Thin-film waveguiding mode light extraction in organic electroluminescent device using high refractive index substrate

The redistribution of thin-film waveguiding emitted light from an organic electroluminescent (EL) layer using high refractive index substrates has been investigated. The internally emitted light can be classified in terms of three modes: external, substrate, and indium-tin-oxide (ITO)/organic modes. According to classical ray optics theory, the ITO/organic mode emission can be completely redistributed to the substrate mode when the refractive index of the substrate becomes larger than that of the emitting layer. The redistributed substrate mode emission should be able to be easily extracted by an uneven surface, such as a microlens array. With this understanding, we prepared organic EL devices in which the substrate refractive index was varied over a range. However, redistribution of the ITO/organic mode emission to the substrate mode was not observed experimentally in our EL devices. In addition, there was no difference in the luminous intensity improvement between glass (n=1.52) substrate and high refractive index (n=1.72) resin substrate using identical microlens array. The improvement in luminous intensity was around a factor of 1.5 in both cases. (C) 2005 American Institute of Physics.