Substrate effect on the interfacial electronic structure of thermally-evaporated CH3NH3PbI3 perovskite layer

The impact of substrate work function on the interfacial electronic structure of thermally-evaporated CH3NH3PbI3 perovskite films on various substrates have been systematically investigated using in-situ ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). On substrates with work function lower than ∼4.43 eV, a Fermi level pinning effect of the lowest unoccupied molecular orbital (LUMO) is observed, resulting in the near zero electron extraction barrier for the CH3NH3PbI3 perovskite solar cells. On the other hand, when substrates with high work function are used, even exceed the highest occupied molecular orbital (HOMO) of CH3NH3PbI3, an almost constant hole extraction barrier of ∼0.88 eV is observed, indicating that the efficiency of hole extraction at these interfaces are low. In order to understand the low hole extraction efficiency at interfaces between CH3NH3PbI3 and these high work function electrodes, the evolution of electronic structures at the interface between CH3NH3PbI3 and MoO3 is further investigated. The charge transfer and dipole formation between CH3NH3PbI3 and MoO3 are deduced from the UPS and XPS results, and the energy level alignment between CH3NH3PbI3 and MoO3 is discussed.