Application of nanoporous metasurfaces to organic semiconductor materials

Organic semiconductor materials exhibit many advantages compared to commonly-used inorganic semiconductor materials in optoelectronic applications, such as, low cost, light weight, mechanical flexibility, solution and large-area processability, and chemically tunable bandgaps. In particular, organic polymer semiconductor (i.e., conjugated polymer) materials are more advantageous compared to conjugated small molecules in terms of their better thin-film solution processability, lower process energies and better bio-compatibility. However, optoelectronic devices based on organic semiconductor materials usually exhibit lower efficiency and stability than inorganic semiconductor optoelectronic devices. The non-negligible non-radiative decay rates of the excitons, the significant formation proportion of triplet excitons (that have extremely low probability of radiatively decay to ground states) under electrical excitation, and the low light extraction efficiency of thin-films next to metallic electrode surfaces, are all factors that limit the performance of organic semiconductor materials in light-emitting applications. Metallic plasmonic metasurfaces with periodic (nano)structures have shown excellent light management ability in optoelectronic applications due to their strong, resonant plasmonic light scattering and their localized optical near-field properties in addition to their ability to function as electrodes. However, the luminescence enhancement or reduction of a light-emitting molecule by an adjacent plasmonic metasurfaces is strongly dependent on the molecule’s optical transition dipole orientation. Furthermore, the complexity and anisotropy of conjugated polymer thin film morphology have been neglected in previous literature that use metallic plasmonic metasurfaces for better light management in conjugated polymer thin films. Therefore, a comprehensive understanding of chain alignment in close proximity to confined metallic nanostructures, and of the relationship between polymer chain alignment and exciton-plasmon coupling would provide a knowledge foundation on which high-performance organic polymer semiconductor-based optoelectronic devices can be developed. The objective of the thesis work is to understand how plasmonic nanostructures interact with the excited states (excitons) in conjugated polymer materials to enhance their light emission, and to understand the role that conjugated polymer morphology plays in these interactions. In this thesis, we focus on understanding the influence of plasmonic nanostructures, specifically, nanoporous silver plasmonic metasurfaces, on the morphology of conjugated polymer thin films (Chapter 2), and the influence of polymer chain orientation on the ability of plasmonic nanostructures to modify the polymer thin film emission intensity, direction and quantum efficiency (Chapter 3). We prove that by controlling the pore size and porosity of nanoporous metal metasurfaces, the chain alignment of the polymer can be influenced in ways not possible using planar metal substrates. This knowledge is critical for optimization of plasmon-enhanced organic polymer thin-film optoelectronics, as the device’s performance is strongly dependent on the polymer morphology. Photoluminescence spectroscopy of conjugated polymer layers on aperiodic nanoporous silver metasurfaces showed pronounced fluorescence enhancements (up to factors of 26) relative to layers on glass. We identify the mechanisms of the fluorescence enhancements and identify the role that molecular orientation plays in enhancing or diminishing the light management ability of the metasurface. Based on these findings, in optimizing the performance of organic-polymer-based light-emitting devices, one can decide if adding plasmonic nanostructures into electrodes is practicable and cost-worthy based on the morphology of the polymer. We also investigate the origins of photoluminescence spectral shape changes from conjugated polymer thin films on metallic surfaces (Chapter 4) using experimental optical spectroscopy and electromagnetic theory. We prove that the origins of the red shifts of the short-wavelength edges observed in the photoluminescence spectra of conjugated polymer thin films on metallic surfaces arise from enhanced absorption of the metal at the polymer absorption red-edge. This is attributed to the large real refractive indices of conjugated polymers at their absorption edge, the enhanced reabsorption in the polymers due to local electric fields of plasmonic nanostructures, and the different morphology of conjugated polymer thin films on different metallic substrates. Finally, we investigate the potential of the application of metallic plasmonic metasurfaces to harvesting the triplet exciton energy of a heavy-atom-free, organic small molecule doped in a conjugated polymer film (Chapter 5). Our findings indicate that rather than applying external magnetic fields or implanting heavy atoms in the organic molecules, plasmonic structures can also be used to harvest triplet exciton emission from heavy-atom-free organic molecules. Further improvement in triplet emission from heavy-atom-free organic molecules is expected to be achieved by optimizing the localized surface plasmon resonance wavelength of plasmonic structures, either to better spectrally overlap with the excitation of the polymer host, or to better overlap with the triplet emission.Ph.D.Includes bibliographical referencesby Zeqing She