TOWARDS UNDERSTANDING AND CONTROLLING SPONTANEOUS ORIENTATION POLARIZATION IN ORGANIC SEMICONDUCTING FILMS

Abstract

Spontaneous orientation polarization (SOP) is inherent in evaporated films of many organic semiconducting molecules with a permanent dipole moment (PDM). A significant electric field is formed in the film due to SOP. Consequently, the properties of organic light-emitting diodes (OLEDs) incorporating such films are influenced. However, the formation mechanism of SOP has been insufficiently understood. First of all, we examined more than 15 OLED materials for SOP towards understanding and controlling SOP. The results indicate that SOP is very common in evaporated films of OLED materials. Furthermore, the results imply that the strong electrostatic interaction between PDMs acts as a negative factor of the SOP formation. Next, SOP of co-evaporated films was also studied. In the co-evaporated films of TPBi and CBP, where TPBi is polar while CBP is non-polar, we found that the orientation degree of PDM increases with diluting PDM density. This result is consistent in terms of the electrostatic interaction. Since the concentration of PDM is diluted by the nonpolar molecule, the electrostatic interaction energy is reduced. As suppressing negative factor, the orientation degree of PDM is enhanced. Finally, we examined the surface potential characteristics of TPBi films deposited on a gold substrate with/without a self-assembled monolayer (SAM) of 1-dodecanethiol. The surface potential of the SAM treated sample was lower than that of the untreated one. The result can also be explained by considering electrostatic interactions. By reducing surface free energy, van der Waals interaction on the film surface is suppressed and PDM interaction becomes relatively dominant, consequently SOP is suppressed. In summary, the SOP formation likely results from a balance between positive and negative factors, where the positive factor is the van der Waals interaction between the molecule and film surface, and the negative factor is the PDM interaction between molecules on the film surface.