An image displayed by the OLED
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An image displayed by the reflective LCD
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OLED and transmissive LCD displays, which are currently used for mobile devices, can be seen well indoors but not so in sunlight outdoors. In addition, they account for a large percentage of the power consumption of mobile devices.

To solve such issues, Semiconductor Energy Laboratory (SEL) Co Ltd developed an OLED/reflective LCD hybrid display and exhibited it at SID 2016, an academic conference on display technologies (announcement number: 7.2). SEL calls it "TR-Hybrid Display." The letters "T" and "R" represent "transmissive OLED" and "reflective LC," respectively.

The hybrid display is made by making holes in part of the reflecting electrodes of a reflective LCD. Light emitted from OLEDs attached to the back side of the reflective LCD is transmitted through the holes.

As a production technology for the display, SEL used a delamination technology for flexible OLED displays. First, a peeling layer is applied to a glass substrate for carriage, and TFTs are formed on it. Then, bottom-emission OLED devices are formed to extract light to the TFT side and covered with a flattening film, and a glass plate is placed on it.

After that, the peeling layer is detached, and the glass substrate is removed. Finally, a color filter substrate is attached to the TFT side (without the glass substrate), and a liquid crystal material is injected.

The screen size, pixel count and resolution of the prototyped display are 4.38 inches, 768 x 1,024 and 292ppi, respectively. The LCD and OLED aperture ratios are 76% and 3.9%. To reduce its power consumption as much as possible, SEL employed the IGZO oxide semiconductor, which enables low-frequency operation and can drive OLEDs, for its TFTs. For example, the power consumption of the reflective LCD can be reduced to about 1/180 by lowering drive frequency from 60Hz to 1/3Hz.

Moreover, SEL optimized the driving method of the display and improved the display contrast of the OLED from 1,111:1 to 2,850:1. The OLED and reflective LCD are driven independently from each other. Each sub-pixel of the OLED has two transistors and one capacitor, and each LCD sub-pixel has one transistor and one capacitor.