A new method to increase the resilience of inverted perovskite solar cells has been created by an international team of researchers. This is a crucial step toward the commercialization of a cutting-edge photovoltaic technology that could dramatically lower the price of solar energy.
Perovskite solar cells are constructed from nanoscale crystals, as opposed to conventional solar cells, which are constructed from wafers of extremely high-purity silicon. Using low-cost, tried-and-true methods, these perovskite crystals can be distributed into a liquid and spin-coated onto a surface.
The thickness and chemical make-up of the crystal layer can also be changed in order to fine-tune the wavelengths of light that are absorbed by the perovskites. Perovskite layers tailored to various wavelengths can even be stacked on top of one another or conventional silicon cells to create “tandem” cells, which are capable of absorbing a wider range of solar spectrum than current technology.
The latest work, published in the journal Science, included researchers from the University of Toronto, Northwestern University, the University of Toledo and the University of Washington.
“Perovskite solar cells have the potential to overcome the inherent efficiency limitations of silicon solar cells,” says study co-author Ted Sargent, who recently joined the department of chemistry and the department of electrical and computer engineering at Northwestern University but remains affiliated with U of T Engineering, where he has a research lab.
“They are also amenable to manufacturing methods that have a much lower cost than those used for silicon. But one place where perovskites still lag silicon is in their long-term durability. In this study, we used a rational-design approach to address that in a new and unique way.”
I think what we’ve done is to show a new path forward that DFT simulations and rational design can point the way toward promising solutions. But there may be even better molecules out there. Ultimately, we want to get to a place where perovskite solar cells can compete commercially with silicon, which is the state-of-the-art photovoltaic technology of today. This is an important step in that direction, but there is still further to go.
Chongwen Li
Sargent and his associates have made a number of developments recently that enhance the functionality of perovskite solar cells. However, a large portion of this earlier work concentrated on improving efficiency, whereas their most recent work examines the issue of durability.
“One key point of vulnerability in these types of solar cells is the interface between the perovskite layer and the adjacent layers, which we call carrier transport layers,” says Chongwen Li, a post-doctoral researcher who recently moved to U of T Engineering from the University of Toledo and is one of the paper’s lead co-authors.
“These adjacent layers extract the electrons or holes that will flow through the circuit. If the chemical bonding between these layers and perovskite layer gets damaged by light or heat, electrons or holes can’t get into the circuit this lowers the overall efficiency of the cell,” Li says.
To address this issue, the international research team went back to first principles. They determined what kinds of molecules would work best at building a bridge between the perovskite layer and the charge transport layers using computer simulations based on density functional theory (DFT).
“Previous research has shown that molecules known as Lewis bases are good for creating strong bonding between these layers,” says Bin Chen, a post-doctoral researcher in Sargent’s lab who is now a research assistant professor at Northwestern University and a co-author on the paper.
“This is because one end of the molecule bonds to the lead atoms in the perovskite layer and the other bonds to the nickel in the carrier transport layers. What our simulations predicted was that Lewis acids, which contained the element phosphorus, would have the best effect.”
In the lab, the team tried out various formulations of phosphorus-containing molecules. Their experiments showed the best performance with a material known as 1,3 bis(diphenylphosphino)propane, or DPPP.
The team built inverted perovskite solar cells that contained DPPP, as well as some without. Scientists put both kinds of solar cells through tests that mimicked the conditions they would encounter in the real world by shining them with light that was roughly equivalent to the sun’s strength. They also tested subjecting them to intense heat in both light and darkness.
“With DPPP, under ambient conditions that is, no additional heating the overall power conversion efficiency of the cell stayed high for approximately 3,500 hours,” says Li.
“The perovskite solar cells that have been previously published in the literature tend to see a significant drop in their efficiency after 1,500 to 2,000 hours, so this is a big improvement.”
Li says the team has applied for a patent for the DPPP technique and has already received interest from commercial solar cell manufacturers.
“I think what we’ve done is to show a new path forward that DFT simulations and rational design can point the way toward promising solutions,” he says. “But there may be even better molecules out there. Ultimately, we want to get to a place where perovskite solar cells can compete commercially with silicon, which is the state-of-the-art photovoltaic technology of today. This is an important step in that direction, but there is still further to go.”
