An AMOLED pixel circuit for high image quality of 1000 ppi mobile displays in AR and VR applications

Abstract

In this paper, an active‐matrix organic light‐emitting diode pixel circuit is proposed to improve the image quality of 5.87‐in. mobile displays with 1000 ppi resolution in augmented and virtual reality applications. The proposed pixel circuit consisting of three thin‐film transistors (TFTs) and two capacitors (3T2C) employs a simultaneous emission driving method to reduce the number of TFTs and the emission current error caused by variations in threshold voltage (Vth) and subthreshold slope (SS) of the low‐temperature polycrystalline silicon TFTs. Using the simultaneous emission driving method, the compensation time is increased to 90 μs from 6.5 μs achieved in the conventional six TFTs and one capacitor (6T1C) pixel circuit. Consequently, the emission current error of the proposed 3T2C pixel circuit was reduced to ±3 least significant bit (LSB) from ±12 LSB at the 32nd gray level when the variations in both the Vth and SS are ±4σ. Moreover, both the crosstalk errors due to the parasitic capacitances between the adjacent pixel circuits and due to the leakage current were achieved to be less than ±1 LSB over the entire gray level. Therefore, the proposed pixel circuit is very suitable for active‐matrix organic light‐emitting diode displays requiring high image quality.

Correlation between the compensation time and the current deviation of organic LED pixel circuit

“…Two representative pixel circuits of Figures 2 and 3 were employed in the SPICE simulation because the p‐channel TFT (pTFT) circuit of Figure 2 is the most widely proven structure in the commercial products and the operation principle of other pixel circuits is similar to this one 11,12,14 . In order to exclude the dependence of device type, we also consider the n‐channel TFT (nTFT) counterpart of Figure 2 circuit as shown in Figure 3.…”

“…We also checked the effects of very long compensation time such as 100 and 500 μs to find the mechanism between the compensation time and the mura of OLED display. However, such long compensation time requires different pixel circuit structure as in previous studies, 11, 12 which still exhibit some problems, for example, poor contrast ratio, larger circuit area, and higher cross talk, to be used in the commercial products.…”

“…However, the LTPS TFT exhibits poor short‐range uniformity due to its polycrystalline nature. Therefore, a compensation circuit is necessary in each pixel of the LTPS‐based OLED display to compensate for the nonuniform characteristics of the LTPS TFTs 7–12 . However, the mura is not completely removed even with the compensation circuit, and it is still a critical problem especially at low gray levels.…”

“…The number of control lines, TFTs, and capacitors in the pixel compensation circuit should be minimized to implement a high‐resolution mobile OLED display with the pixel density over 500 pixels per inch (ppi). As a result, all the pixel circuits in the commercial products have a common feature, which is that the compensation process takes place while the data voltage is being applied to each pixel 8–12 because such structure minimizes the circuit area. Therefore, the compensation time is limited at most to one horizontal line time (1H), which is determined by the frame rate and the vertical resolution of the display panel.…”

“…If we want to extend the compensation time beyond this limit, we need to adopt a different kind of pixel circuit with more control lines, TFTs, or capacitors, which requires smaller critical dimension in the fabrication process 11 . The mura of OLED display tends to be severe as the compensation time decreases 11,12 . It also deteriorates as the OLED current flowing in each pixel decreases with the efficacy of the OLED and the pixel density enhanced.…”