Pure organic compounds that glow in jewel tones could
potentially lead to cheaper, more efficient, and flexible display screens,
among other applications.
Univ.
of Michigan researcher
Jinsang Kim and his colleagues have developed a new class of material that
shines with phosphorescence—a property that has previously been seen only in
non-organic compounds or organometallics.
Kim and his colleagues made metal-free organic crystals that
are white in visible light and radiate blue, green, yellow, and orange when
triggered by ultraviolet light. By changing the materials’ chemical
composition, the researchers can make them emit different colors.
The new luminous materials, or phosphors, could improve upon
current organic light-emitting diodes (OLEDs) and solid-state lighting. Bright,
low-power OLEDs are used in some small screens on cell phones or cameras. At
this time, they aren’t practical for use in larger displays because of material
costs and manufacturing issues.
The OLEDs of today aren’t 100% organic, or made of carbon
compounds. The organic materials used in them must be spiked with metal to get
them to glow.
“Purely organic materials haven’t been able to generate
meaningful phosphorescence emissions. We believe this is the first example of
an organic that can compete with an organometallic in terms of brightness and
color tuning capability,” said Kim, an associate professor of materials
science and engineering, chemical engineering, macromolecular science and
engineering, and biomedical engineering.
This work is published online in Nature Chemistry.
The new phosphors exhibit “quantum yields” of 55%.
Quantum yield, a measure of a material’s efficiency and brightness, refers to
how much energy an electron dissipates as light instead of heat as it descends
from an excited state to a ground state. Current pure organic compounds have a
yield of essentially zero.
In Kim’s phosphors, the light comes from molecules of oxygen
and carbon known as “aromatic carbonyls,” compounds that produce
phosphorescence, but weakly and under special circumstances such as extremely
low temperatures. What’s unique about these new materials is that the aromatic
carbonyls form strong halogen bonds with halogens in the crystal to pack the
molecules tightly. This arrangement suppresses vibration and heat energy losses
as the excited electrons fall back to the ground state, leading to strong
phosphorescence.
“By combining aromatic carbonyls with tight halogen
bonding, we achieve phosphorescence that is much brighter and in practical
conditions,” said Onas Bolton, a co-author of this paper who recently
received his Ph.D. in Materials Science and Engineering.
This new method offers an easier way to make high-energy blue
organic phosphors, which are difficult to achieve with organometallics.
Organic light emitting diodes are lighter and cheaper to
manufacture than their non-organic counterparts, which are made primarily of
ceramics. Today’s OLEDs still contain small amounts of precious metals, though.
These new compounds can bring the price down even further, because they don’t
require precious metals. They’re made primarily of inexpensive carbon, oxygen,
chlorine, and bromine.
“This is in the beginning stage, but we expect that it
will not be long before our simple materials will be available commercially for
device applications,” Kim said. “And we expect they will bring a big
change in the LED and solid-state lighting industries because our compounds are
very cheap and easy to synthesize and tune the chemical structure to achieve
different colors and properties.”
Former doctoral student Kangwon Lee discovered the unique
properties of these materials while developing a biosensor—a compound that
detects biological molecules and can be used in medical testing and
environmental monitoring. The phosphors have applications in this area as well.
After Lee’s discovery, Bolton developed the
metal-free pure-organic phosphors.
The paper is titled “Activating efficient
phosphorescence from purely-organic materials by crystal design.” In
addition to Kim, Bolton, and Lee, other
contributors are: former postdoctoral researcher Hyong-Jun Kim in the
Department of Materials Science and Engineering and recent Chemical Engineering
graduate Kevin Y. Lin. This work is partly funded by the National Science
Foundation and the National Research Foundation of Korea.
The university is pursuing patent protection for the
intellectual property, and is seeking commercialization partners to help bring
the technology to market.
