Upconversion for silicon solar cells: Material and system characterisation

Frequency upconversion of sub-bandgap photons is one approach to push the efficiency limit of solar cells with one single bandgap. A highly efficient upconversion system is Erbium doped NaYF4. In this paper we investigate NaYF4 with 10%, 20% and 30% Erbium content. Time-resolved photoluminescence is applied to gain insight into the excited state dynamics of the upconverter. The dependence of upconversion luminescence on excitation intensity is analysed. We develop an analytical model to analyse the effect of loss mechanisms on the intensity dependence. An upconversion solar cell system is built and characterised. The system exhibits a quantum efficiency of 0.01% at 1523nm wavelength at an illumination with 104 Wm-2s-1. We model the enhancement of upconversion luminescence by resonant plasmon amplification and show that plasmons should be tuned to the absorption transitions.