A New Pixel Circuit Compensating for a Decrease in Luminance of the Stretchable Active Matrix Organic Light Emitting Diode Display
Authors: Bae Byung Seong , Lee Hyuk Su , Yu Eun Seong , Kang Seo Jin 
Published: November 2022

The stretchable display is a next-generation flexible display because it is possible to implement free-form with free change, and various studies are being conducted. The most likely display technology for implementing a stretchable display is the organic light emitting diode (OLED), which is suitable for forming on flexible substrates because it can emit light on its own without using the back light unit (BLU). In addition, through advantages such as high contrast ratio, fast response time, and wide viewing angle, OLED is applied from small size panels such as mobile to large size panels such as televisions. Thin film transistor, a device applied to drive active matrix of OLED, is represented by the amorphous silicon (a-Si), low temperature poly silicon (LTPS), and oxide TFT such as amorphous In-Ga-Zn-oxide(a-IGZO). The a-IGZO, which has advantages such as high filed effect mobility and low leakage current compared to a-Si:H, is used as a backplane of commercial OLED panels. In addition, it is suitable for application to flexible substrates due to low process temperature, and high transmittance in the visible area due to band gap energy of about 3.1 eV can be applied to transparent display. In research to minimize damage to TFTs or OLEDs when the area of flexible substrates increases, rigid island structures protect devices from damage by forming rigid structures on soft substrates. Devices protected from damage caused by increasing the area can maintain performance, and each pixel can maintain brightness. However, when the area of the panel is increasing, the gap between the pixels increases, which leads to a decrease in the luminance of the panel. Among the studies to compensate for the decrease in brightness by increasing the area of the stretchable display, Kang et al. proposed a stretchable pixel circuit for AMOLED displays, which compensates for the reduced luminance by bootstrapping. The proposed circuit increases luminance through a variable capacitor with a strain-dependent capacity change. However, compensation for TFT's threshold voltage and voltage drop, which are possible causes of luminance reduction in OLED were not considered. The threshold voltage or power voltage of a different drive TFT for each pixel affects the current supplied from the backplane to the OLED electrode, and the current supplied differently for each pixel affects the light intensity of the OLED device, so compensation should be considered. In this study, we propose a novel pixel circuit consisting of seven n-type transistors and two capacitors. In order to evaluate the compensation ability of the proposed pixel circuit, a simulation using the RPI Level = 35 model was conducted. Fig. 1 (a) shows the simulation result of the output current of the proposed pixel circuit by the change in the capacitance of C2. Before the capacitance of C2 increased, the output current was 7.65 μA, but when the capacitance of C2 increased by 10%, the output current increased by 2.22% to 7.82 μA, and when the capacitance of C2 increased by 4.31%.

Figure 2 (b) shows the output current caused by the change in VDD voltage of the proposed pixel circuit and the conventional pixel circuit. As the VDD of the conventional pixel circuit changed by 3.0 V, the output current decreased by 40.90% from 8.02 μA to 4.74 μA. In the case of proposed pixel circuit, output current decreased by 21.83% from 7.65 μA to 5.98 μA as VDD changed by 3 V, and decreased by 0.77% to 7.60 μA when VDD changed by 1.1 V. Figure 3 shows the output current due to the threshold voltage change of TFT in the proposed pixel circuit and conventional pixel circuit. In the conventional pixel circuit, output current changed from 10.13 μA to 5.53 μA as the threshold voltage of TFT changed by 3 V. In the proposed pixel circuit, output current changed from 7.12 μA to 6.45 μA with a 3 V change in the threshold voltage. As a result of the simulation, the proposed pixel circuit can compensate for luminance change by an increased area, voltage drop, and the threshold voltage shift of the TFT.

Figure 1