Engineering Of Ag Graded Structure And Bulk/Interface Defects Elimination For CAZTSSe Solar Cells

Due to their similar crystalline structure as Cu（In,Ga）Se₂（CIGS）and presence of earth-abundant component elements,kesterite Cu₂ZnSn（S,Se）₄（CZTSSe）photovoltaic materials have been considered as an ideal substitute with particular potential for next-generation photovoltaics.To date,the highest photoelectric conversion efficiency of 12.6%has been achieved,however,it is still quite lower than that of CIGS（22.6%）.It is commonly recognized that the large open-circuit voltage(V_oc)deficit remains the dominant issue for the further development of CZTSSe photovoltaic technology.Generally,the intrinsic factors of this V_oc deficit in CZTSSe solar cells can be ascribed to two aspects:large carrier recombination in the bulk and at the absorber/Mo interface and a non-ideal band alignment at the absorber/buffer interface.The carrier recombination in the bulk absorber is mainly induced by high concentrations of Cu_Zn antisite defects due to the similar ionic radius and small chemical difference between Cu and Zn atoms.Moreover,the driving force for the diffusion of photo-generated electrons towards ITO becomes weaker in the quasi-neutral region near the absorber/Mo interface,and the recombination of electrons and holes at the Mo substrate finally leads to a notable V_oc loss.More importantly,high concentration of Cu_Zn defects at absorber/buffer will pin the Fermi level at the middle of the band gap,leading to an ineffective p-to-n type inversion near the CZTSSe/CdS interface and only a small band bending can be found in the absorber lead to a large V_oc deficit.One possible strategy to increase the V_oc of CZTSSe solar cells is partial substitution of Cu by Ag to decrease the concentration of Cu _Zn antisite defects.Different from the case in CZTSSe,the formation energy of the p-type Ag_Zn defects is higher than those of donor defects,such as Zn_Ag,Sn_Zn,and V_S,and the easier formation of these weak n-type defects in(Cu_1–xAg_x)₂ZnSn（S,Se）₄（CAZTSSe）will facilitate to retard Fermi level pinning at CAZTSSe/CdS interface.However,it has been proven that the electrical conductivity of CAZTSSe with high substitution content is always poor and the low carrier concentration cannot support an expected cell performance.On the other hand,a low substitution content of Ag can depress recombination process in the absorber,but it will have little influence on the Fermi level pinning improvement and the reported CAZTSSe devices still have a large V_oc deficit so far.Based on the present research status,we prepared the uniform Ag substituted CAZTSSe absorber via solution method to eliminate the formation of Cu_Zn antisite defect in bulk layer,efficiently depress the recombination process and improve the V_oc.Following,we developed a V-shaped Ag composition grading CAZTSSe solar cells with a higher Ag content on both the back and front surfaces of the CAZTSSe layer,efficiently depress the formation of Cu_Zn antisite defect and introduction n-type Zn_Agg defect,and finally lead to a higher V_oc.The main contents can be summarized as follows:（1）Engineering of bulk defects for Ag elemental precursor method CZTSSe solar cells:Low-cost elemental Cu,Ag,Zn,Sn,S,and Se powders were used as the starting materials and dissolved in a mixture of 1,2 ethanedithiol（edtH2）and 1,2-ethylenediamine（en）.The as-prepared CAZTSSe films with different Ag/（Ag+Cu）ratios were prepared subsequently.And the CAZTSSe thin films can be prepared with large grain and compact stacking when selenization at 480°C for 20 min.It is clear that the low annealing temperatures can obviously reduce the Sn loss during the selenization process.The depletion width（W_d）at the heterojunction interface of CZTSSe/CdS was found to be improved by Ag substitution,and the Cu_Zn defect concentration was gradually decreased.When Ag/（Ag+Cu）ratios was 3%,the V_oc is successfully enhancement about～50 mV,and the power conversion efficiency of 10.4%was obtained.（2）Engineering of bulk defects for Ag₂O precursor method CZTSSe solar cells:To obtained CAZTSSe solar cells with a high concentration of Ag,the precursor molecular solution of CAZTS was prepared by dissolving Ag₂O,Cu₂O,ZnO and SnO into ethanol,CS₂,1-butylamine and thioglycolic acid,and the CAZTSSe films were prepared with different Ag/（Ag+Cu）ratios.The bandgap of CZTSSe thin films were decreased initially and then increased due to band edge potential fluctuations and electrostatic potential fluctuations.In addition,the defect concentration and depletion width as a function of Ag content in CAZTSSe devices were systematically investigated.Finally,the V_occ of the solar cells was improved from 379 mV to 445 mV,and the efficiency of the CAZTSSe solar cells was achieved 10.7%。While as the electrical conductivity of CAZTSSe with high Ag substitution content is always poor,the performance of the devices was gradually degraded.（3）Engineering of Ag graded structure and bulk/interface defects elimination for CZTSSe solar cells:We developed a composition grading strategy to achieve a V-shaped Ag-graded structure with a higher Ag content on both the back and front surfaces of the CAZTSSe layer.The key advantages of this Ag-graded structure are as follows:the higher content towards the CdS/absorber interface can create weak n-type donor defects,leading to an effective p-to-n type inversion near the CZTSSe/CdS interface and retard Fermi level pinning,this effective type inversion finally leads to a large band bending and a higher V_oc maximum;whereas the lower content at the interlayer maintains the conductivity and light absorption;moreover,the other higher content towards Mo back contact can effectively suppress the recombination and improve the utilization of long-wave incident light.By appropriately adjusting the Ag gradient,we demonstrated a significant increase in V_oc,and an unexpected conversion efficiency of 11.2%was achieved.