Insights into the Interfacial Properties of Low-Voltage CuPc Field-Effect Transistor

The interfacial transport properties and density of states (DOS) of CuPc near the dielectric surface in an operating organic field-effect transistor (OFET) are investigated using Kelvin probe force microscopy. We find that the carrier mobility of CuPc on high-k Al₂O_y/TiO_x (ATO) dielectrics under a channel electrical field of 4.3 × 10² V/cm reaches 20 times as large as that of CuPc on SiO₂. The DOS of the highest occupied molecular orbital (HOMO) of CuPc on the ATO substrate has a Gaussian width of 0.33 ± 0.02 eV, and the traps DOS in the gap of CuPc on the ATO substrate is as small as 7 × 10¹⁷ cm^–3. A gap state near the HOMO edge is observed and assigned to the doping level of oxygen. The measured HOMO DOS of CuPc on SiO₂ decreases abruptly near E_VGS = VT, and the pinning of DOS is observed, suggesting a higher trap DOS of 10¹⁹–10²⁰ cm^–3 at the interface. The relationships between DOS and the structural, chemical, as well as electrical properties at the interface are discussed. The superior performance of CuPc/ATO OFET is attributed to the low trap DOS and doping effect.