NREL Study Could Ramp Up Carbon Solar Cell Research

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The National Renewable Energy Laboratory has come up with firm evidence for the feasibility of a new generation of carbon-based solar cells. The finding is significant because carbon offers a cheap, abundant source of raw materials that could drive down the cost of solar cells while maintaining a relatively high degree of efficiency.

That would be the renewable energy equivalent of having your cake and eating it, especially when you recall that the first solar cells in use were so expensive that practically the only one who could afford them was NASA.

Roots Of The New Solar Study

The new solar cell study is also noteworthy because it riffs on the work of Marcus Rudolph, who won the 1992 Nobel Prize in Chemistry for his theory of electron transfer in chemical systems.

We’ll pause here for a group hug for US taxpayers. According to the Energy Department Rudolph’s theory was first confirmed for sure in tests run at the agency’s Brookhaven National Laboratory.

Here’s the rundown from the Energy Department:

Marcus’s Nobel Prize-winning work is a mathematical analysis of how the overall energy in a system of interacting molecules changes and induces an electron to jump from one molecule to another. It sheds light on many complex chemical reactions, including photosynthesis, corrosion and electrical conductivity in polymers.

In his Nobel biography, Rudolph explains how his early career on the experimental side influenced his approach to theory:

…I realized that this experimental background heavily flavored my attitude and interests in theoretical research…This interaction of experiment and theory, each stimulating the other, has been and continues to be one of the joys of my experience.

Electron transfer is the meat and potatoes of solar energy conversion, but for whatever reason, Rudolph’s work has not had much impact on the solar field — until now, that is.

Carbon Nanotubes Never Lost Their Coolness Factor

That finally brings us around to carbon nanotubes. Carbon nanotubes were all the rage in next-generation materials when they first emerged in the 1990s. When research dollars started flooding into clean tech research, carbon nanotubes were front and center.

If you’re expecting another #thanksobama story, guess again. The significance of carbon nanotubes was so compelling that in 2008, the last year of the Bush Administration, the Energy Department developed a report titled New Science for a Secure and Sustainable Energy Future that featured a stylized image of carbon nanotubes on the cover with this explanation of the “remarkable and versatile material:”

Composed of hexagonal sheets of carbon atoms rolled into a tube with a diameter on the order of one nanometer, carbon nanotubes have 100 times the strength of steel but only one-sixth the mass density. They can be made single wall or multi-wall, metallic or semiconducting, and are poised to play central roles in catalysis, conversion of sunlight to electricity, and electrical energy storage…

So, #thanksbush!

Other emerging materials have since grabbed the spotlight — that would be graphene and synthetic perovskites — but carbon nanotubes still hold a lot of promise for solar cells and other clean tech applications.

Carbon Nanotubes For Better, Cheaper Solar Cells

According to the National Renewable Energy Laboratory, the new solar cell study is the first time ever that the Marcus formulation has been deployed to study electron transfer in organic semiconductors for use in photovoltaic devices.

The study pivots on the measurement of reorganization energy. In a solar cell, reorganization refers to the structural changes that molecules undergo when light enters. The materials in the solar cell separate photons into two charges to create an electrical current, during which the molecular structure changes and then reorganizes. This process results in energy loss.

The more closely you tune your solar cell material to prevent loss, the more efficient it will be. The challenge is to get the least amount of energy loss using the least expensive materials. Today’s go-to material is silicon, and carbon offers a cheaper alternative if the reorganization thing can be worked out.

The research team studied single walled carbon nanotubes paired with fullerene molecules (fullerene refers to a carbon molecule in a variety of shapes) and to make a long story short, they found that the combination resulted in “exceptionally low” reorganization energy.

The results for carbon nanotubes without the extra molecule were not quite as good, but the team reports “very, very low” reorganization energy for that configuration, too.

You can find the whole study in the journal Nature Chemistry under the title “Tuning the driving force for exciton dissociation in single-walled carbon nanotube heterojunctions” to get the full rundown on how the team deployed the Marcus formulation. Here’s a teaser from the abstract:

Here we investigated the influence of the thermodynamic driving force for photoinduced electron transfer (PET) between single-walled carbon nanotubes (SWCNTs) and fullerene derivatives by employing time-resolved microwave conductivity as a sensitive probe of interfacial exciton dissociation.

For the first time, we observed the Marcus inverted region (in which driving force exceeds reorganization energy) and quantified the reorganization energy for PET for a model SWCNT/acceptor system.

Speaking of abstracts, this may all seem rather abstract, but it’s an important confirmation of ongoing research into the use of carbon nanotubes in solar devices.

Work is already well under way on carbon solar cells, and the new study could help steer more research dollars into the field.

Back in 2012, researchers made an all-carbon solar cell by pairing nanotubes with buckyballs. Among many other examples, researchers in Australia are working on a solar cell that pairs carbon nanotubes with graphene.

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Image (cropped): via NREL.