Fabrication Of Highly Efficient Silver Copper Zinc Tin Sulfur Thin Film Solar Cells From Precursor Solution: Research On Reaction Path,film And Device Properties

Kesterite copper zinc tin sulfoselenides（CZTSSe）are a non-toxic semiconductor materials with earth-abundant elements composition,high absorption coefficient(greater than 10⁴ cm^-1)and tunable band gap（in the range of 1.0～1.5 e V by adjucting S/Se ratio）.The theoretical efficiency of CZTSSe solar cells can reach 32.2%according to Shockley-Queisser calculation,so CZTSSe are ideal absorber materials for thin film solar cells.However,the current world power conversion efficiency of CZTS solar cells is only 12.6%,which is much lower than that of copper indium gallium selenium（CIGS）thin-film solar cells（23.35%）,mainly due to large open circuit voltage loss(V_OC,def).The main factors leading to large V_OC,def are deep level defects,secondary phases,band tailing and interface recombination.In CZTS absorber,Cu _Zn defects are easily formed due to similar ionic radii of Cu⁺and Zn²⁺.When Cu _Zn and Sn _Zn defects form highly concentrated[2Cu _Zn+Sn _Zn]defect clusters,serious band tailing will occur.The Ag alloying strategy can suppress Cu _Zn defects and band tailing by partially replacing Cu with Ag,and can improve the grain growth and properties of the absorber,and reduce V_OC,def.In this thesis,silver copper zinc tin sulfoselenide solid solution((Ag_x,Cu_1-x)₂Zn Sn（S,Se）₄,ACZTSSe,x=0～1)thin films with a whole concentration Ag were fabricated from dimethyl sulfoxide（DMSO）precursor solution with Sn Cl₄ as Sn precursor and Ag NO₃ as Ag precursor.High efficiency ACZTSSe solar cells with low V_OC,def are obtained.Further,the chemical reactions in DMSO solution,the reaction path from the precursor solution to the ACZTSSe solid solution film,the properties of the absorber and the performance of Ag alloying devices were systematically studied.Thesis is mailnly devicded into the folloing three parts.（1）The chemical reaction of precursor solution and the reaction path from the precursor solution to ACZTSSe solid solution were studied.The grain growth mechanism of the film is related to the chemical reactions in the precursor solution.Results show that Ag⁺coordinates with TU in DMSO solution,which is the similar to that of Cu⁺and TU.The precursor solution with Ag⁺forms silver copper zinc tin sulfide((Ag_x,Cu_1-x)₂Zn Sn S₄,ACZTS)precursor film after spin-coating and annealing,which directly converted to ACZTSSe absorber film by substitution of S by Se in the subsequent selenization process.The direct phase transformation grain growth achieves high quality ACZTSSe absorber.（2）The ACZTSSe solar cells with different Ag concent were fabricated and their photovoltaic performance was compared.It is found that the band tailing of ACZTSSe gradually decreases with the increase of Ag concentration.Best device performance was achieved with 5%Ag incorporation with open circuit voltage(V_OC)improved by 30 m V.Finally,the champion ACZTSSe device with a total area efficiency of 12.5%was successfully fabricated,and its V_OC reached 64.2%Shockley–Queisser limit.（3）Comparison of ACZTSSe（x=5%）with CZTSSe absorber.Through the analysis of the film morphology and element composition depth profile,it is found that Cd S entered the film inside during the chemical bath deposition（CBD）through the voids in CZTSSe absorber,which was blocked by the flat and dense grains of ACZTSSe.Further analysis of the defect characteristics of the two absorbers reveal that the concentration of Cu _Zn defects in the ACZTSSe film is significantly lower than that in the CZTSSe film.The high concentration of[2Cu _Zn+Sn _Zn]defect clusters is the reason accounting for the band tailing.Ag alloying suppresses the band tailing,indicating that the significant reduction of Cu _Zn defects also leads to the reduction of the defect clusters.In addition,Ag alloying also reduces deep-level Cu _Sn defects,which also reduces recombination.In this thesis,the ACZTSSe solid solution films with a whole range of Ag content have been fabricated from DMSO precursor solution with a controllable reaction path and a high efficiency ACZTSSe solar cells have been successfully achieved.The strategy developed in this thesis can also be applied to the doping or alloying of other metal ions into kesterite,which is significant for kesterite research community.