Further Investigations of Photovoltaic Cells using Plant Pigments
Hunter Porter, Brian Tetreault, Baylee Tatum, and Jim Bidlack
University of Central Oklahoma, Edmond, OK 73034
Abstract
Additional experiments are being pursued to determine the viability of using
various plant pigments and parts, from the same plant, in dye-sensitized solar
(photovoltaic) cells. Cells were created using glass doped with a thin film of tin
oxide, one side with titanium dioxide annealed onto the plate and then treated with
various plant-derived materials, serving as the anode, and another was coated with
graphite, acting as the cathode. Anodes were treated drop-wise with chlorophyll,
chloroplasts, and anthocyanin extracted from the leaves of Purple Heart
(Tradescantia pallida) in order to impregnate the titanium dioxide with dye.
Lugol’s solution (KI/I2) was added as an electrolyte and the two cell halves were
fixed together using super glue. Cells were then connected to a voltmeter and
output was measured. Output of various cell treatments are currently being
evaluated in a longevity study using a Pico Recorder. At this time, treatment cells
are producing at least 200 millivolts (open circuit) with some cells boasting over
1,000 millivolts of output for periods of time.
Introduction
Dye-sensitized solar cells, or Gratzel cells, are a rather recent object of study. These
cells are based on a key relationship between a semiconductor and a pigment used to
attract light, similar to that of the chloroplast’s structure and the different pigments
involved, consisting of one compound drawing light in and the other using the
energy to release electrons into a circuit (Grätzel, 2003). The photoreceptive
pigment is able to be manipulated in order to improve quality and/or price of
production. One such avenue of pigment research is focused on organically derived
pigments from plants. These pigments are widely available and cheap to produce.
Multiple plant-based dyes are tested in solar cells, such as black rice and rosa
xanthia (Hao, et. al. 2006). The viability of the pigments directly involved in
photosynthesis (chlorophylls) are also tested, but the test pigments used are based
on structures related to or derived from chlorophyll rather than the actual pigment
(Kay & Gratzel, 1993). Past research at UCO has also tested the viability of whole
chloroplasts being used as a photoreceptor (Porter, Tetreault & Bidlack, 2013).
However, testing the viability of naturally complementary pairs of chlorophylls,
chloroplasts and other plant pigments is a novel idea. Experiments examining the
effects of anthocyanin, crude chlorophyll extract, and whole chloroplasts
individually and in combination of chloroplasts/chlorophylls and the natural cooccurring anthocyanin attempt to determine the yield possible and overall yield-tocost ratio of dye-sensitized solar cells utilizing the aforementioned photoreceptors.
The Purple Heart plant (Tradescantia pallida) was chosen due to possession of
accessory pigment and availability. See Figure 1.
Materials and Methods
Solar Cell Construction
Cells construction was performed starting with two glass plates coated on one side with
tin oxide. These plates were then separately treated to create anodes and cathodes.
Anode construction involved spreading an even layer of titanium dioxide mixture,
consisting of a ratio of 0.5 grams of titanium dioxide to 1 ml of acetic acid solution (ph
3) to 0.25 ml of Triton X, into a square shaped mold onto the conductive tin-coated side
of a glass plate (this ratio seems to provide the best liquid consistency for application of
anode layer). This layer was then heated to over 500 degrees Celsius in order to anneal
the titanium dioxide onto the glass, finishing the anode construction. Cathode
construction consisted of crosshatching the tin-coated glass plates with a pencil, leaving
traces of graphite behind. The cell halves, after treatment with dye, then had a
potassium iodide/iodine solution added to facilitate electron transfer and are finished by
being glued together using superglue. Cell halves were glued at a slight offset in order
to allow the attachment of wires to measure and utilize generated energy.
Treatment Isolation and Application
Chloroplasts were isolated by homogenizing 5 grams of finely diced purple heart leaves
leaves in a sugar grinding solution, which is then centrifuged multiple times to pellet
chloroplasts and allow removal of other cell parts (Bidlack 2012). Chloroplasts were
examined under a light microscope to ensure limited damage and lack of free pigments.
Anthocyanin extraction consisted of boiling Purple Heart leaves in a beaker of
deionized water over a hot plate. See Figure 2. The boiling water was a quality solvent
for anthocyanin and the pigment moved into solution. Chlorophyll extraction was
performed crudely by mixing leaf parts vigorously with acetone until the solution was
saturated with chlorophyll. All treatments were mixed with adequate amounts of sugar
grinding solution to control for the extra energy that would only be present in the
chloroplast solution due to its solvent of choice. See Figure 3.
Treatments were applied randomly using a number sequence generator from
Random.org. The conditions included chlorophyll, chloroplasts, anthocyanin,
chlorophyll with anthocyanin, chloroplasts with anthocyanin, and a sugar grinding
solution control. Treatments received an arbitrary number categorization in order to be
randomly assigned to each arbitrarily numbered cell. The cells were then treated dropwise with 50 microliters of treatment solution every hour for 5 hours. See Figure 4.
After the final drop treatment, cells were left to dry for an hour and then final cell
construction proceeds as described above.
Electrical Measurement and Recording
Attaching constructed cells to a voltmeter using alligator clips allowed for voltage
measurement. See Figure 5. This is then added to a circuit including a rheostat in order
to vary resistance. This varied resistance along with voltage readings were used for
calculation of current and power output across multiple resistances, which is useful for
selecting resistors.
After 1 week had elapsed, cells were measured for open circuit voltages using a
voltmeter. See Table 1. All cells were measured in an environmental chamber with
artificial sunlight and controlled temperature. See Figure 6
Treatment
Number of
cells
Mean Voltage
Standard
Deviation
Control
4
137.25
82.56
Chlorophyll
5
158.20
89.16
Chloroplast
6
449.16
319.22
Chlorophyll &
Anthocyanin
3
297.67
87.51
Chloroplast &
Anthocyanin
5
220.40
52.45
Anthocyanin
6
163.17
124.34
Table 1 – Cell treatments and voltages
Figure 5 – Voltmeter connected to cells in the environmental chamber
Results and Discussion
Cells that recorded an error value on the voltmeter (produced no voltage) were
excluded from analysis. ANOVA found significant difference between cell
treatments (F = 2.665, p < .048). Levene's test indicated that the assumption of
homogeneity of variance was violated, p < .001. After correcting for the violation of
homogeneity of variance, no significant difference was found between treatments.
Cell construction requires many procedures that are hard to replicate and qualitative
in nature. The massive differences in standard deviation despite meticulous attempts
at replication point to the need for a less human-influenced construction method.
Methods explored include spray pyrolysis and electrophoretic deposition. These
methods involve high cost equipment and were not feasible for such a project, but
future research will require a more controllable approach. Figure 3 demonstrates one
issue with the current construction recipe, as some charred tape residue can be seen
on the sides of cells. This effect has not been verified, but it is suspected to hinder
conductance by impeding contact of conductive material.
The large standard deviation values, occurring even in the control condition, make
claims about treatment quality invalid when considering this study. A repetition with
better control for cell quality is necessary to accurately test the validity of dyes.
Figure 6 - Hunter Porter and
Baylee Tatum working in the
environmental chamber
Acknowledgements
Figure 2 – Extraction of anthocyanin from Purple Heart Leaves.
Funding for this project was provided by a Research, Creative, and Scholarly Activities
(RCSA) grant from the Office of Research and Grants at the University of Central Oklahoma.
Literature Cited
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Bidlack, James E. (2012). Plant Physiology Laboratory Manual Spring 2012.
Grätzel, M. (2003). Dye-sensitized solar cells. Journal of Photochemistry and Photobiology
C: Photochemistry Reviews, 4, 145-153.
Hao, S., Wu, J., Huang, Y. & Lin, J. (2006). Natural dyes as photosensitizers for
dye-sensitized solar cell; Solar Energy, 80(2), 209-214.
Kay, A. & Grätzel, M. Artificial photosynthesis. 1. photosensitization of titania solar cells
with chlorophyll derivatives and related natural porphyrins. Journal of Physical
Chemistry, 97(23), 6272-6277.
KENPEI's Photo (2007). Tradescantia pallida 'Purpurea.' [Photo]. Retrieved from
upload.wikimedia.org/wikipedia/commons/d/dc/Tradescantia_pallida_Purpurea1.jpg
Porter, H., Tetreault, B. & Bidlack, J. (2013). Use of chloroplasts and anthocyanin in
photovoltaic cells. Poster presented at Oklahoma Research Day 2013.
Figure 1 - Purple Heart plant growing – Photo: By KENPEI (KENPEI's
photo)[GFDL (http://www.gnu.org/copyleft/fdl.html)
Figure 3 – Prepared cell treatments. Left to right: anthocyanin, chloroplasts,
chlorophyll, chlorophyll mixed with sugar solution, anthocyanin, chlorophyll and sugar
mixture, anthocyanin with sugar solution, and anthocyanin, chloroplast and sugar
mixture.
Figure 4 – Treated cells drying with labels. Anode side only.