Research On Preparation And Properties Of Counter Electrode For Dye-sensitized Solar Cells

In the past decades, a series of developments have been made in dye-sensitizedsolar cell (DSSC) counter electrode. Nowadays, exploiting Pt-free counter electrodecatalytic materials and discussion the electrochemical mechanism of counter electrodein-depth are two key directions in this field. This dissertation focuses on synthesismultinary sulfide/selenide and their applications as counter electrode in DSSC. Onone hand, in order to overcome technical bottle-neck of synthesis multinarycompound, many efforts have been made to exploit new synthetic strategies. On theother hand, for the sake of achieving applications of multinary sulfide/selenidenanomaterials as counter electrode in DSSC, valuable investigations have beencarried out on the relationship between technical parameter and their properties.Furthermore, with the purpose of obtaining the accurate result of electrochemistryproperty of the as-prepared sample, a new method was established to characterize theelectrochemical property of the counter electrode with this narrow bandsemiconductor compound. At last, the essence of electron transport of these materialswere investigated to provide theoretical direction for selecting new catalytic materialsand building new structure for potential counter electrode.A simple synthetic method for multinary sulfide/selenide nanocrystals has beendeveloped. A system with a single component of ethanediamine as both the solventand the surfactant was designed and three different kinds of sulfide/selenide,Cu2ZnSnSe4, Cu2SnSe3and Cu2ZnSnS4, in this system were prepared using ahydrothermal synthesis method. Many efforts have been made to overcome technicalbottle-neck of synthesis multinary compound and the materials were obtained withregular shape, pure crystal phase and nearly-stoichiometric ratio. The as-preparednano-particles were dissolved into isopropanol to form “ink”, then the “ink” wasdeposited onto FTO as catalytic materials of counter electrode with drop-casting orspin-coating method. Technological parameters including film’s thickness andannealing temperature, which decided the photoelectric property of DSSC andelectrochemical property of counter electrode were investigated. Optimum technologyroute was obtained to prepare Cu2ZnSnSe4, Cu2SnSe3and Cu2ZnSnS4counterelectrode. A facile one-pot method for measureing semiconductor nanocrystals ascounter electrode has been established. This method was carried out with dummy cellstructure composing of counter electrode-electrolyte-counter electrode which was testing in light. The method can avoid the mutual effect between photocatalysis andelectrocatalysis meanwhile it can provide electrochemical property parameters moreaccurately. The electron transfer mechanism about this semiconduc tor compound wasinvestigated. Different from the traditional Pt counter electrode, the electron transferprocess in these materials are made up of three parts: receiving the electrons fromexternal circuit, the excited electron jumped from valence band to conduction bandand the electron passed through the interface between the counter electrode andelectrolyte. The investigating of electron transfer mechanism about these materialscan provide theoretical basis for prepareing catalytic materials and improving thestructure of the counter electrode.New layer-by-layer structures were designed as composite counter electrodes inDSSC. The electrodes were composed of Cu2ZnSnSe4and Cu2ZnSnS4, which havethe similar crystalline structures and electron structures. The layer-by-layer structureof counter electrode was established by a spin-coated-drying-spin-coated method.With the different light-absorbing characteristics, the layer-by-layer compositeelectrode can capture more light for improving the efficiency o f the electrons exciting.This will be beneficial for electron transfer from counter electrode to electrolyte. Theexperimental result confirmed that layer-by-layer structure composing of Cu2ZnSnSe4and Cu2ZnSnS4counter electrode can improve the efficiency of DSSC evidently.A new composite nano material Cu2ZnSnS4/MWCNT was developed as counterelectrode catalytic material. In order to improve the electron conductivity ofCu2ZnSnS4used as the counter electorde in DSSC, three-dimensional interconnectedconductive mesoporous structure was established. It consisting of electrocatalyticactivity nanocrystals/conducting phase, which can enhance the conductivities ofelectrons in micro/nano levels. MWCNT, as the framework in this system, can offerfast track for electron transferred to the interface of electrode/electrolyte whilemesoporous structure with huge surface area can provide more catalyze site. Theobtained Cu2ZnSnS4/MWCNT composite nano material has excellent electrochemicalproperty with high conductivity and electrocatalytic activity. Considering the simplemethod and mild conditions, the Cu2ZnSnS4/MWCNT as a candidate is verypromising to replace Pt-based CE by virtue of its stability and great cost advantage.