Emerging fix for LID in multi PERC

In the never-ending competitive race between mono and multicrystalline PV technology, mono appears to be making up ground on its lower-cost rival, and fast. At this year’s SNEC conference, the mono versus multi competitive dynamic was very much front and center in discussions. While various manufacturers and researchers at the conference introduced arguments for both strains of crystalline silicon PV, down on the trade show floor it was clear that the momentum behind mono was palpable.

“We are going to see a big shift towards mono PERC very soon,” said Andy Klump, the CEO of Clean Energy Associates (CEA). “On an R&D basis, mono PERC cells are already above 22%, so if you see fully utilized and ramped up capacity above 20%, that will get a lot of attention from downstream developers, particularly in the DG [distributed generation] and rooftop segments.”

Passivated emitter rear contact (PERC) solar cells have been the major technology trend within the PV cell and module segment for the last 18 – 24 months. The technology allows manufacturers to upgrade existing cell lines with tested tooling, and achieve multi-cell efficiencies of over 19% and on mono, as Klump indicates, of above of 21%.

As this investment cycle continues through 2017, Switzerland’s Meyer Burger has found itself in the enviable position of providing its MAiA to meet much of the PERC upgrade demand. Its German MAiA production facility, which also houses its heterojunction pilot line, is currently turning out up to four MAiA tools a week, with a “theoretical” potential to increase that up to five if demand remains robust, according to new CEO Hans Brändle. He reports that as of the end of 2016, Meyer Burger had shipped more than 120 MAiA systems, with a capacity of some 15 GW.

Adam Ge, the Commercial General Manager of Meyer Burger, China, describes the PERC platform as a “next phase” technology for manufacturers, due to its applicability as an upgrade to existing crystalline silicon (c-Si) lines. “Starting from 2017 we see the path even getting quicker and quicker for people to adopt the PERC technology, not only from the tier-1 and tier-2, but even expanding to some smaller companies,” said Ge.

Enter LID

As ever, however, the devil is in the details. While an upgrade tool and technology, delivering an immediate efficiency boost may seem a ‘no brainer’ for manufacturers looking to achieve the efficiency increases required to stay competitive, it is not as straightforward as it sounds.

PERC cells have exhibited light-induced degradation (LID). The CEA’s Klump notes that the company is privy to “some data” that reveal “some challenges” resulting from LID in modules deploying PERC cells. At best LID in PERC can cancel out the efficiency boost delivered by the PERC technology, at worst it can result in an efficiency below that of standard aluminum back surface field (ASF) cell.

“It’s a particularly nasty defect,” declared Stuart Wenham, a director of the University of New South Wales’ (UNSW) Photovoltaics Center of Excellence, while speaking at the SNEC conference. The UNSW is spearheading research into the LID effect in PERC cells and has brought together a group of 12 industry partners to help fund its work in the area. Solutions to the LID in mono PERC cells have now been developed and commercialized although the LID in multi PERC is much more complicated. Elevated temperature also plays a role in the LID that has been observed in multi PERC cells, resulting in the term LeTID (light and elevated temperature-induced degradation), although LID has emerged as the most commonly used nomenclature. Wenham’s research group at the UNSW has worked to identify what is causing the LID in multi PERC cells and has identified two key defects (see chart above). The first causes degradation within the first 100 hours of exposure to light, after which time the cell partially, however slowly, recovers. The second, however, is even “nastier,” to draw on Wenham’s descriptor, and evolves from the first.

“One of the things that the industry struggles with is to identify that second type of defect – because most people don’t even know that it exists,” said Wenham. The type-2 defect occurs once a module has been in the field for around 5 – 10 years, or more than 100,000 hours of test light soaking, according to the UNSW team.

Testing

Because of this delayed onset, it is necessarily difficult to design tests for the type-2 defect (see chart below). The UNSW team has developed a process through which the type-2 defect can be provoked, involving a dark anneal stage followed by light soaking. Wenham explains: “When someone makes a solar module, they use a laminator at about 150°C to do the lamination process. What we would encourage them to do is to leave the module in the laminator for an additional 10 hours [at that temperature], in the dark, and then do the standard light soaking. What those 10 hours in the dark do is greatly accelerate the onset of that type-2 defect – that is the really nasty one. Then they will see how susceptible their solar module is to that type-2 defect.”

UNSW has tested many PERC modules using this method, including those promoted as being ‘LID free’ and found the type-2 defect present. For project developers and investors it is an ominous development, and the prospect of the type-2 defect promoting recombination, and therefore reduced output, some years into an array’s lifetime is worrying.

However, if something can be tested, it’s likely it can be solved – at least in this case, according to Wenham. “If it appears, the answer is to use advanced hydrogenation to passivate the problem [the type-2 defect] and eliminate it altogether,” he said.

Mono versus multi

In discussing LID in PERC cells with PV experts right across the manufacturing, equipment supply, and research sectors, one of the first points they all make is that LID in PERC is very different in mono and multi cells. In mono, the problem has been understood for longer, and the fix is now mature.

LID in mono PERC cells is caused by the boron-oxygen complex within the crystal. Today it is known as the BO-defect. Researchers from University of Konstanz, Germany, working under Giso Hahn were the first to identify the BO-defect and develop partial solutions to this problem that had plagued the PV industry for decades.

Hahn’s team continued its work on the problem, making particular headway with firing step parameters and the role of hydrogenation. The UNSW subsequently developed its hydrogen passivation technique for rendering the BO-complex recombination inactive. German equipment supplier centrotherm began the development of the first production tool to resolve LID in mono PERC cells, the c.REG, in 2014. The company delivered the first of its c.REG machines in June 2015.

The process of identifying the BO-defect as the cause of LID in mono cells, discovering the importance of hydrogen in preventing the recombination, and then transferring these findings into production took roughly ten years. While this may have been the case earlier this decade, the present-day momentum behind the PERC upgrade cycle has seen the mono PERC LID fix rapidly adopted – and today there are alternatives to the c.REG tool available.

The PERC upgrade cycle is also driving researchers and R&D departments within the PV sector to search for a fix for LID in multi PERC. Such a fix is likely to come in the form of the advanced hydrogenation process to which the UNSW’s Wenham refers. However, a production solution is not yet on the market.

“With the expansion of this cell technology [PERC] within the industry, our understanding of the LID process in both mono and multi is progressing very rapidly now,” said BT Imaging’s Thorsten Trupke, “and also approaches as to how to mitigate this effect.”

Trupke, who is also the Deputy Director of the UNSW’s PV Centre of Excellence, did caution that some manufacturer claims that LID in mono and multi PERC cells has been “solved” should be treated with caution. He noted that “the reduction of efficiency has been strongly suppressed, but if people claim that it is fully understood and fully resolved then I would argue against that.”

Charge state of hydrogen

Wenham’s team is currently working with five production equipment partners to develop processes and equipment to deploy its ‘advanced hydrogenation’ technology at scale (see the box at bottom right of p. 93). One of the keys to the potential LID fix for multi PERC is the ability to control the charge state of the hydrogen atoms during the process.

“Stuart [Wenham] said [at the SNEC conference] the LID problem has been solved at UNSW, but it’s a bit more complicated than it first appeared,” summarized Martin Green, who heads up the UNSW’s PV Center of Excellence. “The second type of [LID-causing] defect depends on the details on the hydrogenation process itself.”

“Manufacturers need to control the charge state of the hydrogen first to enable it to move quickly through the silicon material and second to be most effective in neutralizing the defect once it arrives there.”

Wenham reports that a process to control the hydrogen charge state and therefore its diffusivity and reactivity has been developed by the UNSW team and its equipment partners, although production tooling by which the process can be carried out is still at least 6 – 12 months away.

If a cost-effective tooling to perform the advanced hydrogenation and affect the multi-LID fix can be rolled out, the competitive dynamic between mono and multicrystalline technology will shift once again. Solar Media’s Finlay Colville made the bold prediction last month that in 2018 the split between mono and multi will approach 50/50. Independent analyst Corrine Lin, who is also a part of pv magazine’s Investigations and Insight team, is more skeptical of mono achieving such market share with such alacrity. Lin noted that with the challenges to the texturing of diamond wire [DW] cut multi wafers being cost-effectively solved, either through a wet chemical or dry texturing process, then “really cheap” multi is unlikely to lose ground to mono quite so quickly.

“The mono wafer price was too high during 1H 2017, leading some module producers to cancel some mono orders, or switch them to multi,” said Lin. “I think 2017 [mono share] will be around 33% – next year maybe around 40%, or even a little lower if DW multi wafers really become strong.”

Schmid presented its DW PreTex solution during this year’s SNEC, with the company’s VP of Business Unit Photovoltaics Christian Buchner saying that with a cost of only $0.01/wafer and a benefit of DW sawing of $0.10/wafer the solution is both “very exciting and very convincing.”

“With all of the [multi] capacity on the market – 40 GW of multi furnaces – they need to be used,” said Buchner. “Manufacturers need to do something on the cost side again, to make that product competitive. So I think we will see changes coming very soon. People will push multi to higher efficiencies and push against mono.” If the UNSW’s proposed multi-LID fix successfully moves from lab to fab, then PERC may be just the efficiency boost the technology is looking for.