Quantum dots for modern display devices

36
Quantum dots for modern
display devices
Swadesh Kumar Guptaa
, Pawan Kumarb
and
Dharmendra Pratap Singhc,d
a
Department of Physics, DBS PG College, Govind Nagar, Kanpur, Uttar
Pradesh, India, b
Department of Chemical and Petroleum Engineering,
University of Calgary, NW Calgary, Alberta, Canada, c
Unité de Dynamique et
Structure des Matériaux Moléculaires (UDSMM), Université du Littoral Côte
d’Opale, Calais, France, d
Department of Industrial Engineering, EIL Côte
d’Opale, La Malassise, Longuenesse, France
36.1 The magical journey of displays: Big CRT screens to
foldable ones
In the world of growing technology, human beings acquire information around the
globe and entertainment visually via several hardware interfaces such as display
devices which together render the audio/video visuals. Thus, displays have become
an important part of life. In general, displays are the electrically operated devices that
show pictures and text on their screens. The cathode ray tube (CRT) was the first
electronic display device that served our society for many decades until it was displaced
by the plasma TV, liquid crystal display (LCD), TFT display, inorganic light-emitting
diode (LED), and organic light-emitting diode (OLED) based displays, and so on.
Fig. 36.1 depicts the timeline of the evolutionary history of displays. The first demon-
stration of CRT took place in 1897 and it was commercially available in 1922. The
earliest CRT device was monochrome in nature, and it was used in oscilloscopes and
black and white televisions. The images and text are formed on their phosphor-coated
screens due to the continuous projection of electrons via an electron gun. Later on, the
commercial color CRTs were produced in 1954. This technology was used in computer
screens and televisions for almost a half-century. From 1957 to 1961, split-flap and flip-
disc displays were used for unveiling information at the airports and railway stations.
In 1964, the monochrome plasma displays came into existence. In 1968, LEDs were
utilized in the displays, whereas in 1967 vacuum fluorescent displays were used in
consumer electronics. In 1971, the first twisted nematic liquid crystal display (TN
LCD) using nematic LC material came into society [1] followed by the super-twisted
nematic display (STN LCD) in 1984 which was an improved version of TN LCD
having higher resolution panels with 540 × 270 pixels. In 1986, a color thin-film-
transistor liquid crystal display (TFT LCD) was developed which became very popular
for almost a decade.
After two important display developments of the digital light processing (DLP)
imaging device in 1987 by Texas Instruments and full-color plasma display in 1995;
Graphene, Nanotubes and Quantum Dots-Based Nanotechnology: Fundamentals and Applications.
DOI: https://doi.org/10.1016/B978-0-323-85457-3.00013-X
Copyright c
2022 Elsevier Ltd. All rights reserved.

900 Graphene, Nanotubes and Quantum Dots-Based Nanotechnology
Figure 36.1 Evolution of different displays. Individual figures are taken from open web
sources of corresponding display companies to redraw this image.
OLED display appeared into the market in 2003 as a new display device that attracted
the attention of people due to its low power consumption. In the same year, active-
matrix OLED (AMOLED) also came into existence which was preferentially been
used in mobile and tablet screens. The ultrahigh definition (UHD) display devices,
developed in 2013, possess 4x more pixels than a full HD that accommodates more
content on the screen with higher resolution. Such displays allow us to access multiple
windows at the same time. The curved UHD was an advanced version of the UHD
display that was developed a year later. The curved screens provide an “immersive”
experience by allowing a wider field of view.
Quantum dots (QDs)-based backlight has greatly enhanced the color contrast, wider
color gamut, and better optical efficiency for the LC-based displays [2]. QDLED
display was developed in 2015 in which semiconductor nanocrystals (i.e., QDs) were
used to produce monochromatic light of red, green, and blue color [3]. A photoemissive
QD layer is introduced in such a device that uses the blue light as a backlight in order to
emit desired basic colors (i.e., RGB) which enhance the brightness, contrast, and color
gamut by minimizing the light losses and color crosstalk in filters [4]. In a particular
case of QD-enhanced blue phase, LC display combines a major advantage of fringing
field switching and submillisecond response time leading to a unified display solution.
QDLED displays were adopted by the market as an alternative to OLED displays. Most
recently, few mobile displays with foldable features have emerged into the market.
Samsung Company has launched the Galaxy Z Fold 2 in 2020 which is the second-gen
Samsung foldable display. Besides, Samsung Galaxy Z Flip is another foldable device
currently available in the market. Similar to the above-mentioned Samsung products,
Moto Razr, Huawei Mate X, and LG G8X ThinQ are some of the other examples of
foldable displays.
The display market has included different display types (like flat, flexible and
transparent panels, etc.), technology (i.e., OLED, QD, LED, LCD, etc.), applica-
tions (computer and laptop, smartphone, wearable gadgets, vehicle displays, etc.)
and industrial use (healthcare, automotive, defense, etc.). The overall commer-
cial display market size is valued at USD 300 billion at present which is ex-
pected to reach ≈ USD 450 billion by the year 2025 in the Asia pacific only

Quantum dots for modern display devices 901
Figure 36.2 Asia Pacific electronic display market revenue by technology, 2012–2022 (USD
Billion) Reproduced from open source webpage [5].
(Fig. 36.2). This evinces that the display industry is one of the most important
sectors of an industrial revolution which share a vital part of the global budget.
According to a new survey of GLOBE NEWSWIRE, the smart display market is
expected to grow from USD 2.3 billion in 2022 to USD 9.7 billion by 2027.
36.2 Current perspective and challenges in displays
Display in the modern age is not only limited to home and wall screen display
but also gain the inclusion of augmented reality and virtual reality digital world.
Progressive improvement in cutting edge technology of TFT, LEDs, and LCD can
provide solutions to many display applications when combined. In view of different
display applications, three major sections of displays can be realized; large area wall
screens for advertising, home television/computer/mobile/display gadgets, and the
newcomer augmented/virtual reality (AR/VR) displays. These three displays fulfill
different goals and thus require different display parameters. However, one common
necessity for these displays is to produce a large color gamut and high brightness with
good efficiency. Currently, LCD, LED, and OLED are the major display technologies
where the LCD has the most market capture worldwide (Fig. 36.3).
LCD technology has seen several advances from TN displays to in-plane switching
displays [6]. In the current scenario, LCD offers the most cost-effective method for
display demands in various applications with the improved viewing angle, efficiency,
and color performance. The addition of QDs in the backlight unit provides a high
color gamut and thus makes it able to compete with OLED technology on the color
performance scale. Two methods of integrating QDs with LCDs are already in a mature
state, one is the use of QD deposited optical film with backlight unit [7] and the other
is the insertion of QDs directly to the individual LEDs of backlight [8]. The next stage

Figure 36.3 Value propositions of various display technologies.
is the insertion of the QDs in place of color filters to improve the efficiency and color
performance together. However, the flexibility, zero dark level, and the viewing angle
is still been an issue with current LCD technology.
OLED technology, on the other side, provides low power operation, color saturation,
infinite contrast, and most prominently a flexible display solution [9]. The employment
of OLED technology in smartphones and TV devices enables the development of
thinner designs as it has fewer layers in comparison to its LCD counterpart. However,
OLED degrades faster than LCD due to the low life span of different OLEDs and
their high susceptibility toward water and oxygen causing damage to OLED material
[10]. Therefore, OLED requires barrier films with very low water vapor and oxygen
transmission rates for the production of OLED devices and thus is very costly. The
OLED display also fails to provide good images under a bright environment due to
reflection from the OLED display and thus reduces the image quality. Manufacturing
large panel display also require accurate and high precision technology which also
increases the cost and thus OLED are currently popular in mobile and smart watches.
LED display technology has attracted significant attention of researchers and
technologists for different display applications due to its long life and low power
consumption ability [11,12]. Big outdoor marketing displays require high brightness at
reasonable efficiency; hence LEDs are the best option for this purpose. Inorganic LED

displays are the most popular technology for such applications due to low cost, easy
maintenance, high color saturation, and stable operation under varying environmental
conditions. However, using LEDs as pixel requires lowering of LED sizes according to
the required resolution. As manufacturers treat the LEDs as the next-generation display
technology for all sizes, the research on the mini- and micro-LED technology has been
prompted. Based on the size of the LED chip, the technology is divided into two parts:
mini-LEDs and micro-LEDs. High dynamic range is one of the important features for
next-generation displays which require high peak brightness and excellent dark state
of the display system simultaneously [13,14]. This can be achieved in LCD by local
dimming. The multi-zone local dimming for LCD can be inclusively realized via a
direct-lit mini-LED backlight [15]. Fabrication of mini-LED is easy using existing
fabrication tools; however, the large pixel size restricts its use as displays.
On the other hand, the micro-LED display is a new generation LED display
technology, which can make LED units less than 100 microns [16]. Micro-light
emitting diodes (μ-LEDs) have become the focus of display research because of their
excellent properties in terms of brightness, lifetime, resolution, and efficiency. Sony’s
first 55-inch full high definition (HD) μ-LED TV panel with 1920 × 1080 resolution
in 2012 and Samsung’s first consumer modular μ-LED 146-inch TV, (named “The
Wall”), in 2018 have shown broad anticipations for μ-LED applications and strong
interest of manufacturers. However, assembling the RGB LEDs again requires costly
high precision technology. Also, the green and red micro LEDs show poor efficiency
[17,18]. Thus, the down-conversion technology using QDs with blue μ-LED can serve
the purpose of bright and power-efficient displays with a wide color gamut. Although
having promising prospects, micro-LED displays still face technological challenges.
Therefore, the relatively mature mini-LED is expected to be commercialized first while
the micro-LED display technology is still growing.
36.3 Quantum dots: A toolbox for future of display
technologies
The nanostructured material has led to the opportunity to tailor the electrical, optical,
and optoelectronic properties of these materials owing to the quantum confinement
effect (QCE). The semiconductor QDs inherently exhibit the QCE, providing conve-
nient method to tune the physical and optical properties with size variation. The global
structure of QDs includes the core, shell, and ligands. The absorption and emission of
the light attribute to the core, whereas the shell part is responsible for the confinement
of emission and surface passivation within the structure. The ligand layer at the QD
surface provides stability to QDs. QCE together with the certain material composition
allows the emission wavelength (i.e., color) of QDs to be tuned for the entire visible
range. This unique wavelength-selective emission from QDs is the primary advantage
for their promising application in display technology to fulfill true-color with a wide
color gamut [19]. As a second key advantage, QDs offer a thermodynamically stable
single-crystalline lattice that exhibits pure color emission with a narrow emission band
having good stability upon photo-irradiation and external heating effects. Low-cost

solution processability over a large surface area and their compatibility over different
surfaces is the third key of advantages. Though the emission profile of QDs looks
fascinating; some of the major challenges are required to be addressed for better
utilization of QDs for display application.
The first important parameter is the emission wavelength and the full width at half
maxima (FWHM) of the emission curve of QDs which determines the overall color
gamut and color purity provided by the display device. The color gamut defines the
visual characteristic of a display which comprehensively renders the range of colors
that are produced by a display device. Color gamut is defined as the area occupied in
the delimitated color space, e.g., CIE1931, CIE1976 on the chromaticity diagram. As
the current market is aiming to achieve Rec.2020 (BT2020) color gamut, QDs serve
the purpose by tuning their emission accordingly, especially the green emission. The
selectivity of emission wavelength depends on the selected material type, excitonic
bandgap and size of QD. As an example, CdSe shows a Bohr radius ∼5.3 nm and
bandgap of 2.87 eV [20] while for ZnSe; the Bohr radius is ∼4.5 nm and the bandgap is
2.67 eV [21]. Thus, the same size QDs of two different materials will provide different
emissions. However, their FWHM can be different depending upon the size distribution
and surface defects of QDs. As the QDs with FWHM ≤30 nm is considered to be the
best candidate for high color purity and optical efficiency, choosing the correct material
composition is an essential for QD synthesis. The selection of QD material can provide
the desired emission wavelength and FWHM; nevertheless, photoluminescent quantum
yield (PLQY) is significantly affected by the selection of material. PLQY is defined
as the ratio of the number of emitted photons to the number of absorbed photons of
excitation light. The higher the yield, the larger will be the efficiency of QD-based de-
vices. Besides the high PLQY, QDs should exhibit a high optical density for excitation
wavelength to allow low material consumption for the fabrication of a device. The QDs
absorption band should possibly not be overlapped with the emission band to avoid self-
quenching. This can be achieved by obtaining sufficient stoke shift in QD material.
In electroluminescent QD display, the external quantum efficiency (EQE) is a rather
more important parameter that relates the required electric power to the number of
emitted photons. The performance of electroluminescence QD-LEDs remained lower
compared to photoluminescence QD-LEDs due to their charge injection problem and
require high charge mobility with high charge injection density. The long-life span and
display performance of QDs are analogous to their barrier properties against moisture
and oxygen. Presently in QDs-based devices, an external organic and inorganic barrier
layer are used to protect QDs; however, in the last decade, tremendous efforts have
been made on encapsulation of ligand shell that shows a remarkable improvement in
water vapor transmission rate (WVTR) and oxygen transmission rate (OTR) having
least side-effect on emission behavior of QDs [22].
36.4 Quantum dots in display technologies
36.4.1 Quantum dot enhancement film
The very first commercial use of QDs in displays appeared in the form of quantum dot
enhancement film (QDEF) in the backlight unit of LCDs [7,23]. QDEF adds up the new

(A) (B)
Figure 36.4 (A) Schematic diagram of an LCD system with QDEF backlight. The wide-view
compensation films are not shown, (B) Schematic diagram of LCD with QDCFs.
heights to LCD to compete with OLEDs in contrast to the wide color gamut. QDEF
is a semitransparent polymer film with a QD deposited emissive film of thickness
∼100 μm which emits light when excited with blue backlight and produces a white
spectrum together with blue light. The QD film is encapsulated between two barrier
films. Barrier films are used to encapsulate the coated QD film from the ambient
moisture and oxygen. QDEF is fabricated using a polymer-dispersed QDs matrix
coated on the top of the transparent polymer film. QDEF consists of green and red
QDs mixed with suitable polymer compatible with QD ligand shell to provide good
dispersion of QDs. In the most common methods, long alkyl chain ligand units are
attached with QD surface and acrylic polymers are used for film preparation. Usually,
an optically activated photoinitiator is used for the polymerization purpose of QDEF
(Fig. 36.4A).
Mixing a proper amount of red and green-emitting QDs in a polymer matrix is
an utmost important factor to determine the performance of QDEF. High quantum
efficiency with a wide color gamut of the white spectrum from QDEF is the ultimate
goal which depends on the quantum yield and optical density of fabricated QDEF.
QDs in QDEF are isotropic emitters and emit light when excited by the blue light of
the wavelength ∼460 nm. However, only a small part of excitation light is absorbed by
the QD film and most of the blue light leaks out due to the low optical density of the thin
QDEF layer. Thus, the white emission spectrum is not possible with the low absorption
of blue light by QD film. To achieve higher absorption of blue light, scattering particles
of insulating material are also mixed in the polymer matrix. These particles scatter the
blue light and increase the path length inside the film. Hence, the absorption of blue
light by QD film increases and thus improves the efficiency of QDEF. QDEF fits well
within the current production line for LCD units to adopt. QDEF is simply replaced
with the diffuser film and blue LEDs replace white LEDs in the conventional backlight
unit. Hence, no extra effort and investment are sought for adopting this technology
in the LCD line. Samsung and LG have already launched QD displays in the market
entailing QDEF with a blue backlight unit.

LCDs require polarized light to produce images. Though QDs provide a wide
color gamut to LCD, the optical losses due to polarizers cannot be compensated.
Another approach of using aligned quantum rod enhancement film (QREF) shows
some promises on this side by providing polarized light from the backlight unit
[24,25]. Quantum rods emit polarized light due to dipolar emission and if aligned
unidirectionally, it can provide polarized light with a high degree of polarization. Thus,
more efficient LCDs can be realized due to reduced losses through polarizers. Studies
show that the employment of QREF in LCDs can enhance the optical efficiency of
LCD from 4.8% to 8% providing similar color performance as in QDEF based LCD
[26]. However, alignment of QR in the thick film is still a challenging task, which is
needed for high brightness LCDs and requires several new techniques, for example,
ligand modification and alignment method; to achieve a high order of QR alignment.
36.4.2 Quantum dot color filters
The idea of quantum dot color filters (QD CFs) came from the use of QDEF in
LCD backlight [27,28]. QDEF expands the color gamut; nevertheless, the total color
performance and efficiency suffer due to color filters. In QD CFs, the QD material
is used in place of or with the color filter and is excited with a blue backlight. This
provides no subtle change in the working phenomenon of LCD and provides the
different colors and gray scale alike to normal LCD. Since the use of QD CFs facilitates
only a single color to pass through each color filter subpixels rather than the complete
white spectrum as in normal LCD; high efficiency is obtained for LCD due to less light
loss amid absorption by filters (Fig. 36.4B).
Though the idea looks fascinating, the fabrication of QD CFs is challenging and
requires extensive research work. The successful operation of LCD using QD CFs
requires an in-cell polarizer to be developed [29]. Controlled and precise deposition
of QDs is also required for the deposition of QD on micron size pixel format and
cannot be performed manually. Therefore, inkjet printing technology is used to deposit
QD material precisely on the micron size bank structure (Fig. 36.5A). The QD CFs
require sufficient thickness to absorb most of the blue backlight to provide true color.
However, the self-quenching phenomenon increases with increasing thickness in QD
films and thus reduces efficiency. The high thickness of QD CFs also causes a reduced
viewing angle due to color cross-talk between neighboring pixels. Fortunately, the rise
of cadmium-free perovskite QDs with multi-fold absorbance properties, if compared to
traditional cadmium and indium-based QDs, has re-accelerated the initiatives to bring
QD CFs to the mainstream commercial market [30].
36.4.3 Quantum dot emissive displays
Quantum dot emissive display can be divided into two categories; color conversion
quantum dots LED displays (QLED) and self-emissive QD-LED display. QD-LED
should not be confused with QLED display which uses QDs as color conversion
material for LCD’s LED backlight units. QLEDs are based on the photoluminescence
properties of QDs. To utilize the QDs for emissive display, deposition of QDs on top

(A) (B)
Figure 36.5 (A) Schematic diagram of the color conversion-QLED display, (B) QD-LED
basic structure and exciton generation.
of the blue inorganic LED chip is required. Incorporation of QDs into μ-LEDs and
mini LEDs can lead to higher color rendering and saturation to achieve wide color
gamut requirements and different levels of mixed color by independently controlling
different RGB pixels [4,31]. Since the size of pixels is in the micron dimension, it
requires inkjet printing technology to deposit QD precisely on micron-size LEDs. Alike
QDCFs, QLEDs also requires a high thickness of QD emitting layer in order to absorb
sufficient blue light for color conversion. The use of selective reflection coating on top
of the color conversion layer (CCL) using a distributed Bragg reflector (DBR) has been
adopted to overcome this issue [32]. DBR offers a selective reflection for blue light
which is used to excite the CCL that leads to an enhancement in the optical efficiency
with true color. Though, the issue of precisely coated DBR on top of each pixel remains
a challenging task along with its cost-effectiveness.
Self-emissive quantum dots are becoming the next goal of display and received huge
attention in recent years [33,34]. QDs can support large, flexible displays and would not
degrade as readily as OLEDs, theoretically making them good candidates for different
display applications. QDLEDs exhibit a lower efficiency in comparison to OLEDs;
nevertheless, pure color emission, easier tunability of emission color by controlling the
size of QDs and lower emitter cost make them interesting to study. In a self-emissive
display, emission of light by QDs takes place due to electroluminescence leading to
the production of image and color together. In such a device, a QD layer is sandwiched
between the electron and hole-transporting layers where the electrons and holes get
accumulated and recombine under the application of the applied field followed by the
emission of narrow spectrum of photons (Fig. 36.5B).
In the simplest design, QD-LED uses indium tin oxide (ITO) and aluminum (Al) as
anode and cathode layers respectively. Polyvinyl(N-carbazole) (PVK) is used as a hole
transport layer (HTL) and ZnO nanoparticles make an electron transport layer (ETL).
QDs are coated between ETL and HTL layers and work as an emitting layer. HTL
and ETL provide high mobility to charge carriers, facilitate increased recombination
efficiency of holes and electrons in the emitting layer. Balanced electron/hole injection
is an important parameter for efficient QD-LED. This balancing of the carriers is tough,
as most QDs are considered as n-type materials. Hence, the efficiency of these devices
will be low due to lower mobility. Thus, the p-type conductivity and hole injection
barriers of the organic hole transport layer are necessary to improve the efficiency

of QD LEDs. Currently, two main issues remain to be solved before such displays
can be produced commercially: brightness and the effective operating life of the QDs
themselves. At present the blue QDs, in particular, lack sufficient brightness and have
an operating life well short of that required in a consumer display [35,36]. To further
complicate matters, the brighter they become, the shorter will be their operating life.
This leads to a hybrid technology where blue OLED is used to excite the green and red
QDs which is currently being developed by Samsung and TCL [37]. However, other
issues of this hybrid technology are still remained to solve.
36.5 Quantum dot family for displays
36.5.1 Inorganic group II-VI (CdSe) and III–V QDs (InP)
quantum dots
Conventional YAG:Ce3+
phosphor-based LED backlighting devices followed by im-
plementation of β-SiAlON:Eu2+
and K2SiF6:Mn4+
based phosphors are plagued due
to poor color gamut (i.e., YAG:Ce3+
∼72% NTSC), low PLQY, and attainment
of improvement limit. On the other hand, QDs due to the confinement effect can
reach a narrow and tunable emission width, color saturation, good CRI. The initial
transition in QD-based technology was the development of QD-LED-backlit LCD
devices (QLCDs) possessing either polymer sandwiched QD emitting films (QDEF) or
brightness enhancement film (BEF). The first introduction of QLCD TV by Samsung
in 2010 using core-shell-structured CdSe QDs (CdSe//ZnS/CdSZnS) with 104.3%
NTSC has revolutionized the QLCD technology and expected to occupy half of the
displays by 2025. Chalcogenides-based QDs have envisaged the future of the next
generation emitters due to their structural and emission tunability. Group II-VI and
III-V semiconductor QDs are representative of QDs used in display technology. Metal
chalcogenides such as CdSe-based QLCD are already commercialized due to their high
PLQY (∼100%), EQE above 20%, color quality (FWHM ∼20 nm), and excellent
resiliency. In the CdSe QDs based devices, the efficiencies for red, blue, and green
colors have been reached up to 20.5%, 21.0%, and 19.8%, respectively [38]. Due
to a broad visible range spectrum (i.e., 470–640 nm), CdSe QDs have been proven
to be perfect luminophores that could be utilized in a full-color flat panel display.
These QDs render more than 90% PLQY and narrow FWHM (30 nm) leading to
better color quality. Despite these merits, the Cd has negative environmental effects.
Consequently, InP QDs are accepted as a replacement of CdSe QDs. Heavy metal-
free, group III–V QDs based InP QDs are becoming popular alternatives of Cd-based
QDs which possess comparable efficiency (PLQY-green, 95%; PLQY-red, ∼100%)
and color purity (FWHM ∼35 nm) [39,40]. To get optimum performance in the InP
system, it is essential to make a core-shell structure with a large thickness to prevent
nonradiative Förster resonant energy transfer energy loss and Auger recombination
(AR) [41]. To percolate the charge in and out in dense core-shell, the lattice match
(epitaxial match) is an indispensable restriction. The most common and efficient core-
shell architecture is InP/ZnSe/ZnS where ZnS shell thickness determines the PLQY

[42]. The lattice mismatch between the InP core and ZnS shell is high (∼8.5%) which
compromises EQE and stability due to strain [43]. To overcome these issues, Cao
et al. have grown large-size InP/ZnSe/ZnS QDs with ∼15 nm size by layer-by-layer
deposition of ZnS shell under periodic heating and precursor deposition (Fig. 36.6I)
[44]. The resulting QDs exhibit a high QY of 73%, narrow emission width of up
to 40 nm, wide spectrum tunability (from 549 to 617 nm), and excellent stability.
The fabricated InP/ZnSe/ZnS QDs demonstrated a high wavelength tuning (549 to
617 nm), PLQY of 73%, FWHM of 40 nm, with a benchmark EQE of 6.6% for
InP based QDs. Several other modifications like changing the reaction temperature
and precursors, making magic size clusters have been demonstrated to improve the
core and shell quality [45]. Rational design and stoichiometric control of both core
(III–V) and shell (II–VI) can afford near-unity PLQY. To further improve the efficiency,
Won et al. synthesized extremely size-controlled (3.6 nm) InP/ZnSe/ZnS QDs by
oxidative HF etching of InP core and growth of the ZnSe shell at sequential low and
high temperatures (Fig. 36.6II) [46]. For the fabricated InP/ZnSe/ZnS QD-LEDs the
EQE was reached up to 21.4%, with a FWHM of 35 nm at 630 nm. Furthermore
these QDs also displayed highest reported brightness (100,000 Cd m−2
) for InP-based
QDs. Besides, narrowing FWHM (∼15 nm) and improving EQE gives hope for the
development of efficient, heavy metal free QDs, the cost of raw materials, especially
phosphorus precursor is despondent.
36.5.2 Perovskite-based quantum dots
Even though, remarkable progress has been made on QDs-based display technology
with a wide color gamut, harsh reaction conditions, long reaction time, nonuniform
size distribution, unscalable production from earth abundant materials and low PLQYs
remain a challenge. Recently, a new class of QDs called perovskite QDs (PQDs) has in-
vigorated the display technology due to their excellent properties such as color-tunable
narrow-band emissions, high PLQYs, and facile synthesis at low temperature [47–49].
Halide perovskite QDs with a general formula ABX3 where, A is a monovalent cation
such as Cs+
, methyl ammonium CH3NH3
+
, formamidinium (HC(NH2)2
+
) and B is a
divalent cation such as Pb2+
, Ge2+
, Sn2+
Sn2+
, Cd2+
, Zn2+
, Mn2+
, and X is a halogen
usually Cl, Br, or I possess a wide wavelength tunability (400–800 nm) and narrow-
band emission (FWHM ∼20 nm). PQDs have shown great promise for vast numbers
of applications such as LEDs, low-threshold laser, photodetection, phototransistor,
solar cell, and colorful display [50–52]. Based on the presence of organic cation or
inorganic cation in A site the PQD are referred as organometal halide perovskites QDs
(OPQDs) and all-inorganic halide perovskites QDs (IPQDs). The exceptional defense
tolerance system unbiased from intrinsic defects provides excellent optical properties
to PQDs. Interestingly, the emissive wavelength and optical properties of PQDs can
be easily tuned by controlling the halides ratio, temperature-driven quantum size. The
first report on the synthesis of OPQDs CH3NH3PbX3 nanoparticulate appeared in 2014
showing bright green PL [53]. It is observed that IPQDs are superior over OPQDs due
to relatively high stability, easy synthesis, and improved optical properties. Then, first
hot injection synthesis of IPQDs nanocubes (4–15 nm edge length) by Protesescu et al.

910
Graphene,
Nanotubes
and
Quantum
Dots-Based
Nanotechnology
Figure 36.6 (I) (A) Schematic of the synthetic procedure of InP/ZnSe/ZnS core-shell QDs and fluorescent image of QDs under UV light, (B) Band
structure of InP/ZnSe/ZnS QDs and lattice mismatch of InP, ZnSe, and ZnS layers. (C) PL spectra of the resulting InP/ZnSe/ZnS QDs. (D) The
device structure of multilayered InP QLED. (E) Energy levels of individual layers of device (F) CE–EQE–L characteristics (G) Normalized EL and
PL spectra of the device Reproduced with permission from ref. [44] Copyright 2018 ACS. (II) (A) InP/ZnSe/ZnS QDs with different morphology
and shell thickness. (B and C) Electron diffraction spectroscopy mapping of In, Zn, P, Se and S for QD-3R (scale bar, 10 nm) (D) Photoluminescence
spectra of QD-1
(prepared without HF addition), QD-1, QD-2, QD-3, QD-1R, QD-2R and QD-3R. Inset, photograph of QD-1
(no HF) and QD-3
taken under 365 nm illumination (E) UV-visible spectra of the aliquots, taken during the InP core synthesis. (F) EQE–luminance profile. Inset,
photographs of four-pixel QD-LED and text-patterned QD-LED. Reproduced with permission from ref. [46] Copyright 2019 Nature.

using Cs-oleate with a Pb(II)-halide displayed that compositional change of halides in
CsPbX3, X = Cl, Br, I, and mixed Cl/Br and Br/I systems can tune the spectral emission
in the wavelength range 410–530 nm [54]. The fabricated CsPbX3 NCs displayed a
narrow emission line width of 12–42 nm, high quantum yields of 50%–90%, and a wide
color gamut covering up to 140% NTSC (Fig. 36.7C) [54]. After that several variants of
OPQDs/IPQDs have been synthesized by chemical, compositional and morphological
changes showing improved optoelectronic properties. The use of PbX2 salt during the
hot injection PQD synthesis supplies both cation and anion; which restricts precise
tuning of the composition of final nanocrystals. To overcome this issue, Imran et
al. injected benzoyl halide in the metal cation precursor so the number of cations
and anion can be regulated precisely and high-quality PQDs with improved PLQY
(up to 92%), bandwidth (FWHM of 15 nm), and stability were afforded (Fig. 36.7A
and B) [55]. Recently, developed fast anion-exchange methods employing poly-lactic
acid (PLA), Grignard reagents (MeMgX), or oleylammonium halides (OAmX), etc.,
can tune the composition and concomitantly the emission wavelength of CsPbX3
[56,57]. Not only halogens are exchangeable, but A and B sites can also be exchanged
in PQDs. For example, partial cation exchange by doping with Sn2+
, MA/or both to
form CsPb1-xSnxBr3, MA0.5Cs0.5Pb1–xSnxBr3 (i.e., 0 ≤ x ≥ 0.5) has been reported
to enrich photophysical properties of PQDs [58,59]. Intriguingly, metal ions doping
such as Cr3+
, Yb3+
, Ce3+
tri-doped in CsPbCl3 PQDs has been found to increase
the PLQY up to 188% [60]. Unfortunately, perspective applications of PQDs restrain
due to chemical and optical instability. Various stabilization protocols such as surface
passivation using organic ligand, adding thin protection shells, embedding in polymer,
etc., have been developed. Coating PQDs with transparent insulating/semiconductive
materials such as Al2O3, SiO2, TiO2, ZnS, SnO2, α-Zr(HPO4)2·H2O, trioctylphosphine
oxide, alkyl phosphate has been realized using alkoxide precursors showing increased
stability even under harsh water splitting conditions [61–65]. However, compromised
PLQY, the requirement of strict oxygen and water-free environment for some cases
and essentiality of trace water in other cases (TiO2, SiO2) are detrimental for delicate
iodide-based PQDs. An intelligent shelling of magnesium silicate on PQDs was done
by Zhenfu et al. in ambient conditions, where they used already fabricated magnesium
silicate hollow spheres (MSHSs) for the in-situ crystallization of CH3NH3PbX3 PQDs
[66]. The PLQY of the coated QDs-MSHSs was comparable to bare PQDs and retain
80% PL after 72 h compared to 30% for bare PQDs. The CIE color coordinate values
of green nanocomposites (0.141, 0.08), blue nanocomposites (0.127, 0.745), and red
nanocomposites (0.658, 0.29) cover a ∼92% color gamut at NTSC. In operating
perovskite-based LEDs (PeLEDs) devices, the “efficiency roll-off” is a serious issue
characterized by the efficiency declines with the increase of brightness and current
density due to unbalanced charge injection and low ion-migration activation energy
in perovskite active layers, trap-assisted recombination [67]. The efficiency roll-off
value for most of the perovskite QDs-based LEDs with more than 15% EQE, is in
the range of 33%–93% which is much higher and restricts their outdoor application
that requires high brightness over 5000 cd m–2
. In this view, II–VI semiconductor QD-
based LEDs (CdSe/ZnSe QDs LEDs) outperform exhibiting only a 4% efficiency roll-
off. Zhang et al. suggested that the formation of thick semiconductor-coated QDs can

Figure 36.7 (A) PL spectra of CsPbBr3 NCs synthesized by the addition of benzoyl
chloride/benzoyl iodide. (B) Picture of the different CsPbX3 NC solutions obtained by anion
exchange under a UV lamp. Reproduced with permission from ref. [55] Copyright 2018 ACS
(C) Emission from CsPbX3 NCs (black data points) plotted on CEI chromaticity coordinates
and compared to most common color standards (LCD TV, dashed white triangle, and NTSC
TV, solid white triangle). Reproduced with permission from ref. [54] Copyright 2015 ACS (D)
Illustration of Cl vacancy-induced Coulomb trap site formation, electron trapping, and
self-assembly of organic thiocyanate (RSCN) on the defect sites. Reproduced with permission
from ref. [72] Copyright 2020 ACS (E) Practical luminescence peak ranges of different QDs.
Reproduced and modified with permission from ref. [73] Copyright 2020 Wiley. (F) Polyhedral
model of MAPbI3, orthorhombic phase. PbI6 octahedra: gray; the I: brown, and the large
spheres in the cavities represent the CH3NH3
+
cations (N: blue, C: black). (G) Polyhedral
model of Cs2BiAgBr6, cubic phase. Cs: yellow, and Br: pink; the BiBr6 and AgBr6 octahedra
are in dark red and gray. (H) Photograph of a single crystal of Cs2BiAgBr6 (Courtesy of A. A.
Haghighirad, University of Oxford). Reproduced with permission from ref. [74] Copyright
2016 ACS.

overcome the issue of efficiency roll-off due to the creation of an electron barrier that
will suppress carrier imbalance and prove QD’s thermal stability [68]. They further
argued that the CsPbI3/MgxZn1–xTe core structure will be appropriate because it meets
the demand for lattice matching and tuned energy level. With the increased regulation
on color reproducibility and Rec. 2020 standard implemented in 2012, it has become
crucial to develop QD LED TVs which accurately reproduce object colors, for example,
the RGB in wavelength in ultrahigh definition television (UHDTV) should be red:
630 nm, green: 532 nm, and blue: 467 nm with an emission linewidth (20 nm) [69].
Since human eyes are very susceptible to green color and a minute variation can be
noticed, this became even more challenging to satisfy the Rec. 2020 standard requiring
ultrapure green emission in the range of 525–535 nm with narrow FWHM (25 nm).
CsPbBr3 has a green emission lower than 520 nm which requires I−
doping to red-shift
PL emission wavelength. However, I−
doping decreases the stability of PQDs. Yang
et al. stabilized CsPb(Br/I)3 PQDs with tailored I-to-Br ratio inside inorganic glass
via an in-situ nucleation/growth [70]. The compositionally tuned CsPb(Br/I)3 PQDs
displayed exceptional stability with 90% luminescence retention after immersing in
water for one month and no phase separation. Additionally, the CsPb(Br/I)3@glass
exhibited fine-tuned emission 506–532 nm with narrow FWHM of ∼21 nm, covering
up 123% and 92% color gamut of NTSC and Rec. 2020 standards in the CIE 1931 color
space, respectively. Later, Erol et al. embedded CsPbBr3 PQDs in a tellurite glass and
demonstrated that controlled heat-treatment can manipulate the size and concomitantly
PL emission wavelength of PQDs [71]. Size-controlled PQDs in the range of 2.36–
4.89 nm show a wavelength shift in the range of 469 and 520 nm with a narrow FWHM
(16–23 nm) reaching a color purity up to 95.3%. The reported EQE of blue PeLEDs
in the PL wavelength range 460–480 nm (Rec. 2020 standard is 467 nm) is below 5%
and 11% at 480–490 nm which is far less than for green and red PeLEDs the value
is greater than 20%. Efforts to make mixed halide (Br/Cl) perovskites (MHPs) lead
to the generation of Cl vacancies trap centers resulting in poor efficiency and short
operational half-life. Incorporation of pseudohalogens such as thiocyanate (SCN–
) in
the halogen vacancies using n-dodecylammonium thiocyanate (DAT) with excellent
toluene solubility can afford a PLQY of ∼100% at 468.4 nm while pristine MHP QDs
displayed a PLQY of only 83% (FWHM ∼15 nm) (Fig. 36.7D) [72]. Due to the filling
of trap state ∼0.2 eV below the conduction band, a narrow FWHM of ∼17 nm at
471 nm and LED color coordinate at (0.129, 0.087), was obtained complying with
Rec. 2020 specifications.
Figure 36.7E displays the absorption ranges of various II-VI and III–V groups quan-
tum dots and their comparison with PQDs. Interestingly, spectral absorption of PQDs
can be tuned to cover the entire visible spectrum merely by structural modification (i.e.,
halides/ligands exchange) [73]. The basic structure of lead halide-based perovskite
with ABX3 stoichiometry is given in Fig. 36.7F, where PbX6 octahedron surrounds
monovalent cation (such as CH3NH3
+
and Cs+
). Though, organic cations display high
spectral refinement and PLQY, the large cationic radii of organic molecule poses a
severe instability issue [74]. In addition to this, other approaches to increase the charge
transfer dynamic and stabilization of PQDs is functionalizing with ligands such as
benzoic acid (BA), phenylacetic acid (PAA) [75] dodecyl dimethylammonium chloride

(DDAC) [76], etc., and growth in the metal-organic framework (MOFs) [77]. The
use of multiple ligands such as tri-n-octylphosphine, didodecyldimethylammonium
bromide, tetraoctylammonium bromide, and oleic acid with CsPbBr3 (RT-CsPbBr3
PQDs) coupled with K2SiF6:Mn4+
phosphor can synergistically achieve a color gamut
of 124% NTSC for the backlight applications [78]. Other challenges associated with
PQDs are their narrower size distribution and scalable synthesis. The hot injection
colloidal growth of IPQDs strictly obeys the classical LaMer mechanism, which results
in undesirable aggregation and regrowth processes leading to an average FWHMs over
20 nm. Other synthesis approaches such as ligand-assisted reprecipitation, ball-milling
and ultrasonication are crippling due to low-quality defect-rich QDs with minimum
control over size distribution. Furthermore, the use of polar solvents compromised the
stability of IPQDs, and sometimes additional halide ion exchange step is necessary
to make I and Cl containing IPQDs. Tong et al. were able to synthesize IPQDs
nanoplatelets using tip sonication of the precursors in nonpolar solvent, but low yield,
long reaction time restrict their scalable use [79]. Enthusiastically, some reports claim-
ing gram-scale synthesis of PQDs in nonpolar solvent raising hope for mass production.
For example, Li et al. devised a heterogeneous nucleation method to produce gram scale
(≈1.8 g) IPQDs (including blue CsPbCl1.5Br1.5 and red CsPbBrI2) in a nonpolar solvent
using monodispersed silica spheres as the heteromaterial to facilitate the nucleation
process [80]. The plentiful heterogeneous nucleation sites on silica consume a large
proportion of precursors at the nucleation stage resulting in uniform growth of IPQDs.
The CsPbBr3 QDs prepared using the above approach displayed ultranarrow PL and
FWHM of 15.5 nm. Overall, through the novel strategy, blue, green, and red LEDs
achieved good optical properties contributing to a color gamut of 140% NTSC, which is
among the widest color gamut in the field of QLED. The fabricated device structure us-
ing indium-tin-oxide (ITO), poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)
(PEDOT:PSS), poly(9-vinylcarbazole) (PVK), CsPbX3 QDs, 1,3,5-tris(1-phenyl-1H-
benzimidazol-2-yl)benzene (TPBi), LiF (1 nm), and Al (100 nm) layers displayed high
color purity with a color gamut of 140% NTSC standard demonstrate potential use in
high definition displays.
36.5.3 Heavy metal-free quantum dots
Cd-QDs-based electronics with higher than 100 ppm concentration are banned in the
EU (will follow soon in many countries) due to environmental toxicity and health
concerns while Cd-free InP QDs need significant FWHM improvement to enhance
their color purity. So, it is vital to search for some “heavy metal free” QDs with narrow
FWHM and high color gamut [33]. As an alternative of groups II–VI CdSe and group
III–V InP, the group I–III–VI QDs covering the UV to NIR absorption such as CuInS2,
CuInSe2, CuAlTe2, AgGaSe2, and AgInS2 might be a safe option due to their less
toxicity [81]. Additionally, core-shell assemblies such as Cu–In–Se/ZnS, possessing
a tunable emission in the 500–900 nm region have been developed with PLQYs up
to 30% [82]. Further, careful selection composition and core-shells architecture can
increase PLQY, that is, Cu–In–Zn–Se and Cu–In–Zn–Se NCs QDs with ZnS shells
display a PLQY of PLQYs up to 70% [83]. Although QD-LEDs devices with a

maximum luminance of 2100 cd m−2
using ZCIS QDs as emitting materials has been
reported [84], the creation of impurities such as Cu2S, In2S3, and/or ZnS, uneven size
distribution, instability of small particles, presence of high numbers mediated nonra-
diative recombination reduce their PLQY. Alternatively, ZnSe either in standalone form
or with Cu, Mn, S doping and sometimes Al3+
co-doping has also been investigated but
the FWHM and QY remain in the regime of ∼80 nm and ∼60%, respectively [85,86].
Wang et al. reported monodispersed ZnSe/ZnS core/shell QDs can reach PLQYs of
80%, with high color purity (FWHM ∼ 12–20 nm) in the violet-blue range (400–
455 nm). The maximum luminance of these ZnSe/ZnS core/shell QDs was found
2632 cd m−2
while EQE reached as high as 7.83%. Recently, ZnSeTe/ZnSe/ZnSeS/ZnS
QDs with PLQY of 83% and EL outcome of 18,420 cd m−2
has been suggested as a
substitute of InP QDs where PL can be shifted by changing the Te/Se shell ratio [87].
Silicon QDs have also been explored as a replacement for heavy metal-based QDs
[88,89]. Flaig et al. fabricated silicon light-emitting diodes (SiLEDs) device made up
of sized separated silicon nanocrystal (ncSi) with a tunable wavelength emission in
the range of deep red (680 nm) to the orange/yellow (625 nm) with an EQE of 1.1%
[90]. However, stability due to fast oxidization of surface and low quantum efficiency
of SiLEDs remains an issue. Microencapsulation, doping, and surface passivation with
organic ligand found to improve the performance but remains too far to compete with
modern Cd and perovskites-based QDs [91,92]. For example, after hydrosilylation of
SiQDs with 1-decene the EQE was increased up to 3.1% while the brightness was
measured to be 5000 cdm−2
[93]. No QD materials compete with PQDs because
of their excellent photophysical properties. Most of the research in the perovskite
QDs field is limited to the lead-based perovskite halides which are detrimental to the
environment and human exposure causes severe nervous, reproductive, hematopoietic,
renal damages. The use of Pb-based PV modules is allowed in solar panels because
of the exemption from the European Restriction on Hazardous Substances; however,
proper encapsulation and recycling should be ensured to prevent Pb exposure. Indeed,
lead-based PQDs displayed excellent EQE in the green and red region, however, for
fine-tuning the wavelength emission they require partial cations/anions substitution to
form mixed PQDs that further intensify the stability issues. The good news is that
the toxic Pb2+
in lead halide perovskites QDs can be replaced with earth-abundant
nontoxic group 14 metal such as Sn2+
and Ge2+
[94]. After the first report by Jallicose
et al. showing emission spectra can be shifted 470–950 nm by transition from CsSnCl3
to CsSnI3 (PLQE ∼0.14%), several high yields (more than Pb-based PQDs) Sn-based
PQDs have been realized [95]. Indeed, Sn2+
based PQDs such as FASnI3, CsSnI3, and
CsSnBr3, CH3NH3Sn(I1–xBrx)3, with small bandgap has been successfully used for
perovskite solar cells [96,97]. For display application, red emitter phenethylammonium
tin iodide (PEA2SnI4, PSI) perovskite with an emission located at 633 nm, is an ideal
candidate. Unfortunately, Sn-based PQDs suffer from self-doping effects and structural
instabilities due to oxidation of Sn2+
to Sn4+
, leaving undesired Sn2+
vacancies that act
as nonradiative recombination centers and quench emission. Several attempts such as
using SnF2 as Sn2+
compensator, using N2H4, ascorbic acid as reducing agent during
fabrication, compositional change with a more stable structure has been investigated
[96]. Recently, Liang et al. reported the use of H3PO2 (HPA) as a mild reducing agent

which also prevents the formation of the SnI4 complex (phase separation). The HPA
incorporated Pb free PeLED exhibited an FWHM of 24 nm, and CIE x, y coordinates of
the PeLEDs are (0.706, 0.294), close to the Rec. 2020 red standard of (0.708, 0.292).
Additionally, the maximum luminance of 70 cd m−2
was obtained; higher than the
best previously reported red Pb-free PeLEDs [98]. Manipulating structure of PQDs to
A2SnX6 (Cs2SnI6, Cs2Sn(I,Br)6, and Cs2SnX6) with [SnI6]2–
octahedra were found
to be more stable as Sn is present in 4+ oxidation state. Other more stable variants
of PQDs have been developed by incorporating Bi3+
, Sb3+
cations in B-site, that is,
lead-free Cs3Bi2Br9, MA3Bi2Br9, Cs3Sb2I9, MA3Sb2I9 showed significant stability
even in water. Though observed PLQY for Cs3Bi2Br9 (∼19%) and MA3Bi2Br9
(∼12%) is low which can be improved by Eu3+
-doping (∼42.4%) [94,99,100]. Low in-
direct bandgap Cs2BiAgBr6, Cs2BiAgCl6, (CH3NH3)2KBiCl6 with double perovskite
structure has also developed which are showed most promising stability in PQDs family
but their application in display devices is sparse (Fig. 36.7G and H) [74,101]. 2D
copper-based PQDs with an ion stoichiometry of A2CuX4 are evolving candidates
of the PQDs family. The OPQDs such as (CH3NH3)2CuClxBr4–x, C6H4NH2CuBr2I
exhibited low performance while IPQDs Cs2CuX4, X= Cl, Br was found to show
excellent PLQY [102–104]. Yang et al. reported the synthesis of blue–green emissive
(385–504 nm) Cs2CuCl4 Cs2Cu(Br/I)4 QDs with a narrow size distribution of 3.5–3.8
nm, PLQE as high as 51.8% and stability PL retention of 92% after 30 days under
ambient environment [105].
36.5.4 Carbon quantum dots
Since the first accidental discovery of carbon quantum dots (CQDs ∼10 nm) in 2006
during the purification of carbon nanotubes, CQDs have become frontier candidates
for various applications such as bioimaging, sensing, energy conversion, display
technology, etc. [106]. CQDs are spherical or quasi-spherical nanoparticles of carbon
present in graphitic/turbostratic form and possess astonishing optoelectronic properties
[107]. Recently, CQDs have received significant attention for display technology due
to their luminescence properties (PLQY as high as 75%), easy synthesis from earth-
abundant chemicals, and the possibility to tune the PL wavelength and PLQY via
materials genomics [108,109]. The sp2
C core and sp3
C containing surface provide
a blue emission that can be tuned by changing the size of sp2
domains. Additionally,
the bandgap and concomitantly the PL of CQDs can be easily manipulated by N
doping as N atoms contribute 2p electrons in conjugation which shifts the Dirac point
[110,111]. Other doping such as S, F, P, etc., has been realized to improve the PL
behavior of CQDs [112–114]. The most promising approach to tune the wavelength
and PL is controlling the size and shape of CQDs which can be achieved by using
appropriate precursors. Citric acid and urea are the most common precursors used
widely to synthesize blue, green to red-emitting CQDs where urea supplies nitrogen
to the system [115,116]. It is worthy to note that mere doping is not enough but the
nature of doping in the carbon sp2
framework greatly influences the PL emission
wavelength. Hola et al. demonstrated that graphitic Ns triggers red fluorescence in the

CQDs and by controlling the numbers of graphitic Ns, a full spectrum of visible light
can be covered [117]. Additionally, hydrothermal growth and resulting sp2
domain
size of CQDs are also greatly influenced by the nature of the solvent. Tian et al. were
able to control the size and resulting fluorescence of CQD by using different solvents
c.a. water, glycerol, and DMF [118]. They observed that the particle size of CQDs
increase in the order of waterglycerolDMF and the maximum PLQY of 30%–40%
in solid-state was obtained. Substituting, the use of citric acid and urea, several other
high PL and bandwidth yielding precursors have been recently used. Benzene ring
containing precursors are specifically important as by introducing a range of groups,
the PL can be tuned. Wang et al. prepared red CQDs (R-CQDs) with a QY up to 53%
by sequential dehydrative condensation and dehydrogenative planarization (DCDP) of
1,3-dihydroxynaphthalene [119]. The solubility of QDs is another issue that can be
solved by using appropriate precursors. For example, red-emitting CQDs (607 nm)
prepared by polymerization of nitrofunctionalized phenanthrene showed significant
organic solubility and yielded PLQY of 65.93% [120]. In another report, Wang et
al. reported that synthesis of CQDs using o-phenylenediamine (oPD) precursor and
specific acid reagents such as 4-aminobenzenesulfonic acid (4-ABSA), folic acid (FA),
boric acid (BA), acetic acid (AA), terephthalic acid (TPA), and tartaric acid (TA)
in solvothermal reactions can afford a whole visible spectrum emitting QDs [121].
These QDs displayed a PLQY as high as 72% rarely reported in CQDs. Since the
triangle is the most stable structure in nature and theoretical investigations on the
168 and 132 sp2
-hybridized C atoms model suggested that triangular graphene QDs
(T-GQDs) can have a significant advantage over the conventional spherical shapes
[122,123]. Motivated from these findings, Yuan et al. synthesized triangular QDs
(T-CQDs) by controlled polymerization of phloroglucinol and its trimer as a precursor
followed by careful separation covering the whole visible range [124]. Getting violet
luminescence is particularly important because it has wide use in medical science,
solid-state lighting, high-density information storage, display technology. The main
source of violet light is the conventional incandescent source, toxic Cd2+
or Pb2+
based
QDs, and expensive high bandgap gallium nitride or indium gallium nitride. CQDs are
an attractive alternative to replace these undesirable violet light sources. Wang et al.
demonstrated an easy synthesis of violet, fluorescent CQDs using perylene-3,4,9,10-
tetracarboxylic dianhydride (PTCDA) via a hydrothermal approach [125] showing a
PLQY of 23.9%. The LED device fabricated by blending with poly(vinyl carbazole)
(PVK) displayed wavelength emission centered at 408 nm with CIE color coordinate
of (0.180, 0.121). Although the obtained EQE was only 0.831% with a luminance of
163 cd cm−2
but comparing with perovskites QD-based LED this performance was
almost twice.
36.5.5 Importance of surface modification of QDs for
display purpose
The QD-LEDs packaging is done by mixing of QDs with a polymer matrix followed
by film fabrication on a chip and encapsulation. With the development of printing

technology including 3D printing, it became easier to fabricate smooth and uniform
films on the device [2,126]. Mixing of QDs with polymer matrix is the most vital
step which needs high compatibility of QDs with the polymer matrix. As synthesized
QDs remain suspended in some organic solvent and solvent removal step could lead to
agglomeration. Further, the hydrophobic surface of typical chalcogenide QDs makes
it difficult to blend with polymer and can also react adversely, and lead to aggregation,
polymer modification, and deactivation of PL [127]. In the case of PQDs, these effects
further intensify due to their instability and easy structural change via ligand exchange.
To increase the polymer-QDs compatibility, surface modification of QDs is the best
approach that improves dispersibility and resiliency of film [22]. Various compatible
ligands are investigated for the surface functionalization of QDs such as long alkyl
chain, aromatic groups (phenyl, pyridine), and polymeric ligands (polystyrene, etc.).
The attachment of ligands on the surface of QDs reduces fragile dangling bonds,
increases dispersibility in the solvent, and sometimes also facilitates better charge
transport. The two most common category of ligands are: (1) L type which coor-
dinates to metal using lone pairs such as alkyl phosphines (R3P), alkyl phosphine
oxides (R3PO), and alkylamines (RNH2), and (2) X-type which covalently bind to
metal through negative ions such as alkyl carboxylic acids (RCOOH), alkyl thi-
ols (RSH), and alkyl phosphonic acids (RPO3H2). X-type ligands are more robust
than L-type due to strong bonding and increase optical and thermal stability [128].
During PQDs synthesis, surface capping agents such as oleic acid and oleylamine
are added to stabilize the QDs which increase their stability and dissolution in or-
ganic solvents. However, these ligands can be easily dissociated from the surface
when polar antisolvents are used rendering their stability a huge issue. Addition-
ally, the hydrophobicity of the ligand tail promotes undesirable behaviors. The OA
ligand in the PQDs can be easily replaced with other ligands such as zwitterionic
3-(N,N-dimethyloxtadecylammonio)-propane sulfonate, Didodecyl Dimethylammo-
nium Sulfide, NH2-polyhedral oligomeric silsesquioxane which increase the perfor-
mance and stability [129]. Furthermore, introducing steric hindrance in ligands by
using bulky ligands such as tri-n-octylphosphine, tri-n-octylphosphine oxide, tetrade-
cylphosphonic acid (TDPA), and 2,2-iminodibenzoic acid also increase the stability
of QDs. Luo et al. observed that when OA ligands in MAPbBr3 is replaced with
branch-capping ligands, (3-aminopropyl)triethoxysilane) APTES, and PSS- [3-(2-
aminoethyl)amino]propylheptaisobutyl-substituted POSS (NH2–POSS) the function-
alized CsPbBr3 showed higher stability due to steric hindrance while APTES prevents
protic solvents from reacting with the core due to their hydrolytic properties [130].
Under optimized conditions, organic ligands can also promote the reduction of noble
metal (Au, Ag) on the perovskite interface which works as a charge capturing agent
[131]. Koscher et al. demonstrated that a small thiocyanate ligand can replace 10%–
15% of the surface ligands and fill the trap sites while balancing the excess of Pb
resulting in trap-free crystal with PLQY reaching almost unity [132]. Further stability
of PQDs can be increased by functionalization with polymers which prevents access
of air and moisture to the surface of QDs. For example, polystyrene coated hollow

CsPbBr3 PQDs were synthesized by using amphiphilic star-like poly(acrylic acid)-
block-polystyrene (PAA) and PS blocks copolymers followed by ATRP polymer-
ization. Interestingly, the PL emission of PQDs is polymer chemical and polymer
thickness-dependent [133]. Coupling with blue and red-emitting QDs the optimum
CIE coordinate of (0.31, 0.30) and correlated color temperature (CCT) of 7000 K was
obtained which was fairly close to the standard white color (0.33, 0.33). PMMA is also
frequently used for the surface functionalization of PQDs reducing the anion exchange
and stability under long-term light illumination [134]. CsPbBr3 PQD functionalized
with ethyl vinyl acetate as a hydrophilic polymer displayed a color purity of 92% PLQY
of 40.5% and extreme water stability [135]. Sadly, a large fraction of PL is lost during
the fabrication of film from colloidal solution due to the introduction of defects states
promoting nonradiative recombination. The use of proper ligands in the solids state can
increase the stability of film and also facilitate charge transfer. While fabricating the
film to make a device, usually one side is passivated from ligand while the second side
is neglected and can work as a trap site. In a recent report, Xu et al. fabricated an ECL
device by passivating both sides of PQDs film with organic molecules (phosphine oxide
molecule, diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1)) resulting in an
increase of PLQY from 43 to 79% [136]. At the same time, the electro-optic conversion
efficiency of QLEDs was increased to 75 cd A−1
than the initial value, while EQE
jumped to 18.7% from 7.7%. The use of the conductive polymer is also explored widely
which enables fast charge carrier transport. For example, CsPbBr3 PQDs encapsulated
with polypyrrole (PPy) afforded enhanced charge transport properties while showing
improved water stability [137].
36.6 LCD vs OLED vs QLED
LCDs and OLEDs are currently the most popular display technologies which have
acquired the majority of the market. LCD suffers from low color gamut and backlight
leakage. OLED overcomes these problems on the cost of high barrier requirements
and reduced display life. LCDs require a backlight source or reflector to generate
light propagating through a liquid crystal matrix and color filters to produce images
in color. OLED displays are self-emissive, using a thin film transistor (TFT) backplane
to drive each pixel ON and OFF. QLEDs on other hand promise superior quality;
however, the pricing is still under investigation for further research and development.
Self-emissive QD-LED display can resolve all such issues belongs to LCD and OLEDs
providing high color gamut, true dark level, low barrier requirement with longer display
life. However, low efficiency of QD-LEDs is still an issue to be solved. QDs with μ-
LED, that is, QLED can provide even better solutions with unmatchable brightness
at lower power to display with wide applications from AR and VR displays to big
wall displays. With the advancement of QDs properties, micro-LED displays using
QD color conversion (QDCC) materials have been populated where the red and green
QDs are placed at the front of the pixels and excited by the blue light. QDCC has
replaced the inefficient absorptive color filter and provides a highly saturated color
and viewing angle. QD-LED technology can optimally take advantage of all intriguing

Table 36.1 A comparative chart for LCD, OLED, and Q-LED display [138,139].
Display technology LCD OLED QLED
Mechanism Color filter + back light Self-emissive Down-conversion
QDs + μLEDs
Luminance (Cdm−2
) Medium Low High
Luminous efficacy Low Medium High
Power consumption Medium Medium Low
Contrast ratio low High High
Lifetime Long Medium Long
Response time ms μs ns
WVTR requirement 10−2
gm−2
d 10−5
gm−2
d 10−2
gm−2
d
Flexibility Low High High
Cost Low High Under RD
properties of QDs and achieve excellent color performance, elegant pixel design, and
color patterning, device lifetime approaching commercial level and can be printed at
low-cost. Although, OLED exhibits an organic color emitter and renders initial success
but wide linewidths (50 nm) limit color gamut and their further expansion. So,
developing emitters with narrow bandwidth and wide color gamut is crucial to develop
efficient and UHD devices. A relative comparison of these display technologies is given
in Table 36.1.
36.7 Future opportunities and recycling of
display devices
With the development of new technologies, the display industry is anticipated to grow
significantly at a compound annual growth rate of 6.0% from 2019 to 2025. The key
factors that are mainly responsible for the growth of commercial display market include
quick urbanization in developing economies and the growing adoption of commercial
displays for digital advertisements. To produce cheaper displays with better legibility
(such as high luminance, high contrast, low specular reflectance, nonglare, and wide
viewing angle) and low power consumption would be new future challenges for wear-
able display devices, transparent and foldable displays. The current trends of display
requirements in different modern applications will bring new business opportunities
and smart implementation in the global screenless display market. QDs-based micro-
light-emitting diodes (μ-LEDs) are presumed as the foundation of next-generation
display devices that can fulfill the demands of advanced applications, such as micro-
projectors, mobile phones, virtual/augmented reality, wearable watches, ultrahigh-
definition TVs, and so on; however, the LED chip size and poor luminance and
resolution of displays due to the low absorption cross-section remain the challenging
tasks of such display devices. The use of QDs could be a plausible solution to overcome
such demerits of μ-LEDs displays. Of course, the QD characteristics on their μ-LED
display performance are necessary to reach this goal.

ITO is used at a large scale to manufacture transparent conductive coatings for touch
and flat-screen LCD, LED, and other types of display panels. After their life cycle in
display devices, ITO is discarded than recycling. The indium and tin compounds are
not environmentally safe, and it could damage the human organs like the heart, kidney,
and liver, and probably also to the teratogenic. Therefore, it is essential to recycle
ITO for national strategies about resource conservation and to make the environment
healthy. As far as the environmental issues to recycle the display devices are concerned;
the scientific community has started their efforts by recycling the ITO [140,141]. In
general, ITO consists of a mixture of indium (III) oxide In2O3 (90 mass%) and tin (IV)
oxide SnO2 (10 mass%), in which individual percentages of In, Sn, and oxygen are to
be 74, 8, and 18 mass%, respectively. Recycling of indium and ITO from discarded
LCD screens has been carried out using several methods such as pyrometallurgy [142],
hydrometallurgy [141], vacuum pyrolysis [143], mechanochemical treatment [144],
solvent extraction [145], electrochemical method, and acid treatment [146], using
macroporous resins [147], ultrasound-assisted acid leaching [140], etc. In 2007, a basic
hydrometallurgical method was developed using which the LCD panels are chemically
treated to recover indium; whereas, a pyrometallurgical process for recovery of metallic
indium and tin from ITO Scrap was reported in 2011 [142]. The pyrometallurgical
process is a two-step process that includes the reduction of ITO into In-Sn alloy by
CO at a low temperature as a first step followed by the second step of vaporization of
In-Sn alloy at a higher temperature. Due to the difference in vapor pressure of Indium
and tin, Indium vapors are cooled and recovered as metallic indium. Again in 2011,
the solvent extraction method was developed that includes dissolving ITO into 1 M of
H2SO4, then extracting indium and tin to D2EHPA followed by selective stripping of
indium into 1.5 M of HCl. This method was superior in comparison to the dissolution
of ITO in acidic media like 1 M of either H2SO4 or HCl or HNO3 took a long time.
In addition to this, the concentration of acid was also found to have a major effect on
both the amount and rate of leaching [145]. Later, Yang et al. have jointly examined
the leaching kinetics and solvent extraction. They performed a screening test for
the extraction and separation of indium from HCl or H2SO4 with DEHPA, TBP, and
Cyanex 272 or Cyanex 923. They obtained more than 99% of indium from the aqueous
feed having a purity of 90%, by extracting metal ions from 1 M or 0.1 M H2SO4 to 0.1
M DEHPA diluted in kerosene and back-extracting with 1 M HCl [148]. A modified
leaching method that includes the conventional grinding and electrical disintegration
was proposed by Dodbiba et al. in 2012 [149]. This method claimed that the electrical
disintegration is the most effective liberation method, since it fully liberated the
indium containing-layer, ensuring a leaching capacity of 968.5 mg-In/kg-LCD. This
technique also ensures the highest leaching capacity for indium along with the lowest
environmental burden. Hasegawa et al. [144] have introduced a new mechanochemical
treatment technique using aminopolycarboxylate chelants (APCs) to extract indium
from end-of-life LCDs. In this process, APCs form stable complexes with the indium
deposited on the ITO-glass and the mechanochemical treatment induces the destruction
of ITO crystalline structure by facilitating the increased indium dissolution with the
chelants. The extraction of indium was found to be better at the acidic pH condition.

Figure 36.8 Graphical representation of hydrometallurgical recovery process of indium from
waste LCD panels. Reproduced with permission from Ref. [141].
By using an electrochemical method followed by acid treatment, the recovery
of ITO and glass substrate from discarded TFT-LCDs, without crushing the glass
substrate was developed by Choi et al. in 2014 [146]. In this process, they recovered
75% ITO via oxygen evolution lifting of the ITO layer from the glass substrate.
Following this method, the authors were succeeded to recover the glass substrate
by removing the color filter and black matrix by using an acid solution. In another
study [147], the crushed ITO was subjected to a mechanical treatment followed by
its mobilization into an acidic solution (such as HCl:HNO3) under the continuous
propagation of the ultrasonic waves. Three macroporous polystyrene-divinylbenzene
resins (Lewatit TP 208, Lewatit TP 260, and Amberlite IRA 743) were used to adsorb
the indium. The adsorbed In(III) onto the resins was effectively desorbed in an acidic
medium to prepare concentrated indium solution. Authors have also optimized various
processing parameters like pH, the weight of resin, contact time, temperature and
type of resin. In 2018, Souada et al. [140] have proposed an ultrasound-assisted acid
leaching method to extract indium-tin-oxide from waste LCDs. This technique has
many advantages such as fast and controllable kinetics, high extraction yield of indium
and tin, selective recovery of these two metals possible, etc., as compared to the
previously reported methods. They obtained a nearly quantitative indium yield by using
an acid concentration of 18 mol L-1
under the application of ultrasonication. Most
recently, Lahti et al. [141] have explained a recovery process of indium from waste
LCD Panels using an enhanced hydrometallurgical method. In this method, the authors
have first crushed the LCD panels and leached them with 1 M H2SO4. They obtained
around 97.4% yield under fast kinetics of 2 min. They used an ultrafiltration process
to remove the dissolved organic material from the leachate which was concentrated
with nanofiltration before liquid-liquid extraction for indium purification. In Fig. 36.8,
the schematic of the enhanced hydrometallurgical recovery process of indium from the
waste display panel is presented. In the future, plenty of opportunities are yet to develop

for recycling not only the indium or ITO but also the liquid crystalline compound, QDs,
and other chemicals from the waste display screens.
36.8 Conclusions
In summary, the present chapter has described a comprehensive overview of various
display technologies and their principles of working. As far as modern display tech-
nology is concerned, we presented QDs as futuristic materials for display devices.
The unique size-dependent optical and optoelectronic properties of QDs made them
pertinent for modern displays exhibiting high-power efficiency, wide color gamut,
better contrast, etc. Rigorous efforts are continuously in progress to make QDs as
perfect display materials for next-generation devices. This chapter provides the key
aspects of the current display technology and the challenges which can be tackled
by the use of QDs using different fabrication methods. QDs show their prominent
role in future displays when combines with current display technology, for example,
LCD, OLED, and micro-display, as well standalone self-emissive QD display. Each
technology has its own merits depending upon the fabrication cost, durability, lifetime,
brightness, contrast, and most importantly the energy consumption. All such display
applications of QDs can survive only if the QDs have high QY/EQE with well-defined
emission and absorption spectrum. Along with these material properties; QD material
selection is utmost important factor for its stability against temperature and ambient
conditions. Cd based QDs show high QY with well-defined narrow emission spectra
which suits for different display application, however toxicity of such materials is of
big concern. Therefore, researchers are moving to other materials like Indium based
QDs and different perovskites. A brief detail of such effort on different QD materials
has been included in this chapter which shows a remarkable improvement in QY and
emission properties to match with the requirement of QD based displays. Though the
QDs promises the optimistic view of advanced future displays, the further challenges
still remain to tackle are; highly durable QDs free from defects, minimum aggregation,
high barrier properties, maximum quantum efficiency and their biodegradability.
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