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.

Halide perovskites display outstanding photoluminescence quantum yield, tunable emission, and simple deposition, which make them attractive for optoelectronics. At the same time, their facile ion migration and transformation under optical, electrical, and chemical stress are seen as a major limitation. Mixed halide perovskites are particularly problematic since optical excitation can cause changes in the bandgap that are detrimental for solar cell and light-emitting diode efficiency and stability. In this work, instead of preventing such changes, photo-induced halide segregation in perovskites is exploited to enable responsive, reconfigurable, and self-optimizing materials. The mixed halide perovskite film is trained to give directional light emission using a nanophotonic microlens; through a self-optimized process of halide photosegregation, the system mimics the training stimulus. Longer training leads to more highly directional emission, while different halide migration kinetics in the light (fast training) and dark (slow forgetting) allows for material memory. This self-optimized material performs significantly better than lithographically aligned quantum dots because it eliminates lens-emitter misalignment and automatically corrects for lens aberrations. The system shows a combination of mimicking, improving over time, and memory, which comprise the basic requirements for learning, and allow for the intriguing prospect of intelligent optoelectronic materials.

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