**5. World's first UHD OLED TV products**

**4.4. OLED degradation compensation**

48 Green Electronics

Other quality issues include image sticking. Firstly, being current-driven, OLED pixels generate heat when they emit light, and there may be a luminance change because of high temperature. Secondly, luminance will drop according to total driving time because of OLED degradation, like any other self-luminous device. For the former issue, we have designed a mechanical structure to release heat efficiently, and we use real-time temperature compensation. For the latter issue, we use a known correlation between current efficiency decrease and electric characteristic change of OLED [30]. OLED voltage–current characteristics change according to degradation, and we need a higher voltage to have the same current after OLED usage. We sense voltage for the same predetermined current to estimate OLED degradation at each subpixel and use a lookup table to translate OLED voltage shift to luminance compensation value. **Figure 15** compares time-dependent luminance curves for low stress and high stress, without and with repeated OLED voltage sensing and luminance compensation. Luminance difference can be minimized between high-stress and low-stress subpixels through the OLED degradation compensation. We sense OLED degradation with a predetermined interval because OLED degrades much slower than TFT. We do not compensate OLED luminance completely to maintain initial luminance, because OLED compensation rather accelerates degradation. Instead, we match OLED luminance to target degradation curve. **Figure 16(a)** shows the image sticking

**Figure 16.** Image sticking test results (a) Before compensation, (b) After compensation.

**Figure 15.** Image sticking compensation method due to OLED degradation.

**Figure 17** is a photograph of the 55-, 65-, and 77-inch UHD OLED TVs, which are world's first products. These OLED TVs are employing TFT backplane composed of coplanar a-IGZO TFTs, three-stack three-color tandem WOLED and advanced compensation technologies. Our technology platform allows a panel size scalability, a product reliability, and a high aperture ratio. **Table 4** shows specifications of our UHD OLED TVs with high color gamut, high contrast ratio, and thin thickness.

The reason why OLED TV is known to have the best quality of display is that it realizes high contrast ratio since OLED at each subpixel can be completely and individually turned off by oxide TFTs when the subpixel displays zero signal. Low-leakage current of oxide TFTs contributes to the high contrast ratio of OLED TV. Namely, OLED TV can easily achieve high dynamic range (HDR) without raising up the peak luminance, contrary to LCD TV. Accordingly, OLED

**Figure 17.** Photograph of LG's 55-, 65- and 77-inch UHD OLED TV products.


**Table 4.** Specifications of the LG UHD OLED TV (\*DCI: Digital Cinema Initiatives).

TV has an advantage to realize wide-span HDR with low power consumption. New compensation technologies can extend life span of OLED TV by relieving image sticking which seems inevitable for the spontaneous emission-type displays. The ultra-slim thickness of OLED TV not only offers premium values in view of design but also entitles OLED TV as green electronics by eliminating several plastic sheets in LCD TV.

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