Highly efficient multilayer blue-light-emitting diodes based on carbazole fluorophores

Two carbazole molecular derivatives have been used as blue emitters in multilayer light-emitting-diodes i.e. the dimer of N-ethylcarbazole (bicarbazyl, DEC) and the 1,4,5,8,9-pentamethylcarbazole (PMC). Various devices with transporting layers for both electrons and holes and hole blocking material, have been achieved and tested. The influence of the device configuration and of the electronic structure of the organic materials on electroluminescent performance are presented and discussed. It has been found that a five-layer configuration with pentamethylcarbazole as an active layer associated to bathocuproine (BCP) as a hole blocking layer exhibits a very pure and bright-blue light (λ_peak=456 nm). A luminance-to-current efficiency of 4.7 cd/A, a power conversion efficiency of 1.3 lm/W and an external quantum optical-to-electrical conversion efficiency as high as 3.3% have been obtained for DPVBi doped with PMC. Moreover, all the designed devices were working under ambient atmosphere and exhibited a satisfactory long-lived emission.