Technical innovations in light emitting diodes (LEDs) and photovoltaics have consistently reflected the similarities between the two technologies. In fact, research groups at many universities, such as the University of Michigan (Ann Arbor, Mich.; www.umich.edu) and the University of Southern California (USC; Los Angeles, Calif.; www.usc.edu), are studying both simultaneously. With pressing consumer interest in affordable, energy-efficient and environmentally sustainable lighting, displays and energy sources, market demand is now leading industry toward synergistic breakthroughs.
Lighting and displays
In some ways, as Brian D’Andrade of Universal Display Corp. (Ewing, N.J.) points out, commercial advances in white LEDs and OLEDs “can’t be slowed down” and are, in fact, “already here.” Firms like Guangzhou Bright Lighting LED (Vancouver, Canada) are currently offering ‘omni-directional’ 9-W LED bulbs with the comparable output of a 50-W incandescent bulb, available in color temperatures between 3,000K (incandescent) and 6,000K (cool daylight). Others are offering equally competitive, recessed downlights, which in the case of LED Light Fixtures, Inc. (Morrisville, N.C.) are dimmable like conventional units. Rapid improvements in those LED technologies brought to market has led to the necessity of the U.S. Department of Energy’s (DOE; Washington, D.C.) Commercially Available LED Product Evaluation and Reporting (or CALiPER) Program (www.netl.doe.gov/ssl/comm_testing.htm). Tasked with providing “unbiased product performance information,” CALiPER has developed standards for accurately gauging luminous flux from LEDs, as well as accounting for ambient temperature changes and thermal effects specific to the device in which the bulb is installed. As can be seen from its most recent round of product testing, white LEDs have made prodigious leaps over the past year in recessed downlight applications.
According to research firm NanoMarkets LC (Glen Allen, Va. www.nanomarkets.net), new developments in silicon nanocrystals and printed silicon are expected to challenge the role of organic materials in flexible optoelectronics and photovoltaics. But industry experts are doubtful in the case of OLEDs. “OLED technology is already very mature,” says Ansgar Werner, senior manager in Novaled’s (Dresden, Germany) OLED R&D group. “Solution-processed or printed silicon is certainly no material for LED (too-low band-gap, hardly emissive, and so on).” In addition, many familiar with the two technologies also foresee OLEDs and ‘regular’ inorganic-LEDs etching out their own separate and lucrative niches in the marketplace, due to their divergent form factors and relative strengths.
OLEDs can be made as broad luminescent surfaces that are transparent when inactive, flexible and lightweight. These features lend them well to back-lighting for displays, ePaper applications, and architecturally integrated lighting such as windows that shine in the evening. In an early application, General Electric’s GELcore subsidiary, which is now known as Lumination (Valley View, Ohio), marketed its OLED materials as self-lighting displays for museum artifacts. Apple’s recent patent for an OLED keyboard promises keys that change to suit different alphabets, characters and application shortcuts. The same properties that allow for transparent OLEDs also facilitates sharper image quality in OLED displays, because red, blue and green diodes can be stacked on top of one another instead of clustered side by side. Although the first stacked RGB OLED node was patented by USC’s Thompson group in the mid-90’s, a flat-panel OLED TV only hit the market this past year. One barrier to market has been that, while blue phosphorescents had proven themselves to be theoretically 100% light-emission efficient compared to blue fluorescent’s 25%, they tended to degrade quickly over time. Konica Minolta (Tokyo) resolved this issue in June 2006, with the development of a phosphorescent blue that gave the firm’s white-OLEDs a 10,000-h lifetime and a luminous efficiency to rival compact fluorescent bulbs (64 lm/W compared to an average compact fluorescent’s 60-100 lm/W).
Traditional inorganic-LEDs, by contrast, deliver sharper less-diffused beams of light, making them ideal for point-lighting applications like street lamps and automotive headlights. This year, three major automobiles will hit the market featuring full LED headlamps: the Audi R8, the Lexus LS600h and the Cadillac Escalade Platinum. In anticipation of the growing demand, Showa Denko K.K. (Tokyo) is investing $10.6 million to scale up the production of its aluminum-gallium-indium-phosphide (AlGaInP) ultrabright LED chips from 100 to 200 million units-per-month by the end of 2008. In street lamps, LEDs would replace high-pressure sodium lamps approximately quadrupling the time between replacements to an amount in excess of 50,000 hours (or 10 years). LED street lights would also contain no environmentally hazardous mercury and could help to reduce light pollution as a consequence of the sharp, focused quality of their beams. Controlled dimming of streetlights has also been considered since LEDs warm-up radically faster than high-pressure sodium lamps.
There are thermal and electrical obstacles to be overcome before the wide adoption of LEDs takes place in these high luminosity (<12,000 lm) contexts, however. Though LEDs produce more light-per-unit energy than traditional bulbs, their waste heat is highly localized around the miniscule semiconductor components that convert electricity to photons. Due to that concentration, it is not as easily transferred as the heat radiating off a traditional lamp’s large exposed surface. These thermal issues evolve into environmental challenges when the devices are intended for outdoor use — where something like the possibility of a bird nesting over a vital heat exhaust can threaten to permanently damage an LED fixture. Recently, Gordon Routledge of Dialight Lumidrives (York, U.K.) and Roger Shuttleworth of the University of Manchester’s (www.manchester.ac.uk) electrical engineering department have partnered to address these challenges. As Paul Drzaic president-elect of the Society for Information Display (San Jose, Calif., www.sid.org) and chief technology officer for Unidym, Inc. (Menlo Park, Calif.) has noted, “a lot of smart people are worrying” about the thermal management problems.
LEDs smaller size relative to OLEDs also makes them likely candidates for retrofitting into ‘Edison bulb’ sockets, thus competing directly with incandescent and compact fluorescent bulbs — a fact not lost on large players in the lighting industry. In 2007 alone, Royal Philips Electronics (Amsterdam, Netherlands) spent $4.3 billion in the purchasing of five LED lighting companies (including Genlyte Group Inc. and TIR Systems Ltd.), surpassing General Electric as the largest supplier of lighting to the U.S. market. The Netherlands-based firm supplied Times Square with a 9,000-bulb LED ball for its 2008 New Year’s celebration and was also competing to supply color-tuning LEDs for the top of the Empire State Building. (The firm was competing against Color Kinetics, which it also bought in 2007.)
Adding to these high-profile PR-spectacles, Philips has been vocal in its call to institute a federally legislated phase-out of incandescent bulbs. With plans to develop and market new generations of energy-efficient incandescent bulbs, GE has publicly been advocating a more free-market approach to such a phase-out. As one researcher phrased it, it would seem that emerging LED technologies are “more competing with the incumbent” than with OLEDs.
With respect to monitors and displays, however, other emerging technologies are vying with OLEDs for position, namely hybrid organic/inorganic LEDs featuring quantum dot (QD) lumiphores. First developed by Vladimir Bulovic at MIT (Cambridge, Mass.), “Quantum dots (as emitter) are the inorganic components closest to application” in hybrid displays, according to Novaled’s Werner. QD Vision (Watertown, Mass.), a firm developing the technology with Bulovic on its Scientific Advisory Board, estimates that it could produce a marketable QD-LED cell phone display within three years, promising more vivid colors and greater energy efficiency than OLED displays. While “hybrid structures are realistic,” Werner emphasizes that, “today many printed inorganic materials need high temperature annealing, rendering them incompatible with organic layers and flexible substrates.” The sentiment is common among other OLED researchers as well. Due to the required high-temperature annealing, inorganics must be deposited first in any hybrid process, affecting the cost, the flexibility of the manufacturing sequence and what substrates can be used.
These are the kind of obstacles that have encouraged OLED researchers to pursue fully organic methods. While the prospect of paper-thin, flexible monitors, self-illuminating plastic surfaces and wearable solar cells all have an obvious marketing appeal, the real benefit of flexible electronics from an industrial standpoint has consistently been roll-to-roll processing. Traditionally, when indium tin oxide (ITO) is used as an electrode in OLED devices or photovoltaic cells, the placement of the material on glass or plastic requires a vacuum chamber, sputtering tools, and roughly a 1-ft/min processing speed. In effect, the slow processing requirements and the expense of the equipment itself combine to significantly lower the amortization rate on the equipment and reduce the return on investment. Flexible electrode alternatives like carbon nanotubes (CNT) can be deposited at room temperature, with faster and cheaper printing techniques akin to conventional newspaper publishing.
Organic and inorganic solar cells
In 1997, phosphorescent OLED technology had not developed efficiencies that were comparable to those of fluorescents, though many paths toward improvement had been proven at a research level. Within three years, rapid advances in new materials were facilitating innovative design architectures for OLED devices. According to USC’s Mark Thompson, “where we are in organic photovoltaics, today, in terms of intellectual development is not far off from where we were with OLEDs ten years ago.”
Currently, organic photovoltaic (OPV) cells operate at efficiencies between 2/3 and 1/2 of dye-sensitized or Grätzel solar cells and about 1/3 to 1/4 the efficiency of the average, commercially available silicon-based solar panel. However, OPVs carry with them the promise of simple and cost-effective tandem-polymer solar cells. Tandem solar cells are a series of linked solar cells, each with different absorption characteristics capable of collecting the most energy from photons in a different range of the solar spectrum. Silicon-based solar cells can absorb a wide range of photons within the visible spectrum of light, but the material translates them all into red wavelength, lower energy photons. This energy is effectively lost as heat in the cell.
The breakthrough development in stacked OPV research came from a team lead by Kwanghee Lee and Alan J. Heeger who were able to fabricate a two-layer OPV system composed of semiconducting polymers and fullerene derivatives via all-solution processing. Though only power-conversion efficiencies of over 6% were achieved at illuminations of 200 mW/cm2, each tandem cell was absorbing more energy from photons within its respective wavelength range. The team also noted that the OPV materials could “be abricated to extend over large areas by means of low-cost printing and coating technologies that can simultaneously pattern the active materials on lightweight flexible substrates.”
The Lee/Seeger technology is now one of the key patents for Konarka (Lowell, Mass.), which his begun entering into deals with other firms to market its flexible OPV cell technology. Having been awarded a grant from the National Institute of Standards and Technology (Gaithersburg, Md., www.nist.gov), the firm joined with Air Products (Allentown, Pa.) in October to create a translucent OPV window — seen more as competition with ordinary windows than with lower cost-per-watt cells. “We’re not selling high efficiency and not lower prices,” Konarka’s chairman and cofounder Howard Berke told the Lux Research conference on nanotechnology. “It’s the patterns, colors, the aesthetic attributes that make a product more valuable than just the power it produces.”
This past October, Konarka also entered into licensing agreements with Dupont Displays (Wilmington, Del.) for the sole rights within the OPV field to use key patents that Dupont and the University of California at Santa Barbara (www.ucsb.edu) had developed for OLED displays. The patent licensing is reminiscent of a decision made that same month by Applied Materials (Santa Clara, Calif.) to convert its LCD-display processing equipment for the processing of amorphous silicon solar cells, selling equipment to solar industry giant Q-Cell (Bitterfeld-Wolfen, Germany).
Though OPV technology promises economical manufacturing, thin-film techniques for traditional crystalline silicon and amorphous-silicon PV cells still hold major energy conversion records and are the most mature technologies on the market. Nanomarkets has forecast that the thin-film PV market will grow from $1 billion in 2007 to $7.2 billion in 2015, with 75% of that total attributed to large projects and utilities, commercial and industrial facilities, and residential buildings. These figures are echoed by Photon Research Associates (San Diego, Calif.), which estimates that thin-film PV will have a 63% compound annual growth rate into 2010.
Last summer a consortium headed by DuPont and the University of Delaware (Newark, Del.; www.udel.edu) developed a silicon PV with a record efficiency of 42.8% by optically separating the light into three groupings of similar photons before being absorbed by the cell. The technique, funded under DARPA’s Very High Efficiency Solar Cell (VHESC) program, would allow silicon cells to compete in potential future efficiency with OPVs, because the novel optical design circumvents the need to mechanically stack a Si PV to fully absorb energy from a range of photons.
R&D synergies
Optoelectronic materials, whether they are emitting light or absorbing light, find themselves in a highly competitive field today with many promising new technologies. Research into new materials has created more efficient designs for existing technologies like solar cells, LEDs and displays. Furthermore, the structural similarities between these display, energy-collecting and lighting devices have facilitated a ‘cross-pollination’ of technical innovations and unexpected economies of scale. While other factors such as resource markets will be a determining factor in how the field develops, the scope of research interest in academia and industry suggests a high potential for many significant breakthroughs to come.