Tetracene films

Pentacene and tetracene films can be oriented by using friction-transferred PTFE polymer substrates (see Section 3.3). The films consist of both crystallographic... 

The photopotentials of dye-gas-metal systems have reached an order of 0.1 mV 55> and those of alkali metal-aromatic junctions 0.2 to 1.0 V 53). Whereas the photo-emf of noble metal-aromatic junctions were of the order 1—15 mV 53>, thin tetracene films sandwiched between two different evaporated metal electrodes (Au, Al) showed photovoltaic effects with an open-circuit photovoltage up to... 

Figure 9 The emission spectrum of a tetracene film evaporated onto a glass substrate kept at 89 K and the emission monitored at 180 K (full circles). Its decomposition into Gauss profiles (II, III, IV, V) is shown by solid lines. The dashed curve is the sum of the gaussians. The lacking band I (=540 nm) is characteristic of the monomer emission from crystalline films formed at T > 140 K. Adapted from Ref. 72.

The emission spectra of non-crystalline tetracene films have been interpreted in terms of emission from a molecular pair in a sandwich configuration.32 Substitutional and surface fluorescence in pentacene-anthracene crystals33 and the changes in luminescence of single crystals of pure anthracene, pure tetracene, and pentacene-doped tetracene as a result of electrode-induced changes in electrical charge carriers 34>35 have been examined. Hot bands in the fluorescence and phosphorescence of coronene have been analysed.38... 

Fig. 6 Absorbance spectra for solution (dotted trace) and thin film (solid trace) samples of rubrene in the upper panel and tetracene in the lower panel. Molecular aggregation in the tetracene thin film gives rise to the splitting of its absorption bands. Inset are chemical structures for rubrene and tetracene...

Comparing single crystal and vapor-grown devices for these two compounds is difficult, because reports on evaporated tetracene OTFTs are rather scarce [99-101], and despite several (unpublished) attempts, fabrication of an operating thin-film device from rubrene has not yet been successfully achieved. For both compounds the problem seems to arise from an improper deposition mechanism, which, in contrast with experience with pentacene and sexithiophene, does not favor two-dimensional growth. 

Figure 111 Emission spectra from the SL LEDs based on anthracene (A) and tetracene-doped anthracene (T A) films with two different concentrations of tetracene. Violet anthracene emission disappears with increasing concentration of tetracene. The spectra were taken with the lpm-thick films sandwiched between Au anode and A1 cathode at the applied field F 10fiV/cm. After Ref. 212. Reprinted from Ref. 212. Copyright Springer-Verlag, with permission.

Analysis of the vibrational bands revealed that at earliest growth stages the film is amorphous. In particular, a broad band at 1373 cm-1 proves the amorphous nature of the film. On the other hand, the mode at 1606 cm-1, usually an infrared active band, proves a symmetry breakdown of the molecule at this growth stage. Additionally, the amorphous phase lacks of vibrational activity at the phenyl groups, and tetracene backbone. Therefore, it is likely that the geometry of the rubrene molecule is dramatically distorted. 

Rubrene. This organic semiconductor consists of a tetracene core with four additional phenyl groups connected by single bonds. In contrast to the desired crystalline phase the amorphous one is not stable against oxidation [10], The two states can be distinguished easily, since the amorphous phase lacks the typical red color found for crystalline films. 

In spite of the high carotenoid protective effect In natural biopolymers as well as In man-made polymer matrices [demonstrated, e.g., by Inhibition of self-sensltlzed oxygenation of tetracene In polystyrene (127) or by measured rate constant kq = 4.3 X 10 M 1 sec l In polystyrene (135)], B-carotene proved to be Ineffective as a UV stabilizer In polypropylene films under conditions of xenonarc Irradiation (136). The possible explanation Is that the polyene chain of B-carotene Is quickly... 

The photooxldatIon rates of polypropylene films, containing anthracene (a sensitizer, generating 02 within the polymer) and some Nl(II) chelates Is appreciably reduced, as compared with polypropylene films containing anthracene only (29, 286). (Ill) Self-sensltlzed 62 oxygenation of tetracene In polystyrene films containing 82 was considerably... 

Better resolution of an excimer intermediate in fission was achieved in a more recent study of TIPS-tetracene solutions (Fig. 7). In this work, the diffusion-limited dynamics, endothermic energetics and unusually sharp triplet exciton absorption features enabled identification of a spectroscopically distinct intermediate state in transient absorption and photoluminescence. In contrast to TIPS-pentacene, TIPS-tetracene represents a typical tetracene system where singlet fission is endothermic by 200 meV. This endothermicity is well known to have a drastic effect on the rate of fission in the solid state where triplet formation occurs three orders of magnitude slower in films of tetracene than in pentacene. Recent work suggests that fission in tetracene may not require thermal activation. Notably, the decay of singlet excitons and the rise of triplet exciton absorption have been shown to occur independent of temperature. A low-lying, dark intermediate state in tetracene was invoked to explain these observations, however it had been difficult to isolate such a state experimentally. ... 

Lim, S.H., T.G. Bjorklund, EC. Spano, and C.J. Bardeen. 2004. Exciton delocalization and superradiance in tetracene thin films and nanoaggregates. Phys Rev Lett 92 107402. 

A. Hepp, H. Heil, W. Weise, M. Ahles, R. Schmechel and H. von Seggern, Light-emitting field-effect transistor based on a tetracene thin film, Phys. Rev. Lett, 91, 157406 (2003). 

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