Electronic structure of the clean interface between single crystal CH3NH3PbI3 and an organic hole transporting material spiro-OMeTAD

Methylammonium lead triiodide (CH3NH3PbI3) is a fundamental material used for prototypical perovskite solar cells. The electronic properties of the interface between CH3NH3PbI3 and hole transporting materials play a crucial role in the efficient performance of these solar cells. However, the intrinsic characteristics of the interfaces where these materials directly come into contact with each other have not yet been defined since previous studies were performed using polycrystalline thin films of CH3NH3PbI3, which were confirmed to contain a considerable amount of impurities. In this study, x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy were conducted to determine the interfacial electronic structure between CH3NH3PbI3 and 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) on the clean interface formed on the impurity-free single crystal surface of CH3NH3PbI3. Spontaneous hole injection from CH3NH3PbI3 to spiro-OMeTAD occurred at the direct contact sites between these materials, a phenomenon that was confirmed to be hindered by the presence of impurities at the interface.