Promise of Organic Photonics Looms Large

MARIE FREEBODY, CONTRIBUTING EDITOR, marie.freebody@photonics.com

Organic materials could supersede their inorganic cousins in many applications thanks to some unique and intriguing properties. From lasers, lighting and Li-Fi (light fidelity) to OLED TVs and solar cells, organic photonics offers a thin, flexible and easy-to-manufacture substance that is carving out new niches in some highly competitive markets.

Materials composed of organic photonics are inexpensive, have low environmental impact and can be solution-processed, offering fast, simple and cheap manufacturing onto flexible, even wearable substrates. What’s more, organic materials allow for exact tuning of photonic properties, which opens up new applications in optical transmission, lasing and emissive displays.

This is an artist’s interpretation of tomorrow’s world: artificial organic plants converting solar energy to blooming flowers. The bright electroluminescent light attracts a butterfly with wings made of organic field effect transistor sheets. The robotic butterfly, luminescent flowers and solar garden have co-evolved as an interlinked e-community. Courtesy of Sigma-Aldrich Materials Science from MilliporeSigma.

With so much to offer, it’s not surprising that researchers are clamoring to be the first to bring their take on the technology to the mainstream with features that consumers have never before seen. A case in point is the first foray of OLEDs into the automotive industry last year by lighting specialists Osram GmbH of Munich, initially in the form of taillights. The taillights were developed as part the collaborative R2D2 project sponsored by the German Federal Ministry of Education and Research (BMBF). In the consortium, Osram partnered with Fraunhofer FEP, Audi AG, Diehl Aerospace GmbH, Hella KGaA Hueck & Co., Novaled GmbH and Von Ardenne GmbH to make flexible OLEDs in order to give automotive clients even more design freedom.

A rear light module from a single flexible OLED produced as part of the R2D2 project in cooperation with Audi, Osram and Hella. Two such modules are installed in a complete rear light unit. Courtesy of Osram OLED GmbH.

“OLED is considered the next evolutionary step in automotive lighting and can be used both inside and outside the car,” said Marc Lünnemann, CEO of Osram OLED GmbH.

Freedom of design is a key advantage for OLEDs. Organic photonics allow for unique 2D and 3D light shapes that are not only thin and uniform but also require less space in an assembly compared with traditional lighting. The OLED light shapes can be divided into several segments and controlled independently. The result can offer dynamic sequences with different levels of brightness, which helps to add dimensionality.

While indoor and fixed outdoor lighting pose rather static and predictable environmental challenges, the same cannot be said for automobiles. Cars and trucks face harsh environments — such as extreme temperatures, moisture, oil and salt — that engineers at Osram had to overcome.

A curved indoor organic photovoltaic module that harvests energy from light could be useful for electronic shelf labels and other point-of-sale signs. Courtesy of Eight19.

In 2016, these efforts came to fruition in the BMW M4 GTS, which features 15 OLEDs per taillight, and the Audi TT RS, which contains four OLED panels. The next step in the evolution of organic lighting is to better utilize the flexibility the material offers, which Lünnemann expects will lead to significant growth in the next years.

From displays to laser sensors

The rumors were around for a decade before the first OLED televisions finally appeared on the market. But they were met with mixed reaction by consumers. While the displays were impressively thin, uniquely curved and exceptionally clear, the issue was cost. Today, all but one TV manufacturer has completely withdrawn from the OLED market.

Currently, the market is dominated by LED TVs, which are actually based on LCD technology with LED backlighting. LG Display is the only company to offer a range of commercial OLED TV panels, with various models offering flat or curved screens and prices ranging from $2000 to $20,000.

Unlike LED versions, OLED displays are emissive, which leads to vivid bright displays, and since only the lit pixels draw power, they are more efficient. What is more, they can be fabricated as very thin panels, opening up the potential to manufacture flexible or even rollable TVs.

“These TVs have been qualified as the best ever produced, with practically perfect image quality,” said professor María Díaz-García at the University of Alicante in Spain. “[Other advantages include a] faster refresh rate, better contrast and color reproduction.”

A researcher operating thin-film organic lasers in professor María A. Díaz-García’s Laboratory of Organic Electronics and Photonics. Courtesy of María Díaz-Garcí.

Just as TVs utilize the emissive and color-changing capability of organic photonics, lasers are so too being engineered. There is now great interest in using strong light-matter coupling to make novel devices such as polariton lasers, and to control the properties of materials using optical cavities.

“Organic materials show high optical cross sections for absorption and luminescence, broad spectral tunability, and ultrafast responses,” Díaz-García said.

By taking advantage of the absorption and luminescence properties, Díaz-García and colleagues have developed an organic laser sensor that can reveal a protein in the blood that is associated with the presence of cancer. (Details of how the cancer marker is detected can be found in the February 2016 Sensors and Actuators B).

With research in its early stages, the next step is to improve operational lifetime by exploring more active organic materials.

Light harvesting

The numerous solar cell companies that specialize in organic photonics are bold in their conviction of the advantages that organic components offer: They’re nontoxic, inexpensive and flexible, and they can be made into a slim profile not possible with any other material.

Roll-to-roll manufacturing of organic photovoltaics could one day lead to the printing of solar cells as cheaply as newsprint. Courtesy of Eight19.

Despite challenges in performance and stability, an unwavering belief in organic photonics has scientists and engineers doggedly fighting for a spot in the solar cell market. Once their niche is established — in the next few years — they fully expect that the technology will have all the elements it needs to flourish.

Millions have so far been spent in faith of this technology, and while funding is dwindling around the world — and absent in Europe — many believe the time has come to see if this investment was made in vain.

In Europe, current volume cost is less than 1 €/W, however, organic photovoltaics (OPVs) are still typically seen as a curiosity due to a high initial outlay. What’s more, it is not yet a proven technology and many believe it will be difficult to maintain market thrust against silicon PVs.

But the lure of possibility is strong: If explored correctly, OPVs could go where silicon cannot — it is this crucial window of opportunity that PV providers are determined to exploit.

Imagine printing solar cells as easily as newsprint. This is the big driver at life sciences specialist MilliporeSigma, an organization comprising Merck Millipore and Sigma-Aldrich, where the ability to manufacture organic solar cell (OSC) materials using highly scalable and eco-friendly roll-to-roll manufacturing is a major pursuit. Roll-to-roll manufacturing uses orders-of-magnitude less energy than conventional solar cell manufacturing processes.

“The overall aim is to develop technologies that use environmentally friendly, abundant materials and low-cost manufacturing processes offering solutions adding value beyond today’s standard PV technologies,” said Klaus-Reinhard Bischoff, MilliporeSigma’s head of research solutions business area promoting materials for OSC research facilities.

Recent advances include the emergence of nonfullerene polymer solar cells, where the photoactive layer is composed of a donor-acceptor conjugated polymer as an electron donor and a new small-molecule electron acceptor like ITIC — a new material that enables higher performance and greater design freedom.

Such materials also enable higher-power conversion efficiencies compared to traditional fullerene acceptors. As a result, OSC efficiencies are now reaching over 12 percent — a value that has long been considered a significant threshold in efficiency for commercial applications.

OSCs can be produced in different colors, transparencies and shapes, which allow for unique design flexibility and enable new ways of thinking about solar. This offers the opportunity to unlock the huge potential for solar windows and facades in the architectural sector.

Despite recent improvements regarding power conversion efficiency and stability, there are still challenges ahead and more work to be done.

“Researchers are continuously progressing in increasing the stability of OSC by studying the factors limiting the stability, designing new material and device engineering,” Bischoff said. “This will provide a solution to move OSC technologies from today’s niche market toward mass adoption in the near future.”

At U.K.-based Eight19, the target gap in the market is environmentally friendly indoor PV efficiency lighting. Unlike crystalline silicon solar cells, which are typically found on rooftops and large-scale installations, OPV materials perform very well under indoor lighting conditions.

At the Eight19 laboratory, researchers have found that organic solar cells perform very well under indoor lighting conditions. Courtesy of Eight19.

Recently, the company demonstrated efficiencies of over 14 percent under 1000 Lux LED lighting, and the potential is there to take this to over 25 percent or more in the very near future. Harvesting energy from indoor lights opens up interesting options in areas such as sensor networks, the Internet of Things and the retail sector. In the retail sector, for example, energy could be provided for electronic shelf labels and other point-of-sale signs.

“The option of using just the energy harvested from lighting in combination with a small rechargeable battery or even a capacitor is very attractive in situations where comparatively small amounts of power are required in environments where replacing batteries or main electricity connection are costly and challenging,” said Jurjen F. Winkel, technology manager at Eight19.

Even outdoors, the flexibility, robustness and low weight-to-performance could one day offer some interesting opportunities in developing countries, where shipping costs and basic infrastructural challenges make transportation and mounting of traditional silicon panels costly and impractical.

Experts are confident that the next couple of years will see OPV products in the marketplace and that once niche positions are established, continuous improvement will lead to the wider adoption of the technology.

Li-Fi goes organic

Move aside Wi-Fi, the future of wireless communication could be Li-Fi, as scientists in the U.K. make promising initial demonstrations thanks to organic photonics. Researchers from the Universities of Edinburgh, Oxford and Strathclyde worked together to show that light can be used to send and receive communication through the air.

“The general attractive features of organic materials for photonics are the ability to tune the properties by adjusting the structure and simple fabrication of devices by evaporation or spin coating,” said professor Ifor Samuel of the University of St. Andrews.

In the setup, LED lighting is modulated to send information to receivers such as phones and laptop computers. Transmitters are currently challenged by the fact that white LEDs actually consist of a blue LED combined with a phosphor color converter. The phosphor has a long “excited state” lifetime, which limits the modulation bandwidth and hence the data rate.

“We have shown that the short excited state lifetime of organic semiconductors can overcome this limitation, enabling greater than 1-gigabit-per-second communication rates,” Samuel said.

On the receiver side, the challenge is to be sensitive but fast. Here the group showed that fluorescent concentrators can provide fast large-area receivers.

Currently the idea is to use optical communication to supplement Wi-Fi, but in the future, as the impending radio frequency spectrum “crunch” really begins to bite, Samuel predicts greater interest and subsequent advances in Li-Fi.