The Effect of Oxygen Exposure on Pentacene Thin Film Electronic Structure
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The Effect of Oxygen Exposure on Pentacene Thin Film Electronic Structure A. Vollmer1, O. D. Jurchescu2, I. Arfaoui2, I. Salzmann3, T. T. M. Palstra2, P. Rudolf2, J. Niemax4, J. Pflaum4, J. P. Rabe3, and N. Koch3 1
Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung m.b.H., AlbertEinstein-Str. 15, D-12489 Berlin, Germany 2 Materials Science Center, Rijksuniversiteit Groningen, NL-9747 AG Groningen, Netherlands 3 Humboldt-Universität zu Berlin, Institut f. Physik, Newtonstr. 15, D-12489 Berlin, Germany 4 Universität Stuttgart, 3. Physikalisches Institut, D-70550 Stuttgart, Germany
ABSTRACT We use ultraviolet photoelectron spectroscopy to investigate the effect of oxygen and air exposure on the electronic structure of pentacene thin films. It is found that O2 and water do not react noticeably with pentacene, whereas singlet oxygen/ozone readily oxidize the organic compound. Also, we obtain no evidence for considerable p-type doping of pentacene by O2 at low pressure. However, oxygen exposure lowers the hole injection barrier at the interface between Au and pentacene by 0.25 eV, presumably due to a modification of the Au surface.
INTRODUCTION Pentacene is a prototypical conjugated organic molecule for use as active material in novel electronic devices, such as thin film field effect transistors [1,2]. The interplay between thin film structure/morphology and measured charge carrier mobility is of particular interest [35]. However, a reliable comparison between experimental results obtained in different laboratories is often hampered by the variation of sample preparation and the environmental conditions for mobility measurements [1,6,7]. Another issue is long term stability of devices made from organic materials, as oxidation is believed to be a major reason for early device failure [8,9]. It has been observed that the measured conductivity of pentacene single crystals depends on the residual oxygen or air pressure [10]. It was shown that oxygen can diffuse into pentacene, and readily diffuses out again after re-evacuation [10]. When exposing a pentacene single crystal to dry air at atmospheric pressure the current through the crystal increased by a factor of 1.6 (measured in dark) or even 4 (measured under illumination) compared to the same crystal kept in a vacuum of 10-5 mbar. On a timescale of ca. 150 min, the maximum effect is observed and afterwards a plateau is reached, as equilibrium is established [10]. Still to be addressed is the nature, on a molecular scale, of oxygen-induced changes in charge transport properties. It needs to be established to what extent and under which experimental conditions these changes are reversible. Additionally, it has been proposed that oxygen may create new (chemical) species in pentacene films, (e.g., 6,13-pentacenequinone or an endoperoxide). Consequently, a movement of the Fermi-level (EF) in the organic film towards the highest occupied molecular orbital (HOMO) is predicted, displaying p-type behavior of the organic semiconductor [11,12]. Our app
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