Improving the Understanding of CZTS-Se as a Solar Absorber Material through Single Crystals Formed Using Phase Diagram Analysis

该项目的目的是研究低成本四元半导体CZTS-Se（化学式Cu 2 ZnSn（S，Se）4）的电子和结构特征。这旨在告知CZTS-Se在薄膜太阳能光伏应用中的使用。该项目旨在更深入地了解CZTS-Se在不同溶剂中的相图结构和结晶过程，以及沿着对电子性能的影响。该分析的核心是制造大的CZTS-Se单晶，这将允许在没有表面效应的情况下表征材料的本体性质。这是为了通知CZTS-Se太阳能电池制造的溶液处理方法，以达到比当前记录更高的效率。本研究的动机是CZTS-Se器件可以由廉价，丰富和无毒的元素制造。由于无毒，原材料价格低廉，加上处置成本低，这意味着CZTS-Se器件有可能形成非常便宜的太阳能模块。这些器件的另一个优点是，作为固体多晶半导体，CZTS-Se不会受到目前限制有机和钙钛矿器件的可行性的不稳定性问题的影响。这使得创建持续足够长时间以提供必要回报的模块变得更加可行。由于这些理想的功能，如果CZTS-Se模块能够进入能源市场，它们可以显着降低全球太阳能发电的价格。然而，尽管是一种带隙接近最佳的直接带隙半导体，但CZTS-Se太阳能器件目前的效率记录仅为12.6%，而冠军硅模块的效率仅为25%以上。这种低效率意味着CZTS-Se器件的回报率目前太低而不经济。光伏器件中的器件效率可以理解为取决于3个参数：短路电流JSC;开路电压VOC;和填充因子FF。CZTS-Se器件已经能够证明与由CIGS（一种已经建立的四元半导体）制成的器件类似的JSC值，但是CZTS-Se的VOC显著低于CIGS的等效水平。因此，解决CZTS-Se中的“挥发性有机化合物赤字”是该项目的主要目标。电荷载流子复合中心的存在被认为有助于VOC缺陷，其中用于复合的潜在位点包括晶粒边界和/或体材料中的缺陷。另一个导致器件性能不佳的因素是形成次级成分和/或结构相。因此，本项目的重点是通过形成大的CZTS-Se单晶来完全去除晶界。CZTS的单晶生长在过去已经被成功地证明。因此，我们的目标是开发一种生长单晶并测量其光伏特性的方法。然后可以将这些性质与多晶CZTS-Se进行比较，以评估晶界的影响。CZTS-Se的单晶也将允许研究体缺陷和第二相的影响，与表面效应分开。然后可以探索元素组成和制造条件的变化，以减少有害的体缺陷和二次相。单晶的生产和表征需要对CZTS-Se与一系列溶剂形成的溶液有深入的了解。因此，本项目的大部分工作将是绘制CZTS-Se/溶剂体系在不同温度和组成比范围内的相图。本项目的主要关注点是CZTS-Se作为材料的基本分析。然而，我们打算将该项目的见解应用于生产高质量的CZTS-Se吸收层。

The aim of this project is examine the electronic and structural characteristics of the low-cost quaternary semiconductor CZTS-Se (chemical formula Cu2ZnSn(S,Se)4). This is intended to inform the use of CZTS-Se for thin-film solar photovoltaic applications. The project aims to gain a deeper understanding of the phase diagram structure and crystallisation processes of CZTS-Se in different solvents, along with the resultant effect on electronic properties. Central to this analysis is the fabrication of large, single crystals of CZTS-Se which would allow characterisation of the material's bulk properties in the absence of surface effects. This is in order to inform solution processing methods of CZTS-Se solar cell fabrication to reach higher efficiencies than the current record.The motivation for this study is the fact that CZTS-Se devices can be fabricated from cheap, abundant and non-toxic elements. The low price of raw materials coupled with a low disposal cost due to non-toxicity means CZTS-Se devices have the potential to form very cheap solar-power modules. Another advantage to these devices is that as a solid, polycrystalline semiconductor, CZTS-Se does not suffer from the instability issues that limit the viability of organic and perovskite devices at present. This makes it more feasible to create modules that last long enough to provide the necessary returns. Due to these desirable features, if CZTS-Se modules were able to enter the energy market they could significantly reduce the price of solar power around the world. However, despite being a direct-gap semiconductor with a band-gap that is close to optimal, the current record of efficiency for a CZTS-Se solar device is only 12.6%, compared to just over 25% for the champion silicon modules. This low efficiency means that the rate-of-return for CZTS-Se devices is currently too low for them to be economical.Device efficiency in photovoltaic devices can be understood as a dependence upon 3 parameters: the short circuit current, JSC; the open-circuit voltage, VOC; and the fill factor, FF. CZTS-Se devices have already been able to demonstrate similar values of JSC to devices made of CIGS, an already-established quaternary semiconductor, but the VOC for CZTS-Se is significantly below the equivalent level for CIGS. Addressing the 'VOC- deficit' in CZTS-Se is therefore a major aim of this project. The presence of charge-carrier recombination centres is thought to contribute to the VOC- deficit, where potential sites for recombination include crystal grain boundaries and/or defects in the bulk material. Another factor found to contribute to poor device performance is the formation of secondary compositional and/or structural phases.As a result, this project is focussed upon removing grain-boundaries altogether by forming a large, single crystal of CZTS-Se. Single crystal growth for CZTS has been successfully demonstrated in the past. Therefore our aim is to develop a process to grow single crystals and measure their photovoltaic properties. These properties can then be compared with polycrystalline CZTS-Se to assess the effect of grain boundaries. A single crystal of CZTS-Se will also allow the effects of bulk defects and secondary phases to be investigated, separate from surface effects. Variations in the elemental composition and fabrication conditions to reduce detrimental bulk defects and secondary phases can then be explored.Production and characterisation of single crystals requires a deep understanding of the solutions formed from CZTS-Se with a range of solvents. Therefore much of the work of this project will be to characterise phase diagrams of CZTS-Se/solvent systems across a range of temperatures and compositional ratios.The primary concern of this project is the fundamental analysis of CZTS-Se as a material. However we intend that the insights from this project will be applied to the production of high-quality CZTS-Se absorber layers.