The conductivity of a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film can be enhanced by more than two orders of magnitude by adding a compound with two or more polar groups, such as ethylene glycol, meso-erythritol (1,2,3,4-tetrahydroxybutane), or 2-nitroenthanol, to an aqueous solution of PEDOT:PSS. The mechanism for this conductivity enhancement is studied, and a new mechanism proposed. Raman spectroscopy indicates an effect of the liquid additive on the chemical structure of the PEDOT chains, which suggests a conformational change of PEDOT chains in the film. Both coil and linear conformations or an expanded-coil conformation of the PEDOT chains may be present in the untreated PEDOT:PSS film, and the linear or expanded-coil conformations may become dominant in the treated PEDOT:PSS film. This conformational change results in the enhancement of charge-carrier mobility in the film and leads to an enhanced conductivity. The high-conductivity PEDOT:PSS film is ideal as an electrode for polymer optoelectronic devices. Polymer light-emitting diodes and photovoltaic cells fabricated using such high-conductivity PEDOT:PSS films as the anode exhibit a high performance, close to that obtained using indium tin oxide as the anode.