1. Solar Asia 2011 Int. Conf., Institute of Fundamental Studies, Kandy, Sri Lanka ( 28-30 July 2011)
229
NANOCRYSTALLINE TIO2 PHOTO-SENSITIZED WITH NATURAL DYES
P. ABEYGUNAWARDHANA1
, S. PALAMAKUBURA2
, C.A. THOTAWATTAGE 2
,
M.A.K.L. DISSANAYAKE2
AND G.K.R. SENADEERA1,2*
1
Department of Physics, The Open University of Sri Lanka, Nawala, Nugegoda, Sri Lanka.
2
Institute of Fundamental Studies, Hantana Road, Kandy, Sri Lanka
* Corresponding Author, e-mail: gkrsena@yahoo.com
ABSTRACT
Dye sensitized solar cells (DSSCs) are one of the most promising and fast developing photovoltaic
technology, specially due to its low cost comparing with the other photovoltaic technologies. Generally
DSSCs consist of ruthenium (II) polypyridine complexes as sensitizers of wide band gap semiconductors
like TiO2. However the cost in synthesis of these dyes with rare metal complexes is still an expensive
process. Therefore, many of the researchers including us have been investigating the possibilities to
replace these expensive dyes with various materials like conducting polymers, natural dyes, etc.
Therefore, in this work we extended our investigations employing natural dyes extracted from plants
widely spread over the Sri Lankan territory.
Among the 65 studied natural dyes, Hibiscus sabdariffa ,Fire fern (Oxalis hedysaroides), Begonia
(Black velvet) ,and Ficus salicifolia sensitized solar cell showed comparatively higher photo responses.
The short circuit current densities (Jsc) were ranging from 2.4 to 0.9 mA cm-2
, while the open circuit
voltages varied from 565 mV to 425 mV. The highest efficiency obtained from Hibiscus sabdariffa,
was ≈ 0.5 % . However , dramatic enhancement in efficiency in these cells were obtained from the acid
treated dye extracts with con HCl. The DSSCs fabricated with acid treated dye solution obtained from
fire fern leaves showed ≈ 1% efficiency showing its promising usage in low cost DSSCs for novel
disposable bio sensor applications at room temperature under low light conditions.
1. INTRODUCTION
Dye sensitized photoelectrochemical solar cells (DSSCs) are devices for the conversion of
visible light into electricity based on sensitization of wife band gap semiconductors like TiO2.
DSSC is assembled with a cathode of mesoporous nanocryastalline TiO2 film on a conductive
glass substrate anchored a monolayer of dyes, an anode of conductive glass coated with platinum
and an electrolyte of certain organic solvents containing a redox couple such as iodide/triiodide.
The adsorbed dye absorbs visible light and injects electrons into the conduction band of TiO2.
Since dye plays an important role in absorbing visible light and transferring photon energy into
electricity, much attention has been paid to survey the effective sensitizer dyes. No doubtedly, one
of the most efficient sensitizers are synthetic dyes, such as ruthenium(ii) polpyridyl complexes
with carboxcilic ligands, since these compounds present intense and wide range absorption of
visible light as well as suitable ground and excited state energy levels with respect to the band
positions of TiO2 and the redox potential of electron donor I-
. (1). Apart from that many organic
and inorganic dyes including natural dyes have employed in these devices but the performances of
them in DSSCs were poor due to the week binding nature to the semiconductor TiO2 (2-6).
However, in nature, fruits, flowers and leafs of plants show various colours from red to purple and
contain various natural dyes. Because of the simple preparation techniques, widely source and low
cost, natural dye as an alternative senstizer for DSCs is promising , specially for some practical
applications such as usage of DSSCs in portable, disposable bio sensors (7-9). Therefore, the
present work extends our investigations involving natural dyes as semiconductor sensitizers and
reports some of the dyes which can be used successfully in DSSCs as sensitizers aiming to use
them in fabrication of low cost bio sensors.

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2. EXPERIMENTAL
2.1 Extraction of pigments
Petals of flowers and barks of trees chosen were cut into small pieces and extracted into
ethanol (Fluka, 96%(v/v)), by keeping for overnight. Then residual parts were removed by
filtration and the filtrate was washed with hexane several times to remove any oil or chlorophyll
present. The ethanol fraction was separated and divided into two solutions and one fraction was
used as it was (without any further treatments) and the other fraction was treated with few drops of
conc. HCl acid so that the solution color becomes more dark (pH < 1. These solutions were
directly used as the dye solutions for preparation of photovoltaic devices.
2.1 Preparation of TiO2 electrodes
Fluorine doped tin oxide (FTO) glass slides were cleaned well. The dimension of each
glass slide was 2 cm x 0.5 cm. TiO2 (Degussa) powder of average particle size 25 nm was
grounded well with Triton, acitic acid and ethanol. A thin film of the resultant mixture was
deposited on FTO glass slides using the ‘doctor blade method’. After leaving sometime for drying,
only an area of 0.5 cm x 0.5 cm was left by scratching off the rest. Films were then sintered at
450°C for 45 minutes and cooled down to room temperature.
2.3 Fabrication of photo-electrochemical cells (PECs)
Above TiO2 electrodes were dipped in each dye solutions for 12 hours. Dyed electrodes
were then removed and rinsed with ethanol and dried using an airflow. DSSCs were constructed
by introducing the redox electrolyte containing tetrabutylammonium iodide (TBAI)(0.5M) / I2
(0.05M), in a mixture of acetonitrile and ethylene carbonate (6:4, v/v) between the dyed TiO2
electrode and Platinum counter electrode.
2.4 Characterization
UV–visible absorption measurements of dyes in ethanol were carried out with a
Shimadzu dual wavelength/double beam spectrophotometer (model UV-3000). Current voltage (
I-V) characteristics of the cells at 100 mW cm-2
(Am 1.5) were measured using a home made I-V
measuring setup coupled with Keithly 2000 multimeter in to HA-301 Potentiostat via computer
controlled software. Xenon 500 W lamp was used with AM 1.5 filters to obtain the simulated
sunlight with the intensity of 100 mW cm-2
. The I-V curves were used to calculate the short-circuit
photocurrent density (JSC), the open-circuit voltage (VOC), the fill factor (FF), and the energy
conversion efficiency (η) of DSSC.
3. RESULTS AND DISCUSSIONS
Figure 1 (A) shows the UV-visible absorption spectra of different dye extracts in ethanol.
Since the purpose of this study is to use the dye extracts without any pigmential isolation,
elucidation of exact structures were not carried out in this stage. However, according to the
literature (10,11 ) the red extracts obtained from fire fern, begonia black velvet and hibiscus
sabdariffa, which have the main absorption wave bands at 500 to 580 nm, it would be possible to
presume that all these dyes contain cyanidin, peonidin, pelargonidin, malvidin, delphinidin,
quercetin and flavonol co-pigments. Further, since the red-coloured varieties would contain lower
content of delphinidin derived pigments, but a more variable amount of flavonolglycosides, the
extracts obtained from above species should contain higher concentrations of above pigments.

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Figure 1. (A) UV-visible absorption curves of (a) Fire fern (b) Begonia black velvet (c) Hibiscus sabdariffa, (d) Ficus
salicifolia (B) and (C) Basic chemical structures of the most abundant anthocyanidins .
The other species employed in this study, orange-red coloured Ficus salicifolia , which
mainly absorb in the region below 475 nm , would contain a large amount of cyanidin 3,5-O-
diglucoside and relatively lesser amount of flavonol-glycosides (7,10,11 ). However, in general,
can be seen from the figure, they all absorb in the visible part of the spectrum. In the case of
extracts from fire fern the absorption is border than that of the others and it absorbs light from 400-
700 nm with an absorption in higher wavelengths.
While figure 2 shows the current –voltage (I-V) curves of the DSSCs employed with
different pigments as indicated, table 1 presents the performances of DSSCs in term of short-
circuit photocurrent densities (Jsc) , open circuit voltage (Voc) , fill factor (FF) and the power
conversion efficiencies (η). The efficiency of cell sensitized by the fire firm was significantly
higher than DSSCs sensitized by the other pigments. This is in good agreement with the broader
range of the light absorption of fire firn on TiO2 , and higher interaction between TiO2 and
anthocyanin as depicted in the figure 4. The DSSCs employed with Ficus salifcifolia dye has the
lowest photoelectric conversion, which again agrees with the weaker absorption spectrum of the
dye.
Figure 2. Current –voltage (I-V) characteristics of, (i) Fire fern (ii) Hibiscus sabdariffa (iii) Begonia black velvet (iv) Ficus
salifcifolia
i
ii
iii
iv
(B)
(A)
(C)

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Table 1. photovoltaic performances of DSSCs employed with extracted flower pigments
Name Jsc
(mA
cm-2
)
Voc
(mV)
FF η% Flower/leave/bark
(a) Fire fern 2.354 422 49 0.49
(b) Hibiscus sabdariffa 1.968 457 50 0.45
(c) Begonia black velvet 1.140 408 56 0.26
(d) Ficus salifcifolia 0.864 565 52 0.25
The effect of pH of these dye solutions were also investigated. The original pH of these
extracted were found to be in the range from 3.00 to 4.5. However, in this preliminary studies pH
of these extracts were changed to the values lower than 1 (pH <1) by adding trace amounts of conc
HCl. Figure 3 (A) shows the chelation mechanism of anthocyanidins with TiO2 while figure
3(B) presents the possible reaction of the pigments with HCl as proposed by Hao et al (7 ).
According to the literature, in an acidic solution, the oxonium ion results in an extended
conjugation of double bonds through three rings of the aglycone moiety, which helps in the
absorption of photons in the visible spectra. Addition of a base disrupts the conjugation of double
bonds between the second and third rings and results in absorption of photons in the UV range,
rather than in the visible range (7,10,11 ). Therefore, the change in pH on increasing the number of
conjugated double bonds in the molecule, lowers the energy level of the electronic transition
between the ground state and the excited states and in turn results in the absorption of photons at
greater wavelength (Figure not shown).

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233
Figure 3 (A) Chelation mechanism of anthocyanidins with TiO2, (B) Two chemical structures of anthocyanidins in acidic
and basic media.
Figure 4 presents the I-V characteristics of the DSSCs employed with acidified pigment
extracts. The photoelectric parameters of DSSCs sensitized with acidified extracts under the
irradiance of 100 mW cm-2
were summarized in table 2. As can be seen from the table, the acidity
of dye solution is found to affect the resulting photocurrent values. Again, it can be seen from the
data presented in the table 2, fire firn exhibits the best photoelectrochemical performances among
the others. The photoelectric power conversion efficiency ( η) of DSSCs sensitized with acidified
fire fern is two times higher than that of the non acidified dye.
Figure 4. Current –voltage characteristics of pigments after acidification (i) Fire fern (ii) Begonia black velvet (iii) Fiscus
salifcifolia (iv) Hibiscus sabdariffa
(i)
(ii)
(iii)
(iv)
(A)
(B)
BasicAcidic

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Table 2. photoelectrochemical parameters of the DSSCs sensitized by acidified pigment extracts
Jsc (mA cm-2
) Voc (mV) FF η%
(i) Fire fern 3.09 438 61 0.82
(ii) Begonia black velvet 2.03 426 59 0.506
(iii) Ficus salifcifolia 1.48 442 52 0.339
(iv) Hibiscus sabdariffa 1.12 443 56 0.298
The Highest Occupied Molecular Orbital (HOMO) level and the Lowest Unoccupied
Molecular Orbital level (LUMO) of three extracts were calculated using cyclic voltammograms
and the UV-visible absorption spectra, according to the method described elsewhere (12 ) and
tabulated in the table 3.
Table 3. Estimated physical parameters of the dye extracts obtained from cyclicvoltametry and UV0visible absorption
spectroscopic techniques.
Name of the dye solution Band Gap (eV)
HOMO (eV) LUMO (eV)
Fire Fern 2.13 -5.01 -2.88
Begonia(Black velvet)
3.10 -4.90 -1.80
Hibiscus sabdariffa
3.43 -4.99 -1.57
As it is evident from the , table 3, the excited state energy levels for the extracts are higher
than the energy level of TiO2 conduction band edge (–4.40 eV) ( 1,2,19) showing that the electron
injection should be possible energetically. Therefore, upon illumination of the cell, dye extracts
absorb light and get excited from the HOMO level to the LUMO level and eject electrons to the
conduction band of TiO2, which transports these electrons to the transparent bottom electrode
(FTO) and then to the other end of the cell via external load and being received by the redox
mediator (I3
–
/I –
) in the electrolyte. Finally the electrolyte (I3
–
/I –
) regenerates the dyes to their
ground state, completing the cell reaction
4. CONCLUSION
Under the present preparation and irradiation conditions, it was found that the DSSCs
fabricated with fire fern extracts possesses the best photosensitized effect in the extracts studied
in this study. Therefore, fire fern extracts should be an alternative anthocyanin source for DSSCs
fabrication in geographical regions that fire fern is widely available. Moreover, the DSSCs
fabricated with acidified fire fern extracts having ≈1% efficiencies, could easily employed in low
cost , disposable DSSCs which can be employed in DNA bio- sensors specially under room
temperature low light intensity conditions. Therefore, the use of a natural dye for the
semiconductor sensitizer with a straight forward preparation would provide alternative to
commonly used synthetic dyes in many environmentally friendly applications.

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