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.