UCSB researchers uncover fundamental limits on optical transparency in conducting oxides

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Contact information: Hartwin Peelaers, Materials Department, University of California, Santa Barbara, CA 93106-5050 peelaers [at] engineering [dot] ucsb [dot] edu

 

UCSB researchers uncover fundamental limits on optical transparency in conducting oxides

Santa Barbara, California, January 8, 2011

 

Conducting oxides are used as transparent contacts in a wide range of optoelectronic devices, such as solar cells, light-emitting diodes, and LCD touch screens. These materials can conduct electricity while being transparent to visible light. Researchers in the Computational Materials Group at the University of California, Santa Barbara (UCSB) have now uncovered the fundamental limitations on optical transparency in this class of materials.

Transparent conduction oxides are good conductors of electricity while at the same time allowing visible light to travel through them. 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. While metals are the best electrical conductors available, they are also opaque. Most optically transparent materials, on the other hand, are insulating and cannot be used as electrodes. Conducting oxides strike the best balance between optical transparency and electrical conductivity. Their wide band gaps prevent absorption of visible light by excitation of electrons across the gap.  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. Since both the electrical conductivity and the absorption coefficient are proportional to the number of free electrons and compete with each other, there is a fundamental limit to the optical transparency that can be attained in conducting oxides.

In a paper published in Applied Physics Letters [APL 100, 011914 (2012)] a team of researchers at UCSB used cutting-edge calculations to investigate these effects in tin dioxide (SnO2), a widely used conducting oxide. “Direct absorption of visible light cannot occur, 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 Materials Department at UC Santa Barbara and head of the research group.

Fortunately, the absorption of visible light is weak enough for SnO2 still to be a useful transparent contact. The researchers found, however, that transparency quickly gets worse when moving to other wavelength regions: the absorption is 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.”

 

 

Artist’s impression of key results from first-principles calculations for light absorption in a transparent conductor.  Three beams of light (red for infrared, yellow for visible light, and violet for ultraviolet) travel through a layer of SnO2.  Absorption by the conduction electrons in the oxide reduces the intensity of the beams.  The calculations showed that absorption is 5 times stronger for ultraviolet and 20 times stronger for infrared, compared to absorption of visible light.

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