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Keywords:

  • organic light-emitting diodes (OLEDs);
  • transition dipole moment;
  • exciplex forming hosts;
  • phosphorescent dyes

Abstract

High-efficiency phosphorescent organic light-emitting diodes (OLEDs) doped with Ir(ppy)2(acac) [bis(2-phenylpyridine)iridium(III)-acetylacetonate] in an exciplex forming co-host have been optically analyzed. This emitter has a preferred orientation with the horizontal to vertical dipole ratio of 0.77:0.23 as compared to 0.67:0.33 in the isotropic case. Theoretical analysis based on the orientation factor (Θ, the ratio of the horizontal dipoles to total dipoles) and the photoluminescence quantum yield (qPL) of the emitter predicts that the maximum external quantum efficiency (EQE) of the OLEDs with this emitter is about 30%, which matches very well with the experimental data, indicating that the electrical loss of the OLEDs is negligible and the device structure can be utilized as a platform to demonstrate the validity of optical modeling. Based on the results, the maximum EQE achievable for a certain emitting dye in a host can be predicted by just measuring qPL and Θ in a neat film on glass without the need to fabricate devices, which offers a universal plot of the maximum EQE as a function of qPL and Θ.