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Graduate Student Seminar - 03/16/2018

"Nanoscale Imaging of Perovskite Solar Cell Functionality"

by Elizabeth Tennyson

Advisor:  Professor Marina Leite

Friday, March 16, 2018 -- 12:00 p.m.
Large Conference Room, 1207 Energy Research Facility

Abstract: Next generation photovoltaic technologies scalable for global deployment must exhibit two characteristics: (i) high power-conversion efficiency and, (ii) low-cost/W. Extensive efforts have been made to boost the efficiency of high performance and low-cost photovoltaic materials, such as hybrid organic-inorganic perovskites. However, improvement in the overall performance is still limited by the open-circuit voltage (Voc). Perovskites are composed of grains and grain boundaries on the order of micro- and nanometers, respectively, and their nanoscale interfaces could potentially induce charge carriers to recombine nonradiatively, reducing the Voc. The electronic properties of individual grains and the interfaces between the grains can be fully resolved by using nanoscale spatial resolution characterization approaches. For this, I implement Kelvin probe force microscopy (KPFM) and demonstrate a universal method to directly map the Voc of any photovoltaic material. Next, we extend KPFM to rapidly image (16 sec/map) the real-time dynamics of perovskite solar cells, which are notorious for their slow and unstable electrical output. Through fast-KPFM imaging, we discover regions within a single grain that show a residual Voc response which pervades for ~9 min, likely caused by a slow ion migration process. Finally, to understand how different perovskite compositions influence the behavior of the nanoscale electrical response, I utilize KPFM to realize both irreversible and reversible Voc signals. I foresee the functional imaging methods developed in this thesis to be widely implemented as a diagnostic tool for the rational design of photovoltaics with enhanced electrical performance and lower cost.

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