Engineering the optical properties of luminescent solar concentrators at the molecular scale

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 121-128).Luminescent Solar Concentrators (LSCs) concentrate solar radiation onto photovoltaic (PV) cells using an inexpensive collector plate to absorb incoming photons and waveguide fluorescently re-emitted photons to PVs at the edge. This thesis addresses the two main energy loss mechanisms in LSCs, namely transport losses and trapping losses. We used phycobilisomes, a biological light-harvesting complex, as dyes in the LSC collector to circumvent transport losses caused by photon re-absorption. The selfassembled structure of phycobilisomes couples numerous donor chromophores to a handful of acceptor chromophores through an internal F6rster energy pathway that isolates the absorption and emission spectra. We established that energy transfer within intact phycobilisomes reduces LSC self-absorption losses by approximately (48±5)% by comparing intact and partly decoupled phycobilisome complexes. To reduce trapping losses in LSCs, we leveraged the anisotropic emission pattern of dichroic dye molecules. We aligned their dipole moments normal to the face of the waveguide by embedding them in a liquid crystal host. Vertical dye alignment increased the fraction of the power emitted below the critical angle of the waveguide, thereby raising the trapping efficiency to 81% from 66% for LSCs with unaligned dyes. The enhanced trapping efficiency was preserved for geometric gains up to 30, and an external diffuser can enhance absorption in LSCs with vertically-aligned dyes. This thesis also explores an energy harvesting strategy for portable electronics based on LSCs with dye molecules that are aligned in-plane. The purely absorptive polarizers used to enhance contrast ratios in displays can be replaced with two linearly polarized luminescent concentrators (LSCs) that channel the energy of absorbed photons to PVs at the edge of the display. We coupled up to 40% of incoming photons to the edge of a prototype LSC that also achieved a polarization selection ratio of 3. Finaly, we investigated the contribution of self-absorption and optical waveguiding to triplet exciton transport in crystalline tetracene (Tc) and rubrene (Rb). A timeresolved imaging technique that maps the triplet distribution showed that optical waveguiding dominates over diffusion and can transport energy several micrometers at the high excitation rates commonly used to probe the exciton diffusion constants in organic materials.by Carlijn Lucinde Mulder.Ph.D