Giving graphene – atom-thick sheets of carbon – a good brush could be the key to boosting the efficiency of cheap solar cells.

US chemists have developed a novel graphene-based dye that acts as a source of photoelectrons, making it suitable for use in solar cells known as dye-sensitised solar cells (DSSC) – a low cost alternative to silicon ones. They believe the dye could help boost the efficiency of such cells.

DSSCs use photosensitive solutions on a titanium-dioxide-coated surface to absorb energy from photons. The light excites electrons in the dyes and these higher-energy electrons generate electricity.

Mind the gap

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.

“With graphene, we have just reached that optimal value,” says Li. That’s because it is easy to adjust the band gap of electrons in a graphene dye by altering the size of graphene particles used.

To achieve the ideal 1.4 eV gap, the graphene flakes in the dye must be around 2 nanometres across. Until now, the problem has been that flakes of this size tend to clump together to form insoluble graphite. To be any use in the solar cells, the carbon must remain as soluble graphene so it can form a dye.

Brushed surface

Li and his team found they could prevent clumping by attaching molecular “brushes” to each graphene flake. Each brush contains 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 they are chemically bonded to the flakes, “the phenyl groups don’t change the band gap properties of graphene”, says Li. They also allow the graphene to dissolve in a common organic solvent such as toluene. When a conductive plate coated with titanium dioxide is dipped into the solution, the graphene sticks to the titanium dioxide to create the DSSC.

“Right now our best cell with graphene has a solar-to-electricity efficiency of 2 per cent, which is a sixth of the value of the best DSSCs,” says Li. But he is confident of big improvements.

Efficiency drive

Craig Grimes at Pennsylvania State University in University Park has another idea, however. He thinks that lead sulphide nanoparticles look more promising than graphene for DSSCs. Experimental lead sulphide solar cells currently have double the efficiency of the graphene attempts.

One reason is that graphene doesn’t stick readily enough to the titanium dioxide to transfer the current – to do so efficiently the molecular brushes attached to each flake must carry a carboxylic group, says Grimes.

Li agrees and says that adding carboxylic groups is a necessary next step. He says that should be possible, and in the end this will allow graphene to outperform lead sulphide. “And of course lead is an environmental concern, but carbon is not,” he says.

Journal reference: Nano Letters, DOI: 10.1021/nl101060h