Studies Of Cu₂MnSnS₄ Solar Energy Materials And Solar Cells

Copper manganese tin sulfide（Cu₂MnSnS₄,CMTS）all constituents are abundant and non-toxic elements in the earth’crust,and has high absorption coefficients(>10⁵cm^-1).Additionally,CMTS has crystal structure analogous to Cu（In,Ga）Se₂（CIGS）and Cu₂ZnSnS₄（CZTS）,which indicates that it has the possibility of being applied in the photovoltaic field.Therefore,CMTS material has been attracting much attention.So far,the reported highest power conversion efficiencies of the pure sulfide CMTS and mixed sulfo-selenide Cu₂MnSn（S,Se）₄（CMTSSe）thin film solar cells are 0.91%and 1.8%,respectively.However,they are much lower than the recorded efficiencies of CIGS and CZTS solar cells.Because there are many unknown basic physical properties and difficulties of the CMTS material preparation.Therfore,the basic properties and preparation method of CMTS thin films should be further explored.In this paper,the CMTS thin films are prepared by the sulfurization process of electroplated Cu-Sn/MnO₂ precursors.Based on the problems of the prepared CMTS thin films,the preparation paramaters are optimized gradually.At the same time,the effect of preparation parameters on the phase generation,composition,grain growth,structural distortion and optical properties of CMTS thin films and the performance of solar cells are investigated systematically.Finally,we succeeded in solving the problems of the uneven distribution of Sn and S elements,the Bi-layer structure and poor-Mn and rich-Sn component in the CMTS films,which increases the efficiency of solar cell to 1.44%.The innovative results are displayed as follows:1.The CMTS thin films are prepared by sulfurization process of electroplated Cu-Sn/MnO₂ precursors.Investigating the electrodeposition mechanism of Cu-Sn/MnO₂ precursors.Co-electroplating the Cu-Sn alloy layers to solve the problem of rough suface morphology which can be resulted in via electroplating Sn metal layer.To avoid the hydrogen evolution reaction,the MnO₂ layers are electroplated positive potential.The CMTS thin films are prepared by sulfurization process of the electroplated Cu-Sn/MnO₂ precursors.Using cyclic voltammetry（CV）to investigate the electrodeposition mechanism of Cu-Sn/MnO₂ precursor.The reduction potential of Cu²⁺shifts negatively with adding sodium citrate and potassium tartrate complexing agent into Cu-Sn system electrolyte.Co-electroplating the Cu-Sn alloy layers to solve the problem of rough suface morphology which can be induced by electroplating Sn metal layer.To avoid the hydrogen evolution reaction,the MnO₂ layers are electroplated in positive potential.In addition,it is found that the absolute value of the Cu-Sn layer deposition potential must be larger than that of MnO₂ layer or else the Cu-Sn layer would fall off.2.The effect of electrodeposition parameters（deposition potential,the deposition time of MnO₂ layer and Cu-Sn layer）on the electrochemical behavior of Cu-Sn/MnO₂ precursors deposition are studied.The composition,microstructure and optical property of CMTS thin films are investigated systematically,as well as the performance of solar cells.The research results show that the current density of Cu-Sn layer deposition decreases with the deposition potential negatively shifting,which suppress the surface reaction to some extent.The time-current（i-t）curves of MnO₂ layer deposition show the nucleation characteristics.The deposition potential of Cu-Sn/MnO₂ precursor and deposition time of MnO₂ layer and Cu-Sn layer significantly change the phase structure,microstructure and optical properties CMTS films,as well as the performance of solar cells.Finally,the optimal deposition potential,deposition time of MnO₂ layer and Cu-Sn layer are-1.20 V/+1.0 V,6 min and 12 min,respectively.The photoelectric conversion efficiency of CMTS thin film solar cells prepared under the optimal process parameters is 0.76%.However,all the CMTS thin films show the Bi-layer structure because of insufficient sulfurization,which is harmful to device performance.3.To solve the Bi-layer structure problem,the influence of post-sulfurization process parameters（temperature,heating rate and pressure）on the microstructure,optical and electrical properties of CMTS films are systematically investigated.Successfully solving the problem of the uneven distribution of Sn and S elements and the Bi-layer structure in CMTS thin film,which greatly improves the performance of solar cells.It is found that the increase of sulfurization temperature and decrease of heating rate can suppress the Cu₂MnSn₃S₈ phase formation,expand crystal plane spacing,release internal stress,reduce dislocation density and decrease the thickness of fine grains layer in the CMTS thin films.However,the secondary ion mass spectrometry depth analysis（SIMS）results of the solar cell shows the uneven distribution of Sn and S elements in CMTS absorber.By increasing the sulfurization pressure,the crystallinity of the bottomed film can be improved.The SIMS results of device shows that the Sn and S elements distribute evenly and the Bi-layer structure disappears in the CMTS absorber.After optimizing the post-sulfurization process,the efficiency of solar cell increases by 47%.The efficiency of CMTS solar cells prepared under the optimal sulfurization process parameters（560°C of temperature,3°C/min of heating rate and 300 torr of pressure）reaches to 1.12%.However,CMTS films shows the poor Mn and rich Sn composition,which damages the performance of soalr cells.4.The metal element ratio of CMTS films are controlled by tuning the Mn²⁺and Cu²⁺concentration of electrolyte,which solves the problem of poor-Mn and rich-Sn composition.And the influence of Mn²⁺and Cu²⁺concentration on the microstructure and optical property of CMTS thin films and the performance of devices are systematically investigated.Using CV and i-t curves method to study the electrochemical behavior of Cu-Sn/MnO₂ precursor electrodeposition with different Mn²⁺and Cu²⁺concentration.The increasing Mn²⁺concentration promotes the deposition of MnO₂ layer,increases the Mn content and reduces the Cu and Sn content in CMTS films.Moreover,as the Mn²⁺concentration increased,the surface morphology became smoother and denser.However,too high concentration of Mn²⁺would induce the formation of MnS phase and poor crystalline quality.When Mn²⁺concentration is optimized to 25 mM,the CMTS thin film presents the poor-Cu and rich-Mn composition and the solar cell shows the best performance.In addition,the increase of Cu²⁺concentration increases Cu content while decreases Mn and Sn content in CMTS films.On the premise of poor-Cu composition,optimizing Cu²⁺concentration can further improve the crystallinity of CMTS films.When the Cu²⁺concentration is 17.5 mM,the CMTS thin film shows poor-Cu and rich-Mn composition（Cu/（Mn+Sn）=0.88,Mn/Sn=1.05）,and the highest efficiency increases to 1.44%.5.Cu₂MnSn（S,Se）₄（CMTSSe）films are prepared by selenizing the Cu-Sn/MnO₂ precursors under（Se+S）atmosphere.The effects of selenization parameters on the microstructure and optical property of CMTSSe films are studied.Exploring the dependency between the Se/（Se+S）ratio and the CMTSSe films properties and devices performance.When the selenization temperature increases from 480°C to 540°C,the SnSe,MnSe and Cu_xSe phase gradually change into CMTSSe phase,and the crystalline quality of the films can be improved.However,when the selenization temperature increases to 570°C,the CMTS thin film decomposes and generates the Cu_xSe,CTSe,Cu₂MnS₂ phase.Additionally,the increase of selenization pressure suppresses the volatilization loss of S,Se and Sn,to some extent.When selenization pressure increases to 300 torr,the CMTSSe film decomposes which reduces the crystallinity.The Se/（Se+S）ratio of CMTSSe films is tuned by selenizing the Cu-Sn/MnO₂precursors with different contents S and Se powder,and the dependency between the Se/（Se+S）ratio and the CMTSSe absorber property has been investigated.It is found that the lattice constant enlarges linearly according to Vegard law with the increase of Se/（Se+S）ratio,which results in the expansion of cell volume,increases the interplanar spacing and decreases the grain size and band gap of the CMTSSe thin films.And when the Se/（Se+S）ratio is optimized to 0.31,the soalr cell exhibits best performance with the efficiency of 0.71%.