Packing tiny solar cells together, like micro-lenses in the compound eye of an insect, could pave the way to a new generation of advanced photovoltaics, say Stanford University scientists.

In a new study, the Stanford team used the insect-inspired design to protect a fragile photovoltaic material called perovskite from deteriorating when exposed to heat, moisture or mechanical stress. The results are published in the journal Energy & Environmental Science (E&ES).

A compound solar cell illuminated from a light source below. Hexagonal scaffolds are visible in the regions coated by a silver electrode. The new solar cell design could help scientists overcome a major roadblock to the development of perovskite photovoltaics. Credit: Dauskardt Lab/Stanford University

“Perovskites are promising, low-cost materials that convert sunlight to electricity as efficiently as conventional solar cells made of silicon,” said Reinhold Dauskardt, a professor of materials science and engineering and senior author of the study. “The problem is that perovskites are extremely unstable and mechanically fragile. They would barely survive the manufacturing process, let alone be durable long-term in the environment.”

Most solar devices, like rooftop panels, use a flat, or planar, design. But that approach doesn’t work well with perovskite solar cells.

“Perovskites are the most fragile materials ever tested in the history of our lab,” said graduate student Nicholas Rolston, a co-lead author of the E&ES study. “This fragility is related to the brittle, salt-like crystal structure of perovskite, which has mechanical properties similar to table salt.”

Eye of the fly

To address the durability challenge, the Stanford team turned to nature.

“We were inspired by the compound eye of the fly, which consists of hundreds of tiny segmented eyes,” Dauskardt explained. “It has a beautiful honeycomb shape with built-in redundancy: If you lose one segment, hundreds of others will operate. Each segment is very fragile, but it’s shielded by a scaffold wall around it.”

Using the compound eye as a model, the researchers created a compound solar cell consisting of a vast honeycomb of perovskite microcells, each encapsulated in a hexagon-shaped scaffold just 0.02 inches (500 microns) wide.

“The scaffold is made of an inexpensive epoxy resin widely used in the microelectronics industry,” Rolston said. “It’s resilient to mechanical stresses and thus far more resistant to fracture.”

Tests conducted during the study revealed that the scaffolding had little effect on the perovskite’s ability to convert light into electricity.

“We got nearly the same power-conversion efficiencies out of each little perovskite cell that we would get from a planar solar cell,” Dauskardt said. “So we achieved a huge increase in fracture resistance with no penalty for efficiency.”

Durability

But could the new device withstand the kind of heat and humidity that conventional rooftop solar panels endure?

To find out, the researchers exposed encapsulated perovskite cells to temperatures of 185 degrees Fahrenheit (85 degrees Celsius) and 85 percent relative humidity for six weeks. Despite these extreme conditions, the cells continued to generate electricity at relatively high rates of efficiency.

Dauskardt and his colleagues have filed a provisional patent for the new technology. To improve efficiency, they are studying new ways to scatter light from the scaffold into the perovskite core of each cell.

“We are very excited about these results,” he said. “It’s a new way of thinking about designing solar cells. These scaffold cells also look really cool, so there are some interesting aesthetic possibilities for real-world applications.”

Imagine repeatedly lifting 165 times your weight without breaking a sweat — a feat normally reserved for heroes like Spider-Man.

Rutgers University-New Brunswick engineers have discovered a simple, economical way to make a nano-sized device that can match the friendly neighborhood Avenger, on a much smaller scale. Their creation weighs 1.6 milligrams (about as much as five poppy seeds) and can lift 265 milligrams (the weight of about 825 poppy seeds) hundreds of times in a row.

Its strength comes from a process of inserting and removing ions between very thin sheets of molybdenum disulfide (MoS2), an inorganic crystalline mineral compound. It’s a new type of actuator – devices that work like muscles and convert electrical energy to mechanical energy.

The Rutgers discovery — elegantly called an “inverted-series-connected (ISC) biomorph actuation device” — is described in a study published online today in the journal Nature.

“We found that by applying a small amount of voltage, the device can lift something that’s far heavier than itself,” said Manish Chhowalla, professor and associate chair of the Department of Materials Science and Engineering in the School of Engineering. “This is an important finding in the field of electrochemical actuators. The simple restacking of atomically thin sheets of metallic MoS2 leads to actuators that can withstand stresses and strains comparable to or greater than other actuator materials.”

Actuators are used in a wide variety of electromechanical systems and in robotics. They have applications such as steerable catheters, aircraft wings that adapt to changing conditions and wind turbines that reduce drag, the study notes.

The discovery at Rutgers University-New Brunswick was made by Muharrem Acerce, study lead author and a doctoral student in Chhowalla’s group, with help from E. Koray Akdo?an, teaching assistant professor in Department of Materials Science and Engineering, said Chhowalla, senior author of the study.

Molybdenum disulfide — a naturally occurring mineral — is commonly used as a solid-state lubricant in engines, according to Chhowalla, who also directs the Rutgers Institute for Advanced Materials, Devices and Nanotechnology. It’s a layered material like graphite, with strong chemical bonding within thin layers but weak bonding between the layers. Thus, individual layers of MoS2 can be easily separated into individual thin sheets via chemistry.

The extremely thin sheets, also called nanosheets, remain suspended in solvents such as water. The nanosheets can be assembled into stacks by putting the solution onto a flexible material and allowing the solvent to evaporate. The restacked sheets can then be used as electrodes — similar to those in batteries – with high electrical conductivity to insert and remove ions. Inserting and removing ions leads to the expansion and contraction of nanosheets, resulting in force on the surface. This force triggers the movement — or actuation — of the flexible material.

Chhowalla and his group members found that their MoS2-based electrochemical device has mechanical properties such as stress, strain and work capacity that are extraordinary considering the electrodes are made by simply stacking weakly interacting nanosheets.

“The next step is to scale up and try to make actuators that can move bigger things,” Chhowalla said.