In this thesis, new pixel circuits for AMOLEDs have been proposed. First, a new poly-Si TFT pixel circuit composed of four TFTs and two capacitors for an AMOLED has been proposed to provide uniform current of OLED over display area regardless of poly-Si TFT variation. The non-uniformity in the output current of an OLED is 3-6%, simulated with combined $V_{th}$, μ, and $S_t$ variations $(＼Delta V_{th} = ＼pm 0.33V, ＼Delta μ = ＼pm 22 ㎠/vㆍsec, and $＼Delta S_t = ＼pm 0.03V/dec)$. These simulation results based on measured data indicate that the proposed pixel circuit is effective to provide a uniform output current against the threshold voltage and mobility variations in the poly-Si TFT performances. A new a-Si:H TFT pixel circuit for AMOLEDs has been proposed. The threshold voltage shifts after the operating time of 10,000 h, were estimated from experimental data and, based on these results, the feasibility of the proposed pixel circuit for AMOLED has been investigated with the circuit simulation. The circuit simulation results indicate that the proposed circuit is suitable for large-size and high-resolution displays because it can provide only 6% reduction in output light intensity after 10,000 h operation without OLED degradation.
Degradation phenomena of a-Si:H TFTs and OLEDs has been investigated in order to extract main factors required to estimate more exactly the lifetime of a-Si:H TFT OLEDs. In instability of OLED, there are reduction of luminance efficiency and rise of anode voltage over operating time. And instabilities of a-Si:H TFTs were investigated under different BTS conditions. It has been demonstrated that main factor to estimate $＼Delta V_{th}$ of a-Si:H TFT by stress current is effective stress voltage. Calculation method for $＼Delta V_{th}$ of a-Si:H TFT by constant current stress has newly proposed. Under consideration of these instabilities, a novel pixel circuit for a-Si:H TFT OLEDs has been proposed, enhancing the display lifetime. It is composed of 5 TFTs and 2 capacitors. This pixel circuit can compensate for the threshold voltage shift and mobility variation of a-Si:H TFT as well as the turn-on voltage shift and luminance-efficiency degradation of OLED. Good performance and long lifetime of the circuit has been extensively examined by SPICE and numerical simulation based on measured data. Maximum reduction of luminance after 10000 h is estimated at 39 %.
To demonstrate the feasibility of a-Si:H TFT OLED, a small size color displays was fabricated. The display size is 1.72 inch (diag.) and the resolution is 100(×RGB) ×125. The unit pixel pitch is 273 ㎛. In order to develop the active-matrix display using a-Si:H TFTs, each pixel includes circuit to compensate for threshold voltage shift of a-Si:H TFT. Since the pixel circuit is composed of 5 TFTs and utilizes large-sized TFTs, top-emission type OLED is used in pixel to have a high aperture ratio (62%). The processes to design the pixel and fabrication have been demonstrated. The display has been being fabricated.
