In Figure 2, HBL1, HBL2, and HBL3 represented the HBL that was close to the anode side, in the middle, and close to cathode, respectively, which forms the so-called THBL that can control the carrier recombination in the EML to adjust the emission color of devices.
Thus, the thicknesses of DPVBi and CBP layer must be properly controlled to achieve the stable color purity and high efficiency of WOLED in THBL.
In addition, from the turn-on voltages of devices B and C, there is a high turn-on voltage in device C due to inserting a thin BCP layer in the emitting layer to form the THBL structure.
The enhancement of the luminance in device C as compared with device B can be attributed to three factors: (1) THBL has better excitons confinement ability, (2) an increase in excitons formation in EML, and (3) there is formation both emitting layers (DPVBi) in THBL of device C.
As a result, the balance of electron and hole can be improved by inserting a thin BCP layer in device C, forming the THBL structure, and thus more carriers will be confined within both the DPVBi layers.
This is because the emission region of the OLEDs with a THBL consisting of BCP layer and CBP layer is confined within both DPVBi layers.