The New Voltage Programming Pixel Circuit with LTPS-TFTs Driving for AMOLED Displays

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Active matrix organic light-emitting diode (AMOLED) has recently attracted much attention due to its high brightness, light weight, fast response-time, and wide viewing angle. Low-temperature polycrystalline-silicon (LTPS) thin-film transistors (LTPS-TFTs) have been widely considered as pixel elements for AMOLED due to their high current-driving capability. The high electron mobility of LTPS-TFTs can achieve larger aperture ratio for a given pixel size of AMOLED and a higher resolution display to get the better image quality. Furthermore, a very light-weight display with few external interconnections is possible, because the peripheral circuits and the pixel driver circuits both using LTPS-TFTs can be integrated onto the same substrate. However, the LTPS-TFTs still have the issue of non-uniformity of threshold voltage due to the excimer laser annealing (ELA) process. In the conventional two-TFTs pixel circuit for AMOLED, various threshold voltages of driving TFT (DTFT) cause the non-uniform gray-scale over the display area. In addition, the long time operation for OLED will cause the brightness to drop off as the OLED threshold voltage degradation. Therefore, in order to overcome the above-mentioned issues, we have proposed three novel voltage-modulated structures for AMOLED pixel circuit as 5T1C, 5T1C* and 6T1C. These circuits can compensate both the DTFT threshold voltage deviation and the OLED degradation. The proposed circuits are also verified by SPICE simulator. In the 5T1C simulation, results show that the average OLED current error rate under ΔVTH = ±0.33 V is 0.5%, the average OLED current error rate underΔVTH_O = +0.33 V is 10% and the average current error rate under IR Drop voltage = 0.3 V is 8%. It can effectively reproduces almost identical OLED current regardless of the DTFT threshold voltage deviation and OLED degradation. In the 5T1C* simulation results show that the average OLED current error rate under ΔVTH = ±0.33 V is 0.8%, the average OLED current error rate underΔVTH_O = +0.33 V is 4.7% and the average current error rate under IR Drop voltage = 0.3 V is 5.8%. It can effectively reproduces almost identical OLED current regardless of the DTFT threshold voltage deviation and OLED degradation. In the 6T1C simulation results show that the average OLED current error rate under ΔVTH = ±0.33 V is 3.7%, and the average current error rate under IR Drop voltage = 0.3 V is 8.9%. It can effectively reproduces almost identical OLED current regardless of the DTFT threshold voltage deviation and OLED degradation. The above results show that the image non-uniformity of AMOLED can be effectively improved at the same time by compensating the DTFT threshold voltage deviation and the OLED threshold voltage degradation. Thus, we believe that the proposed pixel circuit design has very high driving capability and is a promising candidate for the large size, high resolution AMOLED panels.