Enhancement of the molecular orientation of TPBi in coevaporated films of UGH-2 host molecules

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.