Power/Performance Bits: Sept. 19

Healing perovskites; silicon anode for li-ion batteries; electricity from the bloodstream.


Healing perovskites
A team from the University of Cambridge, MIT, University of Oxford, University of Bath, and Delft University of Technology discovered a way to heal defects in perovskite solar cells by exposing them to light and just the right amount of humidity.

While perovskites show promise for low-cost, efficient photovoltaics, tiny defects in the crystalline structure, called traps, can cause electrons to get “stuck” before their energy can be harnessed. The easier that electrons can move around in a solar cell material, the more efficient that material will be at converting photons into electricity.

In a previous study, researchers found that when perovskites were exposed to illumination, iodide ions migrated away from the illuminated region and in the process swept away most of the defects in that region along with them. However, these effects, while promising, were temporary because the ions migrated back to similar positions when the light was removed.

In the new study, the team made a perovskite-based device, printed using techniques compatible with scalable roll-to-roll processes, but before the device was completed, they exposed it to light, oxygen and humidity. Perovskites often start to degrade when exposed to humidity, but the team found that when humidity levels were between 40% and 50%, and the exposure was limited to 30 minutes, degradation did not occur. Once the exposure was complete, the remaining layers were deposited to finish the device.

The concoction of light with water and oxygen molecules leads to substantial defect-healing in metal halide perovskite semiconductors. (Source: Dr. Matthew T. Klug)

When the light was applied, electrons bound with oxygen, forming a superoxide that could very effectively bind to electron traps and prevent these traps from hindering electrons. In the accompanying presence of water, the perovskite surface also gets converted to a protective shell. The shell coating removes traps from the surfaces but also locks in the superoxide, meaning that the performance improvements in the perovskites are now long-lived.

“It’s counter-intuitive, but applying humidity and light makes the perovskite solar cells more luminescent, a property which is extremely important if you want efficient solar cells,” said Sam Stranks, of Cambridge’s Cavendish Laboratory. “We’ve seen an increase in luminescence efficiency from one percent to 89%, and we think we could get it all the way to 100%, which means we could have no voltage loss – but there’s still a lot of work to be done.”

Silicon anode for li-ion batteries
Researchers at the Okinawa Institute of Science and Technology designed an improved anode for lithium-ion batteries, using silicon rather than graphite. Graphite is commonly used to insulate the lithium ions which improves the safety of the battery, but is limited structurally to a small scale.

Silicon promises a greater battery capacity over graphite. Six atoms of carbon are required to bind a single atom of lithium, but an atom of silicon can bind four atoms of lithium at the same time, multiplying the battery capacity by more than 10-fold. However, being able to capture that many lithium ions means that the volume of the anode swells by 300% to 400%, leading to fracturing and loss of structural integrity.

To overcome this issue, the researchers designed an anode built on layers of unstructured silicon films which are deposited alternatively with tantalum metal nanoparticle scaffolds, resulting in the silicon being sandwiched in a tantalum frame that prevents physical collapse.

“We used a technique called Cluster Beam Deposition,” said Marta Haro, a staff scientist at OIST. “The required materials are directly deposited on the surface with great control. This is a purely physical method, there are no need for chemicals, catalysts or other binders.”

The porosity of the nanostructured Tantalum (in black) enables the formation of silicon channels (in blue) allowing lithium ions to travel faster within the battery. The rigidity of the tantalum scaffold also limits the expansion of the silicon and preserve structural integrity. (Source: OIST Nanoparticles by Design Unit)

The anode provides higher power but restrained swelling, and excellent cyclability – the amount of cycles in which a battery can be charged and discharged before losing efficiency. By looking closer into the nanostructured layers of silicon, the scientists realized the silicon shows important porosity with a grain-like structure in which lithium ions could travel at higher speeds compared to unstructured, amorphous silicon, explaining the increase in power. At the same time the presence of silicon channels along the tantalum nanoparticle scaffolds allows the lithium ions to diffuse in the entire structure. On the other hand, the tantalum metal casing, while restraining swelling and improving structural integrity, also limited the overall capacity.

The design is currently at the proof-of-concept stage, and the team sees opportunities to improve capacity along with the increased power.

“It is a very open synthesis approach, there are many parameters you can play around,” commented Dr. Haro. “For example, we want to optimize the numbers of layers, their thickness, and replace tantalum metal with other materials.”

Electricity from the bloodstream
Researchers at Fudan University developed a lightweight power generator based on carbon nanotube fibers that can generate electrical power when surrounded by a flowing saline solution, or even by flowing blood in blood vessels.

To construct the “fiber-shaped fluidic nanogenerator” (FFNG), an ordered array of carbon nanotubes were continuously wrapped around a polymeric core. Carbon nanotubes are electroactive and mechanically stable, and can be spun and aligned in sheets. In the as-prepared electroactive threads, the carbon nanotube sheets coated the fiber core with a thickness of less than half a micron.

For power generation, the FFNG was connected to electrodes and immersed into flowing water or repeatedly dipped into a saline solution. “The electricity was derived from the relative movement between the FFNG and the solution,” the scientists said. According to the theory, an electrical double layer is created around the fiber, and then the flowing solution distorts the symmetrical charge distribution, generating an electricity gradient along the long axis.

(Source: Huisheng Peng/Fudan University)

The power output efficiency of this system was high compared with other types of miniature energy-harvesting devices. The FFNG showed power conversion efficiency reaching 23.3%, and has the advantages of elasticity, tunability, lightweight, and one-dimensionality. The FFNG can be made stretchable by spinning the sheets around an elastic fiber substrate, and performance was maintained after deformation over 1,000,000 cycles.

The team sees potential in weaving the FFNG into fabrics, as well as harvesting electrical energy from the bloodstream for medical applications.