Since the low work function of ITO, introducing hole injection layer (HIL) with controllable and efficient property is an effective way to improve the performance of organic light-emitting diodes (OLEDs). In this work, films, containing molybdenum oxide units by employing H₂O₂ treated ammonium phosphomolybdate (AMP) as precursors, are fabricated through solution-processed procedure. The hole injection property of the HILs can be modulated by the gap-state density, which is negatively correlated with the molar ratio of AMP/H₂O₂. Green fluorescent and phosphorescent OLEDs based on the HILs are endowed with reduced turn-on voltage from 3.4 to 2.5 V and 4.1 to 3.4 V, respectively. Furthermore, fluorescent OLEDs based on the HIL with AMP/H₂O₂ of 1:14 show a maximum current efficiency of 6.82 cd/A, 1.2-flod higher than that of PEDOT:PSS based devices. This method opens up a new path to obtain HIL with adjustable property in mild prepare condition for various electroluminescent devices.