Structural Cutting of Non-fullerene Acceptors by Chlorination: Effects of Substituent Number on Device Performance

Effects of chlorination on photovoltaic performance of organic solar cells are yet largely unclear though it is emerging as a special yet effective strategy to design highly efficient non-fullerene acceptors (NFAs). Herein, a bi-chlorine-substituted NFA with regioregularity, namely, bichlorinated dithienothiophen[3.2-b]- pyrrolobenzothiadiazole (BTP-2Cl-δ), is synthesized and compared to the non-chlorinated BTP and tetra-chlorine-substituted BTP-4Cl to study the effects of Cl number on the photovoltaic performance. From BTP to BTP-2Cl-δ and BTP-4Cl, the three molecules show gradually red-shifted absorption peaks, narrowed band gaps, and lowered highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs). Polymer solar cells are fabricated using PM6 as the donor and the three small molecules as the acceptors. From BTP to BTP-2Cl-δ, efficiencies (8.8 vs 15.4%) are significantly enhanced due to the better film morphology and strong crystallization of the BTP-2Cl-δ-based device, giving rise to boosted fill factors (FFs) and short-circuit current densities (J_SC's). From BTP-2Cl-δ to BTP-4Cl, although J_SC's (24.3 vs 25.0 mA cm^-2) are slightly elevated due to the higher crystallinity of BTP-4Cl, leading to improved exciton dissociation and collection efficiencies, FFs (71.1 vs 68.0%) are obviously decreased owing to the unfavorable film morphology, unbalanced hole-electron mobilities, and higher charge recombination in BTP-4Cl-based devices. As such, the efficiency of the BTP-2Cl-δ-based device (15.4%) is superior to that of the BTP-4Cl-based device (14.5%). This work elucidates a design strategy by cutting the numbers of substituent chlorine to obtain desired energy levels and crystallization with optimal performance.