Organic single crystals as tools for the rational design of materials for organic electronics.

摘要

Organic electronic materials hold great promise for the development of technologies either technically impossible or economically unviable with traditional inorganic components. The ability to process them from solution in a grossly simplified manner through additive processes such as screen and inkjet printing has afforded the possibility of affordably fabricating circuits, displays, lighting, and even photovoltaics on large, flexible substrates. Furthermore, their diversity of composition and their unique solid-state properties have borne the concept of true rational material design for specific applications. By tuning light absorption and emission for photovoltaics and light-emitting diodes, for example, the solar absorption spectrum may be optimally harvested and the gamut of natural lighting colors covered with a small toolbox of organic building blocks. Similarly, chemical sensors' sensitivity and specificity may be chosen based on targeted analyte binding. In order to realize these functionalities, however, a fundamental understanding of several aspects of the relationship between molecular functionality and macroscopic optoelectronic properties must be further developed. In particular, the correlation between molecular proximity and orientation within the solid state organic thin film or single crystal and charge transport efficiency must be clearly understood. Furthermore, effective strategies for specifying packing motif based on molecular structure are essential to effect the desired intermolecular interactions. Here, we develop experimental techniques targeted at elucidating explicitly the relationship between molecular proximity and orientation in the solid state. We investigate the physics of the tool---the single-crystal field-effect transistor---in order to both aid the understanding of parasitic and scaling effects and also to ensure the results are free of experimental artifacts. We then explore the relationship between molecular structure and packing motif within a series of organic analogues, and correlate quantum mechanical calculations describing their intermolecular interactions and their observed electrical properties.