Design and Characterisation of Hybrid Organic-Inorganic Materials for Luminescence Downshifting Devices

Abstract

Spectral converters can enhance the performance of photovoltaic devices by converting incident sunlight into wavelengths that better match the spectral response of an attached PV cell, for example by converting UV to visible light through luminescence downshifting. Downshifting devices typically consist of a luminescent component, such as organic lumophores, and a host matrix. The host matrix is commonly composed of organic polymeric materials; however, these can suffer from poor processability and durability to long-term UV exposure. An alternative to this is the use of hybrid organic-inorganic materials, which owing to their combined inorganic and organic components show improved properties such as mechanical and thermal stability, processability, tunability, transparency and flexibility. Herein, two classes of hybrid organic-inorganic materials are investigated for their use in luminescence downshifting devices: a system based on siliceous nanoparticles housed in an organic polymer blend, and another based on organosilicon photopolymerisable sols. A special focus is placed on the interplay between host matrix and lumophore, as this can directly impact the final photoluminescence properties and material structure. Hence, understanding these interactions is crucial for the design of highly efficient downshifting devices. Firstly, composites of a polyethylene-co-glycidyl methacrylate organic polymer and octaglycidyl polyhedral oligomeric silsesquioxane (POSS) nanoparticles are investigated. POSS nanoparticles consist of a silica-based nanocage with highly tunable organic substituents on each Si vertex, and their organic polymer composites have previously exhibited reinforced mechanical and thermal properties. These composites are advantageous over other organic-inorganic hybrids in their low cost, tunability and ease of fabrication, as the highly soluble nanoparticles can be directly mixed into the polymer matrix. Chapter 3 describes the fabrication these blended and crosslinked composite materials, as well as the effect of POSS on the key material properties for luminescent downshifting devices, namely the optical, structural, thermal, mechanical, and wetting properties of the composite films. Furthermore, the use of POSS in photonic applications has previously shown improvements in photoluminescence quantum yield and photostability of lumophores. Despite the reported benefits of POSS nanoparticles in material and photoluminescence enhancements, their use in luminescent downshifting devices is yet to be reported. Chapter 4 investigates the effect of POSS nanoparticles on the photoluminescence characteristics of an organic lumophore, Coumarin 1, which has suitable properties for luminescence downshifting. The photoluminescence properties in the presence and absence of POSS are characterised by steady-state and time-resolved photoluminescence techniques. A mechanistic understanding of the effect of POSS on the lumophore behaviour is obtained, and the optimum POSS concentration to maximise downshifting performance is determined. Additionally, a system composed of photopolymerisable methacrylate- and epoxy-siloxane homopolymer and blended sols is investigated. These materials benefit from the enhanced mechanical and thermal properties characteristic of hybrids, while their photopolymerisation allows for the generation of light-guiding architectures known as waveguide-encoded lattices (WELs). WELs are a novel class of photonic material which has previously been shown to increase light collection at wide incident angles. The combination of lumophores and WELs is expected to have a dual light-trapping and spectral-tuning effect, resulting in a new class of concentrating downshifting material. However, the addition of lumophores can significantly alter the photopolymerisation kinetics, which are key to the waveguide photopatterning step and consequently the material structure. Chapter 5 describes the effect of the lumophore species on the photopolymerisation kinetics of the homopolymers and blended materials. Moreover, the fabrication of photopatterned luminescent devices is demonstrated. Finally, conclusions are drawn based on the results presented, and future directions in the field of luminescent downshifting materials are discussed.