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：硫化铜锌锡硒化（CZTSSe）是一种很有希望用于太阳能电池的候选材料，因为它强烈吸收可见光，主要由地球上丰富的无毒元素组成。然而，由于制备高质量薄膜的困难，对CZTSSe的基本科学认识受到限制。在这个项目中，德雷塞尔大学和特拉华大学的研究人员培养了CZTSSe的块状晶体，并表征了它们对光的反应。这种方法可以确定元素组成和光伏响应之间的关系，这可以在短期内提高效率，并改善对由这种新兴材料制成的太阳能电池的实际性能限制的估计。多名研究生和本科生研究人员参与该项目。此外，研究人员使用移动太阳能模块，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，特别是那些代表性不足的群体。技术描述：CZTSSe薄膜的光伏效率高达12.6%，但仍远低于30%以上的理论极限。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。进一步提高效率需要充分了解材料成分、内在点缺陷和界面如何影响超快光激发载流子动力学。然而，复杂的缺陷化学和薄膜生长的高度非平衡条件导致了高密度的晶界和二次相，这对基本的理解构成了重大障碍。在这个项目中，合作研究团队培育出高质量、准平衡的CZTSSe单晶，并使用超快光谱探针对它们进行研究，以了解载流子动力学如何依赖于CZTSSe单晶中的成分、缺陷和界面。这项工作有望导致对超快载流子动力学，加工，材料和界面特性以及光伏性能之间关系的新理解。具体来说，该项目依靠太赫兹光谱和瞬态反射光谱以及有限元输运-重组模型来确定光激发载流子的寿命、迁移率和主要的重组机制。寿命和迁移率是作为Cu:Zn:Sn和S:Se比率的函数来测量的，并且与器件性能相关。此外，研究CZTSSe-CdS异质结的表面/界面复合以及准平衡多晶体中晶界的影响，可以为薄膜光伏器件提供新的基础科学认识。

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.