The hole injection layer (HIL) plays a significant role in determining the performances of organic light-emitting diodes (OLEDs), especially when hole transport materials with deep highest occupied molecular orbital levels (HOMOs) are employed. Intensive efforts have been devoted to exploring novel hole injection materials with good solution-processing abilities in recent years. In this study, the solution-processed molybdenum trioxide (s-MoO3) is prepared via an ultra-facile method. Three different s-MoO3 layers prepared by three different methods, viz. layers annealed at 150 ℃ (s-MoO3 (150)), layers annealed at 150 ℃ and then processed in UV-ozone for 15 min (s-MoO3 (150, UVO)), and layers processed in UV-ozone for 15 min without annealing (s-MoO3 (UVO)), are obtained to investigate their influences on hole injection. The device with the s-MoO3 (150) layer has the lowest current density and the largest driving voltage, showing poor hole injection ability. In contrast, with the s-MoO3 (150, UVO) layer as HIL, the OLED produces a greatly enhanced current and sharply reduced driving voltage, comparable to the device using vacuum-evaporated MoO3. Similar results are obtained for the device with the s-MoO3 (UVO) film, suggesting that high-temperature annealing is not essential for the s-MoO3 film with UV-ozone treatment. Hole injection efficiencies of MoO3 films are quantitatively characterized by analyzing the space-charge-limited current of hole-only devices; the hole injection efficiencies of s-MoO3 (150, UVO) and s-MoO3 (UVO)-based devices are ~0.1, far exceeding that of the s-MoO3 (150)-based device (10−5). XPS analysis is performed to detect the impact of the above treatments on the surface electronic properties of the s-MoO3 films. A typical characteristic of Mo5+ species is obtained for the s-MoO3 (150) film and a high-binding-energy shoulder appears in the O 1s peak of the s-MoO3 (150) film, indicating the existence of oxygen vacancies and oxygen adsorbed at the surface of s-MoO3 (150) film. When UV-ozone treatment is applied to this s-MoO3 (150) film, it produces a decrease of Mo5+ state and elimination of oxygen-rich adsorbates, resulting in MoO3 stoichiometry similar to that of the vacuum-evaporated MoO3 film. Consequently, a maximum current efficiency of 48.3 cd∙A−1 is realized with the optimized UV-ozone treated s-MoO3 HIL. It This UV-ozone treated s-MoO3 should have widespread applications in low-cost solution-processed OLEDs as an excellent hole injection layer.