Argonne offers integrated approach

Gary Boas, Contributing Editor, gary.boas@photonics.com

Researchers anticipate that the world’s energy needs will double by 2050. To help meet these ballooning needs, the US Department of Energy (DoE)’s Argonne National Laboratory in Illinois recently launched an Alternative Energy & Efficiency Initiative. The initiative seeks, in part, to achieve advances toward the large-scale implementation of solar energy by drawing on the laboratory’s strengths in basic and applied research and in collaborating with industry and other research organizations.

“Solar energy has more potential than any other renewable source,” said Seth B. Darling, a researcher with the laboratory. But realizing that potential is always going to be challenging – particularly with respect to cost. “Government subsidies can artificially lower the cost of a technology, but you need the unsubsidized cost to be competitive.”

The US Department of Energy’s Argonne National Laboratory in Illinois has launched an Alternative Energy & Efficiency Initiative, through which it seeks to develop new technologies, working toward large-scale implementation of solar energy, for example. Photos courtesy of Argonne National Laboratory.

And then, it must be competitive in the long term. Consider cadmium telluride. Using this, we could reach grid parity – the point at which renewable energy costs as little as or less than grid power – in a relatively few years. But tellurium, which comprises half of cadmium telluride, is one of the rarest materials on Earth. “It will be great as a stopgap,” said Darling, “but, ultimately, it cannot supply the world’s needs.” Therefore, we need new materials and new technologies.

This much is generally understood. The trick, of course, is to identify the technologies that will successfully address the energy security issue. “What we believe here,” Darling said, “is that having a basic researcher sitting in his or her lab developing a new technology, hoping to serendipitously hit upon something that will solve the problem, is not the most efficient approach. Likewise, endlessly tweaking an existing technology may not be the answer.”

Argonne is working toward an integrated approach combining its materials design, device science and process engineering. This will help to determine early in the process, for example, whether a technique can be scaled up to the necessary extent. Systems analysis is another strength that Argonne will add to the mix, studying the impact and interplay of resource limitations, the variability of sunshine, consumer behavior and more, with respect to a potential technology.

Argonne National Laboratory is exploring the potential of atomic layer deposition, among other techniques. Here, researcher Jeffrey W. Elam examines solar cell materials prepared using the method at various stages of fabrication.

This approach is not necessarily unique, but applying it to technologies that look promising can help us to achieve energy security more quickly than we would otherwise, Darling said. And that, of course, is the ultimate goal.

One technology the researchers are exploring is atomic layer deposition (ALD), which allows them to prepare extremely thin layers of transparent conductors, essential components of most solar cells. With conventional dye-sensitized solar cells, performance is limited by sluggish charge transport through the photoanode. “ALD offers the potential to improve DSSC [dye-sensitized solar cell] efficiency by enabling the fabrication of high-surface-area photoanodes with high conductivity,” said Jeffrey W. Elam, director of the atomic layer deposition program at Argonne.

The technique has further advantages: It produces highly conformal coverage, even on three-dimensional structures, and allows synthesis of a broad range of materials relevant to dye-sensitized solar cells, including titanium dioxide, indium oxide and tin oxide.

Elam noted a significant challenge in working with the technique: Every new material one might want to prepare requires research to develop the appropriate atomic layer deposition “recipe,” providing the necessary self-limiting surface chemistry.

Early in its DSSC development work, the Argonne group encountered the lack of an appropriate atomic layer deposition chemistry for indium oxide, the main ingredient in indium tin oxide (ITO), one of the best transparent conductors known. To address this, he said, “we invented a method for preparing highly conformal ALD ITO coatings on nano-porous structures and used this method to fabricate DSSCs with excellent charge-transport properties.”

The researchers believe that applying the technique to the deposition of ITO will help usher in the next generation of photovoltaics, enhancing the performance and reducing the cost. Also, extending it to alternative, Earth-abundant transparent conductors ultimately could enable production of efficient solar energy devices on a massive scale.