Substoichiometric Mixing of Metal Halide Powders and Their Single-Source Evaporation for Perovskite Photovoltaics

High-vacuum, single-source thermal evaporation is an appealing deposition process for perovskite photovoltaics. It promises the homogeneous and precisely controlled growth of very pure and homogeneous films on large areas in a conformal way. In this work, we study mechanically synthesized substoichiometric cesium bromide–lead iodide precursors, the single-source evaporation of the resulting mixes, subsequent deposited metal halide thin films, and converted perovskite thin films. Diffraction pattern analysis reveals the absence of new phases formed during ball-milling. Using synchrotron in situ grazing-incidence wide-angle X-ray scattering and energy-dispersive X-ray spectroscopy, we demonstrate that halide exchange between precursors occurs during the evaporation process. It is also found that a higher cesium bromide content results in films with more impurities. The proof-of-concept photovoltaic devices with the optimized cesium bromide content reach an efficiency of 15% for an optical bandgap of 1.7 eV.