Tandem solar cells are a very promising technology that would allow to overpass the Shockley-Queisser efficiency limit for photovoltaic devices. They can be manufactured by stacking two solar devices, one of low band-gap (to absorb low-energy photons), on top of which is placed a high band-gap absorber (that would absorb the high-energy photons, therefore reducing the heat losses inside the solar cell). Mixed halide perovskite are particularly interesting for the assembly of those tandem solar cells, because compositional arrangements can allow acute band-gap tuning.

In perovskite of the form ABX₃, where X is a mixture of halide ions, generally iodide and bromide, researchers have observed a phase segregation under illumination. In fact, the halide ions form clusters inside the perovskite, leading to a hysteresis cycle in the J-V characteristic curves and other instabilities. This issue is currently one of the most important to address before perovskite can hit the market.

Here is a collection of a few papers on the subject, that I have found particularly interesting:

• Eric T. Hoke et al. reported first the photo-induced phase segregation effect. (October 2014)
• Christopher Eames et al. proposed a mechanism explaining the ion migration in perovskites. (February 2015)
• Dan Slotcavage, from the same group, continued research on effect. (September 2016)
• Sergiu Draguta et al. propose a model suggesting that the phase segregation is driven by the band-gap reduction of the iodide-rich phase. (September 2016)
• Connor G. Bischak et al. proposed that the effect originates in a polaron. (October 2016)
• Alex J. Barker et al. reported that the role of defects is of utter importance. They hypothesise that the phase segregation is driven by the generation of charge carrier gradients through the thickness of the film. (March 2017)
• Felix Lang et al. proposed that radiation introduce phase-segregation. (May 2017)
• Ute B. Cappel et al. measured enrichment of bromide ions at the surface of thin films upon illumination. (July 2017)
• Gergely F. Samu et al. reported that light-soaking influenced the segregation. (July 2017)
• Carolin M. Sutter-Fella et al.: review about phase segregation (February 2018)
• John M. Howard et al. probed a water-induced phase segregation (April 2018)
• Connor G. Bischak et al. proposed that the photo-induced phase segregation is controlled by tunable polaron distortion, suggesting that phase segregation may be an intrinsic effect of mixed-halide perovskites. (May 2018)