Atomic Movie of Perovskite Solar Cells

Atoms in perovskites respond to light with unusual rotational motions and distortions that could explain the high efficiency of these next-generation solar cell materials. (Source: G. Stewart, SLAC)

In recent years, perovskites have taken the solar cell industry by storm. They are cheap, easy to produce and very flexible in their appli­cations. Their effi­ciency at converting light into elec­tricity has grown faster than that of any other material – from under four percent in 2009 to over twenty percent in 2017 – and some experts believe that perovskites could eventually outperform the most common solar cell material, silicon. But despite their popu­larity, researchers don’t know why perov­skites are so efficient. Now experiments with a powerful electron camera at the Depart­ment of Energy’s SLAC National Acce­lerator Labora­tory have discovered that light whirls atoms around in perov­skites, poten­tially explaining the high effi­ciency of these next-generation solar cell materials and providing clues for making better ones.

“We’ve taken a step toward solving the mystery,” said Aaron Lindenberg from the Stanford Insti­tute for Materials and Energy Sciences SIMES and the Stanford PULSE Insti­tute for ultra­fast science, which are jointly operated by Stanford Uni­versity and SLAC. “We recorded movies that show that certain atoms in a perov­skite respond to light within trillionths of a second in a very unusual manner. This may faci­litate the transport of electric charges through the material and boost its effi­ciency.” When light shines on a solar cell material, its energy displaces some of the material’s nega­tively charged electrons. This leaves behind electron holes with a positive charge where the electrons were origi­nally located. Electrons and holes migrate to opposite sides of the material, creating a voltage that can be used to power elec­trical devices.

A solar cell’s effi­ciency depends on how freely electrons and holes can move in the material. Their mobility, in turn, depends on the material’s atomic structure. In silicon solar cells, for example, silicon atoms line up in a very orderly fashion inside crystals, and even the smallest structural defects reduce the material’s ability to effi­ciently harvest light. As a result, silicon crystals must be grown in costly, multistep proce­dures under extremely clean conditions. In contrast, “perov­skites are readily produced by mixing chemicals into a solvent, which evaporates to leave a very thin film of perov­skite material,” said Xiaoxi Wu from SIMES at SLAC. “Simpler proces­sing means lower costs. Unlike silicon solar cells, perov­skite thin films are also lightweight and flexible and can be easily applied to virtually any surface.” But what exactly is it about perov­skites that allows some of them to harvest light very effi­ciently? Scientists think that one of the keys is how their atoms move in response to light.

To find out more, Wu and her colleagues studied these motions in a proto­type material made of iodine, lead and an organic molecule called methyl­ammonium. The iodine atoms are arranged in octohedra. The lead atoms sit inside the octohedra and the methyl­ammonium molecules sit between octo­hedra. This archi­tecture is common to many of the perov­skites inves­tigated for solar cell applications. “Previous studies have mostly explored the role of the methylammonium ions and their motions in trans­porting electric charge through the material,” Wu said. “However, we’ve discovered that light causes large defor­mations in the network of lead and iodine atoms that could be crucial for the effi­ciency of perov­skites.”

At SLAC’s Acce­lerator Structure Test Area ASTA, the researchers first hit a perov­skite film, less than two millionths of an inch thick, with a 40-femto­second laser pulse. To determine the atomic response, they sent a 300-femto­second pulse of highly energetic electrons through the material and observed how the electrons were deflected in the film. This ultrafast electron diffrac­tion (UED), allowed them to reconstruct the atomic structure. “By repeating the experi­ment with different time delays between the two pulses, we obtained a stop-motion movie of the lead and iodine atoms’ motions after the light hit,” said Xijie Wang, SLAC’s lead scientist for UED.

The team expected that the light pulse would affect atoms evenly in all directions, causing them to jiggle around their original positions. “But that’s not what happened,” Linden­berg said. “Within 10 trillionths of a second after the laser pulse, the iodine atoms rotated around each lead atom as if they were moving on the surface of a sphere with the lead atom at the center, switching each octa­hedron from a regular shape to a distorted one.” The surprising defor­mations were long-lived and unexpec­tedly large, similar in size to those observed in melting crystals. “This motion could alter the way charges move,” Wu said. “This response to light could enhance effi­ciency, for instance by allowing electric charges to migrate through defects and protec­ting them from being trapped in the material.”

“The results from the Linden­berg group provide fasci­nating first-time insights into the pro­perties of hybrid perov­skites using ultrafast electron dif­fraction as a unique tool,” accor­ding to Felix Deschler, an expert in the field of light-induced physics of novel materials and a researcher at Cam­bridge Uni­versity’s Caven­dish Lab. “Knowledge about the detailed atomic motion after photo­excitation yields new infor­mation about their perfor­mance and can provide new guide­lines for material develop­ment.” (Source: SLAC)

Reference: X. Wu et al.: Light-induced picosecond rotational disordering of the inorganic sublattice in hybrid perovskites, Sci. Adv. 3, e1602388 (2017); DOI: 10.1126/sciadv.1602388

Link: Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, USA

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