Enhanced hole injection and transport in molybdenum-dioxide-doped organic hole-transporting layers

We have found that molybdenum dioxide (MoO(2)) is an excellent dopant for enhancing electrical conductivities in organic hole-transporting layers. We fabricated hole-only devices with an alpha-sexithiophene (alpha-6T) layer doped with MoO(2) at various concentrations to investigate how doping MoO(2) into the alpha-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 MoO(2) doping. The 30-mol % MoO(2)-doped alpha-6T layer had a high electrical conductivity of 8.9 +/- 1.3x10(-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 MoO(2) and alpha-6T, which leads to an increase in free hole concentration in the doped layers and the formation of an ohmic contact at an electrode/alpha-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 % MoO(2)-doped alpha-6T hole-transporting layer and a 30-mol % Cs-doped phenyldipyrenylphosphine oxide (POPy(2)) electron-transporting layer. We achieved an extremely low driving voltage of 3.1 V required for a current density of 100 mA/cm(2) in the doped OLEDs owing to the use of the alpha-6T and POPy(2) layers with high carrier mobilities and the excellent p-type MoO(2) and n-type Cs dopants. We demonstrated the enhancement of power efficiencies by approximate to 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/m(2) at 2.3 V, 1000 cd/m(2) at 2.7 V, and 10 000 cd/m(2) at 3.3 V. (c) 2008 American Institute of Physics.