CZTS Thin Film Solar Cells Prepared By Magnetron Sputtering Alloy Target

CZTS thin film solar cells have been attracted more and more attention in recent years owing to its abundant component elements,low cost and environmental friendly.The CZTS thin film is a direct band gap semiconductor material and has the optimum band gap of 1.45 eV for photovoltaic cells.In this paper,CuZn and CuSn alloy targets were used to prepare CZT precursor,and then the CZTS films were obtained by sulfurization.Then the CdS buffer layer was deposited on the CZTS absorber layer by chemical bath deposition.The intrinsic ZnO and ITO window layers were prepared by magnetron sputtering.Finally,a complete CZTS thin film solar cell was fabricated.The crystal structure,photoelectrical properties,the surface and cross-section morphology and composition of CZTS thin films were characterized by X-ray diffractometer,UV-Vis spectrophotometer,Raman scattering spectrometer,Hall measurement,field emission scanning electron microscope and energy spectrometer.The performance of CZTS thin film solar cells was tested by I-V test system and external quantum efficiency measurement.Finally,the quality of CZTS thin films and the performance of CZTS thin film solar cells are analyzed by combining the experimental data and theoretical analysis.The main contents of this paper are as follows:（1）The effect of sputtering period on CZTS films was investigated.The effect of sputtering period on the crystal quality,the surface and cross section morphology and the bandgap of CZTS thin films is discussed in detail.（2）The effects of different sulfurization conditions on CZTS films and their solar cells were investigated.The effect of different sulfurization conditions on the crystal quality,the surface and cross section morphology and electrical properties of CZTS thin films and the performance of CZTS solar cells are discussed.（3）The effects of different Zn contents on CZTS thin films and their solar cells were investigated.The effect of Zn content on the surface and cross-section morphology,the crystal quality and electrical properties of CZTS films and the performance of CZTS solar cells were investigated.Finally,chemical reagents have been used to etch CZTS films to optimize the absorber layer.The results of the study show that:（1）The secondary phase of Cu₂S exists in the CZTS thin films prepared by single sputtering period,and the CZTS films prepared by four sputtering periods have the best crystallinity.There are many pin-holes on the surface of CZTS thin films prepared by single period,while CZTS thin films prepared by four periods are denser and have similar grain size.There are many voids in the cross section of CZTS films prepared by single sputtering period.The longitudinal grain of CZTS thin films prepared by double period sputtering is relatively compact and uniform,but there are still large voids between Mo/CZTS.The grain of the CZTS thin films prepared by four sputtering periods is the most compact and uniform.In addition,the（αhν）2^hνcurves of CZTS thin films show that the band gap of CZTS films prepared by three different sputtering periods is 1.37 eV in single period,1.4 eV in double periods and 1.49 eV in four periods,respectively.（2）The crystal quality of the CZTS films prepared under the AS5 sulfurization condition is the best,while the Cu₂S secondary phase exists in the CZTS films prepared under the conditions of AS2,AS3 and AS7.The surface of CZTS films prepared by AS2,AS3 and AS7 has pin-holes and copper-rich CZTS grains.The carrier concentration of the CZTS films prepared under six different conditions is in the order of 10¹⁸¹0¹⁹.Finally,the CZTS thin film solar cells with an efficiency of 2.8%,3.15%,5.12%,5.44%,4.7%and 3.87%have been fabricated by using different sulfurization conditions（AS2-AS7）.（3）When the Zn/Sn ratio is controlled at 1.2 in the CZTS thin film,the CZTS thin film has a better crystal quality.When the ratio of Zn/Sn is greater than 1.04,the characteristic peak of ZnS secondary phase appears in the 348 cm^-1 position of the Raman spectrum.There are a lot of pin-holes on the surface of tin-rich CZTS thin films,but white fine grains and dots appear on the surface of CZTS films with the increase of Zn/Sn ratio.And the carrier concentration of CZTS films increases with the increase of Zn/Sn ratio.Finally,when the Zn/Sn content of the film is 1.2,the efficiency of the prepared CZTS thin film solar cells reaches 5.07%,which is the highest efficiency of the five groups of solar cells.After etching the absorber layer with dilute hydrochloric acid,the solar cell efficiency was raised to 5.41%.The band gap of CZTS thin film solar cell with two different treatments made by EQE spectra was 1.43 eV and 1.41 eV,respectively.