Study On Photovoltaic Performance Of CZTSSe Thin Film Solar Cells Based On Element Ratio Regulation And Buffer Layer Doping

The conversion of solar to electricity by particular devices shows tremendous potential in the settlement of energy crisis.Among these devices,Cu₂ZnSn（S,Se）₄ thin film solar cell has attracted extensive attention,owing to its direct band gap with various range from 1.0 to 1.5 eV,high light absorption coefficient up to 10⁴～10⁵ cm^-1,and the abundant element reverses in earth.At present,the maximum laboratory photo-electrical conversion efficiency already reached to 13.6%,however,which still far less than the Shockley-Queisser ultimate theory efficiency.Therefore,in order to elevate the device performance,it is important research topics in the domain all the time for CZTSSe solar cell to optimize the preparation method of device absorber and buffer layer.In this paper,based on the brief overview for the research status of the preparation and modification to the absorber and buffer layer of the CZTSSe thin film solar cell at home and abroad,the author systemically summaries the research work of"Study on photovoltaic performance of CZTSSe thin film solar cells based on element ratio regulation and buffer layer doping",during the study of the master’s degree,which mainly consists of the following parts:（1）Aiming at the metal chloride precursors,a novel strategy was put forward to improve the photoelectric performance of CZTSSe absorption layer by adjusting Zn/Sn Ratio,which ultimately promotes the photoelectric conversion performance of solar cell.Selecting the metal chloride as precursor solution and keeping the contents of Cu and Znconstant,the CZTSSe films with the various Zn/Sn ratio 0.8,1.0,1.2,1.4 and 1.6,respectively,were prepared by regulating the content of Sn.The research results suggest that as Zn/Sn=1.2,the secondary phases can be significantly inhibited or even eliminated in the absorber,and the growth of large grains of CZTSSe film was improved effectively.What’s more,compared with the stoichiometric ratio of Zn/Sn（Zn/Sn=1.0）with device performance PCE=7.49%,J_SC=28.15 mA/cm²,V_OC=454mV,FF=58.14%respectively,all the performance parameters of non-stoichiometric devices with Zn/Sn=1.2 were improved obviously(PCE=9.08%,J_SC=30.34 mA/cm²,V_OC=479 mV,FF=63.84%),and the maximum photoelectric conversion efficiency can reach 10.65%,V_OC increase to about 481 mV,and V_OC deficit reduce to 585 mV.The detail analysis to various characterization shows that the optimized Zn/Sn ratio is not only helpful to reduce the number and type of deep-level defects and defect clusters in the film,thus reducing the recombination of carriers,but also favourable for the formation of beneficial defects,thus making the concentration of carriers in the absorption layer increase and the width of depletion layer reduction.Meanwhile,the optimized Zn/Sn ratio can effectively reduce the tail effect.In addition,the Urbach energy and theV_OC deficit are reduced,thus improving the comprehensive photovoltaic performance of the device.This research can provide a solid experimental basis for further understanding of the influence of element ratio on the comprehensive performance of solar devices.（2）Element doping is one of the effective methods to optimize and modify buffer layer for the improvement of device performance.On account of the Cd S buffer layer,the thought of alkali metal doping was proposed and further studied the effect of Na doping on device performance.The Na-doped Cd S buffer layers were prepared by adding different concentrations of Na element into the chemical bath.The photovoltaic performance measurement shows that the relation of these devices parameters are strongly correlated with the Na amount doped.Compared with that with the undoped Cd S buffer layer,whose PCE=8.60%,J_SC=33.98 mA/cm²,V_OC=440 mV,FF=57.55%respectively,the performance of the assembled solar cells with Na doping all decreased in some extent.Moreover,with the Na doping concentration increasing from 0.01 mol/L to 0.05 mol/L,the photoelectric conversion efficiency decreased from 7.46%(J_SC=32.82 mA/cm²,V_OC=440 mV,FF=51.68%)to 6.64%(J_SC=31.81 mA/cm²,V_OC=420mV,FF=49.68%),and V_OC deficit increased from 620 mV to 640 mV.Furthermore,the particular analysis shows that Na element can lead to the depletion layer narrowing,at the same time,enhance the band tail effect,increase Urbach energy and carrier recombination,and deteriorate the device performance.This work is helpful to further understand the effect of Na doped Cd S buffer layer on the photovoltaic performance of CZTSSe thin film solar cell.