Leakage and charge injection optimization in a-Si AMOLED displays

In this paper, we examine the effect of switch thin-film transistor (TFT) leakage and charge injection on the operation and driving of amorphous silicon (a-Si) active matrix organic light-emitting diode (AMOLED) displays. Charge injection causes an undesirable and immediate drop in the data voltage stored on the storage capacitor CS when the switch TFT is turned off, and the leakage of the switch TFT causes the charge on CS to gradually leak out over the frame time. While making the row line negative helps reduce the leakage, it increases the voltage swing on the row line and causes more charge injection. We have demonstrated that for a given V_DD, there is an optimal negative gate drive voltage on the switch TFT that minimizes the overall drop in data voltage on CS over the frame time. In addition, we have also shown that even though this optimal driving point changes with aging of the display since both leakage and V_T increase over time, it is possible to keep the voltage drop on C_S constant irrespective of aging. The analysis provides the designer with a means to improve the long term grey-scale performance of the AMOLED display