Improve Hole Collection by Interfacial Chemical Redox Reaction at a Mesoscopic NiO/CH₃NH₃PbI₃ Heterojunction for Efficient Photovoltaic Cells

Organometal-trihalide-perovskite-based solar cells have exhibited high efficiencies when incorporated into mesoscopic NiO (NiO_nc) hole-transport layers. The integration of a NiO_nc-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 NiO_nc-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 NiO_nc-perovskite heterojunction such that PbI₂ is oxidized to PbO with subsequent formation of hole-dopant CH₃NH₃PbI_3–2δO_δ at the heterojunction. The generation of hole-doping CH₃NH₃PbI_3–2δO_δ induced by the redox reaction at the NiO_nc/perovskite heterojunction plays a significant role to facilitate the carrier transport, and thus enhances the photovoltaic efficiencies.