Highly Efficient IR to NIR Upconversion in Gd2O2S: Er3+ for Photovoltaic Applications

Upconversion (UC) is a promising option to enhance the efficiency of solar cells by Conversion of sub-bandgap infrared photons to higher energy photons that Can be utilized by the solar cell. The UC quantum yield is a key parameter for a successful application. Here the UC luminescence properties of Er3+-doped Gd2O2S are investigated by means of luminescence spectroscopy, quantum yield measurements, and excited state dynamics experiments. Excitation into the maximum of the I-4(15/2) --> I-4(13/2) Er3+ absorption band around 1500 nm induces Very efficient UC emission from different Er3+ excited states with energies above the silicon bandgap, in particular, the emission originating from the I-4(11/2) state around 1000 nm. Concentration dependent studies reveal that the highest UC quantum yield is realized for a 10% Concentration. The UC luminescence is compared to the well-known Er3+-doped beta-NaYF4 UC material for which the highest UC quantum yield has been reported for 25% Er3+. The C intern quantum yields were measured in this work for Gd2O2S: 10%Er3+ and beta-NaYF4: 25%Er3+ to be 12 +/- 1% and 8.9 +/- 0.7%, respectively, under monochromatic excitation around 1500 nm at a power of 700 W/m(2). The UC quantum yield reported here for Gd2O2S: 10%Er3+ is the highest value achieved so far under monochromatic excitation into the I-4(13/2) Er3+ level. Power dependence and lifetime measurements were performed to understand the mechanisms responsible for the efficient UC luminescence. We show that the m I-4(11/2) UC emission is energy transfer UC.