Band structure of rutile SnO2, illustrating free-carrier absorption. Visible light does not carry enough energy to excite carriers across the band gap (a) or to excite free carriers directly to the next conduction-band states (b). However, additional momentum provided by a phonon enables indirect free-carrier absorption (c) for any visible or infrared wavelength. States in the energy range of the visible spectrum are indicated. Credit: Peelaers et al.; Applied Physics Letters.
Transparency versus conductivity: The tug between those properties is often on the mind of those who engineer the components of optoelectronic devices, such as photovoltaic cells, photodetectors and LEDs. For example, contact materials for PV cells ideally would not absorb much light in the solar spectrum. Otherwise, efficiency gets reduced. Conductivity can be enhanced through doping, but that can affect transparency; conversely, optimal transparency can decrease conductivity.
Where’s the sweet spot? To begin answering that question, researchers in the Computational Materials Group at the University of California, Santa Barbara wanted to wrap their brains around the specific mechanisms for the absorption of light in simple transparent conducting oxide. Working completely from first principles and focusing on the mechanisms of free-carrier absorption for a widely used TCO material, n-type tin dioxide, they were able to predict the limits on optical transparency of SnO2.
According to a UCSB press release, the researchers explain that SnO2 works well as a conducting oxide because it only weakly absorbs visible light. The wide band gaps of transparent conducting oxides prevent absorption of visible light by excitation of electrons while dopant atoms provide additional electrons in the conduction band that enable electrical conductivity.
However, dopant-provided free electrons can also absorb light by being excited to higher conduction-band states. Thus, the transparency of SnO2 declined when moving to other wavelength regions. The UCSB group reports that absorption was five-times stronger for ultraviolet light and 20-times stronger for the infrared light. Thus, SnO2 is not optimal for devices that depend on the ultraviolet and infrared light, such as those used in some telecommunications applications.
In the release, Hartwin Peelaers, a postdoctoral researcher and the lead author of a paper published in Applied Physics Letters (doi:10.1063/1.3671162), is quoted as saying, “Direct absorption of visible light cannot occur in [SnO2] 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.”
Chris Van de Walle, a professor in the UCSB Materials Department and head of the research group, notes, “Every bit of light that gets absorbed reduces the efficiency of a solar cell or LED. Understanding what causes the absorption is essential for engineering improved materials to be used in more efficient devices.”
Van de Walle’s Computational Materials Group explores semiconducting binary oxides, nitride semiconductors, novel channel materials and dielectrics, materials for quantum computing, photochemical hydrogen generation and metallic nanoparticles.
The UCSB group’s computational methods work and its investigations into TCOs — part of the school’s efforts as a Energy Frontier Research Center — is expected to lead to significant improvements of the energy efficiency of optoelectronic devices. Their work is also supported by the Belgian–American Educational Foundation, and by the UCSB Materials Research Laboratory, one of the NSF’s Materials Research Science and Engineering Centers.