Synthesis Of ZnO/SnO₂Core-shell Array Films And Their Application In Dye-sensitized Solar Cell

As the representative of the third generation solar cells, dye sensitized solar cells (DSSCs) have received abundant attention for their high photoelectric conversion efficiency, low production cost and commercially available. Band gap semiconductor, including TiO2, ZnO and SnO2act as an active host material in DSSCs. At present, the total effciences of DSSCs based on nanoporous TiO2electrodes has the highest conversion effciency of11%. However, the problem that the dye molecules is easy to be photodegraded and the TiO2film’s relatively slow electron transport rate will lead to the high interface recombination reactions and limit the device efficiency. The bandgap and conduction band edge of ZnO is close to that of TiO2(anatase).ZnO (115-155cm2V-1S) has a higher electron mobility than TiO2(10-5cm2V-1S-1), which should favor electron transport. As a result, ZnO is so far the runner up to TiO2in terms of number of publications. With less surface states and defects,1D nanostructures are able to provide a direct pathway for the collection of photogenerated electrons. Therefore,1D ZnO nanoneedles could reduce the degree of charge recombination and thus improve the photoelectric conversion efficiency of DSSCs. However, the chemical stability of ZnO is rather poor that it dissolves under both acidic and basic conditions. Also Zn3+forms on the surface of the ZnO particles due to the reaction between the ZnO and the dye molecules when DSSC works. Therefore, the efficiency of these DSSCs based on ZnO nanorods was still at a relatively low level. Coating the ZnO with a thin layer of another metal oxide to form a core-shell structure has been considered as a way of inprovement in effciency, resulting from the decrease of ZnO corroding.Based on the above point of view, the main contents focus on ZnO which include:1. ZnO nanoneedles array films were successfully synthesized on FTO substrate by a simple hydrothemal method at low temperature, using Zn (NO3)2·6H2O, NH3·H2O and NH4F as the reactant. The thickness of the film could be easily adjusted by controlling the reaction time and the diameter of ZnO nanoneedles are multi-sized distribution. The crystallinity of the ZnO nanoneedles is increased by annealing treatment. The multi-sized distributed needle-like structure is beneficial to the light harvesting and the highest photo-electrical conversion efficiency (η) of ZnO nanoneedles array based DSSC is 2.87%. In the process of th hydrothemal synthesis, NH4F play a key role in selective etching ZnO to form multi-sized distributed nanoneedles which enhance the light harvesting. In addition, surface states of ZnO nanoneedles were modified by F’ which increase the dye-adsorption.2. A ZnO/SnO2core-shell structure has been prepared by a sol-sel method with the SnO2uniformly coated on the ZnO nanoneedles surface. Amorphous SnO2shell seperate ZnO core from electrolyte and dye, which form tunnel fence of the electronic transmission to the basement, promote the transmission rate of the carriers and restrain carriers recombination, is considered to be an important reason of enhancement of ZnO/SnO2photoanode’s conversion effciency, which is4.71%.