Visible light enhanced TiO2 thin film bilayer dye sensitized solar cells

Visible light enhanced nitrogen-sulfur (N-S) doped titanium dioxide (TiO2) thin films were prepared by the sol-gel method using thiourea as a dopant. The physical and chemical properties of the TiO2 thin films were greatly influenced by the amount of thiourea added to the sol-gel solution. The greatest shift to longer wavelengths for visible light absorption was observed with 0.6 g of thiourea in the precursor solution, while 0.4 g yielded the largest particle sizes. These single-cycle dip-deposited N-S doped TiO2 thin films were used as visible light harvesters as well as blocking layers in dye sensitized solar cells. When deposited directly on conducting fluorine doped tin oxide electrodes, photo-conversion efficiencies were reduced. However, the opposite configuration, with N-S doped thin films on top of nanoporous TiO2, yielded an increased open-circuit voltage of 0.84 V, a short-circuit current density of 9.86 mA cm−2, and an overall conversion efficiency of 5.88% greater than that of a standard cell. The effectiveness of the blocking layer on the cell efficiencies was analyzed by electrochemical impedance spectroscopy.