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

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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.