The doping level of spin-coated poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDT:PSS) films was adjusted quantitatively by an electrochemical doping/dedoping process using p-toluenesulfonic acid in acetonitrile as supporting electrolyte. Higher oxidation states relative to as-prepared layers remain stable under dry condition, and the electrochemical treatment does not affect the surface morphology and homogeneity of the polymer film. The work function of PEDT:PSS increases with the doping level after adjusting the layer at electrode potentials higher than the value of the electrochemical equilibrium potential (E-eq) of as-prepared films. The ohmic behavior for hole-injection is nearly achieved at anode/NPB interfaces for a PEDT:PSS anode with very high doping level (E-eq of up to 0.59 V vs Ag/AgCl). This indicates a further reduction of the hole-injection barrier at the contact of polymeric anode/hole transport layers.