Electronic and optical excitations of the PTB7 crystal: First-principles GW-BSE calculations

Given the recent success in achieving efficient organic photovoltaic solar cells based on thieno[3,4-b]thiophene/benzodithiophene polymers (PTB7) and growing efforts to further improve the power conversion efficiency of the PTB7-based devices, a detailed atomic-scale picture of the electronic structure and the excitonic properties of PTB7 crystal is highly desirable. We report electronic and optical properties of PTB7 on the basis of first-principles density functional theory and GW many-body plus Bethe-Salpeter equation (GW-BSE) calculations. It is established that the first two highest valence bands (HVBs) and the first two lowest conduction bands (LCBs) originate from the benzodithiophene and thieno[3,4-b] thiophene functional units, respectively, thus confirming the donor-acceptor nature of PTB7. A significant difference of band splitting between HVBs and LCBs is found and its origins are explained. Our results strongly suggest that the strength of the interchain pi-pi interaction is not only a function of interchain distance, but is also highly dependent on the nature of the fused rings. The experimental optical absorption spectrum of PTB7 is well reproduced and explained by our GW-BSE calculations. Further analysis shows that the nature of the lowest singlet (triplet) excitons in polymeric crystals such as PTB7 differs from that of organic molecular crystals. A possible reason is explored by combining BSE calculations with a simple Hamiltonian model.