In a groundbreaking advancement for flexible photovoltaics, researchers have unveiled a novel approach to significantly enhance the efficiency of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells and modules. Leveraging a sophisticated alkali-metal regulation strategy, the team has addressed the longstanding challenge of phase segregation that has hindered performance in these lightweight and portable solar technologies. The discovery highlights the complementary roles of sodium (Na) and lithium (Li) during the crystallization process, opening new avenues for high-efficiency flexible solar devices.
Flexible CZTSSe solar cells have garnered significant interest due to their potential for low-cost, lightweight, and versatile power generation suitable for a wide range of applications, including wearable technology and portable energy sources. However, despite their appeal, these devices have historically delivered suboptimal efficiencies. This limitation has primarily been attributed to uncontrolled alkali-metal incorporation during fabrication, which disrupts phase stability and electronic properties. The newly published research elucidates the dynamics underlying the interaction of alkali metals with CZTSSe crystallization, providing a roadmap for precise compositional and structural control.
Central to the study is the finding that the incorporation of Na into CZTSSe promotes crystal growth by facilitating grain boundary mobility and grain coalescence. However, this advantage is offset by the inadvertent enrichment of selenium (Se), which drives extensive phase segregation of tin selenide (SnSex), a detrimental secondary phase known to impede charge transport and increase recombination losses. The phase segregation has been a major bottleneck in achieving uniform and defect-free films necessary for high-performance photovoltaic devices.
The scientific team discovered that introducing Li alongside Na effectively reshapes the free-energy landscape governing the formation of copper (Cu)-related phases within the crystallization environment. In particular, Li incorporation drives the preferential formation of copper selenide (CuxSe), a phase that acts as a selenium sink, thereby consuming excess Se and suppressing the growth of harmful SnSex secondary phases. This kinetic competition mechanism balances the crystallization process, minimizing deleterious phase formation while supporting ordered phase evolution critical for optimal photovoltaic operation.
Such intricate control over phase formation marks a significant departure from traditional approaches that often focus solely on the thermodynamic stability of the final material. By exploiting kinetic competition between Cu- and Sn-containing phases mediated by alkali metals, the researchers achieved high-quality CZTSSe films characterized by large grain sizes, reduced defect density, and enhanced crystal ordering. This intricate balance consequently reduces charge recombination losses, a critical factor limiting the efficiency of kesterite photovoltaics.
The practical impact of these findings is underscored by the impressive power conversion efficiencies realized in flexible solar cells and modules fabricated using the dual-alkali-metal method. The flexible solar cells demonstrated a record certified efficiency of 14.2%, while larger-area shingled modules achieved certified efficiencies of 12.0%—figures that significantly advance the performance benchmarks for CZTSSe-based flexible photovoltaics. These achievements signal a new era of competitive, lightweight solar technologies with tangible commercial potential.
Furthermore, the study offers mechanistic insights with broad implications for multinary chalcogenide materials beyond the CZTSSe system. By demonstrating how alkali-metal-mediated kinetic competition can regulate complex crystallization pathways, the research lays foundational principles for tailoring material properties in other energy conversion and optoelectronic devices. This method can be generalized to control phase segregation and crystal quality in a range of multicomponent compound semiconductors, heralding transformative impacts across materials science.
The researchers employed a comprehensive suite of advanced characterization techniques to unravel the intricate interplay between Na and Li and their influence on CZTSSe film formation. These included high-resolution electron microscopy, compositional and phase mapping, as well as electronic property measurements to correlate structural features with photovoltaic performance. This multi-modal analysis was essential to reliably attribute the observed improvements to the dual alkali metal strategy.
Importantly, the role of Li in reshaping the local chemical environment extends beyond merely suppressing SnSex. By facilitating the formation of desirable CuxSe phases, Li alters the kinetics of elemental diffusion and phase nucleation, influencing the overall microstructural evolution of the film. This orchestrated modification highlights the nuanced interplay of competing reactions that can be strategically harnessed to fine-tune material properties at the nanoscale.
The research also sheds light on the fundamental thermodynamics and kinetics underpinning phase competition in complex solar absorber materials. It reveals that phase segregation, a commonly perceived thermodynamic inevitability, can be dynamically controlled through kinetic intervention. This challenges conventional paradigms and offers a powerful design principle for materials synthesis where phase purity and composition control are paramount.
Besides pushing efficiency records, the flexible CZTSSe modules fabricated in the study demonstrated excellent mechanical robustness, maintaining performance under bending stresses. This attribute is critical for real-world applications where flexibility and durability are required. The integration of dual alkali metals did not compromise mechanical properties, underscoring the viability of this approach in practical device architectures.
Moreover, the improved charge carrier dynamics enabled by reduced recombination translate into enhanced open-circuit voltages and fill factors, critical performance parameters for any photovoltaic technology. The suppression of detrimental secondary phases effectively mitigates non-radiative recombination pathways, boosting the overall device efficiency. These enhancements underscore the direct link between chemical phase control and electronic device performance.
Looking forward, this work sets the stage for further optimization and scaling of flexible kesterite technologies. Future research may explore the influence of other alkali or earth-abundant metals, multi-element doping strategies, and integration with complementary fabrication techniques to push efficiency and stability even higher. Additionally, the principles discovered here could inspire innovations in tandem solar cells and other novel devices combining CZTSSe absorbers with other functional layers.
This breakthrough exemplifies the power of combining chemical insights with materials engineering to tackle long-standing challenges in sustainable energy technologies. By harmonizing phase energetics and crystallization kinetics through targeted alkali metal mediation, the research offers a transformative pathway toward lightweight, efficient, and affordable solar power solutions. Such advances bring flexible kesterite photovoltaics closer to widespread deployment, with profound implications for energy generation in off-grid and mobile applications.
Ultimately, the findings highlight a promising kinetic competition strategy that transcends traditional materials design. This strategy, grounded in fundamental chemical physics and demonstrated through practical device fabrication, exemplifies a new paradigm for managing complexity in multinary chalcogenide semiconductors. Its successful application to flexible kesterite solar cells heralds a new chapter in photovoltaic materials research with far-reaching impacts on renewable energy technologies worldwide.
Subject of Research:
Flexible kesterite Cu2ZnSn(S,Se)4 (CZTSSe) photovoltaics, alkali-metal regulation, phase segregation control, crystal growth dynamics.
Article Title:
Alkali-metal-mediated control of phase segregation for flexible kesterite solar cells and modules with improved efficiency.
Article References:
Xu, X., Wang, J., Jiao, M. et al. Alkali-metal-mediated control of phase segregation for flexible kesterite solar cells and modules with improved efficiency. Nat Energy (2026). https://doi.org/10.1038/s41560-026-02018-5
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