Dye-sensitized solar cell using natural dyes extracted from Morus atba Lam fruit and Striga hermonthica flower

Dye-sensitized solar cell using naturaldyes extracted from Morus atba Lamfruit and Striga hermonthica flower

Abebe RedaSisay TadesseTeketel Yohannes

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Dye-sensitized solar cell using natural dyesextracted from Morus atba Lam fruit

and Striga hermonthica flower

Abebe Reda,a Sisay Tadesse,b,* and Teketel YohannescaBahir Dar University, College of Sciences, Department of Chemistry, P.O. Box 79,

Bahir Dar, EthiopiabHawassa University, College of Natural and Computational Sciences, Department of Chemistry,

P.O. Box 05, Hawassa, EthiopiacAddis Ababa University, College of Natural Sciences, Department of Chemistry,

P.O. Box 1176, Addis Ababa, Ethiopia

Abstract. This study employs anthocyanin extracts from mulberry (Morus atba Lam) fruit andAkenchira (Striga hermonthica) flower as the natural dyes for a dye-sensitized solar cell (DSSC).The electrodes, electrolyte (I−∕I3−), and dyes were assembled into a cell and illuminated by alight with an intensity 100 mW∕cm2 to measure the photoelectrochemical parameters of theprepared DSSCs. According to the experimental results, the maximum conversion efficiencyof the DSSCs prepared from anthocyanin dyes of Morus atba Lam fruit extract is 0.42%,with an open-circuit voltage (Voc) of 0.54 V, a short circuit current density (Jsc) of1.38 mA∕cm2, and a fill factor (FF) of 0.56. The maximum conversion efficiency of theDSSCs prepared by anthocyanin dye from the flower of S. hermonthica extract is 0.304%with a Voc of 0.52 V, Jsc of 1.01 mA∕cm2, and an FF of 0.58. © 2014 Society of Photo-Optical Instrumentation Engineers (SPIE) [DOI: 10.1117/1.JPE.4.043091]

Keywords: natural dyes; anthocyanin; solar cells; titanium dioxide.

Paper 14013 received Mar. 18, 2014; revised manuscript received Jul. 12, 2014; accepted forpublication Jul. 15, 2014; published online Sep. 3, 2014.

1 Introduction

Dye-sensitized solar cells (DSSCs) are the third generation of photovoltaic devices for the con-version of visible light to electrical energy. One aspect of DSSCs that is particularly attractive isthe low cost of such devices for solar energy conversion into electricity. This is possible mainlydue to the use of inexpensive materials and the relative ease of the fabrication processes.1–3

So far, because of their strong visible light absorption, long excitation lifetime, and efficientmetal-to-ligand charge transfer, transition metal coordinated compounds (for instance, rutheniumpolypyridyl complexes) were used as sensitizers. Because of these, such types of DSSCs are greatlysuccessful and have a maximum efficiency of 11%.4,5 The high cost of synthesis and an unlimitedenvironmental consequence of those models provide cheaper, simpler, and safer dyes as alterna-tives. Natural pigments, including chlorophyll, carotene, and anthocyanin, are widely accessible inplant leaves, flowers, and fruits and fulfill these requirements. Experimentally, natural dye-sensi-tized TiO2 solar cells have reached an efficiency of 7.1% with a high stability.6 A higher efficiencyof over 8.0% has been achieved using similar synthetic organic dyes.4

In this study, anthocyanin pigments from flowers of Straiga hermonthica (locally calledAkenchira; the color of the flower is light pink), which has been studied so far as a parasite ofcrops such as sorghum, maize, rice, etc., and for medicinal uses,7,8 and the fruits of Morus atbaLam (Mulberry; the color of the fruit is deep red) were extracted and used as a sensitizer withTiO2 as a wide bandgap semiconductor. The photoelectrochemical performances of the DSSCsfabricated from these dyes were measured and characterized.

*Address all correspondence to: Sisay Tadesse, E-mail: [email protected]

0091-3286/2014/$25.00 © 2014 SPIE

Journal of Photonics for Energy 043091-1 Vol. 4, 2014


http://dx.doi.org/10.1117/1.JPE.4.043091





2 Experimental

2.1 Preparation of Natural Dye Sensitizers

M. atba Lam and S. hermonthica flowers were collected from different sites of Ethiopia (Campusof College of Natural Science, Addis Ababa University situated in Addis Ababa and Alamata,which is situated in the northern part of Ethiopia, 600 km from Addis Ababa, respectively). Bothof these samples were dried at room temperature in open air and a dark place. After drying forabout 2 weeks, the samples were completely dried in an oven at 70°C for 30 min to remove anyremaining moisture content. Afterward, the samples were crushed with a plant microgrindingmachine to produce a powder from the respective plant materials. These materials were extractedfrom water, 0.1 M HCl, ethanol, and acidified ethanol (in an HCl-ethanol mixture) at room tem-perature. Two grams of powdered M. atba Lam and S. hermonthica were extracted in 50 ml ofwater, 0.1 M HCl, ethanol, and acidified ethanol (in HCl-ethanol mixture). The solution wasstored at room temperature for about 6 h to completely extract the dyes from the powder.Solid residues were filtrated out using a filter glass to obtain clear dye solutions. The UV-visabsorption spectrophotometry (Spectronic Genesys 2PC Model 336003, Rochester, New York)of each sample was run and the wavelength of each sample was scanned from 300 to 800 nm.The effects of the dye extracted from those different solvents were studied.

2.2 Preparation of Conductive ITO Glass

Pieces of the indium doped tin oxide (ITO) conductive glass sheets [(1.5 cm × 2.5 cm), Aldrich,Steinheim, Germany] were successively cleaned with ultrasonication in acetone (Aldrich), iso-propanol (Riedel-de Haen, Seelze, Germany), and ethanol (Fluka, Switzerland), respectively;then the samples were sonicated using an ultrasonic bath (Decon, Malmesbury, Wiltshire,United Kingdom) for 20 min at 20°C for each solvent. Finally, the conductive glass wasdried using an air gun.

2.3 Preparation of Photoanode and Counter Electrode

Preparation of the mesoporous TiO2 paste was done with the method described in Ref. 9. A TiO2

film electrode (photoanode) was prepared by blending 3 g of commercial TiO2 powder (P25,Degussa AG, Düsseldorf, Germany, a mixture of about 30% rutile and 70% anatase) in 1 ml ofdistilled water containing 0.1 ml of acetic acid, and then diluted with 4 ml of distilled water and0.05 ml Triton X-100 (Aldrich). A piece of conductive glass was selected and placed on a glasssheet. With the conducting side up, the ITO-coated glass was covered on four sides of the pasteusing a Scotch tape which was used to monitor the film thickness. A small amount of titaniumdioxide paste was added at the edge of the taped ITO-coated glass and spread quickly by pushingit with a glass rod before the paste dried. The tape was removed carefully without scratching theTiO2 coating. The glass was sintered at 450°C to enhance the thin film compactness for 30 minunder the thermal furnace model (Carbolite, parson’s lane, Hope, Hope valley, UnitedKingdom). The surface became smooth as the film was sintered at 450°C for 30 min to producea white sintered titanium dioxide coating on the ITO. The glass was allowed to slowly cool. TheTiO2 film was immersed into the extracted dyes for about 6 h until the monolayer of the TiO2

film became covered with the dye molecules. The dye adsorbed TiO2 film was taken out of theaqueous dye solution and was gently rinsed with ethanol to remove excess dye and water fromthe porous TiO2. The dye adsorbed TiO2 film was also rinsed using ethanol extract in order toremove the excess dye. The film was then dried with an air gun.

The counter electrode was formed by electrochemical polymerization of 3,4-ethylenedioxy-thiophene (EDOT), in a three electrode one compartment electrochemical cell. In an electrochemi-cal cell, a precleaned ITO-coated glass was used as the working electrode, a platinum foil as thecounter electrode, and a quasi-Ag/AgCl as the reference electrode then it was dipped into a sol-ution of EDOTand ðC2H5Þ4NBF4 (Aldrich) in acetonitrile (Sigma-Aldrich, Steinheim, Germany)solution. The solution used for the polymerization contained 0.2 M EDOT and 0.1 MðC2H5Þ4NBF4 (Aldrich) in acetonitrile (Sigma-Aldrich). The monomer was used as received.The polymerization was carried out potentiostatically at þ1.8 V for 2 s. At this potential, the

Reda et al.: Dye-sensitized solar cell using natural dyes extracted. . .



electrode surface becomes covered with a blue-doped PEDOT film. The cell was then rinsed withacetonitrile and dried in air.

2.4 DSSC Assembling

The polymer gel electrolyte was prepared as reported elsewhere.10 About 0.9 M of 3-ethyl-2-methylimmidazolium iodide (EMIM-I) was added to acetonitrile under stirring to form a homo-geneous liquid electrolyte. In order to obtain better conductivity, 0.5 M of sodium iodide (BDH)was dissolved in the above homogeneous liquid electrolyte, and then 0.12 M of iodine (Aldrich)and 35 wt. % of polyvinyl pyrrolidone, PVP, (Aldrich) were added. The resulting mixture washeated at 70 to 80°C under vigorous stirring to dissolve the PVP polymer, followed by coolingdown to room temperature to form a gel electrolyte. Finally, the gel electrolyte was deposited inthe form of thin film on top of the dye-coated TiO2 electrode. The photoelectrochemical cell(PEC) was completed by pressing against PEDOT-coated ITO glass counter electrode. The PECwas then mounted in a sample holder inside a metal box with an area with a 1 cm2 opening toallow light from the source to enter. All experiments were carried out at ambient temperature.

2.5 Characterization and Measurement

The absorption spectra of dye solutions and dyes adsorbed on the TiO2 surface were recordedusing a UV–vis spectrophotometer (Spectronic Genesys 2PC Model). Photoelectrochemicalmeasurement was done by electrochemical analyzer (CH Instruments 630A, Austin, Texas).A 250 W tungsten-halogen lamp regulated by an oriel power supply (model 68830) was usedto illuminate the PEC. The open-circuit voltage, Voc (V), short-circuit current density, Jsc(mAcm−2), fill factor (FF), and photoelectric conversion efficiency (%) of each DSSC werealso determined. In addition, a grating monochromator (model 77250) placed into light pathwas used to select a wavelength between 300 and 800 nm. The measured photocurrent spectrawere corrected for spectral response of the lamp and the monochromator by normalization to theresponse to silicon photodiode (model S 1336-8BK, Hamamatsu, Japan) whose sensitivity spec-trum was known. No correction was made for the reflection from the surface of the sample. Thewhite light intensity from the sample was measured with a Gigaherz-Optik X11 Optometer(Newburyport, Massachusetts).

3 Results and Discussion

3.1 Optical Absorption Measurements

Before preparation of the cell, the UV-vis absorption spectra of the dyes extracted from plantmaterials were measured. Different solvents extract different components of the natural dyes. Forinstance, water as a solvent can extract anthocyanin, ethanol can extract anthocyanin and chloro-phyll, etc.3 Absorption around 500 nm is mainly due to the anthocyanin that is extracted bywater, 0.1 M HCl, and acidified ethanol. Absorption at around 660 nm is due to the chlorophyllextracted by ethanol. Anthocyanin dyes absorb in the region between 500 nm and 600 nm,1,11

while chlorophyll dyes absorb from 400 to 500 nm and 600 to 750 nm.11 Chlorophylls absorbvery little from 500 to 600 nm since this is green region of the spectrum and is reflected. For thisreason, plants appear green. Chlorophyll absorbs so strongly that it can mask other less intensecolors. Some of these more delicate colors (from molecules such as carotene) are revealed whenthe chlorophyll molecule decays or dries in the autumn, and the woodlands turn red, orange, andgolden brown. But in the range from 500 to 600 nm, the absorbance of anthocyanin is better thanthat of chlorophyll dye. The absorption band of S. hermonthica extracted with 0.1 M HCl isbetween 500 and 600 nm, which indicates that the major component is anthocyanin, whilethe dye extracted from water does not show an absorption peak beyond 400 nm. This doesnot mean there is no anthocyanin; rather there may be a large amount of other componentsin addition to the anthocyanin. Koua et al.12 studied the composition and antioxidant propertiesof the aqueous extract, which exhibits antioxidant activity using 2,2-diphenyl-1-picrylhydrazyl(DPPH). This antioxidant property was attributed to the intense presence of polyphenolic




compounds with 4% flavonoids (mainly as luteolin), 2.26% tannins, and 0.14% anthocyanins.This ascertains the presence of other aromatic compounds such as flavonoids in S. hermonthica.

From Fig. 2, an absorption peak of the M. atba Lam extracted from water is about 512 nmwhich is a weak peak. However, it has strong peaks at 515 nm when extracted with 0.1 M HCl(Fig. 1). This indicates both water and 0.1 M HCl mainly extract the anthocyanin components,but the 0.1 M HCl extract has a strong absorption. This is because the equilibrium shifts from thequinonoidal to the flavylium form.

In the absorption spectrum of the S. hermonthica dye extract of water and 0.1 M HCl, thelatter exhibits absorption peak at 516 and 334 nm (Fig. 1) and no absorption was observed for theformer (Fig. 2). This phenomenon is ascribed to its identical components, namely, anthocyanin, agroup of natural phenolic compounds. The absorption peak at 516 nm occurred after the waterbecame acidified (0.1 M HCl). This is because acidification leads to a protonation reaction,which increases the extraction of anthocyanin. This variation of absorption spectra with differentsolvents ascertains that different solvents extract different components from plant pigments.Chlorophylls and anthocyanins are the major components of S. hermonthica extracted with etha-nol and acidified ethanol. The absorption spectra of its extract from acidified ethanol (Fig. 1)look wider when compared to its ethanol extract (Fig. 2) and show no peak at 663 nm. This isbecause ethanol extracts the chlorophyll components, and has an absorption spectra at 663 nm,while acidified ethanol causes the protonation reaction with the anthocyanin. As a resultthis means an enhanced absorption intensity for the anthocyanin. The absorption peak ofS. hermonthica extract in ethanol has a wavelength at 663 nm (Fig. 2), which indicates that

Fig. 1 Light absorption spectra of dye solutions of S. hermonthica extracted with (a) acidified etha-nol, (b) 0.1 M HCl and M. atba Lam extracted with (c) acidified ethanol, and (d) 0.1 M HCl.

Fig. 2 Light absorption spectra of dye solutions ofM. atba Lam extracted with (a) water (b) ethanoland S. hermontica extracted with (c) water, and (d) ethanol.




the major component is chlorophyll; whereas, its extract in acidified ethanol has an absorptionmaxima of 513 nm (Fig. 1), which tells us that the major component of this extraction is antho-cyanin due to the protonation reaction with HCl. In general, the absorption spectrum of the dyeextracted from the S. hermonthica shows that its constituents should be a mixture of anthocyaninand chlorophyll.

In the absorption spectra of the natural dyes extracted from M. atba Lam in two differentsolvents, i.e., ethanol and acidified ethanol, the wavelengths of the maximum absorption of itsextract in ethanol were 665 and 536 nm (Fig. 2), which indicates the presence of both chlorophylland anthocyanin. Therefore, the extract in acidified ethanol has strong absorption spectra ataround 525 nm (Fig. 1), which shows that anthocyanin is a dominant component. As a result,the absorption spectra of M. atba Lam extracted from these two solvents are also a mixture ofchlorophyll and anthocyanin like that of its extract in acidified ethanol, but has stronger absorp-tion spectra from 500 to 580 nm, which differ from its extract in ethanol.

The structure presented in (Fig. 3) depicts the positively charged flavylium cation, which isthe leading equilibrium form in acidic solutions. The relative ease of deprotonating for the struc-ture (Fig. 3) of the two OH groups at positions 4′ and 7 contributes to the color changes of theanthocyanin. One of these hydroxyls loses a proton with a basic media (ethanol), producing thequinonoidal bases (Fig. 3) that exhibit a chromatic deviation toward longer wavelengths relativeto the flavylium cation. Oppositely, one of these hydroxyls gains a proton with acidic media,producing a flavlyium cation (Fig. 3), which is the only more stable form.

Some of the wavelengths of these absorbance peaks show a red and blue shift of a few nano-meters among various pigments, depending on the structure of each and the solvent used for theextraction of pigments. But, in general, the dyes used in this study are effective photoreceptorsbecause they contain networks of alternating single and double bonds. When light is absorbed bya molecule of the dye, the energy from the light excites an electron from its ground state energylevel to an excited state.

Fig. 3 The pH dependence of anthocyanin, the general proposed binding mechanism for cyaninand delphinidin: (a) flavylium cation (Aþ), and (b) quinonoidal.




3.2 Current Density-Voltage (J-V) Measurements for TiO2 Based DSSCs

The FFs of these DSSCs at those different solvents are mostly higher than 50%. The Voc variesfrom 0.49 to 0.54 V and the Jsc ranges from 0.44 to 1.38 mA cm−2. A Voc of 0.54 V and Jsc of1.38 mA cm−2 were obtained from the DSSC sensitized byM. atba Lam extracted from acidifiedethanol, and the efficiency of the DSSC reached 0.420% with an FF of 56%. As was reported, itsanthocyanin dye extract showed a power conversion efficiency of 0.548%, Voc of 0.555 V, Jsc of1.89 mA cm−2, and FF of 53%,11 with an ionic liquid electrolyte. However, in this work, a qua-sisolid-state electrolyte was employed and an ITO was used instead of the FTO for the photo-anode. The photoelectrochemical performance determination of chlorophyll dyes extracted fromethanol was not done. However, the DSSCs sensitized by natural dyes mainly composed ofchlorophyll did not offer high conversion efficiencies. This is because there are no availablebonds between the dye and TiO2 molecules through which electrons can transport from theexcited dye molecules to the TiO2 film.13

Figure 4 shows the J-V (current density-voltage) curve for all the sensitizers. The powerconversion efficiency of the cell sensitized by Morus atba Lam extracted from acidified ethanolwas significantly higher than that sensitized by the S. hermonthica. This is due to the broaderrange of light absorption of the extract for TiO2, and the higher interaction between TiO2 and theanthocyanin in the M. atba Lam extract that resulted in a better charge transfer. The related J-Vcurve is shown in Fig. 4, which has the better cell parameters, i.e., a power conversion efficiencyof 0.420%, Voc of 0.54 V, Jsc of 1.38 mA cm−2, and FF of 56%, and its extract in 0.1 M HClshown the following cell parameters, i.e., a power conversion 0.130%, Voc of 0.51 V, Jsc of0.47 mA cm−2, and FF of 52%. This is because of the lower intensity and narrower rangeof absorption in the visible light absorption spectrum relative to the efficiency obtained withacidified ethanol extraction. The DSSCs using M. atba Lam fruit dye extracted from acidifiedethanol have the best power conversion efficiency.

Among the DSSCs of different extracts of S. hermonthica (i.e., in acidified ethanol and 0.1 MHCl), a better performance was recorded with acidified ethanol as given in Table 1. However, itsDSSCs from a 0.1 M HCl extract had a lower performance compared to the dye extracted fromacidified ethanol. From the results, it is proven that dye extracts of S. hermonthica flower areapplicable for DSSCs. Therefore, if improvement of the power conversion efficiency is attained,the S. hermonthica flower extract could be an alternative anthocyanin source for DSSC prepa-ration, especially in the tropical country, sub-Saharan Africa. As mentioned previously thisflower was studied before as it is a parasitic plant in crops and is used for medicinal purposes,as was reported.14–16 Further investigation on the technique of extraction of the S. hermonthicaflower dye is crucial in order to get better efficiency. The pH and extracting solvent are alsofactors affecting the efficiency of DSSCs using natural dyes.1

Fig. 4 J-V curve of DSSC sensitized by S. hermonthica with (a) 0.1 M HCl,M. atba Lam extractedwith (b) 0.1 M HCl, S. hermonthica extracted with (c) acidified ethanol, and M. atba Lam with(d) acidified ethanol.




The photoelectrochemical parameters of the DSSCs sensitized with the M. atba Lam and S.hermonthica are summarized in Table 1.

3.3 Effect of Sources of Anthocyanin and Extracting Solvents on DSSCsEfficiency

The effect of the DSSC performance was studied for different extracting solvents. Four differentsolvents; water, 0.1 M HCl, ethanol, and acidified ethanol were used as extracting solvents. Thehigher solubility of anthocyanin in ethanol suppresses possible dye attachment on TiO2, which isnot preferable for electron injection and dye regeneration. Unfortunately, ethanol destabilizes theflavylium cation and accelerates UV bleaching as reported in Ref. 17. It was also reported thatthe extracting solvent has an effect on the efficiency of DSSCs.18,19 The efficiency of the DSSCsof both M. atba Lam and S. hermonthica was found to greatly increase when acidified ethanolwas employed as an extracting solvent for both dyes. As shown in Table 1, the DSSCs using asolvent acidified ethanol, for both dyes (M. atba Lam and S. hermonthica) show a higher effi-ciency than that of other solvents. This is due to the fact that after the anthocyanin solubilizes inethanol, it forms a quinonoidal form with TiO2 (Fig. 3). When this quinonoidal form becomesacidified the flavylium cation forms (Fig. 3) as studied by Dai et al.,17 which is the more stableform. However, the DSSCs prepared from those dyes extracted in ethanol may operate for a shorttime. Hence, to prevent this defect it is preferable to use the acidified ethanol that changes thequinonoidal form to a flavylium cation form.

3.4 Incident Monochromatic Photon to Current Conversion Efficiency (IPCE)

The dye extracted from acidified ethanol has a higher IPCE. M. atba Lam extracted with acidi-fied ethanol has an IPCE of 27% while S. hermonthica extracted with the same solvent has anIPCE of 17%. However,M. atba Lam and S. hermonthica extracted with 0.1 M HCl have IPCEsof 15%, in which both dyes have the same IPCE at maximum wavelength, which demonstratesthat the electron injection from the dye LUMO to the conduction band of the semiconductor issimilar. S. hermonthica extracted with acidified ethanol and 0.1 M HCl has a slight difference inabsorption spectra, resulting in a slight varation in the IPCE (Table 2).

Figure 5 shows the incident photon to current conversion efficiency (IPCE) for a DSSC sensi-tized with anthocynin of S. hermonthica extracted with 0.1MHCl and acidified ethanol. As depicted

Table 1 Photoelectrochemical parameters of the quasisolid state DSSCs sensitized with naturaldyes of M. atba Lam and S. hermonthica extracted from different solvents.

Natural dye Solvent Vm (V) Jm (mA) V oc (V) Jsc (mAcm−2) FF (%) η (%)

M. atba Lam 0.1 M HCl 0.36 0.350 0.51 0.470 52 0.130

M. atba Lam Acidified ethanol 0.41 1.020 0.54 1.380 56 0.420

S. hermonthica 0.1 M HCl 0.34 0.290 0.49 0.440 46 0.099

S. hermonthica Acidified ethanol 0.38 0.800 0.52 1.010 58 0.304

Table 2 The incident photon to current conversion efficiency of quasisolid state DSSC both M.atba Lam and S. hermonthica extracted from different solvents.

Naturel dye Solvents Max. wavelength (nm) IPCEmax (%)

Mulberry Acidified ethanol 645 27

Mulberry 0.1 M HCl 641 15

S. hermonthica Acidified ethanol 646 17

S. hermonthica 0.1 M HCl 643 15




in Fig. 5, the dye extracted with acidified ethanol has an IPCE ¼ 17%, whereas, the dye extractedfrom 0.1 M HCl has a slightly lower IPCE of 15%, as shown in Table 2. This isbecause the electron injection rate from the dye LUMO to the semiconductor conduction bandhas an immense contribution in converting the incident photon to a photocurrent at a specific wave-length. When acidified ethanol was used as the exracting solvent, the Jsc became 1.01 mA cm−2.However, when 0.1MHCl was used as an extracting solvent, the Jsc was reduced to 0.44 mA cm−2.This results in decreasing the IPCE (Table 1). From this it is possible to deduce that the dye with thehighest Jsc and Voc has both the highest power conversion efficiency and IPCE.

The IPCEs of M. atba Lam extracted from acidified ethanol and 0.1 M HCl (Fig. 5) are 27%and 15%, respectively. Fortunately, the dye extracted with the highest conversion efficiency alsohas the highest IPCE, i.e., the M. atba Lam fruit extracted with acidified ethanol has the highestefficiency and an IPCE 0.420% and 27%, respectively, while the dye extracted with 0.1 M HClhas a relatively smaller efficiency and IPCE of 0.130% and 15%, respectively. This is becauseaggregation of the dyes may lead to intermolecular quenching or molecules residing in the sys-tem but not being functionally attached to the semiconductor surface and thus acting as filters.20

Therefore, in the case of 0.1 M HCl extraction, there are dyes which are not attached very well tothe semiconductor surface, and thus serve as filters. In the case of acidified ethanol, the dyeattaches to the semiconductor which is preferable for electron injection and dye regeneration.This indicates that acidified ethanol as an extraction solvent is more preferable because it pro-vides better efficiency and IPCE values. This is because the dye extracted from ethanol attains aquinonoidal form. After it becomes acidified, the quinonoidal form immediately changes to fla-vylium cation, which is the only more stable form. In addition, the dye extracted from acidifiedethanol has a faster electron injection from the excited dye LUMO to the conduction band of thesemiconductor. The pristine TiO2 based PEC does not produce any photocurrents in the visibleregion, as can be seen from Fig. 5(e). This is because TiO2 is a wide bandgap semiconductor andonly absorbs light in the ultraviolet region and hence shows a strong peak below 400 nm. Theincident photon-to-current conversion efficiencies of M. atba Lam and S. hermonthica extractedfrom different solvents are summerized in Table 2.

Generally, natural dyes suffer from low Voc and Jsc which leads to a lower power conversionefficiency than an equivalent commercial N719 sensitized solar cell. This is because most of thenatural dyes do not strongly attach to the semiconductor nanoparticle, even upon washing by asolvent to avoid aggregation from the socked film. The sensitizers may leave the surface of thefilm, since most of the natural dye attaches to TiO2 nanoparticles by physical adsorption or aweak bonding force.

4 Conclusion

An investigation on anthocyanin pigments of S. hermonthica andM. atba Lam as natural photo-sensitizers with a TiO2 semiconductor, describing and comparing their sensitization activity with

Fig. 5 IPCE curves of S. hermonthica extracted from (a) 0.1 M HCl, (b) acidified ethanol, and M.atba Lam extracted from (c) 0.1 M HCl, (d) acidified ethanol, and (e) pristine TiO2 PEC.




respect to their extraction from different solvents, was made. Anthocyanin raw pigments simplyextracted in acidic condition from the fruits of M. atba Lam and the flower of S. hermonthicaachieved IPCEs higher than 15%. The natural dyes extracted in acidified ethanol have the highestIPCEs and highest solar energy conversion efficiency, i.e., the IPCEs of M. atba Lam and S.hermonthica are 27% and 17%, and the conversion efficiencies are 0.420% and 0.304%, respec-tively. Overall, natural dyes as sensitizers of DSSCs are promising because of their environmen-tal friendliness, low-cost production, and simple and energy-efficient manufacturing.

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