【作者】 贺显聪；

【导师】 沈鸿烈；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2012， 博士

【摘要】 太阳能是可再生、廉价和清洁的能源之一，如果把太阳能转化为电能可以缓解甚至彻底解决人类面临的能源短缺和环境污染两大问题，太阳电池正是实现该目标的重要手段。因此，太阳电池材料的研究受到人们广泛关注。Cu₂ZnSnS₄（CZTS）化合物直接带隙约1.5eV，具有很高的光吸收系数(104cm^-1-10⁵cm^-1)及光电转换效率，被公认为是最具潜力的清洁、安全和环保的太阳电池吸收层材料。但是，目前对CZTS化合物的制备和性能研究尚处于初期阶段，如何降低其制造成本和提高其光电转换效率是当前的研究热点。采用电沉积方法制备CZTS薄膜具有所需设备简单、原材料成本低和容易大面积成膜等优点，极具工业化前景。因此，本文重点研究了电沉积预制层的机理、溶液配方、电沉积工艺参数及后续硫化或退火工艺对合成CZTS薄膜材料的影响，并应用第一性原理的方法计算了其电子结构、光学、力学和热力学等性质。采用分步电沉积法先制备层状金属预制层，然后通过后续硫化处理成功合成了CZTS薄膜。通过电沉积机理分析三种金属预制层最佳电沉积顺序为Cu/Sn/Zn。Cu、Sn和Zn预制层在FTO衬底上比在Mo衬底上电沉积电位都更负。通过工艺优化，在Mo衬底上Cu、Sn和Zn分别用-0.6V、-1.2V和-1.35V电位分别电沉积5min、2min和10min得到了较好成分比例和均匀的层状预制层；在FTO衬底上Cu、Sn和Zn分别用-0.9V、-1.35V和-1.6V电位分别电沉积5min、0.5min和4min也得到了较好成分比例和均匀的层状预制层。金属预制层低温下合金化易生成Cu₆Sn₅和CuZn相，硫参与反应后这些合金化合物分解并形成二元硫化物CuS、SnS和ZnS，随着温度的升高二元硫化物相互反应形成三元硫化物Cu2SnS3，最终二元和三元硫化物间相互反应转变为四元的Cu₂ZnSnS₄。研究发现预制层于550℃硫化1h合成的CZTS薄膜晶粒呈多面体形态，沿（112）晶面择优取向生长，且平均Cu/（Zn+Sn）和Zn/Sn分别为0.97和1.0，与CZTS化学计量比接近。对比不同衬底的结果发现在FTO衬底上比在Mo衬底上合成CZTS所需温度提高，时间延长。研究结果发现金属预制层在H2S气氛下比在纯硫气氛下硫化合成CZTS的温度要高。纯硫气氛中550℃硫化1h合成的CZTS薄膜禁带宽度约为1.54eV；H2S气氛中550℃硫化1h合成的CZTS薄膜禁带宽度约为1.52eV，两者基本是一致的。使用三元共电沉积法先制备均匀的金属预制层，然后通过后续硫化处理成功合成了CZTS薄膜。通过控制变量法优化出溶液配方及工艺参数。利用含0.40g CuSO₄·5H₂O、0.96gZnSO₄·7H₂O、0.18g SnCl₂·2H₂O、1.34g NaOH、3.26g C₆H₅Na₃O₇和2.28g C₄H₆O₆的配方溶液，在Mo衬底上用-1.62V电沉积5min得到了较好成分比例和均匀的三元共沉积金属预制层。其它成分不变，调整CuSO₄·5H₂O为0.56g的配方溶液，在FTO衬底上用-2.2V电沉积5min也得到了较好成分比例和均匀的三元共沉积金属预制层。三元金属预制层在低温下元素间合金化易生成Cu₃Sn、Cu₆Sn₅和Cu₄Zn等化合物，与硫蒸气反应先形成二元硫化物CuS、SnS和ZnS，随着温度的升高二元硫化物相互反应形成三元硫化物Cu4SnS6，最终二元和三元硫化物间相互反应完全转变为四元的Cu₂ZnSnS₄。纯硫气氛中预制层经550℃硫化1h合成的CZTS薄膜平均Cu/（Zn+Sn）为0.96，平均Zn/Sn为1.10，与CZTS的化学计量比相接近，其禁带宽度约为1.62eV。利用四元共电沉积法先制备预制层，然后将预制层退火成功合成了CZTS薄膜。通过控制变量法优化出四元溶液配方及工艺参数。利用含0.30g CuSO₄·5H₂O、0.40g ZnSO₄·7H₂O、0.31gSnCl₂·2H₂O、0.40g Na₂S₂O₃·5H₂O、0.34g NaOH、3.26g C₆H₅Na₃O₇和2.28g C4H₆O₆的配方溶液，在Mo衬底上用-1.2V电沉积5min和在FTO衬底上用-1.3V电沉积5min均得到了较好成分比例的四元预制层。溶液中Cu²⁺和Sn²⁺浓度不仅影响其自身的电沉积速度，还影响溶液中其它金属元素的电沉积速度，而Zn²⁺浓度仅影响其自身沉积速度。预制层二元硫化物随着退火温度的升高逐渐相互反应转变为四元硫化物。预制层经550℃退火1h合成的CZTS膜层原子比为Cu:Zn:Sn:S=23.72:12.22:13.07:50.99，与CZTS的化学计量比相接近，禁带宽度约1.6eV。综合比较三种合成方法，对预制层成分比例控制方面分步电沉积比三元共电沉积更简单和更稳定，最终合成的CZTS薄膜晶粒度也较大，有利于提高其光电性能。四元共电沉积预制层的溶液不稳定，制备的预制层均匀性和致密性相对较差。因此，采用制备金属预制层及后续硫化工艺更具有应用前景。利用第一性原理的方法和准谐德拜模型理论计算了KS型和ST型结构CZTS化合物的电子结构、光学性质、力学性质和热物理性质。理论计算得出CZTS为直接带隙半导体材料，在可见光区，吸收系数平均高于10⁴cm^-1及较低的反射率和电导率，与实验结果基本一致。能量损耗接近0。通过力学稳定性判据验证两种结构的CZTS化合物理论计算的弹性常数满足其力学稳定性标准。沿[100]和[010]方向的键合强度与沿[001]方向的键合强度相同；在{001}面的剪切弹性性质存在各向异性。根据计算的B/G值判断，CZTS化合物表现为较好的韧性。CZTS化合物的热容在300K以上接近200J/mol·K。在相同压强下，热膨胀系数随温度升高而缓慢升高，熵随温度的升高呈指数变化，内能则几乎以线性变化，吉布斯自由能随温度升高而降低。在室温时，KS和ST型的CZTS德拜温度分别为338K和297K；Grüneisen因子分别为2.50和2.36。CZTS化合物的物理性能理论计算结果为其制备和应用提供了理论依据。更多还原

【Abstract】 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.更多还原