Physics and Technology Forefronts

Waiting in the Wings: OLED-Based Sensors Advance Towards Commercialization

A new sensor platform has the potential to revolutionize the science and technology of chemical and biological sensors, ultimately leading to very cheap, disposable microarrays of sensors. The key ingredient: organic light-emitting diodes (OLEDs), according to Joseph Shinar, a condensed matter physicist at Iowa State University (ISU). Together with Ruth Shinar, a Senior Scientist at the Microelectronics Research Center of ISU, he is developing structurally integrated OLED-based luminescent sensors for numerous potential applications, such as monitoring inorganic gases and biological compounds and organisms.

The Shinars’ lab has helped blaze a path for the commercialization of OLED technology in general over the years, and since 2000 or so, he has turned his attention to integrating an OLED light source with luminescent chemical and biological sensors, receiving his first patent in 2001. “Integration and miniaturization of photoluminescence (PL)-based chemical and biological sensors is highly desirable, as it is the first step towards the development of field-deployable PL-based sensor arrays that could be used for simultaneous analyses of multiple analytes in a single sample, including organisms,” says Shinar.

Conventional PL-based sensors use lasers or inorganic light-emitting diodes (LEDs) as light sources, but they are not easily integrated with the other sensor components, and the resulting devices are consequently expensive. OLEDs enable a uniquely simple integration, and consequently have the potential to be more versatile, flexible and cost-effective, and enable high-density microarrays with hundreds of sensors on a single chip. OLED-based sensors have advanced to the point that they are an important part of a new conference on organic electronics-based sensors, chaired by Ruth Shinar and George Malliaras of Cornell University, that will convene at the annual SPIE meeting this year in San Diego, August 26-30.

OLEDs function like inorganic LEDs: they are solid state devices mounted on a substrate of clear plastic, glass or foil. Typically, there is a transparent anode layer that injects “holes,” and a cathode layer that injects electrons when a forward bias is applied across the device. Sandwiched between the anode and cathode layers are hole- and electron-transporting layers, and between them, the emissive layer. The emissive layer emits light when a forward voltage is applied; the color of the light depends on the type of organic molecule used.

The Shinars’ OLED-based sensor has a sandwich structure that typically includes the sensing element (usually a thin film), the thin OLED light source, and the photodetector (PD) that responds to the sensor’s PL. Individually addressable OLED pixels and a sensor film are fabricated on opposite sides of a common substrate (e.g, glass slide) or on different substrates that are attached back-to-back, generating a compact module with a thickness determined by that of the substrate. As the OLED light source (typically pulsed) excites the sensor film, the latter luminesces. In the presence of an analyte of interest, the PL changes, depending on the analyte’s concentration in the sample, and the PD, typically positioned behind the OLED light source, detects that change. The latter’s placement is an important enabling design, known as back-detection geometry. In this geometry, light passes through the gaps between the OLED pixels and is recorded by the PD below, making it easier to handle the analyte. In the other “front-detection” geometry, the PD is placed on top, with the OLED light source at the bottom and the sensing element in between.

The OLED/sensing element integration also paves the way for the next step in the development process: integrating additionally the PD. Such a device would be more compact, and would permit the development of an array of PL-based sensors that could be driven by an array of OLED pixels and tracked by an array of thin film-based PDs. Ultimately, the Shinars envision that the entire device would be the size of a silver dollar.

On Display

OLED technology is already big business, with a market estimated at $1.4 billion. That is expected to increase to $10.9 billion by 2012. The technology has found its way into the small screens in cell phones, PDAs, digital cameras, and portable music players. To date, OLEDs are not being used in full sized flat panel displays (apart from demonstration prototypes), such as computer monitors or television screens, although several companies are investing heavily developing the technology to do so, including Kodak, Sony, Dupont, and Universal Display Corporation.

OLEDs offer several advantages for full-sized display applications. For instance, OLEDs do not require a backlight, thereby drawing less power and able to operate longer on the same battery charge. Since the OLED pixels emit light directly, they have a greater range of colors, brightness and viewing angle than with LCDs even if the viewing angle is shifted as much as 90 degrees from the axis perpendicular to the display.

Furthermore, they are thinner, lighter and more flexible. In fact, polymers OLEDs can be printed on any suitable substrate using inkjet printer technology, including flexible plastics, making OLEDs ideal for future technologies like roll-up displays, or even displays embedded in clothing. Other future applications include OLED-based heads-up displays, car dashboards, billboards, and home and office solid state lighting.

However, because they are organic, OLEDs degrade over time, and they do so at different rates, depending on their composition. They still cost more to produce, and are easily damaged by exposure to water. Much of the applied research being done in this area is focused on extending OLED lifetimes and improving the manufacturing processes to make them more competitive with standard LED technology.

Sensory Overload

The Shinars face many of the same technical challenges with their sensor technology as those faced by the display industry, most notably extending the lifetime of OLEDs. The challenge is a bit different than for display technologies, where the OLEDs must function at a medium brightness for a long period of time, or for solid state lighting, where they must function continually at more than ten times the brightness of display monitors. OLED-based sensors fall somewhere in between: they must have longer lifetimes for high brightness, but the brightness only needs to be high during the pulse, and the OLED only needs to last about a year. “We’re not there yet, but we’re very close,” says Shinar.

The Shinars have good reason to be optimistic, having already secured one patent and filed two more on their integrated sensor platform technology. They have formed their own start-up company, Integrated Sensor Technologies, Inc. (ISTI), with two Small Business Innovation Research Phase I grants. The first grant was from the National Institutes of Health to develop a gas phase oxygen sensor for monitoring oxygen levels in, e.g., surgery requiring general anesthesia and in the homes of patients with respiratory conditions–an enormous potential market. However, such a sensor requires FDA approval, which is a lengthy and costly process. Therefore, the first commercial product to hit the market will most likely be a dissolved oxygen sensor, which is the focus of the second grant, from the National Science Foundation (NSF). Such a sensor would be especially useful to wastewater treatment facilities, which need it in order to maintain an adequate dissolved oxygen level and thus cut the electricity use by as much as 40%.

There are more potential applications further on the horizon, because the technology can be so easily tailored to suit many different needs. For instance, there is a multi-analyte sensor under development capable of simultaneously detecting glucose, alcohol, and lactate levels, of interest to the sports industry. Additionally, the sensors could be used for high-throughput drug discovery, or for point-of-care medical testing.

NSF also awarded an exploratory grant for the Shinars’ research at ISU to develop an OLED-based sensor to detect anthrax, while an OLED-based sensor for hydrazine, a very toxic substance, was developed in a NASA-funded project. The Shinars’ hydrazine sensor is so sensitive, that it can detect levels of hydrazine that are 80 times lower than the OSHA regulations currently require (< 10 parts per billion over 8 hours of exposure).

And because most of the prototype sensor components are so cheap, apart from the PD, when production is scaled up to mass market volumes, the price could be very low, with most of the cost stemming from the PD array. “A whole new paradigm in sensor technology could emerge from this very basic idea of integrating the very low-cost OLED light source with the photoluminescent sensor in this uniquely simple design,” says Shinar.