Study On Fabrication Of Highly Efficient Cu₂ZnSnS₄ Solar Cells From DMSO Molecular Solution

As multi elements composed semiconductor materials,kesterite-structured Cu₂Zn Sn S₄（CZTS）,Cu₂Zn Sn Se₄（CZTSe）,and their alloy Cu₂Zn Sn（S,Se）₄（CZTSSe）have the characteristics of earth abundant elements composition,high absorption coefficient(10⁴ cm^-1)and tunable band gap（1.0-1.5e V）,which are considered to be promising thin-film solar cell absorber materials.Pure sulfide kesterite（CZTS）is more green and environmentally friendly than CZTSe and CZTSSe because it does not contain toxic element Se.In recent years,this semiconductor has received more and more attention.There are two methods for preparing CZTS thin film solar cells:vacuum method and solution method.Compared with the vacuum method,solution method has many advantages including lower preparation cost,precisely control of composition.In this thesis,we use the solution method to prepare CZTS thin film solar cells.The solution was made by using dimethyl sulfoxide（DMSO）as solvent,simple metal salts and thiourea as precursor compounds.The optimal sulfurization conditions of the precursor film were explored,and the grain growth mechanism of the CZTS film was studied.The CZTS was doped with Cd²⁺and Ag⁺through the precursor solution,and the effect of cation substitution on the grain growth mechanism of the absorber film was studied.The specific research content and results of this thesis are as follows:（1）CZTS thin film solar cells were prepared by DMSO solution method.The precursor solution was made by using DMSO as solvent and simple chemicals Cu Cl,Zn（OAc）₂,Sn Cl₄ and thiourea（Tu）as Cu,Zn,Sn and S sources.Through exploration of sulfurization time,sulfurization temperature and the amount of sulfur powder,a CZTS device with a photoelectric conversion efficiency of 7.0%was obtained,with a short-circuit current density of 18.8 m A cm^-2,an open-circuit voltage of 0.731 V,and a fill factor of 49.8%.The efficiency of this device is the highest value of CZTS solar cells for DMSO solution processed absorber.The phase analysis shows that the CZTS absorber film prepared by this method has a structure of densely packed large grains with a smooth and flat surface.（2）The grain growth mechanism of the DMSO solution processed CZTS absorber film.The grain growth mechanism of CZTS absorber films with different element compositions was investigated.It is found that under Cu-poor condition,Sn S₂ forms in early stage of the sulfurization which assists grain growth and leads to film with a large dense and large grain morphology.With the increase of Cu content,the Sn content decreases,and the Sn S₂ phase was not detected during sulfurization.The film takes direct phase transition path and forms smaller grain film with poor crystallinity.The effect of the element composition on device performance was further investigated and an optimal composition with Zn/Sn=1.2 and Cu/（Zn+Sn）=0.85 was obtained.After heterojunction heat treatment,the efficiency of CZST solar cell was increased from 6.4%to 9.0%under this composition.（3）The effect of Cd²⁺and Ag⁺cation substitution on the grain growth mechanism of absorber film.First,the effect of partial substitution of Zn by Cd on grain growth and device performance was investigated.It is found that Cd doping can not only improve the crystallinity of the film,but also reduce the formation of Zn S phase.With the increase of Cd doping concentration,the grain size first increases and then decreases.When the doping concentration is 20%,the grain size reaches the best,and the PCE of the device prepared at this concentration reaches 10.4%,an 35%enhancement compared with undoped one.When doping the absorber with Ag element,it was found that Ag doping did not change the grain growth process,but significantly improved the crystallinity of the film.The growth rate accelerates doping which reduces the sincering time required to achieve optimum device performance.In summary,in this thesis,CZTS thin film solar cells were fabricated by using DMSO solution as solvent and metal salt and thiourea as precursor compounds.Through the optimization of the sulfurization conditions and investigation of grain growth mechanism,CZTS solar cell with an efficiency of 9.0%was obtained.After Cd cationic substitution,device with and PCE of 10.4%was achieved.The results of this thesis only demonstrate a green solvent approach for CZTS thin film solar cells but also provides new strategies on further improving the performance of CZTS thin film solar cells.