|Physics Department | Center For Optical Technologies | Lehigh University|
The optical absorption and photoluminescence spectra of organic molecular crystals depend on the optical properties of the molecules, on the way the molecules are arranged in the crystal matrix, and how the molecules interact with each other. The rubrene single crystal has a large optical anisotropy that has a strong influence on the absorption and luminescence spectra that are observed under different experimental conditions. Although transport properties of rubrene single crystals have been extensively studied, considerably fewer studies have explored their optical properties.
Figure 1: A rubrene single crystal's ab plane
The rubrene molecule is a polycyclic aromatic hydrocarbon which has a backbone structure made of benzene rings. Rubrene is different from other organic molecules such as anthracene, tetracene, and pentacene because it has phenyl side groups (see figure 2.)
Rubrene crystals grown by physical vapor transport have an orthorhombic unit cell with lattice parameters a = 14.4A, b = 7.18A, and c = 26.9A (see figure 3.) The most common crystal shapes are platelets with extended c-surfaces and crystals elongated in the b direction but with short dimensions in the a and c directions. As-grown rubrene crystals have facets that form a typical angle of 63.5 degrees to the b axis when observing the ab plane, and 75 degrees with respect to the b axis from the bc plane (see figure 3.)
Figure 2: A single rubrene molecule with defined molcular axes (top.) Facets of single-crystal rubrene (bottom.)
Figure 3: Two facets of the rubrene single crystal, abplane view seen top left, and bc plane on top right. Molecular packing structure of rubrene is seen to be a herring bone pattern (bottom image.)
Rubrene crystals have several compelling properties, including one of the highest room-temperature charge carrier mobilities ever observed in an organic material (10 - 40 cm^2/Vs for holes in field-effect transistors) and a high photoconductivity. The high hole mobility values in rubrene crystals are found along the b axis and is characterized by the direction in which the herringbone pattern has an efficient pi-orbital overlap. In addition, recent photoconductivity studies in rubrene have been interpreted in terms of exciton diffusion lengths of several micrometers that allow excitons to migrate to the crystal surface.
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