Polyoxometalates Controlled Sulfide Double-layer Counter Electrodes Materials And Their Applications In Quantum Dot Sensitized Solar Cells

Quantum-dot sensitized solar cells are a kind of low-cost as well as high efficient third generation solar cell.Quantum dots have the advantages of low cost,easy production at room temperature,adjustable band gap,and high possibility of producing excitons.In order to improve the efficiency of the battery,the components of the battery have been continually optimized:photo-anodes,quantum dot sensitizer,electrolyte and counter electrodes.As one of the important components of Quantum Dot Sensitized Solar Cells?QDSSCs?,counter electrodes exert great influence in the performance of the battery.Theoretically,the counter electrodes mainly affect the filling factor?FF?of QDSSCs.However,through a large number of studies,it has been confirmed that the influence of the counter electrodes is not only on the FF of the battery,but also on the open circuit voltage(V_oc)and short-circuit current density(J_sc).Therefore,it is of great significance to study the improvement of the performance of the whole QDSSCs by the counter electrodes.In the past,researchers have focused on counter electrodes made of one material.At present,the counter electrode materials mainly include precious metals,carbon materials,polymers and metal sulfides,but there are relatively few studies on composite counter electrodes.Composite counter electrodes are prepared by using two or more materials which are catalytically active on a polysulfide system electrolyte.In the composite counter electrodes,various counter electrode materials work synergistically,which can greatly improve the catalytic activity of the electrode,thereby improving the performance of QDSSCs.The composite counter electrode can combine the advantages of two or more electrode materials to exhibit good catalytic activities.Carbon materials are characterized by a wide range of sources,low cost,strong electrical conductivity and good chemical stability.Currently,carbon materials such as graphite,carbon black,graphene and carbon nanotubes are continuously introduced into the QDSSCs based on polysulfide electrolytes as counter electrodes.Among all the materials,the application of graphene has attracted much attention.Graphene is a kind of two-dimensional nanomaterial composed of sp2 hybridized carbon atoms,which has a large specific surface area,good stability and super conductivity of honeycomb hexagonal lattice structure.Transition metal sulfides have always been the focus of widespread attention due to their special two-dimensional layered structure.MoS₂ is a representative of transition metal sulfide layered materials.Each layer is composed of a covalent bond S-Mo-S hexagonal two-dimensional structure,and the layers are formed by a combination of weak van der Waals forces.In recent years,MoS₂ has been proven to be a potential CE material for DSSCs and QDSSCs due to its high conductivity and catalytic activity.Polyoxometalates?POMs?are molecular metal oxides that act as an effective electron acceptor to capture photo-generated electrons in a semiconductor,and then transport the electrons to adjacent regions.In particular,POMs can still maintain their structural integrity after a reversible multi-electron oxidation-reduction reaction.Therefore,POMs can promote the transfer of charge in a semiconductor and delay the recombination of electron-holes.For the above reasons,the following three composite counter electrodes are designed and applied to QDSSCs:1)H₄SiW₁₂O₄₀/MoS₂?S-M?;2)Cu₂S-H₄SiW₁₂O₄₀/MoS₂(Cu₂S-SiW₁₂/MoS₂);3)Cu2S-Graphene/MoS2?Cu2S-RGO/MoS2?.What's more,their photo-electric properties were studied.The details are as follows:1.Five kinds of complexes of S-M1,S-M4,S-M5,S-M6 and S-M6.5 were prepared by hydrothermal synthesis.It was found by SEM test that the addition of SiW12 can affect the morphology of molybdenum disulfide.When the amount of SiW12 added is small,the influence on the size of molybdenum disulfide is small;when the SiW12 content is sharply increased,the size of molybdenum disulfide will decrease to 200-300 nm.The J-V curve test,EIS test and Tafel polarization curve test all prove that S-M6 has higher battery efficiency and better electro-catalytic activity,and its efficiency can reach 2.18%.Moreover,the comparative test results show that the improvement of S-M6 battery efficiency is the result of the interaction of cation and anion in the SiW12.2.Two-layer Cu₂S-SiW₁₂/MoS₂ counter electrodes with high electro-catalytic performance are prepared by a convenient screen printing method.The efficiency of the solar cell composed of the Cu2S-SiW12/MoS2 counter electrodes can reach 4.28%.The data obtained by EIS is fitted to reach R_ct:Cu₂S-SiW₁₂/MoS₂<Cu₂S,SiW₁₂/MoS₂<MoS_2.The results may be due to the fact that the SiW12 conduction band?-4.5eV,AVS?is located between the Cu2S?-4.5eV,AVS?and MoS2?-4.73eV,AVS?conduction bands.SiW12 can capture electrons in Cu2S and then transfer it to MoS2.Therefore,SiW12 plays the role of electron transporter.This is also a new strategy to improve QDSSCs PCE by using polyacids.3.Two-layer Cu2S-RGO/MoS2 counter electrodes were prepared,and a series of researches on its performance are carried out.The Tafel and EIS tests show that the Cu2S-RGO/MoS2 electrode has good electro-catalytic activity,and the battery efficiency of the battery can reach 3.15%,which is higher than that of Cu2S and RGO/MoS2.