Distribution of radiation from organic light-emitting diode structures with wavelength-scale gratings as a function of azimuth and polar angles.

The angular distribution of radiation emitted from organic electroluminescent diodes fabricated on substrates with wavelength-scale gratings was measured using an optical Fourier transform instrument. A simple geometrical model is derived that specifies the polar angle of the exiting photon as a function of the azimuth angle, the grating pitch, the wavelength of light, and the effective index of the refraction of the light emitted by the fluorescing excitons. The radiation pattern of the extracted light is shown to fit that predicted by the model if one assumes that it comes from surface plasmon polaritons and bound TE waveguide modes.

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

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.

Waveguide analysis of organic light-emitting diodes fabricated on surfaces with wavelength-scale periodic gratings.

Numerical techniques for the analysis of multilayer waveguide structures were used to study the modes that exist in organic light-emitting diode (OLED) devices. The analysis revealed that waveguide modes of the OLED structure could be grouped, according to the behavior of modal-field profiles in the air cover and the glass substrate, into one of four different "families": (i) bound mode, (ii) semibound modes, (iii) leaky modes, and (iv) nonphysical modes. Four different OLED samples were fabricated on glass substrates on which photoresist gratings had been previously formed. The theory was used to compute the angles at which light from these devices should exit into the air. Theory and data agreed well for the semibound modes for all samples; however, they did not agree so well for the leaky modes. Further investigation revealed that better agreement between theory and data could be obtained with these modes being analyzed as Fabry-Perot cavity modes. The theoretical relation between leaky waveguide modes and Fabry-Perot cavity modes is discussed.