Online

Wan, Jing

Graduate Program in Materials Science

2021

Ph. D.

Due to the weak van der Waals forces between organic semiconductor molecules, the molecular packing depends sensitively on the processing methods and conditions. Thus, understanding the crystallization mechanisms during solution deposition are essential for fundamental studies and reproducible fabrication of electronic devices.The performance of Organic field effect transistors (OFETs) also depends heavily on extrinsic factors such as contact resistance and interfacial defects, which can produce a different kind of transient effect at the metal-semiconductor contact. We have observed structural transient effects during the crystallization process of two small molecule organic semiconductors made from solution. We report in situ X-ray scattering studies of the crystallization of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) and 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10) during its deposition from solution. Multiple transient phases are observed for both materials during the crystallization before the final stable phase when substrate temperatures were 2 - 35C below the bulk phase transition temperature. Below the transition temperature the crystalline phases are stable. However, the high temperature smectic liquid-crystalline phases are observed first, but they exist for a limited time. In addition, for Ph-BTBT-C10 complex dynamics occur in the first few seconds as the transient liquid crystal phase is forming, possibly due to residual solvent. Transient phases are of potential interest for applications, since they can act as a route to self-assembly of organic thin films. Based on these results, we demonstrate a method to produce extremely large grain size and high carrier mobility (3.0 cm2/V·s) during high-speed processing. The other transient effect is observed in the top contact OFETs made from solution processed C8-BTBT thin films when stored in ambient conditions. We report observations of time-dependent carrier transport in field-effect transistors through intermittent characterization of current-voltage characteristics. This effect is characterized by a large increase of drain current and near-elimination of large negative threshold (Vth), which progressively reduces over a period of hours. Our results show that this elimination of non-ideality is predominantly caused by light exposure. We attribute these effects to a Schottky Barrier at the metal / semiconductor interface, modulated by charging of gold particles that form during gold vapor deposition onto organic semiconductor thin films. We also confirm that bottom contact transistors are free of the anomalous effect, consistent with the absence of metal particles near the interface. A detailed understanding is developed for the mechanisms contributing to non-ideal carrier injection, aging, and instability of C8-BTBT top contact transistors.