The development of perovskite solar cells has revolutionized photovoltaics, offering high efficiencies and low-cost fabrication. However, the toxicity of lead (Pb) in conventional perovskite materials like methylammonium lead iodide (MAPbI3) raises environmental and health concerns. Lead-free alternatives, such as tin (Sn), bismuth (Bi), and antimony (Sb)-based perovskites, have emerged as promising candidates for sustainable photovoltaics. These materials aim to retain the advantageous optoelectronic properties of Pb-based perovskites while mitigating toxicity risks.
**Optoelectronic Properties of Lead-Free Perovskites**
Sn-based perovskites, particularly formamidinium tin iodide (FASnI3), exhibit favorable bandgaps (~1.2–1.4 eV) and high charge carrier mobilities, making them suitable for single-junction and tandem solar cells. However, Sn perovskites suffer from rapid oxidation of Sn2+ to Sn4+, leading to high p-type doping and reduced device performance. Bi and Sb-based perovskites, such as Cs3Bi2I9 and Cs3Sb2I9, offer greater stability but wider bandgaps (~1.9–2.2 eV), limiting their absorption range.
Compared to Pb-based perovskites, which achieve power conversion efficiencies (PCEs) exceeding 25%, Sn-based devices have reached PCEs of up to 14%, while Bi and Sb-based variants lag behind at 3–5%. The lower efficiencies stem from higher defect densities, non-ideal band alignment, and poor charge extraction.
**Toxicity Profiles**
The primary advantage of Sn, Bi, and Sb perovskites is their reduced toxicity. Pb is a cumulative neurotoxin with severe environmental persistence, whereas Sn is considered biocompatible and widely used in food packaging. Bi and Sb exhibit lower acute toxicity than Pb, though their long-term environmental impact requires further study. Regulatory restrictions on Pb in electronics and consumer products drive the push for lead-free alternatives.
**Stability Challenges**
Sn-based perovskites degrade rapidly due to oxidation in ambient conditions, necessitating encapsulation or inert atmosphere processing. Strategies such as additive engineering (e.g., reducing agents like SnF2) and mixed cation/anion compositions (e.g., incorporating MA or Cs) have improved stability. Bi and Sb perovskites are more chemically stable but suffer from phase segregation and poor film morphology. Moisture resistance remains a universal challenge for all lead-free variants.
**Synthesis Routes**
Solution processing is the most common method for fabricating Sn, Bi, and Sb perovskites, offering scalability and low-cost deposition. Techniques like spin-coating, blade-coating, and slot-die coating are employed, with solvent engineering (e.g., dimethyl sulfoxide or gamma-butyrolactone mixtures) to control crystallization. Vacuum deposition is also explored for high-purity films.
For Sn perovskites, reducing agents and antioxidant additives are critical to suppress Sn2+ oxidation. Bi and Sb perovskites often require higher annealing temperatures due to their inorganic nature, complicating integration with flexible substrates.
**Defect Passivation Strategies**
Defect passivation is essential to improve carrier lifetimes and reduce non-radiative recombination. For Sn perovskites, Lewis base additives (e.g., thiophene derivatives) bind to undercoordinated Sn ions, while bulk incorporation of larger cations (e.g., guanidinium) reduces defect formation.
Bi and Sb perovskites benefit from surface passivation with organic halides or 2D perovskite capping layers, which mitigate interfacial recombination. Doping with alkali metals (e.g., Na or K) has also shown promise in improving conductivity and film quality.
**Progress in Efficiency**
Recent advances in Sn-based perovskites include mixed-halide compositions (e.g., FASnI3-xBrx) to optimize bandgaps and reduce defect density, achieving PCEs above 10%. Tandem devices combining Sn perovskites with silicon or CIGS absorbers have demonstrated potential for efficiencies beyond 20%.
Bi and Sb perovskites, while less efficient, are being explored for semi-transparent photovoltaics and UV-selective applications due to their wider bandgaps. Efforts to reduce trap states through nanocrystal engineering or interfacial layers could unlock higher efficiencies.
**Industrial Adoption Barriers**
The main hurdles for commercialization include:
- **Scalability:** Reproducing high-quality films over large areas remains challenging.
- **Stability:** Meeting industry-standard lifespans (20+ years) requires breakthroughs in encapsulation and material design.
- **Cost:** While lead-free materials reduce regulatory costs, the need for additives or specialized processing may offset savings.
- **Performance Gap:** Pb-based perovskites still outperform lead-free alternatives, making it difficult to justify switching without further efficiency gains.
**Life-Cycle Analysis Comparisons**
Life-cycle assessments (LCAs) of Pb-based perovskites highlight concerns over Pb leakage during production, use, and disposal. While encapsulation mitigates risks, end-of-life management remains problematic. Sn-based devices show lower environmental impact in LCAs, but the energy-intensive purification of Sn and the use of hazardous solvents (e.g., DMF) present trade-offs. Bi and Sb perovskites, though less studied, are expected to have lower toxicity burdens but may face resource scarcity issues.
**Conclusion**
Lead-free perovskite nanomaterials represent a critical step toward environmentally sustainable photovoltaics. While Sn-based systems offer the highest efficiencies among Pb-free options, stability and oxidation resistance require further improvement. Bi and Sb perovskites, though less efficient, provide alternative pathways for niche applications. Defect passivation, advanced synthesis techniques, and tandem device integration are key focus areas for bridging the performance gap with Pb-based perovskites. Industrial adoption will depend on solving scalability and stability challenges while ensuring competitive costs and performance. Continued research into eco-friendly processing and recycling will be essential to realize the full potential of these materials.