Mass producing pocket labs


mix-id328072.jpg(Nanowerk News) There is certainly no shortage of  lab-on-a-chip (LOC) devices, but in most cases manufacturers have not yet found  a cost-effective way to mass produce them. Scientists are now developing a  platform for series production of these pocket laboratories.
Ask anyone to imagine what a chemical analysis laboratory looks  like, and most will picture the following scene: a large room filled with  electrical equipment, extractor hoods and chemical substances, in which  white-robed researchers are busy unlocking the secrets behind all sorts of  scientific processes. But there are also laboratories of a very different kind,  for instance labs-on-a-chip (LOCs). These “pocket labs” are able to  automatically perform a complete analysis of even the tiniest liquid samples,  integrating all the required functions onto a chip that’s just a few centimeters  long. Experts all over the world have developed many powerful LOC devices in  recent years, but very few pocket labs have made it onto the market.
Scientists at the Fraunhofer Institute for Production Technology  IPT in Aachen want to find out why so many LOCs are not a commercial success.  They are working with colleagues from polyscale GmbH & Co. KG, an IPT  spin-off, and ten other industrial partners from Germany, Finland, Spain, the  United Kingdom, France and Italy on ways to make LOCs marketable. Their ML²  project is funded by the EU’s Seventh Framework Programme (FP7), which is  providing a total of 7.69 million euros in funding through fall 2016.
“One of the main reasons LOCs don’t make it to market is that  the technologies used to fabricate them are often not transferrable to  industrial-scale production,” says Christoph Baum, group manager at the IPT.  What’s more, it is far from easy to integrate electrical functions into pocket  labs, and of the approaches taken to date, none has yet proved suitable for mass  production.
Microfluidic negative for structuring films
Microfluidic negative for structuring films. (© Fraunhofer IPT)
Platform for series production
The ML² project aims to completely revise the way pocket labs  are made so they are more suited to series production. “Our objective is to  create a design and production platform that will enable us to manufacture all  the components we need,” says Baum. This includes producing the tiny channel  structures within which liquids flow and react with each other, and coating the  surfaces so that bioactive substances can bond with them. Then there are optical  components, and electrical circuits for heating the channels, for example. The  experts apply each of these components to individual films that are then  assembled to form the complete “laboratory”. The films are connected to one  another via vertical channels machined through the individual layers using a  laser.
The first step the researchers have taken is to adapt and modify  the manufacturing process for each layer to suit mass-production requirements.  When it comes to creating the channel structures, the team has moved away from  the usual injection molding or wet chemical processing techniques in favor of  roll-to-roll processing. This involves transferring the negative imprint of the  channels onto a roller to create an embossing cylinder that then imprints a  pattern of depressions on a continuous roll of film. The electrical circuits are  printed onto film with an inkjet printer using special ink that contains copper  or silver nanoparticles.
Each manufacturing stage is fine-tuned by the researchers in the  process of producing a number of demonstrator LOCs – for instance a pregnancy  test with a digital display. These tests are currently produced in low-wage  countries, but with increased automation set to slash manufacturing costs by up  to 50 percent in future, production would once again be commercially viable in a  high-wage country such as Germany. The team aims to have all the demonstrators  built and the individual manufacturing processes optimized by 2014. Then it will  be a case of fitting the various steps in the manufacturing process together,  making sure they match up, and implementing the entire sequence on an industrial  scale.
Source: Fraunhofer-Gesellschaft

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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

 

Plastic electronics made easy


QDOTS imagesCAKXSY1K 8(Nanowerk News)  Scientists have discovered a way to  better exploit a process that could revolutionise the way that electronic  products are made.
The scientists from Imperial College London say improving the  industrial process, which is called crystallisation, could revolutionise the way  we produce electronic products, leading to advances across a whole range of  fields; including reducing the cost and improving the design of plastic solar  cells.
The process of making many well-known products from plastics  involves controlling the way that microscopic crystals are formed within the  material. By controlling the way that these crystals are grown engineers can  determine the properties they want such as transparency and toughness.  Controlling the growth of these crystals involves engineers adding small amounts  of chemical additives to plastic formulations. This approach is used in making  food boxes and other transparent plastic containers, but up until now it has not  been used in the electronics industry.
The team from Imperial have now demonstrated that these  additives can also be used to improve how an advanced type of flexible circuitry  called plastic electronics is made.
The team found that when the additives were included in the  formulation of plastic electronic circuitry they could be printed more reliably  and over larger areas, which would reduce fabrication costs in the industry.
The team reported their findings this month in the journal  Nature Materials (“Microstructure formation in molecular and polymer  semiconductors assisted by nucleation agents”).
Dr Natalie Stingelin, the leader of the study from  the Department of Materials and Centre of Plastic Electronics at Imperial, says:
“Essentially, we have demonstrated a simple way to gain control  over how crystals grow in electrically conducting ‘plastic’ semiconductors. Not  only will this help industry fabricate plastic electronic devices like solar  cells and sensors more efficiently. I believe it will also help scientists  experimenting in other areas, such as protein crystallisation, an important part  of the drug development process.”
Dr Stingelin and research associate Neil Treat looked at two  additives, sold under the names IrgaclearÒ XT 386 and MilladÒ 3988, which are  commonly used in industry. These chemicals are, for example, some of the  ingredients used to improve the transparency of plastic drinking bottles. The  researchers experimented with adding tiny amounts of these chemicals to the  formulas of several different electrically conducting plastics, which are used  in technologies such as security key cards, solar cells and displays.
The researchers found the additives gave them precise control  over where crystals would form, meaning they could also control which parts of  the printed material would conduct electricity. In addition, the  crystallisations happened faster than normal. Usually plastic electronics are  exposed to high temperatures to speed up the crystallisation process, but this  can degrade the materials. This heat treatment treatment is no longer necessary  if the additives are used.
Another industrially important advantage of using small amounts  of the additives was that the crystallisation process happened more uniformly  throughout the plastics, giving a consistent distribution of crystals.  The team  say this could enable circuits in plastic electronics to be produced quickly and  easily with roll-to-roll printing procedures similar to those used in the  newspaper industry. This has been very challenging to achieve previously.
Dr Treat says: “Our work clearly shows that these additives are  really good at controlling how materials crystallise. We have shown that printed  electronics can be fabricated more reliably using this strategy. But what’s  particularly exciting about all this is that the additives showed fantastic  performance in many different types of conducting plastics. So I’m excited about  the possibilities that this strategy could have in a wide range of materials.”
Dr Stingelin and Dr Treat collaborated with scientists from the  University of California Santa Barbara, and the National Renewable Energy  Laboratory in Golden, US, and the Swiss Federal Institute of Technology on this  study. The team are planning to continue working together to see if subtle  chemical changes to the additives improve their effects – and design new  additives.
They will be working with the new Engineering and Physical  Sciences Research Council (EPSRC)-funded Centre for Innovative Manufacturing in  Large Area Electronics in order to drive the industrial exploitation of their  process. The £5.6 million of funding for this centre, to be led by researchers  from Cambridge University, was announced earlier this year. They are also  exploring collaborations with printing companies with a view to further  developing their circuit printing technique.
Controlling crystals
Here are some of the technologies that could benefit from Drs  Treat and Stingelin’s research:
Improving drugs
Most drugs work by blocking or activating proteins in our  bodies. To develop better drugs, scientists must understand what these proteins  look like. The work carried out by the Imperial team could enable researchers in  the future to develop more accurate models of proteins, by converting them into  a crystalline form.
More efficient solar technology
Solar cells are made from a solid mixture of electrically  conducting crystalline chemicals. Currently these cells only convert about 10%  of the Sun’s energy into electricity. Dr Treat and Stingelin’s additives may  provide a way of improving crystal growth in solar cells, which could improve  the amount of energy they convert.
New flexible electronics
Flexible semiconductor films can be made by methods such as  inkjet printing. Using additives that control how inkjet-printed droplets of  semiconductors crystallise will mean they crystallise in evenly distributed  patterns that conduct electricity efficiently. This means industry can produce  these printed electronics more easily and cheaply.
Source: By Joshua Howgego, Imperial College London

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