UV nanoimprint lithography for fabrication of 1-D photonic crystal slabs and their application in OLEDs
Organic light-emitting diode (OLED) technology is developing and emerging rapidly since its introduction in 1987. In an OLED device, the light generated from the recombination process may internally reflect in the substrate and be waveguided either in the organic film or high refractive index oxide that serves as an anode. These optical phenomenona reduce the amount of the light exiting the device. One method to increase the efficiency is by embedding periodically nanostructured 1-D photonic crystal slabs into the OLED device. One-dimensional photonic crystal slabs (Bragg gratings) allow the extraction of waveguide modes in OLED devices. This dissertation employs ultraviolet nanoimprint lithography (UV-NIL) technology to fabricate 1-D photonic crystal slabs made from high refractive index hybrid nanoparticles-photoresist. High refractive index hybrid nanoparticles-photoresist is a blend of pristine UV-NIL photoresist with TiO2 NP. As an alternative approach, dry etching technique is performed to transfer the UV-imprinted nanostructure to the Ta2O5 film. Hybrid nanoparticles-photoresist is characterized by thickness, surface morphology, and optical characteristics. The thickness increased as a function of TiO2 NP results for identical spincoat parameters. Surface characterization shows the imprint ability still remains to the pristine photoresist for up to 30% NP content. Optical characterization results are fitted using the Swanepoel method. As a result, the refractive index of hybrid nanoparticles-photoresist containing 30% TiO2 is 1.869 at 552 nm wavelength. One-dimensional photonic crystal slabs made from hybrid nanoparticles-photoresist 0%, 3%, and 30% are integrated with a rigid planar OLED device. The emission spectrum from the Bragg grating made from hybrid nanoparticles-photoresist 30% shows an additional extraction mode. This mode depends on the emission angle. For grating periodicity of 370 nm, the effective refractive index in the device is 1.59 at 591.8 nm wavelength. The study also demonstrates the use of hybrid nanoparticles-photoresist 30% in flexible OLEDs. The results show that hybrid nanoparticles-photoresist is also successful in extracting the waveguide mode from the devices. Here, the emission characteristic is recorded while the device is under bending conditions. It suggests that geometrical changes in the films due to mechanical loads under bending conditions may alter the emission characteristics.
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