A highly efficient ultraviolet to near-infrared converter to improve efficiency of Si solar cells: Yb³⁺-doped BaGd₂(MoO ₄)₄

Efficient conversion from ultraviolet (UV) light to near infrared (NIR) emission has been demonstrated in a series of BaGd_2-xYb _x(MoO₄)₄ (x=0.001-2.0, i.e., Yb³⁺ concentration=0.05-100 mol%) polycrystals. The samples presented enhanced photosensitivity below 375 nm. Under UV light, an intense NIR emission around 1000 nm from ²F_5/2→²F_7/2 transitions of Yb³⁺ was observed, which just corresponds to spectral response of Si solar cells. The emission intensity in NIR region showed a dependence on Yb³⁺ contents. Taking into account the photoluminescence quantum efficiency, the optimal doping to increase the conversion efficiency of Si-solar cells was suggested to be 20 mol%. Diffuse reflectance and luminescence spectra were also systematically investigated to propose a reasonable mechanism for energy conversion process from the photo-excited charge transfer states.