G2V and NREL collaborate to validate LED solar simulators and improve multi-junction testing

When expert researchers, technology influencers and manufacturers gather at the annual IEEE Photovoltaics Specialists Conference (PVSC), it’s an exciting time for sharing developments and getting invigorated by the progress in the field of solar cell technology. As much as it’s a sharing of growth, however, it’s also a time for sharing problems, with the formation of partnerships to bridge gaps that would otherwise remain deep rifts.

The seed of such a partnership was planted at PVSC46 in 2019 when members of the National Renewable Energy Laboratory (NREL) met G2V Optics for the first time. NREL was intrigued by G2V’s Class AAA LED-based solar simulator technology as a path toward enhancing their testing capabilities and remedying some of their biggest pain points in solar cell testing.

NREL & G2V Optics Solar Simulation Collaboration

The initial ties were formed, and information passed back and forth. Eventually Dr. Nikos Kopidakis took over NREL’s PV Cell & Module Performance group, where he began looking for a large area solar simulator. More than this, however, Nikos was looking further into NREL’s future. He was interested in the longer-term opportunity for LEDs in solar simulation. Nikos believed in the promise of LED technology’s ability to activate and probe specific wavelength bands for validating increasingly-prevalent and rapidly-developing multi-junction solar cells. With this in mind, Nikos was eventually introduced by his predecessor to Dr. Ryan Tucker, CEO of G2V Optics, whose technology could do exactly what Nikos was after. For his part, Ryan believed there was nowhere else better in the world to test G2V’s solar simulators and correct any remaining issues to make their instruments the best in the industry.

Nikos and Ryan formulated a plan for NREL to evaluate one of G2V’s Sunbrick large-area solar simulators. This proposal promised to validate the performance of LED solar simulators against NREL’s calibration and characterization standards—the highest in the world.

While the COVID-19 pandemic introduced major delays in their plan, Nikos and Ryan nevertheless maintained contact and found a way to get safely back on the road to progress.

Testing with a Sunbrick™ Large-Area LED Solar Simulator

NREL has now received a Sunbrick that replicates sunlight from 350 nm to 1100 nm in a 625 cm2 area.

NREL staff will integrate the instrument into one of their photovoltaic characterization testbeds, as well as validate the temporal stability, spatial uniformity and spectral mismatch—all the aspects of what makes a solar simulator Class AAA. The first outcome of testing is to verify performance, lifetime and design updates of the Sunbrick solar simulator against two new standards governing the classification of solar simulators: ASTM E927-19 and IEC 60904-9:2020.

The IEC 60904-9:2020 standard, in particular, introduces a new A+ rating for every metric which requires a two-fold improvement in every area. This standard is expected to have a wide-ranging impact on the baseline expectations and caliber of solar simulators in the photovoltaics industry.

Before receiving the Sunbrick, Nikos and his team used a custom-built, gas-discharge-lamp-based solar simulator, known as the One-Sun Multisource Solar Simulator (OSMSS). The instrument uses two types of lamps as sources (xenon and tungsten), and a series of filters and free-space optics that couple nine bands via fibre optic into a homogenizer. While this delicate instrument has allowed them to construct custom spectra to test multi-junction cells, it is very large (requiring housing and optics for four separate gas-discharge lamps), contains many moving parts (apertures) that are prone to failure, and requires regular bulb replacement—for bulbs that are becoming increasingly difficult to source.

For Nikos, changing to G2V’s LED-based solar simulator is a paradigm shift.

“It feels like moving from an old-school CRT TV to flat screen technology. We have this massive volume of instrumentation for our old solar simulator, and in front of it sits the Sunbrick, tiny by comparison, but nominally able to achieve the same or better results,” says Nikos.

Beyond having a much smaller size, G2V’s LED-based solar simulator has bulbs with a much longer lifetime (on the order of 10,000s of hours compared to 1,000s for gas-discharge lamps), has no moving parts aside from cooling fans, and most importantly, has a design and base software technology perfectly suited to finely adjust the spectrum for custom illumination outputs.

G2V and NREL aren’t the only ones who see value and promise in the partnership. Their ties were strengthened through the Canada/NREL Cleantech Accelerator run by the Trade Commissioner Service with the objective to provide high-potential Canadian technology companies the opportunity to access NREL’s unique resources to accelerate growth to a global

3rd Party Evaluation of LED Solar Simulators

For G2V, the third-party evaluation of their technology is invaluable in determining whether the high standards they’ve set for themselves carry through to the most scrutinous end users. NREL is, after all, a calibration lab, publishing their regularly-updated chart of world-record holders for solar cell efficiencies of all technology types. They must hold themselves, and all the equipment they use, to a higher standard than anyone else in the world. They’re going to use G2V’s Sunbrick to understand the intricacies and opportunities that precision LED light sources have to offer, and G2V couldn’t be more excited for such an in-depth review as stated by Ryan:

“LED-based technologies have been a major positive disruption in conventional lighting and G2V has championed their use in the most demanding and precise applications, like aerospace testing, photovoltaics characterization, and advanced photobiology. The scientists at NREL are the absolute leaders in photovoltaic testing in our part of the world and we’re really excited to be working with them to validate our technology to a new level.”

For NREL there are many advantages to using LED solar simulators, but the prospect of moving toward a standalone multi-junction solar cell test station is the most exciting in the near term.
Once the Sunbrick’s performance and lifetime have been verified and validated against the newest standards, NREL can implement the Sunbrick in their multi-junction cell testing lab, taking representative solar cells and building spectra using the Sunbrick, ultimately assessing how well the system carries out the rigorous solar cell evaluations NREL does daily.

Multi-Junction Solar Cells

In multi-junction solar cells, you have between two and six absorbers, and to properly test each one, you need to ensure that each junction sees the same flux it would normally see in the standard solar spectrum. To carry out this test, you need to provide background illumination to turn on all junctions except for the one being measured, then probe with a separate light tuned for the particular junction of interest. In short, you need to fine-tune the output spectrum of your solar simulator, something that isn’t easy to do with gas-discharge-lamp technology.

For decades gas-discharge lamps have been the de facto standard for solar simulators. To do the above work with a gas discharge lamp, however, Nikos and his team carried out a painstaking process of adding optical filters sequentially until they mapped out the spectral response of the device under test. With LED technology, this level of filtering and mapping isn’t necessary—instead, individual LEDs can simply be turned on, off, or dimmed through software communication with the Sunbrick.

Summary

Extensive testing will be tremendously useful for both NREL and G2V, and the results will have an impact on the wider solar cell development and manufacturing community. The results will validate at the highest level the benefits that LED solar simulators have when replicating sunlight compared to gas discharge lamps, while doing away with some of gas discharge lamps’ major disadvantages such as the costly replacement and degradation of bulbs, the long warmup times, the need for expensive optical filters, and the excess heating of substrates that arises from undesired infrared peaks in the spectrum. Nikos and Ryan couldn’t be happier to start this next phase of their partnership. While this plan has been in development for a few years, the collaboration is only the beginning of what holds the promise to accelerate the development of all types of solar cells and other forms of sunlight-driven renewable energy conversion.
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