Design Of High-performance Heterojunction Nanostructure And Their Applications In Dye-sensitized Solar Cells

Rational use of solar energy will provide an important choice to solve the present energy crisis in modern society.So far,solar cells,as an effective utilization way of solar energy via directly converting solar energy into electric energy,has attracted more and more attention and become more and more popular in our life As one typical representative of the third-generation solar cells,dye-sensitized solar cells(DSSCs)have received widespread attention in the past two decades because of their simple preparation processes and abundant raw materials.Generally,DSSCs are mainly composed of three components:a photoanode,electrolyte,and a counter electrode.Among them,the photoanode acts as the heart of DSSCs,and simultaneously is responsible for the collection of sunlight,the generation,separation and transport of photogenerated carriers;while the counter electrode for the adsorption and catalytic reduction of I₃^-ions and the conduction of electrons.It is desmonstrated that heterojunction nanostructures,due to their flexible and diverse compositions and chemical stoichiometric ratios,can be well matched many real requirements according to energy level structures or physical and chemical performances,which makes them have broad application prospects in photoanodes and counter electrodes of optoelectric devices.Therefore,in view of the separation and transport of photogenerated carriers in the traditional TiO₂ photoanode materials and the cost of Pt counter electrode materials used in DSSCs,band-matched TiO₂/SnO₂ side-by-side heterojunction nanofibers and WS₂/CoS₂ hierarchical heterojunction nanofibers were proposed to use as photoanode and counter electrodes,respectively.In addition,their effects on light's scattering,carriers'separation,electrons'transport,and I₃^-ions'catalytic reduction were further investigated in this dissertation.(1)Fabrication and optimization of TiO₂/SnO₂ side-by-side nanofibers photoanodeCore-shelled SnO₂/TiO₂ type II heterojunction nanostructures are often employed as the photoanode materials in optoelectrochemical devices,due to their excellent performances in separation of photogenerated carriers.However,the low conductance of the TiO₂ shell often hinders the carrier transport in the composite electrodes,which greatly limits the application of SnO₂@TiO₂ core-shell heterostructures in DSSCs.Thus,in this work,a type of SnO₂/TiO₂ side-by-side bi-component nanofibers(SBNFs)with tunable composition ratios have been prepared by a novel needleless electrospinning technique with two V-shape connected conductive channels(V-channel electrospinning).Results show that the DSSCs based the optimal SnO₂/TiO₂ SBNFs(R_Sn-Ti=0.24)show the maximum power conversion efficiency(PCE)of 8.3%,which is improved by 61%and 52%compared to those of the ones based on the pure TiO₂NFs and SnO₂@TiO₂ core-shell NFs,respectively.Series of analyses indicate that the enhancements in PCE could mainly be due to the improved electron transport via SnO₂NFs and the enhanced carrier separation via dissymmetric SnO₂/TiO₂ heterojunction interface.(2)Lotus-leaf-like WS₂/CoS₂ hierarchical heterojunction nanofibers(HHNFs)used as counter electrodes for DSSCsThe noble metal Pt was widely used as the counter electrode for DSSCs due to its excellent catalytic performance on redox reactions between I₃^-/I^-ions;while its high price and rarity characteristics often increase the manufacturing cost of the device.Thus,it is of great practical significance to develop cheap and high-performance non-noble metal materials to replace Pt.So far,transition metal disulfides,such as WS₂,CoS₂,Mo S₂,were often used in field of electrocatalysis due to their good catalytic properties.At the same time,these materials are rich in raw materials and low in price.Here,in this dissertation,CoS₂ with high electron transport rate and WS₂ with excellent catalytic performance were chosen to construct WS₂/CoS₂ HHNFs,which can synergistically realize the catalytic reduction of I₃-ions and the transport of electrons in the counter electrode.In this work,a type of lotus-leaf-like WS₂/CoS₂ HHNFs was prepared via V-channel electrospinning technique combining with a post-sulfidizing process.The characterizations on microscale morphologies show that the lotus-leaf-like WS₂nanosheets were uniformly grown on the CoS₂ stems.Cell tests indicated that the assembled DSSCs using WS₂/CoS₂ HHNFs show a PCE of 8.8%,which is comparable to that of the ones based on Pt(8.1%)and far higher than those of the one bassed on pure WS₂(0.5%)and CoS₂(3.1%).Furthermore,series of electrochemical characterizations suggest that the improvement of photoelectric conversion efficiency should be attributed to the synergistic effect of WS₂ and CoS₂,in which the electrons can be quickly transported via CoS₂ facilitated transport channels to the WS₂ nanosheets,and the I₃^-ions on the lotus-leaf-like WS₂ nanosheets will can be reduced into I^-ones quickly under high catalytic activity of WS₂.