Nanocrystalline Dye Sensitized Solar Cells or Gratzel Cell
The nanocrystalline dye sensitized solar cell is a photoelectrochemical cell. It resembles
natural photosynthesis in two respects: (1) it uses an organic dye to absorb light and
produce a flow of electrons (2) it uses multiple layers to enhance both the light absorption
and electron collection efficiency. Like photosynthesis, it is a molecular machine that goes
beyond microelectronics technology into the realm of nanotechnology.
Nano – A SI prefix meaning 10-9
of a unit. One nanometer is one
billionth of a meter
Sensitization – The process by
which a transparent substance
is made to respond to light. This
involves the transfer of energy,
or electron, to the transparent
substance by a compound that
absorbs light
How is a DSSC made?
To create the nanocrystalline solar cell, a suspension of nanometer size particles of
titanium dioxide, TiO2, is placed on a conductive glass plate. The TiO2 film is dried and then
heated on the glass to form a porous, high surface area TiO2 film. When magnified, it looks
like a thin sponge or membrane.
The TiO2 film on the plate is then dipped into a solution of a dye. Many dyes can be used,
but they must possess a chemical group that can attach (adsorb) to the surface and must
have energy levels at the proper positions necessary for electron injection and
sensitization. A single layer of dye molecules adsorbs to each particle of the TiO2 and acts as
the adsorber of the light. To complete the device, a drop of liquid electrolyte containing
iodide is added and a counter electrode, coated with a thin catalytic layer of platinum or
carbon, is placed on top. The sandwich should be illuminated through the TiO2 side.
How does it work?
Photons strike the cell and their energy is absorbed by the fruit dye. Depending on the dye
used, different energy levels of photons are absorbed. This is similar to the ways in which
the pigment chlorophyll preferentially absorbs light of specific wavelengths. The goal is to
maximize absorption over the visible solar spectrum to produce the maximum energized
electrons.
The recommended fruit dyes contain anthocyanin
pigments of which there are many. Anthocyanins are
pigments that absorb photons around the 520-550
nm range. The dye must be attached or chelated to the
TiO2 and it must be able to absorb the photons’
energy, exciting and freeing some of its electrons. The
TiO2 nanoparticle acts as a scaffold to hold the dye
molecules into its 3-D array. Additionally, the small
size of the TiO2 nanoparticles (10-300 nm) allows
many dye molecules to attach after staining, providing
an opportunity to produce many photoelectrons.
These nanoparticles increase the available surface
area 100-1000 times. At Washington University in St. Louis, researchers are working to
create columns of TiO2 to further increase the surface area available attachment of dyes
and/or photosynthetic organisms and photoelectron production.
The excited electrons from the dye are transferred or injected into the conduction band
nanoparticle TiO2. The TiO2 acts as an n-type semiconductor. The injected photoelectrons
move along the nanoparticles toward the top conducting plate (anode). Once the
photoelectrons migrate through the electrical pathway and the extra energy is converted to
electrical energy by devices (loads) in the circuit. The amount of electrons flowing through
the load is the current and the available energy per electron is the voltage or electrical
potential. The triiodie electrolyte serves as a mediator by supplying electrons to replenish
the electron deficient dye molecules back to their original states. The triiodide electrolyte
does this by migrating toward the cathode (counter electrode). At the counter electrode,
electrons that migrated through the circuit reach the counter electrode and recombined
with the oxidized triiodide electrolyte. The triodide electrolyte liquid acts as a catalyst and
is not consumed in the reactions taking place.
Comparison between DSSC and Photosynthesis
In aerobic photosynthesis, photons, carbon dioxide and water combine to produce 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 excites
electrons. Then 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.
In a DSSC the interconnected TiO2 particles serve as the electron acceptor, the iodide serves
as the electron donor, and the dye functions as the pump that excites the electrons to
higher energy levels by using the energy of the light that is absorbed. This energy
configuration is like that found in natural photosynthesis in which the electron acceptor is
carbon dioxide, water is the electron donor, and chlorophyll is the photochemical pump
(see Table 1).
Table 1. Comparison between DSSC and Natural Photosynthesis
Subsystem
Dye Sensitized Solar Cell
Photosynthesis
Electron Acceptor
Nanoparticle TiO2
Carbon Dioxide
Electron Donor
Triiodide Electrolyte
Water
Photon Absorber
Fruit dye
Chlorophyll
Unlike photosynthesis, the oxidation and reduction processes in the dye sensitized solar
cell produce electricity in an external electrical circuit. The voltage produced is obtained
from the difference in energy levels between the TiO2 and the triiodide mediator and
depends on the dye used as well as the condition of the TiO2 and number of iodide
molecules oxidized. The current produced by the cell is directly proportional to the amount
of light (photons) absorbed by the dye, which depends on the intensity of the illumination.
To calculate the power output, measured in watts, of any solar cell multiply the current and
the voltage flowing through the load.