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International Journal of Computer Networks and Communications Security
VOL. 8, NO. 9, September 2020, 80–83
Available online at: www.ijcncs.org
E-ISSN 2308-9830 (Online) / ISSN 2410-0595 (Print)
Structural Analysis of Enhanced Performance Organic Light
Emitting Diodes (OLEDs)
REHAN YOUNAS1 and AMMAR YOUNAS2
1 PhD Candidate, University of Notre Dame, USA
2 PhD Candidate, School of Humanities, University of Chinese Academy of Sciences, China
1reh.younas@gmail.com, 2doctorammaryounas@gmail.com
ABSTRACT
We present a detailed study on structure of Organic LEDs (OLEDs) that promise flexibility and enhanced
performance. Ordinary LEDs fail when it comes to need of ultra-smart size, thin, flexible smart screens and
high efficiency light sources. With electroluminescent layer made of organic compounds, OLEDs promise
all such features. We did a comprehensive analysis to find what structural features distinguish OLEDs from
semiconductor LEDs. We found that it is the special six layered structure with organic emissive layer and
delocalized charges due to weak pi bonds that enable OLEDs to perform better. We dis-cuss a few
limitations related to production and life of these LEDs and suggest possible solutions to overcome these
challenges. A rigorous, in-depth analysis of this structure is imperative to further comprehend the working
of this device in order to make future devices cheaper and more efficient.
Keywords: (160-4890) Organic Materials; (230-3670) Light-emitting-diodes; (310-4165) Multilayer
Design.
1 INTRODUCTION
This paper discusses the structural features that
enable OLEDs to perform better than conventional
semiconductor LEDs. We discuss how inherent
nature of organic materials enable us to fabricate
them on flexible substrates making bendable
screens possible [1-4].
With the invention of first visible LED in 1962-
“The Magic One” – LED found its use in
communication and electronics. At the same time,
large size and very low efficiency limitations
restricted its use to only a limited set of
applications. As the technology evolved and
progressed to be more sophisticated, a need of
more compact and efficient LED arose. This is
when Organic LEDs came into existence. Finding
its use in more sophisticated applications, such
LEDs are lightweight, portable and ensure
promising future [1]. Major contribution of this
technology is towards OLED based television
screens. Though quest for OLEDs begun in late
1960s where scientist strived to utilize organic
materials to produce light, real work begun in early
1990s when the idea of such bendable screens
stroked [2].
OLED technology, though much efficient than
conventional LED, is still very expensive for
commercial applications. The reason being
availability of a limited set organic materials that
can serve to make LEDs. Researchers have long
been utilizing a number of organic materials to
make light generation possible [2][5], what is still
unknown to us is how we can make these LEDs
cheaper and long lasting. The question of choice of
other organic materials and efficiency
improvement is still unanswered. If, somehow, we
could be successful in overcoming these barriers,
entire scientific community in general and
electronic industry in particular would revolution-
ize [1-4].
This letter explores the structure of organic light
emitting diodes to enhance our understanding of
how this structure works in order to enable us to
further explore this domain for future
improvements. We explore the inner structure of
OLED [5], [7] and investigate how different organ-
ic layers work for light generation. We found that
the inherent structure of organic materials used
make efficient charge recombination right at the
interface region resulting in higher efficiency at
lower power consumption. High mobility of holes
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in the hole transport layer (HTL) was one of the
major contributor towards excellent charge
transport and high efficiency [1], [4].
We now discuss rest of the paper. In section I we
discuss the working principal behind an LED. In
section II we discuss the structure of OLED
followed by detailing out the composition of
organic layers used in these LEDs. In section III
we discuss the characteristics comparison of
OLEDs with normal LEDs. We conclude our paper
in section IV with outlining our findings.
2 WORKING PRINCIPAL
The working principal behind an LED is simple-
electron and hole recombine to emit light. Voltage
is applied across a p-n junction, formed by joining
two layers of semiconductors one rich with
electrons (n-type) and other with holes (p-type),
causing electrons and holes to flow in opposite
direction [4-6]. These electrons and holes
recombine to emit photons (figure.1). Depending
upon the wavelength of emitted photon, we see
different colors of light. The emitted wavelength is
a materials property which can be controlled by
using different type of materials depending upon
our requirement [4].
λ = Eg
hν (1)
Here, Eg is the bandgap of the material and 𝜆 is the
wavelength of the emitted light. By controlling the
bandgap of the material we see different colors of
emitted light.
Fig. 1. Working principal of an LED. Under the
influence of applied potential (forward biasing) electrons
from N-type material and holes from P-type material
move to recombine across the interface region to emit
light.
3 OLED STRUCTURE
OLED comes to our rescue offering lower power
consumption, smaller size and better efficiency.
The key feature that distinguishes OLEDs from
conventional ones is the structure which is the key
parameter we are interested exploring. OLEDs are
different on the fact that here instead of using p-
type and n-type semiconductor layers to form a
junction diode [1], [6], [7], we use organic
materials to achieve the same goal of electron-hole
recombination. The most common OLED structure
comprises six layers as shown in figure 2.
Top and bottom layer is made of glass, plastic or
any other coating that serves as protective layer. In
many designs, this bottom layer serves as substrate
as well and the entire structure of LED is fabricated
on it using bottom up approach [1]. The layer
below the top layer is of cathode contact serving as
electron contact. The layer above the bottom layer
is anode commonly known as hole contact. In
between these contacts are two layers made of
organic material [5]. The layer right under cathode
is called emissive layer where light is produced
under which is the other layer called conduction
layer. The biggest advantage of OLEDs comes
from the fact that these organic layers can be
manipulated chemically, providing us bandgap
control and enabling us with huge coloring options.
In addition to that, fabrication process, for
example, inkjet printing on a simple plastic
substrate is extremely simple [2].
For light generation, a potential is applied across
cathode and anode. Source adds electrons to the
cathode and similar positive charge appears across
anode. Cathode being negatively charged terminal
pumps electrons to emissive layer. Opposite
happens at anode end where holes are added to
conductive layer. Now holes being more mobile
than electrons move to the emissive layer where
they recombine with electrons. This recombination
results in photon emission that generates light [4-
6].
Fig. 2. Six layered structure of organic LED. Glass layer
serves as substrate as well as a window for generated
light. Anode is made of transparent material like Indium
Tin Oxide for efficient light emission.
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4 ORGANIC COMPOSITION
In OLED, electrons are pumped to the conduction
band while holes to the valence band of some
organic material- for this case it’s the emissive
layer as shown in figure 3. An unequal distribution
of electrons and holes result in recombination of
particles to produce excitons. These excitons decay
to result in photon emission. Since the whole
process results due to induction of current through
metal electrodes, this process is called as
electroluminescence. Depending upon types of
organic layers used, OLEDs fall in two major
categories.
1. Small molecular structure (SMOLED)
2. Large polymer structure (PLED).
Our general understanding suggests, since polym-
ers are plastics they should not conduct. A polymer
is a long chain of carbon atoms with occasional
boding of oxygen, hydrogen and nitrogen where
electrons occupy the lowest energy states [1], [5].
Fig. 3. Charge movement inside an OLED. Cathode
pumps electrons to emissive layer (pink) and anode
pumps holes to the conductive layer (blue). Holes from
conductive layer moves to the emissive layer due to their
higher mobility and recombine at interface to emit light.
Under such conditions, polymers do not conduct
electricity and are often find use for insulation
purposes. At the same time, in a polymer, a carbon
carbon double bond encompasses a weakly
localized pi (𝜋) bond. This pi bond results in
delocalization of electrons that can contribute to
conduct electricity under the influence of potential.
Here, 2Pz orbitals have same probability of being
closer to either carbon atom resulting in electron
delocalization. This results in splitting of pi bond in
𝜋 and 𝜋∗ band. This delocalization distributes in
two bands similar to the conduction and valence
band of a semiconductor. Here 𝜋 corresponds to
the bonding orbital or conduction band while
𝜋∗ corresponds to antibonding orbital or valence
band of the semiconductor [1-3]. These bands are
also known as Lowest Unoccupied Molecular
Orbital or LUMO and Highest Occupied Molecular
Orbital or HOMO [1]. Depending upon symmetry
of such polymer structures, these polymers can
exhibit semiconducting or even metallic
characteristics.
Fig. 4. Bond model of commonly used organic material
Alq3 with small molecular structure (SMOLED).
Fig. 5. A single monomer of organic polymer PPV
(PLED). Such monomers repeat to form a long chain of
polymer.
The OLED structure comprises two such organic
layers. One working as hole transport layer
(usually naphthyl substituted benzidine derivative)
and Alq3 as electron transport layer. Both these
organic layers are sandwiched between metal
electrodes. Thickness of these layers is about 10-
100nm. When a potential is applied, electrons
from the cathode are transported to the LUMO of
the elctron transport layer and holes are transported
to the HOMO of the holes transport layer. Here,
holes due to their higher mobility drift towards
emissive layer ETL where they recombine with
electrons to emit light. Generated light passes
through anode which is made of transparent
material ITO (indium Tin Oxide) that further adds
to the output optical power [3][5]. The reason
behind high efficiency and superior functionality of
OLEDs is this two layer design that provides
sufficient energy barriers to localize recombination
of charges right at the interface. Since OLEDs
utilize organic materials, they suffer severe
degradation of material. A major setback for initial
models of OLEDs was this short lifetime problem.
However with recent technological advancement,
choice of organic material and fabrication
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R. Younas and A. Younas / International Journal of Computer Networks and Communications Security, 8 (9), September 2020
refinement, 10,000 hours lifetime of OLED has
been achieved[1][3].
5 CHARACTERISTIC COMPARISON
OLED find its most use in making television
screens. Conventional LED displays utilize the fact
that pixels are illuminated by LEDs to produce
image on screen. In case of OLED based displays,
each OLED works as a pixel itself and provides
self-illumination. Comparison of OLED displays
with simple semiconductor LED displays reveals
striking differences with OLED beating
semiconductor LED in efficiency, power consump-
tion and compatibility while semiconductor LED
beating OLED in terms of cost, lifetime and
manufacture ease[4][7]. Table 1 summarizes
comparison of OLED based television screens with
semiconductor LED displays.
6 CONCLUSION
Structural analysis of organic light emitting diodes
reveals that organic layers with high interface
recombination- due to delocalization of charge in
carbon chains- and high mobility of charge carriers
result in enhanced performance OLEDs. This in-
depth structural analysis helps us comprehend
basic device working and enables us to experiment
in order to improve its performance. The
possibility to manufacture organic materials on
large scale ensures a bright future for this
technology. High fabrication cost of OLED
narrows down its use to only limited applications.
In addition to that, degradation of organic materials
with time restricts OLEDs lifetime to be nearly five
times less than semiconducting LEDs which is the
biggest drawback of this technology to this date.
With introduction of less expensive fabrication
technologies and choice of better organic materials
which do not degrade that fast, OLED technology
can be cheap and long lasting.
Table 1: Characteristic comparison of semiconductor
based LED displays with organic LED displays.
Technology
LED
OLED
Power
Consumption(W)
60-300
24–150
Resolutions
(pixels)
1920х1080
1920х1080
Colors
16.7
million
16.7 million
Brightness
(cd/m2)
350-500
1000
Contrast
350:1-
1.000:1
1.000.000:1
Response Time
8-12ms
0,05ms
Viewing Angle
170/170
178/178
Lifetime (hrs)
50.000-
60.000
10.000
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