PORTLAND, Ore. Efficiency is the name of the game for flat-panel display technologies. This is especially important for extending the battery life of cellphones, digital cameras, personal digital assistants and other portable devices that use organic LED displays.
Now, Oak Ridge National Laboratory (ORNL) claims it can make OLEDs 30 percent more efficient by doping them with magnetic nanoparticles. As a bonus, the introduction of magnetism into the OLED material enables brightness to be controlled without the addition of electrical contacts.
"What we did was to enhance the lighting efficiency of an OLED by doping the organic polymers with a very low concentration of magnetic nanoparticles," said ORNL senior researcher Jian Shen. "Doping also allows us to control the OLED-s intensity with a magnetic field, whereas conventional OLED intensity is tuned by an electric field, which needs [electrical] contacts."
Conventional OLEDs are nonmagnetic, depending only on electrical fields to create excitons (electron-hole pairs), the recombination of which emits the photons that make an OLED glow. By mixing magnetic nanoparticles into the polymer matrix (at concentrations of less than one-tenth of 1 percent) Shen's team found they could increase OLED efficiency by 27 percent. And by applying an external magnetic field to the doped OLEDs, an additional 5 percent was achieved, for a total increase in efficiency of 32 percent over conventional OLEDs, Shen said.
Light emission in solid-state LEDs occurs when high-energy injected electrons and holes recombine, dropping their energy levels and causing a single photon to be emitted to compensate. An LED with 100 percent efficiency would recombine every single injected electron and hole. In real devices, 100 percent efficiency is never achieved, but by confining them in a small region, designers can achieve the greatest efficiency possible, Shen said.
When electrons and holes pair up, but before they recombine, they are called excitons. Ordinarily, the magnetic spin of each member of an exciton is random, accounting for their variable efficiency in recombining. To increase the efficiency of recombination, Shen's group doped the organic LED's polymer with nanoparticles made magnetic with cobalt and iron (CoFe). In the presence of the magnetic nanoparticles, a larger number of excitons with opposite spins accumulate, called singlet excitons. Oppositely polarized charge carriers are much more likely to recombine, accounting for the higher efficiency of the magnetically doped OLEDs, Shen said.
"The presence of CoFe magnetic nanoparticles enhances the efficiency of electro luminescence, their fluorescence, by increasing the fraction of the so-called singlet excitons among the total excitons," said Shen.
Next, Shen's group will experiment with different doping levels and methods of mixing the magnetic nanoparticles with polymers to achieve ultra uniform concentrations, in hopes of further enhancing efficiency.