Modification Of Cu₂Zn Sn（S,Se）₄ Semiconductor Photovoltaic Thin Film By Cd Doping And Optimization Of Device Heterojunction Interface

Cu₂ZnSn（S,Se）₄（CZTSSe）material is considered as an ideal material for photovoltaic absorber layer because of its green element composition,non-toxic reserves,adjustable band gap and high absorption coefficient.However,the record power conversion efficiency of CZTSSe solar cells（13.0%）remains significantly lower than that of CIGS（23.4%）and Cd Te（22.1%）devices,with the performance gap primarily being attributed to the defect of CZTSSe material and the CZTSSe/CdS interface.In this paper,the sol-gel method was used to make CZTSSe thin film solar cells,and the deposition process of CdS buffer layer was systematically studied.The CZTSSe absorption layer was doped with Cd in a variety of ways.Harmful defects were suppressed and the heterojunction interface quality was enhanced to improve the efficiency of solar cell devices.The following results were obtained.（1）The deposition process of CdS buffer layer was optimized,and it was found that thick CdS would absorb more photons at short wave below500 nm,and thin CdS could not completely cover the CZTSSe absorber layer.The optimal deposition time of CdS was 7 min and the thickness was50 nm.The uniformity of solar cell performance would be affected by the deposition temperature,and the optimale deposition temperature was 80°C.（2）Cd could be doped into CZTSSe absorption layer film by adding Cd directly into the precursor solution.It was found that Cd successfully replaced Zn doping into CZTSSe,resulting in lager grains on the absorber surface,and reducing the generation of small grains and harmful secondary phase ZnSe.With the increase of Cd doping,the band gap of CZTSSe solar cell decreased and the short-circuit current density increased,but the open-circuit voltage would decrease.Cd doping reduced the content of defects and defect clusters,which could alleviate the voltage loss caused by the band tail state.The optimal Cd doping concentration was 10%.The optimized Cd doping strategy by forming Cd concentration gradient precursors in the prefabricated layer could effectively improve the absorber layer quality,broaden the depletion width.In addition the compound resistance of the device was greatly increased,so that the recombination probability of carrier was reduced.The best efficiency of the solar cell was10.79%.（3）Selenization after deposition of CdS on the prefabricated layer could effectively achieve Cd doping in the CZTSSe absorber layer.The sputtering of Cu on the prefabricated layer before depositing CdS could effectively change the deposition growth pattern of CdS,and the CdS deposited on the Cu film showed a particle aggregation distribution,which increased the surface roughness of the prefabricated layer.On the one hand,the presence of Cu layer regulated the element ratio of the prefabricated layer more precisely,and on the other hand,it served as a carrier for CdS to better integrate Cd elements into the CZTSSe absorber layer.This condition could significantly increase the current,and the best device short-circuit current density reached up to 36.64 m A/cm².