Preparation And Application Of ZnO-based Composite Photoanode For Dye-sensitized Solar Cells

Dye sensitized solar cells（DSSCs） have attracted extensive research attention as one of important potential alternatives to Si-based photovoltaic cells primarily due to their low manufacturing costs, rich source of raw materials, relatively high theoretical energy conversion efficiencies, convenient fabrication and remarkable stability and environmentally friendly. The main parts of the DSSCs are composed of semiconductor oxide anode, dye-photosenitizer, electrolyte and the electrode. As one of the most important parts in dye sensitized solar cells, the specific surface area and the rate of charge collection of size semiconductor oxide anode have direct impact in the photoelectric conversion efficiency（PCE）. Therefore, the band gap of semiconductor oxide materials, the stability of the oxide semiconductor materials, the specific surface area of the materials and the industrial production conditions are important conditions for the selection of excellent semiconductor oxide materials.As one of the excellent semiconductor materials, the band gap and physical properties of Zinc Oxide（ZnO） are similar to that of the TiO₂ which is the most commonly used in DSSCs. Moreover, compared to TiO₂, it has a faster electron mobility, which can reduce the recombination of electrons thus leading to reduce the loss of energy. Although the photoelectric conversion efficiency（PCE） of ZnO based DSSCs is only up to 7.5%, which is still low compared to that of the TiO₂ based DSSCs（13%）, it is still one of the most promising anode materials for photovoltaic cells which can be used to replace TiO₂ because it has more excellent crystallinity,varied morphologies（flower nanoparticles, nanowires, nanorods, nanotubes, nano rings, nanometer nails, nano combs, etc）, relatively mild and simple synthesis conditions and much more variety of preparation methods. And the main restrictions to further improve the photoelectric conversion efficiency on ZnO based dye-sensitized solar cells are the chemical stability of ZnO in the medium acid dye and the low rate of electrons from the dye molecules into the ZnO semiconductor material injection. Therefore, how to improve the chemical stability and electroninjection efficiency of ZnO in acidic medium are very important to improve the effenciency of ZnO based dye sensitized solar cells.This paper is based on the fundamental method by using solid state synthetic method to prepare Zinc Oxide nano film, and then the ZnO film was composed with other materials to prepare composite photoanode which is to provide effective channels for fast electron transfer and increase the electron transfer rate and enhance the charge separation. Secondly, by using organic reagents to spray on the surface of ZnO film to make the ZnO surface grafted with organic functional groups, thereby ZnO surface was modified, and the reaction characteristics was adjusted at the interface of the dye/electrolyte/semiconductor material, thus to improve the stability of ZnO in the acid dye chemical and the electron injection efficiency, leading to the improvement of the effenciency of ZnO based dye sensitized solar cells.The main research contents of this paper are as follows:（1） The preparation of ZnO/MWCNTs-ZnO composite anode and the study on the performance of dye sensitized solar cells. The multiwalled carbon nanotubes（MWCNTs） were uniformly dispersed in the acetone/ethanol mixed solution and by ultrasonic technique. The homogeneous dispersion of MWCNTs and Zn（OH）2precursor were fully mixed before annealing treatment. The ZnO/MWCNTs composite photoanode was successfully prepared after annealing treatment. After carefully analysis, results showed that the pure ZnO had a high dye adsorption, but its electron transport efficiency was low. In the meanwhile, MWCNTs could provide the electron transfer orbit which would be helpful for improving the electron transport efficiency. Thus, a double-layer of ZnO/MWCNTs-ZnO photoanode had been prepared and then was used in the dye sensitized solar cell, finally an efficiency of6.25% had been achieved.（2） Preparation of ZnO/ZnFe₂O₄ P-N heterojunction composite anode and the study on the properties of dye sensitized solar cells.By using low temperature solid state method, the P type semiconductor of ZnFe₂O₄ was prepared and then after fullymixed ZnFe₂O₄ with the precursor Zn（OH）2, the ZnO/ZnFe₂O₄ composite thin film anode had be obtained after annealing treatment. On one hand, the rod-shaped ZnFe₂O₄ formed by the accumulation of particles could provide a directional electron transfer orbit which could improve the current density, on the other hand, after a good combination, a p-n junction could be formed at the interface between the p-type semiconductor ZnFe₂O₄ and the n-type semiconductor ZnO, which could effectively enhance the ZnO surface charge separation degree, thus leading to a great improvement in the open circuit voltage value of the solar cell.（3） The study on the surface modification of ZnO. By using the pneumatic gun,the Thioacetamide（TAA） neutral aqueous solution was sprayed homogeneously on the surface of the prepared ZnO film, moreover, the high power transmission showed that TAA had been attached on the surface of the ZnO particles. Using the high negative elements N and S in TAA to increase the probability of hydrogen bonding with the oxygen functional groups in N719, the adsorption capacity of the dye could be increased. By using this method, the light current density had been increased to nearly twice, and an photoelectric conversion efficiency over 8% had been achieved.