Electrodeposition Preparation And Physical Properties Of Cu₂ZnSnS₄Photovoltaic Films

Solar energy is one kind of renewable, inexpensive and clean energy resources. It can helphumankind to relieve or even solve energy shortage and environmental pollution on condition that thesolar energy can be transformed to electric power by means of solar cells. Therefore, the solar cellmaterials have attracted extensive attention. Cu₂ZnSnS₄（CZTS）with clean, safe and environmentallyfriendly components, is a highly promising absorber for solar cells, which possess direct energy gap（about1.5eV）, extraordinary high absorption coefficient (104cm^-1-10⁵cm^-1) and high photoelectricconversion efficiency. However, study of preparation and performance for CZTS is still at primarystage by now. Key factors on CZTS are how to reduce its production cost, meanwhile, improve thephotoelectric conversion efficiency. Electrodeposition method for preparing CZTS thin films has a lotof advantages, such as simple equipments, low-cost raw materials, large-area preparation and so on.Then, CZTS thin film solar cells can be industrialized widely in the future. Therefore, this workfocuses on investigating the electrodeposition mechanism for precursors, electrolyte formula,electrodeposition process parameters, and effects of the sulfidizing or annealing process on thesynthesis of CZTS thin film. Moreover, electronic structures, optical, mechanical and thermodynamicproperties of the CZTS compound are calculated using the First-principles calculstions.The CZTS films are successfully synthesized by using a process of sequentially electrodepositedCu-Sn-Zn precursors on Mo and FTO substrates, succeeded by annealing in saturated sulfuratmosphere. The Cu/Sn/Zn sequence is found to be the best one through the electrodepositionmechanism analysis. The layer and uniform precursor with good proportion of ingredient can beachieved on Mo substrate using-0.6V for5min in Cu electrolyte,-1.2V for2min in Sn electrolyte,and-1.36V for10imn in Zn electrolyte, and on FTO substrate using-0.9V for5min in Cu electrolyte,-1.35V for0.5min in Sn electrolyte, and-1.6V for4min in Zn electrolyte, respectively. Layerprecursors firstly alloy into Cu₆Sn₅and CuZn at low temperature. Then Cu₆Sn₅and CuZn alloysdecompose in sulfur atmosphere, and CuS, SnS and ZnS are formed. Cu₂SnS₃forms through reactionbetween CuS and SnS with increasing temperature. Finally, the CZTS film is synthesized throughreaction among binary and ternary sulfides. CZTS films synthesized in sulfur atmosphere at550℃for1h have morphology of polyhedral crystals, grain along the （112） crystal plane orientation growth,and average concentration ratio of0.97and1.0for Cu/（Zn+Sn） and Zn/Sn, respectively, which is inagreement with CZTS stoichiometry. The synthesized temperature and time are higher and longer on FTO substrate than that on Mo substrate under the same condition. Temperature of synthesized CZTSis higher in H2S atmosphere than in sulfur atmosphere on FTO substrate. The energy gap （1.54eV） ofCZTS film synthesized in pure sulfur atmosphere is consistent with that （1.52eV） in H2S atmosphereat550℃for1h.The CZTS films are successfully prepared using a process of co-electrodeposited Cu-Sn-Znprecursors on Mo and FTO substrates, succeeded by annealing in saturated sulfur atmosphere.Electrolyte formula and process parameters are optimized by controlling variables method. The goodternary precursor of suitable proportion of ingredient can be achieved on molybdenum substrate at-1.62V for5min using an100ml electrolyte containing0.40g CuSO₄·5H₂O,0.96g ZnSO₄·7H₂O,0.18gSnCl₂·2H₂O,1.34g NaOH,3.26g C₆H₅Na₃O₇and2.28g C₄H₆O₆. The ternary precursor of satisfactoryproportion of ingredient can also be deposited on FTO substrate at-2.2V for5min using a100mlelectrolyte containing above-mentioned same compositions except for0.56g CuSO₄·5H₂O. Cu3Sn,Cu₆Sn₅and Cu4Zn alloys are firstly synthesized at low annealing temperature. Then, these alloysdecompose in sulfur atmosphere and CuS, SnS and ZnS are formed. Cu4SnS6is formed throughreaction between CuS and SnS with the temperature rise. Finally, the CZTS films are synthesizedthrough reaction among binary and ternary sulfides. CZTS films synthesized in pure sulfuratmosphere at550℃for1h have an average concentration ratio of0.96and1.1for Cu/（Zn+Sn） andZn/Sn, respectively, which is in agreement with CZTS stoichiometry, and energy gap with about1.62eV.The CZTS films are successfully synthesized using a process of co-electrodeposited Cu-Sn-Zn-Sprecursors on Mo and FTO substrates, succeeded by annealing in pure nitrogen atmosphere.Electrolyte formula and process parameters are also optimized by controlling variables method. Thequaternary precursors with fine proportion of ingredient can be deposited on Mo and FTO substrate at-1.2V and-1.3V for5min by using a100ml electrolyte containing0.30g CuSO₄·5H₂O,0.40gZnSO₄·7H₂O,0.31g SnCl₂·2H₂O,0.40g Na2S2O3·5H₂O,0.34g NaOH,3.26g C₆H₅Na₃O₇and2.28gC4H6O6. Concentrations of Cu²⁺and Sn²⁺not only influence deposition rate of themselves, but alsochange deposition rate of other elements in quaternary electrolyte. However, Zn²⁺concentrations onlyinfluence itself the deposition rate in quaternary electrolyte. The binary sulfides in precursortransform into ternary and quaternary sulfides during annealing. Precursor annealed in nitrogenatmosphere at550℃for1h transform into CZTS film withan average concentration atom ratio of23.72:12.22:13.07:50.99for Cu:Zn:Sn:S, and energy gap of about1.6eV.Through comprehensive comparison of the three kinds of synthetic methods, metal composition using sequentially electrodeposited method is more stability than that using ternaryco-electrodeposited method. The crystal grain of CZTS synthesized using a process of sequentiallyelectrodeposited Cu-Sn-Zn precursors, succeeded by sulfurizing is bigger than that using a process ofco-electrodeposited Cu-Sn-Zn precursors, succeeded by sulfurizing, which is favorable to improvephotoelectric performance of CZTS films. Electrolyte used for co-electrodeposition of Cu-Sn-Zn-Sprecursors is usually unstable, and uniform and compactless, resulting at the quality of preparedCu-Sn-Zn-S precursors are relatively poor. Therefore, the preparation process using metal Cu-Sn-Znprecursors, and succeeded by annealing in saturated sulfur atmosphere has more industrial prospectsthan the process using co-electrodeposited Cu-Sn-Zn-S precursors, and succeeded by annealing inpure nitrogen atmosphere.The electronic structures, optical, mechanical and the thermodynamics properties of the KS-typeand ST-type CZTS have been calculated by using First-princinples calculations and the quasiharmonicDebye model. The theoretical calculations show CZTS is a direct bandgap semiconductor material.The average absorption coefficient is more than104cm1, reflectivity and electrical conductivity islow in the visible wavelength ranges, which are all in agreement with the experimental value. Andenergy loss is close to0for CZTS. The elastic constants of the KS-type and ST-type CZTS obtainedfrom calculation meet both their mechanical stability conditions. The bonding strength along the [100]and [010] direction is the same to that along the [001] direction and the shear elastic properties of the{001} plane are anisotropic for CZTS. Both compounds exhibit ductile behavior due to their high B/Gratio. The value of thermal capacity is close to200J/mol·K at above300K, and the thermal expansioncoefficients decrease with increasing pressure at same temperature. The entropy is variable bypower-exponent, and the internal energy is almost linear when increasing temperature for CZTS. TheGibbs energy of CZTS decreases with increasing temperature at same pressure. The Debyetemperatures are338K and297K, and Grüneisen parameters are2.50and2.36for KS-type andST-type CZTS at300K, respectively.