Backsheet products are based on different structures using a variety of polymers. These compositions have evolved over time, in tune with the changing needs of module makers – and there is plenty available, as shown in the graphic.
The backsheet has two key roles, which polyester (PET) alone can fulfill and that too in a very cost-effective way.
However, PET's rather poor UV stability means that it is limited in its ability to become a self-sufficient backsheet material. Thus, this core layer of the backsheet is sandwiched between two UV resistant films leading to the typical three-layer construction of a backsheet; and UV protection is where fluoropolymers come in.
To provide some historical context, the early PV modules used backsheets based on an FPF structure with the core PET layer sandwiched between two layers of fluoropolymer. The fluoropolymer here is nothing but the proprietary polyvinyl fluoride (PVF), commercially known as Tedlar from US-based chemical giant DuPont. Such a backsheet structure was often referred to as TPT, which was also the first backsheet composition used for PV module applications. TPT dominated the backsheet market for many years.
PVDF, also belonging to the fluoropolymer stream and used in other outdoor UV-proof applications, has found a footing for itself in the backsheets segment as well. Chemical company Arkema, headquartered in France, developed a special three-layer PVDF film with the Kynar brand name. The backsheets based on Kynar also follow a similar structure as TPT, abbreviated as KPK, which is also a registered trademark of the company. Even today, every fluoropolymer film based backsheet supplier has at least one product representing this symmetrical structured backsheet.
As module prices continued to decrease, the cost pressure on the backsheet producers was growing. At some point, they realized that making symmetrical structured backsheets with fluoropolymers on both sides was a bit of over-engineering, especially when the cumulative UV exposure of the inner film is only about 1/10th of the outer layer of the backsheet. During this time, backsheet manufacturers replaced the rather expensive fluoropolymer films with a more economical polyethylene, which is usually referred to as "E" or "Primer" layer. This initiated the first configuration change in backsheets. All fluoropolymer backsheets then shifted to a polyethylene-based inner layer, leading to new backsheet abbreviations such as TPE and KPE. All of the suppliers involved with fluoropolymer film based backsheets then started offering the symmetrical variants as well as products with an inner "E" layer.
What also led to this shift to the "E" layer was the fact that Tedlar was in short supply during this period of time, pointing the research in the direction of fluorine-free backsheets. Benefiting from the successful implementation of the polyethylene inner layer and the PET core, which are both fluorine-free anyway, the only effort needed here was to develop a fluorine-free outer layer. Coveme, the development leader in this field then, as they still are now, came out with a UV-stable PET in close cooperation with DuPont Teijin Films( DTF), which has provided the base material. This backsheet configuration is denoted as PPE, which is still Coveme's main product. The fluorine-free aspect of these backsheets has not only been used to market this product as "environmentally friendly", but has also made them less costly than their fluoropolymer-based counterparts.
Some companies followed this template to redefine the backsheet structure by replacing the films with coatings. Such backsheets are commonly abbreviated as CTC, where "C" stands for coatings of both types – with and without fluorine. Hangzhou First and Jolywood are both offering fluorine-containing coatings, while Aluminium Féron from Germany is supplying backsheets coated with non-fluoropolymers.
While the inner layer of the backsheet is not exposed to UV as much as the outer layer, the radiation cannot be neglected altogether. Cybrid worked on this idea and developed a so-called fluorine skin, which is essentially a fluorine coating denoted as KPf. Cybrid's inner coating solution fully addresses the UV protection concerns on the cell side. At the same time, the coating is far less expensive than a fluorine film. The idea sold so well that the then young Chinese company soon became the market leader of the segment.
Cybrid is supplying its backsheets predominantly based on PVDF as outer layers, while Kynar-based backsheets are also a part of its portfolio. Cybrid's success with this approach was enough to lure other companies to inner coatings. This move was actually very easy for companies that had already been working on coating-based solutions, such as Hangzhou First, to use their expertise for cell-side coatings.
Hangzhou First is also offering backsheets in the PVDF/PET/coating format. Jolywood, another coating expert and an important laminator for Tedlar-based products, introduced yet another structure called TPC, adapting Cybrid's template into its proprietary coating technology. For module manufacturers that are not comfortable with coatings, backsheet suppliers a few backsheet suppliers started offering backsheets with polyolefin as inner layer, denoted as KPO. Cybrid and Hangzhou First are promoting this structure. On the other hand, Lucky Film has started offering a Tedlar variant, characterized as TPO.
What's common about all the backsheet structures mentioned above is that they are all based on PET as the core layer. Endurans, taking a different route, developed backsheets that use polyolefins as core layers – coextruded instead of the typically followed lamination method. The company in past was marketing a backsheet made of polyamide top layer, with a polyolefin inner layer and core with slightly modified chemistries between them denoted as APPO However, the company has changed its focus on to the all-PP based backsheets. Borealis is yet another strong proponent of all-PP coextrusion solution, however, the resin supplier is cooperating with backsheet makers instead of offering the final product directly.
To summarize, the major backsheet configurations in today's market are TPT, TPE, TPC, TPO, KPK, KPE, KPf, KPO, PPE, CPC, APPO and all-PP. The infographic lists the most popular backsheet structures and their abbreviations.
The Text is an excerpt from TaiyangNews' recent Market Survey on Backsheet and Encapsulation Materials, which can be downloaded for free here
An overview of the survey was presented during TaiyangNews Conference on Reliable PV Module Design. To learn more about the conference and view the presentations click here
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