Photosyntheis in a test tube-Dye sensitized solar cells (USPseminar)

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  • Photosyntheis in a test tube-Dye sensitized solar cells (USPseminar)

    1. 1. PHOTOSYNTHESIS IN A TEST TUBE Dye-Sensitized Solar Cells Atul K. Raturi Division of Physics The university of the south pacific
    2. 2. About 1.8 Billion people without electricity Earth at Night
    3. 3. Pacific Scene <ul><li>PNG:~ 85% </li></ul><ul><li>Solomon Islands: ~98% </li></ul><ul><li>Vanuatu:~ 78% </li></ul><ul><li>Fiji: ~ 51% ( mainly rural) </li></ul>Population without electricity
    4. 4. Global warming & Climate change <ul><li>Recent research shows that the global warming in the New Guinea Island was occurring at the rate 20 times higher than previously thought </li></ul><ul><li>Temperatures have increased on average 0.3 degrees every decade (among the fastest in the world) </li></ul><ul><li>Glaciers on Mount Jaya (highest peak on the Island-Indonesian side) have retreated by 300 m in 30 years </li></ul><ul><li>The priceless flora and fauna (some yet to be discovered) will be lost forever </li></ul>Photo:www.rbgkew.org.uk Source: New scientist, March 2006)
    5. 5. Carteret Atoll, Papua New Guinea The entire population of 2000 is to be moved to higher grounds. “ First Climate Change Refugees” By 2015, all the six islands will be submerged. Saltwater has already destroyed the gardens and crops Two uninhabited islands of Kiribati disappeared in 1999. Tuvalu, Marshall Islands are under threat. Global warming effects in the Pacific
    6. 6. FEA Electricity generation 1990: 94.5 % from hydropower 2003: 67.3 % from hydropower Currently almost 50% is produced using diesel ( Fuel bill ~ 98 M$)
    7. 7. FEA generation forecast Wind farm at Butoni, Sigatoka- Commissioned 2007; Capacity 10 MW
    8. 8. Solar Insolation (Fiji)
    9. 9. All our traditional energy sources ( except wind energy & hydropower) are products of Photo synthesis Photosynthesis produces 8 times the current energy needs of the world Coal Oil Gas Solar Energy Photosynthesis Biomass
    10. 10. Photosynthesis An electrochemical Process: A redox reaction of excited Chlorophyll molecules Chlorophyll, an organic dye, absorbs the light photons to produce excited electrons. Carbon dioxide acts as electron acceptor while Oxygen is produced as oxidation product Light   nCO 2 + n H 2 O  ------(CH 2 O)n + n O 2 (STI)
    11. 11. - + - Thylacoid Membrane 0.1 µm Oxidation Products Reduction Products Sunlight Chlorophyll Electron-hole pairs are generated when light is absorbed by the chlorophyll molecule . The membrane separates the two charges A Thin Section Of A Leaf Electron
    12. 12. <ul><li>A very thin layer is required to minimize electron losses </li></ul><ul><li>A thin layer absorbs very little light </li></ul><ul><li>Nature has solved this problem by </li></ul><ul><li>Providing successive layers </li></ul><ul><li>Antenna molecules </li></ul><ul><li>Can afford to recycle the whole system on annual basis </li></ul>Duplication of Photosynthesis has not been very successful Reasons:
    13. 13. Artificial Photosynthesis <ul><li>What is needed? : </li></ul><ul><li>1. Something to absorb solar energy </li></ul><ul><li>2. A mechanism to separate the two types of charges </li></ul><ul><li>3. A mechanism to pickup one type of charges </li></ul><ul><li>4. A medium to transport the other type of charges </li></ul>
    14. 14. Materials Requirements <ul><li>Semiconductor with appropriate band-gap (1-1.2 eV) and High absorption coefficient </li></ul><ul><li>A non-reactive electrolyte . </li></ul><ul><li>A special relationship between the band energies of the semiconductor and the redox potential of the electrolyte is required to drive the reactions </li></ul><ul><li>For n type: A donor level above or equal to the valence band edge of the semiconductor must exist in the electrolyte </li></ul><ul><li>For p type: An acceptor level below or equal to the conduction band edge of the semiconductor must exist in the electrolyte. </li></ul>
    15. 15. Reduction Oxidation Electrolyte Metal: Cathode N-type Semiconductor: Photo anode LOAD External Connection Hole Electron Light Electron A PHOTOELECTROCHEMICAL CELL- The Next Best Thing Fermi Level V B I
    16. 16. PEC OPERATION <ul><li>Electron and hole pairs are created by photoexcitation of the semiconductor electrode . </li></ul><ul><li>Electron and holes drive chemical reactions: For n-type semiconductor </li></ul><ul><li>Photoanode (SC) : Hole injection- Oxidation reaction </li></ul><ul><li>Metal Cathode: Electron injection – Reduction reaction </li></ul><ul><li>Red + h + --------------Ox + ( At the anode) </li></ul><ul><li>Ox + + e - ---------------Red ( At the cathode) </li></ul><ul><li>No Change in the electrolyte composition. Light energy is converted into electrical energy </li></ul>
    17. 17. Energy Levels <ul><li>E red ---- Ionization energy of the reduced species </li></ul><ul><li>E ox ====Electron affinity of the oxidized specie </li></ul><ul><li>E Fredox – the average of E red and E ox </li></ul>Semiconductor Redox System E Fredox E ox E red E F The energy levels in the electrolyte belong to the localized states bound to the component of the redox system Valence band Conduction band
    18. 18. The Band bending at the junction is given by V B = U F – U FB Where U F is the Fermi Level and the U FB is the Flat Band Potential We need the light to be absorbed in the depletion layer so that the charges can be separated efficiently. The width of the depletion layer is For larger W , N (no. of donors/acceptors/Vol) should be low but that results in higher resisitivity. Find materials with large minority carrier lifetime and hence low recombination.
    19. 19. PHOTOELECTROCHEMICAL CELLS <ul><li>Advantages </li></ul><ul><li>The junctions can be made very easily unlike in the case of solid state cells </li></ul><ul><li>Polycrystalline materials can be used (lower cost) </li></ul><ul><li>Diffusion and doping processes are eliminated </li></ul><ul><li>No front metallization required </li></ul><ul><li>Problems </li></ul><ul><li>Stability of the semiconductor electrodes in the electrolyte is the biggest problem: holes oxidize the semiconductor </li></ul>Examples of PEC systems : nCdSe/S -2 /S, nCdTe/S -2 /S, nCdS/{Fe(CN) 6 } 4+ / {Fe(CN) 6 } 3+
    20. 20. NEW GENERATION PECs DYE SENSITIZED NANOCRYSTALLINE SOLAR CELL <ul><li>Third generation Solar Cells </li></ul><ul><li>Mimics the natural photosynthesis process </li></ul><ul><li>Light absorption process is separated from charge transport process (unlike in older cells) </li></ul><ul><li>An organic dye absorbs the sunlight and a nanocrystalline wide band-gap semiconductor is used to transport the charges </li></ul><ul><li>Also called “Graetzel Cells” </li></ul>
    21. 21. ElectrolyteI/I 3 - TiO 2 Conducting glass Sunlight LOAD Carbon electrode Organic Dye: Light harvester (sensitizer) Nanocrystalline TiO 2: Transports electrons Electrons reach the carbon electrode via external connection and a current flows Reactions: I 3 - + e - -- I - ( at carbon electrode) I - +h + ---- I 3 - + e - ( at the photoelectrode) Max. Photovoltage= Difference between the Redox Potential of the electrolyte and the Fermi level of the semiconductor Electron DSSC Organic Dye: e.g. Cyanin I e -
    22. 22. Efficiency achieved ~ 10.5 % Commercial Solar Cell efficiency ~ 15-17% Tropical Forest Ecosystem efficiency ~ 1% Theoretical limit for natural Photosynthesis ~ 13% Commercial Production : Sustainable Technologies International (Australia)- Under license from EPFL DSSC Façade Panel
    23. 23. Various colours Transparent Hitachi’s 9% efficient Building integrated Source: M.Gratzel:Nano Roadmap conference DSSCs
    24. 24. DSSC Photosensitizers (specially prepared Dyes) (i) Ru Dye (ii)N3 Dye (iii) Black Dye Coumarin Dye Properties: 1: Must contain a chemical group that can attach to the TiO 2 surface 2. Must have energy levels at the proper positions
    25. 25. The dye is obtained from the berries (Rubiaceae family) Experimenting with natural dyes Step1: A nanocrystalline (porous) film of TiO 2 was coated on a conducting glass slide. Step2: Sensitizer dye was derived from the berries. Step3: TiO 2 film was coated with the dye Step4: A cell was constructed by putting a second conducting glass slide on top of the coated one with few drops of electrolyte between them Step4: Cell characterization: V oc , I SC, ,FF, Efficiency
    26. 26. DSSC commercialization <ul><li>DSSC Market </li></ul><ul><li>- 3.5 billion Yen </li></ul><ul><li>2010- 58.1 billion Yen </li></ul><ul><li>Manufacturing costs ; ½ to 1/10 of crystalline cells </li></ul>
    27. 27. CONCLUSION DSSCs have great potential for providing low-cost Photovoltaic power to billions of people around the world , living in remote areas without any access to electricity. These cells can be produced easily without the expensive set-ups needed for conventional solid-state solar cells. This is an example of BIOMIMICRY proving that Nature is our greatest teacher.
    28. 28. Plant Power Green Energy DSSC: BIOMIMICRY

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