A reversible photo-induced instability has been found in mixed-halide photovoltaic perovskites that limits the open circuit voltage in solar cells.

Two-dimensional hybrid perovskites are used as absorbers in solar cells. Our first-generation devices containing (PEA)2(MA)2[Pb3I10] (1; PEA=C6H5(CH2)2NH3(+), MA=CH3NH3(+)) show an open-circuit voltage of 1.18 V and a power conversion efficiency of 4.73%. The layered structure allows for high-quality films to be deposited through spin coating and high-temperature annealing is not required for device fabrication. The 3D perovskite (MA)[PbI3] (2) has recently been identified as a promising absorber for solar cells. However, its instability to moisture requires anhydrous processing and operating conditions. Films of 1 are more moisture resistant than films of 2 and devices containing 1 can be fabricated under ambient humidity levels. The larger bandgap of the 2D structure is also suitable as the higher bandgap absorber in a dual-absorber tandem device. Compared to 2, the layered perovskite structure may offer greater tunability at the molecular level for material optimization.

We report on the second family of layered perovskite white-light emitters with improved photoluminescence quantum efficiencies (PLQEs). Upon near-ultraviolet excitation, two new Pb-Cl and Pb-Br perovskites emit broadband "cold" and "warm" white light, respectively, with high color rendition. Emission from large, single crystals indicates an origin from the bulk material and not surface defect sites. The Pb-Br perovskite has a PLQE of 9%, which is undiminished after 3 months of continuous irradiation. Our mechanistic studies indicate that the emission has contributions from strong electron-phonon coupling in a deformable lattice and from a distribution of intrinsic trap states. These hybrids provide a tunable platform for combining the facile processability of organic materials with the structural definition of crystalline, inorganic solids.