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Sunlight causes large changes to the underlying network of atoms that make up perovskites, a promising material for solar cells. Before being hit with light, six iodine atoms rest around a lead atom. Within 10 trillionths of a second after being hit with light, the iodine atoms whirl around each lead atom.
Image caption: Iodine atoms (gold) in perovskites respond to light with unusual rotational motions and distortions around a lead atom (white). These changes could explain the high efficiency of these next-generation solar cell (bottom) materials. (Image: Greg Stewart, SLAC National Accelerator Laboratory)
These first atomic steps distort the structure and result in long-lived changes, similar in size to those observed in melting crystals. Further, the atoms’ motions alter the way electricity moves and may help explain the efficiency of perovskites in solar cells.
In recent years, perovskites have become superstars in the solar cell industry. They are cheap and easy to produce. Despite their popularity, scientists don’t know why perovskites are so efficient. This work shows how atoms in perovskites respond to light and could explain the high efficiency of these next-generation solar cell materials.
Although perovskite solar cell efficiencies have climbed above the 20 percent mark, the fundamental mechanism responsible for these efficiencies is not understood.
To gain insights into the mechanisms, researchers created stop-motion movies of the atoms involved just after the light hits the hybrid perovskites, made from lead, iodine, and methylammonium.
The iodine atoms are arranged in octohedra, eight-sided structures that look like two pyramids joined at their bases. The lead atoms sit inside the octohedra; the methylammonium molecules sit between octohedra. This architecture is common to many of the perovskites investigated for solar cell applications.
At SLAC, researchers hit a perovskite film with two bursts from ultrafast lasers. The technique, called ultrafast electron diffraction, lets them reconstruct the atomic structure. By repeating the experiment with different time delays between the first and second pulse of electrons, the team created a stop-motion movement of the iodine atoms whirling around the lead atoms. That is, a rotationally disordered halide octahedral structure formed in the picoseconds after the light struck.
This work shows the important role of light-induced structural deformations within the lead-iodine lattice. These structural changes could alter the way that charges (electrons and their associated holes) move in hybrid perovskites and provide new information about solar cell efficiencies.