Preparation And Properties Of Multinary Metal Chalcogenides Counter Electrode For Dye-sensitized Solar Cells

Dye-sensitized solar cells（DSSCs）have attracted many attentions and been regarded as the representive of the low-cost and efficient novel solar cell,due to their characteristic of simple preparation technology,raw materials abudance,environmental friendliness,low-cost materials and being suitable for fabrication of flexible or semi-transparent device.The typical DSSCs have a sandwich-type structure,consisting of a porous-structured oxide film adsorbed with dye molecules as the photosensitized anode,a platinized fluorine-doped tin oxide（FTO）glass as the counter electrode（CE）,and a liquid electrolyte that traditionally contains I^-/I₃^-redox couples as a conductor to electrically connect the two electrodes.As an essential and crucial component in DSSCs,platinum（Pt）deposited on a FTO is usually employed as the CE for DSSCs.However,the high cost and low natural abundance of Pt have been a major concern,limiting the commercial application of DSSCs.Hence,it is highly meaningful to develop and synthesis stable,efficient and low-cost Pt-free CE catalysis.In this thesis,Cu₂ZnSnS₄（CZTS）,multinary metal chalcogenides or selenide have been investigated as counter electreode catalysis materials.Several effective and convenient fabrication technology of thin film have been designed and proposed,in order to lower the DSSCs manufacture cost and promote the efficiency and competitiveness of DSSCs.The main contents of this thesis were summarized as follows:（1）The CZTS nanoparticles were syhthesized by solvothermal methoed using the low-cost inorganic metal sault as raw material.CZTS was introduced into DSSCs as the CE materials via spin coating and post sulphur annealing.Furthermore,the influence of the reaction time,reaction solvent,mole ratio,and sulphur kinds on the phase composition was investigated.The CZTS particle apparently got bigger and the band gap for the CZTS film was about 1.5eV after sulfurization.It was found that the CZTS thin film with post sulphur annealing presented catalytic abilities towards the reduction of I₃^-to I^-,but the corresponding power conversion efficiency（η）of DSSCs based on CZTS CE（3.14%）was lower than that of Pt electrode（4.96%）.The inferior performance was owing to the bad quality of the CZTS thin film where lots of microcracks spreaded.（2）A novel high-temerature（400^oC）solvothermal method was proposed to synthesize high-crystalline and single-scystal CZTS submicron particles.The CE was acquired through spin coating the synthesized CZTS particles on FTO substrate and diresctly used to assemble the DSSCs.Theηwas improved to 4.24%,vertifying the potential of the CZTS as the low-cost and effective CE material.（3）Through introducing the pretreated seed layer,a CZTS nanostructure thin film was directly in-situ grown on FTO glass substrate as Pt-free CE for DSSCs,via solvothermal method.The influence of different seed layer and solvothermal conditions,on thin film morphology,phase structure,elemental composition,electrocatalytic feature and photovalitic performance,was systematically investigated and analysied.It was clearly seen that when the substrate was pretreated by TiCl₄ solution,a homogeneous and compact CZTS thin film was achieved in comparison with the untreated FTO glass.Moreover,the lower surface tension for the reaction solvent was more beneficial to the homogeneous growth of CZTS nanostructure on FTO substrate.Based on the ethanol solvent,porous CZTS thin film presented the morphology of single-crystal interconnected nanosheets and polycrystalline nanoparticles covering the entire top surface,contributing to extend the specific surface area for the catalytic site.The thicker film was acquired with the CZTS precursor concentration increasing.The current density-voltage（J-V）curves of DSSCs indicated that,when the CZTS thin film thickness was enlarged from 350nm to 580nm,the fill factor（FF）was increased from37 to 63,and the current density(J_SC)was extended from 10.74 to 12.45mA/cm²,with the highesetη（5.65%）obtained,which was superior to Pt electrode.Nevertheless,the FF droped shaply and the solar cell performance got worsen when the film went on thicken.The stability test proved the long-term chemical stability and mechanical strength of the acquired CZTS thin film.The cyclic voltammetry（CV）,electrochemical impedance spectroscopy（EIS）and Tafel-polarization measurements demonstrated that,the obtained CZTS thin film had better catalytic abilities,good bonding strength between catalytic materials and FTO substrate,and an acceleration of high electron transfer at the interface of CE/electrolyte.（4）Varieties of low-conductive pretreatment layer were chosen to improve the R_Ct value between the catalytic material and conductive substrate.The DSSCs efficiency based on Carbon/Graphite-Pre,Sn（E）-Pre and Cu（E）-Pre CE was increased to 6.26,7.35 and 6.78%,respectively.The higherηwas due to the considerable improvement in FF value（60%）for Carbon/Graphite-Pre CE and J_SC value（15.38 and 15.69mA/cm²respectively）for Sn（E）-Pre and Cu（E）-Pre CE.The EIS and Tafel curves indicated that the conductive layer can promote the elctron transfer at the interface of catalytic material and conductive substrate.Thus,the pretreated layer possessing higher conductivity was beneficial to improve that catalytic ability of the CZTS CE and it was a smart strategy to combine the high conductive and big specific surface area catalytic materials together through in-situ loading.（5）A facile solvothermal treatment combination of electrodeposition metal precursor was developed to in-situ grow semi-transparent CZTS nanostructure on FTO glass substrate and used directly as CE to assemble DSSCs.The pure kesterite-structure CZTS CE is obtained through solvothermal treatment at 200^oC for 6 h.The CZTS nanostructures can be adjusted by changing the solvothermal condition,in order to get different specific surface area and catalytic ability of CE.J-V curves demonstrated that a 7.09%efficiency is successfully achieved with“leaf-like”curly nanoplateCZTS CE,which is about 18%times higher than that of Pt CE-based DSSCs（η=6.01%）.The stability and mechanical peel-off test results showed that the efficiency retained 97 and 91%of the initial values,respectively,after 7 days storage and 10 times peeling off,suggesting the good chemical and mechanical stability of the prepared CE.The CV,EIS and Tafel polarization curves indicated that the synthesized CZTS nanostructure presented remarkable catalytic reduction activity,good chemical stability,good adhesion strength with substrate,abundant catalytic active site and fast electron migration at the interface of CE and substrate.Furthermore,this green and effective approach can be applied to construct metal sulfide or selenideon arbitrary substrates under mild condition and promotes the development in solar cells,photo/electro-catalysis,and electrochemical energy storage applications.（6）Inspired by the high conductivity of flexible carbon cloth and superior catalytic ability towards I₃^-of CZTS,the CZTS nanoplate arrays（NPLAr）were in-situ grown on flexible carbon cloth（CC）backbone uniformly,first via an electrodeposition method followed by solvothermal treatment.After solvothermal treatment,the well-alligned,interconnected and single-crystal CZTS NPLAr was well adhering to everycarbon fibre uniformly and employed as flexible CE directrly.The CZTS/CC CE-based DSSCs showed much better cell performance than blank CC substrate,with theηas high as7.53%as compared to theηof 2.91%demonstrated by the carbon cloth CE-based DSSCs.The synergistic enhancement of the cell performance based on this novel composite electrode is related to the unique structural features.Furthermore,it was important to note that the composite electrode demonstrated the advantages of flexibility and chemical stability toward the electrolyte.The designed three-dimensional CZTS/Carbon Cloth electrode would contribute to the fundamental research and technologies of flexible energy devices used in area of photovoltaics,photocatalytic hydrogen generation,supercapacitor,and lithium ion batteries.（7）The Cu₂ZnSnSe₄（CZTSe）thin film was prepared on FTO substrate by solvothermal treatment（200^oC for 24h）,while substituting sulfur with selenium powder.The thin film was composed of irregular plates and cluster particles,with plates and partcles presenting single crystal and polycrystal structure,respectively.Compared with CZTS CE,the J_SC of DSSCs assembled with the CZTSe CE was boosted to 17.32mA/cm²,and theηwas increased to 7.56%.The remarkable cell performance was due to the better conductivity of CZTSe material,lower electron transfer resistance between the catalytic material and liquid electrolyte,and better catalytic property.（8）Electronic structures of[112]-oriented Cu₃SnS₄（CTS4）and the adsorption of I₃^-complex on CTS4（112）surfaces were investigated by density-functional theory（DFT）calculations.The CTS4material exhibited a similar metallic characteristicwith Pt,and CTS4（1?1?2?）had higher surface and catalytic activity to let the I₃^-→I^-redox transfer.The precursor CuSn film was transformed to CTS4phase under solvothermal treatment for 24 h.The resulting CTS4 film presented a homogeneous and compact layer,consisting of polycrystalline CTS4 nanosheet.The CTS4 24h CE-based DSSCs showed much better cell performance than others,with theηas high as 7.80%as compared to theηof6.52%indicated by the Pt CE-based DSSC.The significantly enhanced percormance of the CTS424h CE was ascribed from the considerable enhancement in charge transfer at the CE/electrolyte interface,catalytic ability towards reduction of I₃^-,and bonding strength,indicating CTS4 an efficient and ideal cathode materal in DSSCs.