Thin Films Solar Cells on Flexible Substrates


Carbon Nanotube

Background

Thin film silicon solar cells are classified into p-i-n and n-i-p configurations which refer to their deposition sequence; n-i-p processing starts with the n-layer which is normally grown on a metallic back contact. Historically this configuration is connected to flexible substrates because it was used on opaque substrates or poorly transparent substrates like steel foils or high temperature polymers. However, the configuration is not limited to this choice, it is in fact compatible with any kind of substrates, such as rigid or flexible, transparent or opaque. Nevertheless, flexible substrates have remained the main application of n-i-p cells because roll-to-roll processing makes them very interesting to reduce the production costs as well as the energy payback time, particularly when low cost substrates like poly-ethylene are used.

The activities of the n-i-p group combine general aspects of thin film silicon solar cells with special requirements that are imposed by the deposition sequence and the desired compatibility with low temperature substrates. Two main lines of work can be distinguished: .    Substrate texturing .    Light scattering and absorption enhancement

These two combined lines result in, for example, high efficiency triple junctions cell on innovative flat light scattering substrate presented in the last section.

Towards a more fundamental understanding of absorption enhancement in solar cells, we fabricate cells on periodic gratings that permit the study of coupling into guided modes [1].

We obtained fully flexible solar cells on a low cost poly-ethylene substrate with a stabilized efficiency of 9.8% for 0.25cm2 laboratory cells [2].

Research highlights

Substrate texturing

Amorphous and microcrystalline silicon are poor absorbers, particularly for light with energies just above their respective band gaps. Some means of absorption enhancement is required which is commonly called light trapping. It can be achieved by texturing of the interfaces. A common approach for n-i-p cells are back contacts made from so called “hot silver”, which is the texture that silver develops by partial recrystallization during growth on a heated substrate. Unfortunately this is too hot for poly-ethylene, we have to devise other ways. We investigate the incorporation of texture into the substrate itself, during cell fabrication this texture is carried into the other interfaces because of conformal coverage.

Periodic substrate textures

We obtained promising results on low cost poly-ethylene substrates like the one shown below. The substrate texture has been manufactured by a commercial manufacturer.

 

The used texture is periodic with a simple sinusoidal wave pattern, thus the incoming light is diffracted into the rainbow colours. For use in solar cells the substrate is coated with back reflector consisting of silver and zinc-oxide. The right figure shows that the back reflector reproduces the substrate texture, but silicon with a thickness comparable to the period already modifies significantly the sinusoidal wave into something that resembles an inverse cycloid.
More information

Embossing

In addition to the substrate shown above, we also investigate embossing processes in order to manufacture novel substrates textures.

 

The figure above illustrates the embossing process; starting from a master substrate, a negative mold is formed in a polymer (PDMS), then the mold is brought into contact with a UV-sensible lacquer on a substrate. After curing by UV expose it is demolded and the initial texture is reproduced on the substrate.

 

The process permits, among other options, to reproduce textures that require high temperature processes like hot silver on any other substrate, for example poly-ethylene substrate.     This process reproduces the initial texture with high fidelity. The image above compares AFM surface morphologies of a ZnO master (left) and a replica (right). Features with size below 100 nm are well reproduced.
More information: articles K. Söderström et.al. and J. Escarré et.al.

Light scattering and absorption enhancement

Absorption enhancement in silicon by light scattering at textured interfaces has been proposed as early as 1982 by E. Yablonovitch. The idea is the following; take a slab of silicon with textured surface and shine weakly absorbed light on it. The transmitted light will be scattered at the surface roughness, some of it into angles above the Brewster angle. This part will bounce back and forth within the slab by total internal reflection. It would thus be trapped until its complete absorption, except that each bounce scatters a certain part out of the slab. The amount of light trapping thus depends on the angular width of this so-called escape cone. This can be related to an average light path enhancement of 4 times the square of the refractive index which is about 60 for silicon. There are a few underlying assumptions that are quite difficult to realize. Despite a significant amount of research over the years, the path enhancement in current cells is more likely to be between 20 and 30, and it is still an open question how Yablonovitch’s limit can be reached. Part of the work in the n-i-p group is devoted to the investigation of such fundamental questions, but always keeping in mind the application in real devices.

Plasmons and guided modes

A light beam that bounces back and forth within a slab by total internal reflection is not an unknown concept in optics, in fact this a simplified description of waveguides. An alternate view on light trapping is thus simply the question of how efficient we can couple an incoming plane wave to guided modes in the silicon layer. So far there appears little relation with plasmons, but remember that plasmon polaritons (as they should be called in this context) are just waves that propagate parallel to the interface in a multilayer structure; therefore the title of this section.

   

Waveguides are often discussed in terms of dispersion diagrams where the photon energy is plotted against the momentum p (or the wave vector k). For photons these two quantities are related by the speed of light, thus they are represented by straight lines in such a diagram. Note that perpendicular incidence would mean a line that falls on the energy axis. The indicated light lines represent grazing propagation parallel to the interfaces; there is a slight curvature because the refractive index depends on energy.
The diagram to the left shows the modal structure of a 200 nm thick a-Si “waveguide” between air and a zinc-oxide substrate. Such an asymmetric structure is known to have a cut-off, i.e. no guided wave can propagate at energies below 0.25 eV. Between 0.25 and 0.75 eV, only the fundamental mode of s-polarization (s0) can be guided, between 0.75 and 1.25 it can guide two modes (s0 and p0), and for higher energies more and more modes appear. All of these modes are confined between the light lines of silicon and zinc-oxide.
The diagram to the right shows a yet more unusual configuration consisting of a 200 nm thick silicon “waveguide” between a silver “cladding” and air. Most of the modes resemble the waveguide modes of the left image, only that they extend a little further to the left, going as far as the light line of air. Only the lowest energy mode behaves a little strange; it is p-polarized, it has no cut-off, and it runs below the light line of silicon. This particular p0 mode is called plasmon polariton. As mentioned above, it does not propagate in a guiding medium but on the interface between two media.
More information

Light trapping and guided modes

More evidence for the correspondence between light trapping and guided modes was produced in the following experiment: Solar cells were fabricated on a substrate textured with a 1D sinusoidal grating with known period. Such a periodicity folds the above diagrams into Brillouin zones and perpendicular incidence can be represented by vertical lines emerging from the centre of each Brillouin zone. Whenever the characteristics of guided modes and such a vertical line intersects, coupling becomes possible. The excitation of guided modes should thus be visible for specific energies in the form of sharp resonance phenomena. For the 1D grating there should be an additional dependence on the polarization of the incident light.

 

The figure shows the external quantum efficiencies of cells on the grating and a flat reference substrate. Note that there are sharp resonances between 600 and 750 nm. The observed polarization dependence and their variation with changes of the angle of incidence further support the idea of guided mode excitation.
More information

High efficiency triple junctions cell on innovative flat light scattering substrate

To reconcile the opposing requirements of layer growth and light scattering which need flat and rough interfaces, respectively, the separation of the light-scattering interface from the growth interface would be of high interest. With this new approach, light scattering is promoted by a textured layer with a low index of refraction filled with a material with a higher refractive index. This stack is then polished to obtain a flat substrate onto which the cell is grown.

We first fabricated this type of substrate as shown in the figure above and experimentally studied them in single-junction, thick µc-Si:H solar cells [Söderström Solmat 2012]. In second we have been able to fully exploit the potential of these substrates to lead to high efficiency solar cells by growing triple-junctions a-Si:H/µc-Si:H/µc-Si:H in nip configuration. This solar cell exhibits efficiencies of 13.7% in the initial state and 12.5% after degradation as shown in the below. The efficiency after degradation is among the highest reported to this date for purely silicon based n-i-p thin film solar cells [Söderström accepted for publication in JAP].

 

Key publications

[1] K. Söderström, G. Bugnon, F.-J. Haug, S. Nicolay and C. Ballif, Experimental study of flat light-scattering substrates in thin-film silicon solar cells, Solar Energy Materials and Solar Cells, vol. 101, p. 193-199, Elsevier, 2012
[2] K. Söderström, F.-J. Haug, J. Escarré, O. Cubero, C. Ballif, Photocurrent increase in n-i-p thin film silicon solar cells by guided mode excitation via grating coupler, Applied Physics Letters 96, 213508, 2010
[3] T. Söderström, F.-J. Haug, X. Niquille, V. Terrazzoni, and C. Ballif, Asymmetric intermediate reflector for tandem micromorph thin film silicon solar cells, Appl. Phys. Lett., Vol 94 , pp. -063501, 2009
[4] F.-J. Haug, T. Söderström, O. Cubero, V. Terrazzoni-Daudrix, X. Niquille, S. Perregeaux, and C. Ballif, Periodic textures for enhanced current in thin film silicon solar cells, Presented at the MRS Spring Meeting, San Francisco, 2008
[5] T. Söderström, F.-J. Haug, V. Terrazzoni-Daudrix, X. Niquille, M. Python and C. Ballif, N/I buffer layer for substrate microcrystalline thin film silicon solar cell, Journal of Applied Physics, Vol 104, pp. -104505, 2008

 

Quantum Dots that Assemble Themselves


QDOTS imagesCAKXSY1K 8A paper on the new technology, “Self-assembled Quantum Dots in a Nanowire System for Quantum Photonics,” appears in the current issue of the scientific journal Nature Materials. Quantum dots are tiny crystals of semiconductor a few billionths of a meter in diameter. At that size they exhibit beneficial behaviors of quantum physics such as forming electron-hole pairs and harvesting excess energy.

The scientists demonstrated how quantum dots can self-assemble at the apex of the gallium arsenide/aluminum gallium arsenide core/shell nanowire interface. Crucially, the quantum dots, besides being highly stable, can be positioned precisely relative to the nanowire’s center. That precision, combined with the materials’ ability to provide quantum confinement for both the electrons and the holes, makes the approach a potential game-changer.

Electrons and holes typically locate in the lowest energy position within the confines of high-energy materials in the nanostructures. But in the new demonstration, the electron and hole, overlapping in a near-ideal way, are confined in the quantum dot itself at high energy rather than located at the lowest energy states. In this case, that’s the gallium-arsenide core. It’s like hitting the bulls-eye rather than the periphery.

The quantum dots, as a result, are very bright, spectrally narrow and highly anti-bunched, displaying excellent optical properties even when they are located just a few nanometers from the surface – a feature that even surprised the scientists. “Some Swiss scientists announced that they had achieved this, but scientists at the conference had a hard time believing it,” said NREL senior scientist Jun-Wei Luo, one of the co-authors of the study. Luo got to work constructing a quantum-dot-in-nanowire system using NREL’s supercomputer and was able to demonstrate that despite the fact that the overall band edges are formed by the gallium Arsenide core, the thin aluminum-rich barriers provide quantum confinement both for the electrons and the holes inside the aluminum-poor quantum dot. That explains the origin of the highly unusual optical transitions.

Several practical applications are possible. The fact that stable quantum dots can be placed very close to the surface of the nanowires raises a huge potential for their use in detecting local electric and magnetic fields. The quantum dots also could be used to charge converters for better light-harvesting, as in the case of photovoltaic cells.

The team of scientists working on the project came from universities and laboratories in Sweden, Switzerland, Spain, and the United States.

More information: http://www.nature.com/nmat/journal/vaop/ncurrent/fig_tab/nmat3557_F4.htmlJournal reference: Nature Materials Provided by National Renewable Energy Laboratory

Read more at: http://phys.org/news/2013-04-team-quantum-dots.html#jCp

Nanotechnology helps packaging smarten up


Christian Raaflaub, swissinfo.ch (translated from German text by Simon Bradley) Mar 25, 2013 – 11:00

QDOTS imagesCAKXSY1K 8In the near future packaging will be much more than a simple external protective wrapper or support for product advertising. Biotechnologist Christoph Meili outlines the major role nanotechnology will play.

 

 

The Swiss packaging industry, which represents about 250 firms and 19,000 workers, accounts for about 1.1 per cent of national annual gross domestic product (GDP), or CHF586 billion – a much higher figure than in other countries. Of this, CHF3.6 billion comes from plastics, while CHF1 billion is from cardboard sales. Swiss firms include the Model Group, Bourquin and SIG.

swissinfo.ch: You argue that the future of packaging will be closely tied to advances in nanotechnology. What additional properties can nanotechnology offer?

Christoph Meili: Nanotechnology is all about enhancement. Certain properties that are very much in demand can be introduced into packing using nano-materials. Here I’m talking mostly about the shelf life of food, which can be extended. The amount of information and the quality of information on packaging will also increase. The consumer will learn about the state of the product, whether the food is still edible or if there is oxygen present in the packaging, for example. Hopefully, this will also lead to a conservation of resources so that better biodegradable packaging is developed.

Possible applications in packaging

– anti-microbial properties: nano-silver, nanoparticles with zinc, calcium magnesium oxide or titanium dioxide, essential oils or wasabi-coated film. – gas barrier properties: “Nanoclay” layers to better seal PET plastic bottles. – brand protection: traceability  – nanometre barcodes made of nano-silver or gold particles. – environmentally friendly packaging materials: biodegradable packaging based on natural polymer compounds or nano-sized starches from corn. (Source: Christoph Meili)

swissinfo.ch: Are there any smart alternatives to packaging so that we use less in the future?

C.M.: Edible packaging is something on our radar screens. On the other hand it’s important for consumers to be able to differentiate clearly between the product and the packaging. Compostable or biodegradable packaging would be a major step forward. I have my doubts whether this could also be edible. But a product without packaging would be the very best solution.

swissinfo.ch: Nowadays many products are tossed away although they are still usable.

C.M.: Nanotechnology should help us reduce the amount of food that is thrown away. Around one third of all purchased food is thrown away as consumers believe it’s no longer any good or because the sell-by-date has passed. In Switzerland this represents around two million tonnes of food, or 1.3 billion tonnes worldwide. That’s a tremendous waste.

swissinfo.ch: How can ‘smart’ packaging help solve this problem?

C.M.: Smart packaging is able to react to ambient conditions, for example, when the gas mixture in a product changes such as when carbon dioxide or nitrogen are released and escape from the packaging’s protective atmosphere. The second example is self-protecting packaging, which protects itself against oxygen for example. Then there are indicators about the levels of moisture or bacterial and microbial decomposition, which show when the product is no longer usable. Nano-silver, titanium dioxide or zinc oxide slow the growth of bacteria on products, thus extending their shelf-life and freshness.

Christoph Meili

Christoph Meili is founder and head of the Innovation Society.
He studied Biotechnology at the Swiss Federal Institute of Technology (ETH) in Zurich and Business Administration at the University of St.Gallen, where he achieved his PhD. He was head of the Competence Center for Risks of biotechnology, genetic engineering, and nanotechnology at the Institute of Insurance Economics of the University of St.Gallen.
In addition he has considerable consulting experience in the risk management of emerging technologies. He is an innovation and risk expert in applied nano- and micro technology.
He is also a senior lecturer in business administration and management at the University of St.Gallen.

swissinfo.ch: But how practical and realistic are all these new ideas?

C.M.: They really do offer something extra. For example, you have beer bottles with a ‘nano-clay’ plastic layer which slow the leakage of gas and entry of oxygen. This allows products to be kept much longer.  This is really interesting for manufacturers as they can lower costs. On the one hand fewer products are withdrawn and on the other consumers can keep them longer.

swissinfo.ch: What will the packing of the future look like? Will every product contain a chip with accessible information on the packaging?

C.M.: The ability to identify products using a Radio-frequency Identification Device (RFID) is definitely an issue right now. It’s practical for retailers. There are several options already available. The problem is the price. Packaging must be very cheap. RFID is currently only offered on higher-priced items or on large quantities, which enable products to be traced. Trademark protection becomes easier.

swissinfo.ch: You are a biotechnologist and a molecular biologist but you also work as a risk researcher. What are the possible future health threats of integrating minute nanoparticles in packaging?

C.M.: The question we have to ask ourselves is: can nanoparticles escape from packaging? If so, where do they go? Into food, or are they dispersed into the environment? Or do they transform into something else in the biological cycle? In active packaging, where a considerable part involves active elements escaping and interacting with foodstuffs, we have to look at what is happening. The migration and dispersion of low-molecular substances is an issue. Substances that are dangerous for your health shouldn’t end up in the product. But it’s a difficult issue. Where should the limit be set? What level is still safe? What is problematic? Also, alongside respiration, it’s possible for nanoparticles to be absorbed into the digestive system. A lot of research is still needed on this. We have to honest about that.

Christian Raaflaub, swissinfo.ch (translated from German text by Simon Bradley)

NREL and Partners Demonstrate Quantum Dots that Assemble Themselves


Surprising breakthrough could bolster quantum photonics, solar cell efficiency

February 8, 2013

QDOTS imagesCAKXSY1K 8Scientists from the U.S. Department of Energy’s National Renewable Energy Laboratory and other labs have demonstrated a process whereby quantum dots can self-assemble at optimal locations in nanowires, a breakthrough that could improve solar cells, quantum computing, and lighting devices.

 

A paper on the new technology, “Self-assembled Quantum Dots in a Nanowire System for Quantum Photonics,” appears in the current issue of the scientific journal Nature Materials.

Quantum dots are tiny crystals of semiconductor a few billionths of a meter in diameter.  At that size they exhibit beneficial behaviors of quantum physics such as forming electron-hole pairs and harvesting excess energy.

The scientists demonstrated how quantum dots can self-assemble at the apex of the gallium arsenide/aluminum gallium arsenide core/shell nanowire interface. Crucially, the quantum dots, besides being highly stable, can be positioned precisely relative to the nanowire’s center. That precision, combined with the materials’ ability to provide quantum confinement for both the electrons and the holes, makes the approach a potential game-changer.

Electrons and holes typically locate in the lowest energy position within the confines of high-energy materials in the nanostructures. But in the new demonstration, the electron and hole, overlapping in a near-ideal way, are confined in the quantum dot itself at high energy rather than located at the lowest energy states. In this case, that’s the gallium-arsenide core. It’s like hitting the bulls-eye rather than the periphery.

The quantum dots, as a result, are very bright, spectrally narrow and highly anti-bunched, displaying excellent optical properties even when they are located just a few nanometers from the surface – a feature that even surprised the scientists.

“Some Swiss scientists announced that they had achieved this, but scientists at the conference had a hard time believing it,” said NREL senior scientist Jun-Wei Luo, one of the co-authors of the study. Luo got to work constructing a quantum-dot-in-nanowire system using NREL’s supercomputer and was able to demonstrate that despite the fact that the overall band edges are formed by the gallium Arsenide core, the thin aluminum-rich barriers provide quantum confinement both for the electrons and the holes inside the aluminum-poor quantum dot. That explains the origin of the highly unusual optical transitions.

Several practical applications are possible. The fact that stable quantum dots can be placed very close to the surface of the nanometers raises a huge potential for their use in detecting local electric and magnetic fields. The quantum dots also could be used to charge converters for better light-harvesting, as in the case of photovoltaic cells.

The team of scientists working on the project came from universities and laboratories in Sweden, Switzerland, Spain, and the United States.

NREL is the U.S. Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for DOE by the Alliance for Sustainable Energy, LLC.

###

Visit NREL online at www.nrel.gov

The nanomechanical signature of breast cancer


Using ARTIDIS to feel the tissue structure of a tumor biopsy by a nanometer-sized atomic force microscope tip. Image: Martin Oeggerli

 

 

 

 

 

 

Using ARTIDIS to feel the tissue structure of a tumor biopsy by a nanometer-sized atomic force microscope tip. Image: Martin Oeggerli

The spread of cancer cells from primary tumors to other parts of the body remains the leading cause of cancer-related deaths. The research groups of Roderick Lim and Cora-Ann Schoenenberger from the Biozentrum of the University of Basel, reveal in the journal Nature Nanotechnology how the unique nanomechanical properties of breast cancer cells are fundamental to the process of metastasis. The discovery of specific breast cancer “fingerprints” was made using breakthrough nanotechnology known as ARTIDIS. Lim’s team has now been awarded about 1.2 million Swiss francs from the Commission for Technology and Innovation (CTI) to further develop ARTIDIS.
Breast cancer is the most common form of cancer in women with 5,500 patients being diagnosed with the disease in Switzerland each year. Despite major scientific advancements in our understanding of the disease, breast cancer diagnostics remains slow and subjective. Here, the real danger lies in the lack of knowing whether metastasis, the spread of cancer, has already occurred. Nevertheless, important clues may be hidden in how metastasis is linked to specific structural alterations in both cancer cells and the surrounding extracellular matrix. This forms the motivation behind ARTIDIS (“Automated and Reliable Tissue Diagnostics”), which was conceived by Dr. med. Marko Loparic, Dr. Marija Plodinec and Prof. Roderick Lim to measure the local nanomechanical properties of tissue biopsies.

Fingerprintingbreast tumors
At the heart of ARTIDIS lies an ultra-sharp atomic force microscope tip of several nanometers in size that is used as a local mechanical probe to “feel” the cells and extracellular structures within a tumor biopsy. In this way, a nanomechanical “fingerprint” of the tissue is obtained by systematically acquiring tens of thousands of force measurements over an entire biopsy.

Subsequent analysis of over one hundred patient biopsies could confirm that the fingerprint of malignant breast tumors is markedly different as compared to healthy tissue and benign tumors. This was validated by histological analyses carried out by clinicians at the University Hospital Basel, which showed a complete agreement with ARTIDIS. Moreover, the same nanomechanical fingerprints were found in animal studies initiated at the Friedrich Miescher Institute.

Plodinec, first author of the study, explains: “This unique fingerprint reflects the heterogeneous make-up of malignant tissue whereas healthy tissue and benign tumors are more homogenous.” Strikingly, malignant tissue also featured a marked predominance of “soft” regions that is a characteristic of cancer cells and the altered microenvironment at the tumor core. The significance of these findings lies in reconciling the notion that soft cancer cells can more easily deform and “squeeze” through their surroundings. Indeed, the presence of the same type of “soft” phenotype in secondary lung tumors of mice reinforces the close correlation between the physical properties of cancer cells and their metastatic potential.

ARTIDIS in the clinics
“Resolving such basic scientific aspects of cancer further underscores the use of nanomechanical fingerprints as quantitative markers for cancer diagnostics with the potential to prognose metastasis,” states Loparic, who is project manager for ARTIDIS. On an important practical note, a complete biopsy analysis by ARTIDIS currently takes four hours in comparison to conventional diagnostics, which can take one week. Based on the potential societal impact of ARTIDIS to revolutionize breast cancer diagnostics, Lim’s team and the Swiss company Nanosurf AG have now been awarded about 1.2 million Swiss francs by the Commission for Technology and Innovation (CTI) to further develop ARTIDIS into a state-of-the-art device for disease diagnostics with further applications in nanomedicine.

Over the next two years, Lim and colleagues will engage and work closely with clinicians to develop ARTIDIS into an easy-to-use “push-button” application to fingerprint diseases across a wide range of biological tissues. As a historical starting point, the first ARTIDIS demo-lab has already been established at the University Hospital Eye Clinic to collect data on retinal diseases with the goal of improving treatment strategies.

The nanomechanical signature of breast cancer

Source: University of Basel