Photostable Wide Bandgap Metal Halide Perovskites

Bandgap tuning is a crucial characteristic of metal-halide perovskite, with benchmark lead-iodide compounds having a bandgap of 1.6 eV. To increase the bandgap up to 2.0 eV a straightforward strategy is to partially substitute iodine with bromine in so-called mixed-halide lead perovskites. Such compounds are prone, however, to defect-induced halide segregation resulting in bandgap instabilities, which limits their application in tandem solar cells and a variety of optoelectronic devices. The optimization of the perovskite composition and surface passivating agents can effectively slow down, but not completely stop, such light-induced instabilities. Here we identify the defect and the intra-gap electronic state that triggers the material transformation and bandgap shift. It allows us to engineer the perovskite band edge energetics to radically deactivate the photo-activity of such defects and stabilize the perovskite bandgap over the entire spectral range above 1.6 eV. Then, I will discuss how the generation of I2 in mixed-halide perovskites under illumination has a crucial role in halide segregation. In fact, I2 reacts with bromide (Br−) in the lattice to form a trihalide ion I2Br-, which mediates a Br−/I− ion exchange reaction and the formation of an I-rich phase. Importantly, we observe that the efficiency of the process is highly dependent on the binding strength of the bromide within the crystalline structure. Based on this, we propose a general paradigm where an iodine redox based model rationalizes the initiation and growth of halide photo-segregation. Thus, we show how the design of the chemical composition of the metal-halide perovskite crystalline unit can stabilize the semiconductor bandgap, and eventually the performance of related solar cells, not only acting on the reduction of the density of native defects, but stabilizing the elemental composition of the perovskite crystalline unit. This is demonstrated by the fabrication of photo-stable purely inorganic CsPbI1.5Br1.5 solar cells.