Left: This is a 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. Credit: Brookhaven National Laboratory
Scientists’ use of glass to optimize graphene’s electronic properties could could improve technologies from touch screens to solar cells. Scientists have developed a simple and powerful method for creating resilient, customized, and high-performing graphene: layering it on top of common glass. This scalable and inexpensive process helps pave the way for a new class of microelectronic and optoelectronic devices. “We believe that this work could significantly advance the development of truly scalable graphene technologies,” said Matthew Eisaman, physicist at Brookhaven Lab and professor at SBU.
The scientists built the proof-of-concept graphene devices on substrates made of soda-lime glass – the most common glass found in windows, bottles, and many other products. “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,” said Nanditha Dissanayake of Voxtel, Inc. Crucially, the effect remained strong even when the devices were exposed to air for several weeks -a clear improvement over competing techniques.
The graphene tweaks in question revolve around doping, where the electronic properties are optimized for use in devices. This adjustment involves increasing either the number of electrons or the electron-free “holes” in a material to strike the perfect balance for different applications. For successful real-world devices, the local number of electrons transferred to the graphene should not degrade over time.
The team initially set out to optimize a solar cell containing graphene stacked on a high-performance copper indium gallium diselenide (CIGS) semiconductor, which in turn was stacked on an industrial soda-lime glass substrate. They conducted preliminary tests of the novel system to provide a baseline for testing the effects of subsequent doping. But these tests exposed something strange: the graphene was already optimally doped without the introduction of any additional chemicals. “To our surprise, the graphene and CIGS layers already formed a good solar cell junction!” Dissanayake said. “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.”
The scientists now need to probe more deeply into the fundamentals of the doping mechanism and more carefully study material’s resilience during exposure to real-world operating conditions. The initial results, however, suggest that the glass-graphene method is much more resistant to degradation than many other doping techniques.
APPS: Graphene’s high conductivity and transparency make it a very promising candidate as a transparent, conductive electrode to replace the relatively brittle and expensive indium tin oxide (ITO) in applications such as solar cells, organic light emitting diodes (OLEDs), flat panel displays, and touch screens. In order to replace ITO, scalable and low-cost methods must be developed to control graphene’s resistance to the flow of electrical current by controlling the doping strength.
https://www.bnl.gov/newsroom/news.php?a=11814
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