Researchers decipher the dynamics of electrons in perovskite crystals

September 20, 2018, University of Erlangen-Nuremberg
This is a POV ray drawing of a small section of the lattice of an imaginary perovskite. The red atoms are oxygen anions while the the green atom represents the larger cation, and the blue central atom the smaller cation, typically with a higher oxidization state. I created this file by writing a XYZ file using a spreadsheet after reading cotton and wilkinson, this was edited using the text editor of ORTEP. ORTEP was used to write the pov file, then POVray was used to draw it. Credit: Wikimedia Commons.

Physicists at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) have proven that incoming light causes the electrons in warm perovskites to rotate, thus influencing the direction of the flow of electrical current. They have thus found the key to an important characteristic of these crystals, which could play an important role in the development of new solar cells. The results have now been published in Proceedings of the National Academy of Sciences.

The sun is an important source of renewable energy. Its radiation energy provides heat, and sunlight can be converted into electricity thanks to photovoltaics. Perovskites, crystalline compounds that can be simply manufactured using chemical processes, are considered a promising material for photovoltaics. Under laboratory conditions, prototypes have achieved surprising levels of efficiency.

There is little knowledge about precisely why perovskites are so powerful. "Two factors are decisive for generating electrical energy cost-efficiently from sunlight," says Dr. Daniel Niesner from the Chair of Solid State Physics at FAU. "On the one hand, the light must excite as many 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."

Researchers suspect that perovskites make particularly good use of the rotation of electrons for efficient current flow. "Each electron has spin, similar to the intrinsic rotation of a billiard ball," explains Niesner. "As is the case with billiard balls, where left-hand or right-hand spin when they are hit with the cue leads to a curved path on the table, scientists have suspected that rotation and forward movement in electrons in perovskites could also be linked."

Orderly atomic structure

Physicists at FAU in Erlangen have now confirmed this suspicion for the first time. In their experiments, they used a laser whose light also has spin or a direction of rotation. The result: If a crystal is exposed to light with a left-hand spin, the electrons move to the left. If the direction of the light is reversed, the direction of the flow of electrons also reverses. "The experiments clearly demonstrate that the direction of rotation of the electrons and the direction of flow of current are linked."

Until now, scientists presumed that the atomic structure of perovskites was too 'orderly' for such behaviour. In actual fact, experiments with cooled crystals show only a very weak link between the direction of rotation of the electrons and the direction of current flow. "This changes, however, when the crystals are heated to room temperature because the movement of the atoms leads to fluctuating deviations of the highly-ordered structure," says Nieser. "The heat enables the crystals of perovskite to link the of rotation and flow of the electrons. A 'normal' crystal couldn't do that."

The discovery of the connection between heat and spin in electrons means that the FAU researchers have uncovered a vital aspect of the unusual flow of current in perovskites. Their 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.

Explore further: Revealing the microscopic mechanisms in perovskite solar cells

More information: Daniel Niesner et al, Structural fluctuations cause spin-split states in tetragonal (CH3NH3)PbI3 as evidenced by the circular photogalvanic effect, Proceedings of the National Academy of Sciences (2018). DOI: 10.1073/pnas.1805422115

Related Stories

Revealing the microscopic mechanisms in perovskite solar cells

March 21, 2017

A material with the perovskite crystal structure has become very popular for solar cells. While most perovskites are inorganic compounds, this new material is a hybrid of relatively inexpensive organic and inorganic materials. ...

Slow 'hot electrons' could improve solar cell efficiency

January 16, 2018

Photons with energy higher than the band gap of the semiconductor absorbing them give rise to what are known as hot electrons. The extra energy in respect to the band gap is lost very fast, as it is converted into heat and ...

Why perovskite solar cells are so efficient

April 25, 2018

Solar cells with efficiencies above 20% and produced at low costs – perovskites make this possible. Now, researchers of Karlsruhe Institute of Technology (KIT) have gained fundamental insight into the function of perovskite ...

Recommended for you

Copper compound as promising quantum computing unit

December 12, 2018

Quantum computers could vastly increase the capabilities of IT systems, bringing major changes worldwide. However, there is still a long way to go before such a device can actually be constructed, because it has not yet ...

Researchers make shape shifting cell breakthrough

December 11, 2018

A new computational model developed by researchers from The City College of New York and Yale gives a clearer picture of the structure and mechanics of soft, shape-changing cells that could provide a better understanding ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Sep 21, 2018
Congrats, perovskites, ie ferroelectrics one venue, have fascinating potential for molecular manipulation.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.