In order to improve the performance of organic light emitting diodes (OLED), a water-dispersed polyaniline-poly(styrene sulfonate) (PANI-PSS) was used as a bole injection layer (HIL). The current-voltage and luminescence-current characteristics of the OLED devices were investigated as functions of PANI:PSS weight ratio and the kind of dopants. The PANI-PSS film coated on the indium-tin oxide showed a reduced surface roughness, higher work function, and improved transparency, compared to poly(3,4-ethylenedioxythiophene) (PEDOT)-PSS. In addition, PANI-PSS HIL led to a better operation voltage reduction (ca. 15%) and displayed a higher level of luminance performance (ca. 30%) than the commercial PEDOT-PSS system. (c) 2007 Elsevier B.V. All rights reserved.