[16p-E402-1~9] Control and device application of spontaneous orientation of polar molecules

▼ [16p-E402-7] Molecular Designs through Intramolecular Interactions for Large and Stable Spontaneous Orientation Polarization

〇WeiChih Wang^1,2, Kyohei Nakano¹, ChainShu Hsu², Keisuke Tajima¹ (1.RIKEN, 2.NYCU)

Keywords:Spontaneous Orientation Polarization, Giant Surface Potential

Spontaneous orientation polarization (SOP) observed in glassy organic films is characterized by large change of electrostatic potential at the surface. The potential changes are originated form the directional selectivity of the polar molecules (i.e., up or down) during the vapor deposition, which leads to the large net polarity in the films. Vibration-based electret generators and charge transport/injection layers in organic light emitting diodes have been proposed as the possible applications of SOP. However, understanding of SOP in relation to the molecular structures still falls behind. In addition, low stability of SOP under the ambient light irradiation can be an issue for the applications. To establish the molecular design principle for the large and stable SOP, we synthesized a series of molecules based on a prototypical material, TPBi. Packing and orientation of the molecules in the films were analyzed by variable angle spectroscopic ellipsometry and 2D grazing-incidence wide-angle X-ray scattering. Density functional theory calculations and single-crystal structure analysis revealed the stable molecular conformations. Through the comparisons of the results between TPBi and the derivatives, we draw the following conclusions: (1) the stable molecular conformations and the molecular permanent dipole moment can be manipulated through the intramolecular interaction, which largely affects SOP in thin films, (2) in addition to the packing order and the orientation of the molecules, the subtle directional selectivity in the films are important for the large SOP, and (3) wide optical band gap by breaking π-conjugation in the molecules can enhance the photostability of SOP in thin film. These understandings in the combination with recent developments in the control of the directional selectivity would lead to the promising design principle of the SOP materials.