Lead-Free Halide Double Perovskites: Fundamentals, Challenges, and Photovoltaics Applications

Metal halide perovskites (MHPs) have gained tremendous interest in photovoltaics applications reaching the power conversion efficiency (PCE) of 25.8%. Despite the rapid development of MHP-based solar cells, the existence of toxic lead (Pb) and their operational instability are seen as key roadblocks for commercialization. A viable strategy for developing lead-free optoelectronic devices nests in addressing the toxicity issues in these compounds by carefully and strategically replacing lead while maintaining comparable stability and PCE. In this regard, halide double perovskite with structural flexibility, as well as lower toxicity and higher stability have caught the attention of the scientific community, surging the research of lead-free halide double perovskites with A(2)B(I)B(III)X-6 composition. A critical analysis of such compounds is presented from a fundamental point of view considering their orbital characteristics, bonding interaction, and optical transitions and relate these properties with their solar cell performance. This review focuses on the efficiency limiting factors and critically discusses recent advancements and strategies which have been demonstrated to boost their photovoltaic performances. Finally, starting from existing double perovskite materials, prospective research directions are identified toward enhanced optoelectronic properties and device performances.