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

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