A chemical tune up for better perovskites

Scientists in the United States took a closer look at the way charges move through a perovskite material, and found that replacing hydrogen in the material with a heavier isotope can both reduce thermal conductivity and increase carrier lifetime, potentially leading to more efficient solar cells. And the group expects continuing study of charge states and molecule dynamics to yield further important discoveries.

Despite their recent popularity among researchers in photovoltaics, there is still plenty that we don’t understand about perovskite materials. And an understanding of interactions and behaviors deep within the material could add further possibilities the already impressive list of what is theoretically possible with perovskites.

Scientists led by Oak Ridge National Laboratory in the United States took a deep dive into methylammonium lead iodide, the perovskite most commonly investigated in PV applications. Using various spectroscopic techniques, the group investigated the role of phonons – a type of excited state affecting vibrations within the material, and the dynamics of the organic molecule (methylammonium) in the material.

The key finding, presented in the paper Giant isotope effect on phonon dispersion and thermal conductivity in methylammonium lead iodide, published in Science Advances, was that replacing hydrogen in the organic molecule with its heavier isotope deuterium served to both reduce thermal conductivity and slow carrier relaxation.

Both of these factors have the potential to make it easier to capture charges generated by the photovoltaic effect and transport them out of the cell, leading to better-performing solar cells. The group also calculated that these processes within the material should be little affected by light, and the properties would likely remain the same in operation.

The group suggests that it could build on this discovery and develop methods to ‘tune’ vibrational transport properties of the perovskite and further increase hot carrier lifetimes, and that further understanding and ability to control phonons could lead to better-performing perovskites. “…understanding and controlling the phonon properties in this class of materials is critically important,” the researchers state. “Our results reveal how tuning the organic molecule dynamics enables control of phonons important to thermal conductivity and the hot-phonon bottleneck.”

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