Thin-film Transistors And Their Applications On Active-matrix Organic Light-emitting Diodes

Thin-film transistor (TFT) is a kind of field-effect device that composed of deposited semiconductor, metal and insulator. It is the center element in active matrix organic light-emitting diode (AMOLED). However, the traditional a-Si: H TFT has low carrier mobility, so it is hard to meet the large current demand in the OLED; and p-Si TFT has higher mobility, but the uniformity is bad. Organic TFT also has low carrier mobility, but it is a new concept, and it is not mature. So it has the potential to be improved in the future. Furthermore, Organic TFT has the advantages of low-temperature process, large-scale fabrication, low-cost, flexible etc. Therefore, organic TFT will be very useful in the future. In recent years, the research on oxide TFT has gained huge progress. Oxide TFT is famous for its high mobility, good uniformity and good stability. It is one of the most competitive TFTs in AMOLED.This work focuses on organic TFT and oxide TFT. In organic TFT, the operating voltage is high because of the low mobility. So we use high dielectric constant Ta₂O₅ as the insulator. But Ta₂O₅ insulator has the disadvantages of high leakage current, plenty of traps. Thus, we propose two methods to improve the performance for TFT based on Ta₂O₅ insulator: one is using hydrogen treatment; the other is using high polarity PFN-PBT copolymer to modify the surface of Ta₂O₅ insulator. As for p-channel OTFT, the hydrogen treatment of Ta₂O₅ insulator can improve the carrier mobility, increase the on/off current ratio, and lower the threshold voltage. Experiments shows that the hydrogen treatment of Ta₂O₅ insulator can fill the dangling bond in the Ta₂O₅, reduce the traps in the Ta₂O₅, and improve the contact between Ta₂O₅ insulator and p-type semiconductor.The hydrogen treatment of Ta₂O₅ insulator can improve the performance of p-channel OTFT, but it does not work in n-channel OTFT. This is because Ta₂O₅ insulator has plenty of electron traps, and the electron carriers are easy to be trap into Ta₂O₅ insulator causing the declining of the performance of n-channel OTFT. Therefore, we propose using high polarity PFN-PBT copolymer to modify the surface of Ta₂O₅ insulator to improve the performance of n-channel OTFT. Evidence shows that OTFT performance has been improved after modification. And the performance of OTFT based on PFN is best, and then PFN-PBT1, and then PFN-PBT5. This indicates that the higher polarity of insulator, the better performance of n-channel OTFT. The reason for the improvement was discussed to be ascribed to the formation of self-assembly dipole moments in the polymers. The polarity of PFN-PBT can strongly influence on the performances of OTFTs. Strong polarity of the polymers can decrease both electron-trap density at the interface of insulator/ PTCDI and the gate bias stress effect, and finally increase electron mobilities of OTFTs. The highest electron mobility of 0.55 cm²V^-1s^-1 was obtained by this method, which is very high in n-channel OTFTs.In the application of OTFT in the AMOLED, we choosing p-channel OTFT based on pentacene semiconductor, because we can not obtain air-stable n-type organic semiconductor materials. And we successfully driving green OLED fabricated by printing.As for oxide TFT, we have studied on the material, film deposition condition, heat treatment condition, stability, etc. In order to reduce the operating voltage, we propose to use anodic Al₂O₃ as the insulator, and attaining better performance (mobility: 18.7 cm²V^-1s^-1,threshold voltage: 7V,on/off current ratio: 107) compared with TFT based on SiO₂ insulator. It is found that Al2O3 was crystallized during anodizing, and it is very compatible with IGZO semiconductor.Based on the research on the oxide TFT, we design a 50×50-pixel AMOLED display. And we implement it in our lab. The luminance of the panel is 150 cd/m², and the highest luminance in one pixel is 900 cd/m², higher than design. Thus, we can see the future of the oxide TFT.