Interfacial Chemical Redox Reaction at a Mesoscopic NiO/CH3NH3PbI3 Heterojunction
Ming-Wei Lin1*, Kuo-Chin Wang2, Jeng-Han Wang3, Ming-Hsien Li2, Yu-Ling Lai1, Takuji Ohigashi4, Nobuhiro Kosugi4, Peter Chen2, Der-Hsin Wei1, Tzung-Fang Guo2, Yao-Jane Hsu1
1Nano science group, National Synchrotron Radiation Research Center, Hsinchu, Taiwan
2Department of Photonics, National Cheng Kung University, Tainan, Taiwan
3Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan
4Institute for Molecular Science, Okazaki, Japan
* presenting author:mingwei lin,
The mesoscopic NiO (NiOnc) and the high extinction coefficient of CH3NH3PbI3 perovskite are combined to obtain a solar cell with a power conversion efficiency of 9.51%. To study the origin of such highly efficient carrier transport in terms of electronic, chemical and transport properties of a NiOnc-perovskite heterojunction, X-ray photoelectron spectra (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectra, a scanning transmission X-ray microscope (STXM) and calculations of electronic structure were employed. We found a pronounced chemical redox reaction at a NiOnc-perovskite heterojunction such that PbI2 is oxidized to PbO with subsequent formation of hole-dopant CH3NH3PbI3-2O at the heterojunction. The compound CH3NH3PbI3-2δOδ has upper VB (about 0.1 eV) than CH3NH3PbI3, facilitating hole injection from CH3NH3PbI3 (-0.94 eV) to NiOnc (-0.84 eV). In summary, the generation of hole-doping CH3NH3PbI3-2O 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.

Keywords: CH3NH3PbI3, mesoscopic NiO, CH3NH3PbI3-2O