13 Apr 2016
Combining quantum dots and organic molecules can enable solar cells to capture more of the sun’s light.
Light from the sun is our most abundant source of renewable energy, and learning how best to harvest this radiation is key for the world’s future power needs. Researchers at KAUST have discovered that the efficiency of solar cells can be boosted by combining inorganic semiconductor nanocrystals with organic molecules.
Quantum dots are nano-crystals that only measure roughly 10 nanometers across. An electron trapped by the dot has quite different properties from those of an electron free to move through a larger material.
“One of the greatest advantages of quantum dots for solar cell technologies is their optical properties’ tunability,” explained KAUST Assistant Professor of Chemical Science Omar Mohammed. “They can be controlled by varying the size of the quantum dot.”
Mohammed and his colleagues are developing lead sulfide quantum dots for optical energy harvesting; these tend to be larger than dots made from other materials. Accordingly, lead sulfide quantum dots can absorb light over a wider range of frequencies. This means they can absorb a greater proportion of the light from the sun when compared to other smaller dots.
To make a fully functioning solar cell, electrons must be able to move away from the quantum dot absorption region and flow toward an electrode. Ironically, the property of large lead sulfide quantum dots that makes them useful for broadband absorption—a smaller electron energy bandgap—also hinders this energy harvesting process. Previously, efficient electron transfer had only been achieved for lead sulfide quantum dots smaller than 4.3 nanometers across, which caused a cut-off in the frequency of light converted.
The innovation by Mohammed and the team was to mix lead sulfide quantum dots of various sizes with molecules from a family known as porphyrins. The researchers showed that by changing the porphyrin used, it is possible to control the charge transfer from large lead sulfide dots; while one molecule switched off charge transfer altogether, another one enabled transfer at a rate faster than 120 femtoseconds.
The team believe this improvement in energy harvesting ability is due to the interfacial electrostatic interactions between the negatively charged quantum dot surface and the positively charged porphyrin.
“With this approach, we can now extend the quantum dot size for efficient charge transfer to include most of the near-infrared spectral region, reaching beyond the previously reported cut-off,” stated Mohammed. “We hope next to implement this idea in solar-cells with different architectures to optimize efficiency.”
Explore further: Quantum dots with built-in charge boost solar cell efficiency by 50%
More information: Ala’a O. El-Ballouli et al. Overcoming the Cut-Off Charge Transfer Bandgaps at the PbS Quantum Dot Interface, Advanced Functional Materials (2015). DOI: 10.1002/adfm.201504035
The right blend of polymers enables rapid and molecule-selective filtering of tiny particles from water.
A method of fabricating polymer membranes with nanometer-scale holes that overcomes some practical challenges has been demonstrated by KAUST researchers.
Porous membranes can filter pollutants from a liquid, and the smaller the holes, the finer the particles the membrane can remove. The KAUST team developed a block copolymer membrane with pores as small as 1.5 nanometers but with increased water flux, the volume processed per hour by a membrane of a certain area.
A nanofilter needs to be efficient at rejecting specific molecules, be producible on a large scale, filter liquid quickly and be resistant to fouling or the build-up of removed micropollutants on the surface.
Block copolymers have emerged as a viable material for this application. Their characteristics allow them to self-assemble into regular patterns that enable the creation of nanoporous materials with pores as small as 10 nanometers.
However, reducing the size further to three nanometers has only been possible by post-treating the membrane (depositing gold, for example2). Moreover, smaller holes usually reduce the water flux.
Klaus-Viktor Peinemann from the KAUST Advanced Membranes & Porous Materials Center and Suzana Nunes from the KAUST Biological and Environmental Science and Engineering Division formed a multidisciplinary team to find a solution.
“We mixed two block copolymers in a casting solution, tuning the process by choosing the right copolymer systems, solvents, casting conditions,” explained Haizhou Yu, a postdoctoral fellow in Peinemann’s group. This approach is an improvement on alternatives because it doesn’t require material post-treatment.
Peinemann and colleagues blended polystyrene-b-poly(acrylic acid) and polystyrene-b-poly(4-vinylpyridine) in a ratio of six to one. This created a sponge-like layer with a 60 nanometer film on top. Material analysis showed that nanoscale pores formed spontaneously without the need for direct patterning1.
The researchers used their nanofiltration material to filter the biological molecule protoporphyrin IX from water. The filter simultaneously allowed another molecule, lysine, to pass through, demonstrating its molecular selectivity. The researchers were able to filter 540 liters per hour for every square meter of membrane, which is approximately 10 times faster than commercial nanofiltration membranes.
The groups teamed up with Victor Calo from the University’s Physical Science and Engineering Division to develop computer models to understand the mechanism of pore formation. They showed that the simultaneous decrease in pore size and increase in flux was possible because, while the pores are smaller, the pore density in the block copolymer is higher.
“In the future, we hope to optimize membranes for protein separation and other applications by changing the copolymer composition, synthesizing new polymers and mixing with additives,” said Nunes.
The above post is reprinted from materials provided by KAUST – King Abdullah University of Science and Technology. Note: Materials may be edited for content and length.
- Yu, H., Qiu, X., Moreno, N., Ma, Z., Calo, V. M., Nunes, S. P. & Peinemann, K.-V. Self-assembled asymmetric block copolymer membranes: Bridging the gap from ultra- to nanofiltration. Angewandte Chemie International Edition, December 2015
- Haizhou Yu, Xiaoyan Qiu, Suzana P. Nunes, Klaus-Viktor Peinemann. Self-Assembled Isoporous Block Copolymer Membranes with Tuned Pore Sizes. Angewandte Chemie International Edition, 2014; 53 (38): 10072 DOI: 10.1002/anie.201404491
29 Oct 2015
An environment-friendly method for synthesizing a microporous material that can adsorb carbon dioxide emitted from fossil fuel-driven power plants has been developed by researchers at KAUST1.
Burning carbon-based energy sources to meet the world’s energy demands is recognized to have a negative impact on our planet: global warming and ocean acidification could leave an indelible mark on Earth. The slow development process and low efficiency of alternatives such as nuclear fusion and solar power makes it difficult to wean ourselves off the use of conventional fossil fuels.
An alternative strategy is to develop technologies that mitigate the deleterious effects of fossil fuels. Carbon capture is one such approach, and proposes to use porous materials that can adsorb and store emitted carbon dioxide at the end of the energy generation process to prevent it from entering the atmosphere.
Metal–organic frameworks (MOFs) are one promising class of porous solid-state materials. These crystalline networks are made up of metal ions or clusters interconnected by organic molecules.
“The periodic arrangement of these organic and inorganic molecular building blocks gives MOFs one of their most defining properties: a functional and tunable pore system,” said KAUST Professor of Chemical Science Mohamed Eddaoudi. “The deliberate control of the available and accessible space shape, size and functionality enables adsorbing and storing select gases.”
The translation of a prospective MOF that selectively captures carbon dioxide from a laboratory scale to industrial scale settings requires the development of economical synthetic approaches. The manufacturing process frequently involves organic solvents that can also have a negative impact on the environment.
Eddaoudi and colleagues from KAUST’s Advanced Membranes & Porous Materials Research Center have developed a simple and solvent-free method to create a MOF adsorbent that selectively captures carbon dioxide.
The reported MOF structure, which they call SIFSIX-3-Ni, was made by dry mechanical mixing the organic component pyrazine with the inorganic solid NiSiF6 at a molar ratio of four to one, and then wetting with a few drops of water. This was heated for four hours at 65 degrees Celsius and then at 105 degrees Celsius for an additional four hours.
The team confirmed the efficient adsorption of carbon dioxide, even in an environment with very low carbon dioxide content. The authors also proved that the material is tolerant to the acidic gas hydrogen sulfide that is present in natural gas.
- Shekhah, O., Belmabkhout, Y., Adil, K., Bhatt, P. M., Cairns, A. J. & Eddaoudi, M. A facile solvent-free synthesis route for the assembly of a highly CO2 selective and H2S tolerant NiSIFSIX metal–organic framework. Chemical Communications 51, 13595-13598 (2015). | article
08 Sep 2015
A point-of-use solar distillation device that can clean up saltwater and wastewater without producing greenhouse gases has been constructed by a research team from King Abdullah University of Science and Technology (KAUST)1.
The key to the new technology is a floating membrane coated with a special light-absorbing polymer that repairs its hydrophobic “skin” when damaged.
For centuries, attempts have been made to use the sun’s heat to distill clean water from polluted sources. Simple solar stills, such as a glass plate placed over a water-filled box, are inexpensive to operate but are notoriously inefficient. This is because water is a poor light absorber, and any captured heat tends to distribute uniformly through the still instead of localizing at surfaces where evaporation occurs.
To combat these problems, researchers are developing floating “solar generator” materials that heat up quickly in sunlight and then trap heat at air–water interfaces for steam production.
A polypyrrole (PPy)-coated device that absorbs sunlight and releases it as heat can rapidly purify water through distillation Reproduced with permission © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
These devices are usually coated with water-repellant waxy molecules, such as fluorinated alkyl chains, for better floating. However, damage from ultraviolet rays and oxidative chemicals can degrade the hydrophobic layers, causing the generator to sink.
Inspired by the lotus flower, a plant that restores damage to its hydrophobic leaves through the migration of waxy molecules, KAUST Associate Professor Peng Wang and colleagues from the University’s Biological and Environmental Science and Engineering Division developed a self-healing solar generator.
The researchers coated a tightly woven stainless steel mesh with polypyrrole (PPy), a light-absorbing polymer with high photothermal conversion efficiency and bumpy surface microstructures. The team modified the PPy film with fluoroalkylsilane chains, enabling it to act as a reservoir that supplies additional hydrophobic chains to damaged regions through biomimetic self-migration.
The new device nearly tripled the output of freshwater from typical solar stills, thanks to a significant jump in temperature at the air–water interface and a conversion efficiency of close to 60 percent. It also exhibited remarkable damage resistance: after the team used a plasma source to oxidize the mesh and make it sink to the bottom of a beaker, they found a simple one-hour treatment in sunlight was sufficient to restore its self-floating capability.
The team’s first prototype — a transparent plastic condensing chamber and solar fan mounted on top of a PPy-coated mesh — floats lightly on the surface of seawater and distills a steady stream of water for more than 100 consecutive hours.
“Careful material selection allowed us to integrate two types of functions into one distillation device,” Wang said. “This has great potential to be employed in point-of-use potable water production.”
- Zhang, L., Tang, B., Wu, J., Li, R. & Wang, P. Hydrophobic light-to-heat conversion membranes with self-healing ability for interfacial solar heating. Advanced Materials advance online publication, 17 July 2015 doi: 10.1002/adma.201502362 | article
20 Aug 2015
The triplet state lifetime varies with the distance and the strength of binding between the porphyrin and the surface of the quantum dot.
Nanotechnology could improve the efficiency of organic photovoltaic technology, researchers at King Abdullah University of Science and Technology (KAUST) have demonstrated1.
In general, solar cells made from organic materials offer a cheap, simple and sustainable approach to harvesting light from the sun. But there is an urgent need to improve the efficiency of these organic cells.
The performance of these devices is limited by the re-emission of light that has been absorbed, thus detracting energy that should be converted to electricity. When an organic material absorbs light, it can create an exciton — an electron paired to a positively charged equivalent called a hole. This exciton exists for a very short period before recombining radiatively or non-radiatively. So, for a useful current to be produced, the electron and hole must separate before they recombine.
Research by Omar Mohammed and his colleagues from the KAUST Solar and Photovoltaics Engineering Research Center show how the lifetime of excitons in an organic material can be extended by using quantum dots.
Quantum dots are nanometer scale particles. Their advantage in solar-cell technologies is their tunability: the optical properties, such as absorption wavelength, can be changed by varying the size of the dot. Additional molecules attached to the surface of the nanostructure can tailor the functionality of the dots even further.
Mohammed’s team investigated a family of organic compounds (commonly used in solar applications) known as porphyrins. The electron-hole pair generated in porphyrin by light absorption forms a high-energy exciton, which then relaxes to one of two different lower-energy excitons know as a singlet and a triplet.
“The photo-generated singlet excitons exhibit very short lifetimes and consequently they have short diffusion lengths, which is one of the greatest challenges for achieving high power-conversion efficiencies in solar-cell devices,” explains Mohammed. “Triplet excitons with their long lifetimes are an alternative way to overcome this problem.”
The researchers showed that cadmium telluride quantum dots can improve not only the path from excited exciton to triplet exciton — so-called intersystem crossing, but also the elongation of the triplet exciton lifetime. They were able to tune the intersystem crossing and the triplet state lifetime by changing the size of the quantum dots in the solution.
“We are currently testing other absorber materials and other semiconductor quantum dots,” says Mohammed. “In addition, we are planning to fabricate solar cell devices from these nano-assemblies.”
© 2015 KAUST
Ahmed, G. H., Aly, S. M., Usman, A., Eita, M. S., Melnikov, V. A. & Mohammed, O. F. Quantum confinement-tunable intersystem crossing and the triplet state lifetime of cationic porphyrin–CdTe quantum dot nano-assemblies. Chemical Communications 51, 8010—8013 (2015). | article
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25 Jun 2015
Electrical contacts made of branched titanium dioxide nanowires developed by developed by researchers from the King Abdullah University of Science and Technology (KAUST) could improve the efficiency of solar cells.
Titanium dioxide is a white pigment commonly used in paint and a versatile electronic material commonly used in solar cells. It is a cheap and abundant compound and has similar electronic states to those of the light-absorbing compounds used in photovoltaics.
This match ensures that electrical charges are efficiently funnelled away from the active region of a solar cell into the titanium dioxide and from there towards the electrical contacts of the device. “It is the most successful electron transporting material in hybrid organic/inorganic photovoltaics,” explains research leader Aram Amassian.
Schematic of the low-cost fabrcation of titanium dioxide nanomaterials by electrospinning.
© 2015 Wiley
To ensure good contact between the titanium dioxide and the solar cell material, the titanium dioxide needs a very large surface area for maximum capture of electrical charges. The surface area can be expanded through the use of nanostructured materials such as meshes made from nanowires. These meshes have not been able to effectively transport the electrical charges across the nanowires.
Different architectural structures could help improve charge transport — for instance branched structures would have a much stronger electrical connection. But, established techniques for growing such branched structures is inefficient and produces nanostructures with many impurities and defects.
KAUST researchers have now established a two-stage process for an efficient fabrication of branched titanium dioxide materials. The first step is to deposit nanofibers using an electrospinning technique, where a narrow jet of a titanium dioxide solution is ejected from a needle using electrostatic charges (see image), resulting in a network of electron highways. “Electrospinning of metal oxide nanofibers has emerged as a potentially low cost, rapid and useful technique to grow one-dimensional nanostructures on a variety of substrates,” says Amassian.
In the second step these structures are heated further which results in the hydrothermal growth of branched nanostructures. The hyperbranched materials perform better than conventional nanofibers in solar cells, which indicates their potential viability in other devices such as batteries, or catalytic applications.
The team reduced the processing temperature substantially — down to 300 degrees Celsius — but the demand for a high processing temperature remains a practical barrier to the technology, says Amassian. “Future work to halve this temperature could help implement hyperbranched electron transport materials for solar cell fabrication on flexible and stretchable plastic or textile substrates.”
- Mahmood, K., Swain, B. S. & Amassian, A. Highly efficient hybrid photovoltaics based on hyperbranched three-dimensional TiO2 electron transporting materials. Advanced Materials 27, 2814 (2015). | article
Enabling researchers to take their ideas from the earliest research stages right through to the development and commercialization of a final product is a foundation goal at King Abdullah University of Science and Technology (KAUST). One route is for KAUST to provide active support to startup companies that have strong connections with the university’s research activities.
Nicola Bettio, manager of the funding scheme, says seed funding and early-stage capital are made available to startup companies through the KAUST Innovation Fund. “We invest in companies launched by our faculty, researchers and students, alongside international startups that are willing to move their operations to KAUST and Saudi Arabia,” Bettio explains. “By encouraging the transition of science into innovation and new ventures we can help to establish a fertile ecosystem for early-stage technology-based companies in Saudi Arabia.”
Efficient and cost-effective solar power generation will bring significant economic benefits and drastically reduce the carbon footprint of Saudi Arabia and the rest of the world. For the past several years, KAUST has been supporting research into new technologies using ‘colloidal quantum dots’ – a potential alternative to the established design of solar cells.
With an ideal design for solar panels the paint made from quantum dots is highly flexible.
© 2015 KAUST
Colloidal quantum dots are tiny semiconductor particles with the capacity to harvest energy from both the visible spectrum and the previously-untapped near-infrared portion of the sun’s light. Edward Sargent, a senior researcher at the University of Toronto, has been working on the development and application of colloidal quantum dots for the past ten years, and helped found QD Solar, a Canadian startup company aiming to commercialize the new technology.
“When KAUST was founded back in 2009, there was a very strong emphasis on advancing solar technology that led to the creation of KAUST’s Solar and Photovoltaics Engineering Research Center (SPERC),” explains Sargent. “The university employed top professors in the field who have been a key asset during the development of the new quantum dot technique I have been working on. I was fortunate enough to be awarded a KAUST Investigators grant around that time, enabling me to pursue my research in collaboration with these top scientists.”
In 2009, Sargent’s team at the University of Toronto received a grant from KAUST to advance their research into colloidal quantum dots specifically for solar power applications. Since then, the team’s advances have evolved in leaps and bounds. “Our latest design incorporates the dots into a solution, or ‘paint’, which can be applied to flexible surfaces very easily and cheaply,” Sargent explains. “We have also vastly improved the power conversion efficiency of the dots over the past six years, thanks in part to the funding from KAUST.”
Sargent and his team, in collaboration with KAUST researchers, are now investigating ways of combining the dots with existing silicon-based solar cells to create new hybrid structure solar panels. This will allow them to harness a far greater fraction of the sun’s energy than ever before, improving the panels’ efficiency and capacity to generate more power.
Marc Vermeersch, the SPERC managing director, points out that current solar cell technologies are reaching their physical limits. He explains that a key advantage of creating QD Solar’s hybrid system is that existing solar panel producers will be able to modify their manufacturing practices relatively easily to incorporate the new design, rather than starting from scratch. Manufacturers will be able to achieve significant economic and efficiency improvements using QD Solar’s quantum dot-based solar products, Bettio adds.
KAUST anticipates welcoming a QD Solar presence to Saudi Arabia in the near future, as Bettio explains: “QD Solar is working with the KAUST Innovation Fund to raise capital to establish a significant development facility in our Research & Technology Park. Ultimately, this may lead to the establishment of a Saudi player in the photovoltaic industry and the creation of a hybrid solar panel manufacturing facility in the Kingdom.”
29 Oct 2013
(Nanowerk Spotlight) Colloidal quantum dot (CQD) nanocrystals are attractive materials for optoelectronics, sensing devices and third generation photovoltaics, due to their low cost, tunable bandgap – i.e. their optical absorption can be controlled by changing the size of the CQD nanocrystal – and solution processability. This makes them attractive candidate materials for cheap and scalable roll-to-roll printable device fabrication technologies.
One key impediment that currently prevents CQDs from fulfilling their tremendous promise is that all reports of high efficiency devices were from CQDs synthesized using manual batch synthesis methods (in classical reaction flasks).