Studies of Polymer Field-Effect Transistors
Kim, Felix Sunjoo
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The era of plastic electronics is rapidly emerging due to the increasing development and application of low-cost, printable, shape-conforming, and large-area devices, such as organic field-effect transistors (OFETs) and circuits, organic solar cells, and organic light-emitting devices. Deepening our understanding of charge-carrier dynamics in polymer semiconductors is critical to the future advances in organic electronics. This dissertation focuses on studies of OFETs and aims to better understand the charge transport properties of polymer semiconductors and factors that influence the performance of OFETs. Case studies of structure-morphology-property relationships in unipolar p- and n-channel polymer OFETs as well as ambipolar OFETs reveal that variations in molecular structure and processing affect electronic energy levels, solid-state morphology and crystallinity, and thus the magnitude and polarity of charge carriers. The studies resulted in achievement of high-performance OFETs with high charge-carrier mobility of up to 0.3 cm<super>2</super>/Vs. The morphology and electronic energy levels are also related to ambient stability and durability of polymer OFETs through kinetics and thermodynamics of interaction between the semiconductor and extrinsic molecules in ambient air. Air-stable ambipolar OFETs were realized by utilizing unipolar p- and n-type polymer semiconductors as the active channel elements. Complementary digital logic circuits such as inverters and NAND- and NOR-gates, were also demonstrated using the unipolar and ambipolar OFETs. Device engineering studies show that electron mobility and electrical stability of n-channel polymer OFETs can be significantly enhanced by inserting a low-dielectric-constant polymer dielectric buffer layer at the semiconductor/dielectric interface. Electron mobility was found to increase exponentially with decreasing dielectric constant of the buffer layer. Finally, poly(3-butylthiophene)-nanowire/polystyrene nanocomposites were also investigated as a means of controlling the solid-state morphology of active thin films in OFETs. High dc conductivity and high hole mobility were obtained throughout a wide range of the nanowire compositions (2-100 wt%) due in part to the very low percolation threshold (0.5 wt%).
- Chemical engineering