Scientists have uncovered the fundamental limitations of optical transparency in tin dioxide (SnO2), a common conducting oxide. The discovery could lead to more energy efficient photovoltaics, LEDs and LCD touch screens. Transparent conducting oxides are used as contacts in a variety of optoelectronic devices. These materials are unique in that they conduct electricity while being transparent to visible light. For optoelectronic devices to emit or absorb light, the electrical contacts at the top of the device must be optically transparent. Opaque metals and most transparent materials lack the balance between these two characteristics to be functional for use in such technology. Scientists in the Computational Materials Group at the University of California, Santa Barbara, have used cutting-edge calculation methods to identify the optical transparency limitations of SnO2. Three beams of light (red for infrared, yellow for visible and violet for ultraviolet) travel through a layer of SnO2. Absorption by the conduction electrons in the oxide reduces the intensity of the beams. Courtesy of Hartwin Peelaers, University of California, Santa Barbara. Conducting oxides strike a balance between transparency and conductivity because their wide bandgaps 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,” said Hartwin Peelaers, a postdoctoral researcher and the lead author of the paper. “But we found that more complex absorption mechanisms, which also involve lattice vibrations, can be remarkably strong.” The group observed that SnO2 only weakly absorbs visible light, letting most light pass through and making it a useful transparent contact. In their study, the transparency of SnO2 declined when moving to other wavelength regions. Absorption was five 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,” said Chris Van de Walle, head of the research group and a professor of materials science. “Understanding what causes the absorption is essential for engineering improved materials to be used in more efficient devices.” The findings appeared in Applied Physics Letters (doi: 10.1063/1.3671162). The research was supported as part of the UCSB Center for Energy Efficient Materials, an Energy Frontier Research Center funded by the US Department of Energy, the Belgian American Educational Foundation and the UCSB Materials Research Laboratory.