Challenges and Progress in Lead‐Free Halide Double Perovskite Solar Cells

Lead‐free halide double perovskites (HDPs) with a chemical formula of A 2 B + B 3+ X 6 are booming as attractive alternatives to solve the toxicity issue of lead‐based halide perovskites (APbX 3 ). HDPs show excellent stability, a wide range of possible combinations, and attractive optoelectronic features. Although a number of novel HDPs have been studied, the power conversion efficiency of the state‐of‐the‐art double perovskite solar cell is still far inferior to that of the dominant Pb‐based ones. Understanding the fundamental challenges is essential for further increasing device efficiency. In this review, HDPs with attractive electronic and optical properties are focused on, and current challenges in material properties and device fabrication that limit high‐efficiency photovoltaics are analyzed. Finally, the promising approaches and views to overcome these bottlenecks are highlighted.