High-resolution Technology for OLED Microdisplays | Display | Technology

Overview

Sony Semiconductor Solutions Corporation’s (SSS) OLED Microdisplays have a proprietary frontplane and backplane design that enables high resolution performance without sacrificing characteristics such as luminance uniformity or viewing angle characteristics.
The frontplane uses an OCCF (on-chip color filter) structure and a new color filter array, while the backplane uses a proprietary pixel compensation circuit and a new driving circuit.

Technical Features

Issues and countermeasures in achieving higher resolution

Smaller pixel pitch and higher resolution can reduce the size of the transistor, causing variance in characteristics or reduced withstand voltage, which can result in reduced luminance uniformity. In addition to the proprietary SSS's compensation circuit, we have optimized the layout and processes of each transistor in order to successfully achieve high image quality with high resolution yet high luminance uniformity, fast response, and a wide color gamut, We have also introduced a new driving method to achieve both high resolution and high-frame-rate display. Further, while molding the color filter directly onto the silicon substrate to reduce the distance from the luminous layer, we have used an innovative color arrangement for the color filter to minimize the viewing angle degradation associated with high resolution performance.

OLED Microdisplay image comparison:
high-resolution panel (0.5-type UXGA) (Left) and the conventional product (0.5-type QVGA) (right)

Achieving both smaller pixel pitch and luminance uniformity

To reduce the luminance variance in the display area associated with a smaller pixel pitch, we have integrated into the backplane pixel luminance compensation circuits originally developed by SSS, composed of four P-channel transistors (T1, T2, T3, and TD) and two capacitors (C1 and C2).^*1 By optimizing the design and process for each of these four transistors by function, we have been able to achieve equivalent or higher luminance uniformity than with conventional 7.8 μm pixel pitch, despite the smaller pixel pitch (6.3 μm).

*1) With a 0.5-type UXGA

Pixel luminance compensation circuit for high-resolution.

Proprietary high-frame-rate driving with less-resolution degradation

Integrating a newly designed drive circuit into the backplane made it compatible with a high frame rate of 240 fps, twice^*2 that of the conventional product. In addition to normal drive of 120 fps, we have also achieved special drive of 240 fps with added processing to shift each frame for two simultaneously scanning vertical lines and for the combination of those lines. Simply having two vertical lines scan simultaneously would result in the same image displaying on both lines. This presents the drawback of halving the resolution in the vertical direction. But the driving method used in SSS’s OLED Microdisplays shifts the combination of the two simultaneously scanning vertical lines for each frame so it can compensate the displayed image in the temporal direction. This allows it to run at double the speed while reducing the sense of resolution degradation in the vertical direction.

As with EVFs (electronic viewfinders) for cameras, this high-frame drive makes them ideal to more accurately capture fast-moving objects in the viewfinder. And for head mounted display devices, they can reduce misalignment when superimposing images over the real world in AR, which means they can also help to reduce VR motion sickness.

*2) Compared with SSS’s OLED Microdisplay ECX337A (0.5-type QVGA (1280 x 960))

Proprietary high-frame-rate driving with less-resolution degradation

On-chip color filter structure and original color array

In general, making the pixel pitch smaller can cause deteriorated color in the angle of view due to interference in adjacent color filters. One cause of this is the position precision of light-emitting pixels on the silicon substrate and color filter. To improve the position precision, SSS’s OLED Microdisplays alter the conventional structure to mold the color filter directly onto the silicon substrate. This enables higher position precision with no effect on the layering process precision of the backplane and color filter.

The second cause of deteriorated color in the angle of view due to smaller pixel pitch is said to be the color filter array. Given this, SSS’s OLED Microdisplays minimize the effect of color mixing in adjacent color filters by changing the color filter arrangement from the conventional asymmetrical one to the symmetrical color array.

On-chip color filter structure and original color array2

As you can see from the figure right side, introducing these two approaches made it possible to moderately overlap the tristimulus value, even when viewed from the oblique direction, and to reduce coloration. This technology enables both high resolution and a wider viewing angle.

On-chip color filter structure and original color array3

■ Reference literature
SID 2018 Distinguished Paper, Takuma Fujii et al.
4032ppi High-Resolution OLED Microdisplay

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