This article is the second about HJT module making, after we covered metallization as the last part of solar cell processing in this series of articles on HJT solar manufacturing (see Silver-Free Contacts Via Plating For HJT)
HJT solar technology requires changes at the module level. This optimization is done at two levels mainly at interconnection and lamination. We discussed interconnection in our previous article on HJT module manufacturing ((see Connecting The HJT Contacts). While soldering is state of art which is widely used in china and ECA based interconnection is followed in Europe. Now in this section of the article we look into role of lamination in hjt module making which involves encapsulation and backsheets .
Encapsulation
Within the scope of encapsulation, given the hydrophobic nature of the deposited amorphous silicon layers, the encapsulation material needs to be carefully selected to have high water vapor barrier properties. Risen in fact strongly recommends the optimization of the encapsulation material, especially from a TCO compatibility standpoint. "We observed peel-off between the encapsulation and ITO layer and white spots near the ribbon area," said Risen's Yang. To address these issues, Risen has also developed a special encapsulation solution that promotes adhesion between the ITO and encapsulation, while Yang declined to provide any details of the polymer chemistry (see Reducing Indium In HJT TCO Processing).
A few innovations to enhance the optical characteristics of the module through encapsulation are also under development. Maxwell, in cooperation with leading backsheets and encapsulation materials supplier Cybrid, has developed a light conversion film that improves the optical absorption of the module, thereby increasing power by 5W. As discussed above, the TCO and amorphous silicon layers absorb UV light, thus the typical HJT structure suffers from low currents. To mitigate this loss, Maxwell and its partners have developed a photoelectric conversion film. With the use of such a film, ultraviolet light with a low photon response for HJT solar cells can be converted into blue light or red light with a higher photon response, thereby increasing the power of a 60-cell M6 HJT solar module by more than 5W.
Backsheets
When it comes to backsheets, using a layer of aluminum in the backsheet structure was mainly promoted for HJT. However, such a construction limits its usage for monofacial modules. In order to qualify for the transparent backsheet based HJT module structures, Coveme has recently introduced backsheets with a barrier film, replacing the opaque aluminum layer. Also part of the DyMat range, this backsheet is integrated with a barrier layer which gives the backsheet a low WVTR value. They are available in white, black or transparent versions for installations up to 1500 VDC.
Overall, there is a lot of effort being put into a module station of HJT. Not only the process, Risen has also optimized the equipment and materials used in module making, and has even developed a few materials such as encapsulants that are exclusive to the company, according to Yang.
The industry-wide opinion, however, is that the development effort at the module level is less than optimal, especially compared to the cell level. The major problem with the HJT technology has been the low cell-to-module performance ratio (CTM); ironically, the CTM reduces with an increase in cell efficiency. The CTM losses are mainly governed by a few factors – high finger resistivity, dark degradation, high cutting losses and exclusion of ITO at the edges (about 0.6 mm) to avoid shunting, lists Huasun's CTO Wenjing Wang. Increasing the number of busbars and the finger cross-section is one way to reduce finger resistivity. It is well known that HJT has higher cutting losses, i.e., when the cell is cut into two pieces for a half-cell configuration. There are three main reasons for it – edge shunting, non-uniformity of the cells and passivation damage on the cutting edge. Edge shunting results from the fact that – unlike the other three edges – the TCO at the cut edge extends till the point it's cut (no edge exclusion). Huasun observed that the lifetimes of the two half slices of a cell are different, meaning their electrical properties are also different, which causes mismatch issues at the module level. Processing half wafers can avoid most of these issues (see TCO Deposition First Foreign, Increasing Locally).
Turning to degradation, upon the cell's exposure to operating conditions, Huasun observed a decrease in the FF, thereby efficiency, while the current and voltage are not affected. Wang characterizes it as dark degradation. The mechanism behind it is that the oxygen enters the cell through TCO and reacts with amorphous silicon to form amorphous silicon oxide, increasing resistance as a result.
Huasun also found a way to recover the losses completely by subjecting the cell to light and heat treatment, in which the annealing step breaks the bond between silicon and oxygen.
All this implies that factors concerning CTM losses at the module level have remedies at the cell level, especially optimizing the cell processing. This also implies how the cell and module making steps are interdependent in case of HJT. That's why Huasun has opted for a cell-and-module integrated factory and Wang highly recommends having such inhouse cooperation till HJT production is evolved enough to follow standard manufacturing practices.
The text is an excerpt from 3rd edition of TaiyangNews' Heterojunction Technology 2022 report, which provides an overview on the most recent HJT developments as the technology is entering the GW scale production level and can be accessed free of charge here.
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