Double-layer coating of SrCO3/TiO2 on nanoporous TiO2 for efficient dye-sensitized solar cells

Surface modification plays a crucial role in improving the efficiency of dye-sensitized solar cells (DSSCs), but the reported surface treatments are in general superior to the untreated TiO2 but inferior to the typical TiCl4-treated TiO2 in terms of solar cell performance. This work demonstrates a two-step treatment of the nanoporous titania surface with strontium acetate [Sr(OAc)(2)] and TiCl4 in order, each step followed by sintering. An electronically insulating layer of SrCO3 is formed on the TiO2 surface via the Sr(OAc)(2) treatment and then a fresh TiO2 layer is deposited on top of the SrCO3 layer via the TiCl4 treatment, corresponding to a double layer of Sr(OAc)(2)/TiO2 coated on the TiO2 surface. As compared to the typical TiCl4-treated DSSC, the Sr(OAc)(2)-TiCl4 treated DSSC improves short-circuit photocurrent (J(sc)) by 17%, open-circuit photovoltage (V-oc) by 2%, and power conversion efficiency by 20%. These results indicate that the Sr(OAc) 2-TiCl4 treatment is better than the often used TiCl4 treatment for fabrication of efficient DSSCs. Charge density at open circuit and controlled intensity modulated photocurrent/photovoltage spectroscopy reveal that the two electrodes show almost same conduction band level but different electron diffusion coefficient and charge recombination rate constant. Owing to the blocking effect of the SrCO3 layer on electron recombination with I-3 ions, the charge recombination rate constant of the Sr(OAc) 2-TiCl4 treated DSSC is half that of the TiCl4-treated DSSC, accounting well for the difference of their V-oc. The improved J(sc) is also attributed to the middle SrCO3 layer, which increases dye adsorption and may improve charge separation efficiency due to the blocking effect of SrCO3 on charge recombination.