Electronic structures at the interface between Au and CH$_{3}$NH$_{3}$PbI$_{3}$*

Organometal trihalide perovskite (CH$_{3}$NH$_{3}$PbI$_{3})$-based solar cells have been developed rapidly in decades. The electronic properties of interfaces formed between Au and CH$_{3}$NH$_{3}$PbI$_{3}$ are investigated with ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS) and inverse photoemission spectroscopy (IPES). The two-step method prepared CH$_{3}$NH$_{3}$PbI$_{3}$ film, coated onto the poly (3,4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT:PSS)/indium tin oxide (ITO) substrate, 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 (E$_{\mathrm{F}})$. There is an interface dipole of 0.1 eV at CH$_{3}$NH$_{3}$PbI$_{3}$/Au interface. The energy level of CH$_{3}$NH$_{3}$PbI$_{3}$ is lifted ca.0.4 eV with Au coverage of 64 {\AA} upon it, resulting in band bending and a built-in field in CH$_{3}$NH$_{3}$PbI$_{3}$ that encourages hole transport to the interface. Hole accumulation near the interface facilitates the hole transfer from CH$_{3}$NH$_{3}$PbI$_{3}$ to Au. Furthermore, the decreasing offset between the VB maximum of CH$_{3}$NH$_{3}$PbI$_{3}$ and the E$_{\mathrm{F}}$ indicates a decrease of energy loss as extracting holes from CH$_{3}$NH$_{3}$PbI$_{3}$ to Au coverage.