TiO₂ coated urchin-like SnO₂ microspheres for efficient dye-sensitized solar cells

Urchin-like SnO₂ microspheres have been grown for use as photoanodes in dye-sensitized solar cells (DSSCs). We observed that a thin layer coating of TiO₂ on urchin-like SnO₂ microsphere photoanodes greatly enhanced dye loading capability and light scattering ability, and achieved comparable solar cell performance even at half the thickness of a typical nanocrystalline TiO₂ photoanode. In addition, this photoanode only required attaching ~55% of the amount of dye for efficient light harvesting compared to one based on nanocrystalline TiO₂. Longer decay of transient photovoltage and higher charge recombination resistance evidenced from electrochemical impedance spectroscopy of the devices based on TiO₂ coated urchin-like SnO₂ revealed slower recombination rates of electrons as a result of the thin blocking layer of TiO₂ coated on urchinlike SnO₂. TiO₂ coated urchin-like SnO₂ showed the highest value (76.1 ms) of electron lifetime (τ) compared to 2.4 ms for bare urchin-like SnO₂ and 14.9 ms for nanocrystalline TiO₂. TiO₂ coated SnO₂ showed greatly enhanced open circuit voltage (V_oc), short-circuit current density (J_sc) and fill factor (FF) leading to a four-fold increase in efficiency increase compared to bare SnO₂. Although TiO₂ coated urchin-like SnO₂ showed slightly lower cell efficiency than nanocrystalline TiO₂, it only used a half thickness of photoanode and saved ~45% of the amount of dye for efficient light harvesting compared to normal nanocrystalline TiO₂.