Improved Light Extraction Efficiency of Organic Light Emitting
Diode using Photonic Crystals
Chaya B. M., Venkatesha M., Ananya N. and Narayan K.
Department of Electronics and Communication Engineering, Sai Vidya Institute of Technology,
Rajanukunte, Bangalore, India
Keywords: Organic Light Emitting Diode, Photonic Crystals, Light Extraction Efficiency.
Abstract: In this work modelling of two dimensional of a fluorescence based Organic Light Emitting Diode (OLED)
using plastic as flexible substrate is presented. The Finite Difference Time Domain (FDTD) mathematical
modelling has been used to analyse the light extraction efficiency from fluorescence based Organic Light
Emitting Diode (OLED). The OLED structure has been simulated by using 2D Hexagonal photonic crystal
lattice. The Finite Difference Time Domain (FDTD) method is used to model and simulate the OLED
structure. An enhancement of Internal Quantum Efficiency (IQE) and Light Extraction Efficiency (LEE) has
been achieved by inserting Photonic Crystal above the emissive layer. The improvement in the extraction
efficiency of OLED structure is achieved by increasing the radiative decay rate and by optimizing the
angular distribution of light through the substrate.
1 INTRODUCTION
Organic Light Emitting Diode is an electrolumi-
nescence device which is formed using double layer
structure of organic layers to produce light emission.
This is achieved by driving voltage as dc source
below 10 Volts (Tang and VanSlyke, 1987). If the
radiative decay is high due to singlet exciton, then
the process is said to be Fluorescence. In order to
improve the extraction efficiency, the Photonic
crystals is used upon the glass substrate to realize
low power consumption using Nano imprint litho-
graphy technique which showed better performance
than conventional OLEDs (Lee et al., 2003).
The state-of-art OLED stack is reviewed to
determine radiative quantum efficiency and device
efficiency during electrical operation which showed
the significant results by varying electron transport
layer. The efficiency is increased by incorporating
various carrier transport layers in the OLED with
different work functions (Do et al., 2003). The
Silicon Nitride Photonic Crystals (PC) are used to
control light which is acting as a dielectric medium
to extract maximum amount of photons which is
trapped in high index guided structures .
However different experiments on Organic LEDs
are carried out using different structures of the
Photonic crystals, substrates and the materials of the
substrate may affect the thermal resistance (Kim et
al., 2004). In this paper Poly (ethylene terephthalate)
(PET) is used as a Plastic Substrate. PET has greater
flexibility, robustness and is less expensive
compared to glass substrate. The PET is a polymer
electrode with high transmission in visible range of
about 87% (Faraj et al., 2011).
Propitious research work is being carried out
aiming at increasing the light extraction efficiency of
OLED. In this paper an OLED with photonic
crystals using plastic as flexible substrate with a
point dipole source to increase the number of
excitons in the emissive layer has been presented.
2 OLED STRUCTURE
2.1 Proposed Design
Figure 1, shows the structure of two dimensional
OLED which is modelled using Lumerical FDTD
(Finite Difference Time Domain). The proposed
structure uses plastic as a flexible substrate. The
modelled device structure consists of thin active
organic layers which are integrated with the
transport layers. The radiative recombination of
injected electrons and holes is taken place in the
organic layers. These transport and organic Layers
256
B. M. C., M. V., N. A. and K. N.
Improved Light Extraction Efficiency of Organic Light Emitting Diode using Photonic Crystals.
DOI: 10.5220/0006153902560259
In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 256-259
ISBN: 978-989-758-223-3
Copyright
c
2017 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
which is about 200nm is placed between anode and
cathode layer placed on a plastic substrate.
Plastic Substrate=500nm
Photonic crystals: Lattice
Constant(a)=350nm,
Radius of crystal=150nm
Cover layer SiN=700nm
Anode ITO =120nm
HIL =CuPc=15 to 30nm
HTL= TPD =40nm
α-NDP =30nm
Alq3=60nm
HBL=BCP=30nm
Cathode =Al=100nm
Figure 1: Fluorescence based OLED.
This device has been simulated using materials
described in Table 1, green light is simulated having
a peak wavelength of 540nm.
2.2 Modelling of Photonic Crystals in
OLED
Figure 2, shows the modelled Photonic Crystal (PC),
used in OLED. The PC used in this work has lattice
constant of 350nm and radius is of 150nm. The
simulation is done using Photonic crystal made up of
Silicon Nitride which has refractive index of 1.9.
Figure 2: Modelled Photonic crystal Structure.
The Brillouin zone chosen is in the form of a
hexagon as shown in Figure 2, Hence it is called as
Hexagonal Lattice Brilloin Zone (Joannopoulos et
al., 2008). The photonic crystals are placed in order
to achieve better light confinement into the substrate
without undergoing Total Internal Reflection as
discussed in (Boroditsky, M et al., 1998).
3 OLED MATERIALS
The samples used in the structure are described in
Table 1. The materials are chosen depending on the
energy levels at metal organic interface abiding by
Mott Schottky limit. The work function of
electrodes, thickness variations and Organic Layers
with the HOMO (Highest Occupied Molecular
Orbital) and LUMO (Lowest Unoccupied
Molecular Orbital) levels of organic molecules are
given.
Table 1: Materials used in the OLED Structure.
Materials Work Refractive
Function index(n)
Indium Tin Oxide 4.7eV 1.806
Aluminium 4.1eV 1.031
Hole Blocking Layer
(HBL)-BCP
3.2eV 1.686
Hole Injection
layer(HIL)-CuPC
3.1eV 0.47
Hole Transport
layer(HTL)-TPD
2.6eV 1.67
Alq
3
-Tris(8-
hydroxyquinoline)
aluminium
HOMO-5.62eV
LUMO- 2.85eV
1.68
α-NDP- N,N`-
diphenyl-benzidine
2.5eV 1.82
Cover layer –SiN ---- 1.9
Substrate-Plastic ---- 1.53
The most commonly used HIL is CuPC (Copper
(II) phthalocyanine) is used to improve the carrier
injection efficiency. The HTL used here is TPD (N,
N’-Bis (3-methylphenyl)-N, N’-diphenylbenzidine).
The hole transport layer and hole injection layer
placed above organic layers. The hole injection layer
is used to improve the carrier injection efficiency,
and serves two purposes, first, it provides a path for
smooth travel of injected holes up to emitting
layer. Second, it functions like electron blocker to
confine electrons within an emitting layer. The HBL
used is BCP (2, 9 Dimethyl-4, 7-diphenyl-1, 10
phenanthroline with a work function 3.2eV. The
organic layers used in the structure are α-NPD (N,
N’-Di [1-napthyl)-N, N’-diphenyl-(1, 1’-bipheny)-4,
4’diamine) and Alq3- (Tris-(8-hydroxyquinoline)
aluminium). The effective double injection is
possible when the work function of metal electrodes
Improved Light Extraction Efficiency of Organic Light Emitting Diode using Photonic Crystals
257
is close to Lowest Unoccupied Molecular Orbital
(LUMO) and Highest Occupied Molecular Orbital
(HOMO) for Organic materials (Narayan et al.,
2013).
4 METHODOLOGY
The Finite difference Time Domain (FDTD)
method is used for solving Maxwell’s equations in
complex geometries. The Maxwell’s equations are
time dependent hence, FDTD simulations has high
performance optical solver which can capture using
wavelength scale structure to improve the device. In
order to achieve the maximum radiative decay, the
derivation is published in (Novotny and Hecht
,2006), where the quantum radiative decay is
proportional to classical dipole power radiated, the
relationship as in equation (1)
raddecay
(1)
This relation is shown to relate the radiative
decay rate to Fermi’s golden rule about the density
of photonic modes which is represented in equation
2, as represented in (Joannopoulos et al., 2008),

ij
ij
ij
vM
2
(2)
where
ij
= transition rate from higher energy
state i to lower energy state j,
ij
M
related to wave
function overlap of excited states,
)(
ij
v
is
photonic mode density of transition.
In this work two results have been interpreted,
Internal Quantum Efficiency (IQE) and Light
Extraction Efficiency (LEE) (Chutinan et al., 2005).
The Internal Quantum efficiency is the radiative
decay process achieved by relating decay rate to the
power radiated by the single dipole source. With the
dipole source implementation, we can formulate
IQE. From, Fermi’s Golden rule, we can relate
decay rate to density of states and is related to
Classical EM power emitted by a dipole to
imaginary part of green’s function (Novotny and
Hecht ,2006).
The decay rate enhancement is given by,
power source
power dipole
0
decay
decay
(3)
The Light Extraction Efficiency (LEE) is defined
as the fraction of optical power generated in the
active layer of the OLED that escapes into the air
above the OLED within a desired range of angles.
patternno
pattern
lossrad
rad
LEE
LEE
LEE
_
(4)
where,
rad
=Electro Magnetic decay to Far-
field radiation,
loss
= EM decay trapped by Total
Internal Reflection
The light escaping to the glass substrate within a
particular solid angle (e.g. bounded by the TIR
critical angle) is considered. Therefore, the total
extraction efficiency (TEE) is given by, combination
of internal quantum efficiency (IQE) and Light
extraction efficiency (LEE).
LEEIQETEE
(5)
5 RESULTS
5.1 Extraction Efficiency Analysis
For the proposed OLED structure, the far field into
air with Photonic crystals (PC) patterning and with-
out PC patterning is simulated. The Improvement in
the light extraction within bounded critical angle for
a wavelength of 540nm is observed in Figure 3.
Figure 3: Light extraction efficiency (far-field in air, with
and without PC.
5.2 Angular Distribution of Light at
540nm
Figure 4: Angular Distribution of light.
PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology
258
Figure 4, shows the Far field intensity observed
for the bounded critical angle of about 3.2 μm
Volts/m with the presence of photonic crystal in the
OLED structure and 2.1μm Volts/m for structure
without Photonic crystal at a wavelength of 540nm.
The proposed model having photonic crystals
inserted above the emissive layer, if implemented in
the organic light emitting diode will improve the
light extraction efficiency.
5.3 Internal Quantum Efficiency
Figure 5: Dipole power versus Wavelength.
Figure 5 shows a plot between dipole power versus
wavelength. It can be seen from figure 5, that dipole
power is highly dependent. The exponential decay in
the figure 5 indicates that the maximum decay
occurs when the wavelength is 540nm. This implies
that there is an increase in number of excitons
produced at this wavelength. The dipole power
consumed at this wavelength is 5.71e-009 Watt.
6 CONCLUSIONS
In this work Finite Difference Time Domain
(FDTD) modelling of an fluorescence based OLED
using plastic as flexible substrate has been
presented. A high radiative decay rate has been
achieved at 540 nm by inserting a photonic crystal
above the emissive layer. A high decay rate not only
enhances the internal Quantum efficiency but also
light extraction efficiency, it has been shown that an
Green emitting OLED on the plastic substrate has
maximum internal quantum efficiency and light
extraction efficiency at an wavelength of 540 nm.
We have shown that Green emitting OLED on
plastic substrate has maximum Light Extraction
efficiency of 3.2 µm Volts/meter at a wavelength of
540 nm using the photonic crystals at 550THz.
Fabrication of such OLED structures can find future
application as a monolithically integrated light
source for integrated optical Lab-on-a-Chip based
bio-sensors.
ACKNOWLEDGEMENTS
The authors would like to thank Science and
Engineering Research Board, Department of Science
and Technology (DST-SERB) Government of India
for funding this research work. File No.
YSS/2015/000382
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