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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.

In this study, we investigated the spontaneous orientation polarization (SOP) in coevaporated films of polar and nonpolar molecules. SOP has been commonly observed in the evaporated films of organic light‐emitting diode materials, and it influences the device performances. However, the mechanism of the SOP formation is yet to be clarified. Herein, we characterized the molecular orientation of polar molecules of 1,3,5‐tris (1phenyl‐1H‐benzimidazol‐2‐yl) benzene (TPBi) in coevaporated films with nonpolar molecules of 1,4‐bis‐(triphenylsilyl) benzene (UGH‐2) or 4,4′‐bis (N‐carbazolyl)‐1,1′‐biphenyl (CBP). We measured the surface potential characteristics of the coevaporated films. We found that the molecular orientation of TPBi in both UGH‐2 and CBP hosts was enhanced, though the apparent enhancement factor was small in the UGH‐2 host. The enhancement of the molecular orientation is attributed to the reduction of the electrostatic interaction between polar molecules (TPBi), which deteriorates SOP as previously reported. In addition, our results suggest that the SOP of TPBi is robust even in the UGH‐2 host, in contrast to the random orientation of Ir complexes in the UGH‐2 host. Considering a polyhedral shape of Ir complexes, the robust SOP of TPBi in the host molecules with a weak van der Waals interaction, such as UGH‐2, is possibly due to its disk‐like molecular shape.

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