The document describes an experiment where students created dye-sensitized solar cells using blackberry juice. They coated conductive glass with titanium dioxide to create the anode and graphite to create the cathode. The titanium dioxide was sensitized with blackberry dye then assembled into cells with the cathode and iodine electrolyte. The three cells were connected in series to a multimeter and lightbulb. The circuit generated 172mV and powered the lightbulb, demonstrating the cells' ability to convert light to electricity through the anthocyanin dye.
1. Pappas, McKee, Hartheimer 1
NJ Chemistry Olympics 2014
Application of Anthocyanins in Blackberries:
A Dye-Sensitized Solar Cell
Samantha Pappas
Emma McKee
Joline Hartheimer
Northern Highlands Regional High School
2. Pappas, McKee, Hartheimer 2
Background:
Solar cells produce energy by employing the principles of the photoelectric effect,which says
that electrons are ejected from a metal surface when light is shined on it. This radiation consists of
packets of energy, today called photons. According to Planck's formula, equation 1, the energy of these
photons is equal to the product of planck's constant (h) and the frequency of the light (ν).
Equation 1 E = hν
If a photon of light strikes a low energy electron in an atom, the electron becomes excited and
moves to a higher energy level; however the unstable electron quickly returns to its ground state,
releasing energy to its surroundings in the process. In conventional solar cells, such as silicon and thin-
film solar cells, a hole is created in the electron’s initial location when that electron is excited from its
ground state; this hole and the electron formerly occupying its space separate completely. However,in
excitonic cells, the electron remains bound to its positively charged hole, forming an electron-hole pair
called an exciton that requires an interface between an “electron transfer material” and a “hole transfer
material” to split and separately migrate to different electrodes.1
There are two types of excitonic solar cells: dye-sensitized, whose light-active component is a
molecular dye2
, and organic, whose light-active component is an organic polymer3
. The focus of this
research will be on dye-sensitized solar cells (DSSCs), also called Grätzel cells after their creator Michael
Grätzel. These photovoltaic cells employ low to medium purity materials and use low-cost processes,
creating cells that are both more efficient and more cost effective than traditional cells: the combination of
the high surface area of the semiconductor film and the ideal spectralcharacteristics of the dye allows for
high efficiencies of over 80% for the conversion of light energy from photons to electrical current.4
An n-
type material, a semiconductor that has been doped by a donor impurity that adds an excess valence
electron to its lattice causing the amount of free electrons to outnumber the available holes,5
such as TiO2
allows current to be generated when a photon absorbed by a dye molecule leads to an electron being
excited and projected into the conduction band, the band energy where positive mobile charge carriers, or
3. Pappas, McKee, Hartheimer 3
holes, exist,6
of the semiconductor4
. The rough semiconductor surface created by the nanometer-sized
TiO2 particle film allows for a larger number of dye molecules to be adsorbed onto it due to the increased
surface area4
,further increasing the efficiency of the cell. Sintering the TiO2 films deposited onto the
conductive glass in the form of colloidal solutions allows for greater electronic contact between particles.4
For the circuit to be complete, the dye needs to be regenerated through electron transfer with redox
species in solution that then is reduced at the cathode.4
In this experiment, blackberry juice was chosen as the dye. Blackberries contain plant pigments
called anthocyanins, which are water-soluble phenolic compounds belonging to the group of plant
flavonoids; however the content of anthocyanins, even in fruits of the same type, varies due to many
genetic and environmental factors.7
The anthocyanins attach themselves to the titania molecules due to
due to their hydroxyl bonds, which the titanium dioxide molecules also contain, as seen in figure 1.
Figure 112
When light strikes the semiconductive glass, it kicks off a cycle of redox reactions, which mimics
those of photosynthesis. The electrolyte iodine solution acts as a salt bridge for the reaction inducing an
equilibrium process that allows for the transfer of ions, and the graphite as a catalyst, while the redox
reaction occurs between the TiO2 and anthocyanins in the dye. Figure 2 shows how the electrons flow
through a typical dye sensitized solar cell when struck by light.
4. Pappas, McKee, Hartheimer 4
Figure 213
The efficiency of a solar cell can be calculated determining the ratio of power produced per
square inch. Power (P) in watts is equal to voltage (V) in volts squared over resistance (R) in ohms, as
seen in equation 2.
Equation 2 P = V2
/R
Solar cells are a source of energy that the world, most notably the United States, has recently
started to explore. In the year 2013, 29% of new energy generation capacity came from solar power,
which is greater than the amount produced by every other source besides natural gas. The amount of
megawatts of photovoltaic cells installed in 2014 is expected to be 26% greater than that of 2013, and the
PV cells to be implanted in 2014 are predicted to harness enough solar energy to power 1.13 million
American homes. The prices of solar cells have been decreasing, which makes them more feasible for
families or organizations to purchase them.8
2013 also brought about a 20% increase in the amount of
people employed for solar jobs.9
5. Pappas, McKee, Hartheimer 5
Materials and Methods:
The anode was created by coating a piece of conductive glass with a prepared titanium dioxide
paste and covering the dry titanium dioxide film with a prepared blackberry dye. The titanium dioxide
paste was made with 0.682 g of titanium dioxide powder which was first massed on a weighing dish and
then placed into a 50 mL beaker. 1 M nitric acid was then diluted to 0.001 M using 1 mL of the 1 M nitric
acid, transferred with a volumetric pipet to a 100 mL volumetric flask. The flask was filled with distilled
water to the 100.00 mL line. Approximately 10 drops of the 0.001 M nitric acid were added with a
disposable pipet to the powder, which was stirred with a glass stirring rod until the texture resembled the
consistency of white-out and no clumps of powder remained. One piece of the conductive glass was
obtained and wiped with a Kimwipe wet with isopropanol, followed by a Kimwipe wet with distilled
water. The glass was allowed to dry before the slide was tested with a multimeter on the resistance setting
to determine the conductive side. With that side face up, the edges of the slide were taped with Scotch
tape to the table to add stability. The tape was smoothed out to remove air bubbles by pushing a glass
stirring rod across its surface. A disposable pipet was used to place 3 drops of the titanium dioxide paste
along the top tape border of the cell. Then the stirring rod was used to spread and coat the paste evenly
over the exposed glass. After waiting a few minutes for the paste to dry, the pieces of tape holding the
cell to the table were carefully removed and the cell was transferred to a hot plate. It was heated at 450ºF
for approximately twenty minutes until the heat was turned off and the cell was allowed to cool resting on
the hot plate. While the titanium dioxide paste was being sintered, the blackberry dye was prepared. Ten
blackberries were placed into a 250 mL beaker and about 10 drops of distilled water were added with a
disposable pipet. The bottom of a 150 mL beaker was used to crush the fruit into a dye. Once cool, the
glass slide was removed from the hot plate and a disposable pipet was used to place enough drops of dye
on top of the dry titanium dioxide film to completely saturate it. The slide was allowed to rest on the table
for 10 minutes before the excess dye was rinsed off by pouring both distilled water and isopropanol over
the surface of the glass. The completed anode was then blotted dry with another Kimwipe.
6. Pappas, McKee, Hartheimer 6
The cathode was prepared by coating another piece of conductive glass with a layer of graphite to
function as a catalyst. The second piece of glass was cleaned with a Kimwipe wet with isopropanol
followed by another Kimwipe wet with distilled water. Both sides of the glass were then tested for
conductivity using the resistance setting on the multimeter. It was ensured that the slide lay with the
conductive side face up and that side was “colored in” thoroughly with a pencil to add a complete layer of
graphite to the surface. The procedure up to this point was repeated to produce 3 anodes and 3 cathodes.
Before the cells were assembled,an iodine electrolyte solution was prepared using about 10 g of
solid potassium iodide pellets and about 0.5 g of solid iodine. Both amounts were massed and placed into
a 50 mL beaker which was then filled with distilled water until the total volume of the solution was about
30 mL. The solution was mixed with a glass stirring rod until the solid iodine pieces dissolved.
The solar cells were constructed by placing the graphite cathode on the bottom and the dye-
covered titanium dioxide anode on the top, both pieces slightly offset so that an edge of glass stuck out on
each side. 2 small binder clips per cell were used to hold the slides in place, and alligator clip electrodes
were clamped to the offset edges, as seen in figure 3.12
Figure 3
The 3 fully assembled solar cells were then connected in series with alligator wires to each other and to
both a multimeter and a small LED light bulb. To activate the cells, a disposable pipet was used to place a
few drops of electrolyte solution along an offset edge of each cell. The binder clips were then opened and
closed to draw the solution into the center of the cell.
8. Pappas, McKee, Hartheimer 8
Picture:
Data:
Voltage produced by circuit 172 mV
Resistance 0.01 KΩ
Calculations:
P = (0.172 V2
)/100 Ω = 0.000296 watts/6 inches2
= 0.000049 watts/inch2
9. Pappas, McKee, Hartheimer 9
Works Cited
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3. The Solar Spark. Organic and Hybrid PVs. http://www.thesolarspark.co.uk/the-
science/solar-power/excitonic-solar-cells/opvs-hpvs/ (accessed April 9, 2014).
4. O’Regan, B; Grätzel, M. A low-cost, high efficiency solar cell based on dye-sensitized
colloidal TiO2 films. Letters to Nature 1991, 353, 737-740.
5. UC Davis SolarWiki by University of California, Davis. ChemWiki. I. P-Type, N-Type
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6. UC Davis GeoWiki by University of California, Davis. ChemWiki. Band Theory of
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http://chemwiki.ucdavis.edu/Physical_Chemistry/Quantum_Mechanics/Electronic_Struct
ure/Band_Theory_of_Semiconductors (accessed April 9, 2014).
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