Effect of surfactants on the morphologies of TiO₂particles with high-performance scattering layer in dye-sensitized solar cells

A series of TiO₂powders with various morphologies were synthesized using surfactant-assisted hydrothermal method by hydrolyzing the titanium tetra-isopropoxide (TTIP) precursor. Four types of tetraalkylammonium hydroxide (TANH) surfactants such as tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide (TEAH), tetrapropylammomium hydroxide (TPAH), and tetrabutylammonium hydroxide (TBAH) were used before hydrothermal crystallization. The results showed that the surfactants with different alkyl chain length of tetraalkylammonium cations (TAN⁺) had a great influence on the morphology, particle size, and crystalline phase of TiO₂particles. Under the same experimental conditions, the surfactants of TMAH, TEAH, TPAH, and TBAH resulting in the star-, flower-, rod-, and scree-shaped TiO₂particles (TM-, TE-, TP-, and TB-TiO₂), respectively. The particle sizes were decreased in the range from ∼3.6 μm to ∼220 nm while increasing the alkyl chain length of the surfactants. All the TiO₂powders showed an anatase phase besides the TiO₂prepared from TMAH surfactant. The photovoltaic investigations of these TiO₂samples as the scattering layer in DSSCs indicated that the TB-TiO₂based cell exhibited the excellent power conversion efficiency of 9.88%, which was also higher than that of a device which fabricated without a scattering layer (8.83%). Among these scattering layers, TB-TiO₂has the optimum particle size (∼220 nm) with the highest surface area leading to the largest dye adsorption value and an excellent efficiency. In order to further increase the dye adsorption of the photoanodes, the scattering layer was prepared by mixing the large (TB-TiO₂) and small sized (a-TiO₂) particles as composites with different weight ratios, leading to the highest efficiency of 10.53% at TB-/a-TiO₂(2:3).