Many high-performing hybrid perovskite solar cells contain water-dissolvable lead, raising concerns over potential leakage from damaged cells. A new technique to sequester the lead used to make the cells could make them more practical for use in photovoltaic technologies. Developed by researchers at Northern Illinois University (NIU) and the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL), the technique involves the application of lead-absorbing films to the front and back of the solar cell to minimize potential toxic leakage. “In the event of a damaged cell, our device captures the great majority of the lead, preventing it from leaching into groundwater and soils. The films that we use are insoluble in water,” professor Tao Xu said. Two laboratory solar cell samples, one (right) with a protective lead-absorbing film applied to the back side. Courtesy of Northern Illinois University. The researchers first applied a coating of lead-absorbing material to the front and back sides of the device stack. On the glass side of the front transparent conducting electrode, they used a transparent lead-absorbing molecular film containing phosphonic acid groups that bind strongly to lead but do not hinder cell capture of light. On the back metal electrode side, which has no need for transparency, the researchers placed a less expensive polymer film blended with lead-chelating agents between the metal electrode and a standard photovoltaic packing film. The lead-absorbing films on both sides swell to absorb the lead, rather than dissolve, when subjected to water soaking, thus retaining structural integrity for easy collection of lead after damage. Simplified graphic depiction (not to scale) of a perovskite cell with lead-absorbing films. Courtesy of Northern Illinois University. “The materials are off the shelf, but they were never used for this purpose,” Xu said. “Light must enter the cell to be absorbed by the perovskite layer, and the front-side film actually acts as an antireflection agent, improving transparency just a bit.” NIU solar cell researchers Xun Li (left), a graduate student in chemistry and biochemistry, and Tao Xu, a professor of chemistry and biochemistry. Courtesy of Northern Illinois University. Tests for lead leakage included hammering and shattering the front-side glass of 2.5- × 2.5-cm cells, and scratching the back side of the solar cells with a razor blade, before submerging them into water. The films were able to absorb the vast majority of the lead in cells severely damaged due to water ingress. Under conditions of severe solar cell damage in a lab setting, the lead-absorbing films sequestered 96% of lead leakage. The experiments further indicated that the lead-absorbing layers did not negatively affect cell performance or long-term operation stability. The newly developed “on-device sequestration approach” can be incorporated with current perovskite solar cell configurations, Xu said. “It is worth noting that the demonstrated lead-sequestration approach is also applicable to other perovskite-based technologies such as solid-state lighting, display, and sensor applications," Kai Zhu, a senior scientist at NREL, said. The research was published in Nature (www.doi.org/10.1038/s41586-020-2001-x).
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