Substrate Dependent Initial Stage of Coevaporated Perovskite Film Formation

The energy level alignment that occurs at the interfaces in planar-hetero structured perovskite photovoltaic devices strongly influences the charge transport across the interface and thus plays a role in overall device performance. To directly observe the energy level alignment requires pristine homogeneous surfaces that are free of contamination. Coevaporation has been employed to grow perovskite films. The method involves thermally evaporating the perovskite precursors such as PbI₂ and CH₃NH₃I, and some early reports have shown that the perovskite film formation and stoichiometry are problematic at ultralow coverages. Using photoemission spectroscopy, we investigated the perovskite precursors PbI₂ and MAI on gold and highly oriented pyrolytic graphite (HOPG) surfaces. Results show that the nature of the surface and the deposition conditions can strongly influence the film formation. Excessive iodine observed in the initial evaporation stages appears to be substrate dependent, and this may influence the overall energy level alignment.