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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.

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Tape casting was used to fabricate a flexible indium tin oxide layer in an electroluminescent lamp. Credit: Roosen; JACerS

Transparent conducting oxides constitute an important class of electronic materials and are used for applications such as flat panel displays, solar cells and touch panels. TCOs are doped semiconducting oxides and example compositions include tin-doped indium oxide, aluminum-doped zinc oxide and indium-doped cadmium oxide. The most-used commercial composition is tin-doped indium oxide, or indium tin oxide.

ITO films are usually synthesized by sputtering or physical vapor deposition. Such films have about 85 percent optical transparence in the visible range and low electrical resistivities. However, these are high-energy, vacuum-based technologies, which means they are expensive. In multilayers, the films are brittle.

The Andreas Roosen group at the University of Erlangen-Nuremberg (Germany) published a paper in the Journal of the American Ceramic Society (available now via Early View) that investigates a well-established forming technique—tape casting—to ITO fabrication. Tape casting is economical, scalable and is a proven way to fabricate other multilayer devices.

An electroluminescent lamp, which can also be thought of as a “luminescent capacitor,” is a multilayer device comprising a protective layer, transparent front electrode, luminescent layer, reflective dielectric layer, opaque rear electrode and a final protective layer. The corresponding materials stack is PET, ITO, ZnS:Cu, BaTiO₃, Ag, PET. The light is generated in the luminescent zinc sulfide layer and emits from the device through the ITO layer.

The Roosen group made ITO slurries with varying ITO particle loads and amounts of binders, plasticizers, etc. Slurries were also prepared of the other functional layers (ZnS:Cu, BaTiO₃ and silver). ITO tapes were cast using a fixed casting head with two doctor blades onto a silicon-coated PET carrier tape. Several thicknesses were made, and the green tapes were very flexible because of the binder and plasticizer contents. Binders were burned out at 650°C for two hours, but the tapes were not fired.

Electroluminescent lamps were fabricated by laminating the layers in the stack in a uniaxial hot press. The optical transmission of the device was 60-70 percent and was found to be dependent on the thickness of the ITO layer. The electrical conductivity of the devices was found to depend on the ratio of ITO to organic additives. The organics, though contributed to the flexibility of the tapes.

The authors conclude, “Bright electroluminescence of the lamps could be observed even under bending, thus proving the functionality and applicability of the ITO tapes manufactured in the unfired state.”

Not addressed in the paper is whether the process might be useful for other TCO materials. Between 2004 and 2006 the price per kilogram of indium increased from $700 to $900, but dropped to $600 by 2008. With the cost of indium being high and volatile, there are economic incentives to developing aluminum-doped zinc oxide into an acceptable alternative.

Full details are in the paper: “Tape Casting of ITO Green Tapes for Flexible Electroluminescent Lamps,” by Nadja Straue, Martin Rauscher, Martina Dressler and Andreas Roosen (doi:10.1111/j.1551-2916.2011.04836.x).

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