Unique Characteristics and Benefits of DSC

Transcription

1 Unique Characteristics and Benefits of DSC By Hans Deslivestro, Dyesol Limited Chief Scientist Dye Solar Cell technology (DSC) has been widely recognised as a technology of the future because DSC has a number of outstanding characteristics and benefits, which include: Low energy manufacturing process, Toxic materials are not used in the DSC manufacturing process, DSC performs well in real world sun conditions shade, dawn, dusk, dappled light, haze, cloud, etc., DSC works in diffuse light conditions, meaning it is an ideal technology for Building Integrated Photovoltaic (BIPV) applications for the supply of energy at the point of consumption, DSC mimics nature s process of photosynthesis, and DSC s simple and elegant design allows for many aesthetically pleasing applications, including the option of transparency. 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. This article summarises these unique characteristics and benefits. 1. Unique chemistry and materials architecture - DSC as the ultimate thin layer technology Small quantities of product saves resources DSC is the technology with the thinnest-possible photoactive absorbing layer: one single molecular layer of a sensitiser or dye spread out over a high surface area, low cost material such as titanium dioxide (TiO 2 ). Added up, the sensitiser layers on any DSC panel amount to a thickness corresponding to around 1 micrometer, i.e 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 during processing, at the wafer sawing stage in particular. Today, mainly ruthenium-based dyes are employed because of their superior performance and stability. In the future it is likely that metal-free dyes will be employed, in which case even smaller sensitiser quantities are required. Nanotechnology processed at low temperatures means less energy used in production Another important aspect of DSC, which distinguishes this technology from all other photovoltaic (PV) systems, is its nanotechnology basis. Nanostructured titanium dioxide (TiO 2 ), which provides the host matrix for the photoactive dye, offers unique electronic properties, optical properties (such as transparency) and mechanical properties. With nanomaterials, surface characteristics become more and more important in comparison to bulk properties. This latter feature enables nano-tio 2 to be processed at much lower temperatures. TiO 2 is a ceramic material, where processing temperatures of material based on larger (micrometer-sized) particles are well above 1,000 o C. However, because nano-tio 2 is employed in DSC it can be processed around 500 o C only and in the future probably at even Dyesol Ltd Page 1 of 7

2 lower temperature, which saves considerable amounts of energy. Readily available material inputs The major chemical materials used today for DSC are carbon, oxygen, nitrogen, hydrogen, titanium, silicon or iron (steel) for substrates and 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. An annual production of 20 million m 2 of DSC panels producing close to 1.5 GW at maximum power would today require about 2 tonnes of ruthenium, which corresponds to about 6-7% of the annual worldwide ruthenium production or to only about 0.03% of the estimated mineable world resources. The situation 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. And Dyesol has strategies in place to at least partially substitute ruthenium in the future and has substantial expertise with non-platinum catalysts. In comparison, CIGS (copper gallium indium diselenide), another thin film PV technology, requires significant amounts of indium (notably because the active CIGS layers are not as thin as with DSC). Assuming again an annual production of 20 million m 2 of PV panels, based on CIGS, about 120 tonnes of indium would annually be required, which corresponds to about 20% of the annual worldwide indium production or about 1% of the estimated mineable world resources, which is a very significant amount. With all the other applications for indium such as LCD displays, it is likely that indium will become harder and harder to source and thus even more expensive than today. 2. Low toxicity raw materials The major DSC materials mentioned above, such as carbon, oxygen, nitrogen, hydrogen, titanium, silicon or iron (steel) are all elements we are in daily contact with through our food, through the air we breathe, the toothpaste or sunscreen (1) we use, the plates, cups and glasses we eat and drink from and the cutlery we daily use as food-tomouth interface. Toxicity is always relative and always a matter of quantity and exposure. Too much water after intense sports activity or certain food can be deadly! 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. 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 (see above) thin layer PV. US drinking water regulations stipulate a Maximum Contaminant Level (MCL) of 5 ppb for cadmium and 50 ppb (2) for selenium. (1) Both are based on small titanium dioxide particles suspended in a cream or paste (2) ppb = parts per billion, 5ppb = 5 micro gram or g per litre of water. In comparison the US MCL value for arsenic is 10 ppb! 3. Low manufacturing costs for DSC Since traditional PV technologies heavily rely on vacuum processing and require extremely high purity of 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 spacesuit -type work gear. Dyesol Ltd Page 2 of 7

3 In contrast, manufacture of DSC 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. More detail on DSC costs and on the cost of energy produced by DSC are available in the Dyesol article What is Levelised Cost of Energy?. 4. Low embodied energy Since most DSC materials are used in very small quantities and mostly processed at relatively low temperatures, the embodied energy of the DSC system [ 1 ] is the lowest of any PV technology [ 2 ]. Interestingly, glass, if used for encapsulation will always be the highest contributor to embodied energy of DSC and thus for energy payback. The latter quantity specifies the time it takes for a solar panel in the field to produce the energy amount which was required for its production. The Dutch institute ECN published the following figures for energy payback in Southern Europe for DSC panels only, i.e. not counting any roof mounting fixtures or cabling: Energy payback in Southern Europe for DSC panels Substrates Glass-Glass Steel-Polymer Polymer-Polymer Energy payback time (months) In comparison, energy payback times for polycrystalline silicon panels, CIGS and CdTe have been assessed [ 2 ] as 2-5, 2 and 0.5 years, respectively. A forthcoming enewsletter will focus in more detail on embodied energy. 5. Better performance in diffuse light power all day, every day Most standard solar panels suffer from significant loss of efficiency at lower light levels because losses due to electron-hole pair recombination within the semiconductor phase becoming relatively more important. With DSC in contrast, photogenerated positive and negative charges are effectively separated into two different phases: the electron is injected very rapidly, i.e. within fractions of picoseconds (1 ps = 1 trillionth of a second) into the solid phase of titanium dioxide, while the positive charge is chemically neutralised within nanoseconds (1 ns = 1 billionth of a second) from the liquid electrolyte phase. This is where chemists and material scientists come in to assure that any new dye or other cell component offers exceptionally fast photo- and electrochemical reactivity. Speed - kinetics in tech speak - is (almost) everything with DSC. The DSC working principle is much closer to aspects of photosynthesis than to standard semiconductor photovoltaics. In photosynthesis, chlorophyll, which is located in highly organised nanostructured chloroplasts, absorbs the light and photogenerated high energy electrons are shuttled physically in one direction and the remaining positive charge gets neutralised through a cascade of chemical reactions. In contrast to photosynthesis, which converts CO 2 and H 2 O to glucose (fuel) and oxygen, DSC directly makes use of the excited electrons to generate electricity. Dyesol Ltd Page 3 of 7

4 Two years ago, Dr Gavin Tulloch, Dyesol s Director of Technology said Consider how a leaf functions in photosynthesis. It works in all light levels. It does not have to face directly at the sun for photosynthesis to occur. It operates in shade and this is true for DSC also. Based on its principle of operation, DSC perform particularly well under conditions corresponding to ~0.2 to 0.5 AM 1.5G (3) and under diffuse light conditions. Under non-ideal angle of incidence of the solar irradiation, such as on the side of a building, DSC can outperform other technologies with nominally higher efficiencies as determined at AM 1.5G and at 25 o C. Note that no solar panel will operate at 25 o C under AM 1.5G (= 1 sun ) illumination and that today s standards unfairly favour the traditional technologies and disfavour DSC, see also Section 6 below. A recent article in a Japanese electronics magazine highlights the fact, that the efficiency gap between DSC significantly decreases at lower light levels [ 3 ]. The following graph shows Dyesol data obtained during the course of a day, where a DSC, a crystalline silicon and a CIGS panel were compared in vertical orientation facing North. This simulates a North façade of a building. The output is expressed in percentage of the nominal power specified under AM 1.5G conditions, corresponding to 1 sun, i.e. for perpendicular incidence of solar radiation. The data was taken on a clear sunny day close to the Southern autumn equinox in Queanbeyan close to Canberra. The data shows that over the entire day, but particularly in the morning and late afternoon hours, DSC panels of the same nominal power rating deliver much more performance, double and beyond, compared to the other technologies, which lose much more performance due to the nonperpendicular angle of incidence onto vertically oriented panels. Based only on performance between 08:15 and 17:00, below data shows that over a sunny day on a façade DSC delivers at least 31% more energy than c-si and 49% more than CIGS. On hazy or cloudy days, differences are even larger. These features make DSC particularly attractive for building-integrated applications and for usage in cities and areas with relatively hazy conditions. (3) AM 1.5G stands for Air mass 1.5 global illumination. It corresponds to the amount of solar energy striking the earth s surface after passing through nominally 1.5 times the thickness of earth s atmosphere, or said differently, after passing the atmosphere at an angle of 48.2o from perpendicular. AM 1.5G is often also referred to as 1 sun. Dyesol Ltd Page 4 of 7

5 6. Better relative performance at higher temperatures DSC performance depends much less on panel temperature than power output from crystalline silicon. The following graph shows power vs voltage curves for a typical commercial DSC design at temperatures varying from -10 o C to 70 o C, i.e. over a temperature range spanning 80 o C, in comparison with a crystalline Si cell where the temperature was only varied over a temperature range of 52 o C [ 4 ], i.e. between 28 o C and 80 o C. All data was normalized to 100% at 25 o C. For DSC, the power P mpp at the maximum power point is rather constant and varies by only +/-12% over the entire temperature range tested. Such a small influence of temperature on power output from DSCs is in sharp contrast to Si solar cells, which show a much more pronounced decrease in performance with increasing temperatures, i.e. under real life sun light illumination. The following table shows the same data numerically as a comparison between DSC and Si for two temperature ranges and exemplifies that DSC loses much less power when solar panels heat up under real world conditions. More technical detail can be found on Dyesol s website [ 5 ]. Temperature Increase P mpp drop for DSC P mpp drop for c-si From 20 o C to 50 o C 5% 19.5% From 20 o C to 70 o C 15% 32.5% The red area in the two graphs above show that the maximum power point voltage for DSC varies much less with temperature compared to silicon. This is the basis of Dyesol s SureVolt TM technology, which assures much higher voltage stability over a large variation of panel temperatures and light levels. This is important because inverters, i.e. the devices which convert solar DC current into AC grid power, can work with DSC much closer to their optimum voltage, where inverter conversion is highest. In comparison, traditional PV technologies require a much larger voltage range for DC conversion, especially when exposed to non-ideal light conditions, where the inverters work less effectively or simply just shut down. Dyesol Ltd Page 5 of 7

6 7. Architectural appeal and other added benefits of DSC No other PV technology offers nearly as much flexibility in terms of colouration and transparency 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 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 - yellow, orange, green, grey-to-black and brown colours offer attractive efficiencies as well. DSC windows for office buildings can easily 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. DSC panels can also be considered for noise barriers, e.g. along motorways. Such structures are visually much more attractive than non-see through concrete barriers and provide energy as well. A few years ago, Dyesol developed, in collaboration with the Australian Defence Department, a flexible DSC panel offering full camouflage in the field. DSC is the only PV technology known today offering such versatility in coloration and visual appearance. Photo: Semi-transparent DSC windows by Dyesol in the House of the Future, Sydney Olympic Park, internal view. Photo by Thomas Bloch reproduced with permission of Innovarchi PTY LTD, Tel: Conclusions The main features of DSC have been reviewed and discussed. Based on its working principles and materials employed, DSC technology is unique in comparison to other PV systems since: DSC is a true nano-technology, thereby offering unique optical, electronic and processing characteristics; DSC relies mainly on abundantly available and non-toxic materials, with substitutes in sight for ruthenium or platinum, if required; DSC is the PV technology with the lowest embodied energy; DSC works particularly well under non-ideal light environment, such as hazy conditions or flat angles of incidence of radiation, where other PV technologies show their deficiencies; DSC is characterised by much smaller variations of maximum power point voltage as function of panel temperature and/or light levels; DSC has unique aesthetic appeal in terms of transparency and colouration and can offer additional benefits, such as see-through light and/or noise filter and thermal insulation. These characteristics make DSC particularly attractive for applications in the built environment, i.e. where most the electric energy is required and consumed. In addition, DSC is the most sustainable of all PV technologies because it offers the lowest energy payback time and does not require toxic materials. Dyesol Ltd Page 6 of 7

7 [ 1 ] P. Sommeling, M. de Wild-Scholten, T. Veltkamp, and J. Krohn, Life cycle assessment and testing of dye solar cells, 2nd Dye Solar Cell Industrialisation Conference, St. Gallen, Switzerland, September, Presentation accessible on [ 2 ] M. Raugei, S. Bargigli, and S. Ulgiati, Life cycle assessment and energy pay-back time of advanced photovoltaic modules: CdTe and CIS compared to poly-si, Energy, 32, (2007). [ 3 ] T. Nozawa, Organic Solar Cells Now Produced in Volume, Nikkei Electronics Asia, July 2008, [ 4 ] E. Radziemska, The effect of temperature on the power drop in crystalline silicon solar cells, Renewable Energy, 28, 1 (2003). [ 5 ] H. Desilvestro, Section 4 in Voltage_Characteristics_of_Dye_Solar_Cells Dyesol Ltd 3 Dominion Place Queanbeyan, NSW 2620 Australia Tel: Dyesol Ltd Page 7 of 7