Atomic-scale structure of kesterites

光伏能量转换是实现可持续能源供应的主要途径之一，可持续能源供应是二十一世纪的重大全球挑战之一。CdTe和Cu（In，Ga）Se 2薄膜太阳能电池最近达到了超过20%的创纪录效率，接近了与基于硅的技术的差距。除了降低材料成本外，这种薄膜太阳能电池还可以在柔性基板上生产，在建筑和产品设计方面提供诱人的新应用。然而，CdTe的使用受到其组分的毒性的阻碍，而Cu（In，Ga）Se 2的大规模实施可能受到In的可用性的限制。因此，正在努力寻找合适的材料替代品。钾镁矾，如Cu 2 ZnSn（S，Se）4，由在地壳中广泛存在的无毒元素组成。近年来，锌黄锡矿薄膜太阳能电池的效率不断提高，目前的记录为12.6%，相信将来可以实现与CdTe或Cu（In，Ga）Se 2类似的转换效率。目前，锌黄锡矿薄膜太阳能电池的性能受到难以用衍射技术检测并且不利地影响材料的电子性质的第二相和结构修改的出现的限制。此外，混合半导体中的局部原子排列经常显示出与长程晶体学有序的显著偏离。亚纳米尺度上的这种结构不均匀性也对材料带隙产生强烈影响，从而对器件性能产生强烈影响。因此，本项目的目的是研究一套全面的锌黄锡矿材料的组成，结构和电子方面，以提高对这些复杂半导体结构-性能-关系的理解，并指出高效薄膜太阳能电池的优化制备条件。所研究的材料包括四种不同的化合物及其混合物，化学计量和非化学计量，如粉末，薄膜或纳米颗粒。使用X射线吸收光谱（XAS），可以定量确定第二相的存在作为材料组成和生长条件的函数，提供有关优化制备路线的有价值的信息。XAS还产生原子尺度的结构参数，如元素特定的键长，这提供了原始的洞察长程和短程秩序之间的相关性相比，从衍射技术获得的结果。结合局部结构参数与价力场模拟和从头计算进一步产生独特的结构和电子性质之间的关系的信息。为了充分利用这组有前途的材料的潜力，对这些结构-性能-关系的详细了解是必不可少的。

Photovoltaic energy conversion is one of the major routes towards a sustainable energy supply which is one of the big global challenges of the twenty-first century. CdTe and Cu(In,Ga)Se2 thin film solar cells have recently reached record efficiencies of more than 20% closing the gap to silicon based technologies. Apart from reduced material costs, such thin film solar cells can be produced on flexible substrates offering tantalizing new applications in architecture and product design. However, the use of CdTe is hindered by the toxicity of its constituents while the large-scale implementation of Cu(In,Ga)Se2 may be limited by the availability of In. Consequently, strong efforts are being made to find suitable material substitutes. Kesterites, such as Cu2ZnSn(S,Se)4, consist of nontoxic elements that are widely available in the earth crust. The efficiency of kesterite thin film solar cells has continuously increased over recent years up to the current record of 12.6% and it is believed that conversion efficiencies similar to those of CdTe or Cu(In,Ga)Se2 may be achieved in the future. Currently, the performance of kesterite thin film solar cells is limited by the occurrence of secondary phases and structural modifications that are difficult to detect with diffraction techniques and that adversely affect the electronic properties of the material. Furthermore, the local atomic arrangements in mixed semiconductors often show a striking deviation from the long-range crystallographic order. This structural inhomogeneity on the sub-nanometer scale also has a strong impact on the material bandgap and thus on the device performance. Therefore, it is the aim of this project to study compositional, structural and electronic aspects of a comprehensive set of kesterite materials in order to improve the understanding of structure-property-relations in these complex semiconductors and to point out optimized preparation conditions for high-efficiency thin film solar cells. The materials studied include four different compounds and their mixtures, stoichiometric and non-stoichiometric, as powders, thin films or nanoparticles. Using X-ray absorption spectroscopy (XAS), the presence of secondary phases can be determined quantitatively as a function of material composition and growth conditions providing valuable information about optimized preparation routes. XAS also yields atomic-scale structural parameters, such as element-specific bond lengths, which provide original insight into the correlation between long-range and short-range order when compared to results obtained from diffraction techniques. Combining local structural parameters with valence force field simulations and ab initio calculations further yields unique information about the relation between structural and electronic properties. A detailed understanding of these structure-property-relations is indispensable in order to exploit the full potential of this promising group of materials.