We use the electroabsorption technique to measure the electric field in the emitting layer of ITO/MEHPPV/ionomer/Al light-emitting. Sodium sulfonated polystyrene (SSPS) was used as the ionomer. it is observed that the built-in potential in the emitting layer is lowered when the ionomer layer is inserted. On the contrary, the current density increases with the ionomer layers. These results indicate that the ionomer enhances the electron injection from the cathode to improve the balanced injection of electrons and holes into the light-emitting devices. Therefore, the quantum efficiency increases significantly in the EL device with the ionomer layer.