Nonequilibrium processes occurring in functional materials can significantly impact device efficiencies and are often difficult to characterize due to the broad range of length and time scales involved. In particular, mixed halide hybrid perovskites are promising for optoelectronics, yet the halides reversibly phase separate when photo-excited, significantly altering device performance. By combining nanoscale imaging and multiscale modeling, we elucidate the mechanism underlying this phenomenon, demonstrating that local strain induced by photo-generated polarons promotes halide phase separation and leads to nucleation of light-stabilized iodide-rich clusters. This effect relies on the unique electromechanical properties of hybrid materials, characteristic of neither their organic nor inorganic constituents alone. Exploiting photo-induced phase separation and other nonequilibrium phenomena in hybrid materials, generally, could enable new opportunities for expanding the functional applications in sensing, photoswitching, optical memory, and energy storage.Photovoltaic and light-emitting devices typically operate under conditions far from equilibrium. As such, elucidating the response of functional materials to nonequilibrium driving forces is vital to understanding their fundamental properties and to determining their suitability for device applications. In particular, photo-induced dynamic processes are of major importance to the performance of hybrid perovskite-based devices.1-3 Hybrid perovskites are low-cost, solution processable materials that are promising for many device applications, including photovoltaics 4-9 and light-emitting diodes (LEDs). 10The high device efficiencies have been attributed to their high brightness, long charge carrier migration lengths, + 2 , FA), and X is either iodide, bromide, chloride, or iodide/bromide or bromide/chloride mixtures. By varying the halide ratios in hybrid perovskites, the bandgap can be tuned across the visible spectrum.1,14,15 Precise control of the bandgap presents promising opportunities for color tuning perovskite-based LEDs and lasers, and for incorporating hybrid perovskites in tandem solar cells. 9,16Light-induced effects, however, restrict the practical use of mixed halide hybrid perovskites.1,17,18 Photoluminescence (PL) and X-ray diffraction (XRD) measurements suggest that MAPb(I x Br 1−x ) 3 (0.1 < x < 0.8) undergoes reversible phase separation into iodide-rich and bromiderich regions when photo-excited.1 Such demixing is detrimental to photovoltaic performance, since it leads to charge carrier trapping in the iodide-rich regions. Determining the microscopic mechanism behind phase separation is essential for furthering approaches to mitigate adverse photo-induced effects in devices and should expand the range of their functional applications into areas such as optical memory storage and sensing.19,20 Unfortunately, the microscopic mechanism behind this effect has been elusive because of the multiple length and time scales involved in characterizing the ...

The interfaces of neat water and aqueous solutions play a prominent role in many technological processes and in the environment. Examples of aqueous interfaces are ultrathin water films that cover most hydrophilic surfaces under ambient relative humidities, the liquid/solid interface which drives many electrochemical reactions, and the liquid/vapor interface, which governs the uptake and release of trace gases by the oceans and cloud droplets. In this article we review some of the recent experimental and theoretical advances in our knowledge of the properties of aqueous interfaces and discuss open questions and gaps in our understanding.

Smart photovoltaic windows represent a promising green technology featuring tunable transparency and electrical power generation under external stimuli to control the light transmission and manage the solar energy. Here, we demonstrate a thermochromic solar cell for smart photovoltaic window applications utilizing the structural phase transitions in inorganic halide perovskite caesium lead iodide/bromide. The solar cells undergo thermally-driven, moisture-mediated reversible transitions between a transparent non-perovskite phase (81.7% visible transparency) with low power output and a deeply coloured perovskite phase (35.4% visible transparency) with high power output. The inorganic perovskites exhibit tunable colours and transparencies, a peak device efficiency above 7%, and a phase transition temperature as low as 105 °C. We demonstrate excellent device stability over repeated phase transition cycles without colour fade or performance degradation. The photovoltaic windows showing both photoactivity and thermochromic features represent key stepping-stones for integration with buildings, automobiles, information displays, and potentially many other technologies.