Fabrication of double layer photoelectrodes using hierarchical TiO2 nanospheres for dye-sensitized solar cells

Double layer photoelectrodes consisting of the TiO2 nanoparticles (npTiO2) with 30–40 nm in diameter and the TiO2 nanospheres (nsTiO2) with 100–200 nm as a bottom and a top layer, respectively, were fabricated for dye-sensitized solar cells (DSSCs) employing a polymer electrolyte. The nsTiO2 with hierarchical pores were prepared via the combined process of atom transfer radical polymerization (ATRP) and a sol–gel. Use of nsTiO2 resulted in the remarkable enhancement of cell performance due to light scattering and dye adsorption capability. The overall energy conversion efficiency (η) of 5.5% was achieved by the formation of npTiO2–nsTiO2 double layer photoelectrode, which was higher than those formed by npTiO2 alone (4.6%) or nsTiO2–nsTiO2 double layer (4.4%). Electrochemical impedance spectra (EIS) analysis showed that the npTiO2–nsTiO2 double layer had lower electron transfer resistance and longer electron lifetime, leading to facilitation of reduced recombination and consequently improvement of cell performance.