Passivating traps in perovskites: Low-temperature processing of planar organic-inorganic perovskite solar cells made through solution processing would allow for simpler manufacturing and the use of flexible substrates. However, materials currently in use form interfaces with charge carrier trap states that limit performance. Tan et al. used chlorine-capped TiO ₂ colloidal nanocrystal films as an electron-selective layer, which limited interface recombination in solution-processed solar cells. Such cells achieved certified efficiencies of 19.5% for active areas of 1.1 cm ² . Science , this issue p. 722

Perovskite light-emitting diodes (PeLEDs) have shown excellent performance in the green and near-infrared spectral regions, with high color purity, efficiency, and brightness. In order to shift the emission wavelength to the blue, compositional engineering (anion mixing) and quantum-confinement engineering (reduced-dimensionality) have been employed. Unfortunately, LED emission profiles shift with increasing driving voltages due to either phase separation or the coexistence of multiple crystal domains. Here we report color-stable sky-blue PeLEDs achieved by enhancing the phase monodispersity of quasi-2D perovskite thin films. We selected cation combinations that modulate the crystallization and layer thickness distribution of the domains. The perovskite films show a record photoluminescence quantum yield of 88% at 477 nm. The corresponding PeLEDs exhibit stable sky-blue emission under high operation voltages. A maximum luminance of 2480 cd m⁻² at 490 nm is achieved, fully one order of magnitude higher than the previous record for quasi-2D blue PeLEDs.

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

Closer matching of the energy levels of transparent electrodes and active materials in organic light-emitting diodes improves efficiency.