Eu³⁺-Doped NaGdF₄ Nanocrystal Down-Converting Layer for Efficient Dye-Sensitized Solar Cells

We present for the first time the synthesis of Eu(3+)-doped β-phase sodium gadolinium fluoride (NaGdF4:Eu) nanocrystals (NCs) using a hydrothermal method and the application of down conversion (DC) NaGdF4:Eu NCs to efficient dye-sensitized solar cells (DSSCs). The as-prepared NaGdF4:Eu(3+) NCs were characterized by X-ray diffraction, photoluminescence spectrometry, and scanning and transmission electron microscopy. DC layers consisting of poly(methyl methacrylate) (PMMA) doped with luminescent NaGdF4:Eu(3+) were prepared and attached onto the back of a prefabricated TiO2 anode to form a more efficient DSSC, compared with a device based on a pure TiO2 electrode. The influences of both doped and undoped NaGdF4 NC layers on the photovoltaic devices were compared and evaluated by the measurement of the device's incident-photon-to-current efficiency (IPCE). An obvious increase in IPCE was observed when the DC layer was added in the device. As the down-converted photons can be reabsorbed within DSSCs to generate photocurrent, the DSSC with a 100 nm thick NaGdF4:Eu(3+) DC-PMMA layer improved photoelectric conversion efficiency by 4.5% relative to the uncoated solar cell. The experiments conclude that NaGdF4:Eu(3+) nanocrystals mainly act as luminescent DC centers and light scatterers in the ultraviolet and visible domains, respectively, for enhancing the spectral response of the device in the measured spectral regime.