Electron blocking and hole injection: The role of N,N8-Bis„naphthalen-1-y...- N,N8-bis„phenyl...benzidine in organic light-emitting devices

The current density-luminance-voltage characteristics of organic light-emitting devices ~OLEDs! with N ,N8-Bis~naphthalen-1-yl)-N ,N8-bis~phenyl! benzidine ~NPB! of various thicknesses as the hole transport layer have been investigated. It is found that for conventional structures of indium– tin–oxide/NPB/tris~8-hydroxyquinoline! aluminum (Alq3) ~60 nm!/LiF ~0.5 nm!/Al the optimal hole injection and luminescence efficiencies appear at NPB thicknesses of 5 and 20 nm, respectively. The large difference between the two optimal thicknesses suggests that the effective block of the NPB layer against electrons from across the Alq3 /NPB interface is essential for high-efficiency operation of the OLEDs. The electron blocking effect of NPB is further confirmed by the electroluminescence ~EL! behavior of devices with the structure of ITO/NPB~5 nm!/ Alq3 :4-~dicyanomethylene!-2-methyl-6-(p-dimethylaminostyryl!-4H-pyran ~DCM! ~30 nm!/NPB/ Alq3~60 nm!/LiF~0.5 nm!/Al. The proportion of DCM EL to the whole EL decreases with increasing NPB thickness. This suggests that the NPB layer blocks electron transport to the Alq3 :DCM layer. The Förster energy transfer from the 60 nm Alq3 layer to the DCM molecules is ruled out by the EL behavior observed after quenching excitons in the Alq3 layer. The origin of the difference in the optimal N ,N8-Bis~3-methylphenyl!-N ,N8-bis~phenyl!benzidine ~TPD! thicknesses reported by other two different groups is also discussed. © 2004 American Institute of Physics. @DOI: 10.1063/1.1699472#