Published on January 1st, 2014 | by Tina Casey2
Graphene Heats Up Race For Cheap Organic Solar Cells
Does Robin Thicke’s notorious hit “Blurred Lines” contain secret references to graphene? We don’t know, but we are interrupting our New Year’s Day Walking Dead marathon to bring you news that graphene, the notoriously hard-to-handle nanomaterial of the new millennium, has been domesticated to the point where it could help drive down the cost of solar cells.
Here’s the deal: a team of researchers at the A*STAR Institute of High Performance Computing in Singapore has calculated that with today’s graphene production technology, you can manufacture graphene electrodes that could almost match the efficiency of electrodes made from indium tin oxide (ITO) in organic solar cells.
The Graphene Road To Low Cost Solar Cells
That’s a significant breakthrough because although we’re big fans of ITO for its many applications in next-generation electronic devices (here, here and especially here for example), when used in organic solar cells ITO is relatively brittle. It is also expensive and vulnerable to price spikes on the global market.
The search has been on for cheaper, more durable substitutes that would result in lower up-front costs for organic solar cells, plus the added bonus of a longer lifecycle.
To rewind just a bit, organic refers to a relatively new class of solar cells made from polymers (aka plastic). Though less efficient than conventional solar cells based on silicon, organic solar cells lend themselves to low cost, high volume manufacturing processes. They are also light, flexible, and potentially transparent, which provides the opportunity for a broader range of applications including see-though solar windows.
Graphene comes into all this because, being only one atom thick, it is virtually invisible and it weighs practically nothing, making it a perfect fit for organic solar cells. Despite its svelte form graphene also posses superior strength and unique conductive properties that make it ideal for electronic devices including photovoltaic cells.
Solar Cells And Supply Security
The Singapore breakthrough is also significant because in its natural state graphene is nothing more than a nano-layer of graphite, which is a common form of carbon found in many parts of the world. In other words, once the manufacturing process is refined, graphene could be priced competitively in global markets.
That’s in contrast to indium. Aside from cost issues, the US Department of Energy has targeted indium as a supply risk, so you can bet that our friends over in Singapore aren’t the only ones looking at indium substitutes.
The A*STAR Graphene Breakthrough
Now let’s get back to the A*STAR research. Lead researcher Wee Shing Koh and his team based their calculations on sheets of graphene that were manufactured by an existing process called chemical vapor deposition. Loosely speaking, that process involves blowing a mist doped with nanoparticles of material over a substrate, resulting in a super-thin film.
The trick was to get a graphene sheet to function with about the same electrical resistance of a sheet of ITO, and the problem was that one sheet of graphene has about four times the resistance of ITO.
The solution was a kind of mathematical balancing act. You can bring down the resistance of graphene simply by layering on more sheets, but the downside is that you block more light.
The team started with a baseline organic solar cell and calculated that if you keep the layers of graphene down to four, you can get 92.3 percent of the power delivered by an equivalent ITO electrode.
That sounds decent enough, but the team also found that the four layers could come much closer to ITO’s effectiveness when used in next-generation organic solar cells that absorb a broader range of the light spectrum.
Koh also predicts that with improvements in the manufacturing process, the resistance-per-sheet of graphene could become lower, so that only one or two sheets would be needed to match the performance of ITO.
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