Efficiency improvement of yellow organic light-emitting devices by using mixed hole transporting layer

Organic light-emitting devices (OLEDs) with a novel metal iridium complex of bis[2-(4-tertbutylphenyl) benzothiazolato-N,C2'] iridium (acetylacetonate) [(t-bt)2Ir(acac)] doping into a carbazole-based material 4,4'-bis[N-1-napthyl-N-phenyl-amino]biphenyl (CBP) as light-emitting layer (ETL) were fabricated. The optimum doping concentration of (t-bt)2Ir(acac) phosphor was 8 wt%. The maximum power efficiency (ηp) of the OLED was 8 lm/W. Furthermore, to improve the balance of charge carriers and enhance the ηp of the device, a doping system consisted of NPB hole transporting material and CBP host material with a concentration proportion of 1:3 was employed as the mixed hole transporting layer. OLEDs with a structure of indium-tin oxide (ITO)/N,N'-bis-(1-naphthyl)-N,N'- biphenyl-1,1'-biphenyl-4,4'-diamine (NPB):CBP(1:3)/CBP:(t-bt)2Ir(acac)/2, 9-dimethyl-4, 7-diphenyl-phenanthroline (BCP)/Mg:Ag were fabricated. The results showed that current density was decreased and the maximum ηp of the device was 10.6 lm/W. Compared with the conventional device with NPB as hole transporting layer, the improved maximum ηp of the device with mixed hole transporting layer was increased 32.5 %. This is attributed to the reduction of hole injection amount and transporting mobility by doping material CBP in hole transporting layer, which significant enhances charge carrier balance and electron-hole recombination probability.