Authors: Iman Yahyaie, Kevin McEleney, Michael Walter, Derek R. Oliver, Douglas J. Thomson, Michael S. Freund, and Nathan S. Lewis
Affiliation: California Institute of Technology
Journal: Journal of Physical Chemistry Letters
A holy grail of chemical energy technology is the development of an artificial photosynthetic cell designed to split water into oxygen and hydrogen using only the energy provided by sunlight. The authors propose such an artificial photosynthetic system (pictured above), which relies on silicon microwires embedded in a polymeric membrane. The silicon microwires serve two purposes: 1) they collect sunlight, and 2) act as supports for catalysts designed to use energy from sunlight to either oxidize water or reduce protons into molecular oxygen and hydrogen, respectively. When the silicon absorbs sunlight, an electron is excited to a higher energy level, leaving behind a “hole” where it was. In the proposed photosynthetic device, the electrons would move toward the cathode where they could reduce protons to hydrogen gas, and the holes would move toward the anode where they would accept electrons from the oxidation of water. The silicon microwires could be embedded into a proton-permeable membrane made from a conducting polymer to separate the anode from the cathode. Before this device could be realized, however, the silicon microwires and their interaction with the polymer membrane need to first be examined.
In this paper the authors develop a technique to measure the various resistances in the system, including the contact resistance between the microwire and the electrode probe use to measure it, the resistance of the microwire itself, and the resistance between the microwire and the polymer membrane. After growing the silicon microwires, the authors etch away the oxide layer on the surface by exposing the microwires to a solution of hydrofluoric acid. Afterward, they contact tungsten (W) probes to the silicon wires to make resistivity measurements. The contact between the W probe and the silicon is reported to be ohmic (that is, following Ohm’s law where a voltage applied is directly proportional to current) rather than a Schottky contact (where current and voltage are not linearly related). This is presumed to result from the fact that the resistivity of silicon changes with pressure, so supplying sufficient pressure to the microwire allows an ohmic contact to be formed. The fact that ohmic contacts were achieved without essentially melting the W probes onto the silicon means this method can be used to make contact to electrical devices sensitive to heat, which is an important characteristic of a measurement system for these types of devices where the features are small.
Using this W probe/Si microwire system, the authors were able to investigate the resistance along the microwires and show that both the contact resistance and the resistance per unit length of wire were constant for a particular microwire no matter where it was contacted. They use this information to show the robustness of the method, as well as gather information about the specific resistances of microwires.
After proving the method with individual microwires, the authors used this contacting method to measure the resistances associated with a microwire-polymer composite material where the silicon nanowires were embedded within a polymer layer. The W probe method could be used to measure the contact resistance between the silicon microwire and the polymer as well as the resistances of the polymer and wire themselves. The authors use this method to set electrical properties standards for the devices they hope to make in the future for splitting water.
Overall, this paper presented a new method for contacting microwires at low temperatures, and showed the first steps of progress toward the proposed water-splitting solar cell.