on the market. OLED displays can have contrast ratios of 1,000,000:1, but LED technology has not been standing still, and experts continue to argue about which technology produces better displays.
One application of OLED with great potential in the market is the use of white OLED (WOLED) in lighting panels. Potentially, these devices can be more efficient than fluorescent lights and produce more accurate color rendering. In addition, WOLED devices can be flexible and can even become mirrors when turned off. Today’s state-of-the-art devices produce 87 lumens/watt (lumens/W) at 1,000 candelas/m2. The goal is to produce devices that generate 300 lumens/W.
ITO is essential to all of this work because it is the preferred transparent conducting electrode in thin-film photovoltaics, and it is the only transparent conducting electrode in all LCDs and OLEDs. Researchers are making advances in developing transparent zinc oxide and aluminum oxide electrodes, but those efforts are not yet close to producing a commercially viable product.
One of the most promising alternatives is poly(3,4-ethylenedioxythiophene): poly(styrenesulfanate) (PEDOT:PSS). Though this material has been studied for over 10 years, the recent development of a fabrication process that creates multilayered PEDOT:PSS devices has produced a breakthrough in organic photovoltaic and OLED performance (Kim et al., 2011). The new process involves blending these polymers with polyethylene glycol (PEG), immersing the blend in PEG, and then annealing. Even though transmission goes down with each additional layer, sheet resistance also goes down, and that, said Shinar, is important. These PEDOT:PSS sheets are smoother than ITO, which is good for OLEDs, and their refractive index is lower, which reduces internal reflection and increases light output.
In fact, said Shinar, multilayer PEDOT:PSS OLEDs are up to twice as efficient as a standard ITO OLED. And in more recent work, which his group has not yet published, a two-layer PEDOT:PSS OLED produced a maximal luminous power efficiency of 100 lumens/W without coupling enhancement tricks. “With microlens arrays, which typically can double the out-coupling enhancement, these devices would be beyond 200 lumens/W,” said Shinar.
It is important to remember, though, that ITO devices are a moving target with continually improving performance. Recently, for example, chlorinated ITO-based OLEDs showed impressive efficiencies (Helander et al., 2011). The power efficiency reported for these devices exceeded 200 lumens/W, which is approaching the Department of Energy’s goal of 300 lumens/W. Not too long ago, this was considered a pipe dream, said Shinar. The external quantum efficiency for chlorinated ITO is “an amazing 53 percent,” he added. “For every two electrons you inject into the OLED you get one photon out, and not just out, but in the direction you want.”
Shinar noted that heavy and rare earth metal atom chelates, using palladium, platinum, iridium, and europium, are going to continue to be important in the display and SSL industries because they produce the best phosphorescent emitters. As he explained, “In all OLEDs, injection into the emitting layer results in 25 percent singlet excitons and 75 percent triplet excitons, and in fluorescence only the singlet excitons are radiative.” As a result, the maximum internal quantum efficiency is only 25 percent. Heavy metal chelate-based phosphors, however, are radiative with triplet excitons, so their use is critical to achieve high internal quantum efficiency for any OLED device.
In fact, when researchers from the Universal Display Corporation successfully fabricated phosphorescent OLEDs using heavy metal chelates, they boosted internal quantum efficiencies into the 90 to 100 percent range. This success, said Shinar, explains why this company is now worth between $500 million and $1 billion.
Shinar noted that iridium, because of its use in these phosphors, should be considered a critical metal. It is the least abundant of the platinum-group elements, yet today it is priced lower than gold. Major commercial sources of iridium are found in South Africa, Russia, and Canada. Iridium is difficult to refine and is produced in small quantities, but supplies have increased in response to a four-fold increase in demand to 334,000 ounces in 2010, largely a result of the inclusion of iridium crucibles in backlit LED televisions and the increased demand for iridium-tipped automobile sparkplugs. “The sharp increase in demand and the small, relatively illiquid market for iridium had a significant impact on price,” said Shinar. In August 2011, iridium was priced at $1,050 per ounce (eBullionGuide.com, 2011).
Efforts to develop room-temperature phosphors free of heavy metals have begun, but the best results so far still fall short of the mark. Shinar wondered if more research in this direction should be initiated. One possibility would be to exploit triplet-triplet annihilation to produce singlet excitons that could increase the internal quantum efficiency well beyond 25 percent.
In closing, Shinar noted that “for optoelectronics, the critical in critical resources is questionable. There is no single silver bullet because the situation has improved, and instead there are many potential silver bullets for different problems.”
One reason that PV technology is such an exciting area today, said Ken Zweibel, is that solar energy is such an abundant resource (Figure 5-1). The sheer size of the solar energy “reserve” dwarfs all other potential sources of renewable energy and is more than an order of magnitude larger than all coal, uranium, petroleum, and natural gas reserves combined.