Conjugated organic molecules and polymers hold significant promise for use as active materials in electronic devices. Employing such "soft" materials can decrease cost of device fabrication, while enabling unique properties such as mechanical flexibility, large-area coverage, and highly tunable materials properties. A primary roadblock to the realization of this dream has been stability of mobile electrons in organic thin-films. In fact, very few semiconductors exhibit electron transport (n-channel) activity under ambient conditions in field effect transistors (FETs), a standard test-bed device for semiconductor thin-films. Even fewer organic semiconductors have desirable processing characteristics. This work addresses these challenges via computationally-aided rational design of novel electron transporting (n-channel) materials yielding new n-channel air-stable semiconductors, record-setting figures-of-merit for solution processed films, and two examples of rare n-channel FET polymeric semiconductors. Further characterization of charge trapping in organic semiconductors is studied by variable temperature transistor measurements and a direct correlation between charge trapping and mobility is observed. A powerful and general approach to materials design is presented in the first two chapters of this work. DFT-level electronic structure calculations are employed to screen synthetically relevant candidate materials for desirable semiconductor properties. This rational molecular engineering method yields a family of phenacyl-thiophene and quinone-based semiconductors exhibiting mobilities up to ~ 0.3 cm^2V^-1s^-1 for solution processed films and temporally air-stable mobilities of 0.015 cm^2V^-1s^-1 with high current modulation > 106. Additionally, two of the first n-channel polymers are developed, exhibiting mobilities up to 0.012 cm^2V^-1s^-1 and remarkable polymer thin-film crystallinity. The charge transport mechanism for a series of semiconductors is probed by variable temperature FET experiments, the first such study employing semiconductors with diverse molecular and device properties. Analysis of temperature activated FET behavior reveals an inverse correlation between increased charge trap filling and ambient-temperature mobility. Furthermore, there is no evidence for a relationship between conduction orbital energy and the mobility-limiting shallow trap states. This combination of rational design employing appropriate quantum chemical methods and synthetic practicality coupled with an enhanced understanding of trap-limited charge carrier mobility evidences the vibrant progress and continued promise of organic electronic materials.

Last modified

• 08/29/2018

Creator

• Joseph Adam Letizia

DOI

• https://doi.org/10.21985/N2ZH9S

Subject

• Chemistry

Keyword

• n-channel FET
• multiple trapping and release
• computationally-aided
• organic semiconductor

Date created

• 2007-12-11

Resource type

• Dissertation

Rights statement

• In Copyright