Electronic structures at the interface between Au and CH₃NH₃PbI₃

The electronic properties of interfaces formed between Au and organometal triiodide perovskite (CH3NH3PbI3) are investigated using ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES) and X-ray photoemission spectroscopy (XPS). It is found that the CH3NH3PbI3 film coated onto the substrate of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)/indium tin oxide (ITO) by a two-step method presents n-type semiconductor behavior, with a band gap of 1.7 eV and a valence band (VB) edge of 1.0 eV below the Fermi energy (EF). An interface dipole of 0.1 eV is observed at the CH3NH3PbI3/Au interface. The energy levels of CH3NH3PbI3 shift upward by ca. 0.4 eV with an Au coverage of 64 Å upon it, resulting in band bending, hence a built-in field in CH3NH3PbI3 that encourages hole transport to the interface. Hole accumulation occurs in the vicinity of the interface, facilitating the hole transfer from CH3NH3PbI3 to Au. Furthermore, the shift of the VB maximum of CH3NH3PbI3 toward the EF indicates a decrease of energy loss as holes transfer from CH3NH3PbI3 to Au.