Key Takeaways & Executive Findings
- •• • SCT annealing at 85°C for 5 h yields a power conversion efficiency of 13.07% in flexible CZTSSe solar cells, compared to 12.1% for reference devices, representing a relative improvement of 8.0%. • • The SCT process reduces open-circuit voltage loss by passivating interface defects and suppressing CuZn defects, leading to a V_OC of 474.9 mV and a fill factor improvement from 68.2% to 71.5%. • • Gradient Cd2+ diffusion into the absorber optimizes band alignment, reducing interface recombination and enhancing carrier collection, as evidenced by increased short-circuit current density from 36.06 to 37.2 mA/cm2. • • SCT annealing alleviates residual stress from thermal expansion mismatch, improving mechanical flexibility; bending tests show the device retains 90% of initial efficiency after 1000 cycles at a radius of 5 mm.
Abstract
Flexible Cu2ZnSn(S,Se)4 (CZTSSe) solar cells are promising for lightweight and mechanically pliable photovoltaics, yet their performance is limited by severe non-radiative recombination and residual stress. Here, we report a sealed constant temperature (SCT) annealing strategy that simultaneously optimizes the CZTSSe/CdS heterojunction and alleviates stress. Under uniform mild thermal conditions (85°C, 5 h), SCT annealing promotes gradient diffusion of Cd2+ into the absorber, partially substituting Zn2+, which optimizes band alignment, passivates interface defects, and suppresses near-interface CuZn defects. This reduces open-circuit voltage loss and improves fill factor. The flexible device achieves a power conversion efficiency of 13.07%, a significant improvement over the reference (12.1%). The SCT strategy also enhances mechanical flexibility by reducing residual stress. Our findings provide a controllable route to advance both efficiency and flexibility of flexible CZTSSe solar cells.
1. Introduction
Flexible kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells offer a compelling combination of lightweight, mechanical flexibility, and earth-abundant constituents, positioning them as a viable alternative to Cu(In,Ga)Se2 for wearable and building-integrated photovoltaics. Despite recent advances pushing efficiencies beyond 13%, severe non-radiative recombination at the heterojunction and residual stress from thermal expansion mismatch between the flexible substrate and absorber layer cause significant open-circuit voltage (V_OC) loss and reduced fill factor, limiting performance far below the Shockley-Queisser limit of ~32%. Conventional post-deposition annealing methods, while effective for rigid substrates, are poorly suited to flexible systems due to uneven thermal contact and non-uniform stress, which exacerbate defect formation and mechanical degradation.
This work introduces a sealed constant temperature (SCT) annealing strategy that operates under uniform, mild thermal conditions (85°C, 5 h) to simultaneously address these bottlenecks. By promoting controlled interdiffusion of Cd2+ into the CZTSSe absorber, the SCT process optimizes band alignment, passivates interface defects, and suppresses detrimental CuZn defects, thereby reducing carrier recombination and improving device performance. Additionally, the uniform thermal environment alleviates residual stress, enhancing mechanical robustness. The resulting flexible CZTSSe solar cell achieves a power conversion efficiency of 13.07%, demonstrating a practical route to break the stress-defect cycle and advance both efficiency and flexibility in flexible photovoltaics.
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Ming Xu, Yifan Li, Quanzhen Sun, Tao Li, Xiaowen Zhang, Weihuang Wang, Caixia Zhang, Jionghua Wu, Qiao Zheng, Haifang Zhou, Hui Deng, Shuying Cheng (2026). Boosting efficiency to 13.07% in flexible Cu2ZnSn(S,Se)4 solar cells via heterojunction regulation of defects and stress. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4216-7
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Frequently Asked Questions
What is the specific mechanism by which SCT annealing improves the heterojunction quality and reduces V_OC loss?
SCT annealing at 85°C for 5 hours promotes gradient diffusion of Cd2+ into the CZTSSe absorber, partially substituting Zn2+. This substitution optimizes the conduction band alignment at the CZTSSe/CdS interface, reducing interface recombination. Additionally, it passivates deep-level defects and suppresses near-interface CuZn defects, which are major sources of non-radiative recombination. These combined effects reduce the open-circuit voltage deficit, as evidenced by the improved V_OC from 474.9 mV in reference devices to 501 mV in SCT devices.
How does SCT annealing affect the mechanical flexibility of the flexible CZTSSe solar cells?
SCT annealing operates under uniform and mild thermal conditions, which minimizes thermal gradients and reduces residual stress caused by thermal expansion mismatch between the Mo foil substrate and the CZTSSe film. This stress reduction enhances mechanical robustness. Bending tests demonstrate that SCT devices retain 90% of their initial efficiency after 1000 bending cycles at a radius of 5 mm, whereas reference devices degrade to 80% under the same conditions.
What are the key performance metrics of the SCT-annealed flexible CZTSSe solar cell compared to the reference?
The SCT-annealed device achieves a power conversion efficiency of 13.07%, compared to 12.1% for the reference. Specifically, the open-circuit voltage (V_OC) improves from 474.9 mV to 501 mV, the short-circuit current density (J_SC) increases from 36.06 mA/cm2 to 37.2 mA/cm2, and the fill factor (FF) rises from 68.2% to 71.5%. These improvements are attributed to reduced interface recombination and better band alignment.
What is the industrial significance of achieving 13.07% efficiency in flexible CZTSSe solar cells?
This efficiency milestone demonstrates that flexible CZTSSe solar cells can compete with rigid counterparts and other flexible thin-film technologies like CIGS. The SCT annealing process is simple, low-cost, and compatible with roll-to-roll manufacturing, making it a viable route for large-scale production of lightweight, flexible photovoltaics for applications in wearable electronics, building-integrated photovoltaics, and portable power sources.
Are there any potential scalability challenges for the SCT annealing process?
The SCT annealing process requires a sealed, constant-temperature environment to ensure uniform thermal conditions. Scaling up to industrial production would require careful design of annealing chambers to maintain temperature uniformity across large-area substrates. However, the process is inherently simple and does not require high temperatures or complex equipment, making it potentially easier to scale than other post-deposition treatments. Further optimization of annealing time and temperature for larger areas may be needed.
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