An international team led by the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) has applied a new ionic salt at the electron transport layer (ETL) interface in inverted perovskite solar cells to overcome weaknesses in commonly used optimized buckminsterfullerene (C60) ETLs.
Inverted perovskite cells have a device structure known as “p-i-n”, in which hole-selective contact p is at the bottom of intrinsic perovskite layer i with electron transport layer n at the top. Conventional halide perovskite cells have the same structure but reversed – a “n-i-p” layout. In n-i-p architecture, the solar cell is illuminated through the electron-transport layer (ETL) side; in the p-i-n structure, it is illuminated through the HTL surface.
“One big challenge for the conventional device stack is the weak interface between the perovskite absorber layer and the ETL,” Kai Zhu, corresponding author of the research, told pv magazine.
To address the issues, the researchers developed an ionic salt synthesized from C60, referred to in the study as CPMAC. The CPMAC acronym is short for N-methylglycine, and tert-butyl 4-formylbenzylcarbamate molecules and hydrochloric acid. It react with C60, to form an ionic salt called 4-(1′,5′-dihydro-1′-me-thyl-2’H-[5,6] fullereno-C60-Ih-[1,9-c]pyrrol-2′-yl)phenyl-methanaminium chloride.
“We are excited that our new CPMAC ionic salt can significantly strengthen this interface by about a factor of three, and it does not negatively affect device operation,” said Zhu.
In the study, the researchers used the CPMAC salt as the “electron shuttle” in inverted perovskite solar cells (PSC) and minimodules. “This ionic salt layer fundamentally addressed the disadvantages of molecular C60 layer,” said the researchers.
Test results for the unencapsulated perovskite solar cell with CPMAC exhibited a 26% power conversion efficiency (PCE) with 2% degradation after 2,100 hours under standard 1-sun operation at 65 C. The PSC had a 25.5% efficiency with about 5% degradation after 1,500 hours of operation at 85 C.
Mini-modules measuring 6 cm2 achieved a PCE of 23% with less than 9% degradation after 2,200 hours of operation at 55C, according to the research.
The work is described in “C60-based ionic salt electron shuttle for high-performance inverted perovskite solar modules,” published in Science.
The team included researchers from CubicPV, University of Colorado Boulder, Arizona State University, and the University of Toledo, all based in the U.S., as well as from King Abdullah University of Science and Technology in Saudi Arabia and the U.K’s Newcastle University.
The scientists plan to explore the potential for solution deposition based on the research results. “This CPMAC layer can be deposited by solution processing, which is attractive for large-scale manufacturing. We plan to further develop the solution processing conditions and integrate this new material in large module platforms,” said Zhu.
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So we at this point Perovskite based panels are up to an operational life of maybe 2 years. There is a long, long way to go chasing this rainbow. Researchers might more profitably focus on longevity than yield.