Sterically Engineered Perylene Dyes for High Efficiency Oriented Fluorophore Luminescent Solar Concentrators

L solar concentrators (LSCs) collect and concentrate sunlight for use in solar power generation. First proposed over 30 years ago, LSCs are simple devices, consisting of a planar waveguide coated or impregnated with a fluorophore. The fluorophore absorbs sunlight, re-emitting it into the waveguide where it travels to a device edge for conversion by photovoltaic (PV) cells. LSCs have attracted attention due to their high theoretical concentration factors, their ability to provide wavelength-to-bandgap matched photons, and because they function equally well under diffuse and direct illumination. Unlike other concentrator designs, LSCs selectively harvest light in a particular spectral band (determined by the absorption range of the fluorophore) allowing other wavelengths to pass through for secondary applications such as thermal generation or interior lighting, increasing the combined-cycle efficiency. Accordingly, LSC performance is expressed in terms of the optical quantum efficiency (OQE), defined as the fraction of absorbed photons concentrated at the edge. Despite several decades of research, LSC OQEs remain too low for most practical applications and decrease rapidly with LSC size. For dyes possessing high fluorescent quantum yield (FQY), light trapping efficiency is limited primarily by waveguide escape cone (EC) losses defined by Snell’s Law. An approach to circumvent such losses involves orientation of the dye’s emission transition moment perpendicular to the plane of the concentrator, which leads to preferential emission into guided modes. Ballistic photon Monte Carlo simulations predict alignment that can reduce EC losses from about 25% to under 10% per emission in an air-clad waveguide with a refractive index of 1.5, depending on the degree of orientational order. Since EC losses compound due to repetitive reabsorption and re-emission caused by overlap of fluorophore absorption and emission spectra, fluorophore alignment, combined with reduced self-absorption, has the potential to significantly improve LSC efficiency. Here, we focus on the former approach. Previous attempts to prepare oriented fluorophore LSCs demonstrated modest performance gains, limited by the degree of fluorophore orientational order, solubility, or FQY. Here we report a new family of perylenebisimide (PBI) dyes designed for oriented fluorophore LSCs which address these limitations. Incorporation into optically transparent polymerizable liquid crystal (LC) waveguides results in devices with OQE = 74% at a geometric gain G = 10, where G is the ratio of facial to perimeter area. We studied a series of PBIs of the type N,N-bis(2,6diisopropyl-4-octylaniline)-perylene-3,4,9,10-tetracarboximide, containing pendant orthoalkylated anilines with long alkyl tails (Chart 1). The PBI luminophore is widely used for LSC