Substrate Selection for Full Exploitation of Organic Semiconductor Films: Epitaxial Rubrene on β-Alanine Single Crystals

Rubrene (RUB) is one of the most studied organic semiconductors because, in the orthorhombic single-crystal phase, it exhibits a record exciton diffusion length and one of the highest charge carrier mobilities ever reported. Here, thin films of oriented crystalline RUB are successfully grown in vacuum on millimeter-sized (010)-β-alanine (β-ala) single crystals with a step-growth protocol, exploiting organic epitaxy. The experimental characterization demonstrates that these RUB films grow in the orthorhombic polymorph with the (100)RUB plane in contact with the (010)β-ala surface and with precise azimuthal orientations. A complementary study of the RUB(100)/β-ala(010) interface, performed by computational simulations, confirms the epitaxial relations expected by considering the molecular scale corrugations of the surfaces. Moreover, thanks to the wide transparency region of β-ala, the RUB absorption bands in the UV range are directly detected for the first time. Finally, removal of the water-soluble substrate enables the integration of the films in field effect transistors as high quality active organic layers. The characteristics of such RUB-based devices confirm the quality and versatility of epitaxial thin films for use in organic electronics.