Sequential Evaporation of Inverted FAPbI₃ Perovskite Solar Cells – Impact of Substrate on Crystallization and Film Formation

Recent advances in sequential evaporation of perovskite solar cells (PSCs) have culminated in a rapid increase in reported power conversion efficiencies (PCEs), now on par with those of the best solution-processed counterparts. This development triggered vast interest from industry and academics. To date, however, very few studies addressed sequentially evaporated PSCs in the p-i-n architecture, and an in-depth process understanding is lacking. Here, we investigate the impact of the hole transport layer (HTL) on the formation of formamidinium lead triiodide (FAPI) perovskite thin films fabricated via an evaporated two-step process. We find that the crystal orientation of lead iodide (PbI2) changes significantly for different HTLs, thereby affecting the subsequent conversion and crystallization process. Adjusting the amount of deposited FAI reveals an unexpected correlation of the PbI2-to-perovskite X-ray diffraction peak intensity ratio to final PSC performance that depends on the employed HTL. Our approach enables PCEs of more than 17%, the highest reported for fully vacuum-processed pure FAPI PSCs in the p-i-n architecture.