Control of Charge Pathways for Performance Improvement of Organic Photo-Transistors

Abstract

Organic photo-transistors (OPTs) have attracted the growth of interest as fundamental building blocks for optoelectronic systems adopting light as an information carrier. The OPT is a type of optical transducer where the functions of light detection, switching, and signal amplification are integrated into a single device. In addition to the intrinsic advantages of organic materials such as low cost, mechanical flexibility, and chemical versatility, the OPTs exhibit higher photosensitivity and signal-to-noise ratio than two-terminal organic photodiodes owing to the signal amplification capability by the field effect. Due to the advantages, they have widespread technological applications, from telecommunications to sensors in industrial, medical, and civil environments such as information processing, X-ray medical imaging, and position detection. In terms of the operation of the OPTs, charges are accumulated and transported within the first few monolayers of the organic semiconductor (OSC) films adjacent to the gate dielectric surface due to the bias of the third terminal and then extracted to the drain, contributing to the output signal. As flowing in the channel, the charges interact with surrounding conditions and all of the interactions are reflected in output signals. In this regard, the charge pathways play a critical role in determining the performance of the OPTs. Especially for the planar type OPTs, since the charges laterally transport along the OSC/gate insulator interface from the source to the drain electrodes, the performance of the device is highly affected by the interfacial characteristics at OSC/gate insulator. It is important to obtain understanding of the relationship between the interfacial characteristics of the gate insulator and the charge carrier dynamics of the OPTs under light illumination. Another issue is on the restriction of unnecessary charge path to reduce the leakage current in the OPTs. The leakage current is regarded as noise which deteriorates the photoresponse of the OPTs. In this regard, patterning processes of OSC layers are required. Particularly for the vertical-type OPTs, the devices suffer from high leakage current from the source to the drain which cannot be controlled by the gate bias and exhibit considerable current flow even in off state. This thesis primarily aims to demonstrate control of charge pathways for improvement of photoresponse in OPTs which are categorized into the planar and the vertical types. Firstly, the enhanced optical memory effect of OPTs based on polymer gate insulator is demonstrated by investigating the effect of the interfacial properties between a polymer dielectric and an OSC layers on the photoresponse properties of OPTs. The type and the density of functional group of the dielectric material were found to more dominantly govern the optical memory effect of the OPT than the morphological effect. They can be properly tailored for specific applications of the OPTs ranging from optical memory to optical sensing devices. Secondly, photosensitivity was improved through the solution-based patterning of OSC layers. The OPTs based on small-molecule OSC/polymer blends suffer from relatively high off current and thus low photosensitivity due to undesirable current pathways over a whole substrate. By using selective contact evaporation printing based on wetting difference, high-fidelity patterns were obtained and the patterned OPTs exhibited the improved photosensitivity due to the reduction of parasitic leakage current. Lastly, vertical OPTs with reduced off current was developed using a self-aligned source insulator on the source electrode. The protruded part of the self-aligned source insulator blocked the considerable current flowing from the edge of the source to the drain in the off state and thus the optimized OPTs exhibited high photosensitivity. In addition, the devices showed higher photoresponsivity in comparison with the planar types due to the short channel length of the thickness of the OSC layer. With the fabrication method demonstrated here, source insulator coverage can be precisely controlled and thus the devices can be highly integrated. In summary, control of charge pathways is investigated for improvement of photoresponse in the OPTs within the framework of the interfacial phenomena involved in different layers and the development of the novel structure. The work presented in thesis is expected to open a new route to the delicate interfacial modification of multi-layers and the integration of basic building blocks for constructing advanced optoelectronic systems.