The full UV-visible dielectric tensor and the corresponding directions of the principal axes of triclinic tetracene crystals are reported as deduced either by polarized absorption and ellipsometry measurements or by calculations based on the molecular and crystallographic data. The results allow the attribution of the polarized bands observed in both absorption and photoluminescence emission spectra. In particular, the spectral line shape and polarization of the emission are found to depend on the sample thickness, and the effect is attributed to the modification of the state of polarization of the emitted light during its propagation inside the crystal. Indeed, the directions of polarization of the lowest optical transitions and the directions of the principal axes of the dielectric tensor are demonstrated not to coincide, in contrast to the assumptions typically made in the literature, thus causing the mixed transverse/longitudinal character of light propagation.
[Show abstract][Hide abstract] ABSTRACT: Rubrene, perylene, -quaterthiophene, and tetracene single crystals were grown and placed between indium-tin-oxide and Al electrodes; the current-voltage characteristics of the so-fabricated two-terminal organic devices were measured at ambient conditions under solar simulator illumination. While rubrene single crystals do not exhibit any pho-tovoltaic effect, open-circuit voltages reach values up to 1.8 V in devices based on perylene, -quaterthiophene, and tetra-cene single crystals.
[Show abstract][Hide abstract] ABSTRACT: The development of a thin film technology based on the outstanding organic semiconductor rubrene is seriously prevented by its tendency to solidify in an amorphous state when deposited on foreign substrates. Here, a crystalline thin film of rubrene was successfully grown by vapor deposition on the surface of a tetracene single crystal. Atomic resolution imaging performed with scanning force microscopy revealed the high crystallinity of the film and its unique orientation achieved through a line-on-line epitaxial relation.
The Journal of Physical Chemistry C 09/2008; 112(42). DOI:10.1021/jp806877e · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Kramers-Kronig relations between the real and imaginary parts of a response function are widely used in solid-state physics to evaluate the corresponding quantity if only one component is measured. They are among the most fundamental statements since only based on the analytical behavior and causal nature of the material response [Phys. Rev. 104, 1760-1770 (1956)]. Optical losses, for instance, can be obtained from the dispersion of the dielectric constant at all wavelengths, and vice versa [Handbook of optical constants of solids, Vol. 1, p. 35]. Although the general validity was never casted into doubt, it is a longstanding problem that Kramers-Kronig relations cannot simply be applied to anisotropic crystalline materials because contributions from different directions mix in a frequency-dependent way. Here we present a general method to identify frequency-independent principal polarizability directions for which the Kramers-Kronig relations are obeyed even in materials with lowest symmetry. Using generalized spectroscopic ellipsometry on a single crystal surface of triclinic pentacene, as an example, enables us to evaluate the complex dielectric constant and to compare it with band-structure calculations along the crystallographic directions. A general recipe is provided how to proceed from a macroscopic measurement on a low symmetry crystal plane to the microscopic dielectric properties of the unit cell, along whose axes the Kramers-Kronig relations hold.
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