Mixed-halide hybrid perovskites such as CH3NH3Pb(BrxI1-x)3 are a promising family of photovoltaic absorber materials that have achieved power conversion efficiencies of over 17%. By varying the halide composition, the optical bandgap can be tuned over the range 1.6-2.3 eV, making this family of materials a suitable candidate for both single-junction solar cells as well as the large bandgap absorber of a tandem solar cell. However, reports of mixed CH3NH3Pb(BrxI1-x)3devices with higher bromine content have so far not been able to achieve the increase in open circuit voltage that may be expected from the larger bandgap of these materials.
We observe photo-induced halide segregation in bromine-rich CH3NH3Pb(BrxI1-x)3 and other mixed-halide perovskites as evidenced by the appearance of intense photoluminescence and absorption features from a new iodide-rich phase upon continuous illumination and the disappearance of these features with time in the dark. We suggest that photoexcitation may induce halide migration, resulting in iodide-rich domains that act as traps and pin the open circuit voltage at a lower energy. The kinetics of this process have a similar temperature dependence to the hysteretic behavior in planar CH3NH3PbI3-xClxsolar cells which is suggestive of a prominent role of halide migration in perovskite photovoltaic hysteresis. These observations are reminiscent of photo-initiated halide migration in lead halides and other metal halides, which has been proposed to occur via a halide-vacancy diffusion mechanism from surface sites. This suggests that improved control of the perovskite stoichiometry, crystallinity and surface passivation are potential strategies towards reducing halide migratory effects and improving the stability of halide perovskite optoelectronic devices.