Graphene-based solar cells and LEDs take step closer

March 18, 2015 // By Paul Buckley
Caltech has invented a technique to produce graphene at room temperature which could help pave the way for commercially feasible graphene-based solar cells and light-emitting diodes, large-panel displays and flexible electronics.

"With this new technique, we can grow large sheets of electronic-grade graphene in much less time and at much lower temperatures," explained Caltech staff scientist David Boyd, who developed the method.

Boyd is the first author of a study, published in the journal Nature Communications, which details the manufacturing process and the novel properties of the graphene it produces.

Graphene has a tensile strength 200 times stronger than steel and an electrical mobility that is two to three orders of magnitude better than silicon.

But achieving these properties on an industrially relevant scale has proven to be challenging. Existing techniques require temperatures that are much too hot - 1,000 degrees Celsius - for incorporating graphene fabrication with current electronic manufacturing. Additionally, high-temperature growth of graphene tends to induce large, uncontrollably distributed strain - deformation - in the material, which severely compromises its intrinsic properties.   

"Previously, people were only able to grow a few square millimeters of high-mobility graphene at a time, and it required very high temperatures, long periods of time, and many steps," explained Caltech physics professor Nai-Chang Yeh, the Fletcher Jones Foundation Co-Director of the Kavli Nanoscience Institute and the corresponding author of the study. "Our new method can consistently produce high-mobility and nearly strain-free graphene in a single step in just a few minutes without high temperature. We have created sample sizes of a few square centimeters, and since we think that our method is scalable, we believe that we can grow sheets that are up to several square inches or larger, paving the way to realistic large-scale applications."

Boyd hit upon the new manufacturing process by deciding to use a system first developed in the 1960s to generate a hydrogen plasma to remove the copper oxide at much lower temperatures. The initial experiments revealed not only that the technique worked to remove the copper oxide, but that it simultaneously produced graphene as well.

At first, Boyd could not figure out why the technique was so successful but later discovered that two leaky valves were letting in trace amounts of methane into the experiment chamber. "The valves were letting in just the right amount of methane for graphene to grow," explained Boyd.

Images of early-stage growth of graphene on copper. The lines of hexagons are graphene nuclei, with increasing magnification from left to right, where the scale bars from left to right correspond to 10 μm, 1 μm, and 200 nm, respectively. The hexagons grow together into a seamless sheet of graphene. (Courtesy of Nature Communications)