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Applied Optics

Applied Optics


  • Editor: James C. Wyant
  • Vol. 45, Iss. 27 — Sep. 20, 2006
  • pp: 7151–7165

Excitation of waveguide modes in organic light-emitting diode structures by classical dipole oscillators

Joseph F. Revelli  »View Author Affiliations

Applied Optics, Vol. 45, Issue 27, pp. 7151-7165 (2006)

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Analytical techniques known in the literature are used to (i) identify all the planar waveguide modes in four top-emitting organic light-emitting diode (OLED) structures over the visible spectrum, and (ii) compute both TM and TE power spectra for classically radiating dipoles in the emissive layers of these OLED structures. Peaks in the computed power spectra are identified with the waveguide modes in the OLED devices, and areas associated with these peaks are used to estimate the excitation probability of the waveguide modes. In cases where ambiguities arise because of overlapping peaks, it is shown that computed power spectra can be approximated as sums of Lorentzian line shapes. It is found that for all four structures, the dipoles couple almost 80% of their radiant energy into TM modes with only about 20% going into TE modes. Furthermore, except for a narrow spectral band, the excited TM modes are primarily short-range surface plasmon polaritons. Excitations in the narrow spectral band correspond to TM and TE Fabry–Perot microcavity modes. Finally, the analysis shows that, in the absence of grating couplers, only light in the microcavity modes escapes into the air cover.

© 2006 Optical Society of America

OCIS Codes
(250.3680) Optoelectronics : Light-emitting polymers
(310.2790) Thin films : Guided waves

Original Manuscript: February 23, 2006
Manuscript Accepted: April 18, 2006

Joseph F. Revelli, "Excitation of waveguide modes in organic light-emitting diode structures by classical dipole oscillators," Appl. Opt. 45, 7151-7165 (2006)

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  11. Justification for this assumption is based on computations carried out for a series of OLED structures in which (i) the imaginary part of the index of refraction of the ETL was allowed to be nonzero, and (ii) an additional layer was introduced between the ETL and the HTL. The additional layer was assumed to have an index of refraction equal to that of the real part of the ETL, and the emitting dipoles were assumed to be in the middle of this layer. It was found that as the thickness of the additional layer decreased, a new peak emerged in the power spectrum for large values of u (i.e., u > 10), and the area under this peak increased as d−3 where d is the thickness of the additional layer. This new channel is presumably due to dipole-dipole energy transfer and is known in the literature as the Förster transfer, as has been confirmed by numerically comparing the results obtained to Eq. (2.52a) of Ref. 2. Given that virtually all of the dipole energy goes to Förster transfer energy as d approaches zero, almost no light could be extracted from OLED devices if this channel were a nonradiative loss. Because this is not seen experimentally, it is assumed that the dipole-dipole transfer energy is not lost and that the imaginary part of the medium in which the dipoles are embedded is best treated as being effectively zero.
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