A comparison of the outcoupling characteristics of laterally emitting thin-film electroluminescent devices

This paper presents a study of the light output characteristics of laterall y emitting thin-film electroluminescent (LETFEL) devices. The limitations o f the conventional non-etched LETFEL outcoupling mechanism have been theore tically and experimentally analysed. The theoretical approach assumes the d evice behaves as a perfect optical waveguide and studies the scattering and bend radiation losses responsible for outcoupling at the aperture location s. This study leads to outcoupling efficiency values under 2%. Additionally , the angular profile has been experimentally determined and illustrates tw o maxims at +/-50 degrees. As a result, we conclude that the conventional n on-etched LETFEL output characteristics are not optimum for the two main LE TFEL applications, which are head mounted displays (HMD) and electrophotogr aphic printing (EP). Consequently, an alternative outcoupling mechanism is introduced, which is referred to as the etched LETFEL device. This has been fabricated for the first time, using ion milling. The resultant light outp ut characteristics have been experimentally determined and subsequently com pared with the conventional non-etched structure properties. A 400% enhance ment in the total emitted Light intensity has been obtained for a 600 V pea k to peak driving voltage, using the etched structure. Additionally, the an gular profile is observed to redistribute towards narrower angles of view. This introduces further enhancement of the coupling efficiency of HMD and E P applications, with acceptance angles under 30 and 6 degrees respectively, by 12 and 5% respectively.