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Characterisation of CuInS2 Films for Solar Cell Applications by Raman Spectroscopy

机译:拉曼光谱法表征用于太阳能电池的CuInS2薄膜

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摘要

[eng] CuInS2 is a good candidate as photovoltaic absorber for the fabrication of high efficient and low cost solar cells. In this work, the vibrational properties of CuInS2 films have been studied by means of Raman Spectroscopy, and other techniques such as XRD, TED, SEM and AES. Moreover, the CuInS2 absorber films used for the fabrication of solar cell devices have been characterised, and the sulphurisation reaction of the metallic precursors for the preparation of baseline absorbers investigated. Epitaxial and polycrystalline films were grown by a coevaporation process in a vacuum chamber containing sulphur vapour. The structural analysis of the epitaxial samples revealed that in these films two different crystallographic orderings coexist. These two polytypes of the CuInS2 compound correspond to the chalcopyrite equilibrium phase, and the "Cu-Au" metastable phase. It was found, the later to be responsible for the appearance of characteristic bands in the Raman spectra at 305 cm-1 and 60 cm-1, which had not been identified before this work. This assignment is based in both experimental results and theoretical calculations of the phonon spectra of both polytypes performed using a VFF model. On the other hand, polycrystalline films grown at different conditions were prepared in order to investigate the influence of the growth parameters upon the structural properties of the films. Cu excess conditions and growth temperatures above 470ºC lead to good quality absorbers. Moreover, the Cu excess segregates at the surface of the film as CuS, and can be easily removed by a chemical treatment in KCN. The good quality of the CuInS2 films obtained by this recipe and the wide window process (Cu/In between 1.2 and 1.8) justify this additional step for the preparation of CuInS2 films. This constitutes an important advantage with respect to the processing of CuInSe2 absorbers, for which the effect of the stoichiometric deviations is critical. The efficiency of the cells fabricated from absorbers grown under these conditions is in the range of 12%. On the contrary, films grown under Cu poor stoichiometry and/or lower temperatures present very bad crystalline quality, and lead to photovoltaic devices with very low efficiencies. It was proved that these conditions favour the formation of Cu-Au ordered domains in the films, thus deteriorating the crystalline quality of the chalcopyrite phase. Furthermore, Cu-poor conditions favour the formation of precipitates of CuIn5S8 at the bulk of the films. Baseline sequentially processed films are obtained by the sulphurisation reaction of the metallic precursors, which are previously sputtered onto the Mo-coated glass substrate. Cu excess conditions and substrate temperatures around 500ºC are selected for the growth of these baseline absorbers. The crystalline quality of these films is similar to that of the coevaporated films grown under Cu excess conditions. Furthermore, the effect of the incorporation of Ga in the metallic precursors was also studied. The aim of introducing Ga is to increase the VOC of the final device. However, the sulphurisation of the Ga-containing precursors leads to films presenting a bilayer structure, characterised by the existence of an In rich Cu(In,Ga)S2 quaternary at the surface region of the sample, and a Ga rich Cu(In,Ga)S2 quaternary at the back region of the film. Although the homogeneity of the films could be improved by using higher sulphurisation temperatures and a Rapid Thermal Process in H2S, it was not possible to avoid the accumulation of Ga close to back region of the films. The explanation for the structure of these films was provided by the analysis of samples quenched at different stages of the sulphurisation reaction, which allow establishing the fundamental steps of the sulphurisation reaction of the metallic layers.
机译:[In] CuInS2是制造高效,低成本太阳能电池的光伏吸收剂的良好选择。在这项工作中,已经通过拉曼光谱和其他技术(例如XRD,TED,SEM和AES)研究了CuInS2薄膜的振动特性。此外,已经表征了用于制造太阳能电池装置的CuInS 2吸收剂膜,并且研究了用于制备基线吸收剂的金属前体的硫化反应。通过共蒸发工艺在含有硫蒸气的真空室中生长外延膜和多晶膜。外延样品的结构分析表明,在这些膜中两种不同的晶体学顺序共存。 CuInS2化合物的这两个多型对应于黄铜矿平衡相和“ Cu-Au”亚稳相。人们发现,后者是造成拉曼光谱中305 cm-1和60 cm-1处特征谱带出现的原因,而在这项工作之前还没有发现。该分配基于实验结果和使用VFF模型进行的两种多型声子谱的理论计算。另一方面,制备了在不同条件下生长的多晶膜,以研究生长参数对膜结构性能的影响。过量的铜条件和高于470ºC的生长温度可形成优质的吸收器。此外,过量的Cu以CuS的形式偏析在膜的表面,并且可以通过在KCN中的化学处理容易地去除。通过该配方获得的CuInS2薄膜的良好质量和宽窗口工艺(Cu / In在1.2至1.8之间)证明了该额外步骤可用于制备CuInS2薄膜。对于CuInSe 2吸收剂的加工而言,这构成了重要的优点,对此化学计量偏差的影响至关重要。由在这些条件下生长的吸收体制成的电池的效率在12%的范围内。相反,在Cu化学计量差和/或较低温度下生长的膜呈现非常差的晶体质量,并且导致光伏器件的效率非常低。证明了这些条件有利于在膜中形成Cu-Au有序域,从而使黄铜矿相的晶体质量劣化。此外,贫铜条件有利于在大部分薄膜上形成CuIn5S8沉淀。通过金属前驱物的硫化反应获得基线顺序处理的薄膜,该金属前驱物预先溅射在涂Mo的玻璃基板上。为这些基线吸收剂的生长选择了铜过量的条件和约500ºC的基板温度。这些膜的结晶质量类似于在铜过量条件下生长的共蒸发膜的结晶质量。此外,还研究了在金属前体中掺入Ga的作用。引入Ga的目的是增加最终器件的VOC。但是,含Ga的前体的硫化会导致薄膜呈现双层结构,其特征在于样品表面区域存在四价富In的Cu(In,Ga)S2四元系,而Ga富的Cu(In Ga)S2在膜的背面区域为四级。尽管可以通过使用更高的硫化温度和在H2S中进行快速热处理来改善薄膜的均匀性,但无法避免Ga堆积在薄膜背面附近。这些膜的结构的解释是通过对在硫化反应不同阶段淬火的样品的分析来提供的,这可以确定金属层硫化反应的基本步骤。

著录项

  • 作者

    Alvarez García Jacobo;

  • 作者单位
  • 年度 2002
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  • 原文格式 PDF
  • 正文语种 eng
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