Improved performance of organic light-emitting devices with ultra-thin hole-blocking layers

Tris-(8-hydroxyqunoline) aluminum (Alq(3))-based organic light-emitting devices (OLEDs) using different thickness of 2,9-Dimethyl-4,7-diphenyl-1,110-phenanthorline (BCP) as a hole-blocking layer inserted both in the electron- and hole-transport layers have been fabricated. The devices have a configuration of indium tin oxide (ITO)/m-MTDATA (80 nm)/BCP (X nm)/NPB (20 nm)/Alq(3) (40 nm)/BCP (X nm)/Alq(3) (60 nm)/Mg: Ag (200 nm), where m-MTDATA is 4, 4', 4 ''-Tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine, which is used to improve hole injection and NPB is N,N'-Di(naphth-2-yl)-N,N'-diphenyl-benzidine. X varies between 0 and 2 nm. For a device with an optimal thickness of 1-nm BCP, the current and power efficiencies were significantly improved by 47% and 43%, respectively, compared to that of a standard device without a BCP layer. The improved efficiencies are due to a good balance between the electron and hole injection, exciton formation, and confinement within the luminescent region. Based on the optimal device mentioned above, the NPB layer thickness influences the properties of the OLEDs.