Crystal Orientation Imaging of Organic Monolayer Islands by Polarized Light Microscopy

The initial stage of organic semiconductor film formation greatly affects the properties of films, which are used in organic devices including thin-film transistors and light-emitting diodes. Organic monolayer islands that are formed on a suitable substrate can be observed with a conventional optical microscope. Furthermore, the use of a polarized microscope allows the determination of the refractive index and crystal orientation of islands. Here, we report organic monolayer islands of 2,9-diphenyl-dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DPh-DNTT) deposited on a Si substrate with thermally grown SiO₂ to investigate the crystal orientation of islands by polarized light microscopy. The observation of DPh-DNTT islands under polarized quasi-monochromatic light reveals that reflection intensity depends on both the crystal orientation and irradiation wavelength. A comparison between experimental and calculated reflection intensities provides an estimate of an anisotropic complex refractive index in the plane. The crossed-polarized microscopy image of a SiO₂/Si substrate with DPh-DNTT islands shows that the contrast between the islands and SiO₂ surface is sensitive to the angle between the polarizer and analyzer and depends on the direction of crystal orientation. The dependence of reflection contrast, which can be explained by the anisotropic extinction coefficient, is used to confirm crystal orientation.