Spintronic OLED promises brighter displays
The Utah physicists made a prototype of the new kind of LED – known technically as a spin-polarized organic LED or spin OLED – that produces an orange color. Valy Vardeny, University of Utah distinguished professor of physics expects it will be possible within two years to use the new technology to produce red and blue as well, and he eventually expects to make white spin OLEDs.
However, it could be five years before the new LEDs hit the market because right now, they operate at temperatures no warmer than about -33°C, and must be improved so they can run at room temperature, Vardeny adds.
The study was funded by the U.S. National Science Foundation, the U.S. Department of Energy, the Israel Science Foundation and U.S.-Israel Binational Science Foundation. The research was part of the University of Utah’s new Materials Research Science and Engineering Center, funded by the National Science Foundation and the Utah Science Technology and Research initiative.
The device invented by the Utah physicists uses an organic semiconductor, but isn’t simply an electronic device that stores information based on the electrical charges of electrons. Instead, it is a “spintronic” device – meaning information also is stored using the “spins” of the electrons. A low voltage is used to inject negatively charged electrons and positively charged “electron holes” through the organic semiconductor. When a magnetic field is applied to the electrodes, the spins of the electrons and electron holes in the organic semiconductor can be manipulated to align either parallel or antiparallel.
In the new study, the physicists report two crucial advances in the materials used to create “bipolar” organic spin valves that allow the new spin OLED to generate light, rather than just regulate electrical current. Previous organic spin valves could only adjust the flow of electrical current through the valves.
The first big advance was the use deuterium instead of normal hydrogen in the organic layer of the spin valve. Deuterium is “heavy hydrogen” or a hydrogen atom with a neutron added to regular hydrogen’s proton and electron. Vardeny says the use of deuterium made the production of light by the new spin OLED more efficient.
The second advance was the use of an extremely thin layer of lithium fluoride deposited on the cobalt electrode. This layer allows negatively charged electrons to be injected through one side of the spin valve at the same time as positively charged electron holes are injected through the opposite side. That makes the spin valve “bipolar,” unlike older spin valves, into which only holes could be injected.
It is the ability to inject electrons and holes at the same time that allows light to be generated. When an electron combines with a hole, the two cancel each other out and energy is released in the form of light.
“When they meet each other, they form ‘excitons,’ and these excitons give you light,” Vardeny says.
By injecting electrons and holes into the device, it supports more current and has the ability to emit light, he says, adding that the intensity of the new spintronic OLEDs can be a controlled with a magnetic field, while older kinds require more electrical current to boost light intensity.
Existing OLEDs each produce a particular color of light – such as red, green and blue – based on the semiconductor used. Vardeny says the beauty of the new spin OLEDs is that, in the future, a single device may produce different colors when controlled by changes in magnetic field.
Vardeny developed the Spintronic OLED with Tho D. Nguyen, a research assistant professor of physics and first author of the study, and Eitan Ehrenfreund, a physicist at the Technion-Israel Institute of Technology in Haifa.