The first impression of TOPCon cells on a general note is that the passivation and metallization schemes are inverse to that of PERC, which means the aluminum oxide passivation is on the front So, the front side requires the highest degree of protection from PID. The metallization paste used on the emitter side of the cells is aluminum doped, thus is more prone to corrosion, requiring high barrier from moisture ingression. This is also applicable for the rear side of the TOPCon structure as it features a complex passivation stack of amorphous silicon and tunneling oxide, which are also moisture sensitive. Thus, it also requires some optimized solution for the rear side.
According to Cybrid's International Marketing Director Eric Yang, the choice is between POE or EPE on the front side and EPE or EVA on the rear side in glass-glass modules. For the glass-backsheet structure, it mainly depends on the WVTR barrier properties of the backsheet. With a standard backsheet, the choice is only pure POE film, as only such modules are able to pass the testing protocols, according to Yang. However, using the latest generation of high barrier backsheets recently developed by several backsheet suppliers or employing backsheets inserted with an additional aluminum film may enable the use of EVA or EPE. However, Yang emphasizes that the industry is not willing to use aluminum backsheets, one for bad processing, and then also for higher costs (see Trends In Encapsulation Solutions).
Anti-slip solutions for POE
Nevertheless, the current practice is to use pure POE film, at least on the front side. Addressing the common problems with POE, slipping of busbars, Cybid has developed an anti-slip solution by adjusting the process of mixing and film making, improving the embossing, and method for inhibiting the precipitation of auxiliaries, the latter being the ultimate, Gong emphasized in his talk at the latest TaiyangNews Reliable PV Module Design Conference (see Stephen Gong's presentation: Innovations In Solar Module Wraps).
Since basically all top tier companies are investing heavily in TOPCon and the leaders of the segment such as JinkoSolar have already installed 2-digit capacities, the market share of the pure POE film was a little higher in 2022 compared to 2021. The pure POE films still suffer from the sliding issue due to the fact that the additive is flowing out of the film. There are some solutions under development to block the additive flowing out of the film, according to Cybrid's Yang. There are even more burning issues because of which the industry is seriously evaluating other options. As it is, the POE resin is quite a bit expensive. Then there are looming fears of POE being pushed into short supply. There is an industry-wide belief that the current supply of the POE resin can cater to less than 100 GW, especially when using the pure POE film, and several marketresearch firms as well as the industry are quite upbeat about the TOPCon technology and expect it to exceed 80 GW already in 2023
The leading suppliers of polyolefin resins are Dow, Mitsui Chemicals and LG. Segment leader Dow's manager Grand, while not revealing the production capacity for its POE resin, says that solar is definitely a priority for the company. Grand further emphasized that the advantage of the company is in its ability to serve different markets locally, with production facilities in Asia, Europe and North America.
The first obvious approach to POE resin consumption is to again employ the EPE structure. A few companies have started R&D efforts in this direction. However, according to HIUV's Senior Marketing Manager Peter Peng, the standard EPE structure used in PERC modules is not sufficient in itself to pass the reliability tests for damp heat and PID. HIUV, on the other hand, has found a solution that is also based on coextrusion but with a little tweak. Peng says the solution involves using only 2 layers of EVA+POE instead of EPE called EXP, where the 'X' layer is for strengthening the bonding between coextruded EVA and the POE stack. The solution is based on the analysis that when POE is working but the EPE isn't, it indicates that the stack should have more POE content over the EPE level. Peng says that the ratio is currently a secret, but emphasizes that the POE content can be reduced in the future through further optimization.
As mentioned above, since the TOPCon uses aluminum pastes, the POE side faces the cell side and the EVA side is attached to glass. The structure would become the mainstream for TOPCon in 2023, according to Peng. Cybrid has also developed EP solution it is offering. (see: Peter Peng's presentation: Reliable Encapsulation Of TOPcon & HJT Modules).
The article is an excerpt from the TaiyangNews Market Survey on Backsheets and Encapsulation 2022-23, which can be accessed free of charge here.