Micro-nano Structure Design And Interfacial Engineering Of TiO₂ Photoanode For Dye-sensitized Solar Cells

Environmental pollution and energy shortages have become two serious challenges to the sustainable development of human society.Therefore,solar energy,the most important environment-friendly renewable energy,has attracted more and more attention.Reasonable and effective use of solar energy is the key to solving these problems.Solar cell,which can convert solar energy into electricity directly through physical and chemical effects,is one of the important technologies that people use solar energy.The research of solar cells has been continued for more than half a century,and has developed several types of solar cells,including monocrystalline silicon,polysilicon,amorphous silicon,compound semiconductors,polymers,dye sensitization,quantum dot sensitization and perovskite.Because of low cost,low power consumption,simple production process and high theoretical efficiency,dye-sensitized solar cells?DSSCs?gradually become a formidable competitor for the new generation of efficient solar cell.DSSCs are typically composed of photoanode?including conductive substrate and photoanode film?,dye sensitizer,electrolyte and counter electrode.The photoanode film is mainly responsible for dye adsorption and electron transport,hence is the most important part of DSSCS to determine the cells' electrical and optical properties.It is an important strategy to improve the photoelectric conversion efficiency of DSSCs by designing and preparing high-performance semiconductor photoanode films,which are characterized by large dye adsorption amount,fast electron transfer,low recombination rate and high light utilization efficiency.In this thesis,the novel structured photoanode films were studied systematically in order to improve the photoelectric conversion efficiency of DSSCs by designing and preparing hierarchically structured TiO₂ materials and their composites.The main research contents are listed as follows:?1?The effect of photoanode with double-layer film structure on the performance of DSSCs was investigated.TiO₂ nanorod-flower?TNRF?thin film with a thickness of about 16 ?m was directly grown on a fluorine-doped SnO₂ transparent conductive glass?FTO?by a one-step hydrothermal method.The TNRF consists of one-dimensional vertical-oriented TiO₂ nanorod arrays and threedimensional nanorod-based nanoflower.Due to its high crystallinity and special microstructure,the cells assembled with TNRF showed good electron transport properties.However,due to the small specific surface area of one-dimensional nanostructures,the dye adsorption capacity is limited,resulting in low utilization of light.Then we synthesized TiO₂ hollow spheres?THS?with a large specific surface area,which has a diameter of 400-600 nm matching with the visible light band?400-800 nm?.The THS thin film has a good light scattering ability,and the THS based cell showed a photoelectric conversion efficiency?PCE?of 5.48%.A novel double-layer TiO₂ photoanode was fabricated by using THS as the light-scattering layer and TNRF as the electron transport layer.The PCE of the DSSCs was increased to 7.50%.?2?The electron transport properties of DSSCs can be improved by controlling the microstructures of TiO₂ nanocrystalline thin films.Based on the TNRFs,we synthesized three kingds of films with different morphologies?TNRF-2,TNRF-3 and TNRF-4?by changing the amount of added precursor?tetrabutyl titanate,TBT?.The porous nanoflower layer in TNRF-4 showed good dye adsorption and light scattering ability,leading the improved light utilization efficiency of the whole film,and the PCE of TNRF-4 based cell was increased by 90% compared with TNRF based cell.A new type of TiO₂ nanorod-sheet-flower?TNRSF?was prepared by Na OH-assisted hydrothermal method.The nanosheets throughout the nanoflower layer were produced by the corrosion and recrystallized effect of the nanorods of TNRSF,and thus had good connection with the nanoflowers and nanorod arrays,providing efficient electron transport path,which acted like electron collectors.The longer electron lifetime?28.4 ms?also demonstrated that the recombination rate of the photoelectrons produced by the TNRSF film decreased during transmission process,which means more electrons were transferred to the external circuit,causing the significant increase of photoelectric current.?3?We studied the properties of photoanode based on SnO₂/TiO₂ composite.The porous SnO₂/TiO₂ material?PSTM?was prepared by one-step hydrothermal method using P25 as the titanium source,and was used for the dense bottom layer of the bilayered film.To obtain the proper scattering layer material?SnO₂@TiO₂ hollow spheres,STHS?,the SnO₂ hollow spheres?SHS?were firstly prepared by a hard template method,then the TiO₂ layer was grown on the spherical shell of SHS through the hydrolysis of Ti Cl4.The double-layer structured photoanode made full use of the energy band matching characteristics of SnO₂ and TiO₂,so that the photogenerated electrons can be effectively transferred from the TiO₂ to SnO₂,then can be spread to the conductive substrate faster due to the larger electron mobility of SnO₂,so as to improve the photoelectric properties of DSSCs.?4?The application of carbon nanofibers/TiO₂ composite?CNF-TNF?in the DSSCs photoanode transport layer was investigated.TiO₂ nanoflower?TNF?were grown on the surface of CNF by a simple water bath method.This porous surface made of needle-like TiO₂ nanosheets had a large specific surface area of 336.5 m2 g-1,so CNF-TNF exhibited good dye adsorption capacity.At the same time,the PCE of DSSCs based on monolayer CNF-TNF film was increased to 8.57%,which was ascribed to the good electronic conduction and light absorption properties of CNF.Furthermore,we applied the urchin-like TiO₂ hollow sphere?UTHS?as light scattering material to construct a double-layer structured photoanode with CNFTNF,and improved the efficiency of DSSCs to 9.21%.?5?A new kind of Zn O hollow sphere?ZHS?was synthesized and applied as the photoanode material of DSSCs.The single-layer ZHS film showed excellent electron transport properties,and had a long electron lifetime??e=94.7 ms?.We used ZHS and urchin-like TiO₂ microspheres?UTS-x?with large specific surface area to construct DSSCs that finally obtained a PCE of 8.67%,which was mainly ascribed to functional compound of ZHS and UTS-x.?6?Designed and prepared Ag@C@SnO₂@TiO₂ microspheres?ACSTS?which was applied as photoanode material in DSSCs.ACSTS adopted a layer-by-layer construction strategy: The Ag particles were wrapped by layers of diferent materials through the water bath method.The ACSTS-based DSSCs achieved a short-circuit current of up to 18.68 m A cm-2,which resulted in an enhanced PCE of 8.62%.And this enhancement was mainly contributed by the unique structure of the composite material which provided sufficient dye adsorption,effective light-harvesting capacity and fast electron transport.At the same time,the double-layer film constructed with P25 and ACSTS showed a higher PCE of 9.59%,indicating that ACSTS had considerable potential in DSSCs application.