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Samantha Hill1 Ryan Connell2 Vivian Ferry2 Uwe Kortshagen1

1, Mechanical Engineering, University of Minnesota Twin Cities, Minneapolis, Minnesota, United States
2, Chemical Engineering & Materials Science, University of Minnesota Twin Cities, Minneapolis, Minnesota, United States

The near-infrared emission and large effective Stokes’ shift of Si quantum dots (Si QDs) are attractive properties for light-management technologies like downshifting layers on photovoltaics or luminescent solar concentrating windows. In the case of the latter, the Si QDs must be finely dispersed in a transparent matrix, like a polymer, in order to achieve high light concentration factors. Our Monte Carlo models show that increasing the average light scattering length from 1 mm to 1 m in a 1 m2 device would provide a 4-fold increase in collected light for devices using Si QDs with a quantum yield of 50%. This gain only increases in the case of more efficient Si QDs. However, obtaining low-scattering dispersions of Si QDs in relevant, cost-effective polymers remains a challenge, and few reports of Si QD / polymer composites exist. Here, we studied the effect of surface ligand choice on the light scattering properties of Si QDs incorporated within mass polymerized poly(methyl methacrylate). Nanocomposites were made using Si QDs capped with a typical, nonpolar ligand or an ester-ended analog. We find the two surface treatments lead to significantly different dispersion characteristics in methyl methacrylate: either a primarily singly dispersed mixture is achieved, or agglomeration produces strongly scattering Si QD clusters at least 10 times larger than a single QD diameter. Fully polymerized nanocomposites exhibit increased light scattering corresponding to the agglomeration state in the monomer, especially at higher QD loading fractions. UV-Vis measurements of these composites indicate that choosing ligands similar in structure to the composite monomer can help reduce scattering losses within the nanocomposites. These results offer a strategy to improve Si QD luminescent solar concentrator performances in a low-cost, structural polymer suitable for window applications.

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