Strong Fermi-level pinning at metal contacts to halide perovskites

Abstract

The performance of halide perovskite-based electronic and optoelectronic devices is often related to interfacial charge transport. To shed light on the underlying physical and chemical properties of CH₃NH₃PbI₃ (MAPbI₃) in direct contact with common electrodes Al, Ti, Cr, Ag, and Au, the evolution of interfacial properties and Fermi level pinning is systematically studied. Given a unique experimental facility, pristine interfaces without any exposure to ambient air were prepared. We observe aggregation of substantial amounts of metallic lead (Pb⁰) at the metal/MAPbI₃ interface, resulting from the interfacial reaction between the deposited metal and iodine ions from MAPbI₃. It is found that the Schottky barrier height at the metal/MAPbI₃ interface is independent of the metal work function due to strong Fermi level pinning, possibly due to the metallic Pb⁰ aggregates, which act as interfacial trap sites. The charge neutrality level of MAPbI₃ is consistent with the energy level of Pb⁰-related defects, indicating that Pb⁰ interfacial trap states can be nonradiative recombination sites. This work underlines that control of chemical bonding at interfaces is a key factor for designing future halide perovskite-based devices.