven QD luminescence quenching in QD-LEDs
74
Figure 5-1: Heat-induced QD PL quenching. PL spectra of a CdSe-ZnCdS QD
spin-cast thin film at temperatures ranging from room temperature up to 100 ∘ C.
recombination rate comes from surface traps and the local environment (Fig. 5-2,
left). But trap-assisted quenching may also be exacerbated under bias as the device
heats up, for example due to thermally-activated charge transfer from a QD core to a
defect state. Heat-induced PL quenching can be seen by monitoring the PL spectrum
of a film of our QDs on a temperature-controlled substrate. As plotted in Fig. 51, a temperature rise of just a few tens of degrees Celsius is enough to cause QD
𝜂PL to begin to fall. Indeed, the impact of thermal efficiency droop on the EQE of
GaN-based LEDs can be as detrimental as that of current density-related efficiency
droop [200] (Fig. 4-1a).
Studies of single QDs and QD ensembles also demonstrate another bias-dependent
contribution to 𝑘nr ; namely, free charge carriers (Fig. 5-2, middle). For example, with
current densities in QD-LEDs routinely exceeding the order of 1 A cm-2 , imbalanced
charge injection can lead to charge accumulation on a QD, increasing Auger nonradiative recombination, whereby an exciton dissipates its recombination energy as
Chapter 5. Bias-driven QD luminescence quenching in QD-LEDs
75
kinetic energy to a third charge rather than as a photon [6, 12, 67, 87, 136, 138, 209].
Because the rate of Auger recombination is orders of magnitude faster than the rate of
radiative recombination (few to hundreds of picoseconds, versus nanoseconds), PL is
completely quenched in charged QDs. Over the years, numerous specific mechanisms
by which Auger recombination proceeds in QDs have been shown. These include
coupling between an exciton and a carrier that is in a QD [12,209], on its surface [12],
or in a deep trap state [210], coupling between multiple charge-neutral excitons, such
as biexcitons and triexcitons [209], and coupling between a carrier and an exciton
with one of its charges in a deep trap state [211].
Application of bias in a QD-LED also imposes an electric field across the QD film
on the order of 1 MV cm-1 , providing the third mechanism for PL quenching shown in
Fig. 5-2. The electric field spatially polarises the electron and hole wavefunctions of
the exciton, |𝜓e ⟩ and |𝜓h ⟩, respectively, to opposite hemispheres of the QD, reducing
their overlap integral. As a result, 𝑘r , which is proportional to the optical matrix
element of the electron and hole states (𝑘r ∝ |⟨𝜓e | p |𝜓h ⟩|2 , where p is the momentum
operator), is reduced by the field [11]. From Eq. 5.1, a decrease in 𝑘r can therefore
reduce 𝜂PL and with it, EQE. These polarisation effects are also seen, for example, in
InGaN LEDs, where piezoelectric and spontaneous polarisation leads to large electrostatic fields (> 1 MV cm−1 ) in the quantum well active regions, spatially separating
electron and hole wavefunctions and reducing 𝑘r . Interestingly though, this is not
thought to be the cause of efficiency roll-off, but rather is suspected as (partly) responsible for the ‘green-yellow gap’ in LED efficiencies [39, 41]. Additionally in QDs,
the shift of the electron and hole wave functions from the QD’s center breaks its local
neutrality and increases the exciton’s interaction with polar optical phonons. This
interaction leads to significant phonon broadening of the PL line [213]. A closely
related characteristic of the electric field’s effect is the quantum confined Stark Effect
(QCSE) [214], which is a shift in the bound state energies of the QD leading to a
change in the effective band-gap of the material. Described in terms of second order
perturbation theory, the shift in effective band-gap of a QD in the presence of an
electric field, E, is given by [214],
Chapter 5. Bias-driven QD luminescence quenching in QD-LEDs
76
Figure 5-2: Zero-bias and bias-driven QD PL quenching mechanisms.
Schematic of an archetypal QD-LED under bias, with an electric field across the
QD film and free charge carriers present. Even at zero bias, excitons in QDs are exposed to PL quenching pathways such as (box, leftmost) relaxation via surface defect
states and (box, second from left) energy or charge transfer to/from neighbouring
trap states (represented by the blue arrow), for example in conductive charge transport layer materials. As bias increases, PL quenching by free charge carriers also
becomes possible via Auger recombination (box, second from right). In these three
cases, QD 𝜂PL is reduced due to an increase in the non-radiate recombination rate,
𝑘nr . However, according to Eq. 5.1, 𝜂PL can also be reduced due to a decrease in
𝑘r , and this can occur under exposure to an electric field that polarises the exciton
(box, rightmost). Alternative processes due to an electric field could include exciton
dissociation, phonon coupling, or coupling with surface state defects. In these cases
quenching would again occur, but due instead to an increase in 𝑘nr . Adapted from
ref. [212]
Chapter 5. Bias-driven QD luminescence quenching in QD-LEDs
∆𝐸𝑛 = ⟨𝜓n | 𝑒r · E |𝜓n ⟩ +
∑︁ |⟨𝜓k | 𝑒r · E |𝜓n ⟩|2
𝑘̸=𝑛
𝐸𝑛 − 𝐸𝑘
77
(5.2)
or, in simplified form,
1
∆𝐸𝑛 = 𝜇𝐸 + 𝛼𝐸 2
2
(5.3)
where 𝜇 is the projection of the dipole moment of the QD in the direction of the
electric field, and 𝛼 is the QD’s polarisability in the same direction. In QD ensembles,
𝜇 averages to zero and one observes a quadratic red-shift in QD PL with increasing
electric field [214].
However, a reduction in QD 𝜂PL by electric field is not necessarily dominated by
the reduction in 𝑘r described above [215]. An electric field may also reduce 𝜂PL by
enhancing phonon coupling [213], exciton coupling to surface trap states [12, 216], or
exciton dissociation [135,164,217–220]. In all three cases, 𝜂PL is reduced by an increase
in 𝑘nr . For example, the dissociation of excitons can occur when the Coulombic
binding strength of the electron-hole pair is overcome by the polarising electric field,
causing one of the charges to be ionised out of the QD [221]. The expected (roughly
exponential) field-dependence of the exciton dissociation rate given by the OnsagerBraun model would result in a proportional increase in 𝑘nr [11]. Analogously, one
prominent explanation for roll-off in nitride-based LEDs is that polarisation fields
drive electron leakage out of the active region of these devices [39, 205].
In this chapter, we seek to determine which of the three bias-dependent quenching
mechanisms - heating, Auger recombination, or electric-field-inducing quenching - is
primarily responsible for the QD PL quenching found to be responsible for EQE
roll-off in the previous chapter.
Chapter 5. Bias-driven QD luminescence quenching in QD-LEDs
5.2
Electric field-induced quenching in QD-LEDs
5.2.1
Introduction
78
As discussed above, reduction of PL efficiency in QD thin films has been previously
measured when QDs are heated [222], charged (Auger recombination) [219], or placed
under a strong electric field [216,217]. We preliminarily eliminate temperature effects
on QD 𝜂PL , as measurement of the operating temperature of our QD-LEDs with an
infrared camera shows a change of no more than a few degrees (∼5-6 ∘ C rise over
∼10 seconds at 0.1 A cm-2 , corresponding to ∼8 V, as shown in Fig. 5-3). Consulting
Fig. 5-1, this temperature rise is not sufficient to affect 𝜂PL enough to explain the
roll-off, especially on the short timescales (on the order of a second) of each of our
measurements. In QD-LEDs, like in OLEDs, the device temperature will likely rise
by several tens of degrees Celsius, but this temperature rise has a time constant on
the order of tens to hundreds of seconds [223] - incompatible with the fast, repeatable
nature of our measurements. Tentatively speaking, charging effects also seem less
than likely because QDs generally have long charge retention times (on the order of
minutes to hours [219]), whereas the efficiency roll-off curve (Fig. 4-4) is measured
within seconds and was reproduced multiple times successively, with the peak EQE
unchanged. Moreover, we consistently observe a red-shift in EL with increasing biases
applied to our QD-LEDs (Fig. 5-4), qualitatively consistent with the red-shift one
would expect from a QCSE. From these observations, we hypothesise that it is the
electric field associated with the applied bias that is quenching the QD PL at high
voltages and set about testing this hypothesis.
5.2.2
Methods
To characterize the QCSE in our QD-LED structure, we first measure the PL spectra
of the QD films in our devices under reverse bias. The experimental setup is shown
in Fig. 5-5b. Reverse biasing allows the effect of electric field on the QDs in the
QD-LED to be studied in situ, in absence of any charge injection. To avoid damaging
Chapter 5. Bias-driven QD luminescence quenching in QD-LEDs
79
Figure 5-3: Thermal image of QD-LED under constant current bias. Collected with an infrared camera, the thermal image shows the temperature distribution
across a half-inch substrate (delineated by a large grey box) resulting from biasing of
a single QD-LED pixel (1.21 mm2 , small grey box) at 0.1 A cm-2 for ∼10 seconds.
Figure 5-4: QD-LED EL spectra. Normalised spectra of QD-LED EL biased at:
(black) 2 V; (orange) 5 V; and (red) 8 V exhibit a gradual red-shift, qualitatively
consistent with the quantum confined Stark Effect.
Chapter 5. Bias-driven QD luminescence quenching in QD-LEDs
80
the device by prolonged reverse biasing, we apply sawtooth-like voltage waveforms
(Fig. 5-5a) with a 500 Hz repetition rate. The sawtooth amplitude peaks at -18 V
and is followed by a duration of positive voltage (1.6 V) to reduce stress on the device
by minimising the average net applied voltage, but without turning on EL. QD PL
is induced by a 530 nm wavelength LED emitting 100 𝜇s long pulses synchronised
with the voltage waveform and QD PL spectra are collected with a spectrometer.
By sweeping the time delay (phase shift) between the voltage waveform and the
illumination pulse, PL spectra of the QDs under different electric-field strengths can
be collected while keeping all other conditions unchanged. To assess the degree to
which the QCSE occurs while the QD-LED is in operation, EL spectra are monitored
as the device is forward biased, again with all other experimental variables held
constant. The experimental setup for this measurement is shown in Fig. 5-6. This
combined approach allows the study of QD PL and EL from the same active device
structure.
5.2.3
Results and discussion
The resulting PL and EL spectra are normalized and their peak emission energies are
compared. Fig. 5-7 shows EL spectra obtained at 5, 11.6, and 13.8 V overlaid with PL
spectra with coincident peak energies (PL at 1.6, -8.6, and -16 V, respectively). Both
PL and EL spectra are approximately Gaussians at low biases and redshift at higher
biases. However, EL does not exhibit the same spectral broadening that is observed
in the PL. In particular, a shoulder begins to appear on the low energy side of the
PL spectra. The inset of each panel in Fig. 5-7 shows a double-Gaussian fit to each
asymmetrically broadened PL spectrum. We attribute the double-Gaussian profile
to emission from QD subpopulations that are subject to two different environments;
for example, a layer of QDs next to ZnO and a layer of QDs away from ZnO. QDs
placed adjacent to the ZnO are expected to exhibit energy levels that differ from that
of QDs placed adjacent to the CBP, which has a lower dielectric constant [224]. The
difference between EL and PL spectra (even when the peaks are matched) can be
explained by the fact that the electric-field distribution is generally different between
Chapter 5. Bias-driven QD luminescence quenching in QD-LEDs
81
Figure 5-5: QD PL electric field-dependence measurement technique. Reverse biasing our QD-LED allows measurement of PL spectra of QDs in situ in our
devices, exposed to varying electric fields in absence of charge injection. a, The relative timing (phase) of the 530nm LED optical excitation square wave pulse (green)
and a reverse bias sawtooth-like voltage waveform (black), synchronised at 500 Hz
repetition rate, determine the magnitude of the field at which each PL spectrum is
recorded by the spectrometer setup shown in b.
Chapter 5. Bias-driven QD luminescence quenching in QD-LEDs
82
Figure 5-6: QD-LED EL measurement setup. EL spectra from our QD-LED are
measured as increasing increments of forward bias are applied.
Chapter 5. Bias-driven QD luminescence quenching in QD-LEDs
83
forward and reverse biased diodes [225], thus affecting the two populations differently.
Each PL and EL spectrum is decomposed into two Gaussians, and their intensities
and peak energies as a function of device voltage are shown in Figs. 5-8a and b,
respectively. The black solid line in Fig. 5-8a is a fit to the PL intensity data
assuming a simplified version of the model described in Ref. [216]. In Fig. 5-8b, PL
and EL peak energies for both subpopulations redshift under high bias. In particular,
the PL peaks of subpopulation A show a quadratic dependence on voltage (black fit),
which is a signature of the QCSE. There is no clear fit for the EL peak shift because
the distribution of the electric field inside the diode is voltage dependent. The peak
energies are maximum when the device is slightly forward biased, indicating the
presence of a built-in electric field, which is expected in a diode structure.
Assuming that the EQE of the QD-LED is predominantly governed by the QCSE
at high forward biases, we should be able to predict the EQE by comparing the
forward-bias EL to the reverse-bias PL from Fig. 5-8a and b. A QD film exposed
to an electric field will undergo a QCSE, which is manifested as a shift in the QD
PL or EL emission spectra and a concomitant decrease in its PL or EL efficiency.
Because the emission spectrum is a function of the applied field, whenever the forwardbias QD EL emission spectrum of subpopulation A (subpopulation B) matches the
reverse-bias QD PL spectrum of the same subpopulation, the QDs in subpopulation A
(subpopulation B) are experiencing the same local electric field under those particular
EL and PL biasing conditions. Therefore, for each subpopulation, the EL efficiency
at each forward bias can be predicted by finding the corresponding PL spectrum in
reverse bias with peak energy matching that of the EL peak, and assigning the PL
efficiency at that electric field to the EL efficiency. We emphasise that the choice of
physical model used to fit the data in Fig. 5-8a does not affect the predicted EQE,
which is calculated directly from the PL and EL spectra and the corresponding PL
intensities.
For example, subpopulation A (Fig. 5-8b, red) shows an EL peak shift from 1.990
to 1.984 eV between 5 and 10 V. This shift corresponds to a PL peak shift from -4.5
to -11.3 V and indicates that the luminous efficiency is reduced by about 37 % (Fig.
Chapter 5. Bias-driven QD luminescence quenching in QD-LEDs
84
Figure 5-7: Comparison of QD PL spectra (black lines) and QD EL spectra (orange diamonds) at corresponding peak emission energies, for three
different biases. At high biases, the PL spectra exhibit a red shoulder that is not
observed at lower biases or in the EL spectra. Insets: PL spectra (black) are reconstructed (green) using two Gaussians, which correspond to emission from two QD
subpopulations, A and B (red and blue, respectively).
Chapter 5. Bias-driven QD luminescence quenching in QD-LEDs
85
Figure 5-8: Electric field-dependence of QD-LED PL and EL. a, Relative
intensities of subpopulations A (red) and B (blue). The PL data are fitted to a simplified version of the model presented in ref. [216]. b, Peak energies of subpopulation
A (red) and subpopulation B (blue). Quadratic fits (black lines) to the PL data are
made assuming that the shifts are due to the quantum confined Stark effect.
Chapter 5. Bias-driven QD luminescence quenching in QD-LEDs
86
Figure 5-9: Measured EQE and predicted EQE as a function of voltage. EQE
is predicted through the comparison of PL and EL data (Fig. 5-8) as described in the
text. The agreement between the data and the prediction shows that the quantum
confined Stark effect can self-consistently account for the QD-LED efficiency roll-off.
5-8a) for subpopulation A as a result of the QCSE. The relative number of excitons
formed on the two subpopulations (A and B) of QDs is calculated by dividing their
EL intensities in Fig. 5-8a by their respective PL efficiencies. The overall EQE is
then the weighted average of the PL efficiencies of the two subpopulations. This
analysis is applied to EL data between 2.5 and 14 V and the resulting predicted
EQE, which is scaled to match the maximum of the measured EQE, is shown in Fig.
5-9. Predicted and measured EQEs are in good agreement, with EQE rolling off by
up to 40 % at 13 V. The match between the EQE behavior predicted by the QCSE
and the experimentally observed efficiency roll-off is evidence that the electric field
strength alone - and not carrier leakage or QD charging (Auger recombination) - is
sufficient to model the efficiency roll-off.
To further understand the effect of the electric field on the QD PL efficiency,
we measured transient PL of the QDs in the QD-LED. Based on our discussion in
Section 5.1, it may be possible to distinguish between different possible microscopic
Chapter 5. Bias-driven QD luminescence quenching in QD-LEDs
87
mechanisms behind electric field-induced quenching in our QD-LEDs by observing
whether the field causes an increase in 𝑘nr or a decrease in 𝑘r . Reduction of QD
𝜂PL has previously been attributed to a decrease in radiative exciton recombination
rate (e.g. reduced electron-hole wave function overlap [226, 227]), an increase in
non-radiative exciton recombination rate (e.g. exciton dissociation [221, 228]), or a
decrease in the probability of forming thermalized excitons (e.g. hot charge carrier
trapping by QD surface traps [12, 13]).
We perform this measurement using the same reverse biasing scheme as in Fig.
5-5a, but with a 100 ps laser pulse train at 540 nm wavelength replacing the green
excitation LED. PL was detected with a Si avalanche photodiode and timing information was obtained via a time-correlated single photon counting module. The resulting
transient PL at four different voltages reveals a lifetime of 4 ns for all of the voltages
applied while the initial intensity decreases with higher applied voltage (Fig. 5-10).
The inset indicates the times at which the QD PL intensity I has decreased from its
initial value of 𝐼0 so that 𝐼/𝐼0 = 𝑒−1 and 𝐼/𝐼0 = 𝑒−2 (𝜏𝑒−1 and 𝜏𝑒−2 , respectively).
Because 𝜂PL of our QD film is 8 %, PL lifetime is dominated by the non-radiative rate.
Therefore, the voltage independent PL lifetime observed suggests that the cause is either a decrease in the radiative rate or a decrease in the thermalized-exciton formation
efficiency.
5.2.4
Conclusion
In conclusion, we have identified the electric field-induced PL quenching of QDs to be
responsible for the efficiency roll-off in QD-LEDs. We use the relationship between
PL peak shifts and PL quenching of QDs subject to the QCSE - observed while
reverse biasing a QD-LED - to predict the efficiency roll-off in forward bias. The rolloff predicted by this analysis is in excellent agreement with our experimental data
and correctly traces an EQE reduction of nearly 50 %. Transient PL measurements
suggest that the reduced QD luminescence efficiency is not the result of an increased
non-radiative recombination rate.
This was the first study offering detailed insights into the efficiency roll-off in
Chapter 5. Bias-driven QD luminescence quenching in QD-LEDs
88
Figure 5-10: Transient PL of QDs under electric fields in a QD-LED. Timeresolved PL spectra of QDs in our QD-LED, reverse biased at 0, -8, -12, and -16 V.
Time constants of the decays (inset) are independent of the applied voltage, suggesting
that the non-radiative exciton recombination rate is independent of the electric field.
Chapter 5. Bias-driven QD luminescence quenching in QD-LEDs
89
QD-LEDs - a must for designing high-brightness QD-LEDs. It demonstrated definitively the contribution of electric field-induced QD PL quenching on QD-LED EQE,
a conclusion that has since been corroborated [11]. Subsequently, others have reported evidence of roll-off dominated instead by Auger recombination in their QDLEDs [6,229,230]. While this apparent dichotomy has yet to be explicitly resolved in
detail, we hypothesise that it can probably be broadly interpreted as a manifestation
of variations in the electronic confinement of the QDs used in different experiments
by different groups. By "electronic confinement", we mean the degree of spatial confinement of an exciton’s electron and hole wavefunctions within the QD’s core. It is
dictated by the energy levels of the valence and conduction bands of the core relative
to the shell. For example, whereas overcoating a CdSe core with a ZnS shell tightens an exciton’s confinement in the core, overcoating with a CdS shell reduces the
confinement [231]. In the case of CdSe-ZnS QDs, the Type-I energy offset between
CdSe and ZnS presents large barriers to both electrons and holes, confining both to
the core. In contrast, the quasi-Type-II band diagram of CdSe-CdS constrains the
hole to the CdSe core while presenting only a small energetic barrier to delocalisation
of the electron into the shell.
It has been shown that PL quenching by multicarrier Auger recombination is
very significant in highly confined QDs because strong spatial confinement promotes
carrier-carrier interactions, and that it is reduced in less confined QDs [232, 233].
As an analogous form of ‘confinement’, it has been seen experimentally [229, 233]
and theoretically [234] that ‘alloyed’ QDs (graded compositions of cores or of coreshell interfaces) negate Auger recombination due to a ‘smoothing’ of confinement
potentials. These confinement effects were shown to affect the roll-off of some QDLEDs [6, 229, 230]. But there is a tradeoff between mitigation of charge- and electric field-induced PL quenching in QDs. Lower confinement simultaneously enables
electric fields to more easily polarise the exciton wavefunction [11], a phenomenon
particularly evident along the long (low-confinement) axis of nanorods, which exhibit
pronounced Stark shifts in electric fields [235–237]. This tradeoff is illustrated in Fig.
5-11, which organises the different QD PL quenching mechanisms discussed above in
Chapter 5. Bias-driven QD luminescence quenching in QD-LEDs
90
Figure 5-11: Influence of QD electronic confinement on bias-driven PL
quenching mechanisms. Schematic shows the qualitative dependence of electric
field-induced quenching and Auger recombination on QD electronic confinement. The
example band diagrams show that whereas confinement increases in the Type-I energy
alignment of CdSe-ZnS QDs, it is decreased by the quasi-Type-II offsets in CdSe-CdS
QDs. Adapted from ref. [212]
terms of the degree of electronic confinement. It is likely that this tradeoff may be
responsible for the attribution of roll-off to electric field by some and to charging by
others. For example, observations of charge-induced roll-off in QD-LEDs are often
based on devices employing QDs with Type-I band alignments [6], which are therefore
more susceptible to Auger recombination than electric field effects. This is not always
the case [229], however. Additional, more systematic investigations to correlate QD
chemistry (which may, without intention, vary subtly but importantly with synthesis
procedures) with roll-off mechanisms are needed, as discussed in Section 7.2.
Chapter 6
Zero-Bias QD Luminescence
Quenching in Infrared QD-LEDs
Chapters 4 and 5 examined the effect on QD-LED performance of in situ QD photoluminescence (PL) quenching that occurs as a function of applied bias. In this chapter,
we turn our attention instead to zero-bias (occurring even at zero bias) contributions
to PL quenching and QD-LED performance. By chemically passivating the surface
of near-infrared (NIR)-emitting QDs, we are able to significantly mitigate in situ PL
quenching and realise devices of record efficiency.
6.1
Bias-independent QD quenching mechanisms
In the previous chapters, we have seen how bias can induce quenching both by a
reduction in the radiative recombination rate of QDs (𝑘r ) and by an increase in their
non-radiative rate (𝑘nr ), per Eq. 5.1. At zero bias, past QD spectroscopy work and
a limited number of QD-LED studies suggest that the primary contribution to PL
quenching is by way of an increase in 𝑘nr , via electronic trap states on QDs and their
neighbouring environment (Fig. 5-2, left). For example, if a defect state exists on the
surface of a QD, trap-assisted non-radiative recombination can occur, whereby either
the electron or hole of an exciton relaxes to the trap state, before recombining with the
other carrier without photon emission [238]. (One specific bias-dependent instance of
91
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
92
this, relevant to quenching in operating devices, is heat-assisted quenching, whereby
heat excites a carrier out of the QD into a surface trap state [222, 239]. This was
discussed in Section 5.1.) A diversity of non-radiative interactions between excitons
in QDs and neighbouring materials is also possible in QD-LEDs. For example, nonradiative energy transfer of an exciton to surface plasmon modes or non-radiative
mid-gap states in adjacent transport layers are possible quenching pathways that
will increase 𝑘nr [139] (Fig. 5-2). Exciton dissociation at charge transport layer
(CTL)/QD interfaces is another possibility [207, 240, 241]. We note that the presence
of highly conducting substrates can result in zero-bias Auger recombination (discussed
in Chapter 5) due to equilibrium charge transfer [242], but this is unlikely to occur
with typical QD-LED materials.
In thinking about how materials engineering may help characterise and control PL
quenching processes, it is instructive to think about the influence of QD composition
[212]. In addition to the electronic confinement effects depicted in Fig. 5-11 in Section
5.2.4, we can also expect the extent of surface passivation of a QD to impact the degree
of trap-assisted non-radiative recombination, represented by an additional axis in Fig.
6-1. By "surface passivation", we refer to both: the chemical passivation of QD surface
defect states (for example, by treatment with passivating ligands); and the physical
separation of an exciton in a QD core from surface trap states [71, 120] and from
the QD’s neighbouring environment (again, using long-chain organic ligands and, for
example, by overcoating QD cores with a shell material). Several chemical families of
ligands have been tailored to reduce surface defect densities while improving charge
transport through QD films [243], though the tradeoff between defects and transport
is less severe in QD light emitting applications, which therefore tend to make use of
insulating long-chain surfactants (8-18 carbons). The strict distance-dependence of
the rate of Förster Resonant Energy Transfer from QD cores to neighbouring QDs [82]
and adjacent layers means that long ligands and thick shells help reduce the quenching
effect of local defect states on adjacent QDs (‘self-quenching’, see Section 2.1.2) and
on transport layer materials. Likewise, the rate of QD-to-QD carrier tunneling has
previously been shown to depend roughly exponentially on the width and height
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
93
Figure 6-1: Influence of QD surface passivation and electronic confinement
on zero-bias and bias-driven PL quenching mechanisms. Schematic shows the
qualitative dependence of trap-assisted quenching, electric field-induced quenching,
and Auger recombination on both QD surface passivation and electronic confinement.
The example shown is overgrowth of a CdSe core by a CdS or ZnS shell. Both shells
confer passivation, but as the corresponding band diagrams show, whereas confinement increases in the Type-I energy alignment of CdSe-ZnS QDs, it is decreased by
the quasi-Type-II offsets in CdSe-CdS QDs. Adapted from ref. [212]
((∆𝐸)1/2 ) of the potential barrier separating adjacent QDs [244], such that ligands and
shells can also impede PL quenching via charge transfer to/from the QD [229]. Fig.
6-1 portrays the common practice of adding a wide band-gap shell of ZnS or CdS to a
CdSe core-only QD. The passivating effect is experimentally reflected by an increase in
𝜂PL of almost one order of magnitude compared to native CdSe cores [74]. Passivation
is also evidenced by the recent advent of "giant" shelled CdSe QDs [110, 245]; shell
thicknesses more than double the diameter of cores confer resilience to PL quenching
even upon removal of passivating ligands.
Fig. 6-1 delineates the two variables - confinement and passivation - and the
quenching processes we postulate to dominate in each regime. We note that the
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
94
quenching mechanisms may be distinguished by their bias-dependence: whereas Auger
recombination and electric field-induced quenching, by definition, require the application of a device bias (‘bias-driven’), trap-assisted recombination exists even in its
absence (‘zero-bias’). They can also be distinguished by their contribution to PL
quenching: either increasing 𝑘nr (trap states, Auger recombination, and electric-field
induced phonon coupling, surface state coupling, or exciton dissociation) or decreasing
𝑘r (electric field-induced exciton polarisation).
In this chapter, we ask if improved QD passivation (left to right in Fig. 6-1) can
indeed mitigate zero-bias PL quenching induced by surface trap states and the QDs’
local environment. Can this strategy be used to enhance the performance of infraredemitting QD-LEDs, which until now have been based almost solely on core-only QDs?
6.2
Efficient infrared QD-LEDs using core-shell QDs
6.2.1
Overview
Near-infrared (NIR) light sources integrated at room temperature with any planar
surface could be realised by harnessing the broad spectral tunability, high brightness
and solution-processability of colloidal QDs. Yet the performance of NIR QD-LEDs
has remained low compared with visible-emitting devices until now [1]. One notable
difference to date is that while visible-emitting QD-LEDs have employed core-shell
QDs (QD cores overcoated by a wider band-gap inorganic shell) almost since their
inception [132, 246], NIR QD-LEDs have, with one exception [151, 179], been based
solely on core-only QDs. As we have discussed above, the use of QDs without shells
can compromise their intrinsic 𝜂PL , increase their susceptibility to extrinsic (zero-bias)
PL quenching processes, and also reduce their photostability [75]. In this chapter, we
show that PbS-CdS core-shell QDs enhance the peak EQE of PbS core-only control
devices by 50- to 100-fold, up to 4.3±0.3 %. ‘Turn-on’ voltages are lowered by 0.6±0.2
V and per-amp radiant intensities are increased by up to 150 times. Peak EQEs and
power conversion efficiencies are more than double those of previous QD-LEDs [64]
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
95
emitting at wavelengths beyond 1 𝜇m, and are comparable with those of commercial
NIR LEDs. We demonstrate that the primary origin of the performance enhancement
is passivation of PbS cores by CdS shells against in situ PL quenching, suggesting
that core-shell QDs may become a mainstay of high-efficiency NIR QD-LEDs.
6.2.2
Introduction
Electrically-driven and wavelength-selectable NIR and shortwave infrared (SWIR)
light sources that can be deposited on any substrate and at lower cost [1, 2] than existing (usually epitaxially grown) IR-emitters have the potential to enhance existing
technologies and to trigger the development of new ones. Beneficiaries could include
optical telecommunications and computing [77,167]; bio-medical imaging [77,171,247];
on-chip bio(sensing) and spectroscopy [55, 77, 172]; and night-time surveillance and
other security applications. Among the candidate large-area planar light-emitting
technologies, colloidal QD-LEDs and organic LEDs (OLEDs) stand out, as they enable room-temperature processing and non-epitaxial deposition. Between them, QDLEDs should perform better at wavelengths beyond 𝜆 = 1 𝜇m because in this spectral
range the luminescent organic dyes used in OLEDs exhibit lower 𝜂PL (<5 %) [166])
than the colloidal QDs used in QD-LEDs. The PL emission wavelength of QDs can
be precisely tuned through quantum-confinement effects, with high 𝜂PL maintained
throughout the visible and NIR (𝜂PL > 95% [120] and 𝜂PL > 50%, respectively, in
solution). This can be appreciated directly in Fig. 6-2b, which is a false-colour
InGaAs camera image of six different PbS QD samples emitting at evenly spaced
wavelength intervals between 1.2 and 1.7 𝜇m under ultraviolet illumination. Furthermore, solution processability of QDs enables their epitaxy-free integration with
Si-electronics [94]. NIR QD-LEDs with peak EQEs of 8.6 % at 𝜆 > 1 𝜇m [183] and 2
% at 𝜆 > 1 𝜇m [64] have so far been demonstrated, compared with 6.3 % [174] and
<0.3 % [166] for OLEDs, respectively.
In this work, we significantly raise the performance of 𝜆 > 1 𝜇m NIR QD-LEDs
by adopting strategies used in visible-emitting QD-LEDs (which have demonstrated
EQEs of over 20 % in the red [140]). Based on the discussion above, our primary
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
96
Figure 6-2: Infrared QD PL, as seen by eye and with an infrared camera. Six
vials of colloidal PbS QDs illuminated with an ultraviolet lamp (475 nm), as seen:
a, by eye, captured with a regular camera; and b, with an infrared camera. The
samples emit at roughly evenly spaced wavelengths intervals between 1.2 and 1.7 𝜇m.
In b, three images were collected with an InGaAs camera (sensitive from 900-1700
nm): one with a 1300 nm bandpass filter; a second with a 1475 nm bandpass filter; a
third with a 1600 nm bandpass filter. The three images were each assigned a different
primary colour and then overlaid. Courtesy of Jessica Carr.
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
97
strategy is to emulate visible-emitting QD-LEDs in their use of QDs overcoated by
wide band-gap shells [132, 246]. NIR QD-LEDs have, with one exception [151, 179],
employed solely core-only QDs, exposing the QDs to the quenching pathways outlined above. Secondly, the use of ‘type-IV’ (organic-QD-inorganic hybrid) QD-LED
architectures (as defined in Section 2.2), which have shown the highest EQEs, has
heretofore been restricted almost exclusively to visible-emitting devices [1]. Here we
report a novel NIR ‘type-IV’ QD-LED and demonstrate that the use of core-shell
PbS-CdS QDs results in an enhancement of peak EQEs of one to two orders of magnitude over PbS core-only controls. This is shown to be a consequence of increased
in situ QD 𝜂PL accrued by virtue of the passivating CdS shell. The peak EQEs and
power conversion efficiencies obtained surpass those of any previously reported 𝜆 > 1
𝜇m NIR QD-LED [64].
6.2.3
Methods summary
Core-only (control) oleic acid-capped PbS QDs are synthesised using the procedure
described in Section 6.2.6 at the end of this chapter. The QDs are then subjected
to a partial Pb2+ -to-Cd2+ cation-exchange reaction [75] (Fig. 6-3a, Section 3.2, and
Section 6.2.6), whereby the addition of a large excess of cadmium oleate yields coreshell PbS-CdS QDs, also capped with oleic acid. Growing wide band-gap shells
like CdS on PbS cores by cation-exchange likely circumvents mismatch of crystal
structures encountered in traditional shell growth techniques.
The core-only PbS QDs have a mean diameter of 4.0 nm, deduced by transmission
electron miscroscopy and from the spectral position of the 1S excitonic absorption
peak. This corresponds to a peak PL wavelength of 𝜆 = 1315 nm in solution. By
cation-exchange we synthesised two types of thin-shelled PbS-CdS QDs and one type
of thicker-shelled PbS-CdS QDs. The thin-shelled QDs have core diameters of 3.6 nm
and shell thicknesses equivalent to 0.2 nm (assuming uniform shell coverage), but are
distinguished by their synthesis injection temperatures of 80 ∘ C (PbS-CdS(0.2 nm,
Sample A)) and 100 ∘ C (PbS-CdS(0.2 nm, Sample B)), respectively. The thickershelled PbS-CdS QDs have a core diameter of 2.6 nm and a shell thickness equivalent
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
98
Figure 6-3: QD synthesis and QD-LED design. a, Schematic illustration of the
cation-exchange reaction used to convert core-only PbS QDs into core-shell PbS-CdS
QDs. b, Device architecture (left) and cross-sectional scanning electron microscope
(SEM) image (right) of the ‘type-IV’ QD-LED based on these QDs. c, The QDLED’s flat-band energy level diagram. Band energies are in eV (referenced to the
vacuum level) and are taken from literature [122, 248–253]. Based on the model in
ref. [249], the electron affinity of PbS is tuned from approximately 3.8 eV to 3.9 eV
by the reduction in core size (from 4.0 nm to 3.6 nm) accompanying cation-exchange.
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
99
to 0.7 nm (PbS-CdS(0.7 nm)) (again assuming uniform shell coverage). These dimensions were determined by wavelength dispersive spectroscopy (see Section 6.2.6). 𝜂PL
of these QDs, measured in chloroform solution, are shown in Fig. 3-2b.
Using these QDs, we fabricate QD-LEDs comprising a zinc oxide (ZnO) solgel-derived, solution-processed electron-transport layer (ETL) and an organic small
molecule hole-transport layer (HTL) of 4,4-bis(carbazole-9-yl)biphenyl (CBP), sandwiching a film of QDs in an inverted ‘type-IV’ device architecture (as defined in Section 2.2). Thin films of indium tin oxide (ITO), gold (Au), and molybdenum oxide
(MoO3 ) serve as cathode, anode, and hole-injection layers [253, 254], respectively. A
schematic diagram of the QD-LED and a scanning electron microscope (SEM) image
of its cross-section are shown in Fig. 6-3b. Its corresponding energy band-diagram is
depicted in Fig. 6-3c. As previous reports have shown [254], MoO3 plays the vital role
of mediating efficient hole-injection from the Au anode into the deep highest occupied
molecular orbital (HOMO) of CBP.
6.2.4
Results and discussion
Fig. 6-4a presents EL spectra for the PbS and PbS-CdS QD-LEDs described above.
Their overall agreement with corresponding PL spectra from the same locations confirms pure NIR QD EL at a wavelength of ∼1.2 𝜇m. In the absence of a QD layer,
high voltages (>4 V) induce low-intensity visible EL characterized by two peaks at
𝜆 = 400 nm and 𝜆 = 670 nm (see Fig. 6-4a, Inset), resulting from CBP and/or ZnO
emission. However, upon addition of a QD layer, this parasitic emission is entirely
suppressed, indicative of efficient carrier recombination solely within the QDs. Fig. 64a shows that a blue-shift in EL and PL of more than 100 nm is achieved by adding a
0.7 nm CdS shell to core-only PbS. This is indicative of increased exciton confinement
due to a reduction in core size during cation-exchange [75]. By fabricating similar
devices with other batches of PbS and with thicker CdS shells, we have so far tuned
EL peaks from PbS-CdS QDs between 𝜆 = 1163 nm and 𝜆 = 1341 nm. NIR EL and
current-dependent brightness is visualized with an infrared camera (Fig. 6-4d, Inset;
a corresponding video is available as online Supplementary material to the original
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
100
publication of this work, ref. [15]).
The effect of shell growth on NIR QD-LED performance is shown in Fig. 6-4b-d,
which compare core-only PbS QDs with PbS-CdS(0.2 nm, Sample B) QDs. Results
for the other core-shell QDs are summarised in Fig. 6-4b and in Appendix A, Fig.
A-1 to A-4. Fig. 6-4b shows that the current density-voltage (J -V ) behavior for both
core-only and overcoated QDs is described by 𝐽 ∝ 𝑉 𝑚+1 , a signature of trap-limited
space-charge conduction [94, 255]. The corresponding radiance-voltage (L-V ) plot
(Fig. 6-4c) shows that replacing the PbS QDs with PbS-CdS(0.2 nm, Sample B) QDs
results in a reduction in ‘turn-on’ voltage (measured at 3 × 10-4 W sr-1 m-2 ) from
2.0 ± 0.1 V to 1.4 ± 0.1 V. This may be due to more efficient QD charge injection,
suggested by the simultaneous reduction in ‘turn-on’ current density of more than
an order of magnitude with increasing shell thickness [246] (Appendix A, Fig. A5). Taken together, the J -V and L-V data yield an average peak power conversion
efficiency of 2.4 %; more than double that of any previous 𝜆 > 1 𝜇m NIR QD-LED [64]
(Appendix A, Table A.1).
EQE-current density data is shown in Fig. 6-4d. Core-shell PbS-CdS(0.2 nm,
Sample B) QD-LEDs exhibit average peak EQEs of 4.3 ± 0.3 %. To our knowledge,
this surpasses that of any previously reported 𝜆 > 1 𝜇m NIR QD-LED [64] (Fig. 6-5
and Appendix A, Table A.1). This EQE is also commensurate with those typical of
commercial NIR LEDs (3 % to 12 %) [256], though at significantly lower emission
powers. In comparison, the average peak EQE of control core-only PbS QD-LEDs is
approximately 50 to 100 times lower: 0.05±0.01 %; consistent with the performance of
QD-LEDs previously reported using core-only PbS with similar long-chain carboxylic
acid ligands [64]. As shown in Fig. 6-6b (black columns), EQE is sizably enhanced for
all core-shell QDs investigated. A direct consequence of this is that radiant intensity
efficiencies (radiant intensity per amp) are increased at all current densities above
‘turn-on’ by between 35 and 150 times by switching from core-only to core-shell QDs
(Appendix A, Fig. A-5 and A-6). The grey columns in Fig. 6-6b represent the average
peak EQEs observed from an anomalous (not included in any averaged performance
data) ‘champion’ set of devices: up to 8.3 ± 1.0 % for PbS-CdS(0.2 nm, Sample B)
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
101
Figure 6-4: QD-LED performance using core-only versus core-shell QDs. a,
Normalised EL (solid, 3 V applied bias) and PL (dashed) spectra of QD-LEDs containing: (black) PbS; (red) PbS-CdS(0.2 nm, Sample A); (green) PbS-CdS(0.2 nm,
Sample B); and (blue) PbS-CdS(0.7 nm) QDs. Inset: Normalised EL spectra of otherwise identical LEDs (with the structure shown in Fig. 6-3) with and without a film
of QDs (8 V and 6 V applied biases, respectively). b, Average current density-voltage
(J -V ) behavior of: (black) core-only PbS; and (green) core-shell PbS-CdS(0.2 nm,
Sample B) QD-LEDs. The dashed grey lines are fits to 𝐽 ∝ 𝑉 𝑚+1 , with 𝑚 = 0 and
𝑚 > 1 corresponding to Ohmic and trap-limited space-charge conduction regimes,
respectively. The arrows indicate EL ‘turn-on’, which coincides with the upturn in
slope of the J -V curves. c, Radiance-voltage characteristics of these devices. Solid
lines represent average radiances and dotted lines correspond to minimum and maximum ‘turn-on’ voltages recorded for all measured devices. d, EQE-current density
performance of these devices. Black and green error bars indicate experimental measurement errors while the standard deviation of device-to-device variations are shown
in grey (Appendix A, Section A.1). Inset: photograph of an array of five QD-LEDs
taken with an infrared camera, with the middle device turned on (active area of 1.21
mm2 ) and emitting at a centre wavelength of 𝜆 = 1242 nm.
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
102
Figure 6-5: Progression in efficiencies over time of visible- and near-infraredemitting QD-LEDs, and comparison with this work and with commercial
near-infrared LEDs. Visible-emitting QD-LED peak EQEs (a substantial but nonexhaustive selection from the literature, reproduced from our recent review [1]) are
represented by filled black circles. Peak EQEs of near-infrared (NIR) QD-LEDs (𝜆 > 1
𝜇m), represented by filled green circles, are taken from Table A.1 in Appendix A. Solid
lines connect new record values. The 4.3 % average peak EQE of the core-shell NIR
QD-LEDs reported here is shown by a green star. The range of EQEs of typical
commercially-available NIR LEDs are also indicated.
QD-LEDs (Appendix A, Fig. A-7). Repeated measurements of these ‘champion’
devices manifested persistently stable operation, suggesting that NIR QD-LEDs with
EQE >10 % may be within reach.
Since the EQE of a QD-LED is proportional to 𝜂PL of its emitting QDs (Eq. 2.1),
we can better understand the dramatic increase in EQE that comes from replacing
core-only QDs with core-shell QDs by quantifying the correlation between changes
in EQE and in 𝜂PL of the QDs in our devices. As we discussed in Section 6.1, even
at zero-bias, 𝜂PL can be subject to in situ PL quenching mechanisms such as exciton
energy transfer to conductive charge-transport layers (CTLs) [137, 257, 258], exciton
dissociation at CTL/QD interfaces [207, 240, 241], and Auger recombination [242].
We therefore measure relative 𝜂PL of each type of QD in glass/ZnO/QD/CBP
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
103
and glass/ZnO/QD structures (Fig. 6-6b, Inset) that replicate the QDs’ in situ
environment in our QD-LEDs. Relative 𝜂PL is quantified by comparing the integrated
areas under the steady-state PL spectra of the QDs in these structures (see Section
6.2.6). The results for each type of QD are shown in Fig. 6-6b relative to 𝜂PL of
core-only PbS, which has been normalised to unity. Whereas shell growth produces
only a three-fold rise in 𝜂PL of QD-only (glass/QD) samples (blue in Fig. 6-6a,b)
(and similarly when measured in solution, Fig. 3-2b), relative 𝜂PL of these QDs in
situ (orange and green in Fig. 6-6b) increases by 30 to 100 times, indicating that the
CdS shell significantly passivates PbS QDs against in situ non-radiative pathways.
The similarity of the results for test structures with and without CBP (orange versus
green in Fig. 6-6b) implies that ZnO is mostly responsible for the PL quenching. The
generally good agreement between relative in situ 𝜂PL and average peak EQE (black
columns) indicates that the shell-induced increases in 𝜂PL predominantly explain the
efficiency enhancements achieved. This is further substantiated by the reproducibility
of this correlation for EQE data corresponding to all measured current densities;
examples at ∼10−4 A cm-2 and ∼10−2 A cm-2 are shown in Appendix A, Fig. A-8.
Time-resolved PL spectroscopy offers further insights into the shell-dependent QD
PL quenching by ZnO. Fig. 6-7 shows the normalised transient QD PL decay curves
for the glass/ZnO/QD samples. Compared with glass/QD samples (Appendix A,
Fig. A-9), PL lifetimes are reduced for all of the QD types in the presence of ZnO,
indicative of PL quenching; similar changes have previously been seen for PbS QDs
with other metal oxides [259]. The longer PL lifetimes of core-shell versus core-only
QDs (Appendix A, Table A.2) corroborate our conclusion that the former are more
effective at mitigating PL quenching [242, 259, 260].
What specific mechanisms, symbolised by the blue arrow in the inset of Fig. 6-7,
underpin the ZnO-induced QD PL quenching in our devices? Non-radiative energy
transfer from visibly-coloured QDs to surface plasmon modes in conductive metal
oxide CTLs of QD-LEDs has previously been postulated [137], but is unlikely here
given the low carrier density of our undoped ZnO [261, 262]. Indeed in general,
energy transfer from PbS QDs to ZnO - for example, to ZnO mid-gap states - seems
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
104
Figure 6-6: Correlation of QD-LED EQE with QD PL quantum yields. a,
Absolute PL quantum yields (𝜂PL ) of drop-cast films of core-only (PbS) and core-shell
(PbS-CdS) QDs on glass as a function of shell thickness, measured using an integrating
sphere (described in Section 6.2.6). Cation-exchange serves as a facile post-synthesis
approach to increasing 𝜂PL of NIR-emitting QDs to unprecedented values (𝜂PL = 33%
in thin-film for a 0.7 nm shell). Error bars show standard deviations of the values
measured at four locations per sample. b, Corresponding dependence of QD-LED
average peak EQE on QD shell thickness is indicated by black columns. These EQEs
correlate well with average relative in situ 𝜂PL of spin-cast films of these QDs (𝜂PL
of core-only QDs is normalised to unity) in: (green spots) glass/ZnO/QD; and (orange triangles) glass/ZnO/QD/CBP optical structures, depicted schematically in the
inset. In contrast, relative intrinsic QD 𝜂PL (blue squares), taken from a, greatly underestimate the EQE enhancements. Average peak EQEs obtained from a ‘champion’
set of devices are shown as grey columns. The EQE error bars for the main devices
(black columns) and the ‘champion’ devices (grey columns) indicate device-to-device
variations (see Section 6.2.6 and Section A.1 for details). The QD 𝜂PL error bars show
standard deviations between values measured at roughly five locations per sample.
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
105
Figure 6-7: Transient spectroscopy of shell-dependent QD PL quenching.
Normalised time-dependent PL intensity of spin-cast films of: (black) PbS; (red)
PbS-CdS(0.2 nm, Sample A); and (green) PbS-CdS(0.2 nm, Sample B) QDs, all on
ZnO. Inset: Illustrations depicting the passivation by CdS shells of ZnO-induced PbS
PL quenching (blue arrow).
at odds with the very low extinction coefficients reported for sol-gel ZnO in the
NIR [262, 263]. Alternatively, electron transfer from PbS QDs to ZnO, which is
widely observed in photovoltaics [207], could be responsible. As has been noted in the
context of CdSe-CdS QD quenching by ZnO nanocrystals [142], however, the energetic
favourability of the ZnO/QD dissociative interface for electron transfer is expected
to be voltage-dependent (forward biasing mitigates electron transfer) whereas the
ZnO-induced PL quenching that we observe is, at least in order-of-magnitude terms,
voltage-independent (Appendix A, Fig. A-8). Hole transfer to ZnO mid-gap states
[142] is therefore more plausible, yet the challenge in reconciling either energy or
charge transfer models with the extremely short length scales on which PL quenching
occurs (∼0.2 nm of CdS shell) points to a second possibility; quenching by defect
states arising from the oxidation of PbS QDs by ZnO.
The roles of the QD core-shell structure and of the QD-LED ‘type-IV’ architec-
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
106
ture are inextricably linked. While the former largely explains the enhancement in
EQEs observed, the latter is probably central to their high absolute magnitudes.
The ‘type-IV’ design is conducive to efficient carrier recombination because it allows the processes of charge-transport and light-emission to be decoupled. Strikingly,
the only two previously reported NIR QD-LEDs not possessing a ‘type-I’ structure
(a more rudimentary architecture in which QDs play dual roles of CTL and emission centre, see Section 2.2) have also been the highest performing [64,183]. With the
present demonstration we propose that together, cation-exchanged core-shell QDs and
a ‘type-IV’ device architecture offer a new paradigm for the design of high-efficiency
NIR QD-LEDs.
6.2.5
Conclusion
This work demonstrates that the efficiency of NIR QD-LEDs can be enhanced by up
to 100 times by replacing NIR core-only QDs with core-shell QDs, as a result of a
corresponding increase in in situ QD 𝜂PL . As the demand for high brightness - and
therefore high current density - QD-LEDs continues to necessitate the use of metal
oxide charge-transport materials [162] (that commonly quench QD PL, even at zero
bias), the use of core-shell QDs can serve as a general strategy by which to maximise
the performance of future NIR QD-LEDs.
6.2.6
Detailed Experimental Methods
QD and zinc acetate synthesis and characterisation
Based on previously published procedures, core-only PbS QDs were synthesised using
a large-scale method [264] and core-shell PbS-CdS QDs were synthesised by cationexchange [75, 265], as described below. QD shell thicknesses were deduced from elemental analysis of drop-cast films on glass substrates by wavelength dispersive spectroscopy [265].
PbS: A lead precursor is prepared by heating 30 mmol of lead acetate trihydrate
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
107
(Sigma Aldrich), 125 mL of octadecene (Sigma Aldrich), and 175 mL of oleic acid
(TCI) in a round-bottomed flask for 12 hours under vacuum at 100 ∘ C. A sulphur
precursor is prepared by adding 15 mmol hexamethyldisilithiane (Fluka) to 150 mL of
octadecene. The lead precursor is placed under nitrogen and heated to 150 ∘ C before
injection of the sulphur precursor. After cooling to room temperature, this solution
is transferred into a nitrogen-filled glove box without air exposure. The synthesised
PbS QDs are purified by adding methanol and 1-butanol until the solution is turbid,
followed by centrifugation to precipitate the QDs. The supernatant is discarded and
the QDs are redissolved in hexane [266].
PbS-CdS: PbS-CdS core-shell QDs are prepared by cation-exchange [75, 265]. To
control shell thickness, exchange is performed at 80 to 100 ∘ C for between 5 min and
2 hours under nitrogen using an excess of cadmium oleate. Also see Section 3.2.
Zinc acetate dihydrate solution: The ZnO precursor is prepared according to
a variant of well-established procedures [252, 267], dissolving zinc acetate dihydrate
(Zn(CH3 COO)2 ·2H2 O, Aldrich, 99.999 %, 12.56 g) and ethanolamine (NH2 CH2 CH2 OH,
Aldrich, >99.5 %, 3.2 mL) in anhydrous 2-methoxyethanol (CH3 OCH2 CH2 OH, Aldrich,
99.8 %, 76.8 mL) under agitation in an ultrasonic bath for 4 hours. The solution was
stored in a nitrogen-filled glovebox.
QD-LED and optical sample fabrication
Prior to device fabrication, the QDs undergo three rounds of precipitation and centrifugation with methanol and 1-butanol and are then redissolved in chloroform at 40
mg mL-1 and filtered (0.45 𝜇m PTFE). Devices are fabricated on glass substrates that
are pre-patterned with a 150 nm thick film of indium-tin-oxide (ITO) (obtained from
Thin Film Devices) and cleaned with solvent and oxygen-plasma. First, a zinc acetate
dihydrate solution is spin-cast onto the ITO at 2000 rpm for 30 s in a nitrogen-filled
glovebox. The xerogel is then sintered on a hot-plate at 300 ∘ C for 5 min in the
10-20 % relative humidity environment of a glove-bag continuously flushed with dry
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
108
air. This process results in the crystallisation of 55 ± 1 nm (by optical ellipsometry
(Gaertner Scientific)) thick ZnO films and mitigates the sensitivity of ZnO to variations in atmospheric conditions. QD films are then deposited by spin-coating ∼ 40𝜇L
of ∼ 5 mg mL-1 PbS (or PbS-CdS) QDs in chloroform at 1500 rpm for 60s; film
thicknesses of 8 ± 1 nm are obtained, as determined by thickness-concentration calibration curves measured by profilometer (Tencor P-16) (Appendix A, Section A.2).
This level of accuracy is necessary in order to allow the effect of shell thickness to be
studied independently from the effects of QD film thickness. A 150 nm thick HTL of
4,4-bis(carbazole-9-yl)biphenyl (CBP) (purified before use via thermal gradient sublimation), a 10 nm thick hole-injection layer of molybdenum oxide (MoO3 ), and a
100 nm thick Au anode (shadow masked) are then successively deposited via thermal
evaporation at a rate of ∼0.1 nm s-1 at a base pressure of 1 × 10-6 Torr. Overlap of
anode and cathode defines active area pixels of 1.21 mm2 . The glass/ZnO/QD/CBP
and glass/ZnO/QD structures are fabricated by the same methods as the QD-LEDs.
QD-LED characterisation
Current density-voltage (J -V ) characteristics are recorded in a nitrogen-filled glovebox using a computer-controlled Keithley 2636A current/voltage source meter. Simultaneously, front face EL power output through ITO is measured using a calibrated
Newport 818-IR germanium photodiode and recorded with a computer-interfaced
Newport Multi-Function Optical Meter 1835-C. The photodiode’s active area is aligned
with the emissive pixel and a diaphragm between the two prevents collection of waveguided EL from the glass substrate. Total radiated power (from which radiance-voltage
(L-V ) characteristics and power efficiency are obtained) is then calculated by assuming Lambertian emission and by accounting for the wavelength dependence (weighted
by the device’s EL spectrum) of the photodiode’s responsivity (Appendix A, Section
A.1.2). EQE is calculated as the ratio of the number of forward-emitted photons to
the number of injected electrons, per unit time (Appendix A, Section A.1). Power
conversion efficiency is defined as the radiated optical power divided by the input
electrical power. For each QD type, L-J -V and EQE data are taken from about
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
109
three pixels per device, and from devices fabricated on two to five separate runs; the
only exceptions are the ‘champion devices’, for which data comes from three to five
pixels on one ‘champion’ device (see Appendix A, Section A.1). In all cases, variations
between data for a given QD type are referred to as "device-to-device" variations in
this chapter.
Optical measurements
NIR EL and PL spectra, transient PL data, and 𝜂PL from the integrating sphere are
collected in air, before which active sample areas are hermetically encapsulated by a
glass coverslip in a nitrogen-filled glovebox using a solvent-free self-curing two-part
epoxy (Torr Seal) rated to 10-9 Torr.
NIR EL and steady-state PL data are collected through a 𝜆 = 850 nm long-pass
filter with a Princeton Instruments Spectra Pro 300i spectrometer coupled to a liquid
nitrogen-cooled Princeton Instruments OMA V InGaAs CCD array detector. The excitation source for steady-state NIR PL is a 𝜆 = 655 nm diode laser (Thorlabs), which
ensures that in the relative 𝜂PL measurement only the QDs, and not ZnO or CBP, are
significantly excited. The fixed geometry of the setup maximises the comparability
of relative PL intensities. A fiber-coupled Ocean Optics SD2000 spectrometer is used
to monitor for visible EL.
Transient PL data is measured by exciting glass/ZnO/QD samples at 𝜆 = 633 nm
and collecting resulting PL using superconducting nanowire single photon detectors,
as previously reported [268].
For both steady-state and transient measurements of relative in situ QD 𝜂PL , care
was taken to record data in a low excitation-intensity regime (0.5 mW cm-2 for the
steady-state experiments) where PL intensity was not decaying with time (due to
irreversible photobleaching), and at many sample locations to confirm homogeneity.
To extract relative 𝜂PL , steady-state PL spectra were scaled to account for the spectral
responsivity of the InGaAs CCD and for differences in relative absorbance between
QD samples.
Absolute 𝜂PL measurements and calculations are performed using an integrating
Chapter 6. Zero-Bias QD Luminescence Quenching in Infrared QD-LEDs
110
sphere and calibrated reference detectors as described previously [266], except that
here we account for the wavelength dependence (weighted by the PL spectra of the
drop-cast QD films) of the photodiode’s responsivity using an approach analogous to
that for calculating EQE and radiance (Appendix A, Section A.1). It was necessary
to measure 𝜂PL for drop-cast (rather than spin-cast) films in order to detect sufficient
signal. 𝜂PL is measured at four locations per sample, giving the means and standard
deviations plotted in Fig. 6-6a.
NIR EL photographs are taken with a Xenics Xeva-2.5-320 HgCdTe short-wave
infrared camera.
Chapter 7
Conclusion
7.1
Thesis summary
The mainstream commercialisation of colloidal QDs for light-emitting applications
has already begun. Sony televisions emitting QD-enhanced colours have been on sale
since 2013 [2], and optical down-conversion in QDs has been used to manufacture
solid state light (SSL) bulbs with enhanced colour quality and reduced power consumption [1]. The bright and uniquely size-tunable colours of solution-processable
semiconducting QDs also highlight the potential of electroluminescent QD-LEDs for
use in energy-efficient, high-colour-quality thin-film display and SSL applications.
Chapter 1 described how the threats of climate change and the drive for more sustainable global development create an imperative for improvements in society’s energy
efficiency and carbon intensity, and explained why lighting technologies - including,
potentially, QD-LEDs - have an important role to play. Chapter 2 discussed the key
advantages of using QDs as luminophores in LEDs and outlined the two-decade evolution of four ‘types’ of QD-LEDs that have seen efficiencies rise from less than 0.01
% to more than 20 %. Indeed, the efficiencies of state-of-the-art electrically driven
QD-LEDs now rival those of the most efficient molecular organic LEDs (OLEDs), and
full-colour QD-LED displays have now been realised, foreshadowing QD technologies
that will transcend the optically excited QD-enhanced products already available. A
cost-analysis of industrial-scale QD synthesis was also conducted, showing that QD111
Chapter 7. Conclusion
112
LEDs may be competitive with OLEDs in the SSL market. Chapter 3 provided an
overview of the synthesis of colloidal QDs and the quantum confinement effects that
dictate their remarkable size-tunable optical properties.
At the start of Chapter 4, we make the observation that whereas significant attention has been paid to engineering QD-LEDs with improved charge carrier recombination efficiencies, the influence of QD photoluminescence quantum yield (𝜂PL ), which is
also directly proportional to a device’s EQE, has not been carefully addressed. Subdividing QD PL quenching mechanisms into those that affect QD-LEDs even at zero
bias and those that are bias-dependent, Chapter 4 then focuses on bias-driven QD
PL quenching in QD-LEDs. We simultaneously measure the intensity of PL and EL
as a function of applied bias, demonstrating that the roll-off in EQE observed in QDLEDs at high biases can be fully explained by a commensurate reversible reduction
in QD 𝜂PL . This allows us to discount the possibility that roll-off results from charge
leakage in our devices. For two decades, efficiency roll-off has been - and continues
to be - one of the most persistent hurdles facing the realisation of high efficiencies
at high brightness in all LED technologies. Ours is the first dedicated study of this
effect in QD-LEDs, serving to narrow down the mechanisms that may be responsible.
Chapter 5 builds on the previous chapter by uncovering the specific process causing
QD PL quenching, and therefore efficiency roll-off, in our QD-LEDs. Preliminarily
ruling out heating and charge-induced Auger recombination in QDs, we hypothesise
that electric field may be the primary quenching pathway and test that hypothesis.
Using the shift in wavelength of QD PL from the quantum confined Stark effect
as a proxy for the QDs’ local electric field, we measure QD PL under reverse bias
(i.e. electric field without charge injection) to determine the field-dependent PL
quenching. This allows us to predict the forward bias-induced EQE roll-off in our QDLEDs. Excellent agreement with observed EQE confirms that electric field-induced
quenching is indeed sufficient to account for roll-off in our QD-LEDs - an important
guide to the engineering of future high efficiency, high brightness QD-LEDs.
In Chapter 6, we turn our attention to mitigating the influence of zero-bias quenching via electronic trap states on QDs and their local environment in QD-LEDs. We
Chapter 7. Conclusion
113
choose to use NIR-emitting QD-LEDs as a testbed, since these devices boast unique
potential. One can envision NIR light sources that can be deposited on any substrate
and at lower cost than existing (usually epitaxially grown) IR-emitters finding application in optical telecommunications and computing (solution-processability may enable
Si-compatibility), bio-medical imaging (utilising biological transparency windows between 800 nm and 1700 nm), on-chip bio(sensing) and spectroscopy, and night-time
surveillance and other security applications. Yet to date, the performance of NIRemitting QD-LEDs has trailed far behind their visible counterparts. By overcoating
PbS core-only QDs with CdS shells via cation exchange, we demonstrate through
in situ steady-state and transient PL spectroscopy that even atom-thickness surface
passivation dramatically suppresses zero-bias PL quenching by the neighboring ZnO
electron transport layer in our devices. 𝜂PL is enhanced by two orders of magnitude,
yielding a record-performance NIR QD-LED with an average EQE of ∼4.3 % and
emission wavelengths in the technologically important 1.1-1.3 𝜇m range.
7.2
Outlook
This thesis has been dedicated to understanding the role of QD PL quenching on
QD-LED performance with and without bias. However, while our work on NIR QDLEDs yielded a solution to overcoming some of these challenges at zero bias, we have
not yet engineered a solution to the electric field-induced roll-off mechanism that
we have identified. Indeed, further work is required to deepen our understanding of
this process and its interplay with charge-induced Auger recombination in QD-LEDs,
which has since been observed by others to sometimes also be involved. Moreover,
based on our reviews of the QD-LED field and observation of its progress with respect
to the SSL and display markets [1, 2], key research hurdles facing QD-LEDs include
better understanding and optimising blue emission, outcoupling, and device lifetime.
We briefly discuss directions each of these topics could take.
∙ Understanding QD-LED efficiency roll-off through QD composition:
Based on our discussion in Section 5.2.4, our understanding of the interplay
Chapter 7. Conclusion
114
Figure 7-1: Progression of blue-emitting LED EQEs over time. EQEs over
time of blue-emitting OLEDs (black) compared with QD-LEDs (blue). OLEDs are
distinguished as either PhOLEDs (circle) or TADF OLEDs (square). Filled symbols
show peak EQEs whereas open symbols show EQEs as high brightness (1000 cd m-2 ),
illustrating greater efficiency roll-off at high biases in OLEDs than QD-LEDs.
between electric field and Auger recombination in causing EQE roll-off in QDLEDs may be tested by investigating the effect of QD composition - specifically
electronic confinement - on roll-off. Klimov et al. have recently presented
evidence of roll-off due to Auger recombination and its correlation with QD shell
chemistry [6,229], yet similar efforts should be undertaken to better understand
and mitigate electric field-induced roll-off in our devices. We observe that, for
example, EQE roll-off is pronounced in our NIR-emitting QD-LEDs (Fig. 6-4)
but, based on an initial review of the literature, is apparently less so in blueemitting devices (Fig. 7-1). It is possible that the different conclusions we and
others have arrived at as to the cause of roll-off in QD-LEDs (electric field versus
charge) can be ascribed to subtle but systematic differences in the QDs used.
∙ Mitigating QD-LED efficiency roll-off: We propose three approaches to at
least partly address roll-off in QD-LEDs, but there are surely others.
Chapter 7. Conclusion
115
Firstly, one could investigate the use of QDs that are optimised to negate
Auger recombination (for example, with an alloyed shell) [229,233,234] but
whose emission derives from a localised trap state that is less sensitive to
the quantum confined Stark Effect. For example, QD-LEDs might employ
QDs comprising a luminescent impurity like Mn+ [269–271] (note these
QDs have already been successfully employed in a type-III QD-LED [162]).
Another approach, proposed by Bozyigit et al. [212], is to reduce the electric field felt by the QDs in a QD-LED by embedding the QDs in a material
of high dielectric constant. For example, atomic layer deposition might be
used to in-fill the QD film. Simulations suggest that the use of a matrix
material such as TiO2 (𝜀 ≥ 86) could reduce the effective electric field by
a factor of 10 [212].
An alternative strategy could be to operate QD-LEDs in an altogether
different way so as to avoid the deleterious effects of charge and field, and
in fact use them to our advantage. For example, as we have previously
proposed [272], emission from a light emitting device might be based on
optical excitation of QD PL (a "PL-LED"), with electric field (in absence
of charge injection, by way of a capacitor structure, say) applied only to
intentionally dim or turn off QD PL emission. Particularly for applications
that do not require large ON-OFF ratios, this approach leverages the most
invaluable properties of QDs - their optical ones - while avoiding the difficulties and challenges of exciting them electrically. Since thin dielectric
films can then replace thicker, often absorbing charge transport layers, this
opens the door to realising a transparent light-emitting technology.
∙ Thermal efficiency roll-off in QD-LEDs: Our thermal imaging experiments
indicated insufficient short-term temperature rises to explain the roll-off in our
QD-LEDs (Fig. 5-3). However, just as with OLEDs, prolonged operation may
nevertheless lead to a substantial rise in temperature (many tens of degrees
Celsius) in QD-LEDs, threatening both efficiency and material integrity. Sys-
Chapter 7. Conclusion
116
tematic QD-LED junction temperature measurements as a function of current
density, akin to those performed in OLEDs [223], may be informative of their
commercial high-brightness performance limits.
∙ Blue QD-LEDs: The performance of blue-emitting QD-LEDs has improved
dramatically over the past few years [273]. Fig. 7-1 shows the progression in
record efficiencies of hybrid deep-blue (CIE 𝑦 < 0.1) QD-LEDs, which are seen
to be quickly approaching those of state-of-the-art OLEDs using phosphorescent
and thermally-activated delayed fluorescence (TADF) emitters. As Fig. 7-1
also demonstrates, blue QD-LEDs exhibit much less severe efficiency roll-off
at high biases compared to OLEDs, such that QD-LED and OLED EQEs are
comparable at high brightness. While enabling true blue EL emission, these
QD-LEDs have sub-bandgap turn-on voltages of 2.5-3 V [60, 61, 140, 141, 273]
that are lower than those of corresponding OLEDs (3.3-5.4 V [274–276]); a key
factor in ensuring small energy losses during electron-photon conversion, and
therefore in achieving high power-efficiencies [277]. As we described in Section
1.2, efficient, stable blue-emission from OLEDs remains elusive, presenting an
impediment to OLED commercialisation as a white light source.
Blue-emitting QD-LEDs have received limited attention to date compared with
red-emitting devices, so we propose a course of research investigating the two
limiting factors to the performance of blue QD-LEDs: efficiency and lifetime.
By developing understanding in these two largely unexplored areas, it may be
possible to develop efficient and stable blue QD-LEDs with the potential to
enable a new generation of white SSL sources.
To our knowledge, there has been no systematic study to uncover the factors
limiting EQEs and lifetimes in blue QD-LEDs. The substantial rise in efficiencies of blue QD-LEDs in just the last few years (Fig. 7-1) has been largely
attributed to apparent advances in QD synthesis chemistry. At the same time,
it is most frequently cited that blue QD-LEDs exhibit inferior performance to
their red and green counterparts because of the larger hole injection barrier at
Chapter 7. Conclusion
117
the QD/hole transport layer (HTL) interface resulting from the deeper highest
occupied molecular orbital (HOMO) level of the higher-energy QDs [60, 273].
This argument is certainly plausible, but has not yet been validated by systematic investigation of the impacts of hole injection and charge balance on EQE.
Building on successes with OLEDs [118,278], and with the suspicion that excess
electron injection may detrimentally impact today’s blue QD-LEDs, one might
investigate the impact of charge (im)balance on device performance through
the use of doped charge transport layers, conductive metal oxides, and charge
blocking layers.
∙ Outcoupling in QD-LEDs: While many of the structural modifications successfully implemented in OLEDs and GaN-based LEDs to enhance the outcoupling of generated light may be translatable to QD-LEDs, it is emerging that
many of the outcoupling strategies reported in the academic literature may
not be compatible with commercial applications. Outcoupling techniques must
be low cost, scalable to large area devices and manufacturing, compatible with
OLED device architectures, and robust with respect to lifetime and environmental degradation [40]. For example, plasmon losses can most easily be mitigated
by increasing the distance between the emissive layer and metal cathode, yet
this can increase device operating voltages. Solutions to this specific tradeoff
may include doped charge transport materials or unconventional transport materials such as perovskites. Interestingly, ongoing investigations of large QD
PL enhancements (between ∼2- and 50-fold) through QD-metal nanostructure
interactions suggest that carefully tailored plasmonic templates may be used to
enhance the efficiency of QD-LEDs [93, 279].
∙ Lifetimes of QD-LEDs: Intrinsic QD PL lifetimes of >14,000 hours and respectable PL thermal stabilities (12 % fall-off at 140 ∘ C) are reported by QD
Vision, Inc [280, 281]. These are comparable with inorganic phosphors and
render QDs already commercially viable in many optical down conversion applications. However, lifetimes of electrically driven QD-LEDs remain relatively
Chapter 7. Conclusion
118
short. QD-LED half-lives, 𝑇50 , from an initial luminance of 100 cd m-2 , are
typically on the order of 102 -104 hours, compared to 105 -107 hours for today’s
OLEDs, which have undergone more than a decade of rigorous commercial-scale
refinement [282].
Fig. 7-2 depicts the evolution of QD-LED operational lifetimes, with all devices
compared to the same initial luminance of 100 cd m-2 [60–62, 93, 94, 131, 132,
140, 140, 141, 281, 283]. Red-emitting QD-LEDs, which are the colour most
commonly reported, clearly demonstrate a steady and substantial improvement
in lifetimes over the past several years. In fact, encouragingly, people have
recently reported red- and green-emitting QD-LEDs with half-lives of nearly
105 hours when operated at 100 cd m-2 initial brightness (see Fig. 7-2).
However, to date, discussion of the lifetime limitations of QD-LEDs has been
sparse - in almost every case a side note to other kinds of device studies. Indeed,
to our knowledge, there has been no systematic study of the degradation of statistically significant populations of QD-LEDs operating over extended periods
of time in ambient room conditions; for example, no detailed characterisation
of the physical parameters of degradation or attempts to assign specific mechanisms to this process. In contrast, in the more established field of OLEDs,
substantial effort over the past 30 years has gone into discovering and overcoming their failures and degradation. Initial research topics in this area might
include:
Establishing consistent benchmarks and testing procedures for QD-LED
lifetime measurement and reporting: device encapsulation, testing conditions, data fitting, and extrapolation are often not consistent between
studies.
Investigating whether QD-LED ambient stability is primarily limited by
intrinsic or extrinsic degradation processes. In particular, are non-emissive
‘dark spots’ the dominant factor in short QD-LED lifetimes, as was previously found to be the case in OLEDs?
Chapter 7. Conclusion
119
Studying whether colloidal QDs in fact confer additional stability compared to organic luminophores on overall light-emitting device performance, as is commonly supposed. Is QD-LED lifetime primarily limited
by degradation of the QDs themselves, or by their surrounding materials?
Chapter 7. Conclusion
120
Figure 7-2: QD-LED operating lifetimes. a, Progression in reported lifetimes
(𝑇50 ) over time of electrically-driven visible-emitting QD-LEDs, all compared at the
same initial luminance of 100 cd m-2 , driven by constant current. The red-emitting
QD-LED reported in 2007 (hollow red triangle) was tested under low vacuum (5×10−3
Torr) [62], whereas all other devices were tested in ambient conditions. b, Required
OLED/QD-LED luminance for certain applications. Overlaid are the reported lifetimes of several green phosphorescent OLEDs, which are compared to the long-lived
green QD-LED developed by Yang et al. [141] (unfortunately this is the only instance
in which QD-LED lifetimes have been measured for more than one initial brightness).
In the cases where two lifetime data points are available, a straight line extrapolation offers a rough projection of the device half-life at low luminances, as previously
shown [284]. It is widely accepted that for indoor display applications, the required
brightness for individual pixels (depending on size, color, and device architecture) is
about 200-600 cd m-2 [285, 286]. For other applications like outdoor display, lighting
and window integrated transparent devices, higher brightness is required. The industrial community now demands a device lifetime as high as 20,000 h at 500 cd m-2
for displays, where the lifetime is defined to reach 80 % (𝑇80 ) or even 95 % (𝑇95 ) of
the initial brightness; that is, exhibiting a drop of only 20 % and 5 %, respectively,
instead of the 50 % (𝑇50 ) metric typically reported by researchers.
121
Appendix A. Supplementary information to Chapter 6
122
Appendix A
Supplementary information to
Chapter 6
Figure A-1: Raw EQE-current density data with measurement errors and
device-to-device variations. EQEs of individual QD-LED pixels using: a, PbS; b,
PbS-CdS(0.2 nm, Sample A); c, PbS-CdS(0.2 nm, Sample B); and d, PbS-CdS(0.7
nm) QDs are indicated by black lines, with blue dots showing their averages. Measurement errors and device-to-device variations are represented by blue and grey error
bars, respectively.
2000
2003
2004
2005
2005
2008
2010
2011
2012
2012
2014
Tessler et al. [179]
Steckel et al. [170]
Koktysh et al. [181]
Konstantatos et al. [63]
O’Conner et al. [182]
Bourdakos et al. [168]
Choudhury et al. [169]
Cheng et al. [183] (𝜆 < 1𝜇m)
Sun et al. [64]
Ma et al. [95]
This work [15]
4.3
0.72
2.0
6.7 (3 nm QDs); 8.6 (5 nm QDs)
0.83
1.15
0.02
0.27
<0.001
0.001
0.5
Peak EQE (%)
2.4
0.11
1.0
0.4 (3 nm QDs); 8 (5 nm QDs)
0.08
0.66
1.2 × 10-4
0.01
-
<0.1
0.02
Peak Power Conversion Efficiency (%)
All peak external quantum efficiencies (EQEs) are quoted directly from the references. Unless provided, peak power efficiencies are approximated by computing (𝑃EL /𝐼𝑉 ) × 100% as a function of I or V. Here, 𝑃EL is maximum emitted power, which, unless given, is approximated
by extracting plotted data (Plot Digitizer) and computing 𝑃EL = (𝐼 · EQE · ℎ𝑐)/(|𝑒| · 𝜆) (where 𝜆 is peak emission wavelength) as a function of I or V so as to find its maximum. Dashes (-) indicate unknown values not calculable by these methods. Note that 𝜆 > 1𝜇m for all
references except Cheng et al.
Year
Ref.
Table A.1: Summary of reported NIR QD-LED peak external quantum efficiencies and power efficiencies. Peak
performance metrics for a number of reported near-infrared QD-LEDs are tabulated in chronological order.
Appendix A. Supplementary information to Chapter 6
123
Appendix A. Supplementary information to Chapter 6
124
Figure A-2: Current density-voltage behavior. Average performance of QDLEDs based on: (black) PbS; (red) PbS-CdS(0.2 nm, Sample A); (green) PbS-CdS(0.2
nm, Sample B); and (blue) PbS-CdS(0.7 nm) QDs.
Appendix A. Supplementary information to Chapter 6
125
Figure A-3: Test-to-test variations in peak EQE. Peak EQEs of individual QDLED pixels (arbitrarily numbered) tested multiple times using: a, PbS; b, PbSCdS(0.2 nm, Sample A); c, PbS-CdS(0.2 nm, Sample B); and d, PbS-CdS(0.7 nm)
QDs. Peak EQEs are shown after multiple test cycles: (black) Test 1; (red) Test 2;
(green) Test 3; (blue) Test 4; and (turquoise) Test 5. Successive test cycles yield peak
EQE values between 84 % and 104 % of the initial values for all pixels and all QD
types.
Appendix A. Supplementary information to Chapter 6
126
Figure A-4: Day-to-day variations in raw EQE-current density data. EQEs
for: (black and red) two pixels of PbS-CdS(0.2 nm, Sample A) QDs; and for (dark
green, light green, blue) three pixels of PbS-CdS(0.2 nm, Sample B) QDs (all ‘champion’ devices). For each pixel, data was recorded one day after fabrication (Day 1,
open circles) and again (solid lines) either two (Day 2) or four (Day 4) days after
fabrication. We see that EQE either remains unchanged or increases slightly over the
course of at least a few days.
Appendix A. Supplementary information to Chapter 6
127
Figure A-5: Radiance-current density characteristics. Average radiance for
QD-LEDs based on: (black) PbS; (red) PbS-CdS(0.2 nm, Sample A); (green) PbSCdS(0.2 nm, Sample B); and (blue) PbS-CdS(0.7 nm) QDs. Upon shell growth,
‘turn-on’ current densities (measured at 3 × 10-4 W sr-1 m-2 ) are reduced by 8, 16
and 31 times for PbS-CdS(0.2 nm, Sample A), PbS-CdS(0.2 nm, Sample B), and
PbS-CdS(0.7 nm) QDs, respectively. For any given current density above core-only
QD-LED ‘turn-on’, the radiance of the core-shell QD-LEDs is between 35 and 150
times higher than that of the core-only QD-LEDs.
Appendix A. Supplementary information to Chapter 6
128
Figure A-6: Enhancement in radiant intensity efficiency. The enhancement
in radiant intensity efficiency (radiant intensity per amp) compared with that of the
control core-only PbS QD-LED is shown as a function of current density for QDLEDs using: (red) PbS-CdS(0.2 nm, Sample A); (green) PbS-CdS(0.2 nm, Sample
B); and (blue) PbS-CdS(0.7 nm) QDs. ‘Turn-on’ current densities are indicated by
dashed lines. Once the core-only PbS QD-LED (black dashed line) has turned on,
the enhancement is between 35 and 150 times for all core-shell devices.
Appendix A. Supplementary information to Chapter 6
129
Figure A-7: EQE-current density characteristics of anomalous ‘champion’
QD-LEDs. Record performance curves for one set of QD-LEDs using: (black) PbS;
(red) PbS-CdS(0.2 nm, Sample A); (green) PbS-CdS(0.2 nm, Sample B); and (blue)
PbS-CdS(0.7 nm) QDs. Peak EQEs are 1.7 %, 7.7 %, 9.4 % and 5.3 %, respectively,
as plotted in Fig. 6-6b.
Appendix A. Supplementary information to Chapter 6
130
Figure A-8: Dependence of QD-LED EQE-shell thickness correlation on
current density. Average EQE data measured at current densities of ∼10−4 A
cm-2 and ∼10−2 A cm-2 , extracted from Fig. A-1, are represented by green and
blue columns, respectively, and compared with the average peak EQE data (black
columns) shown in Fig. 6-6b. The error bars indicate device-to-device variations in
EQE. For all data sets a similar correlation with shell-thickness is reproduced.
Appendix A. Supplementary information to Chapter 6
131
Table A.2: PL lifetimes of spin-cast films of core-only (PbS) and core-shell
(PbS-CdS) QDs on ZnO, calculated from Fig. 6-7. 𝜏𝑒−1 and 𝜏𝑒−2 are the times
at which QD PL intensity, I, has decreased from its initial value of 𝐼0 to 𝐼 = 𝐼0 /𝑒 and
𝐼 = 𝐼0 /𝑒2 , respectively.
PbS
PbS-CdS(0.2 nm, A)
PbS-CdS(0.2 nm, B)
𝜏𝑒−1 (ns)
0.4
1.2
3.6
𝜏𝑒−2 (ns)
1.1
8.8
17.6
Figure A-9: Transient photoluminescence of QDs on glass. Normalised timedependent PL intensity of spin-cast films of: (black) PbS; (red) PbS-CdS(0.2 nm,
Sample A); and (green) PbS-CdS(0.2 nm, Sample B) QDs, all on glass.
Appendix A. Supplementary information to Chapter 6
A.1
132
Calculation and error estimate of EQE and radiance
A.1.1
EQE calculation
EQE is calculated as the ratio, per unit time, of the number of forward-emitted
photons to the number of injected electrons, 𝐼(V)/ |𝑒|, where 𝐼(V) is the current
passing through the QD-LED at an applied bias, V. We can express this as,
EQE(V) [%] =
𝑁phot (V)) |𝑒|
· 𝑔 · 100
𝐼(V)
(A.1)
where 𝑁phot (V) is the number of forward-emitted photons actually collected by the
photodiode, and the geometric factor, g, accounts for the solid angle of the EL profile
(assumed to be Lambertian) subtended by the photodiode, Ω = 𝜋/𝑔:
𝑔=
𝑎2 + 𝐿 2
𝑎2
(A.2)
Here a is the diameter of the active area of the photodiode and L is the distance
between the emitting QD-LED pixel and this active area.
meas
) of the phoWe calculate 𝑁phot (V) from the current output (‘photocurrent’, 𝐼pc
todiode in response to detected EL at each applied bias, accounting for the wavelength
dependence (weighted by the device’s EL spectrum) of the photodiode’s responsivity,
𝑅(𝜆).
To do this, first note that the area below a wavelength-resolved EL spectrum
measured from a QD-LED, 𝐸𝐿(𝜆), is proportional to the number of photons emitted per unit time. Then, multiplying this by the energy per photon ℎ𝑐/𝜆 and by
[︀
]︀
𝑅(𝜆) AW−1 , we obtain values proportional to measured photocurrent,
′
𝐼pc (𝜆) = 𝐸𝐿(𝜆) ·
ℎ𝑐
· 𝑅(𝜆)
𝜆
(A.3)
Integrating between the wavelength limits of the EL spectrum (𝜆𝑖 and 𝜆𝑓 ), a figure
proportional to total measured photocurrent is then given by,
Appendix A. Supplementary information to Chapter 6
prop
𝐼pc
(𝜆) =
∫︁ 𝜆𝑓
′
𝐼pc (𝜆)d𝜆
133
(A.4)
𝜆𝑖
prop
meas
Scaling 𝐸𝐿(𝜆) to satisfy the requirement that 𝐼pc
(𝜆) equals 𝐼pc
yields a revised
EL spectrum, 𝐸𝐿(𝜆)′ , the area below which is now equal to the number of photons
emitted per unit time.
meas
𝐼pc
𝐸𝐿(𝜆) = 𝐸𝐿(𝜆) · prop
𝐼pc (𝜆)
′
∫︁ 𝜆𝑓
Thus, 𝑁phot =
𝐸𝐿(𝜆)′ d𝜆
(A.5)
(A.6)
𝜆𝑖
Repeating this procedure for all applied biases and substituting 𝑁phot into Eq. A.1
gives voltage (or current density)-dependent EQE, as plotted in Fig. 6-4d. We note
that in the limit - often assumed for relatively narrow QD EL spectra - that emission
meas
is monochromatic, with wavelength 𝜆0 , Eq. A.6 simplifies to 𝐼pc
𝜆0 /ℎ𝑐𝑅(𝜆0 ) and
therefore Eq. A.1 can be rewritten as,
EQE(V) [%] =
meas
𝐼pc
𝜆0 |𝑒|
·
·
· 𝑔 · 100
𝑅(𝜆0 ) ℎ𝑐 𝐼(V)
(A.7)
Eq. A.7 is the simplified expression for EQE often quoted [7].
A.1.2
Radiance calculation
Analogously, the ‘true’ emitted power is 𝑃EL =
𝐿(V) =
∫︀ 𝜆𝑓
𝜆𝑖
𝑃EL (V)
𝐴·Ω
𝐸𝐿(𝜆)′ · ℎ𝑐
d𝜆, giving radiance as,
𝜆
(A.8)
where A is the QD-LED pixel area. Again, this is evaluated for all applied biases,
leading to Fig. 6-4c.
Appendix A. Supplementary information to Chapter 6
A.1.3
134
EQE measurement error
Eq. A.1 indicates that EQE is subject to experimental measurement errors in 𝑁phot ,
I and g, namely 𝜎𝑁 phot , 𝜎𝐼 , and 𝜎𝑔 , respectively. 𝜎𝐼 results from Keithley 2636A
equipment resolution and 𝜎𝑔 from geometric errors in a and L in Eq. A.2. From Eq.
meas
meas , 𝜎𝐸𝐿 ,
A.6, 𝜎𝑁 phot in turn arises from errors in 𝐼pc
, 𝐸𝐿(𝜆), and 𝑅(𝜆), namely 𝜎𝐼pc
and 𝜎𝑅 , respectively. 𝜎𝑅 results from Newport 818-IR photodiode calibration error
meas is due to background noise recorded by the photodiode/Newport Multiwhereas 𝜎𝐼pc
Function Optical Meter 1835-C measurement setup. 𝜎𝐸𝐿 is taken to be negligible.
Total measurement errors, represented by black and green error bars in Fig. 6-4d,
meas , 𝜎𝑅 , 𝜎𝐼 , and 𝜎𝑔 in quadrature, accounting for
are calculated by combining 𝜎𝐼pc
multiplication factors implicit in Eq. A.1. Measurement errors for QD-LEDs based
on all QD types are shown in Fig. A-1, indicated by blue error bars.
A.1.4
Device-to-device EQE variation
Fig. A-1 shows raw EQE-current density data (from about three pixels per device,
and from devices fabricated on two to five separate runs) for QD-LEDs based on all
QD types. These device-to-device variations strictly arise due to systematic errors
because they exceed the measurement errors calculated above. However, as a useful
guide, device-to-device variations are approximated by the standard deviations of raw
data from their mean, shown as grey error bars in Figs. 6-4d, 6-6b, and A-1.
The EQE errors for the ‘champion’ QD-LEDs, indicated by grey error bars in
Fig. 6-6b, represent the standard deviations in EQE of three to five pixels on single
‘champion’ devices.
A.2
QD film thickness calibration
In order to separate the effect of QD shell thickness from that of QD film thickness it
is necessary to accurately control the latter. We do this by varying the concentration
of the spin-cast QD chloroform solution. QD film thickness-concentration calibration
Appendix A. Supplementary information to Chapter 6
135
curves are determined for each QD batch by spin-coating QD solutions of known concentrations onto ZnO (prepared identically to that in the QD-LEDs) and measuring
their film thicknesses with a profilometer. To do this, an edge of each QD film is
wiped-off with a chloroform-moistened polyurethane swab (VWR). Because this procedure does not detectably affect ZnO film thickness, the step-height measured by
profilometer corresponds to QD film thickness. Roughly 15 line-scans per sample (5
line scans (𝜇m separation) at 3 positions (mm separation)) yield the data plotted in
Fig. A-10. Orthogonal distance regressions, accounting for errors in both QD concentration and film thickness, are then used to calculate concentrations required for
the 8 ± 1 nm films used in all QD-LEDs.
Appendix A. Supplementary information to Chapter 6
136
Figure A-10: QD film thickness-concentration calibration curves on zinc
oxide. Black dots indicate average thicknesses (as measured by profilometer) of spincast films of QDs as a function of the QD solution’s concentration in chloroform for:
a, PbS; b, PbS-CdS(0.2 nm, Sample A); c, PbS-CdS(0.2 nm, Sample B); and d, PbSCdS(0.7 nm) QDs. Horizontal error bars indicate uncertainties in concentrations and
vertical error bars show standard deviations in measured thicknesses from multiple
locations per sample. Solid lines are orthogonal distance regressions (Igor Pro 6.1,
WaveMetrics, Inc.) from which concentrations used in QD-LED fabrication were
determined.
Bibliography
137
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