EA-05

Light extraction efficiency of high refractive index glass substrate for
OLED lighting(Nippon Electric Glass, Co., Ltd) ○T. Yanase, C. Lee, T. Murata, K. Choju

OLED lighting is expected as a high efficiency lighting source. However, compared to thefluorescent lamp, the efficiency of commercial OLED lighting is not high. The main reason for thelow efficiency of the OLED lighting is mismatch of refractive index between organic layers(nd=1.8-2.0) and glass (nd=1.52). High refractive-index glass substrate for OLED lighting namedHX-1(nd=1.63) has been newly developed. The glass, made by over flow down draw process, hassmooth surface and high chemical resistance required for OLED glass substrate. Light extraction
efficiency of OLED lighting can be improved only by changing to HX-1 from common glass
substrate.
e-mail address of corresponding author: tyanase@neg.co.jp

Introduction
OLED light is expected as a next generation high efficiency bendable lighting source. However, theefficiency of a commercial OLED lighting has not reached the fluorescent lamp. Standard structureof OLED shows in Figure 1.The main reasons for the low efficiency of the OLED lighting arereflections at each interface, which is caused by difference of refractive index between each layer.
Because of the reflections, only about 25% of emitted light from organic layer is taken out to air.
About 20% of the emitted light trapped in the glass substrate (substrate mode) and about 50% of
the emitted light is trapped in the organic layers (waveguide mode) and electrode (evanescentmode). The various developments have been performed to increase the light which can be taken outin air.
Textured structure on the surface of glass substrate is well known as a technique to takes out thelight trapped in glass. Generally, since incident angle of the ray of the light from the glass is larger
than critical angle, the light is reflected at the interface and is impossible to be taken out light in air,but if there is texture structure at the interface of air and glass, the light can be taken out to air.
Total reflection is ideally decreased by attaching a hemispherical lens to the air side of substrate,
however, using large hemispherical lens on the OLED device is not practical. Alternatively, MicroLens Array (MLA) and out-coupling film has been proposed as the technique to form the shape on
the glass. They has been introduced to OLED lighting devices because of the simple process.
Next technique to improve the efficiency of the light emission is to reduce the mismatching of refractive
index between organic material and glass substrate. Since the refractive index of organic materials are
1.8-2.0, the light, extracted into a glass substrate from the organic layer, increases by using the highrefractive index glass (1.8-). However, conventional high refractive index glass (1.8-) reported so far haspractical issues on productivity and chemical durability. Generally since optical glass have low viscosity
in liquidus temperature, it is produced by casting method. It requires grindings and polishing process
after slicing the glass block to supply the glass sheet.

HX-1 glass
High refractive index glass substrate HX-1 is developed. The glass can be produced by overflow
down draw method which is famous for the mass-production process of the glass substrate for LCD.
The refractive index of HX-1 is 1.63. Though the value is lower than that of ITO or organicmaterials, it is higher than commercially conventional available glass substrate. The higher
refractive index of glass reduces the index mismatch of ITO and glass. That make it possible toextract more light from the organic layer into HX-1, consequently it realize the improvement of theefficiency of OLED device. Furthermore, HX-1 glass substrates have very high productivity in theoverflow down draw process.
Light extraction efficiency
To investigate the relationship between the light extraction efficiency of the OLED and therefractive index of the glass substrate, two kinds of glass with different refractive index wereprepared. The refractive index of two glass substrates are 1.63(HX-1), and 1.52(general refractiveindex glass). Generally the structure of a green phosphorescent OLED is shown in Figure 1, Glass /ITO, 100nm/HIL, 40nm/NPD, 50nm/Ir (ppy) 3 + CBP [6%], 30nm/BAlq, 10nm/Alq, 30nm/LiF,0.8nm/Al, 150nm. This structure is used for the simulation.
As a result of optical simulation, the proportion of waveguide mode decreased from 11% to 5%, andthe proportion of substrate mode increased from 21% to 28% by changing the glass substrate fromgeneral refractive index (nd1.52) glass to HX-1(nd1.63). Consequently, proportion of the external
mode was almost the same. Namely, the light extraction efficiency was not improved. This is
attributed to increasing the difference in refractive index between glass substrate and air.
On the other hand, when a hemisphere lens, which has the same refractive index with the glass
substrate, is used at the air side of the glass substrate, the light extraction efficiency was improvedfrom 44% to 50%. This is assumed that the light of substrate mode in the glass substrate was takenout to the air.

Electrode
Organic layer
ITO

Evanescent mode
Waveguide modeglass

Substrate mode

Hemisphere lensExternal mode
airFigure 1. Standard OLED’s structure and optical mode of OLED.
Table 1. Result of optical mode simulation.

Hemisphere lensEvanescent mode (electrode)
Waveguide mode (organic layer/ITO)
Substrate mode (glass)
External mode (air)nd1.52
without lens with lens
45%
45%
11%
5%
21%
44%
23%

nd 1.63
without lens with lens
45%
45%
5%
5%
28%
50%
22%