Lead-free perovskite semiconductors have emerged as a promising alternative to lead-based perovskites due to growing environmental and toxicity concerns. The most studied candidates include tin (Sn), germanium (Ge), bismuth (Bi), and double perovskites such as Cs2AgBiBr6. These materials exhibit unique electronic structures, optoelectronic properties, and stability challenges that distinguish them from their lead-based counterparts.

Tin-based perovskites, particularly MASnI3 and FASnI3 (MA = methylammonium, FA = formamidinium), are among the most investigated lead-free alternatives due to their favorable bandgaps and strong light absorption. The electronic structure of Sn perovskites is characterized by a smaller bandgap (1.2–1.4 eV) compared to lead-based perovskites (1.5–1.6 eV for MAPbI3), making them suitable for near-infrared applications. However, the Sn 5s orbital contributes to the valence band maximum, leading to a higher tendency for oxidation from Sn²⁺ to Sn⁴⁺. This instability results in high intrinsic p-type conductivity and rapid degradation in ambient conditions. Encapsulation and chemical doping strategies, such as incorporation of reducing agents like SnF2, have been employed to mitigate oxidation.

Germanium perovskites, such as MAGeI3, exhibit even narrower bandgaps (~1.6 eV) but suffer from severe stability issues due to the high reactivity of Ge²⁺. The smaller ionic radius of Ge²⁺ compared to Sn²⁺ and Pb²⁺ induces lattice strain, leading to phase instability and rapid decomposition. Despite their theoretically high absorption coefficients, the practical performance of Ge-based perovskites remains limited by these stability challenges.

Bismuth-based perovskites, including A3Bi2I9 (A = MA, FA, Cs), adopt a defect-ordered structure rather than the traditional perovskite lattice. The electronic structure of Bi³⁺ perovskites features an indirect bandgap (~2.1 eV) due to the lone-pair 6s² electrons, which reduces radiative recombination efficiency. While these materials exhibit better stability against moisture and oxygen compared to Sn and Ge perovskites, their optoelectronic performance is hindered by poor charge carrier mobility and short carrier lifetimes (<10 ns).

Double perovskites, such as Cs2AgBiBr6, offer a more stable and less toxic alternative by replacing Pb²⁺ with a combination of monovalent (Ag⁺) and trivalent (Bi³⁺) cations. The electronic structure of Cs2AgBiBr6 features an indirect bandgap (~2.0 eV), limiting its absorption efficiency compared to direct-bandgap lead perovskites. However, double perovskites exhibit superior stability against heat, light, and humidity, making them viable for long-term applications. The carrier lifetimes in double perovskites are typically in the range of 1–100 ns, significantly shorter than those of lead-based perovskites (100–1000 ns).

The optoelectronic performance of lead-free perovskites is generally inferior to lead-based counterparts due to several factors. Absorption coefficients for Sn perovskites (~10⁵ cm⁻¹) are comparable to lead perovskites, but non-radiative recombination losses are higher due to defect states. Ge and Bi perovskites exhibit lower absorption coefficients (~10⁴ cm⁻¹) and higher exciton binding energies, further reducing their photovoltaic efficiency. Double perovskites show moderate absorption but suffer from inefficient charge extraction due to indirect bandgap transitions.

Synthesis challenges for lead-free perovskites vary by material system. Sn perovskites require inert atmospheres and reducing conditions to prevent oxidation, while Ge perovskites demand ultra-low oxygen environments due to their extreme sensitivity. Bismuth perovskites are more tolerant but require precise stoichiometric control to avoid secondary phases. Double perovskites, though more stable, often suffer from incomplete crystallization and phase segregation during solution processing.

Toxicity trade-offs must also be considered. While lead-free perovskites eliminate the use of toxic Pb, Sn and Ge compounds still pose environmental risks if not properly managed. Bismuth is relatively benign, but its extraction and processing carry ecological concerns. Double perovskites are the least toxic but may incorporate scarce elements like silver, raising sustainability questions.

In summary, lead-free perovskite semiconductors present a complex landscape of electronic structures, stability challenges, and performance trade-offs. While they offer a path toward environmentally benign optoelectronics, significant material improvements are needed to match the efficiency and reliability of lead-based perovskites. Advances in defect passivation, compositional engineering, and synthesis techniques will be critical for their future development.