Carbon nanotubes (CNTs) have had a bit of a hard time of it lately. A few years back the National Institute of Standards and Technology (NIST) reported that CNTs have a major reliability problem when applied to electronics. In photovoltaics the prognosis hasn’t been much better. Despite efforts from some research teams to use CNTs instead of silicon as the basic element for converting light to energy for a solar cell, they simply haven’t proven themselves to be very efficient in energy conversion.
Now researchers at Northwestern University may have turned around the fortunes of CNTs, at least for photovoltaic applications, by demonstrating that they can make solar cells based on CNTs that are twice as efficient at energy conversion than its predecessors.
"The field had been hovering around 1 percent efficiency for about a decade; it had really plateaued," said Mark Hersam, a professor at Northwestern, in a news release. "But we've been able to increase it to over 3 percent. It's a significant jump."
In research published in the journal Nano Letters, the Northwestern team was able to focus on the chirality of the CNTs, which involves both the CNT’s diameter as well as its twist. While several hundred chiralities are possible, in previous work with CNTs in photovoltaics, researchers just picked a chirality that had the best semiconductor properties and went with that for the entire solar cell.
“The problem is that each nanotube chirality only absorbs a narrow range of optical wavelengths,” Hersam said. “If you make a solar cell out of a single chirality carbon nanotube, you basically throw away most of the solar light.”
Hersam and his team used a mixture of multiple chirality CNTs to maximize the amount of photocurrent produced by absorbing a broader range of solar-spectrum wavelengths. The result was that CNT-based solar cells were able to absorb near-infrared wavelengths, which had previously been inaccessible to many leading thin-film technologies.
Of course, three percent energy conversion efficiency is nowhere near silicon, which is currently around 15-20 percent efficiency. However, when this research is combined with work performed last year in Finland that developed a process that provided far greater control over determining the chirality of CNTs, perhaps the numbers could continue to rise as research progresses.
“If you look at our performance, there’s certainly a big jump,” Hersam said. “But there’s more work to be done. We still have to advance this technology by a factor of three to five.”
The Northwestern team will be looking to create polychiral CNT solar cells that have multiple layers with each layer being optimized for a particular solar spectrum.
Hersam noted ambitiously: “What we’d like to do is absorb every photon from the sun and convert it into electricity,” he said. “In other words, we’d like to have a solar cell that has an absorption spectrum perfectly matching solar light. We’re on a path toward that goal.”
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.