Researchers from the University of Northern Illinois and the National Renewable Energy Laboratory (NREL) of the US Department of Energy (DOE) in Golden, Colorado, reported on February 19, 2020 in the journal Nature on a possible breakthrough in the development of hybrid perovskite solar cells.
Perovskite solar cells, which are regarded as rising stars in the field of solar energy, convert light into electricity. They are potentially cheaper and easier to manufacture than conventional silicon-based solar cells and have shown comparable efficiencies, at least on a small scale in the laboratory. But there are still crucial challenges to be overcome before they can become a competitive commercial technology.
One major challenge is the use of lead. Most high-performance hybrid perovskite solar cells contain water-soluble lead, raising concerns about possible leakage of damaged cells.
Under the leadership of Tao Xu of NIU and Kai Zhu of NREL, a team of scientists has developed a technique to sequester the lead used to manufacture perovskite solar cells and minimize potential toxic leakage by applying lead-absorbing films to the front and back of the solar cell.
“The problem of lead toxicity was one of the most annoying challenges facing perovskite solar cells over the last mile,” said Xu, a NIU professor of chemistry. “We believe we have a promising cure for this problem – and it could change the game.
“In the event of a damaged cell, our device captures most of the lead, preventing it from seeping into the groundwater and soil. The films we use are water-insoluble.”
Under conditions of severe solar cell damage in a laboratory environment, the lead-absorbing films retained 96% of the lead that leaked out, the scientists said. Their experiments also show that the lead-absorbing films do not negatively affect cell performance or long-term operational stability.
Perovskite solar cells are so called because they use a class of crystal structures similar to the mineral perovskite. The perovskite structured compound within these solar cells is usually a hybrid organic-inorganic material based on lead halide.
Scientists began studying these crystal structures for use in solar cells only about a decade ago and have rapidly increased their efficiency in converting solar energy. While conventional silicon solar cells are manufactured with precise processes at high temperatures, perovskites can be produced with chemical solutions at room temperature.
The newly developed “on-device sequestration approach” can be easily integrated into current perovskite solar cell configurations, said Xu.
A transparent, lead-absorbing film is applied to a conductive glass on the front of the solar cell. The sequestration film contains strong permanent phosphonic acid groups, but does not hinder the cell from capturing light. A less expensive polymer film mixed with lead chelating agents is used on the back of the metal electrode, which does not need to be transparent.
“The materials are off-the-shelf, but they have never been used for this purpose,” Xu said. “Light has to penetrate the cell to be absorbed by the perovskite layer, and the front film actually acts as an anti-reflective agent and only slightly improves transparency.
The tests for lead leakage included hammering and shattering the front glass of 2.5 x 2.5 cm cells and scratching the back of the solar cells with a razor blade before immersing them in water. The films can absorb most of the lead in heavily damaged cells due to water ingress.
“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,” said Zhu, a senior scientist at NREL.