Design And Synthesis Of Silver-based Sulfide Nanocrystals And Heterodimers For Counter Electrodes In Dye-sensitized Solar Cells

Dye-sensitized solar cells?DSSCs?have recently attracted intense research attention owing to their easy fabrication and cost-effectiveness compared with the traditional silicon-based photovoltaic devices.Traditionally,a DSSC consists of a dye-sensitized TiO₂ photoanode,a counter electrode?CE?,and an electrolyte with the I₃^-/I^-redox couple.As a crucial component,counter electrode?CE?plays an important role in the performance of dye-sensitized solar cells?DSSCs?,which collects electrons from external circuit and fulfills them transfer from CE to electrolyte by catalyzing the reduction of triiodide?I₃^–?to iodide?I^–?.Currently,platinum-loaded?Pt?conducting glass is widely exploited as a CE for DSSCs.However,Pt could dissolve and create by-products because of the corrosive I^-/I₃^-redox electrolyte,such as PtI₄ and H₂PtI₆,which would deteriorate the long term stability of DSSCs.Therefore,tremendous research focus on the development of efficient and stable alternatives to Pt.To date,several kinds of new CE materials,such as carbon-based materials,polymers,oxides,nitrides,and metal chalcogenides,have been developed.However,taking the electrocatalytic activity and stability into consideration,only a few of these materials are able to compete with Pt completely over the past decades.It remains a great challenge to seek suitable CE materials for DSSCs.Hence,this thesis mainly focuses on the design and synthesis of silver-based sulfide nanocrystals and their metal-semiconductor heterodimers as electrocatalysts for efficient DSSCs.The results are summarized as follow:?1?Ag₂S nanocrystals was prepared via a colloidal synthesis process.The obtained nanocrystals can be easily dispersed into chloroform to form a stable ink.To fabricate different thicknesses of Ag₂S CE,nanocrystal inks with various concentrations were spin-coated onto an FTO glass substrate.CE manufactured from Ag₂S nanocrystals exhibited efficient electrocatalytic activity in the reduction of I₃^-to I^-in DSSCs.Moreover,the devices with Ag₂S CE showed better stability compared with Pt CE in iodine based electrolytes.Chemical equilibrium constant of Ag₂S and Gibbs free energy of Pt in electrolyte were also calculated,and they revealed the better chemical stability of Ag₂S CE than Pt CE.?2?Ag₈GeS₆?AgInS₂?Ag₈SnS₆?Ag₉GaS₆?Ag₅SbS₄,and AgBiS₂ nanocrystals were synthesized through a cation-exchange reaction process.CEs fabricated from various nanocrystal inks exhibited efficient electrocatalytic activity in the reduction of I₃^-to I^-in DSSCs.Extension of the CE materials to ternary compounds could greatly expand this research platform.On the other hand,ternary metal sulfides would provide more alternatives and tune physicochemical properties due to the existence of two cations.Moreover,based on the electrochemical measurements of different CEs,the relationship between the electrochemical properties and average effective cationic potential of the silver-based ternary sulfides were presented for the first time.?3?Monodispersed pyramidal Ag-Ag₈SnS₆ heterodimers were synthesized via a simple route for the first time.Based on our experimental results,it is believed that its intrinsic orthorhombic structure and reaction conditions including the hot-injection temperature,sulfur precursor,reaction solvent as well as the capping agent,exert synergetic effect on the morphology.Electrochemical measurements confirmed the formation of Mott-Schottky heterojunction could promote the electron transfer from Ag₈SnS₆ to Ag,which can further accelerate the reducing of I₃^–to I^–.Thus,the obtained heterodimers show catalysis performance comparable to Pt as an I₃^–reduction catalyst.In addition,this work provided a facile and mild approach to prepare heterodimers,and can be extended to other systems,such as Ag₈GeS₆ and their selenides.?4?A general synthesis method for Ag-ternary sulfide heterodimers are demonstrated utilizing sample inorganic compounds as precursors in an octadecene solvent.Instead of forming core-shell or multimer structures,only one Ag nanoparticle located at the vertex could be found in the herterodimeric nanostructure.A serial of Ag-ternary sulfide heterodimers,including Ag-Ag₈GeS₆,Ag-Ag₈SnS₆ and Ag-Ag₉GaS₆,were sucessfully synthesized utilizing similar reaction conditions.Unlike Ge⁴⁺,Sn⁴⁺and Ga³⁺ions,when InCl₃ was used as the precursor as opposed to GeCl₄,Ag₂S-AgInS₂hybrid NCs were obtained.Furthermore,Ag₅SbS₄ or AgBiS₂ NCs could be prepared if SbCl₃ or BiCl₃ were used as the precursor.The results can be attributed to the different polarization effects caused by the polarizability between cations and anions.A plausible mechanism involving cation-exchange and ionic polarization was proposed based on the stepwise experimental analyses.It is predicted that the synthesis method demonstrated in this work could initiate new routes to the further development of metal-semicoductor heterodimers preparation.