| Organic light emitting device is a new display device which has many advantages such as thin layer thickness, light weight, wide angle of view, wide operating temperature range. Passive driving and active driving are two main driving modes for OLED. Passive driving requires high pulse voltage and current, resulting in low working efficiency. The passive driving mode is mainly used in small size, low information capacity display field. Active driving keeps the pixels continuously lighted in one frame time which meets the needs of large area and high information capacity display.The initial active driving circuit is the traditional 2T-1C pixel circuit composed of two thin film transistors and a capacity which has simple structure and it is easy to control. However, threshold voltage of TFTs shifts with long working time, resulting in the change of the OLED current. Therefore, pixel circuits with threshold voltage compensation function have appeared. However, the structure of pixel circuits with threshold voltage compensation function are more complex compared to traditional 2T-1C pixel circuit, which reduces the aperture ratio and affects the display. The effect of reduced aperture ratio is serious especially in high resolution display. Therefore, a multiplexing pixel driving circuit is designed adopting several pixels sharing a driving circuit method in this paper, which has both threshold voltage compensation function and high aperture ratio.A kind of equivalent model of OLED circuit applied to the pixel driving circuit is selected firstly because there is no OLED simulation model in HSPICE simulation software library. Specific parameters of each equivalent element are obtained by using origin to fit the experimental I-V curve. The equivalent model has small errors proved by HSPICE software simulation. So the equivalent model parameters are reasonable and suitable for circuit simulation.A current written circuit and a voltage written threshold voltage compensation circuit are adopted in this paper. The pixel driving circuits based on the IGZO-TFT are simulated and analysed. The simulation results show that the OLED current deviation is small when the threshold voltage drifts with time, proving good compensating effect. However, increased number of elements make the pixel aperture ratio very low proved by layout design, which affects the display and operational life.In this paper, a current written multiplexing threshold voltage compensation pixel driving circuit adopting several pixels sharing a driving circuit method is designed. The proposed circuit realizes 16 gray scales with the brightness range of 0.988~201.224cd/m~2. The display pixel array is 320×240 with the area of single pixel 159×35?m~2, realizing the resolution of 160 ppi. The current written pixel circuit layout design shows that the aperture ratio increases from 20% to 69%, proving the aperture ratio improved greatly.Because the voltage written circuit and current written pixel driving circuit have their own advantages, a voltage written multiplexing pixel driving circuit is also designed based on IGZO-TFT in this paper. The simulation work has been completed and layout design proves the improved aperture ratio. |
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