Post CSPV 2021: Silicon cell, module developments and trends

Every year, China SoG Silicon and PV Power Conference (CSPV) introduces the latest solar technologies and applications. The event brings together professionals from China and abroad to share their research results of silicon, solar cells, and bill of materials, as well as system applications, inspection and certification, and applications. The article delves into silicon cell and modules featured at the CSPV 2021.
 

Granular silicon 

While the whole market opts for silicon chunk, GCL Silicon has been engaging in granular silicon for a decade. In February 2021, the company announced to raise FBR (fluidized bed reactor) capacity from 6,000 MT to 10,000 MT, and then in November, it added and commissioned another 20,000 MT of new lines. With leading manufacturers including Longi, Zhonghuan, and JA Solar signing long-term contracts with GCL Silicon, FBR silicon again received wide attention. At the conference this year, GCL Silicon explained the FBR process, which can reduce total electricity consumption by 70% compared with the conventional Siemens process. FBR process enjoys advantages of shorter process, higher conversion rates, and lower energy consumption, helping the PV sector move toward a low-carbon era.

During the ingot process, hydrogen embrittlement and scrapping issues caused by granular silicon make it unable to put a large amount in the furnace. GCL Silicon offers a solution to the issues and equally well efficiency in the cell segment and shared the results of replacing 75% to 100% of silicon rod with granular silicon, which can deliver compatible performance in the ingot segment. 

Despite limited production of FBR silicon, granular silicon is mostly treated as a material for filling up gaps in the furnace. As GCL Silicon plans to add 200,000 to 300,000 MT of granular silicon during 2022 to 2023 and the fact that the energy consumption of granular silicon production is only one third of that of Siemens process, cost optimization is worth noting and looking forward to.
 

Faster wafer thinning process

After 19 years of large wafer evolvement, wafer thickness had hovered at 170-180um for at least three years. In 2021, short polysilicon supply, surged polysilicon prices, and newly installed equipment for large format accelerated wafer thinning. According to Longi’s presentation at the conference, the mainstream thickness of p-type M10 wafers has been thinned down to 160-165um, and demand for thinner wafers is growing. The wafer thinning technology roadmap presented by Zhonghuan indicates that the thickness of p-type G12 wafers was 160um only in 2021. It is reportedly that both wafer and cell manufacturers are accelerating the transition to 160um for the G12 format.
 

Large wafer competition

As technology for large format advances across the industry chain, large format products significantly drove down costs. Currently, the industry believes that large format can reduce costs by RMB 0.07-0.09/W. Meanwhile, increasing concentration in the industry prompted leading manufacturers to form alliances. Upgrading to larger format is inevitable. In the second half of 2021, the share of large wafers exceeded 50% for the first time and continues rising.

After 2019, Zhonghuan introduced 210mm wafers, while Longi unveiled 18xmm. The selection of formats has been a buzz topic in the industry, and the competition on large wafers escalated after the alliance promoting 210mm introduced modules rated beyond 600 W. Recently, BOM, inverters and junction box, as well as issues revolving room temperature and hot spot have been less discussed, whereas shipping method and reliability were still debated.

Before CSPV took place, Zhonghuan unveiled the 218.2mm wafer. Although the company did not mention much about the new format at the conference, many asked about it. During a Q&A session, a representative from Longi said “we do not object large wafer, but we oppose large format module.” Since the width of 218.2mm modules featuring 5 cell strings is the same as 182mm modules featuring 6 cell strings, the two formats can share most materials in the module segment. However, cell strings of 218.2mm modules are arranged in odd number, and so manufacturers are concerned about the module appearance with more busbars and modification of work tool for producing such modules. Cell and module manufacturers do not wish to see another size shift, and so no one announces they are producing 218.2mm products thus far. It remains to be seen whether the format will become the next standard.
 

N-type cells

As PERC cell efficiencies hit bottleneck, the industry pursues higher efficiencies, shifting its focus from p-type to n-type cells. So far, n-type cell technology has made breakthroughs in research and mass production, with noticeable progress in industrialization. At present, TOPCON and HJT are two major n-type roadmaps; which one can overtake PERC depends on cell efficiency and cost reduction. 
 

TOPCon

During the conference, Jolywood, Jinko, Risen, Longi and Astronergy shared their achievement of reaching an efficiency higher than 24%. TOPCON equipment suppliers also showcased their technology routes, which are listed as follows: 

 

Overall, LPCVD technology is currently more mature among all, but PEVCD has more potential. Regarding issues incurred by PECVD such as higher hydrogen content, Tongwei and Ningbo Material both offered solutions to reduce or even resolve the PECVD thin film issues, including optimization the proportion of SiH4/H2 and deposition temperature, and replace poly-Si with SiCx.

Conventional boron doping utilizes boron diffusion techniques such as BBr3 or BCl3, which delivers poorer uniformity and easier to form boron-rich later and requires longer time. To these issues, Ningbo Material proposed to improve uniformity of doping through PEVCD boron diffusion; however, the technique is still in the R&D phase as the process takes longer time and no equipment is available for industrialization so far. HAC Tech proposed to utilize thermal CVD boron doping to form a doped later to achieve better uniformity.
 

HJT

HJT cell technology has gradually matured, with each manufacturer claiming to reach 24% to 24.5% of cell efficiencies in mass production. Huasun and Maxwell shared their research of replacing the existing a-Si thin film with nc-Si made by VHF-PECVD, which will bring HJT cell efficiency to 25.0%+ in mass production. Huasun and Golden Glass have announced to engage in the technology and will enter mass production in 2022.

With HJT cell capacity growing gradually, issues revolving module assembly emerged. Since module manufacturers mostly apply welding and assembly techniques for PERC cells, they lack of a test standard for HJT cells, resulting in higher deviation of efficiencies and CTM than that of PERC. Lowe CTM level can be also ascribed to the fact that existing welding and assembly techniques are not compatible with HJT cells.
 

Cell to Module loss, CTM loss

Major reasons that caused lower CTM:

1. Edge loss: Edges of HJT cells do not have ITO deposition and thus the active area is smaller.
2. Laser slicing loss: The passivation layer and film layer are damaged during slicing, leading to more than 0.15% higher loss than PERC. This problem can be solved, as newly installed HJT lines will use pre-cut large format cells.
3. Dark sector decay loss: Light soaking can remarkedly improve open-circuit voltage and fill-factor. However, there’s still 0.1-0.2% of dark sector decay after standing. Special light soaking technique can improve the issue to some extent.
4. Welding and lamination

In addition, HJT manufacturers are trying to promote SMBB, which could drive up cell efficiencies and CTM through thinning busbars, increasing its number, and combine high precision string welding technique.

Copper electroplating

With higher efficiency and simpler manufacturing technique, HJT has been in the limelight in the PV industry, but advancing slowly towards mass production thanks to higher production costs, especially costs of metallization. Copper electroplating is considered the most likely way to lower production costs for HJT cells. It is simpler and cheaper but require more procedures to fabricate seed layers and metal masks. The industry continues pursuing easier manufacturing process and lower costs for mass-production. Meanwhile, the reliability of copper in replace of silver needs further confirmation as well.  

Source: 2021 CSPV_ Copper metallization of electrodes for silicon heterojunction solar cells
 

Back-contact cells

Back-contact cells, either TOPCon or HJT, have drawn increasing attention lately, as Aiko introduced Aiko Back-contact Cell (ABC cell), with 300 MW of pilot lines under construction, and other leading manufacturers purportedly develop back-contact cells. During the conference, Zhejiang University presented the non-doped full-area back-contact flexible mono-Si HJT solar cell with MoOx passivation and antireflection thin film. Dr. Wen-jing Wang provided detailed analysis of the pros and cons of heterojunction back-contact (HBC) cells. Even though the front of HBC cell is not metallized, and thus blocks no sunlight, they do not necessarily generate more electricity than HJT cells, due to inferior bifaciality. Yet, HBC cells are compatible with indium-free technology and have better durability for base metals, making it easier to reduce production costs. Therefore, HBC might not be a technology that brings efficiency gains but could be one that reduces production costs. 

Additionally, addressing to HJT’s lofty production costs, the conference saw various academic attempts to replace a-Si:H (p) with dopant-free transition metal oxides that have wider band gap, and thus lower absorption coefficient, such as MoOx and WOx, or replace a-Si:H (n) with alkali/alkaline fluorides or metallic oxides that have lower work function, such as TiOx and LiFx. Some metal oxides can serve simultaneously as the passivation layer and the antireflection layer and are still under R&D for the time being. We expect the product to commercialize as soon as possible, further reducing production costs for HJT products. 
 

Module assembly

As energy transition accelerates, PV power generation projects have more and more application scenarios. Uncertainties in outdoor environments raise requirements on weather resistance of modules, especially that and the ability to block air of encapsulation materials, such as EVA and backsheet. Diversified application scenarios lead to the segmentation of demand in EVA and backsheet markets.
 

Encapsulation

Modules choose EVA differently, taking account of three major factors: anti-PID performance, module encapsulation yield rate, and price-performance ratio. 

Source: 2021 CSPV_public information

1. Subject to POE’s slower cross-linking process and inferior encapsulation performance, more manufacturers choose EPE, a coextruded polyolefin, over POE. 
2. During the conference, Sveck promoted its gap reflective film. The product raises power output 1% lower than grid glass does,and requires automated film pasting equipment. Few module makers are using such product, as they find its ROI low. 
3. Analytic institutions all forecast short EVA particle supply next year, suggesting that more module makers will switch to thinner ribbons. The adoption of SMBB will prevail faster, whilst more manufacturers give segmented ribbon a try to further minimize the weight of EVA on the back side of modules. 
    

Backsheet

Backsheet still holds 50% of market share among encapsulation materials for the back side of modules despite increasing production of glass-glass modules, as the number of residential and C&I distribution power stations rises, whilst transparent backsheet being an encapsulation material of bifacial modules. Backsheet has evolved from KPK to KPC/KPf, CPC/FPf, PPE/PPC/PPf, and polyolefin backsheet (coextruded polyolefin backsheet, E/O film-based backsheet). Presently, more than 95% of backsheet come from China. Cell structure and module layout evolve, resulting in market segmentation, such as transparent backsheet, grid backsheet, black backsheet, eco-friendly backsheet, fluorine-free backsheet, and backsheet with aluminum layer that is developed specifically for HJT products. 

1. Fluorine-free backsheet is only used by some distributed generation stations due to concerns over weather resistance and lack of outdoor testing. Fluorine backsheet still dominates the market.
2. TPT/KPK have been gradually eliminated due to high costs. Only some state-owned businesses use lamination type backsheet.
3. At present, KPC/KPf PVDF backsheet dominates approximately 35% of market share.TPC PVF backsheet took up around 15% of market share. Cybrid Technologies predicted in its report that R142b, the raw material for PVDF, will saw supply slow in the third quarter of 2022. Prices for KPf structure backsheet will decline to around RMB 1/m2 than CPC/ PPf structure backsheet. 
4. According to Jolywood’s report, the share of CPC/FPf backsheet with fluorine on both sides accounted for 30% in 2021. The share of such backsheet is expected to rise further in 2022 due to supply of raw material for PVDF runs short and costs go up.
5. Factors including glass shortage at the end of 2020, concerns over weight of large format modules, and price reduction of transparent backsheet caused more module makers to produce bifacial modules with transparent backsheet. In 2021, the share of transparent backsheet is expected to exceed 20% for the first time in the bifacial market, with total production reaching around 13 GW.

Conclusion

Having pledged to hit peak emissions by 2030 and carbon neutrality by 2060, China will need to mobilize all its resources to increase its deployment of renewable energies to meet the “3060” goals. After China published the “Proposals of the Central Committee of the Communist Party of China on Formulating the Fourteenth Five-Year Plan for National Economic,” the country put more emphasis on renewables. As solar technology advances, the global solar sector is expected to boom.