The document summarizes the process of investigating different natural dyes as photosensitizers for dye sensitized solar cells. It describes how dyes were extracted from 48 different plant species using ethanol or acetonitrile. TiO2 electrodes were prepared and immersed in the dye solutions. Two types of liquid electrolytes, with and without 4-tert-butylpyridine, were also prepared. Dye sensitized solar cells were fabricated by sandwiching the dyed TiO2 electrode and a platinum cathode with the liquid electrolyte in between. The cells were then characterized to analyze their photoelectrical parameters.

1. Investigation of Different Natural Dyes as
Photosensitizers for Dye Sensitized Solar Cells
S.Palamakumbura
2009/07/31

2. Acknowledgements
My period of training at the Institute of fundamental studies would not be
a successful one if not for the guidance and encouragement given by many
outstanding professionals. The most enjoyable part of this project report is
to thank them all from the depths of my heart.
I am very grateful to Prof.C.B.Dissanayake, Director of the Institute of
fundamental studies for granting me permission to work as a volunteer/trai-
nee. A special person to me is Dr.G.K.R.Senadeera under whom I worked
throughout my period at IFS. Thanks you sir, for your guidance. Its truly
an honor to work under your supervision.
My heart filled gratitude also goes out to Mr.C.A.Thotawatthage. Like a
teacher you advised me in my work at IFS. I appreciate your helpfulness. It
was a pleasure to work with Mr.T.R.C.K.Wijayarathna and Mr.Y.P.Y.P.Ari-
yasinghe. Their vast knowledge and experience gave strength to my work.
I am truly indebt to Mr.I.P.L.Jayarathna and Mr.I.G.C.K.Kumara for the
valuable time they spent in helping me.
I am grateful to my beloved parents whose help cannot be expressed in
words. They are always behind me in my work. Throughout my training
period there were many friends whose company I whole heartedly enjoyed.
Their generosity and kindness would always be remembered.
i

3. Contents
0.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
0.2 Experimental techniques and method . . . . . . . . . . . . . . 3
0.2.1 Preparation of natural dye sensitizers . . . . . . . . . . 3
0.2.2 Preparation of TiO2 electrode . . . . . . . . . . . . . . 4
0.2.3 Preparation of liquid electrolytes . . . . . . . . . . . . 4
0.2.4 Fabrication of dye sensitized solar cel . . . . . . . . . . 5
0.2.5 Characterization and measurement . . . . . . . . . . . 5
0.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . 6
0.3.1 The effect of 4-tert-butylpyridine (TBP) on the effi-
ciency of dye sensitized solar cells using natural dyes . 6
0.3.2 Photoelectrical parameters of dye sensitized solar cells
fabricated using natural dyes . . . . . . . . . . . . . . . 7
0.3.3 Methods to improve the conversion efficiency of dye
sensitized solar cells fabricated using Oxalis hedyseroides
as photo sensitizer . . . . . . . . . . . . . . . . . . . . 11
0.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
0.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
ii

4. 0.1. INTRODUCTION 1
0.1 Introduction
Fossil fuel is the main energy supplier of the modern world having an 86%
share in primary energy production. It had been estimated the deposited
fossil fuel is sufficient for only fifty years. To meet the increasing demand of
energy, fossil fuel is no longer a cheap and reliable energy source. In that
context new methods are being innovated and implemented in recent times.
One such attempt is the dye sensitized solar cell invented by Michael Gratzel
and Brian ORegan in 1991.
Dye sensitized solar cell is a new class of solar cells which converts solar
light into electrical energy. Comparing to the semiconductor solar cells, dye
sensitized solar cells are made through a simple and low cost manufacturing
procedure. This is the main reason for the interest it has gained during the
past few years.
As depicted in Figure 1 dye sensitized solar cell mainly consists of three
components.
1. The anode is a glass plate coated with a conducting material (Fluorine
doped Tin Oxide or Indium doped Tin Oxide) on one side. On top of
the conducting surface a highly porous TiO2 layer is coated.
2. A glass plate coated with platinum metal as the cathode.
3. The electrolyte which is mainly an organic solvent containing a redox
couple. (Usually I−
/I−
3 )
The anode is immersed in the dye solution to absorb a thin layer of dye
molecules into the TiO2 surface. Then the electrolyte is spread over the
cathode. The two electrodes are then joined together.
When the dye sensitized solar cell is exposed to sunlight photons which
enter through the transparent anode strike the dye molecules so as to create
an excited state of the dye. (See Figure 2)
D + hv −→ D∗
The excited dye molecules will release electrons to the conduction band of
TiO2.
D∗
−→ D+
+ e
The dye cations would quickly regain their electrons from Iodide ions in the
electrolyte oxidizing them into Tri iodide ions.
3I−
−→ I−
3 + 2e

5. 0.1. INTRODUCTION 2
Figure 1: Schematic Diagram of Dye sensitized solar cell
D+
+ e −→ D
This reaction occurs quickly preventing the recombination of dye cations and
electrons which would short-circuit the solar cell. The ejected electrons flow
through the external circuit and combine with Tri iodide ions reducing them
into iodide ions.
I−
3 + 2e −→ 3I−
As seen from the above reactions Iodide and Tri iodide ions are kept at
equilibrium thus keeping the cell active as long as there are Photons with
sufficient energy to excite dye molecules.
From the time dye sensitized solar cells were invented; improving the ef-
ficiency had been the primary objective. Cell performance mainly depends
on the dye used. Synthesized various photosensitizes were tried out to im-
prove the efficiency of dye sensitized solar cells. Among them Ruthenium
polypyridyl complexes were found out as one of the most effective sensi-
tizers. However, Ruthenium polypyridyl complexes contain heavy metals.
Therefore Ruthenium polypyridyl complexes are not suitable in terms of en-
vironmental concerns. In addition the complicated process that has to follow
in synthesizing Ruthenium polypyridyl complexes makes them costly thus
eliminating from commercial use. As a result interest is focused on natural
dyes as sensitizers. In this study dye sensitized solar cells were fabricated us-
ing dyes extracted from forty eight different plants which are widely available
in tropical countries.

6. 0.2. EXPERIMENTAL TECHNIQUES AND METHOD 3
Figure 2: Working mechanism of dye sensitized solar cells.
0.2 Experimental techniques and method
0.2.1 Preparation of natural dye sensitizers
Dyes were extracted from plants using the following methods.
Betel leaves (Piper betle) and Areca nuts (Areca catechu) were mixed
together in similar portions and grinded thoroughly. Afterwards the mix-
ture was kept in 95%wt Ethanol for twenty four hours. A clear dye Betel
and Areca nut mixture was obtained via removing the solid residues after
centrifuging.
Powdery form of Red Sandalwood (Pterocarpus santalinus), Turmeric
(Curcuma longa), Coscinium tenestratum (locally called Venivalgata), Com-
mon Madder/Walmadata (Rubia cordifolia) and Ebony (Diospyros ebenum)
were kept in 95%wt Ethanol for twenty four hours. Dye solutions were ex-
tracted by removing solid residues through centrifuging.
Mustard (Brassica nigra) was grinded vigorously and then kept in 95%wt
Ethanol for twenty four hours. Solid residues were filtrated out.
The following plant samples were cut into small pieces and kept in 95%wt
Ethanol and or Acetonitrile for twenty four hours. Then solid residues were
filtrated out. Garden balsam (Impatiens balsamina), Hibiscus rosa sinensis
(orange colour), Brunfelsia gandiflora (locally known as yesterday,today and
tomorrow), Begonia Rex(Black beauty), Nerium Oleander (locally known as
Kaneru), Parrot beak (Heliconia Pendula), Hibiscus schizopetalus, Crown
of thorns (Euphobia milii), Butterfly pea (Clitoria ternatea), Cockscomb

7. 0.2. EXPERIMENTAL TECHNIQUES AND METHOD 4
(Celosia cristata), Canna indica (colour red), Canna indica (yellow with red
colour dots), Grapes (Vitis vinifera), Mangoostin (Garcinia mangostana),
Fire fern(Oxalis hedysaroides), Orange (Citrus sinensis), Madagascar peri-
winkle (Catharanthus roseus), Prosopis juliflora, Heliconia bihai, Creeping
woodsorrel (Oxalis corniculata), False shamrock (Oxalis regnellii), Averrhoa
bilimbi (locally known as Biling), Starfruit (Averrhoa carambola), Eupho-
bia caracasana, Peacock flower (Caesalpinia pulcherrima), Fiddlehead jat-
ropha (Jatropha pandurifolia), Kings mantle (Thunbergia erecta), Rose apple
(Syzygium samarangense), Hibiscus mutabilis(pink colour), Nutmeg (Myris-
tica fragrans), Rangoon creeper (Quisqualis indica), Egyptian starcluster
(Pentas lanceolata), Nephelium lappaceum (locally known as Rambutan),
Poinciana (Delonix regia), Garcinia cambogia (locally known as Goraka),
Lily of the incas (Alstroemeria aurantiaca), Flame lily (Gioriosa superba),
Exacum trinervium (locally known as Binara)
0.2.2 Preparation of TiO2 electrode
TiO2 paste was prepared by blending 0.2g of TiO2 power, 12 drops of Acetic
acid (99.74%wt), 1 drop of Triton X-100 and 2ml of Ethanol (95%wt). The
mixture was ground vigorously for 30 minutes until a uniform TiO2 paste
was formed. Conductive glass plates (FTO, Fluorine doped Tin Oxide) of
2cm x 0.5cm area were used to apply the TiO2 paste. TiO2 paste was spread
on about 0.5cm x 0.5cm area (doctor blade method). Then the glass plates
were sintered at 450C for 45 minutes. After cooling to room temperature
the TiO2 electrodes were immersed in the dye solutions for appropriate time.
Dyed films (electrodes) were then removed from the dye solutions and dried
with a hot air flow.
0.2.3 Preparation of liquid electrolytes
In this study two different electrolytes were prepared.
1. Electrolyte with 4-tert-butylpyridine.
0.783g of Tetrapropylammonium Iodide, 0.06g of Iodine, 3.6ml of Ethy-
lene carbonate, 0.35ml of 4-tert-butylpyridine and 1ml of Acetonitrile
were mixed together and kept without exposing to the direct sunlight.
2. Electrolyte without 4-tert-butylpyridine.
0.783g of Tetrapropylammonium Iodide, 0.06g of Iodine, 3.6ml of Ethy-
lene carbonate and 1ml of Acetonitrile were mixed together and kept
without exposing to the direct sunlight.

8. 0.2. EXPERIMENTAL TECHNIQUES AND METHOD 5
Figure 3: Typical I-V curve of a solar cell.
0.2.4 Fabrication of dye sensitized solar cel
Dye sensitized solar cells were fabricated by sandwiching the liquid electrolyte
between the TiO2 electrode (anode) and counter electrode (cathode). Then
the two electrodes were clipped using crocodile clips.
0.2.5 Characterization and measurement
Current-Voltage (I-V) curves of the cells were plotted under standard solar
irradiation of 100mWcm-2 supplied by a Xe-lamp. A multimeter (Keithley
2000) and a Potentiostat (Hokuto Denko, HA301) coupled with a computer
was used for data acquisition. The absorption spectrums were acquired by us-
ing a UV-3000 UV-VIS spectrophotometer. (Shimadzu Corporation, Japan)
Based on the I-V curve (Figure 3) the fill factor (FF) is defined as follows.
FF =
Imax × Vmax
Isc × Voc
Where Imax and Vmax are current and voltage at the maximum power output
level. Isc and Voc are short circuit current density and open circuit voltage
respectively.
When the Intensity of the incident light is Pin (Wm−2
), the power con-
version efficiency of the cell (η) can be defined by the following equation.
η =
Jsc × Voc × FF
Pin

9. 0.3. RESULTS AND DISCUSSION 6
Figure 4: Performance of dye sensitized solar cells fabricated using natural
dyes based on two different electrolytes. (Electrolyte with TBP and elec-
trolyte without TBP) 1) Fire fern 2) Mangoostin skin 3) Mangoostin pulp
4) Grapes 5) Orange 6) Mangoostin/Orange/Grapes)
Where Voc is open circuit voltage (V) and Jsc is short circuit current density
(Am−2
).
0.3 Results and Discussion
0.3.1 The effect of 4-tert-butylpyridine (TBP) on the
efficiency of dye sensitized solar cells using nat-
ural dyes
Six dye sensitized solar cells were fabricated using the following natural dyes;
Fire fern (Oxalis hedysaroides), Grapes ((Vitis vinifera), Mangoostin skin
((Garcinia mangostana), Mangoostin pulp ((Garcinia mangostana), Orange
((Citrus sininsin) and a mixture of Mangoostin / Grapes / Orange in similar
proportions. Two cells were fabricated for each dye using electrolyte with
TBP and electrolyte without TBP. Then conversion efficiency of solar power
to electrical power was measured and depicted in Figure 4.
It was observed that higher efficiencies can be achieved by cells with
electrolyte containing no TBP. Therefore electrolyte without TBP was used
hereafter in the purpose of testing natural dye sensitized solar cells.

10. 0.3. RESULTS AND DISCUSSION 7
0.3.2 Photoelectrical parameters of dye sensitized so-
lar cells fabricated using natural dyes
Table 1 shows the photoresponses of dye sensitized solar cells prepared with
different dyes. Betel leaves (Piper betle) and Areca nuts (Areca catechu) mix-
ture, Red Sandalwood (Pterocarpus santalinus), Turmeric (Curcuma longa),
Coscinium tenestratum (locally called Venivalgata), Common Madder (Ru-
bia cordifolia), Ebony (Diospyros ebenum), Mustard (Brassica nigra), Gar-
den balsam (Impatiens balsamina), Hibiscus rosa sinensis (orange colour),
Brunfelsia gandiflora (locally known as yesterday,today and tomorrow), Be-
gonia Rex(Black beauty), Nerium Oleander (locally known as Kaneru), Par-
rot beak (Heliconia Pendula), Hibiscus schizopetalus, Crown of thorns (Eu-
phobia milii), Butterfly pea (Clitoria ternatea), Cockscomb (Celosia cristata),
Canna indica (colour red), Canna indica (yellow with red colour dots), Grapes(Vitis
vinifera), Mangoostin (Garcinia mangostana), Fire fern (Oxalis hedysaroides),
Orange (Citrus sinensis), Madagascar periwinkle (Catharanthus roseus), Prosopis
juliflora, Heliconia bihai, Creeping woodsorrel (Oxalis corniculata), False
shamrock (Oxalis regnellii), Averrhoa bilimbi (locally known as Biling), Star-
fruit (Averrhoa carambola), Euphobia caracasana, Peacock flower (Caesalpinia
pulcherrima), Fiddlehead jatropha (Jatropha pandurifolia), Kings mantle
(Thunbergia erecta), Rose apple (Syzygium samarangense), Hibiscus muta-
bilis(pink colour), Nutmeg (Myristica fragrans), Rangoon creeper (Quisqualis
indica), Egyptian starcluster (Pentas lanceolata), Nephelium lappaceum(locally
known as Rambutan), Poinciana (Delonix regia), Garcinia cambogia (locally
known as Goraka), Lily of the incas (Alstroemeria aurantiaca), Flame lily
(Gioriosa superba), Exacum trinervium (locally known as Binara) were used
to fabricate dye sensitized solar cells.
Table 1: Conversion efficiency (η), Fill factor (FF), Short circuit
current density (Jsc) and Open circuit voltage (Voc) of dye solar
cells sensitized with different dyes
Dye Voc(mV ) Jsc(mAcm−2
) FF% η%
Fire fern (Oxalis hedysaroides) 422 1.528 60.78 0.392
Egyptian starcluster (Pentas lanceolata) 562.7 0.88 62.04 0.307
Begonia Rex (Black beauty) 467.3 0.804 71.17 0.267
Turmeric (Curcuma longa) 579.7 0.56 70.2 0.229
Hibiscus schizopetalus 575.4 0.137 72.59 0.229
Poinciana (Delonix regia) 574 0.444 70.08 0.178
Mangoostin skin (Garcinia mangostana) 609.9 0.404 68 0.167
Garcinia cambogia (locally known as Goraka) 419.1 0.804 47.91 0.162
Hibiscus rosa sinensis (orange colour) 621.7 0.396 64.76 0.16
Common Madder(Rubia cordifolia) 517.2 0.4 69.93 0.144
Continued on the next page

11. 0.3. RESULTS AND DISCUSSION 8
Dye Voc(mV ) Jsc(mAcm−2
) FF% η%
Crown of thorns (Euphobia milii) 544 0.101 65.76 0.145
Parrot beak (Heliconia Pendula) 602.5 0.324 72.43 0.141
Hibiscus mutabilis(pink colour) 599.6 0.081 70.56 0.137
Betel leaves (Piper betle) 625 0.3 67.9 0.127
and Areca nuts (Areca catechu) mixture
Mangoostin pulp (Garcinia mangostana) 625.3 0.296 66.9 0.123
Butterfly pea (Clitoria ternatea) 618.2 0.244 71.27 0.108
Orange (Citrus sinensis) 627.1 0.232 74.06 0.108
Euphobia caracasana 551.7 0.28 67.84 0.105
Rangoon creeper (Quisqualis indica) 561.4 0.264 65.74 0.097
Madagascar periwinkle (Catharanthus roseus) 575.9 0.228 72.8 0.096
Canna indica (colour red) 591.7 0.272 58.16 0.093
Averrhoa bilimbi leaves (locally known as Biling) 663.5 0.208 61.86 0.085
Cockscomb (Celosia cristata) 602.1 0.2 69.12 0.083
Rose apple (Syzygium samarangense) 519.2 0.22 73.26 0.083
Averrhoa bilimbi flowers (locally known as Biling) 531.1 0.204 71.24 0.077
Exacum trinervium (locally known as Binara) 614.4 0.18 68.88 0.077
Grapes(Vitis vinifera) 492.8 0.244 63.2 0.076
Lily of the incas (Alstroemeria aurantiaca);(Red colour) 601.6 0.172 68.95 0.071
Lily of the incas (Alstroemeria aurantiaca);(Pink colour) 595.8 0.156 73.48 0.069
Fiddlehead jatropha (Jatropha pandurifolia) 527.5 0.204 62.27 0.067
Flame lily (Gioriosa superba) 562.5 0.168 70.53 0.067
Canna indica (yellow with red colour dots) 616.1 0.192 50.66 0.06
Coscinium tenestratum (locally called Venivalgata) 531.3 0.16 68.67 0.059
Nephelium lappaceum(locally known as Rambutan) 495.6 0.196 60.9 0.059
Red Sandalwood (Pterocarpus santalinus) 426.6 0.276 47.99 0.057
Prosopis juliflora 592.2 0.136 66.9 0.054
Garden balsam (Impatiens balsamina) 553.7 0.12 73.95 0.048
Starfruit (Averrhoa carambola) 627 0.116 65.24 0.048
Kings mantle (Thunbergia erecta) 631.1 0.096 79.88 0.048
Peacock flower (Caesalpinia pulcherrima) 600.4 0.12 65.75 0.047
False shamrock (Oxalis regnellii) 382.3 0.168 66.51 0.043
Ebony (Diospyros ebenum) 502.9 0.116 67.47 0.04
Brunfelsia grandiflora 618.4 0.096 68.01 0.04
(locally known as yesterday,today and tomorrow)
Mustard (Brassica nigra) 689.2 0.12 45.46 0.038
Heliconia bihai 598.7 0.072 69.4 0.03
Nutmeg (Myristica fragrans) 580.7 0.06 61.52 0.022
Creeping woodsorrel (Oxalis corniculata) 378.4 0.064 66.87 0.016
Nerium Oleander (locally known as Kaneru) 641.1 0.004 17.51 0
Among the dye solar cells tested, cells fabricated with Oxalis hedysaroides,
Pentas lanceolata and Begonia Rex (Black beauty) showed best photoelec-
trical characteristics. The current-voltage curves obtained from these cells
were shown in Figure 5.
The highest conversion efficiency was recorded from Oxalis hedysaroides

12. 0.3. RESULTS AND DISCUSSION 9
Figure 5: Current-Voltage characteristics of the dye sensitized solar cells
fabricated with different dyes. (a) Oxalis hedysaroides (b) Pentas lanceolata
(c) Begonia Rex (Black Beauty)
(Table 1). Figure 6 show the absorption spectra of dye solutions (a) Oxalis
hedysaroides (b) Begonia Rex (Black Beauty). Both dyes show absorbance
in the visible region. But Oxalis hedysaroides absorbs more light than Bego-
nia Rex (Black Beauty) especially in the wavelength range from 550 nm to
700 nm. The relatively high conversion efficiency of Oxalis hedysaroides is
probably due to this higher absorption in the above region than other dyes
(other absorption spectra were not included for clarity). The dark I-V curve
of the cell fabricated using Oxalis hedysaroides as photo sensitizer is shown
in Figure 7.

13. 0.3. RESULTS AND DISCUSSION 10
Figure 6: Absorption spectrums of (a) Oxalis hedysaroides (b) Begonia Rex
(Black Beauty)
Figure 7: Dark I-V curve of solar cell sensitized with Oxalis hedysaroides

14. 0.3. RESULTS AND DISCUSSION 11
0.3.3 Methods to improve the conversion efficiency of
dye sensitized solar cells fabricated using Oxalis
hedyseroides as photo sensitizer
The following methods were investigated to improve the efficiency of cells
fabricated using Oxalis hedysaroides dye.
1. ZrO2 was added to the TiO2 paste in the molar ratio [TiO2] : [ZrO2] =
95 : 5
2. Added 10 drops of 0.025M CuCl2 to the TiO2 paste.
3. Five drops of 1.55x10−3
M Trisodium citrate was added to the TiO2
paste.
4. Addition of 0.2M Guanidinium Thiocyanate to the electrolyte.
5. Increasing the thickness of the TiO2 paste by coating several times.
6. Immersing time of the TiO2 electrode in Oxalis hedysaroides dye solu-
tion is changed.
Addition of ZrO2, CuCl2, Trisodium citrate, Guanidinium Thiocyanate, in-
creasing the thickness of the TiO2 paste, did not have expressive impact on
cell performance. It was observed that changing the immersing time of TiO2
electrode in Oxalis hedysaroides dye solution significantly affects cell effi-
ciency. As shown in Figure 8 the highest conversion efficiency was achieved
when the TiO2 electrode was immersed for two minutes.
The current-voltage curve of the cell which was immersed for two minutes
in the dye solution is shown in Figure 9.
Considering the above results it can be concluded that, the highest effi-
ciency of dye solar cell sensitized with Oxalis hedysaroides can be obtained
by immersing the semiconducting film in the dye solution for two minutes.

15. 0.3. RESULTS AND DISCUSSION 12
Figure 8: Variations of cell efficiency due to the immersing time of TiO2
electrode.
Figure 9: Current-Voltage curves of Oxalis hedysaroides when the immersing
time of TiO2 electrode is 2 minutes.

16. 0.4. CONCLUSION 13
0.4 Conclusion
Dye sensitized solar cells were fabricated using forty eight different natural
dyes as photosensitizes. Among them Fire fern (Oxalis hedysaroides), Egyp-
tian starcluster (Pentas lanceolata) and Begonia Rex (Black beauty) reported
best photoelectrical characteristics, with conversion efficiencies of 0.392%,
0.307%, 0.267% respectively. Dye solar cells sensitized with Turmeric (Cur-
cuma longa), Hibiscus schizopetalus, Poinciana (Delonix regia), Mangoostin
skin (Garcinia mangostana), Garcinia cambogia (locally known as Goraka)
and Hibiscus rosa sinensis (orange colour) showed moderate performance.
Oxalis hedysaroides which had the highest conversion efficiency was further
improved by changing the immersing time of TiO2 electrode in the dye so-
lution. An optimum short circuit current density of 3.088mAcm−2
and a
conversion efficiency of 0.823% were recorded when the immersing time was
adjusted to two minutes.
Moreover the effect of 4-tert-butylpyridine (TBP) in the electrolyte, on
the performance of dye sensitized solar cells was investigated. The results
indicated that electrolyte without TBP is more suitable for use with natural
photosensitizes as it gives higher conversion efficiencies than electrolyte with
TBP.
Further studies should be carried out to enhance the cell performance of
Fire fern (Oxalis hedysaroides), Egyptian starcluster (Pentas lanceolata) and
Begonia Rex (Black beauty) sensitized solar cells. Especially investigations
should be carried out to improve film morphologies of the semiconductor
TiO2 layer.

17. 0.5. REFERENCES 14
0.5 References
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