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NREL Scientists Use In-Situ Characterization to Explain Perovskite Solar Cell Growth and Processing

June 6, 2016

NREL scientists have helped to establish research capabilities to address several important scientific and practical questions concerning perovskite solar cells. Perovskites are an important class of next-generation photovoltaic materials exhibiting impressive light absorption, high device efficiencies, and low-cost, industry-scalable processing. However, further development of successful, scalable technologies requires an in-depth understanding of the complex correlations among various physical, chemical, and electro-optical properties, as well as their impacts on device characteristics.

Observing formation dynamics across various successful perovskite materials has provided a framework for understanding process design of these materials. However, what has been lacking are direct insights into the mechanisms of perovskite formation from the inorganic and organic components.

A recent study with significant contributions from NREL scientists addresses this need for direct observation of fundamental processes that occur during the synthesis and crystallization of perovskite-based materials. Published in Energy & Environmental Science, "In-situ investigation on the formation and metastability of formamidinium lead tri-iodide perovskite solar cells" was authored by NREL's Jeffery AguiarMengjin YangJoseph BerryMowafak Al-Jassim, and Kai Zhu, and others from the University of New Orleans, Los Alamos National Laboratory, Arizona State University, and the University of Tennessee. In this study, scientists combined and applied advanced in-situ characterization techniques to understand and elucidate the material properties of formamidinium lead-triiodide, an organic-inorganic hybrid photovoltaic material.

In-situ STEM characterization provides detailed resolution of lead (Pb) mobility to and from the grain boundaries of perovskite (FAPBI3) solar cells; controlled annealing to redistribute Pb-containing species improves cell performance.

The fundamental study used characterization techniques such as atomic contrast scanning transmission electron microscopy (STEM) and high-resolution electron energy-loss spectroscopy (EELS). STEM and EELS measurements revealed that differences in thermal and environmental processing significantly impact the combined microstructure and chemistry of grain boundaries. In the lab, sequentially annealing formamidinium lead tri-iodide in an inert dry environment provides one way to redistribute the lead-containing species along grain boundaries, which scientists suspect promotes crystallization and subsequent charge-carrier transport.

Thus, state-of-the-art characterization allows direct in-situ observation of material structure and chemistry over relevant spatial, temporal, and temperature scales. And it has enabled identifying key formation and degradation mechanisms related to grain evolution and interface chemistry. But it also offers one perspective into how to develop a processing routine to gain control over grain-boundary chemistry-and ultimately, how to apply this knowledge to assemble low-cost, high-efficiency solar cells.

Visit the NREL website to learn more about its solar research areas.

—Linh Truong