Real-time observation of oxidation and photo-oxidation of rubrene thin films by spectroscopic ellipsometry

Abstract

We follow in real-time and under controlled conditions the oxidation of the organic semiconductor rubrene grown on SiO2 using spectroscopic ellipsometry. We derive the complex dielectric function ε1 + iε2 for pristine and oxidized rubrene showing that the oxidation is accompanied by a significant change of the optical properties, namely the absorption. We observe that photo-oxidation of rubrene is orders of magnitude faster than oxidation without illumination. By following different absorption bands (around 2.5 eV and 4.0 eV for pristine rubrene and around 4.9 eV for oxidized rubrene) we infer that the observed photo-oxidation of these films involves non-Fickian diffusion mechanisms.Many organic materials with delocalized π-electron systems exhibit significant potential for electronic and optoelectronic applications [1]. But despite progress in the development of encapsulation strategies [2] one of the important issues in this area remains the change of the electronic properties upon exposure to ambient gases. Rubrene (C 42 H 28 , 5,6,11,12-tetraphenylnaphthacene, see inset of Fig. 1) belongs to a group of small organic molecules with promising properties which found use in organic light emitting diodes (as a red dopant) [3] and organic field effect transistors [4]. However, as also other molecules [5], rubrene exhibits strong reactivity and affinity to (photo-)oxidation which reduces the stability and lifetime of devices. While it is well known that rubrene tends to undergo (photo-)oxidation [6], the understanding and control of degradation due to oxidation of these materials still is a key challenge in organic electronics. For this purpose we studied in real-time and under controlled conditions the kinetics of oxidation of rubrene thin films using spectroscopic ellipsometry.The rubrene material used was purchased from Acros and purified by gradient sublimation. The Si(100) substrates with ∼ 20 nm thermal oxide were cleaned with acetone and propanol in an ultrasonic bath, transfered into the vacuum chamber (base pressure p = 3 × 10 −8 mbar) and heated at temperatures T ≥ 400 • C for several hours. The rubrene films were grown with the substrate at room temperature by evaporation from a Knudsen cell with a typical growth rate of 0.85 nm/min. The experimental data were acquired in situ using a spectroscopic ellipsometer (Woollam M-2000) with a broad band 75 W Xe-lamp (250 nm to 1000 nm) and CCD-based detection system with a resolution of about 1.6 nm. The light spot on the sample was ∼ 2 × 6 mm 2 . The light source and detector were mounted to the vacuum chamber which provides a pair of strain-free windows at a * Electronic address: frank.schreiber@uni-tuebingen. de FIG. 1: Dielectric function ε2 of the rubrene film before and during photo-oxidation (initial film thickness 25.1 nm). The absorption bands around 2.5 eV and 4.0 eV correspond to pristine rubrene, whereas the feature at 4.95 eV originates from oxidized rubrene. The relatively small difference between the pristine and '0 min' spectrum shows effect of...

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