We report on the employ of several kinds of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) dispersions as a hole injection layer to increase the stability and the charge injection in organic light emitting diodes (OLEDs). The PEDOT:PSS dispersions have been characterized in solution and thin film by means of dynamic viscosity, Dynamic Light Scattering, profilometer, contact angle measurement, UV Vis-NIR transmittance and current-voltage characteristics. Then, the dispersions have been employed to manufacture OLEDs with structure: indium tin oxide (ITO)-PEDOT:PSS-poly(9,9-dihexyl-9H-fluorene-2,7-diyl-Tris (8-hydroxy)quinoline aluminium-Al. Device electrical and optical properties have been extensively investigated and discussed in function of PEDOT:PSS ratio. We have found that the hole barrier at the ITO-PEDOT:PSS interface plays a key role in electrical transport and in setting the external quantum efficiency. (c) 2008 Elsevier B.V. All rights reserved.