Fabrication And Optimization Of TiO₂-based Quantum Dot Sensitized Solar Cells

In recent decades,solar cells have received great attention due to the extensive development of new renewable energy.In various types of solar cells,quantum dots sensitized solar cells（QDSSCs）with numerous advantages make them standout,such as high theoretical efficiency,bandgap controllable,low-cost and so on.However,the photoelectric conversion efficiency（PCE）has a certain gap between the laboratory one and the theoretical value,so there still exist some challenges.There are many factors that affect cell PCE,such as the type of quantum dots,the nanostructure of photoanode,the electrolyte and the counter electrode etc.Based on the factors which affect the performance of QDSSCs,the doped TiO₂ semiconductor film,CdS/CdSe quantum dots with series structures,SiO₂ passivation layer,the concentration of polysulfide electrolyte and several kinds of the counter electrodes have been studied in this thesis.This thesis discusses these factors to improve the PCE of QDSSCs.The work which has been done is summarized as follows:（1）Two-dimensional g-C₃N₄ nanosheets were doped into TiO₂ nanoparticles to prepare the mesoporous layers of photoanodes.The CdS sensitizers were prepared by the method of successive ionic layer adsorption and reaction（SILAR）,and the deposition time of quantum dots was eight times after experimental optimization.By measuring the photoelectric properties of different doping amounts of g-C₃N₄ in cells,it could be seen that the efficiency increased first and then decreased with the increase of doping amount.When the doping amount was 15%,the solar cell was in the best condition and the photoelectric conversion efficiency was 2.91%.It was increased by 29.3%.（2）The chemical bath deposition（CBD）method was used to deposit CdSe quantum dots outside of CdS sensitized photoanode.The preparation of CdS/CdSe co-sensitized photoanode with series structure has the advantages from both broad band gap of CdS which is easy to produce electron and narrow band gap of CdSe which has the wide range of absorption wavelength.Thus it makes the solar cell obtain a high current density of 14.7mA/cm²,which is 79.3% higher than the one which has CdS as a single sensitizer（8.18mA/cm²）.At the same time,the electrocatalytic activities of four kinds of counter electrodes in CdS/CdSe sensitized solar cells have been compared.The results show that CuS/FTO prepared by acetic acid as reliever agent has the best performance,the photoelectric efficiency of which has reached 3.87%.（3）By using the high concentration of electrolyte and the introduction of passivation layer,it is able to further improve the energy efficiency of QDSSCs.The efficiencies ofcells,with different concentrations of electrolyte,are 3.42%（1 mol/L）and 4.02%（2mol/L）.The increase of the concentration leds to the fast recovery of the holes which are left by the excited electron,so the photoelectric conversion efficiency has enhanced 18%.The optimum deposition conditions of the SiO₂ passivation layer are as follows: the concentration of tetraethyl silicate solution is 0.01 mol/L;the deposition time is 2 h;and the deposition position is the outer layer of quantum dots.The existence of the passivation layer effectively is able to suppress the charge recombination at the interface between photoanode and electrolyte,resulting in an efficiency of 5.6%,which is 33.3% higher than the one without the passivation layer.