We report the fabrication and characterization of solution-processed organic bilayer field effect transistors with a middle-contact configuration. P3HT and PCBM were chosen for a hole and electron transporting material, respectively. The
P3HT:PCBM bilayer FET with a middle contact structure showed only p-type behavior with a hole mobility (μ<sub>h</sub>) of ~ 10<sup>-3</sup> cm<sup>2</sup> V<sup>-1</sup> s<sup>-1</sup>, which is a different result compared to the conventional top-contact ambipolar device. Electron injection
was enabled by using a thin CPE layer beneath the PCBM layer. The thickness of the CPE layer is critical for achieving
balanced hole and electron mobilities and provides an important variable for consideration in future optimization studies.
More detailed mechanisms on the role of CPE layer and the middle-contact structure are currently under investigation.
We report the characterization of the interface formation of fullerene (C<sub>60</sub>) on hafnium (Hf) using x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). The valence band, C 1s and Hf 4f spectra were measured during the deposition of C<sub>60</sub> on a clean Hf surface in a stepwise manner. After enough deposition of C<sub>60</sub> layers on Hf, XPS measurements indicate that there is no chemical reaction between C<sub>60</sub> on Hf, and band bending exists at the C<sub>60</sub>/Hf interface. From UPS measurements, the energy level difference between Fermi level of Hf and the onset of the highest occupied molecular orbital of C<sub>60</sub> is 2.01 eV. The vacuum level of Hf was shifted toward low binding energy (0.95eV) as the C<sub>60</sub> deposition. These valence spectra results indicate that there exist an interface dipole and small electron injection barrier at the C<sub>60</sub>/Hf interface. We provide the complete energy band diagram of the C<sub>60</sub>/Hf interface and confirm that a small electron injection barrier can be achieved by inserting a low work function metal in a C<sub>60</sub> field-effect transistor.
The surface structure of C<sub>2</sub>H<sub>4</sub> on Si(001) has been investigated by coaxial impact collision ion scattering spectroscopy (CAICISS). To determine the adsorption structure of the C<sub>2</sub>H<sub>4</sub> molecules definitely, the computer simulation with the two-dimensional trajectory count method has been applied to the recently proposed most possible two single molecular adsorption configurations (di-σ on-top and di-σ end-bridge). The CAICISS spectra and simulation results show that the di-σ on-top structure is better fitted with the experimental results rather than the di-σ end-bridge.