Emerging photovoltaic materials such as copper zinc tin sulfide (CZTS, kesterite) and antimony selenide (Sb₂Se₃) have garnered significant attention as potential alternatives to conventional solar technologies. These materials offer advantages in terms of earth-abundant elemental composition, defect tolerance, and environmental sustainability, making them promising candidates for next-generation photovoltaics. While their current efficiencies lag behind established technologies like silicon and perovskites, ongoing research aims to overcome their limitations and unlock their full potential.

CZTS, with its chemical formula Cu₂ZnSnS₄, belongs to the kesterite family and shares structural similarities with the more widely studied copper indium gallium selenide (CIGS). Unlike CIGS, which relies on scarce and expensive indium and gallium, CZTS is composed of abundant and low-cost elements—copper, zinc, tin, and sulfur. This makes it an attractive option for large-scale deployment without supply chain constraints. The bandgap of CZTS is tunable between approximately 1.0 and 1.5 eV, which is close to the ideal range for single-junction solar cells.

One of the key challenges with CZTS is its complex defect chemistry. The material is prone to the formation of secondary phases, such as ZnS and Cu₂SnS₃, which can act as recombination centers and degrade device performance. Additionally, disorder in the cation sublattice (Cu, Zn, Sn) leads to band tailing and potential fluctuations, reducing open-circuit voltage. Despite these issues, CZTS exhibits a degree of defect tolerance, meaning that moderate defect densities do not catastrophically impair performance. The current record efficiency for CZTS-based solar cells stands at around 12.6%, achieved through careful control of composition and annealing conditions. Further improvements may come from alternative synthesis methods, such as hydrazine-based solution processing or vacuum deposition techniques, which can enhance phase purity and crystallinity.

Antimony selenide (Sb₂Se₃) is another emerging material with favorable properties for photovoltaics. It has a near-ideal bandgap of approximately 1.1–1.3 eV, strong optical absorption, and excellent carrier transport characteristics. Like CZTS, Sb₂Se₃ is composed of earth-abundant elements, reducing concerns over material scarcity. The material crystallizes in a one-dimensional ribbon-like structure, which facilitates anisotropic charge transport. However, this anisotropy also presents challenges in device fabrication, as grain orientation must be carefully controlled to maximize charge collection.

Sb₂Se₃ has demonstrated promising defect tolerance, with experiments showing that grain boundaries are less detrimental to performance compared to other thin-film materials. The current record efficiency for Sb₂Se₃ solar cells is approximately 10.6%, achieved through optimized deposition techniques such as close-spaced sublimation and thermal evaporation. One of the primary limitations is the relatively low carrier lifetime, which restricts the achievable open-circuit voltage. Strategies to improve performance include interface engineering, doping, and the use of advanced buffer layers to minimize recombination losses.

Both CZTS and Sb₂Se₃ face common challenges in terms of scalability and stability. While laboratory-scale devices have shown reasonable efficiencies, translating these results to industrial production remains a hurdle. Stability under prolonged illumination and environmental exposure is another critical factor that requires further investigation. For CZTS, sulfur loss during high-temperature processing can degrade performance, while Sb₂Se₃ devices may suffer from oxidation or phase segregation over time.

Despite these challenges, the potential advantages of these materials make them compelling for future photovoltaic applications. Their reliance on abundant elements ensures long-term sustainability, while their defect-tolerant nature simplifies manufacturing compared to more demanding materials like perovskites. Continued research into compositional tuning, interfacial engineering, and advanced characterization techniques will be essential to push efficiencies closer to the theoretical limits.

The development of CZTS and Sb₂Se₃ also highlights broader trends in photovoltaics research, where the focus is shifting toward materials that combine performance with sustainability. As the demand for renewable energy grows, the ability to produce efficient solar cells without relying on scarce or toxic elements will become increasingly important. While silicon and perovskites currently dominate the market, emerging materials like CZTS and Sb₂Se₃ represent viable pathways toward a more diverse and resilient solar technology landscape.

In summary, CZTS and Sb₂Se₃ offer a compelling combination of earth-abundant composition and defect tolerance, positioning them as promising candidates for future photovoltaic applications. Although their efficiencies currently trail behind established technologies, ongoing research efforts aim to address their limitations and unlock their full potential. Advances in synthesis, defect control, and device architecture will be critical in determining whether these materials can transition from laboratory curiosities to commercially viable solar technologies.