Sequentially doped blue electrofluorescent organic light-emitting diodes

We report on a blue fluorescent [4,4′-bis(9-ethyl-3-carbazovinylene)-1-1′-bisphenyl] organic light-emitting diode with a sequentially doped device architecture introduced recently. The emission layer consists of a few repeating cells, similar to a multiple quantum well structure, which are made of sequentially evaporated host and guest layers. An external quantum efficiency as high as 2.8% photons/electron was obtained, comparable with that fabricated using the conventional doping method. Without degrading the efficiency, devices with varied emission spectra (peak wavelength shifted from 472 to 488 nm) were produced by simply varying the guest layer thickness. The spectrum shift is due to changes in local order or aggregate state, or both.