Field Effect Transistors With Polythiophene Active Layers

Polythiophene is a stable conjugated polymer with a lower band-gap in comparison to many other organic materials and a high degree of intermolecular order leading to high carrier mobilities. It is widely used in solar cells, light-emitting diodes, field-effect transistors and biosensors.In this thesis, we fabricated and tested organic phototransistors and organic field effect transistors (OFETs) with polythiophene as active material. Our study includes P3HT phototransistors, photoinduced P3HT field-effect transistors and light doped P3HT field-effect transistors. The qualitative operating modes in these devices used for explaining the performance were proposed in detail.During the research of phototransistors, we fabricated two kinds of channels with channel lengths of shorter than 100 nm and 50μm, respectively. Then we fabricated the phototransistors by depositing a P3HT thin film as active layer on top of the channels. The output characteristics of the devices were tested with different light intensity. The results show that the light has a function of controlling the current. The current enhanced with a strengthened light intensity. The amplification factors of the light to the current increase then decrease as the source-drain current increases. By comparing the results of the devices with different channel lengths, we found that the short channel device has a larger current and a larger source-drain current needed to the regime of saturation region.During the research of P3HT OFETs, we used two methods to improve the performance of the devices. One is applying an additional optical field to the device. By stimulating more free carriers in the active layers, the light had significant influences on the devices. The drain current and carrier mobility were increased, the threshold was decreased. The 500 nm monochromatic light make the device resulting in the largest carrier mobility of 3.1×10^-2 cm²V^-1s^-1, and the operation process of the photoinduced OFET is depicted at last.Another method is doping a small quantity of PCBM to increase the free carriers in the active layers. We fabricated three devices, including the device without PCBM and with PCBM of 1% and 2% to P3HT. The results show that the none-doped devices operate in enhancement mode, but the doped devices operate in both enhancement mode and depletion mode. As the doping concentration increased, the threshold was decreased and the on-off ratio was increased.