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International Conference on Display Technology 2022 (Volume 53, Issue S1)

The study on minimizing angular color shift in high resolution Micro-OLED

with Color Filter
Dong-Wang Kang, Seung-Hyun Cho, Sang-Min Shim, Seung-Ho Jeon
Optic Business Unit, LinkGlobal21 Co., Ltd.,
7-11, Baumoe-Ro 27-Gil, Seocho-Gu, Seoul, Korea.

Abstract transparent metal electrode (5nm)/hole transport layer (140

In high-resolution and wide-viewing-angle Microdisplay nm)/emitting organic layers (60 nm)/electron transport layer (50
OLED(Micro-OLED) with Color Filter (CF), the control of the nm)/the reflective Al electrode (100nm), which are separated by
color mixture between adjacent sub-pixels is a method to reduce organic planarization layer(1.0um) from the CF layer (2.0 um).
the color shift. With optic simulation, we analyze the dependency The adjacent CFs are placed at the same level, and the
of the color mixture on design parameters of the pixel structure corresponding pixel define layer (PDL) is also formed between
such as mainly pixel size, CF size, and CF location. Additionally, sub-pixels with the width of 1um.
we propose the design methodology to minimize the angular color
shift.
Author Keywords
Micro-Display, OLED, Angular Color Shift, Color Mixture.
1. Introduction
Micro-OLED in many types of microdisplays have received
strong attention as main part of augmented reality/ virtual reality
(AR/VR) devices due to their small size and high resolution [1-
2]. Micro-OLED have been used in various electronic devices
such as TVs, mobile phones, tablets, laptops, and automotive
displays due to their fast response time, small thickness, light
weight, high contrast ratio rich colors, and small form factor.
However, even though Micro-OLED have been released
commercially, they need much higher luminance, resolution, and
efficiency to improve the device quality. To increase the
applications and market share of Micro-OLED, their device Figure 1. The top view of Micro-OLED with 4 sub-pixels.
architectures, materials, and fabrication processes should be When the emitted light at a pixel propagates along the optical
improved continuously for improving a wide field of view in path formed between the adjacent sub-pixel and the PDL denoted
AR/VR glasses and for high resolution. The sub-pixel size of by the dotted red lines in Figure 2, the color mixture of Micro-
Micro-OLED is as small as a few micrometers. This scale of pixel OLED is stronger because of propagating light from emission
can’t avoid the color mixture between sub-pixels. position to CF on adjacent sub-pixels. As the distance (𝑤𝐵𝐺 )
In this study, the color shift free OLED structure was necessary between Green CF (G-CF) and EML of bule sub-pixel decreases,
and was found through the optimization of the thickness of it is expected that the amount of color mixture becomes larger,
multilayers since main interest is CF size, CF width, and pixel and the emission position close to the PDL can increase optical
size. and the finite difference time domain (FDTD) method has path into the adjacent CF. Thus, it is necessary to characterize the
been adopted to quantitatively investigate the color mixture amount of color mixture between the adjacent pixels considering
between sub-pixels in several micro pixel.[3] The estimated the characteristics of OLED spectrum and CF transmittance.
results show that the amount of color mixture at increased 2𝜋𝐾𝐵 (𝜆) 2𝜋𝐾𝐺 (𝜆)
𝑇𝐶𝐹 = exp(− 𝑑𝐵 − 𝑑𝐺 ) (1)
viewing angle strongly depend on various parameters of the pixel 𝜆 𝜆
structure like the thickness of Regin layer, the distance between
CFs of adjacent pixel and the edge of emission layer (EML), and Where K_B (λ) and K_G (λ) are the imaginary refractive index
the thickness of CFs. The quantified color shift variation of Blue CF (B-CF) and Green CF (C-CF) at wavelength (λ), 𝑑𝐵
depending on the amount of color mixture levels provides an and 𝑑𝐺 are traveling distance of light in B- and G-CF, and T_CF
effective guideline to improve the performance of high-resolution is the transmittance of light propagating through B- and G-CF.
Micro-OLED comprising white OLEDs and a CF array. The traveling distance of light between two sub-pixels determines
the amount of color mixture. Since incoherent emission happens
2. Device Structure and Method randomly in OLEDs, we need to integrate EL spectrum of
Figure 1 represents the optical microscope top-view image of a emitting lights over whole emitting area (𝑤𝐵 ). It is necessary to
typical Micro-OLED with CF pattern. Here, the CFs are arranged choose the number of emitting points.
in such a way that blue and red sub-pixels are alternatively placed 780
along the vertical direction, while blue and green along the 𝐸𝐿(𝑥𝑖 , 𝑦𝑖 ) = ∫380 𝑃𝐿(𝜆)𝑃𝑜𝑢𝑡(𝜆)𝑑𝜆 (2)
horizontal one. We assume that the efficiency of R/G/B sub-
pixels in a pixel is the same. The pixel size is 36𝜇𝑚2 𝐸𝐿 = ∫𝐴 𝐸𝐿(𝑥, 𝑦)𝑑𝐴 (3)
(𝑤𝐵 + 𝑤𝐺 ) × (𝑤𝐵 + 𝑤𝑅 ) in Figure 1. The cross-sectional view 𝐵

of an OLED display with a CF array is also schematically drawn

in Figure 2. Here, the white OLED is constructed with the Where 𝑥𝑖 and 𝑦𝑖 are the location of dipole emission, 𝜆 is

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International Conference on Display Technology 2022 (Volume 53, Issue S1)

wavelength, 𝐸𝐿(𝑥𝑖 , 𝑦𝑖 ) is electro-luminance at 𝑥𝑖 and 𝑦𝑖 , 𝑃𝐿(𝜆) with CFs. That is, the width of PDL is so large that color mixture
is photo-luminance at 𝜆 , 𝑃𝑜𝑢𝑡(𝜆) is out power at 𝜆 , and between subpixels is negligible (𝑤𝐵𝐺 ≫ 0). It keeps below 0.02
𝐴𝐵 (𝑤𝐵 × 𝑤𝐵 ) is the active area of blue sub-pixel. color difference at viewing angle lower than 30. Normally the
emissive properties of OLEDs are related to not only the
properties of EML but also the multilayer structure that can
produces complex interference patterns. PL is the intrinsic
properties of EML. The output power (Pout) of emitting light is
determined by the interference of multilayer. Pout can be
optimized by changing the structure of cavity like thickness. By
multiplying PL with Pout from the OLEDs, the EL spectrum of
OLEDs has been estimated as shown in Figure 4a. The peak
location of PL is not changed even at high viewing angle. but the
peak shift of Pout at increased viewing angles always stands out
as a side effect of the microcavity resonance. When PL peak is
located at the shorter wavelength than the peak of Pout, the color
shift is stronger (Figure 4b). In contrast, when the peak of PL is
located at shorter wavelength than that of output power, color
shift is minimized (Figure 5b) [5].

Figure 2. The cross-sectional view of Micro-OLED

In OLEDs, electrons and holes recombine to generate excitons
mostly in EML. Since the radiative decay of an exciton can be
described by a dipole light source radiating into the OLED
structure [4], the light propagation is given by the solution of the
Maxwell equations for incoherently oscillating dipoles in the
layered structure. Thus, the structure was excited by multiple
single dipole sources located at the emitting layer, and the output
power were estimated by placing the detector just above CF layer.
Since the dipole direction is randomly oriented in OLEDs, the
dipole direction is considered as equally distributed in the x-, y-,
and z-directions. The behavior of emitted light in multilayers
were calculated with FDTD. These dipoles were defined as
current source in this method (Figure. 2). The amount of output
power along the vertical or horizontal direction, respectively, was
monitored by the detector placed above whole G-, B-, and G-CFs
layer of pixel, or whole R-, B-, and R-CF. Here, the permittivity
of all materials in this structure was described with a modified
Drude-Lorenz model. Each side of the structure was modelled by
a perfectly matched layer absorbing boundary condition.
3. Result and Discussion

Figure 4. (a) The peak of PL is located at the right side of that

of Pout. (b) The peak of EL is shift to short wavelength at
increased viewing angle.
The role of CFs in EL spectrum is the similar like that of PL
because CFs is also intrinsic property of material. However, since
Figure 3. Optimization Flow for minimizing color shift of the transmittance of CF depends on the optical path (Eq.1), the
Micro-OLED with CFs. intensity of EL is reduced at high viewing angle. As the result,
The optimization flow in Figure 3 was followed to design Micro- OLEDs with CFs gives much smaller color shift than without
OLED with minimum color shift. As the first step, we need to CFs. The color shift is estimated with the below equation.
prepare the structure of White OLED(WOLED) without CF layer Through the optimization, the structure of WOLED with/without
before investigating the amount of color mixture in Micro-OLED CF is optimized to be below 0.02 at 30 as color shift (Figure 6).

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International Conference on Display Technology 2022 (Volume 53, Issue S1)

Table 1. The Split Condition of Pixel Structure

Parameters
Parameter 𝐖𝐁𝐆 𝐭𝐁 𝐭 𝐑𝐞𝐠𝐢𝐧
−∆ m m m
±0 m m m
+∆ m m m

The analysis of the behavior of light emitting in a sub-pixel can

be applied to that in other sub-pixels is analyzed. Thus, blue sub-
pixel was focused. Table 1 shows the split table about three
parameters such as 𝑤𝐵𝐺 , 𝑡𝐵 , and 𝑡𝑅𝑒𝑔𝑖𝑛 (Figure 2). 𝑤𝐵𝐺 is the
distance between the edge of EML in blue sub-pixel and G or R-
CF layer of adjacent sub-pixel, 𝑡𝐵 is the thickness of B-, G-, and
R-CF. 𝑡𝑅𝑒𝑔𝑖𝑛 is the thickness of Regin layer. We choose
𝑤𝐵𝐺 =0.3μm, 𝑡𝐵 =2.0μm, 𝑡𝑅𝑒𝑔𝑖𝑛 =1.0um as reference pixel
structure. This reference structure is to show that the color
difference is 0.02 at viewing angles below 30° in Figure 6. 0.02
as color shift is the acceptable color shift.

Figure 5. (a) The peak of PL is located at the right side of

that of Pout. (b) The peak of EL has the same wavelength.

Figure 6. Color shift @ optimized blue sub-pixel of

conventional WOLED.
The structure of OLEDs with minimum color shift are used for Figure 7. (a) Angular EL vs. CF layer thickness(top) (b)
Micro-OLED with four sub-pixels. This approach can estimate The color difference vs. CF layer thickness. (bottom)
obviously the color mixture between two sub-pixels. While Under 𝑤𝐵𝐺 = 0.3𝜇𝑚 and 𝑡𝑅𝑒𝑔𝑖𝑛 = 1.0𝜇𝑚 , we extracted the
emitting light at the center of sub-pixel causes small color viewing angles at Δu′ v ′ = 0.02 with the split of CF thickness
mixture, emitting lights near PDL experience long propagation ( 1.5𝑢𝑚 , 2.0𝑢𝑚 , and 2.5𝑢𝑚 ). The thicker CF layer ( 𝑡𝐵 =
though CF of the adjacent sub-pixel. In the latter case, the optical 2.5𝜇𝑚) gives large color shift at the increased viewing angle in
path (𝑑𝐺 ) in adjacent sub-pixel becomes longer. With Eq.2 and Figure 7b since the optic path of light at G- or R-CF layer
Eq.3, all EL of emitting sources(dipole) in whole position of EML becomes longer. however angular EL at the thicker CF layer is
are integrated. smaller in Figure 7a. In the condition that 𝑤𝐵𝐺 = 0.3𝜇𝑚 and

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International Conference on Display Technology 2022 (Volume 53, Issue S1)

𝑡𝐵 = 2.0𝜇𝑚, the thickness of Regin layer is split into 0.50, 1.00,

and 1.50𝑢𝑚 respectively. The viewing angles with Δu′ v ′ =
0.02 in Figure 8b are about 37.8/29.8/25.6 at 1.5/ 2.0 /2.5𝜇𝑚.
The thicker Regin layer gives large color shift because emitting
light in blue sub-pixel propagates deeply to G- or R-CF. While 𝑡𝐵
and 𝑡𝑅𝑒𝑔𝑖𝑛 are 1.5𝑢𝑚 and 1.0𝑢𝑚, 𝑤𝐵𝐺 is split to 0.00, 0.30, and
0.60𝑢𝑚. The variation range of PDL width must be smaller than
the thickness of CF layer and Regin layer because Micro-OLED
keeps the minimum emitting region for the performance. Figure
8a shows the narrower PDL causes large color shift. The wider
PDL means that emitting region is far from G- or R-CF.
Therefore, EL at the front view becomes larger. The viewing
angle with Δu′ v ′ = 0.02 in Figure 9b are 21.5/29.8/39.9
respectively at 0.0/0.3/0. 6𝜇𝑚. In three parameters, PDL width is
the most sensitive to color shift. Table 2 is the summary of the
upper investigation.

Table 2. Viewing Angle @ 𝚫𝐮′ 𝐯 ′ = 0.02 about Split

Condition.
Parameter 𝐖𝐁𝐆 𝐭𝐁 𝐭 𝐑𝐞𝐠𝐢𝐧
−∆ 21.5 30.1 37.8
±0 29.8 29.8 29.8
+∆ 39.9 27.1 25.6

Figure 9. (a) Angular EL vs. PDL layer Thickness(top) (b)

The color difference vs. PDL layer thickness. (bottom)
4. Conclusion
We have investigated the amount of color mixture between the
adjacent color sub-pixels of Micro-OLED comprising the CF-
array stacked WOLED by using FDTD method. Three facts are
shown for reducing color difference. (i) the width of CF layer is
wider than pixel active region. (ii) CF layer is closer to EML. (iii)
The CF layer is thicker. Additionally we found that the most
sensitive parameter to color shift is PDL width.
5. References
[1] T. Fujii et al., “4032 ppi High-resolution OLED
microdisplay”, Journal of the Society for Information
Display,26, 3, 178-186 (2018)
[2] Y. Onoyama et al., “High-Efficiency OLED Microdisplay
with Microlens Array”, SID'19 Digest, 52, 721–724 (2019)
[3] Taflove, S.C. Hagness, Computational Electrodynamics:
The Finite Difference Time-Domain Method, Artech
House, Norwood, Mass, USA, 2005.
[4] H. Benisty et al,” Method of source terms for dipole
emission modification in modes of arbitrary planar
structures” , J. Opt. Soc. Am. A 15 (1998) 1192–1201.
[5] E. Kim et all, “A systematic approach to reducing angular
color shift in cavity-based organic light-emitting diodes,”
Org. Electron. 48, 348–356 (2017)
Figure 8. (a) Angular EL vs. Regin layer thickness(top)
(b) The color difference vs. Regin layer thickness. (bottom)

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