We fabricated ambipolar organic thin-film transistors (OTFTs) using C60 and pentacene. The electronic structure of the interface was investigated by using ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy. The magnitude of the interface dipole and the band bendings at the interface was determined, and the complete energy level diagram for C60 on pentacene (C60/pentacene) was obtained. The lowered band offsets, due to the enhanced charge redistribution in C60/pentacene relative to pentacene on C60, are favorable for the ambipolar OTFTs. The measured field-effect mobilities were 0.017cm2∕Vs and 0.007cm2∕Vs for the p-channel and the n-channel operations, respectively. The threshold voltages were −2V for the p channel and 15.6V for the n channel, comparable to those of unipolar OTFTs using C60 or pentacene.

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To reduce the hole-injection barrier, an additional indium–tin–oxide (ITO) sublayer with high oxygen concentration is introduced between the ITO anode layer and hole-transporting layer. A thin ITO layer with a high oxygen concentration is prepared by increasing oxygen partial pressure during deposition of the last few tens Å of ITO layer via rf magnetron sputtering. The workfunction and chemical composition of the resulting film are determined by using ultraviolet photoemission spectroscopy (UPS) and x-ray photoemission spectroscopy. UPS results indicate the increment of the workfunction by 0.19 eV. The measured luminescence–voltage and current–voltage characteristics of the final organic electroluminescent devices with an oxygen-rich ITO sublayer demonstrate enhanced characteristics. Our result suggests that the film resistivity, as well as the workfunction of the ITO layer, should be optimized for the best device performance.

The electronic structures of tris-(8-hydroquinoline) aluminum (Alq3)∕Li2O∕Al interfaces were studied using in situ ultraviolet and x-ray photoelectron spectroscopies (UPS and XPS). The UPS and XPS spectra allowed us to evaluate the complete energy level diagrams and to analyze the chemical interactions at the interfaces. Inserting Li2O between Al and Alq3 led to the highest occupied molecular orbital (HOMO) of Alq3 shifting to a higher binding energy compared to that without Li2O, which resulted in an improved electron injection. We also observed that the magnitude of the secondary cutoff shift was almost identical to that of the HOMO shift with the insertion of Li2O. This implies that the energy level alignment depends on the interface dipole and ionization energy of the adsorbate. Additionally, a gap state was observed in the gap of Alq3, which is related to the interfacial reaction. The N 1s spectra revealed that there were destructive chemical reactions between Alq3 and Al, which could be prevented by inserting Li2O between them.

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