The electrical and structural properties of poly (3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) thin films deposited from aqueous dispersion using different concentrations of selected secondary dopants are studied in detail. An improvement of the electrical conductivity by three orders of magnitude is achieved for dimethyl sulfoxide, sorbitol, ethylene glycol, and N,N-dimethylformamide, and the secondary dopant concentration dependence of the conductivity exhibits almost identical behavior for all investigated secondary dopants. Detailed analysis of the surface morphology and Raman spectra reveals no presence of the secondary dopant in fabricated films, and thus the dopants are truly causing the secondary doping effect. Although the ratio of benzenoid and quinoid vibrations in Raman spectra is unaffected by doping, the phase transition in PEDOT:PSS films owing to doping is confirmed. Further analysis of temperature-dependent conductivity reveals 1D variable range hopping (VRH) charge transport for undoped PEDOT:PSS, whereas highly conductive doped PEDOT:PSS films exhibit 3D VRH charge transport. We demonstrate that the charge-hopping dimensionality change should be a fundamental reason for the conductivity enhancement.

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.

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