Tailoring hole injection of sol-gel processed WOx and its doping in PEDOT:PSS for efficient ultraviolet organic light-emitting diodes
Hole injection governs efficiency of ultraviolet organic light-emitting diode (UV OLED) due to its deep highest occupied molecular orbital level of emissive molecule. Transition metal oxide of tungsten oxide (WOx) with high work function and good stability casts light on shooting this problem. For meeting low-cost, scalable and high-throughput manufacturing of solution process, herein a facile synthesis of WOx solution and its doping in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS+WOx) is systematically investigated for assembling efficient UV OLEDs. The X-ray diffraction, atomic force microscope, scanning electron microscope and X-ray photoelectron spectroscopy measurements confirm that WOx and PEDOT:PSS+WOx behave good film morphology and exceptional electronic properties such as oxygen deficiency dominated non-stoichiometry of WOx. With PEDOT:PSS+WOx tailoring hole injection and 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole as emitter, the UV OLED shows outstanding electro-optic performance of radiance of 3.98 mW/cm2, external quantum efficiency of 2.30%, electroluminescence peak of 400 nm and full width at half maximum of 47 nm, which are superior to the corresponding references. The mechanism of charge transfer from PEDOT polycation to WOx enhancing conductivity is responsible for the robust hole injection/transport and further elucidated by ultraviolet photoelectron spectroscopy and impedance spectroscopy, contributing to optimization of carrier balance and recombination zone. Our results pave an alternative approach for boosting UV OLED performance and advancing organic electronics.