Eosin Yellowish Dye-Sensitized ZnO Nanostructure-Based Solar Cells Employing Solid PEO Redox Couple Electrolyte

Kanmani, S. S.; Ramachandran, K.; Umapathy, S.

doi:https://doi.org/10.1155/2012/267824

International Journal of Photoenergy

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Abstract Introduction Results and Discussion Conclusion Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2012 | Article ID 267824 | https://doi.org/10.1155/2012/267824

Eosin Yellowish Dye-Sensitized ZnO Nanostructure-Based Solar Cells Employing Solid PEO Redox Couple Electrolyte

S. S. Kanmani,¹K. Ramachandran,¹and S. Umapathy¹

Academic Editor: Leonardo Palmisano

Received06 Oct 2011

Accepted02 Jan 2012

Published11 Mar 2012

Abstract

ZnO nanostructures are synthesized by low-temperature methods, and they possess polycrystalline hexagonal wurtzite structure with preferential c-axial growth. Morphological study by SEM shows the presence of ~30 nm sized spherical-shaped ZnO nanoparticle, the branched flower-like ZnO composed of many nanorods (length: 1.2 to 4.2 μm and diameter: 0.3 to 0.4 μm), and ~50 nm diameter of individual ZnO nanorods. Reduction in photoemission intensity of nanorods infers the decrease in electron-hole recombination rate, which offers better photovoltaic performance. The dye-sensitized solar cell (DSSC) based on ZnO nanorods sensitized with Eosin yellowish dye exhibits a maximum optimal energy conversion efficiency of 0.163% compared to that of nanoparticles and nanoflowers, due to better dye loading and direct conduction pathway for electron transport.

1. Introduction

As a novel renewable and clean solar to electricity conversion system, dye-sensitized solar cells (DSSCs) offer the hope of fabricating photovoltaic devices showing high efficiency at low cost with simple fabricating process, as an alternative to conventional p-n junction photovoltaic devices [1]. Generally, DSSCs comprise of four main parts such as large band gap and porous nanocrystalline semiconductor electrode, sensitizer, counter electrode, and electrolyte. Each part has its own importance in DSSC operation, among which the role of semiconductor is crucial, and the overall cell performance strongly depends on the surface and electronic properties of semiconductors. Among the various semiconductors used in DSSC, such as TiO₂, ZnO, SnO₂, and Nb₂O₅, TiO₂ is the best choice (11% efficiency) until now due to its excellent properties [2]. In case of TiO₂-based DSSC, the electron injection process is ultrafast (femto seconds), but the electron recombination is high due to low electron mobility and transport properties [3]. Recently, ZnO is expected to be an active promising alternative material and assembly for TiO₂ due to its large band gap (3.37 eV), large exciton binding energy (60 meV), high electron mobility (115–155 cm² V⁻¹ s⁻¹), lower recombination rate, and also easy to tailor the morphology as per our need. In addition, the flat band potential of ZnO higher than TiO₂ is also beneficial in enhancing open circuit voltage. Electron transport in nanoparticle- (NP-) based DSSCs occurs either by a series of hopping events between trap states on neighboring particles or by diffusive transport within extended states slowed down by trapping/detrapping events [4, 5]. Therefore, a noticeable way for achieving higher efficiency is to use 1D nanostructure such as nanorods (NRs), nanowires (NWs), branched nanoflowers (NFs), and nanotubes (NTs), which will provide direct conduction pathway for electron transport from the point of generation to the collection electrode and maintain high surface area for dye adsorption [6]. Several techniques such as electrodeposition, thermal evaporation technique, and solution phase synthesis, have been followed to develop nanostructures of different oxide materials. However, nowadays, especially ZnO nanostructures can be grown by chemical precipitation method [7–9] and simple solution synthesis followed by hydrothermal treatment, which provide the most simple and effective way to prepare sufficiently crystallized, well-defined shape and sized materials at relatively low temperature. Indeed various forms of ZnO nanostructures have been synthesized but efficiency values reported so far are 0.25% for ZnO NP/N3 or N179 dye electrodes [10], 2.4% for ZnO NP/eosin Y electrodes [11], 1.5% for ZnO/squaraine electrodes [12], 1.6% for ZnO NT [13], 1.9% for ZnO NF/N719 [14], and 2.55% for ZnO NR with microflowers/N719 [15], and so forth. However, such efficiency values have been achieved only using a liquid electrolyte as the redox medium of the cell, which may suffer from some drawbacks such as leakage, instability at high temperature, evaporation of solvents, and corrosion of the electrodes. Therefore several attempts have been made to replace the liquid electrolyte by a solid-state charge transport redox medium such as polymer gel electrolyte, organic hole transport materials, room temperature molten salts, inorganic p-type semiconductors, and solid-state polymer electrolyte. Very recently, Deng et al. [3] reported a PEO-based solid-state electrolyte in ZnO aggregate DSSC using N719 sensitizer exhibiting a maximum efficiency of 1.8%. As reported by Hyung et al. [16] solid PEG electrolyte-based ZnO NW sensitized using N719 dye experienced a conversion efficiency of 0.24%. Rani et al. [17] showed the influence of pH value of ZnO sol on the performance of DSSC and achieved the efficiency of 1.11% for pH-9 under gel electrolyte medium. But to date, studies on solid-state ZnO DSSC are still rare, and many efforts have to be put to improve the efficiency further. In this context, ZnO NP, NR, and NF are synthesized and characterized for structural, optical, and morphological properties. Furthermore, as an application of the synthesized material, eosin yellowish (EY) dye-sensitized DSSCs based on it are fabricated successfully and the cell performance is characterized by employing PEO-based solid-state electrolyte.

2. Experimental Details

2.1. Growth Procedure

2.1.1. ZnO Nanoparticles

ZnO nanoparticles (ZnO NPs) are prepared by a coprecipitation method involving the reaction between Zn²⁺ and OH⁻ ions in an alcoholic medium (methanol) [18]. In a typical preparation, one solution containing 100 mM of potassium hydroxide (KOH) in 200 mL of methanol is added dropwise to the other solution containing zinc acetate di-hydrate (Zn(ac)₂·2H₂O, 50 mM) in 200 mL of methanol aided by constant magnetic stirring at 50°C for 2 h. The mixture is allowed to cool to room temperature and then aged for two days. The resulting precipitate is separated, washed several times with absolute ethanol and distilled water, and dried at room temperature, and the final products are annealed at 250°C for 2 h in atmosphere to obtain good crystalline ZnO NP.

2.1.2. ZnO Nanoflowers

ZnO nanoflower-like nanorods (ZnO NF) are prepared by a very simple hydrothermal process [19]. In a typical procedure, 100 mL aqueous solution of zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, 0.054 M) and ammonia (NH₃·H₂O, 0.5 M) are mixed well and sonicated for 10 min, and then 0.15 g of poly ethylene glycol (PEG) is added to it and stirred constantly for 30 min. The final solution is transferred into teflon-lined autoclave and maintained at 80°C for 24 h. After it is cooled down to room temperature naturally, the white precipitate is collected, washed several times with deionized water to remove impurities, dried at room temperature, and then finally annealed at 250°C for 2 h in atmosphere.

2.1.3. ZnO Nanorods

Here ZnO nanorods (ZnO NRs) are synthesized following the approach of Cheng and Samulski [20], as 65 mL methanol solution of 0.1 M, Zn (ac)₂·2H₂O is mixed with 130 mL methanol solution of 0.5 M, NaOH to get a clear solution, which is transferred to teflon lined stainless steel autoclave and maintained at 150°C for 24 h. The resulting white precipitate is washed with water and methanol for several times and dried in ambient temperature, which is further annealed at 250°C for 2 h in atmosphere.

2.1.4. ZnO Electrode and Treatment with Dye

Thin nanostructured photoanode films are fabricated using synthesized ZnO nanomaterials by employing doctor blade technique [21]. ZnO nanomaterials are ground by a mortar and pestle with addition of appropriate amount of distilled water and acetylacetone. After making a viscous paste, it is further diluted with distilled water and then few drops of triton X-100 are added for better adhesion of paste on conducting substrate. The paste is spread on the conducting substrate with a glass rod using an adhesive tape as spacers. After drying in air, the films are sintered for 30 min at 450°C in air. After sintering, the ZnO films are immersed in 0.5 mM of eosin yellowish (EY) ethanol dye solution as sensitizers for 24 h. To minimize the adsorption of impurities from moisture in the ambient air, the electrodes are dipped in the dye solution while they are still warm at 80°C. The dye-sensitized electrodes are then rinsed with ethanol to remove excess unanchored dye molecules on the surface.

2.1.5. Assembly of ZnO-Based DSSC

The platinum-coated counter electrode is prepared by spreading a drop of 5 mM chloroplatinic acid hexahydrate (H₂PtCl₆) in isopropyl alcohol on separate ITO substrate and calcinated it at 450°C for 15 min under air ambient. For two-electrode measurement, the solid-state electrolyte prepared as reported by Stergiopoulos et al. [22] is sandwiched between photoanode and counter electrode, pressing firmly. A thin layer of parafilm is used as a spacer to avoid short-circuiting between two electrodes. A binder clip is fixed externally to maintain the mechanical grip of the cell without any further sealing, which finalized the assembly of the DSSC.

2.2. Characterizations

Crystallinity and the phase purity of the materials are determined by X-ray diffraction (XRD) recorded at room temperature using PANalytical X’Pert X-ray diffractometer with Cu-K radiation (wavelength: 1.54056 Å). The morphology, size distribution, and the elemental composition of nanostructures are determined by scanning electron microscope (SEM, JEOL JSM-6390) along with energy dispersive X-ray spectroscopy (EDS, Oxford Instruments, Model No. 7582) operating at an accelerating voltage of 20 kV. UV-Vis absorption measurements are carried out at room temperature by using UV-Vis absorption spectrometer (Shimadzu-2450). The room temperature photoluminescence (PL) spectra are recorded by using spectrofluorophotometer (Shimadzu RF-5000) with excitation wavelength at 280 nm. Raman measurements are performed by LABRAM HR 800 model (in via laser Raman microscope) with He-Ne laser (632 nm) with the power of 17 mW. The performance of the DSSC is evaluated, from manually recorded photocurrent-photovoltage curves, by using the electronic circuit designed with a 10 kΩ potentiometer as a variable load, 15 W fluorescent lamp as light source, and the position of the DSSC from source is 1.5 cm, as shown in Figure 1.

3. Results and Discussion

3.1. X-Ray Diffraction Analysis

Figure 2 shows the XRD pattern of synthesized ZnO nanomaterials. All the diffraction peaks can be well indexed to polycrystalline hexagonal wurtzite-structured ZnO with three pronounced peaks (100), (002), and (101), appearing at 2θ = 31.84°, 34.53°, and 36.35°, which match well with those of the standard ZnO XRD pattern (JCPDS 89-7102). No characteristic peak is observed for other impurities such as metallic Zn and Zn(OH)₂, indicating the purity of the products. All the peaks in ZnO NF are high intense and narrower than ZnO NP and NR, which indicate that the former has higher crystallinity than latter. It is noticed that the (002) diffraction peaks are extremely strong compared to standard diffraction intensity, which is probably related to the preferential c-axis growth of ZnO NF and NR [10], as evident from relative intensity shown in Table 1. The crystallite size is estimated using Debye Scherrer’s equation, by measuring the line broadening of (101) main intensity peak, as shown in Table 1.

3.2. UV-Vis Absorption Analysis

The absorption spectra of synthesized ZnO nanomaterials are presented in Figure 3. All the samples show strong UV absorption and high transparency in visible region. The absorption edge of ZnO NP, NF, and NR is found out as 398, 399 and 403 nm, respectively, which is red shifted from bulk ZnO. The theory of optical absorption provides the dependence of the absorption coefficient on the photon energy for direct allowed transition as [23] The is plotted against using the data obtained from optical absorption spectra and extraplotted to to get optical band gap, termed as Tauc’s plot. Inset in Figure 3 shows an -absorption plot, from which the band gap is estimated as 3.12, 3.11, and 3.08 eV for ZnO NP, NF, and NR, respectively.

3.3. Photoluminescence Studies

In order to investigate the information about material quality, structural defects such as surface oxygen vacancy, and Zn interstitials, present in the synthesized samples, room temperature PL measurements are carried out. As shown in Figure 4, PL spectra exhibit strong UV emission peak at 385, 384, and 388 nm with a corresponding band gap of 3.22, 3.23, and 3.20 eV for ZnO NP, NF, and NR, respectively, also called near band edge emissions, which should be attributed to the radiative annihilation of excitons [24, 25]. It could be seen here that the emission intensity of NR is lower than NP and NF (Figure 4), which infers the decrease in electron-hole recombination. Visible emission band occurred at around 495 nm corresponds to the presence of various point defects such as oxygen vacancy either extrinsic or intrinsic. As reported by Rai et al. [26], in this case also, visible emission may take place due to low-temperature synthesis (growth temperature (80°C) comparable with reported literature) of NF, where some of the Zn(OH)₂ are adsorbed on the surface of ZnO. In this work, the visible emission of ZnO NF is much stronger than that of NP and NR, suggesting that the content of oxygen vacancy is higher in NF. Since this longer wavelength emission is due to recombination of photogenerated hole with the oxygen vacancy sites, this morphology will deform the performance of solar cell. It is evident that the luminescent properties can be controlled by changing the morphologies though the level of lattice orientation perfection without defects plays a vital role in cell performance. The weak broad band near UV emission at 407 nm comes from the recombination of free excitons [27].

3.4. Raman Measurements

Raman spectrum is a key tool to study the vibrational properties of materials and to identify the crystallization, structural disorder, and material defects and now used to study the optical properties also. Figure 5 shows the typical Raman spectra of ZnO nanostructures observed at room temperature. According to group theory, for hexagonal wurtzite ZnO, the optical phonons at the Γ point of Brillouin zone are A₁ + 2B₁ +E₁ + 2E₂. A sharp and dominant E₂ (high) mode located at 438 cm⁻¹ is the intrinsic Raman active mode of wurtzite hexagonal ZnO [28], which is a nonpolar mode associated with oxygen displacement. The suppressed peaks at 581 and 404 cm⁻¹ are attributed to E₁ (LO) and E₁ (TO) mode, respectively, which may probably have occurred due to oxygen vacancy and zinc interstitial [29]. Two weak peaks occurred at 331 and 380 cm⁻¹ may correspond to multiphonon scattering process of (E₂ (high)-E₂ (low)) and A₁ (TO), respectively. The Raman mode observed at 547 cm⁻¹ can be assigned to A₁ (LO) mode. Thus the presence of high intense E₂ (high) mode and suppressed E₁ (LO) mode indicates that the synthesized ZnO nanomaterials possess crystalline nature with hexagonal wurtzite structure, which further testifies the results of XRD pattern.

3.5. SEM with EDS Analysis

Figure 6 presents the SEM images of ZnO NP, NF, and NR for different magnification, in which Figures 6(a1) and 6(a2) demonstrate that ZnO NP is distinctly spherical in shape without any aggregation of nanoparticles and its size varies from 23 to 30 nm. Typical SEM images of flower-like ZnO nanorods are shown in Figures 6(b1)–6(b3) at different magnifications. SEM image here shows that the sizes of single flower-like ZnO are about ~10 μm. From the enlarged image shown in Figure 6(b1), it is interesting that an individual flower like ZnO is composed of many sword-like nanorods with a sharp tip and they are radiated through the center to form flower-like structure. These radially oriented ZnO NRs possess nonuniform length and diameter, which varies from 1.2 to 4.2 μm and 0.3 to 0.4 μm, respectively, which agrees with the results of Rajkumar et al. [8]. They synthesized similar flower-like nanorods under two-step seed-mediated growth process and concluded that the typical length and diameter of nanorods can be controlled by the precursor’s concentration. Along with flower-like structure, few individual nanorods and rarely some 8 to 10 nanorods (diameter: 0.3 μm and tip: 0.1 μm) aligned together at common center without forming complete flower-like structure are also observed. Images in Figures 6(c1)–6(c3) comprised of both short- and long-sized hexagonal shaped ZnO NR with a typical diameter and length of 30–50 nm and 0.2–0.5 μm, respectively, which shows the improvement in the synthesis of smaller dimension nanorods as compared to already reported literature [7]. EDS analyses are also carried out and the spectra shown in Figures 7(a)–7(c) indicate the presence of Zn and O elements alone in the samples without any other impurities. The Pt signal is attributed to platinum coating done during SEM with EDS analysis.

(a)

(b)

(c)

3.6. Photovoltaic Properties of ZnO-Based DSSC

The current-voltage (J-V) characteristics for DSSC constructed using ZnO NP, NF, and NR sensitized with EY dyes under illumination condition (41.399 mW/cm²) are shown in Figure 8. Table 2 summarizes the measured and calculated values obtained from typical J-V curve. From Figure 8, it can be seen that the cell constructed using ZnO NR film gives a clear improvement in and over ZnO NP- and NF-based DSSC. The fill factor of the fabricated ZnO-based DSSC is calculated as follows: where and are the current density and voltage, respectively, under maximum output power. According to (2), FF is calculated to be maximum of 0.4 for ZnO NP with of 0.339 V. It is to be noted that the increase in FF suggest that there is reduction in recombination between the photoexcited carriers in the electrodes and tri-iodide ions in electrolyte. In our case, for ZnO NF and NR, this higher recombination rate results in lower and FF than NP.

The power conversion efficiency of the fabricated cell is given as where is the power density of the incident radiation. According to (3), the solar to electric conversion efficiency is obtained as 0.163% for ZnO NR-based DSSC. The film consisting of NPs has an interconnected porous structure, so during solar cell operation, the electron transport occurs through hopping event between trap states on neighboring particles. The effect of morphology on the performance of the cell has been studied. However, in an in situ constructed interconnected nanostructure (like nanorods), one can easily transport electron by providing direct conduction pathway from the place of generation to the collector without sacrificing high surface area for dye adsorption. Similarly, the increment in is mainly due to an effective interaction of randomly oriented NRs with the incoming photons without much loss and also due to more dye adsorption with respect to higher surface area. Rani et al. [17] reported the pH effect on the performance of DSSC with maximum efficiency of 1.1% for ZnO NP (pH = 9) sensitized with EY dye by employing gel electrolyte. For ZnO nanowire (NW) DSSC sensitized with ruthenium complex by adopting a solid-state PEG electrolyte, they showed the conversion efficiency of 0.24% [16]. Recently in 2010, Liu et al. [10] reported that the power conversion efficiency of ZnO NP DSSC sensitized by N3 dye with liquid electrolyte is only 0.2%. Akhtar et al. [30] believed that ruthenium dye-sensitized ZnO NF offers 0.3% as conversion efficiency, which is much lower than that of nanospheres (2.61%) and nanoplates (1.9%) due to lower surface area and dye adsorption site in NF. Comparing the overall performance of NF with NP and NR, we can conclude that defect-related visible recombination observed in PL spectra has high impact on the performance of the cell, even though its crystallinity is as good as other nanostructures. Generally, we can conclude that the lower efficiency is mainly due to the contribution of two things: (i) lower ionic conductivity of used PEO matrix in redox coupled electrolyte and (ii) as formation of Zn²⁺-dye complex aggregates, which limits the efficient electron injection process. Our results are comparable with that reported by Rani et al. [17] as 0.45% for EY dye-sensitized 8 nm sized ZnO NPs by using PEG-based electrolyte, which can promote electron transport efficiently than that of PEO due to its higher ion transport rate.

4. Conclusion

In summary, ZnO nanomaterials synthesized via low-temperature approach are used as photoanodes in DSSC. Material characterization by XRD, Raman, and SEM analyses show that the synthesized nanostructures possess high crystalline hexagonal wurtzite-structured ZnO with preferentially c-axis growth. Room temperature PL measurements exhibit strong UV emission for all samples and particularly ZnO NRs show reduction in overall PL intensity that is inferring the decrease in recombination process. Samples exhibit strong UV absorption and high transparency in visible region, and its absorption edge is slightly red shifted from bulk. The solid state DSSC fabricated using ZnO NR sensitized with EY dye achieves of ~564 μA/cm² and overall conversion efficiency of 0.163%, which is higher than ZnO NP and NF. This increase in is mainly attributed to the high interaction of NRs with the incident photons and due to high dye loading content.

Acknowledgments

One of the authors (S. S. Kanmani, S. S. K.) acknowledges DST for the award of INSPIRE Fellowship and also thanks UGC-UPE and DRS for the financial support. S. S. Kanmani also thanks Professor K. Pithchumani and his scholar Mr. K. Kanagaraj, School of Chemistry, Madurai Kamaraj University, for providing autoclave facility and also Mr. A. Raja, Central Research Facility Lab, Karunya University, Coimbatore, for SEM analysis.

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Copyright © 2012 S. S. Kanmani et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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