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Natural dye as light-harvesting pigments for quasi-solid-state dye-sensitized solar cells

Natural dye as light-harvesting pigments for quasi-solid-state dye-sensitized solar cells Mater Renew Sustain Energy (2016) 5:13 DOI 10.1007/s40243-016-0077-x ORIGINAL PAPER Natural dye as light-harvesting pigments for quasi-solid-state dye-sensitized solar cells 1 1,2,3 2,3 • • Negese Yazie Delele Worku Abebe Reda Received: 8 October 2015 / Accepted: 12 July 2016 / Published online: 25 July 2016 The Author(s) 2016. This article is published with open access at Springerlink.com Abstract In this paper, quasi-solid-state dye-sensitized and Euphorbia cotinifolia pigments as natural sensitizers solar cell has been constructed based on natural photo- along with the use of quasi-solid electrolyte and PEDOT sensitizers extracted from the bracts of Bougainvillea coated FTO counter electrodes could be a possible alter- spectabilis and the leaves of Euphorbia cotinifolia using native for the production of low-cost and environment acidified (0.1 M HCl) distilled water and ethanol sepa- friendly DSSCs. rately. The absorption spectra of the extracts were per- formed in the spectral range from 395 to 750 nm. The cells Keywords Dye-sensitized solar cell  Titanium dioxide were assembled using commercial TiO powder film and Natural dyes  Quasi-solid state electrolyte  Counter PEDOT coated FTO glasses as working and counter elec- electrode trodes, respectively, and also the quasi-solid electrolyte sandwiched in between. The Photovoltaic parameters such as short circuit current density (J ), open circuit voltage Introduction sc (V ), fill factor (FF), and overall conversion efficiency (g) oc for the as-prepared DSSC were determined under 100 mW/ Currently, the solar cells available commercially are based cm illuminations. The highest open circuit voltage on inorganic silicon semiconductors, made of p-n junc- (V = 0.549 V) and short circuit current density tions, which are relatively expensive to manufacture and oc (J = 0.592 mA/cm ) were obtained from the DSSCs also the manufacturing process releases harmful emissions sc assembled by natural dye extracted with the acidified to the environment that cause pollution. Hence, hybrid ethanol of Bougainvillea spectabilis bracts and the leaves solar cells appear to be highly promising and cost-effective of Euphorbia cotinifolia, respectively. The highest power alternatives for photovoltaic energy sectors due to its rel- conversation efficiency (g) of the as-prepared DSSC atively cheapness to produce, and promising efficiency. In assembled with natural dye extracted from Bougainvillea this regard, dye sensitized solar cells (DSSCs) have been spectabilis bracts using acidified ethanol as extracting given considerable attention in recent years [1]. DSSC, solvent was 0.175 %. The use of Bougainvillea spectabilis which was invented by Michael Gra¨tzel and Brian O’Re- gan in 1991, is a device that converts visible light energy into electrical energy based on the sensitization of the wide & Delele Worku band gap semiconductors [2, 3] by suitable regenerative delelew@bdu.edu.et; delelewww@yahoo.com dyes. A DSSC consists of a porous wide band gap semi- conductor thin film layer like (TiO , ZnO, SnO2, Nb O ) 1 2 2 5 Material Science and Engineering Program, College of coated on a fluorine-doped tin oxide (FTO) or indium- Science, Bahir Dar University, P.O. Box 79, Bahir Dar, Ethiopia doped tin oxide (ITO) photoanode electrode, a dye, a platinum (Pt) or carbon or conductive polymer thin film Department of Chemistry, Bahir Dar University, P.O. Box 79, counter electrode and an electrolyte normally containing Bahir Dar, Ethiopia - - I /I redox couple. Titanium dioxide (TiO ) is most 3 2 Energy Research Center, Bahir Dar University, P.O. Box 79, commonly used as photoanode in DSSC application since Bahir Dar, Ethiopia 123 13 Page 2 of 7 Mater Renew Sustain Energy (2016) 5:13 it is nontoxic, inert, and has a large energy bandgap as well In this paper, we have extracted natural dyes from the as good optical and electrical properties and, thus, can be bracts of a shrub Bougainvillea spectabilis and the leaves efficiently sensitized by a dye [4]. In DSSC, the dye plays of Euphorbia cotinifolia and studied their potential appli- an important role in harvesting solar energy and converting cation to be employed as sensitizers for DSSC manufac- it to electrical energy with the aid of a semiconducting turing using TiO as wide band gap semiconductor. photoanode. Therefore, the cell performance is mainly Furthermore, quasi-solid state electrolyte and PEDOT films dependent on the type of dye used as a sensitizer in which on FTO glass as a counter electrode were used. the absorption spectrum and the anchorage of the dye to the surface of TiO are important parameters in determining the efficiency of the cell [5, 6]. Several metal complexes Experimental sections and organic dyes have been synthesized and used as sen- sitizers. Among these, ruthenium based complexes are Chemicals Ethanol (CH CH OH, 97 %) was purchased 3 3 considered as the most efficient sensitizers because of their from (Fluka). Fluorine doped tin oxide (FTO— intense charge -transfer absorption over the entire visible 1.5 cm 9 3 cm, 15 X), TiO (70 % anatase and 30 % range and highly efficient metal-to-ligand charge transfer rutile—P25-Degussa AG, Germany), EDOT, Tri-ton— [2, 3, 7]. However, these advantages are offset by their 1009, polyvinyl pyrrolidone (PVP), acetonitrile, (C H ) 2 5 4- rarity, high cost, complicated synthetic routes, and envi- NBF and acetone all were purchased from Aldrich. ronmental threat. Thus, an alternative organic dye such as Hydrochloric acid (HCl, 35 %) was purchased from India. natural dyes is suggested with similar characteristic with an acceptable efficiency [8–10]. Recently, research has Preparation of natural dye sensitizers focused on the easily available dyes extracted from natural sources because of their large absorption coefficients, high Enough amounts of the bracts of a shrub Bougainvillea light-harvesting efficiency, complete biodegradation, low spectabilis and the leaves of Euphorbia cotinifolia were cost, simple and safe preparation and eco-friendly. Thus collected in Bahir Dar city, Ethiopia. Before drying, the far, several dyes such as anthocyanins, carotenoids, beta- bracts and leaves were washed with tape water and then lains and chlorophylls extracted from parts of different dried in laboratory for 6 weeks without light exposure. plants (such as Jathropha curcas, Citrus aurantium, red After drying, the bracts and leaves were crushed into fine cabbage and etc.) have been used as sensitizers in DSSC powder using an electrical blender. Figure 1 shows the [11, 12]. photographic picture of tree plants, the respective dried Fig. 1 Plant leaves used in this study a Bougainvillea spectabilis and b Euphorbia cotinifolia with their corresponding dried leaves, powders and extracted dye solutions 123 Mater Renew Sustain Energy (2016) 5:13 Page 3 of 7 13 bracts and leaves and the powder of the dried bracts of three electrode system with one-compartment electrochem- Bougainvillea spectabilis and leaves of Euphorbia cotini- ical cell. The electrochemical cell consisted of a precleaned folia. 2 g powder of the dried bracts of Bougainvillea FTO-coated glass working electrode, platinum foil counter spectabilis was separately extracted in 50 ml ethanol and electrode and quasi-Ag/AgCl reference electrode. The 50 ml distilled water solvents that were acidified with solution used for the polymerization contained with 0.2 M 0.1 M HCl at room temperature. EDOT and 0.1 M (C H ) NBF in acetonitrile. The mono- 2 5 4 4 Similarly, 2 g powder of the dried leaves of Euphorbia mer was used as received. The polymerization was carried cotinifolia was separately extracted in 50 ml of ethanol and out potentiostatically at ?1.8 V for 2 s. At this potential, the 50 ml of distilled water solvents that were acidified with electrode surface was covered with blue-doped PEDOT film. 0.1 M HCl at room temperature. The glass containers were The cell was then rinsed with acetonitrile and dried in air. covered with aluminium foils to prevent damage from light PEDOT improves the charge transfer between the FTO and - - exposure and were left for 24 h in dark conditions. The the I /I redox couple [13, 14]. It is chosen as a counter solids were filtered out first by decantation and then filtered electrode, because of less costly and can easily be prepared by using glass filter. Finally, the dye was ready for optical electrochemically to a desired transparency [15]. characterization and to be used as sensitizers. Preparation of electrolyte Preparation of working electrode The quasi-solid electrolyte, which had been used in this Fluorine doped tin oxide conductive glass sheets were first work, was prepared as follow: 0.9 M of 3-ethyl-2- methyl cleaned with distilled water, acetone and then ethanol for Immidazolium iodide (EMIM-I) was added into acetoni- 10 min using ultrasonic bath at each step and then dried. trile under stirring to form a homogeneous liquid elec- The nanocrystalline TiO paste was prepared using com- trolyte. To obtain a better conductivity, 0.5 M sodium mercial titanium dioxide powder. 3 g of commercial TiO iodide was dissolved in the above homogeneous liquid powder was ground in a porcelain mortar with 1 ml dis- electrolyte, and then 0.12 M iodine and 35 % (m/m) of tilled water containing 0.1 ml acetic acid to prevent reag- polyvinyl pyrrolidone were added. The resulting mixture gregation of the particles. Once the powder was dispersed was heated at 70–80 C under vigorous stirring to dissolve by the high shear forces in the viscous paste, it was diluted the PVP polymer, followed by cooling down to room by slow addition of 4 ml distilled water under continued temperature to form a gel state electrolyte. Finally, the gel grinding in a porcelain mortar for half an hour. Finally, a electrolyte was deposited in the form of thin film on top of nonionic surfactant (0.05 mL Triton X-100), was added the dye coated TiO electrode. The DSSC was completed and the mixture was grinding for additional half an hour by pressing gel electrolyte coated TiO electrode against until a homogeneous paste was obtained. The surfactant PEDOT-coated FTO glass counter electrode. was used to ease adhesion of TiO particles to FTO con- Owing to its unique hybrid network structure, quasi- ductive glass substrate layer. Then, the conductive side of solid-state electrolytes always possess, simultaneously, the FTO glass was covered on two parallel edges with both the cohesive property of a solid and the diffusive adhesive tape to control the thickness of the TiO paste and transport property of a liquid [16]. Namely, quasi-solid- to provide non-coated areas for electrical contact. The state electrolytes show better long-term stability than liquid paste was spread on one of the free edges of the conducting electrolytes do and have the merits of liquid electrolytes glass using a glass rod–doctor blade method. After drying including high ionic conductivity and excellent interfacial in air, the electrode was sintered for 30 min at 450 Cina contact property [17, 18]. furnace (Carbolite Model ELF11/14 B). After cooling down, the electrode was immersed in the natural extracts Assembling of DSSC for 24 h. Finally, the electrode was withdrawn from the solution and rinsed with ethanol to remove the residues left The prepared photoelectrode and counter electrode were on the TiO film and dried in air which would be used as placed one over the other face-to-face and hold with a photoelectrode. clamps for performance evaluation as can be seen from the following schematic diagram, so that the electrolyte sand- Preparation of counter electrode wiched between the titanium dioxide covered area of the working electrode and the PEDOT coated area of coun- The counter electrode was prepared by coating the conduc- terelectrode. The schematic representaion of the as pre- tive side of FTO glass with poly (3,4-ethylenedioxythio- pared DSSC has been shown in Fig. 2. As seen in Fig. 2, phene) (PEDOT) film which was formed by electrochemical the as prepared DSSC were attached to potentiostat polymerization of 3,4 ethylenedioxythiophene (EDOT), in a equipment by means of cords and crocodile clips. 123 13 Page 4 of 7 Mater Renew Sustain Energy (2016) 5:13 Fig. 2 A schematic representation of DSSCs Optical characterization and DSSC performance measurements The absorption spectra of the extracted pigments in acidi- fied (0.1 M HCl) ethanol and distilled water solution sep- arately were obtained using a UV–Vis spectrophotometer (PerkinElmer lambda 35). The absorption spectra analysis was carried out in the wavelengths ranging from 250 to Fig. 3 UV-visible absorption spectrum of natural dye extracted from 850 nm. The photoelectrochemical measurements of the Bougainvillea spectabilis using acidified (with 0.1 M HCl) a distilled cells under illumination were performed using a computer water and b ethanol as extracting solvents controlled CHI630A Electrochemical Analyzer. A 250-W tungsten–halogen lamp regulated by an Oriel power supply (Model 68830) was used to illuminate the DSSC. The measured photocurrent was corrected for the spectral response of the lamp and the monochromator by normal- ization to the response of a calibrated silicon photodiode (Hamamatsu, Model S1336-8BK) whose sensitivity spec- trum was known. No correction was made for the reflection from the surface of the sample. The illumination light intensity was measured in the position of the sample cell with Gigahertz-Optik X11 Optometer and the intensity of the incident light was 100 mW/cm . Results and discussion Absorption of natural dyes Fig. 4 UV-visible absorption spectrum of natural dye extracted from The representative UV–Vis absorption spectra of the Euphorbia cotinifolia leaves using acidified (with 0.1 M HCl) extracts of Bougainvillea spectabilis and Euphorbia a distilled water and b ethanol as extracting solvents cotinifolia have been investigated. Figures 3 and 4 show the UV–Vis absorption spectra of these extracts dissolved in acidified (0.1 M HCl) ethanol and distilled water sepa- 420 nm which might indicate the extract contains chloro- rately. Figure 3 shows the absorption spectra of phyll mixture which shows an absorption peak in between Bougainvillea spectabilis extracted by water and ethanol, 400–500 nm and 600–700 nm [21] and also the extract has acidified with 0.1 M HCl. As can be seen from Fig. 3,in two small peaks between 476 and 545 nm which could be acidified distilled water extract, one peak was found around associated with the presence of betalains [22]. 545 nm maximum absorbance which can be associated to Figure 4 shows the absorption spectra of Euphorbia the presence of betalain, betalains in acidic environments cotinifolia extracted by distilled water and ethanol, acidi- have strong absorption in the 400–600 nm range due to the fied with 0.1 M HCl. As can be seen from Fig. 4,in color combination of yellow–orange betaxanthins and red– acidified ethanol extract of Euphorbia cotinifolia, the pink betacyanins [19, 20]. Differently, acidified ethanol extracted pigment had three absorption peaks at about 418 extract of Bougainvillea spectabilis shows two absorption and 655 nm which show the characteristics absorption peak peaks around 655 nm (close to 660 nm) and around of chlorophyll mixture and the third absorption peak was at 123 Mater Renew Sustain Energy (2016) 5:13 Page 5 of 7 13 about 532 nm which might be associated with the presence of anthocyanin [9]. Differently, distilled water extract of Euphorbia cotinifolia had an absorption peak around 520 nm which might indicate the absorption of antho- cyanins, a group of natural phenolics compounds. Antho- cyanin is the core component of some natural dyes and is often found in the fruits, flowers, and leaves of plants [2]. Because anthocyanin shows color in the range of visible light from red to blue, it is predicted to become a highly efficient sensitizer for wide band gap semiconductors. In the acidified distilled water extracts, the concentration of dyes (betalain and anthocyanin) is expected to be higher than in acidified ethanolic extracts as can be seen from absorption spectra in Figs. 3 and 4, respectively, probably because of a higher solubility in water-polarity effect. But Fig. 5 J–V characteristic curves of Bougainvillea spectabilis bracts water extracts did not contain chlorophyll absorption since using acidified ethanol (EtOH) and distilled water (H O) as extracting chlorophylls are relatively polar and are thus normally solvents extracted with methanol, ethanol, acetone or other organic solvents miscible with water [20, 23]. The acidity of the extract likely influences the solubility of various dyes, leading to extracts with different compositions. Generally, the chlorophylls exhibit strong absorption in the blue and red regions of the solar spectrum [24]. How- ever, they show poor absorption in the green region, which is consistent with our results. Photoelectrochemical properties Photovoltaic tests of the fabricated DSSCs using these natural dyes as photosensitizers were performed by mea- suring the J–V characteristics of each cell under 100 m W/ cm irradiation from a tungsten–halogen lamp. The per- formance of natural dyes as sensitizers in DSSCs was evaluated by short circuit current density (J ), open circuit sc voltage (V ), fill factor (FF) and energy conversion effi- oc Fig. 6 J–V characteristic curves of Euphorbia cotinifolia leaves ciency (g). Based on J–V and P–V results the fill factor using acidified ethanol (EtOH) and distilled water (H O) as extracting solvent (FF) and energy conversion efficiency (g) were calculated using Eqs. (1) and (2). FF  J  V sc oc depicted in Table 1, the fill factor of the fabricated DSSCs g ¼  100% ð1Þ in ranges between 0.552 to 0.603. The V changes from oc 0.411 to 0.549 V, and the J varies from 0.541 to 0.592 m where P is the power of incident light, V is open circuit sc in oc A/cm . Specifically, a high V = 0.549 V and voltage; J is short circuit current density oc sc J = 0.592 mA/cm were obtained from the DSSCs sen- sc J  V m m FF ¼ ð2Þ sitized by the dyes of Bougainvillea spectabilis and J  V sc oc Euphorbia cotinifolia, respectively, that were extracted by where J and V are the photocurrent density and photo- m m acidified ethanol. These data are significantly higher than voltage for maximum power output P . m those of the DSSCs sensitized by other natural dyes in this Figures 5 and 6 show the typical J–V curves of the as work. prepared DSSCs using the sensitizers extracted from The highest output power and energy conversion effi- Bougainvillea spectabilis and Euphorbia cotinifolia. All ciency were obtained for the DSSC sensitized with acidi- the photoelectrochemical parameters of the DSSCs fied ethanolic extract of Bougainvillea spectabilis where assembled with these natural dyes are listed in Table 1.As the efficiency of the cell reached 0.175 %. This might be 123 13 Page 6 of 7 Mater Renew Sustain Energy (2016) 5:13 Table 1 The photoelectrochemical parameters of the as prepared DSSCs Plant Solvent V (V) J V (V) J P (mW/ FF P g (%) Remarks oc sc max max in max 2 2 2 2 (mA/cm ) (mA/cm ) cm ) (mW/cm ) Bougainvillea spectabilis Water 0.541 0.545 0.361 0.454 100 0.552 0.164 0.164 Current work Ethanol 0.549 0.574 0.371 0.471 100 0.555 0.175 0.175 Euphorbia cotinifolia Water 0.411 0.553 0.295 0.464 100 0.602 0.137 0.137 Ethanol 0.417 0.592 0.299 0.498 100 0.603 0.149 0.149 Commercial carrot Ethanol 0.010 35.4 75.4 29.3 0.61 0.015 [30] Hibiscus flower Ethanol 0.268 0.96 0.43 0.11 [31] Lawsonia inermis Ethanol 1.60 0.91 0.96 1.39 [32] associated with the combination presence of chlorophyll energy conversion efficiency of the DSSC will be reduced. and betalain pigments in the extract to absorb photons—a Despite the low efficiencies, the as prepared DSSCs exhibit synergistic effect of two sensitizing pigments. Although the applicability of Bougainvillea spectabilis and Euphor- there may not be a single pigment that can act as a highly bia cotinifolia dyes for photovoltaic energy conversion. efficient sensitizer, it is possible that a combination of pigments could provide the absorbance necessary to increase the efficiency of a DSSC. In fact, some research Conclusion has demonstrated a synergistic effect of two sensitizing pigments, meaning the absorption spectra of two pigments The Bougainvillea spectabilis and Euphorbia cotinifolia combined to increase light absorption and increase the extracts were used as low-cost sensitizers for commercial incident photon to current conversion efficiency of the TiO nanoparticles photoanode based dye sensitized solar solar cell [25, 26]. Chlorophyll plays an important role in cell. The UV–Vis absorbance measurement of the plant photosynthesis; the DSSCs using chlorophyll Bougainvillea spectabilis and Euphorbia cotinifolia derivatives as sensitizers obtained a relatively high con- extracts in acidified distilled water and ethanol as a solvent version efficiency [15, 16]. This is because there are were carried out and the extracts had absorption peaks in available bonds between the dye and TiO2 molecules the visible light regions due to the presence of plant pig- through which electrons can transport from the excited dye ments such as chlorophyll and betalain in Bougainvillea molecules to the TiO2 film [17]. This result indicates that spectabilis and chlorophyll and anthocyanin in Euphorbia the interaction between the sensitizer and the TiO2 film is cotinifolia. The photovoltaic performance of the extracts as significant in enhancing the power conversion efficiency of sensitizers for the constructed DSSCs was evaluated under DSSCs. The DSSC output power was calculated as simulated solar light irradiation. The solar cell sensitized P = J 9 V using the J–V data. The maximum power with these extracts delivered short-circuit photocurrent (P ) of the DSSCs for each cell is then calculated. The densities (J ) ranging from 0.545 to 0.592 mA/cm , open max sc current (I ) and the voltage (V ) corresponding to the circuit voltages (V ) varied from 0.411 to 0.549 V and the max max oc maximum power point are then obtained. Generally, the fill factors varied from 0.552 to 0.603. The highest open natural dyes employed as sensitizers in solar cell deliver circuit voltage (V = 0.549 V) and short circuit current oc very low efficiencies when compared to synthetic organic density (J = 0.592 mA/cm ) were obtained from the sc and inorganic dyes, due to the absence of specific func- DSSCs sensitized by the acidified ethanol extracts of tional attachment groups and low absorption in the visible Bougainvillea spectabilis bracts and Euphorbia cotinifolia region of the solar spectrum [27, 28]. The conversion leaves, respectively. The highest power conversation effi- efficiencies of the as prepared DSSCs in this study were ciency (g) of the prepared DSSC was 0.175 %. better as compared to some DSSC values reported as The overall results of the study suggest that the shown in Table 1. But, the obtained conversion efficiencies exploitation of Bougainvillea spectabilis and Euphorbia were lower than those reported [32] for natural dye based cotinifolia pigments as natural sensitizers along with the DSSCs may be due to several reasons. Of them one use of quasi-solid electrolyte and PEDOT coated FTO potential reason for the low efficiencies is due to the counter electrodes could be a possible alternative for the inconsistency of the TiO film thickness in the layer [29]. production of low-cost and environment friendly DSSCs. The uniformity of the TiO film thickness affects the Acknowledgments The authors gratefully acknowledge the financial conversion efficiency of the solar cell. If the film thickness support provided by Bahir Dar University, Energy Research center for is not uniform, charge recombination can occur and the the study undertaking. 123 Mater Renew Sustain Energy (2016) 5:13 Page 7 of 7 13 Open Access This article is distributed under the terms of the 16. 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Natural dye as light-harvesting pigments for quasi-solid-state dye-sensitized solar cells

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Material Science; Materials Science, general; Renewable and Green Energy; Renewable and Green Energy
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Mater Renew Sustain Energy (2016) 5:13 DOI 10.1007/s40243-016-0077-x ORIGINAL PAPER Natural dye as light-harvesting pigments for quasi-solid-state dye-sensitized solar cells 1 1,2,3 2,3 • • Negese Yazie Delele Worku Abebe Reda Received: 8 October 2015 / Accepted: 12 July 2016 / Published online: 25 July 2016 The Author(s) 2016. This article is published with open access at Springerlink.com Abstract In this paper, quasi-solid-state dye-sensitized and Euphorbia cotinifolia pigments as natural sensitizers solar cell has been constructed based on natural photo- along with the use of quasi-solid electrolyte and PEDOT sensitizers extracted from the bracts of Bougainvillea coated FTO counter electrodes could be a possible alter- spectabilis and the leaves of Euphorbia cotinifolia using native for the production of low-cost and environment acidified (0.1 M HCl) distilled water and ethanol sepa- friendly DSSCs. rately. The absorption spectra of the extracts were per- formed in the spectral range from 395 to 750 nm. The cells Keywords Dye-sensitized solar cell  Titanium dioxide were assembled using commercial TiO powder film and Natural dyes  Quasi-solid state electrolyte  Counter PEDOT coated FTO glasses as working and counter elec- electrode trodes, respectively, and also the quasi-solid electrolyte sandwiched in between. The Photovoltaic parameters such as short circuit current density (J ), open circuit voltage Introduction sc (V ), fill factor (FF), and overall conversion efficiency (g) oc for the as-prepared DSSC were determined under 100 mW/ Currently, the solar cells available commercially are based cm illuminations. The highest open circuit voltage on inorganic silicon semiconductors, made of p-n junc- (V = 0.549 V) and short circuit current density tions, which are relatively expensive to manufacture and oc (J = 0.592 mA/cm ) were obtained from the DSSCs also the manufacturing process releases harmful emissions sc assembled by natural dye extracted with the acidified to the environment that cause pollution. Hence, hybrid ethanol of Bougainvillea spectabilis bracts and the leaves solar cells appear to be highly promising and cost-effective of Euphorbia cotinifolia, respectively. The highest power alternatives for photovoltaic energy sectors due to its rel- conversation efficiency (g) of the as-prepared DSSC atively cheapness to produce, and promising efficiency. In assembled with natural dye extracted from Bougainvillea this regard, dye sensitized solar cells (DSSCs) have been spectabilis bracts using acidified ethanol as extracting given considerable attention in recent years [1]. DSSC, solvent was 0.175 %. The use of Bougainvillea spectabilis which was invented by Michael Gra¨tzel and Brian O’Re- gan in 1991, is a device that converts visible light energy into electrical energy based on the sensitization of the wide & Delele Worku band gap semiconductors [2, 3] by suitable regenerative delelew@bdu.edu.et; delelewww@yahoo.com dyes. A DSSC consists of a porous wide band gap semi- conductor thin film layer like (TiO , ZnO, SnO2, Nb O ) 1 2 2 5 Material Science and Engineering Program, College of coated on a fluorine-doped tin oxide (FTO) or indium- Science, Bahir Dar University, P.O. Box 79, Bahir Dar, Ethiopia doped tin oxide (ITO) photoanode electrode, a dye, a platinum (Pt) or carbon or conductive polymer thin film Department of Chemistry, Bahir Dar University, P.O. Box 79, counter electrode and an electrolyte normally containing Bahir Dar, Ethiopia - - I /I redox couple. Titanium dioxide (TiO ) is most 3 2 Energy Research Center, Bahir Dar University, P.O. Box 79, commonly used as photoanode in DSSC application since Bahir Dar, Ethiopia 123 13 Page 2 of 7 Mater Renew Sustain Energy (2016) 5:13 it is nontoxic, inert, and has a large energy bandgap as well In this paper, we have extracted natural dyes from the as good optical and electrical properties and, thus, can be bracts of a shrub Bougainvillea spectabilis and the leaves efficiently sensitized by a dye [4]. In DSSC, the dye plays of Euphorbia cotinifolia and studied their potential appli- an important role in harvesting solar energy and converting cation to be employed as sensitizers for DSSC manufac- it to electrical energy with the aid of a semiconducting turing using TiO as wide band gap semiconductor. photoanode. Therefore, the cell performance is mainly Furthermore, quasi-solid state electrolyte and PEDOT films dependent on the type of dye used as a sensitizer in which on FTO glass as a counter electrode were used. the absorption spectrum and the anchorage of the dye to the surface of TiO are important parameters in determining the efficiency of the cell [5, 6]. Several metal complexes Experimental sections and organic dyes have been synthesized and used as sen- sitizers. Among these, ruthenium based complexes are Chemicals Ethanol (CH CH OH, 97 %) was purchased 3 3 considered as the most efficient sensitizers because of their from (Fluka). Fluorine doped tin oxide (FTO— intense charge -transfer absorption over the entire visible 1.5 cm 9 3 cm, 15 X), TiO (70 % anatase and 30 % range and highly efficient metal-to-ligand charge transfer rutile—P25-Degussa AG, Germany), EDOT, Tri-ton— [2, 3, 7]. However, these advantages are offset by their 1009, polyvinyl pyrrolidone (PVP), acetonitrile, (C H ) 2 5 4- rarity, high cost, complicated synthetic routes, and envi- NBF and acetone all were purchased from Aldrich. ronmental threat. Thus, an alternative organic dye such as Hydrochloric acid (HCl, 35 %) was purchased from India. natural dyes is suggested with similar characteristic with an acceptable efficiency [8–10]. Recently, research has Preparation of natural dye sensitizers focused on the easily available dyes extracted from natural sources because of their large absorption coefficients, high Enough amounts of the bracts of a shrub Bougainvillea light-harvesting efficiency, complete biodegradation, low spectabilis and the leaves of Euphorbia cotinifolia were cost, simple and safe preparation and eco-friendly. Thus collected in Bahir Dar city, Ethiopia. Before drying, the far, several dyes such as anthocyanins, carotenoids, beta- bracts and leaves were washed with tape water and then lains and chlorophylls extracted from parts of different dried in laboratory for 6 weeks without light exposure. plants (such as Jathropha curcas, Citrus aurantium, red After drying, the bracts and leaves were crushed into fine cabbage and etc.) have been used as sensitizers in DSSC powder using an electrical blender. Figure 1 shows the [11, 12]. photographic picture of tree plants, the respective dried Fig. 1 Plant leaves used in this study a Bougainvillea spectabilis and b Euphorbia cotinifolia with their corresponding dried leaves, powders and extracted dye solutions 123 Mater Renew Sustain Energy (2016) 5:13 Page 3 of 7 13 bracts and leaves and the powder of the dried bracts of three electrode system with one-compartment electrochem- Bougainvillea spectabilis and leaves of Euphorbia cotini- ical cell. The electrochemical cell consisted of a precleaned folia. 2 g powder of the dried bracts of Bougainvillea FTO-coated glass working electrode, platinum foil counter spectabilis was separately extracted in 50 ml ethanol and electrode and quasi-Ag/AgCl reference electrode. The 50 ml distilled water solvents that were acidified with solution used for the polymerization contained with 0.2 M 0.1 M HCl at room temperature. EDOT and 0.1 M (C H ) NBF in acetonitrile. The mono- 2 5 4 4 Similarly, 2 g powder of the dried leaves of Euphorbia mer was used as received. The polymerization was carried cotinifolia was separately extracted in 50 ml of ethanol and out potentiostatically at ?1.8 V for 2 s. At this potential, the 50 ml of distilled water solvents that were acidified with electrode surface was covered with blue-doped PEDOT film. 0.1 M HCl at room temperature. The glass containers were The cell was then rinsed with acetonitrile and dried in air. covered with aluminium foils to prevent damage from light PEDOT improves the charge transfer between the FTO and - - exposure and were left for 24 h in dark conditions. The the I /I redox couple [13, 14]. It is chosen as a counter solids were filtered out first by decantation and then filtered electrode, because of less costly and can easily be prepared by using glass filter. Finally, the dye was ready for optical electrochemically to a desired transparency [15]. characterization and to be used as sensitizers. Preparation of electrolyte Preparation of working electrode The quasi-solid electrolyte, which had been used in this Fluorine doped tin oxide conductive glass sheets were first work, was prepared as follow: 0.9 M of 3-ethyl-2- methyl cleaned with distilled water, acetone and then ethanol for Immidazolium iodide (EMIM-I) was added into acetoni- 10 min using ultrasonic bath at each step and then dried. trile under stirring to form a homogeneous liquid elec- The nanocrystalline TiO paste was prepared using com- trolyte. To obtain a better conductivity, 0.5 M sodium mercial titanium dioxide powder. 3 g of commercial TiO iodide was dissolved in the above homogeneous liquid powder was ground in a porcelain mortar with 1 ml dis- electrolyte, and then 0.12 M iodine and 35 % (m/m) of tilled water containing 0.1 ml acetic acid to prevent reag- polyvinyl pyrrolidone were added. The resulting mixture gregation of the particles. Once the powder was dispersed was heated at 70–80 C under vigorous stirring to dissolve by the high shear forces in the viscous paste, it was diluted the PVP polymer, followed by cooling down to room by slow addition of 4 ml distilled water under continued temperature to form a gel state electrolyte. Finally, the gel grinding in a porcelain mortar for half an hour. Finally, a electrolyte was deposited in the form of thin film on top of nonionic surfactant (0.05 mL Triton X-100), was added the dye coated TiO electrode. The DSSC was completed and the mixture was grinding for additional half an hour by pressing gel electrolyte coated TiO electrode against until a homogeneous paste was obtained. The surfactant PEDOT-coated FTO glass counter electrode. was used to ease adhesion of TiO particles to FTO con- Owing to its unique hybrid network structure, quasi- ductive glass substrate layer. Then, the conductive side of solid-state electrolytes always possess, simultaneously, the FTO glass was covered on two parallel edges with both the cohesive property of a solid and the diffusive adhesive tape to control the thickness of the TiO paste and transport property of a liquid [16]. Namely, quasi-solid- to provide non-coated areas for electrical contact. The state electrolytes show better long-term stability than liquid paste was spread on one of the free edges of the conducting electrolytes do and have the merits of liquid electrolytes glass using a glass rod–doctor blade method. After drying including high ionic conductivity and excellent interfacial in air, the electrode was sintered for 30 min at 450 Cina contact property [17, 18]. furnace (Carbolite Model ELF11/14 B). After cooling down, the electrode was immersed in the natural extracts Assembling of DSSC for 24 h. Finally, the electrode was withdrawn from the solution and rinsed with ethanol to remove the residues left The prepared photoelectrode and counter electrode were on the TiO film and dried in air which would be used as placed one over the other face-to-face and hold with a photoelectrode. clamps for performance evaluation as can be seen from the following schematic diagram, so that the electrolyte sand- Preparation of counter electrode wiched between the titanium dioxide covered area of the working electrode and the PEDOT coated area of coun- The counter electrode was prepared by coating the conduc- terelectrode. The schematic representaion of the as pre- tive side of FTO glass with poly (3,4-ethylenedioxythio- pared DSSC has been shown in Fig. 2. As seen in Fig. 2, phene) (PEDOT) film which was formed by electrochemical the as prepared DSSC were attached to potentiostat polymerization of 3,4 ethylenedioxythiophene (EDOT), in a equipment by means of cords and crocodile clips. 123 13 Page 4 of 7 Mater Renew Sustain Energy (2016) 5:13 Fig. 2 A schematic representation of DSSCs Optical characterization and DSSC performance measurements The absorption spectra of the extracted pigments in acidi- fied (0.1 M HCl) ethanol and distilled water solution sep- arately were obtained using a UV–Vis spectrophotometer (PerkinElmer lambda 35). The absorption spectra analysis was carried out in the wavelengths ranging from 250 to Fig. 3 UV-visible absorption spectrum of natural dye extracted from 850 nm. The photoelectrochemical measurements of the Bougainvillea spectabilis using acidified (with 0.1 M HCl) a distilled cells under illumination were performed using a computer water and b ethanol as extracting solvents controlled CHI630A Electrochemical Analyzer. A 250-W tungsten–halogen lamp regulated by an Oriel power supply (Model 68830) was used to illuminate the DSSC. The measured photocurrent was corrected for the spectral response of the lamp and the monochromator by normal- ization to the response of a calibrated silicon photodiode (Hamamatsu, Model S1336-8BK) whose sensitivity spec- trum was known. No correction was made for the reflection from the surface of the sample. The illumination light intensity was measured in the position of the sample cell with Gigahertz-Optik X11 Optometer and the intensity of the incident light was 100 mW/cm . Results and discussion Absorption of natural dyes Fig. 4 UV-visible absorption spectrum of natural dye extracted from The representative UV–Vis absorption spectra of the Euphorbia cotinifolia leaves using acidified (with 0.1 M HCl) extracts of Bougainvillea spectabilis and Euphorbia a distilled water and b ethanol as extracting solvents cotinifolia have been investigated. Figures 3 and 4 show the UV–Vis absorption spectra of these extracts dissolved in acidified (0.1 M HCl) ethanol and distilled water sepa- 420 nm which might indicate the extract contains chloro- rately. Figure 3 shows the absorption spectra of phyll mixture which shows an absorption peak in between Bougainvillea spectabilis extracted by water and ethanol, 400–500 nm and 600–700 nm [21] and also the extract has acidified with 0.1 M HCl. As can be seen from Fig. 3,in two small peaks between 476 and 545 nm which could be acidified distilled water extract, one peak was found around associated with the presence of betalains [22]. 545 nm maximum absorbance which can be associated to Figure 4 shows the absorption spectra of Euphorbia the presence of betalain, betalains in acidic environments cotinifolia extracted by distilled water and ethanol, acidi- have strong absorption in the 400–600 nm range due to the fied with 0.1 M HCl. As can be seen from Fig. 4,in color combination of yellow–orange betaxanthins and red– acidified ethanol extract of Euphorbia cotinifolia, the pink betacyanins [19, 20]. Differently, acidified ethanol extracted pigment had three absorption peaks at about 418 extract of Bougainvillea spectabilis shows two absorption and 655 nm which show the characteristics absorption peak peaks around 655 nm (close to 660 nm) and around of chlorophyll mixture and the third absorption peak was at 123 Mater Renew Sustain Energy (2016) 5:13 Page 5 of 7 13 about 532 nm which might be associated with the presence of anthocyanin [9]. Differently, distilled water extract of Euphorbia cotinifolia had an absorption peak around 520 nm which might indicate the absorption of antho- cyanins, a group of natural phenolics compounds. Antho- cyanin is the core component of some natural dyes and is often found in the fruits, flowers, and leaves of plants [2]. Because anthocyanin shows color in the range of visible light from red to blue, it is predicted to become a highly efficient sensitizer for wide band gap semiconductors. In the acidified distilled water extracts, the concentration of dyes (betalain and anthocyanin) is expected to be higher than in acidified ethanolic extracts as can be seen from absorption spectra in Figs. 3 and 4, respectively, probably because of a higher solubility in water-polarity effect. But Fig. 5 J–V characteristic curves of Bougainvillea spectabilis bracts water extracts did not contain chlorophyll absorption since using acidified ethanol (EtOH) and distilled water (H O) as extracting chlorophylls are relatively polar and are thus normally solvents extracted with methanol, ethanol, acetone or other organic solvents miscible with water [20, 23]. The acidity of the extract likely influences the solubility of various dyes, leading to extracts with different compositions. Generally, the chlorophylls exhibit strong absorption in the blue and red regions of the solar spectrum [24]. How- ever, they show poor absorption in the green region, which is consistent with our results. Photoelectrochemical properties Photovoltaic tests of the fabricated DSSCs using these natural dyes as photosensitizers were performed by mea- suring the J–V characteristics of each cell under 100 m W/ cm irradiation from a tungsten–halogen lamp. The per- formance of natural dyes as sensitizers in DSSCs was evaluated by short circuit current density (J ), open circuit sc voltage (V ), fill factor (FF) and energy conversion effi- oc Fig. 6 J–V characteristic curves of Euphorbia cotinifolia leaves ciency (g). Based on J–V and P–V results the fill factor using acidified ethanol (EtOH) and distilled water (H O) as extracting solvent (FF) and energy conversion efficiency (g) were calculated using Eqs. (1) and (2). FF  J  V sc oc depicted in Table 1, the fill factor of the fabricated DSSCs g ¼  100% ð1Þ in ranges between 0.552 to 0.603. The V changes from oc 0.411 to 0.549 V, and the J varies from 0.541 to 0.592 m where P is the power of incident light, V is open circuit sc in oc A/cm . Specifically, a high V = 0.549 V and voltage; J is short circuit current density oc sc J = 0.592 mA/cm were obtained from the DSSCs sen- sc J  V m m FF ¼ ð2Þ sitized by the dyes of Bougainvillea spectabilis and J  V sc oc Euphorbia cotinifolia, respectively, that were extracted by where J and V are the photocurrent density and photo- m m acidified ethanol. These data are significantly higher than voltage for maximum power output P . m those of the DSSCs sensitized by other natural dyes in this Figures 5 and 6 show the typical J–V curves of the as work. prepared DSSCs using the sensitizers extracted from The highest output power and energy conversion effi- Bougainvillea spectabilis and Euphorbia cotinifolia. All ciency were obtained for the DSSC sensitized with acidi- the photoelectrochemical parameters of the DSSCs fied ethanolic extract of Bougainvillea spectabilis where assembled with these natural dyes are listed in Table 1.As the efficiency of the cell reached 0.175 %. This might be 123 13 Page 6 of 7 Mater Renew Sustain Energy (2016) 5:13 Table 1 The photoelectrochemical parameters of the as prepared DSSCs Plant Solvent V (V) J V (V) J P (mW/ FF P g (%) Remarks oc sc max max in max 2 2 2 2 (mA/cm ) (mA/cm ) cm ) (mW/cm ) Bougainvillea spectabilis Water 0.541 0.545 0.361 0.454 100 0.552 0.164 0.164 Current work Ethanol 0.549 0.574 0.371 0.471 100 0.555 0.175 0.175 Euphorbia cotinifolia Water 0.411 0.553 0.295 0.464 100 0.602 0.137 0.137 Ethanol 0.417 0.592 0.299 0.498 100 0.603 0.149 0.149 Commercial carrot Ethanol 0.010 35.4 75.4 29.3 0.61 0.015 [30] Hibiscus flower Ethanol 0.268 0.96 0.43 0.11 [31] Lawsonia inermis Ethanol 1.60 0.91 0.96 1.39 [32] associated with the combination presence of chlorophyll energy conversion efficiency of the DSSC will be reduced. and betalain pigments in the extract to absorb photons—a Despite the low efficiencies, the as prepared DSSCs exhibit synergistic effect of two sensitizing pigments. Although the applicability of Bougainvillea spectabilis and Euphor- there may not be a single pigment that can act as a highly bia cotinifolia dyes for photovoltaic energy conversion. efficient sensitizer, it is possible that a combination of pigments could provide the absorbance necessary to increase the efficiency of a DSSC. In fact, some research Conclusion has demonstrated a synergistic effect of two sensitizing pigments, meaning the absorption spectra of two pigments The Bougainvillea spectabilis and Euphorbia cotinifolia combined to increase light absorption and increase the extracts were used as low-cost sensitizers for commercial incident photon to current conversion efficiency of the TiO nanoparticles photoanode based dye sensitized solar solar cell [25, 26]. Chlorophyll plays an important role in cell. The UV–Vis absorbance measurement of the plant photosynthesis; the DSSCs using chlorophyll Bougainvillea spectabilis and Euphorbia cotinifolia derivatives as sensitizers obtained a relatively high con- extracts in acidified distilled water and ethanol as a solvent version efficiency [15, 16]. This is because there are were carried out and the extracts had absorption peaks in available bonds between the dye and TiO2 molecules the visible light regions due to the presence of plant pig- through which electrons can transport from the excited dye ments such as chlorophyll and betalain in Bougainvillea molecules to the TiO2 film [17]. This result indicates that spectabilis and chlorophyll and anthocyanin in Euphorbia the interaction between the sensitizer and the TiO2 film is cotinifolia. The photovoltaic performance of the extracts as significant in enhancing the power conversion efficiency of sensitizers for the constructed DSSCs was evaluated under DSSCs. The DSSC output power was calculated as simulated solar light irradiation. The solar cell sensitized P = J 9 V using the J–V data. The maximum power with these extracts delivered short-circuit photocurrent (P ) of the DSSCs for each cell is then calculated. The densities (J ) ranging from 0.545 to 0.592 mA/cm , open max sc current (I ) and the voltage (V ) corresponding to the circuit voltages (V ) varied from 0.411 to 0.549 V and the max max oc maximum power point are then obtained. Generally, the fill factors varied from 0.552 to 0.603. The highest open natural dyes employed as sensitizers in solar cell deliver circuit voltage (V = 0.549 V) and short circuit current oc very low efficiencies when compared to synthetic organic density (J = 0.592 mA/cm ) were obtained from the sc and inorganic dyes, due to the absence of specific func- DSSCs sensitized by the acidified ethanol extracts of tional attachment groups and low absorption in the visible Bougainvillea spectabilis bracts and Euphorbia cotinifolia region of the solar spectrum [27, 28]. The conversion leaves, respectively. The highest power conversation effi- efficiencies of the as prepared DSSCs in this study were ciency (g) of the prepared DSSC was 0.175 %. better as compared to some DSSC values reported as The overall results of the study suggest that the shown in Table 1. But, the obtained conversion efficiencies exploitation of Bougainvillea spectabilis and Euphorbia were lower than those reported [32] for natural dye based cotinifolia pigments as natural sensitizers along with the DSSCs may be due to several reasons. Of them one use of quasi-solid electrolyte and PEDOT coated FTO potential reason for the low efficiencies is due to the counter electrodes could be a possible alternative for the inconsistency of the TiO film thickness in the layer [29]. production of low-cost and environment friendly DSSCs. The uniformity of the TiO film thickness affects the Acknowledgments The authors gratefully acknowledge the financial conversion efficiency of the solar cell. If the film thickness support provided by Bahir Dar University, Energy Research center for is not uniform, charge recombination can occur and the the study undertaking. 123 Mater Renew Sustain Energy (2016) 5:13 Page 7 of 7 13 Open Access This article is distributed under the terms of the 16. 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Materials for Renewable and Sustainable EnergySpringer Journals

Published: Jul 25, 2016

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