The search for ever higher conversion efficiency has led solar researchers to focus on back-contact cell approaches, and efforts to develop more cost-effective manufacturing are bringing technologies such as interdigitated back contact (IBC) into the solar energy mainstream. Mark Hutchins reports.
IBC solar cells were designed in the 1970s and the idea behind them is simple: Move all the contacts, metallization and other internal functions of the cell to the back, leaving the front, the active layer unobstructed and open to more sunlight.
This approach ensures that IBC and other back contact type devices can reach the practical limit of single junction silicon cell conversion efficiency than other cell architectures. This goal has kept them a focus for researchers, as many of the highest efficiencies in the lab have come from back-contact structures.
However, processing rear contact cells at scale has proven difficult and expensive. The resulting lack of adoption means that comparison with mainstream approaches is difficult. “Because the rear contacts are small in scope and mostly used in distributed generation projects that aim for better looks, it has a higher premium,” says Corrine Lin, principal analyst at renewable energy market intelligence firm InfoLink. “Based on current manufacturers’ product pricing, the premium is usually $0.01/W or even higher compared to a PERC (passivated emitter, back contact solar) system. However, there is no clear premium index because trading volumes and different target markets are small.
Most major manufacturers took the simpler route to boost efficiency, but more cost-effective means are nearing exhaustion – and researchers at Germany’s Solar Research Institute Hamelin last year lowered the efficiency of back contacts to 29.1% – a new effort to commercialize back contact cells is underway.
Advantage of IBC
The US-based SunPower Corporation has been manufacturing IBC cells for over 20 years. Its products have been particularly successful in the housing market, where aesthetic appearance and superior efficiency and reliability are valued, leading to a high price range. In 2020, SunPower spun off its manufacturing business into a new company, Maxeon Solar Technologies, which will continue to manufacture IBC products under the SunPower and Maxeon brands.
Doug Rose, Maxeon’s vice president of technology strategy, explains that the back contact design enables full optimization of the front surface, maximizing light into the cell and minimizing recombination, rather than having to optimize metal coverage and lateral conductivity to metal fingers. . On the back side, IBC offers the option of thicker metal, resulting in lower series resistance.
Rose adds that along with efficiency, superior reliability is also a focus for Maxeon, and the premium product allows it to make design and material choices that lead to better-than-standard reliability – offering up to a 40-year warranty on some products. on the market.
The main advantage of Maxeon’s IBC products is that they have a unique, undamaged reverse bias behavior. This prevents hot spots – which can damage cells and modules – even with partially shaded failed module diodes. “We believe this provides added value and differentiation, and we have done a lot of work to extend the advantage in this area in our next generation.” Rose goes on to explain that the ability to do this is another inherent advantage of back-touch technology, although not all back-touch architectures have it.
Back contact solar energy has great expectations. Radovan Kopecek, founder and director of the German International Solar Research Center (ISC) Konstanz, said. pv magazine in November, IBC modules could overtake TOPCon products (tunnel-oxide, passivod contact) by 2028 and account for half of the market in 2030. Other market observers are less bullish, but agree that rear contact technologies are likely to grow over the next five years.
“Incremental improvements in manufacturing processes and efficiency are leading to incremental cost reductions in most rear contact technologies,” said Karl Melkonyan, senior research analyst at S&P Global Commodity Insights. “However, significant cost savings require more time to fully develop. This could be a significant reduction in silver consumption; or complete replacement with copper or aluminum; or further improvements to soldering and cell interconnect processes and other efficiency, yield and performance improvements.”
S&P Global predicts that rear-contact solar could reach 15 percent market share in the next five years. InfoLink expects growth to be around 5% over the next two to three years, but little after that. “In the long term, its market share is unlikely to reach the scale of PERC or TOPCon due to its complex process and higher manufacturing costs,” says InfoLink’s Lin. “Although there is still demand for the rear contact – due to its high efficiency and better appearance – its market share is unlikely to reach 10%.”
Maxeon has worked to improve the manufacturing process. “Maxeon has a fully proprietary process with a huge number of patents protecting the approach,” says David Smith, a scientist at Maxeon’s California R&D center. “Today, our pilot production process uses a lot of mainstream equipment, so we can drive the coat tails of the TOPCon industry. We’re using these large-scale powerful tools as much as possible, and our new generation is using more of them than ever.”
Saving and scaling
However, Maxeon remains focused on customers who pay a premium for performance and aesthetics, and says it will continue to focus on its various products even as mainstream IBC technology begins to emerge.
ISC Konstanz believes that the cost savings necessary to support its rising IBC expectations can be achieved. The research center has developed “Zebra” IBC cells, which are on the market through manufacturing partners in Europe and Asia, and continue to reduce production costs and increase efficiency.
“Our goal is to be as close as possible to standard technology from the point of view of the cost structure,” says Joris Libal, ISC’s technology transfer project manager. “We want to start with cheap processes directly from the toolkit and reach high efficiency.”
Valentin Mihailetchi, head of the n-type (negatively doped) cell R&D group at ISC Konstanz, further explains that zebra cells are produced using conventional processes such as tube diffusion, screen-printed metallization, plasma-enhanced chemical vapor deposition and laser ablation. , “It’s about using already established processes, just in a slightly different way.” This approach could bring broader benefits, as the back contact architecture can be combined with passivating contacts and other cell structures, providing another route to mass production.
In addition to handling costs, IBC cells require silver contacts for both polarities. “Compared to p-type (positively doped) PERC, the cost of the wafer is maybe 5% more and there is a small difference in device depreciation because there are slightly more devices compared to PERC or TOPCon,” says Libal. “The main difference is the price of the metal.”
However, ISC Konstanz uses copper paste instead of silver in its screen printing, because the fingers on the back side do not need to be made as thin as possible in the rear contact cells. Traditional manufacturers, on the other hand, have to make more progress in copper plating processes to reduce the silver content.
Although ISC Konstanz’s IBC cells still require some silver for the contacts—copper cannot be in direct contact with silicon—Libal says silver consumption can be reduced below 5 mg/W of cell production capacity. That’s a fraction of the amount used in PERC and TOPCon cells, and would help ease the industry’s worries about silver supplies.
The research institute is making progress in its copper silk-screen printing process and reports that it has so far obtained good results in accelerated testing. “The copper-metallized Zebra IBC cell is a breakthrough that will ultimately bring the cost of IBC below PERC and still significantly higher efficiency,” says Libal. “When those key cost and performance parameters are met, that’s when this really flies.”