The function of electronically functional organic materials often originates at an interface, one example being organic electroluminescent devices (the Figure shows a typical energy diagram). Therefore, elucidation of the electronic structure at interfaces will lead to a better understanding of these devices, enabling their performance to be improved. Basic concepts are reexamined and recent progress in the area is reviewed.

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The effect of the method used to clean indium–tin–oxide (ITO) on its work function was investigated by ultraviolet photoemission spectroscopy (UPS) and x-ray photoemission spectroscopy. With only ultrasonic cleaning in the organic solvent, considerable carbon contamination remained on the ITO surface and the work function was low (4.5 eV). In contrast, ultraviolet (UV)–ozone treatment removed significant carbon contamination, with an increase in the work function to 4.75 eV, which improves the hole-injection efficiency into the organic hole-transport layer in organic electroluminescent devices. Although carbon contamination on the ITO surface was also removed by Ar+ sputtering, it was accompanied by the removal of oxygen from ITO, and the work function was reduced (4.3 eV). Three factors, i.e.,: (i) C-containing contaminants, (ii) the O/In ratio, and (iii) the In/Sn ratio on the ITO surface affect the work function. The present results and those of other workers suggest that these three factors affect the work function in the order: (ii)>(i)>(iii), and (i) is the main cause of the increase in the work function in the UV–ozone or O2 plasma treatments.

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