Nanostructured ZnO for solution-based transparent electrodes
- L. Goris, M. Donovan
Transparent conducting oxides have gained great industrial importance. The popularity of flat-screen technology (television, computer, phones, i-pods,…) and the recent greentech boom -which put photovoltaic modules at the forefront of the quest for efficient renewable energy sources- have greatly increased the use of Indium Tin Oxide (ITO). As a result, the price of In has recently surpassed that of silver. It is widely believed that the price volatility of In will become an extremely detrimental factor in the fabrication cost of photovoltaic cells. We are currently seeking to replace ITO with a low-cost materials set that is compatible with large-area, low-cost processing techniques, such as printing.
Zinc based oxides, can be easily processed from solution in order to produce thin, homogeneous films that can be used as electrodes in the aforementioned applications. Because of the abundance and low toxicity of Zn, these materials are a very attractive replacement for ITO. Pure ZnO is an intrinsic semiconductor but it can be made conductive by doping it with Al, Ga or F. Several methods have been described to form polycrystalline, highly doped ZnO, the most successful being Chemically Vapor Deposition (CVD), sputtering and spray pyrolysis of Zn based salts. These techniques, however, have many disadvantages, such as the use of expensive vacuum equipment and high processing temperatures, which makes them incompatible with flexible substrates. Moreover, effective doping can be problematic due to the high reactivity of Al with residual oxygen, preventing its uniform incorporation into the ZnO lattice.
In our group, we use a colloidal synthesis in an organic solvent for the synthesis of ZnO nanostructures (particles, rods and wires). Based on previous work by Yang et al.,1 we produced nanoparticles and nanowires (W~100 nm, L~1-2 μm). The size and aspect ratio of the nanostructures depends on the concentration of the Zn salt and the presence of surfactants (Fig 1).

Fig 1: Left: ZnO nanoparticle synthesis, without surfactant Right: ZnO nanowire synthesis, with surfactant
These nanostructured ZnO powders can be easily resuspended in ethanol and spincoated into very uniform films that are both diffusing and transparent (Fig 2).

Fig 2: Nanostructured ZnO “ink” (a). Spin-coating of the ZnO “ink” on a glass slide produces a uniform film with a milky-white appearance (b). The spin-coated film is highly diffusing yet transparent (insert).
By decomposing Al salts during the ZnO nanowire growth, we try to actively dope the wires and achieve a low sheet resistance. Studies on the effect of temperature and relative salt concentration are being performed to optimize the doping process. To understand the growth kinetics of the process, we plan on starting a study where the growth is monitored in-situ under synchrotron radiation. This work is performed in collaboration with Dr. M. Toney and the Stanford Synchroton Radiation Laboratory (SSRL).
|