Solution processing of thin films for solar cell applications : CuIn(S,Se)2, Cu(In,Ga)(S,Se)2 and ZnO:Al
Solution processing of thin films for solar cell applications : CuIn(S,Se)2, Cu(In,Ga)(S,Se)2 and ZnO:Al
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发表时间：
2016
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通讯作者：
Panagiota Arnou
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作者：
Panagiota Arnou
铜(In，Ga)(Se，S)2(CIGS)太阳电池因其高性能和比传统单晶硅太阳电池更低的制造成本而备受关注。所有最近创纪录的CIGS吸收体都是使用真空工艺沉积的，这带来了高昂的制造成本。CIG还可以兼容低成本的常压处理，从而显著降低制造成本。最近，在为CIGS开发基于大气溶液的过程方面取得了一些进展。在不同的溶液方法中，沉积分子前体在简单和直接的成分控制方面可能是有利的。尽管如此，开发的方法涉及剧毒试剂或设备中的大量杂质含量，限制了商业化的潜力。本论文描述了一种新的基于溶液的方法来沉积CIGS吸收层。使用金属硫化物作为起始前驱体，不含有害杂质。这些化合物含有很强的共价键，因此不溶于普通溶剂。直到最近，具有剧毒和爆炸性的联氨是唯一有效溶解这些类型前体的溶剂，限制了这种方法在工业应用中的可行性。在这项工作中，金属硫化物被溶解在1，2-乙二硫醇和1，2-乙二胺的更安全的溶剂组合中，完全消除了工艺中的联氨。通过使用该溶剂体系，可以形成具有长期稳定性的光学透明溶液。前驱体溶液被干净地分解，并在硒化后转化为单相CIGS。采用喷射沉积法制备了CuIn(S，Se)2太阳电池，其功率转换效率高达8.0%。这一进展是通过不断优化沉积、干燥，特别是硒化配置来实现的。在其他参数中，硒处理过程中的工作压力被发现对材料的结晶质量有显著影响。此外，还探索了快速热处理作为管式炉退火的一种替代方法，结果表明，快速热处理可以改善背接触/吸收界面。已经证明，Ga可以很容易地掺入到吸收体中，用于带隙调谐，从而增强太阳能电池的VOC。研究了薄膜的结构性质与Ga含量的关系，以及相应太阳电池的光电特性。通过简单地调整前驱体比例，可以方便地改变材料的带隙，从而实现精细的成分控制。利用这种技术，获得了转换效率高达9.8%的Cu(In，Ga)(Se，S)2太阳电池。太阳能电池在本工作中的性能受到吸收体的孔隙率和背接触质量的限制。尽管在这项工作中有了很大的改进，但吸收体的剩余孔隙率会导致硒向背面扩散，形成厚厚的MoSe2层，并导致器件中的高串联电阻。还开发了一种低成本、基于溶液的技术来制备铝掺杂氧化锌薄膜，该薄膜可用作薄膜太阳能电池的透明导电氧化层。这种方法包括使用超声波喷雾热解系统，这是一种非常通用且易于控制的沉积技术。虽然氧的存在使薄膜更接近化学计量比(更少的氧空位)，但通过工艺优化获得了良好的电子和光学性能。在最佳工艺条件下制备的薄膜的方阻为23Ω/sq，可以在最小的透过率损失的情况下通过增加薄膜厚度来进一步降低方阻。简单、低毒和直接的控制使所提出的方法在低成本和可扩展的薄膜太阳能电池沉积方面具有极大的潜力。
Cu(In,Ga)(Se,S)2 (CIGS) solar cells have attracted a lot of attention due to their high performance and the prospect for lower manufacturing costs over conventional crystalline silicon solar cells. All recent record efficiency CIGS absorbers have been deposited using vacuum processing which introduces high manufacturing costs. CIGS can also be compatible with low cost, atmospheric processing which can significantly reduce manufacturing costs. Recently, there has been some progress in developing atmospheric solution-based processes for CIGS. Among different solution approaches, deposition of molecular precursors can be advantageous in terms of simplicity and straightforward compositional control. Nonetheless, the developed methodologies involve highly toxic reagents or large impurity content in the device, limiting the potential for commercialisation. This thesis describes the development of a novel solution-based approach for the deposition of CIGS absorber layers. Metal chalcogenides are used as the starting precursors, which are free from detrimental impurities. These compounds contain strong covalent bonds and, consequently, they are insoluble in common solvents. Until recently, hydrazine, which is highly toxic and explosive, was the only solvent to effectively dissolve these types of precursors, limiting the feasibility of this approach for industrial applications. In this work, metal chalcogenides are dissolved in a safer solvent combination of 1,2-ethanedithiol and 1,2-ethylenediamine, completely eliminating hydrazine from the process. By using this solvent system, optically transparent solutions are formed which exhibit long-term stability. The precursor solutions are decomposed cleanly and they are converted to single phase CIGS upon selenisation. CuIn(S,Se)2 solar cells with power conversion efficiencies up to 8.0% were successfully fabricated by spray depositing the precursor solution, followed by a selenisation step. This progress has been made by continuously optimising the deposition, drying, and especially the selenisation configuration. Among other parameters, the working pressure during selenisation was found to have a dramatic effect on the material crystalline quality. Rapid thermal processing was also explored as an alternative selenisation configuration to tube furnace annealing and it was shown to improve the back contact/absorber interface. It has been demonstrated that Ga can easily be incorporated in the absorber for band-gap tuning and, consequently, for VOC enhancement of the solar cells. The structural properties of the films were investigated with Ga content, as well as the opto-electronic characteristics of the corresponding solar cells. The band-gap of the material was conveniently varied by simply adjusting the precursor ratio, allowing for fine compositional control. By using this technique, Cu(In,Ga)(Se,S)2 solar cells with conversion efficiencies of up to 9.8% were obtained. The solar cell performance in this work is limited by the porosity of the absorber and the back contact quality. Despite a significant improvement during the course of this work, the remaining porosity of the absorber causes selenium to diffuse towards the back forming a thick MoSe2 layer and causing a high series resistance in the device. A low cost, solution-based technique was also developed for the deposition of aluminium-doped zinc oxide films that can be used as the transparent conductive oxide layer in thin film solar cells. This methodology involves the use of an ultrasonic spray pyrolysis system, which is a very versatile and easily controlled deposition technique. Although the presence of oxygen makes the film closer to stoichiometric (fewer oxygen vacancies) good electronic and optical properties have been obtained by process optimisation. Films deposited with optimum conditions exhibited a sheet resistance of 23 Ω/sq, which can be further reduced by increasing the thickness with minimal transmittance losses. The simplicity, low toxicity and straightforward control make the proposed methodologies extremely potential for low cost and scalable deposition of thin film solar cells.