How does the Grätzel Solar Cell Work?

As discussed in more detail in one of the other links, a traditional solar cell is based upon two types of silicon sandwiched together. One type of silicon is doped with an element or substance (phosphorus, for example) having an extra electron (n-type) and the other is doped with an element or substance (boron, for example) having fewer electrons than silicon (p-type). When these two types of silicon are sandwiched together, a p-n junction is formed. This p-n junction creates an electric field acting as a diode permitting electric current to flow in one direction. When a photon strikes the solar cell, an electron-hole pair is created. Before the electron-hole can be re-united, the electron is attracted to the n-type silicon and the hole is attracted to the p-type silicon. The energized electron is forced to travel through a complete circuit if available, giving up some of its extra energy. This electron eventually recombines with a hole in the p-type silicon.

The Grätzel Solar Cell (nano-crystalline dye sensitized) solar cell is a photoelectrochemical cell. Instead of creating electron-hole pairs, solar photons give up their energy to excite electrons found in the fruit dye. These energized electrons give up in an electrical circuit.

Here is a picture of the sandwiched components of the Grätzel solar cell:

As indicated in the picture, the sandwich is bordered by the two conductive glass slides. To one of the glass slides, we painted the nanoparticle titanium dioxide and then stained it with the dye. On the other glass slide, we carbonized it with the graphite pencil and the candle soot. Between the glass slide we added the liquid triiodide electrolyte.

The Grätzel solar cell is then placed into the sun:

 

Photons strike the cell and their energy is absorbed by the dye. The dye has several important properties. It must be complexed or chelated (attached) to the titanium dioxide and it must be able to absorb the photons' energy, exciting and freeing some of its electrons. The basic structure of the anthocyanin pigment (those pigments found in the fruit dyes we were using) was found on the internet.

 

The nanoparticle titanium oxide acts as a scaffold to hold the dye molecules into its 3 dimensional array. The following image was reproduced from the manual delivered with the kit to build the cells:

There are many different variations of anthocyanins but these are the pigments that produce the red, blue, violet, and orange colors we see in fruits and flowers. The different variations of anthocyanins aborbs different wavelengths of photons of the visible and ultraviolet spectrum.

 

Because of the small size of the titanium dioxide nanoparticles (10-300 nanometers), many dye molecules are attached after staining providing many photoelectrons produced. The nanoparticles increase this available surface area 100-1000 times (relative to the area of the glass squares) enhancing dye attachment, porosity, and consquently, photoelectron production. The following image is a rollover picture of the titanium dioxide nanoparticles (pictures courtesy of University of Washington researchers).

These excited electrons from the dye are transferred or injected into the conduction band nanoparticle titanium dioxide. The titanium dioxide acts as a n-type semiconductor (like n-type silicon). The injected photoelectrons move along the nanoparticles towards the top conducting plate (anode). With the thin layer of titanium dioxide (on the order of microns) , the excited electrons do not need to travel far to reach the anode.

Once the photoelectrons reach the anode, the photoelectrons migrate through the electrical pathway and the extra energy is converted to electrical energy by devices in the circuit (loads).

What is the purpose of the triiodide electrolyte? The dye has given up electrons and is now deficient of electrons, being oxidized by the titanium dioxide. The triiodine electrolyte supplies electrons to replenish the deficiency reducing the dye molecules back to their original states. The triiodide electrolyte is now oxidized and electron-deficient but recovers its missing electrons by migrating toward the cathode (conducting glass plate at the bottom of the cell also called the counter electrode). Electrons migrating through the circuit reach the counter electrode and recombine with the oxidized triiodide electrolyte. The triiodide electolyte liquid acts as a true catalyst as it is not consumed in the reactions taking place.


 

There has been discussion about the similarities between the mechanism of action of the Grätzel solar cell and photosynthesis in green plants. In aerobic photosynthesis, photons, carbon dioxide, and water combine to produce carbohydrates (glucose) and oxygen. In the case of photosynthesis, pigments such as chlorophyll a, chlorophyll b, xanthophylls, and carotenoids absorb energy from photons. This absorbed energy excite electrons; these electrons are moved around inside the chloroplasts found in plant cells and through many reactions, ATP and NADPH molecules are formed. Through additional reactions glucose and carbohydrates are produced.

Here is a particularly good animation of photosynthesis and the many reactions that take place. Several more are here.

In essence, we can come up with similar thoughts between the Grätzel solar cell and photosythesis

subsystem Grätzel solar cell Photosynthesis
Electron Acceptor: Nanoparticle Titanium Dioxide carbon dioxide
Electron Donor triiodide electrolyte water
Photon Absorber Fruit dye chlorophyll

Resources:

http://www.rsc.org/Education/EiC/issues/2007Sept/HarnessingSolarEnergyGratzelCells.asp

http://www.elp.uji.es/juan_home/research/solar_cells.htm

http://www.newscientist.com/article/mg16121705.200-sun-traps.html

http://www.smm.org/buzz/buzz_tags/nano

http://jcwinnie.biz/wordpress/?p=2386

http://en.wikipedia.org/wiki/Graetzel_cells

http://en.wikipedia.org/wiki/Anthocyanins

http://science.nhmccd.edu/biol/bio1int.htm#photo (biology tutorials)

nanocrystalline solar kit model instructions


Alan D. Gleue

Lawrence High School

Lawrence High School Science Department

back to Mr. Gleue's home page


Created: June, 2008.
Modified: July, 2008.