We reveal an iodine vapor-induced degradation mechanism in formamidinium–lead-iodide-based perovskite solar cells stressed under combined heat and light illumination.

High-bandgap (>1.7 eV) mixed halide perovskites for multijunction solar cells are usually affected by photoinduced phase segregation, which triggers subbandgap defects that are detrimental to the open-circuit voltage. While this effect may be reversed, e.g., when leaving the cells in the dark, new perovskite compositions that exhibit enhanced stability may be required. In this Perspective, the compositional space beyond the conventional methylammonium-and formamidinium-based mixed halide compounds is reviewed in light of multijunction applications. These alternative absorber compositions include: (1) layered or quasi-2D perovskites, where larger organic cations are incorporated into the structure; (2) inorganic perovskites (i.e., when the organic components are removed altogether); and (3) lead-free structures, where the toxic lead is substituted by one or more elements. The development perspectives of highefficiency and stable perovskite materials based on these compositions are discussed in view of an integration in multijunction solar cells.

The mechanisms controlling the degradation of p–i–n perovskite solar cells in reverse bias are identified using various optoelectronic and microstructural characterization techniques.

Cesium−formamidinium-based mixed-halide perovskite materials with optical band gaps ranging from 1.5 to 1.8 eV are investigated by variable-angle spectroscopic ellipsometry. The determined complex refractive indices are shown to depend on the fabrication procedure and environmental conditions during processing. This data is complemented by additional optical and structural characterization, as well as the demonstration of efficient perovskite solar cells. Finally, the data is used in optical simulations to provide guidelines for the optimization of perovskite/silicon tandem solar cells.