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Er 3+ -doped TeO 2 –PbF 2 glasses with and without Ag nanoparticles (NPs) were prepared using a melt-quenching method. Ag precipitation was subsequently induced by heat treatment. The influence of Ag nanoparticles on the optical properties of Er 3+ -doped glasses was determined using optical absorption and photoluminescence spectroscopy. Upconversion (UC) emission properties were mainly dominated by the structural properties of the glass matrix. Ag + doping and NP formation contributed to the enhancement of the total UC emission intensities. However, the green emission property of the glasses closely depended on the Ag NP size, and transition from quenching to enhancement occurred when the NP size of the Er 3+ –Ag co-doped glasses was increased. This size-dependent effect originated from the trade-off between the absorption and local field enhancement of Ag NPs with respect to the green emission of Er 3+ ions.

PbF2:Yb3+,Er3+ nanoparticles were synthesized using the hydrothermal method. The upconversion (UC) luminescence properties of products obtained under different Yb3+ doping concentrations and annealing temperatures were then investigated. As the Yb3+ concentration increased, the product structures transformed from a mixed phase to a cubic phase. Their total emission intensities, however, were remarkably low. After annealing, the UC emission of the products was enhanced because of their increased grain size. The maximum emission was obtained with 7% Yb3+. The products were applied in a hydrogenated amorphous silicon solar cell, and an external quantum efficiency of 0.04% was measured under the illumination of a 980 nm laser at 4.8 W/cm2.

The upconversion (UC) properties of β-NaYF4:Yb3+(10%),Er3+(1%) microprisms synthesized using a hydrothermal method were studied under different excitation conditions. It was found that the UC emission of β-NaYF4:Yb3+(10%),Er3+(1%) was more sensitive to the 980 nm photons than the 1560 nm photons, because of the high mole ratio of Yb3+/Er3+ in crystals. Applying this material at the back of a hydrogenated amorphous silicon (a-Si:H) solar cell, a short-circuit current density of 64 μA/cm2 was measured under 0.53 W/cm2 980 nm laser excitation. This corresponds to an external quantum efficiency of 0.015% of the solar cell. When co-excited with the 980 nm and 1560 nm lasers, a very weak enhancement in the current was shown, which resulted from the slightly increased red emission. This indicates that co-excitation with multiple wavelengths accessible to the β-NaYF4:Yb3+(10%),Er3+(1%) microprisms is not an effective method to enhance the efficiency of a-Si:H solar cells.

NaYF4:Yb(20%),Er(1%) microcrystals were synthesized using a hydrothermal method, and the effects of the reaction time and the particle size on the up conversion (UC) emission properties were systematically studied. The UC emission intensities of the samples are directly related to the particle size and phase structure. The increase in the reaction time resulted in the phase transformation from cubic nanoparticles to hexagonal microprisms, leading to the enhancement of the UC emission. Moreover, the emission intensities and the intensity ratios of the green and red emissions (Fg/r) of the microprisms, in which a normal two-photon UC process was displayed under excitation with a 980 nm laser, remarkably increased with the increase of grain sizes, resulted from the increase of Yb3+ concentrations in crystals.

β-NaYF 4 :Er 3+ (10%) microprisms, synthesized using a hydrothermal method, were applied to the back of a thin film hydrogenated amorphous silicon (a-Si:H) solar cells to investigate response to sub-band gap near-infrared irradiation. Currents of 0.3 μA and 0.01 μA were measured during single-illumination with 60 mW (80 mW/cm 2 ) 980 nm and 1560 nm diode lasers, respectively, due to frequency upconversion (UC). Under co-excitation by 60 mW 980 nm and 100 mW 1560 nm lasers, a current improvement to 0.54 μA was obtained, resulting from enhancements in red emission. The finding indicates that co-excitation with multiple wavelengths accessible to UC materials is very effective in enhancing the efficiency of solar cells.