Electroluminescence enhancement in ultraviolet organic light‐emitting diode with graded hole‐injection and ‐transporting structure

Abstract

Electroluminescent intensity and external quantum efficiency (EQE) in ultraviolet organic light‐emitting diodes (UV OLEDs) have been remarkably enhanced by using a graded hole‐injection and ‐transporting (HIT) structure of MoO 3 / N , N ′‐bis(naphthalen‐1‐yl)‐ N , N ′‐bis(phenyl)‐benzidine/MoO 3 /4,4′‐bis(carbazol‐9‐yl)biphenyl (CBP). The graded‐HIT based UV OLED shows superior short‐wavelength emis‐ sion with spectral peak of ∼410 nm, maximum electroluminescent intensity of 2.2 mW/cm 2 at 215 mA/cm 2 and an EQE of 0.72% at 5.5 mA/cm 2 . Impedance spectroscopy is employed to clarify the enhanced hole‐injection and ‐transporting capacity of the graded‐HIT structure. Our results provide a simple and effective approach for constructing efficient UV OLEDs. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)