Alloying Improves The Electron Transport Properties Of Cu₃SnS₄ Films

Photovoltaic power generation is an important aspect of solar energy utilization.At present,the energy payback time of photovoltaic modules is continuously shortening,and the annual installation volume of the modules keeps increasing.Dye sensitized sol ar cells(DSSCs),which are low-cost,easy fabrication,and low requirement in production equipment,have received extensive attention and research.However,althrough the currently commonly used noble metal Pt counter electrodes(CEs)exhibit excellent electrocatalytic activity and electron transport performance,the mineral resources of Pt are scarce,and the CEs fabricated by it account for 50%of the cost of the entire DSSCs device,which severely limits the large-scale production and application of DSSCs device.In order to achieve Pt-free DSSCs,the research works were carried out on optimizing Cu₃SnS₄(CTS)CEs,which were systematically studied by motheds of XRD,Raman spectroscope,UV-vis spectrophotometer,SEM,TEM,AFM,solar simulator,electrochemical measurement system,simulation calculation of XRD,etc.The synthesis temperature and the formation process of orthorhombic petrukite CTS were studied in detail.It is found that the orthorhombic CTS takes the monoclinic Cu ₇S₄nanocrystals formed at low temperature(130?)as the nucleation center for nucleation and growth.The formation temperature is between 160 and 180?,and the secondary phase of Sn_xS_y is available at about 200?.The reaction solution is heated at 130?for30 min,then heated up to 180?for 30 min,or directly heated it at 160?170?for 60min,to obtain the orthorhombic CTS single phase.The synthesized orthorhombic CTS precursor powder was annealed at 550?for 15min,which transformed into tetragonal kuramite CTS film.The tetragonal CTS nanomaterial was used as CE of DSSCs to substitute noble metal Pt.It presented good electrocatalytic reduction activity for I₃^-in the electrolyte,but its electron transport performances of conductivity,electron transfer etc.,are lower than those of Pt CE,which leads to a low cell efficiency.In order to improve the performance of CTS materials in DSSCs,we learned some research from the similar Cu₂Zn SnS₄ materials system,and determined the relevant research on alloying CTS materials with Mn and Se elements.In the study of Mn alloyed Cu₂Cu_1-XMn _XSnS₄(CCMTS)CEs,it is found that the effects of Mn²⁺on the orthorhombic CTS crystals caused the precursor nanosheets to appear grain refinement,amorphization,and energy band narrowing.After high temperature annealing at 550?for 15 min,the orthorhombic CTS without Mn alloying transformed into a tetragonal structure,while CCMTS with different Mn content maintained the orthorhombic structure.Among the DSSCs,Mn alloyed orthorhombic CCMTS CEs showed better electrocatalytic activity.In addition,the amorphous layer on the outer layer of the precursor grains was converted into crys tals after high temperature annealing,which effectively improves the electron transport between grains(or at the crystal boundaries).As the thickness of the outer amorphous layer of the precursor grain increases,the series resistance of the corresponding DSSCs gradually decreases,and the electron lifetime gradually increases.As a consequence,the use of orthorhombic CCMTS CEs improved the power conversion efficiency of DSSCs by more than 20%,than that of the tetragonal CTS CE.In the study of selenium alloyed Cu₃SnS_4-XSe _X(CTSSe)CEs,after selenization annealing at 500?for 20 min,it is found that the Sn_xS_y impurity in the precursor can be effectively eliminated,and the pure tetragonal CTSSe CEs can be obtained.The CTSSe CEs of different thicknesses showed good electron transport performance and efficient electrocatalytic ability for the reduction of I₃^-in the DSSCs.The continuous and uniform CTSSe film was used as CEs,and its excellent electron transport performance enables the fill factor of the DSSCs to reach more than 70%,and the highest cell efficiency reaches7.57%.After the continuity of the film degrades,its electron transport performance becomes worse,and the fill factor decreases accordingly.