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

Using density functional theory (DFT)-based calculations, we explore the extent to which achievable modes of atomic layering can tune the near-gap electronic structure of tin- and lead-halide perovskites with applications in dye-sensitized solar cells. We show that regardless of how atomic layering is achieved—whether by the growth of layered inorganic phases such as the Ruddlesden-Popper series, hybrid perovskites connected by organic linker molecules, or layered perovskite heterostructures—their band gaps can similarly be widened by several tenths of an eV or more. Because these classes of compounds are known to have band gaps spanning much of the visible region of the solar spectrum, the ability to controllably tune their near-gap electronic structure could further optimize their performance in solar energy conversion applications. Throughout this work, trends in band gap are explained based on the effects of atomic layering and quantum confinement on the character and energy of band-edge crystal orbitals.