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 θ_p as the angle between the PDM and surface normal, we estimated the average and variance of cos θ_p, ⟨cos θ_p⟩ and Vcosθp$V_{\cos {\mathit{\theta }}_{\mathrm{p}}}$, respectively, based on Kelvin probe, X-ray reflectometry, impedance, and IR absorption measurements, and uniquely determined the angular distribution of θ_p assuming a normal distribution of cos θ_p. 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 θ_p⟩ and Vcosθp$V_{\cos {\mathit{\theta }}_{\mathrm{p}}}$, 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.

中文翻译：

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大自发取向极化非晶有机半导体薄膜中永久偶极矩的角分布分析

当分子永久偶极矩（PDM）各向异性取向时，真空沉积非晶有机薄膜中会发生自发取向偏振（SOP）。最近在许多类型的有机发光二极管（OLED）材料中观察到这种现象，作为显着影响器件性能的因素。尽管SOP的形成机制可以用表面平衡来解释，但基于分子结构的SOP定量估计仍然具有挑战性，部分原因是缺乏PDM角分布的信息。在本研究中，我们分析了常用基质材料 9-(3-(9 H-咔唑-9-基)苯基)-9 H-咔唑-3-甲腈 (mCPCN)真空沉积非晶薄膜中的 PDM 角分布）。将θ _p定义为 PDM 与表面法线之间的角度，我们估计 cos θ _p、 ⟨cos θ _p ⟩ 的平均值和方差以及电压余弦θp​$V_{\cos {\mathit{\theta }}_{\mathrm{p}}}$分别基于开尔文探针、X 射线反射计、阻抗和红外吸收测量，并在假设 cos θ _p呈正态分布的情况下唯一确定了θ _p的角度分布。角度分布与随机分布的微小偏差导致 mCPCN 薄膜的 SOP 较大。我们的角度分布分析方法，基于获得 ⟨cos θ _p ⟩ 和电压余弦θp​$V_{\cos {\mathit{\theta }}_{\mathrm{p}}}$，通常可用于实现 OLED 材料的 PDM 角分布可视化，从而进一步了解大 SOP 的形成机制。