Enhanced charge collection efficiency by thin-TiO₂-film deposition on FTO-coated ITO conductive oxide in dye-sensitized solar cells

We have investigated effect of thin TiO₂ layers deposited on an indium tin oxide (ITO)/fluorine-doped tin oxide (FTO) double-layered transparent conductive oxide on the performance of dye-sensitized solar cell. FTO is deposited on an ITO-coated glass, followed by TiO₂ deposition using a radio frequency magnetron sputtering technique. The thicknesses of the sputtered-TiO₂ layers are varied from 10 nm to 20 nm, while the ITO and FTO are fixed to be 150 nm and 70 nm, respectively. Atomic force microscopy (AFM) shows that the surface roughness is similar but the surface morphology is altered by thin TiO₂ layer deposition. The sheet resistance of the ITO/FTO conductive glass is hardly changed by thin TiO₂ layer deposition. Photovoltaic performance is significantly enhanced after introduction of thin TiO₂ underlayer. The 15 nm thick TiO₂ underlayer leads to the increases of photocurrent density from 9 mA cm ^-2 to 10.3 mA cm^-2 and fill factor from 0.715 to 0.747, as a result, the overall conversion efficiency is improved from 5.28% to 6.37%, corresponding to 20.6% increase. Photovoltage, however, remains almost unchanged. Photocurrent is improved over the entire wavelength. The increased transmittance at wavelength ranging from 300 to 600 nm contributes in part to increase in photocurrent. Improvement of charge collection efficiency from ∼90% to ∼97% is also attributed to the increased photocurrent, where the increased transport rate is responsible for the improved charge collection, indicating that the thin TiO₂ underlayer has influence on opto-electronic property in the dye-adsorbed bulk TiO₂ film.