Lead-free perovskite solar cells enabled by hetero-valent substitutes

Perovskite materials have demonstrated remarkable optoelectronic properties, which have placed them at the crux of the photovoltaic technology. In this era of striving for clean and economical methods of energy production, lead-based perovskites have become the key materials in photovoltaics owing to their facile solution processability and phenomenal performance. Unfortunately, lead toxicity poses a major hurdle to their scalability and widespread commercialization. Hence, to fulfill the indisputable need for energy, lead-free perovskites are considered to be a boon to the future of photovoltaic technology. In this perspective, we provide a comprehensive assessment of lead-free perovskite solar cells enabled by hetero-valent substitutes. These comprise A(3)B(2)X(9)-structured perovskites, halide double perovskites, and mixed metal halide-chalcogenide perovskites. Importantly, we emphasize the effects of cationic-anionic sites, metal substitutions, and solvents on the chemical and structural properties of A(3)B(2)X(9)-structured perovskites. Moreover, we also focus on antimony(v) perovskite-like materials and antimony-based perovskite nanocrystals (NCs) in the fabrication of devices. Two types of halide double perovskites are described, including A(2)B(i)B(iii)X-6 and A(2)B(iv)X-6 double perovskites. Subsequently, chalcogenide perovskite solar cells are also discussed. Overall, the primary purpose of this perspective is to explicitly describe lead-free perovskite solar cells enabled by hetero-valent substitutions more broadly as a category of next-generation optoelectronic materials. Finally, we propose that mixed halide-chalcogenide perovskites offer a promising pathway towards achieving highly efficient and stable perovskite solar cells.