Power/Performance Bits: Oct. 4

New in solar: nighttime batteries; when defects are good; boosting perovskite efficiency.

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Solar battery

Chemists at the University of Wisconsin–Madison and the King Abdullah University of Science and Technology in Saudi Arabia integrated solar cells with a large-capacity battery in a single device that eliminates the usual intermediate step of making electricity and, instead, transfers the energy directly to the battery’s electrolyte.

The team used a redox flow battery, or RFB, which stores energy in a tank of liquid electrolyte.

According to Song Jin, a professor of chemistry at the University of Wisconsin–Madison, discharging the battery to power the electric grid at night could hardly be simpler. “We just connect a load to a different set of electrodes, pass the charged electrolyte through the device, and the electricity flows out.”

Solar charging and electrical discharging, he notes, can be repeated for many cycles with little efficiency loss.

Unlike lithium-ion batteries, which store energy in solid electrodes, the RFB stores chemical energy in liquid electrolyte. “The RFB is relatively cheap and you can build a device with as much storage as you need, which is why it is the most promising approach for grid-level electricity storage,” said Jin.

In the new device, standard silicon solar cells are mounted on the reaction chamber and energy converted by the cell immediately charges the water-based electrolyte, which is pumped out to a storage tank.

This solar-charged device directly transfers energy from sunlight into a liquid battery and stores it in the container at lower right. During the discharge cycle, electricity leaves the device through electrodes at top. (Source: David Tenenbaum/University of Wisconsin–Madison)

This solar-charged device directly transfers energy from sunlight into a liquid battery and stores it in the container at lower right. During the discharge cycle, electricity leaves the device through electrodes at top. (Source: David Tenenbaum/University of Wisconsin–Madison)

Redox flow batteries already on the market have been attached to solar cells, “but now we have one device that harvests sunlight to liberate electrical charges and directly changes the oxidation-reduction state of the electrolyte on the surface of the cells,” said Wenjie Li, graduate student and the first author of the study. “We are using a single device to convert solar energy and charge a battery. It’s essentially a solar battery, and we can size the RFB storage tank to store all the energy generated by the solar cells.”

The unified design suggests multiple advantages. “The solar cells directly charge the electrolyte, and so we’re doing two things at once, which makes for simplicity, cost reduction and potentially higher efficiency,” said Jin.

The team is working on improvements to the system. One would be to match the solar cell’s voltage to the chemistry of the electrolyte, minimizing losses as energy is converted and stored. They are also searching for electrolytes with larger voltage differential, which currently limits energy storage capacity.

Defects lead to increased performance

Scientists at the University of Connecticut and the U.S. Department of Energy’s Brookhaven National Laboratory discovered highly-conductive properties in cadmium telluride, a promising material that could someday surpass the performance of silicon in solar cells.

While studying the effects of a chloride solution treatment on solar cells made of cadmium telluride, the team of researchers noticed that microscopic fault lines within and between crystals in the material acted as conductive pathways that eased the flow of electric current.

Normally, such planar defects – the fault-like misalignments in the arrangement of atoms within a material – are viewed as a bad thing. They can create dead-end traps in materials that interrupt the flow of electric current in solar cells and reduce their efficiency. But the opposite is true, it appears, with cadmium telluride.

This image shows a slice of cadmium telluride with a 3-D cutaway revealing the pathways of conductivity (bright spots) in crystal grains and along planar defects throughout the material. (Source: Justin Luria/UConn Image)

This image shows a slice of cadmium telluride with a 3-D cutaway revealing the pathways of conductivity (bright spots) in crystal grains and along planar defects throughout the material. (Source: Justin Luria/UConn Image)

“Cadmium telluride is a market-ready technology and primary competitor to silicon-based solar cells,” says Justin Luria, a postdoctoral fellow and one of the project’s lead researchers. “This study identifies new paths to optimize the performance of cadmium telluride solar cells and increases our understanding of the conductive properties of this promising material.”

“There is already a billion-dollar-a-year industry making cadmium telluride solar cells, and lots of work exploring other alternatives to silicon,” said Eric Stach, head of the electron microscopy group at the Brookhaven National Laboratory’s Center for Functional Nanomaterials. “But all of these alternatives, because of their crystal structure, have a higher tendency to form defects … In the case of cadmium telluride, we may want to find ways to make more of these defects, or look for other materials in which defects improve performance.”

Improving perovskite efficiency

Meanwhile, a lab at EPFL integrated rubidium cations into perovskites, maintaining exceptional stability over 500 continuous hours in full sunlight at 85°C, while pushing power-conversion efficiency to a reported record value of 21.6%. The lab has submitted a patent based on their innovation.

With perovskites, heat stability is an issue and can significantly limit the solar cell’s long-term efficiency, as the cell’s structure can degenerate over time. One solution has been to mix perovskites with other materials, such as cesium, that can improve the cell’s stability without compromising its efficiency in converting light into electrical current.

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A cross-section of a perovskite solar cell seen through a scanning electron microscope. (Source: M.Grätzel/EPFL)

The project also showed that perovskite cells built with rubidium make available voltage close to the so-called “thermodynamic limit,” which is the theoretical maximum efficiency of converting sunlight to electricity. According to Michael Saliba, a postdoc who led the project, “this paves the way toward an industrially deployable, new generation of perovskite photovoltaics.”