Physical Society Colloquium

Tracking the nanoscale dynamics of ultrafast electronic energy flow and non-equilibrium phase transitions in energy materials

Naomi Ginsberg

Physics Department
UC Berkeley

My research group interrogates dynamic nanoscale processes in energy-related materials, especially those involved in solar light harvesting, as is for example the case in photosynthesis. The most prevalent materials that we consider are intended for next-generation photovoltaics and are all formed through deposition from the solution-phase or using solution-phase self-assembly. Although this approach to material formation is facile and energy efficient, it often results in heterogeneous, kinetically trapped structures far from equilibrium. One of our main goals is therefore to elucidate how these materials' physical structure, including the nature of their heterogeneities and defects, determines their emergent optoelectronic properties. Ultimately, establishing such structure-function relationships will enable us to suggest specific solution-phase approaches to material formation that generate optimally performing functional materials.

Achieving a nanoscale understanding of dynamic processes in heterogeneous energy-related materials is challenging because it requires achieving unprecedented combinations of spatial and temporal measurement resolution. From a practical standpoint, we have therefore had to conceive of and develop multiple new forms of dynamic optical microscopies with sub-diffraction resolution, each tailored to a particular class of materials and their associated femtosecond-to-minutes dynamics. For resolving the dynamics of excitation energy flow we primarily employ ultrafast optical microscopies; to resolve dynamic material structures we primarily extend the applicability cathodoluminescence microscopy to soft materials otherwise too delicate to withstand electron beam irradiation. I will describe a recent example from each category, taking you first on a journey to discover the nature of energy landscapes in disordered, electronically-coupled molecular aggregates, and second, to elucidate the manner by which light can lead to steady-state charge carrier traps by inducing local changes in structure and composition in solid solutions of halides in hybrid perovskite photovoltaics.