BACKSHEETS Selecting the Right Materials for Solar Modules & EVA

From cells to glass to encapsulant to backsheets, each component of a solar panel is relevant to performance and plays an important role in a PV panel. By definition, Backsheet is a film that protects the solar cell from severe environmental conditions. A solar backsheet is the last layer at the bottom of the solar PV panel and is typically made of a polymer or a combination of polymers. One of the lessvisible but essential components of a solar panel to their long-term performance is backsheets. On the other hand, ethylene vinyl acetate also known as EVA provides encapsulation for solar cells.

Why Backsheets Are Important?

Backsheets play a significant role in protecting PV modules and delivering needed module lifetime. They protect the solar panels against environmental damage and ensure that panels remain electrically insulated.

Backsheet failure can result in module failure such as terrible failure, unacceptable power degradation and safety failures. The impact can be very significant, ranging from brand and reputation damage to outright bodily harm. Solar backsheet is not just the simple layer of a plastic film on the back of the surface of the module, however this is the only layer of protection from dangerous DC voltage.

The major purpose of backsheet is to protect PV module from UV radiations, moisture penetration, electrical insulation of the system, and to offer durability to the PV module. Therefore, PV backsheet is extremely important for increasing the durability of a PV module. The mechanical, electrical, optical and chemical properties and durability of backsheets are critical to the long term reliability, durability and safety of the photovoltaic modules. However, not all backsheets are created equal. In order to protect a module for 25 years, the backsheet must have three critical properties: weather ability, mechanical strength and adhesion; however, the most important thing is the optimal balance of these three properties.

Different Categories of Backsheet Layers

With regard to the layers that form the backsheet, there are three basic categories of Backsheets – double fluoropolymers, single fluoropolymers and non-fluoropolymers and various constructions within each category. Double Fluoropolymer: This consists mainly of outer layers of Tedlar polyvinyl fluoride (PVF) films, or of Kynar polyvinylidene fluoride (PVDF) films, and a core layer of polyethylene terephthalate (PET). The molecular structure of fluoropolymers is based on a chain of carbon atoms completely surrounded by fluorine atoms, which are responsible for a better protection of the atom chains present on the layer. In terms of price, these kinds of backsheet are the most expensive.

Single Fluoropolymer: One way of reducing the cost of the backsheet while maintaining satisfactory behaviour and durability is to reduce the number of fluoropolymer layers from two to one. In this case, the layer structure is formed mainly with Tedlar or Kynar on the air side and with PET and primer or EVA layers on the inner side.

Non-fluoropolymer: This consists of two PET and one primer or EVA layers, and is the cheapest option. In the past it was not considered because of the possible degradation under UV exposure or hydrolysis over long periods of time. However, significant advances in polyester chemistry and production engineering have led to the development of highly UV-durable polyester films.

Arkema one of the market players in the backsheet industry manufactures tri-layered Kynar PVDF film which is used in backsheet lamination. Arkema Manager, Sachin V Upadhye stated that PVDF is one of the toughest molecules within fluoropolymer chemistry. By virtue of its chemistry, it gives very high chemical resistance, very good sand abrasion resistance and the longevity to the backsheet. He further said that Arkema’s KPK and KPE give much better performance than the other chemistries. Kynar PVDF is a known and proven product since 1965, whereas Polyolefin is the recent development so it is yet to prove its performance.

Encapsulant for PV Modules

An encapsulant is used to provide adhesion between the solar cells, the top surface and the rear surface of the PV module. The encapsulant should be stable at elevated temperatures and high UV exposure. It should also be optically transparent and should have a low thermal resistance. EVA (ethyl vinyl acetate) is the most commonly used encapsulant material. EVA comes in thin sheets which are inserted between the solar cells and the top surface and the rear surface. This sandwich is then heated to 150 °C to polymerize the EVA and bond the module together.

EVA sheets provide the most optimal solution for packaging solar cells with the required degree of environmental protection. However apart from just long term protection, encapsulants serve also multiple other purposes. They physically hold module components in place, provide electrical insulation, reduce moisture ingress, optically couple superstrate materials (e.g., glass) to PV cells, protect components from mechanical stress and protect materials from corrosion. To achieve this, encapsulants must adhere well to all surfaces, remain compliant, and transmit light after exposure to temperature, humidity, and UV radiation histories.

Quality Assurance and Standard Tests for EVA

The standard tests for determination of quality of various module components and especialy EVA are vastly varied and sometimes ambiguous. However the following tests are generally used to define quality of the EVA encapsulant films.

Peel strength (Adhesion test)

Adhesion of EVA to substrate is key to the longevity of a module throughout its operational lifetime. Low adhesion is a good indication of expired EVA or exposure to moisture during transit/storage or even improper laminating conditions. EN 1895 & ASTM D1876 test standards can be followed to determine the peel strength accurately. Spring balance test is often used in the industry as quick test to validate the incoming material. Values between 75 N/cm and 125 N/cm can be used as a reference for quality control.

Gel Content Test

Gel content of EVA is a measure of degree of cross-linking of the polymer. Lower gel content indicates lower degree of cross-linking, which severely jeopardises the long term mechanical integrity of the module. ASTM D-2765 test standard can be followed to determine the gel content accurately. Variations of the same are often used to approximate the gel content values at shop floor level.

Thermal Shrinkage

EVA films have an inherent tendency to shrink when heated at lamination temperatures, which if not controlled might lead to breakage of solar cells, misalignment of strings, amongst other stability issues. Shrinkage is generally caused due to process induced stresses during the manufacturing of EVA films however the rule ‘lower the shrinkage and better the encapsulant’ is not generally true due to a variety of other trade-offs. Globally, shrinkage levels of 2% or less are acceptable and are known to cause no laminating problems.

Long Term Encapsulation and Protection

A durable encapsulant requires stringent quality control, uniform compositions, and use of the best available raw materials. Even a slight deterioration of any of the properties of the encapsulant over time will impair the electrical output of the module, which is of critical importance to providing a 25 to 30 year working life of a solar module. Thus, a good EVA film should possess high peal strength, sufficient gel content, low shrinkage, and should be able to withstand the module tests and harsh environment conditions with ease.

Solar PV Backsheet Market Research

The surging demand for solar-powered systems across the globe has generated lucrative opportunities for solar PV backsheet market giants. The solar PV backsheet market has gone through several challenges recently. The industry on account of advancements in technologies coupled with the falling component cost has witnessed huge competition. Eminent participants in the solar PV backsheet market include DuPont, Isovoltaic, Coveme, Arkema, 3M, Toyo Aluminium, Madico, Hangzhou, Taiflex, Krempel, Targray, Toray, Dunmore, Astenik, and ZTT International.

In order to make their presence felt in the competitive market well-known players partaking in solar PV backsheet industry share have been adopting numerous business strategies such as joint ventures and M&As. For instance, DuPont, one of the Solar PV suppliers, has partnered up with Saudi Arabia based manufacturer, Desert Technologies in order to capture more revenue from MEA, driven by the growing demand for photovoltaic systems across the region. Under this agreement, Desert Technologies will reportedly implement Dupont’s backsheets in its solar panels and projects.

Dutch-based company, DSM has also recently acquired the Chinese manufacturer of PV backsheets, Suzhou SunShine New Materials Technology to improve its product portfolio in solar systems. With this strategic acquisition, DSM seems to be looking forward to firmly establish its position in China solar PV backsheet market over the years ahead.

Apart from business expansion, most companies in solar PV backsheet market have been observed to be focusing on the improvement of the material properties of PV backsheets. For instance, 3M has invested heavily to develop new PV backsheets endowed with the properties of reliability and long-term performance. The increasing involvement of biggies in new material development is thus slated to propel solar PV backsheet market outlook in the years ahead. Furthermore, the ongoing R&D activities and the growing advancements in the product landscape will generate lucrative opportunities for players in solar PV backsheet industry, thereby impelling its revenue graph by 2024.

Solar PV Backsheet Market Glance

According to the research by Global Market Insights, solar PV backsheet market would surpass more than USD 2.3 billion by 2024. The research includes in-depth coverage of the backsheet industry with estimates and forecast in the following segments:

Solar PV Backsheet Market, By Product

In 2016, TPT accounted for over 12 percent solar PV market share. In the present industrial scenario, the technology acts as an economical substitute to TPT configurations. These products have witnessed an appreciable penetration subject to the availability of cost-effective and efficient auxiliary backsheet technologies. Rapid technological enhancements related to operational flexibility and efficiency are anticipated to drive the product demand.

Solar PV Backsheet Market, By Thickness

Less than 100 micrometer backsheets is predicted to witness strong growth owing to lower product costs and extensive deployment across small-scale solar applications. Utility based deployments along with ongoing adoption of photovoltaic technology to establish a sustainable energy mix across industries will augment the > 500 Micrometer solar PV backsheet market.

Solar PV Backsheet Market, By Material

Fluoropolymers account for over 50 percent of global solar PV backsheet market, subject to their early adoption and unit longevity. These products possess higher efficiency and insulation properties when compared to its non-fluoro counterparts. However, the integration of fluorine for production in the recent years has inclined consumer perception towards the adoption of cleaner auxiliary backsheet technologies.

Solar PV Backsheet Market, By Technology

Crystalline solar PV backsheet market is set to witness a growth of over 4 percent by 2024. The heavier composition of crystalline cells along with large-scale deployment of these units escalates the demand for support and insulation materials. Increasing product efficiencies coupled with the development of compact panel structures will drive the deployment of advanced backsheet technologies across crystalline photovoltaic modules.