Feb 15 2016
Graphene combines transparency, electrical conductivity, and high durability into a one-atom-thick sheet of carbon. Even though graphene is known to be a "wonder material," it has still not been successful in industrial and commercial processes and products.
Left:Schematic of a graphene field-effect-transistor used in this study. The device consists of a solar cell containing graphene stacked on top of a high-performance copper indium gallium diselenide (CIGS) semiconductor, which in turn is stacked on an industrial substrate (either soda-lime glass, SLG, or sodium-free borosilicate glass, BSG). The research revealed that the SLG substrate serves as a source of sodium doping, and improved device performance in a way not seen in the sodium-free substrate. Right: A scanning electron micrograph of the device as seen from above, with the white scale bar measuring 10 microns, and a transmission electron micrograph inset of the CIGS/graphene interface where the white scale bar measures 100 nanometers (Brookhaven National Laboratory).
Scientists have now developed a method to create high-performing, customized, and resilient graphene, by layering it on top of common glass. This new technique is cost-effective and scalable, and it could introduce a whole new class of optoelectronic and microelectronic devices, ranging from solar cells to touch screens.
Results from this work were published in the February 12, 2016, issue of Scientific Reports. This study was led by scientists from the Colleges of Nanoscale Science and Engineering at SUNY Polytechnic Institute, Stony Brook University (SBU), and the U.S. Department of Energy's (DOE) Brookhaven National Laboratory.
We believe that this work could significantly advance the development of truly scalable graphene technologies.
Matthew Eisaman, Physicist, Brookhaven Lab
The scientists constructed the proof-of-concept graphene devices on soda-lime glass-based substrates. This glass material is commonly used for bottles, windows, and a wide range of other products. It was surprising that the sodium atoms present in the glass played a vital role in the electronic properties of the graphene.
The sodium inside the soda-lime glass creates high electron density in the graphene, which is essential to many processes and has been challenging to achieve. We actually discovered this efficient and robust solution during the pursuit of something a bit more complex. Such surprises are part of the beauty of science.
Nanditha Dissanayake, Voxtel, Inc., Brookhaven Lab
In comparison with competing techniques, this new effect was a positive development as it continued to have a strong effect even when the devices were exposed to the atmosphere for a prolonged period.
The majority of this experimental work was carried out at Brookhaven's Sustainable Energy Technologies Department and the Center for Functional Nanomaterials (CFN), which is a DOE Office of Science User Facility.
Using the doping process, the electronic properties of graphene are further improved to be used in devices. This enhancement is achieved by either increasing the electron-free "holes" or the number of electrons in a material, in order to achieve a complete balance for a wide range of applications. For practical devices, it is extremely important to prevent degradation of the local number of electrons transferred to the graphene.
The graphene doping process typically involves the introduction of external chemicals, which not only increases complexity, but it can also make the material more vulnerable to degradation. Fortunately, we found a shortcut that overcame those obstacles.
Matthew Eisaman, Physicist, Brookhaven Lab
Initially the scientists focused on obtaining a solar cell with graphene layered on a high-performance copper indium gallium diselenide (CIGS) semiconductor, which was placed on an industrial soda-lime glass substrate. Next the system went through some preliminary tests to develop a baseline for testing the subsequent doping effects. These tests lead to unusual results as the graphene had been doped previously without the introduction of extra chemicals.
To our surprise, the graphene and CIGS layers already formed a good solar cell junction! After much investigation, and the later isolation of graphene on the glass, we discovered that the sodium in the substrate automatically created high electron density within our multi-layered graphene.
Nanditha Dissanayake, Voxtel, Inc., Brookhaven Lab
The performance of the system was analyzed under varied conditions to highlight the mechanism that allowed sodium to act as a dopant. The team continued to analyze this mechanism by developing devices, and then calculating the doping strength on a wide variety of substrates, both without and with sodium.
Developing and characterizing the devices required complex nanofabrication, delicate transfer of the atomically thin graphene onto rough substrates, detailed structural and electro-optical characterization, and also the ability to grow the CIGS semiconductor. Fortunately, we had both the expertise and state-of-the-art instrumentation on hand to meet all those challenges, as well as generous funding.
Nanditha Dissanayake, Voxtel, Inc., Brookhaven Lab
A major part of this work was carried out at the Brookhaven Lab using techniques developed in-house, including advanced lithography. CFN staff scientists, and study coauthors, Kim Kisslinger and Lihua Zhang lent their skills and ideas for the high-resolution electron microscopy measurements. Coauthors Harry Efstathiadis and Daniel Dwyer, both at the College of Nanoscale Science and Engineering at SUNY Polytechnic Institute, headed the entire work that aimed to grow and characterize the superior quality CIGS films.
Now that we have demonstrated the basic concept, we want to focus next on demonstrating fine control over the doping strength and spatial patterning.
Matthew Eisaman, Physicist, Brookhaven Lab
At this point the scientists focused on initiating an in-depth study on the basic principles of the doping mechanism, and to examine the material's resilience when exposed to actual operating conditions. The results initially obtained highlight that the glass-graphene method is a lot more resistant to degradation when compared with other doping methods.
The potential applications for graphene touch many parts of everyone's daily life, from consumer electronics to energy technologies. It's too early to tell exactly what impact our results will have, but this is an important step toward possibly making some of these applications truly affordable and scalable.
Matthew Eisaman, Physicist, Brookhaven Lab
Graphenes transparency and high conductivity allow it to be used as a conductive and transparent electrode, in place of the costly and comparatively fragile indium tin oxide (ITO), in applications that include touch screens, solar cells, flat panel displays, and organic light emitting diodes (OLEDs). ITO can be replaced by developing cost-effective and scalable techniques that will help to control the resistance exhibited by graphene to current flow, by manipulating the doping strength. The scientists point out that this newly developed glass-graphene system has the possibilities to rise to that challenge.
The study at Brookhaven Lab was financially supported through Brookhaven Lab's Sustainable Energy Technologies Department by the DOE Office of Science and by Brookhaven's Laboratory Directed Research and Development (LDRD) Program. The DOE Office of Science (BES) supports the CFN at Brookhaven.
The Office of Science of the U.S. Department of Energy supports the Brookhaven National Laboratory. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working towards addressing some of the most complex challenges of our time.