We investigate exciton-phonon coupling and exciton transfer in diindenoperylene ͑DIP͒ thin films on oxidized Si substrates by analyzing the dielectric function determined by variable-angle spectroscopic ellipsometry. Since the molecules in the thin-film phase form crystallites that are randomly oriented azimuthally and highly oriented along the surface normal, DIP films exhibit strongly anisotropic optical properties with uniaxial symmetry. This anisotropy can be determined by multiple sample analysis. The thin-film spectrum is compared with a monomer spectrum in solution, which reveals similar vibronic subbands and a Huang-Rhys parameter of S Ϸ 0.87 for an effective internal vibration at ប eff = 0.17 eV. However, employing these parameters the observed dielectric function of the DIP films cannot be described by a pure Frenkel exciton model, and the inclusion of charge-transfer ͑CT͒ states becomes mandatory. A model Hamiltonian is parametrized with density-functional theory calculations of single DIP molecules and molecule pairs in the stacking geometry of the thin-film phase, revealing the vibronic coupling constants of DIP in its excited and charged states together with electron and hole transfer integrals along the stack. From a fit of the model calculation to the observed dielectric tensor, we find the lowest CT transition E 00 CT at 0.26Ϯ 0.05 eV above the neutral molecular excitation energy E 00 F , which is an important parameter for device applications.

We present real-time in situ studies of optical spectra during thin film growth of several prototype organic semiconductors (pentacene, perfluoropentacene, and diindenoperylene) on SiO2. These data provide insight into surface and interface effects that are of fundamental importance and of relevance for applications in organic electronics. With respect to the bulk, the different molecular environment and structural changes within the first few monolayers can give rise to significant optical changes. Similar to interface-driven phenomena in, e.g., magnetism, spectral changes as a function of thickness d are a very general effect, decaying as 1/d in the simplest approximation. We observe energy shifts of 50-100 meV, rather small changes of the exciton-phonon coupling, and new transitions in specific systems, which should be considered as general features of the growth of organics.

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...

In order to investigate the optical properties of rubrene we study the vibronic progression of the first absorption band (lowest π → π * transition). We analyze the dielectric function ε2 of rubrene in solution and thin films using the displaced harmonic oscillator model and derive all relevant parameters of the vibronic progression. The findings are supplemented by density functional calculations using B3LYP hybrid functionals. Our theoretical results for the molecule in two different conformations, i.e. with a twisted or planar tetracene backbone, are in very good agreement with the experimental data obtained for rubrene in solution and thin films. Moreover, a simulation based on the monomer spectrum and the calculated transition energies of the two conformations indicates that the thin film spectrum of rubrene is dominated by the twisted isomer.

Organic thin films frequently exhibit strong anisotropic optical constants, which in many cases are uniaxial with the optic axis oriented along the surface normal. The data analysis presented here to obtain the anisotropic optical constants of thin films on silicon substrates is based on the model system diindenoperylene (DIP), but nevertheless applies to all uniaxial films on SiO2. In addition to variable angle spectroscopic ellipsometry, different substrates are used to perform a multiple sample analysis. This way of analysing data increases the sensitivity to uniaxial anisotropy. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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