Solution-processed aqueous composite hole injection layer of PEDOT:PSS+MoO for efficient ultraviolet organic light-emitting diode

Hole injection layer (HIL) plays a crucial role in governing external quantum efficiency (EQE) of ultraviolet organic light-emitting diodes (UV OLEDs). We develop a solution-processed aqueous composite HIL of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) incorporated MoOx (PEDOT:PSS+MoOx) and cast successful application to UV OLEDs. PEDOT:PSS+MoOx is characterized in detail with scanning electron microscopy, atomic force microscopy, UV–visible absorption spectra, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and impedance spectroscopy measurements. The results show that PEDOT:PSS+MoOx features superior film morphology and exceptional electronic properties such as enhanced surface work function and promoted hole injection capacity. With PEDOT:PSS+MoOx as HIL, the UV OLED gives maximum EQE of 4.4% and radiance of 12.2 mW/cm2 as well as improved durability. The electroluminescence peaks at 376 nm with full width at half maximum of 34 nm and stable voltage-dependent spectra. Our results pave a way for exploring efficient UV OLEDs with solution-processable techniques.