Making Inorganic Solar Cells with an Airbrush Spray


 

Nano Particles for Steel 324x182(Nanowerk Spotlight) There is currently a tremendous  amount of interest in the solution processing of inorganic materials. Low cost,  large area deposition of inorganic materials could revolutionize the fabrication  of solar cells, LEDs, and photodetectors. The use of inorganic nanocrystals to  form these structures is an attractive route as the ligand shell that surrounds  the inorganic core allows them to be manipulated and deposited using organic  solvents.

The most common methods currently used for film formation are  spin coating and dip coating, which provide uniform thin films but limit the  geometry of the substrate used in the process. The same nanocrystal solutions  used in these procedures can also be sprayed using an airbrush, enabling larger  areas and multiple substrates to be covered much more rapidly.

The trade-off is  the roughness and uniformity of the film, both of which can be substantially  higher.    Reporting their findings in a recent online edition of ACS  Applied Materials & Interfaces (“Inorganic Photovoltaic Devices Fabricated Using  Nanocrystal Spray Deposition”), researchers have now attempted to quantify  these differences for a single-layer solar cell structure, and found the main  difference to be a reduction in the open circuit voltage of the device.            deposited films of CdTe nanocrystals SEM  images of the top surface of the deposited films following deposition and  sintering, showing (a) CdTe spin coated and (b) CdTe spray coated. The scale bar  in both images represents 200 nm. (Reprinted with permission from American  Chemical Society)

“Our work was motivated by a desire to coat larger substrate  areas more efficiently,” Edward Foos, a research scientists in the Materials  Synthesis and Processing Section of the Chemistry Division at the Naval  Research Laboratory, and first author of the paper, tells Nanowerk. “Our initial  work indicated that if the layers were thick enough to cover the substrate  completely and avoid pinhole formation that would lead to shorting of the  device, then the increased surface roughness might be tolerable.”

He adds that this is the first time the impact of this surface  roughness on the performance characteristics has been directly compared for  these types of devices.

The team prepared single-layer Schottky-barrier solar cells  using spray deposition of inorganic (CdTe) nanocrystals with an airbrush. The  spray deposition results in a rougher film morphology that manifests itself as a  2 orders of magnitude higher saturation current density compared to spin  coating.   “We’re currently working to improve the spray coating process to  improve the layer uniformity,” says Foos. “If the surface roughness can be  reduced, then the overall device performance should increase.”   The team is confident that further optimization of the spray  process to reduce this surface roughness and limit the Voc suppression should be possible and eventually lead  to comparable performances between the two deposition techniques.   “Importantly” Foos points out, “the spray-coating process  enables larger areas to be covered more efficiently, reducing waste of the  active layer components, while enabling deposition on asymmetric substrates.

These advantages should be of substantial interest as inorganic  nanocrystal-based solar cells become increasingly competitive as  third-generation devices.”   The team’s next step will be the fabrication of more complex  device architectures that incorporate multiple solution processed layers. These  structures will have an even smaller tolerance for variation. In addition, the  deposition chemistry used must not interfere with the material applied in the  previous step.

By Michael Berger. Copyright © Nanowerk

Read more: http://www.nanowerk.com/spotlight/spotid=32458.php#ixzz2fyNZ5tzG

 

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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

 

“Spin States” and Quantum Dots and Why … It’s Important


QDOTS imagesCAKXSY1K 8New experiments show how to optically initialize a specific spin state of a single manganese atom placed inside a quantum dot for spintronics applications.

Spin dynamics of a Mn atom in a semiconductor quantum dot under resonant optical excitation S. Jamet, H. Boukari, and L. Besombes

Published June 10, 2013 | PDF (free)
8447cb8b9e31ef1b The ability to control individual spins (the intrinsic units of angular momentum carried by electrons) in semiconductors is an important requirement for a new generation of devices based on spin rather than charge logic. Single magnetic ions are promising for this type of application because of their long spin coherence times.

 

Figure 1 Scheme of the optical transitions in a quantum dot containing an individual magnetic atom and 0 (Mn ), 1 (X−Mn ), or 2 (X2−Mn ) excitons. The exciton states are split by the exchange interaction with the Mn spin, whereas in the ground (Mn ) and biexciton (X2−Mn ) states, the energy levels result from the fine and hyperfine structure of the Mn spin. Spin-population trapping on the 5/2 level is achieved by carefully tuning a resonant CW laser (green arrow). Spin readout: A direct resonant excitation of the biexciton is performed by a pulsed two-photon absorption through an intermediate virtual state (blue arrows). The biexciton photon emission allows monitoring all six Mn spin levels simultaneously.

The difficulty lies in addressing these stable but isolated spins, a feat that can be achieved by placing a single magnetic ion like manganese (Mn ) inside a semiconductor quantum dot [1]. This arrangement strongly mixes the states of the Mn spin and the charge carriers trapped inside the nano-object. As a result, optical initialization and readout of an Mn spin state can be achieved [2] using resonant laser excitation, as employed previously in single-electron and hole spin-pumping schemes used in quantum dots [3, 4]. A key feature of these established spin-pumping techniques is a depletion of the spin level that is resonantly excited, which may at first seem counterintuitive.

Segolene Jamet and colleagues at the French National Center for Scientific Research (CNRS) and Joseph Fourier University, France, have now experimentally demonstrated a new spin-population-trapping scheme for a single Mn spin state [5] monitored via a new readout technique (Fig. 1). Writing in Physical Review B, they show how the Mn atom is directly pumped into the spin state that is resonantly excited, in stark contrast to existing methods [2, 3, 4]. Tuning the laser energy and power allows varying the strength of the coherent coupling between Mn spin levels that is at the origin of the spin-population trapping.

Researchers have demonstrated efficient optical and electrical control of individual electron and nuclear spin states in semiconductor quantum dots that allow for the integration with standard semiconductor circuits [6]. Placing an isolated Mn ion at a fixed lattice position inside a quantum dot adds important new opportunities for spintronics because of the possibility of manipulating Mn spins in this environment with well-established optical and electrical quantum-dot control techniques [7, 8]. When a single Mn
impurity is introduced into a II-VI quantum-dot material like CdTe, as used by Jamet et al. [5], an interesting situation arises. An Mn atom has five electrons on the d shell, which results in a possible total spin of up to S=5/2 (in units of ħ ). Just as a free electron can have a spin (projected on a given axis) of +1/2 or −1/2 , there are six different spin states for a Mn atom from −5/2 , −3/2 , −1/2 , … up to +5/2 .

Storing information in these six spin states can be thought of as a “quantum die,” as opposed to a two-quantum-level system called “quantum bit.” Therefore the system investigated by Jamet et al. is interesting in the context of quantum computing because 15 pairs of different quantum bits can be defined for a single Mn impurity.

A general advantage of an isolated Mn atom in a semiconductor matrix is that these spin states can be well separated from each other in energy, even in modest magnetic fields, through what is termed the “giant Zeeman effect” [9]. Applying magnetic fields is not always feasible in real-life applications, therefore Jamet et al. lift the spin-state degeneracy through photoexcitation: The optically created electron-hole pair (exciton) strongly interacts with the 5d electrons of the Mn via an effect known as the Coulomb exchange interaction. As a result the quantum-dot emission, usually one single line, is split into six well-separated components. In the absence of Mn spin pumping, such as that applied by Jamet et al., the spin state of the Mn atom following excitation of the dot by a nonresonant laser is completely arbitrary. As a result, all six possible lines are observed in the time-averaged optical spectra [1].

Using laser excitation that is slightly off resonance with respect to the 5/2 spin state (green arrow in Fig.1), Jamet et al. are able to populate mainly this targeted spin state, to which the populations from the 1/2 and −1/2 states have been transferred. In order for this transfer to occur, first, they needed to match the energy of the spin states. Jamet et al. arranged this through resonant excitation with a laser.

The strong coupling between the electromagnetic radiation and the Mn system shifts the states in energy as dressed states are formed [5]. Second, they needed to establish population transfer toward the 5/2
state. Spin systems are, in general, very sensitive to the exact symmetry (spherical, cubic,…) of their environment. CdTe dots in a ZnTe matrix are subject to lattice strain.

This results in an anisotropic local environment for the electronic Mn spin system inside a strained quantum dot. As a direct consequence of the anisotropic strain distribution [10], spin states separated by two units of ħ are coupled (i.e., 5/2 coupled to 1/2 not 3/2 ). As the 5/2 and 1/2 states dressed by the laser field are brought into resonance, this coherent coupling induces a population transfer from the 1/2
to the 5/2 states. This population transfer is irreversible once the photon has left the dot; i.e., the optically dressed state has recombined.

The novel spin-population-trapping scheme introduced by Jamet et al. is controlled by the presence of coherent coupling between different Mn spin states. In future experiments, this coupling can be optimized through strain engineering, i.e., using different dot-barrier material combinations with a variety of lattice parameters. Also the application of a small, external magnetic field in the dot plane will modify the coupling between spin states.

The spin-population trapping for the Mn electron also involves flips with the spin of the Mn
nucleus. Optical pumping of the spin of a single Mn nucleus with long spin-memory times is a natural extension of the current work. In principle, the spin-population trapping introduced by Jamet et al. can be applied to other solid-state and atomic systems provided that a coherent coupling between the spin sublevels is present or can be induced.

Dots, rods and tetrapods: CdSe gets in shape


Jan 31, 2011

QDOTS imagesCAKXSY1K 8Researchers from the South China University of Technology have presented a surfactant-free recipe for fabricating high-quality CdSe nanocrystals (NCs). The morphology, which includes irregular dots, rods, tetrapods and sphere-shapes, can be controlled easily by varying the experimental conditions. More importantly, the preparation techniques involved are simple, low-cost and can be used to fabricate other II-VI group semiconductor NCs.

CdSe Nanocrystals

CdSe nanocrystals: dots, nanorods and tetrapods

The CdSe NCs were produced with a fixed Cd/Se molar ratio of 2:1 and using 2.32 g of trioctylphosphine oxide (TOPO); at the same time, all the trioctylphosphine selenide (TOPSe) injections were kept at 1 ml, but with different concentrations. No other ligands were used in the case study.

Homogeneous CdSe NCs with different morphology were obtained under such experimental conditions. The sample quality (size distribution, optical properties, tetrapod selectivity) is as good as that of the best CdSe NCs synthesized by using extra ligands. As for the growth mechanism, we believed that the decomposition of TOPSe and cadmium myristate at a temperature of 240 or 300 °C would also supply in situ-generated TOP and myristic acid in the reaction mixture, which affected the anisotropic growth of CdSe NCs.

To further investigate the application of this surfactant-free recipe, the group is now optimizing the experimental conditions and has found that well controlled morphology of CdTe and/or CdSexTe1–x NCs can also be successfully fabricated.

Thanks to the easily controllable NC-growth kinetics, such a synthesis route is very promising for low cost, large-scale preparation of CdSe and CdTe NCs for application in solution-processed thin-film solar cells.

More information can be found in the journal Nanotechnology.

About the author

The study was funded by the National Natural Science Foundation of China (nos. 50703012, 50773023 and 50990065), the National Basic Research Program of China (973 program no. 2009CB623600) and SCUT grant (no. 2009ZZ0003). The experiments were performed at the Institute of Polymer Optoelectronic Materials and Devices, Key Laboratory of Special Functional Materials group. Hongmei Liu is a PhD student in materials science and holds a bachelors degree in chemistry. Currently she is exploring the fabrication of high-quality semiconductor nanostructures, together with the measurement and application of the resulting nanostructures in the field of solution processed thin-film solar cell systems and other nano-electronic devices.

Third Generation Solar Technology And DSC Technology – An Interview With Gordon Thompson


 

 

Date Added: Nov 7, 2012 | Updated: Nov 7, 2012

 

 
In the first part of this ‘insight from industry’ interview, Gordon Thompson, CEO & Executive Director at Dyesol Ltd, talks to AZoM about recent innovations in solar technology. Interview conducted by Gary Thomas.
What is meant by 3rd generation solar technology – how has solar technology progressed in the last decade?

Solar technology has progressed significantly over the last 10 years although unlike some industries where older technologies become obsolete quicjkly, consumers and developers still work across all ‘generations’ of solar technology. These generations include:

1st Generation – Crystalline Silicon

By far the most prevalent bulk material in solar cells is silicon. It is separated into multiple categories: monocrystalline, polycrystalline and ribbon silicon. Crystalline silicon cells account for around 90 per cent of the market. The annual growth rate is expected to be 30 per cent. CSi uses higher cost, high energy, super “clean-room” manufacturing environment.

2nd Generation – Thin Film Semiconductor

Categorized by the cell materials: amorphous or nano-crystalline, e.g. CdTe. The thin film share, in terms of actual production, was 13.5 per cent in 2010. Thin Film technologies use more rare materials in manufacture.

3rd Generation – Artificial Photosynthesis, Nanotechnology

Third generation PV includes multiple technologies, including DSC, that seek to improve upon first two generations through a combination of cost reduction, increased energy efficiency, improved aesthetics, and opportunity for product integration. Dye Solar Cell technology uses less energy in manufacture – making it a much more environmentally friendly choice, is cheaper to manufacture, and one of the key benefits is that it works well in low-light real-world solar conditions, such as on cloudy days which are common in the heavily populated northern hemisphere.

Could you briefly explain the term ‘DSC technology’ and how this is manufactured?

DSC are made from a few key materials applied in very thin (many times thinner than a human hair) layers to a substrate (such as glass, steel or plastic).  On top is a transparent anode made of fluoride-doped tin dioxide deposited on the back of a glass plate or other substrate. On the back of this conductive plate is a thin layer of titanium dioxide (TiO2), which forms into a highly porous structure with an extremely high surface area. The plate is then immersed in a mixture of a photosensitive ruthenium-polypyridine dye (also called molecular sensitizers) and a solvent. After soaking the film in the dye solution, a thin layer of the dye is left covalently bonded to the surface of the TiO2.

A separate plate is then made with a thin layer of the iodide electrolyte spread over a conductive sheet, typically platinum metal. The two plates are then joined and sealed together to prevent the electrolyte from leaking. Although DSC uses a number of advanced materials, these are inexpensive compared to the silicon needed for traditional silicon solar cells because they require no expensive manufacturing steps.

TiO2, for instance, is already widely used as a paint base.  The materials are applied to a substrate by screen printing, soaking, and baking and the whole process takes place in a much less stringent manufacturing environment – at standards similar to the food manufacturing industry – not the “space-suit”, high energy/high cost, super clean room environments of Crystalline Silicon technology.
When sunlight, indirect light, indoor/artificial light, dappled light, or low light strikes the DSC, the light excites electrons in the dye which are absorbed by the titanium to generate an electric current.

What are the advantages of using DSC technology over Silicon based photovoltaic technology?

DSC technology has been widely recognised as a technology of the future because DSC has a number of outstanding characteristics and benefits, which include:

  • DSC performs well in real world sun conditions – shade, dawn, dusk, dappled light, haze, cloud, and even indoor light
  • Low energy, low cost manufacturing process,
  • No toxic materials,
  • Small layers/quantities of product saves resources,
  • Aesthetically appealing options for building integration, and
  • Widely available raw materials.

These features combine to make DSC a clean, green technology inherently suitable to application in the built environment where the largest part of human activity occurs and where electricity demand is highest.

Good Performance in Real-World Solar Conditions

DSC technology works well – and relatively better than other solar technologies – in real-world solar conditions, including: cloudy days, hazy days, polluted skies, at higher latitudes (i.e. Europe, Asia, North America), at dawn and at dusk, not just at noon on a sunny day.

Low Embodied Energy & Nanotechnology

An important distinction of DSC, which distinguishes this technology from all other photovoltaic systems, is its nanotechnology basis.  One of DSC’s key materials is a nanostructured titanium dioxide (TiO2), which provides the host matrix for the photoactive dye, offers unique electric properties, unique optical properties (such as transparency), and unique mechanical properties.  Nanomaterials can be processed at much lower temperatures.  For example, micrometer-sized TiO2 particles are processed at temperatures around 1,000 ˚C, but nano-TiO2 particles are processed at temperatures around 500 ˚C.  This  saves considerable amounts of energy and means that Dye  Solar Cells have less embodied energy than competitor technologies.  DSC is truly a clean, green technology.

Low Cost Manufacturing Processes

Since traditional PV technologies rely heavily on vacuum processing and require extremely high purity materials and stringent cleanliness for the manufacturing environment, these technologies are generally based on expensive equipment, including the most sophisticated and energy-hungry clean rooms and all factory workers wearing ‘space-suit’-type work gear.  In contrast, DSC manufacture relies mainly on printing, ‘baking’ and packaging processes.  Only relatively moderate control of atmospheric dust and moisture is required for DSC assembly.  Most production steps are similar to high throughput processes used by the coating, printing, lamination and food packaging industries. Therefore, capital expenditure for manufacturing is much lower for DSC, a fact which is certainly appreciated by our commercial partners.

Flexible Applications & Scalable Production

DSC technology may be applied onto a range of substrates, including glass, metal, and polymeric substrates.  DSC technology can be applied onto rigid and flexible substrates and is bifacial, meaning it can take in light from both sides of a pane of glass for example.  DSC technology is also scalable to high levels and the ability to integrate DSC into roll-to-roll manufacturing lines (such as in DSC enabled steel roofing material) makes high volume product manufacture achievable.

Low Environmental Impact – Non-Toxic Raw Materials

None of the materials used in today’s DSC is known to be toxic according to international standards and regulations.  The main ruthenium-based dye used today has been biologically tested (AMES test) and found not to be mutagenic. As DSC does not utilise toxic raw materials in cell production, there is minimal remediation risk and other additional protective measures in production are minimal. In contrast, some of the potential competitor technologies to DSC rely on very toxic materials such as cadmium and rather toxic materials such as selenium used for CdTe (cadmium telluride) and CIGS photovoltaics.
Small quantities of product saves resources

DSC is the technology with the thinnest-possible photoactive absorbing layer: one single molecular layer of a sensitiser dye spread out over a high surface area and low cost titanium dioxide (TiO2) layer.

Added up, the sensitiser layers on any DSC panel amount to a thickness corresponding to around 1 micrometer, i.e. many times thinner than a human hair.

Thus DSC is the ultimate ‘miser’ when it comes to the usage of natural resources.  In comparison, silicon wafers used for standard solar panels, are more than 100x thicker once in the product, plus there are considerable material losses and waste during processing, at the wafer sawing stage in particular.

Aesthetics

No other photovoltaic technology offers nearly as much flexibility in terms of colouration and transparency as Dye Solar Cell technology due to the very nature of the Dye Solar Cell chemistry.  Many architects are attracted towards DSC for its virtually endless possibilities of colours and transparency for windows, doors, atriums, skylights and internal dividing walls, all whilst producing clean energy.  DSC windows will not only provide electricity, but can also moderate harsh sunlight and provide thermal and noise insulation. While the most efficient dyes used today are red to yellow – orange, green, grey and brown colours offer attractive efficiencies as well.  DSC windows for office buildings can be coloured in a neutral grey, whereas art galleries and music halls may opt for more vibrant and expressive colours.  DSC integrated into glass houses could filter and scatter stark sunlight, whilst converting the part of the solar spectrum – which does not contribute effectively to the growth of plants to electricity – right at the point of use.

Widely Available Raw Materials

The major chemical materials used in DSC are carbon, oxygen, nitrogen, hydrogen, titanium, and silicon (glass) or iron (steel) for substrates, plus very small amounts of platinum and ruthenium.  With the exception of the latter two, these are all very common materials and there is no shortage in sight.  The most critical component in today’s DSC in terms of natural resources is ruthenium.  Annual production of 20 million m2 of DSC panels producing close to 1.5 GW at maximum power would today require about 2 tonnes of ruthenium, which corresponds to 6-7% of the annual worldwide ruthenium production or to only about 0.03% of the estimated mineable world resources.

Material supply availability is even less significant with platinum since this metal is used in even smaller quantities in DSC compared to ruthenium and because natural platinum resources are significantly higher than those of ruthenium. In comparison, second generation solar technology CIGS (which stands for copper, gallium, indium, and diselenide), requires significant amounts of indium.  Assuming again an annual production of 20 million m2 of CIGS PV panels, about 120 tonnes of indium would be required annually.  This corresponds to about 20% of the annual worldwide indium production or about 1% of the estimated mineable worldwide resources, which is a very significant amount.  With all of the other applications for indium, such as LCD displays, it is likely that indium will become harder and harder to source and thus more and more expensive than today.

A ruthenium dye is used in the manufacture of DSC technology. Why is ruthenium used? Could this be substituted by another element? 

In dye sensitised solar cells, the dye is one of the key components for high-power conversion efficiencies.  Ruthenium based dyes are used because of their better performance and stability characteristics.  Yes, other dyes may be used and there is a body of work on organic dyes as alternatives, however, at the moment these alternates do not match the ruthenium based dyes on performance or stability.

 

Solar Panel Makers Need Equipment Upgrades to Survive Shakeout


With overcapacity of 82%, companies need innovative tools to differentiate from cheaper Chinese rivals, says Lux Research.

English: Thin-film PV array

English: Thin-film PV array (Photo credit: Wikipedia)

BOSTON, Oct 25, 2012 (BUSINESS WIRE) — Reeling from a glut of production capacity, makers of solar panels need to acquire innovative production equipment in order to cut costs, increase margins, and offer differentiated products, according to Lux Research.

This year, global capacity utilization is at 55% for crystalline silicon (x-Si) module production, 70% for cadmium telluride (CdTe) and 80% for copper indium gallium (di) selenide (CIGS). Consequently, cell and module manufacturers are turning to core product differentiation to revamp margins and fend off low-cost Chinese competition.

“Across the industry there is recognition that innovation is needed to survive a shakeout,” said Fatima Toor, Lux Research Analyst and the lead author of the report titled, “Turning Lemons into Lemonade: Opportunities in the Turbulent Photovoltaic Equipment Market.” “Equipment suppliers have a vital role to play in enabling that innovation.”

Lux Research analysts examined the PV production equipment landscape to identify opportunities for innovation. Among their findings:

— There’s opportunity in reducing silicon costs. Current wafer sawing techniques waste silicon; in contrast, technologies, such as direct solidification and epitaxial silicon eliminate the need for wafer sawing. Emerging quasi-monocrystalline silicon (qc-Si) ingot growth enables 40% cheaper c-Si wafers.

— In CIGS, standardization is key. CIGS thin-film PV relies on custom equipment today. However, off-the-shelf tools and improved throughput will drive higher efficiencies, performance and yield – lowering capex and helping manufacturers attain scale and competitive production costs.

— New cell designs lead to equipment upgrades. Emerging cell designs, such as selective emitter (SE) and heterojunction with intrinsic thin layer (HIT) present potential for high efficiencies. However, they require new tools, and as a result, 60% to 70% of new equipment sales are for the cell production equipment.

The report, titled “Turning Lemons into Lemonade: Opportunities in the Turbulent Photovoltaic Equipment Market,” is part of the Lux Research Solar Components Intelligence service.

About Lux Research

Lux Research provides strategic advice and on-going intelligence for emerging technologies. Leaders in business, finance and government rely on us to help them make informed strategic decisions. Through our unique research approach focused on primary research and our extensive global network, we deliver insight, connections and competitive advantage to our clients. Visit http://www.luxresearchinc.com for more information.

SOURCE: Lux Research

Note To Readers: We have been following a ‘disruptive nanotechnology’ company, researching and developing a ‘3rd Generation’ of solar cells based in part on low-cost quantum dots and reduced input cost printing techniques. Below is a short excerpt from a website, a link also provided below. Perhaps, with innovation such as this, the U.S. Solar industry can become the clear leader in providing grid competitive renewable energy. Perhaps ….        Cheers!  – BWH-

Solterra Renewable Technologies  

http://www.solterrasolarcells.com/corporate_vision.php?ID=11

“Solterra will be producing and distributing a Thin Film Quantum Dot PV Solar Cell which is differentiated from other PV cells by a unique technology that results in lower cost, higher efficiency, and broader spectral performance.  Solterra’s Quantum Dot Solar Cell achieves a dramatically lower manufacturing cost per watt because no vacuum equipment is required, no expensive silicon is required and low-cost screen printing and/or inkjet techniques are used on inexpensive substrates. Secondly, the Solterra Thin Film Quantum Dot Solar Cell has the potential to generate multiple excitons from each proton providing the potential for exponential improvements in conversion efficiency. Third, Solterra’s PV cell is not only more efficient in the early morning and late afternoon compared to crystalline silicon PV cells, but it also has the potential to harvest light energy in the infrared and ultraviolet spectra.