Removing residual PbI2 on the perovskite surface for efficient solar cells

In preparing the Pb-based perovskite film, excess PbI2 is usually needed in the precursor to promote the perovskite crystallization. However, this generally leads to rich unreacted PbI2 at the formed film surface, which can seriously degrade the final perovskite solar cells (PSCs). Here, we developed a polar-nonpolar co-solvent process to efficiently remove such residual PbI2 and passivate the surface, which was firstly demonstrated in a 3-step process, where methylammonium iodide (MAI) was used as the passivator: 1) removing the excess PbI2 by a 1:4 1-butanol/cyclohexane mixture. 2) Passivating the surface by a 1:8 tert-amyl alcohol/cyclohexane (TC) mixture containing MAI. 3) Cleaning the residual MAI by a 1:8 TC mixture. The strategy was then applied to another passivator p-methoxyphenethylammonium iodide, and the PSC power conversion efficiency was enhanced from 20.56% to 23.69%.