Two molecules needed to address recombination problem

Researchers at Northwestern University have again raised the standards for perovskite solar cells with a new development that has helped the emerging technology hit new records for efficiency.

In a paper in Science, the researchers report a dual-molecule solution to overcoming losses in efficiency as sunlight is converted to energy. This solution involves 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 then a second molecule to disrupt recombination at the interface between layers. In this way, the researchers were able to achieve 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 Ted Sargent at Northwestern University. “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 is the co-executive director of the Paula M. Trienens Institute for Sustainability and Energy (formerly ISEN) and a multidisciplinary researcher in materials chemistry and energy systems. He has appointments in the department of chemistry in the Weinberg College of Arts and Sciences and the department of electrical and computer engineering in the McCormick School of Engineering.

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. In contrast, the size and composition of perovskite materials can be adjusted to ‘tune’ the wavelengths of light they absorb, making them a favorable and potentially lower-cost, high-efficiency emerging tandem technology.

Historically, perovskite solar cells have been plagued by challenges over improving their efficiency because of their relative instability. Over the past few years, however, advances from Sargent’s lab and others have brought the efficiency of perovskite solar cells to within the same range as can be achieved with silicon solar cells.

In the present study, rather than trying to help the solar cell absorb more sunlight, the team focused on increasing efficiency by maintaining and retaining the generated electrons. When the perovskite layer contacts the electron transport layer of the cell, electrons move from one to the other. But the electrons can also move back outward and ‘recombine’ with holes in 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 had uncovered evidence that one molecule, PDAI₂, does a good job at solving interface recombination. But they also needed to find a molecule that would work to repair surface defects and prevent electrons from recombining with them. This led them to narrow in on sulfur, theorizing that it could replace carbon groups — typically poor at preventing electrons from moving — to cover missing atoms and suppress recombination.

In a recent paper in Nature, the same researchers reported developing 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 detailed in the Science paper.

While the team hopes their findings will encourage the larger scientific community to continue moving the work forward, they too will be working on follow-ups.

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

This story is adapted from material from Northwestern University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.