Explicit analysis of functional group orientation in amorphous organic semiconductor films by using deuterated materials†
Regarding the development of high-performance organic light-emitting diodes (OLEDs), the molecular orientation in amorphous organic semiconductor films is a key factor for deeply understanding the higher-order structure of the films and the corresponding effects on device properties. However, in conventional analyses, the axis of “molecular orientation” is often ambiguously defined owing to the variety of conformation structures specific to amorphous organic materials. Because the functional group orientation has not been discussed in detail, the relationship between the electrical properties of the films and the molecular orientation of amorphous organic materials remains unclear. In this study, to solve the ambiguity of molecular orientation, we investigate the functional group orientation of a hole transport material for OLEDs in vacuum-deposited films by an IR absorption analysis (named “IR-QACIOS”) with deuterated materials. The degree of functional group orientation depends on the functional groups of the molecule; the central axis of the molecule is substantially oriented in the horizontal direction, whereas the side wings of the molecule are slightly oriented. In addition, we analyze the face-on orientation of each phenyl ring in the molecule separately. The degree of the horizontal orientation of the side wings strongly depends on the deposition rate and affects the charge transport in the bulk film of devices that incorporated the molecule. By explicitly defining the axes of molecular orientation in a manner that focuses on functional group orientation, one can obtain unprecedentedly detailed information on the molecular orientation in amorphous organic semiconductor films and the corresponding effects on device properties.