Effect of the nano-structure of TiO₂ photoanodes on the performance of dye-sensitized solar cells

Abstract

Dye-sensitized solar cells (DSSCs) are considered as an alternative source in the field of renewable energy resources. In DSSCs, the photoanode based on nanocrystalline TiO₂ plays a key role in achieving excellent photo-electric conversion efficiencies. The surface morphology, surface area, crystalline phase of TiO₂, the dispersion of TiO₂ nanoparticles and the degree of dye absorbance are the most important factors influencing the properties of a photoanode. In this study, the effect and significance of different types of TiO₂ based photoanodes on the performance of dye-sensitized solar cells have been investigated. DSSCs fabricated from Ruthenium (N719) dye adsorbed photoanodes made from commercial grade (P-25 Degussa) TiO₂ nanoparticles (TNP) as the starting material, by doctor-blade technique and the cathodic electrophoretic deposition (EPD) at 25 V, show photoelectric conversion efficiencies of 5.55% and 3.05%, respectively, for liquid based electrolyte containing I₃-/I- (0.5 M tetrapropylammonium iodide + 0.05 M iodine in a 1:3.6 by volume mixture of acetonitrile + ethylene carbonate). The cells assembled with TiO₂ films prepared on FTO by anodic electrophoretic deposition of titania nanotubes (TNT), synthesized via the hydrothermal process, using commercial P-25 nanoparticles as the starting material, at 40 V show an average efficiency of 1.83% for the same electrolyte. Subsequently, two different types of DSSCs were fabricated having different double layers of TiO₂. For this purpose TiO₂ films were prepared by anodic electrophoretic deposition of above titania nanotubes (TNT) either, on a TiO₂ compact layer (~110 nm) which was obtained by spin coating an acidic-alcoholic solution of titanium isopropoxide on FTO (type-1) or on a TiO₂ film prepared by doctor-blading commercially available Solaronix-T nanopaste (type-2). While the DSSCs fabricated with only Solaronix-T nanopaste showed an efficiency of 2.21%, DSSCs fabricated from type-1 and type-2 TiO₂ films were able to enhance the efficiency of 1.83% of the above mentioned pristine TNT solar cells prepared with anodic EPD up to 3.06% and 7.68%, respectively, with photocurrent density increments from 3.41x10⁴ mA m⁻² up to 5.18x10⁴ mA m⁻² and 17.06x104 mA m⁻² respectively. Therefore, it is evident that although TNP-based DSSCs are superior to pristine TNT-based DSSCs, the efficiency of TNT-based DSSCs can be improved further by integrating the TNT electrode with other types of TNP electrodes and constructing composites.