Molecular orientation anisotropy of the emitter molecules used in organic light emitting diodes (OLEDs) can give rise to an enhanced light-outcoupling efficiency, when their transition dipole moments are oriented preferentially parallel to the substrate, and to a modified internal quantum efficiency, when their static dipole moments give rise to a locally modified internal electric field. Here, the orientation anisotropy of state-of-the-art phosphorescent dye molecules is investigated using a simulation approach which mimics the physical vapor deposition process of amorphous thin films. The simulations reveal for all studied systems significant orientation anisotropy. Various types are found, including a preference of the static dipole moments to a certain direction or axis. However, only few systems show an improved outcoupling efficiency. The outcoupling efficiency predicted by the simulations agrees with experimentally reported values. The simulations reveal in some cases a significant effect of the host molecules, and suggest that the driving force of molecular orientation lies in the molecule-specific van der Waals interactions of the dye molecule within the thin film surface. The electrostatic dipole–dipole interaction slightly reduces the anisotropy. These findings can be used for the future design of improved dye molecules.

中文翻译：

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有机发光二极管发射层中各向异性染料取向的分子起源

当有机发光二极管（OLED）的跃迁偶极矩优先平行于基板取向时，有机发光二极管（OLED）中使用的发射体分子的分子取向各向异性可以提高光输出耦合效率，而当它们的跃迁偶极矩优先平行于衬底取向时，则可以提高内部量子效率静态偶极矩会产生局部改变的内部电场。在这里，使用模拟非晶态薄膜的物理气相沉积过程的模拟方法研究了最先进的磷光染料分子的取向各向异性。该仿真揭示了对于所有研究系统的显着取向各向异性。发现了各种类型，包括静态偶极矩偏向某个方向或轴。但是，只有很少的系统显示出更高的输出耦合效率。模拟预测的输出耦合效率与实验报告的值一致。模拟在某些情况下揭示了宿主分子的显着作用，并表明分子取向的驱动力在于薄膜表面内染料分子的分子特异性范德华相互作用。静电偶极-偶极相互作用会稍微降低各向异性。这些发现可用于改进染料分子的未来设计。静电偶极-偶极相互作用会稍微降低各向异性。这些发现可用于改进染料分子的未来设计。静电偶极-偶极相互作用会稍微降低各向异性。这些发现可用于改进染料分子的未来设计。