Professor Guillermo Bazan and a team of postgraduate researchers at UC Santa Barbara’s Center for Polymers and Organic Solids (CPOS)  have announced a major advance in the synthesis of organic polymers for plastic solar cells.

Postdoctoral student Greg Welch removing a sample from the microwave reactor. (Credit: Image courtesy of Tony Rairden / College of Engineering, University of California - Santa Barbara)

Bazan’s team reduced reaction time by 99%, from 48 hours to 30 minutes, and increased average molecular weight of the polymers by a factor of more than 3.

The reduced reaction time effectively cuts production time for the organic polymers by nearly 50%, since reaction time and purification time are approximately equal in the production process, in both laboratory and commercial environments.

The higher molecular weight of the polymers, reflecting the creation of longer chains of the polymers, has a major benefit in increasing current density in plastic solar cells by as much as a factor of more than four. Over polymer batches with varying average molecular weights, produced using varying combinations of the elements of the new methodology, the increase in current density was found to be approximately proportional to the increase in average molecular weight.

The methodology, detailed in a recent Nature Chemistry paper, “will greatly accelerate research in this area,” stated Bazan, “by making possible the rapid production of different batches of polymers for evaluation.” He further noted, “We plan to take advantage of this approach both to generate new materials that will increase solar cell efficiencies and operational lifetimes, and to reevaluate previously-considered polymer structures that should exhibit much higher performance than they showed initially.”

To make these gains, the team:

• replaced conventional thermal heating with microwave heating,
• modified reactant concentrations, and
• varied the ratio of reactants by only 5% from the nominal 1:1 stoichiometric ratio normally employed in polymerization reactions.

Mike McGehee, Director of Stanford’s Center for Advanced Molecular Photovoltaics, hailed Bazan’s work, commenting, “Many synthetic chemists around the world are making copolymers with alternating donor and acceptors to attain low bandgaps. Most of them are having trouble attaining adequate molecular weight, so this new synthetic method that creates longer polymer chains is a real breakthrough. The reduction in synthesis time should also make it easier to optimize the chemical structure as the research moves forward and will ultimately reduce the manufacturing cost.”

Bazan is a Professor of Chemistry and of Materials at UC Santa Barbara, and is co-director of CPOS and a faculty member at the NSF-funded Materials Research Laboratory.

Danish nanophysicists have developed a new method for manufacturing the cornerstone of nanotechnology research — nanowires. The discovery has great potential for the development of nanoelectronics and highly efficient solar cells.

A nanowire made of the two semi- conductors (GaInAs and InAs) with gold (Au) as a catalyst. To the right a schematic illustration of the new cultivation method, where the semi- conductor materials can move both from the top of the gold droplet and from the underside. (Credit: Image courtesy of University of Copenhagen)

It is PhD student Peter Krogstrup, Nano-Science Center, the Niels Bohr Institute at the University of Copenhagen, who developed the method during his dissertation.

“We have changed the recipe for producing nanowires. This means that we can produce nanowires that contain two different semiconductors, namely gallium indium arsenide and indium arsenide. It is a big breakthrough, because for first time on a nanoscale, we can combine the good characteristics of the two materials, thus gaining new possibilities for the electronics of the future,” explains Peter Krogstrup.

We can capture more of the sun’s light

Today only approximately 1 % of the world’s electricity comes from solar energy. This is because it is difficult to convert solar energy into electricity. It is a great advantage for the researchers to be able to combine different semiconductors in the same nanowire.

“Different materials capture energy from the sun in different and quite specific absorption areas. When we manufacture nanowires of gallium indium arsenide and indium arsenide, which each have their own absorption area, they can collectively capture energy from a much wider area.

“We can therefore utilize more solar energy, if we produce nanowires from the two superconductors and use them for solar cells,” explains Peter Krogstrup

The nanowires of gallium indium arsenide and indium arsenide also have great potential in nanoelectronics. They can, for example, be used in the new OLED displays and LEDs. But it requires sharp transitions between the two materials in the nanowire.

No soft transitions

The cultivation of nanowires takes place in a vacuum chamber. The researchers lay a gold droplet on a thin disc comprising of the semiconductor and the nanowire grows up from below. In the transition between the two semiconductor materials in the gold droplet there was previously a mixing between the materials in the gold droplet and there was a soft transition between the materials. With the new method both of the materials can go from the top of the gold droplet or from the underside of the gold droplet. When the material comes from the underside, there is no mixing of the semiconductor materials. There is therefore a sharp transition on the atomic level between the gallium indium arsenide and indium arsenide.

“This sharp transition between the two semiconductors is necessary for the current — in the form of electrons, to be able to travel with high efficiency between the two materials. If the transition is soft, the electrons can easily get caught in the border area. The new mixed nanowire can be beneficial for many areas of nano research around the world,” says Peter Krogstrup, who has been working at the Danish III-V Nanolab, operated in collaboration between the University of Copenhagen and the Technical University of Denmark.

A new collaboration between the company SunFlake A/S and The Danish National Advanced Technology Foundation has recently begun. SunFlake A/S uses nanowires to develop prototypes of solar cells and they can also benefit from the new method in their continuing work. The nanophysicists’ discovery has just been published in the scientific journal Nano Letters.

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Danish nanophysicists have developed a new method for manufacturing the cornerstone of nanotechnology research — nanowires. The discovery has great potential for the development of nanoelectronics and highly efficient solar cells.

A nanowire made of the two semi- conductors (GaInAs and InAs) with gold (Au) as a catalyst. To the right a schematic illustration of the new cultivation method, where the semi- conductor materials can move both from the top of the gold droplet and from the underside. (Credit: Image courtesy of University of Copenhagen)

It is PhD student Peter Krogstrup, Nano-Science Center, the Niels Bohr Institute at the University of Copenhagen, who developed the method during his dissertation.

“We have changed the recipe for producing nanowires. This means that we can produce nanowires that contain two different semiconductors, namely gallium indium arsenide and indium arsenide. It is a big breakthrough, because for first time on a nanoscale, we can combine the good characteristics of the two materials, thus gaining new possibilities for the electronics of the future,” explains Peter Krogstrup.

We can capture more of the sun’s light

Today only approximately 1 % of the world’s electricity comes from solar energy. This is because it is difficult to convert solar energy into electricity. It is a great advantage for the researchers to be able to combine different semiconductors in the same nanowire.

“Different materials capture energy from the sun in different and quite specific absorption areas. When we manufacture nanowires of gallium indium arsenide and indium arsenide, which each have their own absorption area, they can collectively capture energy from a much wider area.

“We can therefore utilize more solar energy, if we produce nanowires from the two superconductors and use them for solar cells,” explains Peter Krogstrup

The nanowires of gallium indium arsenide and indium arsenide also have great potential in nanoelectronics. They can, for example, be used in the new OLED displays and LEDs. But it requires sharp transitions between the two materials in the nanowire.

No soft transitions

The cultivation of nanowires takes place in a vacuum chamber. The researchers lay a gold droplet on a thin disc comprising of the semiconductor and the nanowire grows up from below. In the transition between the two semiconductor materials in the gold droplet there was previously a mixing between the materials in the gold droplet and there was a soft transition between the materials. With the new method both of the materials can go from the top of the gold droplet or from the underside of the gold droplet. When the material comes from the underside, there is no mixing of the semiconductor materials. There is therefore a sharp transition on the atomic level between the gallium indium arsenide and indium arsenide.

“This sharp transition between the two semiconductors is necessary for the current — in the form of electrons, to be able to travel with high efficiency between the two materials. If the transition is soft, the electrons can easily get caught in the border area. The new mixed nanowire can be beneficial for many areas of nano research around the world,” says Peter Krogstrup, who has been working at the Danish III-V Nanolab, operated in collaboration between the University of Copenhagen and the Technical University of Denmark.

A new collaboration between the company SunFlake A/S and The Danish National Advanced Technology Foundation has recently begun. SunFlake A/S uses nanowires to develop prototypes of solar cells and they can also benefit from the new method in their continuing work. The nanophysicists’ discovery has just been published in the scientific journal Nano Letters.

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