Optimization Study Of Novel Three-Dimensional Ordered Photoanode For Quantum Dot Sensitized Solar Cells

Governments over the world have pitched into the development of renewable energy to deal with energy crisis.The fomulation and promotion of Green Energy Policy has greatly enhanced the development of solar cells.In 1991,Michael Gratzel et.al.firstly proposed dye-sensitized solar cells(DSSCs)composed of nano structured titanium dioxide(TiO2)electrode and dye,which could attain cell efficiency of more than 7%,attracted a stirring of research interest in DSSCs.It is found that two or more hot electrons can be excited by a high-energy photon via impact ionization effect of quantum dots(QD),which yields an over 100%quantum productivity.The characteristics of QDs enable the maximum theoretical efficiency of quantum dot sensitized solar cells(QDSSCs)up to 66%,which is twice as high as that of single organic DSSC of 31%.So QDSSCs have great potential in application.However,the current energy conversion efficiency of QDSSCs is still far below that of the traditional solar cells.Based on the structure and principle of QDSSCs,this research mainly focuses on the effects of materials,structures and interface modification methods on the cell performance.The three-dimensional ordered TiO2 inverse opal films are fabricated.Then a series of photoanodes sensitized by different QDs are prepared by successive ionic layer adsorption and reaction(SILAR)and chemical bath deposition(CBD)methods.The interface between TiO2 film and QDs is modified.Solar cells based on these photoanodes are then assembled and investigated.Firstly,three-dimensional ordered TiO2 inverse opal film was fabricated using template method,and its structure and morphology were characterized.CdS,CdSe and PbS QDs was grown in situ by SILAR and CBD method on TiO2 inverse opal film to form the photoanodes.TiO2 inverse opal based CdS/CdSe co-sensitized solar cells were obtained for the first time by assembling the QDSSCs.The combination of TiO2 inverse opal electrode and cascade-structured CdS/CdSe QDs co-sensitizers has fully exploited their advantages to further improve the performance of QDSSCs:CdS/CdSe build a stepwise band-edge level structure to promote the transferring of electrons,and the deposition of the QDs on top of TiO2 inverse opal effectively prevents the clog of photoanode.The triple stepwise band-edge level structure of PbS/CdS/CdSe co-sensitized solar cells are benefit to broaden the absorption spectrum and harvest more light,as well as facilitate the diffusion and transportation of charge carriers inside the devices.However,the lifetime of electrons is reduced,which will limit the cell.Secondly,in order to improve the energy conversion efficiency of QDSSCs,TiO2 nanocrystalline thin films are prepared via sintering,on which CdS,CdSe and PbS QDs are in situ grown by SILAR and CBD method.When preparing the photoanode,a novel interface modification method is utilized,i.e.ZnS inserting layer is introduced at the interface of TiO2/QD.It can be seen from the absorption spectrum that with the insertion of ZnS between TiO2 and CdS QDs,there is an apparent and red shift of the absorption peak with intensity increase,which indicates the enhanced deposition of CdS QDs and larger grain size.The results show that the modification of ZnS layer on TiO2 surface can effectively promote the growth of QDs,which is benefit for the light harvesting and increasing the photoelectric efficiency.Finally,the thesis discusses the effects of interfacial modification on the photoanode based on three-dimensional ordered TiO2 inverse opal.The energy conversion efficiency of the device modified by the ZnS inserting layer is 3.75%,which is 1.79 times of the non-modified one(2.09%),and the fill factor(FF)of the solar cell increases from 0.288 to 0.422.With the introduction of ZnS inserting layer,the reverse recombination of photogenerated electrons toward the interface defects of photoanode and the oxidation state of the electrolyte is reduced.The light absorption effeciency and the photocurrent axe increased.The inserting of ZnS layer promotes the growth of QDs decreases?the internal charge transfer resistance and transmission resistance inside the solar cells.The analysis of the electron transfer and transmission of the TiO2/QDs/electrolyte interface shows that the electron lifetime at the interface is reduced.However,the overall photocurrent and cell efficiency can be improved and the modification is proved to be an effective way to further enhance the performance of QDs co-sensitized solar cells based on three-dimensional ordered TiO2.