The polymer light-emitting diodes (LEDs) were fabricated using poly[2-methoxy-5-(2'-(ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as an emissive material and sodium sulfonated polystyrene (SSPS) ionomer with various ionic mole percents as an electron injecting material. The optimum ion concentration of SSPS was found at 6.7 mol% in the ITO/MEH-PPV/ionomer/Al structure. The quantum efficiency of the device is greatly enhanced near the critical ion concentration (CIC) of the ionomers, which is depending upon by the number of charge carriers and their mobility. The increase of charge carriers in the region below the CIC of ionomers enhances the quantum efficiency of the device by introducing more electrons. On the contrary, in the region above the CIC, the quantum efficiency is reduced by the restricted mobility of the ions due to the cluster formation. (C) 2000 Elsevier Science S.A. All rights reserved.