In a groundbreaking advance that could significantly reshape the future of thin-film photovoltaics, researchers have unveiled a novel strategy to control grain growth in Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells, resulting in a new record for device efficiency in this promising material class. The research centers on the critical selenization reaction stage, a pivotal step that directly impacts the quality, homogeneity, and electronic properties of CZTSSe absorber layers. Achieving balanced migration kinetics of cations during this process has remained a formidable challenge, as imbalanced migration can foster deep-level defect formation, which severely degrades open-circuit voltage (Voc) and overall efficiency.
The breakthrough comes from introducing a Li₂SnS₃ interphase layer, which elegantly orchestrates the migration pathways of metal ions, particularly Zn²⁺ and Sn⁴⁺, during the selenization step. This interlayer specifically encapsulates Cu₂Sn(S,Se)₃ intermediate grains that evolve during selenization, effectively serving as a rate-controlling barrier that harmonizes the ionic fluxes. By modulating these pathways, the researchers were able to substantially reduce the difference in migration energy barriers between Zn²⁺ and Sn⁴⁺ ions from a stark 0.41 eV in Cu₂Sn(S,Se)₃ to a narrowed 0.21 eV in Li₂SnS₃. This balance mitigates the preferential migration of metal ions that typically results in non-uniform grain growth and defect clustering.
The consequences of this finely tuned ionic transport are profound. The presence of the Li₂SnS₃ interphase promotes the formation of larger, more uniform grains endowed with high crystallinity. These pristine grains minimize grain boundary-induced recombination, which often plagues kesterite thin films with lower efficiencies. The result is an absorber film with significantly improved chemical homogeneity and electronic properties, leading directly to enhanced device performance metrics.
One of the most remarkable outcomes reported is the device efficiency leap from 13.86% to 15.45%, with a certification confirming an efficiency of 15.04%. Such an improvement not only demonstrates the effectiveness of the interphase approach but also places CZTSSe solar cells within striking distance of commercial viability. Moreover, the open-circuit voltage achieved reached 602 mV at an optimal bandgap of 1.10 eV, indicating reduced voltage losses and affirming the quality of the absorber.
Understanding the selenization mechanism has been one of the thorniest problems hindering the progress of kesterite solar cells. The multi-cation diffusion process, involving copper, zinc, tin, sulfur, and selenium ions, is notorious for its complexity and susceptibility to kinetic imbalances. These imbalances lead to compositional inhomogeneities and defect formations, especially deep-level defects like antisites and vacancy complexes, which severely impair carrier collection. The introduction of Li₂SnS₃ interphase redefines this paradigm by serving as a controlled diffusion medium, effectively levelling the playing field for cation mobility.
From a materials science perspective, the choice of Li₂SnS₃ as an interphase material is compelling. It possesses a crystal structure compatible with the kesterite framework and exhibits suitable electronic and ionic conductive properties. The interphase does not merely block or slow down ion migration; rather, it selectively moderates the kinetics to facilitate synchronized migration of Zn²⁺ and Sn⁴⁺. This synergy prevents the aggregation tendencies of either cation, thus avoiding the formation of secondary phases or compositional gradients that can degrade device performance.
The research involved comprehensive characterization techniques to elucidate the function of the Li₂SnS₃ interphase. Advanced microscopy allowed the visualization of grain growth dynamics, while spectroscopic analyses confirmed reduced defect densities and improved stoichiometry. Electrical characterization substantiated enhanced carrier lifetimes and reduced recombination losses, correlating directly with the observed increase in open-circuit voltage and fill factor.
This innovation in selenization kinetics represents a crucial step toward resolving long-standing efficiency barriers in kesterite solar cells. The ability to regulate grain growth and transition from irregular to uniform, large grains redefines the microstructural quality achievable in CZTSSe absorbers. Given the earth-abundant and non-toxic elements involved in CZTSSe, such advancements significantly boost the material’s prospects for sustainable and cost-effective photovoltaic technologies.
The methodology developed also opens new avenues for broader application across related multinary chalcogenide systems where ionic migration imbalance limits device performance. By adapting similar interphase control strategies, other thin-film technologies could potentially benefit, enhancing grain growth uniformity and electronic quality to push efficiencies upward.
Beyond efficiency, the robustness and scalability of the Li₂SnS₃ interphase strategy are also promising. Since the Li₂SnS₃ layer forms naturally or can be deliberately engineered during precursor synthesis or early selenization stages, integration into existing manufacturing workflows could be achievable without exorbitant cost or complexity increases. This bodes well for accelerating the commercialization potential of kesterite photovoltaics.
Intriguingly, the study highlights the fundamental interplay between chemical diffusion kinetics and microstructural evolution, spotlighting how precise atomic-level engineering can unlock performance leaps in emerging solar technologies. The Li₂SnS₃ interphase functions as a microscale “traffic controller,” preventing congestion and disorder in the ionic migration highway during selenization, thereby enabling the formation of electronic “superhighways” in the resultant crystal grains.
This work underlines that controlling ion migration kinetics is just as critical as the materials’ intrinsic optoelectronic properties for achieving device-grade absorbers. Future research will likely explore fine-tuning the interphase composition and thickness to further optimize the balance of cation migration. Additionally, complementary doping strategies and interface engineering with adjacent layers may synergistically amplify the observed gains.
As the solar industry strives to move beyond silicon-based modules into innovative, low-cost alternatives, breakthroughs like this provide a crucial leap forward. The reported certified device efficiency exceeding 15% in CZTSSe solar cells stakes a significant claim for this absorber system in the competitive photovoltaic landscape. It paves the way for renewed interest and investment in kesterite solar cells, underlining the viability of earth-abundant, sustainable materials without compromising on performance.
In conclusion, the researchers’ novel Li₂SnS₃ interphase strategy addresses the nuanced challenge of cation migration imbalance during selenization in CZTSSe solar cells. By harmonizing Zn²⁺ and Sn⁴⁺ diffusion barriers, the approach enables superior grain growth, defect mitigation, and ultimately, substantial efficiency improvements validated by certified measurements. This advancement signals a major step toward transforming kesterite photovoltaics from a laboratory curiosity into a commercially competitive renewable energy solution slated for the future energy landscape.
Subject of Research: Kesterite thin-film solar cells, ion migration kinetics, selenization process, grain growth control
Article Title: Regulating grain growth via Li₂SnS₃ interphase in kesterite solar cells with certified efficiencies exceeding 15%
Article References:
Cui, C., Li, Y., Wei, H. et al. Regulating grain growth via Li₂SnS₃ interphase in kesterite solar cells with certified efficiencies exceeding 15%. Nat Energy (2026). https://doi.org/10.1038/s41560-026-01987-x
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