Effect of ion concentration in the ionomers used as an electron injecting layer in polymer light-emitting diodes

The polymer light-emitting diodes (LEDs) were fabricated using poly[2-metho xy-5-(2'-(ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as an emissive material and sodium sulfonated polystyrene (SSPS) ionomer with various ioni c mole percents as an electron injecting material. The optimum ion concentr ation 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 critica l ion concentration (CIC) of the ionomers, which is depending upon by the n umber of charge carriers and their mobility. The increase of charge carrier s in the region below the CIC of ionomers enhances the quantum efficiency o f 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. A ll rights reserved.