Metal-halide perovskites (MHP) based solar cells stand on the brink of revolutionizing the field of photovoltaics through their highly versatile nature, their bandgap tenability making them ideal candidates for tandem applications. Vapor deposition of these materials, where the precursors are heated in ultra-high vacuum such that they sublime onto the substrate [1], is the most straightforward way of depositing multiple layers in succession and hence fabricating tandem solar cells. However, this method still suffers from poor understanding of the sublimation and crystallization dynamics, particularly for the organic precursor [2]. The highest performance co-evaporated devices use the prototypical MHP CH₃NH₃PbI₃ (MAPbI₃) as the absorber, but show grains much smaller than those typically seen in solution processed devices. Understanding the film formation process and what leads to grain nucleation versus additive growth is likely to lead to improved performance and reproducibility of these devices.

In this study, we show that it is possible to control the grain size of vapor deposited MAPbI₃ by tuning the temperature of the substrate on which the precursor vapor sublimes. We find that, surprisingly, it is cold temperatures (-2 °C) that lead to the growth of large, micrometer-sized grains, while films grown at room temperature (23 °C) exhibit much smaller grains of size 100 nm. While the large grain samples show improved crystallographic properties, their performance is significantly worse than the ones with small grains. We find that the smaller grains and enhanced performance are linked to the presence of excess PbI₂, but that the enhanced performance remains even when all excess PbI₂ is converted to MAPbI₃. To more accurately control the sublimation rate of MAI we developed a novel technique based on the sublimation rate measured near the substrate, which is crucial to disentangle the effects of temperature and stoichiometry. As such, substrate temperature significantly affects the rate of adsorption of the organic MAI vapor and hence the rate of conversion of PbI₂ into MAPbI₃, and that both a balanced stoichiometry and high adsorption rate are necessary to form larger grains. However, a small excess of PbI₂ plays a beneficial, passivating role both during the film growth process and in the finished device, and the stoichiometry of the interfaces can be tuned to give optimized performance [3].