Three beams of light (red for infrared, yellow for visible light, and violet for ultraviolet) travel through a layer of tin dioxide. Absorption by the conduction electrons in the oxide reduces the intensity of the beams. Image: Hartwin Peelaers, UCSB |
Researchers
in the Computational Materials Group at the University of California, Santa Barbara (UCSB) have
uncovered the fundamental limits on optical transparency in the class of
materials known as transparent conducting oxides. Their discovery will support
development of energy-efficiency improvements for devices that depend on
optoelectronic technology, such as light-emitting diodes (LEDs) and solar
cells.
Transparent
conducting oxides are used as transparent contacts in a wide range of
optoelectronic devices, such as photovoltaic cells, LEDs, and LCD touchscreens.
These materials are unique in that they can conduct electricity while being
transparent to visible light. For optoelectronic devices to be able to emit or
absorb light, it is important that the electrical contacts at the top of the
device are optically transparent. Opaque metals and most transparent materials
lack the balance between these two characteristics to be functional for use in
such technology.
In a paper
published in Applied Physics Letters,
the UCSB researchers used cutting-edge calculation methods to investigate tin
dioxide, a widely-used conducting oxide.
Conducting
oxides strike an ideal balance between transparency and conductivity because
their wide band gaps prevent absorption of visible light by excitation of electrons
across the gap, according to the researchers. At the same time, dopant atoms
provide additional electrons in the conduction band that enable electrical
conductivity. However, these free electrons can also absorb light by being
excited to higher conduction-band states.
“Direct
absorption of visible light cannot occur in these materials because the next
available electron level is too high in energy. But we found that more complex
absorption mechanisms, which also involve lattice vibrations, can be remarkably
strong,” says Hartwin Peelaers, a postdoctoral researcher and the lead author
of the paper. The other authors are Emmanouil Kioupakis, now at the University of Michigan, and Chris Van de Walle, a
professor in the UCSB Materials Department and head of the research group.
They found
that tin dioxide only weakly absorbs visible light, thus letting most light
pass through, so that it is still a useful transparent contact. In their study,
the transparency of tin dioxide declined when moving to other wavelength
regions. Absorption was 5 times stronger for ultraviolet light and 20 times
stronger for the infrared light used in telecommunications.
“Every bit
of light that gets absorbed reduces the efficiency of a solar cell or LED,”
remarked Chris Van de Walle. “Understanding what causes the absorption is
essential for engineering improved materials to be used in more efficient
devices.”
