NOVELTY - Co-extruded multilayer biaxially oriented polyester film comprises primary polyester layer and dissimilar secondary polyester layer adjacent to the primary polyester layer, where the primary polyester layer and the secondary polyester layer each comprise glycidyl ester of branched monocarboxylic acid, the monocarboxylic acid has 5-50 carbon atoms, and the glycidyl ester is in the form of its reaction product with at least some of the end groups of the polyester.    USE - The co-extruded multilayer film is useful as a backplane in a photovoltaic cell (claimed).    ADVANTAGE - The polyester film: exhibits an interlayer peel strength after 60 hours at 121 degrees C and 1.2 bar pressure and 100% relative humidity of at least 0.5 N, and/or retention of interlayer peel strength (RIPS) after 60 hours at 121 degrees C and 1.2 bar pressure and 100% relative humidity of at least 30%, where RIPS = (IPSt / IPSo) x 100, and IPSo and IPSt are the interlayer peel strength values measured prior to the accelerated aging test and after accelerated aging for 60 hours at 121 degrees C and 1.2 bar pressure and 100% relative humidity, respectively; hydrolysis resistance at 121 degrees C and 1.2 bar pressure and 100% relative humidity such that the elongation to break (ETB) of the film is at least 10% after at least 80 hours under the conditions (all claimed); exhibits excellent hydrolytic stability and improved delamination resistance, preferably while maintaining or improving the ease, efficiency and economy of film manufacture, and without detriment to the mechanical and/or optical properties of the film.    DETAILED DESCRIPTION - Co-extruded multilayer biaxially oriented polyester film comprises primary polyester layer and dissimilar secondary polyester layer adjacent to the primary polyester layer, where the primary polyester layer and the secondary polyester layer each comprise glycidyl ester of branched monocarboxylic acid, the monocarboxylic acid has 5-50 carbon atoms, and the glycidyl ester is in the form of its reaction product with at least some of the end groups of the polyester. INDEPENDENT CLAIMS are also included for:    (1) photovoltaic cell comprising front-plane, at least electrode layers, photovoltaic-active layer and a back-plane, where the back-plane comprises co-extruded multilayer biaxially oriented polyester film;    (2) manufacturing co-extruded multilayer biaxially oriented polyester film comprising: co-extruding, preferably at a temperature of 280-300 degrees C; quenching the co-extrudate; stretching the quenched co-extrudate in two mutually perpendicular directions; and heat-setting the film, preferably at a temperature of 200-225 degrees C; and    (3) improving the delamination resistance of co-extruded multilayer biaxially oriented polyester film comprising providing the polyester of each of the primary and secondary polyester layers such that at least some of the end-groups of the polyester are in the form of their reaction product with glycidyl ester of a branched monocarboxylic acid.