TECHNOLOGY FOCUS - POLYMERS - Preferred Film: Alternatively, the biaxially oriented polyester film comprises polyester and at least one hydrolysis stabilizer selected from a glycidyl ester of a branched monocarboxylic acid, where the monocarboxylic acid has 5-50 carbon atoms, where the hydrolysis stabilizer is present in the film in the form of its reaction product with at least some of the end-groups of the polyester, and where the polyester film further comprises a metal cation selected from Group I metal cations. The metal cations are selected from Group I metal cations, where the metal cations are sodium cations. The amount of the metal cation 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 ppm, preferably at least 65 ppm by weight, and/or preferably not more than about 500 ppm, preferably not more than about 200 ppm by weight, relative to the amount of polyester. The intrinsic viscosity of the polyester in the polyester film is at least about 0.65, preferably 0.8. The hydrolysis stabilizer is present in an amount in the range of 0.1-2.0%, relative to the total weight of the layer. The hydrolysis stabilizers in the polyester film consist essentially of at least one glycidyl ester of a branched monocarboxylic acid. The branched monocarboxylic acid has 5-15 carbon atoms, and/or where the branched monocarboxylic acid is saturated, and/or the branched monocarboxylic acid is a synthetic material. The hydrolysis stabilizer is manufactured by the reaction of epichlorohydrin with the branched monocarboxylic acid. The hydrolysis stabilizer has Formula (I).R1 and R2=independently selected from alkyl, and preferably at least one (and in one embodiment only one) of R1 and R2 are selected from methyl; andR3=selected from hydrogen and alkyl, and preferably from alkyl; and where the total number of carbon atoms in the alkyl groups R1, R2 and R3 is 3-48, preferably 3-13.The R1 is selected from methyl, and R2 and R3 are independently selected from alkyl, where the total number of carbon atoms in the alkyl groups R2 and R3 is 7. The hydrolysis stabilizer is reacted with the polyester by injecting the additive into the molten polymer prior to the polymer being cast into a film. The polyester film further comprises a UV-absorber, preferably an organic UV-absorber, and preferably selected from benzophenones, benzotriazoles, benzoxazinones and triazines, preferably from triazines. The amount of UV-absorber is in the range of 0.1-10% by weight, relative to the total weight of the layer. The polyester film has been stabilized by heat-setting at a temperature within the range of 200-225 degrees C. The polyester of the polyester film exhibits an endothermic high temperature peak at a temperature of (A) degrees C and an endothermic low temperature peak at a temperature of (B) degrees C, both peaks being measured by differential scanning calorimetry (DSC), where the value of (A-B) is in the range of 1-50 degrees C. Moreover, the polyester film exhibits an elongation to break, measured according to ASTM D882, of at least 10% after at least 56-92 hours when aged at 121 degrees C and 1.2 bar pressure. The polyester film exhibits a haze of no more than 30% and/or a total luminous transmission (TLT) of at least 50%. The polyester film is selected from a white film, a black film and an opaque film. The polyester film further comprises an antioxidant. The polyester film has disposed on a first surface an additional polymeric layer, where the additional polymeric layer is 10 a polyester layer optionally comprising one or more additives independently selected from hydrolysis stabilizers, UV-absorbers, anti-oxidants and particulate inorganic fillers, preferably where the second layer exhibits a haze of no more than about 30%. The polyester is polyethylene terephthalate. Preferred Cell: In the photovoltaic cell, the electrode layers and photovoltaic-active layer are encapsulated in a suitable encapsulant, and where the back-plane comprises the film. The film is an opaque or white film, where the film has disposed on a first surface an additional polymeric layer, where the additional layer exhibits a haze of no more than about 30%, and where the film is outermost in the multi-layer assembly. Preferred Process: Manufacturing the biaxially oriented polyester film comprises the further step of 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.