Bifacial PV is a technology that requires specific changes in three stages of the PV manufacturing chain – cells, modules, and systems. The cell level is obviously where it all starts to change for making bifacial PV. The choice is wide, because any move away from standard BSF technology to any of the advanced cell technologies qualifies for bifacial. One can pick from the three main commercial cell architectures – p-type PERC, the mainstream, and high efficiency cell types based on n-type wafers such as heterojunction (HJT), and the relatively new passivated contacts cell architecture, commonly called TOPCon. With some optimization, even the IBC cell structure is an eligible candidate for bifacial solar.
PERC has established itself as the new standard in solar cell technology. It's good news for bifacial as well, as it is very easy to upgrade PERC to bifacial technology. PERC's bifacial capability is perhaps one important feature that has also helped the technology to spread wider and reign longer than many anticipated.
PERC is the most simple and cost-effective candidate for bifacial. But unlike other advanced cell structures, PERC is not bifacial by default. That's because the standard PERC cell structure contains an opaque local BSF-forming layer of aluminum paste on the rear side. However, the step to turn PERC into a bifacial solar cell is rather simple, i.e., apply an aluminum grid instead of spreading paste over the full area. This requires specific aluminum pastes designed for printing fingers rather than the products used for full area prints. Now, there are several paste makers that are offering aluminum pastes that can support bifacial PERC. Japan's Toyo Aluminium (Toyal) was the first paste maker to offer such products and is also the technology leader of the segment. Another advantage of this approach is that it avoids the usage of silver on the rear side, while most of the other bifacial structures need silver on both sides. Shifting from PERC to bifacial also requires some optimization in the areas of silicon nitride capping layer of the rear passivation stack and laser opening of the rear passivation layers.
Unlike Aiko and Tongwei, which are mainly cell manufacturers, other leading PV companies that are fully integrated such as LONGi and JinkoSolar do not supply cells directly. However, a few companies have provided the efficiencies and bifaciality of their PERC cells for reference, which are listed in the table below. Several companies announced typical efficiencies above 23% and the range in bifaciality between 65% and 75%. Bifacial PERC is not just limited to monocrystalline, but also applicable to multicrystalline cells – and Canadian Solar is a frontrunner in this area. The multicrystalline PERC cells of CSI show average efficiencies of 20.8% with a bifaciality of 72%, according to the company.
The text is an excerpt from the TaiyangNews Bifacial Solar 2021 – Part 1: Cells and Modules report. For more details on bifacial solar cells and modules, please download the report, here.
TaiyangNews' Bifacial Solar Conference 2021 Videos are available on YouTube; to view the event recordings click here.
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