Solution-processable Li-doped transition metal oxide hole-injection layer for highly efficient quantum-dot light-emitting diodes

Abstract

Vanadium oxide (V₂O₅) doped with Li (5, 10, and 15 mol%) was synthesized using an easy and low-cost solution process method and was used as a hole-injection layer (HIL) for quantum-dot light-emitting diodes (QLEDs). The optical, electrical, and chemical properties of the Li-doped V₂O₅ films were investigated. UV-visible spectroscopic analysis revealed that the optical band gap of V₂O₅ decreased, while the Urbach energy increased with Li doping, indicating that the defects increased with the addition of the Li dopant. The effective hole-injection characteristics of Li-doped V₂O₅ were confirmed by fabricating and measuring hole-only devices. Consequently, the maximum luminance and current efficiency of the QLED with the V₂O₅:Li (10%) HIL were measured as 152 140 cd m⁻² and 22.07 cd A⁻¹, respectively. This is at least 45% higher than that of the device using pristine V₂O₅. The peak wavelength of the QLED was 538 nm and the full-width at half-maximum was approximately 22 nm, which was closer to achieving a monochromatic green color. To determine the origin of the device-performance improvement, ultraviolet and X-ray photoelectron spectroscopy methods were used. The gap state was observed to approach the Fermi energy level as the amount of Li dopant was increased. Furthermore, we confirmed that increasing the ratio of V⁴⁺ oxidation states at the V 2p_3/2 core level improved the electrical conductivity. Therefore, solution-processable Li-doped V₂O₅ HILs with outstanding hole-injection capabilities are recommended for producing efficient QLEDs.