Silicon quantum dot luminescent solar concentrators

Abstract

Silicon quantum dots (Si QDs) have previously been established as a unique class of quantum-confined materials with potential for a wide variety of optoelectronic applications. In this work, we examine their application to luminescent solar concentrators, or LSCs, for the first time by developing high-quality Si QD / polymer nanocomposites. By encasing Si QDs with in a polymer slab, most of their photoluminescence becomes trapped via total internal reflection and escapes only at the slab edges where solar cells can be placed to harvest the concentrated light. We find that Si QDs are suitable for such LSC devices due to their unique combination of indirect band gap absorption with efficient photoluminescence. The resulting low overlap between the absorption and photoluminescence spectra yields low reabsorption losses in large-area LSCs without the use of rare or toxic elements in the luminophore. We demonstrate effective Si QD LSC prototypes consisting of flexible and rigid bulk nanocomposites as well as films on glass using methacrylate-based polymers. We find the Si QDs maintain their optical properties throughout radically-initiated polymerization processes but are prone to forming light scattering agglomerates in the solid phase. These agglomerates drastically reduce the LSC waveguiding efficiency due to their light scattering properties. We find that light scattering from these nanocomposites increases with Si QD concentration. One approach for improving the dispersion of the Si QDs within solid polymers is to choose surface ligands which mimic the structure of the encasing polymer. We demonstrate this with ester-capped Si QDs compared to alkane-capped Si QDs in poly(methyl methacrylate), or PMMA. Furthermore, we find that fast polymer solidification rates also reduce the formation of light scattering agglomerates. We show ester-Si QD / PMMA films cast from prepolymer solutions have an order of magnitude higher concentration limit before the onset of light scattering compared to their bulk-polymerized counterparts. Overall, this work establishes Si QDs as a promising luminophore for visibly transparent LSCs which may be used in the future for solar harvesting windows and architectural elements or in concert with other LSCs to form more efficient tandem structures.