In the past decades, organic semiconductors (OSC) has attracted interest of researchers for their possible applications in organic field effect transistors, organic photovoltaic (OPV), and organic light emitting diodes (OLEDs) [1, 2]. In many of these applications, several molecular components, typically donor-acceptor (D/A) combinations, are needed and their mixing behaviour plays a key role for the performance of the devices. As an example, in OPV devices, the size of the D/A domains for partially mixing OSCs has to be comparable to the effective charge carrier diffusion length for the effective charge carriers separation and their withdrawal towards electrodes. Recent research on OSCs mixing, stimulated by the device related importance of the topic, has also shown some new and unexpected results at the fundamental level.
The presentation will review recent findings in the field of OSCs mixing behaviour. We will focus on blends of small OSCs molecules in thin films prepared by co-evaporation on weakly interacting silicon substrates. At the very beginning, we shorty introduce some most studied OSCs and highlight the importance of the OSCs blends for device applications. Thereafter, we will mention the process of OSM thin film growth via organic molecular beam deposition and the X-ray scattering techniques employed in the structural studies on them, including X-ray specular reflectivity and grazing incidence X-ray diffraction [3].
The main part of the presentation will deal with crystal structure of binary mixtures of OSCs and how it is influenced by molecular interactions. Some structures of OSC blends find direct counterparts in the well explored elemental systems, such as binary alloys, while other mixing scenarios are completely new and surprising. Similar to the elemental systems, phase separation , statistical mixing of molecular components, and formation of a new molecular compound phase, respectively, are observed for OSC mixtures [4]. However, in contrast to the elemental blends, not only interaction between molecular components of mixtures determines the realized mixing scenario but also the steric, i.e. shape, compatibility of the molecules plays an important role [4, 5]. Additionally, anisotropy of the shape and of the interaction potential of the OSC molecules can lead to an ordering anisotropy, where the mixed system shows periodicity only along a certain preferential direction [6].