While both the crystalline silicon solar cells and amorphous silicon solar cell examples shown in Fig. 15.3 are optimized to have a broad spectral response, it is known that photovoltaic devices would work with a higher efficiency if they only had to absorb monochromatic light (Sark et al. 2008) or light from a very narrow spectral range. One way to provide a limited portion of the spectrum to a solar cell, to make the most of its peak efficiencies, is to use a luminescent solar concentrator (LSC). A LSC is a flat-plate solar concentrator made from a thin transparent polymer (such as acrylic) containing a luminescent material (Sark et al. 2008). They work by accepting light from the AM1.5 solar spectrum and directing a portion of the light toward the edges of the flat polymer sheet. Solar cells are located at the edge of this flat sheet for converting the light into electricity. Photons incident upon the polymer sheet with enough energy will excite the luminescent materials. These materials will then re-emit a photon with a longer wavelength. As the photons are emitted in random directions, a portion of these will be captured by total internal reflection in the flat sheet and transmitted to the edge where it can be collected by a solar cell (Sark et al. 2008). With the photons being re-emitted in random direc­tions, there will be some that are transmitted out of the concentrator, and however, a portion is captured and directed toward the edge for collection. Photons without insufficient energy to excite the luminescent material will be transmitted through the concentrator with very low loss. The beauty of these flat-panel concentrators is that increasing the size of the polymer sheet will directly increase the number of photons captured via total internal reflection and able to be converted to electricity. That is, the concentration factor increases with the area of the concentrator. This increase in output from the concentrator can be achieved without increasing the size of the solar cell itself.

Although the efficiency of LSC has historically been fairly low (Sark et al.

2008) , there have been devices reported with efficiencies of up to 7.1 % (Sark et al. 2008) which bodes well for the technology. This style of concentrator relies on the use of only a small portion of the solar spectrum. For example, if a luminescent material that emitted green photons was used, all incident light with longer wavelengths would be transmitted through the concentrator. The re-emitted green photons would be directed to a solar cell with a high efficiency in this part of the spectrum. This would potentially be a good match for a system that would convert solar energy into electricity and grow microalgae.