  Mixed-cation Cesium-Methylammonium lead halide perovskite (CsxMA1-xPbI3-xBrx)
thin-films have been used to demonstrate stable and efficient perovskite
devices. However, a systematic study of the Cs incorporation on the properties
of the perovskite films has not been reported. In this report, Impact of Cesium
incorporation on the minority carrier recombination lifetime of
Cesium-Methylammonium lead halide perovskite thin-films is studied. The
lifetime for the as-deposited perovskite films decreases with increasing
concentration of cesium. However, mixed cation perovskite film is more stable,
showing higher lifetime (15-20 micro seconds) after 9 hours of ambient exposure
than just after deposition (6-13 micro seconds). Methylamine Vapor Exposure
(MVE) technique was used to improve the morphology of the as-deposited film.
MVE treated films are more oriented along (110) direction and were even more
stable in ambient, with Cs0.10MA0.90PbI2.90Br0.10 films showing lifetime of
almost 50 micro seconds after 9 hours of ambient exposure, twice the lifetime
of a comparable MAPbI3 film. These results throw light on why mixed-cation
cesium-methylamine lead halide perovskite films are better for highly efficient
and stable perovskite solar cells.
