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The highest solar cell efficiencies are obtained with concentrating systems. However, these systems have two major drawbacks: solar tracking is needed and diffuse sunlight is poorly absorbed. Here, a system that overcomes both limitations is presented, by exploiting light‐induced halide segregation in mixed halide perovskite films. A monolayer of silica microspheres focuses direct sunlight in a mixed halide perovskite film below. Through light‐induced phase segregation a low bandgap region forms in the focal point. Together with funneling of excited states toward the low bandgap regions, this results in a voltage increase characteristic for concentrating systems. Diffuse sunlight is still absorbed by the high bandgap material, avoiding the loss of diffuse sunlight characteristic for conventional concentrators. The formation of low bandgap regions in the perovskite is dynamic, and can follow the position of the focus as the sun moves throughout the day, making solar tracking unnecessary. With detailed optical and electrical simulations, it is shown that this concept can lead to an increase in both voltage and current. This leads to a 6.6% absolute increase in power conversion efficiency compared to the film without microspheres. Absorption and emission measurements confirm the concentration effect, showing the promising potential for a self‐optimizing concentrating system. [ABSTRACT FROM AUTHOR]

Lead halide perovskites are among the most exciting classes of optoelectronic materials due to their unique ability to form high‐quality crystals with tunable bandgaps in the visible and near‐infrared using simple solution precipitation reactions. This facile crystallization is driven by their ionic nature; just as with other salts, it is challenging to form amorphous halide perovskites, particularly in thin‐film form where they can most easily be studied. Here, rapid desolvation promoted by the addition of acetate precursors is shown as a general method for making amorphous lead halide perovskite films with a wide variety of compositions, including those using common organic cations (methylammonium and formamidinium) and anions (bromide and iodide). By controlling the amount of acetate, it is possible to tune from fully crystalline to fully amorphous films, with an interesting intermediate state consisting of crystalline islands embedded in an amorphous matrix. The amorphous lead halide perovskite has a large and tunable optical bandgap. It improves the photoluminescence quantum yield and lifetime of incorporated crystalline perovskite, opening up the intriguing possibility of using amorphous perovskite as a passivating contact, as is currently done in record efficiency silicon solar cells. [ABSTRACT FROM AUTHOR]