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 Gd2O2S doped with Er3+ was investigated under 4I15/2→4I13/2 excitation around 1500 nm. The main 4I11/2→4I15/2 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.