In Situ Determination of the Orientation of the Emissive Dipoles in Light‐Emitting Electrochemical Cells

The orientation of the emissive dipoles in thin‐film devices is important since it strongly affects the light outcoupling and thereby the device emission efficiency. The light‐emitting electrochemical cell (LEC) is particularly interesting in this context because its emissive dipoles are located in a high electric‐field p‐n junction, which is formed in situ by redistribution of bulky ions. This implies that the dipole orientation could be distinctly different in the driven LEC compared to the pristine device. This study develops the destructive‐interference microcavity method for the accurate in situ determination of the orientation of the emissive dipoles during LEC operation and apply it on a common LEC device comprising an amorphous conjugated polymer termed Super Yellow as the emitter. It is found that ≈95% of the emissive dipoles are oriented in the horizontal direction with respect to the thin‐film plane in both the pristine LEC and during steady‐state light emission. This finding is attractive since it enables for efficient outcoupling of the generated photons, and interesting because it shows that a horizontal orientation of the emissive dipoles can remain despite the existence of a strong perpendicular electric field and the nearby motion of bulky ions during LEC operation. A method for the in situ determination of the orientation of the emissive dipoles in light‐emitting electrochemical cells (LECs) is reported. It reveals that ≈95% of the dipoles remain oriented parallel to the device plane during long‐term operation in a common polymer LEC. This finding is surprising since the dipoles are positioned in a strong vertical electric field and in proximity of redistributing bulky ions, but also attractive since it enables for efficient outcoupling.