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.
