Influence of intermolecular interactions on the formation of spontaneous orientation polarization in organic semiconducting films

Spontaneous orientation polarization (SOP) has been frequently observed in the evaporated films of organic light-emitting diode materials. Because SOP modifies the charge injection and the accumulation properties of the device, understanding and controlling SOP is crucial in optimizing the performance of the device. In this study, we investigated the dominant factors for SOP formation by focusing on intermolecular interactions. We examined the giant surface potential characteristics of coevaporated films incorporating 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) that is a typical polar molecule exhibiting SOP. In the coevaporated films of TPBi and nonpolar molecules such as 4,4′-bis(N-carbazolyl)-1,1′-biphenyl and 4,4′,4″-tris (carbazol-9-yl)triphenylamine, the orientation degree of the permanent dipole moment (PDM) of TPBi is significantly enhanced with diluted TPBi density, though the enhancement is weak on the film with N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine. The results indicate that the PDM interaction between polar molecules results as a negative factor for SOP formation. Furthermore, we found that SOP formation is suppressed by the surface treatment of the self-assembled monolayer on the gold substrate, indicating a positive effect of the van der Waals interaction between the molecule and the substrate surface.