Making solar cells more efficient is a target of any physicist or chemist around the globe. Dye-sensitized solar cells (Graetzel), made of a layer of dye sprayed over a layer of titanium dioxide look promising in terms of costs and possibility of development – even more promising than thin film – not to mention silicon solar cells.
Their weak point is that scientists haven’t discovered high-efficiency versions of dye-sensitized solar cells so far.
Graphene, a single layer of carbon atoms – can play an important role in dye-sensitized solar cells, as researchers from the University of Indiana in Bloomington discovered. Until now, graphene could have theoretically been incorporated into the dye used to make the Graetzel cell to improve its efficiency, but what stopped the scientists to do that was that it always clumped together, forming graphite, which is of no use for a solar cell.
The difference between the energy level that a molecule’s electrons normally occupy and the level they jump to when excited is known as the band gap. The energy of the gap is measured in electronvolts (eV) and the optimal value for basic solar cells is 1.4 eV, says Liang-shi Li at Indiana University in Bloomington.
That’s the energy of infrared photons just beyond the visible spectrum, and a molecule with such a band gap will absorb photons at this energy and most photons at higher energies, meaning the molecule will absorb most visible light to generate electricity.
To be able to be used in solar cells, the graphene flakes should have around 2 nanometers across to pass the ideal 1.4 eV bandgap. The Indiana researchers came up with the solution of attaching molecular “brushes” to each graphene flake, with each brush containing three carbon chain “bristles” that meet at a central phenyl ring which chemically bonds to carbon atoms on the edge of the graphene flake. The lack of space around the graphene forces these bristles away from the surface, which in turn prevents graphene flakes from coalescing into graphite.
Although chemically-bonded to the graphene flakes, “the phenyl groups don’t change the band gap properties of graphene”, says Liang-shi Li, one of the scientists. He also mentions that the dye-sensitized solar cells made so far with this method only reached 2 percent efficiency, but they’re optimistic to higher values as research continues.
A researcher from the Pennsylvania State University, Craig Grimes, says that graphene is not the best candidate for this job. His experiments showed that lead sulphide nanoparticles were 2 times better. Still, he encourages Li’s work, as graphene, made of carbon atoms, is by far better for the environment than lead. He says, and Li agrees, that they need to attach a carboxylic group to each of the graphene flakes to make their cells more efficient.