Novel Cation-substituted Quarternary Chalcogenide Nanocrystals Cu2XSnS4 (X = Zn, Ni, Fe, Co): Properties and Potential Applications

Cu2ZnSn(S,Se)4 （CZTSSe）作为一种很有前途的光伏替代材料，在过去的十年中得到了广泛的研究。然而，将其应用到实际生活中受到其对非化学计量、缺陷和二次相的高耐受性的挑战。在CZTSSe结构中取代阳离子在克服这些挑战和实现新性质方面显示出很大的希望。Ag和Ba是目前研究最多的取代基，但它们只引起部分光伏和其他性质的改变。非常有趣的铁族元素（Fe, Ni, Co）在这方面还没有被探索过。对于Cu2XSnS4 （X = Zn, Ni, Fe, Co）化合物的胶体纳米晶体（NCs）的系统研究尤其缺乏，尽管这种NCs是通过印刷在指定的衬底和区域上形成能量转换和存储装置的有希望的选择。已有报道成功合成了CZTSSe NCs，并在较小程度上合成了其他I2-II-IV-VI4 NCs。在太阳能电池、热电、催化和其他具有竞争力的应用中推进这些nc需要填补其结构和电子特性方面的知识空白。本项目的目的是建立Cu2XSnS4 (X = Zn, Ni, Fe, Co) NCs的晶格结构（包括缺陷）、电子态分布和光谱之间的关系。我们期望在这个项目中提出的阳离子取代可以(i)由于某些缺陷的钝化而导致宿主材料性能的改善，（ii）有助于确定CZTS本身某些天然缺陷或电子态的起源，以及（iii）诱导CZTS晶体和nc中不存在的新特性，特别是磁性能。计算筛选将有助于合成部分，使我们能够专注于最有希望的CXTS成分，并了解阳离子取代引起的电子，光学和声子实验光谱的变化。此外，我们将研究材料性能的演变，从合成的单个nc的系综到退火形成的多晶薄膜。基于获得的结果，我们将在简单的光伏或热电器件中对获得的材料进行初步调查。在申请者之间16年的成功合作中，有100多篇联合出版物证实了这一点，其中大部分与胶体硫属碳纳米化合物有关，我们已经获得了足够的经验和工具来成功完成当前的项目。除了为该项目选择的既定表征技术外，乌克兰合作伙伴可以使用的相当独特的深层光谱和电子顺磁共振方法，对于在Cu2XSnS4 nc等复杂材料中建立电子态的完整图像是必不可少的。

Cu2ZnSn(S,Se)4 (CZTSSe) has been intensively studied over the last decade as a promising alternative material for photovoltaics. However, bringing it to real-life applications is challenged by its high tolerance to non-stoichiometry, defects, and secondary phases. Substitution of cations in the CZTSSe structure showed great promise in overcoming these challenges and enabling new properties. Ag and Ba are the substituents mostly studied so far, but they cause only a partial amendment of the photovoltaic and other properties. The very interesting iron group of elements (Fe, Ni, Co) are rather unexplored in this respect yet. Systematic investigations are especially lacking for colloidal nanocrystals (NCs) of the Cu2XSnS4 (X = Zn, Ni, Fe, Co) compounds, although such NCs are a promising option for forming energy conversion and storage devices on deliberate substrates and areas by printing. The successful synthesis of CZTSSe NCs, and to a lesser extent of other I2-II-IV-VI4 NCs, has been reported. Advancing these NCs to solar cells, thermoelectrics, catalysis, and other application at a competitive level demands filling the gap in the knowledge about their structure and electronic properties. The aim of the project is to establish the relation between the lattice structure (including defects), distribution of electronic states, and optical spectra of Cu2XSnS4 (X = Zn, Ni, Fe, Co) NCs. We expect that the cation substitutions proposed in this project can (i) lead to an improvement of the properties of the host material due to the passivation of certain defects, (ii) help to establish the origin of certain native defects or electronic states in CZTS itself, and (iii) induce new properties, not existing in CZTS crystals and NCs, in particular magnetic properties. Computational screening will be performed to facilitate the synthetic part, enabling us to focus on the most promising CXTS compositions and to understand the changes in the electronic, optical, and phonon experimental spectra caused by cationic substitution. Furthermore, we will investigate the evolution of the material properties from the ensembles of as-synthesized individual NCs to polycrystalline films formed by annealing. Based on the results obtained, we will perform a preliminary investigation of the obtained material in simple photovoltaic or thermoelectric devices. In 16 years of successful collaboration between the applicants, as confirmed by more than 100 joint publications and most of them related to colloidal chalcogenide NCs, we have acquired sufficient experience and instrumental tools for the successful fulfillment of the current project. Besides established characterisation techniques, selected for the project, rather exclusive methods of deep level spectroscopies and electron paramagnetic resonance, which are available for the Ukrainian partners, are indispensable for building the complete picture of electronic states in such a complex materials as Cu2XSnS4 NCs.