Enhancement of Spontaneous Orientation Polarization in Glassy Organic Semiconductor Mixtures

The alignment of permanent dipole moments and the resulting spontaneous orientation polarization (SOP) are commonly observed in evaporated neat films of polar organic molecules and lead to a so-called giant surface potential. In the case of mixed films, often enhanced molecular orientation is observed, i.e., a higher degree of alignment, in comparison to neat layers, if it is diluted into a suitable (nonpolar) host. So far, different possible influences on molecular orientation have been discussed, with the most prominent probably being the so-called surface equilibration model. In this contribution, we discuss how surface equilibration can influence orientation in mixed layers and which other intermolecular interactions have to be considered to explain the observed enhancement of SOP in mixed layers.