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The outcoupling efficiency of OLEDs can be significantly enhanced by orienting the emitter molecules horizontally within the film. While some guidelines have been developed to control molecular orientation in OLEDs[1], our understanding of the factors at play remains limited by the fact that current techniques only obtain average values of what in reality is a distribution of orientations. Here, we develop a method to obtain orientation distributions of emitters in thermally evaporated films of host-guest emissive layers relevant to OLED displays[2]. We achieve this by adapting out-of-focus fluorescence imaging of individual molecules for use in thermally evaporated systems. We show how the orientation distribution of emitters depends on processing conditions and on the nanoscale environment of the emitters. Crucially, we also show that emitters can adopt different orientation distributions that would be indistinguishable in ensemble-averaging measurements.
[1] F. Tenopala-Carmona, … M. C. Gather, Adv. Mater. 2021, 33, 2100677
[2] F. Tenopala-Carmona, … M. C. Gather, Nat. Comm. 2023, 14, 6126
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