Investigation On The Fabrication And Performance Of Cu₂SnS₃-Based Solar Cells

In recent years,world has focused on renewed and clean resource to resolve the increasing energy crisis and environment pollution.Solar energy has drawn worldwide attentions as an efficient and promising clean resource.As the disadvantages of Si,such as indirect band gap and small light absorption coefficient,people focus on the inorganic compounds thin film solar cells.GaAs,CdTe,Cu(InxGa1-x)Se2(CIGS)and Cu2ZnSn(S,Se)4(CZTSSe)based solar cells have a long-term development because of their properties of suitable band gap and high absorption coefficient.The efficiency of CZTSSe has achieved 12.6%recent years.In the research process of Cu2ZnSnS4,a new ternary Cu based conpounds,Cu2SnS3(CTS)has attracted extensive interest.It is noticed that Cu2SnS3 is also suitable for the absorption layer materials in solar cells because of the appropriate properties such as p-type conductivity,higher carrier mobility,high optical absorption coefficient(104-105)and direct band gaps tunable in(0.8-1.7eV).In addition,it is easier to control the composition of CTS film relative to the quaternary compound CZTS.As a result,CTS has been expected to be a promising absorption material for thin film solar cells.Three methods are developed to investigate the fabrication of CTS thin film solar cells and the properties of CTS thin film solar cells are studied in this thesis.In the first chapter,we review the history,development and mechanism of solar cells.The current research situations and problems in CTS thin film solar cells are also discussed in this chapter.In the end,we come up with the research topic and contents of this thesis.In the second chapter,we discuss the fabrication of CTS thin film solar cells by Chemical Bath Deposition(CBD)and the effects of sulfurization condition on the properties of CTS based solar cells.Especially,the grain growth mechanism of CTS thin film is investigated by rapid thermal process.We find that binary sulfide Cu2-xS forms at low temperature and decomposes to Cu8S5 with the increase of annealing temperature.Gradually,top binary sulfide layer disappears along with the generation of tetragonal Cu2SnS3 between 300? and 400?.The tetragonal Cu2SnS3 transforms to monoclinic CTS at about 430?,which indicates the monoclinic CTS is stable at high temperature.In the third chapter,a conventional method is introduced to deposit the CTS thin film by reusing the Cu2O and SnS powders obtained from the precipitate in CBD reaction.Cu2O and SnS powders are collected by centrifuging the CBD solution and cleaned by de-ionized water several times.Subsequently,they are dissolved in ethanol-ammonium solvent to obtain the Cu2O-SnS slurry.The precursor films obtained by doctor blade method are compressed under the pressure of 4MPa for 90s,which is beneficial to obtain compact and smooth films.Finally we investigate the sulfurization process and find that the morphology gets more compact and smooth with the increasing of annealing temperature and time.A compact and crack free CTS film is obtained under 600? for 20min with the grain size of micrometer scale.In the fourth chapter,two main researches are discussed:(1)Fabrication of the mixed oxides target and the CTS thin film solar cells;(2)Investigation of the properties of CTS thin film solar cells obtained by Pulsed Laser Deposition(PLD).In the first part,to get the CTO target,the resulted CuO/SnO2 powder after baking is compressed into pellet under the pressure of 10 MPa and sintered in air at high temperature.The deposition time by PLD is the first factor.If the time is too short,CTS films will be full of cracks and the thickness of annealed film is not enough.In the second part,the annealed conditions are studied and the optoelectronic performance of final solar cells are characterized by EIS and Hall measurement.The best efficiency of CTS solar cell is 0.69%obtained under 600? for 20min.However,the Voc is decreased while the annealed temperature is 600?.According to the data of EIS and SEM,there are some cracks in the films annealed under 600?,which would result in the increasing leakage current as a result of the direct contact between CdS and CTS layer.