Researchers at Northwestern University and University of Toronto have developed a way to improve the efficiency of inverted perovskite solar cell using a combination of molecules to address different issues. They reported a dual-molecule solution to overcoming losses in efficiency as sunlight is converted to energy.
By incorporating a molecule to address surface recombination, in which electrons are lost when they are trapped by defects — missing atoms on the surface, with a second molecule to disrupt recombination at the interface between layers, the team achieved a National Renewable Energy Lab (NREL) certified efficiency of 25.1% where earlier approaches reached efficiencies of just 24.09%.
“Perovskite solar technology is moving fast, and the emphasis of research and development is shifting from the bulk absorber to the interfaces,” said Northwestern professor Ted Sargent. “This is the critical point to further improve efficiency and stability and bring us closer to this promising route to ever-more-efficient solar harvesting.”
In their recent study, rather than trying to help the cell absorb more sunlight, the team focused on the issue of maintaining and retaining generated electrons to increase efficiency. When the perovskite layer contacts the electron transport layer of the cell, electrons move from one to the other. But the electron can move back outward and fill, or “recombine” with holes that exist on the perovskite layer.
“Recombination at the interface is complex,” said first author Cheng Liu, a postdoctoral student in the Sargent lab. “It’s very difficult to use one type of molecule to address complex recombination and retain electrons, so we considered what combination of molecules we could use to more comprehensively solve the problem.”
Past research from Sargent’s team has found evidence that one molecule, PDAI2, does a good job at solving interface recombination. Next they needed to find a molecule that would work to repair surface defects and prevent electrons from recombining with them. By finding the mechanism that would allow PDAI2 to work with a secondary molecule, the team focused on sulfur, which could replace carbon groups — typically poor at preventing electrons from moving — to cover missing atoms and suppress recombination.
"In addressing the core inefficiencies found in inverted perovskite solar cells, which are predominantly due to nonradiative recombination losses, a new standard in solar cell efficiency is being set," said Northwestern professor Mercouri Kanatzidis. "This is a prime illustration of how the field of advanced materials chemistry can significantly enhance the energy conversion efficiency and longevity of emerging perovskite photovoltaic technologies."
"We are excited that our bimolecular strategy shows applicability to a range of perovskite compositions, including those that are promising for tandem solar cells," said Bin Chen, research assistant professor of chemistry and a co-author on the paper.
“We have to use a more flexible strategy to solve the complex interface problem,” Chen said. “We can’t only use one kind of molecule, as people previously did. We use two molecules to solve two kinds of recombination, but we are sure there’s more kinds of defect-related recombination at the interface. We need to try to use more molecules to come together and make sure all molecules work together without destroying each other’s functions.”