Scientists squeeze more light from organic LEDs

Only a small portion of the electricity flowing into common incandescent light bulbs is emitted as visible light; the rest is given off as heat. The typical light-emitting diode (LED) faces a similar problem. With conventional inorganic LED materials, which are unsuitable for white-light lighting, only about 10% of incoming electricity converts to light. Until recently, conventional thought was that this yield could perhaps be boosted to 25%, no more.

Now scientists at the University of Utah (Salt Lake City, UT) have proven that this limit doesn't extend to all possible LED materials. Their tests indicate that some electroluminescent polymer and oligomer materials—which do show promise as white-light LEDs—should be able to convert 41% to 63% of incoming electricity into light.¹

According to Valy Vardeny, physics chairman at the University of Utah, these findings could fuel the development of more-efficient light emitters for lasers, displays, room lighting, computer screens, and televisions.

Vardeny and University of Utah postdoctoral physicist Mark Wohlgenannt conducted the study with Sumit Mazumdar at the University of Arizona (Tucson, AZ) and S. Ramasesha and Kunj Tandon at the Indian Institute of Science (Bangalore, India).

LEDs produce light when incoming negative and positive electrical charges (electrons and holes) combine to form singlet excitons that subsequently decay. The assumed 25% limit for quantum yield was based on the theory that light is emitted only one of every four times the electron and hole combine. This was thought to be an immediate consequence of the fact that electrons and holes each carry a unique spin characteristic. To examine this premise, Wohlgenannt and colleagues placed thin films of 10 different p-conjugated polymers and oligomers in a magnetic field at supercold temperatures and then used a laser to make the materials emit light. By also bombarding the thin films with microwaves, the scientists were able to demonstrate that some of the materials—particularly those that emit red and blue-violet light—could emit more light than they would otherwise.

The tests relied on both continuous-wave photoinduced absorption (PA) and photoinduced absorption-detected magnetic resonance (PADMR) to compare the formation cross-section of singlet excitons to that of any one of three equivalent nonemissive combinations of holes and electrons. The resulting ratio provided information about the quantum efficiency for electroluminescence in organic LEDs.

The continuous-wave PA tests used by Wohlgenannt and colleagues measured the excited-state absorption spectrum of electrons and holes. The experimental excitation beam was produced with an argon-ion laser modulated with a chopper. The probe-transmission came from a tungsten halogen lamp. The pump-beam-induced changes in the probe-beam transmission were then measured with a monochromator and a combination of solid-state detectors to provide data for determining the PA intensity.

The PADMR technique was used to measure the effect of strong microwave absorption on the electron and hole combinations. In addition to providing a PA setup, this part of the experiment included a magnetic resonance source and resonator. By placing the sample inside the resonator and cooling it with liquid helium, the scientists were able to ensure that both PA and PADMR data were measured under identical conditions.

In addition to disproving the previously assumed ceiling for quantum yield, the researchers believe this is the first time that such a combination of spin-dependent spectroscopy techniques has been used to measure this parameter for p-conjugated polymers and oligomers. According to Vardeny, the technique is very general and could work for other organic or inorganic materials that have long-lived charge excitations—particularly if someone would like to determine the maximum electroluminescence possible from each class of material.

Vardeny admits that it would be both expensive and impractical to use microwaves to improve the efficiency of commercial LEDs. For this reason, the University of Utah is seeking a patent on a method of doping light-emitting plastics with iron compounds and chemicals to produce the same effect as the microwaves have in boosting quantum yield of these materials.

REFERENCE