Understanding the Cubic Phase Stabilization and Crystallization Kinetics in Mixed Cations and Halides Perovskite Single Crystals

The
spontaneous α-to-δ phase transition of the formamidinium-based
(FA) lead halide perovskite hinders its large scale application in
solar cells. Though this phase transition can be inhibited by alloying
with methylammonium-based (MA) perovskite, the underlying mechanism
is largely unexplored. In this Communication, we grow high-quality
mixed cations and halides perovskite single crystals (FAPbI3)1–x(MAPbBr3)x to understand the principles for maintaining pure
perovskite phase, which is essential to device optimization. We demonstrate
that the best composition for a perfect α-phase perovskite without
segregation is x = 0.1–0.15, and such a mixed
perovskite exhibits carrier lifetime as long as 11.0 μs, which
is over 20 times of that of FAPbI3 single crystal. Powder
XRD, single crystal XRD and FT-IR results reveal that the incorporation
of MA+ is critical for tuning the effective Goldschmidt
tolerance factor toward the ideal value of 1 and lowering the Gibbs
free energy via unit cell contraction and cation disorder. Moreover,
we find that Br incorporation can effectively control the perovskite
crystallization kinetics and reduce defect density to acquire high-quality
single crystals with significant inhibition of δ-phase. These
findings benefit the understanding of α-phase stabilization
behavior, and have led to fabrication of perovskite solar cells with
highest efficiency of 19.9% via solvent management.