Mixed-halide perovskites are attractive candidates as wide-bandgap absorber layers in tandem solar cells. However, photoinduced phase segregation leads to the formation of Br-rich and I-rich domains with uncontrolled composition and bandgap, which is considered as the main obstacle for achieving stable solar cells. Herein, we tune the grain size of CsPbBr1.5I1.5 films derived from nanocrystals and investigate the corresponding charge transport properties. The correlation between phase segregation, grain size, and the corresponding solar cell performance is unravelled. Specifically, phase segregation takes place in CsPbBr1.5I1.5 films with an average crystal size being equal to or larger than 43 nm, which consequently leads to significantly degraded performance in the solar cells with such large grains under continuous light soaking. These findings provide the highly sought-after guidelines for achieving phase-segregation-free mixed-halide perovskite films for stable optoelectronic applications.
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