Flash Evaporation of Perovskite Powders—Angstrom-Precision Growth for Single-Source Solar Cell Fabrication

Vacuum-based deposition of halide perovskites has received a lot of attention for its proven scalability and conformal depositions. Co-evaporation in particular has had remarkable success, but efficiencies continue to lag behind its solution-based counterparts. This is in part attributed to complex sublimation behavior of some organic ammonium precursor salts and the increased complexity of the absorber materials, requiring the use of four or more sources in a conventional co-evaporation setup. The latter, as well as the promise of scalability, has driven work into single-source or sequential deposition solutions. Flash evaporation has been presented as one such single-source method, enabling the stoichiometric transfer of preformed perovskite powders or precursors. This is possible with temperatures sufficiently high to overcome the relative volatility or degradation reactions of the constituent components. However, flash evaporation suffers from poor process control, largely because all the necessary precursors are evaporated in a single batch (i.e. all the precursor powders are placed at once into/onto a heated crucible, boat, or foil). This imposes a trade-off between the deposition time and temperature; short deposition times with high temperatures are beneficial to avoid degradation reactions, but sputter effects seen at high temperatures drive researchers towards low temperatures where prolonged exposure causes degradation. Using a patented method, we obtain stable flash-evaporation rates for >1 hour, with controlled rates of around 1.5 Å per second, enabling film thicknesses >500 nm. We demonstrate the feasibility of this method for three different perovskite compositions FAPbBr₃, MAPbI₃, and FA_1-xMA_xPbI₃. Furthermore, we have integrated FAPbBr₃ and MAPbI₃ into all vacuum processed thin-film solar cells leading to power conversion efficiencies of ~4% and 10% respectively.