In a development that could hasten the advent of low-cost, high-efficiency LEDs and solid-state lighting, a group of University of Michigan researchers has developed a new process that can improve the efficiency of the of metal-free organic phosphors that could be the key to phosphorescent LEDs.
Today’s LEDs emit light in what’s called a singlet energy state, which only converts about 25% of the electricity that’s consumed into light. But phosphors convert energy into a triplet state, which in theory enables them to emit nearly 100% of consumed energy as light. The trouble is, getting bright light has traditionally required makers to dope the phosphors with heavy metals. These organo-metallic phosphors are expensive and sometimes toxic. And blue phosphors–which are essential in the production of white light–don’t last long enough to be viable for use in commercial devices.
The Kim group introduced bright, metal-free, purely organic phosphors that could be tuned to any color in 2011, using a rational molecular design. But their efficiency was still much less than that of their organo-metallic counterparts because vibration at the molecular level caused them to waste much of their energy as heat.
Now, the group has used a different approach to create a new, phosphor-doped polymer with a more rigid molecular structure. They used rational chemical design to covalently link metal-free purely organic phosphors and paired polymer matrixes. The self-assembling, metal-free polymers waste far less energy as heat through vibration, dramatically increasing their efficiency compared to the same phosphors-doped polymers without the covalent linkage.
The team’s phosphor-doped polymer achieved a phosphorescence efficiency of 28 percent. While this figure is only slightly better than today’s widely-used fluorescent LEDs, the developed strategy and thorough understanding of the correlation between molecular motions and emission intensity could be readily applicable to further development of organic phosphors and variety of polymers, potentially leading to big efficiency gains in future all-organic phosphorescent LEDs. The technology is also useful in bio-imaging and sensing applications.
The team’s findings are detailed in a new paper published in the journal Nature Communications entitled “Suppressing Molecular Motions for Enhanced Room Temperature Phosphorescence (RTP) of Metal-free Organic Materials.” Funding was provided by the National Science Foundation, a Samsung GRO grant and the Spanish Science Ministry.