We investigate electroluminescent characteristics of gradiently doped organic light-emitting diodes, which were gradiently doped in both the hole and the electron-transporting layer to form a double emitting zone. The device structure was ITO/(15nm) CuPc/(60nm) NPB:rubrene/(30nm) Alq3:rubrene/(20nm) Alq3/(0.5nm) LiF/Al. We observed that charge carriers were well trapped by the dopant molecules and the main emitting zone was localized at the NPB:rubrene side close to the interface of NPB:rubrene/Alq3:rubrene. The quantum efficiency (cd/A) was enhanced to 5.89cd/A at 6V. We attributed this improvement to the charge carriers trapping and the emitting of the double emitting zone.
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