| Summary: | Lead halide perovskites have gained more and more attention because of their ease of synthesis and excellent photoelectric properties including a large absorption coefficient, long carrier lifetime, long carrier diffusion length, and high carrier mobility. However, their toxicity, instability, and phase degradation in ambient environments impede their large-scale applications. To address these concerns, it is desirable to find stable alternative halide perovskites without toxicity and with comparable optoelectronic properties to lead-based perovskites. Over the years, a considerable number of lead-free halide perovskites have been added to this family of materials, including A<sub>2</sub>B’B’’X<sub>6</sub>, A<sub>2</sub>BX<sub>6</sub>, and A<sub>3</sub>B<sub>2</sub>X<sub>9</sub> type perovskites. Among these, double perovskites with the general formula A<sub>2</sub>B’B’’X<sub>6</sub> are deemed to be a potential alternative to lead halide perovskites as they possess good stability under ambient conditions and excellent optoelectronic properties. In this review, recent progress in exploring Pb-free halide double perovskites is highlighted. The synthesis, composition-tuning, physical properties, and applications of representative 3D, 2D, and nanocrystal A<sub>2</sub>B’B’’X<sub>6</sub> double perovskites are introduced. In addition, perspectives about current challenges and solutions in this field are also provided.
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