Solar Cells

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By conservative estimates the world demand for energy is projected to more than double by 2050 to 27 TW (TW = 1 trillion joules per second). Currently, nearly all energy production comes from the burning of fossil fuels (oil, coal and natural gas) which produces carbon dioxide, CO2, as a direct consequence. In order to keep CO2 levels in check, to slow the consequences of global warming, the the world will have to generate more than 10 TW of power from carbon-free sources by 2050. Harnessing solar energy is an attractive option amongst the different carbon-free alternatives (wind, geothermal, hydroelectric, and nuclear). Approximately 120,000 TW of solar energy strikes the Earth’s surface, capturing only a fraction could supply all of our energy needs. Although incremental improvements in silicon based solar cells have lowered cost and improved efficiency the technology has little room to gain in order to be competitive against relatively plentiful and cheap fossil fuels. In order for large scale adaptation of this technology to occur the cost needs to drop by a factor of 10 in order to be at on par with current means of electricity generation. The following is a demonstration of an alternative type of solar cell made from simple, common and relative cheap materials. Science and technology has yet to be discovered and developed in order to meet this tremendous challenge. We invite you to take a moment and build your own solar cell. Photosynthetic Electricity: A Cheap Sustainable Energy Solution presented by The Barnard College Department of Chemistry and The Barnard Chemical Society

Dye Sensitized Solar Cell (DSSC): 

Dye Sensitized Solar Cell (DSSC)

Titanium Dioxide (TiO2): That Sounds Expensive: 

Titanium Dioxide (TiO2): That Sounds Expensive TiO2 provides whiteness and opacity to products such as paints, coatings, plastics, papers, inks, foods, and most toothpastes. TiO2 is used in sunscreens to block harmful UV B radiation from the sun. Small particles (~ 10-1 m or 1/100th the thickness of a human hair) are dispersed in the sunscreen solution.

Porous TiO2 Network: 

Porous TiO2 Network To the left is a scanning electron microscope image of TiO2 in a DSSC. The scale bar is 60 nm. 10 nm particles are fused together to form a 10 mm thick film of porous TiO2. 10 mm thick film of TiO2 A paste of nanometer TiO2 particles and viscous organic compounds is spread on to transparent conductive glass (F doped SnO2). The film is then heated in an oven to 450 °C burning off the organic paste leaving behind a fused network of TiO2 particles.

Blueberry Dye: 

Blueberry Dye Anthrocyanin is a highly colored molecule that naturally occurs in raspberries, blueberries and beetroot. It binds to the TiO2 film. When visible light is absorbed by the dye an excited distribution of electrons is formed. This in turn transfers an electron to the TiO2 which leads to the generation of electricity in the DSSC.

How does a solar cell work?: 

How does a solar cell work? A solar cell requires a material that acts as a semiconductor, neither a conductor (like a copper wire) or an insulator (like a grant counter top). In a solar cell, upon absorption of light electrons are promoted to the conduction band (CB) leaving behind holes in the valence band. One of two processes can occur (1) the electrons and holes make it to the solar cell contacts and the energy is converted into electricity or (2) the electrons and holes recombine insode the semiconductor to generate heat. The maximum power (current x voltage) is at Pmax. The open circuit volatage, Voc, is the maximum volatage obtained in the system and the short curcuit current, Isc, in the largest current that can be obtained by the system.

Putting It All Together: 

Putting It All Together Blueberry stained TiO2 glass slide Conductive glass slide PARAFILM® (acts as a spacer) Electrolyte (iodide/triiodide in ethylene glycol (antifreeze) Two clips Two alligator clip connectors Voltmeter (ampmeter)

Method – Part 1: 

Method – Part 1 Obtain two slides of conductive glass and coat a small square section of TiO2 on one slide Allow the paste to dry and heat in an oven at 45o oC for 1 hour. Allow to cool. Crush blueberries using mortar and pestle Make up a solution of 25mL methanol, 4mL acetic acid and 21mL water in a beaker Add the methanol/water/acid solution to fruit and crush fruit Filter solution into a second beaker Using tweezers, place glass slide, titanium dioxide side up, and leave for an hour NOTE: DO NOT TOUCH TITANIUM DIOXIDE PART WITH FINGERS!

Method – Part 2: 

Method – Part 2 Prepare iodide electrolyte from 0.5M potassium iodide mixed with 0.05M iodine in anhydrous ethylene glycol. Remove glass slide from dye solution using tweezers and wash in water, then isopropanol. Dry, using paper towels Place spacers (PARAFILM®) around the blueberry dye and put 3 drops of electrolyte solution on top of the dye. Place conductive glass, conductive side down, partially over titanium dioxide slide and squeeze edges together with fingers. Secure the glass slides together with clips Connect cell to multimeter and measure short circuit current and open circuit voltage - use aluminium foil to make contacts To increase conductivity, try scribbling with a graphite pencil on the top piece of glass before covering the TiO2 slide.

Where to find it: 

Where to find it 1.) Smestad G.P., Gratzel. M., Demonstrating Electron Transfer and Nanotechnology: A Natural Dye–Sensitized Nanocrystalline Energy Converter. Journal of Chemical Education, 75 (6), 1998. 2.) O'Regan, B., and M. Gratzel, 1991. A low-cost, high efficiency solar cell based upon dye-sensitized colloidal TiO2 films. Nature 353, 737-740. 3.) 4.) 5.) You can also contact Professor James McGarrah at Barnard College.