Abstract: 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)_xto  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 isx=  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 FAPbI₃single  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.

Links:

http://pubs.acs.org/doi/abs/10.1021/jacs.6b12432

College of Chemistry and Chemical Engineering

Zhong-Qun TIAN