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
The problem of charge carrier photogeneration in conjugated polymers is related to the question concerning the singlet excitation binding energy. Measurements of the cw- photoconduction in several conjugated polymers as a function of photon energy, electric field, and temperature under different cell geometries indicate that excitons can dissociate at an electrode as well as via sensitization in the bulk. Intrinsic photogeneration can also occur at higher photon energies via dissociation of vibrationally excited singlet excitons. Charge transport, monitored via time-of- flight signals, can be rationalized in terms of a disorder concept except for a ladder-type poly-(para-phenylene) film in which built-in disorder is weak.
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