Cu₂ZnSn?S,Se?₄ Thin Film Ultrasonic Spray Pyrolysis Preparation And Properties Study

Cu₂ZnSnS₄?CZTS?is considered as one of the most promising solar cell absorber materials owning to its optimal band gap and high absorption coefficient.Meanwhile,it has noticeable advantages of low material cost,non-toxic elements and earth-abundant sources for its photovoltaic application.At present,more and more attention has been paid to the preparation process of CZTS thin film.In view of the problems existing in the preparation of CZTS film by direct solution process,a safe,non-toxic,homogeneous and pure CZTS precursor ink was proposed in this thesis,and the ink properties,the thin films preparation process and properties were studied systematically,the main research work is as follows:Novel inorganic non-hydrazine polysulfided-based CZTS precursor ink was proposed,in which polysulfide ammonium was used as the single solvent,cuprous sulfide?Cu₂S?,elemental zinc?Zn?and tin disulfide?SnS₂?as metal source.The dissolution mechanism of the raw materials was analyzed,and the dissolution products of each component were extracted and identified.The result shows that Cu₂S,Zn and SnS₂ dissolved in ammonium polysulfide to form ammonium copper sulfide?NH₄CuS₄?,Zinc sulfide?ZnS?and tetraammonium thiostannate?IV?trihydrate??NH₄?₆Sn₃S₉·1.3H₂O?respectively.Then optimized CZTS ink was proposed by using NH₄CuS₄,ZnS nanocrystals with polysulfide ligands and SnS₂ as metal sources,ammonium sulfide aqueous solution as the single solvent.The result reveals that NH₄CuS₄,ZnS nanocrystals with polysulfide ligands,SnS₂ and CZTS precursor have low decomposition temperature,the corresponding thermal decomposition products are CuS,ZnS,SnS₂ and CZTS,which comfirms that the proposed ink is feasible for CZTS preparation.According to the characteristics of the CZTS ink,an ultrasonic spray pyrolysis deposition system was set up for CZTS films deposition.The physical process of spray pyrolysis deposition was analyzed,and the effect of substrate temperature and carrier gas flow rate on the deposition morphology was investigated.The result shows that,due to the surface tension of the droplet decreased with the increasing of temperature,the spreading area of the droplets enlarged with the increasing of substrate temperature,which contributed to the evaporation of the droplets,timely decomposition of the precursor and finally densification of the precursor thin films.When the substrate temperature was too high,the spreading area of the droplets decreased because of Leidenfrost effect,and resulted in poor compactness of the precursor thin films.Low carrier gas flow rate was conducive to droplets spreading and precursor thin films densification,while due to the cooling effect of strong jet,excessive flow rate resulted in small contacting area of the droplets,and poor compactness of the precursor thin films emerged.The effects of substrate temperature,spray scanning rate and annealing atmosphere on the properties of CZTS films were investigated.The study of substrate temperature shows that CZTS precursor lost its weight mainly from 240°C to 360°C,the crystallization and densification of the films were obviously improved by increasing the substrate temperature in this temperature range.As the substrate temperature further increased,the crystallinity and the compactness of the film were deteriorated due to the rapid thermal decomposition of the ink.The study of the scanning rate shows that the crystallinity and compactness of the films were improved with the increase of nozzle scanning rate.In the low scanning rate range,the crystallinity and densification of the films were improved with the increase of scanning rate.When the scanning rate reached a certain value,the improvement of crystallinity and compactness became no longer obvious although the increasing of scanning rate.The study of annealing atmosphere indicates that although the precursor thin film had single phase structure of CZTS,high saturated vapor pressure of SnS and S resulted in the loss of Sn and S elements when sulfur-free atmosphere annealing was employed.On the contrary,sulfur atmosphere annealing could reduce the loss of Sn and S,which could inhibit the decomposition of CZTS,so sulfur atmosphere annealing is necessary for crystallinity improvement and composition maintenance.Finally,novel CZTSSe ink was proposed,which was developed from the proposed CZTS ink.The ink was applied to CZTSSe precursor thin films deposition through the ultrasonic spray pyrolysis deposition system,and Se-free atmosphere annealing mechanism for CZTSSe preparation was discussed.In this ink,sulfide ammonium was used as the single solvent,NH₄CuS₄,ZnS nanocrystals with polysulfide-based ligand and SnS₂ as the metal source,while elemental selenium as the Se source.Since the ink is Se-containing and the Se element of the precursor films can be retained after pre-annealing,the CZTSSe film can be obtained by Se-free atmosphere annealing process.When the Se concentration of the ink increases from 0.025 M to 0.125 M,the S/?S+Se?ratio of the CZTSSe thin films drops from0.88 to 0.39,and the optical band gap dropped from 1.43 eV to 1.16 eV.