Solution-Processable Molecular and Polymeric Semiconductors for Ambient-Stable Organic Field-Effect Transistors

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π-Conjugated oligomeric and polymeric semiconductors have been the focus of intense research over the past few decades as alternatives to inorganic semiconductors for low-cost electronic applications such as organic field-effect transistors (OFETs). These materials enable vapor- or solution-phase fabrication of large-area, light-weight electronic devices, and are compatible with plastic substrates for mechanically flexible, conformable, and wearable electronics. Two primary challenges to realizing these applications are the ambient stability of organic thin films and solution-processability. Although several synthetic and device processing strategies have led to air-stable semiconductors having charge mobilities ranging from 10-5 to > 0.1 cm2/Vs, there are still very few p-type polymeric and n-type or ambipolar molecular/polymeric semiconductors exhibiting truly high performance (charge mobility ≥ 0.1 cm2/Vs, Ion/Ioff ratio of >105). Thus, the development of new solution-processable, p-conjugated semiconductors exhibiting high field-effect carrier mobility and good stability under ambient conditions is of great interest. Furthermore, the fundamentals governing device environmental stability and FET performance with respect to the molecular/polymeric structure should be understood in greater depth. This study addresses these challenges via theory-aided rational design, synthesis, and characterization of novel hole- and electron transporting molecules and polymers as air-stable and solution-processable semiconductors for high performance p-channel, n-channel, and ambipolar OFETs. Significant correlations are established between molecular/polymeric structures, physicochemical properties, and OFET device performance, providing detailed insight into charge transport characteristics and ambient stability. In the first part, a series of dibenzosilole- and dithienosilole-based homo- and copolymers were developed for air-stable solution-cast p-channel FETs with high hole mobilities up to 0.08 cm2/Vs and current modulations of 105 - 106. In the second part, a new family of carbonyl-/dicyanovinylene-functionalized bis(indenofluorene) compounds and their corresponding homo- and copolymers were synthesized and characterized. Thin films of these semiconductors yield OFETs with high electron mobility (up to 0.16 cm2/Vs) and high Ion/Ioff ratio (107 - 108), one of the highest to date for a solution-cast air-stable n-channel semiconductor. Additionally, we reported the first examples of polymeric and molecular ambipolar semiconductors to function in air. Detailed analysis of the operational air-stabilities of a series of thin-films shows that air stability is principally governed by LUMO energetics with minimal contribution from thin film microstructure. The onset LUMO energy for carrier electron stabilization is estimated as -4.1 - -4.0 eV. Density functional theory calculations provide detailed insight into molecule/polymer physicochemical and charge transport characteristics. These results, in total, affirm the possibility of achieving low-cost microelectronic devices through organic materials that enable simple solution fabrication processes under ambient conditions.

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  • 10/01/2018
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