Investigation of Er³⁺-Doped BaF₂ Single Crystals for Infrared Emission and Photovoltaic Efficiency Enhancement

Er³⁺-doped BaF₂ single crystals were investigated with two primary aims: first, to probe the infrared emissions from the ⁴I_11/2 level (around 1.0 μm) under 1500-nm excitation and, second, to use the crystal to enhance the efficiency of silicon-based solar cells through upconversion mechanism. Upon excitation at 1500 nm, the upconversion emission spectrum of the Er³⁺-doped BaF₂ single crystals, recorded in the range of 480-1080 nm, exhibited two well-structured visible bands at 538 and 650 nm, along with a strong near infrared emission at 971 nm. This strong 971-nm emission has an emission cross-section of approximately 0.23 × 10^-20 cm². As with any phenomenon inherent to energy transfer by upconversion, the ⁴I_11/2 fluorescence decay exhibits a rise time followed by a long decay of approximately 15 ms and a positive optical gain from the low values of the population inversion coefficient, which could potentially give rise to laser emission from this level. When we place our crystal on a photovoltaic device illuminated by 1500-nm wavelength radiation, we record a photocurrent of 300 μA at an illumination power of 85 mW. This indicates that the Er³⁺-doped BaF₂ crystal is highly suitable for significantly enhancing the efficiency of silicon-based solar cells.