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

Abstract

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 I₃⁻ 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.