Organic light-emitting diodes (OLEDs) are attractive for next-generation displays and lighting applications because of their potential for high electroluminescence (EL) efficiency, flexibility and low-cost manufacture. Although phosphorescent emitters containing rare metals such as iridium or platinum produce devices with high EL efficiency, these metals are expensive and their blue emission remains unreliable for practical applications. Recently, a new route to high EL efficiency using materials that emit through thermally activated delayed fluorescence (TADF) was demonstrated. However, it is unclear whether devices that emit through TADF, which originates from the contributions of triplet excitons, are reliable. Here we demonstrate highly efficient, stable OLEDs that emit via TADF by controlling the position of the carrier recombination zone, resulting in projected lifetimes comparable to those of tris(2-phenylpyridinato)iridium(III)-based reference OLEDs. Our results indicate that TADF is intrinsically stable under electrical excitation and optimization of the surrounding materials will enhance device reliability.

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Although some azo-carbazole derivatives attached on or doped into inert polymers are known to show photorefractive effect without external electric field, the origin of their asymmetric energy transfer in two-beam coupling experiments were unknown. We made the two-beam coupling experiment followed by sample translation and one-beam diffraction at 633 nm for thick films composed of 3-[(4-nitrophenyl)]azo-9H-carbazole-9-ethanol (NACzEtOH) and poly(methylmethacrylate), finding that photoinduced gratings grew in several minutes accompanied with phase displacement of the gratings, but the phase shift was not always synchronized with the refractive index modulation. We reformulated the Kogelnik's coupled-wave theory with strict energy conservation law for analysis. Comparison of the grating growth and erasure at 532 nm to Disperse Red 1 (DR1), the most well-known azo dye showed that the photoisomerization was dominant at this wavelength and that the azo-carbazole dyes were faster in response time and more resistive to erasure than DR1.

We prepared photorefractive composites using poly(methyl methacrylate) (PMMA) as a host material doped with 3-[(4-Nitrophenyl)azo]-9H-carbazole-9-ethanol (NACzE) as an NLO chromophore. Photorefractive properties were characterized by means of a two beam coupling (2BC) and a degenerated four wave mixing (DFWM) methods performed at 632.8 nm and room temperature. We successfully observed asymmetric 2BC under non-electric field: photorefractive characteristics obtained are a net 2BC gain of 426 cm À1 and a diffraction efficiency of 35%. We could also demonstrate its high performance by DFWM image processing experiment. In order to reduce the glass transition temperature T g of photorefractive composites we selected poly(ethyl methacrylate) (PEMA) as a host material. Photorefractive properties obtained are a net 2BC gain of 398 cm À1 and a diffraction efficiency of 40%.

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