Double-ion-doping Strategy To Improve The Efficiency Of Cu₂ZnSn（S,SE）₄ Solar Cells

Cu₂ZnSn（S,Se）₄（CZTSSe）with Kesterite structure is a new type absorber layer material for solar cells.Compared with other types of materials,it has following advantages: first,it consists of earth abundant non-toxic elements which existed in the crust;second,CZTSSe is a direct bandgap semiconductor and the bandgap can be adjusted between 1 and 1.5 eV by changing the ratio of S/Se,which is closely to the optimal bandgap of solar cells;finally,the light absorption coefficient of CZTSSe exceeds 10⁴ cm^-1.The theoretical photoelectric conversion efficiency（PCE）of CZTSSe solar cell is as high as 33%,but the record efficiency of CZTSSe solar cell is only 12.6%,which is far from the theoretical value.Poor microstructure performance of the light absorbing layer and undesirable Cu_Znantisite defect have severely limited the further improvement of efficiency of CZTSSe solar cells.Ion doping strategy is an effective method to solve these shortcomings,and Li⁺,Ag⁺ and Cd²⁺doping can significantly improve the efficiency of CZTSSe solar cells.However,most of the previous reported works are focused on studying the effects of single ion doping in CZTSSe solar cells,and only a few works have reported double ion doping strategies.Therefore,we study the Li⁺,Ag⁺,Cd²⁺,Li⁺&Cd²⁺,and Li⁺&Ag⁺ ions doping in CZTSSe solar cells,and obtain the following research results:（1）The effects of Li⁺,Ag⁺ and Cd²⁺single ion doping on the efficiency of CZTSSe solar cells were studied.N,N-dimethylformamide was used as the solvent,CuCl and Cu（AC）₂?H₂O（1:1）as the copper source,ZnCl₂ and SnCl₂ as the zinc and tin sources,and thiourea as the sulfur source.These materials were used to prepare the CZTS precursor solutions.The CZTS as-prepared films were prepared by repeating the spin-coating and annealing processes in a glove box,and then the CZTSSe films were fabricated by a selenization process.After that,CdS,ZnO,ITO,and Ag were deposited to assemble the CZTSSe solar cells.The PCE of undoped CZTSSe solar cells was 6.18%,which was comparative with the reported works.Afterwards,the precursor solutions were doped by Li⁺,Ag⁺,Cd²⁺ions with a doping concentration of 1%（Li/（Cu+Zn+Sn））,5%（Ag/（Cu+Ag））,and 5%（Cd/（Zn+Cd））,respectively.Through XRD,Raman,SEM,and J-V tests,we find that ions doping can significantly affect the qualities of absorber layers and the grain size of CZTSSe became larger.In addition,the Li⁺ and Ag⁺ doping can eliminate the fine-grain layers of the CZTSSe films.At the same time,the device performances of the CZTSSe solar cells have also been largely improved.The efficiency of the devices doped by Li⁺,Ag⁺,Cd²⁺ions were 7.60%,7.99%,and 7.11%,respectively.Compared with the undoped CZTSSe solar cells,the efficiency has been improved by 22.9 %,29.3%,and 15.0%,respectively.（2）The effects of Li⁺&Ag⁺ and Li⁺&Cd²⁺co-doping on the efficiency of CZTSSe solar cells have been studied.According to the previous reported works,Li⁺ions doped CZTSSe on glass/Mo substrates can passivate grain boundary defects,Ag⁺ doping can reduce nonradiative bulk recombination,and Cd²⁺doping can alter the defect characteristics of acceptor states near the valence band.Therefore,Li⁺&Ag⁺ and Li⁺&Cd²⁺co-doping strategies can suppress grain boundary defects and improve the internal properties of the crystal at the same time.Through XRD,Raman,SEM and J-V tests,we find that the double ions co-doping has more significant effects on the CZTSSe light absorbing layer.The microstructure performances of the light absorbing layers have become better,and the efficiency of corresponding device is higher.The efficiency of Li⁺&Ag⁺ and Li⁺&Cd²⁺ions co-doped CZTSSe solar cells are 8.87% and 8.39%,respectively.Compared with the undoped CZTSSe solar cells,the efficiency of CZTSSe solar cells with Li⁺&Ag⁺ and Li⁺&Cd²⁺ions co-doping was increased by 43.5% and 35.8%,respectively,which was also significantly higher than that of single ion doped CZTSSe solar cells.