The authors discussed the luminant properties of the organic electroluminescent diode (organic ELD) with multi layers and the conventional organic ELD with two layers (a hole transporting layer and an emitting layer) by means of the dye-doping method. For the organic ELD with five narrow, 5nm-thick emitting layers, the emission peak was shifted to the short wavelength region as compared with that of the conventional organic ELD. In addition, the FWHM of the EL spectrum was narrower than that in the conventional organic ELD. The emitting region in the organic ELD with multi layers was found to be a few emitting layers near the cathode, and not a few emitting layers near the anode side.
The shift of emission peak could be explained by the interference of a direct light and a reflected light onto the metal cathode mirror. But the decrease of FWHM could not be done by the interference effect. The decrease of FWHM was due to the suppression of EL with lower energy than the emission peak. The authors thought the following as the EL model: the ecitons which are generated in the TPD/Alq3 interface diffuse in the emitting layer. As the diffusing excitons lose their energy, these will come close to the ground state of excitons. In the organic ELD with multi layers, as the excitons cannot diffuse to more than the thickness of emitting layers, the component of EL with high energy relatively increases. Therefore, the FWHM of EL spectrum becomes narrow.