Conjugated Polymer Light-Emitting Diodes

Abstract

The development of organic light-emitting diodes (OLEDs) has attracted considerable interest in innovations for our daily life and future. They have promising applications in flat panel displays, energy-saving, eco-friendly, thinner and smaller in size, lightweight, and cost-effective fabrication process. For the development of OLEDs, numerous conjugated polymers have been studied due to their semiconductor nature, which is being associated with pi bond delocalization along the backbone of the polymer chain. Conjugated polymers are highly recommended for electroluminescent devices such as OLEDs. The chapter comprises basic knowledge of polymer light-emitting diodes, their construction, device function, and use of conjugated polymers in blue, red, green, and multicolored light-emitting diodes along with challenges and their future perspectives.

Full text

77
Inamuddin, Rajender Boddula, Mohd Imran Ahamed and Abdullah M. Asiri (eds.) Polymers for
Light-Emitting Devices and Displays, (77–98) ©2020 Scrivener Publishing LLC
4
Conjugated Polymer Light-Emitting Diodes
Sapana Jadoun1,2* and Ufana Riaz2†
1School of Basic & Applied Sciences, Department of Chemistry,
Lingaya’sVidyapeeth, Faridabad, Haryana, India
2Material Research Laboratory, Department of Chemistry, Jamia Millia Islamia,
New Delhi, India
Abstract
e development of organic light-emitting diodes (OLEDs) has attracted consid-
erable interest in innovations for our daily life and future. ey have promising
applications in at panel displays, energy-saving, eco-friendly, thinner and smaller
in size, lightweight, and cost-eective fabrication process. For the development
of OLEDs, numerous conjugated polymers have been studied due to their semi-
conductor nature, which is being associated with pi bond delocalization along the
backbone of the polymer chain. Conjugated polymers are highly recommended for
electroluminescent devices such as OLEDs. e chapter comprises basic knowl-
edge of polymer light-emitting diodes, their construction, device function, and
use of conjugated polymers in blue, red, green, and multicolored light- emitting
diodes along with challenges and their future perspectives.
Keywords: Conjugated polymers, organic electronics, OLEDs, PLEDs, band gap,
blue, green, and red emission region
4.1 Introduction
Light-emitting diode (LED) is a semiconductor light source that emits light
in response to an electric current [1]. In semiconductors, electron recom-
bine with holes, releasing energy in the form of photons [2]. e color of
the light (corresponding to the energy of the photons) is determined bythe
*Corresponding author: sjadoun022@gmail.com; sapana@lingaysuniversity.edu.in
†Corresponding author: uriaz@jmi.ac.in

78 Polymers for Light-Emitting Devices and Displays
energy required for electrons to cross the band gap of the semiconduc-
tor [3]. Nowadays, organic light-emitting diodes (OLEDs) have attracted
considerable attention of researchers, scientists, and industrial aspirants
and bring innovation to our lives due to its unique properties such as low-
cost, wide viewing angle, exibility, lightweight, low power consumption,
and thin panel thickness [4–8]. OLED device architecture includes the use
of an organic carbon-based lm which is made up of some conjugated
polymers, sandwiched between two charged electrodes. ese electrodes
are the transparent anode (generally glass) and the other is a metallic
cathode [9]. OLEDs have smart features of ultra-thin and ne image-
quality along with lightweight as self-emitting devices [10]. Nowadays,
they are very familiar to general consumers and civilians due to their uses
in smartphones like Samsung’s Galaxy series as well as in the manufac-
turing of large TV displays [11]. OLED can be fabricated by small mol-
ecules or polymers. e former can be fabricated by vacuum deposition
method and polymer organic light-emitting diodes (PLEDs) are generally
developed by solution deposition method [12, 13]. e latter method has
attracted wide attention due to its ease of preparation and low cost [14].
e process of fabrication of PLEDs can be carried out at room tempera-
ture so that organic exible substrate can be a platform for device fabri-
cation [15]. is technology allows polymers to fabricate full color PLED
pixels by various printing methods or techniques like dye diusion [16],
laser-induced thermal transfer [17], screen printing [18], inkjet printing
[19, 20], and patterning with the photolithographic process [21, 22]. In the
comparison of small molecules OLEDs, these patterning methods make
PLEDs more useful in the larger size and high-resolution display [23]. For
PLEDs, three colors, i.e., red [24], green [25], and blue [26] used for full-
color display for which various types of conjugated polymers have been
proposed and synthesized.
Conjugated polymers are commonly referred as “Organic Macro-
molecules” having conjugation of the single and double bond in their poly-
mer backbone chain that makes them intrinsically conducting in nature
[27]. In these organic macromolecules, the overlapping of p-orbitals takes
place resulting delocalized π-electrons system which is responsible for
unique optical and electronic properties [28]. In conjugated polymeric
chain, injection of hole and electron via π (bonding) and π* (antibo-
nding) can create a self-localized excited state by delocalization of
charge in valence and conduction bands which on emission can decay
radiatively and revealed outstanding potential in electroluminescent
devices [29]. e generation of charge carriers such as polarons, bipo-
larons, and solitons are responsible for the conduction mechanism [30].

Conjugated Polymer Light-Emitting Diodes 79
ese conjugated macromolecules show changes in electrical and opti-
cal properties when doped or functionalized by chemical species [31].
A numerous variety of conjugated polymers have been developed and
evaluated since 1977, since the discovery of electrical conductivity in
polyacetylene discovered [32]. ese materials include polyanilines
[31, 33] and their derivatives such as poly(o-phenylenediamine) [34,
35], polyluminol [36, 37], and some other polymers like polypyrroles
[38], polynaphthylamines [39–41], polythiophenes [42], polyanisidine
[43], polycarbazoles [44–46], polyphenylenevinylene (PPV) [47], and
their derivatives [48], polyphenylenes [49], polyuorenes [50], and pol-
yaryleneethynylenes [51]. ese have attracted numerous industries and
academics for a dierent type of optoelectronic applications. Conjugated
polymers have drawn considerable attention due to their strong uores-
cence emission [52], excellent hole transport ability [53], exibility [54],
and solution processability [55–57]. Numerous reports have been pub-
lished regarding enhancement of processability of conjugated polymers
by blending or composite formation [58–61].
Conjugated polymers which are processable by solubility are the
most promising candidate for optoelectronic devices such as LEDs [62].
Enhanced solubility and processability of conjugated polymers help in
preparing an emissive electroluminescent thin lm of organic compound
which emits light in response to an electric current in optoelectronic
devices [63, 64]. Chemical structure and extended conjugation in such
type of polymers provides controlled morphology, enhanced solubility
and processability for using them in OLEDs via preparing an emissive
electroluminescent thin lm of organic compound which emits light in
response to an electric current [65]. In addition, extended conjugation
also provides a low band gap as a functional property for these applica-
tions [66].
4.2 History, Classication, and Characteristics
ofPolymer OLED Material
In Cambridge University, a group worked on conjugated polymers and
found electroluminescence in these materials in 1989 [67]. In this nding,
the device had a very short lifetime of some minute and very weak exter-
nal quantum eciency (EQE) of 0.1%. Some of the companies worked on
the progress of polymer OLEDs material and optimization of these devices
fastly such as Sumitomo Chemical Co. Ltd, Cambridge Display Technology
(CDT), Dow Chemical and Covion, etc., and the result of the research was

80 Polymers for Light-Emitting Devices and Displays
achieved as several tens of thousands of hours long lifetime and high EQE
of about 5%~10%.
OLEDs material can be broadly classied into two groups on the basis
of emissive materials which includes small molecules based OLEDs and
polymer-based OLEDs [68, 69]. Small molecules-based OLED can be fab-
ricated by the vacuum deposition method [70] and polymer-based OLED
can be developed by the solution deposition method, Table 4.1 [71]. e
polymer material can be conjugated or non-conjugated further for the
fabrication of OLED while dendrimers play the role of intermediate of
polymers and small molecules, Figure 4.1. e multilayer device struc-
ture is used in small molecule OLEDs, and so there is a requirement of
more amendments in chemical structures in addition to solubility while
polymeric material to form OLEDs is willingly soluble by ink solvents and
makes it a useful material for wet process or printing process discussed
earlier in Section 4.1.
Copolymerized or modied conjugated polymeric materials work with-
out a multilayer structure in OLEDs [72]. In these devices, one of the most
important parameters in emission color and this is controlled by incorpo-
ration of emissive moieties into the polymeric backbone, which is generally
a wide band gap structure [73].
Table 4.1 Comparison of small molecule LED and polymer LED (Reprinted from
Chizu Sekine et al., 2014 Sci. Technol. Adv. Mater. 15, 034203).
Small Molecule
Cathode Cathode
Electron injection layer
Electron transporting layer
Emissive layer
Hole transporting layer
Hole injection layer
Anode (ITO) Anode (ITO)
Buer layer
Multi function
Emissive layer
Glass substrate Glass substrate
Polymer
Process Dry process
(Vacuum
evaporation)
Wet process
Source
Printing (IJ etc.)
Simple layer structure (2-3)
Simple process, scalable
Integrated function
Performance (esp. LT)
Patterning technology
Shadow mask
Complex layer structure (5-6)
Complex process
Separated function
Layer structure complexity
Diculty in mask patterning
Patterning
Structure
Material
Issue

Conjugated Polymer Light-Emitting Diodes 81
4.3 Polymer OLED Device Construction
andWorking
An OLED is a 100- to 500-nm-thick solid-state semiconductor device and
about 200 times smaller to human hair [74]. e structure of OLED com-
prises a cathode, an emissive layer, a hole injection layer and an anode. In
OLED there is an interlayer sandwiched in between cathode and anode
electrodes [75]. e interlayer is composed of an organic layer in which the
delocalization of pi electrons takes place and make this layer conductive in
nature [76]. ese materials are known as organic semiconductors due to
the conductivity between conductors and insulators [77]. In organic semi-
conductors, their HOMO (highest occupied molecular orbitals) and LUMO
(lowest unoccupied molecular orbitals) are considered as valence and con-
duction band present in the inorganic semiconductor [78]. An interlayer
between hole injection and emissive layer makes an improvement in the
device structure as well as in its performance due to cross-liking properties
of the interlayer [79]. In polymer OLEDs, an interlayer is responsible for
the enhancement in the emission eciency. is layer is placed in between
the hole injection layer and the emission layer. e function of this layer
is hole-transporting as well as electron and exciton blocking. In summary,
this layer is responsible for the separation of the hole injection layer and
emission zone, as well as due to its electron blocking property, it accu-
mulates electrons at the interface of the interlayer [80]. However, modern
Small
Molecule
Dendrimer
Polymer
Conjugated type
Non-conjugated type
Emissiv
e
Ma
terials
Pendant type
Host-guest type Polyvinyl carbazole
(+ uorescent/phosphorescent
emitters)
Poly(phenylene vinylene)
(PPV)
nnn
n
CH
N
CH2
RR
Polyuorene (PF) polyphenylene
(PPP)
Fluorescent
Phosphrescent
(Single component)
(Host-guest type)
Figure 4.1 Classication of polymers according to emissive materials. (Reprinted from
Chizu Sekine et al., 2014 Sci. Technol. Adv. Mater. 15, 034203).

82 Polymers for Light-Emitting Devices and Displays
OLEDs are fabricated by a very simple bilayer structure having a layer of
emissive material and other is conductive material [9]. e basic OLED
structure is shown in Figure 4.2.
Polymer OLEDs device performance is based on its emission eciency
and its lifetime. Its emission eciency was discussed by Tokito and his
labmates [81] and expressed as:
Ψ = Υ.ne−h.Ψph.(1 – Q) (4.1)
In the above equation, Ψ stands for electroluminescence eciency, Υ
stands for carrier balance for electron and hole, Ψph stands for photolu-
minescence eciency, and Q stands for quenching factor. It can be stated
by the equation that by improving carrier balance for electron and hole,
by enhancing photoluminescence eciency and recombination rate or by
suppressing cathode quenching, device performance can be improved in
terms of electroluminescence eciency.
4.4 Blue Light-Emitting Diodes
Various types of conjugated polymers have been proposed for blue
light-emitting diodes which were highly soluble and reveled strong blue
uorescence or emit blue light in 350–440 nm range without any excimer
OLED Structure
Cathode
Emissive
Layer
Conductive
Layer
(Organic
Molecules or
Polymers)
(Organic
Molecules or
Polymers)
Anode
Substrate
Figure 4.2 e basic OLED device architecture. Source: https://electronics.
howstuworks.com/oled1.htm.

Conjugated Polymer Light-Emitting Diodes 83
formation on longer wavelength [82–85]. For blue OLED, some phenylene
chains have been polymerized by meta linkage and displayed 4.69cd A−1
eciency [86].
In this regard, carbazole is a rigid plane biphenyl monomer having a wide
band gap along with its high exibility and high luminescent eciency which
helps in modication of the molecule skeleton for its use as a key chromo-
phore in OLEDs [87, 88]. ese are extensively used in the development of
highly competent blue light-emitting diodes as a key role of host and charge-
transporting materials [89]. By derivatives of carbazole, Morin [90] and his
coworkers developed light-emitting diodes by electron and hole transport
molecules, i.e., 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole and
N,Ń-diphenyl-N,Ń-bis(3-methylphenyl)-1,1 -biphenyl-4,4 -diamine, respec-
tively, which showed electroluminescence in 424–432 nm wavelength (blue
region) with Al and indium tin oxide as the electrodes. ey also developed 2,7
carbazole based blue light-emitting copolymers with some other highly aro-
matic comonomers by Yamamoto or Suzuki cross-coupling reactions. ese
high molar mass modied with substituent species were highly water-soluble
and amorphous in nature as well as these showed good redox properties and
thermal stability for their application in layer blue light-emitting diodes [91].
Numerous works have also been reported on polyuorenes that is also
a blue light-emitting material [92]. Although, polyuorenes exhibited
promising features, literature suggested the tendency of forming exci-
mers and aggregation which caused shiing of emission spectra towards
longer wavelengths or showed bathochromic shi and decreases uores-
cence quantum yields [93]. To remove such type of problems, scientists
and research have done a number of studies and done functionalization
by bulky side substituents or copolymerization with other monomers or
by introduction of lithe spacers amid the chromophores [94–98]. Oligo(9,
9-dihexyl-2,7-uorene ethynylene)s were synthesized by Tsutsui et al. [99]
for extraordinary photo luminescence having high quantum yields as well as
high oxidative and thermal stability for their use in OLED application. Chan
et al. [100] developed poly(9,9-dihexyl-2,7-dibenzosilole) by copolymer-
ization of monomers dibromo and bis(boronate) which showed better e-
ciency in a single layer light-emitting device in comparison of polyuorene.
4.5 Green Light-Emitting Diodes
Seino and his labmates developed a green organic light-emitting device
(OLED) whose power eciency was too high (100 lm W−1). is was

84 Polymers for Light-Emitting Devices and Displays
achieved via energy transfer from an exciplex and it was 1.6-times higher
than green thermally activated delayed uorescence (TADF) OLEDs [101].
Tanaka et al. [102] developed highly ecient green organic light-emitting
diodes (OLEDs) using a conjugated polymer dipyridylphenyl moieties and
fac tris(2-phenylpyridine)iridium, Ir(ppy)3 as a phosphorent emitter and
yield 29% at 100 cd/m2 and 26% at 1000 cd/m2 external quantum ecien-
cies which were too high and lead to 133 lm/W and 107 lm/W ultra high
power eciencies at 100 cd/m2 and 1000 cd/m2 respectively. A highly e-
cient green OLED was fabricated by using 9,9-diaryluorene-terminated
2,1,3-benzothiadiazole derivative in Ku’s lab [103] which revealed EQE
(ηext) of 3.7% and maximum brightness at 168,000 cd m−2. Zhu and his
coworkers [104] proposed green and blue-green phosphorescent OLEDs
based on tetraphenylimidodiphosphinate and iridium complex. ese
were highly ecient, due to shorter excited stated lifetime and better car-
rier transport ability along with good electroluminescence performance.
3,5 -di(carbazol-9-yl)-[1,1 -biphenyl]-3,5-dicarbonitrile was used as a
host material for green thermally activated delayed phosphorescent and
uorescent OLEDs by Cho et al. [105]. is materials showed high quan-
tum eciency close to 25%. Chen [106] and his coworkers achieved highly
ecient green phosphorescent OLEDs by 1,3,5-Triazine derivatives as
new electron transport-type host materials. A dicarbazole-triazine hybrid
bipolar host material was developed in which dicarbazole moiety is elec-
tron donor and triazene is electron acceptor. is device was highly e-
cient green phosphorescent OLEDs having maximum eciency of up to
20.0%[107].
4.6 Red Light-Emitting Diodes
Red OLEDs have an emitting layer comprising of one material during their
initial development. Okada et al. [108] developed a highly ecient com-
plex for red OLED by using 1-phenylisoquinoline. e complex revealed
emission peak in the red region, i.e., 598–635 nm, the quantum yield of
0.17–0.32 and lifetime of those complexes ranged 1.07–2.34 µs, respec-
tively. Jung and his labmates [109] reported red organic light-emitting
devices (OLEDs) by incorporating various donor-acceptor moieties in
polymeric backbone which showed emission at 637–677 nm and highest
brightness of 8,300 cd m−2 with 4.46% maximum EQE at 7V, the current
eciency and the power eciency was found 3.43cd A−1 and 1.64lm W−1,
respectively. 3-2-(3,3-dicyanomethylene-5,5-dimethyl-1-cyclohexylidene)

Conjugated Polymer Light-Emitting Diodes 85
vinyl-N-naphthy l-carbazole (NCz-2CN) and 3,6-bis(2-(3,3-dicyanomethylene-
5,5-dimethyl-1-cyclohexylidene)vinyl-N-phenyl-carbazole (PCz-4CN)
which are derivatives of carbazole were used as donor-Л-acceptor for
obtaining red OLEDs by Fu et al. [110]. ey emit in red region 630–666
nm and maximum luminance of 4,110 cd/m2 attained at 15V. e current
eciency was found 2.09 cd/A while maximum luminous eciency was
found at 0.49 lm/W.
4.7 Multicolor Light-Emitting Diodes
Bouillud et al. [111] presented conjugated polymers based on uorene
with the incorporation of thiophene and phenylene moieties in their copo-
lymer having outstanding properties of tunability of the electrolumines-
cent properties. By changing the composition of comonomers, it has the
ability to change their emission from blue to green or yellow. ey also
suggested hole injection and hole transport phenomena by the addition
of an insulating buer layer as well as the hole transporting material into
the material. is electroluminescence eciency was increased from 4.5 to
125 cd/m2 by incorporation of N,N’-diphenyl-N,N’-bis(3-methylphenyl)-
1,1-biphenyl-4,4’-diamine as the hole transporting layer and LiF as an
insulating buer layer.
Wong and his coworkers [112] studied the use of conjugated and
non-conjugated polymeric backbone in terms of synthesis, physical prop-
erties, sample quality, and device performances for blue, green, and red
OLED materials (Figure 4.3), while Chou et al. [113] prepared universal
bipolar host by phosphine oxide and two carbazole groups for blue, green,
and red phosphorescent OLEDs which was highly ecient.
4.8 Advantages of OLEDs over Other Liquid
CrystalDisplay
OLEDs revealed many advantages with other conventional liquid crys-
tal displays (LCDs) such as polymer OLEDs that show dierent colors
such as red, green, blue (RGB) as well as white too which is important
to many applications [114, 115]. OLEDs are biodegradable. An OLED
display can be many times lighter, more exible, and thinner compared
to LCD, as well as it can display deep black levels because it works with-
out a backlight [116]. ese are much brighter as compared to LCD.

86 Polymers for Light-Emitting Devices and Displays
esehave the active light-emitting capability as well as the OLEDs are
driven by the direct current with 10-V approximate voltage [117]. ey
are also highly ecient in a light color so that full-color display can
be realized easily and in place of early vacuum display devices, these
are all-solid-state devices [118]. In addition, they have a lithe informa-
tion display technology with low energy consumption and high light-
emitting eciency [119]. ese devices have outstanding light-emitting
properties and temperature features which least aected by temperature
uctuations and faster response time. ey are easy to prepare at a very
low cost [120].
Donor
(5 mol%)
C8H17
C8H17
N
N
Acceptor
(50 mol%)
Backbone
(45 mol%)
TADE-P1
N
NN
C12H25
Figure 4.3 Functionalized conjugated polymer having donor (5mol.% triphenylamine),
acceptor (50mol.% 1,3,5-triazine) and backbone (45mol.%) with an insulating
n-butyl link (Reprinted from Michael Y. Wong, 2017, Journal of Electronic Materials. 46,
6246–6281).

Conjugated Polymer Light-Emitting Diodes 87
4.9 Applications of OLEDs
In 2003, Kodak has used OLED in the fabrication of its rst digital cam-
era [121]. OLEDs are widely being used in the latest smartphones, DVD
players, digital cameras, digital watches, etc., due to its high exibility
and foldability which helps in saving space and weight [122]. Aer this,
numerous companies have used this technology in their products includ-
ing Nokia [123], Samsung [124], etc. Since the year 2019, OLED screen is
readily used in many mobile phones including Honor view 10, Nokia 6.1
plus, Samsung Galaxy A8+, Vivo V15 Pro, Gionee S11S, OnePlus 7 Pro,
etc., and in TV such as Sony Bravia Android Smart OLED TV KD-55A8G,
LG Smart TV OLED55C8PTA, METZ Android OLED TV M55S9A, etc.
OLEDs have also applicable in high-end television systems, at-panel
displays, computer monitors, some pocket-size systems like digital cam-
eras, portable gaming consoles, media players, android phones, and mini-
screens [125–127].
ese are also widely used in various multiple-input output wireless
optical channels [128]. Some other applications of OLED are shown in
Figure 4.4.
4.10 Challenges and Future Possibilities
OLED has a wide area of applications in different areas, but still, there
are many challenges involved in this technology such as its is costly in
Back Lighting
Future
Driver
Information System
Cellular Phones
Pagers
SmartCard
PDA/PC
Interior
Car Lighting
Airport Runway
Lighting
Light Sources
Advertising
Panels
Automotive
Lighting
OLED Technology
Figure 4.4 Applications of OLED. Source: https://www.elprocus.com/oled-display-
technology-architecture-applications/.

88 Polymers for Light-Emitting Devices and Displays
comparison to LCD or LEDs. There is still a lack of a broad range of
products incorporating OLEDs. Light efficiency is still low and com-
pared to other display devices, their lifetime is shorter. Red, green, and
white LEDs give longer lifetime but the blue OLED revealed a limited
lifetime of 1.6 years. Technology is highly sensitive to water so can be
damaged by moisture. When OLEDs compared with LCD in direct sun-
light, these show worse scenario. In the future, these problems can be
resolved by using various types of technologies and conjugated poly-
mers. OLEDs can be used for curved display in the future as well as
transparent displays entrenched in windows. The property of flexibil-
ity of OLEDs maybe uses in roll to roll manufacturing process which
allows flexible display architecture. Flexible, stretching, and self- healing
materials need to incorporate for OLED device fabrication that can also
improve barrier layers to protect OLEDs from moisture and oxygen. By
removal of triplet exciton from conjugated polymer backbone could
expressively enhance the device stability, specifically at the initial stage
of pouring, i.e., via the introduction of additive which quenches lead
the high efficiency and device lifetime. For this, various features of
polymers could be introduced in single polymer via its copolymeriza-
tion, doping, medication, or via functionalization of monomers then
copolymerization.
4.11 Conclusion
Great progress has been done in the synthesis of conjugated polymers for
OLED device fabrication. By modication, copolymerization, doping,
and other techniques, those materials are being prepared which helps in
applications of electron transport materials to improve the device fabri-
cation and performance of OLEDs. e eciency of the OLED device
is dependent on charge injection, charge transport, and emission. e
combination of phosphorescent emitters and conjugated polymers is
very important to achieve high eciency. For this point of view, many
functionalized or copolymerized conjugated polymers have been inves-
tigated having various features in a single polymeric material as conju-
gated polymers having delocalized pi electrons in their backbone possess
brilliant charge carrier transport properties so by incorporation of vari-
ous moieties conjugation length could be controlled or tuned for further
improved modications in OLEDs.

Conjugated Polymer Light-Emitting Diodes 89
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