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.
We demonstrated that the diffusive layer laminated to glass substrate surface increased the light output of organic electroluminescent devices by extracting and reemitting light trapped in the substrate of such devices. Lamination of the diffusive layer improved the efficiency of the coupling-out factor, which was also changed by the thickness of the electron transporting layer (ETL). High total emitting flux for the sample with ETL thickness around quarter wavelength optical thickness (QWOT) was not improved significantly by lamination of the diffusive layer. Conversely, low total emitting flux for the sample with ETL thickness around 2QWOT without the diffusive layer was largely increased by the lamination of the diffusive layer, which was due to the extraction of waveguided light from the substrate mode. As the results, large dependence of total emitting flux on ETL thickness was significantly minimized by the lamination of diffusive layer. In addition, lamination of the diffusive layer also significantly reduced the variation of emission color and luminous intensity, which were strongly dependent on viewing angle and ETL thickness for the sample without the diffusive layer.
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