A polar transition of spontaneous orientation polarization in organic amorphous thin films

Certain organic molecules autonomously form dipole orientation during a thin-film formation, leading to spontaneous orientational polarization (SOP) even in an amorphous film. Although the polarity of SOP can be controllable by molecular design itself, we demonstrate that a guest-host system also provides a unique tool for controlling SOP. Here, we report that the transition of SOP polarity occurs depending on the concentration of a polar guest molecule in a non-polar matrix. While a 2,8-bis(diphenylphosphoryl)dibenzo-[b,d]thiophene (PPT) film shows negative SOP in the neat film, the polarity could be changed to positive with a concentration increase of the non-polar molecule.