Triplet-Triplet Annihilation Upconversion in Solid-State Organic-Inorganic Ureasil Polymer Hosts

Abstract

Triplet–triplet-annihilation upconversion (TTA-UC) has attracted significant attention as an approach to harvest low energy solar photons that cannot be captured by conventional photovoltaic devices. However, device integration requires the design of solid-state TTA-UC materials that combine high upconversion efficiency with long-term stability. Herein, we investigated solid-state TTA-UC systems based on organic-inorganic hybrid polymers known as ureasils as hosts for a range of sensitizer/emitter pairs over the solar spectrum. Ureasils consist of organic poly(oxyalkylene) chains covalently linked to an inorganic siliceous network via urea bridges. A library of ureasil structures were synthesised and characterised to reveal structure-function trends. The role of the ureasil structure on the TTA-UC performance was probed by varying the branching and molecular weight of the organic precursor to tune the structural, mechanical, optical, and thermal properties. Solid-state photon upconversion was observed, and in some cases UC emission could be retained over months without any special treatment, including deoxygenation. Detailed analysis of the structure-function trends revealed that while a low glass transition temperature is required to promote TTA-UC molecular collisions, a higher inorganic content is the primary factor that determines the UC efficiency and stability, due to the inherent oxygen barrier provided by the silica nanodomains. Methods to improve upconversion efficiency were further investigated, such as employing the use of oxygen scavengers to protect excited triplets and covalently grafting emitters to the polymer backbone. Ureasils were co-condensed with a methacrylate-functionalised siliceous network to form a set of new generation hosts. A methacrylate-functionalised emitter was synthesised to enable grafting to the ureasil host using polymerisation. The thesis concludes with applications of solid UC hosts and directions for future investigations with the goal of further enhancing the efficiency and long-term stability of TTA-UC in the solid-state.