Study On Fabrication Of Highly Efficient Cu₂ZnSn（S,Se）₄ Solar Cells From DMF Molecular Solution

With the aggravation of the energy problem,the development of new energy has been paid more and more attention.Solar cells are widely studied as a way of utilizing solar energy.Non-toxic and earth-abundant elements composed kesterite semiconductors（Cu₂Zn Sn（S,Se）₄,CZTSSe）are promising alternative absorber materials to CIGS and Cd Te for low cost and large area photovoltaic technology.Kesterite absorbers can be fabricated by both vacuum and solution methods.The solution method has the advantages of low-cost and large-area preparation,which can further reduce the cost.This thesis uses N,N dimethylformamide（DMF）as solvent and simple metal salts and thiourea（Tu）as precursor compounds to make molecular solution.The chemical reactions in the DMF solution,the reaction path from the solution to the precursor film and from precursor film to absorber film,and the grain growth of the absorber film during the selenization process were systematically studied.A CZTSSe device with an efficiency of 11.5%was obtained.The specific research contents are as follow:（1）The chemical reactions of the precursor compounds in the DMF solution and the reaction path from the solution to the precursor film.By using DMF as the solvent,Cu Cl,Zn（OAc）₂,Sn Cl₄and Thiourea（Tu）as Cu,Zn,Sn and S precursors,the chemical reactions of metal salts,especially Sn Cl₄,in DMF solution were studied.The results show that Sn Cl₄ reacts with DMF and forms complex Sn（DMF）₂Cl₄ whereas Cu Cl and Zn（OAc）₂ form complexes with Tu.The coordination bond（Sn-O）between Sn⁴⁺and DMF（（CH₃）₂NCO）in Sn（DMF）₂Cl₄ is weak and easily break during the subsequent heat-treatment,which results in an amorphous sulfide kesterite（CZTS）structured precursor film.（2）The reaction path and the grain growth from precursor to absorber film.Monitoring of the selenization process of the precursor film by X-ray diffraction,Raman spectra and scanning electron microscopy reveals the precursor film took a direct substitution reaction path and a top-down and bottom up bi-direction grain growth,leading to a CZTSSe absorber film with clean surface,large grain and few defects.The results show that the formation of kesterited structured CZTS precursor film is the key for the direct phase transformation reaction from precursor to absorber film.（3）CZTSSe solar cell from DMF.The effects of selenization time,selenization temperature,absorber thickness and heterojunction heat temperature on device performance and the stability of devices were studied.A champion CZTSSe solar cell with a photoelectric conversion efficiency of11.5%was achieved under the optimal conditions.The device is the highest efficiency CZTSSe solar cell fabricated from DMF molecular solution.In this thesis,several important issues on the preparation of CZTSSe absorber from DMF solution have been studied.The results demonstrate the coordination of Sn⁴⁺with a weak ligand that can easily de-coordinate is the key for the formation of a kesterite structured precursor film that can facilitate direct phase transformation grain growth and thus fabrication of a high quality kesterite absorber.The high-efficiency CZTSSe device obtained revelas the great potential of DMF molecular solution approach for commercial application.