CZTSSe Solar Cells: A Game-Changing Efficiency Milestone from China's CAS Researchers

In a landmark achievement for sustainable energy research, a team from the Chinese Academy of Sciences (CAS) has pushed the boundaries of thin-film photovoltaic technology. Led by Prof. CUI Guanglei at the Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT), the researchers reported a power conversion efficiency (PCE) of 15.45% for Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells, with independent certification confirming 15.04%. This breakthrough, detailed in Nature Energy on February 25, 2026, marks the first time open-circuit voltage (Voc) has exceeded 600 mV at a bandgap of 1.10 eV, addressing a persistent challenge in kesterite solar cell development.

This surge in efficiency underscores China's dominance in advancing earth-abundant materials for photovoltaics, positioning CZTSSe as a viable alternative to conventional silicon and toxic thin-film technologies. For academics and students exploring materials science and renewable energy, this development highlights the pivotal role of CAS institutes in fostering cutting-edge higher education research.

Understanding CZTSSe and Kesterite Solar Cells: The Basics

CZTSSe solar cells are based on kesterite-structured semiconductors, where copper (Cu), zinc (Zn), tin (Sn), sulfur (S), and selenium (Se) form Cu₂ZnSn(S_1-x,Se_x)₄. Unlike silicon panels, which dominate over 95% of the market with efficiencies around 22-26%, or cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) thin-films that rely on scarce or toxic elements, CZTSSe uses abundant, non-toxic materials. This makes it ideal for large-scale, low-cost production while minimizing environmental impact.

The photovoltaic process in CZTSSe involves absorbing sunlight to generate electron-hole pairs. These carriers are separated by the built-in electric field at the p-n junction, producing current. Efficiency is calculated as PCE = (Voc × Jsc × FF) / Pin, where Jsc is short-circuit current density, FF is fill factor, and Pin is incident power. Historically, CZTSSe lagged due to defects, but recent innovations are closing the gap.

Historical Challenges and the 'Efficiency Stagnation' Era

Kesterite solar cells emerged prominently around 2013 with efficiencies near 12%, but progress stalled at 12.6% for nearly a decade. Key issues included deep-level defects from Cu-Zn anti-site disorders (Cu_Zn), band tail states, and uncontrollable ion migration during selenization—the high-temperature step converting precursors to CZTSSe crystals.

During selenization (typically 500-550°C in Se atmosphere), metal ions like Zn²⁺ and Sn⁴⁺ migrate at different rates, leading to secondary phases like Cu₂Sn(S,Se)₃ (CTSSe) and poor grain uniformity. This caused severe Voc deficits—Voc far below the Shockley-Queisser limit—and recombination losses, capping PCE below commercial thresholds (usually >15-18% for thin-films).

• Cu-Zn disorder: Low formation energy promotes random cation placement, creating trap states.
• Bandgap-voltage offset: ~0.6-0.7 V loss, compared to 0.3 V in silicon.
• Scalability issues: Solution or vacuum deposition struggles with uniformity over large areas.

China's CAS teams have led efforts, with records progressing: 13.6% (IOP CAS, 2022), 13.8% (2023), 14.6% (2024), building to this 15%+ milestone.

The CAS Innovation: Li₂SnS₃ Interphase Strategy

The QIBEBT-CAS team introduced a Li₂SnS₃ (LTS) interphase to regulate grain growth. Here's the step-by-step process:

1. Precursor deposition: Solution-processed Cu-Zn-Sn-S precursors on Mo substrate.
2. LTS formation: Introduce Li-Sn-S via controlled annealing, forming LTS that encapsulates CTSSe grains.
3. Selenization: At 550°C in Se + SnSe vapor; LTS acts as rate-determining layer, slowing Sn⁴⁺ migration while facilitating Zn²⁺.
4. Result: Migration barrier difference drops from 0.41 eV (CTSSe) to 0.21 eV (LTS), enabling uniform, large grains (>1 μm) with fewer defects.

This 'interfacial phase equilibrium' balances kinetics, reducing deep defects by 50% and boosting minority carrier lifetime.

Affiliated with University of Chinese Academy of Sciences (UCAS), the team exemplifies how CAS labs collaborate with higher education institutions to drive innovation.

Photo by Ling App on Unsplash

Impressive Performance Metrics and Verification

The champion cell boasts:

• PCE: 15.45% (certified 15.04% by Fraunhofer ISE).
• Voc: 602 mV (record at Eg=1.10 eV).
• Jsc: 37.2 mA/cm².
• FF: 68.9%.
• Active area: 0.12 cm².

Stability tests show <5% degradation after 1000 hours damp heat (85°C/85% RH). Compared to prior CAS records (e.g., 14.9% certified from IOP CAS vacancy strategy), this LTS method excels in Voc and grain quality.

China's Leadership in Kesterite Research and Higher Education Ties

CAS institutions like QIBEBT and IOP, partnered with UCAS and universities such as Henan University and Qingdao University, form China's powerhouse for PV R&D. China produces 80%+ of global solar panels, and kesterite aligns with 'dual carbon' goals (carbon peak 2030, neutrality 2060). This record reinforces CAS's role in training PhD students via UCAS, producing experts in optoelectronics.Explore research positions in China's booming solar sector.

Recent papers from Nanjing University of Posts & Telecom (14.3% solution-processed) highlight university contributions.

Implications for Global Photovoltaics and Sustainability

At 15%+, CZTSSe nears CIGS (23% lab) viability. Potential: <$0.20/Wp manufacturing cost vs. silicon's $0.25/Wp, with flexibility for BIPV. Reduces reliance on Ag/In/Ga (supply risks). For China, scales 'Made in China 2025' green tech exports.

Stakeholders: Manufacturers eye pilots; policymakers support via subsidies. Challenges remain: large-area uniformity, encapsulation.

Future Outlook: Pathways to 20%+ and Commercialization

Team projects tandem cells (CZTSSe-perovskite, >25%). IP portfolio (LTS patents) paves industrialization. CAS aims for modules by 2028. Students: Join UCAS programs in materials science for hands-on PV research.China university opportunities

Photo by Noble Mitchell on Unsplash

Career Insights and Opportunities in Solar Research

This breakthrough opens doors for postdocs, faculty in thin-film PV. China's CAS labs offer competitive salaries (~¥500k/year for PIs), funding via NSFC. Explore higher-ed jobs, research roles, and career advice. Rate professors via Rate My Professor.