'Sweet spot' for mass-producing polymer solar cells may be far larger than dictated by the conventional wisdom
For all the promise they have shown in the lab, polymer solar cells still need to "get on a roll" like the ones employed in printing newspapers so that large sheets of acceptably efficient photovoltaic devices can be manufactured continuously and economically. Polymer solar cells offer advantages over their traditional silicon-based counterparts in numerous ways, including lower cost, potentially smaller carbon footprint and a greater variety of uses.
A demonstration solar park based on polymer solar cells at the Technical University of Denmark in Roskilde, Denmark.
Credit: DTU Energy
New research results reported by an international team led by the National Institute of Standards and Technology (NIST) indicate that the "sweet spot" for mass-producing polymer solar cells--a tantalizing prospect for decades--may be far larger than dictated by the conventional wisdom. In experiments using a mock-up of a high-volume, roll-to-roll processing method, the researchers produced polymer-based solar cells with a "power conversion efficiency" of better than 9.5 percent, just shy of the minimum commercial target of 10 percent.
That's almost as good as the small-batch devices made in the lab with spin-coating, a method that produces high-quality films in the laboratory but is commercially impractical since it wastes up to 90 percent of the initial ink.
Somewhat surprising to the researchers, their mass-produced versions exhibited molecular packing and texture that only slightly resembled that of lab-made varieties, which at their best convert about 11 percent of incident sunlight into electrical energy.
"The 'rule of thumb' has been that high-volume polymer solar cells should look just like those made in the lab in terms of structure, organization and shape at the nanometer scale," said Lee Richter, a NIST physicist who works on functional polymers. "Our experiments indicate that the requirements are much more flexible than assumed, allowing for greater structural variability without significantly sacrificing conversion efficiency."
"Efficient roll-to-roll fabrication is key to achieving the low-cost, high-volume production that would enable photovoltaics to scale to a significant fraction of global energy production," explained He Yan, a collaborator from Hong Kong University of Science and Technology.
The team experimented with a coating material composed of a fluorinated polymer and a fullerene (also known as a "buckyball"). Going by the technical name PffBT4T-2OD, the polymer is attractive for scaled production--achieving a reported power conversion efficiency of more than 11 percent. Importantly, it can be applied in relatively thick layers--conducive to roll-to-roll processing.
However, the top-performing solar cells were produced with the spin-coating method, a small-batch process. In spin coating, the fluid is dispensed onto the center of a disk or other substrate, which rotates to spread the material until the desired coating thickness is achieved. Besides generating lots of waste, the process is piecemeal--rather than continuous--and substrate size is limited.
So the research team opted to test commercially relevant coating methods, especially since PffBT4T-2OD can be applied in relatively thick layers of 250 nanometers and more, or roughly the size of a large virus. They started with blade-coating--akin to holding a knife edge at a fraction of a hair's breadth above a treated glass substrate as it slides by, painting the PffBT4T-2OD onto the substrate.
A series of X-ray-based measurements revealed that the temperature at which the PffBT4T-2OD was applied and dried significantly influenced the resultant coating's material structure--especially the orientation, spacing and distribution of the crystals that formed.
The substrates blade-coated at 90 degrees Celsius (194 degrees F) were the highest performing, achieving power conversion efficiencies that topped 9.5 percent. Surprisingly, at the nanometer level, the end products differed significantly from the spin-coated "champion" devices made in the lab. Detailed real-time measurements during both blade-coating and spin-coating revealed the different structures arose from the rapid cooling during spin-coating versus the constant temperature during blade-coating.
"Real-time measurements were critical to developing a proper understanding of the film formation kinetics and ultimate optimization," said Aram Amassian, a collaborator from Saudi Arabia's King Abdullah University of Science and Technology.
Encouraged by the results, the team performed preliminary measurements of PffBT4T-2OD coating formed on the surface of a flexible plastic sheet. The coating was applied on NIST's slot-die roll-to-roll coating line, directly mimicking large-scale production. Measurements confirmed that the material structures made with blade-coating and those made with slot-die-coating were nearly identical when processed at the same temperatures.
"It's clear that the type of processing method used influences the shape of the domains and their size distribution in the final coating, but these distinctly different morphologies do not necessarily undermine performance," said Harald Ade, a collaborator from North Carolina State University. "We think these findings provide important clues for designing polymer solar cells optimized for roll-to-roll processing."
Article: H.W. Ro, J.M. Downing, S. Engmann, A.A. Herzing, D.M. DeLongchamp, L.J. Richter, S. Mukherjee, H. Ade, M. Abdelsamie, L.K. Jagadamma, A. Amassian, Y. Liu and H. Yan. 2016. Morphology changes upon scaling a high-efficiency, solution-processed solar cell. Energy & Environmental Science. Published August 2, 2016. DOI: 10.1039/c6ee01623e
Mark Bello | EurekAlert!
Interfacial Superconductivity: Magnetic and superconducting order revealed simultaneously
17.01.2017 | Sonderforschungsbereich 668
Manchester scientists tie the tightest knot ever achieved
13.01.2017 | University of Manchester
Researchers from the University of Hamburg in Germany, in collaboration with colleagues from the University of Aarhus in Denmark, have synthesized a new superconducting material by growing a few layers of an antiferromagnetic transition-metal chalcogenide on a bismuth-based topological insulator, both being non-superconducting materials.
While superconductivity and magnetism are generally believed to be mutually exclusive, surprisingly, in this new material, superconducting correlations...
Laser-driving of semimetals allows creating novel quasiparticle states within condensed matter systems and switching between different states on ultrafast time scales
Studying properties of fundamental particles in condensed matter systems is a promising approach to quantum field theory. Quasiparticles offer the opportunity...
Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.
As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...
At TU Wien, an alternative for resource intensive formwork for the construction of concrete domes was developed. It is now used in a test dome for the Austrian Federal Railways Infrastructure (ÖBB Infrastruktur).
Concrete shells are efficient structures, but not very resource efficient. The formwork for the construction of concrete domes alone requires a high amount of...
Many pathogens use certain sugar compounds from their host to help conceal themselves against the immune system. Scientists at the University of Bonn have now, in cooperation with researchers at the University of York in the United Kingdom, analyzed the dynamics of a bacterial molecule that is involved in this process. They demonstrate that the protein grabs onto the sugar molecule with a Pac Man-like chewing motion and holds it until it can be used. Their results could help design therapeutics that could make the protein poorer at grabbing and holding and hence compromise the pathogen in the host. The study has now been published in “Biophysical Journal”.
The cells of the mouth, nose and intestinal mucosa produce large quantities of a chemical called sialic acid. Many bacteria possess a special transport system...
10.01.2017 | Event News
09.01.2017 | Event News
05.01.2017 | Event News
17.01.2017 | Earth Sciences
17.01.2017 | Materials Sciences
17.01.2017 | Architecture and Construction