One interesting development in the stream of backsheets is the adoption of the co-extrusion process. Endurans has been a longtime advocate of such polyolefin backsheet technology. According to Imco Goudswaard, Commercial Manager from Endurans, polyolefins are widely available, and co-extrusion can skip intermediate film production steps, resulting in cost-effective manufacturing. Polyolefins are also highly inert, durable, and proven in similar applications, such as roof membranes, which endure environmental conditions comparable to those faced by backsheets. Additionally, polyolefins offer excellent moisture barrier properties and allow for the easy release of degradation byproducts, like acetic acid. From a sustainability perspective, polyolefins have the advantage of being thermoplastic and recyclable, offering a lower carbon footprint than fluoropolymers and enabling circular manufacturing processes.
Endurans developed a transparent backsheet based on the coextrusion process designed specifically for bifacial TOPCon modules. The product is based on HPO co-extruded polyolefin technology and features a light-trapping design to enhance energy capture. Traditionally, transparent backsheets or glass, often with a mesh, have been used in module construction. However, as cell gaps continue to shrink, the effectiveness of the mesh has diminished. Endurans’ Goudswaard explained that by eliminating the mesh and integrating its light-trapping technology, the company’s latest transparent backsheet helps improve production efficiency, as there is no need for alignment during module assembly. In addition to the well-known weight advantage compared to glass, Endurans’ product is fully recyclable and offers a significantly lower carbon footprint due to the absence of titanium dioxide. Above all, its translucent nature enhances its light-trapping ability. Goudswaard emphasized that beyond front-sidelight trapping, the backsheet also improves the transmittance of diffuse light. Since the light from the rear side is never perpendicular and is always scattered and diffused, the high level of diffuse transmittance results in a greater light-trapping effect compared to typical fully transparent backsheets.
Providing a comparison, Goudswaard noted that while glass typically has around 92% transparency and commercial transparent backsheets achieve approximately 82% to 83%, the company’s backsheet offers about 78% transparency. This lower transparency contributes to better rear-side light diffusion, ultimately leading to an overall energy gain of around 3%, which is particularly advantageous for bifacial modules. The company also pointed out that the product is manufactured in the US, aligning with domestic production incentives.
Endurans also presented the results of reliability tests, which demonstrated minimal optical and structural degradation. The company reported that modules featuring its backsheet produced 0.3% higher power compared to PET transparent backsheets when flashed on the front side. However, the most significant benefit came from increased bifacial energy yield in real-world conditions. Field tests conducted on both tracking and fixed-tilt configurations confirmed a consistent energy gain of approximately 3% over conventional transparent backsheets, with the improvements being particularly noticeable on the rear side due to superior light diffusion (see Growing Preference For Glass Backsheets In N-Type Technology).
Coveme has also developed a special high-barrier transparent backsheet for TOPCon modules. It is based on the company’s well-established transparent backsheet structure, featuring a 3-layer composition – one UV-resistant PET layer, one hydro-resistant PET layer, and a special high-density primer. Alessandro Anderlini, head of the photovoltaic division at Coveme, emphasized that rather than developing an entirely new backsheet structure, Coveme focused on enhancing its existing proven design by increasing the primer thickness and utilizing a specialized barrier material. This thicker primer, which also increases the overall thickness of the backsheet, improves the moisture barrier properties by 4 times, according to Anderlini. This significantly enhances module durability, providing a strong alternative to glass-glass modules for bifacial solar panels, he adds. The product was introduced last year and is already in use by several customers, with more currently testing it, underscores Anderlini. Early reports indicate strong resistance when paired with TOPCon modules, and Coveme has successfully conducted 2,000-hour damp heat tests, showing only 1.5% to 3% power degradation, which Anderlini characterizes as “excellent in the industry.” Further testing is being conducted in collaboration with Fraunhofer Institute, along with additional customer trials (see Coveme's Italian-Engineered PV Backsheets Featured At REI 2024).
Cybrid has also developed a high water-barrier backsheet, details of which were presented by Jacob Harris, the company’s overseas account manager, at the TaiyangNews Cell & Module Production Equipment & Processing Materials Virtual Conference. The top layer of this backsheet consists of UV-resistant PE, followed by an adhesive and a specially formulated water-barrier primer, all laminated onto a PET core layer featuring Cybrid’s proprietary coating on the inner side. This innovative backsheet achieves an impressively low water vapor transmission rate of 0.05 g/m2/d, making it particularly well-suited for TOPCon and HJT modules, according to Harris. Cybrid mainly promotes the transparent variant of this solution as the water-barrier primer technology does not compromise transparency (watch Jacob Harris’ presentation on Advanced Backsheet Structures here).
