Henna (Lawsoniainermis L.) Dye-Sensitized Nanocrystalline Titania Solar Cell
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Hindawi Publishing Corporation
Journal of Nanotechnology
Volume 2012, Article ID 167128, 6 pages
doi:10.1155/2012/167128
Research Article
Henna (Lawsonia inermis L.) Dye-Sensitized Nanocrystalline
Titania Solar Cell
Khalil Ebrahim Jasim,1 Shawqi Al-Dallal,2 and Awatif M. Hassan3
1 Department of Physics, College of Science, University of Bahrain, P.O. Box 32038, Bahrain
2 College
of Graduate Studies and Research, Ahlia University, P.O. Box 10878, Bahrain
3 Department of Chemistry, University of Bahrain, P.O. Box 32038, Bahrain
Correspondence should be addressed to Khalil Ebrahim Jasim, khalilej@sci.uob.bh
Received 15 June 2011; Revised 21 October 2011; Accepted 24 October 2011
Academic Editor: Thomas Stergiopoulos
Copyright © 2012 Khalil Ebrahim Jasim et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
Low-cost solar cells have been the subject of intensive research activities for over half century ago. More recently, dye-sensitized
solar cells (DSSCs) emerged as a new class of low-cost solar cells that can be easily prepared. Natural-dye-sensitized solar cells
(NDSSCs) are shown to be excellent examples of mimicking photosynthesis. The NDSSC acts as a green energy generator
in which dyes molecules adsorbed to nanocrystalline layer of wide bandgap semiconductor material harvest photons. In this
paper we investigate the structural, optical, electrical, and photovoltaic characterization of two types of natural dyes, namely, the
Bahraini Henna and the Yemeni Henna, extracted using the Soxhlet extractor. Solar cells from both materials were prepared and
characterized. It was found that the levels of open-circuit voltage and short-circuit current are concentration dependent. Further
suggestions to improve the efficiency of NDSSC are discussed.
1. Introduction The overall DSSC efficiency was found to be propor-
tional to the electron injection efficiency in wide bandgap
A solar cell is a photonic device that converts photons with nanostructured semiconductors. This finding has escalated
specific wavelengths to electricity. Materials presently used research activities over the past decade. ZnO2 nanowires,
in photovoltaics (PVs) are mainly semiconductors includ- for example, have been developed to replace both porous
ing, among others, crystalline silicon, III–V compounds, and TiO2 nanoparticle-based solar cells [6]. Also, metal
cadmium telluride, and copper indium selenide/sulfide [1– complex and novel man-made dyes have been proposed [7–
3]. Low-cost solar cells have been the subject of intensive 9]. However, processing and synthesization of these dyes is
research work for the last three decades. Amorphous semi- complicated and costly process [10–16]. Moreover, devel-
conductors were announced as one of the most promising opment or extraction of photosensitizers with absorption
materials for low-cost energy production. Most recently dye- range extended to the near IR is greatly desired. We found
sensitized solar cells (DSSCs) emerged as a new class of low- that our environment provides natural, nontoxic, and low-
cost energy conversion devices with simple manufacturing cost dyes sources with high absorbance level of UV, visible,
procedures. Incorporation of dye molecules in some wide and near IR. Examples of such dye sources are Bahraini
bandgap semiconductor electrodes was a key factor in Henna (Lawsonia inermis L.). This work provides further
developing photoelectrochemical solar cells. O’Regan and investigation on the first reported operation of Henna
Grätzel [4] and Nazerruddin et al. [5] succeeded for the first (Lawsonia inermis L.) as a dye sensitizer of nanostructured
time in producing a dye-sensitized solar cell (DSSC) using a solar cell [17, 18].
nanocrystalline TiO2 film with novel Ru bipyridl complex. It In this work we describe first the preparation of
was shown that DSSCs are promising class of low-cost and nanostructured TiO2 layer, followed by the process of dye
high-efficiency solar cell based on organic materials [1]. extraction and staining of the nanostructured TiO2 layer.
2 Journal of Nanotechnology
In the second part we describe the solar cell preparation Table 1: Measured and calculated parameters of assembled
using two types of Henna extracts, namely, the Bahraini and Bahraini and Yemeni Henna solar cells.
Yemeni Henna extracts. In the third part we examine the Henna extract concentration Voc (V) Isc (mA) FF % η%
structural, optical, and electrical characterization of the dye-
Bahraini 80.0 g 0.426 0.368 24.6 0.128
sensitized solar cells manufactured from the above extracts.
Bahraini 8.0 g 0.410 0.906 36.3 0.450
Bahraini 0.8 g 0.419 0.620 33.0 0.286
2. Structure and Operation of Dye-Sensitized Yemeni 84.0 g 0.306 0.407 28.1 0.117
Solar Cells Yemeni 21.0 g 0.326 0.430 37.1 0.174
Yemeni 4.2 g 0.500 0.414 27.6 0.191
Following the description in [18, 19] the operating prin-
ciple of dye-sensitized solar cells is shown schematically in
Figure 1. The cell is composed of four elements, namely, the
In fact, a smaller energy separation between the HOMO
conducting and counter conducting electrodes, the nanos-
and LUMO is desired to ensure absorption of low energy
tructured TiO2 layer, the dye molecules, and the electrolyte.
photons in the solar spectrum. This is analogous to inor-
The transparent conducting electrode and counterelectrode
ganic semiconductors energy bandgap (Eg ). Therefore, the
are coated with a thin conductive and transparent layer of
photocurrent level is dependent on the HOMO-LUMO
tin dioxide (SnO2 ). Nanocrystalline TiO2 is deposited on the
levels separation. To enhance electron injection into the
conducting electrode (photoelectrode) to provide the neces-
conduction band of TiO2 , one must use a sensitizer with
sary large surface area where dye molecules are adsorbed.
the largest energy separation of LUMO and the bottom of
Upon absorption of photons, dye molecules are excited
the TiO2 conduction band. Furthermore, for the HOMO
from the highest occupied molecular orbitals (HOMOs) to
level to effectively accept the donated electrons from the
the lowest unoccupied molecular orbital (LUMO) states as
redox mediator, the energy difference between the HOMO
shown schematically in Figure 1. This process is represented
and redox chemical potential must be more positive. Finally,
by (1). Once an electron is injected into the conduction
the maximum potential produced by the cell is determined
band of the wide bandgap semiconductor nanostructured
by the energy separation between the electrolyte chemical
TiO2 film, the dye molecule (photosensitizer) becomes
potential (Eredox ) and the Fermi level (EF ) of the TiO2 layer
oxidized (2). The injected electron is transported between
as shown in Figure 1.
the TiO2 nanoparticles and then gets extracted to a load
where the work done is delivered as an electrical energy
(3). Electrolyte containing I− /I3 − redox ions is used as an 3. Experimental
electron mediator between the TiO2 photoelectrode and
the carbon electrode. Therefore, the oxidized dye molecules Nanostructured TiO2 films were prepared following the
(photosensitizer) are regenerated by receiving electrons from procedure detailed in [18]. A suspension of TiO2 is prepared
the I− ion redox mediator that get oxidized to I3 − (Tri- by adding 9 mL of nitric acid solution of PH 3-4 (1 mL
iodide ions). This process is represented by (4). The I3 − increment) to 6 g of colloidal P25 TiO2 powder in mortar
substitutes the internally donated electron with that from the and pestle. While grinding, 8 mL of distilled water (in 1 mL
external load and gets reduced back to I− ion, (5). Therefore, increment) is added to get a white-free flow-paste. Finally, a
generation of electric power in DSSC causes no permanent drop of transparent surfactant (any clear dishwashing deter-
chemical change or transformation: gent) is added in 1 mL of distilled water to ensure coating
Excitation process, uniformity and adhesion to the transparent conducting glass
electrode. The ratio of the nitric acid solution to the colloidal
S + photon −→ S∗ (1) P25 TiO2 powder is a critical factor for the cell performance.
If the ratio exceeds a certain threshold value the resulting
Injection process, film becomes too thick and has a tendency to peel off. On
the other hand, a low ratio reduces appreciably the efficiency
S∗ + TiO2 −→ e− (TiO2 ) + S+ (2) of light absorption.
Soxhlet Extractor is used for the extraction of dyes
Energy generation, solution from 80 g of Bahraini Henna and 84 g of Yemeni
Henna (powder) where 100 mL of methanol is used in
e− (TiO2 ) + C.E. −→ TiO2 + e− (C.E.) + electrical energy (3) each extraction process. Different concentrations have been
prepared from the collected extract. The light harvesting
Regeneration of dye, efficiency (LHE) for each concentration has been calculated
from the measured absorbance using dual-beam spectropho-
3 1
S+ + I− −→ S + I3 − (4) tometer. The electrical characteristic and parameters of the
2 2
assembled solar cells were determined and presented in
e− Recapture reaction, Table 1.
Doctor blade method was employed by depositing the
1 − − 3 TiO2 suspension uniformly on a cleaned (rinsed with
I3 + e (C.E.) −→ I− + C.E. (5)
2 2 ethanol) electrode plate. The TiO2 film was allowed to dry
Journal of Nanotechnology 3
Transparent conductive Transparent
electrode (SnO2 ) counterelectrode
Dye
molecule
Conduction
band
e−
S∗ /S+
e− LUMO
kinj
e−
krad/non
EF
HOMO
3I − qV
Sunlight
e−
e− e−
S Eredox
Valence
band I3−
Electrolyte
e−
TiO2
e− e− e−
Load
Figure 1: Schematic diagram illustrating the structure and operation principle of a dye-sensitized cell.
for few minutes and then annealed at approximately 450◦ C demonstrate the effectiveness of sensitizing the nanocrys-
(in a well ventilated zone) for about 15 minutes to form talline TiO2 layer. Both the Bahraini and Yemeni Henna
a porous, large surface area TiO2 film. The film must be extract dye solutions of different concentrations have
allowed to cool down slowly to room temperature. This been optically characterized by measuring the absorbance
is a necessary condition to remove thermal stresses and using the dual-beam UV-VIS spectrophotometer (Shimadzu,
avoid cracking of the glass or peeling off the TiO2 film. model UV-3101). Typical examples of light harvesting effi-
Investigation of the formation of nanoporous TiO2 film ciency (LHE = 1 − 10−A , where A is the absorbance) measure-
was confirmed by scanning electron microscope SEM. After ments are shown in Figure 2. Bahraini Henna extracts were
that, the TiO2 nanocrystalline layer was stained with the found to exhibit higher LHE than those of corresponding
dye for approximately a day and then washed with distilled concentration of Yemeni Henna. The high light harvesting
water and ethanol to ensure the absence of water in the efficiency of Henna extracts in the UV, visible, and near
film after removal of the residual dye. The counterelectrode IR region of the spectrum suggests that they are promising
is coated with graphite that acts as a catalyst in redoxing natural dye sensitizer for single-junction DSSC. The color of
the dye. Both the photo- and the counterelectrodes are the highly concentrated extract (80 g in 100 mL of methanol)
clamped together and drops of electrolyte are applied to fill is dark green, and when it is diluted the degree of the green
the clamped cell. The electrolyte used is iodide electrolyte color enhanced. Moreover, as the concentration of Henna
(0.5 M potassium iodide mixed with 0.05 M iodine in water- extract is increased the film texture becomes more greasy and
free ethylene glycol) containing a redox couple (traditionally sticky. The color of the highly concentrated Yemeni Henna
the iodide/triiodide I− /I3 − couple). The measurements of extract (84 g in 100 mL of methanol) is dark green golden,
open-circuit voltage and short-circuit current have been and when it is diluted the golden degree enhanced.
performed under direct sun illumination at noon time. The TiO2 films were first annealed, and their nanostruc-
Neither UV or IR cutoff filters nor antireflection (AR) ture properties were then examined by SEM measurements.
coatings on the photoelectrode have been used. Figure 3 shows the SEM measurements carried out on the
TiO2 layer. It shows that after sintering the TiO2 film
4. Results and Discussion becomes nanocrystalline. X-ray diffraction measurements
on our samples confirmed the formation of nanocrystalline
Bahraini and Yemeni Henna extracts have been prepared TiO2 particles of sizes less than 50 nm [17]. The formation of
at different concentrations. Optical measurements show nanostructured TiO2 film is greatly affected by TiO2 suspen-
that these extracts posses interesting optical properties and sion preparation procedures and the annealing temperature.
4 Journal of Nanotechnology
Yemeni Henna Bahraini Henna
100 100
Light harvesting efficiency (%)
Light harvesting efficiency (%)
80 80 g
80
60 84 g 60 8g
0.8 g
8.4 g
40 40
0.84 g
20 20
21 g
0.08 g
0 0
200 400 600 800 1000 1200 200 400 600 800 1000 1200
Wavelength (nm) Wavelength (nm)
(a) (b)
Figure 2: Light harvesting efficiency versus wavelength for Bahraini Henna extracts and Yemeni Henna extract of different concentrations.
Data are given in g per 100 mL of methanol.
better operation, a much lower series resistance effect, and
a higher efficiency. Dye concentration has remarkable effect
on the magnitude of collected photocurrent. Highly diluted
extracts reduce the magnitudes of photocurrents and cell
efficiencies.
Table 1 illustrates the electrical properties of Henna
photovoltaic cells. Due to light reflection and absorption
by the conductive photoelectrode and the scattering nature
of the nanostructured TiO2 , the measured transmittance of
the photoelectrode shows that only an average of 10% of
the solar spectrum (AM 1.5) is useful. Despite the variation
of Bahraini Henna extract concentration the cells produced
almost the same open-circuit voltage VOC . However, the
short-circuit current Isc reflected variations due to Henna
extract concentration. The Yemeni Henna extracts solar cells
Mag WD HV 500 nm
250000 x 4.2 mm 5 kV Henna
produced almost the same level of the short circuit current,
but open circuit voltage varies with the concentration. It
Figure 3: SEM images of TiO2 film after annealing. turns out that highly concentrated Henna extracts do not
exhibit ideal I-V characteristics even though they possesses
100% light harvesting efficiency in the UV and visible parts
It was found that a sintered TiO2 film at temperatures of the electromagnetic spectrum.
lower than the recommended 450◦ C resulted in solar cells Investigations showed that there are many factors affect-
that generate unnoticeable electric current even in the μA ing the performance of the natural-dye-sensitized photo-
domain. Moreover, TiO2 film degradation in this case is fast voltaic cells. Dye structure must own several carbonyl
and cracks form after a short period of time when the cell is (C=O) or hydroxyl (−OH) groups to enable dye molecule
exposed to illumination. chelating to the Ti (IV) sites on the TiO2 surface [20]. For
Figure 4 shows the I-V characteristics of NDSSC sensi- example, extracted dye from California blackberries (Rubus
tized with Yemeni Henna and Bahraini Henna. Because the ursinus) has been found to be an excellent fast-staining
80 g Bahraini Henna and 84 g Yemeni Henna extracts are dye for sensitization. on the other hand, dyes extracted
highly concentrated and posses sticky texture, the electron from strawberries lack such complexing capability and
injection efficiency into the nanocrystalline TiO2 film deteri- hence not suggested to be used as natural dye sensitizer
orated and hence lower values of the measured photocurrent in NDSSCs [10, 21]. Since not all photons scattered by
are obtained. In other words, the use of highly concentrated or transmitted through the nanocrystalline TiO2 layer get
extract introduces series resistance Rs in the solar cell mainly absorbed by a monolayer of chelating Henna dyes molecules,
due to the path traversed by the photogenerated electrons. At the incorporation of energy relay dyes might help enhancing
lower concentrations where Henna extract viscosity is similar the light harvesting efficiency. A remarkable enhancement in
to that of the solvent, the cell I-V characteristics show a absorption spectral bandwidth and 26% increase in power
Journal of Nanotechnology 5
1 researchers have suggested replacing the liquid electrolyte
with a solid state one that provides a better sealing of the
0.8 1
cell [31, 32] and fabrication of large area modules with
efficiency above 12%. The success of Sony in 2010 to present
Photocurrent (mA)
a DSSC module with efficiency close to 10% is one of the
0.6 driving achievements toward fabrication of outdoor larger
area modules that can be integrated in green buildings. Also,
3
0.4 as presented by Dyesol, development of flexible transparent
2 substrate (polymer based) will transform DSSCs industry
4 toward mass production and commercialization of single-
0.2
cell or minimodules for indoor and personal appliances.
5
0
0 0.1 0.2 0.3 0.4 0.5 5. Conclusion
Voltage (V)
In this work we studied the optical and photovoltaic
(1) 8 g Bahraini Henna cell properties of sensitized nanostructured TiO2 with two types
(2) 80 g Bahraini Henna cell
of Henna (Lawsonia inermis L.) extracts. Henna dyes were
(3) 21 g Yemeni Henna cell
(4) 4.2 g Yemeni Henna cell extracted using Soxhlet extractor technique. It was shown
(5) 84 g Yemeni Henna cell that both types of Henna extracts exhibit high level of
absorbance in the UV, visible, and near-infrared region of the
Figure 4: I-V characteristic of Henna extracts cells. solar spectrum. However, a higher light harvesting efficiency
of Bahraini Henna, as compared to Yemeni Henna, was
obtained. This fact is reflected in a higher photocurrent and
open-circuit voltage and consequently a higher efficiency of
conversion efficiency have been accomplished with some the Bahraini Henna based photovoltaic cell. Extract concen-
sensitizers after energy relay dyes have been added [22]. tration was found to influence remarkably the magnitude
The fact of the dependence of both hole transport of the collected photocurrent. High concentration of Henna
and collection efficiency on the dye-cation reduction and extract introduces a series resistance that ultimately reduces
I− /I3 − redox efficiency at counterelectrodes is to be taken the collected photocurrent. On the other hand, diluted
into account [23], and the redoxing electrolyte must be extracts reduce the magnitude of the photocurrent and cell
chosen such that the reduction of I3 − ions by injection of efficiency. Optimization of the preparation conditions of the
electrons is fast and efficient (see Figure 1). As a matter of nanocrystalline TiO2 layer and the choice of the redoxing
fact, besides limiting cell stability due to evaporation, liquid electrolyte are found to be determining factors for the
electrolyte inhibits fabrication of multicell modules, since performance of the dye-sensitized photovoltaic cell.
module manufacturing requires cells be connected electri-
cally yet separated chemically [24, 25]. Another important
factor playing major role in enhancing the cell’s efficiency Acknowledgments
is the thickness of the nanostructured TiO2 layer which
must be less than 20 μm to ensure diffusion length of the The authors are greatly indebted to the University of Bahrain
photoelectrons be greater than that of the nanocrystalline for financial support. Also, they would like to express their
TiO2 layer. To increase photogenerated electron diffusion thanks to Dr. Mohammad S. Hussain (National Nanotech-
length, studies suggest replacing the nanoparticles film with nology Center King Abdulaziz City for Science and Technol-
an array of single-crystalline nanowires or nanosheets in ogy (KACST)) for providing the SEM measurements.
which electrons transport increase by several orders of mag-
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