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-<i>k</i> Al<sub>2</sub>O<sub><i>y</i></sub>/TiO<sub><i>x</i></sub> (ATO) dielectrics under a channel electrical field of 4.3 × 10<sup>2</sup> V/cm reaches 20 times as large as that of CuPc on SiO<sub>2</sub>. 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 × 10<sup>17</sup> cm<sup>–3</sup>. A gap state near the HOMO edge is observed and assigned to the doping level of oxygen. The measured HOMO DOS of CuPc on SiO<sub>2</sub> decreases abruptly near <i>E</i><sub><i>V</i><sub>GS</sub> = <i>V</i><sub>T</sub></sub>, and the pinning of DOS is observed, suggesting a higher trap DOS of 10<sup>19</sup>–10<sup>20</sup> cm<sup>–3</sup> 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.