Integrating Ultrathin Bulk-Heterojunction Organic Semiconductor Intermediary for High-Performance Low-Bandgap Perovskite Solar Cells with Low Energy Loss

Mixed tin (Sn)-lead (Pb) perovskite is considered the most promising low-bandgap photovoltaic material for both pursuing the theoretical limiting efficiency of single-junction solar cells and breaking the Shockley-Queisser limitation by constructing tandem solar cells. However, their power conversion efficiencies (PCEs) are still lagging behind those of medium-bandgap perovskite solar cells (pero-SCs) due to their serious energy loss (E-loss). Here, an ultrathin bulk-heterojunction (BHJ) organic semiconductor (PBDB-T:ITIC) layer is used as an intermediary between the hole transporting layer and Sn-Pb-based low-bandgap perovskite film to minimize E-loss. It is found that this BHJ PBDB-T:ITIC intermediary simultaneously provided a cascading energy alignment in the device, facilitated high-quality Sn-Pb perovskite film growth, and passivated the antisite defects of the perovskite surface. In this simple way, the E-loss of pero-SCs based on (FASnI(3))(0.6)(MAPbI(3))(0.4) (bandgap approximate to 1.25 eV) is dramatically reduced below 0.4 eV, leading to a high open-circuit voltage (V-oc) of 0.86 V. As a result, the best pero-SC showed a significantly improved PCE of 18.03% with negligible J-V hysteresis and high stability. As far as it is known, the PCE of 18.03% and V-oc of 0.86 V are the highest values among the low-bandgap pero-SCs to date.