Amorphous silicon and organic thin film transistors.

Abstract

The main objectives of research work fall into two categories: amorphous silicon (a-Si:H) and organic thin film transistors (TFTs). Effects of temperature and instability on electrical performance of a-Si:H TFTs has been characterized. The output current variation of 4-TFT pixel electrode circuit due to temperature and instability effects was simulated. The simulation results show that output current slightly increases with temperature, and 4-TFT pixel circuit can compensate threshold voltage shift, providing constant current flow through OLED. The measurement of 4-TFT pixel circuit demonstrated that this current programming pixel circuit has capability of supplying constant current to OLED during the whole frame period. The second goal of this thesis work concentrates on the study of organic thin film transistors. A polymer patterning technique, polymer inking and stamping, was developed. This patterning technique is fully compatible for patterning on flexible substrate, and the reel-to-reel pattern transfer of conductive polymers is demonstrated. OTFT channel length of sub-micron scale has been achieved. Organic TFTs with PEDOT electrodes patterned by polymer inking and stamping technique demonstrates excellent electrical characteristics. Electrical performance of top contact pentacene TFTs with PEDOT electrodes is superior than those with gold electrodes, which is due to a lower carrier injection barrier between the PEDOT and the semiconducting pentacene. Electrical performance for bottom contact pentacene TFTs with PEDOT electrodes is very similar to OTFTs with Ti/Au electrodes, because the interface morphology between the electrode and the pentacene deposited on top limits the carrier injection efficiency. In addition, humidity dependence of electrical performance of OTFTs was investigated. All devices showed degraded transistor performance as the relative humidity (RH) was increased. It was found that moisture sensitivity of OTFT saturation current depends on device geometry (bottom or top contact device) and channel length, and OTFTs with short channel length and bottom contact configuration was most affected by humidity compared to top contact and larger channel length OTFTs.