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Abstract
Herein, the color gamut change by optical crosstalk between sub-pixels in high-resolution full-color organic light-emitting diode (OLED) microdisplays was numerically investigated. The color gamut of the OLED microdisplay decreased dramatically as the pixel density of the panel increased from 100 pixels per inch (PPI) to 3000 PPI. In addition, the increase in thickness of the passivation layer between the bottom electrode and the top color filter results in a decrease in the color gamut. We also calculated the color gamut change depending on the pixel structures in the practical OLED microdisplay panel, which had an aspect ratio of 32:9 and a pixel density of 2,490 PPI. The fence angle and height, refractive index of the passivation layer, black matrix width, and white OLED device structure affect the color gamut of the OLED microdisplay panel because of the optical crosstalk effect.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the development of the augmented reality/virtual reality (AR/VR) technology, the demand for high-performance AR/VR devices has been increasing. Humans commonly obtain most information visually, and thus, display is very important for highly immersive AR/VR devices [1,2]. Because most AR/VR devices show images in front of the user’s eye, the resolution of the display should be very high to reduce the screen door effect and reduce eye fatigue [3].
Organic light-emitting diodes (OLEDs) have received great attention for use as the main display panels for AR/VR devices owing to their numerous advantages, such as fast response time, high color gamut, high contrast ratio, and self-emitting properties [4–9]. Complementary metal-oxide-semiconductor (CMOS)-based OLED microdisplays have been widely used in recent AR/VR devices because of their high resolution, which is over 2,000 pixels per inch (PPI) compared to thin-film transistor (TFT)-based conventional OLED displays [10–15]. For example, T. Fujii et al. in Sony reported 4,032 PPI high-resolution OLED microdisplay with 110% of standard RGB (sRGB) color gamut [16].
As the resolution of the display increases, the pixel size and distance between the pixel and adjacent pixels decrease, which results in electrical and optical crosstalk [17,18]. Because typical common layer materials, such as hole injection layer (HIL) materials in OLEDs, have very high sheet resistance, common layers are not patterned and the voltage of one pixel does not affect the adjacent pixel. However, the voltage of one pixel affects an adjacent pixel in a high-resolution display because of the lateral leakage current, resulting in electrical crosstalk and color distortion [19]. Moreover, as the distance between the pixels decreases, light leakage occurs owing to various optical effects, such as a refractive index difference and total internal reflection, and the colors are distorted, which is called optical crosstalk [18,20]. Optical crosstalk can decrease the color gamut of high-resolution, full-color OLED displays.
Red (R), green (G), and blue (B) subpixels are required for full-color OLED microdisplays. High-resolution R, G, B emitting layer (EML) patterning has various issues, such as high-precision fine metal mask (FMM) fabrication, FMM alignment, and hole clogging [21,22]. Therefore, most full-color OLED microdisplays use white OLEDs with color filer (CF) technology [23–26]. Generally, a white OLED panel and CF panel were separately fabricated and combined for full-color OLED TVs. However, the CF patterning process must be conducted on white OLEDs with a passivation layer to reduce optical crosstalk in high-resolution OLED microdisplays. The studies on the optical crosstalk in OLED microdisplays are limited.
In this study, we investigated the optical crosstalk effect using a practical OLED microdisplay panel pixel. We conducted an optical simulation and calculated the color gamut depending on pixel structures, such as pixel size, distance between pixels, thickness of the passivation layer between the OLED and CF, refractive index of the passivation layer, fence angle and thickness, and OLED device structures.
2. Experimental and optical simulation method
2.1 Experimental
A CMOS-process-based silicon (Si) substrate was used for the fabrication of white tandem OLEDs. The substrate size was 2 cm × 2 cm and the active area was approximately 2 mm × 2 mm with a pixel pitch of 10.8 µm × 3.6 µm. The top metal of the Si substrate was aluminum (Al)/titanium nitride (TiN). The substrates were sequentially rinsed with acetone, methanol, and deionized water for 15 min each and then dried in a vacuum oven at 80 °C. Subsequently, the organic and metal layers were deposited using a vacuum thermal evaporator to fabricate the OLEDs. To protect OLEDs from moisture and oxygen, Al2O3 was deposited onto the OLEDs as a thin-film encapsulation (TFE) layer using atomic layer deposition (ALD) [27,28]. The ALD process temperature was 95 °C. The detailed structure of the white tandem OLED is shown in Fig. 1(a) as follows: Si/Al (50 nm)/TiN (3 nm)/HIL (8.5 nm)/hole transport layer (HTL) (45 nm)/yellow-green (YG) EML (20 nm)/electron transport layer (ETL) (25 nm)/n-type charge generation layer (CGL) (20 nm)/p-type CGL (8.5 nm)/HTL (50 nm)/B EML (20 nm)/ETL (30 nm)/electron injection layer (EIL) (2 nm)/Mg:Ag (1:10) (15 nm) as a semitransparent cathode/capping layer (CPL) (80 nm)/LiF (50 nm)/Al2O3 (60 nm). Before TFE, a 50 nm thick LiF layer was used to protect the OLEDs during the ALD process. The R, G, and B CFs were supplied by DONGJIN SEMICHEM CO., LTD.
Fig. 1. (a) Schematic device structure of white tandem OLED for fabrication and optical simulation (b) Measured and optically simulated EL spectra of white tandem OLED.
The electroluminescence (EL) spectra were measured using a spectroradiometer (Konica Minolta CS-2000) equipped with a goniometer for angular analysis. The transmittance of the CFs was measured using a UV-Vis-NIR spectrophotometer (PerkinElmer LAMBDA 750). The photoluminescence (PL) spectra of the yellow-green (YG) and B dopants films were measured using a spectrophotometer (HORIBA Fluorolog3).
2.2 Optical simulation
For the optical simulation, the commercial software SETFOS (Fluxim) and LAOSS (Fluxim) were used. The optical constants of the materials were obtained by spectroscopic measurements, literature, and software. It is assumed that the internal quantum efficiency is 100% and the recombination zone (RZ) of EML B and EML YG are placed at the interface of HTL and EML and 4 nm away from the HTL, respectively, as shown by the red dashed lines in Fig. 1(a). The optically simulated EL spectra matched the measured EL spectra well, as shown in Fig. 1(b). The first peak and the valley at blue and green region in the simulated EL spectra are slightly different from measured EL spectra, which may be due to the difference between a real device and simulation assumptions such as exciton recombination zone distribution, absorption by emitter dopants, and dipole orientation [29]. However, this subtle EL spectra difference has little effect on the calculated color gamut results by optical crosstalk as shown in Fig. S1. For optical crosstalk calculations, the angular reflectance, transmittance, and EL spectra in the visible spectral region are required. We obtained angular EL spectra from the fabricated device measurements and angular reflectance and transmittance from the optical simulation (Fig. S2). To calculate the colors of the pixels, the angular reflectance and transmittance of the R, G, and B CFs were also required. We measured the refractive index (n) and extinction coefficient (k) of each CF color and calculated the reflectance and transmittance of the CFs. The calculated transmittance from B was well matched with the measured value, but those from R and G CFs were not well matched with the measured values (Fig. S3(a)). The transmittances of the R and G CFs provided by LAOSS are well matched with the measured transmittances of the R and G CFs (Fig. S3(b)). Therefore, we obtained the angular reflectance and transmittance of the R and G CFs from LAOSS software and calculated those of the B CF using the measured n and k values of the B CF.
To simulate optical crosstalk, we designed bottom electrodes and top CFs. The pixel structures are based on a previously reported practical OLED microdisplay panel [24]. The aperture ratio of each pixel was fixed at 72.2%, and the resolution of the panel was 1920 × 1080 (full high definition (FHD)).
3. Results and discussion
Figure 2(a) and 2(b) show the pixel dimensions and light leakage concepts, respectively. We investigated the color change depending on pixel structures by calculating the color gamut compared to sRGB when one middle sub-pixel among nine sub-pixels was emitted.
Fig. 2. (a) Pixel array dimension and (b) 3D schematic pixel array structure for optical simulation.
We changed the pixel density by varying the sub-pixel size and distance between the sub-pixels, as shown in Table 1. Figure 3 shows the color gamut with different pixel densities depending on silicon nitride (SiNx) thickness. When the thickness of SiNx was 100 nm, the color gamut of the pixel was nearly the same (approximately 114%), regardless of the pixel density. However, the color gamut changed as the thickness of SiNx increased [30], depending on the pixel density. As the pixel density increased, the color gamut of the pixel decreased dramatically as the thickness of SiNx increased. For example, the color gamut of the pixel with 3000 PPI was 59.6% at a SiNx thickness of 2000 nm, whereas that of the pixel with 100 PPI was 113.4%. The light from the middle sub-pixel can be easily waveguided to adjacent sub-pixels as the passivation layer thickness and pixel density increase, resulting in a reduced color gamut due to optical crosstalk. This result suggests that the passivation layer between the bottom electrode layer and top CF layer should be thin to reduce the optical crosstalk as the pixel density increases.
Fig. 3. Calculated color gamut of pixel depending on pixel densities and passivation layer thicknesses.
Table 1. Pixel dimensions depending on pixel densities
For practical applications, we simulated the optical crosstalk and calculated the color gamut based on a 32:9 OLED microdisplay panel, which has a pixel density of approximately 2,490 PPI [23]. The x, y, and d values of the sub-pixel were 3.40 µm, 10.2 µm, and 0.370 µm, respectively. Figure 4(a) shows optical crosstalk ratios [31] which were calculated as A – B divided by A, where A and B represent the total output light intensity and active emitting sub-pixel output light intensity, respectively, and can be expressed as follows:
The optical crosstalk ratio increases and the color purity of R, G, and B sub-pixels decreases as the thickness of SiNx increases as shown in Fig. 4(a) and 4(b), respectively. For instance, the optical crosstalk ratio and color coordinates of R, G, and B sub-pixels were 0.215%, 0.0294%, 0.00412%, and R(0.664, 0.331), G(0.255, 0.653), B(0.139, 0.120), respectively, at 100 nm of SiNx thickness, whereas those were 52.5%, 12.1%, 4.9%, and R(0.369, 0.294), G(0.244, 0.595), B(0.140, 0.144), respectively, at 3,000 nm of SiNx thickness. In addition, the optical crosstalk ratio of R is higher compared with other colors. Because the red emission ratio is lower compared with blue and green emission in the white emission of 2-wavelength white tandem OLED, the leakage light intensity from B and G sub-pixels are relatively higher compared with light intensity of an emitting R sub-pixel, resulting in deteriorated red color. Therefore, the color gamut of the pixel decreases as the thickness of the SiNx increases as shown in Fig. 4(c). This result indicates that 3-wavelength white OLED with R, G, and B emission can be advantageous for high color gamut compared with 2-wavelength white OLED.
Fig. 4. Calculated (a) optical crosstalk ratio and (b) color coordinates of R, G, and B sub-pixels (arrow direction: SiNx thickness increase from 100 nm to 3,000 nm), and (c) color gamut of a 2,490 PPI OLED microdisplay panel with different passivation layer thicknesses.
The role of the fence was to separate each sub-pixel, as shown in Fig. 5(a), and the dimension of the fence is also crucial for determining the optical crosstalk in the microdisplay panel. We calculated the color gamut by changing the fence angle and height for different SiNx thicknesses, as shown in Fig. 5(b) and 5(c), respectively. When the fence angle was changed, the fence height was fixed at 30 nm. When the thickness of the SiNx was less than 1000 nm, the fence angle did not affect the color gamut of the pixel. However, the color gamut increased as the fence angle increased from 30° to 90° when the thickness of the SiNx was greater than 1500 nm. This result indicates that a fence angle of 90° is much advantageous for reducing the optical crosstalk compared to a low fence angle. The fence height also changed when the fence angles were 45° and 90°. For the fence angle of 45°, the color gamut of the pixel decreased as the height of the fence increased. In contrast, for a fence angle of 90°, the color gamut of the pixel increased as the fence height increased. This result implies that a 45° fence angle with a high fence height can make the emitting light easily waveguide to adjacent sub-pixels, whereas a 90° fence angle with a high fence height can block the light leakage to adjacent sub-pixels.
Fig. 5. (a) Pixel structure with fence. Color gamut with different SiNx thickness depending on (b) fence angle, and (c) fence height.
The refractive index of the passivation layer can also affect the optical crosstalk in the display panel. We calculated the color gamut in the OLED microdisplay pixel structure by changing the refractive index of the passivation layer from 1.2 to 2.8 as shown in Fig. 6, assuming an extinction coefficient of zero. The pixel with a passivation layer with a high refractive index showed a higher color gamut than that with a low refractive index as the thickness of the passivation layer increased. For instance, the color gamut with refractive index of 2.8 is 78.1% at passivation layer thickness of 3000 nm, whereas that with refractive index of 1.2 is 5.34%. This result suggests that a refractive index of passivation layer is very important and a high-refractive-index material is advantageous as a passivation layer compared to a low-refractive-index material as the optical crosstalk becomes much stronger.
To investigate the optical crosstalk effect based on the width of the black matrix (BM), we changed the width of the BM by reducing the width of the CFs, as shown in Fig. 7(a). The width of the CF changed from 3.4 µm to 1.8 µm. As the width of BM increased, the color gamut also increased. For example, when the thickness of the passivation layer was 2000 nm, the color gamut of a pixel with a CF width of 3.4 was 46.13%, whereas that of a pixel with a CF width of 1.8 was 113.05%. This result indicates that an increased width of the BM can effectively reduce optical crosstalk.
Fig. 7. (a) CF dimension and (b) color gamut with different SiNx thickness depending on each CF’s width (a, b, c).
To investigate the optical crosstalk effect according to the white OLED device structure, tandem and single devices were compared. The tandem device was already optically simulated in the above results, and the single device structure is shown in Fig. 8(a), which has no CGL, and the total thickness of 345 nm is much thinner than that of the tandem device, which has a thickness of approximately 487 nm. To conduct the optical crosstalk simulation, we first simulated the EL spectra using SETFOS, and the simulated EL spectra were well matched with the measured EL spectra, as shown in Fig. 8(b). Based on the calculated angular-dependent reflectance, transmittance, and measured angular EL spectra, we calculated the color gamut in a single white device depending on the SiNx thickness. The single device structure showed a higher color gamut compared to the tandem structure device, regardless of the BM. In addition, the color gamut difference increases as the SiNx thickness increases. This result suggests that a longer cavity length of the tandem device structure can increase the optical crosstalk, resulting in a low color gamut compared with a single device structure with a relatively shorter cavity length.
Fig. 8. (a) Schematic device structure of white single OLED for fabrication and optical simulation, (b) Measured and optically simulated EL spectra of white single OLED, and (c) calculated color gamut depending on device structure and SiNx thicknesses.
4. Conclusion
We investigated the optical crosstalk effect in high-resolution OLED microdisplay pixel arrays by calculating their color gamut. The color gamut of the pixel decreased as the pixel density increased owing to the optical crosstalk. The thickness of the passivation layer between the bottom electrode layer and the top color filter layer plays a very significant role in determining the optical crosstalk effect, and a thinner passivation layer is advantageous for reducing the optical crosstalk. In addition, we calculated the color gamut based on a practical OLED micro-display pixel structure. A high fence angle, fence height, refractive index, and wide black matrix width are advantageous for a high color gamut. Moreover, the color gamut of the single-device structure was higher than that of the tandem device structure. We believe that these results can be helpful in designing and improving the performance of full-color high-resolution OLED microdisplays.
Funding
Electronics and Telecommunications Research Institute (ETRI) (22ZB1200); Ministry of Trade, Industry & Energy/Korea Evaluation Institute of Industrial Technology (MOTIE/KEIT) (20015805); Ministry of Science and ICT/National Research Foundation of Korea (MSIT/NRF) (2021R1F1A1045517); Ministry of Science and ICT, South Korea/Institute of Information & Communications Technology Planning & Evaluation (MSIT/IITP) (2022-0-00026); Sookmyung Women's University Research Grants (1-2203-2004).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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