Physical Chemistry Seminar: Professor Aaron Fafarman, Drexel University
Physical Chemistry Seminar: Professor Aaron Fafarman, Drexel University (Host: Professor Steve Boxer)
About the Seminar
"Big advantages of going small: nanostructured perovskite-phase cesium lead iodide"
Perovskite-phase cesium lead iodide (CsPbI3) possess three almost too-good-to-be-true properties for photovoltaic and electroluminesence applications and one achilleas heel: it is not stable under ambient conditions. This talk will provide an introduction to CsPbI3, highlighting our contributions to identifying and understanding some of its remarkable chemical, physical and functional properties. Using time-resolved transient terahertz spectroscopic measurements of the kinetics of charge-carrier recombination, we have constructed a numerical model of the dominant recombination mechanisms. The kinetic parameters thus obtained provide evidence that CsPbI3 shares two features found in its much more well studied cousins, the hybrid organic/inorganic lead iodide perovskites: spin- and momentum-forbidden radiative recombination and an extraordinarily low defect-assisted recombination rate due to apparently benign defect chemistry. The existence of forbidden radiative recombination in an all-inorganic perovskite decisively refutes the importance of the organic component to this phenomenon in the hybrid variants. Using differential scanning calorimetry, we quantitatively measured the thermodynamics of the phase change that defines the problematic instability of the desired metastable perovskite phase, identifying a purely catalytic destabilization effect of atmospheric moisture. We detail a novel approach to stabilizing the functional metastable phase using co-assembly of CsPbI3 and CsPbCl3 nanocrystals, followed by chemical sintering into thin films. "Nanocrystal substitutional doping" in this manner creates a composite material that is more stable by at least two orders-of-magnitude, suggesting that the metastability of CsPbI3 may yet prove surmountable. A hypothesis to account for the stabilization effect of nanostructuring will be presented, with significant implications beyond CsPbI3.
About the Speaker
Aaron Fafarman earned a BS in Chemistry from the University of California, Berkeley in 2000 and a PhD in Physical Chemistry from Stanford University in 2010. After a postdoc with Cherie Kagan at the University of Pennsylvania, he joined the Department of Chemical and Biological Engineering at Drexel as an Assistant Professor in 2013. His lab seeks to develop new, wet-chemical techniques for the fabrication of energy-conversion materials and to deepen our understanding of the coupling between synthesis, nanoscale structure and function. Of particular interest are the effects of nanostructure and compositional heterogeneity on the density of electronic defects in semiconducting materials and the discovery of novel, defect-tolerant materials. Dr. Fafarman has been recognized with an Outstanding Faculty award from the Delaware Valley Section of AIChE, an Outstanding Early-Career Research Achievement Award from Drexel's College of Engineering, an Outstanding STAR Mentor award from Drexel University and the Linus Pauling Chemistry Teaching award from Stanford University. He is currently the chair of the Electronic and Photonic Materials program of AIChE, co-Chair of the Department Safety Committee and is the faculty advisor for the Drexel AIChE Student Chapter. He has authored or co-authored 33 peer-reviewed papers and has five patents issued.