A chemist perspective on charge transport in organic semiconductors
Prof. Yves Geerts, Université Libre de Bruxelles
Charge carrier mobility, µ (cm2/V.s), is commonly used to benchmark organic semiconductors. Reproducible room temperature mobility values in the range of 10 to 20 cm2/V.s have been measured, in OFETs fabricated with single crystals of best performing molecular semiconductors. Importantly, mobility values increases when lowering temperature. This is viewed as an evidence of band-like transport. However, the charge transport mechanism operating in single-crystals is still under debate.
I will report on recent results, obtained in close collaboration with G. Schweicher, Jérôme Cornil, David Beljonne, Paolo Samori and Shu Seki. We have observed that the 2,7-isomer of didodecylbenzothieno[3,2-b]benzothiophene exhibits a peculiar behavior as compared to the corresponding 1,4-, 3,8-, and 4,9-isomers. In a nutshell, the 2,7-isomer has an ionization potential 5.3 eV, whereas the one of the other isomers is around 5.8-5.9 eV. This is rationalized by electrostatic effects but also by a much larger delocalization of charges in the 2,7 isomer. Charge carrier mobility measured by Field-Induced Time Resolved Microwave Conductivity (FI-TRMC) ranges from 0.1 to 0.5 cm2/V.s for 1,4-, 3,8-, and 4,9 isomers. Surprisingly, the 2,7-isomer exhibits a mobility of 170 cm2/V.s. It must be stressed that FI-TRMC measures mobility at the semiconductor dielectric interface, like in a transistor, and that charges travel back and forth, under the influence of the microwave oscillating field, over distances on the order of 1-100 nm.[2,3] FI-TRMC is a promising new method that allows to probe locally the intrinsic charge transport. Energetic disorder and dimensionality of electronic interactions appear as key concepts to understand the results.[5,6] Experimental results have been rationalized by quantum calculations using band-like model.
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 S. Illig, A.S. Eggeman, A. Troisi, L. Jiang, C. Warwick, M. Nikolka, G. Schweicher, S. G. Yeates, Y. H. Geerts, J. E. Anthony, H. Sirringhaus, Nature Communication, Vol. 7, (2016), pp 10736