The Modification Of TiO₂Photoelectrode In Dye-sensitized Solar Cells And Its Influence On Dye Stability

Dye-sensitized solar cell (DSSC) is one of the most promising new generation photovoltaic device, which has the advantages of high cost-efficiency ratio, low weight, and simple fabrication technology. The most efficient DSSC has a conversion efficiency of12.3%. Up to date, the conversion efficiency and the stability of DSSC are the main factors hindering commercialization of DSSC. In a common DSSC, the efficiency-decided electron transfer processes mainly occur at the TiO2/dye/electrolyte interface, including electron injection, recombination of conduction band electron, and dye cation regeneration. These rates of electron transfer processes also determine the lifetime of the excited states and the oxidized states. And the excited states and the oxidized states are the main intermediate states which lead to irreversible degradation. So the TiO2surface properties determine not only the DSSC performance, but also the dye stability in DSSCs. In this thesis, we focus on the effect of TiO2surface treatment on the performance and dye stability of DSSCs. Besides, the thesis has an independent chapter describing the initial work on CuInS2preparation and material characterization.The TiO2photo-electrodes were prepared by sol-gel and spin-coating technology, and the morphology, crystallinity, and crystalline size were characterized. DSSC devices were fabricated based on these TiO2photo-electrodes. Then the effect of different surface properties on DSSC performance was studied in detail by using different surface treatment technology. HCl surface protonation leads to H+adsorption on TiO2surface, and makes the TiO2surface positively charged. This will make the TiO2conduction band level shift downwards and increase the injection rate of excited electrons and thus the Jsc. This effect will also lead to increased interface recombination and slightly decrease the Voc. The much increased Jsc and slightly decreased Voc make the conversion efficiency increase significantly. When the TiO2electrodes are annealed in different atmosphere, i.e., H2, N2, and O2atmosphere, the density of surface defects (O vacancy-Ti3+) increases with the increasing reducibility of atmosphere. The surface defects serve as recombination centers in DSSCs, so the recombination current increases with increasing reducibility of atmosphere, leading to a much decreased Jsc, Voc, and η. When a ZnO barrier layer is introduced to the TiO2surface, it covers a portion of surface defects and decreases recombination current, leading to increased Voc. While the ZnO barrier layer also greatly decreases the injection rate and thus the Jsc. Finally, the ZnO barrier layer doesn’t enhance the η.The in-situ Raman was used to study the dye degradation under continuous exciting laser irradiation (514nm). Different surface treatments were used to investigate the effect of TiO2surface properties on the stability of adsorbed dye. By increasing reducibility of atmosphere, the density of O vacancy-Ti3+surface defects increases, which increases the recombination process at the interface. The increased recombination resupplies some I-ions by reducing the I3-, and make the oxidized dye more stable. The HCl surface protonation shifts the TiO2conduction band level downwards, and increases the injection rate of excited electrons. This makes the excited dye more stable. Besides, surface protonation can also increase the recombination and makes the oxidized dye more stable. The ZnO barrier layer decreases both the injection and recombination rate, so the dye will degrade via excited and/or oxidized dye more easily. The effect of H2O in the electrolyte is similar to that of surface protons. So H2O will increase both the excited and oxidized dye stability.CuInS2thin films with mixed phases, namely the chalcopyrite (CH) and the Cu-Au (CA) phases, were prepared by sulfurization of Cu-In precursor layers. Detailed temperature-dependent Raman scattering was carried out on the CuInS2films. The temperature dependences of both the Raman shift and the linewidth were fitted by using the Ridley model, namely, both the CH A1and the CA A1phonons decayed asymmetrically into one TO and one LA phonons. Excellent fitting results could be obtained for the temperature-dependent Raman shifts of both the CH A1and the CA A1modes. For the CH A1mode, multiple-phonon fitting could match the experimental data of the Raman linewidth quite well, while for the CA A1mode, the fitting could only match the experimental data at high temperature. It was concluded that the CA/CH phase boundary scattering played an important role in determining the CA A1phonon lifetime and consequent the Raman linewidth at low temperature. According to our results, the intensity ratio between the CA A1and the CH A1modes was believed to be more reliable in monitoring the CH crystallinity at different temperatures than the CH A1linewidth, which was temperature dependent. The intensity ratio between the CA A1and the CH A1modes almost maintained constant between83and693K, meaning that no phase transformation took place during the test.