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 MoO(x) hole-injecting layer (HIL) or a MoO(x)-doped 4,4'-bis(carbazol-9-yl)biphenyl (CBP) HIL were studied. The devices having the MoO(x)-doped CBP HIL are superior to the device having the neat MoO(x) 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 MoO(x) 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 MoO(x)-doped CBP HIL is more than two times that of the device with a neat MoO(x) HIL measured at a driven current of 5 mA/cm(2). Moreover, the lifetime of the device with a 20 vol % of MoO(x)-doped CBP HIL is more than three times that of the device with a neat MoO(x) HIL estimated at an initial luminance of 1000 cd/m(2). The MoO(x)-doped HIL further ensures the feasibility of the simplified phosphorescent OLEDs for potential applications.