Enhanced driving performance of organic light-emitting diodes with all carrier ohmic-contacts

This paper elucidates the enhancement of the optoelectronic properties of organic light-emitting diodes (OLEDs) by the p-doping effect of molybdenum oxide (Mo O_x)-doped 4,4′, 4″-tris[2-naphthyl(phenyl)amino] triphenylamine (2-TNATA). The device performance strongly depends on both the thickness and the doping concentration of the Mo O_x-doped 2-TNATA. As the doping concentration is increased from 25 to 75%, the hole ohmic properties of the hole-only device with the glass/indium tin oxide (ITO) /Mo O _x-doped 2-TNATA (100 nm) /Al structure were improved, due to the increase in the p-type doping effect. Also, the photoemission spectra revealed that the p-type doping effects cause the lowering of the hole-injecting barrier height, as well as the improvement of the hole conductivity. This improvement is caused by the formation of the charge transfer complex (Mo O_x ^-/2-TNATA⁺) that is generated by doping Mo O_x into 2-TNATA, markedly increasing the number of hole carriers. With Mo O _x-doped 2-TNATA as a hole ohmic contact and fullerene (C ₆₀) /lithium fluoride (LiF) as an electron ohmic contact, the OLED with the glass/ITO/ Mo O_x-doped 2-TNATA (75%, 60 nm)/NPB (10 nm) /Al q₃ (35 nm) / C₆₀ (5 nm) /LiF (1 nm) /Al (150nm) structure showed both a maximum luminance of 127,600 cd/ m₂ at 6.4 V and power efficiency of 4.7 lm/W at about 1000 cd/ m₂.