Triplet fusion photon upconversion systems: Towards low threshold applications
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Photon upconversion (UC), a promising anti-Stokes process that can convert two or more photons with low energy to a single photon of higher energy, can be readily achieved via triplet fusion or triplet-triplet annihilation (TTA) using low incident power density. To facilitate energy migration required for TTA-based upconversion systems, triplet exciton diffusion of the chromophores within an inert medium is of paramount importance, especially for practical device integration. However, the majority of studies have been carried out where the conditions have limitations such as deoxygenated organic solvents and bulk polymer matrices. The research objective here, therefore, is to find effective ways to apply active UC materials into potential devices powered by sunlight. In this thesis, we will investigate how to improve the diffusion-limited energy transfer in TTA-UC processes and demonstrate diverse approaches to enhance their optical properties in terms of materials science and engineering perspectives. First, we will discuss two microfluidics-based approaches which allow for the controlled formation of uniform microcapsules that contain a chromophore-embedded solution core. One is photo-induced interfacial polymerization and the other is multiple emulsion encapsulation using coaxially focused glass capillaries. These core-shell structures provide high quantum yields due to the preservation of molecular mobility in the fluidic phase, as well as structural rigidity and photochemical stability. In addition, we will address alternative photon energy manipulating approaches using the tunable interference structure of cholesteric liquid crystals and surface plasmon resonance coupling of gold nanorods to increase TTA-UC emission intensities and tailor the resultant optical properties.