Oxalate-assisted morphological effect of NaYF4:Yb3+,Er3+ on photoelectrochemical performance for dye-sensitized solar cells

In this study, we have employed a facile oxalate-assisted hydrothermal approach to tailor the morphology of β-NaYF₄:Er³⁺,Yb³⁺ (NYFEY) powders through the variation of the molar ratio of oxalate ions (Oxa^2?) and rare earth ions (RE³⁺) in the range of 0.5:1, 1:1, 2:1, 5:1, and 10:1. The obtained results show that the crystallinity, particle size and upconversion luminescence intensity of the as-synthesized NYFEY particles are gradually decreased as the Oxa^2?:RE³⁺ molar ratio increases from 0.5:1 to 10:1. For the purpose of photoelectrochemical performance evaluation, the as-synthesized NYFEY particles with different morphologies are incorporated into the nanocrystalline TiO₂ films to form the multifunctional nano- and sub-micrometer composite photoanodes of dye-sensitized solar cells (DSSCs). A short-circuit current density (J_SC) of 14.26 mA/cm² and power conversion efficiency (PCE) of 7.31% are obtained for DSSCs prepared with hexagonal rod-like NYFEY crystals, evidencing an increase of 29.8% compared with DSSCs prepared with only TiO₂ nanoparticles. The demonstrated synthesis approach for tailoring the morphology and size of NYFEY particles and enhancing the performance of DSSCs can also be applied for other types of solar cells.