Synthesis And Photoelectrochemcal Properties Of CdSe/CdS/TiO₂ Nano Structure Composite Films

The energy crisis is one of the main challenges that human faced. At present, fossil energy such as coal, oil and gas are mainly used by human. Large scale use of fossil fuels causes environmental pollution and energy crisis, Therefore, "change the energy structure, develop renewable clean energy" has become a global pursuit. Solar energy is one of green renewable energy which is widely distributed, inexhaustible and environment friendly. Solar cell is one of the main ways to use solar energy. Recently, research on Quantum dot sensitized solar cells（QDSCs） has grown significantly, and the technology has been considered as the most promising photovoltaic devices because of the low cost, simple fabrication process and the high theoretical conversion efficiency. QDSCs is made up of photoanode, counter electrode and liquid electrolyte. As the core component part of QDSCs, the function of photoanode is generating photogenerated charge carriers, effective transportion and collection of photogenerated electrons, determines the final power conversion efficiency.Titanium dioxide（TiO₂） has attracted considerable attention as a promising semiconductor material for applications in solar cells because of its high chemical inertness, non-toxic, low-cost and appropriate energy band position. And TiO₂ is a N-type semiconductor material which has wide band gap of 3.2 e V. However, due to its large band gap, TiO₂ can only harvest light to the ultraviolet region which is only 3-5 % of the whole solar spectrum. In this thesis, we will focus on the preparation of TiO₂ nanosheets array film, and used as photoanode materials. And realized the Cd S and Cd Se quantum dots（QDs） sensitization through chemical bath deposition method（CBD） and successive ionic layer adsorption and reaction（SILAR） method. The samples Cd S/TiO₂ NSs and Cd Se/Cd S/TiO₂ NSs array composite films are characterized and analyzed. Specific work includes below aspects:（1） Vertically aligned two-dimensional（2D） anatase TiO₂ nanosheets array film with large percentage exposed {001} reactive facets have been successfully grown on FTO substrates by hydrothermal method. The formation of Cd S QDs sensitized nanosheets array composite films were carried out by a chemical bath deposition method（CBD）. The sensitization of Cd S QDs has extended the photoresponse of TiO₂ ptotoelectrode into the visible region, the absorption edge is about 550 nm. The photocurrent density of the samples Cd S/TiO₂ NSs array composite films increases first and then decreases with increasing Cd S deposition time. It is found that the optimum deposition time is 30 min, Cd S/TiO₂ NSs shows the highest photocurrent density of 3.44 m A/cm2 at 0 V versus Ag/Ag Cl, and the corresponding photoconversion efficiency is 1.92 %.（2） Cd Se/Cd S/TiO₂ NSs composite films are prepared through SILAR method. The absorption edge is about 740 nm, and the photoelectrochemical properties are studied, we find that the photocurrent density of the samples follow the law of increases first and then decreases with increase of Cd Se cycles. The Cd Se/Cd S/TiO₂ NSs shows the highest photocurrent density of 7.82 m A/cm2 at 0 V versus Ag/Ag Cl, and the corresponding photoconversion efficiency of 4.61%. The photocurrent density of Cd Se/Cd S/TiO₂ NSs photoanodes are higher than that of single Cd S QDs sensitized TiO₂ NSs. The improved photoelectrochemical properties of co-sensitized Cd Se/Cd S/TiO₂ NSs photoanodes are mainly ascribed to the following reasons: the narrow band gap Cd Se QDs further broaden absorption range, and stepwise energy level structure would boost electron injection efficiency and reduce electron recombination rate constant. Therefore, the photoelectrochemical performance is significantly improved.