Building a More Efficient Solar Cell Electrode

Forests of carbon nanotubes have been found to be an efficient alternative for platinum electrodes in dye-sensitized solar cells (DSC), according to research collaboration by Rice University and Tsinghua University. According to Jun Lou, a materials scientist at Rice, the single-wall nanotube arrays (http://news.rice.edu/2012/04/17/nanotube-electrodes-improve-solar-cells/) —grown in a process invented at Rice—are both much more electroactive and potentially less expensive than platinum, a common catalyst in DSCs.

The combination of these nanotube arrays and newly developed sulfide electrolytes synthesized at Tsinghua could lead to more efficient and robust solar cells at a fraction of the current cost for traditional silicon-based solar cells. The work by Lou and co-lead investigator Hong Lin, a professor of materials science and engineering at Tsinghua, is detailed here.

Based on the technology the universities built working solar cells with similar results and achieved a power conversion efficiency of 5.25%—lower than the DSC record of 11% with iodine electrolytes a platinum electrode, but significantly higher a control that combined the new electrolyte with a traditional platinum counter electrode. Resistance between the new electrolyte and counter electrode is “the lowest we’ve ever seen,” Lou said.

Arrays of vertically aligned single-walled carbon nanotubes (VASWCNTs) grown at Rice University are key to making better and cheaper dye-sensitized solar cells, an alternative to more expensive silicon solar cells. The arrays are transferred to conducting glass, topped with a second electrode of titanium oxide and surrounded by iodine-free electrolyte developed at Tsinghua University. (Source: Rice University)

Ferroelectric Fanfare

While some materials naturally do what researchers want them to do—such as semiconducting silicon found in almost every electronic device—sometimes naturally occurring materials need a little nudge. In the case of recent Cornell University research, some materials need a twist to make them useful.

Craig Fennie, assistant professor of applied and engineering physics at Cornell and James Rondinelli from Drexel University have published a method for turning a class of ceramic materials called perovskites into a material that is ferroelectric, which means that the spontaneous electric polarization can be flipped by applying a small electric field.

This material could be useful for low-power memory and switching devices although traditional ferroelectric mechanisms are often chemically incompatible with such phenomena as ferromagnetism, limiting their use in new types of multifunctional devices.

Electronic structure calculations based on density functional theory have uncovered a novel mechanism for inducing ferroelectric polarizations in ordered perovskites. Researchers from Cornell and Drexel have outlined a materials selection strategy for designing this behavior. The guidelines are based on the octahedral rotations found in the two constituent oxides and the way the perovskite building blocks are interwoven to form the superlattice. (Source: Cornell)

The researchers engineered electric polarizations are the result of stacking chemically different perovskites into atomically thin striped-patterns, which allow their normal rotational patterns to induce ferroelectricity.

“In the past, those rotations and tilts didn’t do anything, but by combining them in this way, they can be coupled to an electric field through polarization,” Fennie said. “This is the first step in the broad field of using rotations that couple to an applied electric field to control the properties of materials.”

–Ann Mutschler