Gregory D. Gutierrez
Luminescent solar concentrators (LSCs) are strategic, cost-effective, and scalable solutions for the convenient incorporation of photovoltaic (PV) technology into building infrastructures or other electronic architectures. LSCs consist of transparent slabs of plastic or glass that guide the emission of sunlight-absorbing photoluminescent dyes to their edges, where small and efficient PV devices may be attached. Perylene bisimides (PBIs) are valued as LSC emitters because of their exceptionally high photoluminescence quantum efficiencies and photostability. However, due to their small Stokes shifts, these materials often suffer from non-radiative reabsorption processes that severely curb their optical efficiencies in LSCs of commercially relevant sizes. One approach to minimize reabsorption is to employ very low concentrations of PBI, albeit at the expense of the dye’s overall absorbing ability. This presentation communicates the development of an LSC that employs two luminescent conjugated polymers as surrogate absorbers to amplify the emission of small amounts of a PBI (e.g. 0.5 - 1 wt% relative to the polymers) through an energy transfer cascade. The optical bandgaps of the polymers are designed such that their corresponding absorption spectra cover a significant fraction of the solar spectrum, but are entirely decoupled from the emission of the PBI. Upon excitation by sunlight, a thin film comprised of the polymers and minority PBI emits photons that are guided into a transparent glass slab and consequently directed to its edges through total internal reflection. A key structural feature of the polymers is a pentiptycene repeat unit that prevents solid-state photoluminescence quenching and hampering of energy transfer processes. The high internal free volume induced by this pentiptycene substituent also results in the polymers having high solubility in organic solvents for facile processing into thin films and relatively low refractive indices for waveguide compatibility with commercially available glass substrates. The photophysical properties of the composite blend are presented along with its performance in a model LSC.