Dye Sensitised Solar Cells SSERC Chemistry Summer School Practical Session: Dye Sensitised Solar Cells

Fossil Fuels Estimated reserves: 25-30 TW for several centuries Spending entire annual salary in the first 30 seconds of the year.

Renewables

1 hour of solar radiation on the earth = 1 year’s energy consumption Renewables Theoretical Maximum 1 hour of solar radiation on the earth = 1 year’s energy consumption

But what about Scotland? Edinburgh has 56% of the solar resources of Barcelona 1Kw/m2 Grid parity hit Italy in 2012 and is spreading North 2009: Renewable energy subsidies $42 – 46 billion 2008: fossil fuel subsidies $557 billion

http://www.nrel.gov/ncpv/images/efficiency_chart.jpg

Solar Energy So…..why is it difficult? The sun shines over different parts of the earth at different times. Energy conversion is not 100% efficient. Some energy lost (eg as heat)

Solar Panels Solar panels are not perfect, but they do a pretty good job of converting the light that shines on them into electricity. Solar Panel Solar panels use light to create free electrons in a special material called a semi-conductor – usually Silicon. These electrons are channelled off into a wire and used as an electric current.

Dye Sensitised Solar Cells (DSSCs) Dye sensitised solar cells (DSSCs) were invented in 1991 by Michael Grätzel and his research group at the Swiss Federal Institute of Technology. These cells use a dye which directly converts the energy of sunlight into electrical energy using a process which is similar to photosynthesis in some aspects.

Dye Sensitised Solar Cells (DSSCs) 50 nm Conventional solar cells are rather expensive owing to the cost of producing high grade silicon. In contrast, DSSCs use a layer of nanocrystalline titanium dioxide making them much cheaper to construct. Dye-sensitised solar cells contain two transparent electrode plates, which are internally connected by an iodine/tri-iodide electrolyte.  

How the dye sensitised solar cell works Various dyes can be used to make dye sensitised solar cells. The most suitable plant dyes probably contain a mixture of anthocyanidins which have the general structure shown here: OH HO O+ Anthocyanidins gives the red colour to raspberries and strawberries; the magenta to blackcurrants and the blue colour to delphiniums and violas.

Dye sensitised solar cells and the future Dye sensitised solar cells that work, can be easily built by science students at school. These solar cells containing synthetic dyes are at the cutting edge of science, and will replace expensive silicon solar cells in the near future. Indeed, it has been suggested that within five years it could be possible to print out a solar cell using a computer printer!

Planet Earth Energy Sources and Sustainability I can investigate the use and development of renewable and sustainable energy to gain an awareness of their growing importance in Scotland or beyond. TCH 2-02b Forces, electricity and waves Electricity Using a variety of sources, I have explored the latest developments in chemical cells technology and can evaluate their impact on society. SCN 4-10b Planet Earth Energy Sources and Sustainability By investigating renewable energy sources and taking part in practical activities to harness them, I can discuss their benefits and potential problems. SCN 3-04b Materials Chemical Changes I can collect and analyse experimental data on chemical reactions that result in an obvious change in energy. . . SCN 4-19a Topical Science I have researched new developments in science and can explain how their current or future applications might impact on modern life. SCN 4-20a National 4 Nature's Chemistry -Fuels - Alternative energy sources

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graphite coated electrode titanium dioxide coated electrode How the dye sensitised solar cell works The photo-electrode is a conductive glass or plastic base which is coated by a nanocrystalline layer of titanium dioxide. This acts both as a matrix for the dye and as a wide gap semiconductor. TiO2 ē dye reduced dye electroly te 2ē graphite coated electrode titanium dioxide coated electrode photons of light I3 – 3I – I – 2I – I3 – I2 electrical device

graphite coated electrode titanium dioxide coated electrode How the dye sensitised solar cell works When the dye molecules are exposed to sunlight some of their electrons absorb photons and become ‘excited’. TiO2 ē dye reduced dye electrolyte 2ē graphite coated electrode titanium dioxide coated electrode photons of light I3 – 3I – I – 2I – I3 – I2 electrical device

graphite coated electrode titanium dioxide coated electrode How the dye sensitised solar cell works The excited electrons escape the dye molecules and become free electrons. These free electrons move through the titanium dioxide and accumulate at the -ve plate (dyed TiO2 plate). TiO2 ē dye reduced dye electroly te 2ē graphite coated electrode titanium dioxide coated electrode photons of light I3 – 3I – I – 2I – I3 – I2 electrical device

graphite coated electrode titanium dioxide coated electrode How the dye sensitised solar cell works After flowing through the external circuit, the electrons move through the positive counter electrode, which is coated in graphite, to interact with the electrolyte. TiO2 ē dye reduced dye electroly te 2ē graphite coated electrode titanium dioxide coated electrode photons of light I3 – 3I – I – 2I – I3 – I2 electrical device

graphite coated electrode titanium dioxide coated electrode How the dye sensitised solar cell works The electrolyte contains an iodine/tri-iodide redox couple which form an equilibrium: I – + I2 I3 – TiO2 ē dye reduced dye electroly te 2ē graphite coated electrode titanium dioxide coated electrode photons of light I3 – 3I – I – 2I – I3 – I2 electrical device

graphite coated electrode titanium dioxide coated electrode How the dye sensitised solar cell works At the graphite covered counter electrode the tri-iodide ion (I3 –) is reduced by gaining electrons: I3 – + 2ē 3I – TiO2 ē dye reduced dye electroly te 2ē graphite coated electrode titanium dioxide coated electrode photons of light I3 – 3I – I – 2I – I3 – I2 electrical device

graphite coated electrode titanium dioxide coated electrode How the dye sensitised solar cell works The iodide ions, which are formed, move through the electrolyte and two iodide ions regenerate the reduced dye molecule on the photo-electrode by donating electrons to it:   2I – I2 + 2ē TiO2 ē dye reduced dye electroly te 2ē graphite coated electrode titanium dioxide coated electrode photons of light I3 – 3I – I – 2I – I3 – I2 electrical device

graphite coated electrode titanium dioxide coated electrode How the dye sensitised solar cell works The remaining iodide ion shifts the iodine/tri-iodide equilibrium towards the formation of more tri-iodide ions:   I2 + I – I3 – TiO2 ē dye reduced dye electroly te 2ē graphite coated electrode titanium dioxide coated electrode photons of light I3 – 3I – I – 2I – I3 – I2 electrical device