Perovskite-Silicon Tandem Cells: Longi's 35.5% Record and Qcells' IEC Certification in the Same Week

On July 14, Longi announced a 35.5% certified efficiency for a two-terminal crystalline silicon-perovskite tandem solar cell — the highest ever independently verified by the European Solar Test Installation (ESTI) at the European Commission's Joint Research Centre. Two days later, on July 16, Qcells announced that its perovskite-silicon tandem modules had received certification from TÜV Rheinland for compliance with IEC 61215 and UL 61215, the first time a tandem module has passed the full suite of reliability and safety requirements for commercial solar products.

Longi's 35.5%: The New Efficiency Benchmark

Longi presented the result on July 14 during its 2026 Solar and Storage Innovation Technology Conference at Shanghai Jiao Tong University. The 35.5% efficiency was independently certified by ESTI, improving on Longi's previous directly comparable lab record of 34.85%, announced in April 2025 and certified by the US National Renewable Energy Laboratory (NREL).

The progression is striking. Longi reported certified tandem efficiencies of 31.8% in 2023, 33.9% later that year, 34.6% in 2024, 34.85% in April 2025, and now 35.5% in July 2026 — roughly 0.7 percentage points per year of improvement while the cell architecture remained two-terminal. According to pv magazine, Longi did not disclose the active area or detailed electrical characteristics of the record cell, meaning the figure refers to a laboratory device, not a production-grade product.

Longi has also been developing larger-area tandem devices aimed at commercial formats. In June 2025, the company reported a 33% efficiency rating for a 260.9 cm² two-terminal cell certified by NREL. The gap between the 35.5% small-area result and the 33% large-area result illustrates the scaling challenge: larger cells introduce coating uniformity issues, higher series resistance from the transparent conductive oxide (TCO) layer, and increased recombination losses at defects. The company has not provided a timeline for commercial production of tandem cells; in June 2025, Longi stated it did not have an active mass-production plan for crystalline silicon-perovskite tandem cells.

The efficiency announcement was part of Longi's "Technology Forest" concept, which groups cell, module, and system technologies under a broader development framework. Technologies presented at the event included nano-alloy contact matrix (ACM), integrated conductive backsheet technology, shingled-cell interconnection, hidden busbars, temperature-controlled alloy interconnection, smart module technology, and the Longi One OS digital platform. Longi said ACM technology has already entered volume production at its 21 GW Xixian cell manufacturing facility.

Qcells' IEC Certification: The First Bankable Tandem Module

While Longi pushed the efficiency ceiling, Qcells addressed the other critical barrier: reliability. On July 16, the South Korea-based manufacturer announced that its perovskite-silicon tandem solar modules had received certification from TÜV Rheinland for compliance with both IEC 61215 and UL 61215 — the first time any company has achieved this for tandem technology, according to pv magazine.

The certified cells and modules were manufactured on Qcells' tandem R&D pilot line in Bitterfeld-Wolfen, Germany, using full-area M10 substrates (182 mm wafers). The company's 2-terminal tandem architecture combines a perovskite top device with its proprietary Q.antum silicon bottom cell. In December 2024, a cell built with this configuration reached 28.6% efficiency, independently verified by the CalLab at Fraunhofer ISE. In May 2025, the company revealed that modules built with the same technology had passed multiple IEC stress tests before the July 2026 formal certification.

"Solar today is the most affordable and fastest-to-deploy energy resource on the planet," Fabian Fertig, Head of Tandem R&D for Qcells Germany, told pv magazine. "Achieving these milestones means we are getting closer to delivering a solar product that more immediately addresses the issues we care about most — from cost to carbon."

Qcells announced plans to develop tandem PV technology in 2022, establishing its Thalheim R&D pilot line in partnership with the Helmholtz-Zentrum Berlin (HZB). The ~28.6% cell efficiency and now IEC 61215 certification represent significant progress toward production readiness, though the company has not announced a commercial launch date.

The Efficiency Roadmap: From 35.5% Lab Cells to 28%+ Commercial Modules

A critical distinction separates the Longi and Qcells announcements: Longi's result is a lab cell, while Qcells' certification covers modules built on production-scale pilot equipment. Module efficiency typically trails cell efficiency by 5–7 percentage points due to interconnection losses, encapsulation absorption, and active area ratio (the ratio of cell area to module area, including gaps between cells and frame margins).

Applying this gap to the current landscape:

• Longi lab cell: 35.5% (small area, ~1 cm²)
• Longi large-area cell: 33% (260.9 cm², Q2 2025)
• Estimated Longi commercial module: ~27–29% (using 7–8 pp derate from small-area cell)
• Qcells certified module: ~26–28% (based on 28.6% cell efficiency, ~2–3 pp derate for module-level vs cell-level on same architecture)

For context, the best commercial silicon-only modules — TOPCon and back-contact — currently operate at 23–24.5% module efficiency. A tandem module achieving 27%+ represents a 15–20% relative improvement in power density, which directly reduces balance-of-system (BOS) costs: fewer modules, less racking, less wiring, and lower installation labor per watt. According to BloombergNEF, a 15% improvement in module efficiency can reduce total installed system cost by approximately $0.03–0.05/W in utility-scale applications, primarily through BOS savings.

Closing the Reliability Gap: What IEC Certification Means for Tandem PV

The perovskite layer in tandem cells introduces failure mechanisms that crystalline silicon modules do not face. Perovskites are ionic compounds with relatively low formation energies — they degrade under combined heat, humidity, and light through ion migration, phase separation, and chemical reaction with encapsulant materials. According to research published by NREL, unencapsulated perovskite films can lose 50%+ of their efficiency within 500 hours under damp heat (85°C / 85% RH), though encapsulation strategies have improved dramatically since 2020.

Qcells' certification covers four stress tests that directly address these vulnerabilities:

• UV preconditioning (15 kWh/m²): Tests the perovskite's stability under ultraviolet light, which can excite ion migration and cause halide segregation in mixed-halide perovskites.
• 200 thermal cycles (−40°C to +85°C): Tests the mechanical integrity of the perovskite layer and its interfaces with the TCO and silicon bottom cell. Coefficient of thermal expansion (CTE) mismatch between layers is a primary failure mode.
• 10 humidity-freeze cycles: Tests moisture ingress resistance after freeze-thaw cycling, which can create micro-cracks in the encapsulation that accelerate degradation.
• 1,000 hours damp heat (85°C / 85% RH): The industry-standard accelerated aging test. Perovskite modules historically failed this test within 100–300 hours; passing 1,000 hours is a significant milestone.

TÜV Rheinland confirmed that Qcells' modules did not exceed the power degradation limits defined by the standards, which is typically 5% maximum power loss for IEC 61215 certification. The independent verification provides project financiers and insurers with the data they need to assign bankability ratings to tandem modules, which has been the technology's primary barrier to commercial deployment.

Competitive Landscape: Who Is Where on the Tandem Path

The tandem race is shaping up across three vectors: lab efficiency records, large-area cell development, and module-level reliability certification. Below is the current positioning:

Oxford PV, the UK-based startup that pioneered perovskite-silicon tandem development, commercialized the first tandem modules at ~26.9% in 2025 but at low production volumes. Their M6-based architecture (166 mm wafers) is an older wafer format that the industry is phasing out in favor of M10 (182 mm) and G12 (210 mm). Jinko announced a 33.2% tandem cell efficiency in April 2026 but has not disclosed module-level data or certification progress.

Pathway to Commercialization: 2027–2029 Outlook

The twin announcements in mid-July 2026 — a new efficiency record at Longi and the first module-level IEC certification at Qcells — suggest that tandem PV is following the same S-curve that silicon heterojunction (HJT) and back-contact cells followed in the 2019–2024 period: years of lab records, followed by pilot-scale reliability validation, then production-scale deployment.

Several milestones remain before tandem modules reach gigawatt-scale production:

1. Yield at scale. Perovskite deposition over large areas (G12-size, ~3 m²) with <1% coating variation remains challenging. Slot-die coating, meniscus coating, and vapor deposition are competing approaches; none has demonstrated production-scale yield above 95% at tandem-relevant quality levels.
2. Encapsulation cost. Tandem modules require barrier encapsulants with <10⁻⁸ g/m²/day water vapor transmission rate (WVTR) to protect the perovskite layer. These materials cost 2–3x more than standard EVA/POE encapsulants, adding approximately $0.02–0.03/W to module cost based on industry estimates.
3. Top electrode. The transparent conductive oxide (TCO) used as the top electrode — typically indium-tin oxide (ITO) — creates a cost and supply constraint. Indium is a scarce element, and tandem PV demand could strain supply by 2029 if production reaches 50+ GW annually. Alternative transparent electrodes (aluminum-doped zinc oxide, silver nanowire networks, and graphene-based TCOs) are in development.
4. Lifetime validation. The 25-year degradation guarantee that silicon modules carry requires real-world field data spanning multiple years. Accelerated testing (IEC 61215) is a necessary but not sufficient condition — Qcells' certification is the first step on a path that typically takes 3–5 years to establish full bankability ratings.

Analysts at BloombergNEF and the Fraunhofer ISE project that perovskite-silicon tandem modules will reach 5–10 GW of annual production by 2029, representing approximately 1–2% of total global PV production, with initial applications concentrated in utility-scale installations where higher efficiency translates to significant land and BOS savings (BloombergNEF, Fraunhofer ISE).

The week of July 14–16, 2026, may be remembered as the moment tandem PV passed through the chasm between lab and market. Longi showed the ceiling is still climbing. Qcells showed the floor — reliability — is finally solid enough to build on.

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