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Contact angle measurement: A preliminary diagnostic method for evaluating the performance of ZnO platelet-based dye-sensitized solar cells
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Scripta Materialia 61 (2009) 12–15 www.elsevier.com/locate/scriptamat
Contact angle measurement: A preliminary diagnostic method for evaluating the performance of ZnO platelet-based dye-sensitized solar cells Swapnil B. Ambade,a Rajaram S. Mane,b,1 Anil V. Ghule,b M.G. Takwale,c Aditya Abhyankar,a ByungWon Chod and Sung-Hwan Hanb,* a
Engineering Laboratory for Nanomaterials Devices, Vishwakarma Institute of Information Technology, Kondwa, Pune 411038, India b Inorganic Nano-Materials Laboratory, Department of Chemistry, Hanyang University, Haengdang-dong 17, Sungdong-ku, Seoul 133-791, Republic of Korea c School of Energy Studies, University of Pune, Pune 411007, India d Korea Institute of Science and Technology, 31-1 Hawolgok-dong, Seongbuk-gu, Seoul 130-650, Republic of Korea Received 18 December 2008; revised 6 February 2009; accepted 7 February 2009 Available online 13 February 2009
Contact angle (CA) measurement is used as an empirical diagnostic method to pre-evaluate the performance of N3 related dyesensitized solar cells. The method uses a 30 min sensitization of ZnO nanoplatelet-based electrodes, and the CA value of 48° is optimized for a maximum short-circuit current density of about 6.7 mA cm2. On account of local inhomogeneity, resulting from the Zn2+/dye complex layer, a systematic study of the effect of dye-loading time on the surface wettability and solar-to-electrical conversion performance of dye-sensitized solar cells is undertaken. The wurzite structure and nanoplatelet morphology are confirmed by X-ray diffraction and high-resolution scanning electron microscopy, respectively. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: ZnO nanoplates; XRD; SEM; Surface wettability; J–V
Dye-sensitized solar cells (DSSCs) have recently gained tremendous popularity as a low-cost alternative to conventional solid-state photovoltaic devices [1]. The principle of operation of DSSCs is photoexcitation of dye molecules, efficient separation of photogenerated charges at the dye/oxide interface and injection of electrons into the conduction band of semiconductor oxide; these electron eventually diffuse through the semiconductor oxide to the collecting electrode. A large surface area at the oxide/dye interface is a prerequisite and plays an important role in efficient harvesting of solar energy. The interface between the dye molecules and the semiconductor oxide surface as a mono/bilayer is crucial in inducing a large number of photoexcited electrons into the semiconductor oxide electrode. As diffusion is a dominant electron transport process in the semiconductor oxide, this necessitates the reduction of charge traps
* Corresponding author. Tel.: +82 2 2220 2558; fax: +82 2 2299 0762; e-mail: [email protected] 1 Present address: Department of Physics, Clarendon Laboratory, Oxford University, Oxford OX1 3PU, UK.
at the semiconductor oxide/dye/electrolyte interfaces to ensure faster electron transport. ZnO, a workhorse of technological development, is known for its interesting electronic properties (high electron mobility at room temperature (115–155 cm2 V1 s1), large band gap energy (3.38 eV), low excitonic binding energy (60 meV) and moderate dielectric constant (3.75 1)); and a tunable morphologies such as nanocrystals [2], nanowires [3,4], nanotubes [5,6] and platelets [7,8] which govern its electronic properties, rendering this a versatile material. ZnO nanoparticles with different sizes, size distributions and morphologies have also been explored in DSSCs applications in conjunction with Ru(II) cisdi(thiocyanato)bis(2,20 -bypyridyl-4,40 -dicarboxylic acid) (N3) dye to achieve higher solar cell performance. However, the sensitization time using N3 dye is crucial in obtaining higher cell performance as N3 is known to form a Zn2+/dye complex, a relatively non-conductive/ insulating layer, with increasing sensitization time and thus, affects the cell performance [9,10]. Extensive work and repeated experiments have been carried out by Cao et al. [9] while developing their solar cell systems to optimize the output. Performance optimization is
1359-6462/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2009.02.011
S. B. Ambade et al. / Scripta Materialia 61 (2009) 12–15
crucial; however, fabricating a cell device to evaluate its performance is a tedious and laborious process. Thus, there is a need to develop a simple method which could be correlated with the performance to give a rough prior estimate of the actual cell performance after fabrication. Surface wettability—which involves the interaction between a liquid and a solid in contact—is an important parameter in surface science and its measurement provides a simple and reliable technique for the interpretation of surface engineering. With this motivation, in the present work we have developed a simple and economical approach by establishing an empirical diagnostic relationship between contact angle (CA) measurement (surface wettability) and solar-to-electrical conversion efficiency for electrodeposited ZnO nanoplates (NP)/ N3-based DSSCs. The surface wettability calibrated using Young’s equation (sessile drop method) on the ZnO NP/N3 surface is measured as a function of N3 dye-sensitization time (15–240 min). This is an economical and environmentally acceptable approach that minimizes the time taken and reduces the amount of chemicals used during actual cell fabrication. This method could serve as a diagnostic technique to evaluate the quality or performance of ZnO NP/N3-based DSSCs without actually fabricating the photoelectrochemical cells. We hope that this approach could also be applied to other DSSCs systems as well. The indium-tin-oxide (ITO) substrates were cleaned prior to electrodeposition and the ZnO NP were grown on ITO substrate using cathodic electrodeposition as reported earlier [11]. In particular, a potential of 1100 mV (vs. Ag/AgCl reference electrode) was applied to ITO substrate in an aqueous solution of zinc nitrate (0.01 M, Aldrich) and potassium chloride (0.1 M, Aldrich) maintained at 343 K and the deposition was carried out for 1800 s. The electrodes were then annealed at 553 K for 1 h in a closed furnace under ambient air atmosphere. After annealing, the electrodes were allowed to cool down naturally in the same atmosphere. The vertically grown hexagonal ZnO NP electrode was characterized by X-ray powder diffraction (XRD; Philips Japan MPD 1880) using Cu Ka radiation (V = 40 kV and I = 100 mA) and scanning electron microscopy (SEM; JEOL) prior to its use in further studies. Dye sensitization, i.e. N3 dye-loading time, was varied from 30 to 240 min. A Surface Electro-Optics, Model-Phoenix 150 CA analyzer was used to measure the CA at the contact line of advancing liquid drops. Liquid drops were put onto the electrode using a vertical syringe, and images were recorded with a CCD camera after adjusting contrast, magnification and focus following an initial waiting period of 10 s. Next, the dye-immobilized ZnO NP electrodes and the 100 nm thick Pt-sputtered ITO were sandwiched together using a cell holder, into which an electrolyte solution was infiltrated using a fine 10 ll non-toxic Kovax syringe. 0.6 M 1-hexyl-2,3-dimethyl-imidazolium iodide (C6DMI), 0.1 M lithium iodide (LiI), 0.05 M iodide (I2) and 0.5 M 4-tert-butylpyridine (t-BPy) in 15 ml methoxyacetonitrile (98%) was used as an electrolyte. DSSCs measurements were performed using a 1 kW xenon lamp with a photointensity of 100 mW cm2 with an effective electrode area of 0.28 cm2. In all experi-
13
ments, the samples were illuminated through the conducting glass substrate. Figure 1a shows the XRD pattern in the 2h range of 25°–37° obtained from the ZnO NP electrode with a scan rate of 4° min1. The XRD pattern clearly shows the dominance of the (1 1 0) and (1 0 1) peaks over the (0 0 2) peak, suggesting the formation of ZnO nanoplates. The dominance of the (0 0 2) peak is the characteristic of ZnO rod-like structures [11]. The peak intensity of the (0 0 2) plane in the XRD pattern is sensitive to the concentration of oxygen vacancies. The higher the number of oxygen vacancies, the lower is the resistivity, and therefore, the best conducting electrodes should ideally have a zero peak intensity for the (0 0 2) peak [12]. The presence of the (0 0 2) peak with relatively lower intensity indicates that the electrochemically deposited ZnO NP electrodes can be used as an active working electrode in DSSCs. The peaks marked with filled circular spots in the XRD pattern correspond to those of ITO reflections. Figure 1b presents the representative SEM image obtained from the ZnO NP electrode. The SEM images clearly show that the ZnO NP are grown upright against the ITO substrate surface and are uniformly distributed over the surface. The grown ZnO NP are found to be spaced with approximate void length of 500 nm–1.2 lm. Insets of Figure 1b (right top) show the cross-sectional view of the ZnO NP electrode, indicating an electrode thickness of 5 (±0.5) lm; the width of upright grown ZnO NP is 40–60 nm (right bottom). The shape of liquid drops in the presence of gravity can be described by Laplace’s equation, which relates surface tension of the liquid to the pressure difference across the liquid/air interface. Figure 2 shows the CA variation as a function of N3 dye-sensitization time in comparison with the bare ZnO NP. The inset of Figure 2 shows the CA and the corresponding actual photographs of ZnO NP electrodes sensitized with N3 dye as a function of sensitization time. The presence of micron-sized voids in between the ZnO NP enables adsorption of the liquid droplet completely within the void space, resulting in superhydrophilicity on the surface (CA = 7 ± 2°). After sensitizing the ZnO NP electrode
Figure 1. (a) XRD pattern obtained from the ZnO NP electrode. The relatively lower (0 0 2) peak intensity compared to the other peaks confirms the platelet structure of ZnO. (b) Representative SEM image obtained from the ZnO NP electrode confirming the platelet structure of ZnO. Inset at the top is the SEM image showing the cross-section view of the ZnO NP electrode, indicating the average thickness of electrode. Inset at the bottom is the SEM image showing the width of a single ZnO nano plate.
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S. B. Ambade et al. / Scripta Materialia 61 (2009) 12–15
Figure 2. Surface wettability (CA measurements) of ZnO NP electrodes as a function of N3 dye-sensitizing time. Inset shows the CA and the corresponding actual photographs of ZnO NP electrodes sensitized with N3 dye as a function of sensitizing time.
with N3 dye for a period of 30 min, the CA value increases from 7° to 48 ± 2°. This indicates the adsorption of N3 dye molecules onto the ZnO NPs. However, the CA values show a decreasing trend with increasing N3 dye-sensitization time. Subsequently, the CA value is found to decrease from 48° to 19 ± 2° with increase in sensitization time to 240 min. This is attributed to the change in surface engineering of ZnO NPs in the presence of N3 dye. The surface contamination caused by inhomogeneous Zn2+/N3 dye complex composition (chemical heterogeneity and etched surface structure, discussed later) at the interface [13,14], which is commonly known as a surface aggregation effect in ZnObased DSSCs, is responsible for the observed surface roughness. This is because the change in the surface roughening causes a change in surface energy and interface energy of the solid–liquid interface, and therefore liquid adhesion on a relatively rough surface is effective [15]. Figure 3a shows the variation of the dark current density as a function of applied voltage for samples with varying N3 dye-sensitization time in comparison with the bare ZnO NP. Forward-biased dark current density in bare ZnO NP electrodes, a result of an electrochemical reaction that involves the redox couple, is observed to be higher than that of the electrodes with varying N3 dye-sensitization time. This indicates that leakage current is highest for the bare ZnO NP electrode on account of its higher charge-transfer resistance across
the electrode/electrolyte interface when compared to that of ZnO NP/N3 dye electrodes. Dark current density is higher initially for bare ZnO surface and slowly reduces with N3 dye-loading time. Excess charge leakage from bare ZnO favors higher dark current density which decreases with N3 loading time due to the increase in the insulating Zn2+/dye complex layer thickness, which effectively prevents charge carrier transport ion. Furthermore, the dissolution of Zn2+ by the adsorption of acidic dye followed by the formation of agglomerates with dye molecules eventually blocks the I diffusion pathway into the dye molecule on the ZnO surface [16], and can reduce overall current density in the dark. Figure 3b illustrates current density variation as a function of applied voltage for different ZnO NP/N3 dye electrodes. DSSCs performance in terms of energy conversion efficiency (g%) of individual ZnO NP electrodes is considerably affected by the N3 dyesensitization time (dye-loading time). The performance (g%) of the ZnO NP electrodes with varying sensitization time shows a similar trend to that observed in the CA measurements—a surprising and interesting observation. Table 1 summarizes the series resistance (Rs, measured from the slope at I = 0), the short-circuit current densities (Jsc), the open-circuit voltages (Voc), the fill factors (ff), and g% for the bare ZnO NP electrodes and the ZnO NP electrodes with different N3 dye-sensitization times. Due to the wide band gap energy, bare ZnO NP electrode has lower Jsc, Voc, and g% as expected; however, its fill factor (ff), a function of charge-transfer resistance, is higher than that of the other electrons. The Rs value for bare ZnO NP electrode is comparatively higher than the electrode sensitized with N3 dye for 15 min (limited N3 dye coupling, as seen in Fig. 2) and 30 min (uniform N3 dye coupling, as seen in Fig. 2), respectively. Thereafter, a gradual increase in Rs followed by a decrease in Jsc and ff with N3 dye-sensitization time is attributed to dissolution of Zn surface atoms caused by the acidic carboxylic groups of the N3 dye and formation of Zn2+/dye complexes on the surface [17,18]. Variation in Voc is associated with a change in surface conduction band position due to N3 dye molecule coupling and the surface aggregation effect, as the surface treatment usually induces a change in Voc regardless of the surface-coating materials and substrate used [19]. The CA measurements and sensiti-
Figure 3. (a) Dark and (b) light current density as a function of applied voltage obtained from the bare ZnO NP electrode in comparison with those electrodes obtained after varying the N3 dye-sensitizing time from 30 to 240 min.
S. B. Ambade et al. / Scripta Materialia 61 (2009) 12–15
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Table 1. Effect of CA measurements on ZnO NP-based DSSCs as a function of N3 loading time. Sensitizing time (min)
CA (±2°)
Rs (X)
Jsc (mA cm2)
Voc (V)
ff
g (%)
0 15 30 60 120 180
7 34 48 39 29 24
213 102 66 111 325 540
0.644 1.76 6.70 5.30 3.30 1.80
0.23 0.40 0.62 0.69 0.71 0.63
0.34 0.40 0.32 0.25 0.25 0.19
0.05 0.28 1.32 0.91 0.60 0.22
240
19
549
1.16
0.61
0.19
0.09
zation times show good correlation and a peculiar trend. Initially, for a sensitization time of 15 min, few molecules of N3 dye are coupled with ZnO NP, though this gradually increases with increase in sensitization time to 30 min. A similar trend in CA is observed: the CA is 7 ± 2° for bare ZnO NP electrode, which increases to 34 ± 2° for a sensitization time of 15 min and further increases to 48 ± 2° for a sensitization time of 30 min. However, a further increase in N3 dye-sensitization time shows a decreasing trend in CA values. This indicates initiation of the Zn atom dissolution process from the surface of the ZnO NP electrodes as a result of Zn2+/dye complex formation. This contributes to the increasing surface roughness due to reduced strain energy (formation of the weaker Zn–O bond) [20] thereby reverting the hydrophobic surface of the electrode to a superhydrophilic surface (CA = 19 ± 2°) [15]. In conclusion, a simple cathodic electrodeposition method is used for growing ZnO NP electrodes, which are sensitized with N3 dye for varying time periods from 15 to 240 min. The effect of N3 dye-sensitization time on CA measurements, and in turn on the solar-to-electrical conversion efficiency, was also investigated. Good correlation and a similar trend between the surface tension (surface energy) monitored by the CA measured and the solar-to-electrical conversion efficiency is noted as a function of dye-loading time. A maximum CA value of 48 ± 2° and optimized cell performance of 1.32% is observed for an N3 dye-sensitization time of 30 min. As surface engineering is important to the performance of DSSCs, CA measurement provides qualitative and quantitative information on the chemical nature of the surface and, in particular, on the interfacial bonds between the dye and the ZnO NP electrode. The key feature of the physical property i.e. CA measurement (surface wettability) of the electrode surface is utilized in this work to develop a preliminary diagnostic method for evaluating or estimating cell performance prior to actually fabricating the cell. We hope that this observed correlative trend is of scientific interest and might also be applicable to electrodes with different ZnO morphologies and combinations of different dye-sensitizer molecules.
This work was supported by the Seoul R&BD Program (10919) and Korea Institute of Science and Technology (Development of Basic Technologies for Large-Scale Lithium Secondary Batteries for Plug-in Hybrid Electric Vehicle). [1] H.J. Snaith, R. Humphrybaker, P. Chen, I. Cesar, M. Graetzel, Nanotechnology 19 (2008) 424003. [2] A. Otsuka, K. Funabiki, N. Sugiyama, T. Yoshida, Chem. Lett. 35 (2006) 666. [3] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P.D. Yang, Nat. Mater. 4 (2005) 455. [4] J.B. Baxter, E.S. Aydil, Appl. Phys. Lett. 86 (2005) 53114. [5] M. Gratzel, J. Photochem. Photobiol. C Photochem. Rev. 4 (2003) 145. [6] A.B.F. Martinson, J.W. Elam, J.T. Hupp, M.J. Pellin, Nano Lett. 7 (2007) 2183. [7] R. Liu, A.A. Vertegel, E.W. Bohannan, T.A. Sorenson, J.A. Switzer, Chem. Mater. 13 (2001) 508. [8] T. Yoshida, H. Minoura, Adv. Mater. 12 (2000) 1219. [9] T.P. Chou, Q. Zhang, G. Cao, J. Phys. Chem. C 111 (2007) 18804. [10] H. Horiuchi, R. Katoh, K. Hara, M. Yanagida, S. Murata, H. Arakawa, M. Tachiya, J. Phys. Chem. B 107 (2003) 2570. [11] R.S. Mane, H.M. Pathan, W.J. Lee, S.H. Han, J. Phys. Chem. B 109 (2005) 242540. [12] A. Asadov, W. Gao, Z. Li, J. Lee, M. Hodgson, Thin Solid Films 476 (2005) 201. [13] C.D. Lokhande, A. Barkschat, H. Tributsch, Solar Energy Mater. Solar Cells 79 (2003) 293. [14] R.S. Mane, V.V. Todkar, C.D. Lokhande, J. Ahn, S.H. Han, Nanotechnology 17 (2007) 5393. [15] D.E. Packham, Int. J. Adhes. Adhes. 23 (2003) 437. [16] I. Bedja, P.V. Kamat, X. Hua, P.G. Lappin, S. Hotchandani, Langmuir 13 (1997) 2398. [17] N.A. Anderson, X. Ai, T.Q. Lian, J. Phys. Chem. B 107 (2003) 14414. [18] J.J. Wu, G.R. Chen, H.H. Yang, C.H. Ku, J.Y. Lai, Appl. Phys. Lett. 90 (2007) 213109. [19] Y. Diamant, S.G. Chen, O. Melamed, A. Zaban, J. Phys. Chem. B 107 (2003) 1977. [20] K. Keis, C. Bauer, G. Boschloo, A. Hagfeldt, K. Westermark, H. Rensmo, H. Siegbahn, J. Photochem. Photobiol. A Chem. 148 (2002) 57.
Scripta Materialia 61 (2009) 12–15 www.elsevier.com/locate/scriptamat
Contact angle measurement: A preliminary diagnostic method for evaluating the performance of ZnO platelet-based dye-sensitized solar cells Swapnil B. Ambade,a Rajaram S. Mane,b,1 Anil V. Ghule,b M.G. Takwale,c Aditya Abhyankar,a ByungWon Chod and Sung-Hwan Hanb,* a
Engineering Laboratory for Nanomaterials Devices, Vishwakarma Institute of Information Technology, Kondwa, Pune 411038, India b Inorganic Nano-Materials Laboratory, Department of Chemistry, Hanyang University, Haengdang-dong 17, Sungdong-ku, Seoul 133-791, Republic of Korea c School of Energy Studies, University of Pune, Pune 411007, India d Korea Institute of Science and Technology, 31-1 Hawolgok-dong, Seongbuk-gu, Seoul 130-650, Republic of Korea Received 18 December 2008; revised 6 February 2009; accepted 7 February 2009 Available online 13 February 2009
Contact angle (CA) measurement is used as an empirical diagnostic method to pre-evaluate the performance of N3 related dyesensitized solar cells. The method uses a 30 min sensitization of ZnO nanoplatelet-based electrodes, and the CA value of 48° is optimized for a maximum short-circuit current density of about 6.7 mA cm2. On account of local inhomogeneity, resulting from the Zn2+/dye complex layer, a systematic study of the effect of dye-loading time on the surface wettability and solar-to-electrical conversion performance of dye-sensitized solar cells is undertaken. The wurzite structure and nanoplatelet morphology are confirmed by X-ray diffraction and high-resolution scanning electron microscopy, respectively. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: ZnO nanoplates; XRD; SEM; Surface wettability; J–V
Dye-sensitized solar cells (DSSCs) have recently gained tremendous popularity as a low-cost alternative to conventional solid-state photovoltaic devices [1]. The principle of operation of DSSCs is photoexcitation of dye molecules, efficient separation of photogenerated charges at the dye/oxide interface and injection of electrons into the conduction band of semiconductor oxide; these electron eventually diffuse through the semiconductor oxide to the collecting electrode. A large surface area at the oxide/dye interface is a prerequisite and plays an important role in efficient harvesting of solar energy. The interface between the dye molecules and the semiconductor oxide surface as a mono/bilayer is crucial in inducing a large number of photoexcited electrons into the semiconductor oxide electrode. As diffusion is a dominant electron transport process in the semiconductor oxide, this necessitates the reduction of charge traps
* Corresponding author. Tel.: +82 2 2220 2558; fax: +82 2 2299 0762; e-mail: [email protected] 1 Present address: Department of Physics, Clarendon Laboratory, Oxford University, Oxford OX1 3PU, UK.
at the semiconductor oxide/dye/electrolyte interfaces to ensure faster electron transport. ZnO, a workhorse of technological development, is known for its interesting electronic properties (high electron mobility at room temperature (115–155 cm2 V1 s1), large band gap energy (3.38 eV), low excitonic binding energy (60 meV) and moderate dielectric constant (3.75 1)); and a tunable morphologies such as nanocrystals [2], nanowires [3,4], nanotubes [5,6] and platelets [7,8] which govern its electronic properties, rendering this a versatile material. ZnO nanoparticles with different sizes, size distributions and morphologies have also been explored in DSSCs applications in conjunction with Ru(II) cisdi(thiocyanato)bis(2,20 -bypyridyl-4,40 -dicarboxylic acid) (N3) dye to achieve higher solar cell performance. However, the sensitization time using N3 dye is crucial in obtaining higher cell performance as N3 is known to form a Zn2+/dye complex, a relatively non-conductive/ insulating layer, with increasing sensitization time and thus, affects the cell performance [9,10]. Extensive work and repeated experiments have been carried out by Cao et al. [9] while developing their solar cell systems to optimize the output. Performance optimization is
1359-6462/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2009.02.011
S. B. Ambade et al. / Scripta Materialia 61 (2009) 12–15
crucial; however, fabricating a cell device to evaluate its performance is a tedious and laborious process. Thus, there is a need to develop a simple method which could be correlated with the performance to give a rough prior estimate of the actual cell performance after fabrication. Surface wettability—which involves the interaction between a liquid and a solid in contact—is an important parameter in surface science and its measurement provides a simple and reliable technique for the interpretation of surface engineering. With this motivation, in the present work we have developed a simple and economical approach by establishing an empirical diagnostic relationship between contact angle (CA) measurement (surface wettability) and solar-to-electrical conversion efficiency for electrodeposited ZnO nanoplates (NP)/ N3-based DSSCs. The surface wettability calibrated using Young’s equation (sessile drop method) on the ZnO NP/N3 surface is measured as a function of N3 dye-sensitization time (15–240 min). This is an economical and environmentally acceptable approach that minimizes the time taken and reduces the amount of chemicals used during actual cell fabrication. This method could serve as a diagnostic technique to evaluate the quality or performance of ZnO NP/N3-based DSSCs without actually fabricating the photoelectrochemical cells. We hope that this approach could also be applied to other DSSCs systems as well. The indium-tin-oxide (ITO) substrates were cleaned prior to electrodeposition and the ZnO NP were grown on ITO substrate using cathodic electrodeposition as reported earlier [11]. In particular, a potential of 1100 mV (vs. Ag/AgCl reference electrode) was applied to ITO substrate in an aqueous solution of zinc nitrate (0.01 M, Aldrich) and potassium chloride (0.1 M, Aldrich) maintained at 343 K and the deposition was carried out for 1800 s. The electrodes were then annealed at 553 K for 1 h in a closed furnace under ambient air atmosphere. After annealing, the electrodes were allowed to cool down naturally in the same atmosphere. The vertically grown hexagonal ZnO NP electrode was characterized by X-ray powder diffraction (XRD; Philips Japan MPD 1880) using Cu Ka radiation (V = 40 kV and I = 100 mA) and scanning electron microscopy (SEM; JEOL) prior to its use in further studies. Dye sensitization, i.e. N3 dye-loading time, was varied from 30 to 240 min. A Surface Electro-Optics, Model-Phoenix 150 CA analyzer was used to measure the CA at the contact line of advancing liquid drops. Liquid drops were put onto the electrode using a vertical syringe, and images were recorded with a CCD camera after adjusting contrast, magnification and focus following an initial waiting period of 10 s. Next, the dye-immobilized ZnO NP electrodes and the 100 nm thick Pt-sputtered ITO were sandwiched together using a cell holder, into which an electrolyte solution was infiltrated using a fine 10 ll non-toxic Kovax syringe. 0.6 M 1-hexyl-2,3-dimethyl-imidazolium iodide (C6DMI), 0.1 M lithium iodide (LiI), 0.05 M iodide (I2) and 0.5 M 4-tert-butylpyridine (t-BPy) in 15 ml methoxyacetonitrile (98%) was used as an electrolyte. DSSCs measurements were performed using a 1 kW xenon lamp with a photointensity of 100 mW cm2 with an effective electrode area of 0.28 cm2. In all experi-
13
ments, the samples were illuminated through the conducting glass substrate. Figure 1a shows the XRD pattern in the 2h range of 25°–37° obtained from the ZnO NP electrode with a scan rate of 4° min1. The XRD pattern clearly shows the dominance of the (1 1 0) and (1 0 1) peaks over the (0 0 2) peak, suggesting the formation of ZnO nanoplates. The dominance of the (0 0 2) peak is the characteristic of ZnO rod-like structures [11]. The peak intensity of the (0 0 2) plane in the XRD pattern is sensitive to the concentration of oxygen vacancies. The higher the number of oxygen vacancies, the lower is the resistivity, and therefore, the best conducting electrodes should ideally have a zero peak intensity for the (0 0 2) peak [12]. The presence of the (0 0 2) peak with relatively lower intensity indicates that the electrochemically deposited ZnO NP electrodes can be used as an active working electrode in DSSCs. The peaks marked with filled circular spots in the XRD pattern correspond to those of ITO reflections. Figure 1b presents the representative SEM image obtained from the ZnO NP electrode. The SEM images clearly show that the ZnO NP are grown upright against the ITO substrate surface and are uniformly distributed over the surface. The grown ZnO NP are found to be spaced with approximate void length of 500 nm–1.2 lm. Insets of Figure 1b (right top) show the cross-sectional view of the ZnO NP electrode, indicating an electrode thickness of 5 (±0.5) lm; the width of upright grown ZnO NP is 40–60 nm (right bottom). The shape of liquid drops in the presence of gravity can be described by Laplace’s equation, which relates surface tension of the liquid to the pressure difference across the liquid/air interface. Figure 2 shows the CA variation as a function of N3 dye-sensitization time in comparison with the bare ZnO NP. The inset of Figure 2 shows the CA and the corresponding actual photographs of ZnO NP electrodes sensitized with N3 dye as a function of sensitization time. The presence of micron-sized voids in between the ZnO NP enables adsorption of the liquid droplet completely within the void space, resulting in superhydrophilicity on the surface (CA = 7 ± 2°). After sensitizing the ZnO NP electrode
Figure 1. (a) XRD pattern obtained from the ZnO NP electrode. The relatively lower (0 0 2) peak intensity compared to the other peaks confirms the platelet structure of ZnO. (b) Representative SEM image obtained from the ZnO NP electrode confirming the platelet structure of ZnO. Inset at the top is the SEM image showing the cross-section view of the ZnO NP electrode, indicating the average thickness of electrode. Inset at the bottom is the SEM image showing the width of a single ZnO nano plate.
14
S. B. Ambade et al. / Scripta Materialia 61 (2009) 12–15
Figure 2. Surface wettability (CA measurements) of ZnO NP electrodes as a function of N3 dye-sensitizing time. Inset shows the CA and the corresponding actual photographs of ZnO NP electrodes sensitized with N3 dye as a function of sensitizing time.
with N3 dye for a period of 30 min, the CA value increases from 7° to 48 ± 2°. This indicates the adsorption of N3 dye molecules onto the ZnO NPs. However, the CA values show a decreasing trend with increasing N3 dye-sensitization time. Subsequently, the CA value is found to decrease from 48° to 19 ± 2° with increase in sensitization time to 240 min. This is attributed to the change in surface engineering of ZnO NPs in the presence of N3 dye. The surface contamination caused by inhomogeneous Zn2+/N3 dye complex composition (chemical heterogeneity and etched surface structure, discussed later) at the interface [13,14], which is commonly known as a surface aggregation effect in ZnObased DSSCs, is responsible for the observed surface roughness. This is because the change in the surface roughening causes a change in surface energy and interface energy of the solid–liquid interface, and therefore liquid adhesion on a relatively rough surface is effective [15]. Figure 3a shows the variation of the dark current density as a function of applied voltage for samples with varying N3 dye-sensitization time in comparison with the bare ZnO NP. Forward-biased dark current density in bare ZnO NP electrodes, a result of an electrochemical reaction that involves the redox couple, is observed to be higher than that of the electrodes with varying N3 dye-sensitization time. This indicates that leakage current is highest for the bare ZnO NP electrode on account of its higher charge-transfer resistance across
the electrode/electrolyte interface when compared to that of ZnO NP/N3 dye electrodes. Dark current density is higher initially for bare ZnO surface and slowly reduces with N3 dye-loading time. Excess charge leakage from bare ZnO favors higher dark current density which decreases with N3 loading time due to the increase in the insulating Zn2+/dye complex layer thickness, which effectively prevents charge carrier transport ion. Furthermore, the dissolution of Zn2+ by the adsorption of acidic dye followed by the formation of agglomerates with dye molecules eventually blocks the I diffusion pathway into the dye molecule on the ZnO surface [16], and can reduce overall current density in the dark. Figure 3b illustrates current density variation as a function of applied voltage for different ZnO NP/N3 dye electrodes. DSSCs performance in terms of energy conversion efficiency (g%) of individual ZnO NP electrodes is considerably affected by the N3 dyesensitization time (dye-loading time). The performance (g%) of the ZnO NP electrodes with varying sensitization time shows a similar trend to that observed in the CA measurements—a surprising and interesting observation. Table 1 summarizes the series resistance (Rs, measured from the slope at I = 0), the short-circuit current densities (Jsc), the open-circuit voltages (Voc), the fill factors (ff), and g% for the bare ZnO NP electrodes and the ZnO NP electrodes with different N3 dye-sensitization times. Due to the wide band gap energy, bare ZnO NP electrode has lower Jsc, Voc, and g% as expected; however, its fill factor (ff), a function of charge-transfer resistance, is higher than that of the other electrons. The Rs value for bare ZnO NP electrode is comparatively higher than the electrode sensitized with N3 dye for 15 min (limited N3 dye coupling, as seen in Fig. 2) and 30 min (uniform N3 dye coupling, as seen in Fig. 2), respectively. Thereafter, a gradual increase in Rs followed by a decrease in Jsc and ff with N3 dye-sensitization time is attributed to dissolution of Zn surface atoms caused by the acidic carboxylic groups of the N3 dye and formation of Zn2+/dye complexes on the surface [17,18]. Variation in Voc is associated with a change in surface conduction band position due to N3 dye molecule coupling and the surface aggregation effect, as the surface treatment usually induces a change in Voc regardless of the surface-coating materials and substrate used [19]. The CA measurements and sensiti-
Figure 3. (a) Dark and (b) light current density as a function of applied voltage obtained from the bare ZnO NP electrode in comparison with those electrodes obtained after varying the N3 dye-sensitizing time from 30 to 240 min.
S. B. Ambade et al. / Scripta Materialia 61 (2009) 12–15
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Table 1. Effect of CA measurements on ZnO NP-based DSSCs as a function of N3 loading time. Sensitizing time (min)
CA (±2°)
Rs (X)
Jsc (mA cm2)
Voc (V)
ff
g (%)
0 15 30 60 120 180
7 34 48 39 29 24
213 102 66 111 325 540
0.644 1.76 6.70 5.30 3.30 1.80
0.23 0.40 0.62 0.69 0.71 0.63
0.34 0.40 0.32 0.25 0.25 0.19
0.05 0.28 1.32 0.91 0.60 0.22
240
19
549
1.16
0.61
0.19
0.09
zation times show good correlation and a peculiar trend. Initially, for a sensitization time of 15 min, few molecules of N3 dye are coupled with ZnO NP, though this gradually increases with increase in sensitization time to 30 min. A similar trend in CA is observed: the CA is 7 ± 2° for bare ZnO NP electrode, which increases to 34 ± 2° for a sensitization time of 15 min and further increases to 48 ± 2° for a sensitization time of 30 min. However, a further increase in N3 dye-sensitization time shows a decreasing trend in CA values. This indicates initiation of the Zn atom dissolution process from the surface of the ZnO NP electrodes as a result of Zn2+/dye complex formation. This contributes to the increasing surface roughness due to reduced strain energy (formation of the weaker Zn–O bond) [20] thereby reverting the hydrophobic surface of the electrode to a superhydrophilic surface (CA = 19 ± 2°) [15]. In conclusion, a simple cathodic electrodeposition method is used for growing ZnO NP electrodes, which are sensitized with N3 dye for varying time periods from 15 to 240 min. The effect of N3 dye-sensitization time on CA measurements, and in turn on the solar-to-electrical conversion efficiency, was also investigated. Good correlation and a similar trend between the surface tension (surface energy) monitored by the CA measured and the solar-to-electrical conversion efficiency is noted as a function of dye-loading time. A maximum CA value of 48 ± 2° and optimized cell performance of 1.32% is observed for an N3 dye-sensitization time of 30 min. As surface engineering is important to the performance of DSSCs, CA measurement provides qualitative and quantitative information on the chemical nature of the surface and, in particular, on the interfacial bonds between the dye and the ZnO NP electrode. The key feature of the physical property i.e. CA measurement (surface wettability) of the electrode surface is utilized in this work to develop a preliminary diagnostic method for evaluating or estimating cell performance prior to actually fabricating the cell. We hope that this observed correlative trend is of scientific interest and might also be applicable to electrodes with different ZnO morphologies and combinations of different dye-sensitizer molecules.
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