Study of Hole-Injecting Properties in Efficient, Stable, and Simplified Phosphorescent Organic Light-Emitting Diodes by Impedance Spectroscopy

Simplified phosphorescent organic light-emitting diodes (OLEDs) using only two kinds of hosts and comprising either a neat MoOx hole-injecting layer (HIL) or a MoOx-doped 4,4'-bis(carbazol-9-yl)biphenyl (CBP) HIL were studied. The devices having the MoOx-doped CBP HIL are superior to the device having the neat MoOx HIL in terms of power efficiency and operational lifetime. Impedance spectroscopy studies revealed that both the reduced hole-injecting barrier height at the anode/doped HIL interface and the reduced bulk resistivity in the doped CBP HIL contribute to the improvement in electroluminescence characteristics. When increasing the MoOx volume percentage from 5 to 10% and then to 20%, the hole-injecting barrier height is decreased from 0.63 eV to 0.36 eV and then to 0.18 eV. The power efficiency of the device with a 20 vol % of MoOx-doped CBP HTL is more than two times that of the device with a neat MoOx HIL measured at a driven current of S mA/cm(2). Moreover, the lifetime of the device with a 20 vol % of MoOx-doped CBP HIL is more than three times that of the device with a neat MoOx HIL estimated at an initial luminance of 1000 cd/m(2). The MoOx-doped HIL further ensures the feasibility of the simplified phosphorescent OLEDs for potential applications.