Polymer semiconductors such as the alkyl-substituted polythiophenes have been the focus of interest because of their solution-processable nature and application in devices such as transistors and solar cells. Their carrier mobilities, however, are significantly lower than small-molecule semiconductors such as pentacene and rubrene. The lower mobility is attributed to the structural disorder associated with entangled polymer chains which result in smaller and less-aligned crystals that form grain boundaries in thin films. In order for polymer semiconductors to outperform small-molecule semiconductors and make their way to real world applications, a protocol for fabricating highly crystalline films must first be developed. The objective of this study is to develop a crystallization protocol by employing low molecular weight polymers, or oligomers. Our intent is to develop a structure-property relationship from these materials and determine the electronic and molecular properties of this class of oligomers. The approach to this project is to synthesize a series of oligomers from the benchmark building block, didodecylquaterthiophene, and then employ coupling chemistry to produce low molecular weight oligomers with well-controlled conjugation lengths and narrow polydispersities. The PI will also utilize oligomer single-crystalline films as test structures for measuring intrinsic carrier mobilities in field-effect transistors. The scientific outcome of the program will be a fundamental understanding of chain packing in low molecular weight polymers. This knowledge will help one to understand and ultimately control crystallization in higher molecular weight polymers. This work will also impact on how one can design polymers with controlled molecular weights and narrow polydispersities for growing highly crystalline films.
Fundamental knowledge of how molecules are arranged in polymer semiconductor crystals will be critical for the understanding of electrical transport in polymer thin films and devices. The approaches described in this project combine creation of new materials by chemical means, expertise in crystal growth, device fabrication, and an interdisciplinary way of solving problems, and are aimed toward understanding and optimization of how polymeric/organic electronic devices operate. The success of the proposed research program would be of importance toward our fundamental understanding of polymer crystallization on both the nano- and bulk length scales. The proposed studies will offer opportunities to train postdoctoral, graduate, and undergraduate students in synthesis, polymer science, and device fabrication. Principal Investigator (PI) Briseno will also be engaged in professional activities that have broader impact on society, the scientific community, and underprivileged students across the nation.
Oligothiophenes provide a highly controlled and adaptable platform to explore various synthetic, morphologic, and electronic relationships in organic semiconductor systems. These short chain systems serve as models for establishing valuable structure-property relationships to their polymer analogs. In contrast to their polymer counterparts, oligothiophenes afford high-purity and well-defined materials which can be easily modified with a variety of functional groups. Because oligothiophenes can be precisely tailored, they present an excellent system for examining the role of functionality beyond the thiophene core. In particular, the moieties can have an effect on key properties such as effective conjugation length, crystal packing, intermolecular interaction, polymorphism, and electronic structure. Our work has revealed that oligothiophenes are the up-and-coming generation of advanced materials for organic electronic devices. This grant enabled the synthesis and characterization of linear oligothiophenes with a focus on applications in organic field effect transistors and organic photovoltaics. We have reported key structural parameters, such as crystal packing, intermolecular interactions, polymorphism, and energy levels, which in turn define the device performance. Our results have been transformative in our fundamental understanding of polymer crystallization on both the nano- and bulk length scales. Our polymer funded program offered opportunities to train postdoctoral, graduate, and undergraduate students in synthesis, polymer science, and device fabrication. PI Briseno also engaged in professional activities that have broader impact on society, the scientific community, and underprivileged students across the nation.