Lead-free hybrid perovskites have emerged as a promising alternative to lead-based perovskites due to growing environmental and health concerns associated with lead toxicity. While lead halide perovskites, such as MAPbI3 (methylammonium lead iodide), exhibit excellent optoelectronic properties, their instability and toxicity have driven research into Sn-, Ge-, Bi-, and Cu-based alternatives. These materials aim to retain favorable electronic properties while mitigating environmental risks. This article examines the structural, electronic, and stability characteristics of these lead-free alternatives and compares them with conventional lead-based perovskites.

Tin-based perovskites, particularly MASnI3 (methylammonium tin iodide), are among the most studied lead-free alternatives due to their similar electronic structure. Tin, like lead, belongs to Group 14, enabling comparable bandgap tuning. MASnI3 exhibits a direct bandgap of approximately 1.2–1.4 eV, which is ideal for photovoltaic applications. However, a major drawback is the oxidation of Sn²⁺ to Sn⁴⁺ in ambient conditions, leading to rapid degradation. This instability is exacerbated by the formation of intrinsic defects, such as Sn vacancies, which act as p-type dopants and increase carrier recombination. Strategies to improve stability include compositional engineering with additives like SnF2 and encapsulation techniques to limit oxygen exposure.

Germanium-based perovskites, such as MAGeI3, offer another Group 14 alternative but face significant challenges. While Ge²⁺ has a similar ionic radius to Pb²⁺, the smaller size of Ge results in a more distorted perovskite lattice, leading to poor phase stability. MAGeI3 has a wider bandgap (~1.9 eV), limiting its absorption range compared to lead perovskites. Additionally, Ge²⁺ is even more prone to oxidation than Sn²⁺, making these materials highly unstable under ambient conditions. Despite these issues, germanium perovskites remain of interest due to their potential for defect-tolerant behavior in specific compositions.

Bismuth-based perovskites, such as MA3Bi2I9, adopt a different crystal structure due to Bi³⁺ preferring an octahedral coordination. Unlike the 3D perovskite framework of lead halides, bismuth perovskites often form 0D or 2D structures, which exhibit larger bandgaps (~2.1 eV) and lower charge carrier mobility. The indirect nature of their bandgap further reduces optical absorption efficiency. However, bismuth perovskites demonstrate superior stability against moisture and oxygen compared to Sn and Ge counterparts. The higher formation energy of defects in bismuth-based structures also reduces non-radiative recombination, making them suitable for certain optoelectronic applications where stability is prioritized over efficiency.

Copper-based perovskites, such as Cs2CuX4 (X = Cl, Br, I), represent another class of lead-free materials with distinct electronic properties. These materials typically form 2D layered structures with strong excitonic effects due to quantum confinement. The bandgaps range from 1.4 eV to 2.6 eV depending on halide composition. Copper perovskites exhibit high stability against environmental factors but suffer from low carrier mobility and high exciton binding energies, limiting charge separation efficiency. The presence of Jahn-Teller distortions in Cu²⁺ further complicates their electronic structure, often leading to localized states that impede charge transport.

Comparing toxicity profiles, lead-based perovskites pose significant risks due to lead’s cumulative toxicity and environmental persistence. In contrast, Sn and Ge are less toxic but still require careful handling due to their oxidation products. Bismuth is considered non-toxic and environmentally benign, making it an attractive alternative. Copper is also relatively safe, though excessive exposure can have ecological impacts. The reduced toxicity of these alternatives makes them more suitable for large-scale deployment, provided stability and performance challenges are addressed.

Defect tolerance is a critical factor influencing the performance of hybrid perovskites. Lead-based perovskites exhibit remarkable defect tolerance due to the high dielectric constant and strong spin-orbit coupling of Pb²⁺, which localizes defects without creating deep traps. Sn-based perovskites partially retain this property but suffer from higher defect densities due to oxidation. Bismuth and copper perovskites, while more stable, lack the same level of defect tolerance, leading to higher non-radiative recombination rates.

Oxidation susceptibility remains a primary challenge for Sn and Ge perovskites. The oxidation of Sn²⁺ and Ge²⁺ not only degrades the material but also introduces detrimental defects. Encapsulation and compositional engineering, such as alloying with more stable cations or incorporating reducing agents, are potential mitigation strategies. Bismuth and copper perovskites, while more stable, require optimization of their electronic structure to improve charge transport properties.

In summary, lead-free hybrid perovskites based on Sn, Ge, Bi, and Cu offer viable alternatives to lead-based materials but each comes with trade-offs in electronic properties, stability, and defect tolerance. Tin-based perovskites closely mimic lead’s optoelectronic performance but suffer from instability. Germanium variants face even greater challenges due to oxidation and structural distortions. Bismuth and copper perovskites provide enhanced stability but at the cost of reduced efficiency due to wider bandgaps and lower carrier mobility. Future research must focus on overcoming these limitations through advanced material design and stabilization techniques to realize environmentally sustainable perovskite technologies.