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

Abstract

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 PbI 2 is oxidized to PbO with subsequent formation of hole‐dopant CH 3 NH 3 PbI 3–2δ O δ at the heterojunction. The generation of hole‐doping CH 3 NH 3 PbI 3–2δ O 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.