Investigation On The Effects Of B Interface Passivation And Sr Doping On The Performance Of Cu₂ZnSn（S,Se）₄ Solar Cells

It is well known that thin-film compound solar cells have become a new trend in the photovoltaic solar cell market due to their outstanding advantages such as low manufacturing cost,short energy recovery period and convenience for large-scale continuous production.At present,the most representative Cu（In,Ga）Se₂（CIGS）thin film cell has reached the highest power conversion efficiency of 23.35%.However,since Ga and In are both rare metals and their prices are high,the cost of batteries is high,making it difficult to produce and use sustainably.Cu₂Zn Sn（S,Se）₄（CZTSSe）of Kesterite is a direct band-gap p-type semiconductor,which has similar structure and performance to GIGS.Compared with CIGS,the preparation method of CZTSSe film is simple,the earth reserves of constituent elements are abundant,the price is low,and it is non-toxic.Therefore,it is considered to be an ideal semiconductor material that can replace CIGS to prepare solar cells.However,despite the great progress made in the research of CZTSSe cells in the past two decades,the current maximum certified efficiency is only 12.62%,which is far lower than the 23.35%of CIGS.Therefore,it is still very important to find the methods and technologies to improve the power conversion efficiency of CZTSSe solar cells.Studies have shown that:Compared with CIGS cells,the key scientific problem that limits the efficiency of CZTSSe solar cells is the small open circuit voltage(V_OC).This is mainly because:（1）the lattice mismatch and band arrangement mismatch of the front interface（Cd S/CZTSSe）and non-ohmic contact of the rear interface（CZTSSe/Mo）make photo-generated carriers recombine more at the interface.（2）Due to the small composition region of CZTSSe in single phase,it is inevitable that there will be secondary phase,defect state,tail state and poor crystallization quality,resuiting in short carrier life and large photo-generated carrier recombination.Therefore,looking for the method and technology to reduce carrier recombination is a key problem to improve V_OC.In this thesis,the following work is carried out to address the problem of large carrier recombination caused by lattice mismatch at the front interface and quality of the CZTSSe crystal:1.Improve the performance of solar cells by using boron（B）to passivate surface defects of Cu₂Zn Sn（S,Se）₄ thin films.This paper uses radio frequency magnetron sputtering technology to plate the passivation layer（B/CZTSSe）of boron（B）on the surface of CZTSSe,and control the thickness of the B layer by controlling the sputtering time（1-5min）.The structure prepared by traditional technology is Al/ITO/Zn O/Cd S/B/CZTSSe/Mo/SLG solar cell（denoted as CZTSSe-B solar cells）.The effect and mechanism of B layer on the photovoltaic performance of CZTSSe cells were studied.The results show that compared with the CZTSSe solar cells without B layer,with the increase of sputtering time（that is,the increase of the thickness of the B layer）,the photovoltaic parameters V_OC,FF and J_SC of the CZTSSe-B cell first increase and then decrease,while the electrical parameters J₀ first decrease,then slowly increase,R_Sh first increase and then decrease,R_S first slowly decrease and then rapidly increase.It is worth noting that the J₀ of all CZTSSe-B solar cells is one order of magnitude smaller than that of the CZTSSe solar cells.XPS tests show that B is paired with suspended electrons of metal atoms（Cu,Zn and Sn）and S（Se）atoms on the surface of CZTSSe.It is proved that B passivates the surface dangling bonds and reduces the interface state defects.Therefore,its efficiency increased from 2.17%of the CZTSSe solar cells without B layer to 4.22%.It shows that using appropriate B to passivate the surface of CZTSSe can greatly improve the conversion efficiency of CZTSSe cells.2.The effect of Sr doping on the performance of CZTSSe solar cells.In this thesis,Sr-doped CZTSSe thin films（denoted as CZTSSe-Sr）with different Sr content（0.20-1.32at%）were prepared by sol-gel combined with selenization technology.The traditional structure of CZTSSe and CZTSSe-Sr solar cells were prepared by using CZTSSe and CZTSSe-Sr as absorption layers,respectively.The effect of Sr doping on the photovoltaic performance and power conversion efficiency of CZTSSe solar cells was studied.The results of XRD,Raman and SEM shows that Sr may not be doped into the crystal lattice,but exists in the grain boundary.as the amount of Sr doping increases,the grain size of CZTSSe increases,the crystal quality becomes better,the surface secondary phase Zn Se decreases,and the contact with the back electrode becomes stronger,indicating that Sr may act as a co-solvent.However excessive Sr doping will cause delamination and voids in the CZTSSe film.By optimizing the doping amount of Sr,the highest power conversion efficiency（6.3%）of CZTSSe-Sr cell was prepared when the doping amount of Sr was 0.46at%.Compared with the CZTSSe solar cells prepared under the same conditions,the power conversion efficiency is increased by 2.23%.