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Photosynthetic Electricity A Cheap
Sustainable Energy Solution presented by The
Barnard College Department of Chemistry and The
Barnard Chemical Society
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 Earths 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.
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Dye Sensitized Solar Cell (DSSC)
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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.
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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.
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.
10 mm thick film of TiO2
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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.
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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.

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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)

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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!

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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.

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Where to find it

• 1.) Smestad G.P., Gratzel. M., Demonstrating
  Electron Transfer and Nanotechnology A Natural
  DyeSensitized 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.) http//www.dyesol.com
• 4.) http//www.solideas.com
• 5.)http//www.physics.usyd.edu.au/foundation/Outre
  ach/STW/proceedings/ned.pdf
• You can also contact Professor James McGarrah at
  Barnard College.
• http//www.barnard.edu/chem/Directory_McGarrah_2.h
  tm