Stacking solar, setting standards

Solar photovoltaic installations on the cusp of significant performance boost

By David Appleyard

Rapid development of new materials and production techniques in solar photovoltaics (PV) is pushing both the limits of technology and relevant industry standards. With a swathe of new PV cell types emerging, the solar sector is set for a revolution. International Standards developed by IEC Technical Committee (TC) 82 and the IECEE certification scheme for PV help support this expansion.

First Solar CdTe thin-film module First Solar Cadmium Telluride (CdTe) thin film solar modules (Photo: First Solar)

Multitrack approach to better PV performance at lower cost…

New materials, new processes and new ideas continue to break records for solar photovoltaic technology cost and performance. In many places across the world, solar is already cost-competitive and the advances continue apace. In November, for instance, Germany’s Fraunhofer Institute for Solar Energy System (ISE) working with Austrian company EV Group announced that it had achieved a record 30,2% conversion efficiency with a multi-layered solar cell.

Perhaps though, the biggest breakthroughs are yet to come. A host of novel materials are set to emerge in the coming years that are widely anticipated to transform solar into the lowest-cost energy option on a dollar per kWh basis.

Principal ways in which researchers are addressing PV costs are the use of cheaper materials and achieving higher conversion efficiencies. 

…through different chemistries, the art of stacking…

Currently the solar market is dominated by crystalline silicon technology. A relatively expensive material, alternatives have already been successfully launched. Lower-cost, though typically lower-efficiency, thin-film technologies such as cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) are commercially mature. For example, in October, one of Jordan’s largest solar installations, the 52,5 MW Shams Ma'an plant featuring CdTe modules from First Solar Inc., was commissioned.

However, as the theoretical limits of conversion efficiency come closer for crystalline silicon and other cell chemistries, the next generation of PV technologies are looking at stacking cells in order to more effectively absorb a wider range of the electromagnetic spectrum.

Fraunhofer’s 30%+ efficiency cell is just such an arrangement, effectively a hybrid between a conventional crystalline silicon cell with three-semi-transparent thin-film solar layers above, including commercially emerging materials gallium-indium-phosphide (GaInP), gallium-arsenide (GaAs) cells. 

…and better tuning

By ‘tuning’ each of the cells to a specific range of receptive wavelengths, the overall range of the stacked cell is significantly improved, allowing more of the spectrum to be utilized.

Indeed, there are a host of novel photovoltaic materials and technologies under development. Some of the more promising that are suitable for this kind of stacking approach include perovskites, quantum dot and organic PV.

But, to achieve commercial success, prospective new semiconductor materials require both attractive conversion efficiencies and low-cost primary materials. Such materials must also be reproducible at an industrial scale while maintaining performance and multi-decade longevity. 

Large-scale low-cost manufacturing

Certainly, many of the new materials being explored today offer opportunities through the use of lower-cost primary materials, but there are also advances anticipated in large-scale, and therefore low-cost, manufacturing. Tom Aernouts, R&D team leader at the Belgian research and innovation hub imec, explains: “Mostly those technologies are thin-film technologies, so less materials and also mostly quite low-cost materials and also advantages in processing. Most are looking to solution processing with the idea of going to roll-to-roll processing in the future, but this will probably take some time before it really will be a commercial product.

“Nevertheless, solution processing potentially can make the processing low-cost with high throughputs at low temperatures. These are some of the claims that these technologies make and some of them have even shown that it is possible, and therefore can be at a competitive level – if the efficiency and stability become also competitive with silicon PV, which is still dominating the market.”

Tyler Ogden, Research Analyst at US-based market intelligence firm Lux Research Inc. highlights one emerging prospect: “One of the hottest topics – in terms of emerging technologies in the solar world – is perovskites in regards to their skyrocketing efficiency gains. They are currently at a record efficiency above 20%, which has caught up pretty rapidly to where the record efficiency for more mature thin-film technology such as CdTe currently are.”

“One of the major concerns is wanting to try to maintain a high efficiency in the production of modules, going from cell to module. But also insuring that these cells and modules are produced for commercial installation, meet reliability and durability standards set by crystalline silicon.” 

Scaling up is the challenge

This is a point echoed by Aernouts, who says: “For such a young technology, which has only been out there for a few years, the challenge will be to see if it can be up-scaled, if you can make modules out of that.”

He adds: “We think technologically it’s possible, especially with the fact that [perovskite] cells have already shown that efficiency can go up to 22% and potentially even higher. The potential is definitely there, the scalability is thought to be also possible.”

Most start-ups using perovskite technology are in the process of demonstrating that the technology has the potential to meet the same standards as the industry incumbents.

UK-based Oxford Photovoltaics, for example, is currently one of the more advanced players in the perovskite sector as, following an GBP 8,7 million (USD 10,4 million) funding round, it is now establishing a demonstration production line in Germany in order to showcase their technology in tandem with standard crystalline silicon technology.

This is a trend picked up by Ogden: “A lot of the research is based around pairing perovskite technology with existing silicon or even thin-films. After demonstrating core efficiency of perovskites alone, it has turned to looking at how we can integrate perovskites with industry incumbent technology.

“They’re aiming to pair up the technology and lead to a module achieving greater than 22% upwards to 25% or even higher and breaking the [theoretical] limit for silicon.”

Though positive about the promise of perovskites, Aernouts does sound a note of caution: “Lifetime is the next key issue. The stability is still to be improved. There are signs of improvement, but the initial stability was pretty poor on perovskite devices because of high sensitivity to the humidity, which really can deteriorate the quality of the active layer.” 

IEC Standards and Certification are central to solar development

Inevitably, where key considerations concern the performance and longevity of an electrical product, industry standards are a key requirement. IEC already has a number of appropriate instruments in place. A Technical Specification (TS) setting out general guidelines and recommendations for the design and installation of ground-mounted photovoltaic plants is under development by IEC TC 82: Solar photovoltaic energy systems.

For example, IEC 61215:2016 lays down requirements for the design qualification and type approval of terrestrial photovoltaic modules suitable for long-term operation and apply to all crystalline silicon terrestrial flat plate modules. 

Ogden emphasizes the importance of standards evolution: “These start-ups, they are holding their technology to the same standards as thin-films are held to under the IECEE certification scheme for PV – in terms of things like damp heat testing, UV exposure, temperature cycling. They’re aiming to meet the same type of standards that thin-film technologies like CIGS and CdTe have met in the past and led them to reach maturation.”

IECEE is the IEC System for Conformity Assessment Schemes for Electrotechnical Equipment and Components, it covers 23 categories of electrical and electronic equipment and testing services.

Ogden adds: “There is interest among developers of perovskites, for instance, or really any new or novel material, to develop new standards that better align with these emerging materials that don’t quite correspond with what we see currently in the industry.”

This is a point also noted by Aernouts: “If you look into the stacking aspect, I think that silicon and silicon standards are dominating. If you want to give a warranty on a stacked product, the perovskite part probably has to be able to withstand the same standards and testing also.”

However, he also emphasizes the challenge in developing new standards for emerging photovoltaic technologies: “Understanding the exact type of measurements that you need on the perovskite and on thin-film PV in general, the best way of measuring these devices and how much lifetime you can guarantee from this kind of accelerated lifetime testing, is not fully clear. That’s a crucial point to investigate and come to reasonably good standards, which might deviate quite a lot from conventional ones used for silicon PV.”

Gallery
Fraunhofer ISE silicon-based solar cell Fraunhofer ISE and EVG recently announced a record 30,2% conversion efficiency with a multi-layered solar cell (Photo: Fraunhofer ISE)
First Solar CdTe thin-film module First Solar Cadmium Telluride (CdTe) thin film solar modules (Photo: First Solar)
perovskite solar cells Perovskite technology is in the process of demonstrating that it has the potential to meet the same standards as crystalline silicon technology (Photo: EMPA)