Laser ablation of PERC solar cells

Under sustained pressure to keep costs low while increasing efficiency, over the past few years crystalline silicon PV manufacturers have increasingly turned to passivated emitter rear contact (PERC) PV designs. The technology offers modest efficiency increases with only a few additional tools and minor changes to existing cell processes, and as such PERC has been a popular upgrade for many companies. PERC particularly gained prominence in the market downturn of 2012-2014, as it was a way for PV cell makers to differentiate their products without spending large amounts of capital on equipment.
PERC involves the creation of a passivation layer on the rear side of the PV cell, and as part of this process it is necessary to create an opening on the back side through this layer. No other approach has proven as practical and inexpensive as lasers, and this has created a large market for laser tool makers. IHS Markit estimates that at present there are 13.9 GW of capacity online for PERC and its two related technologies, PERT and PERL. PERC, however, makes up the majority of this, and all of this capacity includes laser tools.

Enter lasers

There have been multiple other approaches to opening up the back side of PERC cells, including mechanical scribing and wet chemical etching processes. Both of these have largely proved impractical. Etching processes create extensive waste streams, and this is part of the reason that they have been unable to compete with lasers on cost. As such, they have never been commercially adopted.
Swiss solar equipment maker Meyer Burger offers complete PERC solutions for solar PV makers, sourcing laser tools from multiple third-party vendors. Dirk Habermann, Chief Innovation Officer at Meyer Burger, notes that one advantage of lasers is that one can freely adjust the size of openings on the rear of the cell, which is more difficult with masking processes. “Laser processes are very fast, very stable, and reliable in the production,” notes Habermann. “And from the cost structure and operations perspectives they perform pretty well.” 3D Micromac makes lasers for not only PV but also semiconductor, glass, and display applications, as one of the most well-established of the European laser makers.
Frederick Bamberg, who serves as Product Manager for PV laser tools at 3D Micromac cites the work that German research institute Fraunhofer ISE did with laser fired contacts as the beginning of practical laser technology for PERC.
“It was known in the lab, they could achieve a new efficiency world record on polycrystalline solar cells with it, but it was not ready for mass production,” recalls Bamberg. “The deposition technique, as well as the metal paste, and all the rest around this laser process wasn’t really developed. It took a long time for the rest to catch up with it, and this was delaying the introduction of PERC I would say.” Concurrently the application of lasers for other uses was developing, such as laser edge isolation selective emitter dop xAdvertisementing, which assisted in the development of laser applications for back side contact opening.
“Some handling techniques from laser edge isolation machine manufacturers developed,” explains Bamberg. “And then it was just a very small step to have some industrial market for laser contact opening for the industry.” Since that time period, PERC and laser ablation have grown together. “Starting production in the PERC technology a couple of years ago, laser technology was the key to being successful, and to reach our targets in time,” states Meyer Burger’s Habermann. “So this helps very much the overall market penetration of PERC that this opening technology gets this advantage compared to the wet chemical process.” Cost remains a major consideration, and an essential advantage of lasers. At this point, laser systems have almost become a commodity, furthering the importance of competitive pricing.
Innolas is a German laser tool maker that develops equipment for both thin film and crystalline silicon PERC PV cells.
“The number one criterion is always the price and throughput of the tool. Second, there are still differences in the cell efficiency achievable with the respective process setup. And last but not least are the accuracy and general hardware features of the tool,” states Innolas Director of Sales and Marketing Ernst Hartmannsgruber.
Additionally, the use of lasers for PERC cells has expanded, and now includes bifacial designs, which necessitate a slightly different approach due to their different architectures.

Short-pulse vs. nanosecond

While the dominant approach to PERC laser ablation has been nanosecond lasers, lasers with even shorter pulses have emerged recently. These shorter pulse lasers have the quality of cutting with less dissipation of heat into the surrounding material, meaning less impact on the passivated surface. However, they are also more expensive than nanosecond lasers.
The industry is divided as to whether these shorter pulse lasers offer a technical advantage. “Shorter pulse lengths such as pico or femtosecond lasers increase cost at no benefit, and can even worsen the efficiency,” states Innolas’ Hartmannsgruber.
“This has to do with the interaction between the Al paste and the Si surface after the laser treatment. The surface treated by a nanosecond laser is a bit more molten, resulting from a picosecond laser, and as such can help the paste to better form the Si-Al alloy during firing.” In the end, even for companies that have seen good results with both short-pulse and nanosecond lasers, many companies make decisions primarily on cost. “In the end, the price makes the decision,” explains Meyer Burger Product Portfolio Manager Mirko Meyer.

Changes and challenges in the market

Western laser makers have had a good run with PERC, which has been animportant source of new business in the PV equipment industry after the disappearance of so many thin film PV makers from the marketplace during the industry downturn that began in 2012. However, they are facing new competition from Asian laser makers.
“Local suppliers from China, Taiwan and elsewhere can supply equipment that is competitive now,” states Meyer Burger’s Meyer, “all equipped with laser sources.” This all started approximately three years ago, and since that time the companies from these regions have moved rapidly. “They had a learning curve in the beginning, but at the moment let’s say they are comparable with others in the world,” says Meyer.
There is some consolation in that these companies have not been as successful in selling products outside of Asia. “The Chinese and Taiwanese laser suppliers are the main competitors in their domestic markets, but weaker competitors when they have to export their products to other markets,” says Hartmannsgruber of Innolas. “The reason is simply the additional cost for exportation and service in foreign countries, as well as the language barrier.” However, as China and Taiwan are where most of the world’s PV cell manufacturing is located, such suppliers are increasingly relevant for much of the industry.

The future of lasers and PERC

Laser technology will be around for the foreseeable future, with PERC designs expected to continue to gain market share in global PV cell production. IHS predicts that PERC/PERT/PERL capacity will more than double to 32 GW by 2020. SEMI has an even more optimistic view, with their 2015 International Technology Roadmap for PV estimating that by 2022 PERC will be the most popular design for crystalline silicon PV, with a roughly 35% market share. Every indication is that laser deployment will grow alongside this.
Western laser makers have a bright future ahead of them in markets where they can sell tools. But like the larger PV industry, there is the question of whether, and for how long, Western companies can compete with Asian entrants, with more parts of the value chain centered in Asia.