EPE: The Two In One Approach For Module Encapsulation

• Addressing the shortcomings of EVA, POE became an interesting alternate encapsulation material, especially for rear of PERC bifacial modules
• Still, POE also suffers from few inherent limitations and longer lamination times -,which are by and large addressed by the multiplayer EVA-POE-EVA (EPE) coextruded film
• The EPE structure finding increased – bubble formation, mis-alignment of busbar in MBB layout EPE, a thin layer of polyolefin sandwiched between two EVA layers is used to overcome few limitations of encapsulation material
• POE structure is finding more and more applications, especially with new cell architectures such as TOPCon and HJT
• Driven by the high prices of polymers across the board, encapsulation materiel suppliers are working on thinning down the film without affecting the performance

With the increased deployment of bifacial modules and EVA having some limitations, especially the low PID resistance, its applications are mostly limited to glass-glass modules. Module makers have started looking for encapsulation solutions beyond EVA – and POE fits the bill well. The polymer with superior water vapor barriers and high PID resistance has proved a good fit for glass-glass bifacial modules. POE is free from acid and free radical formation. While POE was initially used on both the front and rear, due to its higher costs, its application is now rather limited to the rear side, especially for PERC bifacial modules. That's because the rear side of the module contributes most to PID due to the presence of aluminum oxide on the rear side of PERC cells.

However, nothing is perfect and so is POE, which has some practical constraints. If not cared for properly, it is very easy to generate bubbles during the lamination process when using POE film. Moreover, the lamination time is longer, requiring roughly 6 minutes more per lamination cycle. When using polyolefin films for an MBB module, the busbars might be slightly displaced during the cooling cycle of the lamination. The adhesion properties of POE to glass is also inferior to that of EVA. To overcome these limitations, encapsulation material suppliers engineered a multilayer construction called EPE, consisting of a thin layer of polyolefin sandwiched between two EVA layers, manufactured via coextrusion. Using POE in the middle increases the resistance to water vapor significantly and also improves anti-PID performance. This way, the 2 outermost EVA layers provide enhanced adhesion to cells and glass, while the middle layer works as a better barrier to moisture ingression. As for lamination time, the coextruded film finds the middle ground at around 450 seconds compared to about 600 and 300 seconds for EVA and single layer POE, respectively. In order to avoid the acid formation, specially developed acid-free EVA is used in EPE configurations. All leading encapsulant suppliers we talked to are of the opinion that the single layer POE will be replaced with the coextruded EPE structure. Indeed, the structure is finding more and more applications, especially with new cell architectures.

HJT with TCO on both sides, for example, has low adhesivity with POE but good adhesion with EVA. But EVA alone is not sufficient to act as the required moisture barrier for long. Thus, Chinese company HIUV recommends its EPE structure. Cell manufacturer Jinergy, one among the early developers of HJT in China chose our EPE, said HIUV's Vice President Colin Quan. As for TOPCon, world leading solar EVA film supplier Hangzhou First says that the technology is more sensitive to water vapor ingression compared to PERC. So the company recommends the coextruded multilayer EPE for TOPCon bifacial modules, more specifically for the transparent backsheet based one. For glass-glass modules, front side POE and EVA on the back can still be adapted. But a glass-backsheet module requires POE on both sides. Using POE on both sides is not ideal for passing the damp heat test due to poor adhesion to glass, according to Hangzhou First. Also due to the shortage of EVA raw material, several companies are seriously evaluating the possibility of employing an EPE encapsulant also on the front side (see Excerpts Of Trends In Encapsulation Segment).

As for the price of EPE, it completely depends on the raw material prices of EVA and POE. Generally, the raw material price of POE is higher than EVA. But given EVA is in short supply currently, the prices of EVA resin are very high and volatile. As for the manufacturing costs, the coextrusion production process for EPE is more complex compared to making a single film. The waste from edge trims in this process cannot be recycled, which drives the costs up. Overall, the costs for making EPE are slightly higher; however, the final price difference mainly depends on the resin prices of EVA and POE. An interesting development in this context of cost reduction is that encapsulation manufacturers are also weighing the option of reducing the thickness without affecting the performance (see White EVA For Higher Module Output).

The Text is an excerpt from TaiyangNews' recent Market Survey on Backsheet and Encapsulation Materials 2021, which can be downloaded for free here.

An overview of the survey was presented during TaiyangNews Conference on Reliable PV Module Design, where also encapsulation and backhsheet suppliers presented technology updates. To learn more about the conference and view the presentations click here.