Inverted perovskite solar cell achieves 25% efficiency

Researchers from Northwestern University have developed a dual-molecule solution for perovskite solar cells, to overcome losses in efficiency as sunlight is converted to energy. The research findings, published in the journal Science, have helped the emerging technology hit new milestones for efficiency.

By incorporating a molecule to address something called surface recombination, in which electrons are lost when they are trapped by defects (missing atoms on the surface), and a second molecule to disrupt recombination at the interface between layers, the researchers achieved a National Renewable Energy Lab (NREL) certified efficiency of 25.1%.

Professor Ted Sargent said perovskite solar technology is moving fast, with the emphasis of research and development shifting from the bulk absorber to the interfaces. “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,” Sargent said.

Conventional solar cells are made of high-purity silicon wafers that are energy-intensive to produce and can only absorb a fixed range of the solar spectrum. Perovskite materials whose size and composition can be adjusted to ‘tune’ the wavelengths of light they absorb are a favourable and high-energy emerging tandem technology.

In the present research, rather than trying to help the cell absorb more sunlight, the researchers 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 another. The electron can also move back outward and fill (or ‘recombine’) with holes that exist on the perovskite layer. First author Cheng Liu said recombination at the interface is complex. “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,” Liu said.

Previous research from Sargent’s team has found that one molecule, PDAI₂, is good at interface recombination. Next they needed to find a molecule that would work to repair surface defects and prevent electrons from recombining them. By finding the mechanism that would allow PDAI₂ to work with a secondary molecule, the team narrowed in on sulfur, which can replace carbon groups to cover missing atoms and suppress recombination.

The researchers developed a coating for the substrate beneath the perovskite layer to help the cell work at a higher temperature for a longer period. This solution, according to Liu, can work in tandem with the findings from the recent study.

“We have to use a more flexible strategy to solve the complex interface problem. 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,” Cheng said.

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