Suppression and Reversion of Light-Induced Phase Separationin Mixed-Halide Perovskites by Oxygen Passivation

Mixed-halide perovskites (MHPs), such as methyl ammonium lead bromide iodide, CH3NH3Pb(BrxI1-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.