Sol-gel processed vanadium oxide as efficient hole injection layer in visible and ultraviolet organic light-emitting diodes

Low-cost, high-throughput and scalable production currently boosts organic electronic device towards solution processing. Sol-gel processed aqueous vanadium oxide (h-VO_x) is facilely synthesized and proven to be efficient hole injection layer (HIL) in visible and ultraviolet organic light-emitting diodes (OLEDs). Atomic force microscopy and X-ray/ultraviolet photoelectron spectroscopy measurements indicate that h-VO_x behaves superior film morphology and exceptional electronic properties such as oxygen vacancy dominated non-stoichiometry and appropriate surface work function. With tris(8-hydroxy-quinolinato)aluminium as emitter, the visible OLED gives maximum luminous and power efficiencies of 6.3 cd/A and 3.2 lm/W, respectively, which are slightly superior to the counterpart with vacuum thermally-evaporated VO_x (5.6 cd/A and 2.7 lm/W). With 3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole as emitter, the ultraviolet OLED produces attractive short-wavelength emission of 379 nm with full width at half maximum of 40 nm and improved durability. The maximum radiance and external quantum efficiency reach 15.3 mW/cm² and 2.92%, respectively, which are considerably enhanced in comparison with the corresponding reference (11.9 mW/cm² and 2.32%). Current versus voltage characteristics and impedance spectroscopy analysis elucidate that h-VO_x exhibits robust hole injection and accordingly high-performance OLEDs. Our results pave an alternative way for advancing organic electronic devices and VO_x applications with solution process.