Solar cell electrodes could turn market to gold

Marie Freebody, Contributing Editor, marie.freebody@photonics.com

A university spinout company wants to put the Midas touch on the organic solar cell market through the use of gold-coated electrodes. Molecular Solar, the commercializing force behind the University of Warwick’s endeavors in solar cell development, believes that gold is the ideal material to replace the traditional indium tin oxide (ITO) for electrode coating and that it may enable mass production on flexible substrates.

Currently, ITO-coated glass is the most widely used transparent electrode for organic solar cells; however, this is largely due to the absence of a viable alternative.

The problem for organic solar cell applications is that, on the submicron scale, this complex material is poorly defined, so engineering stable contacts with the organic semiconductors responsible for harvesting light and transporting charge is problematic. What’s more, ITO is brittle and, when supported on flexible plastic substrates, prone to cracking.

Gold, on the other hand, which already is widely used in the electronics industry, forms reliable interconnects; moreover, it is the material of choice for electrodes in emerging nanoelectronic and nanophotonic technologies.

“Unlike ITO and other conducting oxides currently used for transparent electrodes, these ultrathin gold electrodes are chemically well defined and very smooth,” said Dr. Ross Hatton, who heads up the research on these novel electrodes as a member of the University of Warwick’s chemistry department. “Roll-to-roll vacuum evaporation of metal films is also a well-established process in the packaging industry, making this electrode amenable to mass production.”

These electrodes can be used with a range of solar cell technologies, but they are particularly well suited to organic solar cells based on small-molecule semiconductors because both gold and small-molecule organic semiconductors can be deposited using vacuum evaporation, which presents the tantalizing prospect of fabricating the entire device without breaking vacuum.

The Warwick group built on previous work by Hatton in which he used ultrathin gold films as an electrode material for organic LEDs. The drawback preventing the electrode from progressing, however, was the need for a solvent in the fabrication process.

Ultrathin gold films on glass may replace traditional ITO electrodes for organic solar cells. Courtesy of the University of Warwick.

Now Hatton and his team have developed a rapid, solvent-free fabrication process that greatly improves the technological feasibility of these electrodes and also makes them suitable for use with plastic substrates.

The current high price of gold could give some people pause, but Hatton explained that, at just 8 nm thick, a sheet of gold costs less than $8 per square meter. Furthermore, the gold could be recouped at the end of the useful life of the device, partially offsetting the cost.

For any solar cell technology, efficiencies are the all-important numbers.

Although the efficiency of organic solar cells fabricated on ultrathin gold electrodes on glass is 10 to 15 percent lower than that achieved on ITO glass (due to the difference in transparency), the preliminary work on plastic substrates is more promising.

Hatton explained that the gold-film electrodes are much more electrically conductive than ITO electrodes when supported on plastic substrates, and if organic solar cells are to realize their full commercial potential, they must be produced on flexible plastic substrates.

While it’s too early to determine the cost per watt, the Hatton group thinks that if the performance of organic solar cells using a gold electrode proves to be significantly better than that of cells using other transparent electrode materials, the gold electrodes may prove economically viable.

The next step for the team is to demonstrate the technology on flexible plastic substrates, Hatton said. “We are also exploring a number of approaches to reducing the materials cost, without compromising the advantages of this transparent electrode.”

This work was supported by the European Regional Development Fund/Advantage West Midlands SCRA AM2 project, the Engineering and Physical Sciences Research Council and the Royal Academy of Engineering.