Hironaka Fujii

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 refra...

Improvement of coupling-out efficiency in organic electroluminescent devices by addition of a diffusive layer

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 significan...

Extraction of waveguided light by anisotropic scattering polarizer in OLED for LCD backlights

An approach to increase the coupling-out efficiency of organic light-emitting diodes (OLEDs) was studied. The approach, especially, is suitable for liquid crystal display (LCD) backlight applications. We demonstrated that the anisotropic scattering polarizer (ASP) laminated to the glass substrate surface increased the optical efficiency of OLED for LCD backlight applications. The ASP was properly prepared by drawing the liquid crystal polymer (LCP) dispersed poly(carbonate) film. The ASP could extract the emitted light trapped in substrate of OLED involved in polarization selectivity. The light extraction ability and the polarization selectivity were changed by the thickness of electron transporting layer (ETL) of OLED. The optical efficiency for 60 nm ETL thickness device through absorbing polarizer was not improved significantly by the lamination of ASP. Alternatively, the low efficiency of 120 nm ETL thickness device was effectively improved by the lamination of ASP, which was due to the extraction of waveguided light as a substantially polarized emission. Although the degree of increase for 120 nm ETL thickness device by the lamination of ASP was substantially large, the final optical efficiency was less than or comparable to that for 60 nm ETL thickness device. However, in the case of 90 nm ETL thickness device, the lamination of ASP increased the optical efficiency by the factor of 1.2-1.3 against 60 nm ETL thickness device.