"We believe that our new test will enable engineers to design more efficient light emitters for lasers and displays, eventually applying them to LEDs, computer screens, TVs and even room lighting," said professor Valy Vardeny, chairman of the physics department at the University of Utah.

Vardeny was assisted in his work by postdoctoral researcher Markus Wohlgenannt and by researchers Sumit Mazumdar at the University of Arizona (Tucson) and Kunj Tandon and S. Ramasesha at the Indian Institute of Science in Bangalore.

The test places potential LED polymer materials inside a super-cooled container and bombards them with microwaves in the presence of a magnetic field. A laser is then used to induce light emission in the material, and its efficiency is measured. Using this method, the team characterized well-known polymers as potentially achieving much higher efficiencies than they do today — as high as 63 percent.

Garden-variety LEDs operate at efficiencies as low as 10 percent, dissipating the other 90 percent of their energy into thermal output — making them barely more efficient than an ordinary light bulb. Quantum physicists originally predicted that any LED's theoretical peak efficiency would never surpass 25 percent, since there is only a one-in-four quantum probability that light emission will result from each electron entering them.

Rule refuted

Last year, however, researchers demonstrated that the so-called 25 percent rule was only a first approximation and that manufacturing conditions could foster as much as 42 percent efficiency using traditional vapor deposition techniques. Now, Vardeny and associates have shown that plastic LEDs made using "ink and printing" manufacturing equipment instead of vapor deposition can achieve even better efficiency: up to 63 percent. Their demonstration also shows development engineers how to test new polymer materials for potential light-emitting efficiency before they are even fabricated into plastic LEDs.

"We hope our test will pave the way to future white LEDs that replace incandescent light bulbs, because they provide more light and last much longer — about 100,000 hours rather than the 1,000-hour lifetime of regular light bulbs," said Vardeny.

LEDs produce light when incoming electrons are attracted to "holes" with which they combine to produce photons. Quantum mechanics teaches that any charge carrier has a "spin" associated with it — either up or down — giving four ways that they can combine, only one of which produces light. This one-in-four quantum probability is what led physicists to predict that the highest efficiency of LEDs would be 25 percent. More recently, researchers found that many different methods can be used to break the 25 percent rule.

"There never really was a 25 percent rule — it's not even a statistical barrier. People just hadn't thought the problem through," said Alan Heeger, a physicist at the University of California, Santa Barbara, who shared the 2000 Nobel Prize in chemistry for developing polymers that conduct electricity. He is the founder of Uniax Corp. (Santa Barbara), a polymer electronics manufacturer.

"With polymers there really is no upper limit to efficiency, and this important new test devised by Vardeny and Wohlgenannt can be used to predict which polymers and oligomers are the best candidates for high-efficiency plastic LEDs," Heeger said.

In theory, the higher efficiency was achieved because the spin of the incoming electrons could be randomized, in effect giving them a second chance at combining with holes to produce light. Usually, the spin of incoming electrons is fixed at up or down when it enters the diode. But the microwave bombardment was suspected of randomizing the spin of electrons inside the device, thereby enabling some electrons to combine with holes to produce light that would otherwise have been wasted, producing heat instead.

In general, conducting polymers offer many more opportunities for getting around the 25 percent rule because of the relative complexity of their molecular makeup. Inorganic solids such as gallium arsenide have a very simple, regular molecular structure — a crystal — which cannot be varied. Thus, technologists are stuck with whatever opportunities that structure offers for electron-hole interactions.

Novel configurations

But polymers with virtually unlimited possibilities for novel molecular configurations can be easily prepared at room temperature. The field of conducting polymers, which began with Heeger's work, adds the semiconductor properties of inorganic compounds to the flexibility of polymers.

Rather than arrange atoms in a regular three-dimensional array, polymers are tangled groups of very long chain molecules. Electronically, therefore, a conducting polymer is essentially a bundle of one-dimensional semiconducting wires.

This provides a general explanation for the higher conversion efficiency of polymers. Electrons in solids emit a photon when they drop from a higher, "excited" energy state to a lower energy state. The photon radiates the lost energy. Thus, solids that make good light emitters have some means of confining electrons in higher energy states. In solids such as GaAs, electrons are confined in the semiconductor's conduction band and travel across the energy barrier to combine with holes, which are just lower energy points in the lattice.

The full story of how electrons are bound in higher energy states in polymers such as poly (p-phenylene vinylene, or PPV), which the Utah group has been studying, has yet to be told. That's because of the complexity of the process, which involves novel electron-hole behavior such as relatively long-lived configurations known as "excitons."

Since electrons and holes have equal and opposite electronic charge, they can couple together in the same manner as an electron and a proton uniting to form a hydrogen atom. In inorganic semiconductors, excitons form but are very short-lived. In polymers they have appreciably longer lifetimes and offer an additional means for electrons to be confined in excited states.

Researchers have recently found that excitons in polymers come in two distinct types, "singlet" and "triplet" excitons, defined by different electron-spin configurations. The Utah team has been studying this aspect of photoemission and the role that the longer-lived triplet excitons play in boosting PPV's performance. Of course, there may be additional complexities in the behavior of electrons in conducting polymers that have yet to be discovered.

That makes the current test method a valuable tool for accelerating the development of high-efficiency plastic LEDs, since it offers a figure of merit that can be experimentally determined, apart from the underlying theory. Practical applications can then be developed.

Stability an issue

Although polymers can achieve very high efficiencies, they have one principal disadvantage over inorganic semiconductors in terms of long-term stability. The atomic bonds that hold inorganic crystals together are much stronger and more stable than polymer bonds. Thus, polymers need to be studied in realistic environmental tests over a long period to verify their stability.

Also, making efficient electrical contact with polymer systems has proven difficult, and that has an impact on building practical devices. As in the Utah studies, the most efficient way to energize a polymer system is with optical "pumping" rather than electrical contacts. Unlike electrons, photons can efficiently enter the system without needing a physical contact.

Vardeny believes that once a material has tested as potentially a high-efficiency LED polymer, it is just a matter of engineering it into a working device to realize that gain. In particular, paramagnetic materials, most likely using iron compounds, could be used to dope the polymer appropriately to realize their high efficiency.