ISSN 0030-400X, Optics and Spectroscopy, 2015, Vol. 119, No. 4, pp. 666–671. © Pleiades Publishing, Ltd., 2015.
CONDENSED-MATTER SPECTROSCOPY
Rare Earth Ion (La, Ce, and Eu) Doped ZnO Nanoparticles Synthesized Via Sol-Gel Method: Application in Dye Sensitized Solar Cells1 Padmini Pandey, Rajnish Kurchania, and Fozia Z. Haque Optical Nanomaterials Lab, Department of Physics, Maulana Azad National Institute of Technology (M.A.N.I.T.), Bhopal (M.P.)-462051, India e-mail:
[email protected] Received April 10, 2015
Abstract—Dye-sensitized solar cells (DSSCs) were fabricated by using ZnO nanoparticles as working electrode material synthesized via simple and cost effective sol-gel method. Crystallography and morphology was investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM), respectively. Among various rare earth ions, 1.0 mol % La, Ce, and Eu doped ZnO nanoparticles based photoanodes were used to test DSSC performance. Lower efficiency (η = 1.14%) for La ion doped ZnO nanoparticles based cell was observed. A much lower photocurrent Jsc = 2.52 mA/cm2 with 0.60% efficiency (η) for the Ce ion doped ZnO nanoparticles based prototype was observed as compared to that (Jsc = 3.86 mA/cm2 with η = 1.24%) of the undoped one which may be due to the formation of opposite internal electric potential difference in the cell. Furthermore, the improvement in efficiency (η = 1.36%) and Jsc = 3.99 mA/cm2 for Eu ion doped ZnO can be attributed to enhanced electron injection and transport abilities. This indicates that 1.0 mol % Eu ion doped ZnO film possesses better electrical conductivity probably due to the existence of high-valance Eu ions in the ZnO matrix which might be promising in ZnO-based dye sensitized solar cell. DOI: 10.1134/S0030400X15100215
1. INTRODUCTION Due to the economic aspects, comparing to renewable energy resources, photovoltaic technology holds great promises for energy generation [1]. As a clean solar-to-electricity conversion system, dye-sensitized solar cells (DSSCs) are considered viable substitutes to the conventional silicon-based photovoltaic devices since the pioneering work reported by O’Regan and Grätzel [2], and then attracted much more attention due to its low cost and simple fabrication process [3, 4]. The porous electrode in DSSC is crucial and the overall energy conversion efficiency strongly depends on its surface and electronic properties [5]. Recently metal oxide nanostructure electrodes have been of interest for potential applications in many fields of technologies, such as electric transistors [6], photovoltaic devices [7, 8] and sensors [9, 10]. To have better performance, the candidate electrodes such as TiO2 [2–5], SnO2, In2O3 [11], Nb2O5 [12] and ZnO [13–15] need to have preferred electron transporting characteristics and large surface, though the working is still the same. 1 The article is published in the original.
Among the various semiconductor oxides, ZnO with bandgap of ~3.37 eV and high excitonic binding energy (60 meV) [16–18] should be an alternative electrode material for the DSSCs due to its electronic properties similar to TiO2. The properties of ZnO depend on its morphology and microstructure. Application of ZnO in solar cells as an optoelectronic material makes it essential to have control on its size, shape, and microstructure [19]. There are various methods available to form ZnO nanostructures from either physical or chemical method. The fabrication costs of the chemical routes which include hydrothermal, chemical bath deposition and sol-gel method are lower than those of physical routes. The advantages of sol-gel method are larger area and feasible to form various ZnO nanostructures with excellent physical properties [20]. In addition sol-gel method allows the hybridization of inorganic, organic, and metallic materials at low-temperature [21]. Studies have shown that the surface of ZnO nanocrystals can play an important role in carrier transport. The unbounded oxygen chemisorbed on nanocrystals surface serves as traps for charge carriers, thus, increasing the interface potential and lowering carrier mobility [22]. However, in order to enhance the versatility and structural prop-
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erty of ZnO to meet the different requirements of application, structural modifications have usually been utilized, among which metal ion doping is the most well known and effective approach. There have been numerous reports on the utilization of several metals such as Ga [23], Mn [24], In [25], Mg [26], Al [27] and so on to tailor the desirable properties of ZnO. In addition to these metal doping reports, rare earth ion doped ZnO has the potential to be a highly multifunctional material with coexisting semiconducting and optical properties. Generally, surface modification or ion-doping of the electrode [28–31] can be used to improve the efficiency of DSSCs. Those surface-modified solar cells have photovoltaic performance related to the nature, porosity and amount of the coating materials, which further influence the electron transport property and the nanostructure of electrode. The efficiency of solar cell has been attributed to the following two factors: First, the wide bandgap overlayer as a barrier layer retards the back electron transfer and suppresses the dark current. Second, the overlayer enhances the dye adsorption, leading to the improved cell performance. Moreover, rare earth ion modification can form new energy barriers on the TiO2 electrode surface, which can effectively inhibit the surface charge combination and improve the conversion efficiency of the TiO2-based DSSCs [32, 33]. Aim of the present study is to investigate the effect of rare earth (La, Ce, and Eu) ion doping on the performance of ZnO nanoparticles based photoanodes in dye sensitized solar cell application. 2. EXPERIMENTAL DETAILS 2.1. Synthesis of Undoped and Rare-Earth Ion Doped ZnO Nanoparticles Zinc acetate dihydrate [Zn(CH3COO)2·2H2O] (0.2M), lanthanum acetate hydrate, cerium acetate and europium acetate hydrate of AR grade having 99.99% purity procured from Sigma Aldrich, used as starting material and dopant sources respectively. The concentration of rare earth ion doping in ZnO was selected as 1.0 mol %. Further procedure has been described elsewhere for pure and europium ion doped ZnO nanoparticles [34] and same was adopted here for all the three cases of doping. The as-synthesized undoped and rare earth (La, Ce, and Eu ion doped ZnO) ion doped ZnO nanoparticles were further subjected to characterizations in terms of their structural property analysis, followed by dye sensitized solar cell prototype fabrication and cell testing. 2.2. Characterizations of Undoped and Rare-Earth Ion Doped ZnO Nanoparticles The X-ray diffraction (XRD) data was collected through Bruker D8 Advance X-ray diffractrometer OPTICS AND SPECTROSCOPY
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with Cu Kα radiation operating at 40 kV and 40 mA and a position sensitive detector (Lyn xEye) based on Bruker AXS compound silicon strip technology was used. Scanning has been performed from 20° to 80° 2θ range. Diffraction peaks of the crystalline phase were compared with those of the standard compound reported in the JCPDS data files No. 751526. The morphology of the samples was determined using Field Emission Scanning Electron Microscopy (FESEM-Nova NanoSEM) and Transmission Electron Microscopy (TEM-TECNAI G2). The photoresponse of DSSC prototypes was measured with a solar simulator (91545 Newport, USA) equipped with an AM1.5G filter, where the light energy was adjusted with Newport-69907, for one-sun light intensity (100 mW/cm2). A computer-controlled Keithley-2420 source meter was employed to collect the current-voltage (I-V) curves. 2.3. Fabrication of Dye Sensitized Solar Cell Doctor-blade technique was adopted to prepare the porous electrodes on fluorinated tin oxide (TCO, 15 Ω cm−2, Solaronix SA, Switzerland), which were rinsed ultrasonically in acetone, ethanol and distilled water for 15 min, respectively. Paste of ZnO: RE0.0mol % (undoped ZnO) was prepared by grinding the sample with acetylacetone and DI water with a subsequent addition of a small amount of Triton X-100 and the same procedure of pasting was adopted for rare earth ion doped ZnO (1.0 mol % La, Ce, and Eu ion doped ZnO) samples. The resulting pastes were dropped onto one large area FTO glass with adhesive tapes served as spacers, and spread evenly using a glass rod. After drying in the air the films were then annealed at 500°C for 1 h. Dye-sensitization was achieved by immersing the electrodes into (0.5 mM) di-tetrabutylammonium cisbis (isothiocyanato) bis (2,2'-bipyridyl-4,4'-dicarboxylato) ruthenium (II) (N719) dye ethanol solution for 4 hrs, followed by rinsing in ethanol. The dye-sensitized electrodes were assembled in typical sandwichtype cells; the identical platinum (Pt) counter-electrodes were placed over the dye-sensitized electrodes, and the electrolyte (Iodolyte AN-50, Solaronix SA), containing 50 mM tri-iodide and 0.5 M 4-tertButylpyridine in anhydrous acetonitrile was sandwiched between the photoanode and the counter electrode by firm press in all the cases. 3. RESULTS AND DISCUSSION 3.1. Crystallographic Analysis of Undoped and Rare-earth Ion Doped ZnO Nanoparticles X-ray diffraction patterns of undoped and rare earth ion doped ZnO samples were depicted in Fig. 1. The peaks observed in XRD patterns were well indexed to pure hexagonal wurtzite-phase ZnO (JCPDS Card, No. 751526). This crystal structure
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Intensity, a.u. 16000
Intensity, a.u. 20000
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(101)
(b)
12000 15000
10000 (100)
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(002)
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(102)
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(103) (112) (200) (201) (004)(202)
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2000 0 20
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16000 (c)
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Fig. 1. X-Ray diffraction patterns of (a) undoped ZnO (ZnO: RE0.0 mol %), (b) 1.0 mol % La ion doped ZnO, (c) 1.0 mol % Ce ion doped ZnO, and (d) 1.0 mol % Eu ion doped ZnO nanoparticles.
belongs to the hexagonal crystal system with space group P63mc. No other diffraction peaks were detected except the ZnO peaks, which indicated the presence of ZnO nanocrystals without any amorphous component and other additional crystalline phase. For undoped ZnO we obtained a = b = 3.197 Å, c = 5.220 Å and unit cell volume ‘V’ = 46.231 Å3. Furthermore, the positions of the main diffraction peaks (100), (002) and (101) evidently shifted towards the lower 2θ angles and show broadening, for rare earth (La, Ce, and Eu) ion doped ZnO samples. This shift in peak position to lower 2θ angle led to an enhancement in lattice parameters can be attributed to doping with rare earth (RE) cations, having large effective ionic radius (1.03–0.96 Å) than Zn2+ (0.74 Å). For La doped ZnO lattice parameters a = b = 3.235 Å, c = 5.283 Å and ‘V’ = 47.91 Å3, for Ce doped ZnO nanoparticles a = b = 3.241 Å, c = 5.293Å, and ‘V’ = 48.19 Å3 and for Eu doped ZnO lattice parameters are a = b = 3.236 Å, c = 5.284 Å, and unit cell volume ‘V’ = 47.92 Å3.
3.2. Field Emission Scanning and Transmission Electron Microscopic Analysis Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were used to investigate surface morphology of as synthesized samples. Figure 2a displayed FESEM image at 120.000 X magnification at 500 μm and Fig. 2b showed TEM image at magnification scale of 50 nm for undoped ZnO nanoparticles, certain agglomerations were observed along with heterogeneous morphology as an effect of high calcination temperature (500°C). The typical micrograph consists of an assembly of ZnO nanoparticles. In sol–gel process, the dissolution of the Zinc acetate dihydrate in ethanol is a key step as it provides homogeneous environment for the reacting species. Zn–O–Zn bridges are believed to have formed in the sol thus producing single phase ZnO as the end product. Tartaric acid with double carboxyl groups is an important anchor for Zn+2 ions, with the capability to participate in the morphologically controlled synthesis.
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11/6/2004 dwell HV HFW pressure mag WD 10:53:06 AM 20 µs 18.00 kV 3.45 µm 2.25e-2 Pa 120000 x 5.0 mm
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Fig. 2. (a) Field emission scanning electron microscopy (FESEM) and (b) Transmission electron microscopy (TEM) image of undoped ZnO nanoparticles.
3.3. Photovoltaic Performance of Undoped and Rare-earth Ion Doped ZnO Nanoparticles The photocurrent density-voltage (J-V) characteristics of dye sensitized solar cells (DSSCs) fabricated using undoped ZnO (ZnO:RE0.0 mol %), 1.0 mol % La, Ce, and Eu ion doped ZnO nanoparticles based photoanode under 100 mW/cm2 light illumination, are shown in Fig. 3. Reproducibility of the photovoltaic results was confirmed by testing 3–5 such prototypes. The fill factor of DSSCs was calculated by using the following equation:
F =
I maxV max , V ocI sc
(1)
where Imax and Vmax are the maximum current and voltage obtained, respectively, at the maximum power point on the photovoltaic power output curve. The solar conversion efficiency (η) was calculated by the following equation:
η=
V oc J scFF , Pin
(2)
where Jsc is the short-circuit current density, Voc is the open-circuit voltage, FF is the fill-factor and Pin is the incident light power. Lower efficiency η = 1.14% with 61.23% fill factor was observed in case of La doped ZnO nanoparticles as compared to undoped ZnO based DSSC. A much lower photocurrent density 2.52 mA/cm2 with 0.60% efficiency for Ce ion doped ZnO nanoparticles based cell was observed as compared to that for the undoped one (Jsc = 3.86 mA/cm2). This may be due to the decomposition of cerium precursor into CeO2; this quardivalent cerium can be easily reduced into trivalent cerium ion. As mentioned, Ce4+ can easily trap electron and change into Ce3+ ions, therefore, the dye excited state electrons are easily captured by CeO2 on the substrate so that the photo-generated electrons cannot be injected effectively in the conduction band of ZnO and then formed an opposite internal electric potential difference in the cell. This is one of the reasons that Ce-ion doped ZnO based dye sensitized solar cell shows markedly decreased photovoltage and power conversion efficiency. After doping with Eu, the Jsc and η significantly increased, reaching a maximum value to 3.99 mA/cm2 and 1.36%, respectively. The
Parameters of dye-sensitized solar cells based on undoped and rare earth (RE) ion doped ZnO nanoparticles DSSC device
Voc, V
Isc, A
Jsc, mA/cm2 Maximum power, mW Fill factor, % Efficiency η, %
I-Undoped ZnO ZnO:La1.0 mol %
0.59
0.00096
3.86
0.296
51.47
1.24
0.54
0.00094
3.16
0.318
61.23
1.14
ZnO:Ce1.0 mol %
0.53
0.00063
2.52
0.146
43.20
0.60
ZnO:Eu1.0 mol %
0.68
0.00099
3.99
0.342
50.10
1.36
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Current density, mA/cm2 3.5
Current density, mA/cm2 4 (a)
(b) 3.0 2.5
3 2 1
Voc = 0.59 V Jsc = 3.86 mA/cm2 FF = 51.47% η = 1.24%
2.0 1.5 1.0
Voc = 0.54 V Jsc = 3.16 mA/cm2 FF = 61.23% η = 1.14%
0.5 0
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Current density, mA/cm2 3.0
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Voc = 0.53 V Jsc = 2.52 mA/cm2 FF = 43.20% η = 0.60%
2
Voc = 0.68 V Jsc = 3.99 mA/cm2 FF = 51.10% η = 1.36%
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Fig. 3. Current density—voltage curves of (a) undoped ZnO (ZnO: RE0.0 mol %), (b) 1.0 mol % La ion doped ZnO, (c) 1.0 mol % Ce ion doped ZnO, and (d) 1.0 mol % Eu ion doped ZnO nanoparticles based DSSCs.
improvement in efficiency (η) and Jsc for Eu-doped ZnO can be attributed to higher amount of dye adsorption, enhanced electron injection and transport abilities. This indicates that the Eu-doped ZnO film posses better electrical conductivity probably due to the existence of Eu ions in the ZnO matrix. As shown in table the power conversion efficiencies of DSSCs are relatively small (except Eu doped ZnO). The surface annihilation of ZnO nanoparticles that occurs in the sensitization process can be attributed to this fact [35]. To solve the above problem, a conventional electrolyte needs to be replaced with a novel one. These issues are being studied, and will be reported in the future. CONCLUSIONS In summary, undoped and rare earth ion (La, Ce, and Eu) doped ZnO nanoparticles were successfully synthesized via efficient and inexpensive sol-gel method and subsequently used as effective photoanode material in DSSC. Among the three rare earth ions tested, the Eu-ion doped ZnO film shows
enhanced power conversion efficiency, whereas, La and Ce ion doped ZnO-based cells show lower efficiencies than the undoped one. The optimal conversion efficiency 1.36% is obtained from the Eu ion doped ZnO-based cell, with an improvement in photocurrent density ~3.99 mA/cm2, indicating that this rare earth ion doping is promising in the ZnO-based solar cells. ACKNOWLEDGMENTS The authors would like to thank Director, MANIT Bhopal for all the support and Dr. Satish Ogale for allowing synthesis and characterization work to perform in his laboratory in CSIR-NCL. The support from UGC-DAE-CSR Indore for providing XRD and TEM-SAED instrument facilities for material characterizations is duly acknowledged. Due acknowledgment is extended to CSIR-HRDG India for providing Senior Research Fellowship (163025/2K13/1) to Padmini Pandey.
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