On January 31, 2024, researchers at the Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) announced that together with perovskite developer Oxford PV, they had produced a full-scale perovskite tandem module with a conversion efficiency of 25%. At 421 W, the power of the double glass module is far from that of the large format modules produced by solar industry giants. Nevertheless, the result was a powerful demonstration of the steps being taken toward commercializing what is widely considered the next generation of solar cell technology.
Announcing the result, the Fraunhofer ISE team noted that scientists from the CalLab PV Modules calibration laboratory used a “multispectral solar simulator” to measure both the crystalline silicon solar cells and the perovskite cells. This allowed different light spectra to be applied to the cell while it was under continuous illumination. This required specialized measuring equipment based on LED light sources that could provide even illumination over the 1.68 m of the module.2 surface.
“The continuous intensity and spectral stability of the light source is of particular importance, especially for tandem devices,” says Johnson Wong, general manager for the Americas at equipment supplier Wavelabs. The researchers from Fraunhofer ISE used the Sinus-3000 Advanced LED module IV tester from Wavelabs for the Oxford PV module.
“Thanks to optimized light distribution over a long working distance, the tester light source is designed to cast a field of light at any point over the large module area that very accurately mimics the sun,” Wong adds. He said the Sinus-3000 LED tester exceeds the A+ class in terms of “spectrum, light uniformity and stability over time, which play a crucial role in measurement accuracy.”
Accurate characterization
The accurate characterization of perovskite solar devices requires not only new equipment, but also new processes. Longer lighting times are required; the temperature impact of the light source must be checked or corrected; IV sweeps should be significantly slower than in crystalline silicon cells; and in tandem cells their current must be aligned so as not to limit the combined power.
The PV research community, prospective manufacturers and equipment suppliers are making progress in overcoming the enormous challenges posed by perovskite solar installations. New, joint research projects are being launched and measurement routines are becoming increasingly advanced. As a result, there is growing confidence that the equipment and processes will be ready as future PV perovskite manufacturers develop their devices toward maturity.
Sunny prospects
Karl Melkonyan, PV technology analyst at S&P Global Commodity Insights, said perovskite tandems “have the best chances for commercialization” among next-generation solar cell technologies. Perovskite PV cells can be coupled with crystalline silicon (c-Si) or thin-film solar cells.
Early perovskite PV devices achieved conversion efficiencies in the low single digits: 3.8% was recorded in 2008. Record efficiencies are now achieved at regular intervals and are well above 25%.
Perovskite tandem devices are extremely promising, especially because the thin-film perovskite cell plus the “base” c-Si, cadmium telluride, or copper-indium-gallium selenide layer can capture different light wavelengths, resulting in small-scale research cells with efficiencies greater than 30% . %.
However, translating laboratory efficiency to larger cells and modules is difficult. “Although many record efficiencies have been achieved with perovskite solar cells reaching 20% and more, the total efficiency of a tandem structure can be much lower than the sum of those individual efficiencies,” says Melkonyan. He noted that the reason for this is often a current mismatch between bottom and top cells.
Measurement challenges
For a PV device to prove its worth, it must be possible to measure the power delivered in a highly accurate, replicable and standardized manner. Ultimately, when a PV module is to be purchased and installed, it is crucial that its rated power can be trusted.
Here, as noted in the recent Fraunhofer ISE and Oxford PV result, perovskite PV devices present a host of new challenges. “Yes, power measurement of a perovskite tandem or multi-junction cell presents challenges and can be quite difficult as it requires very specific spectrally adjustable solar simulators,” says Melkonyan. “Apart from suitable stabilization methods for different perovskite materials, the processes must include standardized protocols for measuring under standard test conditions.”
In late April 2024, Fraunhofer ISE, Oxford PV, Wavelabs and the University of Freiburg completed an eleven-month study into how to accurately characterize large-format perovskite tandem PV cells. Martin Schubert from Fraunhofer ISE led the project – abbreviated to “Katana” in German. He said there are two major differences between the characterization of perovskite tandem devices and regular PV modules.
Two factors
“One is that efficiency can change during illumination,” says Schubert, who leads the quality assurance, characterization and simulation team. “The reason for this is that an ion migration takes place in the perovskite cell in which some ions move. The second complication is the tandem architecture. In itself this means that we have two solar cells – one on top of the other and with different spectral sensitivity. We have to make sure that the top cell gets the right amount of power and the bottom cell gets the right amount of power.”
Ion migration within the perovskite device under continuous illumination means that the measured efficiency can increase or decrease over time. This “metastability” necessitates the long illumination time required to establish stabilized power delivery. Complicating matters further is that different perovskite-PV compositions exhibit different levels of metastability.
The need for prolonged light exposure to enable metastability introduces heat, even when using LEDs. This means that the measurement of perovskite devices is often performed at temperatures higher than standard test conditions (STC).
The output power of a photovoltaic device decreases as temperature increases, a factor described as a device’s temperature coefficient. Different PV technologies mean different temperature coefficients. For example, c-Si solar products have a larger temperature coefficient than thin-film devices. If this is not controlled and taken into account, measurement uncertainty arises.
Testing temperature-controlled equipment – essentially an air-conditioned room – can reduce this uncertainty in best-case scenarios. Such advanced devices, especially those with sufficient scale to house entire modules, come at a cost.
The impact of temperature can be corrected by using mathematical models based on accurate temperature measurements and can take into account the uncertainty that higher temperatures can bring. With tandem devices, the temperature sensitivity of both the top and bottom cells must be taken into account – a complex, if not impossible, equation.
Commercial implications
Currently, perovskite device testing is performed within minutes to account for metastability related to ion migration in the perovskite cell, so that slower IV sweeps, with multiple power point tracking (MPPT), can be performed. This is not suitable for mass production, because many modules have to roll off the production lines every minute.
Wavelabs’ Wong said a “more pragmatic testing routine” would likely first involve preconditioning the module using light exposure from mass-produced light sources. That could then be followed by “a rapid IV sweep using high-quality illumination that should fit within the specifications of spectral match, uniformity and stability,” Wong said. “The rapid IV sweep will likely be performed on the order of 100 milliseconds to one second, during which the ions are ‘frozen’ in their preconditioned distribution and do not redistribute significantly.”
Fraunhofer ISE will launch a three-year research project in May 2024 that will investigate how “fast and accurate measurements” can be developed and performed for perovskite devices, including tandems. The project, abbreviated to “PERLE” in German, will be funded by the German Federal Ministry of Economic Affairs and Climate Action. Fraunhofer ISE’s Schubert said it is possible that the project’s first findings will be published in May 2025.
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