Calibrated photoluminescence measurements of the upconverter NaYF4:20% ER3+ for silicon solar cells

Upconversion (UC) of subband-gap photons is promising to enhance solar cell efficiency. We explore the properties of -NaYF4:20% Er3+ for this application. This material shows efficient UC suitable for semiconductor based solar cells with band gaps up to 1.25 eV. In this work we investigate the optical UC efficiency by calibrated photoluminescence measurements. The values were obtained in dependence of the excitation wavelength as well as the irradiance. The highest optical UC quantum efficiency of 4.3% was obtained for monochromatic irradiance of 1390 Wm- 2 at 1523 nm. Because these data from optical measurements do not account for any losses associated with the application of the upconverter to a solar cell, they represent the upper limit of energy conversion for an optimized solar cell UC device.