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New perovskite solar cell design could outperform existing commercial technologies

Researchers have created a new type of solar cell that replaces silicon with a crystal called perovskite, this design converts sunlight to electricity at efficiencies similar to current technology but at much lower cost

21 Oct 2016 | Editor

Scientists at Stanford University and Oxford University researchers have announced they have created novel solar cells from crystalline perovskite that they claim could outperform existing silicon cells on the market today. They said their design converts sunlight to electricity at efficiencies of 20%, similar to current technology but at much lower cost.

The research has been published in the Oct. 21 edition of Science, the researchers describe using tin and other abundant elements to create novel forms of perovskite.

The new device consists of two perovskite solar cells stacked in tandem. Each cell is printed on glass, but the same technology could be used to print the cells on plastic.

Previous studies showed that adding a layer of perovskite can improve the efficiency of silicon solar cells. But a tandem device consisting of two all-perovskite cells would be cheaper and less energy-intensive to build said the authrors.

Building an all-perovskite tandem device has been a difficult challenge. The main problem is creating stable perovskite materials capable of capturing enough energy from the sun to produce a decent voltage.

A typical perovskite cell harvests photons from the visible part of the solar spectrum. Higher-energy photons 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 but produces a very low voltage. A cell with a larger energy gap generates a higher voltage, but lower-energy photons pass right through it.

The smaller 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 an efficient cell with a small energy gap.

One 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.

To assess stability, the research team subjected both experimental cells to temperatures of 212 degrees Fahrenheit (100 degrees Celsius) for four days.

The researchers found that their exhibited excellent thermal and atmospheric stability, which they beleive is unprecedented for tin-based perovskites.

Michael McGehee, a professor of materials science and engineering at Stanford Univerisyt, said, "Perovskite semiconductors have shown great promise for making high-efficiency solar cells at low cost." Michael added, "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."
Michael, continued, "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."
Henry Snaith, a professor of physics at Oxford Univeristy, said, "The all-perovskite tandem cells we have demonstrated clearly outline a roadmap for thin-film solar cells to deliver over 30% efficiency." Henry added, "This is just the beginning." Henry concluded, "The next step is to optimise the composition of the materials to absorb more light and generate an even higher current. The versatility of perovskites, 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 proven."
Tomas Leijtens, a postdoctoral scholar at Stanford University, 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)." Tomas added, "Perovskite cells can be processed in a laboratory from common materials like lead, tin and bromine, then printed on glass at room temperature." Tomas concluded, "There are thousands of possible compounds for perovskites, but this one works very well, quite a bit better than anything before it.”
Giles Eperon, an Oxford postdoctoral scholar currently at the University of Washington, said, "An efficient tandem device would consist of two ideally matched cells." Giles added, "The cell with the larger energy gap would absorb higher-energy photons and generate an additional voltage.""
Giles continued, "The cell with the smaller energy gap can harvest photons that aren’t collected by the first cell and still produce a voltage. We developed a novel perovskite that absorbs lower-energy 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."

Funding was provided by the Graphene Flagship, The Leverhulme Trust, U.K. Engineering and Physical Sciences Research Council, European Union Seventh Framework Programme, Horizon 2020, U.S. Office of Naval Research and the Global Climate and Energy Project at Stanford.

Perovskite-perovskite tandem photovoltaics with optimized bandgaps

Giles E. Eperon | Tomas Leijtens | Kevin A. Bush | Rohit Prasanna | Thomas Green | Jacob Tse-Wei Wang | David P. McMeekin | George Volonakis | Rebecca L. Milot | Richard May | Axel Palmstrom | Daniel J. Slotcavage | Rebecca A. Belisle | Jay B. Patel | Elizabeth S. Parrott | Rebecca J. Sutton | Wen Ma | Farhad Moghadam | Bert Conings | Aslihan Babayigit | Hans-Gerd Boyen | Stacey Bent | Feliciano Giustino | Laura M. Herz | Michael B. Johnston | Michael D. McGehee | Henry J. Snaith

Science 20 Oct 2016: | DOI: 10.1126/science.aaf9717

Abstract

We demonstrate four and two-terminal perovskite-perovskite tandem solar cells with ideally matched bandgaps. We develop an infrared absorbing 1.2eV bandgap perovskite, FA0.75Cs0.25Sn0.5Pb0.5I3, that can deliver 14.8% efficiency. By combining this material with a wider bandgap FA0.83Cs0.17Pb(I0.5Br0.5)3 material, we reach monolithic two terminal tandem efficiencies of 17.0% with over 1.65 volts open-circuit voltage. We also make mechanically stacked four terminal tandem cells and obtain 20.3% efficiency. Crucially, we find that our infrared absorbing perovskite cells exhibit excellent thermal and atmospheric stability, unprecedented for Sn based perovskites. This device architecture and materials set will enable “all perovskite” thin film solar cells to reach the highest efficiencies in the long term at the lowest costs.

www.stanford.edu    www.ox.ac.uk   

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