Preparation And Photovoltaic Properties Of Flexible Cu₂ZnSn（S,Se）₄ Thin Films By Sputtering Method

In nowadays society,energy has penetrated into every aspect of human life.With long-term consumption of traditional fossil energy,seeking renewable and clean energy has become one of the important issues facing humanity in the 21st century.Solar energy is a kind of clean energy which is inexhaustible,properly collected and not affected by geographical factors.The efficiency of traditional crystalline silicon solar cell is high and its technology is mature.However,the large demand for raw materials and complex processes limited its wide application.Cu₂Zn Sn S₄（CZTS）is a new type of absorber material in thin film solar cell showing desirable optical and electrical property,environmental friendly and component abundant,which has been a new absorber with great potential among thin film solar cells in the future.After decades of development,CZTS thin film solar cells have gained great improvement on their power conversion efficiency.However,there is currently a large gap between theoretical power conversion efficiency and industrialization.The problems are mainly reflected in large open voltage deficit,stress of flexible CZTS thin films and band matching.Based on the above problems,a series of work was carried out in this thesis.In this thesis,Cu₂Zn Sn（S,Se）₄（CZTSSe）thin film was prepared by co-sputtering an element gradient precursor in vertical direction followed by selenization process.After optimizing the weight of selenium,selenization temperature and selenization time,efficiency of 5.31%in CZTSSe thin film solar cell was obtained on glass substrate.After that,mixed solution with the volume ratio of H₂O:HNO₃:HF=5:4:1 was used to chemically polish the flexible Ti to solve the roughness problem of the substrate.It was found that the oxide layer and scratch lines on the substrate surface could be effectively removed by mixed acid solution during polishing process.The surface roughness of substrate decreased from the original 59.20 nm to 8.6 nm after polishing 60 s.On one hand,the remove of oxide layer and the decrease of surface roughness could guarantee the ohmic contact property between thin film and back electrode.On the other hand,the increased grain of CZTSSe could effectively reduce the possibility of carrier recombination.Flexible CZTSSe thin film solar cell exhibited the best performance when the polishing time was 60 s,showing increased efficiency from1.4%to 2.19%.To solve the residual stress and open circuit deficit problem of CZTSSe absorber on flexible substrate,ANSYS finite element analysis software was used to simulate the stress distribution of“flexible Ti substrate/Ge transition layer/Mo electrode layer/CZTSSe”.It was found that the introduction of Ge transition layer could effectively relieve the thermal stress caused by the difference of thermal expansion coefficient.In this thesis,double Ge layers,namely Ge transition layer and Ge doped layer,were introduced into CZTSSe thin film.After calculating the grazing incidence XRD,it was found that the introduction of a Ge transition layer could effectively reduce the residual stress of CZTSSe thin film and increase the bonding force between thin film and substrate.The Ge transition layer would not diffuse into CZTSSe through the Mo layer during the selenization.However,excessively thick Ge transition layer would generate large internal stress during the nucleation process and increase the residual stress of CZTSSe thin film.The introduction of Ge doped layer could effectively promote the grain growth of CZTSSe,increase the optical band gap and suppress the transition from Sn⁴⁺to Sn²⁺in CZTSSe.In addition,the Ge doped layer could also effectively solve the band mismatch problem between CZTSSe and Cd S buffer layer.Finally,the efficiency of flexible CZTSSe thin film solar cell prepared with double Ge layers increased to 3.25%.To further reduce the residual stress of CZTSSe and improve the performance of flexible CZTSSe thin film solar cell,in this thesis,Sb layer was prepared by electron beam evaporation method between back electrode Mo layer and CZTS precursor,and the growth mechanism model of CZTSSe thin film under Sb doping was proposed.It was found that the introduction of Sb doped layer would not replace the lattice position of Sn in CZTSSe.During the selenization process,Sb could react with Se vapor to form quasi-liquid phase Sb₂Se₃.On one hand,Sb₂Se₃ could effectively promote the grain growth of CZTSSe,passivate the grain boundaries and reduce space radiation and non-radiation defects.On the other hand,the nucleation temperature of CZTSSe could also be reduced to achieve low-temperature preparation of CZTSSe and reduce its residual stress.Finally,Sb₂Se₃ completely volatilized with the increased selenization temperature.When the thickness of the Sb thin film was 20 nm,the electrostatic potential fluctuation of the CZTSSe thin film decreased from 115.9 me V to 60.7 me V.Subsequently,it was discovered that compared to Sb thin film,the use of Sb₂Se₃ thin film as a doping source could avoid the generation of heterogeneous phases of Cu₂Sb S₃ and Cu₂Sb Se₃ and the doping effect was better.Finally,flexible CZTSSe thin film solar cell prepared with Sb₂Se₃ doping layer showed the best photoelectric performance and its efficiency reached 4.0%.The antisite defect caused by Cu occupying Zn site in CZTS is the most common defect,which will cause serious open circuit deficit.Aiming at this situation,the substitution of Cd for Zn was realized by introducing a Cd S layer into the precursor to suppress the occurrence of Cu _Zn antisite defect.It was found that the introduction of Cd element could effectively promote the grain growth of CZTSSe,increase the hydrophilic property and improve the band matching between CZTSSe thin film and Cd S buffer layer.The contact angle of the thin film decreased from 67.8°to 51.5°and the CBO between CZTSSe and Cd S buffer layer increased from-0.70 e V to-0.20 e V when the thickness of Cd S thin film was 50 nm.Subsequently,the study of partially replacing Zn with Cd in CZTSSe thin film was carried out by depositing 50 nm Cd S above or below CZTS precursor.It was found that the Cd element was uniformly distributed in absorption layer when Cd S was located above CZTS precursor.However,Cd element will accumulate in the bottom of the absorption layer when Cd S was located under CZTS precursor.Flexible CZTSSe thin film solar cell with the best photoelectric performance can be obtained when the thickness of the Cd S thin film was 50 nm and its highest efficiency reached 4.41%.