Graphene is proving to be time and again one of the most promising new substances ever developed. In the latest application, researchers at MIT are developing photovoltaic (PV) devices using layers of graphene, the atom-thick layer of carbon that won the Nobel Prize for Physics in 2010, coated with nanowires. MIT published its research in Nano Letters, on Dec. 21. Its just the latest way that the researchers are looking to harness the power of the new nanomaterial.
The research was co-authored by MIT postdocs Hyesung Park and Sehoon Chang, associate professor of materials science and engineering Silvija Gradečak, and eight other MIT researchers. In the research published, the researchers show how they produced a new kind of PV cell based on sheets of flexible graphene coated with a layer of zinc-oxide nanowires. According to MIT, the method could result in low-cost, transparent and flexible solar cells. Such cells could be used on windows, roofs, exterior walls and more.
The result is a device that uses a less expensive option than the indium tin oxide (ITO), which has been used as a transparent electrode for flexible PV cells. “Currently, ITO is the material of choice for transparent electrodes,” Gradečak said in a release about the research. The rarity of indium is main cost-driver in such applications.
In their abstract the researchers wrote that they were able to demonstrate growth of highly uniform and well-aligned zinc-oxide nanowire arrays on the graphene by modifying its surface with conductive polymer interlayers. “On the basis of this structure, we then demonstrate graphene cathode-based hybrid solar cells using two different photoactive materials,” they wrote. The cells used PbS quantum dots and the conjugated polymer P3HT. devices had AM 1.5G power conversion efficiencies of 4.2 percent and 0.5 percent, respectively. Such conversion efficiencies, while far below that of conventional silicon PV is close to the performance of ITO-based devices with similar architectures.
The resulting device has additional advantages beyond cost, including its light weight, flexibility, strength and chemical robustness, the institute said.
A chief challenge was dealing with graphene’s slipperiness, otherwise known as its stable and inert structure—without impacting its electricity conductive characteristics, Gradečak said. The polymer coatings modified its properties, allowing the researchers to bond the zinc oxide nanowires to it. The nanowires are layered over with a light reactive material aka, the aforementioned lead-sulfide quantum dots or polymer.
Thus far Gradečak and her team have only made proof-of-concept devices a half-inch in size. But given the processes used to make such devices, she anticipated a short road to making larger-sized devices. “I believe within a couple of years we could see [commercial] devices” based on this technology, she said.
Other researchers—including some at MIT are looking at other means of using graphene in a new generation of photovoltaic devices that could usher in a new generation of less expensive, flexible and more robust solar cells.