Absolute upconversion quantum yield of β-NaYF4 doped with Er3+ and external quantum efficiency of upconverter solar cell devices under broad-band excitation considering spectral mismatch corrections

For applications to harvest solar energy, it is essential to characterize upconverter materials under broad-band excitation at reasonable irradiance levels achievable by concentration of solar radiation. We present a method to determine the absolute upconversion quantum yield (UCQY) under broad-band excitation by photoluminescence measurements. We introduce a spectral mismatch correction that allows calculating the UCQY that can be expected under illumination with the solar spectrum with a certain solar concentration. Applying these methods to β-NaYF4 doped with 25%Er3+, we determine an external UCQY of 2.0% in the spectral range from 1450nm to 1600nm under a comparatively low solar concentration of only 50suns. This value corresponds to a potential increase of the short-circuit current density of 3.89mA/cm2. Subsequently, we measure the external quantum efficiency due to upconversion of sub-band-gap photons for a bifacial silicon solar cell with upconverter attached to its rear side under the same broad-band excitation. We determine an additional short-circuit current density due to upconversion of 4.03mA/cm2 for a solar concentration of only 77suns. This value agrees very well, with the expected additional short-circuit current density due to upconversion, calculated from the external UCQY values as determined by the photoluminescence measurements, by considering the transmittance of the solar cell for sub-band-gap photons and the external quantum efficiency of the solar cell for photons emitted by the upconverter. Finally, a comparison with different literature values based on a set of figures of merit shows that our results currently constitute the largest enhancement of the short-circuit current density due to upconversion. •Upconversion investigated under broad-band excitation presented, which is much more relevant for solar applications than monochromatic lasers.•Upconverter and upconverter solar cell devices measured with broad-band excitation with less than 100suns solar concentration.•Spectral mismatch correction method for upconverter solar cell devices introduced, making results more comparable.•Additional short-circuit current density of 4.03mA/cm2 determined due to upconversion of sub-band-gap photons under a concentration of 77suns.•Highest gain due to upconversion reported in literature.