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

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

-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),





 (2)

where



= transition rate from higher energy

state i to lower energy state j,

related to wave

function overlap of excited states,

)(



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



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











(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

REFERENCES

Tang, C. and VanSlyke, S. 1987. Organic electrolumines-

cent diodes. Applied Physics Letters, 51 (12), p.913.

Lee, Y., Kim, S., Huh, J., Kim, G., Lee, Y., Cho, S., Kim,

Y. and Do, Y. 2003. A high- extraction-efficiency

Nano patterned organic light-emitting diode, Applied

Physics Letters, 82(21), p.3779.

Do, Y., Kim, Y., Song, Y., Cho, C., Jeon, H., Lee, Y.,

Kim, S. and Lee, Y. 2003. Enhanced light Extraction

from Organic Light Emitting Diodes with 2D

SiO2/SiNx Photonic Crystals. Advanced Materials, 15

(14), pp. 1214-1218.

Kim, Y., Song, Y., Y. W., & Lee, Y. H. 2004. Enhanced

light extraction efficiency from organic light emitting

diodes by insertion of a two-dimensional photonic

crystal structure. Journal of Applied Physics. 96(12),

7629.

Faraj, M. G., Ibrahim, K., Ali, M. K. M. 2011. PET as a

plastic substrate for the flexible optoelectronic

applications, Optoelectronics and Advanced

Materials–Rapid Communications. 5(8)

Boroditsky, M., Coccio1i, R., Yablonovitch, Eli. 1998.

Analysis of photonic crystals for light emitting diodes

using the finite difference time domain technique,

SPIE, Vol. 3283.

Novotny, L. and Hecht, B. 2006. Principles of Nano-

Optics, Cambridge, Cambridge University Press.

Chutinan, A. et al., 2005. Theoretical Analysis on light-

extraction efficiency of organic light-emitting diodes

using FDTD and mode expansion methods, Organic

Electronics. 6(1), pp.3-9.

Joannopoulos, J., Johnson, S., Winn, J. and Meade, R.

2008. Photonic Crystals, Molding the Flow of Light.

2nd ed. Princeton: Princeton University Press.

Narayan, K., Mohan Rao, G., Varadharajaperumal, S.,

Manoj Varma, M and Srinivas, T. 2013. Effect of

thickness variation of hole injection and hole blocking

layers on the performance of fluorescent green organic

light emitting diodes, Current Applied Physics 13(1),

pp.18-25.

Improved Light Extraction Efﬁciency of Organic Light Emitting Diode using Photonic Crystals

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