What’s Cool in Solar: Cells

Solar as a technology is full of surprises. When a particular solar component or application is thought to have reached its technical limitations, often some new approach leads to increased optimisation.

SolarPower Europe has examined the latest solar technology developments that can reduce overall system cost and thus lead to increased deployment.

PERC – the workhorse

With basically all the recent gigantic solar cell manufacturing capacities based on Passivated Emitter Rear Contact (PERC), this cell architecture has become the state-of-the-art cell technology. A considerable price drop for PERC production equipment paved the way for multi-GW scale expansion in China. PERC has progressed at a rapid pace in terms of efficiency improvement of greater than 0.5% absolute per year. While the pace has slowed down, several manufactures have already exceeded 23% efficiency in commercial production of PERC cells. With PERC technology being flexible towards production using larger wafers, it has further strengthened its position. Another bonus is its bifaciality, as it is very simple to tweak PERC into a cell that produces power on both sides without any additional costs. With that said, the bifaciality of this technology is on the lower side of crystalline solar cell varieties. Another interesting development that is relevant for PERC is gallium doping technology. Ingots for monocrystalline wafers are historically mostly positively doped (p-type) with boron, which is the root cause for a degradation mechanism, light induced degradation (LID). Negatively doped wafers (n-type) do not suffer from this issue. Employing gallium instead of boron liberates p-type from this inherent disadvantage, which has started to be used more frequently.

Today PERC has everything: a well-established supply chain, high throughput, efficient production equipment, and compatible process consumables. The technology is at its peak in terms of process optimisation, providing the best cost performance ratio today. But PERC is expected to hit its limits; there is no clear pathway to improve the efficiency beyond the current level of a little over 23% in industrial production environment. While record PERC cell efficiencies reach around 24%, these production practices are not cost effective for mass production; at least not yet. Now the big question is: What comes next to bring cell efficiencies to a higher level?

Passivated Contacts – a prominent upgrade

The next evolutionary step in solar cell technology following PERC is likely to be Passivated Contact cells, often referred to as TOPCon (a passivated contacts technology developed at Germany’s Fraunhofer ISE), where a sophisticated passivation scheme is adapted to advance cell architectures with an aim to reduce recombination in the electrical contact region. Implementing TOPCon requires only a few additional processing tools over PERC. Theoretically speaking, TOPCon shows the highest ultimate efficiency potential of all c-Si cells at 28.75%. But in practice research institute ISFH has attained a record level performance of 26.1% by combining its proprietary POLO structure with a back contact architecture on a p-type base wafer in lab scale. In the industrial environment, there has been a tight race for record efficiencies in recent times. In June 2021, JinkoSolar announced a 25.25% world record efficiency for commercial-size cells. When it comes to industrial implementation, the technology has only a handful of followers so far, as there were issues with certain production equipment. Now, not only workarounds and new tools have been developed to overcome those limitations, the machines are also capable of processing larger wafers. With these developments in place, TOPCon is seeing new traction. At the world’s largest solar trade fair SNEC 2021, several leading cell and module makers unveiled TOPCon products. The pioneer in this field, Jolywood, has recently developed a second generation of TOPCon technology that has reached an average cell efficiency of 24.09% in its pilot lines.

Heterojunction – High in Efficiency

Heterojunction technology (HJT) has demonstrated the highest crystalline silicon cell efficiency so far – it holds the overall cell record for silicon solar cells at 26.3%, based on a combination of HJT and IBC. When it comes to the pure HJT structure, the highest efficiencies for commercial sizes were reported this June/July by China’s LONGi and Huasun, both at 25.3%. There has been significant interest expressed to venture into HJT, totalling to about 50 GW globally during 2020, but the actual production capacities are much lower, with output at the single-digit scale. Probably the most important recent development in the context of HJT commercialization is former PV equipment manufacturer Meyer Burger’s decision to become a cell and module manufacturer itself and stop selling its HJT technology. In May 2021, it started production of cells and modules in Germany, each with 400 MW capacity, and has already announced expansion to 1.4 GW in 2022 and 7 GW by 2027. This comes after it sold its technology to REC, which was the first company after Panasonic that successfully commercialized HJT cell/modules in volume manufacturing in Singapore, as well as to ENEL Green Power in Italy and Ecosolifer in Hungary. Another European pure HJT player is Hevel Solar from Russia, which operates a 340 MW cell/module factory based on its own cell technology, while a few Chinese companies are working on HJT as well.

HJT has several advantages over traditional crystalline solar cells, showing a leading low temperature coefficient, the highest bifaciality of all cell technologies and much less production steps, but it requires investment in a completely new line and the capex is considerably higher than for baseline PERC. However, with several Asian tool vendors venturing into the development of deposition equipment for HJT, the capex has already been coming down.

Tandem – generation next

The way improvements in cell efficiencies are progressing, not only PERC but single junction crystalline cell efficiencies as a whole will reach their practical efficiency limits soon. Considering that HJT’s best commercial cells are produced at 24.5% today, the practical limit of around 26% will be reached in a few years. At that time, the industry must be ready with next generation multi-junction technology, where different materials are stacked to harvest a larger part of the light spectrum. There are many different options for choosing materials and combinations. As it looks today, the most promising candidate seems to be a c-Si/Perovskite tandem cell structure, for which Oxford PV demonstrated the latest efficiency world record efficiency of 29.52% at the end of 2020, anticipating a practical efficiency potential of around 35%. Oxford PV is currently setting up its first commercial 125 MW manufacturing unit for c-Si/perovskite tandem cells in Germany, targeting first sales in 2022.

This blog series is extracted from the Global Market Outlook 2021-2025. Download the report here. 

Photo: © Enel Green Power

©Oxford PV

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