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Oxygen (O2) plasma treatment is one of the most widely applied methods for modifying the electrode work function. However, owing to the instability of O2-plasma treatment effects under air-exposed conditions, it is necessary to confirm whether the O2-plasma treatment effects can be continuously maintained at organic semiconductor/electrode interfaces in realistic devices. In the present study, the electronic structures of organic semiconductor/O2-plasma treated electrode interfaces were characterized by using in situ deposition and ultraviolet photoemission spectroscopy analysis. The structures of the corresponding samples were re-analyzed after a 1-week-long exposure to air to confirm the energy-level changes. To achieve this, we inceptively designed the studies of the energy level alignments of air-exposed samples based on the photoemission spectroscopy combined with Ar gas cluster ion beam sputtering process. The results of our studies clearly confirm the consistency of O2-plasma treatment effects at organic semiconductor/electrode interfaces. In addition, we confirmed the preservation of controlled energy-level structures at C60/Au interfaces by examining the relative rates of electron transfer at the C60/Au interfaces, obtained from photoluminescence (PL) measurements.

The valence band structures and ionization energies of pentacene layers grown on poly (3,4-ethylenedioxythiophene) polymerized with poly (4-styrenesulfonate) (PEDOT:PSS) and Au electrodes were characterized using in situ ultraviolet photoemission spectroscopy. The pentacene layers grown on Au films consist of amorphous phase and round oval grains regardless of the pentacene deposition temperature, whereas the pentacene deposited on PEDOT:PSS films showed highly distinctive ordered structures depending on the deposition conditions. The pentacene layer (thickness: 30 nm) grown on PEDOT:PSS at room temperature (RT) showed a highly crystalline thin film phase (001) and typical terrace-like structure, but that grown on PEDOT:PSS at 100◦C consists of a weakly oriented thin film/bulk mixed phase with a disordered grain structure. However, irrespective of the molecular ordering structure, the pentacene layers grown on PEDOT:PSS films showed nearly the same hole-injection barriers (0.40 ∼ 0.45 eV) at both interface (10 nm) and bulk regions (30 nm). On the basis of the correlation between the molecular ordering and device characteristics, we conclude that the carrier transport at pentacene/electrode interface relies more on the hole-injection barrier than on the ordering of the pentacene layer grown on the electrode. © 2013 The Electrochemical Society. [DOI: 10.1149/2.038308jes] All rights reserved.