A team of researchers from Northwestern University, the University of Toronto, and the University of Toledo claim to have developed an all-perovskite tandem solar cell with a conversion efficiency of 27.4%, which is comparable with traditional silicon solar cells.

Additionally, perovskite tandem solar cells are less expensive to manufacture. The prototype shows an emerging photovoltaic technology that bypasses the need for silicon and overcomes key limits associated with silicon-based solar cells.

The cell was independently certified at the National Renewable Energy Laboratory in Colorado, delivering an efficiency of 26.3%. The team used industry-standard methods to measure the stability of the new cell and found that it maintained 86% of its initial efficiency after 500 hours of continuous operation.

Ted Sargent of the University of Toronto said, “Further improvements in the efficiency of solar cells are crucial for the ongoing decarbonization of our economy.” He explained that while silicon solar cells have undergone impressive advances, there are inherent limitations to their efficiency and cost arising from material properties. Perovskite technology can overcome these limitations.

Silicon versus Perovskite Solar Cells 

Traditional solar cells are made from wafers of high-purity silicon, which is energetically costly to produce. In contrast, perovskite solar cells are built from nano-sized crystals that can be dispersed into a liquid and spin-coated onto a surface using low-cost techniques.

While the thickness of perovskites can be ‘tuned’ through the chemical composition of the crystal films, allowing it to absorb different wavelengths of light, silicon always absorbs the same part of the solar spectrum.

The team used two different layers of perovskite, each tuned to a different part of the solar spectrum, to produce a tandem solar cell.

“In our cell, the top perovskite layer has a wider band gap, which absorbs well in the ultraviolet part of the spectrum, as well as some visible light,” says Chongwen Li, a postdoctoral researcher in Sargent’s lab.

“The bottom layer has a narrow band gap, which is tuned more toward the infrared part of the spectrum. Between the two, we cover more of the spectrum than would be possible with silicon,” Li added.

The key innovation came when the team analyzed the interface between the perovskite layer, where light is absorbed and transformed into electrons, and the adjacent layer, known as the electron transport layer.

Co-lead author Aidan Maxwell said that the ability of perovskite technology to hold its own against silicon is encouraging. “In the last ten years, perovskite technology has come almost as far as silicon has in the last 40,” he added.

Addressing Challenges

Maxwell said that the electric field across the surface of the perovskite layer was not uniform. The team coated the surface of the perovskite layer with a substance known as 1,3-propanediammonium (PDA), which resulted in improvement.

“PDA has a positive charge, and it is able to even out the surface potential,” says postdoctoral fellow Hao Chen, another of the co-lead authors.

“When we added the coating, we got much better energetic alignment of the perovskite layer with the electron transport layer, and that led to a big improvement on our overall efficiency,” Hoa Chen added.

The researchers will now focus on further enhancing efficiency by increasing the current that runs through the cell and improves its stability. The aim is to enlarge the area of the cell so that it can be scaled up to commercial proportions.

Recently, China-based solar cell and module manufacturer Jinko Solar announced achieving a record maximum solar conversion efficiency of 26.1% in its 182 mm N-type module, which is Jinko’s latest TOPCon (tunnel oxide passivated contact) cell technology. Earlier in the month, a team of researchers at the University of Alcala in Spain claimed that a temperature reduction in solar modules by up to 20ºC can enhance the net system efficiency by about 14%.