Pulse-Width Modulation with Current Uniformization using Reduced Pixel Circuit for TFT-OLEDs

We have proposed a novel driving concept for TFT-OLEDs, pulse-width modulation with current uniformization. This driving method can simultaneously achieve precise grayscale and exceedingly improve luminance uniformity. Lately, we succeeded to reduce the number of TFTs in a pixel circuit from nine to seven. Especially in this paper, its working and tolerance against characteristic deviations of TFTs and OLEDs are confirmed using circuit simulation. Introduction TFT-OLEDs have been highlighted for a next generation of flat-panel displays [1-2]. One of the most critical issues for TFT-OLEDs is luminance uniformity between neighboring pixels. In order to improve the luminance uniformity, many kinds of driving method were proposed [3-11]. Formerly, we and another group presented pulse-width modulations with self-biased inverter [6-7]. These driving methods can compensate characteristic deviation of TFTs. However, they cannot compensate characteristic deviation of OLEDs. On the other hand, recently, we presented a time-ratio grayscale with current uniformization [11]. This driving method can compensate characteristic deviations of both TFTs and OLEDs. However, it needs high-speed scanning for high-resolution and many-grayscale displays. We have proposed a novel driving concept for TFT-OLEDs, pulse-width modulation with current uniformization [12]. This driving method can simultaneously achieve precise grayscale and exceedingly improve luminance uniformity. Lately, we succeeded to reduce the number of TFTs in a pixel circuit from nine to seven [13]. Especially in this paper, its working and tolerance against characteristic deviations of TFTs and OLEDs are confirmed using circuit simulation [14]. Pulse-width modulation with current uniformization This driving method can simultaneously achieve precise grayscale using the pulse-width modulation and exceedingly improve luminance uniformity using the current uniformization. The pixel circuit consists of the Pwm-part, pulse-width modulation part, and Cu-part, current uniformization part, as shown in Fig. 1. Here, the reduced TFTs and buslines that exist in the Proceedings of 6th Thin Film Materials & Devices Meeting November 2-3, 2009, Kyoto, Japan http://www.tfmd.jp/