Mixed-halide perovskites (MHPs), such as methylammonium lead bromide−iodide, CH 3 NH 3 Pb(Br x I 1−x ) 3 (0 < x < 1), are very promising candidates for tandem solar cells and wavelength-tunable light-emitting diodes (LEDs) due to their tunable bandgaps. However, uniform MHPs undergo phase separation under visible light irradiation or an electric field, and the reason is still under debate. In this work, we report that the phase separation can be dramatically suppressed by oxygen passivation under light illumination. More excitingly, we observe that phase-separated MHPs can be recovered to their original uniform state in an oxygen environment. Theoretical calculations demonstrate that oxygen atoms can effectively passivate traps in MHPs and occupy halide vacancies, thus suppressing halide redistribution and phase separation. Our results show that with sophisticated trap passivation techniques, phase-stable MHPs are attainable.

Ion migration is a notorious phenomenon observed in ionic perovskite materials. It causes several severe issues in perovskite optoelectronic devices such as instability, current hysteresis, and phase segregation.Here, we report that, in contrast to lead halide perovskites (LHPs), no ion migration or phase segregation was observed in tin halide perovskites (THPs) under illumination or an electric field. The origin is attributed to a much stronger Sn-halide bond and higher ion migration activation energy (E a ) in THPs, which remain nearly constant under illumination. We further figured out the threshold E a for the absence of ion migration to be around 0.65 eV using the CsSn y Pb 1-y -(I 0.6 Br 0.4 ) 3 system whose E a varies with Sn ratios. Our work shows that ion migration does not necessarily exist in all perovskites and suggests metallic doping to be a promising way of stopping ion migration and improving the intrinsic stability of perovskites.

Ion migration is a notorious phenomenon observed in ionic perovskite materials. It causes several severe issues in perovskite optoelectronic devices such as instability, current hysteresis, and phase segregation.Here, we report that, in contrast to lead halide perovskites (LHPs), no ion migration or phase segregation was observed in tin halide perovskites (THPs) under illumination or an electric field. The origin is attributed to a much stronger Sn-halide bond and higher ion migration activation energy (E a ) in THPs, which remain nearly constant under illumination. We further figured out the threshold E a for the absence of ion migration to be around 0.65 eV using the CsSn y Pb 1-y -(I 0.6 Br 0.4 ) 3 system whose E a varies with Sn ratios. Our work shows that ion migration does not necessarily exist in all perovskites and suggests metallic doping to be a promising way of stopping ion migration and improving the intrinsic stability of perovskites.

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