Infrared to visible upconversion (UC) is a promising way to enhance the efficiency of silicon based solar cells. In this paper, the spectral conversion and recovery of sub-band gap photons of the solar spectrum, from NIR-IR to the VIS-NIR wavelength region, is investigated in two fluorides hosts doped with trivalent erbium ions (Er3+). The efficiency gain due to upconversion in silicon solar cells is compared for single crystal samples of BaY2F8:Er3+ and LiYF4:Er3+ in a dedicated upconverter solar cell device (UCSCD) with monochromatic excitation in the 1.5 µm spectral region. The highest external quantum efficiency due to upconversion was found for the UCSCD using the BaY2F8:30 at% Er3+ single crystal, reaching an EQE of 6.8±0.2% for (1.10±0.12)·105 W m−2 spectral irradiance at 1494 nm. We present a comprehensive spectroscopic study of the crystal samples also taking into account the effects of the different crystal symmetry as well as the different phonon energies. Our findings enable us to explain the higher efficiency of the BaY2F8:Er3+ compared to the LiYF4:Er3+ upconverter in terms of both static and dynamic properties
