Matthew Eisaman, Ph.D., center, in a laser lab at Brookhaven Lab with co-authors, from left, Nanditha Dissanayake, Yutong Pang and Ahsan Ashraf. PHOTO CREDIT: STONY BROOK UNIVERSITY

Matthew Eisaman, center, in a laser lab at Brookhaven Lab with co-authors, from left, Nanditha Dissanayake, Yutong Pang and Ahsan Ashraf. PHOTO CREDIT: BROOKHAVEN NATIONAL LABORATORY

A research team based at Brookhaven National Laboratory recently discovered unique properties of graphene when paired with glass which could have applications in creating more efficient solar cells and better touch screens on electronics.

The lab of Matthew Eisaman, Ph.D., a physicist at Stony Brook University and Brookhaven National Laboratory, stumbled upon these unique properties of graphene when they set out to design more efficient, lower cost solar cells.

Copper indium gallium diselenide, or CIGS, solar cells typically have a layer of cadmium sulfide or zinc oxide, but these layers can absorb only some of the sunlight the cell needs to make electricity and can also be toxic.

The group set out to investigate if graphene, a single layer of carbon atoms, could be a more effective and safer alternative.

“The reason graphene would be a good candidate for us is because it’s optically transparent but electrically very conductive,” Eisaman said. “So that’s the perfect thing to put on top of a solar cell because the light can go through the graphene into the CIGS and then you can very easily conduct the electricity with that top layer.”

Typically, CIGS solar cells are grown on soda-lime glass because it is an inexpensive, common material with sodium that diffuses into the CIGS, helping it work better.  

Normally, the graphene would be placed on top of the CIGS on the soda-lime glass and then a chemical would be used add extra electrons to the graphene, a process known as doping.

To the scientists’ surprise, the graphene placed on the CIGS and soda-lime glass was already doped without any external chemicals being used.

Through a series of controlled experiments, the team found that the sodium in the soda-lime glass concentrates at the top, next to the graphene, and donates the extra electrons the graphene needs.

“What’s exciting is the fact that you can use soda-lime glass to dope the graphene because when people use external chemicals to dope graphene, that will tend to degrade over time because the chemicals will evaporate or react,” Eisaman said, “whereas the soda-lime glass has an infinite reservoir of sodium that doesn’t really change over time and we can tune and control it.”

These findings are not only important for improving solar cells, but they can also be used to improve consumer electronics like touch screens on iPads.

Eisaman emphasized that these findings are a result of collaboration between several universities and labs.

“There’s a big team of people,” Eisaman said. “It took that kind of collaboration among a lot of different specialties to really figure out what was going on conclusively.”

In the future, the lab plans to further investigate the doping characteristics of soda-lime glass, find ways to control the strength of doping and determine how the doping strength degrades over time. The goal is to use this information to create even more efficient solar cells and electronic elements.