Particle Size Matters – Impact of Particle Size and Crucible Geometry on Sublimation Behavior of Formamidinium Iodide

Vapor phase deposition processes for the fabrication of perovskite solar cells show great potential for transferring from laboratory-scale to continuous industrial-scale production. Precise process control and high process reproducibility are of utmost importance to unlock their full potential. In this regard, the sublimation behavior and rate control of organic precursor materials in thermal evaporation processes are particularly challenging. Here, we investigate in detail how the particle size of formamidinium iodide (FAI) and the crucible geometry influence the directionality of the emitted vapor flux. We show that conical crucibles lead to beam focusing of the vapor flux, while cylindrical crucibles show a broader, less directional emission profile. This leads to differences in the homogeneity of material deposition depending on the lateral source-to-substrate distance. Furthermore, there is a substantial impact of FAI particle size on the directionality of the vapor flux for conical crucibles, affecting the deposited material thickness gradient over the substrate. Analyzing commonly employed inorganic materials reveals the strong material dependence of effusion characteristics, leading to additional complexity for multi-material deposition processes. Our findings emphasize that both homogenization of organic precursor materials and optimization of source geometry and arrangement are critical for achieving uniform deposition and, consequently, improved process reproducibility.