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Crystallization changes in Kesterite solar cells continue to limit solar cell efficiency. A new study has introduced a thermal-decoupled selenization strategy that controls selenium delivery during absorber formation. A new approach has improved charge transport and enabled a certified power conversion efficiency of 15.3%. This significantly advances sustainable thin-film solar technologies. The approach has been written about in Nature Communications.
Study: Thermal-decoupled selenization enables kesterite solar cells with 15.3% certified efficiency. Image Credit: iknowartworks/Shutterstock.com
Crystallization Challenge in Kesterite Solar Cells
Kesterite solar cells have gained significant attention as sustainable thin-film photovoltaics, combining earth-abundant elements with promising photovoltaic performance.
Although advances in defect engineering and interface optimization have steadily improved device performance, heterogeneous crystallization during selenization remains a major obstacle to higher efficiencies.
Kesterite solar cells ideally form large columnar grains that grow from the top surface toward the back contact. In conventional processing, crystallization often begins simultaneously at the molybdenum back contact, creating upward-growing grains that collide with the downward-growing growth front. This interaction produces a bilayer grain structure with horizontal grain boundaries.
Researchers have explored precursor engineering, alkali-metal doping, and intermediate-phase control to improve film quality. These strategies have enhanced absorber properties but have failed to reverse crystallization, and their underlying cause remains poorly understood.
The study identifies the origin of reverse crystallization and proposes a thermal-decoupled selenization strategy to delay selenium delivery to the back contact. This approach alters the crystallization pathway, suppresses reverse crystallization, and enables the formation of high-quality columnar absorber structures.
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Designing a Thermal-Decoupled Selenization Process
The researchers developed a thermal-decoupled selenization strategy to control selenium delivery during absorber formation. In conventional selenization, rapid selenium evaporation exposes the entire absorber to selenium early in the process. Researchers replaced the graphite crucible with a boron nitride crucible to better control selenium delivery and to create a temporary concentration gradient within the film. This provided greater control over crystallization during the initial stages of absorber formation.
Researchers monitored temperature evolution and selenium evaporation in real time to understand how the modified process influenced film formation. They studied the entire process using advanced characterization techniques, including X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectroscopy, and electron microscopy, to track elemental distribution, phase evolution, and microstructural development during selenization.
The experimental work was supported by density functional theory calculations, which helped identify the thermodynamically favored reaction pathways at the CZTS/Mo interface under different selenium conditions. Lastly, solar-cell devices were fabricated using both conventional and thermal-decoupled absorbers. They were evaluated and compared to determine the impact of the new processing strategy on material quality and photovoltaic performance.
Improved Crystallization Delivers Higher-Efficiency Solar Cells
The study showed that selenium distribution during the early stages of selenization plays a decisive role in determining the final absorber structure. Under conventional processing conditions, selenium rapidly reaches the molybdenum back contact, where it promotes the formation of a low-melting Cu(S, Se) phase. This phase drives reverse crystallization, causing grains to grow upward from the back interface and resulting in defect-rich horizontal grain boundaries.
Combining theoretical and experimental analyses, the researchers identified low-temperature Cu(S, Se) formation as the key driver of reverse crystallization and abnormal grain growth. The thermal-decoupled selenization strategy altered this reaction pathway by delaying selenium exposure at the back interface. This prevented Cu(S, Se) formation during the critical early stage and promoted a more stable Cu2(S, Se)-mediated pathway.
As a result, reverse crystallization was suppressed and dense top-down columnar grains formed throughout the absorber layer. The improved crystallization behavior significantly enhanced film quality. The absorbers exhibited a uniform grain structure with minimal elemental segregation, fewer secondary phases, and no detectable horizontal grain boundaries. These structural improvements reduced non-radiative recombination, resulting in a nearly threefold increase in carrier lifetime from 3.3 ns to 8.9 ns.
Electrical measurements further revealed improved charge extraction and collection efficiencies, accompanied by a significant reduction in defect density from 3.4 × 10¹5 cm-³ to 2.0 × 10¹5 cm-³. Together, these improvements enabled more efficient carrier transport and reduced recombination losses within the device.
The improved absorber quality translated into significantly higher device performance.
The best solar cell achieved a power conversion efficiency of 15.7%, with an independently certified efficiency of 15.3%.
Gains in open-circuit voltage and fill factor highlight the effectiveness of controlling crystallization kinetics during selenization. The results rank among the highest reported for CZTSSe solar cells and demonstrate the potential of thermal-decoupled processing to advance sustainable thin-film photovoltaics.
Advancing the Future of Kesterite Solar Cells
The study sheds new light on the crystallization processes that limit kesterite solar-cell performance. The researchers identified low-temperature Cu(S, Se) formation as the trigger for reverse crystallization, resolving a longstanding challenge in CZTSSe absorber fabrication. They further showed that controlling selenium delivery can guide grain growth toward a more favorable structure.
The thermal-decoupled selenization strategy suppresses reverse crystallization, eliminates defect-rich grain boundaries, and produces high-quality absorbers with improved charge-transport properties. Beyond kesterite devices, the approach also improved the performance of Cu(In, Ga)Se2 (CIGS) solar cells, highlighting its broader applicability to chalcogenide thin-film photovoltaics.
The work demonstrates how precise control of reaction kinetics can complement materials engineering to overcome fundamental performance limitations. The findings provide a practical framework for developing more efficient, scalable, and sustainable thin-film solar technologies.
Journal Reference
Wu, Z., Wei, H., et al. (2026). Thermal-decoupled selenization enables kesterite solar cells with 15.3% certified efficiency. Nature Communications. DOI: 10.1038/S41467-026-74180-Z https://www.nature.com/articles/s41467-026-74180-z
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