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Natural Dyes Extracted from Fruits of Phyllanthus reticulatus as Sensitizers in ZnO Nanorods Based Dye Sensitized Solar Cells
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 8 (2019) 284–293
www.materialstoday.com/proceedings
ICMEE 2018
Natural Dyes Extracted from Fruits of Phyllanthus reticulatus as Sensitizers in ZnO Nanorods Based Dye Sensitized Solar Cells P. Sanjaya, K. Deepab, J. Madhavanb, and S. Senthila, * Department of Physics, Government Arts College for Men, Nandanam, Chennai, India.
Abstract DSSCs are a promising class of photovoltaic cells with the capability of generating green energy at low production cost since no expensive equipment is required in their fabrication. Zinc oxide (ZnO) nanorods were synthesized by hydrothermal method and their perfomance as photoanodes in dye-sensitized solar cells (DSSCs) employing I-/I3- as electrolyte and the dye extracts from fruits of Phyllanthus reticulatus as sensitizers was analyzed. the structural and surface characterization of the ZnO nanorods were accomplished using X- ray diffraction (XRD) analysis and high resolution scanning electron microscopy (HRSEM). XRD results revealed that the synthesized ZnO material exhibited hexagonal crystal structure which was found to be highly stable and crystallite. The dielectric properties of the ZnO nanorods were analyzed. The dye extracts were subjected to UV-Visible absorption spectroscopy analysis. The short-circuit photocurrent, the open-circuit photovoltage, and the power conversion efficiency of DSSC were measured using J-V measurement system. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the Materials For Energy and Environment. Keywords: DSSC, ZnO nanorods, HRSEM, XRD, Phyllanthus reticulatus,etc.
1. INTRODUCTION Solar cell is a promising renewable energy technology that converts sunlight to electricity with specific wavelength, with broad potential to contribute significantly to solving the future energy problem that humanity faces [1, 2]. Grätzel reported a new low cost chemical solar cell by the successful combination of nanostructured electrodes and efficient charge injection dyes, known as Grätzel cell or dye-sensitized solar cell which falls under the third generation photovoltaic cell [3]. Titanium oxide (TiO2) is most known material for photoanode, and TiO2 mesoporous based DSSC has exhibited efficiency of more than 12 %. However, it is difficult to further increase the
* Corresponding author. Tel.: 8838974192 E-mail address: [email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the Materials For Energy and Environment.
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current limit of the efficiency. Hence, Zinc oxide (ZnO) is the most attractive and alternative material to TiO2. It is an n-type semiconductor material and has an exciton binding energy of 60 meV with band gap energy of 3.37 eV. ZnO has high electron mobility with similar electronic band structure compared to TiO2 [4]. Most of the research groups have devoted more time to synthesize different novel structures such as nanoparticle (zero dimensional) [5], nanorods or nanowire [6], nanotube (one dimensional) [7], nanosheet (two dimensional) [8] and microsphere particle (three dimensional) [9]. The efficiency of the DSSC depends on morphology of the photoanode [10]. The photoanode should posse high electron mobility, high charge collection, good charge separation, large surface area and high light scattering ability. In this case, the nanoparticle based DSSC has large surface area to more dye loading. Although it has high surface area, the efficiency of the DSSC is low owing to presence of large number of grain boundary, which significantly affects the electron mobility. The poor electron mobility leads to recombination between back electrons transfer and oxidized dye molecules or oxidized species in electrolyte [11]. In order to retard these types of recombination, some researchers have synthesized one dimensional (1D) nanostructures such as nanowires and nanorods which offer direct pathway to injected electron to reach the FTO conductive substrate. The high electron mobility of the 1D nanostructure has inhibited the recombination between back electrons transfer and oxidized dye molecules or oxidized species in electrolyte. 1D nanowire based DSSC has achieved low efficiency compared to nanoparticles based DSSC. This is due to the limited surface area to dye loading, instability, poor coverage of nanowire on FTO substrate which causes recombination between injected electron from uncovered FTO substrate and oxidized electrolyte species or oxidized dye molecules [12]. To prevent this type of recombination, Wang et al introduced TiO2 blocking layer between FTO substrate and ZnO seed layer. Eventhough, they used the blocking as well as seed layer, the efficiency of the DSSC is low [13]. Further to enhance the conversion efficiency, some of the researchers have employed scattering layer to improve the light absorption through multi-scattering of light within photoanode, and they have significantly increased the efficiency of the DSSCs [14-16]. Among the various methods, spin coating and hydrothermal are the best methods to synthesize the blocking as well as seed layer and 1D nanostructure for light scattering on account of low cost and easy scalable. In this work, we have reported our efforts in the synthesis of ZnO nanorods by hydrothermal method which was employed as photoanode for DSSCs. The morphology, crystallite structures and optical properties of the synthesized ZnO nanorods were characterized. The dyes were extracted from P. reticulatus fruits with solvents such as water and ethanol. The optical absorption spectrum of dye extracts was analyzed. The J-V characterization was carried out to analyze the power conversion efficiency of the fabricated dye sensitized solar cells. 2. Experimental methods 2.1 Synthesis of ZnO nanorods ZnO nanorods were prepared by hydrothermal method [9]. Zinc acetate (Zn (CH3COO)2.2H2O) was used as a zinc source. In a typical reaction 3.357 g of Zn(Ac)2.2H2O (5.08x10-7 mol L-1) was dissolved into 30 mL of distilleddeionized water (DDW). 4.0 g NaOH (3.33x10-6 mol L-1) was dissolved in 30 mL distilled-deionized water (DDW). The NaOH solution was added dropwise to Zn(Ac)2.2H2O solution and the stirring was continued for 30 min and Zn(OH)2 was formed. The mixture was transferred into a Teflon lined autoclave and was heated in an oven at 200°C for 7h. Then cooled up to room temperature, the obtained solid product was centrifuged, washed three times with distilled-deionized water (DDW) and once with ethanol in order to remove the residues, and dried at 400°C for 4h to obtain ZnO nanorods. 2.2 Preparation of natural dye The natural dyes were extracted with deionized water and ethanol employing the following procedure. Phllanthus reticulatus fruits were collected from Thiruvannamalai region and washed with running tap water. 10 g of the P. reticulatus fruits were grinded to small particles using mortar and pestle. The residue was soaked in 100 ml of deionised water and 100 ml of ethanol respectively. The solutions were filtered to separate the solid residue from the solvents and the filtrate was used as the light harvesting pigment without further purification (Fig. 1).
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Fig.1. Phyllanthus reticulatus plant with fruits; Dye extracted with a) Water b) Ethanol
2.3 Preparation of electrolyte The I−/I3− is a common electrolyte in organic solvents, such as acetonitrile, which was used in this study. Lithium ion was added to facilitate electron transport. This electrolyte is suitable for ion diffusion and infiltrates well into the ZnO film, exhibiting the highest efficiency among all DSSCs. The electrolyte solution was prepared by dissolving 0.3 M of Lithium Iodide (LiI) and 0.03 M of Iodine (I2) in tert-butyl alcohol and acetonitrile in 1:1 volume ratio, which can be used as charge transport mediator between photoanode and counter electrode in DSSC. 2.4 Fabrication of DSSC The FTO glass (Fluorine doped tin oxide) (sheet resistance 7.5kΩ/cm2) was used as the current collector. The FTO plate was first cleaned using an ultrasonic bath with acetone, ethanol, and water for about 15 min respectively. FTO glasses were rinsed well with distilled water and air dried which are used as anode and cathode in DSSC. Scotch tape was used as a spacer to control the film thickness and to provide non-coated areas for electrical contact. The prepared ZnO nanoparticles were made as paste by mixing with polyethylene glycol binder and coated on FTO by doctor blade technique to prepare DSSC photoanode. The prepared ZnO film was air dried at 70°C for 30 min and the films were annealed at 450°C for 1h to eliminate the polymer binder. The coated glasses were soaked in P. reticulatus dye extracts for 12h. After the dye-sensitization process, the photoanode was washed with ethanol to remove the unanchored dye molecules and air dried. A platinum coated FTO glass plate was used as the counter electrode. The dye-covered ZnO electrode and Pt counter electrode were assembled as a sandwich-type cell. The electrolyte solution was injected in between the cells which act as charge transport mediator between photoanode and counter electrode in DSSC. 2.5 DSSC assembly A schematic representation of DSSC assembly in shown in Fig. 2. The ZnO nanoparticles coated on FTO glass was placed facing upward, and the conductive side of the platinum coated counter electrode faced the ZnO film. A DSSC was assembled by introducing liquid electrolyte (0.3 M of Lithium Iodide (LiI) and 0.03 M of Iodine (I2) in tert-butyl alcohol and acetonitrile in 1:1 volume ratio) into the space between the ZnO electrode (photo anode) and the counter electrode (cathode) by capillary action. The two electrodes were clipped together using two binder clips to prevent the electrolyte from leaking.
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Fig.2. A schematic representation of DSSC assembly
2.6 Characterization and measurements The absorption spectrum of P. reticulatus dye solution was determined using UV-Vis spectrophotometer in the wavelength range of 200–800 nm. The J-V response of the fabricated solar cells was studied using standard solar simulator (Oriel Sol3A Class AAA) of 1 Sun intensity (1000 W/m2) with AM 1.5-G filter. The current–voltage (JV) curve was used to determine short-circuit current (Jsc) and open-circuit voltage (Voc). The solar cell parameters fill factor (FF) and efficiency (η) have been calculated using the following Equations . . η=
J .
.
100%
where Jsc is the short-circuit photocurrent density (mA cm2), Voc the open-circuit voltage (volts), Pin is the intensity of the incident light (W cm-2) and Jm (mA cm-2) and Vm (volts) are the maximum current density and voltage in the J–V curve, respectively, at point of maximum power output. 3. Results and Discussion 3.1 Absorption of natural dyes Figure 3 shows the UV-Vis absorption spectra of dyes extracted from P. reticulatus fruits with water and ethanol. P. reticulatus fruits extracted with water and ethanol shows the broadest absorption level detected between 490- 560nm which confirms the presence of anthocyanin. The structure of anthocyanin is shown in Fig 4. Anthocyanin is a suitable material used as photosensitizer in the visible-light region.
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Fig. 3. UV – Visible spectrum of Phyllanthus reticulatus dye
Fig. 4. Structure of anthocyanin
3.1 X-Ray Diffraction Analysis The crystallinity of the synthesized ZnO nanoparticles was examined by X-ray diffraction (XRD). Figure 5 exhibits the typical XRD pattern of as-synthesized ZnO nanorods. All the diffraction patterns can be indexed to the wurtzite hexagonal phase and can be assigned as ZnO (100), (002), (101), (102), (110), (103), (200), (112), and (201) respectively. The observed diffraction reflections are well-matched with the reported (JCPDS) card no. 0361451. The high-intense and sharp diffraction reflections confirmed that the synthesized nanorods are well-crystallite. The crystallite size was calculated by the Debye scherrer formula D =0.89λ/β cos θ, where D is the crystallite size, λ is the wavelength of X-ray radiation, ‘β’ is the full width half maximum and θ is the diffraction angle. It was found that the average crystallite size of ZnO nanorods were 34 nm.
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Fig.5. XRD pattern of ZnO nanorods
3.2 High Resolution Scanning Electron Microscopy (HRSEM) The grain morphology of ZnO nanorods has been observed by High Resolution scanning electron microscopy and the micrographs are displayed in Fig. 6. HRSEM micrograph shows rod like morphologies of ZnO nanoparticles. The formation mechanism of ZnO nanorods takes place in two steps: nucleation and growth. At the initial step of the reaction between Zn(CH3COO)(OH) and NaOH, Zn(OH)2 precipitates. Now as more NaOH is added to the solution, the precipitated Zn(OH)2 dissolves and forms a homogeneous aqueous solution that contains enough Zn(OH)42- ions. At the onset of super saturation and dehydration of Zn(OH)42- Nzo nuclei are formed. It is also supposed that during this process, Zn(OH)42- ions act as the growth unit of ZnO and directly incorporated into ZnO crystallites. These nuclei grow further to produce rod-like ZnO. The rods are transparent with a length of around 124 nm. Most of the rods have smooth side faces. Such ZnO nanorods with high surface area can enhance dye adsorption for DSSC and expected to give technologically promising photocatalytic applications. Hence, in the present study a large number of sensitizer dye molecules can be adsorbed on the surface of multifaceted hexagonal shaped ZnO nanorods for improving DSSC efficiency.
Fig 6. HRSEM image of ZnO nanorods
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3.3 Energy dispersive X-ray spectroscopy (EDAX) Energy dispersive X-ray spectroscopy (EDAX) was carried out to identify the type of elements present in the samples. The energy dispersive X-ray (EDAX) spectrum of ZnO nanorods showed distinguished peaks of zinc and oxygen in Fig. 7. The weight contribution of Oxygen is 21.35% and Zinc is 78.65%. Both the elements oxygen and zinc together contributes 100% of the total weight. This indicates the purity of ZnO as there is no impure material present in it.
Fig. 7. EDAX spectrum of ZnO nanorods
3.6. I-V characteristics of Dye sensitized solar cell (DSSCs) The J - V characteristics of ZnO based solar cells using P. reticulatus dye extracts are shown in Fig. 8. The photo electrochemical activity is dependent on the morphology of the ZnO photo anode.
Fig. 8. J-V characteristics of ZnO based solar cells sensitized with P. reticulatus dye extracts
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The ZnO solar cell fabricated using P. reticulatus ethanolic dye extract exhibited a power conversion efficiency of 4.62% with a short circuit current density (Jsc) of 8.87 mA/ cm2, open circuit voltage (Voc) of 0.687 V and fill factor (FF) of 67.70%. The power conversion efficiency exhibited by the solar cell fabricated using ZnO sensitized by fruits of P. reticulatus water extract exhibited a power conversion efficiency of 4.12% with a short circuit current density (Jsc) of 7.73 mA/ cm2, open circuit voltage (Voc) of 0.701 V and fill factor (FF) of 64.91%. Table 1. Performance parameter of Phyllanthus reticulatus dyes Dyes Phyllanthus reticulatus
Photoanode ZnO ZnO
Solvent(s)
Jsc (mAcm-2)
Voc (V)
FF (%)
η (%)
Water Ethanol
7.73 8.87
0.701 0.687
64.91 67.70
4.12 4.62
3.7.Dielectric Measurements of ZnO Nanoparticles Figure 10 shows the variation of real and imaginary part of dielectric constant with respect to the frequency of applied electric field in the range of 50 Hz to 5 MHz at room temperature for the prepared ZnO nanorods. It is observed that the real and imaginary part of dielectric constant decreases as frequency increases in respect to the sample, whereas it increases when the concentration of precursor is increased. The high dielectric constant of ZnO nanoparticles at low frequency is attributed due to presence of space charge polarizations, while the decrease in dielectric constant with increase in frequency is natural since any species contributing to polarizability is found to lag behind the applied field at higher and higher frequencies.
Fig.10. Variation of real imaginary part of dielectric constant of ZnO nanorods with frequency
The variation of dielectric loss (tan δ) as function of frequency at room temperature is shown in Fig 11. The dielectric loss is represented as dissipated energy in a dielectric system. It is clear that the dielectric loss decreases with increase in frequency in respect to the sample which exhibit dispersion behavior similar to the dielectric constant. Generally, the low value of dielectric loss indicates that the samples possess good crystalline nature with few defects.
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Fig. 11.Variation of dielectric loss of ZnO nanorods with frequency
Figure 12 shows the variation of AC conductivity with frequency for ZnO nanorods at room temperature. It is clear from the results that the conductivity increases as the frequency of applied AC field increases in the sample. It is found that the AC conductivity increases with increase in the frequency of applied AC field because increase in the frequency enhances the migration of electron. It is well known that the electrical conductivity of ZnO samples at room temperature is due to intrinsic defects created by VO. These defects introduce donor states in the forbidden band slightly below the conduction band and hence resulting in the conducting behaviour of ZnO.
Fig.12.Variation of AC conductivity of ZnO nanorods with frequency
4. CONCLUSION Natural dyes can be safely and economically extracted from its source and do not need any complicated technique for synthesis. As it is synthesized from natural source therefore no need to test its toxicity. This makes natural dye an important entity for development of economically and commercially available DSSC. In this study,
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ZnO photoanode have been investigated for their performance in DSSCs. ZnO nanoparticles were synthesized by hydrothermal method and were further characterized by XRD, HRSEM and EDAX. The dielectric studies reveal that the dielectric constants and dielectric loss support the normal dielectric behaviour. The AC conductivity increases with increase in frequency. The natural dyes were extracted from fruits of P. reticulatus using water and ethanol as solvents and were subjected to UV- Vis spectroscopy. The broad spectrum obtained coincides with that of anthocyanin pigment. The current density obtained by P. reticulatus was 7.73 mAcm-2 and 8.87 mAcm-2 for ethanolic and water extract respectively. However, this investigation showed a great potential for natural sensitizers in improving the DSSCs performance. The results demand additional studies to optimize the DSSCs with natural dyes, and we are planning new experiments to address more scientific photochemical aspects. References [1] O’regan B, Gratzel M, A Nature 353 (6346) (1991) 737-740. [2] Jobin Job Mathen, Madhavan J, Thomas A, Edakkara A J, Sebastin J, Ginson P joseph, Journal of Materials Science: Materials in Electronics, 28(10) 7190-7203.. [3] Sanjay P, Deepa K, Madhavan J, Senthil S, Materials Letters 219 (2018) 158–162. [4]. K. Mahmood, H. W. Kang, R. Munir, H. J. Sung, RSC Adv. 3 (2013) 25136. [5]. EzhilArasi S , Victor Antony Raj M , Madhavan J, Journal of Materials Science: Materials in Electronics, 29 (4), (2018) , 3170-3177. [6]. Jobin Job Mathen, Thomas, Edakkara A J, Sebastin J, Madhavan J, Ginson P Joesph, Journal of Materials Science: Materials in Electronics, 28(11), (2017), 7544-7557. [7]. A. B. F. Martinson, J. W. Elam, J. T. Hupp, M. J. Pellin, Nano Lett. 7 (2007) 2183. [8]. E. Hosono, S. Fujihara, I. Honna, H. S. Zhou, Adv. Mater. 17 (2005) 2091. [9]. C. Zhu, Y. Shi, C. Cheng, L. Wang, K. K. Fung, N. Wang, J. Nanomater. 4 (2012) 212653. [10]. Z. H. Liu, X. J. Su, G. L. Hou, S. Bi, Z. Xiao, H. P. Jia, RSC Adv. 3 (2013) 8474. [11]. X. Kang, C. Jia, Z. Wan, J. Zhuang, J. Feng, RSC Adv. 5 (2015)16678. [12]. Y. Q. Wang, Y. M. Sun, K. Li, Mater. Lett. 63 (2009) 1102. [13]. T. P. Chou, Q. Zhang, G. E. Fryxell, G. Cao, Adv. Mater. 19 (2007) 2588. [14]. K. S. Kim, H. Song, S. H. Nam, S-M. Kim, H. Jeong, W. B. Kim, G. Y. Jung, Adv. Mater. 24 (2012) 792. [15]. J. Qu, Y. Yang, Q, Wu, P. R. Coxon, Y. Liu, X. He, K. Xi, N. Yuan and J. Ding, RSC Adv. 4 (2014)11430. [16]. N. Huang, M. W. Zhu, L. J. Gao, J. Gong, C. Sun, X. Jiang, Appl. Surf. Sci. 257 (2011)2030.
ScienceDirect Materials Today: Proceedings 8 (2019) 284–293
www.materialstoday.com/proceedings
ICMEE 2018
Natural Dyes Extracted from Fruits of Phyllanthus reticulatus as Sensitizers in ZnO Nanorods Based Dye Sensitized Solar Cells P. Sanjaya, K. Deepab, J. Madhavanb, and S. Senthila, * Department of Physics, Government Arts College for Men, Nandanam, Chennai, India.
Abstract DSSCs are a promising class of photovoltaic cells with the capability of generating green energy at low production cost since no expensive equipment is required in their fabrication. Zinc oxide (ZnO) nanorods were synthesized by hydrothermal method and their perfomance as photoanodes in dye-sensitized solar cells (DSSCs) employing I-/I3- as electrolyte and the dye extracts from fruits of Phyllanthus reticulatus as sensitizers was analyzed. the structural and surface characterization of the ZnO nanorods were accomplished using X- ray diffraction (XRD) analysis and high resolution scanning electron microscopy (HRSEM). XRD results revealed that the synthesized ZnO material exhibited hexagonal crystal structure which was found to be highly stable and crystallite. The dielectric properties of the ZnO nanorods were analyzed. The dye extracts were subjected to UV-Visible absorption spectroscopy analysis. The short-circuit photocurrent, the open-circuit photovoltage, and the power conversion efficiency of DSSC were measured using J-V measurement system. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the Materials For Energy and Environment. Keywords: DSSC, ZnO nanorods, HRSEM, XRD, Phyllanthus reticulatus,etc.
1. INTRODUCTION Solar cell is a promising renewable energy technology that converts sunlight to electricity with specific wavelength, with broad potential to contribute significantly to solving the future energy problem that humanity faces [1, 2]. Grätzel reported a new low cost chemical solar cell by the successful combination of nanostructured electrodes and efficient charge injection dyes, known as Grätzel cell or dye-sensitized solar cell which falls under the third generation photovoltaic cell [3]. Titanium oxide (TiO2) is most known material for photoanode, and TiO2 mesoporous based DSSC has exhibited efficiency of more than 12 %. However, it is difficult to further increase the
* Corresponding author. Tel.: 8838974192 E-mail address: [email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the Materials For Energy and Environment.
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current limit of the efficiency. Hence, Zinc oxide (ZnO) is the most attractive and alternative material to TiO2. It is an n-type semiconductor material and has an exciton binding energy of 60 meV with band gap energy of 3.37 eV. ZnO has high electron mobility with similar electronic band structure compared to TiO2 [4]. Most of the research groups have devoted more time to synthesize different novel structures such as nanoparticle (zero dimensional) [5], nanorods or nanowire [6], nanotube (one dimensional) [7], nanosheet (two dimensional) [8] and microsphere particle (three dimensional) [9]. The efficiency of the DSSC depends on morphology of the photoanode [10]. The photoanode should posse high electron mobility, high charge collection, good charge separation, large surface area and high light scattering ability. In this case, the nanoparticle based DSSC has large surface area to more dye loading. Although it has high surface area, the efficiency of the DSSC is low owing to presence of large number of grain boundary, which significantly affects the electron mobility. The poor electron mobility leads to recombination between back electrons transfer and oxidized dye molecules or oxidized species in electrolyte [11]. In order to retard these types of recombination, some researchers have synthesized one dimensional (1D) nanostructures such as nanowires and nanorods which offer direct pathway to injected electron to reach the FTO conductive substrate. The high electron mobility of the 1D nanostructure has inhibited the recombination between back electrons transfer and oxidized dye molecules or oxidized species in electrolyte. 1D nanowire based DSSC has achieved low efficiency compared to nanoparticles based DSSC. This is due to the limited surface area to dye loading, instability, poor coverage of nanowire on FTO substrate which causes recombination between injected electron from uncovered FTO substrate and oxidized electrolyte species or oxidized dye molecules [12]. To prevent this type of recombination, Wang et al introduced TiO2 blocking layer between FTO substrate and ZnO seed layer. Eventhough, they used the blocking as well as seed layer, the efficiency of the DSSC is low [13]. Further to enhance the conversion efficiency, some of the researchers have employed scattering layer to improve the light absorption through multi-scattering of light within photoanode, and they have significantly increased the efficiency of the DSSCs [14-16]. Among the various methods, spin coating and hydrothermal are the best methods to synthesize the blocking as well as seed layer and 1D nanostructure for light scattering on account of low cost and easy scalable. In this work, we have reported our efforts in the synthesis of ZnO nanorods by hydrothermal method which was employed as photoanode for DSSCs. The morphology, crystallite structures and optical properties of the synthesized ZnO nanorods were characterized. The dyes were extracted from P. reticulatus fruits with solvents such as water and ethanol. The optical absorption spectrum of dye extracts was analyzed. The J-V characterization was carried out to analyze the power conversion efficiency of the fabricated dye sensitized solar cells. 2. Experimental methods 2.1 Synthesis of ZnO nanorods ZnO nanorods were prepared by hydrothermal method [9]. Zinc acetate (Zn (CH3COO)2.2H2O) was used as a zinc source. In a typical reaction 3.357 g of Zn(Ac)2.2H2O (5.08x10-7 mol L-1) was dissolved into 30 mL of distilleddeionized water (DDW). 4.0 g NaOH (3.33x10-6 mol L-1) was dissolved in 30 mL distilled-deionized water (DDW). The NaOH solution was added dropwise to Zn(Ac)2.2H2O solution and the stirring was continued for 30 min and Zn(OH)2 was formed. The mixture was transferred into a Teflon lined autoclave and was heated in an oven at 200°C for 7h. Then cooled up to room temperature, the obtained solid product was centrifuged, washed three times with distilled-deionized water (DDW) and once with ethanol in order to remove the residues, and dried at 400°C for 4h to obtain ZnO nanorods. 2.2 Preparation of natural dye The natural dyes were extracted with deionized water and ethanol employing the following procedure. Phllanthus reticulatus fruits were collected from Thiruvannamalai region and washed with running tap water. 10 g of the P. reticulatus fruits were grinded to small particles using mortar and pestle. The residue was soaked in 100 ml of deionised water and 100 ml of ethanol respectively. The solutions were filtered to separate the solid residue from the solvents and the filtrate was used as the light harvesting pigment without further purification (Fig. 1).
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Fig.1. Phyllanthus reticulatus plant with fruits; Dye extracted with a) Water b) Ethanol
2.3 Preparation of electrolyte The I−/I3− is a common electrolyte in organic solvents, such as acetonitrile, which was used in this study. Lithium ion was added to facilitate electron transport. This electrolyte is suitable for ion diffusion and infiltrates well into the ZnO film, exhibiting the highest efficiency among all DSSCs. The electrolyte solution was prepared by dissolving 0.3 M of Lithium Iodide (LiI) and 0.03 M of Iodine (I2) in tert-butyl alcohol and acetonitrile in 1:1 volume ratio, which can be used as charge transport mediator between photoanode and counter electrode in DSSC. 2.4 Fabrication of DSSC The FTO glass (Fluorine doped tin oxide) (sheet resistance 7.5kΩ/cm2) was used as the current collector. The FTO plate was first cleaned using an ultrasonic bath with acetone, ethanol, and water for about 15 min respectively. FTO glasses were rinsed well with distilled water and air dried which are used as anode and cathode in DSSC. Scotch tape was used as a spacer to control the film thickness and to provide non-coated areas for electrical contact. The prepared ZnO nanoparticles were made as paste by mixing with polyethylene glycol binder and coated on FTO by doctor blade technique to prepare DSSC photoanode. The prepared ZnO film was air dried at 70°C for 30 min and the films were annealed at 450°C for 1h to eliminate the polymer binder. The coated glasses were soaked in P. reticulatus dye extracts for 12h. After the dye-sensitization process, the photoanode was washed with ethanol to remove the unanchored dye molecules and air dried. A platinum coated FTO glass plate was used as the counter electrode. The dye-covered ZnO electrode and Pt counter electrode were assembled as a sandwich-type cell. The electrolyte solution was injected in between the cells which act as charge transport mediator between photoanode and counter electrode in DSSC. 2.5 DSSC assembly A schematic representation of DSSC assembly in shown in Fig. 2. The ZnO nanoparticles coated on FTO glass was placed facing upward, and the conductive side of the platinum coated counter electrode faced the ZnO film. A DSSC was assembled by introducing liquid electrolyte (0.3 M of Lithium Iodide (LiI) and 0.03 M of Iodine (I2) in tert-butyl alcohol and acetonitrile in 1:1 volume ratio) into the space between the ZnO electrode (photo anode) and the counter electrode (cathode) by capillary action. The two electrodes were clipped together using two binder clips to prevent the electrolyte from leaking.
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Fig.2. A schematic representation of DSSC assembly
2.6 Characterization and measurements The absorption spectrum of P. reticulatus dye solution was determined using UV-Vis spectrophotometer in the wavelength range of 200–800 nm. The J-V response of the fabricated solar cells was studied using standard solar simulator (Oriel Sol3A Class AAA) of 1 Sun intensity (1000 W/m2) with AM 1.5-G filter. The current–voltage (JV) curve was used to determine short-circuit current (Jsc) and open-circuit voltage (Voc). The solar cell parameters fill factor (FF) and efficiency (η) have been calculated using the following Equations . . η=
J .
.
100%
where Jsc is the short-circuit photocurrent density (mA cm2), Voc the open-circuit voltage (volts), Pin is the intensity of the incident light (W cm-2) and Jm (mA cm-2) and Vm (volts) are the maximum current density and voltage in the J–V curve, respectively, at point of maximum power output. 3. Results and Discussion 3.1 Absorption of natural dyes Figure 3 shows the UV-Vis absorption spectra of dyes extracted from P. reticulatus fruits with water and ethanol. P. reticulatus fruits extracted with water and ethanol shows the broadest absorption level detected between 490- 560nm which confirms the presence of anthocyanin. The structure of anthocyanin is shown in Fig 4. Anthocyanin is a suitable material used as photosensitizer in the visible-light region.
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Fig. 3. UV – Visible spectrum of Phyllanthus reticulatus dye
Fig. 4. Structure of anthocyanin
3.1 X-Ray Diffraction Analysis The crystallinity of the synthesized ZnO nanoparticles was examined by X-ray diffraction (XRD). Figure 5 exhibits the typical XRD pattern of as-synthesized ZnO nanorods. All the diffraction patterns can be indexed to the wurtzite hexagonal phase and can be assigned as ZnO (100), (002), (101), (102), (110), (103), (200), (112), and (201) respectively. The observed diffraction reflections are well-matched with the reported (JCPDS) card no. 0361451. The high-intense and sharp diffraction reflections confirmed that the synthesized nanorods are well-crystallite. The crystallite size was calculated by the Debye scherrer formula D =0.89λ/β cos θ, where D is the crystallite size, λ is the wavelength of X-ray radiation, ‘β’ is the full width half maximum and θ is the diffraction angle. It was found that the average crystallite size of ZnO nanorods were 34 nm.
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Fig.5. XRD pattern of ZnO nanorods
3.2 High Resolution Scanning Electron Microscopy (HRSEM) The grain morphology of ZnO nanorods has been observed by High Resolution scanning electron microscopy and the micrographs are displayed in Fig. 6. HRSEM micrograph shows rod like morphologies of ZnO nanoparticles. The formation mechanism of ZnO nanorods takes place in two steps: nucleation and growth. At the initial step of the reaction between Zn(CH3COO)(OH) and NaOH, Zn(OH)2 precipitates. Now as more NaOH is added to the solution, the precipitated Zn(OH)2 dissolves and forms a homogeneous aqueous solution that contains enough Zn(OH)42- ions. At the onset of super saturation and dehydration of Zn(OH)42- Nzo nuclei are formed. It is also supposed that during this process, Zn(OH)42- ions act as the growth unit of ZnO and directly incorporated into ZnO crystallites. These nuclei grow further to produce rod-like ZnO. The rods are transparent with a length of around 124 nm. Most of the rods have smooth side faces. Such ZnO nanorods with high surface area can enhance dye adsorption for DSSC and expected to give technologically promising photocatalytic applications. Hence, in the present study a large number of sensitizer dye molecules can be adsorbed on the surface of multifaceted hexagonal shaped ZnO nanorods for improving DSSC efficiency.
Fig 6. HRSEM image of ZnO nanorods
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3.3 Energy dispersive X-ray spectroscopy (EDAX) Energy dispersive X-ray spectroscopy (EDAX) was carried out to identify the type of elements present in the samples. The energy dispersive X-ray (EDAX) spectrum of ZnO nanorods showed distinguished peaks of zinc and oxygen in Fig. 7. The weight contribution of Oxygen is 21.35% and Zinc is 78.65%. Both the elements oxygen and zinc together contributes 100% of the total weight. This indicates the purity of ZnO as there is no impure material present in it.
Fig. 7. EDAX spectrum of ZnO nanorods
3.6. I-V characteristics of Dye sensitized solar cell (DSSCs) The J - V characteristics of ZnO based solar cells using P. reticulatus dye extracts are shown in Fig. 8. The photo electrochemical activity is dependent on the morphology of the ZnO photo anode.
Fig. 8. J-V characteristics of ZnO based solar cells sensitized with P. reticulatus dye extracts
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The ZnO solar cell fabricated using P. reticulatus ethanolic dye extract exhibited a power conversion efficiency of 4.62% with a short circuit current density (Jsc) of 8.87 mA/ cm2, open circuit voltage (Voc) of 0.687 V and fill factor (FF) of 67.70%. The power conversion efficiency exhibited by the solar cell fabricated using ZnO sensitized by fruits of P. reticulatus water extract exhibited a power conversion efficiency of 4.12% with a short circuit current density (Jsc) of 7.73 mA/ cm2, open circuit voltage (Voc) of 0.701 V and fill factor (FF) of 64.91%. Table 1. Performance parameter of Phyllanthus reticulatus dyes Dyes Phyllanthus reticulatus
Photoanode ZnO ZnO
Solvent(s)
Jsc (mAcm-2)
Voc (V)
FF (%)
η (%)
Water Ethanol
7.73 8.87
0.701 0.687
64.91 67.70
4.12 4.62
3.7.Dielectric Measurements of ZnO Nanoparticles Figure 10 shows the variation of real and imaginary part of dielectric constant with respect to the frequency of applied electric field in the range of 50 Hz to 5 MHz at room temperature for the prepared ZnO nanorods. It is observed that the real and imaginary part of dielectric constant decreases as frequency increases in respect to the sample, whereas it increases when the concentration of precursor is increased. The high dielectric constant of ZnO nanoparticles at low frequency is attributed due to presence of space charge polarizations, while the decrease in dielectric constant with increase in frequency is natural since any species contributing to polarizability is found to lag behind the applied field at higher and higher frequencies.
Fig.10. Variation of real imaginary part of dielectric constant of ZnO nanorods with frequency
The variation of dielectric loss (tan δ) as function of frequency at room temperature is shown in Fig 11. The dielectric loss is represented as dissipated energy in a dielectric system. It is clear that the dielectric loss decreases with increase in frequency in respect to the sample which exhibit dispersion behavior similar to the dielectric constant. Generally, the low value of dielectric loss indicates that the samples possess good crystalline nature with few defects.
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Fig. 11.Variation of dielectric loss of ZnO nanorods with frequency
Figure 12 shows the variation of AC conductivity with frequency for ZnO nanorods at room temperature. It is clear from the results that the conductivity increases as the frequency of applied AC field increases in the sample. It is found that the AC conductivity increases with increase in the frequency of applied AC field because increase in the frequency enhances the migration of electron. It is well known that the electrical conductivity of ZnO samples at room temperature is due to intrinsic defects created by VO. These defects introduce donor states in the forbidden band slightly below the conduction band and hence resulting in the conducting behaviour of ZnO.
Fig.12.Variation of AC conductivity of ZnO nanorods with frequency
4. CONCLUSION Natural dyes can be safely and economically extracted from its source and do not need any complicated technique for synthesis. As it is synthesized from natural source therefore no need to test its toxicity. This makes natural dye an important entity for development of economically and commercially available DSSC. In this study,
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ZnO photoanode have been investigated for their performance in DSSCs. ZnO nanoparticles were synthesized by hydrothermal method and were further characterized by XRD, HRSEM and EDAX. The dielectric studies reveal that the dielectric constants and dielectric loss support the normal dielectric behaviour. The AC conductivity increases with increase in frequency. The natural dyes were extracted from fruits of P. reticulatus using water and ethanol as solvents and were subjected to UV- Vis spectroscopy. The broad spectrum obtained coincides with that of anthocyanin pigment. The current density obtained by P. reticulatus was 7.73 mAcm-2 and 8.87 mAcm-2 for ethanolic and water extract respectively. However, this investigation showed a great potential for natural sensitizers in improving the DSSCs performance. The results demand additional studies to optimize the DSSCs with natural dyes, and we are planning new experiments to address more scientific photochemical aspects. References [1] O’regan B, Gratzel M, A Nature 353 (6346) (1991) 737-740. [2] Jobin Job Mathen, Madhavan J, Thomas A, Edakkara A J, Sebastin J, Ginson P joseph, Journal of Materials Science: Materials in Electronics, 28(10) 7190-7203.. [3] Sanjay P, Deepa K, Madhavan J, Senthil S, Materials Letters 219 (2018) 158–162. [4]. K. Mahmood, H. W. Kang, R. Munir, H. J. Sung, RSC Adv. 3 (2013) 25136. [5]. EzhilArasi S , Victor Antony Raj M , Madhavan J, Journal of Materials Science: Materials in Electronics, 29 (4), (2018) , 3170-3177. [6]. Jobin Job Mathen, Thomas, Edakkara A J, Sebastin J, Madhavan J, Ginson P Joesph, Journal of Materials Science: Materials in Electronics, 28(11), (2017), 7544-7557. [7]. A. B. F. Martinson, J. W. Elam, J. T. Hupp, M. J. Pellin, Nano Lett. 7 (2007) 2183. [8]. E. Hosono, S. Fujihara, I. Honna, H. S. Zhou, Adv. Mater. 17 (2005) 2091. [9]. C. Zhu, Y. Shi, C. Cheng, L. Wang, K. K. Fung, N. Wang, J. Nanomater. 4 (2012) 212653. [10]. Z. H. Liu, X. J. Su, G. L. Hou, S. Bi, Z. Xiao, H. P. Jia, RSC Adv. 3 (2013) 8474. [11]. X. Kang, C. Jia, Z. Wan, J. Zhuang, J. Feng, RSC Adv. 5 (2015)16678. [12]. Y. Q. Wang, Y. M. Sun, K. Li, Mater. Lett. 63 (2009) 1102. [13]. T. P. Chou, Q. Zhang, G. E. Fryxell, G. Cao, Adv. Mater. 19 (2007) 2588. [14]. K. S. Kim, H. Song, S. H. Nam, S-M. Kim, H. Jeong, W. B. Kim, G. Y. Jung, Adv. Mater. 24 (2012) 792. [15]. J. Qu, Y. Yang, Q, Wu, P. R. Coxon, Y. Liu, X. He, K. Xi, N. Yuan and J. Ding, RSC Adv. 4 (2014)11430. [16]. N. Huang, M. W. Zhu, L. J. Gao, J. Gong, C. Sun, X. Jiang, Appl. Surf. Sci. 257 (2011)2030.