High efficiency non-fullerene organic solar cells without electron transporting layers enabled by Lewis base anion doping

Interfacial charge extraction and recombination play a critical role in photovoltaic characteristics of organic bulk heterojunction solar cells. Here we propose a methodology based on Lewis base n-doping with tetrabutylammonium acetate (TBAA) salts to boost charge extraction and resultant power conversion efficiencies (PCE) in OPVs with non-fullerene acceptors (NFA). Based on the PBDB-T:IT-M model system, we show that the TBAA doping via interfacial electron transfer is advantageous in maintaining the preferential intermolecular stacking and built-in potential necessitated for device operation compared to conventional bulk doping. Benefitting from the melioration of interfacial charge extraction and recombination, gains of photocurrent and fill factor are obtained in PBDB-T:IT-M solar cells. We found that the energy of charge transfer states after doping becomes slightly raised, which promotes the mitigation on radiative losses for open-circuit voltage. Furthermore, the described TBAA doping is found with the generality in modifying the device characteristics in non-fullerene based organic solar cells with different LUMO energies. Based on the PBDBT-2F:IT-4F BHJ blends, the solar cell doped with TBAA yields a PCE exceeding 13% without electron-transporting layer (ETL). This strategy allows for high efficiency ETL-free organic solar cells with potentials of reduced fabrication cost.