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Surface defects influence the dye adsorption on TiO2 used as a substrate in dye-sensitized solar cells (DSSCs). In this study, we have used different Ar+ sputtering doses to create a controlled density of defects on a TiO2 surface exposed to different pre-heating temperatures in order to analyse the influence of defects on the N719 dye adsorption. TiO2 was pre-treated using two different treatments. The first treatment involved heating to 200 °C with subsequent sputtering at different doses. The second treatment included heating only, but at four different temperatures starting at 200 °C. After the pre-treatments, the TiO2 samples were immersed into an N719 dye solution for 24 hours at room temperature to dye the TiO2 substrates. The amount of Ti3+ surface defects introduced by the different pre-treatments and their influence on dye adsorption onto the TiO2 surface were examined by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and metastable induced electron spectroscopy (MIES). Neutral impact collision ion scattering spectroscopy (NICISS) was used to determine the coverage of the TiO2 surface by adsorbed dye molecules. It was found that Ti3+ surface defects were formed by Ar+ sputtering but not by pre-treatment through heating alone. MIES analysis of the outer-most layer and density of states calculations show that the thiocyanate ligand of the N719 dye becomes directed away from the TiO2 surface. Both XPS and NICISS results indicate that the amount of adsorbed N719 dye decreases with increasing density of Ti3+ surface defects. Thus, the generation of surface defects reduces the ability of the TiO2 surface to adsorb the dye molecules. Heating alone as pre-treatment of the TiO2 substrates instead increases the dye adsorption, without causing detectable defects on the TiO2 surface.

Surface defects influence the dye adsorption on TiO

Titanium dioxide (TiO2) is used to form the anode in dye sensitized solar cells (DSSCs) and is thus one of the crucial components of this type of solar cell. A large range of treatments of the titania anode before deposition of the dye have been applied to improve the performance of DSSCs among which the TiCl4 treatment is the most frequently used treatment. The present study investigates how the TiCl4 treatment of the TiO2 changes the anode. For this purpose, different sample sets, nontreated, post-treated, and pre-post treated cells, were fabricated and analyzed with electron and ion scattering spectroscopy to investigate how they change the titania surface and consequently influence the cell performance. After the TiCl4 treatment, the short circuit current density (Jsc), open-circuit voltage (Voc), and efficiency significantly improved. An upward shift in the conduction band of the semiconductor as an effect of TiCl4 treatment was measured using inverse photoemission spectroscopy and UV-photoelectron spectroscopy. An increase in dye adsorption due to the treatment was found through neutral impact collision ion scattering spectroscopy measurement. The change in the energy level alignment of the titania and the adsorbed dye and the increase in dye loading explain the threefold increase of power conversion of the cells.