2D transition metal dichalcogenide (TMD) nanosheets, including MoS2, WS2, and TaS2, are used as hole injection layers (HILs) in organic light-emitting diodes (OLEDs). MoS2, WS2, and TaS2 nanosheets are prepared using an exfoliation by ultrasonication method. The thicknesses and sizes of the TMD nanosheets are measured to be 3.1-4.3 nm and more than 100 nm, respectively. The work functions of the TMD nanosheets increase from 4.4-4.9 to 4.9-5.1 eV following ultraviolet/ozone (UVO) treatment. The turn-on voltages at 10 cd m(-2) for UVO-treated TMD-based devices decrease from 7.3-12.8 to 4.3-4.4 V and maximum luminance efficiencies increase from 5.74-9.04 to 12.01-12.66 cd A(-1). In addition, this study confirms that the stabilities of the devices in air can be prolonged by using UVO-treated TMDs as HILs in OLEDs. These results demonstrate the great potential of liquid-exfoliated TMD nanosheets for use as HILs in OLEDs.