We have found that molybdenum dioxide (MoO2) is an excellent dopant for enhancing electrical conductivities in organic hole-transporting layers. We fabricated hole-only devices with an alpha-sexithiophene (α-6T) layer doped with MoO2 at various concentrations to investigate how doping MoO2 into the α-6T layers influences the hole-injection and hole-transport characteristics of these layers. We observed a marked increase in electrical conductivity as a result of the MoO2 doping. The 30-mol % MoO2 -doped α-6T layer had a high electrical conductivity of 8.9±1.3× 10-6 S/cm. From the results of our visible/near-infrared absorption spectra study of these doped layers, we confirmed that this increase in electrical conductivity is caused by a charge transfer between MoO2 and α-6T, which leads to an increase in free hole concentration in the doped layers and the formation of an ohmic contact at an electrode/ α-6T interface. In the latter part of this paper, we discuss current flow and electroluminescence (EL) characteristics of organic light-emitting diodes (OLEDs) with a 30-mol % MoO2 -doped α-6T hole-transporting layer and a 30-mol % Cs-doped phenyldipyrenylphosphine oxide (POPy2) electron-transporting layer. We achieved an extremely low driving voltage of 3.1 V required for a current density of 100 mA/ cm2 in the doped OLEDs owing to the use of the α-6T and POPy2 layers with high carrier mobilities and the excellent p -type MoO2 and n -type Cs dopants. We demonstrated the enhancement of power efficiencies by ≈2 times in the doped OLEDs compared with undoped OLEDs and observed bright EL at low driving voltages in the doped OLEDs, for example, 100 cd/ m2 at 2.3 V, 1000 cd/ m2 at 2.7 V, and 10 000 cd/ m2 at 3.3 V.
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