Performance Optimization Of Metal Chalcogenides As Counter Electrode In Quantum Dot-sensitized Solar Cells

Owing to their high theoretical conversion efficiency(44%),low cost and easy preparation,quantum dot-sensitized solar cells(QDSSCs)have become an important photovoltaic technology to alleviate present energy problems.Recently,although the power conversion efficiency(PCE)of QDSSCs has jumped from less than 1% to 15%,it is still far below their theoretical conversion efficiency.Therefore,optimization of electrode materials and electrolytes to improve the photovoltaic performance of QDSSCs has already become a hot research topic.Counter electrodes(CEs),as an indispensable component of QDSSCs,are responsible for collecting and transferring electrons from external circuits and catalyzing the reduction of oxidizing electrolytes at the electrolyte/CE interface.Hence,the development of high-performance counter electrode is an effective way to further enhance the performance of QDSSCs.Among various CE materials,metal chalcogenides are recognized as the most commonly used CE materials due to their relatively high stability in polysulfide electrolytes.However,although extensive studies have been conducted,the development of novel metal chalcogenides CEs is still critical,and the limitation of single-component CE materials on the undesirable functionality such as relatively weak conductivity,catalytic activity and stability makes it difficult to realize breakthroughs in the efficiency of QDSSCs.Polyoxometalates(POMs)are a class of nanoscale metal oxide clusters with exceptional physicochemical properties,and their outstanding redox reversibility and strong electron accepting ability make them promising for application in the field of photoelectric materials.Meanwhile,depending on its excellent electrical conductivity,large specific surface area and good structural mobility,reduced graphene oxide(RGO),a typical representative of carbon-based materials,has emerged as a potential candidate for the preparation of composite materials.Consequently,in this work,we screened metal chalcogenides with suitable components and further prepared composites with enhanced properties by improving preparation methods,doping with POM molecules,carbon-based materials and heterostructure construction,and then systematically investigated their application as CE in(Mn)-Cd S/Cd Se/Zn S co-sensitization QDSSCs.The specific studies are as follows:(1)To avoid the persistent corrosion of brass substrates in conventional Cu2S/brass-CE and further enhance the stability and optoelectronic performance of Cu2S-based QDSSCs,Cu2S/X%-Si W11 Co composite counter electrodes were synthesized on FTO substrates by in situ sulfidation at room temperature using porous sponge-like Cu O/Si W11Co(K6Si W11O39 Co II)as precursors(X = 0,1.5,2,2.5,3,X is the mass percentage).Electrochemical and photovoltaic performance tests revealed that the FTO/Cu2S-CE prepared by in situ sulfidation method has higher stability compared with the conventional Cu2S/brass-CE.Furthermore,the addition of an appropriate amount of Si W11 Co further promoted the ability of CE to collect and transfer electrons,effectively suppressing charge recombination and thus boosting the performance of QDSSCs.Thereinto,the PCE of QDSSC(5.94%)assembled with Cu2S-2 CE was nearly 20% higher than QDSSCs(4.96%)based on Cu2S/brass-CE.(2)Compared with binary copper(Cu)-based sulfides,ternary metal sulfides usually possess higher catalytic activity due to the synergetic effect between multivalent transition metals.Highly efficient ternary Cu2WS4(CWS)CE was prepared by a simple one-step solvothermal method and applied to QDSSCs to achieve a PCE of 5.92%,which is 28.7%higher than the commonly used binary Cu2S-based QDSSCs.Additionally,the CWS was further modified with lanthanide metal ion substituted Keggin-type POM([Me2NH2]11[Ce III(PW11O39)2],Ce III(PW11)2)as the “electron transport medium” to obtain CWS/X%-Ce III(PW11)2(X = 0,3,4,5,X represents mass percentage)composite CEs.Study found that the catalytic activity of CWS-X CEs decreased in the order of CWS-4,CWS-3,CWS,and CWS-5,and QDSSCs assembled from CWS-4 could achieve a PCE of 6.63%,which was 12% higher than that of QDSSCs based on a single ternary CWS-CE(PCE =5.92%),indicating that the addition of appropriate amount of POM could promote the collection and transfer of electrons by forming “shallow electron traps” and reduce electron-hole recombination,thus improving the photovoltaic performance of the cell.(3)For solving the limitation of the undesirable function of single metal sulfide CEs,a Co9S8/Cu7S4 heterostructure composite CE was prepared by a one-step solvothermal in situ sulfidation method using copper-cobalt Prussian blue analogue(Cu Co-PBA)as the precursor,and then assembled with Ti O2/Mn-Cd S/Cd Se/Zn S photoanodes to form QDSSCs.The relevant electrochemical tests suggested that the formation of heterojunctions inside the Co9S8/Cu7S4 composites was favorable to improve the catalytic activity,electrical conductivity and stability of the materials,and thus the performance of QDSSCs.Photovoltaic performance characterization demonstrated that QDSSCs constructed from Co9S8/Cu7S4 heterostructure composite CEs could achieve the PCE of 8.43% with Jsc = 23.42 m A cm-2;Voc= 0.672 V;FF = 0.54,which was significantly better than QDSSCs assembled with PM-Co9S8/Cu7S4(PCE = 6.55%),Cu7S4(PCE = 5.60%),Co9S8(PCE = 5.13%)and Cu2S/brass(PCE = 4.96%)CEs.The results illustrated that the composite CE can achieve superior catalytic performance than that of single component CE through the synergistic effect among the components,and the formation of heterojunction can further facilitate the electron transfer from the CE to the electrolyte.(4)Due to their excellent photoelectric properties,metal oxides have been extensively used in the fields of energy storage,energy conversion,and photocatalysis.However,their application as CE materials in QDSSCs is relatively rare.In this work,the porous nanorod-like structure of binary Co O was prepared by a simple hydrothermal and post-pyrolysis method,and attempted to explore its application as a CE material in QDSSC.Compared with the commonly used binary Cu2S-CE,both electrochemical characterization and photovoltaic performance tests confirmed that the prepared Co O electrode also has excellent catalytic activity for the reduction of Sn2-.Besides,the PCE of QDSSC using binary Co O as CE could reach 6.02% accompanied by Jsc = 20.42 m A cm-2,Voc = 0.66 V,and FF =0.45,which is 33.5% higher than QDSSCs based on the frequently used nano-flower Cu2S-CE(PCE = 4.51%),indicating the Co O and other binary transition metal oxides have the potential to act as CE for QDSSCs.(5)Based on the feasibility of binary Co O-CE,to further enhance the catalytic performance of the CEs,stable and efficient hollow microspherical ternary Cu Co2O4 counter electrode was prepared by solvothermal and post-pyrolysis methods.Simultaneously,to reduce the agglomeration phenomenon of Cu Co2O4 nano-microspheres,RGO with good structural mobility and electrical conductivity was used to compound with Cu Co2O4 to obtain Cu Co2O4/RGOx(x = 6,12,24 mg,x is the mass of RGO)composite CEs and constituted different QDSSCs with Cd S/Cd Se/Zn S-modified photoanodes.Both electrochemical and photovoltaic performance tests showed that ternary Cu Co2O4 has better catalytic activity than Ni Co2O4 when used as a CE,and the addition of an appropriate amount of RGO conductive support material could further improve the electrocatalytic activity and stability of the Cu Co2O4/RGO composite CE.Compared with the QDSSC based on ternary Ni Co2O4-CE,the QDSSC with Cu Co2O4-CE exhibited a higher PCE(6.19%).In addition,the optimal PCE of QDSSC based on Cu Co2O4/RGO12 composite CE could reach 7.04%,demonstrating that the introduction of RGO can form a composite CE material with more enhanced performance.