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 (Eloss). 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 Eloss. 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 Eloss of pero‐SCs based on (FASnI3)0.6(MAPbI3)0.4 (bandgap ≈1.25 eV) is dramatically reduced below 0.4 eV, leading to a high open‐circuit voltage (Voc) 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 Voc of 0.86 V are the highest values among the low‐bandgap pero‐SCs to date. A facile way to address the bottleneck of high Eloss of low bandgap perovskite solar cells (pero‐SCs) is developed by incorporating an ultrathin bulk‐heterojunction organic semiconductors (PBDB‐T:ITIC) intermediary. The resulting Sn–Pb‐based pero‐SCs show the highest reported efficiency of 18.03% in the low‐bandgap (below 1.4 eV) pero‐PCs to date.