Breakthrough Achievement at AIST: Setting a New Benchmark in Solar Cell Efficiency
Researchers at Japan's National Institute of Advanced Industrial Science and Technology (AIST) have made headlines with a groundbreaking advancement in thin-film solar technology. Led by Shogo Ishizuka, the team has developed a copper gallium selenide (CGS, chemical formula CuGaSe2) solar cell achieving a certified power conversion efficiency (PCE) of 12.28%. This marks the highest efficiency ever reported for an indium-free wide-bandgap chalcopyrite solar cell in the 1.65 to 1.75 electron volt (eV) bandgap range.
The device, with an active area of 0.499 square centimeters, delivered an open-circuit voltage (Voc) of 0.996 volts, a short-circuit current density (Jsc) of 17.90 milliamps per square centimeter, and a fill factor (FF) of 68.8%. These parameters were independently verified by AIST's Photovoltaic Calibration, Standards and Measurement Team. This record surpasses the team's previous 2024 achievement of 12.25% and sets a new standard for indium-free alternatives to traditional copper indium gallium selenide (CIGS) cells.
Japan's push towards sustainable energy aligns perfectly with this innovation, as AIST continues to lead in photovoltaic research amid the nation's goals for carbon neutrality by 2050. For academics and researchers eyeing opportunities in Japan's vibrant solar sector, platforms like higher-ed research jobs offer pathways to contribute to such pioneering work.
What is Copper Gallium Selenide and Why Does It Matter?
Copper gallium selenide (CGS) belongs to the chalcopyrite family of semiconductors, structurally similar to CIGS but without indium (In). Chalcopyrite materials, named after the mineral CuFeS2, feature a tetragonal crystal structure that enables high absorption coefficients—around 105 cm-1—allowing thin films (1-2 micrometers thick) to capture most visible sunlight. CGS has a direct bandgap of approximately 1.68 eV, ideal for absorbing higher-energy blue and green photons while transmitting lower-energy red and infrared light.
The key advantage lies in its indium-free composition. Indium, a rare earth element, faces supply constraints due to demand in electronics like touchscreens and LEDs. By replacing indium with abundant gallium, CGS reduces costs and geopolitical risks, making scalable production feasible. Additionally, CGS exhibits excellent defect tolerance: even with polycrystalline grains (0.5-2 micrometers), recombination losses remain low, unlike silicon where single-crystal purity is crucial.
In Japan, where resource scarcity drives innovation, CGS supports the transition from silicon-dominated photovoltaics (currently ~95% market share) to diverse thin-film technologies. Universities like the University of Tokyo and Tohoku University collaborate with AIST, fostering a ecosystem rich in research positions—check Japan university jobs for openings in materials science.
The Innovative Fabrication Process Step-by-Step
AIST's record-breaking cell was fabricated on soda-lime glass (SLG) substrates coated with molybdenum (Mo) as the back contact. The CuGaSe2 absorber layer, 1.4-1.8 micrometers thick, was grown via a three-stage co-evaporation process in a vacuum chamber:
- Stage 1 (Early): Deposit Cu, Ga, Se vapors at 350-550°C substrate temperature, introducing aluminum (Al) and rubidium fluoride (RbF) to initiate doping.
- Stage 2: Continue Cu, Ga, Se evaporation, forming the bulk absorber with Cu-poor stoichiometry (Cu/(Ga+Al) ratio ~0.76-0.98).
- Stage 3 (Late): Additional RbF supply to refine surface properties and passivate defects.
A critical innovation is the steep Al concentration gradient: low (~0.06 atomic %) at the front surface for optimal bandgap, rising to ~1.25 atomic % at the rear to create a back-surface field (BSF). This electric field repels minority carriers (electrons) from the rear Mo interface, reducing recombination and boosting Voc and FF. Secondary ion mass spectrometry (SIMS) confirmed this gradient, while transmission electron microscopy (TEM) revealed Rb accumulation at the CdS interface without unwanted secondary phases like RbGaSe2.
Post-growth, a 150-nm cadmium sulfide (CdS) buffer layer forms the p-n junction via chemical bath deposition at 80°C. Sputtered intrinsic ZnO (50 nm) and aluminum-doped ZnO (300 nm) serve as the window layer, topped with Ni/Al grid electrodes and MgF2 anti-reflective coating. Mechanical scribing isolated the cell, minimizing edge recombination.
Performance Breakdown and Comparisons
The 12.28% PCE edges out prior CGS records (e.g., 9.5% in early reports) and aligns with Efficiency Tables Version 67 benchmarks for wide-bandgap chalcogenides. Compared to CIGS (lab record ~23.6%, commercial ~22%), CGS lags due to its wider bandgap reducing Jsc but excels in Voc potential (~1V). Silicon cells top ~27% single-junction, perovskites ~26%, but CGS shines in tandems: its 1.68 eV pairs with ~1.1 eV silicon bottoms for theoretical >30% PCE.
| Technology | Bandgap (eV) | Lab PCE (%) | Key Advantage |
|---|---|---|---|
| CGS (AIST) | 1.68 | 12.28 | Indium-free, tandem top |
| CIGS | 1.1-1.7 | 23.6 | High abs, flexible |
| Perovskite | 1.5-1.8 | 26 | Low-cost solution process |
| Crystalline Si | 1.1 | 27 | Mature, scalable |
External quantum efficiency (EQE) showed strong blue response, with external quantum efficiency (EQE) >80% up to 600 nm. Carrier density (NCV) ~1016 cm-3, depletion width ~0.8 micrometers. For Japan, this supports NEDO-funded projects aiming for 14 yen/kWh solar by 2030.Science Advances paper
AIST's Pivotal Role in Japan's Higher Education and Research Ecosystem
AIST, under the Ministry of Economy, Trade and Industry (METI), bridges academia and industry, collaborating with universities like Tsukuba and Tokyo Tech. Shogo Ishizuka, team leader of AIST's Compound Semiconductor Thin Film Team, has ~6000 citations in photovoltaics. Such institutes train PhD students and postdocs, fueling Japan's 400,000+ international students goal by 2030. Explore postdoc positions or research jobs at AIST-linked programs.
NEDO funds AIST's next-gen solar efforts, including tandems, aligning with Japan's 36-38% renewables by 2030. Universities contribute: e.g., Tohoku's HVPE for low-cost GaAs cells.
Pathway to Tandem Solar Cells and Beyond
CGS's wide bandgap positions it as the top absorber in tandems with silicon or perovskites, potentially exceeding Shockley-Queisser limit (33% single-junction). AIST plans prototypes on transparent substrates. Challenges include heat-light soaking (HLS) degradation—Voc/FF drop at 90°C due to metastable VSe-VCu defects, unlike CIGS's light-soaking gain.
Solutions: deeper defect passivation, edge sealing. For hydrogen production, CGS photoanodes could split water efficiently. Japan's GX strategy invests ¥2 trillion in green innovation.NEDO Next-Gen Solar
Challenges, Stability, and Scalability
While PCE impresses, stability under HLS reveals shorter carrier lifetimes (~1 ns vs. CIGS's 100+ ns) from deep defects (0.5-0.8 eV). Smaller grains increase recombination. Scalability requires roll-to-roll on flexible polyimide, AIST's forte. Cost: CGS uses earth-abundant Cu, Ga (from bauxite), Se (byproduct).
- Defect management: Alkali post-treatment.
- Interface: Thicker CdS reduces recombination.
- Production: Three-stage evaporation adaptable to inline.
Japan's Solar Research Landscape and Global Competition
Japan lags silicon (~10 GW installed 2025) but leads thin-film R&D. Competitors: China's kesterite 15.45% (CZTSSe), Uppsala CIGS 23.64%. AIST/NEDO target 30% tandems by 2030. Universities like Kyushu drive perovskites. For careers, academic CV tips help secure roles.
Future Outlook and Actionable Insights for Researchers
AIST eyes >15% CGS, tandem demos. Students: Pursue materials science at Tsukuba/AIST internships. Industries: Partner for pilots. Japan offers scholarships; see scholarships. This CGS leap accelerates net-zero.
In conclusion, AIST's record underscores Japan's ingenuity. Explore Rate My Professor, higher ed jobs, university jobs, career advice.
