Henna(LawsoniainermisL.)Dye-SensitizedNanocrystalline …downloads.hindawi.com/journals/jnt/2012/167128.pdf · 2019-07-31 · eﬃciency of Henna extracts in the UV, visible, and

Hindawi Publishing CorporationJournal of NanotechnologyVolume 2012, Article ID 167128, 6 pagesdoi:10.1155/2012/167128

Research Article

Henna (Lawsonia inermis L.) Dye-Sensitized NanocrystallineTitania 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, Bahrain2 College of Graduate Studies and Research, Ahlia University, P.O. Box 10878, Bahrain3 Department of Chemistry, University of Bahrain, P.O. Box 32038, Bahrain

Correspondence should be addressed to Khalil Ebrahim Jasim, [email protected]

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 AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Low-cost solar cells have been the subject of intensive research activities for over half century ago. More recently, dye-sensitizedsolar 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 generatorin which dyes molecules adsorbed to nanocrystalline layer of wide bandgap semiconductor material harvest photons. In thispaper we investigate the structural, optical, electrical, and photovoltaic characterization of two types of natural dyes, namely, theBahraini Henna and the Yemeni Henna, extracted using the Soxhlet extractor. Solar cells from both materials were prepared andcharacterized. It was found that the levels of open-circuit voltage and short-circuit current are concentration dependent. Furthersuggestions to improve the efficiency of NDSSC are discussed.

1. Introduction

A solar cell is a photonic device that converts photons withspecific wavelengths to electricity. Materials presently usedin photovoltaics (PVs) are mainly semiconductors includ-ing, among others, crystalline silicon, III–V compounds,cadmium telluride, and copper indium selenide/sulfide [1–3]. Low-cost solar cells have been the subject of intensiveresearch work for the last three decades. Amorphous semi-conductors were announced as one of the most promisingmaterials for low-cost energy production. Most recently dye-sensitized solar cells (DSSCs) emerged as a new class of low-cost energy conversion devices with simple manufacturingprocedures. Incorporation of dye molecules in some widebandgap semiconductor electrodes was a key factor indeveloping photoelectrochemical solar cells. O’Regan andGratzel [4] and Nazerruddin et al. [5] succeeded for the firsttime in producing a dye-sensitized solar cell (DSSC) using ananocrystalline TiO2 film with novel Ru bipyridl complex. Itwas shown that DSSCs are promising class of low-cost andhigh-efficiency solar cell based on organic materials [1].

The overall DSSC efficiency was found to be propor-tional to the electron injection efficiency in wide bandgapnanostructured semiconductors. This finding has escalatedresearch activities over the past decade. ZnO2 nanowires,for example, have been developed to replace both porousand TiO2 nanoparticle-based solar cells [6]. Also, metalcomplex and novel man-made dyes have been proposed [7–9]. However, processing and synthesization of these dyes iscomplicated and costly process [10–16]. Moreover, devel-opment or extraction of photosensitizers with absorptionrange extended to the near IR is greatly desired. We foundthat our environment provides natural, nontoxic, and low-cost dyes sources with high absorbance level of UV, visible,and near IR. Examples of such dye sources are BahrainiHenna (Lawsonia inermis L.). This work provides furtherinvestigation on the first reported operation of Henna(Lawsonia inermis L.) as a dye sensitizer of nanostructuredsolar cell [17, 18].

In this work we describe first the preparation ofnanostructured TiO2 layer, followed by the process of dyeextraction and staining of the nanostructured TiO2 layer.

2 Journal of Nanotechnology

In the second part we describe the solar cell preparationusing two types of Henna extracts, namely, the Bahraini andYemeni Henna extracts. In the third part we examine thestructural, optical, and electrical characterization of the dye-sensitized solar cells manufactured from the above extracts.

2. Structure and Operation of Dye-SensitizedSolar Cells

Following the description in [18, 19] the operating prin-ciple of dye-sensitized solar cells is shown schematically inFigure 1. The cell is composed of four elements, namely, theconducting and counter conducting electrodes, the nanos-tructured TiO2 layer, the dye molecules, and the electrolyte.The transparent conducting electrode and counterelectrodeare coated with a thin conductive and transparent layer oftin dioxide (SnO2). Nanocrystalline TiO2 is deposited on theconducting electrode (photoelectrode) to provide the neces-sary large surface area where dye molecules are adsorbed.Upon absorption of photons, dye molecules are excitedfrom the highest occupied molecular orbitals (HOMOs) tothe lowest unoccupied molecular orbital (LUMO) states asshown schematically in Figure 1. This process is representedby (1). Once an electron is injected into the conductionband of the wide bandgap semiconductor nanostructuredTiO2 film, the dye molecule (photosensitizer) becomesoxidized (2). The injected electron is transported betweenthe TiO2 nanoparticles and then gets extracted to a loadwhere the work done is delivered as an electrical energy(3). Electrolyte containing I−/I3

− redox ions is used as anelectron mediator between the TiO2 photoelectrode andthe carbon electrode. Therefore, the oxidized dye molecules(photosensitizer) are regenerated by receiving electrons fromthe I− ion redox mediator that get oxidized to I3

− (Tri-iodide ions). This process is represented by (4). The I3

−

substitutes the internally donated electron with that from theexternal load and gets reduced back to I− ion, (5). Therefore,generation of electric power in DSSC causes no permanentchemical change or transformation:

Excitation process,

S + photon −→ S∗ (1)

Injection process,

S∗ + TiO2 −→ e−(TiO2) + S+ (2)

Energy generation,

e−(TiO2) + C.E. −→ TiO2 + e−(C.E.) + electrical energy (3)

Regeneration of dye,

S+ +32

I− −→ S +12

I3− (4)

e− Recapture reaction,

12

I3− + e−(C.E.) −→ 3

2I− + C.E. (5)

Table 1: Measured and calculated parameters of assembledBahraini and Yemeni Henna solar cells.

Henna extract concentration Voc (V) Isc (mA) FF % η %

Bahraini 80.0 g 0.426 0.368 24.6 0.128

Bahraini 8.0 g 0.410 0.906 36.3 0.450

Bahraini 0.8 g 0.419 0.620 33.0 0.286

Yemeni 84.0 g 0.306 0.407 28.1 0.117

Yemeni 21.0 g 0.326 0.430 37.1 0.174

Yemeni 4.2 g 0.500 0.414 27.6 0.191

In fact, a smaller energy separation between the HOMOand LUMO is desired to ensure absorption of low energyphotons in the solar spectrum. This is analogous to inor-ganic semiconductors energy bandgap (Eg). Therefore, thephotocurrent level is dependent on the HOMO-LUMOlevels separation. To enhance electron injection into theconduction band of TiO2, one must use a sensitizer withthe largest energy separation of LUMO and the bottom ofthe TiO2 conduction band. Furthermore, for the HOMOlevel to effectively accept the donated electrons from theredox mediator, the energy difference between the HOMOand redox chemical potential must be more positive. Finally,the maximum potential produced by the cell is determinedby the energy separation between the electrolyte chemicalpotential (Eredox) and the Fermi level (EF) of the TiO2 layeras shown in Figure 1.

3. Experimental

Nanostructured TiO2 films were prepared following theprocedure detailed in [18]. A suspension of TiO2 is preparedby adding 9 mL of nitric acid solution of PH 3-4 (1 mLincrement) to 6 g of colloidal P25 TiO2 powder in mortarand pestle. While grinding, 8 mL of distilled water (in 1 mLincrement) is added to get a white-free flow-paste. Finally, adrop of transparent surfactant (any clear dishwashing deter-gent) is added in 1 mL of distilled water to ensure coatinguniformity and adhesion to the transparent conducting glasselectrode. The ratio of the nitric acid solution to the colloidalP25 TiO2 powder is a critical factor for the cell performance.If the ratio exceeds a certain threshold value the resultingfilm becomes too thick and has a tendency to peel off. Onthe other hand, a low ratio reduces appreciably the efficiencyof light absorption.

Soxhlet Extractor is used for the extraction of dyessolution from 80 g of Bahraini Henna and 84 g of YemeniHenna (powder) where 100 mL of methanol is used ineach extraction process. Different concentrations have beenprepared from the collected extract. The light harvestingefficiency (LHE) for each concentration has been calculatedfrom the measured absorbance using dual-beam spectropho-tometer. The electrical characteristic and parameters of theassembled solar cells were determined and presented inTable 1.

Doctor blade method was employed by depositing theTiO2 suspension uniformly on a cleaned (rinsed withethanol) electrode plate. The TiO2 film was allowed to dry

Journal of Nanotechnology 3

S∗/S+

Load

Transparent conductive Transparentcounterelectrode

Dyemolecule

S

Conductionband

Valenceband

Electrolyte

Sunlight

LUMO

HOMO

e−e−

e−e−e−

e−

e−

e−e−

e−

EF

Eredox

krad/non

3I−

I−3

TiO2

electrode (SnO2)

kinj

qV

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(in a well ventilated zone) for about 15 minutes to forma porous, large surface area TiO2 film. The film must beallowed to cool down slowly to room temperature. Thisis a necessary condition to remove thermal stresses andavoid cracking of the glass or peeling off the TiO2 film.Investigation of the formation of nanoporous TiO2 filmwas confirmed by scanning electron microscope SEM. Afterthat, the TiO2 nanocrystalline layer was stained with thedye for approximately a day and then washed with distilledwater and ethanol to ensure the absence of water in thefilm after removal of the residual dye. The counterelectrodeis coated with graphite that acts as a catalyst in redoxingthe dye. Both the photo- and the counterelectrodes areclamped together and drops of electrolyte are applied to fillthe clamped cell. The electrolyte used is iodide electrolyte(0.5 M potassium iodide mixed with 0.05 M iodine in water-free ethylene glycol) containing a redox couple (traditionallythe iodide/triiodide I−/I3

− couple). The measurements ofopen-circuit voltage and short-circuit current have beenperformed under direct sun illumination at noon time.Neither UV or IR cutoff filters nor antireflection (AR)coatings on the photoelectrode have been used.

4. Results and Discussion

Bahraini and Yemeni Henna extracts have been preparedat different concentrations. Optical measurements showthat these extracts posses interesting optical properties and

demonstrate the effectiveness of sensitizing the nanocrys-talline TiO2 layer. Both the Bahraini and Yemeni Hennaextract dye solutions of different concentrations havebeen optically characterized by measuring the absorbanceusing the dual-beam UV-VIS spectrophotometer (Shimadzu,model UV-3101). Typical examples of light harvesting effi-ciency (LHE = 1−10−A, where A is the absorbance) measure-ments are shown in Figure 2. Bahraini Henna extracts werefound to exhibit higher LHE than those of correspondingconcentration of Yemeni Henna. The high light harvestingefficiency of Henna extracts in the UV, visible, and nearIR region of the spectrum suggests that they are promisingnatural dye sensitizer for single-junction DSSC. The color ofthe highly concentrated extract (80 g in 100 mL of methanol)is dark green, and when it is diluted the degree of the greencolor enhanced. Moreover, as the concentration of Hennaextract is increased the film texture becomes more greasy andsticky. The color of the highly concentrated Yemeni Hennaextract (84 g in 100 mL of methanol) is dark green golden,and when it is diluted the golden degree enhanced.

The TiO2 films were first annealed, and their nanostruc-ture properties were then examined by SEM measurements.Figure 3 shows the SEM measurements carried out on theTiO2 layer. It shows that after sintering the TiO2 filmbecomes nanocrystalline. X-ray diffraction measurementson our samples confirmed the formation of nanocrystallineTiO2 particles of sizes less than 50 nm [17]. The formation ofnanostructured TiO2 film is greatly affected by TiO2 suspen-sion preparation procedures and the annealing temperature.


Yemeni Henna

0

20

40

60

80

100

Ligh

t h

arve

stin

g ef

fici

ency

(%

)

200 400 600 800 1000 1200

Wavelength (nm)

0.84 g

8.4 g

21 g

84 g

(a)

200 400 600 800 1000 12000

20

40

60

80

100Bahraini Henna

0.08 g

0.8 g8 g

80 g

Ligh

t h

arve

stin

g ef

fici

ency

(%

)

Wavelength (nm)

(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.

Mag250000 x

WD4.2 mm

HV5 kV

500 nmHenna

Figure 3: SEM images of TiO2 film after annealing.

It was found that a sintered TiO2 film at temperatureslower than the recommended 450◦C resulted in solar cellsthat generate unnoticeable electric current even in the μAdomain. Moreover, TiO2 film degradation in this case is fastand cracks form after a short period of time when the cell isexposed to illumination.

Figure 4 shows the I-V characteristics of NDSSC sensi-tized with Yemeni Henna and Bahraini Henna. Because the80 g Bahraini Henna and 84 g Yemeni Henna extracts arehighly concentrated and posses sticky texture, the electroninjection efficiency into the nanocrystalline TiO2 film deteri-orated and hence lower values of the measured photocurrentare obtained. In other words, the use of highly concentratedextract introduces series resistance Rs in the solar cell mainlydue to the path traversed by the photogenerated electrons. Atlower concentrations where Henna extract viscosity is similarto that of the solvent, the cell I-V characteristics show a

better operation, a much lower series resistance effect, anda higher efficiency. Dye concentration has remarkable effecton the magnitude of collected photocurrent. Highly dilutedextracts reduce the magnitudes of photocurrents and cellefficiencies.

Table 1 illustrates the electrical properties of Hennaphotovoltaic cells. Due to light reflection and absorptionby the conductive photoelectrode and the scattering natureof the nanostructured TiO2, the measured transmittance ofthe photoelectrode shows that only an average of 10% ofthe solar spectrum (AM 1.5) is useful. Despite the variationof Bahraini Henna extract concentration the cells producedalmost the same open-circuit voltage VOC. However, theshort-circuit current Isc reflected variations due to Hennaextract concentration. The Yemeni Henna extracts solar cellsproduced almost the same level of the short circuit current,but open circuit voltage varies with the concentration. Itturns out that highly concentrated Henna extracts do notexhibit ideal I-V characteristics even though they possesses100% light harvesting efficiency in the UV and visible partsof the electromagnetic spectrum.

Investigations showed that there are many factors affect-ing the performance of the natural-dye-sensitized photo-voltaic cells. Dye structure must own several carbonyl(C=O) or hydroxyl (−OH) groups to enable dye moleculechelating to the Ti (IV) sites on the TiO2 surface [20]. Forexample, extracted dye from California blackberries (Rubusursinus) has been found to be an excellent fast-stainingdye for sensitization. on the other hand, dyes extractedfrom strawberries lack such complexing capability andhence not suggested to be used as natural dye sensitizerin NDSSCs [10, 21]. Since not all photons scattered byor transmitted through the nanocrystalline TiO2 layer getabsorbed by a monolayer of chelating Henna dyes molecules,the incorporation of energy relay dyes might help enhancingthe light harvesting efficiency. A remarkable enhancement inabsorption spectral bandwidth and 26% increase in power

Journal of Nanotechnology 5

0 0.1 0.2 0.3 0.4 0.50

0.2

0.4

0.6

0.8

1

5

4

3

2

1

Ph

otoc

urr

ent

(mA

)

Voltage (V)

(1) 8 g Bahraini Henna cell(2) 80 g Bahraini Henna cell(3) 21 g Yemeni Henna cell(4) 4.2 g Yemeni Henna cell(5) 84 g Yemeni Henna cell

Figure 4: I-V characteristic of Henna extracts cells.

conversion efficiency have been accomplished with somesensitizers after energy relay dyes have been added [22].

The fact of the dependence of both hole transportand collection efficiency on the dye-cation reduction andI−/I3

− redox efficiency at counterelectrodes is to be takeninto account [23], and the redoxing electrolyte must bechosen such that the reduction of I3

− ions by injection ofelectrons is fast and efficient (see Figure 1). As a matter offact, besides limiting cell stability due to evaporation, liquidelectrolyte inhibits fabrication of multicell modules, sincemodule manufacturing requires cells be connected electri-cally yet separated chemically [24, 25]. Another importantfactor playing major role in enhancing the cell’s efficiencyis the thickness of the nanostructured TiO2 layer whichmust be less than 20 μm to ensure diffusion length of thephotoelectrons be greater than that of the nanocrystallineTiO2 layer. To increase photogenerated electron diffusionlength, studies suggest replacing the nanoparticles film withan array of single-crystalline nanowires or nanosheets inwhich electrons transport increase by several orders of mag-nitude [6, 26–28]. Significant successes have been achievedin improving the photoconversion efficiency of solar cellsbased on CdSe quantum dot light harvesters supported withcarbon nanotube network [29, 30]. This is accomplished byincorporating carbon nanotubes network in the nanostruc-tured TiO2 film and accordingly assisting charge transportprocess. Consequently, an appreciable improvement in thephotoconversion efficiency of the DSSC was obtained.

Finally, dark current in DSSC is mainly due to the lossof the injected electron from nanostructured TiO2 to I3

−

(the hole carrier in solution electrolyte). Reduction of darkcurrent enhances the open-circuit voltage of the cell, and onesuccessful way to suppress dark current is to use coadsorbateson the nanostructured TiO2 surface. Therefore, to achievefurther improvement of the performance of DSSC cell many

researchers have suggested replacing the liquid electrolytewith a solid state one that provides a better sealing of thecell [31, 32] and fabrication of large area modules withefficiency above 12%. The success of Sony in 2010 to presenta DSSC module with efficiency close to 10% is one of thedriving achievements toward fabrication of outdoor largerarea modules that can be integrated in green buildings. Also,as presented by Dyesol, development of flexible transparentsubstrate (polymer based) will transform DSSCs industrytoward mass production and commercialization of single-cell or minimodules for indoor and personal appliances.

5. Conclusion

In this work we studied the optical and photovoltaicproperties of sensitized nanostructured TiO2 with two typesof Henna (Lawsonia inermis L.) extracts. Henna dyes wereextracted using Soxhlet extractor technique. It was shownthat both types of Henna extracts exhibit high level ofabsorbance in the UV, visible, and near-infrared region of thesolar spectrum. However, a higher light harvesting efficiencyof Bahraini Henna, as compared to Yemeni Henna, wasobtained. This fact is reflected in a higher photocurrent andopen-circuit voltage and consequently a higher efficiency ofthe Bahraini Henna based photovoltaic cell. Extract concen-tration was found to influence remarkably the magnitudeof the collected photocurrent. High concentration of Hennaextract introduces a series resistance that ultimately reducesthe collected photocurrent. On the other hand, dilutedextracts reduce the magnitude of the photocurrent and cellefficiency. Optimization of the preparation conditions of thenanocrystalline TiO2 layer and the choice of the redoxingelectrolyte are found to be determining factors for theperformance of the dye-sensitized photovoltaic cell.

Acknowledgments

The authors are greatly indebted to the University of Bahrainfor financial support. Also, they would like to express theirthanks to Dr. Mohammad S. Hussain (National Nanotech-nology Center King Abdulaziz City for Science and Technol-ogy (KACST)) for providing the SEM measurements.

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Henna(LawsoniainermisL.)Dye-SensitizedNanocrystalline …downloads.hindawi.com/journals/jnt/2012/167128.pdf · 2019-07-31 · eﬃciency of Henna extracts in the UV, visible, and