Angular Distribution Analysis of Permanent Dipole Moments in Amorphous Organic Semiconductor Film with Large Spontaneous Orientation Polarization

Spontaneous orientation polarization (SOP) in vacuum-deposited amorphous organic films occurs when molecular permanent dipole moments (PDMs) are anisotropically oriented. This phenomenon has recently been observed in many types of organic light-emitting diode (OLED) materials as a factor significantly affecting device performance. Although the formation mechanism of SOP can be explained in terms of surface equilibration, quantitative estimation of SOP based on molecular structure remains challenging, partly because the information on PDM angular distribution is lacking. In this study, we analyzed the PDM angular distribution in vacuum-deposited amorphous films of a commonly used host material 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazole-3-carbonitrile (mCPCN). Defining θp as the angle between the PDM and surface normal, we estimated the average and variance of cos θp, ⟨cos θp⟩ and Vcos⁡θp, respectively, based on Kelvin probe, X-ray reflectometry, impedance, and IR absorption measurements, and uniquely determined the angular distribution of θp assuming a normal distribution of cos θp. A very slight deviation in the angular distribution from random distribution resulted in the large SOP of the mCPCN films. Our angular distribution analysis method, based on obtaining ⟨cos θp⟩ and Vcos⁡θp, can be adopted generally to allow visualization of PDM angular distributions of OLED materials, leading to further understanding of the formation mechanism of large SOP.