Nanoparticles for solar spectrum conversion

We review the use of nanometer-sized particles (including quantum dots) in the conversion of parts of the solar spectrum incident on solar cells to more usable regions. The modification of the solar spectrum ideally would lead to a narrow-banded incident spectrum at a center wavelength corresponding to an energy that is slightly larger than the band gap of the semiconductor material employed in the solar cell, which would lead to an enhancement of the overall solar energy conversion efficiency. Modification of the spectrum requires down and/or up conversion or shifting of the spectrum, meaning that the energy of photons is modified either to lower (down) or higher (up) energy. Nanostructures such as quantum dots, luminescent dye molecules, and lanthanide-doped glasses are capable of absorbing photons at a certain wavelength and emitting photons at a different (shorter or longer) wavelength. We will discuss down and up conversion and shifting by quantum dots, luminescent dyes, and lanthanide compounds, and assess their potential in contributing to ultimately lowering the cost per kWh of solar generated power.