Transmission spectra of the MoOx /5% Ga-doped films deposited at (a) 200 °C and 10-3 Torr of oxygen pressure on sapphire; (b) same parameters on glass; and (c) on glass at 400 °C and 10-3. The inset shows the schematic and transmission spectra of (a) without MoOx on glass substrate. Credit: Jay Narayan; NCSU.
Although the world of lighting is moving steadily towards an all-LED future, research still continues in the field to find the most efficient and manufacturable materials. One line of research is based on the notion that the manufacturing of LEDs currently has an Achilles Heel in the form of potential disruptions in the supply chain for indium, a crucial component of indium tin oxide, the most widely used transparent conductor in current LEDs. The use of ITO is not limited to just LEDs, either, and often is found in thin-film photovoltaics.
The search for replacements for ITO has included some possible alternative routes involving carbon nanotubes (with electrically conductive latex and polystyrene) and aluminum-doped oxides, although there are also significant drawbacks to both (see, for example, this 2008 PDF report from NREL on stability issues of transparent conducting oxides).
One person active in the search for an ITO surrogate is North Carolina State University’s Jagdish (Jay) Narayan, who reports that he has discovered (and patented) a new class of zinc oxide-based transparent conductors. Narayan says one key to his ZnO materials is that he alloyed them with optimum concentrations of gallium and aluminum. These alloys, he says, have electrical and optical properties comparable to ITO. Also, the addition of a few of monolayers of molybdenum oxide and nickel oxide can enhance the ZnO-based alloys by improving the ZnO-based composites capacity for contacting and electroding GaN-based LEDs.
An additional important difference, says Narayan, is that the ZnO-based materials are more stable and exhibit superior diffusion barrier characteristics compared to ITO contacts.
All of this means that the ZnO-based contacts can enhance optical power and reduce voltage, and are more stable and less expensive, compared to current ITO-based contacts.
As mentioned earlier, Narayan has received a patent for his discovery, and he says it is already being licensed by major GaN LED manufacturers.