Upconversion quantum yield of Er3+-doped β-NaYE4 and Gd2O2S: The effects of host lattice, Er3+ doping, and excitation spectrum bandwidth

The upconversion luminescence of beta-NaYF4 and Gd2O2S doped with Er3+ was investigated under I-4(11/2) -> I-4(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 beta-NaYF4: 25% Er3+ for broad-band excitation. Both host materials reach similar external UCQYs for large monochromatic irradiance values above 3500 W/m(2). Particularly, the best external (internal) UCQYs are 8.6% (12.0%) for beta-NaYF4: 25% Er3+ and 8.5% (15.1%) for Gd2O2S: 10% Er3+ at irradiances of 4020 W/m(2) and 4070 W/m(2), respectively. Under broadband excitation we found the external UCQY of beta-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 beta-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. (C) 2014 Elsevier B.V. All rights reserved.