Design and characterization of benzotrithiophene-based donor molecules for organic solar cells.

Tuning the electronic, optical and charge transport properties of electron donor materials through functional group substitution and extending the π-conjugation are effective strategies to improve the power conversion efficiency of an organic solar cell (OSC). In the present work, density functional theory (DFT) and time-dependent DFT calculations are used to investigate benzotrithiophene (BTT)-based donor molecules. Benzene, thiophene and dioxane are fused at the terminals of symmetric and asymmetric BTT molecules. It is found that the terminal group substitution and molecular symmetry influence the frontier molecular orbital (FMO) energy, optical absorption spectra, charge transport and donor-acceptor interaction. In particular, the substitution decreases the FMO energy gap and red shifts the absorption spectra relative to the unsubstituted BTT molecules. Their electronic structure provides favourable energy level alignment with the PC61BM acceptor, and thiophene and dioxane-substituted BTT molecules possess suitable FMO alignment with the Y6 acceptor. The calculated optoelectronic properties, donor-acceptor interaction characteristics, open circuit voltage and fill factor show the potential of the studied BTT molecules as electron donors for OSCs. Furthermore, the present study provides significant insights for the rational design of high performance organic photovoltaic materials.