(99a) Nanocrystal Doping Stabilizes the Perovskite Phase of Cesium Lead Iodide

All-inorganic, perovskite-phase cesium lead iodide is a promising material for photovoltaics. However, under ambient conditions it rapidly transforms to a non-functional, so-called yellow phase. Here we demonstrate more than two orders of magnitude improvement in the stability of nanostructured perovskite-phase cesium lead iodide thin films through a nanocrystal doping approach. The doped, functional perovskite phase is synthesized by co-assembling cesium lead iodide nanocrystals with cesium lead chloride dopant nanocrystals. The resulting doped nanocrystal solid is subsequently fused into a polycrystalline thin film by chemically-induced, room-temperature sintering. This process ensures nanometer-scale mixing even at compositions that exceed the bulk miscibility of the two constituents. Spectroscopy and X-ray diffraction indicate that some of this chloride is further dispersed during sintering and a polycrystalline mixed phase is formed. By comparison to DFT-calculated values, the relative change in band gap and the lattice contraction are shown to be consistent with a Cl:I ratio of a few percent in the mixed phase.