Organic single crystals: Addressing the fundamentals of organic electronics
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Introduction Organic semiconductors form the basis of organic electronics and photonics, fields that are engaged in the development of electronic devices such as flexible, low-cost, disposable electronics, organic solar cells, light-emitting displays, smart tags, and molecular sensors.1 Despite the rapid recent progress in applied organic electronics and a long history of basic research on organic materials,2,3 our understanding of the fundamental properties of this important class of semiconductors is still limited. This disparity is mainly rooted in significant disorder that dominates the electrical and optical properties of polycrystalline and amorphous organic thin films, resulting in disorder-limited charge-carrier mobilities and exciton diffusion that complicate attempts to compare experimental results with theory. However, recently developed electronic devices based on single crystals of organic semiconductors, materials with a much higher long-range structural order and chemical purity, now allow experimental access to the intrinsic (i.e., not dominated by disorder and sample morphology) fundamental electronic properties of organic semiconductor devices (see References 4 and 5). Because of this, organic single crystals
are now widely recognized as an important tool for the studies of the fundamental charge and energy excitations (polarons and excitons, respectively) and their transport, intrinsic surface, and interfacial phenomena, as well as intrinsic optical properties of organic semiconductors. This introductory article describes the motivation, a brief history, and the main future goals of the field of organic singlecrystal optoelectronics. It also gives an outline of the review articles that follow, including crystallization and patterning methods, device fabrication, measurements of electrical and optical properties, and theoretical modeling. In addition, this article gives a thorough review of different fabrication methods of high-performance single-crystal organic field-effect transistors (OFETs), their electrical characteristics, and intrinsic and trap-dominated transport regimes and an overview of modern theoretical models of electronic properties of organic semiconductors. The importance of organic single-crystal research stems from several specific benefits that these materials offer in regard to fundamental studies: (1) high structural order (in particular the absence of grain boundaries), (2) high chemical purity,
Vitaly Podzorov, Department of Physics and Astronomy, Rutgers University; [email protected] DOI: 10.1557/mrs.2012.306
© 2013 Materials Research Society
MRS BULLETIN • VOLUME 38 • JANUARY 2013 • www.mrs.org/bulletin
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ORGANIC SINGLE CRYSTALS: ADDRESSING THE FUNDAMENTALS OF ORGANIC ELECTRONICS
and (3) availability of several efficient methods for fabrication of high-quality semiconductor-dielectric interfaces developed during the past decade. The first two properties have been shown to be very important for observations of band-like transport in photoconductors based on molec
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