Upconversion quantum yield of Er³⁺-doped β-NaYF₄ and Gd₂O₂S: The effects of host lattice, Er³⁺ doping, and excitation spectrum bandwidth

The upconversion luminescence of β-NaYF₄ and Gd ₂O₂S doped with Er³⁺ was investigated under ⁴I_15/2→⁴I_13/2 excitation around 1500 nm. The main ⁴I_11/2→⁴I _15/2 upconversion emission around 1000 nm is ideally suited for the excitation of silicon solar cells with a band gap of approximately 1150 nm. The upconversion quantum yields (UCQYs) of these materials were measured under monochromatic and broad-band excitations for different Er³⁺ doping levels. We observed a strong dependence of the UCQY on the Er³⁺ doping, the spectral bandwidth of the excitation, and the irradiance. The best performing samples were Gd₂O₂S: 10% Er³⁺ for monochromatic excitation and β-NaYF₄: 25% Er³⁺ for broad-band excitation. Both host materials reach similar external UCQYs for large monochromatic irradiance values above 3500 W/m². Particularly, the best external (internal) UCQYs are 8.6% (12.0%) for β-NaYF₄: 25% Er³⁺ and 8.5% (15.1%) for Gd₂O₂S: 10% Er³⁺ at irradiances of 4020 W/m² and 4070 W/m², respectively. Under broad-band excitation we found the external UCQY of β-NaYF₄: 25% Er³⁺ to be up to 1.71 times larger than that for Gd₂O₂S: 10% Er³⁺, depending on the spectral bandwidth and the irradiance of the excitation. Thus, the β-NaYF₄ host lattice seems to be more advantageous for broad-band excitation, as required for instance in solar cell applications, whereas the external UCQY of the Gd₂O₂S host lattice is larger under monochromatic excitation at low irradiances.