The Many Faces of Mixed Ion Perovskites: Unraveling and Understanding the Crystallization Process

Understanding the
crystallization process of organic–inorganic
halide perovskites is of paramount importance for fabrication of reproducible
and efficient perovskite solar cells. We report for the first time
on the discovery and interplay of ubiquitous hexagonal polytypes (6H
and 4H) during the crystallization process of mixed ion perovskite,
namely (FAPbI3)x(MAPbBr3)1–x. These polytypes,
the first reported 3D hexagonal lead-halide-based perovskites, orchestrate
a perovskite crystallization sequence revealed as 2H (delta phase)-4H-6H-3R­(3C),
commonly found among inorganic transition metal oxide perovskites
under extreme conditions. We show that the chemical pressure arising
from the incorporation of >3% Cs+ cations into the lattice
successfully inhibits the formation of these environmentally sensitive
polytypes, elucidating the origin of the widely reported improved
device stability and reproducibility of Cs+-containing
mixed ion perovskites.