TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Process: The process further comprises manufacturing the hydrolysis stabilizer by the reaction of epichlorohydrin with the branched monocarboxylic acid. The hydrolysis stabilizer is reacted with the polyester by injecting the additive into the molten polymer prior to extruding the layer. Preferred Components: The metal cations are sodium cations. The metal cation is present in the film, and/or present in the reaction mixture during the reaction of the hydrolysis stabilizer with the end-groups of the polyester, is at least 45 (at least 65) ppm by weight, and/or at least not greater than 500 (preferably not greater than 200) ppm by weight, relative to the amount of polyester. The intrinsic viscosity of the polyester in the polyester film is at least 0.65, preferably 0.8. The hydrolysis stabilizer: is present at 0.1-2 wt.% relative to the total weight of the layer; in the polyester film comprises at least one glycidyl ester of 5-15C branched monocarboxylic acid, where the branched monocarboxylic acid is saturated, and/or synthetic material; is manufactured by the reaction of epichlorohydrin with the branched monocarboxylic acid; is oxiranyl of formula (I); and is reacted with the polyester by injecting the additive into molten polymer prior to the polymer being cast into a film. The polyester film further comprises a UV-absorber, which is an organic UV-absorber including triazines (preferred), benzophenones, benzotriazoles or benzoxazines. The UV-absorber is present at 0.1-10 wt.% relative to the total weight of the layer. The polyester film has been stabilized by heating-setting at 200-225 degrees C. The polyester of the polyester film exhibits an endothermic high temperature at (A1) degrees C and an endothermic low temperature at (B1) degrees C, where both peaks are measured by differential scanning calorimetry, and the value of (A1-B1) is 15-50 degrees C. The polyester film further comprises an anti-oxidant. The polyester film has disposed on a first surface of it an additional polymeric layer, where the additional polymeric layer is a polyester layer optionally comprising at least one additives, which are hydrolysis stabilizers, UV-absorbers, anti-oxidants or particulate inorganic fillers, where the second layer exhibits a haze of not greater than 30%. The polyester is polyethylene terephthalate.R1, R2 = alkyl, where preferably at least one of R1 and R2 are CH3; andR3 = alkyl (preferred) or H, where the total number of C-atoms in the alkyl of R1-R3 is 3-48C, preferably 3-13C.    TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The electrode layers and photovoltaic-active layer are encapsulated in a suitable encapsulate, where the back-plane comprises the film. In the photovoltaic cell, the film has disposed on a first surface of it an additional polymeric layer exhibiting a haze of not greater than 30%, and the film is outermost in the multi-layer assembly. EXAMPLE - Cardura E10P (RTM: Neodecanoic acid 2-oxiranylmethyl ester) (800 ml/hour) was metered directly into polyethylene terephthalate (PET) melt stream in a film line twin-screw extruder, where the flow rate of the PET was 93.3 kg/hour. The PET contained Dispex G40 (RTM: Sodium salt of acrylic copolymer) (250 ppm) was added at the start of the polymerization process, along with the terephthalic acid and ethylene glycol, and further contained titanium dioxide (0.3 wt.%) and silica (0.3 wt.%) of the polyester. The reaction mixture was extruded at 285 degrees C, cast onto a cooled rotating drum and stretched in the direction of extrusion to 2.9 times its original dimensions at 86 degrees C. The cooled stretched film was then passed into a stenter oven at 110 degrees C where the film was dried and stretched in the sideways direction to 3.4 times its original dimensions, followed by heat setting the film at 220-232 degrees C, to obtain the biaxially oriented polyester film.