Back contact solar module manufacturing capacity may reach 1 TW by 2030, says tech expert
In an interview held with pv magazine in late 2022, Radovan Kopecek, the co-founder and director of Germany's ISC Konstanz, predicted that solar modules built with cells based on both-sided contacted technology, such as TOPCon panels, could potentially be driven out of the market by PV modules based on back-contacted (BC) designs by 2028, with the first strong market volumes of BC technologies materializing between 2025 and 2028.
More than two years on from that interview, Kopecek believes more than ever that the prospects he outlined at that time are currently taking shape, especially in light of the fact that large BC module makers such as Longi and Aiko recently announced surprising shipment results. “Both Longi and Aiko are convinced that the scenario I predicted may even materialize in 2027,” Kopecek told pv magazine. “I would rather stick to my 2028 outlook.”
Roadmap
Kopecek states that the PV industry will transition from pure TOPCon products to BC designs that could potentially be combined with TOPCon itself, or other mainstream technologies such as PERC or heterojunction. “TOPCon means that only the rear side of the cells have passivating contacts or carrier selective contacts, while BC technologies, which are leading the module chart and are the future I am talking about, have all contacts passivated,” he explained. “The next step may be a shift from pure TOPCon to a spurious TOPCon technology combined with BC.”
“TOPCon could be improved by placing the passivating contacts on the front side, which is called ‘double poly.' However, this is more complex than what can be done for BC, where manufacturers only have to print the fingers on the front side very shallow. So, the logical consequence is to go to BC, especially to bifacial BC modules, for the utility-scale market,” Kopecek added. “Also the implementation of alternative metallization, like copper (Cu) or aluminium (Al), is much easier on the rear side without cannibalization of front side efficiency.”
According to Kopecek, the processes for manufacturing BC cells and modules was more complicated in the past. “ISC Konstanz initially started to cooperate with Chinese manufacturer SPIC and we developed how to improve the module technology further, at the stringer level. What was really important in the last four years was that companies like SPIC, Aiko and Longi were involved in the development of single processes. It is very important to produce BC panels with standard stringing processes.”
Equipment
Another key step in the uptake of BC products at the industrial level was the development of new, fast lasers. “A simple laser enhanced contact optimization (LECO) or a laser for selective emitters is not fast enough,” Kopecek said. “Additionally, it will be crucial to develop low-cost p-plus poly layers. At the moment, for example, we at ISC Konstanz are depositing poly layers and then we implement doping with boron diffusion. However, an intrinsic p-plus poly layer would be extremely beneficial to the production process. This is expected to happen in the next two years and will be a breakthrough, as it will have a positive impact on both performance and costs.”
Kopecek also explained that, for BC cell production, the in-situ doping in p-plus poly layers deposited via plasma-enhanced chemical vapor deposition (PECVD) has already been improved significantly, which will make the whole manufacturing process faster and capable of higher throughputs at the same cost. Processing of p-doped poly layers is more complicated because of the low solubility of boron in silicon, which is also the reason why TOPCon tech is based on n-type material having the passivating n-doped poly fingers on the rear. However, p-doped polysilicon is being developed at high speed and will be used more effectively in the future.
In terms of power conversion efficiency, ISC Konstant researchers say single-junction silicon solar cells are now approaching their theoretical limits, with cells and modules approaching 27% and 25-26%, respectively. “But most importantly, BC cells may reach almost the same bifacility rate as TOPCon cells,” Kopecek added. “The common belief is that bifaciality and BC don't go well together, but this is not true. TOPCon has a fully covered rear side by an absorbing n-type poly-layer, while BC has p-plus and n-plus regions, in some cases also with some undoped regions which is beneficial. There is more shadowing by the metallization than in TOPCon, but the emitter for BC tech is on the rear side, which contributes to a higher internal quantum efficiency (IQE). If we consider all advantages and disadvantages, the bifacial factor can be almost identical to that of TOPCon, which is about 0.8.”
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