Spectral observations of hole injection with transition metal oxides for an efficient organic light-emitting diode

Transition metal oxides, such as molybdenum trioxide (MoO₃), tungsten trioxide (WO₃) and vanadium pent-oxide (V₂O₅), are well-known hole injection materials used for organic electronic devices. These materials promote work functions of anodes, reduce energy barriers, and facilitate hole transport at the interface between the inorganic anode and organic hole-transporting layer (HTL). In this study, we characterized the transmittance spectra and work function of these materials. Furthermore, we employed a hole-injection layer (HIL) in a blue phosphorescent organic light-emitting diode (OLED) to evaluate their hole-injection capacity by detecting the variation in the emission spectra. Thus, we utilized an OLED structure that has fast electron transporting dynamics to establish the recombination zone located at emitting layer and a partial HTL close to the anode. We used these three transition metal oxides individually as HILs sandwiched between the ITO anode and HTL and concluded that the strength of emissive light from the HTL was determined by their hole-injection capacity, depending on work function. The small amount of HTL emission light of the V₂O₅ OLED was explained by the high work function of 5.8 eV for the V₂O₅ film. However, the V₂O₅ OLED demonstrated the least favorable optoelectrical performance because of its low transmittance and high resistance of the V₂O₅ film. Ultimately, the 5 nm-MoO₃ OLED exhibited the highest device performance because of its high material conductivity and transparency in the visible band. (paper)