On the Orientation Mechanism of Nonpolar Dyes in Light-Emitting Guest–Host Systems

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Alignment of the emissive transition dipoles in organic guest–host systems is a powerful tool to enhance light outcoupling from organic light-emitting diodes. Here, we study the orientation mechanism of four nonpolar dyes in neat films and dye-doped guest–host systems with varying emitter concentrations. We find that the key factors controlling their alignment in thermally evaporated thin films are their shape anisotropy and the ratio between the temperature of the substrate, on which the film is grown, and the glass transition temperature of the guest–host system. However, care has to be taken when a mixed cohost system with largely different glass transition temperatures (T_g) of its constituents is used. In this case, the emitter orientation may not follow the effective T_g,mix of the mixed host. In addition, we suggest using the principal moments of inertia of a molecule for judging its resilience toward reorientation, and the derived aspect ratio to characterize its shape anisotropy.