We report the characterization of the interface formation of fullerene (C60) 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 C60 on a clean Hf surface in a stepwise manner. After enough deposition of C60 layers on Hf, XPS measurements indicate that there is no chemical reaction between C60 on Hf, and band bending exists at the C60/Hf interface. From UPS measurements, the energy level difference between Fermi level of Hf and the onset of the highest occupied molecular orbital of C60 is 2.01 eV. The vacuum level of Hf was shifted toward low binding energy (0.95eV) as the C60 deposition. These valence spectra results indicate that there exist an interface dipole and small electron injection barrier at the C60/Hf interface. We provide the complete energy band diagram of the C60/Hf interface and confirm that a small electron injection barrier can be achieved by inserting a low work function metal in a C60 field-effect transistor.
The interface electronic structure of pentacene and C60 layer was investigated in detail by using ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy. The magnitudes of measured interface dipole were 0.11 eV and 0.07 eV for the C60 deposited on pentacene (C60/pentacene) and the pentacene deposited on C60(pentacene/C60), respectively. The obtained C 1s spectra on pentacene/C60 and C60/pentacene indicate no significant chemical bonds existing at the interface. The offsets of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) at the C60-pentacene interface were 1.29 eV and 0.89 eV for C60/pentacene/Au while it were 1.5 eV and 1.1 eV for pentacene/C60/Au. This resulted in that the HOMO and LUMO offset depend on the deposition order of the organic layers. We fabricated the ambipolar OTFTs used the favorable structure of pentacene and C60, and obtained the field effect mobilities were 0.017 cm2/Vs and 0.007 cm2/Vs for p-channel and n-channel operations.