A novel current-scaling a-Si:H TFTs pixel electrode circuit for AM-OLEDs

Hydrogenated amorphous silicon thin-film transistor (a-Si:H TFT) pixel electrode circuit with a function of current scaling is proposed for active-matrix organic light-emitting displays (AM-OLEDs). In contrast to the conventional current mirror pixel electrode circuit, in this circuit a high data-to-organic light-emitting device (OLED) current ratio can be achieved, without increasing the a-Si:H TFT size, by using a cascade structure of storage capacitors. Moreover, the proposed circuit can compensate for the variations of TFT threshold voltage. Simulation results, based on a-Si:H TFT and OLED experimental data, showed that a data-to-OLED current ratio larger than 10 and a fast pixel programming time can be accomplished with the proposed circuit.

I. Introduction

Since the first observations of the light emission in small molecules based organic light-emitting diodes (OLEDs) [1], there have been increasing interest in their applications to a large area flat panel displays due to their adequate opto-electric properties, versatility of colors, large viewing angle and potentially a low fabrication cost [2]–[4]. At the same time, for a number of years hydrogenated amorphous silicon thin-film transistor (a-Si:H TFT) and a low-temperature polysilicon (LTPS) TFT active-matrix (AM) array have been developed for liquid-crystal displays (LCDs). Recently it was also demonstrated that a combination of the OLEDs with the TFT active-matrix arrays can be used for a high resolution active-matrix OLEDs [5]– [10]. Since the AM-OLED luminance is directly proportional to the driving current passing through it, the pixel electrode circuits must deliver continuous current to OLED during the whole frame period [11].