Theoretical study of the fluorination effect on charge transport properties in fused thiophene derivatives

Abstract

To gain a better understanding of the fluorination effect on charge transport properties, the charge transport properties of the six fused thiophene derivatives 2,6-diphenylbisthieno[3,2-b:2′,3′-d]thiophene (DP-DTT), 6,6′-diphenyl-2,2′-bibisthieno[3,2-b:2′,3′-d]thiophene (DP-BDTT), 2-(pentafluorophenyl)-6-phenylbisthieno[3,2-b:2′,3′-d]thiophene (FPP-DTT), 6,6′-bis(pentafluorophenyl)-2,2′-bibisthieno[3,2-b:2′,3′-d]thiophene (FPP-BDTT), 2,6-dipentafluorophenyl-bisthieno[3,2-b:2′,3′-d]thiophene (DFP-DTT) and 6,6′-dipentafluorophenyl-2,2′-bibisthieno[3,2-b:2′,3′-d]thiophene (DFP-BDTT) were explored by density functional theory (DFT) coupled with the incoherent charge-hopping model at the molecular and crystal levels. The crystal structures of the title compounds are either predicted by the dispersion-corrected density functional method (DFD-D) or retrieved from the Cambridge Crystallographic Database. Introducing electron-withdrawing fluorine atoms to the end phenyl of the DTT and BDTT molecules can decrease the HOMO–LUMO gap, which is beneficial to the conductivity. FPP-BDTT has the largest electron mobility among the six compounds because it has a small electron reorganization energy and large transfer integral. The efficient overlaps of π-orbitals and smaller π–π stacking distance are proved to be the main reasons for the good hole transport property of DFP-DTT. Additionally, FPP-BDTT and DFP-BDTT have shown remarkable anisotropic behaviors and the maximal charge mobilities are along a specific crystal axis direction with strong π–π interactions, which further confirms our finding that the fluorination effect may be an effective way to improve charge mobilities.