Peculiar Steric Hindrance Assists Monoclinic Phase Formation toward High-Quality All-Inorganic Perovskites

Bromine-containing
metal halide all-inorganic perovskite CsPbI2Br exhibits
excellent photoelectric performance and supreme
thermal and structural stabilities; it is thus attractive for use
as photoabsorbing layers in perovskite solar cells (PSCs). However,
when steric hindrance molecules are introduced, the complicated phase
transition mechanism and the difficult-to-control crystallization
process in CsPbI2Br are not well understood. Here, we introduce
a class of sterically hindered cesium naphthenate small molecules
to control the crystallization process of CsPbI2Br films.
Of interest, a new intermediate monoclinic phase has been discovered
which leads to formation of dense and nonporous polycrystalline perovskite
films. This phenomenon was also explained by density functional theory.
The residues of steric hindrance molecules inside the CsPbI2Br film also improve its stability. We further show that as the ring
number of cycloalkanes increases, the hindrance for the crystallization
becomes more significant. Thus, by choosing the suitable steric hindrance,
the optimal photovoltaic efficiency is 15.45%.