An international research team has produced an all-perovskite tandem solar cell with exceptionally high open-circuit voltage and efficiency. The prototype device demonstrates the potential of perovskite, an emerging photovoltaic technology, to overcome some of the limits of silicon solar cells. The prototype solar cell developed by researchers from the University of Toronto, Northwestern University, and the University of Toledo demonstrated an open-circuit voltage of 2.19 eV, setting a new record for all-perovskite tandem solar cells, according to the team. From left, Ph.D. student Aidan Maxwell, postdoctoral fellow Hao Chen, and postdoctoral fellow Chongwen Li from the Department of Electrical and Computer Engineering at the University of Toronto demonstrate their prototype all-perovskite tandem solar cell at their testing facility. Courtesy of Aaron Demeter/University of Toronto Engineering. The device’s power conversion efficiency was measured at 27.4%, which is higher than the current record for traditional single-junction silicon solar cells, the researchers said. The perovskite cell was independently certified at the National Renewable Energy Laboratory in Colorado, where it demonstrated an efficiency of 26.3%. For the prototype, the researchers used two different layers of perovskite, each tuned to a different part of the solar spectrum. The top perovskite layer has a wide band gap to absorb UV and some visible light. The bottom layer has a narrow bandgap, which is tuned toward the IR range of the spectrum. “Between the two, we cover more of the spectrum than would be possible with silicon,” researcher Chongwen Li said. The tandem design enables the cell to produce a high open-circuit voltage, which in turn improves its efficiency. However, when the researchers analyzed the interface between the perovskite layer, where light is absorbed and transformed into excited electrons, and the electron transport layer, they found that they could improve performance further. Quasi-Fermi level splitting (QFLS) measurements showed that recombination at the electron transport layer contact, stemming from inhomogeneous surface potential and poor perovskite-electron transport layer energetic alignment, was limiting open-circuit voltage. “What we found is that the electric field across the surface of the perovskite layer — we call it the surface potential — was not uniform,” researcher Aidan Maxwell said. “In some places, excited electrons were moving easily into the electron transport layer, but in others, they would just recombine with the holes they left behind. Those electrons were being lost to the circuit.” This prototype all-perovskite tandem solar cell measures 1 cm2 and has a power conversion efficiency of 27.4%, which is higher than is currently possible with traditional single-junction silicon solar cells. Courtesy of Aaron Demeter/University of Toronto Engineering. To modify the perovskite surface states and achieve a more uniform spatial distribution of surface potential, the researchers introduced diammonium molecules by coating the surface of the perovskite layer with 1,3-propanediammonium (PDA). “PDA has a positive charge, and it is able to even out the surface potential,” researcher Hao Chen said. “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.” With the use of PDA, QFLS increased by 90 MeV, enabling 1.79 eV perovskite solar cells with a certified 1.33-V open-circuit voltage, and greater than 19% power conversion efficiency. When the researchers incorporated this layer into the monolithic, all-perovskite tandem, they realized the record-breaking open-circuit voltage of 2.19 V and a greater than 27% power conversion efficiency. The team used industry standard methods to measure the stability of the perovskite cell and found that it maintained 86% of its initial efficiency after 500 h of continuous operation. “The team developed a deep chemical understanding of what was limiting a crucial interface — the junction with the electron-extracting layer — in the large-bandgap portion of perovskite solar cells,” professor Alberto Salleo, chair of the Department of Materials Science and Engineering at Stanford University, said. “These insights from basic science, acted on with innovative materials engineering strategies, will continue to drive the field forward.” Unlike silicon solar cells, which are costly to produce, perovskite solar cells are made using low-cost, well-established techniques. Another advantage of perovskites is their tunability. By adjusting the thickness and chemical composition of the crystal films that comprise a perovskite solar cell, manufacturers can selectively tune the wavelengths that the cell absorbs and converts into electricity. The team’s focus will now be on enhancing efficiency by increasing the current that runs through the cell, improving stability, and enlarging the area of the cell so that it can be scaled up to commercial proportions. The team’s prototype solar cell measures 1 cm2. Future improvements will also be guided by the team’s identification of the role of the interfaces between the cell’s layers. “In this work, we’ve focused on the interface between the perovskite layer and the electron transport layer, but there is another important layer that extracts the ‘holes’ those electrons leave behind,” professor Ted Sargent said. “Learning to master one interface doesn’t necessarily teach you the rules for mastering the other interfaces. I think there’s lots more discovery to be done.” The team is optimistic about the ability of perovskite technology to hold its own against silicon, even though silicon solar cells have a multidecade head start. “In the last 10 years, perovskite technology has come almost as far as silicon has in the last 40,” Maxwell said. “Just imagine what it will be able to do in another 10 years.” “Further improvements in the efficiency of solar cells are crucial for the ongoing decarbonization of our economy,” Sargent said. “While silicon solar cells have undergone impressive advancements in recent years, there are inherent limitations to their efficiency and cost, arising from material properties. Perovskite technology can overcome these limitations, but until now, it had performed below its full potential. Our latest study identifies a key reason for this and points a way forward.” The research was published in Nature (www.doi.org/10.1038/s41586-022-05541-z).