Alternating-current white a-C:H thin-film light-emitting diodes with composition-graded carrier injection layers

Abstract The optoelectronic characteristics of hydrogenated intrinsic amorphous carbon (i-a-C:H) alternating-current white thin-film light-emitting diodes (ACW-TFLEDs) with composition-graded (CG) hydrogenated intrinsic amorphous silicon carbide (i-a-SiC:H, CG C) layers had been obviously improved with additionally incorporated CG hydrogenated intrinsic amorphous silicon germanium (i-a-SiGe:H, CG Ge) carrier injection layers. For an ACW-TFLED with CG Ge carrier injection layers, the electroluminescence (EL) threshold voltage and brightness were improved from 9 V 344 cd/m2 of a CG C one to 7.5 V, 1000 cd/m2 under direct-current forward bias, and from 9 V, 200 cd/m2 of a CG C one to 7.8 V, 560 cd/m2 under direct-current reverse bias, respectively, at an injection current density of 300 mA/cm2 for brightness measurement. The enhancement of EL performance with CG Ge carrier injection layers was due to the increased carrier injection efficiency and reduced contact resistance resulting from the lower barrier and partially formed polycrystalline Ge layer between the Al (electrode)/Ge interface. Moreover, the EL intensity of an obtained ACW-TFLED increased with the frequency up to 20 kHz and then decreased rapidly and became very weak as the frequency was increased to about 100 kHz. This frequency-dependent EL behavior would be qualitatively explained with the mobility of charge carriers.