Research On The Influence Of Back Electrode Interface Modification On The Performance Of Cu₂ZnSn?S,Se?₄ Solar Cells

Cu₂ZnSn?S,Se?₄?CZTSSe?with a kesterite structure is a direct bandgap semiconductor.Due to its suitable optical bandgap,high absorption coefficient,earth-abundant composition and environmental friendliness,it is considered as an ideal absorbing layer material for the preparation of highly efficient and low cost solar cells.However,the highest power conversion efficiency?PCE?of CZTSSe-based thin-film solar cells is only 12.62%,which is far lower than its limit power conversion efficiency?32%?and the power conversion efficiency of Cu?In,Ga?Se₂ thin-film solar cells?22.9%?.Studies have shown that one of the main reasons for the low efficiency is that the irrational interface structure and the poor contact quality between the back electrode?Mo?and the absorption layer?CZTSSe?.As we all know,the current CZTSSe solar cells use metal Mo as the back electrode.Mo reacts with Se to form MoSe₂ layer during selenization.On the one hand,an excessively thick MoSe₂ layer causes the electrical contact between CZTSSe and Mo to deteriorate,resulting in an increase in series resistance?R_S?or a decrease in shunt resistant(R_Sh),thereby reducing the power conversion efficiency?PCE?of the cell.On the other hand,due to MoSe₂ is n type semiconductor,which forms a back electric field in contact with p-CZTSSe.This back electric field will hinder the movement of photo-generated holes to the Mo electrode,promote the interface recombination of electrons and holes,and cause the PCE of the cell to decrease.Therefore,finding a suitable back electrode material or a method to avoid the formation of MoSe₂ layer has become one of the vital contents in the research of CZTSSe solar cells.In order to solve this problem of the interface of the back electrode of CZTSSe solar cell,this paper carried out the research work of using p-type WSe₂ as the back electrode passivation layer and using metal W instead of Mo as the back electrode to improve the PCE of the CZTSSe solar cells,and obtained the following research results:?i?We used magnetron sputtering to sputter 20 nm thick metal W on Mo-coated soda lime glass?SLG?substrate,and then in the CZTSSe thin film preparation process,metal W was completely selenized to achieve a p-type WSe₂ passivation layer was inserted between the back electrode Mo and the CZTSSe absorption layer to prepare a solar cell having a structure of Al/ITO/ZnO/CdS/CZTSSe/WSe₂/Mo/SLG.Studies have shown that WSe₂ can effectively inhibit the formation of MoSe₂.Compared with the traditional structure of Al/ITO/ZnO/CdS/CZTSSe/MoSe₂/Mo/SLG solar cells,WSe₂ replaces MoSe₂,which makes the open circuit voltage(V_OC),short circuit current density(J_SC)and fill factor?FF?have all been significantly improved,with PCE from 4.13%to 5.45%.The increase of V_OC and J_SC is mainly due to the increase of photo-generated current density?J_L?and the decrease of reverse saturation current density?J₀?,while the increase of FF is due to the increase of R_Sh.The increase in J_L was proved to be the improvement in the quality of CZTSSe crystals due to the insertion of the WSe₂ layer,leading to a decrease in N_B and an increase in V_bi and W_d.The decrease of J₀ is due to the formation of a passivation electric field caused by the contact between p-WSe₂ and CZTSSe,which promotes the transport of holes to the back electrode and reduces the recombination of holes and electrons.The increase in R_Sh may be due to the higher quality of the CZTSSe crystal,which leads to a decrease in the leakage current of the p-n junction and the back interface.?ii?Because of the poor adhesion of metal W on soda lime glass?SLG?,it is difficult to directly and firmly adhere to the SLG to form W back electrode.Therefore,in this thesis,a 10 nm thick metal Cr is sputtered on the SLG substrate,and then 900nm thick W layer is sputtered on the Cr to prepare W back electrode.A solar cell having a structure of Al/ITO/ZnO/CdS/CZTSSe/WSe₂/W/Cr/SLG was prepared by a method of partially selenizing W in the preparation of CZTSSe.Compared with the solar cell with the conventional structure using Mo as the back electrode,the V_OC,J_SCC and FF of the solar cell with W as the back electrode are increased,thereby increasing the PCE from 3%to 3.84%.The increase in V_OC and J_SC is mainly due to the increase in photo-generated current density?J_L?,while the increase in FF is due to the increase in R_Sh and the decrease in R_S.The increase in J_L was proved to be the improvement of the CZTSSe crystal quality due to the insertion of the WSe₂ layer,resulting in a decrease in N_B and an increase in W_d.The increase in R_Sh can be attributed to the decrease in p-n junction and back interface leakage current caused by the higher quality of CZTSSe crystals,and the decrease in R_S can be attributed to the W resistivity being smaller than Mo.