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Identification of the key parameters for horizontal transition dipole orientation in fluorescent and TADF organic light-emitting diodes

Research output: Contribution to journalReview articlepeer-review


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 (Tg) 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 Tg, 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.


Original languageEnglish
Article number2100677
Number of pages28
JournalAdvanced Materials
VolumeEarly View
Early online date2 Aug 2021
Publication statusE-pub ahead of print - 2 Aug 2021

    Research areas

  • Meta-analysis, Molecular oreintation, Multiple linear regression, ORganic light-emitting diodes, Thermally activated delayed fluorescence

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