Development Of Polyoxometalate-heteropoly Blue Photosensitive Nano-functional Material And Its Application In Photovoltaic Devices

Energy has played a key driving role in the evolution of human society.The huge consumption of fossil energy has led to a serious global energy crisis and environmental pollution.Therefore,there is an urgent need to explore renewable and clean energy sources to solve these problems.Solar energy,due to its inexhaustible and environmental friendly characteristics,is considered to be the most promising renewable energy.Dye-sensitized solar cells?DSSCs?with low cost,easy preparation,high efficiency and environment benignity,are becoming one of the typical representatives of the third generation solar cells.With a focus on the golden triangle issues of photovoltaic technology:efficiency,cost and stability,and on the basis components of DSSCs:photoanodes,electrolytes and counter electrodes,the scientists have devoted a great deal of endeavor and important progress has been achieved.However,some problems still exist,such as the serious electron recombination in the devices,the limited absorption range of photosensitizer and the expensive price of Pt electrode.Polyoxometalates?POMs?is a kind of inorganic anion nanoscale metal-oxygen clusters.They have the rich charge and skeleton structures,excellent physical and chemical properties:such as adjustable band gap energy,strong electron acceptor ability,reversible multi-electron redox activity.Its reduced state heteropoly blue?HPBs?has a wide absorption spectrum.The nano-scale properties and solubility of POMs make them an excellent building block for the preparation of other nanomaterials.In this paper,we will make full use of the excellent properties of POMs to improve the efficiency and stability,ruduce the cost of DSSCs.?1?HPBs could be a kind of excellent photosensitizers due to their wide spectra absorptions,however,most of which are hardly stable in the air.The study on photosensitive properties of HPBs is not deep enough.Here,we employ a strategy of combining the improved vacuum thermal evaporation solid state reduction and layer-by-layer?LBL?to obtain a series of long-term stable HPBs nanocomposite films,including SiW₁₂,PW₁₂,SiMo₁₂,PMo₁₂,P₂W₁₈,P₂Mo₁₈ and P₂W₁₅V₃?HPBs?.The photosensitive properties of the HPBs are revealed by the optical and photoresponse measurements.Particularly,the obtained HPBs have wider visible light absorption,higher exciton dissociation efficiency,which not only improve the photoresponse of TiO₂?28.9?A?,but also generate high photocurrent for the pure HPBs?130?A?.Furthermore,the effects of the structure,the elements composition and the different electron reduction degree on the photosensitive properties have been discussed based on the comparisons of the photoresponse signals of different HPBs.?2?In this chapter,we study in detail the photosensitive properties of HPBs based on different reduction states of the same POMs.The superlattice HPB/rGO heterojunction was constructed by solvent volatilization drive method.Based on the photosensitive property of the HPBs,the superlattice P₂W₁₈?HPB?/rGO heterojunction was first applied to the photoanode of DSSCs.Selecting two kinds of Dawson POMs as the research object,the materials were characterized by IR,UV-vis,TG,XRD and XPS.The TEM and AFM indicated that successfully obtained the monolayer POMs superlattice structure.The superlattice P₂W₁₈?HPB?/rGO heterojunction was introduced into DSSCs photoanode,and the photoelectric conversion efficiency was improved to 8.09%.Researches show that the HPB nanoparticles to be evenly dispersed in the photoanode,and P₂W₁₈?HPB?/rGO superlattice form a dense and highly robust self-assembled monolayer on the surface of mesoporous TiO₂,which not only increases spectral absorption and generates high-light current,but also prevents the electrons transfer to electrolyte.Notably,the DSSCs are stable for six hours under simulated full solar exposure at 60?.?3?This experiment based on the excellent catalytic performance of P₂Mo₁₈,high specific surface area of the 2D MoS₂ and carbonaceous materials with good electric conductivity.We demonstrate the design and synthesis of hierarchical nanoflowers structures,by in situ self-assembly highly ordered arrangement of superlattice POMs on MoS₂ surface(P₂Mo₁₈/MoS₂)and then perpendicular fixing the P₂Mo₁₈/MoS₂ nanosheet on the surface of C nano particle.The hierarchical nanomaterial provides more active sites with large surface area(174.2 m² g^-1),and good electrical conductivity is conducive to rapid charge transfer.The maximum efficiency of DSSCs using P₂Mo₁₈/MoS₂@C composite as counter electrode?CE?reaches 8.85%,which is better than that of Pt electrode battery?7.37%?.We also proposed to improve the structure of DSSCs devices:use an ultra-thin interlayer to prevent short circuit of the DSSCs,and the redox mediator is only diffused in the TiO₂ film,shortening the diffusion path and attenuating the Warburg resistance for increasing the photocurrent.This structure is suitable for both the Pt CE and P₂Mo₁₈/MoS₂@C CE DSSCs.