Strain-stabilized perovskites: The perovskite materials used for solar cells and light-emitting diodes (which are black in color) are generally less stable at room temperature than the electronically inactive nonperovskite phases (which are yellow in color). Steele et al. show that for CsPbI ₃ , strain induced in a thin film after annealing the material to 330°C and then rapidly cooling it to room temperature kinetically trapped the black phase. Grazing-incidence wide-angle x-ray scattering revealed the crystal distortions and texture formation created by interfacial strain. Science , this issue p. 679

Efficient wide-bandgap perovskite solar cells (PSCs) enable high-efficiency tandem photovoltaics when combined with crystalline silicon and other low-bandgap absorbers. However, wide-bandgap PSCs today exhibit performance far inferior to that of sub-1.6-eV bandgap PSCs due to their tendency to form a high density of deep traps. Here, we show that healing the deep traps in wide-bandgap perovskites—in effect, increasing the defect tolerance via cation engineering—enables further performance improvements in PSCs. We achieve a stabilized power conversion efficiency of 20.7% for 1.65-eV bandgap PSCs by incorporating dipolar cations, with a high open-circuit voltage of 1.22 V and a fill factor exceeding 80%. We also obtain a stabilized efficiency of 19.1% for 1.74-eV bandgap PSCs with a high open-circuit voltage of 1.25 V. From density functional theory calculations, we find that the presence and reorientation of the dipolar cation in mixed cation–halide perovskites heals the defects that introduce deep trap states.

The impressive optoelectronic performance and low production cost of metal halide perovskites have inspired applications well beyond efficient so-lar cells. Herein, we widen the materials engineer-ing options available for the efficient and selective photocatalytic oxidation of benzylic alcohols - an industrially significant reaction - using forma-midinium lead bromide (FAPbBr3) and other per-ovskite based materials. The best performance was obtained using a FAPbBr3/TiO2 hybrid photocata-lyst under simulated solar illumination. Detailed optical studies reveal the synergetic photophysical pathways arising in FAPbBr3/TiO2 composites. An experimentally supported model rationalizing the large conversion enhancement over the pure con-stituents shows that this strategy offers new pro-spects for metal halide perovskites in photocatalyt-ic application.

Inspired by the efficient perovskite solar cells, we developed a three-component hybrid perovskite-based solar photocatalyst cell - NiO_x/FAPbBr₃/TiO₂ - for C(sp³)-H bonds activation with high selectivity (~90%) and high conversion rates (3800 µmol g^-1 h^-1) under ambient conditions. Time-resolved spectroscopy on our photocatalytic cell reveals efficient exciton dissociation and charge separation, where TiO₂ and NiO_x serve respectively as the electron and hole transporting layers. The photo-generated charge carriers, injected into TiO₂ and NiO_x, drive the challenging C-H activation reaction via the synergetic effects of their band alignment relative to FAPbBr₃. The reaction pathway is investigated by controlling the free radical formation and we find that C-H activation is mainly triggered by hole oxidation. Besides aromatic alkanes, also C(sp³)-H bond in cycloalkanes can selectively be oxidised. This work demonstrates a generic strategy for engineering high-performance photocatalysts based on the perovskite solar cell concept.