On the floor of the 2018 Solar Power International trade show, manufacturers showed off a wide array of new module designs, with half-cut cells, copper wire interconnections, and bifacial modules all over the place, often in combination.

But one thing was consistent: The majority of the modules on display featured monocrystalline silicon, and there was even a module made of half-cut cells with the distinctive absence of grain at the booth of the world’s largest maker of multicrystalline silicon wafers. And the trend to monocrystalline is global.

For years, monocrystalline technology has been reserved largely for residential and other space-constrained installations, and multicrystalline has dominated the utility-scale sector. But market analysts say that more and more large-scale solar plants are using monocrystalline modules, and two different analysts told pv magazine that the global split between mono and multi product sold will be within a few percent of 50/50 in the second half of this year. This is a sea change from the first half of 2016, when modules using monocrystalline cells made up less than 30% of all product sold.

Price dynamics

Price dynamics underlie this market shift. According to IHS Markit, prices for mono wafers fell to $0.133/W in the first half of 2018, making them only 10% more than multicrystalline wafers. This difference is less noticeable as the products are made into cells and then modules, as there is no inherent higher cost for mono cell production or module assembly. By the time we reach system prices, this is often more than compensated for by a balance of systems advantage due to the higher efficiency of monocrystalline silicon.

So how has monocrystalline silicon been able to reduce prices so sharply, and why is it taking over the global market? In order to answer this question, you have to go to the factories.

Diamond wire and PERC

Monocrystalline silicon makers have benefited from two of the big technology trends in solar wafer and cell manufacturing: diamond wire slicing and PERC.

Chinese manufacturing giant Longi Green Energy Technology, which has led the rise of mono (See China’s monocrystalline boom, pv magazine 04/2018, p. 56), claims to have been the first company to widely deploy diamond wire sawing. These are more expensive than slurry saws but also more efficient, with no need for glycol slurry or grit, and less saw waste, meaning more wafers per kilogram of ingot.

Diamond wire sawing was pioneered by Switzerland’s Meyer Burger, but as has been the case with other equipment, Chinese manufacturing is bringing down the cost of these saws, making a further economic case. At first, multicrystalline wafer makers could not take advantage of diamond wire sawing, as the saw left too smooth of a texture, which did not work with existing wet processes.

However, with multicrystalline wafer and cell makers making modifications to their etching solutions, by the end of 2017, diamond wire had also become the dominant technology for multicrystalline silicon, often combined with black silicon surface treatments.

A potentially longer lasting technological driver to monocrystalline has come with passivated emitter rear contact (PERC) cell technology. PERC requires only a few process additions, and the machinery to do PERC is far less expensive and easier to integrate than switching over to heterojunction or interdigitated back contact (IBC) cell designs. PERC has been the high-efficiency design of choice for many manufacturers during the last six years.

PERC can give both multi and monocrystalline silicon a boost, but the advantage is greater for mono. Charts from the German Mechanical Engineering Industry Association’s (VDMA’s) latest International Technology Roadmap for PV (ITRPV) show the average efficiency of multicrystalline PERC cells at around 1% higher than standard multi, but for monocrystalline cells it is more like 1.7%.

This relative efficiency gain is not the only advantage of mono-PERC over multi-PERC. PERC cells are more susceptible to light-induced degradation (LID) and light and elevated temperature-induced degradation (LeTID), and analysts including IHS Markit Research Director for Solar, Energy Storage and Smart Utilities Edurne Zoco say this is a bigger problem for multi-PERC: “LID is the biggest problem,” notes Zoco.

RCz and CCz

While both PERC and diamond wire have been significant, another factor in the shift to mono is lower costs coming from dramatic improvements in casting monocrystalline ingots through the Czochralski process. This includes the ability to cast more monocrystalline silicon in a single crucible, through the rechargeable Czochralski (RCz) and continuous Czochralski (CCz) processes.

Longi, the world’s largest monocrystalline wafer maker, stresses the advantages of the RCz technology it has widely adopted. Various analysts estimate that the company is getting three ingot pulls from a single crucible; however Longi Chair Zhenguo Li tells pv magazine that his company is getting as many as five.

Not only does this extend the life of crucibles, but also saves energy and reduces downtime. And in the world of ingot casting as elsewhere, time is money. “The main driver is productivity, which leads to lower cost,” explains Simon Price, the CEO and head of research at U.K.-based analysts Exawatt. Price notes that RCz has also benefited from a move to larger crucibles, which means longer ingots and less time loading and unloading, as well as other advantages. “There are little bits of savings all along to doing RCz,” he says.

RCz is not the only technology to get more monocrystalline silicon out of a single crucible. In 2016 GCL-Poly, which has managed to stay just ahead of Longi as the world’s largest wafer maker overall (including both mono and multicrystalline silicon wafers), bought the intellectual property behind SunEdison’s continuous Czochralski process, and in April of this year established a joint venture to build 20 GW of monocrystalline ingot capacity using this technology.

GCL also acquired SunEdison’s fluidized bed reactor (FBR) polysilicon technology, which it says produces the best material for making CCz mono. GCL is currently the only company in the world to own viable versions of both of these technologies. Exawatt’s Price notes that there are some advantages to CCz “in principle,” including the ability to run at a lower temperature, reducing wear on crucibles and extending crucible lifetimes. More significantly, he says that the process allows for better control of doping. Not only does this mean easier production of n-type silicon, but allows for gallium-doped wafers.