Titanium Dioxide Dye Sensitized Solar cells
DISCLAIMER : - The drawings, procedures and words are for information only. No claims are expressed or implied as to the safety, usefulness, or accuracy of this information. I will not accept any liability for any damages caused to people or property from the using of this information or from any associated links. Your actions are your responsibility - VERIFY and CHECK information out before proceeding, and don't attempt anything without the required skills, if you cannot agree to this, leave this page now . . . . . . . Chris.
There has been some fantastic leaps forward in the last 5 years in solar cell technology. Amorphous cells have been produced using "thin film" technology and are also available with "tri-layer" construction. This allows much higher efficiencies because each layer is selective to a particular band of light frequencies which combine to produce greater output. Deposition rates of semiconductor material are about 10% of what used to be laid down and so has greatly brought down manufacturing costs, (and are more "eco-friendly" to produce.)
Another recent development is the "Titania Dye Sensitized Cells" which use titanium dioxide as the substrate in place of silicon. TDS cells use 2 transparent sheets of glass with conductive coatings and an electrolyte sandwiched between, thus allowing them to be used as a window, (A heavily tinted one! - there is similar development in sun-glasses too.) They are at present producing electricity commercially at about 10% efficiency and produce about 50 watts per square metre. When used as a window they have the potential of being able to reduce the heat gain into a building and also provide power to it. When considering the surface area of high-rises etc , a lot of power could be collected (bags more than enough for "additional" lighting - and smart building design could overcome any lighting problems anyway)
Part of the attractiveness of this type of cell is
it's potentially low cost to produce and relatively simple construction.
Titanium dioxide is fairly cheap and has been used in antibacterial
applications, deodorization, waste water treatment and other decomposition
processes of organic compounds and pollutants. (It is an oxidizing agent that
is activated by light). It has a anatase crystalline structure, is white to
semi-transparent, soluble in water or alcohol with a Ph of 2+-0.5. The use in
commercial solar cells requires a higher purity colloidal powder which is more
expensive however. The electrodes and the exposure to light is provided by a
glass sheet sandwich with conductive coatings of Silicon Dioxide. The titanium
dioxide is treated with a synthetic ruthenium bipyridyl based dye on the
incoming light surface and works inconjunction with an electrolyte of
iodide/triiodide to the other conductive surface to produce a voltage
potential. The "back" layer has a catalyst coating upon it's SnO2
layer such as carbon. The photo-excited dye injects an electron through the
TiO2 layer which is passed to the SnO2 surface and out to the external circuit.
The SnO2 layer is conductive because of the existence of oxygen vacancies which
act as donors. Within the iodide electrolyte it (iodide/triiodide) undergoes
oxidation at the dye and regeneration at the catalyst coated SnO2 electrode at
the opposite side, thus maintaining and electrolyte balance and completing the
circuit. This type of solar cell will eventually have superior performance
particularly in lower light levels over the typical PV
"semiconductor" types because it doesn't suffer from the
electron-hole recombination in the semiconductor material which seriously
affects the efficiency of PV cells. (the processes of light absorption and
charge separation are differentiated, in PV, absorbtion of light causes the
charge separation)
Since the cell will transmit light through it, it may be possible to use the
"amorphous tri-layer" idea and provide several (many) cells
sandwiched using thin films and different dyes that are selective to different
wavebands of light. - time will tell?
* Light passes through the glass plate and SnO2
coating to stimulate the organic dye (anthocyanins) to produce free electrons.
* The injected electrons are passed through the TiO2 layer and collected at the
SnO2 conductive surface and out to an external circuit.
* The electrolyte gives up electrons to the dye to regenerate it, and in the
process is converted to triiodide.
* The external circuit flows back to the catalyst coated electrode to balance
the electrolyte again with electrons. (triiodide back to iodide)
More exciting news is that these cells can be easily
manufactured by a home constructor or in a classroom for a school project. The
dye used in the commercial cells is hard to synthesize but can be replaced by
naturally occurring juices from "blackberries, raspberries, pomegranate
seeds, beets, hibiscus tea leaves, citrus leaves" etc , and are known as
flavonoids. Flavonoids have anthocyanins which are present in fruit and plants
and are a robust naturally occurring "dye", the trade off however is
performance.
This makes this an ideal study project because it uses natures processes,
photosynthesis, oxidation, reduction and catalysts etc, and can be tied and
compared to many of the cycles and process on our world, and ties in chemistry,
physics, optics etc.
Conductive glass plates can be purchased or prepared chemically and are
available in any size you want. Care should be taken to minimize contact and
handling of the surfaces as this deposits "oils" on the surface which
will degrade the cells performance. An ohm-meter will help to determine
"which side" has the conductive surface, if you can't see the
difference.
Construction: -
The deposition of nanocrystaline TiO2 upon the
conductive glass plate is done by grinding TiO2 degussa P25 powder in a mortar
while adding solvent (about 20ml nitric or acetic acid - pH 3 - 4 in
demineralized / deionized water to 12g of powder - adding slowly) and spreading
the solution across the conductive plate by rolling a glass rod across the
surface to evenly and thinly spread it. (Mask an area off with tape that will
form the electrical contact). this yields a film about 7-10um thick with a
porous nature which helps with the absorption of the dye and admittance of
light. Allow to air dry and then heat with a 450 deg C flame for about 10 -15
minutes, or use a hot air gun. Allow to cool and then soak in the dye of your
choice for 10 minutes. The cyanin dye extract is absorbed quickly which forms a
substance capable of electron injection and is sensitive to light, but if any
"white" is showing (undyed TiO2) resoak until a uniform surface is
attained. The plate is then washed in deionized water, dried, and then washed
again in isopropanol alcohol. The "back" conductive surface is
completely coated with a graphite rod or with a soft lead pencil to cover it
evenly. ( it won't worry the electrical contact surface) Then heated to anneal
the carbon upon the surface. (platinum could be deposited chemically as an
alternative catalyst) The surface is then washed with alcohol and allowed to
dry. The cell can now be assembled with the electrolyte between. The
electrolyte is a mixture of 0.5M potassium iodide mixed with 0.05M iodine in
water free ethylene glycol. Place the TiO2 coated plate down first , coatings
up, place the catalyst plate on top with it's coatings down, and the plates
offset so the electrical connection to the SnO2 on each plate can be made. Drop
2 or 3 drops (per square inch of plate area) of iodide/iodine electrolyte
solution onto the side of the plates (contact point), lift the catalyst plate
up slightly and the solution will be drawn in evenly between the plates. (Add
electrolyte until the entire surface is "wet"). The cell can be
sealed with a low melting point polymer film such as "Dupont Surlyn
1702" which is widely used in food packaging. The cell is now complete and
should produce power at about 1 to 2 Ma / cm2 and produce about 0.3 to 0.7
volts. Better results can be generated by purifying the cyanin extract or using
a chlorophyllide extracted from chlorophyll (natural dye). { Chlorophyll and Cu
have energy conversion efficiencies of 2.6% and generate 9.4mA/cm2, in the
laboratory optimizing everything, photocurrents of 20mA/cm2 and efficiencies of
10.4% have been achieved}.
Homemade cells may deteriorate over time but can be revived by washing with
alcohol and reassembling with fresh electrolyte.
This has been produced with the hope that more people may become more curious about solar energy and may even decide to flirt with it. A TiO2/dye cell should be able to be produced by anyone willing to be careful and try, and certainly within the capability of the average high school labs. Special thanks must go to "Greg P. Smestad and Michael Gratzel" for producing a highly informative, practical article on this subject which I have relied heavily on. The article can be found at JChemEd.chem.wisc.edu. and provides much more information on procedures, optimization and the chemistry, and is certainly well worth a visit and reading. This technology really should get more support, and hopefully it may even drive solar panel prices down so they are generally more affordable to the average person, and possibly then there will be a more general acceptance of "solar energy" and "sustainable practices" . (Fuel companies have moved to dominate the "solar energy" scene for their own ends, and somehow I don't think it will be necessarily good for you or me, or our planet.)
LINKS to other sites: -
Sol Ideas Technology Development - Greg. P. Smestad site with "kits" available for experimentation.
Article for schools- for the construction and discussion of a cell by Greg Smestad
Index to scientific papers- mainly on dye sensitized solar cells
Curtin engineering and science- dye sensitized solar cells
Swiss Federal institute of
technology- A very good
article on dye sensitized solar cells
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