Graphene is a sheet form of carbon one atom thick with high electron mobility and the scientists have developed a scalable and inexpensive process for the creation of microelectronic and optoelectronic devices. Graphene’s high conductivity and transparency make it a 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.
The scientists built the graphene devices on substrates of soda-lime glass – the most common glass used in bottles and windows – and found that the sodium present in the glass could act as dopant for the graphene. The effect remained strong in the devices even after they had been exposed to air for several weeks.
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. Source Brookhaven National Laboratory.
"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., but formerly of Brookhaven Lab and one of the journal Scientific Reports.
The team initially set out to optimize a solar cell containing graphene stacked on a copper indium gallium diselenide (CIGS) semiconductor, which in turn was stacked on an industrial soda-lime glass substrate. The scientists then 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.
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). Source: Brookhaven National Laboratory
It was consequently found that sodium atoms were doping the graphene and could form a vital part of the creation of transistor devices where the differences in electron-hole densities contribute to the transistor action. Pinpointing the mechanism by which sodium acts as a dopant involved a painstaking exploration of the system and its performance under different conditions, including making devices and measuring the doping strength on a wide range of substrates, both with and without sodium.
The collaboration was led by scientists at the US Department of Energy’s (DOE) Brookhaven National Laboratory, Stony Brook University (SBU), and the Colleges of Nanoscale Science and Engineering at SUNY Polytechnic Institute.
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.
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