Organometal-trihalide-perovskite-based solar cells have exhibited high efficiencies when incorporated into mesoscopic NiO (NiOnc) hole-transport layers. The integration of a NiOnc-perovskite heterojunction provides an inorganic alternative as a p-type contact material with efficient hole extraction for perovskite-based solar cells. Herein the origin of such highly efficient carrier transport is studied in terms of electronic, chemical and transport properties of a NiOnc-perovskite heterojunction with X-ray photoelectron spectra, ultraviolet photoelectron spectra, near-edge X-ray absorption fine structure spectra, a scanning transmission X-ray microscope, and calculations of electronic structure. A pronounced chemical redox reaction is found at an NiOnc-perovskite heterojunction such that PbI2 is oxidized to PbO with subsequent formation of hole-dopant CH3NH3PbI3–2δOδ at the heterojunction. The generation of hole-doping CH3NH3PbI3–2δOδ induced by the redox reaction at the NiOnc/perovskite heterojunction plays a significant role to facilitate the carrier transport, and thus enhances the photovoltaic efficiencies.
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