materials science

Low-cost solar cells from nanotube ‘forests’

RICE (US) — By replacing platinum with carbon nanotubes, researchers hope to make efficient solar cells at a fraction of the current cost for silicon-based solar cells.

Single-wall nanotube arrays, grown in a process invented at Rice University, are both much more electroactive and potentially cheaper than platinum, a common catalyst in dye-sensitized solar cells (DSC), says Jun Lou, a materials scientist at Rice.

When combined with newly developed sulfide electrolytes synthesized at Tsinghua University, the work paves the way for a low-cost, efficient alternative to silicon-based cells. Lou and co-lead investigator Hong Lin, a professor of materials science and engineering at Tsinghua, detailed their work in the open-access Nature journal Scientific Reports.

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DSCs are easier to manufacture than silicon-based solid-state photovoltaic cells but not as efficient, explains Lou.

“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 says.

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

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 lab of Robert Hauge, a co-author of the paper and a distinguished faculty fellow in chemistry at Rice.

“These are very versatile materials,” Lou says. “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 notes.

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 says.

“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.

More news from Rice University: http://news.rice.edu/

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