In this interview, Gordon Thompson, CEO & Executive Director at Dyesol Ltd, talks to AZoM about recent innovations in solar technology.
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.
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.
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