In organic light-emitting diodes (OLEDs), horizontal orientation of the emissive transition dipole moment (TDM) can improve light outcoupling efficiency by up to 50% relative to random orientation. Therefore, there have been extensive efforts to identify drivers of horizontal orientation. The aspect ratio of the emitter molecule and the glass-transition temperature (T_g) of the films are currently regarded as particularly important. However, there remains a paucity of systematic studies that establish the extent to which these and other parameters control orientation in the wide range of emitter systems relevant for state-of-the-art OLEDs. Here, recent work on molecular orientation of fluorescent and thermally activated delayed fluorescent emitters in vacuum-processed OLEDs is reviewed. Additionally, to identify parameters linked to TDM orientation, a meta-analysis of 203 published emitter systems is conducted and combined with density-functional theory calculations. Molecular weight (MW) and linearity are identified as key parameters in neat systems. In host–guest systems with low-MW emitters, orientation is mostly influenced by the host T_g, whereas the length and MW of the emitter become more relevant for systems involving higher-MW emitters. To close, a perspective of where the field must advance to establish a comprehensive model of molecular orientation is given.

中文翻译：

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确定荧光和 TADF 有机发光二极管中水平跃迁偶极子取向的关键参数

在有机发光二极管 (OLED) 中，发射跃迁偶极矩 (TDM) 的水平取向相对于随机取向可将光输出耦合效率提高多达 50%。因此，已经进行了广泛的努力来确定水平方向的驱动因素。发射体分子的纵横比和玻璃化转变温度 ( T _g) 的电影目前被视为特别重要。然而，仍然缺乏系统研究来确定这些和其他参数在与最先进的 OLED 相关的各种发射器系统中控制方向的程度。在这里，回顾了真空处理 OLED 中荧光和热激活延迟荧光发射器的分子取向的最新工作。此外，为了确定与 TDM 方向相关的参数，对 203 个已发表的发射器系统进行了元分析，并结合密度泛函理论计算。分子量 (MW) 和线性被确定为纯系统中的关键参数。在具有低 MW 发射器的主客体系统中，方向主要受宿主T _{g 的影响}，而发射器的长度和 MW 变得与涉及更高 MW 发射器的系统更相关。最后，给出了该领域必须前进以建立分子取向的综合模型的观点。