“Our solar cell does not need to be doped, nor does it require high-quality heterojunctions, which are challenging and expensive to fabricate,” says team member Oscar Vazquez-Mena. “Our work is a novel and promising approach for making photovoltaics with low-cost and abundant materials such as certain phosphides and sulphides that are easy to synthesize and which are environmentally friendly.”

Beside expensive light absorbers like silicon, there are semiconductors like zinc phosphide, copper zinc tin sulphide, cuprous oxide and iron sulphide that are much cheaper. However, for these materials to efficiently convert sunlight into energy, they need to be doped to form homojunctions, or require complementary emitter materials to form high-quality p-n heterojunctions. A team led by Alex Zettl, Harry Atwater, Ali Javey and Michael Crommie has now overcome this problem by making a simple junction with graphene rather than a semiconductor. A voltage applied to a gate over the junction can tune the energy barrier between the graphene and an adjoining layer of zinc phosphide to boost how efficiently solar cells made from these materials convert light into energy.

The devices are relatively simple to fabricate, says Vazquez-Mena. “Jeff Bosco from Harry Atwater’s team at Caltech makes high-quality zinc phosphide films and in our lab at UC Berkeley, we are experts at growing graphene on copper substrates. Basically, we transfer the graphene from the copper onto the zinc phosphide film to form a graphene- zinc phosphide junction. We then add an insulator layer on top of the graphene, prepared by our colleagues in Ali Javey’s team, also at UC Berkeley. Finally we add a thin top gate to the structure.”

Barrier is like a dam

Conventional solar cells normally contain two bulk semiconductors, with their electrons at different energy levels. These semiconductors are brought into contact to form an electric barrier between them that separates the electrons from each side. “This barrier can be likened to the dam in a hydroelectric power plant that separates two reservoirs of water at different heights,” explains Vazquez-Mena. “In a solar cell, the electric charges are the water in the dam and we use energy from the Sun to make the charges jump over the barrier.”

In the new device, the researchers used a layer of graphene in place of one of the semiconductors and added a top gate to it. “Why? Because it is easy to control the energy level of electrons in graphene by doing this,” Vazquez-Mena tells nanotechweb.org. “Such a thing is difficult to do in a bulk semiconductor.”

The top gate can regulate the barrier between graphene and the zinc phosphide, needed for the solar cell to work, he adds. “This is critical for the performance of the device and allows us to optimize the energy extracted from it. Going back to the dam analogy, it is as if we would be controlling the height of the dam.”

The fact that we can manipulate the barrier height in this way means that, in principle, we could make graphene junctions with many other materials, he says.

Modifying the barrier

In bulk semiconductor solar cells, the barrier height depends on the intrinsic properties of the materials making up the barrier. So, once you put the materials together, there is not much you can do to change the barrier, explains Vazquez-Mena.

“Our device is very different in that we can modify this barrier by simply applying an electric field to the top gate and adjusting the strength of the field applied for different materials and light conditions to optimize energy conversion. Our device, which is just a basic graphene-zinc phosphide solar cell, normally has an efficiency of 1% without any applied gate voltage, but we have doubled this to 2% by increasing the gate voltage to 2V. We have thus been able to boost its performance beyond the intrinsic properties of the material it is made up of.”

This type of solar cell might be ideal in climes where the sunlight varies a lot, he says – thanks to the fact that we can adjust the barrier to optimize energy conversion.

The California researchers say that they are looking to improve the efficiency of their devices and improving the quality of the graphene-zinc phosphide junction so that it produces a higher photocurrent. “We also want to apply our technology to other low-cost and readily available materials,” says Vazquez-Mena. “For example, the device we have made can be improved by using graphene itself or a transparent conductor like indium-tin oxide as the top gate.”

The team, reporting its work in Nano Letters, says that it will also test copper zinc tin sulphide, cuprous oxides and copper sulphide. “These materials are less harmful to the environment compared with commonly used solar cell materials like cadmium telluride and are cheaper than pure silicon. We definitely have many ideas to try but we also hope that other research groups will be inspired by our experiments and develop similar strategies to keep improving the efficiencies of alternative photovoltaic materials.”