Effects Of Interface Modification And Absorber Layer Modulation On Cu₂ZnSn（S,Se）₄ Solar Cell Performance And Stability

The rapid development of society necessitates continuous energy support,and solar energy as a renewable source has gained widespread popularity.However,solar energy cannot be directly utilized in social life and must first be converted into electrical energy.The technology that enables this conversion is known as solar cells.The absorber layer is the core component of a solar cell.As a p-type semiconductor material,Cu₂Zn Sn（S,Se）₄（CZTSSe）has excellent optoelectronic physical properties such as the high abundance of constituent elements in the earth’s crust,high absorption coefficient(>10⁴cm^-1)in the visible light region and adjustable band gap（1.0～1.5e V）,which have attracted extensive attention from researchers.The power conversion efficiency（PCE）of CZTSSe thin-film solar cells has significantly increased from an initial value of 0.66%to 13.80%at present.However,this value remains significantly below its theoretical maximum efficiency of 32.8%.The Culn Ga Se₂（CIGS）solar cell with similar structure has been successfully commercialized,and the CZTSSe solar cell has a long way to go.One of the primary obstacles hindering the adoption of CZTSSe solar cells is their low PCE and stability at present.The main problems of low PCE of CZTSSe solar cells are:（1）the carrier recombination in the CZTSSe/Cd S heterojunction interface and CZTSSe/Mo back interface;（2）the high Cu _Zn antisite defect concentration in CZTSSe leads to small energy band bending at the front interface,which is not conducive to the improvement of device open circuit voltage(V_OC).A small amount of Ag doping increases the V_OC of CZTSSe solar cells,but reduces the short circuit current density(J_SC);（3）the deep energy level defects in the CZTSSe absorber layer are easy to form a recombination center,which increases the probability of carrier recombination.In terms of performance stability:Since the current optimal PCE is not very high,the research on the factors affecting the stability of CZTSSe solar cells is relatively lagging.Given the above problems,this paper carries out the following research work,and the specific contents and results are as follows:1.To solve the problem of large carrier recombination at the front interface（CZTSSe/Cd S interface）,we adopt the method of replacing the traditional Cd S film with Zn doped Cd S(Zn_xCd_1-xS)film to adjust the energy band matching at the front interface and reduce the lattice mismatch,thus reducing the interface recombination.It is found that the performance of CZTSSe solar cells is related to the existence of Zn in the Zn_xCd_1-xS buffer layer.When x is within the range of 0-0.26,Zn replaces Cd;In the range of 0.26-0.32,a part of Zn exists in the interstitial site of the Cd S lattice.This makes the lattice mismatch between CZTSSe and Zn_xCd_1-xS decrease with the increase of x in the range of 0-0.26,and increase with the increase of x in the range of 0.26-0.32.The conduction band offset（CBO）at the CZTSSe/Zn_xCd_1-xS interface is a positive"spike"type.With the increase of x from 0 to 0.32,the CBO increases from 0.11 e V to0.43 e V.The PCE of the final CZTSSe solar cell increases with the increase of x between 0-0.26 and decreases with the increase of x between 0.26-0.32.The PCE of CZTSSe solar cells increased from 5.00%to 7.73%,with an increase of 54.6%.The increase in PCE is attributed to the increase in V_OC and filling factor（FF）,while the J_SCdecreased.The increase of V_OC and FF mainly comes from the increase of shunt resistance(R_sh)and the decrease of reverse saturation current density（J₀）.The decrease of J₀ comes from the decrease of lattice mismatch at the front interface,resulting in the decrease of interface recombination.The decrease in J_SCis attributed to the increase in CBO.2.To solve the problem of J_SC reduction caused by Ag doping CZTSSe（CAZTSSe）,we can improve J_SC by adjusting the ratio of different valence Cu elements(Cu²⁺and Cu⁺)in the copper salts used in the preparation of the precursor solution to further improve the PCE of the device.The results show that when the ratio of Cu²⁺/(Cu⁺+Cu²⁺)is 50%,the PCE of CAZTSSe solar cells is improved compared with 0%.The main factor for improving PCE is the increase of J_SC.The capacitance-voltage（C-V）test results of CAZTSSe solar cells prepared under different nominal ratios of Cu²⁺/(Cu⁺+Cu²⁺)show that when the nominal ratio of Cu²⁺/(Cu⁺+Cu²⁺)increases from0%to 100%,the depletion layer width（W_d）first increases and then decreases,and when the nominal ratio is 50%,W_d is the largest.When the ratio of Cu²⁺/(Cu⁺+Cu²⁺)is50%,W_d becomes larger and the photogenerated carrier separation ability becomes stronger,so photogenerated current density（J_L）becomes larger and J_SC also increases.3.To solve the problem of large carrier recombination at the back interface（CZTSSe/Mo interface）,we adopt the method of inserting the Cu Se intermediate layer between CZTSSe and Mo electrodes to weaken the recombination.It was found that PCE increased from 7.52%to 10.09%by optimizing the thickness of the Cu Se intermediate layer.The increase in PCE comes from the increase of FF,J_SC,and V_OC.The increased FF,J_SC,and V_OC come from the decrease of J₀ and the increase of J_L.The experimental characterization and SCAPS-1D simulation results show that the decrease of J₀ is due to the formation of a passivation electric field and a high electron barrier due to the insertion of a Cu Se intermediate layer with appropriate thickness at the back of CZTSSe;The increase of J_L is due to the increase of CZTSSe/Cd S depletion region width.4.At present,Cu⁺salt and Cu²⁺salt have been widely used to prepare CZTSSe solar cells,but the effect of these two copper salts on the stability of the prepared CZTSSe solar cells has not been studied.In this paper,we studied the performance degradation process of CZTSSe solar cells(Cu⁺-cell and Cu²⁺-cell,respectively)prepared with Cu⁺salt and Cu²⁺salt under dry conditions for 249 days.It is found that the valence state of Cu element in Cu salt has an important influence on the stability of CZTSSe solar cells.From 0 to 73 day,the average PCE of Cu²⁺-cell increased by 13%,while the average PCE of Cu⁺-cell decreased slightly.From 73 to 249 day,the average PCE of Cu²⁺-cell decreased slowly to 84%of its initial average PCE,while the average PCE of Cu⁺-cell decreased to 5%of its initial PCE.It can be seen that compared with Cu²⁺-cell,the performance stability of Cu⁺-cell is poor.For Cu²⁺-cell,J_L,series resistance（R_s）,R_sh,and（ideal factor（A）,J₀）have an impact on the PCE decline;For Cu⁺-cell,the decline of PCE mainly comes from the sharp decline of R_sh and the sharp rise of J₀.