Organic field effect transistor fabrication by a novel patterning technique and the study of organic semiconductor crystallization

This thesis is concerned with micro/nano fabrication of polymer materials for organic thin film transistor (OTFT) application and crystallization of solution-processable small molecule and conjugated polymer thin film. A method has been developed to form self-assembled arrays of nano/micro wires by controlling solvent evaporation of a solution film that is trapped in between a substrate and a structured template. Various solution-processable materials were successfully patterned by the developed method, with particular focus on conjugated polymers materials, such as p-type & n-type semiconductor polymers, and conductive polymers. The potential applications of such fine patterned materials were demonstrated on both field effect transistors (FET) and electrochemical transistors (ECT). The polymer FET measurement results demonstrated that the device range of on/off ratio was from 10^3 to 10^5 and the range of charge carrier mobility was from 10^(-4) cm^2/Vs to 10^(-2) cm^2/Vs. For the electrochemical transistor, the device can work under a small applied gate and drain bias (less than 0.5 volts). Pattern formation with complicated geometries, pattern transfer processes, and pattern formation dynamics have been investigated. Further, the recrystallization mechanism of amorphous small molecule thin film spin-coated on different substrate was investigated. It was found that a small molecule film crystallized from different substrates can have different preferential orientations caused by a different scenario of materials-substrate interaction, and this can dramatically influence the conductivity of the crystalline film. The dynamics of recrystallization was studied in detail through examination of both growth and nucleation of crystals from their amorphous matrix. The results showed that both the activation energy the small molecule obtained and the crystal growth preferential orientation on different substrates were different during the crystallization process. The study of both the temperature and annealing time influences on the conjugated polymer crystalline film property were also studied experimentally.