Efficient upconversion systems for silicon solar cells

Frequency upconversion of sub-bandgap photons is one approach to push the efficiency limit of solar cells with one bandgap. A highly efficient upconversion system is Erbium doped NaYF4. In this paper we present photoluminescence studies on NaYF4 with 10%, 20% and 30% Erbium doping. We show experimental results of an upconversion system consisting of an upconverting powder on a bifacial silicon solar cell. The system has a quantum efficiency of 2.5·10-3% at 1520nm wavelength. We model the efficiency enhancement of an upconversion system by spectral concentration with fluorescent dyes. To enhance the upconversion efficiency, fluorescent dyes can be usedfor spectral concentration. Our theoretical calculation show that an eightfold increase in upconversion efficiency is within reach.