This thesis focuses on the development of high-performance phototransistors based on several solution-processed semiconductors. After giving an outline of the relevant theory and experimental methods, the thesis is divided into three experimental chapters.
The first chapter presents the development of an ambipolar organic phototransistor based on [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). The balanced electron and hole transport characteristics are achieved by fine-tuning of the injection electrode work function using a self-assembled monolayer. The ambipolar PC61BM phototransistor exhibits promising photoresponse in the UVA region, with maximum photosensitivity and responsivity of 9×103 and 3×103 A/W respectively. In addition, the phototransistor shows fast temporal response with switching times of <2 ms, validating the potential of ambipolar phototransistor for fast and sensitive photodetection.
The second chapter demonstrates a novel hybrid heterostructure transistor based on indium oxide (In2O3) and copper thiocyanate (CuSCN), which enables ambipolar operation while maintaining a high optical transparency. The ability of visible light detection is then enabled by the incorporation of a light-absorbing organic dye, resulting in an In2O3/dye/CuSCN trilayer phototransistor with remarkably enhanced photoresponse to green illumination. More interestingly, the trilayer phototransistor exhibits high switching speed with a response time of ~1 ms, indicating the hybrid method as an effective way to improve the speed of oxide-based phototransistors for efficient and fast photo-sensing application.
The third chapter is a study of hybrid perovskite-organic heterojunction phototransistors by integrating metal-halide perovskite (MHP) into a series of organic semiconductor transistor channels to form different types of heterojunction. Phototransistor characterisation shows that “straddling-gap” type-I heterojunction structure with preferential edge-on molecular orientation leads to more efficient photocarrier circulation and hence enhanced photoresponse, whilst transforming the heterojunction type by varying the organic composition allows tuneable photoresponse. This study provides important insights into the charge-transfer dynamics which can help the development of advanced device designs with “on-demand” optoelectronic properties. In addition, a novel evaluation method for phototransistors is proposed using a photo-inverter circuitry, which assesses phototransistors’ amplification as well as asserts transistors’ functionality for both linear and saturation operations.