36th EU PVSEC, 09 - 13 September 2019
Marseille Chanot Convention and Exhibition Centre, Marseille, France

21 February 2018

Mono or Multi?

The Crystalline Solar World Seems to Turn Towards Mono Cell Technologies

There is no doubt on the leading solar cell technology today – it is and will stay for the foreseeable future based on crystalline silicon wafers. The jury is still out about the crystallization type - multicrystalline and monocrystalline.

While multicrystalline has been the prime choice for low cost solutions, monocrystalline with its higher efficiency and higher cost was rather used for applications with limited space. As the PV market has been always very price sensitive, multicrystalline has been dominating the solar sector, in particular in the utility-scale segment, where most PV modules has been deployed so far. However, the recent trend towards PERC has been pushing the scale towards mono.

According to the April 2017 released 8th edition of the International Technology Roadmap for PV (ITRPV), an annual survey of leading players in the solar sector, casted multicrystalline (traditional multicrystalline and high performance multicrystalline) solar cells had a share of 65% in the field of crystalline cell technology by the end of 2016, which it anticipates to shrink to below 40% over the next 10 years. The monocrystalline wafer segment – supported by advanced cell technologies, including n-type - is supposed to capture 60% of the crystalline market by 2027, from around 35% end of 2016.

The 8th ITRPV forecasted that mono’s share in crystalline technology would slightly increase to about 38% in 2017, while multi would drop to 62%. By 2019, mono would reach a 42% crystalline silicon share and around 54% in 2024. Solar Media is more optimistic about mono and believes it will almost reach parity with multi in 2018. However, the first solar market analysts even believe that mono could have already surpassed multicrystalline silicon last year. Paula Mints from SPV Market Research was just quoted by Renewable Energy World, saying that she expects monocrystalline to have reached a 49% share in 2017 total module shipments, taking over the lead from multicrystalline, which dropped from 54% in 2016.

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Whatever the actual number, the recent shift from mono to multi has been facilitated, on the one hand, by improvements at the wafer and cell level, which have helped to diminish the cost gap between the two crystallographic variants of silicon PV. On the other hand, the multiple-GW Chinese Top Runner program, which requires certain efficiency levels and module power ratings, has played a crucial role in pulling higher mono demand as well.

Wafers: When looking at the wafer level, one key for cost reduction attained with monocrystalline substrates has been the move from a slurry based wire-sawing process to a fixed abrasive diamond-based process, which enables a higher throughput using thin wires and not only reduces the amount of silicon lost as kerf but also permits producing a higher number of silicon slices per work piece. The promise of low-cost mono, has encouraged world leading monocrystalline wafer makers Longi, for example, to triple its wafer capacity to 45 GW by 2020, a dimension of scale that will further improve the cost structure for producing mono wafers.

However, diamond wire based sawing can also be employed for multicrystalline. First, there were initial hiccups - one being the intrinsic nature of multicrystalline having grain boundaries, which makes the switch more complicated. Another bottleneck for employing diamond wire sawing of multicrystalline is that the process leaves a smooth surface that poses a significant challenge to be textured with standard acidic texturing process. But this has been solved – the magic word is ‘black silicon’, a texturization technology to smoothen the wafer surface -, and most multi wafer companies are in the process of upgrading their tools to fixed abrasive sawing. GCL, the world’s largest multicrystalline wafer maker, for example, had plans to shift 90% of its production to diamond wire based sawing by end 2017.

Cells: When looking at the cell level, mono PERC is on a good way to become the new crystalline workhorse. The majority of PERC producers today are betting on mono wafers for their PERC products. But PERC cells can also be made using multicrystalline wafers, which REC and Hanwha QCells have pioneered. While Light-induced degradation (LID) was the main bottle neck for multi-PERC, researchers seem to have succeeded in keeping LID at low levels. Moreover, it is now possible to combine black silicon technology with multicrystalline PERC products.

While the recent improvements for multi look promising for this silicon variety, mono has a joker – and that is ‘n-type’, something multicrystalline cannot compete with as of now. Except for PERC, all advanced cell architectures rely on mono n-type substrates - including PERT, heterojunction and IBC.

 

In any case, EU PVSEC, the premier platform on solar technology research, is looking forward to discuss the latest research along the process chain of all crystalline silicon based PV technologies and beyond. Latest updates on the whole PV research, technologies and policies will be reported in the Conference that gathers the top players from around the globe in the Photovoltaic community. Be part of that group – and submit your abstract by 09 March 2018.