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High-efficiency Polysilicon, Cast Mono Cells, and Module Technology Applications

2019-04-26   

On March 8, 2019, the 2019 High-efficiency Polycrystalline and PV Advanced Technology and Industry Seminar which was organized by the Jiangsu PV Industry Association was held in Suzhou. Dr. ZHANG Chun, CTO of GCL-SI, gave a keynote presentation with an title of High-efficiency Polysilicon, Cast Mono Cells, and Module Technology Applications. In his presentation, he shared major breakthroughs have been made in cast mono technology within three phases .

 The current average efficiency of mass-produced black polysilicon PERC cells reaches to 21% and the corresponding power output of a 60-piece module reaches to 300W+.

 The current average efficiency of mass-produced GCL Cast Mono PERC cells reaches to to 21.87% and a 72-piece module with power output of 370W+ is achieved.

 The average efficiency in 2019 of mass-produced polysilicon PERC cells reaches to 21.5%, and the average efficiency of GCL Cast Mono PERC cells reaches to 22.3%.

 Technology to control LeTID of polysilicon PERC cells has been commercialized. The technology to control the LeTID of GCL Cast Mono cells requires further improvements.

Today, I will talk about high-efficiency polysilicon, and cast mono cells, and their module technologies. There are three parts in my presentation: the first one is black polysilicon PERC cells, the second one is GCL Cast Mono cells (also referred as GCL-SI’s improved cast monocrystalline ingot PERC cells), and the third one is the application of polysilicon cell LeTID and GCL Cast Mono cell LeTID in the cell industry.

I will start with high-efficiency polysilicon cells. Currently, joint efforts have been made by all staff at GCL-SI and with the basis of GCL-Poly’s high-efficiency polysilicon wafers, we have achieved GCL’s mass-produced black polysilicon PERC cells with an average efficiency of up to 21%.

Here are two research projects have been successfully elected as the National Key R&D Program of China. One is the key technology for the commercialization of high-efficiency P-type polysilicon cell which is led by GCL-SI; the another other is the N-type polysilicon cell project which is led by another well-known company in the PV industry. Both projects have been made known to the public and will be officially announced in the press. Canadian Solar is our important partner in both projects. We will achieve the average efficiency of over 22% for mass-produced, high-efficiency polysilicon cells within three years.  

I will now discuss GCL-SI’s development of GCL Cast Mono within the three aspects of cells, modules, and their advances in LID. In the first phase, we produced about 1.7 million cells using the 5BB technique. The average efficiency was between 21.2% and 21.3% and the module power reached up to about 305W. On top of that, we used the multi busbar technique to push the efficiency up to 22%.

Here we can see that if GCL Cast Mono cells were going to be mass-produced, we must solve the quality related problems such as cost, trailing, appearance, LeTID, and C-class cells. The blue box with a gray bottom indicates how trailing affects the average efficiency of the Cast Mono cells.

GCL Cast Mono cells are made with basically the same technique as high-efficiency mono cells, but the wet black silicon-production technique for making C-class cells significantly improves the appearance. The efficiency of C-class cells in the first phase was less than 21%. But in 2019, the average efficiency will reach to over 21%.

Now let’s take a look at EL. In the first phase, we can see that the defects indicated in the red circle can reduce the cell efficiency. However, much better results have been shown in the second and third phases.

In the first phase, we introduced some essential improvements in silicon wafers. You can see that in the second phase, production has been risen to over 3 million cells by using the multi busbar technique instead of the 5BB technique which was used in the first phase. At the same time, improvements have been made through automatic sorting of silicon wafers. Late in the second phase, we obtained an average efficiency of up to 21.8%. Such improvement is mainly due to our efforts made on the improvement of silicon wafer production, specifically on sorting and production technique for silicon wafers.

Let’s have a look at automatic sorting. In the chart, we can see that the efficiency of the gray main class is between 21.8% and 21.9%. The main-class efficiency has been shifted towards to the high efficiency. By improving wafer quality, the average efficiency was reached to 21.9%.

In producing millions of wafers in the first and second phases, by keeping better quality control over the technique on the production line, we ensured an average yield rate at 95%. Another key point is that in the production of GCL Cast Mono cells, the proportion of flakes of small grains reduced gradually. With good wafer quality, improvements of cell fabrication techniques will produce much better results. You can see that before introducing the SE technique (which is commonly used for monocrystalline PERC cells) in GCL Cast Mono cells fabrication, their average efficiency was 21.95% from the test result from 9,000 experimental cells by using the multi busbar and the high-resistance technique.

After improvements have been made in the cell fabrication technique in the second phase, we saw much better results in the third phase. Here, based on the 7 million cells results, an average efficiency of around 21.9% is achieved. With good wafer quality, our cell fabrication technique is improving with a stable condition. Mass-produced products now have an average efficiency of about 21.9%.

The percentage of poor EL in the production of GCL Cast Mono cells has been reduced to 0.05%, and the percentage of the severely substandard dislocation is reduced. The EL results show that the quality of GCL Cast Mono cells has now reached to a high level.

If we ensure wafer quality and a stable cell fabrication technique, we can see that over 70% of the modules have a main-class power output of over 370W. The next step is to increase cell efficiency and module CTM. We will make continuous efforts in this aspect. GCL Cast Mono cells has great potential to ultimately become as good as, or even better than, Czochralski mono cells.

In 2019, we strongly believe that GCL Cast Mono cells will reach the average efficiency of above 21.8% which  is in direct competition with Czochralski mono cells. The roadmap for GCL Cast Mono cells in 2019 will focus on two aspects: further improvement of wafers and enhancing cell production technique. Currently, the SE technique is not yet introduced in GCL Cast Mono cells. Once this technique is implemented with our current cell fabrication process, we are very confident to reach an average efficiency of 22.2% - 22.3% for mass-produced cells.

Finally, let’s look at LID results for high-efficiency polycrystalline cell and GCL Cast Mono cell LeTID.

After 1,000 hours at a light intensity of 1KW/m2and a temperature of 75 degrees, the LID was kept within 2% for polycrystalline cells. Under the same conditions, the LID for GCL Cast Mono cells was kept within 2.5%. Here, we emphasis that the current technical requirements for the control of the LID in GCL Cast Mono cells and poly cells are different: LID control for GCL Cast Mono cells is processed at a higher temperature to ensure greater efficiency. In future, we will optimize cell efficiency by controlling LID.

As we know, the cell LID testing is done under an open-circuit condition. The LID test in a module level is carried out at its highest-efficiency output level. However, cell degrades more under the open-circuit condition. Thus, LID testing conditions for the cells are more strict. If the cells passed the test, the module itself would pass with better performance and concerns regarding to its long-term performance would be reduced. As a result, GCL Cast Mono cells fully meet degradation ratio standards for the first year and the long term.