“…In the two-step case, it usually needs to form the lead salt· x Sol (Sol = solvent) first. Thus, to achieve that goal, it requires a long time of annealing to completely eliminate the excess of those high boiling point solvents, such as dimethylformamide, dimethyl sulfoxide, γ-butyrolactone, etc. − (III) With annealing for a long time (10 min or more), the pure α-FAPbI 3 film can be decomposed into bright yellow PbI 2 grains, which has detrimental effects on the visible light absorption and photogenerated carrier transport, thus reducing the efficiency, reproducibility, and stability of the solar cells . However, previous reports also found that moderate PbI 2 crystals can passivate the surface and grain boundaries, which means that the annealing time needs to be precisely controlled to obtain moderate PbI 2 crystals, thus enhancing the efficiency of the solar cells.…”