Light-based tech could inspire Moon navigation and next-gen farming

Super-thin chips made from lithium niobate are set to overtake silicon chips in light-based technologies, according to world-leading scientists in the field, with potential applications ranging from remote ripening-fruit detection on Earth to navigation on the Moon.

They say the artificial crystal offers the platform of choice for these technologies due to its superior performance and recent advances in manufacturing capabilities.

RMIT University's Distinguished Professor Arnan Mitchell and University of Adelaide's Dr Andy Boes led this team of global experts to review lithium niobate's capabilities and potential applications in the journal Science.

The international team, including scientists from Peking University in China and Harvard University in the United States, is working with industry to make navigation systems that are planned to help rovers drive on the Moon later this decade.

As it is impossible to use global positioning system (GPS) technology on the Moon, navigation systems in lunar rovers will need to use an alternative system, which is where the team's innovation comes in.

By detecting tiny changes in laser light, the lithium-niobate chip can be used to measure movement without needing external signals, according to Mitchell.

"This is not science fiction -- this artificial crystal is being used to develop a range of exciting applications. And competition to harness the potential of this versatile technology is heating up," said Mitchell, Director of the Integrated Photonics and Applications Centre.

He said while the lunar navigation device was in the early stages of development, the lithium niobate chip technology was "mature enough to be used in space applications."

"Our lithium niobate chip technology is also flexible enough to be rapidly adapted to almost any application that uses light," Mitchell said.

"We are focused on navigation now, but the same technology could also be used for linking internet on the Moon to the internet on Earth."

What is lithium niobate and how can it be used?

Lithium niobate is an artificial crystal that was first discovered in 1949 but is "back in vogue," according to Boes.

"Lithium niobate has new uses in the field of photonics -- the science and technology of light -- because unlike other materials it can generate and manipulate electro-magnetic waves across the full spectrum of light, from microwave to UV frequencies," he said.

"Silicon was the material of choice for electronic circuits, but its limitations have become increasingly apparent in photonics.

"Lithium niobate has come back into vogue because of its superior capabilities, and advances in manufacturing mean that it is now readily available as thin films on semiconductor wafers."

A layer of lithium niobate about 1,000 times thinner than a human hair is placed on a semiconductor wafer, Boes said.

"Photonic circuits are printed into the lithium niobate layer, which are tailored according to the chip's intended use. A fingernail-sized chip may contain hundreds of different circuits," he said.

How does the lunar navigation tech work?

The team is working with the Australian company Advanced Navigation to create optical gyroscopes, where laser light is launched in both clockwise and anticlockwise directions in a coil of fibre, Mitchell said.

"As the coil is moved the fibre is slightly shorter in one direction than the other, according to Albert Einstein's theory of relativity," he said.

"Our photonic chips are sensitive enough to measure this tiny difference and use it to determine how the coil is moving. If you can keep track of your movements, then you know where you are relative to where you started. This is called inertial navigation."

Potential applications closer to home

This technology can also be used to remotely detect the ripeness of fruit.

"Gas emitted by ripe fruit is absorbed by light in the mid-infrared part of the spectrum," Mitchell said.

"A drone hovering in an orchard would transmit light to another which would sense the degree to which the light is absorbed and when fruit is ready for harvesting.

"Our microchip technology is much smaller, cheaper and more accurate than current technology and can be used with very small drones that won't damage fruit trees."

Read more at Science Daily

Built to last: The perovskite solar cells tough enough to match mighty silicon

Researchers at Oxford University and Exciton Science have demonstrated a new way to create stable perovskite solar cells, with fewer defects and the potential to finally rival silicon's durability.

By removing the solvent dimethyl-sulfoxide and introducing dimethylammonium chloride as a crystallisation agent, the researchers were able to better control the intermediate phases of the perovskite crystallisation process, leading to thin films of greater quality, with reduced defects and enhanced stability.

Large groups of up to 138 sample devices were then subjected to a rigorous accelerated ageing and testing process at high temperatures and in real-world conditions.

Formamidinium-caesium perovskite solar cells created using the new synthesis process significantly outperformed the control group and demonstrated resistance to thermal, humidity and light degradation.

This is a strong step forward to matching commercial silicon's stability and makes perovskite-silicon tandem devices a much more realistic candidate for becoming the dominant next-generation solar cell.

Led by Professor Henry Snaith (Oxford University) and Professor Udo Bach (Monash University), the work has been published in the journal Nature Materials and is available here.

Oxford University PhD student Philippe Holzhey, a Marie Curie Early Stage Researcher and joint first author on the work, said: "It's really important that people start shifting to realise there is no value in performance if it's not a stable performance.

"If the device lasts for a day or a week or something, there's not so much value in it. It has to last for years."

During testing, the best device operated above the T80 threshold for over 1,400 hours under simulated sunlight at 65°C. T80 is the time it takes for a solar cell to reduce to 80% of its initial efficiency, a common benchmark within the research field.

Beyond 1,600 hours, the control device fabricated using the conventional dimethyl-sulfoxide approach stopped functioning, while devices fabricated with the new, improved design retained 70% of their original efficiency, under accelerated aging conditions.

The same degradation study was performed on a group of devices at the very high temperature of 85°C, with the new cells again outperforming the control group.

Extrapolating from the data, the researchers calculated that the new cells age by a factor of 1.7 for each 10°C increase in the temperature they are exposed to, which is close to the 2-fold increase expected of commercial silicon devices.

Dr David McMeekin, the corresponding and joint first author on the paper, was an Australian Centre for Advanced Photovoltaics (ACAP) Postdoctoral Fellow at Monash University and is now a Marie Skłodowska-Curie Postdoctoral Fellow at Oxford University.

He said: "I think what separates us from other studies is that we've done a lot of accelerated aging. We've aged the cells at 65°C and 85°C under the whole light spectrum."

The number of devices used in the study is also significant, with many other perovskite research projects limited to just one or two prototypes.

"Most studies only show one curve without any standard deviation or any kind of statistical approach to determine if this design is more stable than the other," David added.

The researchers hope their work will encourage a greater focus on the intermediate phase of perovskite crystallisation as an important factor in achieving greater stability and commercial viability.

This work was supported by the Stanford Linear Accelerator Center (SLAC) and the National Renewable Energy Laboratory (NREL).

Background: About Perovskites

Artificially synthesised in laboratory conditions, semiconductor thin films made up of perovskite compounds are far cheaper to make than silicon solar cells, with greater flexibility and a tunable band gap.

They emerged unexpectedly in the last decade and have reached impressive power-conversion efficiencies of over 25%.

However, too much focus has been placed on creating the most efficient perovskite solar cell, rather than resolving the fundamental problems inhibiting the material from being used in widespread commercial applications.

Compared to silicon, perovskites can degrade rapidly in real world conditions, with exposure to heat and moisture causing damage and negatively impacting device performance.

Read more at Science Daily