Light-emitting diodes, which employ semiconductors to produce artificial light,
could reduce electricity consumption and lighten the impact of greenhouse gas
emissions. However, moving this technology beyond traffic signals and laser
pointers to illumination for office buildings and homes-the single largest
use of electricity-requires materials that emit bright, white light cheaply
and efficiently. White light is the mix of all the colors, or wavelengths, in
the visible spectrum.
Biwu Ma, a staff scientist with the Molecular Foundry, was part of a research team that found a new way to process white OLEDs for solid state lighting. (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs)
Organic light-emitting diodes (OLEDs), based on organic and/or polymer semiconductor
materials, are promising candidates for general lighting applications, as they
can cover large-area displays or panels using low-cost processing techniques.
Indeed, single-color OLED displays are already available commercially. A mix
of red-, green- and blue-emitting materials can be used to generate white light,
but these bands of color often interact with one another, degrading device performance
and reducing color quality.
Using polymer nanoparticles to house light-emitting 'inks', scientists
at the Molecular Foundry, a U.S. Department of Energy nanoscience center located
at Berkeley Lab, and the University
of California, Berkeley, have made a thin film OLED using iridium-based guest
molecules to emit various colors of visible light. The polymer nanoparticle
surrounding a guest light-emitter serves as a 'do not disturb' sign,
isolating guest molecules from one another. Each guest can then emit light without
pesky interactions with neighboring nanoparticles, resulting in white luminescence.
“This simple and bright approach to achieving nanoscale site isolation
of phosphors opens a new door for facile processing of white OLEDs for solid
state lighting,” said Biwu Ma, a staff scientist with the Molecular Foundry's
Organic Nanostructures Facility who contributed to this study. With this proof-of-concept
device under their belts, Ma and his colleagues plan to vary the ratio of each
color nanoparticle in the OLED to enhance efficiency and brightness. White light
from OLEDs can be adjusted from cooler to warmer whites, making these materials
easy to use in office or home environments. Buildings account for more than
40 percent of carbon emissions in the United States, so replacing even a fraction
of conventional lighting with OLEDs could result in a significant reduction
in electricity use.
A paper reporting this research titled, “Site isolation of emitters within
cross-linked polymer nanoparticles for white electroluminescence,” appears
in the journal Nano Letters and is available in Nano Letters online. Co-authoring
the paper with Ma, were Haifeng Gao, Daniel Poulsen and Jean Fréchet
of Berkeley Lab's Materials Sciences Division, plus David Unruh, Xiaoyong
Zhao and Jill Millstone with the UC Berkeley Chemistry Dpeartment.
Portions of this work at the Molecular Foundry were supported by DOE's
Office of Science.
The Molecular Foundry is one of the five DOE Nanoscale Science Research Centers
(NSRCs), premier national user facilities for interdisciplinary research at
the nanoscale. Together the NSRCs comprise a suite of complementary facilities
that provide researchers with state-of-the-art capabilities to fabricate, process,
characterize and model nanoscale materials, and constitute the largest infrastructure
investment of the National Nanotechnology Initiative. The NSRCs are located
at DOE's Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge and Sandia
and Los Alamos National Laboratories.