Effective solvent-additive enhanced crystallization and coverage of absorber layers for high efficiency formamidinium perovskite solar cells

For a high efficiency of planar-type perovskite solar cells, a good crystallization and high surface coverage of the absorber films are required. However, these two key factors are still the main challenges in perovskite film formation to date. Here, 1-chloronaphthalene (CN) is used as a solvent-additive in the precursor HC(NH₂)₂PbI₃ (FAPbI₃) solution to control the crystallization and surface coverage of the FAPbI₃ films by adjusting its concentration. The CN chlorinated monodentate ligand is likely to temporarily chelate with Pb²⁺ during the crystal growth, facilitating homogenous nucleation to form relatively high quality FAPbI₃ films. Meanwhile, the CN additive with a high boiling point delays the growth rate of the FAPbI₃ film, which helps to form homogenous continuous FAPbI₃ films with fewer pin-holes. As a result, with the addition of the CN solvent-additive, the efficiency of the FAPbI₃ planar-type solar cells is enhanced to 16.53%.