Eu³⁺-doped NaGdF₄ nanocrystal down-converting layer for efficient dye-sensitized solar cells

We present for the first time the synthesis of Eu³⁺-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:Eu³⁺ 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:Eu³⁺ 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:Eu³⁺ DC-PMMA layer improved photoelectric conversion efficiency by 4.5% relative to the uncoated solar cell. The experiments conclude that NaGdF₄:Eu³⁺ 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.