Improved Viewing Angle and Light Extraction Efficiency of

Microcavity OLED with Corrugated Capping Layer

Byung Doo Chin

1 a

, Jeong-Yeol Yoo

and Sung Min Jo

1,2

Department of Polymer Science and Engineering, Dankook University, 152 Jukeon-ro, Yongin, Korea

LG Display Co, Ltd, South Korea

Keywords: OLED, Micro-Cavity, Light Extraction, Corrugated, Thermal Expansion, Viewing Angle.

Abstract: Simple and low-cost materials and process for the formation of light extraction layer of organic light emitting

diodes (OLED) are important. However, in case of microcavity-driven OLED, microscale structure for

improved light extraction of devices is not easily applicable due to the sever interference with efficiency and

viewing angle limitation. In this work, various type of periodic and non-periodic corrugated pattern array for

OLEDs were formed by soft lithographic process or spontaneous self-assembly after thermal annealing.

Improvement of top emission OLED with microcavity-driven high efficiency were observed, while the

angular dependence of light emitting spectra (viewing angle characteristics) was reduced at devices with

larger features of corrugated non-periodic capping layer structure. Optical properties of devices were

investigated in terms of the scale of periodic patterns and optical cavity effect. Methods in this work could be

utilized as an effective tool for managing microcavity-driven performance of high efficiency OLEDs.

1 INTRODUCTION

There exist various technologies for a light extraction

(LE) scheme for organic light emitting diodes

(OLEDs). However, for the successful commercial

application for high-performance OLED display with

micro-cavity structure, the restriction on the cost of

manufacturing, directionality of color or viewing

angle, and the requirement to maintain a high-quality

image are severe. For a top-emitting OLED

(TOLED), which is a useful platform of high-aperture

OLED display with high-resolution mobile and non-

transparent flexible substrates, color and spectra are

significantly sensitive to the intrinsic micro-cavity

effects. Recovering of the optical loss from surface

plasmon (SP) is also effective strategy for an

enhancement of light extraction efficiency, and a

method of grating coupling by a generation of

corrugate the organic/metal interface were found to

be effective. Several kinds of periodically corrugated

patterns were proposed, showing a reduction of light

loss by a scattering of SPs (Koo et al, 2010, Koo et

al. 2011, Yin et al, 2016).

https://orcid.org/0000-0002-6414-5239

In this work, we have introduced some of the

simple fabrication processes for the microscale LE

structure formation. Several patterns of regular and

irregular photonic structures were introduced with

controllable scale. In case of the generation of a

quasi-periodic corrugation patterns with broad size

distribution for LE structures of top-emitting OLED,

formation of corrugated patterns is spontaneously

driven by the mismatch of thermal expansion

coefficient of the organic and polymeric layers, which

are easily fabricated by simple deposition process.

The luminous efficiency of device with corrugated

patterns was generally increased, where the change of

the spectral characteristics and color stability depends

on a status of micro-cavity for specific TOLED

devices. Experimental data for different scale of

buckling patterns at top emitting OLED were

investigated with the wavelength-dependent grating

pitch and scattering order and optical simulation,

showing that increased scale/pitch of corrugated

patterns at capping layer resulted in a suppression of

viewing angle variation for microcavity OLEDs.

142

Chin, B. D., Yoo, J.-Y. and Jo, S. M.

Improved Viewing Angle and Light Extraction Efﬁciency of Microcavity OLED with Corrugated Capping Layer.

DOI: 10.5220/0013311700003902

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 13th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2025), pages 142-146

ISBN: 978-989-758-736-8; ISSN: 2184-4364

2 EXPERIMENTS AND RESULTS

2.1 Corrugation Pattern Formation:

Internal and External LE Layers

1D regular photonic structures were generated from

master molds, followed by soft lithographic process

(Fig 1a), while the irregular 2D corrugated patterns

for LE structures capping layer for top-emitting

OLED were described in Fig 1b. More detailed

information of process such as thermal treatment

could be found elsewhere (Koo et al. 2021).

Figure 1: Comparison of the LE structures for TOLEDs in

this work (a) Ordered wavy 1D formed on the bottom-

substrate of TOLED expecting very weak microcavity (b)

Random 2D wavy pattern on the top electrode of TOLED;

maintaining the micro-cavity.

2.1.1 1D Regular Pattern

The pattern of 1D wavy type was fabricated through

thermal reflow process of positive photoresist (PR).

The depth of the pattern was adjusted according to the

spin coating condition of the positive PR. Si wafer-

based master mold with original pattern (height 800

nm, width 800 nm, pitch 1.6 um) was ultrasonically

cleaned using Acetone and IPA and blown with N

Then dried in an oven at 180°C for 1 hour. Positive

PR (ma-P1205, Microresist corp.) was dropped on the

cleaned master mold and maintained for 2 min, then

spin-coated at 2000 rpm for 60 seconds. When the

spin coating was completed, a wavy pattern was

formed by heating treatment for 10 min on a hot plate

of 100°C. The polydimethylsiloxane (PDMS) was

mixed with the curing agent at a ratio of 10:1 and

poured onto the wavy pattern. It was stored at room

temperature for 24 hours and removed bubbles inside.

The PDMS was thermally cured by storing in an oven

at 60°C for 4 hours. The PDMS replica mold was

separated from the master mold and ultrasonically

cleaned for 10 min in a 1:1 mixture of acetone and

isopropyl alcohol (IPA). Then, using N

, it was dried

in an oven at 180°C for 1 hour to produce PDMS

replica mold. The pre-cleaned glass with PR-spin-

coated at 4000 rpm for 60s was pre-baked on a 100°C

hot plate for 1 min. The PDMS replica mold with

wavy pattern was transferred to pre-baked PR by

thermal nanoimprinting lithography (NIL) process.

The thermal NIL process was carried out on a 100

hot plate and the pressure was applied for 1 hour using

a weight of 1kg or 3kg. Wavy patterns for devices B,

C, and D (atomic force microscopy - AFM image as

seen in Fig. 2b) were prepared by controlling the rpm

of the spin coating process at 2000, 2250, and 2500

rpm.

Figure 2: (a) Schematic fabrication process of a wavy

patterned TOLED by thermal imprinting (b) scale of the 1D

wavy patterns for TOLED device fabrication (c) Cross-

sectional SEM image of wavy pattern produced through

thermal reflow process (magnification: 10k), (left) using 1

kg, (right) 3 kg weight.

2.1.2 2D Irregular/Corrugated Pattern

At least two organic layers (typically for 35nm/15nm-

thick bi-layers) were deposited by thermal

evaporation at a high vacuum system with a base

pressure below 1 x 10

-6

torr without breaking the

vacuum on top of the TOLED devices with

transparent top cathode. After deposition of bi-layer,

the sample was thermal treated at 80

C for 10min in

the N

chamber for thermal expansion stage. Because

the glass transition temperature of other organic

materials (charge transporting and emission layers) in

TOLEDs were higher than 80

C, almost no change of

device without 2D pattern after this thermal

annealing. Followed by the heating, the sample was

cooled to ambient temperature by keeping in the N

chamber for 5min. The difference in coefficient of

thermal expansion (CTE) between the bi-layer on top

of transparent electrode of TOLED generated a

spontaneous buckling structure.

2.2 Fabrication of Microcavity OLEDs

Microcavity OLED with top-emission structure were

fabricated by thermal evaporation onto pre-cleaned

glass substrate. All of the following deposition

process were performed through a thermal

Improved Viewing Angle and Light Extraction Efﬁciency of Microcavity OLED with Corrugated Capping Layer

143

evaporation without breaking vacuum. Aluminum

layer was deposited on glass substrate as a reflective

anode electrode having a thickness of 100 nm.

Afterwards, 2 nm-thick MoO

layer for hole injection

and buffer, 28 nm-thick TCTA layer for hole

transport, 30 nm-thick EML layer of doped co-host

system (TCTA : TmPyPB : Ir(ppy)

= 0.465 : 0.465 :

0.07), 25 nm-thick TmPyPB layer for electron

transport, 0.5 nm-thick LiF layer and 1 nm-thick Al

layer for electron injection, and 18 nm-thick Ag layer

for composite semi-transparent cathode electrode

were deposited. Surface morphology of sample was

measured by atomic force microscopy (AFM, Veeco

Instruments) and scanning electron microscope

(SEM), etc. Fabricated TOLEDs were characterized

using a source meter (Keithley 2400) with a

spectrometer (PR655, Photo Research) and custom-

built goniometer for angular dependence measure-

ment of light emission. All electroluminescence

spectra and angular emission patterns were recorded

at a constant current density of 10mA/cm

with a

spectrometer

2.3 Results of Device Data: Improved

LE and Viewing Angle Behavior

Fig 3 (a) illustrates current density - external quantum

efficiency behavior of TOLED with 1D wavy

underneath of entire layers. The devices with wavy

pattern showed slightly higher current density than

references. At the inflection point of these sinusoidal

patterns, the thickness of the organic layer is partially

thinned, which might result in the formation of a

stronger local electric field. Luminance tended to

increase as the depth of the wavy pattern increases,

however, EQE values are only slightly increased at

device B (low-height 1D wavy) compared to

reference device A, showing only similar values of

7.73 and 7.56% for devices with higher 1D patterns

(device C and D, respectively). Fig 3b shows

normalized spectral intensity in the normal direction

of the emitting surface. The sinusoidal wavy pattern

gradually changes the microcavity length of the

TOLED. The thickness of the cavity length (organic

layer) became thinner at the inflection point and

thickens at the trough peak (Liu et al. 2012). With the

height of wavy patterns inside TOLED increases, the

micro-cavity effect starts to be reduced, so that

angular dependence of the spectrum for device D is

less-sensitive compared with reference device A.

However, overall performance of wave LE layer was

not great for these structures.

Fig 4 illustrate the representative image obtained

by AFM for 2D random wavy patterns that organic

bi-layers with thermal expansivity difference

generates. The pitch and depth of patterns could be

systematically controlled, depending on the annealing

temperature and time. The Si wafer and the buckling

structure formed on the top of the TOLED are shown

in Fig 4a and 4b, respectively. For example, 35nm

N,N’-Bis(3-methylphenyl)-N,N’-diphenylbenzidine

(TPD) and 15nm tris(8-hydroxy-quinolinato)

aluminum (Alq

) deposited and annealed at a

temperature of 80°C were shown. The both measured

patterns had 750 nm pitch and 30 nm depth.

Figure 3: (a) Current density v. EQE, (b) spectral response

(c)(d) angular dependence per each viewing angle for

device A and D, without and with 1D micron scale wavy

pattern. Devices are identical with structures in Fig. 2.

Figure 4: AFM image of 2D buckling structure formed by

spontaneous thermal expansion/compression (a) on the Si

wafer substrate and (b) on the TOLED device (scan size: 10

x 10 μm).

Since the formation of buckling 2D wavy patterns

formed on top of the transparent cathode of TOLED

does not affect the thermal state of the underlying

organic layer of devices, micro-cavity of those

TOLEDS were well maintained. Devices in Fig 5a

showed; the layout of LE bucking structures, where

device A (EQE 12.4 represents the reference without

LE, device B shows non-bucking LE with same

organic bi-layer. For devices of C, D, and E, wavy

pattern pitch/depth is 750/30, 1000/45, and 1150/64

(nm), respectively. The improvement of external

quantum efficiencies were significant in case of

TOLEDs with 2D buckling, where the most

optimized devices E showed 21% (reference A and

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144

w/o buckling LE device B shows EQE of 13.5 and

17%; see Fig 5b). More than 50% of EQE was

obtained for device with largest 2D wavy LE

structure. Such a result, observed in case of optimum

buckling structure formed on top of the translucent

electrode of device, can be explained by the

suppression of light loss by the SP mode at semi-

transparent metal/air interface without losing the

control of polarization of light. Moreover, the shift of

the maximum peak of electroluminescence for

buckling patterned device was less than 4nm with the

viewing angle range of 0 to 75 degree (>20nm shift

for control device), securing a negligible change of

the spectral characteristics and color stability (robust

viewing angle behavior as for cavity-based device,

comparable results with Kim et al. 2017) as seen in

Fig 5c.

Figure 5: (a) Layout of TOLEDs; reference, flat capping

layer, and 2D buckled layer (same bi-layer) (b) current

density vs. EQE for each TOLEDs with different

flat/buckling LE structures (c) comparison of the angular

dependence for reference device A and 2D buckling device

3 CONCLUSIONS

As for overall evaluation, corrugated “wavy” patterns

at a micron-scale enhances the viewing angle, which

is a significant advantage for display technologies.

Moreover, luminous efficiency was improved by

more than 50% for green phosphorescent TOLEDs as

long as they maintained less-sensitive viewing angle

dependence and stable color purity. It can be noted

that spontaneous formation of 2D wavy buckling

structures through thermal deposition and annealing

is simple and potentially cost-effective.

For 1D corrugated “wavy” patterns at a general

micron-scale, as the aspect ratio of the pattern

becomes larger, the interference phenomenon became

complicated and the viewing angle was improved.

However, efficiency was increased only at limited

condition of low aspect ratio 50:1 to 17:1, where the

larger patterns break the micro-cavity of TOLEDs.

Therefore, this might be a limitation restricting the

practical application of this method for the robustness

of commercial process.

A simple process for the spontaneous formation

of 2D wavy buckling structures generated patterns

with pitches of 750 ~ 1100 nm. These randomly

directed patterns were formed on top of the cathode

(semi-transparent) of TOLEDs by the thermal

deposition and annealing process, which did not

affect the thermal and transition status of the other

organic layers.

Even though the interference phenomenon is

rather complicated, and larger patterns breaking the

micro-cavity of TOLEDs could lead to structural

integrity issues and affect the overall device

performance, the improvement of the luminous

efficiency more than 50 % of the reference shown at

green phosphorescent TOLED will have a strong

impact on the progress of device optimization. Since

this method ensures less-sensitive viewing angle

dependence and stable color purity as well as

efficiency boost-up, this approach can be applied

various kinds of devices having strong micro-cavity

structures, such as TOLEDs with stable color purity

and OLED lasers.

ACKNOWLEDGEMENTS

This study was supported by the Industrial Strategic

Technology Development Program funded by the

Ministry of Trade, Industry and Energy (MOTIE),

South Korea (RS-2024-00417913) and Basic Science

Research Program Through National Research

Foundation of Korea (NRF) funded by the Ministry

of Education, Science and Technology

(2022R1F1A1074725)

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