Flexible Solar Cells a Step Closer to Reality … Lower Cost and Improved Performance – University of Warwick

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Solar cells that use mixtures of organic molecules to absorb sunlight and convert it to electricity, that can be applied to curved surfaces such as the body of a car, could be a step closer thanks to a discovery that challenges conventional thinking about one of the key components of these devices.

A basic organic solar cell consists of a thin film of organic semiconductors sandwiched between two electrodes which extract charges generated in the organic semiconductor layer to the external circuit. It has long been assumed that 100% of the surface of each electrode should be electrically conductive to maximise the efficiency of charge extraction.

Scientists at the University of Warwick have discovered that the electrodes in organic solar cells actually only need ~1% of their surface area to be electrically conductive to be fully effective, which opens the door to using a range of composite materials at the interface between the electrodes and the light harvesting organic semiconductor layers to improve device performance and reduce cost. The discovery, published today (11 September), is reported in Advanced Functional Materials.

The academic lead, Dr Ross Hatton from the University’s Department of Chemistry, said: “It’s widely assumed that if you want to optimise the performance of organic solar cells you need to maximize the area of the interface between the electrodes and the organic semiconductors. We asked whether that was really true.”

The researchers developed a model electrode that they could systematically change the surface area of, and found that when as much as 99% of its surface was electrically insulating the electrode still performs as well as if 100% of the surface was conducting, provided the conducting regions aren’t too far apart.Nanotechnology-in-Solar-Energy-2

High performance organic solar cells have additional transparent layers at the interfaces between the electrodes and the light harvesting organic semiconductor layer that are essential for optimising the light distribution in the device and improving its stability, but must also be able to conduct charges to the electrodes. This is a tall order and not many materials meet all of these requirements.

Dr Dinesha Dabera, the post-doctoral researcher on this Leverhulme Trust funded project, explains:“This new finding means composites of insulators and conducting nano-particles such as carbon nanotubes, graphene fragments or metal nanoparticles, could have great potential for this purpose, offering enhanced device performance or lower cost.

“Organic solar cells are very close to being commercialised but they’re not quite there yet, so anything that allows you to further reduce cost whilst also improving performance is going to help enable that.”

Dr Hatton, who was interviewed by Serena Bashal of the UK Youth Climate Coalition at the British Science Festival this week, explains: “What we’ve done is to demonstrate a design rule for this type of solar cell, which opens up much greater possibilities for materials choice in the device and so could help to enable their realisation commercially.’’

Organic solar cells are potentially very environmentally friendly, because they contain no toxic elements and can be processed at low temperature using roll-to-roll deposition, so can have an extremely low carbon footprint and a short energy payback time.

Dr Hatton explains: “There is a fast growing need for solar cells that can be supported on flexible substrates that are lightweight and colour-tuneable. Conventional silicon solar cells are fantastic for large scale electricity generation in solar farms and on the roofs of buildings, but they are poorly matched to the needs of electric vehicles and for integration into windows on buildings, which are no longer niche applications. Organic solar cells can sit on curved surfaces, and are very lightweight and low profile.

“This discovery may help enable these new types of flexible solar cells to become a commercial reality sooner because it will give the designers of this class of solar cells more choice in the materials they can use.”

From Nano Magazine.com

Inexpensive, Flexible Solar Cells: Rice & Penn State Collaborate

QDOTS imagesCAKXSY1K 8(Phys.org) —Work by a team of chemical engineers at Penn State and Rice University may lead to a new class of inexpensive organic solar cells.

Work by a research team at Penn State and Rice University could lead to the development of flexible solar cells. The engineers’ technique centers on control of the nanostructure and morphology to create organic solar cells made of block polymers. Credit: Curtis Chan

Most solar cells today are inorganic and made of . The problem with these, Gomez explained, is that inorganic solar cells tend to be expensive, rigid and relatively inefficient when it comes to converting sunlight into electricity.

But offer an intriguing alternative that’s flexible and potentially less expensive.

Not many organic solar cells currently exist. He said, “There are a bunch of prototypes floating around. You see them in places like in solar-powered laptop bags and on the top of some bus depots.”

The problem is that the bulk of organic solar cells employ fullerene acceptors—a carbon-based molecule that’s extremely difficult to scale up for mass production.

Gomez’s approach skips the fullerene acceptor altogether and seeks to combine

The idea of utilizing molecular self-assembly for solar cells isn’t new, but Gomez said, “It’s not been well executed.”

He continued, “It’s like trying to mix oil and water.” The issue is that weak and disorder at junctions of different organic materials limited the performance and stability of previous organic solar cells.

But by controlling the and morphology, the team essentially redesigned the molecules to link together in a better way.

The engineers were able to control the donor-acceptor  through microphase-separated conjugated block copolymers.

“We have not only demonstrated control of the microstructure, but also control of the interface responsible for the initial steps in charge photogeneration in a way never achieved before,” Gomez said.

The result, which was detailed in a recent issue of the American Chemical Society‘s Nano Letters journal, is an organic solar cell made of that’s three percent efficient.

The team included Penn State chemical engineering graduate student Changhe Guo; undergraduate student Matt Witman; Rafael Verduzco, the Louis Owen Assistant Professor of Chemical and Biomolecular Engineering at Rice University; Joseph Strzalka, research scientist at Argonne National Laboratory; and research scientists Cheng Wang and Alexander Hexemer of Lawrence Berkeley National Laboratory.

Though the team’s prototype is not as efficient as some solar cells that are commercially available, Gomez explained the work shows flexible organic solar cells are indeed possible.

“Our cells right now don’t capture a lot of light. We need to look back and redesign the molecule. We think we can do better than 3 percent,” he said.

Read more at: http://phys.org/news/2013-08-chemical-inexpensive-flexible-solar-cells.html#jCpmolecules in a solution.

Regulating Electron ‘Spin’ Key to Making Organic Solar Cells Competitive

3adb215 D BurrisAug. 7, 2013 — Organic solar cells, a new class of solar cell that mimics the natural process of plant photosynthesis, could revolutionise renewable energy — but currently lack the efficiency to compete with the more costly commercial silicon cells.

At the moment, organic solar cells can achieve as much as 12 per cent efficiency in turning light into electricity, compared with 20 to 25 per cent for silicon-based cells.
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This is the laser set-up used to to make the actual measurements reported in the paper. (Credit: Dr. Akshay Rao)

Now, researchers have discovered that manipulating the ‘spin’ of electrons in these solar cells dramatically improves their performance, providing a vital breakthrough in the pursuit of cheap, high performing solar power technologies.

The study, by researchers from the Universities of Cambridge and Washington, is published today in the journal Nature, and comes just days after scientists called on governments around the world to focus on solar energy with the same drive that put a man on the moon, calling for a “new Apollo mission to harness the sun’s power.”

Organic solar cells replicate photosynthesis using large, carbon-based molecules to harvest sunlight instead of the inorganic semiconductors used in commercial, silicon-based solar cells. These organic cells can be very thin, light and highly flexible, as well as printed from inks similar to newspapers — allowing for much faster and cheaper production processes than current solar cells.

But consistency has been a major issue. Scientists have, until now, struggled to understand why some of the molecules worked unexpectedly well, while others perform indifferently.

Researchers from Cambridge’s Cavendish Laboratory developed sensitive laser-based techniques to track the motion and interaction of electrons in these cells. To their surprise, the team found that the performance differences between materials could be attributed to the quantum property of ‘spin’.

‘Spin’ is a property of particles related to their angular momentum, with electrons coming in two flavours, ‘spin-up’ or ‘spin-down’. Electrons in solar cells can be lost through a process called ‘recombination’, where electrons lose their energy — or “excitation” state — and fall back into an empty state known as the “hole.”

Researchers found that by arranging the electrons ‘spin’ in a specific way, they can block the energy collapse from ‘recombination’ and increase current from the cell.

“This discovery is very exciting, as we can now harness spin physics to improve solar cells, something we had previously not thought possible. We should see new materials and solar cells that make use of this very soon” said Dr. Akshay Rao, a Research Fellow at the Cavendish Laboratory and Corpus Christi College, Cambridge, who lead the study with colleagues Philip Chow and Dr. Simon Gélinas.

The Cambridge team believe that design concepts coming out of this work could help to close the gap between organic and silicon solar cells, bringing the large-scale deployment of solar cells closer to reality. In addition, some of these design concepts could also be applied to Organic Light Emitting diodes, a new and rapidly growing display technology, allowing for more efficient displays in cell phones and TVs

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.

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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.

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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