It is well known that HJT requires a completely different fab setup and process sequence. The change in fact starts at the wafer level. HJT typically uses n-type wafers, unlike today's mainstream that relies on p-type base wafers. Not just base polarity, the quality of wafers is also equally important. The advantages of HJT are a high Voc of 740 mV and above, which is a result of particularly good surface passivation. If anything is limiting the voltage, it is the bulk rather than the surface. The advantage of HJT is that there is no involvement of thermal treatment of the wafer during cell processing, which eliminates the risk of any change in bulk lifetime during the process.
However, this also means the technology requires that the initial silicon used be of high quality. According to a presentation held at the 3rd International HJT (SHJ) Workshop on Siliocn HJT Solar Cells of FZ Juelich Research Center in Oct. 2020 by Yichun (YC) Wang, director of application engineering and key customer service at LONGi Silicon, there are four important prerequisites for n-type wafers, which are:
It is experimentally proven that the efficiency of HJT cells is very sensitive to wafer material quality. Specifically, a joint research project by LONGi and Meyer Burger found a direct correlation between the ratio of lifetime and wafer resistivity (Teff/ρbulk). LONGi quotes this ratio at a minimum of 2,000 as the benchmark, while adding the same to the specifications of n-type wafers.
Polysilicon being the raw material for wafers, its quality also has an impact on the final quality. According to Wang, due to the inconsistency in high-quality polysilicon supply, especially from overseas, qualifying more domestic suppliers is especially important. This not only streamlines the supply, but also makes the polysilicon immune from trade barriers. LONGi has been testing and verifying various domestic suppliers for supplying high-quality polysilicon suitable for n-type wafer production. In order to improve the yield, LONGi has reduced the interstitial oxygen content to 14 ppma.
Another topic related to wafer quality are ring patterns, caused by impurities such as dopant, oxygen precipitation, thermal donor and defect distribution. While these are usually thought of as harmful, Wang emphasizes the importance of identifying harmless ring patterns and the need for the market to accept them in order to improve the yield.
Employing larger wafer formats is a simple way of increasing the power per cell independent from efficiency, which has been a very hot topic in 2020 for PV manufacturing. Leading PV manufacturers were successfully able to implement PERC technology on larger wafers such as 182 mm and 210 mm. However, a move to larger wafers is not yet the case with HJT. According to Wang's presentation, the G1 size, which is 158.75 mm full-square, is still the mainstream for HJT. However, Wang said that while a few sample requests did come in for M6 and M4+ formats, there have been no requests for M10 or M12 so far (see graph).
This text is from our recently published HJT Solar Technology 2020 report, which provide the latest details on this promising high-efficiency solar cell technology. Download the report for free here.