The most efficient solar architecture ever demonstrated at commercial scale has spent years trapped between a laboratory breakthrough and a bankable product, blocked not by the cells themselves but by the absence of a measurement infrastructure that investors, insurers, and grid operators require before they will commit capital. On July 7, Germany’s Institute for Solar Energy Research Hamelin announced that its Calibration and Test Center — ISFH CalTec — had formally expanded its accredited calibration services to cover large-area perovskite-silicon tandem solar cells up to the G12 wafer format, measuring 210 by 210 millimeters, according to ISFH CalTec’s expanded calibration service. That announcement removes one of the quietest and most consequential bottlenecks in the global energy transition.
The development directly affects solar manufacturers, project developers, institutional investors financing utility-scale solar, and the insurers who underwrite performance guarantees on projects worth hundreds of millions of dollars. Without independently verified, traceable efficiency data from an ISO/IEC 17025-accredited laboratory, a technology cannot achieve what the solar finance industry calls bankability — the quality of being acceptable to lenders and investors as the basis for long-term project financing. Until now, Europe had no accredited calibration pathway for large-area tandem devices. CalTec has one.
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Perovskite-silicon tandem solar cells work by stacking two absorber layers with different bandgaps — a perovskite top cell that captures high-energy light from the blue end of the spectrum, and a silicon bottom cell that harvests lower-energy photons toward the infrared. Together they capture a far wider slice of the solar spectrum than either material can manage alone, which is why the architecture has produced efficiency figures that break a fundamental constraint of single-junction solar technology.
That constraint is the Shockley-Queisser limit, first calculated by William Shockley and Hans-Joachim Queisser in 1961: a theoretical ceiling of approximately 33.7% for any single-junction solar cell under standard illumination. LONGi’s NREL-certified record of 34.85% was achieved in April 2025 — comfortably above that ceiling. The theoretical maximum for a two-junction tandem is approximately 43%, meaning the technology still has enormous headroom.
Those efficiency numbers carry commercial and legal weight. Project lenders require independently verified efficiency data to model energy yield and calculate financial returns. Insurers writing performance guarantees need the same. Grid operators planning capacity additions depend on certified figures to assess generation projections. The chain from a laboratory record to a financed gigawatt-scale factory runs directly through an accredited calibration laboratory — and the link connecting European tandem manufacturers to that chain did not fully exist before this week.
PV Tech’s report on CalTec’s expansion records Karsten Bothe, head of the ISFH CalTec solar cell calibration laboratory, as saying: “Tandem solar cells are much more complex than conventional silicon cells. Specialised calibration is needed to ensure that reported efficiencies are accurate, comparable, and trusted by researchers, manufacturers, and investors. We provide the measurement confidence needed to move tandem photovoltaics from laboratory breakthroughs to commercial products.”
The reason a standard silicon calibration laboratory cannot simply extend its services to tandem cells is architectural, not administrative. A conventional silicon solar cell is a single p-n junction that responds to one continuous range of photon energies. Measuring its efficiency involves exposing it to a calibrated light source and recording how much electrical power it generates — a well-understood procedure codified in IEC 60904 and performed reliably for decades.
A two-terminal perovskite-silicon tandem is structurally different in every way that matters for measurement. The two absorber layers are connected in series on a single substrate, which means their electrical currents must match — the layer producing less current limits the output of the entire device, regardless of how efficiently the other layer performs. Accurately measuring a tandem cell’s real efficiency therefore requires controlling and correcting the test light source’s spectral distribution separately for each subcell, a process called subcell-specific spectral mismatch correction. Without it, the measurement produces an artifact rather than a true efficiency value.
Scale compounds the problem. A laboratory research cell occupies roughly one square centimeter. The G12 format that CalTec now covers spans 210 by 210 millimeters — more than 400 times larger. At that scale, effects that are negligible on a tiny lab sample become meaningful sources of measurement error: the shading cast across the cell surface by the metal contact grid that carries current away from the device, variations in light intensity across the measurement plane, and temperature gradients that affect performance readings across a large area.
The fifth challenge is intrinsic to perovskite as a material. Perovskite absorbers exhibit metastability — a tendency for their current-voltage characteristics to shift transiently under illumination, so that a measurement taken too quickly or too slowly captures an artifact of the measurement sequence rather than the cell’s true steady-state output. The International Electrotechnical Commission has issued IEC TR 63228 advisory guidance on power-rating procedures for metastable devices, but no consensus standard has yet been codified specifically for tandem formats at commercial scale. CalTec’s expanded service addresses all five of these challenges in a single accredited framework.
ISFH CalTec holds CalTec’s DAkkS accreditation under ISO/IEC 17025 from the German Accreditation Body, DAkkS, which is a signatory to the International Laboratory Accreditation Cooperation’s Mutual Recognition Arrangement. That arrangement means CalTec’s measurement certificates carry the same formal standing in international project finance and insurance workflows as those from NREL in the United States or Fraunhofer CalLab in Germany — the two laboratories most frequently named as required independent verification sources in international solar procurement agreements.
This comparability matters at a moment when the efficiency race for perovskite-silicon tandems is running on multiple national tracks simultaneously. LONGi’s 34.85% record was NREL-certified. JinkoSolar’s June 2026 announcement of a competing 34.82% record was certified by the Shanghai Institute of Microsystem and Information Technology under China’s Academy of Sciences — a nationally accredited body, but one that does not carry the same international procurement standing as NREL or Fraunhofer CalLab. CalTec’s expansion means European manufacturers now have a primary reference laboratory that fills the same role for their efficiency claims in international markets.
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The solar finance industry has a specific requirement for new technology: efficiency data that is traceable to a recognised national metrology institute through an unbroken chain of calibrations, measured by a laboratory whose competence has been independently assessed. That requirement is not optional. Lenders financing a utility-scale project that will operate for 25 years need to know that the energy yield projections underpinning their financial models are based on measurements that can be compared across time, across laboratories, and across jurisdictions.
CalTec’s track record of 27 world-record verifications for conventional silicon provides the institutional credibility behind this expansion. Since receiving its silicon calibration accreditation in 2016, it has served more than 70 companies from 20 countries. Since 2018, when it gained authorisation to independently verify photovoltaic conversion efficiencies, it has certified 27 world-record silicon solar cell efficiencies, including results from Trinasolar, LONGi, and JinkoSolar. The new tandem calibration service builds on that standing rather than starting from scratch.
The methodology presented at the tandemPV Workshop in Berlin last month was designed specifically to address the measurement challenges described above, using a dedicated LED-based solar simulator capable of independently controlling the spectral output for each tandem subcell — the essential hardware requirement for subcell-specific spectral mismatch correction. Academic research in Progress in Photovoltaics published in 2025 confirmed that fully traceable efficiency determination for large-area perovskite-silicon tandem modules requires exactly this type of LED-based multi-spectral approach, and that the measurement procedure must account for transient device effects arising from perovskite metastability to produce reliable results.
The G12 format is not arbitrary. It is the standard wafer size adopted by LONGi, JinkoSolar, and several other volume silicon manufacturers who are now designing tandem production capacity around their existing silicon infrastructure. By calibrating at G12 scale, CalTec’s service is directly applicable to the cell dimensions at which commercial tandem manufacturing is being engineered.
The calibration announcement arrives at an inflection point. Oxford PV’s September 2024 commercial launch announcement marked the world’s first commercial sale of perovskite-silicon tandem panels: 72-cell perovskite-on-silicon panels producing 24.5% module efficiency shipped to a US utility customer from its pilot production line in Brandenburg an der Havel, Germany. The company holds the module efficiency record at 26.9%, certified by Fraunhofer CalLab. Oxford PV targets gigawatt-scale manufacturing to come online in 2026 to 2027.
Hanwha Q CELLS’ December 2024 world-record efficiency reached 28.6% on a full-area M10-sized perovskite-silicon tandem cell, verified independently by the CalLab at Fraunhofer Institute for Solar Energy Systems, and the company subsequently passed stress tests required for IEC and UL certification — becoming the first tandem manufacturer to do so. The company targets mass production in the first half of 2027.
The gap between cell-level records and module-level performance remains one of the defining unsolved challenges for the entire field. Oxford PV’s 26.9% module efficiency illustrates how much efficiency is lost when a record-setting small-area cell is manufactured at the scale of a commercial panel — a gap of 2 to 8 percentage points depending on the device. Closing that gap reliably, while meeting durability requirements, is the manufacturing challenge that stands between current pilot lines and volume commercial deployment.
Durability is the other open question. Silicon solar panels carry 25-year warranties backed by decades of field data. Perovskite cells degrade under heat, moisture, and ultraviolet exposure in ways that silicon does not, and the longest published operational data for perovskite-based tandem devices remains approximately 1,000 hours — roughly 100 times shorter than commercial silicon requirements. Helmholtz-Zentrum Berlin’s June 2026 study identified three primary degradation mechanisms in perovskite cells aged under outdoor conditions: halide phase segregation, copper corrosion, and edge pattern formation — and found that no existing accelerated test standard reliably replicates all three. For all of that to be verified, compared, and ultimately insured, traceable measurement infrastructure is the prerequisite — which is precisely what CalTec’s July 7 announcement provides.
Not entirely. CalTec’s expansion resolves the measurement and calibration gap, but three additional infrastructure layers remain before perovskite-silicon tandem technology achieves the same commercial standing as silicon. A harmonised accelerated stability test standard — the equivalent of IEC 61215 for silicon, but specifically designed to capture perovskite-specific degradation mechanisms including ion migration and halide phase segregation — has not yet been codified by the IEC. Module-level performance certification across multiple product configurations and form factors remains incomplete even for the most advanced players. And the bankability gap that thin-film technologies such as CdTe faced a decade ago — convincing lenders and insurers to accept 25-year performance guarantees on a technology with less than five years of commercial field data — has not yet been closed.
What the CalTec announcement does close is the measurement foundation on which all of those next steps rest. Stability standards require measurements. Module certification requires accredited efficiency data. Bankability arguments begin with independently verifiable performance figures. The laboratory infrastructure that CalTec has now established for large-area European tandem devices is the precondition for each of those subsequent steps to be taken on internationally comparable terms.
ISFH has emphasised that robust and standardised calibration procedures will play an increasingly important role as perovskite tandem technology transitions to industrial manufacturing, providing a basis for comparable measurement results and support for commercialisation globally. What it has also, more quietly, done is ensure that European manufacturers entering the commercial race — whether from Oxford PV’s pilot line in Brandenburg or from facilities yet to be built — now have access to the same quality of independent certification that has anchored investor confidence in silicon PV for three decades.
Oxford PV made the first commercial sale of perovskite-silicon tandem panels in September 2024, shipping 72-cell panels at 24.5% module efficiency to a US utility customer. That represents the start of commercial availability at limited pilot scale. Hanwha Q CELLS has targeted mass production in the first half of 2027, beginning in Korea and later at its US facility in Cartersville, Georgia. Broader commercial availability at volume — with the 25-year warranties that major project developers require — depends on closing two additional gaps: a harmonised IEC stability test standard for perovskite-specific degradation, and enough accumulated field data to underwrite long-term performance insurance. Industry analysts generally expect that threshold to be crossed between 2028 and 2030.
Project lenders and insurers base their decisions on independently verified efficiency data from laboratories with formal accreditation under ISO/IEC 17025 — the international standard for testing and calibration competence. Without that accreditation, a manufacturer’s efficiency claim, regardless of how credible the underlying measurement, is not acceptable as a basis for long-term project finance. CalTec’s expansion means that European manufacturers of perovskite-silicon tandem cells now have access to the same calibration standing — through DAkkS accreditation and the ILAC Mutual Recognition Arrangement — that has supported bankable silicon PV projects for years. An investor can now require an ISFH CalTec measurement certificate for a tandem cell the same way they would for a silicon cell.
Three factors combine. First, a two-terminal tandem has two series-connected subcells responding to different parts of the solar spectrum, so the test light source must be calibrated separately for each subcell — a process called subcell-specific spectral mismatch correction that conventional solar simulators are not designed to perform. Second, scaling from a 1-square-centimeter lab cell to a 210-millimeter commercial wafer introduces error sources — contact grid shading, irradiance non-uniformity, and temperature gradients — that are negligible at small scales but significant at commercial size. Third, perovskite materials are metastable: their electrical output shifts transiently under illumination, so the measurement protocol must be designed to capture a true steady-state value rather than a transient artifact. CalTec’s expanded service addresses all three.
LONGi’s 34.85% world record and JinkoSolar’s 34.82% result are cell-level measurements certified by NREL and a Chinese national lab respectively. CalTec fills the same reference-laboratory role for European manufacturers: its DAkkS accreditation under the ILAC Mutual Recognition Arrangement means its measurement certificates carry the same standing in international procurement as NREL or Fraunhofer CalLab certifications. A future European tandem efficiency record certified by CalTec would carry the same formal weight as a record certified by those laboratories — enabling European manufacturers to compete in global markets on the same measurement terms.
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