Volatile 2-Thiophenemethylammonium and Its Strongly Bonded Condensation Product for Stabilizing α-FAPbI3 in Sequential-Deposited Solar Cells

The perovskite films prepared by the sequential deposition method suffer from incomplete lead iodide (PbI2) reaction, which is manifested by the poor photostability of the prepared perovskite solar cells. Herein, 2-thiophenemethylammonium (TMA), known for the formation of Ruddlesden–Popper layered perovskites, is used to modify the PbI2 film. The introduction of TMA reduces the crystallinity of PbI2 and participates in the formation of the solvent intermediate phase, thus promoting the formation of α-formamidinium lead triiodide (α-FAPbI3) perovskite films in the whole film thickness direction. Most of TMA escapes from the film by deprotonation, and only trace amounts of TMA (0.02%) and the condensation product (TMFA, 0.10%) between TMA and formamidinium (FA) are found to be strongly bonded to the perovskite grain boundaries. The results show that the stability of the perovskite solar cell is improved and the power conversion efficiency reaches 24.5%. This discovery confirms the great value of volatile alkylammoniums in the preparation and stabilization of α-FAPbI3 perovskite films.