New Nanoparticles to make Solar Cells Cheaper to Manufacture

072613solarUniv. of Alberta researchers have found that abundant materials in the Earth’s crust can be used to make inexpensive and easily manufactured nanoparticle-based solar cells.

The U of A discovery, several years in the making, is an important step forward in making solar power more accessible to parts of the world that are off the traditional electricity grid or face high power costs, such as the Canadian North, said researcher Jillian Buriak, a chemistry professor and senior research officer of the National Institute for Nanotechnology, based on the U of A campus.

Buriak and her team have designed nanoparticles that absorb light and conduct electricity from two very common elements: phosphorus and zinc. Both materials are more plentiful than scarce materials such as cadmium and free from manufacturing restrictions imposed on lead-based nanoparticles.

“Half the world already lives off the grid, and with demand for electrical power expected to double by the year 2050, it is important that renewable energy sources like solar power are made more affordable by lowering the costs of manufacturing,” Buriak said.

Her team’s research supports a promising approach of making solar cells cheaply using mass manufacturing methods like roll-to-roll printing (as with newspaper presses) or spray-coating (similar to automotive painting). “Nanoparticle-based ‘inks’ could be used to literally paint or print solar cells or precise compositions,” Buriak said.

The team was able to develop a synthetic method to make zinc phosphide nanoparticles and demonstrated that the particles can be dissolved to form an ink and processed to make thin films that are responsive to light.

Buriak and her team are now experimenting with the nanoparticles, spray-coating them onto large solar cells to test their efficiency. The team has applied for a provisional patent and has secured funding to enable the next step to scale-up manufacture.

The research, which was supported by the Natural Sciences and Engineering Research Council of Canada, is published in the latest issue of ACS Nano.

Water 2.0 2013 Water Management And Nano Energy Summit

Water 2.0 open_img

2013 Water Management And Nano Energy Summit : November 13 & 14, 2013

Rice University – Shell Auditorium Jones Graduate School of Business Rice University 6100 Main Street Houston, Texas 77005

THE SUMMIT is a gathering of the world’s leading experts who are generating cutting-edge technological solutions for challenges in the water and energy sectors.

Produced in partnership with the Water Innovations Alliance, WATER 2.0, the NanoBusiness Commercialization Association, the Rice Alliance, and the Smalley Institute at Rice University, THE SUMMIT will feature prominent speakers from industry, government, finance and academia. THE SUMMIT will address state-of-the-art innovative solutions to decades-old problems in the water and oil and gas sectors. These pioneering technologies are emerging rapidly into the market thanks to revolutionary breakthroughs in material science, nanoscience and computational power.

For Full Details, Sponsors, Presenters and Exhibitors, go here:




Since 2009, Vincent Caprio’s Blog EVOLVING INNOVATIONS has addressed issues on Science & Technology.

About The Water Innovations Alliance Foundation The Water Innovations Alliance Foundation is focused on educating the public and key stakeholders as to new developments in fresh and waste water technologies. The Foundation works to gather data, develop reports, standards, economic analysis, and model training programs for advancing the development and deployment of new water technologies.

The Water Innovations Alliance Foundation is located in Cambridge, MA and Shelton, CT. It is a 501(c)(3) organization that works in conjunction with the Water Innovations Alliance. The Foundation was launched in Spring 2009. It is undertaking a series of initiatives to advance the understanding of new opportunities, technologies, and best practices for the water field.

To learn more about the Foundation and its membership, contact Vincent Caprio,

Nanotechnology Today – Fuel Cells, Buckyballs and Carbon Nanotubes

Nanotubes images 

To celebrate the 25th anniversary of National Chemistry Week, we visited the Maryland Nanocenter at the University of Maryland (UMD) to check out the latest research in nanotechnology — this year’s theme for NCW.

Three UMD researchers explain how their work in the nano-scale could lead to better fuel cells, solar cells, cancer treatments and super strong materials made from carbon nanotubes. Check out the video for a first hand look at the exciting applications of nanotechnology available today, and those that are just around the corner.

Drs. Eichhorn and Reutt-Robey at the University of Maryland ‘illuminate’ for us some of the current nano-technology being developed for commercial applications.

Video by Kirk Zamieroski Produced by the American Chemical Society



Hydrogen fuel from sunlight

3adb215 D Burris*** Note to Readers: I must admit it .. to me [and I have been accused of being “geeky” and “technology smitten” : -) ] … these guys are the true “Rock Stars” of our time.  “Imagine .. the possibilities!” – GNT –

“Great Things … From Small Things!”


DOE/Lawrence Berkeley National Laboratory

*** Note to Readers: I must admit it .. to me [and I have been accused of being “geeky” and “technology smitten” : -) ] … these guys are the true “Rock Stars” of our time.  “Imagine .. the possibilities!” – GNT –      “Great Things … From Small Things!”

DOE/Lawrence Berkeley National Laboratory

Berkeley Lab researchers at the Joint Center for Artificial Photosynthesis have developed a way to interface a molecular hydrogen-producing catalyst with a visible light absorbing semiconductor. With this approach, hydrogen fuel can be produced off a photocathode using sunlight. »


             IMAGE:   For more than two billion years, nature, through photosynthesis, has used the energy in sunlight to convert water and carbon dioxide into fuel (sugars) for green plants.

In the search for clean, green sustainable energy sources to meet human needs for generations to come, perhaps no technology matches the ultimate potential of artificial photosynthesis. Bionic leaves that could produce energy-dense fuels from nothing more than sunlight, water and atmosphere-warming carbon dioxide, with no byproducts other than oxygen, represent an ideal alternative to fossil fuels but also pose numerous scientific challenges.

A major step toward meeting at least one of these challenges has been achieved by researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) working at the Joint Center for Artificial Photosynthesis (JCAP).

“We’ve developed a method by which molecular hydrogen-producing catalysts can be interfaced with a semiconductor that absorbs visible light,” says Gary Moore, a chemist with Berkeley Lab’s Physical Biosciences Division and principal investigator for JCAP. “Our experimental results indicate that the catalyst and the light-absorber are interfaced structurally as well as functionally.”

Moore is the corresponding author, along with Junko Yano and Ian Sharp, who also hold joint appointments with Berkeley Lab and JCAP, of a paper describing this research in the Journal of the American Chemical Society (JACS). The article is titled “Photofunctional Construct That Interfaces Molecular Cobalt-Based Catalysts for H2 Production to a Visible-Light-Absorbing Semiconductor.” Co-authors are Alexandra Krawicz, Jinhui Yang and Eitan Anzenberg.

Earth receives more energy in one hour’s worth of sunlight than all of humanity uses in an entire year.

Through the process of photosynthesis, green plants harness solar energy to split molecules of water into oxygen, hydrogen ions (protons) and free electrons. The oxygen is released as waste and the protons and electrons are used to convert carbon dioxide into the carbohydrate sugars that plants use for energy. Scientists aim to mimic the concept but improve upon the actual process.

             IMAGE:   Gary Moore is a chemist with Berkeley Lab’s Physical Biosciences Division and principal investigator for the Joint Center for Artificial Photosynthesis.

Click here for more information.     

JCAP, which has a northern branch in Berkeley and a southern branch on the campus of the California Institute of Technology (Caltech), was established in 2010 by DOE as an Energy Innovation Hub.

Operated as a partnership between Caltech and Berkeley Lab, JCAP is the largest research program in the United States dedicated to developing an artificial solar-fuel technology. While artificial photosynthesis can be used to generate electricity, fuels can be a more effective means of storing and transporting energy. The goal is an artificial photosynthesis system that’s at least 10 times more efficient than natural photosynthesis.

To this end, once photoanodes have used solar energy to split water molecules, JCAP scientists need high performance semiconductor photocathodes that can use solar energy to catalyze fuel production. In previous efforts to produce hydrogen fuel, catalysts have been immobilized on non-photoactive substrates. This approach requires the application of an external electrical potential to generate hydrogen. Moore and his colleagues have combined these steps into a single material.

“In coupling the absorption of visible light with the production of hydrogen in one material, we can generate a fuel simply by illuminating our photocathode,” Moore says. “No external electrochemical forward biasing is required.”

The new JCAP photocathode construct consists of the semiconductor gallium phosphide and a molecular cobalt-containing hydrogen production catalyst from the cobaloxime class of compounds. As an absorber of visible light, gallium phosphide can make use of a greater number of available solar photons than semiconductors that absorb ultraviolet light, which means it is capable of producing significantly higher photocurrents and rates of fuel production. However, gallium phosphide can be notoriously unstable during photoelectrochemical operations.

             IMAGE:   Grafting molecular cobalt-containing hydrogen production catalysts to a visible-light-absorbing semiconductor exploits the UV-induced immobilization chemistry of vinylpyridine to p-type (100) gallium phosphide (GaP).

Click here for more information.     

Moore and his colleagues found that coating the surface of gallium phosphide with a film of the polymer vinylpyridine alleviates the instability problem, and if the vinylpyridine is then chemically treated with the cobaloxime catalyst, hydrogen production is significantly boosted.

“The modular aspect of our method allows independent modification of the light-absorber, linking material and catalyst, which means it can be adapted for use with other catalysts tethered over structured photocathodes as new materials and discoveries emerge,” Moore says. “This could allow us, for example, to replace the precious metal catalysts currently used in many solar-fuel generator prototypes with catalysts made from Earth-abundant elements.”

Despite its promising electronic properties, gallium phosphide features a mid-sized optical band gap which ultimately limits the total fraction of solar photons available for absorption.

Moore and his colleagues are now investigating semiconductors that cover a broader range of the solar spectrum, and catalysts that operate faster at lower electrical potentials. They also plan to investigate molecular catalysts for carbon dioxide reduction.

“We look forward to adapting our method to incorporate materials with improved properties for converting sunlight to fuel,” Moore says. “We believe our method provides researchers at JCAP and elsewhere with an important tool for developing integrated photocathode materials that can be used in future solar-fuel generators as well as other technologies capable of reducing net carbon dioxide emissions.”


This research was funded by the DOE Office of Science.

Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time.

For more information, please visit the Office of Science website at

Chlorine and Graphene Combine

Nanotubes imagesResearchers at the Massachusetts Institute of Technology in the US are reporting on a new way to p-dope graphene that does not sacrifice its excellent electronic properties too much – something that has proved to be somewhat of a challenge until now. The resulting material could be ideal for making all-graphene integrated circuits on a chip, radio-frequency transistors and nanoelectronic circuit interconnects to name a few examples.

In the lab

Graphene is a flat sheet of carbon just one atom thick – with the carbon atoms arranged in a honeycombed lattice. Since the material was first isolated in 2004, its unique electronic and mechanical properties, which include extremely high mobility, and high strength, have amazed researchers who say that it could be used in a host of device applications. Indeed, graphene might even replace silicon as the electronic material of choice in the future according to some. This is because electrons whiz through graphene at extremely high speeds, behaving like “Dirac” particles with no rest mass, a property that could allow for transistors that are faster than any existing today.

However, unlike the semiconductor silicon, graphene has no gap between its valence and conduction bands. Such a bandgap is essential for electronics applications because it allows a material to switch the flow of electrons on and off. One way of introducing a bandgap into graphene is to chemically dope it, but this has to be done carefully so as not to destroy graphene’s unique electronic properties too much.

Plasma-based surface functionalization technique

A team led by Mildred Dresselhaus and Tomas Palacios has now succeeded in p-doping graphene with chlorine using a plasma-based surface functionalization technique. “Compared with other chemical doping methods, the advantages of our approach are very significant,” says team member Xu Zhang. “First and foremost, the chlorine-doped graphene keeps a high charge mobility of around 1500 cm2/Vs after the hole doping. This value is impressively high compared to those obtained with other chemical species previously.”

The chlorine can also cover over 45% of the graphene sample surface, he adds. This is the highest surface coverage area reported for any graphene doping material until now, according to the researchers.

Density functional theory predicts that a bandgap of up to 1.2 eV can be opened up in graphene if both sides of the sample are chlorinated, and if the amount of chlorine on each side covers 50% of the total sample area. “The 45.3% coverage in single-sided chlorinated graphene observed in our work is thus important and paves the way to ultimately opening up a sizeable bandgap in the material while maintaining a reasonably high mobility,” Zhang told

In their work, the researchers studied both “exfoliated” graphene and that obtained using chemical vapour deposition (CVD). They performed the chlorine plasma treatments in an Electron Cyclotron Resonance Reactive Ion Etcher (ECR/RIE) in which chlorine gas was excited into the plasma state by absorbing energy from an in-phase electromagnetic field at a certain frequency. The chlorine plasma was accelerated by applying a DC bias relative to the sample stage. “We carefully optimized both the ECR power and DC bias to control the reaction conditions,” explained Zhang, “and the experiments were performed at room temperature.”

The p-doped material produced could be used to make all-graphene integrated circuits on a chip and RF transistors, he added. Doping the graphene with chlorine also reduces its sheet resistance, making it suitable for use in electronic circuit interconnects.

The team now plans to dope suspended samples of graphene with chlorine – to access both sides of a sample – and so open up an even bigger electronic band gap.

The present work is detailed in ACS Nano DOI: 10.1021/nn4026756.

Computer Chips Get Smaller .. Cost Less .. with Nanotechnology

Printing Graphene Chips(Nanowerk News) Not so long ago, a computer filled a  whole room and radio receivers were as big as washing machines. In recent  decades, electronic devices have shrunk considerably in size and this trend is  expected to continue, leading to enormous cost and energy savings, as well as  increasing speed.
Key to shrinking devices is Terascale computing, involving  ultrafast technology supported by single microchips that can perform trillions  of operations per second.
Using Terascale technology, semiconductor components commonly  used to make integrated circuits for all kinds of appliances could measure less  than 10 nanometres within several years. Keeping in mind that a nanometre is  less than 1 billionth of a metre, electronic devices have the potential to  become phenomenally smaller and require significantly less energy than today – a  development that will revolutionise the electronics industry.
Despite progress, the technology for producing these ultra-small  devices has a long way to go before being reliable. To advance the work, the  EU-funded project TRAMS (‘Terascale reliable adaptive memory  systems’) sought to improve reliability by improving chip design.
The TRAMS team conducted in-depth variability and reliability  analyses to develop chip circuits that are much less prone to errors. These  circuits feature new designs that yield reliable memory systems from currently  unreliable nanodevices.
The main challenge was to develop reliable, energy efficient and  cost effective computing using a variety of new technologies with individual  transistors potentially measuring below five nanometres in size.
The team investigated a number of technologies and materials  with potential to make Terascale computing a reality. These included:
  • carbon  nanotubes;
  • new  transistor geometries, such as FinFETs;
  • state-of-the-art  nanowires, which offer very advanced transistor capabilities for use in a new  generation of electronic devices.
Using models, the researchers analysed reliability – from the  technology to the circuit level.
These advances are expected to redefine today’s standard  ‘complementary metal-oxide semiconductors’ (CMOS). The team’s results would help  Europe’s manufacturers develop CMOS devices below the 16 nanometre range. The  biggest challenge will lie in reducing CMOS devices to below five nanometres – a  development that now starts to look possible.
From communication and security to transport and industry,  CMOS-based devices of the future promise to redesign the technology we use,  introducing radical energy and cost savings.
The TRAMS consortium includes universities and companies from  Spain, Belgium and the UK. The project was coordinated by Spain’s Universitat  Politècnica de Catalunya, and received almost EUR 2.5 million in EU funding. The  team concluded its work in December 2012.
Source: Cordis

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Printing Ultrafast Graphene Chips for Flexible Electronics

Futurists are always talking about how flexible electronics will change our lives in amazing ways, but we’ve yet to see anything mind-blowing come to market. A team of scientists from the University of Texas in Austin, however, think they’ve found the key to changing that: ultrafast graphene transistors printed on flexible plastic.

Graphene is amazing. Or at least, it could be. Made from a layer of carbon one-atom thick, it’s the strongest material in the world, it’s… Read…

   9 Incredible Uses for Graphene

Graphene is amazing stuff for a lot different reasons. One reason is that it’s the perfect material for chip-making, and conventional graphene chips have broken several electronic speed records. In the past, however, attempts to put graphene transistors on flexible materials have caused that speed to take a dive. Not with this new method.

Indeed, the chips from Texas clock in at a record-breaking 25-gigahertz. The MIT Technology Review explains the manufacturing process:

To make the transistors, the researchers first fabricate all the non-graphene-containing structures—the electrodes and gates that will be used to switch the transistors on and off—on sheets of plastic. Separately, they grow large sheets of graphene on metal, then peel it off and transfer it to complete the devices. …

The graphene transistors are not only speedy but robust. The devices still work after being soaked in water, and they’re flexible enough to be folded up.

And things are only getting better. Earlier this week we learned about a cutting edge technique for making graphene chips developed by a team of researchers from the University of California.

All we need now is a company to take the plunge and start bringing some of this next level technology to market. And you thought Liquidmetal was cool !!     [Technology Review]


Scientists Just Figured Out How to Make Lightning-Fast Graphene CPUs

Graphene has the power to change computing forever by making the fastest transistors ever. In theory. We just haven’t figured out how yet. Sound familiar? Fortunately, scientists have just taken a big step closer to making graphene transistors work for real.

Graphene transistors aren’t just fast; they’re lightning fast. The speediest one to date clocked in at some 427 GHz. That’s orders of magnitude more than what you can tease out of today’s processors.  The problem with graphene transistors, though, is that they aren’t particularly good at turning off. They don’t turn off at all actually, which makes it hard to use them as switches.

New Nanomaterial Increases Yield of Solar Cells


New nanomaterial increases yield of solar cells  6 hours ago

Linked quantum dots – In the new nanomaterial two or more electrons jump across the band gap as a consequence of just a single light particle (arrow with waves) being absorbed. Using special molecules the researchers have strongly linked the …more

Researchers from the FOM Foundation, Delft University of Technology, Toyota Motor Europe and the University of California have developed a nanostructure with which they can make solar cells highly efficient. The researchers published their findings on 23 August 2Researchers from the FOM Foundation, Delft University of Technology, Toyota Motor Europe and the University of California have developed a nanostructure with which they can make solar cells highly efficient. The researchers published their findings on 23 August 2013 in the online edition of Nature Communications.

Smart nanostructures can increase the yield of . An international team of researchers including physicists from the FOM Foundation, Delft University of Technology and Toyota, have now optimised the so that the solar cell provides more electricity and loses less energy in the form of heat.

Solar cells

A conventional solar cell contains a layer of silicon. When sunlight falls on this layer, in the silicon absorb the energy of the (photons). Using this energy the electrons jump across a ‘‘, as a result of which they can freely move and electricity flows.

The yield of a solar cell is optimised if the is equal to the band gap of silicon. Sunlight, however, contains many photons with energies greater than the band gap. The excess energy is lost as heat, which limits the yield of a conventional solar cell.


Several years ago the researchers from Delft University of Technology, as well as other physicists, demonstrated that the excess energy could still be put to good use. In small spheres of a the enables extra electrons to jump across the band gap. These nanospheres, the so-called , have a diameter of just one ten thousandth of a .

If a light particle enables an electron in a quantum dot to cross the band gap, the electron moves around in the dot. That ensures that the electron collides with other electrons that subsequently jump across the band gap as well. As a result of this process a single photon can mobilize several electrons thereby multiplying the amount of current produced.

Contact between quantum dots

However, up until now the problem was that the electrons remained trapped in their quantum dots and so could not contribute to the current in the solar cell. That was due to the large molecules that stabilize the surface of quantum dots. These large molecules hinder the electrons jumping from one quantum dot to the next and so no current flows.

In the new design, the researchers replaced the large molecules with small molecules and filled the empty space between the quantum dots with aluminium oxide. This led to far more contact between the quantum dots allowing the electrons to move freely.


Using laser spectroscopy the physicists saw that a single photon indeed caused the release of several electrons in the material containing linked quantum dots. All of the electrons that jumped across the band gap moved freely around in the material. As a result of this the theoretical yield of solar cells containing such materials rises to 45%, which is more than 10% higher than a conventional solar cell.

This more efficient type of solar cell is easy to produce: the structure of linked nanospheres can be applied to the solar cell as a type of layered paint. Consequently the new solar cells will not only be more efficient but also cheaper than conventional cells.

The Dutch researchers now want to work with international partners to produce complete solar cells using this design.

Read more at: mobilise several electrons thereby multiplying the amount of current produced.


Read more at:

Inkjet printing of graphene for flexible electronics

how-nanotechnology-could-change-solar-panels-photovoltaic_66790_600x450(Nanowerk Spotlight) Graphene has a unique combination  of properties that is ideal for next-generation electronics, including  mechanical flexibility, high electrical conductivity, and chemical stability.  Combine this with inkjet printing, already extensively demonstrated with  conductive metal nanoparticle ink (see for instance: “Low-cost  nanotechnology substitute for gold and silver in printable electronics”),  and you get an inexpensive and scalable path for exploiting these properties in  real-world technologies.

Although liquid-phase graphene dispersions have been  demonstrated (see: “Inkjet-printed  graphene opens the door to foldable electronics”), researchers are still  struggling with sophisticated inkjet printing technologies that allow efficient  and reliable mass production of high-quality graphene patterns for practical  applications. There are several challenges that need to be overcome:  

  • a  good ink should possess proper fluidic properties, in particular the right  viscosity and surface tension;
  • the  graphene concentration in these solvents is often quite low so that several tens  of print passes are required to obtain functional films, reducing efficiency of  the technique;
  • graphene  flakes easily aggregate in inks or during solvent evaporation, which decreases  the ink stability and/ or degrades the film/device performance;
  • ideal  solvents for graphene dispersions are toxic so that their corresponding inks  cannot be used in an open environment;
  • most  studies published thus far on inkjet printing of graphene are actually based on  graphene oxide inks, not graphene inks.

Recent work by researchers at the KTH Royal Institute of  Technology in Sweden has addressed these issues and proposes an approach to  overcome these problems. Reporting their findings in a recent issue of  Advanced Materials (“Efficient Inkjet Printing of Graphene”), a team  led by Max Lemme and Mikael Östling, professors at the School of  Information and Communication Technology at KTH, demonstrates a mature but  simple technology for inkjet printing of high-quality few-layer graphene.

Inkjet printed graphene patterns

Inkjet printed graphene patterns. a–c) Optical images of as-printed  patterns on glass slides: a) droplet matrix, b) lines, and c) a film corner.  (Reprinted with permission from Wiley-VCH Verlag)  


The approach is based on the team’s previously published  distillation-assisted solvent exchange technique to prepare high-concentration  graphene dispersions (“A simple route towards high-concentration  surfactant-free graphene dispersions”). They first  exfoliate graphene from  graphite flakes in dimethylformamide (DMF), and then DMF is exchanged by  terpineol through distillation by virtue of the large difference between their  boiling points.

Therefore, graphene can be significantly concentrated if  terpineol is of much lower volume than DMF. More importantly, the solvent is  changed from low-viscosity and toxic DMF to high-viscosity (about 40 cP at 20°  C) and environmentally-friendly terpineol.   They write, though, that the disadvantages of the technique in  the previous work – a short stable period (the dispersion can only be stable for  about 10 hours) and severe flake aggregation during solvent evaporation –  prevent the dispersions from being practical inks. “In this work, we have  improved the ink formulation mainly through polymer stabilization.

Before  distillation, a small amount of polymer (ethyl cellulose) is added into the  harvested graphene/DMF dispersion to protect the graphene flakes from  agglomeration. After printing, the stabilizing poly mers can be effectively  removed through a simple annealing process.”   The resulting graphene dispersion had a stable period of at  least several weeks.

The researchers point out that the inks provide  well-directed and constant jetting out of all nozzles at an even velocity, which  is comparable to the performance of commercially available inks.   To investigate the quality of the printed graphene, the team  fabricated large-area centimeter-scale graphene thin films with between 1 and 6  printing layers on glass slides.


“Printed transparent conductive films attain a sheet resistance  around 200 /sq at a transmittance of about 90%,” they summarize their results.  “Printed narrow-line resistors exhibit a resistance range from a few kΩ to  several MΩ. Printed few-layer graphene thin film transistors can be modulated by  the electric field effect.

Printed micro-supercapacitors achieve a high specific  capacitance of 0.59 mF cm-2 and a rapid  frequency response time around 13 ms.”   The team concludes that the present technology provides an  efficient and low-cost method to fabricate a variety of graphene electronic  devices with good performance and is a promising alternative for future  commercial applications in printed and flexible electronics.


By Michael Berger. Copyright © Nanowerk
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Solar paint paves the way for low-cost photovoltaics



(Nanowerk Spotlight) Using quantum dots as the basis for solar cells is not a new idea, but attempts to make such devices have not yet achieved sufficiently high efficiency in converting sunlight to power. The latest advances in quantum dots photovoltaics have recently resulted in solar cell power conversion efficiencies exceeding 7% (see for instance: “Graded Doping for Enhanced Colloidal Quantum Dot Photovoltaics”).


Although these performance levels are promising, all high-performing device results to date have relied on a multiple-layer-by-layer strategy for film fabrication rather than employing a single-layer deposition process. The attractiveness of using quantum dots for making solar cells lies in several advantages over other approaches: They can be manufactured in an energy-saving room-temperature process; they can be made from abundant, inexpensive materials that do not require extensive purification, as silicon does; and they can be applied to a variety of inexpensive and even flexible substrate materials, such as lightweight plastics.


In new work, reported in the August 12, 2013 online edition of Advanced Materials (“Directly Deposited Quantum Dot Solids Using a Colloidally Stable Nanoparticle Ink”), a research team from the University of Toronto and King Abdullah University of Science and Technology (KAUST) developed a semiconductor ink with the goal of enabling the coating of large areas of solar cell substrates in a single deposition step and thereby eliminating tens of deposition steps necessary with the previous layer-by-layer method.


“We sought an approach that would achieve highly efficient utilization of CQD materials,” says Professor Ted Sargent from the University of Toronto, who, together with Osman Bakr, an assistant professor in the Solar & Photovoltaics Engineering Research Center at KAUST, led the work.


“To achieve this, we made a solar cell ink that can be deposited in a single step which makes it an excellent material for high-throughput commercial fabrication.” The team’s ‘solar paint’ is composed of semiconductor nanoparticles synthesized in solution – so-called colloidal quantum dots (CQDs). They can be used to harvest electricity from the entire solar spectrum because their energy levels can be tuned by simply changing the size of the particle.


Previously, films made from these nanoparticles were built up in a layer-by-layer fashion where each of the thin CQD film deposition steps is followed by curing and washing steps to densify the film and form the final semiconducting material. These additional steps are required to exchange the long ligands that keep the CQDs stable in solution for short ligands that allow efficient charge transport. However, this means that many steps are required to build a thick enough film to absorb enough sunlight.


“We simplified this process by engineering the CQD surfaces with short organic molecules in the solution phase to enable a stable colloidal solution and reduce the film formation to a single step,” Bakr explains to Nanowerk. “At the same time, the post processing steps are reduced significantly, since the semiconducting material is formed in solution.


Dr Sargent

a) Schematic of the standard layer-by-layer spin-coating process with active materials usage yield and required total material indicated. b) Schematic of the single-step film process with active materials usage yield and required total material indicated. (Reprinted with permission from Wiley-VCH Verlag)


This means that CQD films can be deposited quickly and at low cost, similar to a paint or ink.” colloidal quantum dot solar cell fabrication methods:


Besides the reduction in processing steps, the new process is also much more efficient in terms of materials usage. While the layer-by-layer, solid-state treatment approach provides less than 0.1% yield in its application of CQD materials from their solution phase onto the substrate, the new approach achieves almost 100% use of available CQDs. “This means that for the same amount of CQD material, we could make a thousand-fold larger area of solar cells compared with conventional methods,” Bakr points out.


“Our technology paves the way for low-cost photovoltaics that can be fabricated on flexible substrates using roll-to-roll manufacturing, similar to a printing press,” adds Lisa Rollny, a PhD candidate in Sarget’s group and a co-author of the paper. “Our ink is also useful in biological applications, e.g. in biosensors and tracing agents with an infrared response.” “In previous work, we found new routes of passivating the CQD surface using a combination of organic and inorganic compounds in a solid state approach with large improvements in efficiency,” says Rollny. “We intend to integrate this knowledge with our solar CQD ink to further improve the performance of this material, especially in terms of how much solar energy is converted into usable electrical energy.”


Although the team have developed an effective method for producing a CQD film in a single step, the electronic properties of the resulting films are not optimized yet. This is due to the very small imperfections on the CQD surface that reduce the usable electricity output of a solar cell. Through careful engineering of CQD surfaces in solution, the researchers plan to eliminate these unwanted surface sites in order to make higher quality, higher efficiency CQD solar cells using their single step process.


By Michael Berger. Copyright © Nanowerk

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