Electronic structures of organic materials and their interfaces studied by UV photoemission under very small light irradiation

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The detection sensitivity of UV photoemission is improved by using sensitive sub-pico ampere meter, which reduces the amount of photoelectrons emitted from sample substrate. This advantage is suited to investigate organic materials and interfaces since their conductivity is quite low and tends to make UPS measurements difficult due to sample charging. By using this technique, we investigated the electronic structure of Au/sexiphenyl(6P) interface. We found that Au cluster is formed at this interface and can work as carrier trap to leading anomalous spectral shift of UPS. Similar phenomenon probably related to interfaces charging was also found for fullerene / phthalocyanine interface which is a typical p-n junction.By using very low current measurements technique developed in the above study, we constructed measurement system of photoelectron yield spectroscopy (PYS) which is available in both ambient atmosphere and vacuum. Instead of a special ion counter used in previous works, sub-pico ammeter was used in this study with a specially-designed electrode. Signal-to-noise ration is also much improved due to wide dynamic range of the ammeter. The measurements of ionization potential of several organic semiconductors were successfully demonstrated. This technique is quite suited to investigate the properties of organic materials in atmosphere condition to discuss organic devices.