Perovskite solar cells (PSCs) are a promising candidate for next-generation photovoltaics and showed an incredible increase in performance during the last decade reaching record power conversion efficiencies (PCEs) > 25%. Vacuum deposition techniques are widely used for fabrication of thin-films and have several advantages compared to solution-based fabrication methods. These include conformal deposition of high-quality layers, low material consumption and the ease of scalability to larger areas. However, PCEs of thermally evaporated PSCs have been lacking behind those of solution-processed PSCs for years. While originally most research regarding thermally evaporated PSCs was dedicated to co-evaporation processes reaching maximum PCEs of 20.6% ^[1], recent record PCEs > 21% were achieved via sequential layer deposition approaches in the n-i-p architecture.^[2,3] Here, the individual materials for the perovskite layer are deposited in multiple steps and converted to perovskite during a subsequent annealing step.

In this work, we show the first ever p-i-n all-vacuum processed methylammonium-free PSCs via a sequential layer deposition approach. In the first deposition step, lead iodide (PbI₂) and caesium iodide (CsI) are co-evaporated onto the substrate. The amount of formamidinium (FA) iodide needs to be optimized by tuning its layer thickness in a second evaporation step. During an optimized annealing step under ambient atmosphere, the two layers are converted into a perovskite film with a final composition of Cs_0.15FA_0.85PbI₃ and a bandgap of 1.54 eV. Comparing different vacuum-processed hole transport layers (HTLs) we find a similar microstructure and crystallinity with X-ray diffraction measurements and observe a conformal and homogeneous coverage via scanning electron microscope imaging. Additionally, we studied the optoelectronic properties of the fabricated films and reached high photoluminescence quantum yields above 1%. Using our recently developed evaporated MeO-2PacZ self-assembled monolayer as HTL we achieve PCEs beyond 17%.^[4]

With our work, we pave the way for efficient all-evaporated PSCs and their application to monolithic tandem solar cells with an up-scalable and industrially relevant deposition technique. The next goals are to further push the efficiency and change the perovskite composition to obtain bandgaps relevant for tandem solar cells.