Composition Adjustment In Zinc Rich Copper-Zinc-Tin-Selenide Solar Cells And Its Effect On Photovoltaic Performance

Cu₂Zn Sn（S,Se）₄（CZTSSe）is a promising kesterite absorber material due to its low-toxicity,earth-abundant elements and widely studied in application in solar cells.The mainstream fabrication methods of CZTSSe solar cell are divided into solution method,spray pyrolysis method,magnetron sputtering post-heat treatment method and thermal evaporation method.As so far,CZTSSe solar cells prepared by magnetron sputtering post-heat treatment method have achieved a maximum efficiency of 12.62%and a maximum open circuit voltage of 541.1 m V.Furthermore,Cu₂Zn Sn Se₄（CZTSe）solar cells prepared by thermal evaporation and Cu₂Zn Sn S₄（CZTS）solar cells prepared by heat treatment method have achieved the highest efficiency of 11.6%and 11%,respectively.CZTSe solar cells require obviously further research to improve the photoelectric conversion efficiency.CZTSe absorber as a self-doped semiconductor material has a high concentration of intrinsic defects caused by the low intrinsic defect formation energy and behaves as a hole-conduct P type semiconductor by the main acceptor type Cu _Zn substitution defect.But excess Cu _Zn substitution defect might pin the Fermi level in the middle of band gap and thus stop the effective formation of inversion layer near the p-n junction interface.As a result,it might cause a low open circuit voltage due to a small band bending.Moreover,the Cu _Zn antisite defects could binding with other donor-type defects like Zn _Cu and Sn _Zn to form the defect complexes(Cu _Zn+Zn _Cu),(Cu _Zn+Sn _Zn)and(2Cu _Zn+Sn _Zn)in the Zn-poor and Cu-rich CZTSe thin films due to similar atomic sizes and chemical properties for Cu and Zn elements,leading to an electrostatic potential fluctuation and/or resulting the trapping and recombination of carrier.Moreover,it is reported that the increase of Zn in CZTSe absorber could decrease the(Cu _Zn+Sn _Zn)and(2Cu _Zn+Sn _Zn)to some extent.According to the research report,Ag could be used to replace Cu because Ag atom radius is about 16%larger in addition to the same chemical group as Cu.Ag alloying（Ag,Cu）₂Zn Sn Se₄（ACZTSe）absorbers might modify the disorder in the Cu-Zn sublattice,carrier density,defects,grain structure and morphology.At the same time,the Cu _Zn related defects are decreased and consequently the band tailing and possible Fermi level pinning could be supressed.In addition,further adjusting[Zn]/[Sn]ratios in Zn-rich ACZTSe could be helpful to the performance of solar cells due to a fine modification of the carrier density and defects.Therefore,it is very important to study Ag alloying ACZTSe solar cells to achieving the high efficiency CZTSe thin film solar cells.In this paper,we prepare different[Ag]/（[Cu]+[Ag]）ratios of Ag alloying ACZTSe solar cell by magnetron sputtering post selenization method to demonstrate that Ag alloying ACZTSe absorber has a profound influence on material properties such as carrier density,grain structure and morphology.The carrier density of ACZTSe solar cells could be effectively decreased by Ag alloying,thus increasing the corresponding depletion region width of ACZTSe solar cells.As a result,we obtain the highest photoelectric conversion efficiency of ACZTSe solar cell with[Ag]/（[Cu]+[Ag]）=10%,which increases the photoelectric conversion efficiency of CZTSe solar cells from 6.53%to 8.31%.Moreover,we use sputtering post selenization to prepare 10%Ag alloying ACZTSe solar cells with different[Zn]/[Sn]ratios.The carrier density in Zn-rich CZTSe absorber might increase due to the formation of a high population of Zn _Cu and Zn _Sn defects and thus reduce depletion region width（DRW）,causing a further drop in J_SCof the solar cells due to a possible tunneling effect for the moving electrons.Hence,we modify the carrier density and depletion region width of Zn-rich CZTSe solar cells by using 10%Ag alloying.Finally,we obtain the highest photoelectric conversion efficiency at the ratio of[Zn]/[Sn]=1.14,which increases the photoelectric conversion efficiency of ACZTSe solar cells from 7.68%to 8.68%.