Collaborative Research: SusChEM: Using Ultrafast Carrier Dynamics to Link Structure, Properties, and Performance in Single-Crystal Cu2ZnSn(S,Se)4 for Thin Film Photovoltaics

非技术描述：铜锌锡硫硒化物（CZTSSe）是用于太阳能电池的有前途的候选材料，因为它强烈吸收可见光并且主要由地球丰富的无毒元素组成。然而，CZTSSe的基本科学认识受到制造高质量薄膜的困难的限制。在这个项目中，德雷克塞尔大学和特拉华州大学的研究人员生长了CZTSSe的块状晶体，并描述了它们对光的反应。这种方法能够识别元素组成和光伏响应之间的关系，这可以导致效率的短期增加和对由这种新兴材料制成的太阳能电池的实际性能极限的改进估计。多名研究生和本科生研究人员参加了这个项目。此外，研究人员还利用移动的太阳能模块，通过费城材料日等活动，将太阳能转换的概念带给费城和纽瓦克社区的K-12学生，特别是来自代表性不足的群体。技术说明：CZTSSe薄膜的光伏效率高达12.6%，但仍远低于超过30%的理论极限。CZTSSe太阳能电池中的光电流和光电压受到短（纳秒）光激发载流子寿命的限制。效率的进一步提高将需要充分理解材料成分，本征点缺陷和界面如何影响超快光激发电荷载流子动力学。然而，复杂的缺陷化学和薄膜生长的高度非平衡条件导致晶界和第二相的高密度，对基本理解构成了重大障碍。在这个项目中，合作研究团队生长高质量的准平衡CZTSSe单晶，并使用超快光谱探针对其进行询问，以了解载流子动力学如何取决于CZTSSe单晶中的成分，缺陷和界面。这项工作有望导致新的理解之间的关系超快载流子动力学，加工，材料和界面特性，光伏性能。具体来说，该项目依赖于太赫兹光谱和瞬态反射光谱加上有限元传输复合模型，以确定光激发载流子的寿命，迁移率和占主导地位的复合机制。寿命和迁移率作为Cu：Zn：Sn和S：Se比率的函数来测量，并且与器件性能相关。此外，CZTSSe-CdS异质结中的表面/界面复合和准平衡多晶中晶界的影响的研究，使新的基本科学理解的外推到薄膜光伏器件。

Non-technical Description: Copper zinc tin sulfide selenide (CZTSSe) is a promising candidate material for use in solar cells because it strongly absorbs visible light and is composed primarily of earth-abundant, non-toxic elements. However, fundamental scientific understanding of CZTSSe has been limited by difficulties in fabricating thin films of high quality. In this project, researchers at Drexel University and the University of Delaware grow bulk crystals of CZTSSe and characterize their response to light. This approach enables identification of relationships between elemental composition and photovoltaic response, which can lead to both near-term increases in efficiencies and improved estimates of the practical performance limits of solar cells made from this emerging material. Multiple graduate and undergraduate student researchers participate in this project. Additionally, researchers use a mobile solar module to bring concepts in solar energy conversion to K-12 students in the Philadelphia and Newark communities, especially from under-represented groups, through events such as Philly Materials Day.Technical Description: CZTSSe thin films have shown promising photovoltaic efficiencies up to 12.6%, but they are still far below the theoretical limit of over 30%. Photocurrent and photovoltage in CZTSSe solar cells are limited by short (nanoseconds) photoexcited carrier lifetimes. Further improvements in efficiency will require full understanding of how materials composition, intrinsic point defects, and interfaces affect ultrafast photoexcited charge carrier dynamics. However, complex defect chemistry and highly non-equilibrium conditions of thin film growth result in high densities of grain boundaries and secondary phases, posing a significant impediment to fundamental understanding. In this project, the collaborative research team grows high-quality, quasi-equilibrium CZTSSe single crystals and interrogates them using ultrafast spectroscopic probes to understand how carrier dynamics depend on composition, defects, and interfaces in CZTSSe single crystals. This work is expected to lead to new understanding of the relationships between ultrafast carrier dynamics, processing, material and interface properties, and photovoltaic performance. Specifically, the project relies on terahertz spectroscopy and transient reflectance spectroscopy coupled with finite element transport-recombination models to determine photoexcited carrier lifetimes, mobilities, and dominant recombination mechanisms. Lifetimes and mobilities are measured as a function of Cu:Zn:Sn and S:Se ratios and are correlated to device performance. Additionally, studies of surface/interface recombination in CZTSSe-CdS heterojunctions and the effects of grain boundaries in quasi-equilibrium polycrystals enable extrapolation of new fundamental scientific understanding to thin film photovoltaic devices.