Researchers from the University of Oxford and Stanford University have created all-perovskite tandem solar cells that convert sunlight to electricity at efficiencies above 20%, with the potential to go much higher. According to the scientists, these solar cells could rival and even outperform conventional cells made of silicon – potentially exceeding 30% efficiency.
Writing in the journal Science, the researchers describe using tin and other inexpensive materials to create novel forms of perovskite – a photovoltaic crystalline material they says is thinner, more flexible and easier to manufacture than silicon crystals.
Co-author Michael McGehee, professor of materials science and engineering at Stanford, said, “Perovskite semiconductors have shown great promise for making high-efficiency solar cells at low cost. We have designed a robust, all-perovskite device that converts sunlight into electricity with an efficiency of 20.3 percent, a rate comparable to silicon solar cells on the market today.”
The new device consists of two perovskite solar cells stacked together in tandem. Each cell is printed on glass, but the same technology could be used to print the cells on plastic.
Co-author Henry Snaith, professor of physics at Oxford, said, “The all-perovskite tandem solar cells we have demonstrated clearly outline a road map for thin-film solar cells to deliver over 30 percent efficiency. This is just the beginning.’
The researchers say that although previous studies have shown that adding a layer of perovskite can improve the efficiency of silicon solar cells, a tandem solar device consisting of two all-perovskite cells would be cheaper and less energy intensive to build.
Co-lead author Tomas Leijtens, a postdoctoral researcher at Stanford, said, “A silicon solar panel begins by converting silica rock into silicon crystals through a process that involves temperatures above 3,000 degrees Fahrenheit (1,600 degrees Celsius). Perovskite solar cells can be processed in a laboratory from common materials like lead, tin, iodine and bromine, then printed on glass at room temperature.”
However, building an all-perovskite tandem cell has been a difficult challenge. The main problem is creating stable perovskite materials that can capture enough energy from the sun to produce a suitable voltage.
According to the researchers, a typical perovskite cell harvests photons from the visible part of the solar spectrum. Photons with enough energy can cause electrons in the perovskite crystal to jump across an “energy gap” and create an electric current.
A solar cell with a small energy gap can absorb most photons from the sun but produces a very low voltage. A cell with a larger energy gap generates a higher voltage but allows lower-energy photons to pass right through it.
Co-lead author Giles Eperon, a postdoctoral researcher at Oxford at the time of the study (now at the University of Washington), said, “An efficient tandem device uses two ideally matched solar cells. The cell with the larger energy gap can absorb higher-energy photons and generate a high voltage. The cell with the smaller energy gap can harvest the photons that aren’t collected by the first cell and produce an additional voltage.”
The smaller energy gap has proven to be the bigger challenge for scientists. Working together, Eperon and Leijtens used a unique combination of tin, lead, cesium, iodine and organic materials to create a perovskite cell with a small energy gap.
Eperon said, “We developed a novel perovskite that absorbs infrared light and delivers a 14.8 percent conversion efficiency. We then combined it with a perovskite cell composed of similar materials but with a larger energy gap.”
The result is a tandem device consisting of two perovskite cells with a combined efficiency of 20.3%.
Leijtens added, “There are thousands of possible compounds for perovskites, but this one works very well – quite a bit better than anything before it.”
A concern with perovskites is stability. Rooftop solar panels made of silicon typically last 25 years or more. But some perovskites degrade quickly when exposed to moisture or light. In previous experiments, perovskites made with tin were found to be particularly unstable, according to the researchers.
To assess stability, the research team subjected both experimental cells to temperatures of 212 degrees F (100 degrees C) for four days. The authors wrote, “Crucially, we found that our perovskite cells exhibit excellent thermal and atmospheric stability, unprecedented for tin-based perovskites.”
McGehee added, “The efficiency of our tandem device is already far in excess of the best tandem solar cells made with other low-cost semiconductors, such as organic small molecules and microcrystalline silicon. Those who see the potential realize that these results are amazing.”
The next step is to optimize the composition of the materials to absorb more light and generate an even higher current, Snaith added. He said, “The versatility of perovskites and the low cost of materials and manufacturing, now coupled with the potential to achieve very high efficiencies, will be transformative to the photovoltaic industry once manufacturability and acceptable stability are also proved.”
Photo of perovskite tandem solar cell courtesy of Rongrong Cheacharoe