A polymer film having a conductivity gradient across its thickness is maderom a mixed solution of an insulating polymer, A and a polymer, B that can be made conducting by doping or protonation by a method including the steps of:(A) mixing the solution of the insulating polymer, A and the polymer, B that can be made conducting by doping or protonation,(B) casting the mixed solution together as a solid composite film, and(C) exposing the film to a treating agent that can dope or protonate polymer B and make polymer B conductive and create a conductivity gradient across the thickness of the polymer films.