SusChEM: Atomic Structure and Dynamic Behaviors of Extended Defects in Earth-Abundant Solar-Cell Materials

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Nontechnical Description: The solar-cell material CZTS is made of the low-cost, Earth-abundant elements Cu, Zn, Sn, S, and Se. Challenges to the development of high-efficiency CZTS solar cells include control of impurities and defects. This project is designed to gain a fundamental understanding of the atomic structure and local properties of defects in CZTS under electrical and optical excitations, using state-of-the-art transmission electron microscopy (TEM) in combination with novel in-situ TEM methods developed in the Principle Investigator's lab. These new techniques will allow us to directly probe the atomic structure of individual defects and the responses of those defects to an applied electric field and/or light illumination providing for the determination of the atomic scale structure-property relationships of individual defects. The result will be knowledge needed for the optimization of the material's microstructure and composition, advancing the development of low-cost and sustainable materials with improved properties for solar energy technology. In addition, this project provides a wide range of opportunities for the interdisciplinary education and training of undergraduate and graduate students needed in both industry and academic research today.Technical Description: This SusChEM project is to study the structure and dynamic behaviors of Earth-abundant solar-cell materials using a combination of advanced aberration-corrected transmission electron microscopy (TEM) and novel in-situ TEM techniques. The research primarily focuses on thin films of kesterite Cu2ZnSn(S,Se)4 (CZTS), a candidate material to replace Cu(In,Ga)Se2 (CIGS). Because of the polycrystalline nature and the co-existence of multiple impurity phases in CZTS thin films, it is critical, but very challenging to understand the role of defects and interfaces in controlling the electrical properties and solar conversion efficiency. In this project, the PI combines the state-of-the-art aberration-corrected TEM imaging, spectroscopy, and the novel in-situ techniques recently developed in his lab to study the atomic structure and dynamic behaviors of individual defects, grain boundaries, and interfaces in CZTS materials. Spatially resolved cathode-luminescence and scanning tunneling microscopy holders with optical excitation, combined with holography and electron energy-loss spectroscopy (EELS), are used to identify the active and inactive regions of defects (grain boundaries, interfaces, secondary phase boundaries, etc.), while the atomic structure, chemical composition and local electronic properties of the same regions are determined by TEM imaging and spectroscopy with atomic resolution. In combination with the optoelectronic properties measured from the same material, the role of defects in controlling the materials properties can be understood.