Transparent bi-axially oriented multi-layer film, used in laminate used in backsheet of a transparent photovoltaic panel, comprises polyester layers consisting of core layer and external layer comprising load particles and light stabilizer

The transparent bi-axially oriented multi-layer film comprises two polyester layers consisting of a core layer and an external layer that are identical or different. Each polyester layer includes a transparent bi-axially oriented polyethylene terephthalate film including 1.3-3 mole/ton of an alkaline metal phosphate and phosphoric acid that is 0.4-1.5 times (by mole) that of alkaline metal phosphate. The increase in carboxylic acid in the polyethylene terephthalate film measured before and after a wet hot treatment carried out at 155[deg] C for 4 hours under saturated vapor is = 90 eq./ton. The transparent bi-axially oriented multi-layer film comprises two polyester layers consisting of a core layer (B) and an external layer (A) that are identical or different. Each polyester layer includes a transparent bi-axially oriented polyethylene terephthalate film including 1.3-3 mole/ton of an alkaline metal phosphate and phosphoric acid that is 0.4-1.5 times (by mole) that of alkaline metal phosphate. The increase in carboxylic acid in the polyethylene terephthalate film measured before and after a wet hot treatment carried out at 155[deg] C for 4 hours under saturated vapor is = 90 eq./ton. The external layer comprises particles of load and light stabilizer. The film satisfies an equation of (3.1>= [C]x [T]x [D]>= 0.003), where [C] is concentration of particles by weight in the external layer, [T] is thickness of the external layer containing the particles in mu m, and [D] is median diameter of the particles d50 in mu m or average diameter of the particles in mu m. An average elongation retention of the film after aging for 60 hours at 121[deg] C is in an increasing order of greater than 30%, greater than 40%, greater than 50% or greater than 40%. The film further comprises another external layer (C) arranged in one side of the core layer, while the former external layer is arranged on the other side of the core layer. The particles of load have a median diameter of 0.03-5.5 mu m, an average particle size of 0.03-5.5 mu m or 6 mu m, and a porous volume of greater than 0.9 ml/g. A total concentration of the light stabilizer is in an ascending order of 0.01-35 w/w%, 0.03-25 w/w%, 0.05-20 w/w%, 0.1-10 w/w% and 0.1-5 w/w%. The film has total transmittance of the light of 80-100%, 84-97% and 86-95%, a haze of less than 5%, less than 4% and 0.01-3%, a dynamic friction coefficient of less than 0.55, and an overall planar sonic modulus of 440-465 kgf/mm 2>, 445-465 kgf/