CZTSSe Solar Cells Breakthrough: CAS Researchers Achieve Record 15% Efficiency with Novel Strategy

Revolutionizing Thin-Film Photovoltaics: The Path to Record-Breaking CZTSSe Efficiency

In a landmark advancement for sustainable energy, researchers from the Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT) under the Chinese Academy of Sciences (CAS) have shattered efficiency barriers in CZTSSe solar cells. Copper zinc tin sulfur selenide (CZTSSe), a kesterite thin-film photovoltaic material composed of earth-abundant elements, has long promised low-cost, non-toxic alternatives to conventional silicon panels. The team's novel Li₂SnS₃ (LTS) interphase strategy has propelled device efficiency to 15.45%, with third-party certification confirming 15.04%—marking the first time open-circuit voltage (Voc) exceeded 600 mV at a 1.10 eV bandgap. 44 67

This breakthrough addresses a persistent bottleneck: uncontrollable cation migration during the selenization process, which causes defects and voltage losses. By engineering an interfacial phase equilibrium, the LTS layer acts as a selective barrier, balancing Zn²⁺ and Sn⁴⁺ migration rates. This controlled grain growth yields larger, defect-poor crystals, paving the way for industrial scalability. 44

Published in Nature Energy on February 25, 2026, this work underscores China's dominance in photovoltaic innovation, where the nation installed a record 315 GW of solar capacity in 2025 alone—over half the global total. 78 As thin-film technologies like CZTSSe gain traction for flexible, lightweight applications, this development could accelerate China's transition to carbon neutrality by 2060.

What Are CZTSSe Solar Cells? A Primer on Kesterite Photovoltaics

CZTSSe solar cells belong to the kesterite family of thin-film photovoltaics, with the chemical formula Cu₂ZnSn(S_xSe_1-x)₄. The sulfur-to-selenium ratio (x) tunes the bandgap from 1.0 eV (Se-rich) to 1.5 eV (S-rich), ideal for single-junction or tandem configurations. Unlike crystalline silicon (c-Si), which dominates 95% of the market with ~27% lab efficiency, CZTSSe uses abundant, inexpensive elements—no rare indium/gallium or toxic cadmium/tellurium as in CIGS or CdTe. 58

Deposited via solution processing (e.g., spin-coating precursors followed by selenization), CZTSSe enables low-temperature, roll-to-roll manufacturing suitable for flexible substrates like building-integrated PV (BIPV) or wearables. China's PV prowess, producing 80% of global modules, positions kesterite as a strategic asset amid silicon supply strains. 81

• Earth-abundant: Cu, Zn, Sn reserves exceed demand by orders of magnitude.
• Non-toxic: Avoids heavy metals, easing end-of-life recycling.
• Stable: Kesterite structure resists degradation better than perovskites.
• Cost-effective: Solution-based fabs cut capex vs. vacuum deposition.

Yet, commercialization lags: modules trail silicon's 22-24% efficiency. The CAS breakthrough targets this gap head-on.

The Core Challenge: Metal Ion Migration and Voc Deficit in CZTSSe

During selenization (annealing precursors in Se atmosphere at 500-550°C), CZTSSe forms via solid-state reactions: Cu-Sn-Se intermediates (CTSSe) react with ZnSe. However, Zn²⁺ (smaller, faster diffuser) outpaces Sn⁴⁺, causing Zn-rich surfaces, Sn vacancies, and Cu-Zn anti-sites—deep defects that trap carriers, slashing Voc. Typical Voc deficit (Eg/q - Voc) exceeds 0.7 V, vs. 0.4 V in c-Si. 58 59

This manifests as band tailing, non-radiative recombination, and poor fill factor (FF). Previous mitigations—doping (Ag, Ga), alloying—yielded incremental gains, topping 14.6% (certified 14.2%). 51 The CAS team reframed the problem: not suppress migration, but equilibrate it via interfacial engineering.

Real-world case: Early prototypes suffered 50% FF losses from MoSe₂ overgrowth at back contacts, exacerbating shunt paths.

Unveiling the LTS Interphase: Step-by-Step Novel Strategy

The innovation: Introduce Li₂SnS₃ (LTS)—a stable sulfide phase—as a passivation interphase during precursor deposition. Here's the process:

1. Precursor Prep: Mix Cu, Zn, Sn thiols in eco-friendly solvents; add Li source to form LTS nano-domains.
2. Deposition: Spin-coat on Mo-coated glass; LTS selectively coats CTSSe grains.
3. Selenization: Ramp to 550°C under Se/Ar; LTS raises Zn/Sn migration barrier parity (ΔE from 0.41 eV to 0.21 eV), slowing kinetics for uniform large grains (>1 μm).
4. Device Stack: CZTSSe/i-ZnO/CdS/i-ZnO/Al:ZnO grid.

DFT simulations confirm LTS encapsulates CTSSe, dictating diffusion. Result: Defect density drops 10x, minority carrier lifetime triples. 44

Read the full Nature Energy paper for simulations and JV curves. 67

Record-Shattering Performance Metrics and Validation

Champion cell (0.16 cm²): Voc=618 mV, Jsc=38.2 mA/cm², FF=65.3%, PCE=15.45%. Certified by Newport (15.04%). Large-area (1.05 cm²) hits 14.2%. EQE >80% (400-1100 nm), confirming low recombination.

TRPL shows τ=12 ns (vs. 4 ns baseline). XPS/STEM verify LTS persistence post-selenization. 44

QIBEBT CAS official site details precursor chemistry.

Photo by Markus Spiske on Unsplash

Spotlight on Prof. Cui Guanglei and CAS QIBEBT's PV Legacy

Prof. Cui Guanglei, group leader at QIBEBT's Biomimetic Solid-State Energy Lab, has pioneered defect engineering in chalcogenides. His team holds 20+ patents on LTS processes. QIBEBT, CAS's bioenergy hub, integrates PV with biofuels, aligning with China's "Dual Carbon" goals.

In higher education context, collaborations with Tsinghua University exemplify Sino-academic synergy: Tsinghua's perovskite records (26%+) complement kesterite for tandems. Aspiring researchers can explore research jobs in photovoltaics at top Chinese institutions.

China's PV Dominance: From Silicon to Next-Gen Thin-Films

China's solar ascent is staggering: 2025 capacity hit 1 TWh+, exports $50B. Yet, c-Si module prices crashed 50%, squeezing margins. Kesterite offers diversification—flexible, BIPV niches. CAS/IOP's prior 13.6% (2022) paved the way; this 15% leap signals modules >12% soon. 43

• 2026 forecast: 200-275 GW additions despite dip. 78
• Tsinghua/CAS tandems eyed for 30%+ eff.
• Policy: 14th FYP boosts thin-film R&D.

For career advice, check academic CV tips for PV roles.

Path to Industrialization: Scalability and IP Portfolio

LTS uses hydrazine-free solvents, slashing toxicity/cost. IP covers full chain: precursors to modules. Pilot lines at QIBEBT test 10x10 cm². Challenges: Uniformity over m². Solutions: Inline monitoring, AI-optimized annealing.

Stakeholders: JinkoSolar eyes kesterite lines; govt subsidies via MIIT. Global view: UNSW's 13.2% (2025) lags; EU/US seek supply chain resilience. 48

Global Implications: Accelerating Clean Energy Transition

CZTSSe enables off-grid, rooftop PV in China’s rural west. Tandems with perovskites (Tsinghua expertise) target 30% eff, halving LCOE. Reduces import reliance; aligns with Belt & Road exports. Environment: Avoids 1M tons mining waste/yr vs. CdTe.

Actionable: Universities ramp kesterite courses; explore China higher ed jobs.

Overcoming Hurdles: Future Roadmap for Kesterite Supremacy

Remaining: Jsc boost via back-surface passivation; module eff >13%. Outlook: 18% cells by 2028 via Ga/Ag co-doping. China’s 900 GW pipeline demands innovation; CAS leads with 20% R&D share.

Prospects bright for graduates: faculty positions in PV materials.

Photo by Markus Spiske on Unsplash

Conclusion: A Brighter, Greener Horizon

The CAS LTS breakthrough heralds kesterite's commercial dawn, blending high efficiency with sustainability. As China cements PV supremacy, global academia benefits. Stay ahead with Rate My Professor, higher ed jobs, and career advice. Share insights in comments—what's next for thin-films?