Optoelectronic characteristics of MEH-PPV polymer LEDs with thin, doped composition-graded a-SiC:H carrier injection layers

The optoelectronic characteristics of poly(2-methoxy-5-(2′ethyl-hexoxy)-1,4-phenylene-vinylene) (MEH-PPV) polymer LEDs (PLEDs) have been improved by employing thin doped composition-graded (CG) hydrogenated amorphous silicon–carbide (a-SiC:H) films as carrier injection layers and O2-plasma treatment on indium–tin-oxide (ITO) transparent electrode, as compared with previously reported ones having doped constant-optical-gap a-SiC:H carrier injection layers. For PLEDs with an n-type a-SiC:H electron injection layer (EIL) only, the electroluminescence (EL) threshold voltage and brightness were improved from 7.3 V, 3162 cd/m2 to 6.3 V, 5829 cd/m2 (at a current density J = 0.6 A/cm2), respectively, by using the CG technique. The enhancement of EL performance of the CG technique was due to the increased electron injection efficiency resulting from a smoother barrier and reduced recombination of charge carriers at the EIL and MEH-PPV interface. Also, surface modification of the ITO transparent electrode by O2-plasma treatment was used to further improve the EL threshold voltage and brightness of this PLED to 5.1 V, 6250 cd/m2 (at J = 0.6 A/cm2). Furthermore, by employing the CG n[p]-a-SiC:H film as EIL [hole injection layer (HIL)] and O2-plasma treatment on the ITO electrode, the brightness of PLEDs could be enhanced to 9350 cd/m2 (at a J = 0.3 A/cm2), as compared with the 6450 cd/m2 obtained from a previously reported PLED with a constant-optical-gap n-a-SiCGe:H EIL and p-a-Si:H HIL.