Energy Level Offsets at Lead Halide Perovskite/Organic Hybrid Interfaces and Their Impacts on Charge Separation

Lead halide perovskites are currently attracting a great deal of attentions due to their great promise as light absorbers in high-efficiency hybrid organic-inorganic solid-state solar cells. The reliable information about interface energetics of lead halide perovskite-based interfaces is indispensable to unraveling the photon harvesting and charge separation process for this emerging photovoltaic technology. Here, we provide the direct evidence on energy level alignments at the hybrid interfaces between lead halide perovskite and organic hole-transport materials (HTMs) using in situ ultraviolet and X-ray photoemission techniques. The measured alignment schemes at perovskite/HTM hybrid interfaces reveal four entirely different energy level offsets with respect to the variation of HTMs, including spiro-OMeTAD, NPB, F16CuPc, HATCN, and MoO3, and their impacts on charge separation are also elucidated. It is identified that the staggered-gap heterojunction in contact with a HTM of higher-lying occupied molecular orbital can facilitate the interfacial hole extraction. Our experimental findings provide the guideline of not only understanding the interfacial charge separation mechanisms but also optimizing the HTMs in perovskite-based solar cells.