ERLANGEN, Germany, Sept. 20, 2018 — Physicists at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) have shown that incoming light will cause the electrons in warm perovskites to rotate, thus influencing the direction of the flow of electrical current.

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FAU researchers uncover the importance of electron spin in perovskite crystals. Courtesy of Panthermedia/Franz Metelec.

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“Two factors are decisive for generating electrical energy cost-efficiently from sunlight,” said professor Daniel Niesner. “On the one hand, the light must excite as many electrons as possible in a layer that’s as thin as possible. On the other, the electrons must be able to flow as freely as possible to the electrodes that pick up the current.”

Each electron has spin, and scientists have long suspected that perovskites are efficient at harvesting light energy because they make particularly good use of electron spin for efficient current flow. The FAU team confirmed this suspicion in experiments using a laser whose light had spin. The researchers found that if a perovskite crystal was exposed to light with a left-hand spin, the electrons moved to the left. If the direction of the light was reversed, the direction of the flow of electrons also changed.

“The experiments clearly demonstrate that the direction of rotation of the electrons and the direction of flow of current are linked,” said Niesner.

Experiments with cooled perovskite crystals showed only a very weak link between the direction of rotation of the electrons and the direction of current flow. According to Niesner, this changed when the crystals were heated to room temperature because the movement of the atoms led to fluctuating deviations in the highly ordered structure of the perovskites. The team’s findings support a spin splitting caused by thermally induced structural fluctuations which break inversion symmetry.

“The heat enables the crystals of perovskite to link the direction of rotation and flow of the electrons. A ‘normal’ crystal couldn’t do that,” said Niesner.

The work could contribute to improving the understanding of the high energy efficiency of these crystals and to developing new materials for photovoltaics in the future.

The research was published in Proceedings of the National Academy of Sciences.(doi:10.1073/pnas.1805422115).

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