Methacrylate copolymers as tunable hosts for solid-state triplet- triplet annihilation upconversion Mike Bennison, Abigail R. Collins, Joshua K. G. Karlsson, Bethan E. Charles, Rachel C. Evans Department of Materials Science and Metallurgy, University of Cambridge, U.K. Email: mjb307@cam.ac.uk
Solar photovoltaics offer a huge potential for renewable, clean energy generation a, however current generation silicon-based photovoltaic cells fail to fully utilise the full potential of the solar spectrum, particularly at longer wavelengths. 1 Triplet-triplet annihilation upconversion (TTA-UC) offers a mechanism for converting two low-energy photons to a single higher energy photon by use of a sensitiser/emitter dye pair. 2 For practical device integration, this process needs to move away from the well-studied solution-phase systems and into solid-phase materials. Polymeric hosts have been shown to offer promising efficiencies in the solid-state. 3,4 This is typically achieved using homopolymers with a low glass-transition temperature (T g ) to enable high rates of molecular diffusion to facilitate TTA-UC, but in doing so increase oxygen permeability which can lead to quenching of triplet states. Here, we investigate the use of a tuneable host system, based on binary copolymers of n -hexyl methacrylate and 2,2,2-trifluoroethyl methacrylate as low and high T g monomers (T g s = -5 °C and 74 °C) , with a view to balancing these competing processes. Variation of the monomer ratio allows for control of the T g of the final copolymer, enabling the formation of clear films suitable as hosts for TTA-UC. Incorporation of the standard sensitiser- emitter pair of palladium octaethylporphyrin (PdOEP) and 9,10-diphenylanthracene (DPA) allowed measurement of the TTA-UC activity as a function of monomer ration using a combination of steady-state and time-resolved optical characterisation techniques (UV/Vis transmittance, photoluminescence emission and lifetimes, as well as upconversion quantum yield (UCQY)). While the literature suggests ever-lower T g s lead to beneficial gains in TTA-UC activity, our investigation identified only superficial increases in UCQY once the T g reaches ambient temperatures, potentially due to increased oxygen ingress. This suggests that host optimisation may be achieved through careful tuning of a copolymer system to balance UCQY and oxygen ingress through T g -matching to the end application. References 1. B. McKenna and R. C. Evans, Adv. Mater. , 2017, 29 , 1606491 2. P. Bharmoria, H. Bildirir and K. Moth-Poulsen, Chem. Soc. Rev. , 2020, 49 , 6529
3. T.N. Singh-Rachford and F.N. Castellano, Coordination Chemistry Reviews , 2010, 254 , 2560–2573 4. M.J. Bennison, A.R. Collins, B. Zhang and R.C. Evans, Macromolecules , 2021, 54 (12), 5287–5303
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