MTL Annual Research Report 2011

Photovoltaic solar concentrators aim to increase the electrical power obtained from solar cells.  Conventional solar concentrators track the sun to generate high optical intensities, often by using large mobile mirrors that are expensive to deploy and maintain.  High optical concentrations without excess heating in a stationary system can be achieved with a luminescent solar concentrator (LSC) ^[1] . The LSC consists of a dye dispersed in a transparent waveguide.  Incident light is absorbed by the dye and then reemitted into a waveguide mode. The energy difference between absorption and emission prevents reabsorption of light by the dye, isolating the photon population in the waveguide. The performance of LSCs has been limited by two factors: self-absorption losses and a scarcity of dyes that emit efficiently in the infrared region. We have previously made significant progress on the problem of self-absorption losses ^[2] . Now we address operation in the infrared region.

Neodymium (Nd³⁺) and ytterbium (Yb³⁺) are nearly the optimal infrared LSC materials: inexpensive, abundant, efficient, and spectrally well-matched to high-performance silicon solar cells. These rare earth ions are natural three or four-level systems, therefore reasonably transparent to their own radiation and capable of generating high optical concentrations. Neodymium’s and ytterbium’s disadvantage is their relatively poor absorption overlap with the visible spectrum, meaning that the rare earth ions will require sensitization in the visible spectrum. Transition metals (TMs) can efficiently transfer energy to Nd and Yb ions, a process depicted in Figure 1. With a selection of TMs, broad sensitization of the both the visible and near infrared regions of the solar spectrum is possible, as shown in Figure 2. Finally, these ions can be combined with a high-throughput and chemically robust glass-making process for low cost and stable LSCs. Sensitized neodymium and ytterbium should enable LSCs matched to silicon.