Introduction to organic thin film transistors and design of n-channel organic semiconductors

The development of new organic semiconductors with improved performance in organic thin film transistors (OTFTs) is a major challenge for materials chemists. There is a particular need to develop air-stable n-channel (electron-conducting) organic semiconductors with performance comparable to that of p-channel (hole-conducting) materials, for organic electronics to realize the benefits of complementary circuit design, i.e., the ability to switch transistors with either positive or negative gate voltages. There have been significant advancements in the past five years. In terms of standard OTFT metrics such as the field effect mobility (μ_FET) and on-to-off current ratio (I_ON/I_OFF), n-channel OTFTs have achieved performance comparable both to that of n-channel amorphous silicon TFTs and to that of the best reported p-channel (hole-conducting) OTFTs; however, issues of device stability linger. This review provides a detailed introduction to OTFTs, summarizes recent progress in the development of new n-channel organic semiconductors, and discusses the critical properties that any prospective n-channel material must have. Methods important to semiconductor design such as electronic structure calculations and synthetic structural modifications are highlighted in a case study of the development of a new n-channel material based on a terthiophene modified with electron-withdrawing groups. The review concludes with a discussion of directions for future work in this area.