Organic light-emitting devices with a mixture emitting layer of tris-(8-hydroxyquinoline) aluminum and 4,4′-bis(carbazol-9-yl)-biphenyl

Organic light-emitting devices with a mixture of tris-(8-hydroxyqunoline) aluminum $\left(\mathrm{Al}{\mathrm{q}}_3\right)$ and $4,4^{\prime }$ -bis(carbazol-9-yl)-biphenyl (CBP) as the emitting layer have been fabricated. The devices were fabricated in the same run with a standard device without CBP for comparison, with an identical structure of indium tin oxide $\left(\mathrm{ITO}\right)∕m$ -MTDATA $\left(80\mathrm{nm}\right)∕\mathrm{NPB}$ $\left(20\mathrm{nm}\right)∕\mathrm{CBP}:\mathrm{Al}{\mathrm{q}}_3$ $\left(40\mathrm{nm}\right)∕\mathrm{BCP}$ $\left(10\mathrm{nm}\right)∕\mathrm{Al}{\mathrm{q}}_3$ $\left(60\mathrm{nm}\right)∕\mathrm{Mg}:\mathrm{Ag}$ $\left(200\mathrm{nm}\right)$ , where $m$ -MTDATA is $4,4^{\prime },4^{″}$ -tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine, which is used to improve hole injection; NPB is $\mathrm{N},{\mathrm{N}}^{\prime }$ -di(naphth-2-yl)-$\mathrm{N},{\mathrm{N}}^{\prime }$ -diphenyl-benzidine; and BCP is 2,9-Dimethyl-4,7-diphenyl-1,110-phenanthorline. The ratio of CBP to $\mathrm{Al}{\mathrm{q}}_3$ in mixture was varied from 0 to 2. For device with a ratio of 0.5, the current efficiency and power efficiency were significantly improved by 35% and 32%, respectively, compared to the standard device with $\mathrm{Al}{\mathrm{q}}_3$ only as emitting layer. By increasing the ratio to 2, the current efficiency and power efficiency were dropped by 20% and 11%, respectively. The electroluminescence spectra showed a slight blueshift with the increase of CBP to $\mathrm{Al}{\mathrm{q}}_3$ ratio.