Theoretical Study on Electronic Structure and Photophysical Properties of Low Band Gap Dithiophene Copolymer with Different Bridging Atoms

The excited state characteristics of two low band gap donor-acceptor copolymers with different bridging atoms used in bulk heterojunction solar cells were analyzed through their absorption spectra. The charge transfer state features were investigated to distinguish their abilities of charge separations. The electronic structures and spectral properties of PSBTBT and PCPDTBT(n = 1-4) were investigated based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The results show that the absorption spectrum of PSBTBT is similar to that of PCPDTBT and they have similar abilities to match with the solar spectra. Meanwhile their capabilities of charge separations for the charge transfer state are similar to each other in terms of exciton binding energy. However when the carbon atom was replaced by the silicon atom in the main chain of the conjugated polymer the C-Si bond is significantly longer than the C-C bond which reduces the steric hindrance between the alkyl groups and the thiophene ring. Thus this replacement may be beneficial to the improvement of the crystallinity and increment of the carrier transport ability. These results indicate that PSBTBT may be a promising polymer material for application in polymer solar cells.