Preparation And Properties Of Cu₂ZnSnS₄ Thin Films And Solar Cells By Magnetron Sputtering

CZTS thin film,with the properties such as earth-abundant constitution,environment friendly and high optical absorption coefficient,attracts the attention of photovoltaic researchers.In this paper,Cu-Zn-Sn-S compound target and ZnS/GeSe/CdS target were used as for the preparation of CZTS based material.Cu₂ZnSnS₄?CZTS?,Ge,Se-doped CZTS?CZ?TG??SSe??and Cd-doped CZTS?C?ZC?TS?thin films were prepared by one-step sputtering method.The solar cells were completed by further depositing CdS as buffer layer and i-ZnO/AZO as window layer.Scanning electron microscopy,X-ray diffraction and Raman scattering were used to analyze the morphology and structure of the films.J-V and QE tests were used to analyze the photovoltaic properties of the devices.The optimized conditions for the preparation of the CZTS-based thin films and devices were investigated.The incorporation of zinc sulfide,germanium selenide and cadmium sulfide on the properties of CZTS thin films and performances of the solar cells were analyzed.The specific research content and conclusions are as follows:?1?The effects of Zn content and substrate temperature on the microstructure of CZTS thin films and the photovoltaic properties of solar cells were studied.The experimental results show that the increase of ZnS sputtering power leads to the increase of crystallinity and the increase of grain size.When the sputtering power is low,the prepared CZTS film contains Cu₂S,SnS₂and Cu₂SnS₃ impurities.As the sputtering power increases,the Cu₂S,SnS₂ and Cu₂SnS₃impurities gradually decrease.When the sputtering power is too high,ZnS phase precipitates in the prepared CZTS thin film.The increase of substrate temperature leads to the increase of crystallinity and grain size in the films.When the substrate temperature is low,the film has ZnS secondary phase.The increase of substrate temperature leads to the reduction of the ZnS phase.If the substrate temperature is too high,the CZTS film decomposes,resulting in Cu_2-xS phase in the film.As the substrate temperature increases,the band gap of the film decreases firstly and then increases,while the microstructure order increases firstly and then decreases.When the substrate temperature is 600°C,the film has the highest micro-structure order.The optical band gap of the film is 1.48eV.J-V analysis on the solar cell shows that with the increase of ZnS sputtering power,the open circuit voltage of the battery increased from 22mV to 234mV,and the short-circuit current density increased from 1.07mA/cm² to 1.95mA/cm².As the temperature of the substrate increases,the open circuit voltage of the solar cell increases from83mV to 234mV,and the short-circuit current density increases from 1.2mA/cm² to1.95mA/cm².?2?The effects of Ge and Se incorporation on the microstructure and the photovoltaic performance of the prepared CZ?TG??SSe?thin films and solar cells were investigated by changing the GeSe sputtering power.The microstructure analysis on the thin film reveals that the incorporation of Ge and Se increase the grain size of the CZ?TG??SSe?thin film and significantly improve the compactness and flatness of the thin film.With the increase of Ge and Se incorporation,the film transforms gradually from CZTS to CZGSe.The analysis on the solar cells shows that the incorporation of Ge and Se contributes to the improvement of the performances of solar cells.With the increase of Ge and Se incorporation,the open circuit voltage of the solar cell increases to 264mV and the short-circuit current density increases to1.97mA/cm².The cell quantum efficiency results show that the improvement of the microstructure of the thin film caused by the incorporation of Ge and Se leads to the increase of the carrier extraction in the whole spectrum of the cell,especially in the region of long wavelengthes.?3?The effect of Cd incorporation on the microstructure and the photovoltaic properties of the prepared C?ZC?TS thin films and solar cells were studied.The microstructure analysis shows that the Cd incorporation leads to the increase of grain size and increase of film flatness and compactness.With the increase of CdS sputtering power,the density of the defect clusters[V_Cu+Zn_Cu]in the film increases,and the density of[2Cu_Zn+Sn_Zn]decreases.When the sputtering power is higher than 15W,the CdS impurity phase precipitates in the film.UV-Vis characteristics showed that the optical bandgap of the film gradually decreased with the increase of the Cd incorporation.Under a low sputtering power,the microscopic order of the film first increases and then decreases.The analysis of solar cell performance showed that with moderate Cd incorporation by a sputtering power of 25W,the prepared solar cell has the open circuit voltage and the short-circuit current density of 318mV and 3.47mA/cm²,respectively.By changing the deposition process,such as increasing sputtering power,etching suface defect on the film,and regulating cooling process after film depostion,and by storing the device in the air for several days,the film quality can be improved further.The open circuit voltage of the solar cell was increased to 343mV and the short-circuit current density reached 6.43mA/cm².