Theoretical investigation on the electronic and charge transport characteristics of push-pull molecules for organic photovoltaic cells

Abstract

We theoretically investigated the electronic and charge transport characteristics of ditolylaminothienyl-benzothiadiazole-dicyanovinylene (DTDCTB), ditolylaminophenyl-benzothiadiazole-dicyanovinylene (DTDCPB) and ditolylaminothienyl-pyrimidine-dicyanovinylene (DTDCTP) push-pull molecules consisting of donor-acceptor-acceptor (D-A-A) configurations for a donor photoactive layer having low energy gaps in organic photovoltaic cells (OPVCs). They commonly have a strong electron-withdrawing moiety and hence conspicuously deep-lying highest occupied molecular orbital (HOMO) levels, leading to high open circuit voltages (V-OC) in OPVCs. In addition, they commonly have low hole reorganization energy due to the fully delocalized wave function of the HOMO on over the entire molecule, and high electron reorganization energy due to their contracted wave functions of the lowest unoccupied molecular orbital (LUMO) on a D moiety. However, their theoretically calculated hole and electron mobility (mu(h) and mu(e)) based on Marcus theory in the molecular crystalline is dissimilar: DTDCTB showed the higher mu(h) than mu(e), DTDCPB showed the similar mu(h) and mu(e), and DTDCTP showed the higher mu(e) than mu(h), which can explain the tendency of their short circuit current (J(SC)) in OPVCs. This originates from significantly different electronic coupling, although they have similar pi-pi coplanar molecular packing in appearance. (C) 2014 Elsevier B.V. All rights reserved.