Studies On Synthesis And Properties Of Cu₂MnSnS₄ Thin Film

Stannite copper manganese tin sulfide?Cu₂MnSnS₄,CMTS?materials has high absorption coefficients(>10⁴ cm^-1),and bang gap in the optimal light absorption range?1.0¹.5 eV?,and all constituents are abundant in the crust of the earth.Meanwhile,CMTS derives from Cu?In,Ga?Se₂?CIGS?by elements changing of In and Ga to Mn and Sn,and has similar structure as Cu₂ZnSnS₄,this indicates that it has the possibility of being applied to thin film solar cells.So far,the research on CMTS materials has focused on the bulk and magnetic properties,which is mainly due to the lack of research on the properties of CMTS and the preparation methods.Therefore,in order to apply CMTS materials to thin film solar cells,based on the sol-gel technology we systematically studied the basic key problems such as technology preparation,compositional regulation and microstructure change of CMTS film.And explored the influence of different synthesis methods on the physical properties of CMTS thin films.Finally,successfully fabricated the CMTS thin film solar cells.At the same time,the influence of metal elements doping?Zn and K?on the grain growth,structural distortion and optical properties of CMTS thin films were studied systematically.During these periods,the following innovative achievements has been obtained,as follows:1.Based on the conventional non-hydrolytic sol-gel reaction,use non-toxic or low toxicity organic solvents and metal salts as raw materials,the CMTS thin film was successfully prepared by sol-gel method combined with high temperature annealing process.Studied and revealed the synthesis and decomposition of CMTS sol,and the optimum process parameters of CMTS precursors were obtained on the basis of thermal decomposition.CMTS precursors were prepared by the combination of alkoxide sol and heat treatment.The thermal decomposition analysis of the precursor sol shows that the CMTS sol decomposition can be mainly divided into three stages,in the first two stages?range from room temperature to 280°C?,the solvent volatilization and poly-condensation reactions in the gel are predominant,after which the CMTS phase begins to form.This helps to determine the pre-anneal temperature in subsequent experiments.Pre-annealing temperature too low would lead to inadequate decomposition of the gel,and later inducing holes and gaps exist;pre-annealing temperature too high would lead to the oxidation and further decomposition of the precursor film,thus deteriorating the quality of CMTS film.At the same time,the pre-annealing time,the process parameters and the additive content in sol were analyzed and optimized.The structure characterization confirmed that the CMTS precursor was nano-crystalline,which was in low crystallinity.From the optical characterization,it is found that the band gap of CMTS precursor is about 1.7 eV,which is close to the optimum light absorption range.2.The effects of the main process parameters?annealing temperature and preservation time?of the direct rapid thermal annealing on the CMTS film's growth,structure and optical properties have been investigated.And explored the influence of the direct high temperature annealing process in the film preparation and the components constriction problems of CMTS.The results of sol thermal analysis shows that CMTS precursor films require further high-temperature treatment to satisfy high quality for the solar cell absorber layer.Therefore,a direct rapid thermal annealing process with low energy consumption and convenience was used.It was found that the increase of annealing temperature was conducive to the growth of CMTS thin films,and easy synthesize single-phase CMTS,and help to improve crystalline quality.At the same time,the Cu content in the CMTS thin film increase with increasing the annealing time and leads to the shift of the optical band gap.This is due to the hybridization of the orbital between the Cu atoms and the S atoms thus the valence band top position gradually moves upward as the Cu content increases.In addition,the annealing time impact the growth of CMTS film,which plays an important role in the quality of crystalline film and the release of internal stress.However,the rapid thermal annealing process also makes the samples in copper-rich state,which has a negative impact on the final device.Through the change of the copper content in the film,it is found that the content of Cu metal in the sol has limited ability to control the final film composition and can't completely eliminate the Cu-rich state brought by the direct rapid annealing process.3.The effect of sulfurization annealing on the properties of CMTS thin films and the devices performance were investigated.Through the study of composition,structure and morphology of the CMTS films,the CMTS physical phase can be effectively controlled by the sulfurization process.Finally,CMTS photovoltaic cells were fabricated and showed good photovoltaic properties.In the high temperature annealing process,the vacuum tube system was used for sulfurization,and the sulfurization parameters and heating rate were studied.The copper-rich and sulfur-poor condition of CMTS films can be effectively improved,and the defects of Cu _Sn and V_Sn can be reduced at the same time.Appropriate increase of annealing temperature and time can improve the film quality,excessive growth will have a serious impact on the film properties.The optimization of sulfurization rate can eliminate the Bi-layer structure during the film growth and promote the reaction of the sulfur powder during the process,so that the prepared particles reach the micron order and grown more uniformity and densely.The CMTS film grown on the Mo substrate has the lowest defect density and the maximum internal stress release,so Mo is used as the bottom electrode in preparing device.CMTS thin film solar cells were prepared in lab,which is also the first time reported in the literature and the photovoltaic conversion efficiency reaches 0.49%.At the same time,the characterization of interface and electron spectrum of the device cross-section shows that the contact between the CMTS layer and the bottom electrode is not ideal,which limits the further improvement of the device performance.The study on the composition of the solution shows that the growth of the grain depends on the sol,and the use of additives can stabilize the sol to produce absorber film with better quality.4.Cu2MnSn?SxSe1-x?4?CMTSSe?thin films were successfully prepared by selenizing the CMTS precursor films.The effects of process parameters on the properties of CMTSSe films were investigated in detail.By using different Se powder,the control of S/Se ratio was achieved,it is confirmed that the structure properties of CMTSSe films vary linearly with the S/Se ratio.The CMTSSe film can be synthesized by selenized the pre-prepared CMTS precursor film.The optimization of the selenization process showed that the quality of the CMTSSe films getting better with the increase of the selenization time and temperature,and the final device performance confirmed the improvement of films crystallization with the process parameters.The device with CMTSS film as absorber layer was fabricated and the best conversion efficiency of 0.36%was obtained.However,in the study of selenization time and temperature,it was found that the proportion of Se elements in all the samples did not exceed 10%,resulting in restriction on the properties of CMTSSe films with high Se ratio.Furthermore,the adjustment of sulfur-selenium ratio in the final CMTSSe film can be achieved by varying the content of Se powder and the pressure of selenizing pressure.It is proved that S and Se can exist in different ratios,and change from the CMTS to CMTSe.The properties of CMTSSe thin films are sensitive to the change of selenization process parameters.The increase of selenization pressure will greatly increase the crystallinity and grain size.This means that for CMTSSe,high pressure and optimized process are suitbale for fabricate high quality CMTSSe films.5.The effects of metal dopings?Zn and K?on the structure distortion,film growth and optical properties of CMTS and the device performance were studied.The results show that heavy-doping of Zn ions causes the structural phase transition of CMTS and inhibits the grain growth of Cu2?Mn1-xZnx?SnS4?CMZTS?films.And the band structure of CMZTS shifts due to the change of Zn/Mn ratios.The light-doping of K ions can promote the film growth,and effectively improve the electrical properties of the CMTS photovoltaic device.The doping of CMTS was easily achieved by using the advantage of sol-gel process.Firstly,the heavy-doping of Mn element with Zn in CMTS was studied.The results show that the incorption of Zn can inhibit the film grain growth and decrease the grain size.At the same time,due to the radius of Zn²⁺is larger than that of Mn²⁺,the diffraction peak in the XRD pattern shifts to the high angle,and the Raman A1 vibration mode shifts to high wave number.When the Zn doping proportion reaches 0.5-0.7,the structure of the CMZTS film changes from original CMTS-Stannite to CZTS-Kestrite.As the Zn element increases,the CMZTS band gap can be adjusted within the range of1.23-1.51 eV with an optical bending factor of 0.10,this is caused by the internal electron rearrangement induced by the difference in the outermost electron arrangement of Mn and Zn.Secondly,the structure change of CMTS thin film caused by K⁺doping was studied.Light-doping of K ions can promote the CMTS film growth,reduce the defect density in the layer,and improve the electrical properties of the device.That is,by increasing the grain size of the film to reduce the grain boundary density,thereby increasing the light absorption rate in the P-N junction and increasing the photocurrent.The CMTS highest cell device conversion efficiency reached 0.58%,in which the current density increased dramatically to 6.07mA/cm².However,when the doping amount of K⁺is over 1.0%,cracks and voids would appear during the film growth,and the device performance is deteriorated.