A back-sheet is the layer of the solar panel where the solar cells are laid out. The other side of this back-sheet, specifically at the top middle area, is the junction box. A back-sheet is often made of polyvinyl fluoride or P.V.F. which is similar to P.V.C. or polyvinyl chloride, the material used in conduits and water pipes. P.V.F. is commonly used in airplane and vehicle interiors.
DuPont, an American chemical company founded in 1802, created in the 1940s the polyvinyl fluoride (P.V.F.) polymer. After around 10 years, they started developing products based on their invented material. 20 years after invention, the Tedlar brand name was finally registered by DuPont in the US. Eventually, Tedlar became the leading choice of material for the back-sheets of solar panels and is also recognized as the standard in the other industries it serves.
Polyvinyl Fluoride (P.V.F.) has excellent non-stick properties making it easy to clean. It is also weather resistant and can prevent water from penetrating the solar cells. It also does a good job in keeping water vapor from getting in. Another important feature of P.V.F. is its low thermal resistance which is crucial in the operations of the solar cells. Low thermal resistance means that the material is capable of resisting heat from transferring to the solar cells which can affect the layers that compose it. This is also the reason why some solar panels do not perform as expected when the temperature rises above 25 degrees Celsius (77 degrees Fahrenheit). All solar panels in their data-sheet have a term called "temperature coefficient pMax" and this determines the percentage of reduced performance for each degree added above 25 degrees Celsius. If the "temperature coefficient pMax" is -0.47% then the maximum power of your solar panel is reduced by 0.47% for every degree rise in temperature (Celsius). Conversely, when the temperature is below or at 25 degrees Celsius, the solar cells perform better and can increase above its rated output.
Making the most efficient solar panels requires high quality materials such as Tedlar but if you are making one on your own, an alternative would be just a plain good old plywood. Painting this plywood with a protective coating such as latex acrylic paint should do the job in resisting UV and preventing water from getting in. Plywood is also non-conductive but may not have the low thermal resistance that P.V.F. has.
