Urban centers across the globe are responsible for a significant fraction of energy consumption and CO2 emission. As urban centers continue to grow, the popularity of glass as cladding material in urban buildings is an alarming trend. Dynamic windows reduce heating and cooling loads in buildings by passive heating in cold seasons and mitigating solar heat gain in hot seasons. Here, reduced energy consumption in highly glazed buildings in a mesoscopic building energy model is demonstrated when thermochromic windows are employed. Savings are realized across eight disparate climate zones of the United States. The model is used to determine ideal critical transition temperatures of 20–27.5 °C for thermochromic windows based on metal halide perovskite materials. Ideal transition temperatures are realized experimentally in composite metal halide perovskite films composed of perovskite crystals and an adjacent reservoir phase. The transition temperature is controlled by cointercalating methanol, instead of water, with methylammonium iodide and tailoring the hydrogen‐bonding chemistry of the reservoir phase. Thermochromic windows based on metal halide perovskites represent a clear opportunity to mitigate the effects of energy‐hungry buildings.

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All-back-contact perovskite solar cells promise greater power conversion efficiency compared to conventional planar device architectures. However, the best-performing devices to date use photolithography to fabricate electrodes, which is expensive for deployment and a barrier for research facilities. Herein, we utilize cracked film lithography, a solution-processed micropatterning technique, to form an interconnected, defect-tolerant back-contact electrode network. We introduce a crack widening technique to control the optical transparency and sheet resistance while decoupling the relative areas of the electron and hole contacts in the back-contact network. Wider cracks increase the area of the hole-selective contact, which increases photocurrent and power conversion efficiency.

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