A partnership between the University of Toronto Engineering, Northwestern University, and the University of Toledo has led to an all-perovskite tandem solar cell exhibiting exceedingly high efficiency and record-setting voltage.
The prototype device illustrates the ability of this developing photovoltaic technology to defeat the main limits linked to conventional silicon solar cells. Also, this provides a lesser manufacturing cost.
Further improvements in the efficiency of solar cells are crucial for the ongoing decarbonization of our economy.
Ted Sargent, Engineering Professor, University of Toronto
Sargent added, “While silicon solar cells have undergone impressive advances 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.”
In recent times, Sargent joined the Department of Chemistry and the Department of Electrical and Computer Engineering at Northwestern University.
Conventional solar cells are composed of wafers of very high-purity silicon, which is energetically expensive to produce. On the other hand, perovskite solar cells are constructed from nano-sized crystals that can be dispersed into a liquid and spin-coated onto a surface with the help of affordable and well-established methods.
One more benefit of perovskites is that by adapting the thickness and chemical composition of the crystal films, it is possible for manufacturers to selectively “tune” the wavelengths of light that tend to get absorbed and transformed into electricity. In contrast, silicon frequently absorbs the identical part of the solar spectrum.
In a new study recently reported in Nature, the international research group utilized two diverse perovskite layers. Each layer has been tuned to a diverse 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.
Chongwen Li, Study Co-Lead Author and Postdoctoral Researcher, Sargent’s lab, University of Toronto
Li added, “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.”
The tandem design allows the cell to generate a very high open-circuit voltage, enhancing its efficiency. However, the main innovation came when the team examined the existence of an interface between the perovskite layer, where light could be absorbed and converted into excited electrons. The adjacent layer is called the electron transport layer.
What we found is that the electric field across the surface of the perovskite layer — we call it the surface potential — was not uniform.
Aidan Maxwell, Study Co-Lead Author and PhD Student, University of Toronto
Maxwell added, “The effect of this was that 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.”
For this challenge to be fulfilled, the team coated a substance called 1,3-propanediammonium (PDA) onto the surface of the perovskite layer. Although the coating was measuring only a thickness of a few nanometres, it made a huge difference.
“PDA has a positive charge, and it is able to even out the surface potential. 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 in our overall efficiency,” stated postdoctoral fellow Hao Chen, another of the co-lead authors.
The prototype solar cell of the team quantifies one square centimeter in the area. It generates an open-circuit voltage of around 2.19 eV, considered a record for all-perovskite tandem solar cells. Its power conversion efficiency was quantified at 27.4%, which is higher than the present record for conventional single-junction silicon solar cells.
Also, the cell was separately certified at the National Renewable Energy Laboratory in Colorado, providing an efficiency of 26.3%.
The industry-standard techniques were utilized by the team to quantify the stability of the new cell and discovered that it retained around 86% of its initial efficiency following 500 hours of continuous operation.
“Continuing to advance the efficiency and stability of next-generation solar cells is a crucial priority for decarbonizing the electricity supply,” says Professor Alberto Salleo, Chair of the Department of Materials Science and Engineering at Stanford University. He was not involved in the study.
“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. These insights from basic science, acted on with innovative materials engineering strategies, will continue to drive the field forward.”
At present, the scientists will concentrate on additionally improving efficiency by increasing the current that runs via the cell, enhancing stability, and expanding the area of the cell so that it can be generalized to commercial proportions.
Also, identifying the main role played by the interfaces between layers points the way toward possible improvements in the future.
“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,” says Sargent.
Sargent stated, “One of the intriguing things in my experience with this field is that 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.”
Maxwell states that the potential of perovskite technology to hold its own against silicon, although the latter has had a multi-decade head start, which seems promising.
Sargent added, “In the last ten years, perovskite technology has come almost as far as silicon has in the last 40. Just imagine what it will be able to do in another ten years.”
Chen, H., et al. (2022) Regulating surface potential maximizes voltage in all-perovskite tandems. Nature. doi.org/10.1038/s41586-022-05541-z.