Professors Stephen Forrest and Mark Thompson will receive this year's Jan Rajchman Prize for their pioneering work with PHOLEDs, part of a stellar class of 2006 honorees.

by Jessica Quandt

PROFESSORS Stephen Forrest of the University of Michigan believes in luck, in the certainty that any significant scientific discovery is backed by some amount of good fortune in addition to the intellect and diligence of the scientist. His work with his longtime research partner, Mark Thompson of the University of Southern California, is no exception, he says. Although the pair is responsible for the creation of phosphorescent organic light-emitting diodes (PHOLEDs), which today are being pegged for use in everything from military display applications to consumer lighting, Forrest is quick to credit "serendipity" as a contributing factor to their discovery.

But while Forrest and Thompson are clearly lucky, their good fortune is doubtlessly as much a by-product of their own dedication and creativity as it is of fate. At the 2006 Society for Information Display (SID) International Symposium, Seminar, and Exhibition, which will take place June 4–9 in San Francisco, the hard-working research partners will be honored with SID's Jan Rajchman Prize for their insight into the process of exciton generation and diffusion in organic light-emitting diodes, which led to the discovery of phosphorescent OLEDs and quadrupled OLED efficiency.

"It's a very unusual but productive relationship that we (as a company) have with Mark and Steve," explained Janice Mahon, vice president of technology commercialization for Universal Display Corp. (UDC), which has been working on OLED development with Forrest and Thompson for more than a decade. "They are kind of the 'R' in R&D. We continue to look to Mark and Steve as key inventors who are really thinking outside the box, thinking about things that people say are not possible."

Forrest and Thompson did not begin their work together until about 1993, but the event that inspired both to begin researching OLEDs happened in 1987. That year, Ching Tang and Steven van Slyke, researchers at Kodak Laboratories in Rochester, New York, started a revolution when they published a paper on their creation of the first practical light-emitting diode based on thin films of fluorescent organic small molecules. That 1987 paper planted the initial seed of curiosity in OLEDs in Forrest and Thompson.

"It was really Tang and van Slyke's pioneering work that got us excited and made us say, 'This really is an area that has some real interest,'" Thompson recalled. "The efficiencies (of OLEDs) were good, but they were nowhere near as good as they are today. The understanding of mechanisms of these devices was OK, but not great. So there was a wonderful place to start. They (Tang and van Slyke) had demonstrated exciting stuff and left open a million scientific questions that we're still banging our heads up against."

Tang and van Slyke's light-emitting devices were destined to have a lasting effect on the scientific community, but these devices also had a significant drawback imposed by quantum mechanics: reasonably low efficiency.

To form an excited state on a molecule that eventually emits light, electrons must be injected from electrical contacts made to the film surfaces. From simply statistical considerations, 25% of those electrons excite the light-emitting molecules in their spin "singlet" state, while 75% excite the same molecules in the spin "triplet" state. But due to symmetry considerations imposed by the relaxed (unexcited) molecular state for most organic materials, only singlet states typically emit light or "fluoresce." That implies that of four electrons injected, only one will produce light emission. This leads to relatively low efficiency – a problem that would not be resolved until 1998, when Forrest and Thompson, both working at Princeton University, discovered that including a heavy-metal atom within the organic molecule could quadruple efficiency.

Five years earlier, in 1993, Thompson and new Princeton arrival Forrest had begun collaborating on OLEDs at the university labs. At the time, Thompson had also been working on solar-energy conversion for about a year with Sherwin Seligsohn, who would found UDC in 1994. Seligsohn was looking for the "next significant display technology," according to UDC's Mahon. One day, Thompson suggested to Seligsohn that he, Forrest, and Thompson meet to discuss the Princeton researchers' work on OLEDs. Forrest and Thompson showed Seligsohn a "pretty simple green LED," Thompson said, and a lasting partnership was born.

"(The LED) wasn't the most innovative thing in the world, but it caught his eye," Thompson explained. "Sherwin saw that and his eyes lit up. He got very excited about it and that was really where UDC came from. It was really that initial sort of excitement he gotfrom seeing a device, and we have that happen every day in the lab."

The realization regarding the heavy-metal atom came in 1998, while Forrest and Thompson were working with their partners at UDC. The duo discovered that the inclusion of a heavy-metal atom could lead to a violation of the quantum-mechanical selection of singlet states, allowing 100% of the injected electrons to result in light emission, compared to the 25% Tang and van Slyke had demonstrated 11 years earlier.

Using a platinum-based molecule, Forrest and Thompson were able to achieve what Forrest still calls a "gorgeous, juicy-red" PHOLED, but as they increased the PHOLED's brightness, its efficiency fell rapidly, Forrest said. His team quickly attributed the problem to triplet-triplet annihilation, and on the materials side, Thompson's group promptly suggested swapping platinum for iridium as a solution to the problem. Because iridium compounds have highly reduced triplet-triplet annihilation, they make it possible to create PHOLEDs that are super-bright while maintaining efficiency, Forrest explained. According to Forrest, it was the use of iridium that really made the field "explode."

Otto Schade Prize    Dr. Curtis R. Carlson (top) and Dr. Roger Cohen (bottom) have been named as the winners of the inaugural Otto Schade Prize for outstanding scientific or technical achievement in or contribution to the advancement of the functional performance and/or image quality of information displays. Cohen and Carlson are the co-developers of the Just-Noticeable Difference (JND) image-quality metric, which has been widely adopted as an accurate method for assessing the impact of display-design decisions in perceptual units. First reported in 1978 and later described in a paper in 1980, the JND approach decomposes displayed information into spatial-frequency channels commensurate with those of the human visual system, then quantifies the energy in each channel in units of just-noticeable difference, explained Dr. Albert Pica, who nominated Carlson and Cohen for the Braun Prize. Use of the JND approach extended the domain of image-quality analysis to include the effects of noise, pixelation, and variations in image content, he added. "The pioneering work of Carlson and Cohen laid the basis for a new approach in the design and optimization of displays," Pica said. "Their development of the JND image-quality metric gave researchers and engineers the first direct quantitative method for accurately predicting display performance."

"I think there was a fair amount of serendipity in (the discovery of iridium as a means to higher efficiency). But it was not due to aimless stumbling. (The research) was very directed and we had a very clear idea of where we were going, but we also had a lot of luck along the way," Forrest said. "The most important thing about doing science sometimes is not always to have basic insight but to understand that when you got something that you got something. You can always throw the baby out with the bathwater, but we knew exactly what we had when we had it."

What they had was a new technology and a new field, called electrophosphorescence. Today, electrophosphorescence may allow OLED devices to compete with the established LCD and PDP technologies for high-efficiency full-motion-video full-color displays.

"When they first discovered it (electrophosphorescence and PHOLEDs), a couple of papers were written and no one believed it," recalled UDC's Mahon. "People laughed and said, 'The data can't be right.' So we started to get more materials into the marketplace, and people started to evaluate the technology themselves, and of course, it then took off tremendously."

Through the work of Forrest and Thompson's partners at UDC, the first FPD product using PHOLEDs made its way to the market in 2001 in the form of a full-color subdisplay manufactured by Pioneer Corp. and used in a Fujitsu cell phone. During the next few years, Forrest and Thompson continued to help PHOLEDs evolve into one of UDC's core technologies.

Today, Forrest and Thompson say they are interested in pushing PHOLEDs to the limits of the color spectrum, both in blue and ultraviolet as well as the near-infrared, for use in military applications.

"If you think about displays out in the field, particularly in military situations, a display that doesn't give off light and make you a prime target is an awfully good thing to have," Forrest said. "A lot of our military folks wear night-vision goggles anyway, and so they're very sensitive to the infrared. So it's about making a two-tone display that only people with night-vision goggles can see."

PHOLEDs also have myriad potential applications that extend beyond the realm of displays, according to Dr. Munisamy Anandan, president and chief technology officer of Organic Lighting Technologies who, along with Dr. Julie Brown of UDC, nominated Forrest and Thompson for this year's Rajchman Prize.

"I know full-well the great impact the invention of Steve Forrest and Mark Thompson is going to create for the flat-panel-display industry," Anandan said. "In addition, all inventions in flat-panel-display technologies hitherto created a big impact only in the field of displays. But the invention of Steve Forrest and Mark Thompson is unique in that it is not only significant for flat-panel-display technology, but also equally significant for the consumer-lighting industry. The benefits of their invention will increasingly unfold in the future when we see the 'organic lighting' around us, be it in TVs or laptops or cell phones or decorative lighting."

Thompson agreed, saying he and Forrest are currently working with UDC on white LEDs and high-efficiency light sources that can be used in general illumination.

"The interesting thing is that light sources such as this have never existed," Thompson said. "So the simple solution is to replace the fixtures in your ceiling with panels. But the more interesting thing is, if you start making these on plastic, do rigid panels have to be replaced? I think they can be used as wallpaper, or they could be converted into ribbons and woven into fabric. Or the lampshade could become the light instead of being the shade for the light bulb, or any number of things."

Wherever PHOLEDs are headed, Forrest, Thompson, and UDC say they will continue to work together on the technology for years to come. All involved credit a strong and unique working relationship with producing a lasting partnership and such dramatic results.

"This is a spectacular collaboration with Mark and I. It's just about as good as it gets," Forrest said. "UDC has also been a very important aspect to this partnership. I think it's really been a very good situation in a three-way match. Obviously, UDC's business is based largely on our inventions, and our success is largely based on the fact that they let us work without many restrictions."

The 2006 SID Awards Banquet will take place at 8:00 pm on Monday, June 5, in the Yerba Buena Ballroom at the San Francisco Marriott Hotel. This dinner honors all the SID international award winners. •