Power/Performance Bits: June 13

Theoretical all-carbon circuits; stabilizing perovskite solar cells; modeling quantum dots.

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Theoretical all-carbon circuits
Engineers at the University of Texas at Dallas, the University of Illinois at Urbana-Champaign, the University of Central Florida, and Northwestern University designed a novel computing system made solely from carbon.

“The concept brings together an assortment of existing nanoscale technologies and combines them in a new way,” said Dr. Joseph S. Friedman, assistant professor of electrical and computer engineering at UT Dallas. The resulting all-carbon spin logic proposal is a computing system that Friedman believes could be made smaller than silicon transistors, with increased performance.

In the all-carbon spintronic circuit design, electrons moving through carbon nanotubes create a magnetic field that affects the flow of current in a nearby graphene nanoribbon, providing cascaded logic gates that are not physically connected.

Because the communication between each of the graphene nanoribbons takes place via an electromagnetic wave, instead of the physical movement of electrons, Friedman expects that communication will be much faster, with the potential for terahertz clock speeds. In addition, these carbon materials can be made smaller than silicon-based transistors, which are nearing their size limit due to silicon’s limited material properties.

While the concept is still on the drawing board, Friedman said work toward a prototype of the all-carbon, cascaded spintronic computing system will continue.

Stabilizing perovskite solar cells
Scientists from EPFL and Solaronix have now built a low-cost, ultra-stable perovskite solar cell that has operated for more than a year, running at an efficiency of 11.2% without loss in performance.

While perovskite cells show promise for their high efficiency, they are sensitive to water and moisture, ultraviolet light and thermal stress, and stability has been a major factor preventing commercialization. A marketable product requires a warranty for 20–25 years with <10% drop in performance, according to the researchers.


Source: MK Nazeeruddin/EPFL

The team engineered a 2D/3D hybrid perovskite solar cell. This combines the enhanced stability of 2D perovskites with 3D forms, which efficiently absorb light across the entire visible spectrum and transport electrical charges. In this way, the scientists were able to fabricate of efficient and ultra-stable solar cells, which is a crucial step for upscaling to a commercial level. The 2D/3D perovskite yields efficiencies of 12.9% (carbon-based architecture), and 14.6% (standard mesoporous solar cells).

The scientists built 10×10 cm2 solar panels (with an active area of 47.6 cm2) using a fully printable industrial-scale process. The devices were protected with a glass slide via a very simple sealing method under air conditions. The resulting solar cells have now delivered a constant 11.2% efficiency for more than 10,000 hours, while showing zero loss in performance as measured under standard conditions.

Modeling quantum dots
Researchers at the Department of Energy’s Argonne National Laboratory and the University of Chicago ran a series of atomistic calculations at Lawrence Berkeley National Laboratory’s National Energy Research Scientific Computing Center (NERSC) to find the root cause of defects in two commonly used semiconductor materials, lead selenide (PbSe) and cadmium selenide (CdSe), and provide design rules to avoid them.

“We are interested in understanding quantum dots and nanostructures and how they perform for solar cells,” said Giulia Galli, professor of Molecular Engineering at the University of Chicago.

For this study, the team focused on heterostructured nanoparticles–in this case a colloidal quantum dot in which PbSe nanoparticles are embedded in CdSe. This type of quantum dot, also known as a core-shell nanoparticle, is like an egg, Márton Vörös, Aneesur Rahman Fellow at Argonne, explained, with a “yolk” made of one material surrounded by a “shell” made of the other material.

“Experiments have suggested that these heterostructured nanoparticles are very favorable for solar energy conversion and thin-film transistors,” Vörös said.

For example, while colloidal quantum dot energy conversion efficiencies currently hover around 12% in the lab, “we aim at predicting quantum dot structural models to go beyond 12%,” said Federico Giberti, postdoctoral research scholar at the University of Chicago’s Institute for Molecular Engineering. “If 20% efficiency could be reached, we would then have a material that becomes interesting for commercialization.”


Cross section of the interface between a lead chalcogenide nanoparticle and its embedding cadmium chalcogenide matrix. When integrated into optoelectronic devices, it is enough to have a single atom in the wrong place at the interface (represented by the glowing blue color) to jeopardize their performance. (Source: Peter Allen / Institute for Molecular Engineering, University of Chicago)

To aid in this, the team developed a computational strategy to investigate, at the atomic level, the effect of the structure of the interfaces on the materials’ optoelectronic properties. By using classical molecular dynamics and first principles methods that do not rely on any fitted parameters, their framework allowed them to build computational models of these embedded quantum dots.

Using this model as the basis for a series of simulations run at NERSC, the research team was able to characterize PbSe/CdSe quantum dots and found that atoms that are displaced at the interface and their corresponding electronic states–what they call “trap states”–can jeopardize solar cell performance, Giberti explained. They were then able to use the model to predict a new material that does not have these trap states and should perform better in solar cells.