Formation chemistry of perovskites with mixed iodide/chloride content and the implications on charge transport properties

Abstract

Although Cl-doping is a common technique for achieving high photovoltaic (PV) performance, the Cl content is negligibly small and cannot easily be tuned. Therefore, we herein study the formation chemistry of Cl-doped perovskites by examining the chemical interactions between thermally evaporated MAI and PbCl₂ through X-ray photoemission spectroscopy (XPS). We show that PbCl₂ is not stable at the MAI/PbCl₂ contact surface. The Cl atom readily detaches from the PbCl₂, which subsequently initiates electron transition from Pb to MAI. Via thermal-evaporation, a perovskite with a high PbCl₂ content can be prepared and examined. We found that the presence of metallic Pb, associated with increased Cl content, can quench the photogenerated exciton in PV devices. By optimizing the ratio of MAI : PbCl₂, a perovskite solar cell with ∼6% efficiency was obtained.