POE film is one type of encapsulation film used in photovoltaic modules. Other common encapsulation films include EVA film and EPE film.
According to data, photovoltaic encapsulation materials are integrated between the cover glass/backsheet and the middle cells of the module. The main function is to protect the solar cells, so that the photovoltaic module can operate without being affected by external factors, prolonging its service life, while allowing sunlight to pass through the film to reach the surface of the cells and increasing the photovoltaic module's power generation efficiency. Therefore, photovoltaic encapsulation films need to have features such as high light transmittance, resistance to UV, humidity, and yellowing, and good adhesion with glass and backsheet.
Currently, the main photovoltaic encapsulation materials are EVA film, POE film and EPE film. The cost of the film accounts for about 4-6% of the module cost. Although the proportion is not high, it has a significant impact on the service life and power generation efficiency of the module.
The service life of photovoltaic modules is generally 20-30 years, and they are generally required to have a first-year efficiency degradation of no more than 2.5%, and an average annual degradation of no more than 0.7% in the following years. The failure of photovoltaic modules generally includes initial failure, intermediate-term failure and late failure. PID effect, film yellowing, delamination and cell corrosion caused by water vapor infiltration are directly related to encapsulation films.
Firstly, in terms of volume resistivity, as the operating temperature increases, the volume resistivity of EVA film, which is made by the EVA extrusion process drops rapidly, and at high temperatures, the volume resistivity of POE film is 1-2 orders of magnitude higher than that of EVA film;
Secondly, in terms of water vapor permeability, POE made by the film extruder is a non-polar material, which cannot form hydrogen bonds with water molecules and does not adsorb water vapor like EVA (which has a polar group of vinyl acetate), resulting in a water vapor permeability that is one-tenth that of EVA material;
At the same time, thanks to the saturated structure of the POE macromolecular chain, the POE film contains relatively few tertiary carbon atoms in the molecular structure, giving it better thermal aging and UV resistance.
EVA film was the mainstream material for photovoltaic encapsulation film in the PERC era, and was the most mature film material in the past. Although POE has significant advantages over EVA, the price of POE particles previously limited its market share.
However, whether it is the increase in the market share of dual-side modules, or the photovoltaic industry's move towards N-type cells, higher requirements are being placed on the performance of encapsulation film. Against this background, POE film is vying for the leading position.
Dual-side modules increase power generation by 5.67%, 11.81%, and 24.60% in grass, sandy and high-reflectivity white scenarios compared to their single-side counterparts, with maximum gains of over 30% when used with tracking brackets. Therefore, the penetration rate of dual-side modules has been increasing in recent years. Since POE film does not contain acetate ions, it is more suitable for encapsulation of dual-side modules. Currently, dual-side modules are usually encapsulated with POE film or co-extruded POE film.
At the same time, the next generation of mainstream technology, N-type cells, also require POE film. N-type cells also place higher demands on encapsulation film: firstly, N-type cells use silver paste containing aluminum, which requires higher water resistance of the film; secondly, N-type cells require higher lightweighting, and finally, the PN junction of N-type cells is opposite to that of P-type cells, making the front-side more susceptible to PID. POE film perfectly meets the requirements of N-type cells.