Charge transfer characteristics of fullerene-free polymer solar cells via multi-state electronic coupling treatment†
Recently, non-fullerene (NF) polymer solar cells (PSCs), where new electron acceptor (eA) materials are blended with a donor–acceptor (D–A) copolymer as an electron donor (eD), have shown promising power conversion efficiencies up to 18%. Some of the best-performing NF PSCs use the eD copolymers PBDT-TzBI, PDTB-EF-T, and PBDB-T-2F, and either a D–A copolymer P(NDI2OD-T2) or small molecule acceptors (SMAs) ITIC-4F and ITIC-2Cl as the NF eA compounds. Here we investigate these systems with density functional theory methods and extend our previous study of the multi-state fragment charge difference (FCD) electronic coupling scheme by applying it to the calculations of charge transfer (CT) rates for exciton dissociation and charge recombination (CR) processes at local eD–eA interfaces. Despite similar backbone structures and optical properties, the studied eD copolymers have different conformational, ionization, excitation, and CT characteristics. The electronic couplings and CT rates depend strongly on the relative positioning of the eD and eA compounds in the eD–eA complexes. While the main CT path is from eD to the eA compound, CT from eA to the eD compound is also predicted in the polymer–polymer PBDT-TzBI–P(NDI2OD-T2) system. The multi-state FCD electronic couplings are independent of the number of the excited states included in the calculations when using a dispersion-corrected optimally tuned long-range corrected functional. The calculated CR rates are slower in the polymer–SMA systems than in the polymer–polymer system, which could partly account for their higher experimentally observed efficiencies in devices.