Nanotube electrodes improve solar cells

Engineerblogger
April 18, 2012

Pei Dong, a graduate student at Rice University, holds a lab-built solar cell that combines a carbon nanotube current collector and a sulfide-based electrolyte. The combination could make such solar cells more efficient and less expensive than current dye-sensitized units. Photo by Jeff Fitlow

The single-wall nanotube arrays, grown in a process invented at Rice, are both much more electroactive and potentially cheaper than platinum, a common catalyst in DSCs, said Jun Lou, a materials scientist at Rice. In combination with newly developed sulfide electrolytes synthesized at Tsinghua, they could lead to more efficient and robust solar cells at a fraction of the current cost for traditional silicon-based solar cells.

Lou and co-lead investigator Hong Lin, a professor of materials science and engineering at Tsinghua, detailed their work in the online, open-access Nature journal Scientific Reports this week.

DSCs are easier to manufacture than silicon-based solid-state photovoltaic cells but not as efficient, said Lou, a professor of mechanical engineering and materials science. “DSCs are sensitized with dyes, ideally organic dyes like the juices from berries – which some students have actually used in demonstrations.”

Dyes absorb photons from sunlight and generate a charge in the form of electrons, which are captured first by a semiconducting titanium oxide layer deposited on a current collector before flowing back to the counter electrode through another current collector. Progress has been made in the manufacture of DSCs that incorporate an iodine-based electrolyte, but iodine tends to corrode metallic current collectors, which “poses a challenge for its long-term reliability,” Lou said.

Iodine electrolyte also has the unfortunate tendency to absorb light in the visible wavelengths, “which means fewer photons could be utilized,” Lou said.

So Tsinghua researchers decided to try a noncorrosive, sulfide-based electrolyte that absorbs little visible light and works well with the single-walled carbon nanotube carpets created in the Rice lab of Robert Hauge, a co-author of the paper and a distinguished faculty fellow in chemistry at Rice’s Richard E. Smalley Institute for Nanoscale Science and Technology .

“These are very versatile materials,” Lou said. “Single-walled carbon nanotubes have been around at Rice for a very long time, and people have found many different ways to use them. This is another way that turns out to be very well-matched to a sulfide-based electrolyte in DSC technology.”

Both Rice and Tsinghua built working solar cells, with similar results. They were able to achieve a power conversion efficiency of 5.25 percent – lower than the DSC record of 11 percent with iodine electrolytes and a platinum electrode, but significantly higher than 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. Lou Lab/Rice University

There’s much work to be done, however. “The carbon nanotube-to-current collector still has a pretty large contact resistance, and the effects of structural defects in carbon nanotubes on their corresponding performance are not fully understood, but we believe once we optimize everything, we’re going to get decent efficiency and make the whole thing very affordable,” Lou said. “The real attraction is that it will be a very low-cost alternative to silicon-based solar cells.”

Pei Dong, a graduate student in Lou’s lab, and Feng Hao, a graduate student at Tsinghua, are lead authors of the paper. Co-authors include Rice graduate students Jing Zhang and Philip Loya, Yongchang Zhang of Tsinghua and Professor Jianbao Li of Hainan University, China.

The project was supported by the National High Technology Research and Development Program of China, the Welch Foundation and the Faculty Initiative Fund at Rice.


Source: Rice University

Additional Information:

• Nature journal Scientific Reports this week ("High Electrocatalytic Activity of Vertically Aligned Single-Walled Carbon Nanotubes towards Sulfide Redox Shuttles").