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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trating systems. In most terrestrial applications, the loss in absorption due to the presence of diffuse sunlight completely counteracts the efficiency gain due to concentration. [2] In this work, we present a new concentrating solar cell that automatically tracks the sun without the presence of mechanical systems and is able to collect the majority of diffuse light, overcoming the two major limitations of modern-day concentrators.To overcome the limitations in diffuse sunlight absorption, a system is needed that can absorb and concentrate both direct and diffuse sunlight. In our approach, lenses focus direct sunlight into vertical pillars of low bandgap regions embedded in a high bandgap matrix. Diffuse sunlight is absorbed in the high bandgap matrix and subsequently the excited states are funneled to the low bandgap regions. This electronic concentration of charges results in an effective concentration of diffuse sunlight, which is thermodynamically allowed and driven by the difference in bandgap. [3] This concept is shown schematically in Figure 1.A self-aligning lens-emitter system is desirable in order to simplify fabrication and avoid the need for solar tracking. Our design achieves this goal using absorber films that undergo light-driven phase separation to form low bandgap regions. Microsphere lenses placed on top of the film accept light from any angle of incidence and focus it into different spatial locations depending on the incident angle. Therefore, as the sun moves during the day, the phase-separated (low bandgap) region also moves to automatically stay at the focus, leading to a self-tracking system. Diffuse sunlight, coming from any other angle, will still be absorbed by the high-bandgap material. This approach requires the absorber film to phase separate to form lower bandgap regions under the high intensity focused direct sunlight but also to remix under low intensity diffuse sunlight to reform the higher bandgap matrix. The kinetics of phase separation and remixing also need to be sufficiently fast to track the sun (minutes to hours timescale).Mixed halide perovskites are an excellent candidate material for the absorber film in our self-tracking and diffuse light utilizing solar concentrator. Halide perovskite thin films are in general good candidates for efficient solar cell devices, surpassing all expectations in terms of solar cell efficiency over

Back‐contact perovskite solar cells offer a significant potential to reach high efficiency due to reduced parasitic absorption from the top surface. However, the currently reported efficiencies are considerably lower (<10%) than planar perovskite solar cells (>20%). Herein, back‐contact perovskite solar cells are fabricated to study loss mechanisms that cause low device efficiency. This work spatially resolves the short‐circuit current, open‐circuit voltage, photoluminescence quantum yield, carrier lifetime, and external quantum efficiency of the devices. The results indicate that the front surface recombination, increased nonradiative recombination at hole contact layer/perovskite interface, and the extraction barriers are three main mechanisms limiting devices from achieving high efficiencies.

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.