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Two techniques—co-localized electron back scattered diffraction imaging (left) and ultrafast optical microscopy (center and right)—help determine how local structural defects affect fast electron movement within a single microscopic crystal.
Two techniques—co-localized electron back scattered diffraction imaging (left) and ultrafast optical microscopy (center and right)—help determine how local structural defects affect fast electron movement within a single microscopic crystal.
Image courtesy of Erik Grumstrup

The Science

Researchers used two advanced microscopy techniques to learn how crystal defects in a type of solar power cell affect their performance. The study involved crystalline solar cells called lead halide perovskite cells. The research used two microscopy techniques. Electron backscattering diffraction lets scientists examine crystal quality at scales of 100 nanometers, or 1,000 smaller than the width of a human hair. Ultrafast microscopy uses laser pulses 100 quadrillionths of a second long to examine how electrons move. By combining these techniques, researchers found that microscopic defects that form when the crystals are made can reduce how fast electrons move by a factor of almost 10.

The Impact

Because of their potential for low manufacturing cost, lead halide perovskites have enormous promise as light absorbers in next-generation photovoltaics. A key challenge has been understanding the reasons why electron transport varies so widely in these materials. The conversion of light into electricity requires solar cells to transport electrons, so this process affects how efficient the cells are. This work shows that changes in crystal quality that are an inherent part of the crystal fabrication process are responsible for the variation. Importantly, the crystal defects occur at the smallest scales and even in single crystals. This suggests that control of nanoscale defects is essential to ensure optimal performance in lead halide perovskite devices. The research could ultimately lead to improved solar power devices.

Summary

Previous research indicates that structural disorder derived from solution processing in lead halide perovskites is as an important determiner of their properties involving the physical effects of light. However, scientists have not found a direct correlation between the functional properties of these materials and the local crystal structure in which non-equilibrium states evolve. This is due in part to their structural heterogeneities, which occur on length scales that defy conventional characterization techniques. To address this knowledge gap, researchers used co-localized electron backscatter diffraction and ultrafast transient reflection microscopy on a single crystal lead halide perovskite microcrystal measuring 14 by 50 micrometers. The researchers used transient reflection microscopy to measure the local charge carrier diffusion coefficient on the single crystal. Subsequent collection of the electron backscatter diffraction patterns on a 250 nanometer square sampling grid provided a means to quantify the local crystal quality via the local diffraction pattern contrast. Correlation between these two measurements shows that intra-crystal variations in crystal quality lead to nearly a factor of 10 reduction in the ambipolar diffusion coefficient. This investigation suggests that variability in the crystal quality profoundly affects the efficiency of charge carrier transport in this important class of photovoltaic materials.

Contact

Erik M Grumstrup
Montana State University
erik.grumstrup@montana.edu  

Funding

This research was supported by the Department of Energy Office of Science, Basic Energy Sciences program. The National Science Foundation supported one of the authors through a graduate research fellowship.

Publications

Hickey, C. L. and Grumstrup, E. M., Direct Correlation of Charge Carrier Transport to Local Crystal Quality in Lead Halide Perovskites. Nano letters 2020, 20 (7), 5050-5056. [DOI: 10.1021/acs.nanolett.0c01244]