Device engineering of organic field-effect transistors toward complementary circuits
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Organic complementary circuits are attracting significant attention due to their high power efficiency and operation robustness, driven by the demands for low-cost, large-area and flexible devices. Previous demonstrations of organic complementary circuits often show high operating voltage, small noise margins, low dc gain, and electrical instability such as hysteresis and threshold voltage shifts. There are two obstacles to developing organic complementary circuits: the lack of high-performance n-channel OFET devices, and the processing difficulty of integrating both n- and p-channel organic field-effect transistors (OFETs) on the same substrate. The operating characteristics of OFETs are often governed by the boundary conditions imposed by the device architecture, such as interfaces and contacts instead of the properties of the semiconductor material. Therefore, the performance of OFETs is often limited if either of the essential interfaces or contacts next to the semiconductor and the channel are not optimized. This dissertation presents research work performed on OFETs and OFET-based complementary inverters in an attempt to address some of these knowledge issues. The objective is to develop high-performance OFETs, with a focus on n-channel OFETs through interface engineering both at the interface between the organic semiconductor and the source/drain electrodes, and at the interface between the organic semiconductor and gate dielectric. Through interface engineering, both p- and n-channel high-performance low-voltage OFETs are realized with high mobilities, low threshold voltages, low subthreshold slopes, and high on/off current ratios. Optimization at the gate dielectric/semiconductor also gives OFET devices excellent reproducibility and good electrical stability under multiple test cycles and continuous electrical stress. Finally, with the interfaces and contacts optimized for both p- and n-channel charge transport, the integration of n- and p-channel OFETs with comparable performance are demonstrated in complementary inverters. The research achieves inverters with a high-gain, a low operation voltage, good electrical stability (absence of hysteresis), and a high switching-speed. A preliminary study of the encapsulation of OFETs and inverters with an additional protective layer is also presented to validate the practicality of organic devices containing air-sensitive n-channel transport.