Tag Archives: letter-dd-tech

By Michael Droeger

Are you ready for a shared economy where your transportation needs are no longer met by an automaker, but rather a “mobility service provider”? While smart transportation news has mostly focused on the likes of electrification (Tesla) and autonomy (Waymo), the real changes in transportation may be more fundamental than self-driving electric cars. According to presenters at this week’s Smart Automotive Summit at SEMICON Taiwan, new technologies won’t just make cars smarter: they will transform the way we see and use transportation in myriad ways.

Constance Chen, public relations general manager for forum sponsor Mercedes Benz, opened with a brief overview of parent Daimler’s evolving approach to transportation, dubbed CASE, which stands for Connected, Autonomous, Shared and Services, and Electric.

“The fundamental value of vehicles is changing,” Chen said, and car ownership is one of the biggest changes. Ride-sharing services like Uber and Lyft, and shared car services like ZipCar and DriveNow, are already addressing the transportation needs of a growing urban population that eschews car ownership. Traffic congestion, parking challenges, and a desire to improve air quality are key drivers (no pun intended) moving people away from car ownership to embrace shared transportation solutions.

Indeed, societal considerations are as challenging as some technological hurdles facing autonomous vehicle development. Robert Brown, Taiwan operations manager for Magma Electronics, listed his top five challenges for autonomous transportation:

  1. Perception (vision, sensors)
  2. Assessment (ability of systems to analyze data)
  3. Control (need for faster-than-human response)
  4. Communication (vehicle-to-vehicle, vehicle-to-everything)
  5. Expectations—specifically people’s expectations of the value autonomous transportation should deliver

As people change the way they view transportation and begin to understand what is possible when they can relinquish control of their vehicle, they’re transportation needs and expectations are likely to change. The challenges are, of course, also an opportunity to deliver a wide range of services, including information, entertainment, and retail, which opens the door for traditional carmakers to position themselves more as service providers like Mercedes Benz.

For those who have grown up with traditional car ownership and the perceived freedom that owning allows one to go anywhere at anytime, the idea of giving up their car—one that they drive themselves—might seem beyond the pale. But as ride-sharing services are already showing, a growing portion of our population seems more than ready to embrace a shared and autonomous future.

The SEMICON Taiwan Smart Automotive Summit is part of SEMI’s Smart Transportation initiative focusing on automotive electronics, a top priority for SEMI and its 2,000+ members. SEMI’s industry standards, technology communities, roadmap efforts, EH&S/regulatory activities and other global platforms and communities bring together the automotive and semiconductor supply chains to collaborate, increase cross-industry efficiencies and shorten the time to better business results.

Michael Droeger is director of marketing at SEMI. 

Originally published on the SEMI blog.

Universal Display Corporation (Nasdaq: OLED), enabling energy-efficient displays and lighting with its UniversalPHOLED® technology and materials, announced today the recipients of the UDC Innovative Research Award in Organic Electronics and the UDC Pioneering Technology Award in Organic Electronics. These awards were presented at the 18th International Meeting of Information Display (IMID) conference on August 30, 2018 in Busan, Korea by Dr. Julie Brown, Senior Vice President and Chief Technology Officer of Universal Display.

“Universal Display Corporation is proud to sponsor and support the innovative and brilliant research work in the organic electronics industry,” said Steven V. Abramson, President and Chief Executive Officer. “As a leader in the OLED industry, we believe that it is important to encourage and foster the relentless pursuit of exploration, experimentation and education in the scientific community. We congratulate the award recipients, and commend all the researchers for the important role they play in the field of organic electronics.”

The UDC awards recognize outstanding individuals or teams that have demonstrated innovative ideas or research initiatives impacting the organic electronics industry. The winners were selected by IMID and KIDS (Korean Information Display Society). The recipients for 2018 are:

UDC Innovative Research Award in Organic Electronics

Byung-Jun Kang, Dong-Myung Lee, Chang-Jae Yu, E-Joon Choi (Kumoh National Institute of Technology, Korea), and Jae-Hoon Kim (Hanyang University, Korea)

UDC Pioneering Technology Award in Organic Electronics

Hwang-Beom Kim and Jang-Joo Kim (Seoul National University, Korea)


Most current displays do not always accurately represent the world’s colors as we perceive them by eye, instead only representing roughly 70% of them. To make better displays with true colors commonly available, researchers have focused their efforts on light-emitting nanoparticles. Such nanoparticles can also be used in medical research to light up and keep track of drugs when developing and testing new medicines in the body. However, the metal these light-emitting nanoparticles are based on, namely cadmium, is highly toxic, which limits its applications in medical research and in consumer products–many countries may soon introduce bans on toxic nanoparticles.

These are structures of silver indium sulfide/gallium sulfide core/shell quantum dots and pictures of the core/shell quantum dots under room light. Credit: Osaka University

It is therefore vital to create non-toxic versions of these nanoparticles that have similar properties: they must produce very clean colors and must do so in a very energy-efficient way. So far researchers have succeeded in creating non-toxic nanoparticles that emit light in an efficient manner by creating semiconductors with three types of elements in them, for example, silver, indium, and sulfur (in the form of silver indium disulfide (AgInS2)). However, the colors they emit are not pure enough–and many researchers declared that it would be impossible for such nanoparticles to ever emit pure colors.

Now, researchers from Osaka University have proven that it is possible by fabricating semiconductor nanoparticles containing silver indium disulfide and adding a shell around them consisting of a semiconductor material made of two different elements, gallium and sulfur. The team was able to reproducibly create these shell-covered nanoparticles that are both energy efficient and emit vivid, clean colors. The team have recently published their research in the Nature journal NPG Asia Materials.

“We synthesized non-toxic nanoparticles in the normal way: mix all ingredients together and heat them up. The results were not fantastic, but by tweaking the synthesis conditions and modifying the nanoparticle cores and the shells we enclosed them in, we were able to achieve fantastic efficiencies and very pure colors,” study coauthor Susumu Kuwabata says.

Enclosing nanoparticles in semiconductor shells in nothing new, but the shells that are currently used have rigidly arranged atoms inside them, whereas the new particles are made of a more chaotic material without such a rigid structure.

“The silver indium disulfide particles emitted purer colors after the coating with gallium sulfide. On top of that, the shell parts in microscopic images were totally amorphous. We think the less rigid nature of the shell material played an important part in that–it was more adaptable and therefore able to take on more energetically favorable conformations,” first author Taro Uematsu says.

The team’s results demonstrate that it is possible to create cadmium-free, non-toxic nanoparticles with very good color-emitting properties by using amorphous shells around the nanoparticle cores.

Each issue of the journal Nature Electronics contains a column called “Reverse Engineering,” which examines the development of an electronic device now in widespread use from the viewpoint of the main inventor. So far, it has featured creations such as the DRAM, DVD, CD, and Li-ion rechargeable battery. The July 2018 column tells the story of the IGZO thin film transistor (TFT) through the eyes of Professor Hideo Hosono of Tokyo Tech’s Institute of Innovative Research (IIR), who is also director of the Materials Research Center for Element Strategy.

TFTs using oxides including indium (In), gallium (Ga), and zinc (Zn), or IGZO, made possible high-resolution energy-efficient displays that had not been seen before. IGZO electron mobility is 10 times that of hydrogenated amorphous silicon, which was used exclusively for displays in the past. Additionally, its off current is extremely low and it is transparent, allowing light to pass through. IGZO has been applied to drive liquid crystal displays, such as those on smartphones and tablets. Three years ago, it was also used to drive large OLED televisions, which was considered a major breakthrough. This market is rapidly expanding, as can be seen from the products being released by South Korean and Japanese electronics manufacturers, which now dominate store shelves.

The electron conductivity of transition metal oxides has long been known, but electric current modulation using electric fields has not. In the 1960s, it was reported that modulating the electric current was possible when zinc oxide, tin oxide, and indium oxide were formed into TFT structures. Their performance, however, was poor, and reports of research on organic TFTs were mostly nonexistent until around 2000. A new field called oxide electronics came into existence in the early noughties, examining oxides as electronic materials. A hub for this research was the present-day Laboratory for Materials and Structures within IIR, and research into zinc oxide TFTs soon spread worldwide. However, since the thin film was polycrystalline, there were problems with its characteristics and stability, and no practical applications were achieved.

Application in displays, unlike CPUs, requires the ability to form a thin, homogenous film on a large-sizedsubstrate — like amorphous materials — and a dramatic increase in electric current at a low gate voltage when the thin film is subjected to an electric field. However, while amorphous materials were the optimal choice for forming thin, homogeneous film, high carrier concentration and other issues due to structural disorder arose, for the most part preventing electric current modulation by electric fields. The only exception was amorphous silicon containing a large amount of hydrogen, reported in 1975. TFTs made of this material were applied to drive liquid crystal displays, which grew into a giant 10 trillion-yen industry. However, electron mobility was still lower by two to three orders of magnitude compared to that of crystalline silicon — no better than 0.5 to 1 cm2 V-1 s-1. Amorphous semiconductors, therefore, were easy to produce, but were seen to have much inferior electronic properties.

Hosono focused his attention on oxides with highly ionic bonding nature, the series made up of non-transition metals belonging to the p-block of the periodic table. In this material series, the bottom of the conduction band, which works as the path for electron, is made up mainly of spherically symmetrical metal s-orbitals with a large spatial spread. Because of this, the degree of overlap of the orbitals, which govern how easily electrons can move, is not sensitive to bond angle variation which is an intrinsic nature of amorphous materials.

The professor realized that this characteristic might allow for mobility in amorphous materials that is comparable to that of polycrystalline thin films. He experimented accordingly, and was able to find some examples. In 1995, he presented his idea and examples at the 16th International Conference on Amorphous Semiconductors, and had the paper on its proceedings published the following year. After proving this hypothesis through experiments and calculations, he started test-producing TFTs. Many combinations of elements fulfilled the conditions of the hypothesis. IGZO was selected because it had a stable crystalline phase that is easy to prepare, and its specific local structure around Ga suggested that carrier concentration could be suppressed. In 2003, Hosono and his collaborators reported in Science that crystalline epitaxial thin film could produce mobility of around 80 cm2 V-1 s-1. In the following year, they published in Nature that amorphous thin film could also produce mobility of around 10 cm2 V-1 s-1.

Following these findings, research on amorphous oxide semiconductors and their TFTs began increasing rapidly around the world — not just among the Society for Information Display (SID) and the International Conference on Amorphous Semiconductors. This activity has continued, and Hosono’s two papers have now been cited over 2,000 and 5,000 times respectively. The total citations of the patents associated with these inventions now exceed 9,000. Products with displays incorporating these TFTs have been available to the general consumers since 2012. In particular, large OLED televisions, which appeared around 2015, became possible only due to the unique characteristics of amorphous IGZO TFTs — their high mobility and ability to easily form a thin, homogenous film over a large area. Such displays are installed on the first floor of the Materials Research Center for Element Strategy and the foyer of the Laboratory for Materials and Structures at Tokyo Tech. Application of IGZO TFTs to high-definition large LCD televisions are expected to start soon.

A new manufacturing technique uses a process similar to newspaper printing to form smoother and more flexible metals for making ultrafast electronic devices.

The low-cost process, developed by Purdue University researchers, combines tools already used in industry for manufacturing metals on a large scale, but uses the speed and precision of roll-to-roll newspaper printing to remove a couple of fabrication barriers in making electronics faster than they are today.

Roll-to-roll laser-induced superplasticity, a new fabrication method, prints metals at the nanoscale needed for making electronic devices ultrafast. Credit: Purdue University image/Ramses Martinez

Cellphones, laptops, tablets, and many other electronics rely on their internal metallic circuits to process information at high speed. Current metal fabrication techniques tend to make these circuits by getting a thin rain of liquid metal drops to pass through a stencil mask in the shape of a circuit, kind of like spraying graffiti on walls.

“Unfortunately, this fabrication technique generates metallic circuits with rough surfaces, causing our electronic devices to heat up and drain their batteries faster,” said Ramses Martinez, assistant professor of industrial engineering and biomedical engineering.

Future ultrafast devices also will require much smaller metal components, which calls for a higher resolution to make them at these nanoscale sizes.

“Forming metals with increasingly smaller shapes requires molds with higher and higher definition, until you reach the nanoscale size,” Martinez said. “Adding the latest advances in nanotechnology requires us to pattern metals in sizes that are even smaller than the grains they are made of. It’s like making a sand castle smaller than a grain of sand.”

This so-called “formability limit” hampers the ability to manufacture materials with nanoscale resolution at high speed.

Purdue researchers have addressed both of these issues – roughness and low resolution – with a new large-scale fabrication method that enables the forming of smooth metallic circuits at the nanoscale using conventional carbon dioxide lasers, which are already common for industrial cutting and engraving.

“Printing tiny metal components like newspapers makes them much smoother. This allows an electric current to travel better with less risk of overheating,” Martinez said.

The fabrication method, called roll-to-roll laser-induced superplasticity, uses a rolling stamp like the ones used to print newspapers at high speed. The technique can induce, for a brief period of time, “superelastic” behavior to different metals by applying high-energy laser shots, which enables the metal to flow into the nanoscale features of the rolling stamp – circumventing the formability limit.

“In the future, the roll-to-roll fabrication of devices using our technique could enable the creation of touch screens covered with nanostructures capable of interacting with light and generating 3D images, as well as the cost-effective fabrication of more sensitive biosensors,” Martinez said.

Researchers have demonstrated nanomaterial-based white-light-emitting diodes (LEDs) that exhibit a record luminous efficiency of 105 lumens per watt. Luminous efficiency is a measure of how well a light source uses power to generate light. With further development, the new LEDs could reach efficiencies over 200 lumens per watt, making them a promising energy-efficient lighting source for homes, offices and televisions.

Researchers created nanomaterial-based white LEDs that exhibit a record high efficiency thanks to quantum dots that are suspended in solution rather than embedded in a solid. The new LEDs could offer an energy-efficient lighting source for homes, offices and televisions. Credit: Sedat Nizamoglu, Koç University

“Efficient LEDs have strong potential for saving energy and protecting the environment,” said research leader Sedat Nizamoglu, Koç University, Turkey. “Replacing conventional lighting sources with LEDs with an efficiency of 200 lumens per watt would decrease the global electricity consumed for lighting by more than half. That reduction is equal to the electricity created by 230 typical 500-megawatt coal plants and would reduce greenhouse gas emissions by 200 million tons.”

The researchers describe how they created the high-efficiency white LEDs in Optica, The Optical Society’s journal for high impact research. The new LEDs use commercially available blue LEDs combined with flexible lenses filled with a solution of nano-sized semiconductor particles called quantum dots. Light from the blue LED causes the quantum dots to emit green and red, which combines with the blue emission to create white light.

“Our new LEDs reached a higher efficiency level than other quantum dot-based white LEDs,” said Nizamoglu. “The synthesis and fabrication methods for making the quantum dots and the new LEDs are easy, inexpensive and applicable for mass production.”

Advantages of quantum dots

To create white light with today’s LEDs, blue and yellow light are combined by adding a yellowish phosphor-based coating to blue LEDs. Because phosphors have a broad emission range, from blue to red, it is difficult to sensitively tune the properties of the generated white light.

Unlike phosphors, quantum dots generate pure colors because they emit only in a narrow portion of the spectrum. This narrow emission makes it possible to create high-quality white light with precise color temperatures and optical properties by combining quantum dots that generate different colors with a blue LED. Quantum dots also bring the advantage of being easy to make and the color of their emission can be easily changed by increasing the size of the semiconductor particle. Moreover, quantum dots can be advantageously used to generate warm white light sources like incandescent light bulbs or cool white sources like typical fluorescent lamps by changing the concentration of incorporated quantum dots.

Although quantum dots embedded in a film are currently used in LED televisions, this lighting approach is not suitable for widespread use in general lighting applications. Transferring the quantum dots in a liquid allowed the researchers to overcome the problematic drop in efficiency that occurs when nanomaterials are embedded into solid polymers.

Making efficient white LEDs requires quantum dots that efficiently convert blue light to red or green. The researchers carried out more than 300 synthesis reactions to identify the best conditions, such as temperature and time of the reaction, for making quantum dots that emit at different colors while exhibiting optimal efficiency.

“Creating white light requires integrating the appropriate amount of quantum dots, and even if that is accomplished, there are an infinite number of blue, green and red combinations that can lead to white,” said Nizamoglu. “We developed a simulation based on a theoretical approach we recently reported and used it to determine the appropriate amounts and best combinations of quantum dot colors for efficient white light generation.”

To make the new LEDs, the researchers filled the space between a polymer lens and LED chip with a solution of quantum dots that were synthesized by mixing cadmium, selenium, zinc and sulfur at high temperatures. The researchers used a type of silicone to make the lens because its elasticity allowed them to inject solutions into the lens without any solution leaking out, and the material’s transparency enabled the necessary light transmission.

The researchers showed that their liquid-based white LEDs could achieve an efficiency double that of LEDs that incorporate quantum dots in solid films. They also demonstrated their white LEDs by using them to illuminate a 7-inch display.

“Quantum dots hold great promise for efficient lighting applications,” said Nizamoglu. “There is still significant room for technology development that would generate more efficient approaches to lighting.”

As a next step, the researchers are working to increase the efficiency of the LEDs and want to reach high efficiency levels using environmentally friendly materials that are cadmium- and lead-free. They also plan to study the liquid LEDs under different conditions to ensure they are stable for long-term application.

By Dave Lammers

The semiconductor industry is collecting massive amounts of data from fab equipment and other sources. But is the trend toward using that data in a Smart Manufacturing or Industry 4.0 approach happening fast enough in what Mike Plisinski, CEO of Rudolph Technologies, calls a “very conservative” chip manufacturing sector?

“There are a lot of buzzwords being thrown around now, and much of it has existed for a long time with APC, FDC, and other existing capabilities. What was inhibiting the industry in the past was the ability to align this huge volume of data,” Plisinskisaid.

While the industry became successful at adding sensors to tools and collecting data, the ability to track that data and make use of it in predictive maintenance or other analytics thus far “has had minimal success,” he said. With fab processes and manufacturing supply chains getting more complex, customers are trying to figure out how to move beyond implementing statistical process control (SPC) on data streams.

What is the next step? Plisinski said now that individual processes are well understood, the next phase is data alignment across the fab’s systems. As control of leading-edge processes becomes more challenging, customers realize that the interactions between the process steps must be understood more deeply.

“Understanding these interactions requires aligning these digital threads and data streams. When a customer understands that when a chamber changes temperature by point one degrees Celsius, it impacts the critical dimensions of the lithography process by X, Y, and Z. Understanding those interactions has been a significant challenge and is an area that we have focused on from a variety of angles over the last five years,” Plisinski said.

Rudolph engineers have worked to integrate multiple data threads (see Figure), aligning various forms of data into one database for analysis by Rudolph’s Yield Management System (YMS). “For a number of years we’ve been able to align data. The limitation was in the database: the data storage, the speed of retrieval and analysis were limitations. Recently new types of databases have come out, so that instead of relational, columnar-type databases, the new databases have been perfect for factory data analysis, for streaming data. That’s been a huge enabler for the industry,” he said.

Rudolph engineers have worked to integrate multiple data threads into one database.

Leveraging AI’s capabilities

A decade ago, Rudolph launched an early neural-network based system designed to help customers optimize yields. The software analyzed data from across a fab to learn from variations in the data.

“The problem back then was that neural networks of this kind used non-linear math that was too new for our conservative industry, an industry accustomed to first principle analytics. As artificial intelligence has been used in other industries, AI is becoming more accepted worldwide, and our industry is also looking at ways to leverage some of the capabilities of artificial intelligence,” he said.

Collecting and making use of data with a fab is “no small feat,” Plisinskisaid, but that leads to sharing and aligning data across the value chain: the wafer fab, packaging and assembly, and others.

“To gain increased insights from the data streams or digital threads, to bring these threads all together and make sense of all of it. It is what I call weaving a fabric of knowledge: taking individual data threads, bringing them together, and weaving a much clearer picture of what’s going on.”

Security concerns run deep

One of the biggest challenges is how to securely transfer data between the different factories that make up the supply chain. “Even if they are owned by one entity, transferring that large volume of data, even if it’s over a private dedicated network, is a big challenge. If you start to pick and choose to summarize the data, you are losing some of the benefit. Finding that balance is important.”

The semiconductor industry is gaining insights from companies analyzing, for instance, streaming video. The network infrastructures, compression algorithms, transfers of information from mobile wireless devices, and other technologies are making it easier to connect semiconductor fabs.

“Security is perhaps the biggest challenge. It’s a mental challenge as much as a technical one, and by that I mean there is more than reluctance, there’s a fundamental disdain for letting the data out of a factory, for even letting data into the factory,” he said.

Within fabs, there is a tug of war between equipment vendors which want to own the data and provide value-add services, and customers who argue that since they own the tools they own the data. The contentious debate grows more intense when vendors talk about taking data out of the fab. “That’s one of the challenges that the industry has to work on — the concerns around security and competitive information getting leaked out.” Developing a front-end process is “a multibillion dollar bet, and if that data leaks out it can be devastating to market-share leadership,” Plisinski said.

Early adopter stories

The challenge facing Rudolph and other companies is to convince their customers of the value of sharing data; that “the benefits will outweigh their concerns. Thus far, the proof of the benefit has been somewhat limited.”

“At least from a Rudolph perspective, we’ve had some early adopters that have seen some significant benefits. And I think as those stories get out there and as we start to highlight what some of these early adopters have seen, others at the executive level in these companies will start to question their teams about some of their assumptions and concerns. Eventually I think we’ll find a way forward. But right now that’s a significant challenge,”Plisinski said.

It is a classic chicken-and-egg problem, making it harder to get beyond theories to case-study benefits. “What helped us is that some of the early adopters had complete control of their entire value chain. They were fully integrated. And so we were able to get over the concerns about data sharing and focus on the technical challenges of transferring all that data and centralizing it in one place for analytical purposes. From there we got to see the benefits and document them in a way that we could share with others, while protecting IP.”

Aggregating data, buying databases and analytical software, building algorithms – all cost money, in most cases adding up to millions of dollars. But if yields improve by .25 or half a percent, the payback comes in six to eight months, he said.

“It’s a very conservative industry, an applied science type of industry. Trying to prove the value of software — a kind of black magic exercise — has always been difficult. But as the industry’s problems have become so complex, it is requiring these sophisticated software solutions.”

“We will have examples of successful case studies in our booth during SEMICON West. Anyone wanting further information is invited to stop by and talk to our experts,” adds Plisinski.

Researchers at Kyushu University’s Center for Organic Photonics and Electronics Research (OPERA) in Japan have demonstrated a way to split energy in organic light-emitting diodes (OLEDs) and surpass the 100% limit for exciton production, opening a promising new route for creating low-cost and high-intensity near-infrared light sources for sensing and communications applications.

OLEDs use layers of carbon-containing organic molecules to convert electrical charges into light. In normal OLEDs, one positive charge and one negative charge come together on a molecule to form a packet of energy called an exciton. One exciton can release its energy to create at most one beam of light, or photon.

Illustration of the singlet fission process used to boost the number of excitons in an OLED and break the 100 percent limit for exciton production efficiency. The emitting layer consists of a mixture of rubrene molecules, which are responsible for singlet fission, and ErQ3 molecules, which produce the emission. A singlet exciton, which is created when a positive charge and a negative charge combine on a rubrene molecule, can transfer half of its energy to a second rubrene molecule through the process of singlet fission, resulting in two triplet excitons. The triplet excitons then transfer to ErQ3 molecules, and the exciton energy is released as near-infrared emission by ErQ3. Credit: William J. Potscavage Jr.

When all charges form excitons that emit light, a maximum 100% internal quantum efficiency is achieved. However, the new technology uses a process called singlet fission to split the energy from an exciton into two, making it possible to exceed the 100% limit for the efficiency of converting charge pairs into excitons, also known as the exciton production efficiency

“Put simply, we incorporated molecules that act as change machines for excitons in OLEDs. Similar to a change machine that converts a $10 bill into two $5 bills, the molecules convert an expensive, high-energy exciton into two half-price, low-energy excitons,” explains Hajime Nakanotani, associate professor at Kyushu University and co-author of the paper describing the new results.

Excitons come in two forms, singlets and triplets, and molecules can only receive singlets or triplets with certain energies. The researchers overcame the limit of one exciton per one pair of charges by using molecules that can accept a triplet exciton with an energy that is half the energy of the molecule’s singlet exciton.

In such molecules, the singlet can transfer half of its energy to a neighboring molecule while keeping half of the energy for itself, resulting in the creation of two triplets from one singlet. This process is called singlet fission.

The triplet excitons are then transferred to a second type of molecule that uses the energy to emit near-infrared light. In the present work, the researchers were able to convert the charge pairs into 100.8% triplets, indicating that 100% is no longer the limit. This is the first report of an OLED using singlet fission, though it has previously been observed in organic solar cells.

Furthermore, the researchers could easily evaluate the singlet fission efficiency, which is often difficult to estimate, based on comparison of the near-infrared emission and trace amounts of visible emission from remaining singlets when the device is exposed to various magnetic fields.

“Near-infrared light plays a key role in biological and medical applications along with communications technologies,” says Chihaya Adachi, director of OPERA. “Now that we know singlet fission can be used in an OLED, we have a new path to potentially overcome the challenge of creating an efficient near-infrared OLED, which would find immediate practical use.”

Overall efficiency is still relatively low in this early work because near-infrared emission from organic emitters is traditionally inefficient, and energy efficiency will, of course, always be limited to a maximum 100%. Nonetheless, this new method offers a way to increase efficiency and intensity without changing the emitter molecule, and the researchers are also looking into improving the emitter molecules themselves.

With further improvements, the researchers hope to get the exciton production efficiency up to 125%, which would be the next limit since electrical operation naturally leads to 25% singlets and 75% triplets. After that, they are considering ideas to convert triplets into singlets and possibly reach a quantum efficiency of 200%.

One of the leading challenges for autonomous vehicles is to ensure that they can detect and sense objects–even through dense fog. Compared to the current visible light-based cameras, infrared cameras can offer much better visibility through the fog, smoke or tiny particles that can scatter the visible light.

Artist’s rendering of light interacting with BaTiS3 crystals. Credit: Talia Spencer

Within the air, infrared light –within a specific range called mid-wave infrared– scatter much less compared to other visible or other infrared light waves. Infrared cameras can also see more effectively in the dark, when there is no visible light. However, currently the deployment of infrared cameras is limited by their heavy cost and scarcity of effective materials. This is where materials, which possess unique optical properties in the infrared and can be scalable, might make a difference in providing better object identification in several technologies including autonomous vehicles.

A new material developed by scientists at the USC Viterbi School of Engineering and the University of Wisconsin along with researchers from Air Force Research Laboratories, University of Missouri, and J.A. Woollam Co. Inc, might show promise for such infrared detection applications as autonomous vehicles, emergency services and even manufacturing.

The research group of Jayakanth Ravichandran, an assistant professor of materials sciences at the USC Viterbi School of Engineering has been studying a new class of materials called chalcogenide perovskites. Among these materials is Barium titanium sulfide (BTS), a material rediscovered and prepared in large crystal form by Shanyuan Niu, a doctoral candidate in the Materials Science program at the USC Mork Family Department of Chemical Engineering and Materials Science. Ravichandran’s research group collaborated with the research groups of Mikhail Kats, an assistant professor of electrical and computer engineering at University of Wisconsin-Madison and Han Wang, an assistant professor of electrical engineering and electrophysics in USC’s Ming Hsieh Department of Electrical Engineering to study how infrared light interacts with this material. These researchers discovered that this material interacted differently with light in two different directions.

“This is a significant breakthrough, which can affect many infrared applications,” says Ravichandran.

This direction dependent interaction with light is characterized by an optical property called birefringence. In simple terms, birefringence can be viewed as light moving at different speeds in two directions in a material. Much like sunglasses with polarized lenses block glare, BTS has the ability to block or slow down light depending on the direction in which it travels in the material. The researchers maintain that their material, barium titanium sulfide, has the highest birefringence among known crystals.

“The birefringence is larger than that of any known solid material, and it has low losses across the important long-wave infrared spectrum,” says Kats.

How BTS could improve infrared vision:

The BTS material can be used to construct a sensor to filter out certain polarizations of light to achieve better contrast of the image. It could also help filter light coming from different directions to enable sensing of a remote object’s features. This could be particularly important for improving infrared vision used in autonomous vehicles, which need to see the entire landscape around them even in low visibility conditions.

“The hope is that in the future, a BTS-enhanced sensor in a car would function as retinas do to the human body,” says Niu.

The authors believe these infrared-responsive materials can extend human perception. Beyond autonomous vehicles, there are other possible heat sensing or temperature measurement applications. One application could be in the creation of imaging tools used by firefighters to generate an instant temperature map outside a burning building to assess where a fire is spreading and where emergency responders need to rescue trapped individuals.

At present, the cost of infrared equipment makes it too expensive for all fire stations to have such equipment. BTS, which is made of elements readily abundant in earth crust–could make infrared equipment more affordable and effective. In addition, such materials are safer for the user and the environment, as well as easier to dispose of than the materials that are used now, which contain hazardous elements such as mercury and cadmium.

These materials could also be useful in devices that sense harmful molecules, gases, even biological systems. The applications range from heat sensing, pollution monitoring to medicine.

“To date, the constraint of existing mid-IR materials is a big bottleneck to translate many of these technologies,” says USC’s Wang.

The researchers hope that intense research in this area will make several of these technologies a reality in the near future.

The research on BTS is documented in “Giant optical anisotropy in a quasi-1D crystal” featured in Nature Photonics.

Researchers have demonstrated large-scale fabrication of a new type of transparent conductive electrode film based on nanopatterned silver. Smartphone touch screens and flat panel televisions use transparent electrodes to detect touch and to quickly switch the color of each pixel. Because silver is less brittle and more chemically resistant than materials currently used to make these electrodes, the new films could offer a high-performance and long-lasting option for use with flexible screens and electronics. The silver-based films could also enable flexible solar cells for installation on windows, roofs and even personal devices.

In the journal Optical Materials Express, the researchers report fabrication of a transparent conducting thin-film on glass discs 10 centimeters in diameter. Based on theoretical estimations that matched closely with experimental measurements, they calculate that the thin-film electrodes could perform significantly better than those used for existing flexible displays and touch screens.

“The approach we used for fabrication is highly reproducible and creates a chemically stable configuration with a tunable tradeoff between transparency and conductive properties,” said the paper’s first author, Jes Linnet from the University of Southern Denmark. “This means that if a device needs higher transparency but less conductivity, the film can be made to accommodate by changing the thickness of the film.”

The researchers used an approach called colloidal lithography to create a silver nanopattern that conducts electricity while letting light through the holes. The new transparent electrode films could be useful for solar cells as well as flexible displays and touch screens. Credit: Jes Linnet, University of Southern Denmark

Finding a flexible alternative

Most of today’s transparent electrodes are made of indium tin oxide (ITO), which can exhibit up to 92 percent transparency — comparable to glass. Although highly transparent, ITO thin films must be processed carefully to achieve reproducible performance and are too brittle to use with flexible electronics or displays. Researchers are seeking alternatives to ITO because of these drawbacks.

The anti-corrosive nature of noble metals such as gold, silver and platinum makes them promising ITO alternatives for creating long-lasting, chemically resistant electrodes that could be used with flexible substrates. However, until now, noble metal transparent conductive films have suffered from high surface roughness, which can degrade performance because the interface between the film and other layers isn’t flat. Transparent conductive films can also be made using carbon nanotubes, but these films don’t currently exhibit high enough conductance for all applications and tend to also suffer from surface roughness due to the nanotubes stacking on top of each other.

In the new study, the researchers used an approach called colloidal lithography to create transparent conductive silver thin films. They first created a masking layer, or template, by coating a 10-centimeter wafer with a single layer of evenly sized, close-packed plastic nanoparticles. The researchers placed these coated wafers into a plasma oven to shrink the size of all the particles evenly. When they deposited a thin film of silver onto the masking layer, the silver entered the spaces between the particles. They then dissolved the particles, leaving a precise pattern of honeycomb-like holes that allow light to pass through, producing an electrically conductive and optically transparent film.

Balancing transparency and conductivity

The researchers demonstrated that their large-scale fabrication method can be used to create silver transparent electrodes with as much as 80 percent transmittance while keeping electrical sheet resistance below 10 ohms per square – about a tenth of what has been reported for carbon-nanotube-based films with the equivalent transparency. The lower the electrical resistance, the better the electrodes are at conducting an electrical charge.

“The most novel aspect of our work is that we accounted for both the transmission properties and the conductance properties of this thin film using theoretical analysis that correlated well with measured results,” said Linnet. “Fabrication problems typically make it hard to get the best theoretical performance from a new material. We decided to report what we encountered experimentally and postulate remedies so that this information could be used in the future to avoid or minimize problems that may affect performance.”

The researchers say that their findings show that colloidal lithography can be used to fabricate transparent conductive thin films that are chemically stable and could be useful for a variety of applications.