Study On Properties Of Cu₂ZnSn(S,Se)₄ Thin Films And Solar Cells

Cu₂ZnSn(S,Se)₄ is a five-element compound semiconductor material,which is considered as one of the most promising candidate materials for a new generation of thin-film solar cells due to its abundant and environment-friendly elements,adjustable band gap and high light absorption coefficient.In this paper,Cu₂ZnSn(S,Se)₄ absober and solar cells based on different precursor starting materials and In element doping were systematically studied,and the following research results were obtained:1.The effects of different anions in the starting materials on the properties of Cu₂ZnSn(S,Se)₄ absorber and the performance of the solar cell were systematically studied.The results show that chloride is more suitable to be the starting material of the precursor solution.By comparing the effects of different precursor solutions based on acetates and chlorides on Cu₂ZnSn(S,Se)₄ absorber,the importance of anions in the starting material was revealed.The experimental results show that,in the acetate-based absorber,although the formation of Mo(S,Se)₂ has been inhibited by the carbon residue in the small grain layer,but the growth of grains has also been inhibited.Moreover,the composition distribution of the absorber is not uniform.However,when chlorides is used instead of acetates,the organic residue in the absorber has been greatly reduced,and the distribution of elements also becomes uniform,which resulting in the improvement of the quality of grains.By comparing the photovoltaic performance of Cu₂ZnSn(S,Se)₄ solar cells based on acetates and chlorides,we have found that small grain layer of acetate-based absorber are rich in carbon,which leads to a large carrier recombination loss and high concentrations of defect states,and correspondingly the reduction of the J_SC and FF.The absorber of Cu₂ZnSn(S,Se)₄ based on chlorides has low carbon content and uniform composition,which avoids the reduction of J_SC and FF caused by the carbon-rich small grain layer,and the efficiency of the soalr cell can reach up to 6.58%.2.The effects of copper sources with different valence states on the properties of Cu₂ZnSn(S,Se)₄ absorber and the performance of solar cell were compared.The results show that Cu Cl is more suitable to be the copper source of precursor solution.By comparing the influence of CuCl₂ and Cu Cl as the copper sources in the precursor solution on Cu₂ZnSn(S,Se)₄ absorber,it has been observed that Sn⁴⁺can be found due to the incomplete REDOX reaction between Cu²⁺and Sn²⁺,which leads to the formation of low solubility complex Sn(DMSO)Cl₄ in the DMSO and poor stability of the precursor solution,while using Cu Cl as the copper source can effectively avoid this problem.GIXRD and SEM images show that the quality of Cu Cl-based absorber is significantly improved,and the small grain layer is almost eliminated.The J-V curves of Cu₂ZnSn(S,Se)₄ thin-film solar cells with different copper sources show that the Rs of Cu₂ZnSn(S,Se)₄ solar cells based on Cu Cl is smaller,and the J_SC has been greatly improved.Finally,the efficiency of the solar cell based on Cu Cl can reach 8.48%.3.The effects CdS buffer layer with different thickness on the properties of Cu₂ZnSn(S,Se)₄ solar cells were studied.The experiments show the film has good quality and optical properties when CdS layer is about 50 nm thick.The soalr cell based on 50 nm buffer layer has higher J_SC and lower EQE loss in the wavelength below 500 nm.The thickness of CdS films was controlled by changing the deposition time of CdS films.The morphology and optical characterization of CdS films show that with the increase of deposition time,the thickness and grain size of CdS films are gradually increased,but the optical transmittance is decreased.Through AFM test,it has been found that when the deposition time is 15 min,the thickness of CdS film is about 50nm,and the surface of the film is smooth and compact,with low roughness,no obvious voides and large particles.The J-V curves of Cu₂ZnSn(S,Se)₄ solar cells with different thickness of CdS buffer layer shows that when CdS is deposited for 15 min,the J_SC is the highest,and the EQE loss in the wavelength region below 500 nm is also lower.Eventually,the conversion efficiency of the soalr cell can reach 8.74%.4.The effects of In doping on the properties of Cu₂ZnSn(S,Se)₄ absorbers and the performance of solar cells were studied.The results show that a small amount of In doping can significantly increase the grain size and improve the FF of the solar cell.The concentration of In element diffused into the Cu₂ZnSn(S,Se)₄ absorber is controlled by adding ITO intermediate layer with different thickness between Mo back electrode and Cu₂ZnSn(S,Se)₄ absorber.SEM images show that a small amount of In doping can significantly increase the grain size.The J-V test of Cu₂ZnSn(S,Se)₄ solar cell shows that when the ITO is about 10 nm,a small amount of In doping can increase the carrier concentration,which reduces the R_Sand is beneficial to the improvement of FF.The final efficiency of the solar cell is improved to 9.22%(no antireflection layer).