Hot carrier solar cells: Controlling thermalization in ultrathin devices

In operating hot carrier solar cells, a steady-state hot carrier distribution is established in the absorber in such a way that the excess kinetic energy of carriers can be collected. A high-carrier concentration is normally favorable to the formation of a nonequilibrium hot-carrier population. A small absorber thickness is thus expected to improve the efficiency of hot carrier solar cells, but no quantitative analysis of the impact of the cell thickness on its performance has been done so far. Here, the potential of efficiency improvement using thinned absorber is investigated by simulating the absorption, heat losses, and efficiency of a hot carrier solar cell with varying absorber thickness. Efficiency improvement requires efficient light trapping to maintain absorption in ultrathin layers. Solutions are proposed to achieve strong absorption in a 25-50-nm-thick absorber, resulting in cell efficiencies that are higher than the Shockley-Queisser limit corresponding to the absorbers bandgap.