Sandwich-type silicotungstate modified TiO2 microspheres for enhancing light harvesting and reducing electron recombination in dye-sensitized solar cells

Abstract

Sandwich-type polyoxometalates K₆Na₂Ni₂[Ni₄(H₂O)₂(SiW₉O₃₄)₂]·30H₂O(Ni₄), K₆Na₂Co₂[Co₄(H₂O)₂(SiW₉O₃₄)₂]·30H₂O(Co₄) and K₁₀[Ni(H₂O)₂(γ-SiW₁₀O₃₅)₂]·13.25H₂O(Ni₁) were doped into TiO₂ separately and applied as photoanodes for dye-sensitized solar cells (DSSCs) for the first time, enhancing the current intensity and photoelectric conversion efficiency of the cells. The TiO₂ and POM@TiO₂ used in this paper were synthesized by a modified sol–gel method. The J_sc of pure P25 is 15.84 mA cm⁻², the V_oc is 0.637 V, the FF is 0.564, and the η is 5.69%. The J_sc of the prepared TiO₂ is 16.48 mA cm⁻², the V_oc is 0.68 V, the FF is 0.537 and the η is 6.01%, which is a higher efficiency than that of pure P25, therefore the POMs@TiO₂ were directly used in photoanodes without being formed into composites with P₂₅ in this paper. The J_sc of 0.8% Ni₄@TiO₂ is 18.79 mA cm⁻², the V_oc is 0.71 V, the FF is 0.595 and the η is 7.94%, which is 32.1% higher than that of the prepared TiO₂ based DSSC. 0.8% Co₄@TiO₂ and 0.8% Ni₁@TiO₂ modified photoanodes obtained power conversion efficiencies of 6.22% and 6.17%. The electron lifetimes of the self-prepared TiO₂ and 0.8% Ni₄@TiO₂ are 13.5 ms and 28 ms respectively, calculated from a bode plot. Among the three kinds of sandwich-type POMs mentioned in this paper, the 0.8% Ni₄ modified photoanode in DSSCs exhibited the best performance in enhancing light harvesting and reducing electron recombination. The energy levels of Ni₄ were also investigated.