We report a solution-processed tungsten oxide hole injection layer for polymer light-emitting diodes (PLEDs). Unlike vacuum evaporated tungsten oxide, solution-processed tungsten oxide has a reduced work function due to contamination of the ambient atmosphere. To increase the work function of tungsten oxide layer, an organic ionic solution containing poly(ethylene oxide) and tetra-n-methylammonium tetrafluoroborate was coated on the tungsten oxide layer. Organic ions infiltrated into the tungsten oxide layer and formed an interfacial dipole, increasing the layer's work function by 0.4 eV. As a result, a PLED, which had a hole-injection layer of the tungsten oxide and organic ionic solution, showed a performance equivalent to that of the PEDOT:PSS-based device with even higher maximum luminance of 27,560 cd/m(2) at 10 V.