An Ambipolar Organic Semiconductor with Well-Balanced High Hole and Electron Mobility

Organic light-emitting diode (OLED) devices have received much attention because they provide a next-generation organic display and light source. Simplification of the device component is necessary in order to reduce production costs, since the OLED device usually consists of more than five layers including emissive host material, hole transporter, and electron transporter.¹⁾ The research group of Nakamura and Tsuji developed a dibenzofuran-based organic semiconductor, the so-called CZBDF (1).²⁾ Several characteristic features of 1 were reported: (1) ambipolar organic semiconductor with well-balanced high hole and electron mobility (hole: 3.7 × 10⁻³ cm²/Vs, electron: 4.4 × 10⁻³ cm²/Vs), (2) wide-gap material with HOMO-LUMO gap of 3.3 eV, (3) high glass-transition temperature (T_g = 162 °C), and (4) effectively storing a generated charge for an emissive dopant.

One can fabricate a homojunction OLED device of the ambipolar material 1 by vapor deposition. For instance, V₂O₅ (oxidant) or Cs (reductant) was simultaneously deposited with 1 on ITO (anode) or Al (cathode) for the purpose of p-doping or n-doping, however, 1 was used as a sole host material. The doping of blue (TBP), green fluorophore (C545T) and red phosphorophore (Ir(piq)₃) to the non-charged layer of 1 enabled us to show three primary-colored emissions. In particular, the doping of the green fluorophore (C545T) indicated a high external quantum efficiency of 4.2%.²⁾

In addition, Nakamura and Tsuji et al. also reported a heterojunction OLED using 1 as a host material. They successfully fabricated a multi-colored OLED device by doping TBP, C545T, rubrene and Ir(piq)₃. A device using the dual dopants (TBP and rubrene) showed a white-colored emission with an external quantum efficiency of 1.8%.³⁾