Effects of lateral-chain thiophene fluorination on morphology and charge transport of BDT-T based small molecule donors: A study with multiscale simulations

Thiophene substituted benzo[1,2-b:4,5-b′]dithiophene (BDT-T) is widely used as the building block of promising donor materials in organic solar cells (OSCs). Fluorination on the lateral-chain thiophenes of BDT-T is a considerable strategy to further improve the performance of BDT-T based small molecule (SM) donors. However, how lateral-chain thiophene fluorination affects the morphology and hole mobility (μ_h) of SM donors has not been well understood. In the present work, we systematically study the effects of the positions and numbers of fluorinations on the morphology and μ_h of a promising SM donor named DRTB-T (referred as 0F) with multiple fluorinations via multiscale simulations. We find the crystallinity is weakened because of fluorination. The face-on configurations are dominated in 0F and 1F-substituted molecules. In contrast, the edge-on configurations are dominated in 2F-substituted molecules. The trend of μ_hs in these fluorinated molecules is consistent with that of the proportions of face-on configurations, and matches well with available experimental results. We find that functionalizing the lateral-chain thiophenes of side BDT-Ts (1F-side) or all BDT-Ts (1F-all) with only one F atom are efficient to fine-tune the morphologies and μ_h of DRTB-T based SM donors simultaneously.