Ultra-broadband near-infrared upconversion for solar energy harvesting

Upconversion – the absorption of two or more photons resulting in radiative emission at a higher energy than the excitation – has the potential to enhance the efficiency of solar energy harvesting technologies, most notably photovoltaics. However, the required ultra-high light intensities and the narrow absorption bands of lanthanide ions limit efficient solar utilisation. In this paper, we report results from exciting upconverters with concentrated sunlight at flux densities up to 2300 suns, where the radiation is restricted to photon energies below the bandgap of silicon (corresponding to a wavelength λ = 1200 nm). Upconversion to λ = 980 nm is achieved by using hexagonal erbium-doped sodium yttrium fluoride (β-NaYF₄: Er³⁺) in a fluoropolymer matrix. Upconversion has a nonlinear relation with irradiance, therefore at a high irradiance a threshold occurs where the process becomes linear. For β-NaYF₄:25%Er³⁺, we find a two-photon threshold under concentrated sunlight at 320 suns. Notably, this threshold is lower than under corresponding laser excitation and can be related to all resonantly excited Er³⁺ ion levels and excited stated absorption. These results highlight a pathway that utilises a far broader portion of the solar spectrum for photovoltaics.