Intrinsic charge transport in single crystals of organic molecular semiconductors: A theoretical perspective
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Introduction In spite of extensive efforts over several decades, a complete understanding of the charge-transport mechanism in organic molecular semiconductors still has to be reached.1–5 Theoretical and experimental studies of highly ordered crystalline structures are critical for the determination of the intrinsic charge-transport properties and of the upper limits that carrier mobilities could reach. As described in other contributions in this issue of MRS Bulletin, significant progress has been made recently in synthesizing high-quality single crystals of organic molecular semiconductors and characterizing their electronic and electrical properties by means of various techniques such as electron spin resonance, Hall effect measurements, or angle-resolved ultraviolet photoelectron spectroscopy (ARUPS). Both p-channel and n-channel organic crystals with charge-carrier mobilities above 1 cm2 V−1 s−1 are now available. In particular, much attention has been given to pentacene and rubrene due to their remarkably high hole mobilities around 20–40 cm2 V−1 s−1 measured in single-crystal OFET (organic field-effect transistor) configurations.6–10 While interest to date has largely focused on organic crystals based on a single molecular building block, it
has been shown that bimolecular crystals, where one component acts as an electron donor (D) and the other as an acceptor (A), could open new opportunities for organic electronics.11–15 For instance, ambipolar OFETs using DA bimolecular crystals as active components have been recently reported.14,15 On the theoretical side, significant efforts have been made toward evaluation of the microscopic parameters that govern charge transport using quantum-chemical calculations.16–20 In many systems, the microscopic interactions that determine, on the one hand, electronic couplings and, on the other hand, electron-phonon (electron-vibration) couplings, are found to be of comparable magnitude. Since none of these interactions can be treated as a perturbation, several attempts have been made recently to develop charge-transport models that are able to treat all relevant interactions on the same footing.13,21–26 Here, we review some recent progress made along these lines and illustrate the results in the case of pentacene and DMQtT–F4TCNQ (DMQtT = dimethylquaterthiophene; F4TCNQ = 7,7,8,8tetracyano-2,3,5,6-tetrafluoroquinodimethane), as representative examples of single-component and bimolecular DA crystals, respectively.
Veaceslav Coropceanu, Georgia Institute of Technology; [email protected] Yuan Li, Georgia Institute of Technology; [email protected] Yuanping Yi, Georgia Institute of Technology; [email protected] Lingyun Zhu, Georgia Institute of Technology; [email protected] Jean-Luc Brédas, Georgia Institute of Technology; [email protected] DOI: 10.1557/mrs.2012.313
© 2013 Materials Research Society
MRS BULLETIN • VOLUME 38 • JANUARY 2013 • www.mrs.org/bulletin
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INTRINSIC CHARGE TRANSPORT IN SINGLE-CRYSTAL ORGANIC MOLECULAR SEMICONDUCTORS
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