Microscopic Structure of Perovskite Solar Cells

Perovskite materials showed peak efficiency before the intermediate phase transformation was complete (Source: U Houston)

Perovskite materials showed peak efficiency before the intermediate phase transformation was complete (Source: U Houston)

Researchers from the University of Houston have reported the first expl­anation for how a class of materials changes during production to more effi­ciently absorb light, a critical step toward the large-scale manu­facture of better and less-expensive solar panels. The work offers a mechanism study of how a perovskite thin film changes its micro­scopic structure upon gentle heating, said Yan Yao, assistant professor of electrical and computer engineering. This infor­mation is crucial for designing a manu­facturing process that can consis­tently produce high-efficiency solar panels.

Last year Yao and other researchers identified the crystal structure of the non-stoi­chiometric inter­mediate phase as the key element for high-ef­ficiency perovskite solar cells. But what happened during the later thermal annealing step remained unclear. The work is fundamental science, Yao said, but critical for processing more efficient solar cells. “Otherwise, it’s like a black box,” he said.“We know certain processing conditions are important, but we don’t know why.”

The work also yielded a surprise: the materials showed a peak effi­ciency before the inter­mediate phase trans­formation was complete, suggesting a new way to produce the films to ensure maximum efficiency. Yao said researchers would have expected the highest efficiency to come after the material had been converted to 100 percent perovs­kite film. Instead, they discovered the best-performing solar devices were those for which conversion was stopped at 18 percent of the inter­mediate phase, before full conversion.

“We found that the phase comp­osition and morphology of solvent engineered perovs­kite films are strongly dependent on the processing conditions and can signi­ficantly influence photo­voltaic per­formance,” the researchers wrote. “The strong dependence on processing conditions is attributed to the molecular exchange kinetics between organic halide molecules and DMSO (dimethyl sulfoxide) coordinated in the inter­mediate phase.”

Perovs­kite compounds commonly are comprised of a hybrid organic-inorganic lead or tin halide-based material and have been pursued as potential materials for solar cells for several years. Yao said their advantages include the fact that the materials can work as very thin films. About 300 nanometers, compared with between 200 and 300 micro­meters for silicon wafers, the most commonly used material for solar cells. Perovs­kite solar cells also can be produced by solution processing at tempe­ratures below 150 degrees Centi­grade making them relatively inexpensive to produce.

At their best, perovs­kite solar cells have an efficiency rate of about 22 percent, slightly lower than that of silicon with 25 percent. But the cost of silicon solar cells is also dropping drama­tically, and perovs­kite cells are unstable in air, quickly losing effi­ciency. They also usually contain lead, a toxin. Still, Yao said, the materials hold great promise for the solar industry, even if they are unlikely to replace silicon entirely. Instead, he said, they could be used in con­junction with silicon, boosting efficiency to 30 percent or so. (Source: U Houston)

Reference: Y. Rong et al.: Critical kinetic control of non-stoichiometric intermediate phase transformation for efficient perovskite solar cells, Nanoscale 8, 12892 (2016); DOI: 10.1039/C6NR00488A

Link: Dept. of Electrical & Computer Engineering and Materials Science and Engineering Program, University of Houston, Houston, USA

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