Nanotechnology enhanced organic photovoltaics: Breaking the 10% efficiency barrier


QDOTS imagesCAKXSY1K 8Nanowerk Spotlight) Many researchers are investigating  the development of flexible solar cells in hopes of improving efficiency and  lowering manufacturing costs. Organic solar cells hold the potential for  integration into building facades and windows, due to their optical translucency  and ability to be manufactured on large areas at high-throughput.

As an  important member of the organic photovoltaics (OPV) family, polymer solar cells  draw the most research interest, due to the relatively high power conversion  efficiency (PCE) achieved (read more: “The  state of nanoimprinted polymer organic solar cell technology”).

However,  compared to the high efficiencies (>10%) of inorganic solar cells, the best  polymer solar cells (6-7%) still show a lower efficiency. Organic solar cells are regarded as an emerging technology to  become one of the low-cost thin-film alternatives to the current dominating  silicon photovoltaic technology, due to their intrinsic potential for low-cost  processing (high-speed and at low temperature).

However, it is generally  believed that the PCE needs to be improved to above 10% in order for organic  solar cells to become truly competitive in the marketplace. Currently, the best  reported PCE, achieved in laboratories, lies in the range of 6.7% to 7.6% for  molecular, and 8.3% to 10.6% for polymeric OPVs.

A recent review article in Advanced Materials (“Plasmonic-Enhanced Organic Photovoltaics: Breaking  the 10% Efficiency Barrier”) looks at the recent progress on  plasmonic-enhanced OPV devices using metallic nanoparticles, and one-dimensional  (1D) and two-dimensional (2D) patterned periodic nanostructures. The authors,  Qiaoqiang Gan from The State University of New York, Filbert Bartoli from Lehigh  University, and Zakya Kafafi from Northwestern University, discuss the benefits  of using various plasmonic nanostructures for broad-band,  polarization-insensitive and angle-independent absorption enhancement, and their  integration with one or two electrode(s) of an OPV device.

Organic Solar

2D  nanoaperture arrays with high-order symmetries: SEM images of periodic and quasi  periodic 2D nanoaperture arrays milled in a 300 nm-thick silver film. The right  insets show the 2D discrete Fourier transform power spectra for each array. The  left inset in (b) shows a high-resolution cross sectional SEM image of a polymer  filled hole in the triangular array. (© 2011, AIP)

The authors focus on two main research areas: the broadening of  the absorption band for an OPV material and its extension to the NIR region,  achieving polarization insensitive/independent plasmonic nanostructures; and  omnidirectional absorption enhancement as well as the integration of the  metallic plasmonic nanostructure(s) with one or two of the electrode(s) of an  OPV device. The two main patterned nanostructures frequently used to  introduce plasmonic enhancement in OPVs are randomly distributed metallic  nanoparticles, and 1D and 2D periodic nanopatterned arrays.

Metallic Nanoparticles

Metallic nanoparticles are probably the most popular and  extensively employed nanostructures for enhancing the performance of PV devices,  due to their relative ease of fabrication (see for instance: “Gold nanoparticles boost  organic solar cell efficiency”). Design considerations using metallic nanoparticles of different  materials, concentrations, shapes, sizes, and distributions have been introduced  in various layers and interfaces within the devices. One option introduces  metallic nanoparticles outside the active layer(s) of the OPV device.

A strong  localized plasmon field enhancement and/or increased light scattering when  metallic nanostructures were placed outside the active light-harvesting layer  result in an enhanced PCE. Embedding metallic nanoparticles inside the active layers of  OPVs exploits the strongly confined field of the localized surface plasmon  resonance and more efficient light scattering within the active layers.

It is  generally believed that small metallic nanoparticles (usually <20 nm in  diameter) can act as sub-wavelength antennas in which the enhanced near-field is  coupled to the absorbing OPV layer(s), increasing its effective absorption  cross-section; while large nanoparticles (>40 nm in diameter) can be used as  effective subwavelength scattering elements that significantly increase the  optical path length of the sunlight within the active layers.

High-Order Symmetric Plasmonic Nanostructures

Although the synthesis of plasmonic metal nanoparticles seems  relatively simple, it is quite challenging to control their size, shape, and  monodispersity using solution processing or vacuum thermal evaporation or  electrodeposition. Periodically patterned metallic nanostructures offer another  approach to enhance the optical absorption of the organic active  light-harvesting layers in OPVs. By properly designing plasmonic nanostructures,  light can be effectively coupled to surface plasmon polaritons modes which are  strongly confined at the metal surface next to the thin active layer. Both 1D  and 2D periodic metallic nanostructures have been explored in various OPV  designs to achieve unique and remarkable features. The review summarizes  important results from numerous studies on these nanostructures.

Organic Solar 2

Plasmonic nanostructures for omnidirectional absorption: (a) SEM  image of fabricated crossed trapezoid arrays and (b) a single unit cell of  crossed trapezoid where the scale bars are 500 and 100 nm, respectively; (c)  schematic drawing of a periodically crossed trapezoid array with the angle of  incidence shown as θ; (d) extinction plotted as a function of wavelength and  angle of incidence (note that broadband extinction is preserved even at higher  angles of incidence). (© 2011, NPG)

The authors conclude that, in order to achieve further progress  with OPVs, a renewed focus on the science and technology of nanophotonics for  light management and trapping offers the potential of achieving  higher-efficiency devices. Plasmonic strategies offer several unique features and are one  of the most promising solutions for enhancing the OPV optical absorption and  device performance. By incorporating plasmonic nanostructures in the front and  back metallic electrodes of an OPV device, it is possible to achieve broadband,  polarization- and angle-independent absorption enhancement.

This  plasmonic-assisted OPV has the potential to significantly surpass the 10% PCE  barrier if the enhanced optical absorption can be transferred into excitons and  separated photo-generated charge carriers which are efficiently collected at the  respective electrodes.

By Michael Berger. Copyright © Nanowerk

International partnership between New York State and the State of Israel to grow nanotechnology industry


QDOTS imagesCAKXSY1K 8(Nanowerk News) Governor Andrew M. Cuomo today  announced the signing of a Memorandum of Understanding (MOU) to establish an  international partnership between New York State and the State of Israel,  through a collaboration involving the College of Nanoscale Science and  Engineering (CNSE) and the Israeli Industry Center for Research &  Development (MATIMOP), that will significantly expand business, technology, and  economic relations in the burgeoning field of nanotechnology, while enabling  billions of dollars in new investments and the creation of thousands of  high-tech jobs in New York and Israel.
“I am so proud of the partnership between the State of Israel  and College of Nanoscale Science and Engineering, which continues to be the  leader in the global nanotechnology industry,” said Governor Cuomo. “This  partnership will strengthen our state’s relationship with the State of Israel,  while also investing in a thriving industry that will create jobs and expand the  economy right here in New York.”
Lieutenant Governor Robert Duffy said, “This partnership is yet  another example of how Governor Cuomo has strengthened New York State’s global  reputation as an attractive place to do business and create jobs. I thank the  State of Israel for partnering with New York State to ensure the continued  growth of the global nanotechnology industry. New York State is at the forefront  of this industry, and I commend Dr. Alain Kaloyeros for his leadership and hard  work on this agreement. Through this partnership, the College of Nanoscale  Science and Engineering can continue to drive this emerging and rapidly growing  field.”
Nili Shalev, Israel’s Economic Minister to North America, said, “This agreement is the first significant step to stimulate scientific and  industrial collaboration in areas where both states excel. The partnership will  enable Israeli companies access to CNSE’s renowned facilities and collaborate  with leading American and multinational companies on campus. It introduces many  other opportunities, including industrial R&D and commercialization joint  ventures, natural synergy between the two G450 Consortia of both states, and the  enhancement of academic research in Nano scaling. I would like to congratulate  Governor Cuomo, Lt. Governor Duffy and Dr. Alain Kaloyeros, the CEO of CNSE, for  supporting this initiative.”
Dan Vilenski, Former Chairman of Applied Materials’ Israeli  subsidiary and Board member of the Israeli National Nanotechnology Initiative,  said, “Nanotechnology is one of the major areas in which both Israel and New  York have a great deal to offer. Israel is the leader in metrology and  inspection in the semiconductor market, and the State of New York has built one  of the leading facilities in the world for Nano scaling research and will play a  significant role in shaping the future of this industry.”
State University of New York Chancellor Nancy Zimpher said, “The  governor has fostered an innovation environment in New York that is drawing top  scientists from around the world, and through SUNY’s globally renowned  NanoCollege, the potential for advancement and discovery is limitless. Only a  world-class university system like SUNY can generate an international  collaboration and investment of this magnitude. I want to welcome our new  Israeli partners to New York and to the SUNY system. I am sure our combined  expertise and passion for academic excellence and high tech innovation will  yield tremendous results.”
CNSE Senior Vice President and CEO Dr. Alain Kaloyeros said, “As  further testament to the pioneering leadership, strategic vision, and critical  investments of Governor Andrew Cuomo, which have truly established New York as  the epicenter for the global nanotechnology industry, the NanoCollege is  delighted to enter into this partnership with the most prestigious Israeli  Industry Center. In harnessing the power of nanotechnology innovation to bring  together corporate and university partners from the U.S. and Israel, this  collaboration sets the stage for leading-edge advances in nanoscale  technologies, and opens the door for high-tech growth that will provide exciting  career and economic opportunities for individuals and companies across the New  New York.”
The partnership announced today between CNSE and MATIMOP, acting  on behalf of the Office of the Chief Scientist (OCS) in the ministry of Industry  Trade and Labor, builds on and leverages the multi-billion dollar investments in  New York’s nanotechnology industry under the leadership of Governor Cuomo. This  partnership will facilitate and promote bilateral and multilateral research,  development, and commercialization programs in innovative nanoscale technologies  between corporations and academic institutions in the U.S. and Israel.
Through the agreement, the Israeli government has allocated up  to $300 million a year to fund access for Israeli companies and universities to  CNSE’s state-of-the-art 300mm wafer and 450mm wafer infrastructure, facilities,  resources, and know-how, which are unparalleled worldwide. In addition, a  publicity and marketing campaign is being prepared to generate interest and  participation from Israel’s corporate and academic entities.
The centerpiece of the collaboration is the NanoCollege, the  most advanced nanotechnology education, research, development, and deployment  enterprise in the world. With more than $14 billion in high-tech investments,  over 300 global corporate partners, and a footprint that spans upstate New York,  CNSE is uniquely positioned to support this first-of-its-kind partnership.
The agreement is designed to enable a host of nanotechnology  research and development (R&D), prototyping, demonstration and  commercialization activities, including the facilitation of partnerships to spur  collaborative projects targeting industrial R&D and commercialization;  exchange of technical information and expertise to promote global development of  next-generation nanoscale technologies; and the organization of joint seminars  and workshops to enhance cooperation between corporate and academic entities in  New York and Israel.
Specific technology areas targeted for initial collaboration  include sub-systems, sensors and accessories for deployment in the nanoscale  cleanroom environment; simulation and modeling for next-generation tools and  technologies; and tools, processes, and testing technologies essential to  accelerate critical innovations in the multiple fields enabled by  nanotechnology, including nanoelectronics, energy, and health care, among  others.
Congressman Paul Tonko said, “This partnership between New York  State and Israel is yet further proof that the Capital Region is not only  renowned on a national stage, but indeed on the world stage. Clean energy  innovation jobs and long-term economic growth require investments, and Tech  Valley laid that foundation years ago. As a fast-growing region for high tech  jobs and all the ancillary benefits that follow, these sorts of partnerships led  by Governor Cuomo will ensure we remain a bright spot for continued education,  research, development and deployment by some of the most cutting edge  innovators, entrepreneurs, small businesses and large companies in the world.”
Senator Neil D. Breslin said, “This is a fantastic partnership  between New York State and the State of Israel that will create jobs, further  leverage a proven investment, and continue to let the Capital Region shine as  the forefront of the nanotechnology industry. I commend Governor Cuomo for  championing the growth in nanotechnology in New York, and the State of Israel  for choosing to enter into this great partnership.”
Assemblywoman Patricia Fahy said, “I am pleased that this  partnership between New York State and the State of Israel will not only create  jobs, but add immensely to a much-needed boost of economic development in the  Capital Region. I congratulate the Governor for bringing global attention to New  York State in the field of nanotechnology, and the State of Israel for choosing  to do business in New York.”
Mayor Jerry D. Jennings said, “I applaud Governor Cuomo for his  leadership in developing Albany’s nanotechnology sector, and thank the State of  Israel and the College of Nanoscale Science and Engineering for their hard work  in making this partnership a reality. This is great news for the Capital Region,  which has already seen immense growth in this industry, and I look forward to  ensuring that this progress continues.”
Albany County Executive Daniel McCoy said, “Governor Cuomo has  done a great job leading the way toward greater economic development, and this  partnership between New York State and the State of Israel is just another  example. I applaud the Governor, the State of Israel, and the College of  Nanoscale Science and Engineering for their hard work in developing this  partnership that will spur job creation, economic development, and greater  international attention for our state.”
Source: College of Nanoscale Science and  Engineering

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Nanoparticles Split Water, Power Fuel Cell


QDOTS imagesCAKXSY1K 8Si Nanoparticles Split Water, Power Fuel Cell

by Tim Palucka

Materials Research Society | Published: 29 January 2013

Generating electricity in the field to power a laptop or night vision goggles could someday be just as simple as adding water to a cartridge containing silicon nanoparticles and a base. Researchers at the University at Buffalo (SUNY) have demonstrated that nanoparticles of Si in a basic solution can split water to release hydrogen and power a portable fuel cell to produce electricity. The ability to split water on-demand without adding heat, light, or electricity to the system could be a significant advance in fuel cell technology.

TEM 10 nm SI - Hydrogen“The reaction rate with these very small 10-nm Si particles is so much faster than with the relatively large 100 nm Si particles,” says Mark Swihart, whose team published their results in a recent issue of ACS Nano Letters. “Because of this fast reaction rate and the fact that there’s no delay between when you add water and when the reaction starts, it makes the technology at least practical in terms of being able to power a device instantaneously.”

 

While there was some scant evidence in the scientific literature that Si could perform this feat of splitting water to release hydrogen, it was largely ignored because the reaction rate was so slow as to be uninteresting. Using Al, Zn, or metal hydrides for this purpose looked so much more promising that Si fell by the wayside.

But Swihart and his group have been working with Si nanoparticles for more than a decade, mostly in the realm of quantum dot research. In doing so, they frequently had to use a base such as hydrazine for etching, and they noticed that hydrogen was released when aqueous hydrazine reacted with Si. Investigation showed that the hydrogen came not from decomposition of hydrazine, but from the oxidation of Si to release hydrogen from water.

Further investigation of the reaction using Si particles of different sizes, focusing on 10-nm and 100-nm-diameter particles with aqueous KOH, showed a particle size dependent liberation of hydrogen from water. But the factor of 150 increase in the reaction rate for the 10-nm-diameter particles compared to the 100-nm-particles was well in excess of the factor of 6 difference in their specific surface area. Thus, the increase in rate is much greater than expected based on increased surface area alone.

Swihart believes the difference is caused by geometry, not surface area. The 111 lattice planes etch much more slowly than other planes of Si, so crystals terminated entirely by 111 planes react slowly.  “The 10 nm particles etch isotropically—they just get smaller and go away,” he says. There’s no time for faceting to occur in this case. But the 100 nm particles undergo anisotropic etching. The faster-reacting 100 and 110 planes etch away first, leaving a particle with slower-reacting 111 planes behind in what he describes as a “hollow nano-balloon structure.” “With the bigger particles,” Swihart says, “eventually the unreactive 111 surfaces are the ones that end up being left,” thus slowing the reaction rate.

As a proof-of-concept, the research team tested a small fuel cell with a 20 stack polymer electrolyte membrane, comparing the fuel cell’s power output when fed hydrogen from the Si nanoparticle reaction versus hydrogen from a gas cylinder. Stoichiometrically, two moles of H2 should be generated for one mole of Si. In the tests, the fuel cell powered by H2 generated by reaction with Si produced more current and voltage than when the fuel cell was fed a stoichiometric amount of H2 from a gas cylinder. The difference is due to additional hydrogen, beyond the stoichiometric reaction amount, that terminates the Si surfaces after fabrication of the nanoparticles.

While there is much more work to be done, Swihart believes that if this technology is ever to become practical as a portable electricity generator, the KOH (or other base) would have to be mixed in with the Si in a cartridge, so you would not have to carry around a bottle of KOH solution. Such a device would come with the instructions “just add water.” For a soldier in the field needing to power night vision goggles, water from a nearby stream could be all he needs.

 

Read the abstract in ACS Nano Letters  here.