9th October 2013

Major leap towards graphene for solar cells

Researchers in Germany have discovered that graphene retains its properties even when coated with silicon, paving the way for solar cells with much greater efficiency.

Graphene – a revolutionary new material discovered in 2004 – has extreme conductivity and is completely transparent, while being inexpensive and non-toxic. This makes it a perfect candidate material for layers in solar cells to conduct electricity without reducing the amount of incoming light; at least in theory. Whether or not this holds true in a real world setting is questionable as there is no such thing as "ideal" graphene – a free floating, flat honeycomb structure formed by a single layer of carbon atoms: interactions with adjacent layers can change its properties dramatically.

Now, researchers at Helmholtz-Zentrum Berlin (HZB) have shown that graphene retains its impressive properties when coated with a thin silicon film. These findings pave the way for entirely new possibilities to use in thin-film photovoltaics.

"We examined how graphene's conductive properties change if incorporated into a stack of layers similar to a silicon-based thin film solar cell, and were surprised to find that these properties actually change very little," Dr. Marc Gluba explains.

To this end, they grew graphene on a thin copper sheet, next transferred it to a glass substrate, and finally coated it with a thin film of silicon. They examined two different versions that are commonly used in conventional silicon thin-film technologies: one sample contained an amorphous silicon layer, in which the silicon atoms are in a disordered state similar to a hardened molten glass; the other sample contained poly-crystalline silicon to help them observe the effects of a standard crystallization process on graphene's properties.

The team obtained their measurements on one square centimetre samples. Even though the morphology of the top layer changed completely as a result of being heated to a temperature of several hundred degrees Celsius, the graphene was still detectable.

"That's something we didn't expect to find, but our results demonstrate that graphene remains graphene even if it is coated with silicon," says Norbert Nickel. Their measurements of carrier mobility, using the Hall effect, showed that the mobility of charge carriers within the embedded graphene layer was roughly 30 times greater than that of conventional zinc oxide-based contact layers.

"Admittedly, it's been a real challenge connecting this thin contact layer – just one atom thick – to external contacts. We're still having to work on that," said Gluba.

"Our thin film technology colleagues are already pricking up their ears and wanting to incorporate it," added Gluba.

HZB is the same company which last month achieved a solar cell with 44.7% efficiency, a new record. Third-generation solar cells with graphene could, in theory, reach efficiencies in the range of 60%.

Comments »