Researchers develop organic phosphorescents that could dramatically reduce cost of LEDs
by Andrew Brister –
Incandescent lamps turn only 5% of the electricity they use into light, while fluorescent LEDs can produce light from 25% of the electrons that pass through them. Phosphorescent LEDs offer the potential to turn every electron into a ray of light, however, the materials that are typically used to make phosphorescents can be costly.
LEDs are semiconductors that produce light when an electric current runs through them. Organic semiconductors are much cheaper than inorganic semiconductors, but today’s organic technologies rely on specific metals in the semiconductor to enable phosphorescence. This increases the price and sometimes the toxicity of the material.
Now, a team of researchers at UoM led by professor of materials science and engineering, Jinsang Kim, have developed a range of bright, metal-free, organic phosphorescent LEDs.
Organic semiconductors, if left to their own devices, fritter away 75% of the energy that might produce phosphorescence. Instead of light, they generate vibrations in surrounding molecules, thereby heating the material. Incorporating metals can improve phosphorescence, but going metal-free requires a new solution.
“That’s why phosphorescence from metal-free organic materials in nature is very rare,” explained Min Sang Kwon, a materials science and engineering postdoctoral scholar in Kim’s team. A well-known example is the Hope Diamond. Though famously blue, the diamond glows red after exposure to ultraviolet light. Traces of boron in its carbon structure are thought to produce both the blue colour and the phosphorescence.
Kim and his team have demonstrated that the crystal approach works for their all-organic light emitters. The stiff lattice structure stifles vibrations, allowing more energy to be released as light. The team’s crystals produced light from 55% of the charges, but it is difficult to consistently produce high quality crystals in manufacturing.
Instead, the group are looking into how to provide rigidity with less effort. They have tweaked the design of the molecules so that they form structural bonds with a transparent polymer, similar to the way magnets snap together.
The team heated and dried a solution containing the newly-devised light emitters and polymers, and the molecules self-assembled into a stiff matrix. This design allowed 24% of the charges to produce light. Whilst this is only as good as the output of fluorescent LEDs, the team is working to further improve the efficiency. For more, visit: www.oled-info.com