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Solid-state dye-sensitized hierarchically structured ZnO solar cells
Electrochimica Acta 56 (2011) 4176–4180
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Solid-state dye-sensitized hierarchically structured ZnO solar cells Jie Deng a , Yan-zhen Zheng a,b , Qian Hou a , Jian-Feng Chen b,∗ , Weilie Zhou c , Xia Tao a,∗ a
Key Laboratory for Nanomaterials of the Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China Research Center of the Ministry of Education for High Gravity Engineering & Technology, Beijing University of Chemical Technology, Beijing 100029, China c Advanced Materials Research Institute University of New Orleans New Orleans, LA 70148, USA b
a r t i c l e
i n f o
Article history: Received 22 November 2010 Received in revised form 26 January 2011 Accepted 27 January 2011 Available online 3 February 2011 Keywords: Solid-state Dye-sensitized solar cell Hierarchically structured ZnO Composite electrolyte Photovoltaic performance
a b s t r a c t A novel solid-state hierarchically structured ZnO dye-sensitized solar cell (DSC) was assembled by using TiO2 as filler in polyethylene oxide (PEO)/polyethylene glycol (PEG) electrolytes and ZnO nanocrystalline aggregates as photoanode film. Under optimized composite polyelectrolyte containing PEO/oligoPEG/TiO2 /LiI/I2 the photovoltaic performance of the solid-state ZnO DSCs was significantly better, with an overall conversion efficiency () of 1.8% under irradiation of 100 mW/cm2 , which was higher than those of the cells with PEO/TiO2 /LiI/I2 ( = 1.1%) or PEO/oligo-PEG/LiI/I2 electrolyte ( = 1.5%). Further, the hierarchically structured ZnO-based cell showed a higher value of 2.0% under 60 mW/cm2 radiation. The morphologies, ionic conductivity of three different composite electrolytes and their performance to the DSCs were also studied by FESEM, I–V data, IPCE and EIS. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSCs) have been widely considered as potential inexpensive alternatives to conventional p–n junction photovoltaic devices over the past decades [1–3]. A typical DSC consists of a mesoporous nanocrystalline TiO2 film covered by a monolayer of dye molecules, redox electrolyte, and counter electrode. In TiO2 -based DSCs, the electron injection from a photoexcited dye in the conduction band of TiO2 is ultra-fast, on the order of femto seconds, but the electron recombination is high due to low electron mobility and transport properties. Thus, there has been a considerable interest in search for alternate materials and assemblies. ZnO is a promising wide band oxide material due to its large band gap (3.37 eV) and high electron mobility (115–155 cm2 V−1 s−1 ) [4–7]. An overall energy conversion efficiency based on ZnO DSCs has reached up to 2.4% for the nanowire/nanorod arrays [8], 2.3% for the nanotube arrays [9], 5.0% for nanocrystalline particles [10] and 3.5% for uniform nanocrystalline aggregates [11]. 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, and corrosion of the electrodes. Replacing the liquid electrolyte by a solid-state charge transport medium is a solution to these problems [12–17]. Very recently, Plank et al. reported a
∗ Corresponding authors. Tel.: +86 10 6445 3680; fax: +86 10 6443 4784. E-mail addresses: [email protected] (J.-F. Chen), [email protected] (X. Tao). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.01.099
solid-state ZnO nanowire DSC using organic dyes and core–shell nanostructures exhibiting a maximum efficiency of 0.71% [18]. But, to date, studies on solid-state ZnO DSCs are still rare and their conversion efficiencies also need to be further improved. In our work, three solid-state electrolytes including PEO/TiO2 /LiI/I2, PEO/oligo-PEG/LiI/I2 , and PEO/oligoPEG/TiO2 /LiI/I2 were first introduced into hierarchically structured ZnO DSCs. The hierarchically structured ZnO particles were composed of secondary ZnO colloids with ∼250 nm in diameter that were made up of 10–20 nm nanocrystallites exhibiting sufficient surface area for dye molecule adsorption and light scattering effect for efficient photon-capture. Based on three solid-state electrolyte ZnO DSCs, conversion efficiencies were achieved up to 1.8% and 2.0% under 100 and 60 mW/cm2 illuminations, respectively. Besides, the performance enhancement of the cells was also discussed. 2. Experimental 2.1. Materials Anhydrous lithium iodide (LiI), iodide (I2 ), poly(ethylene oxide) (PEO, Mw = 1,000,000) and H2 PtCl6 were obtained from Sigma. Zinc acetate dihydrate [(CH3 COO)2 Zn], diethylene glycol [(HOCH2 CH2 )2 O], NaOH, HCl, polyethylene glycol (PEG, Mw = 200), acetonitrile and ethanol (analytical grade purity) were purchased from Tianjin Chemical Reagents Co. and were used without further purification. Ru-based N719 dye (cis-bis (isothiocyanato) bis (2, 2-bipyridyl 4, 4-dicarboxylato) ruthenium (II)
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bis-tetrabutylammonium) was obtained from Solaronix (Aubonne, Switzerland). TiO2 nanoparticles (P25, mean particle size 21 nm) were obtained from Degussa (AG, Germany). All solutions used in this work were prepared with 18.2 M cm−1 water produced by a reagent water system (Easy pure II, Barnstead). 2.2. Preparation and characterization of ZnO aggregates The ZnO monodispersed aggregates were synthesized by hydrolyzing 0.02 mol [(CH3 CO2 )2 Zn] in 100 mL [(HOCH2 CH2 )2 O], as described in our previous report [19]. Under vigorous stirring, the mixture was heated to 160 ◦ C in an oil bath at a rate of 5 ◦ C min−1 and then refluxed for 20 h. Subsequently, the as-obtained colloid suspension was centrifuged at a rate of 5000 rpm for 10 min. After centrifugation, the obtained precipitates were redispersed in ethanol for further use. 2.3. Preparation of polymer electrolytes Three solid polymer electrolytes composed of PEO/TiO2 /LiI/I2 (denoted as E1), PEO/oligo-PEG/LiI/I2 (denoted as E2) and PEO/oligo-PEG/TiO2 /LiI/I2 (denoted as E3) were prepared as follows: E1 was obtained according to the literature [20]. Specially, 0.0383 g TiO2 was dispersed in acetonitrile, and then I− /I3 − redox couple (0.1 g LiI and 0.019 g I2 ) was added. Subsequently, 0.264 g PEO was slowly introduced and left under continuous stirring for 24 h. E2 was fabricated by dissolving 0.264 g PEO in acetonitrile and then a predetermined amount of oligo-PEG was added. The PEO:PEG ratio was fixed at 4:6 (w/w), whereas the monomer to cation ratio [EO]:[Li+ ] was fixed at 20:1 (LiI/I2 = 10/1, w/w) [21]. E3 was prepared by introducing 0.0383 g TiO2 powder as nanofiller into E2 electrolyte. 2.4. Fabrication of solid-state ZnO-based DSC Fluorine-doped tin oxide-coated glass (FTO, Hartford, 14 /sq, 80% transmittance) plates were used as the substrates of ZnO photoelectrode and counter electrodes. Prior to fabricating electrode, the FTO substrates was ultrasonically cleaned sequentially in HCl, acetone, ethanol and water each for 15 min. The ZnO photoelectrode films were deposited on FTO substrates using a drop-coat method. Thickness of films was estimated to be 6 ± 0.5 m by exactly controlling the added drops. After the films dried, they were sintered at 350 ◦ C for 60 min and then sensitized with N719 dye for 15 min. Counter electrodes were prepared by spin-coating 0.35 mM H2 PtCl6 solution onto FTO glass and annealed at 400 ◦ C for 15 min. A two step casting method was adopted for casting as-prepared polymer electrolytes on the ZnO layer [22], subsequently, covered by a platinized conducting glass to form a sandwich solar cell device. For convenience, DSCs assembled with E1, E2 and E3 were denoted as device 1, device 2 and device 3, respectively. During the assembly procedures, the active area of the resulting cell exposed in light was approximately 0.25 cm2 (0.5 cm × 0.5 cm). 2.5. Instruments and measurements The morphology of the samples were observed by field emission scanning electron microscope (FESEM, Hitachi S-4700 and JEOL, JSM 6701-F). The photocurrent–voltage characteristics of the DSCs were recorded by an electrochemical workstation (CHI 660C, ShangHai) under one sun condition using a solar light simulator (Newport 69911, AM 1.5, 100 mW/cm2 ). The incident photo to current conversion efficiency (IPCE) curves of DSCs were measured in the range of 400–800 nm by a Keithley model 2000 Source Meter under short circuit conditions using monochromatic light. The ionic conductivity of the solid-state electrolytes was evaluated
Fig. 1. (a) FESEM image of ZnO monodispersed aggregates. Inset showing a single aggregate at 350,000× with a length bar scale of 10 nm. (b) Cross-sectional FESEM images of the ZnO aggregate electrode.
using electrochemical impedance spectra (EIS) method at ambient temperature and humidity. The impedance measurement was carried out on the electrochemical workstation. The sample was sandwiched between two home-made stainless steel blocking electrodes (2.5 cm2 ). 3. Results and discussion 3.1. Morphological characterization of ZnO electrode film Fig. 1a shows that as-synthesized ZnO aggregates have a uniform spherical structure with diameter of ∼250 nm. A high magnification image as evident in the inset shows that a single submicrometer-sized aggregate is composed of ZnO nanosized crystallites with diameter of 10–20 nm. The average pore diameter of ZnO particles are approximately 10.8 nm and 123.9 nm as determined by the Brunauer-Emmett-Teller (BET) method from N2 desoprtion isotherm. The 10.8 nm-sized pores are regarded as the internal pores of the ZnO aggregates, whereas the 123.9 nmsized pores originated from the interstitial channels formed by the close-packed 250 nm-sized aggregates [23]. A cross-sectional FESEM image indicates that the film is well stacked with ZnO aggregates (see Fig. 1b). The thickness of ZnO film is determined to be about 6 m. 3.2. Interfacial contact between dye-adsorbed ZnO aggregates and electrolytes Since deeper penetration of polymer electrolytes into ZnO nanopores lies on the balance of the radius of gyration (Rg ) of
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Fig. 3. EIS analyses of ZnO-based DSSCs employing different polymer electrolytes (a) Nyquist plots (inset showing equivalent circuit model used for the DSC in this study). (b) Bode phase plots.
Fig. 2. Cross-sectional FESEM images of ZnO aggregate electrode incorporating electrolytes (a) E1, (b) E2 and (c) E3.
polymer chains and the pore diameter, it is understandable that oligo-PEG (Rg = 0.9 nm) can penetrate into the nanopores more easily compared to PEO (Rg = 63.0 nm) [21,22]. Fig. 2 shows that cross-sectional FESEM images of nanoporous ZnO aggregate electrode films after coating the composite electrolyte. Many particles can be clearly distinguished (Fig. 2a) and this means poor interfacial contact between E1 and the ZnO particles. Upon oligo-PEG being introduced into the PEO polymer electrolyte system, cross-sections of the particles became apparently smooth and homogeneous (Fig. 2b and c), suggesting better penetration of E2 and E3 into ZnO electrode films [22]. 3.3. EIS analyses The kinetics of electrochemical and photo-electrochemical process occurring in solid-state ZnO DSCs was investigated via EIS in
the frequency range of 0.01 Hz–10 KHz [24–30]. The data were collected under illumination of one sun at open-circuit potential and the Nyquist plots are shown in Fig. 3a. The equivalent circuit employed for the curve fitted the impedance spectra of the DSCs is also provided (see the inset of Fig. 3a). All the EIS spectra exhibit three arcs, which are attributed to electrochemical reaction at the Pt counter electrode (Rct1 ), the charge transfer at the ZnO/dye/electrode (Rct2 ) and the Warburg diffusion process of I− /I3 − (Rdiff ) [31–33]. The ohmic serial resistance (Rs ) can be estimated from the intercept on the real axis at high frequency range. Obviously, device 3 exhibits the lowest Rs among the three devices. This result indicates that the introduction of TiO2 and oligo-PEG improves the charge transfer reaction at the interface of Pt counter electrode and solid-state electrolyte. Fitting the medium-frequency range device 3 gives the smallest Rct2 and this means the lowest hole–electron recombination action, hence leading to high conversion efficiency. Fig. 3b shows the Bode phase plots of EIS spectra for the DSC employing with different polymer electrolytes. The characteristic frequency peaks of device 1, device 2 and device 3 shifted to lower frequency sequentially and the corresponding characteristic low-frequency peaks (fmax ) locate at 25.7 Hz for device 1, 14.4 Hz for device 2 and 11.9 Hz for device 3, respectively. The electron lifetime for recombination ( e ) in ZnO based DSCs is determined by fmax values, where e = 1/ωmin = 1/2fmax [24,26]. The recombination lifetime for device 3 is 13.4 ms, much longer than that of 6.2 ms for device 1, and 11.1 ms for device 2. This implies that device 3 has the longest electron lifetime, which could effectively reduce the electron recombination and consequently lead to a significant enhancement of the cell efficiency.
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Fig. 5. Photocurrent density–voltage curves of ZnO DSCs based on three different polymer electrolytes measured at 100 mW/cm2 illumination. Table 1 The photovoltaic characteristics of DSCs assembled with the composite electrolytes. All measurements were performed under AM 1.5 globe one sun light intensity of 100 mW/cm2 and the active areas were about 0.25 cm2 for all of the cells. Electrolytes
Jsc (mA/cm2 )
Voc (mV)
FF (%)
(a (%)
PEO/TiO2 PEO/oligo-PEG PEO/oligo-PEG/TiO2
5.4 5.9 8.2
532 515 528
39 51 42
1.1 1.5 1.8
a The conversion efficiency of the cell can be calculated from the equation = (Jsc ·Voc ·FF)/Pin , 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 density.
TiO2 nanofillers may decrease the crystallinity of the polymer and build a transfer channel for the redox couples [15,20]. On the other hand, the oligomer may also enhance the ion mobility as well as the penetration of electrolyte into the ZnO layer [21,22]. 3.5. Photovoltaic performance of ZnO-based DSSCs
Fig. 4. Ionic conductivities for solid-state polymer electrolytes: (a) PEO/TiO2 /Li/I2 (b) PEO/oligo-PEG/LiI/I2 and (c) PEO/oligo-PEG/TiO2 /LiI/I2 .
3.4. Ionic conductivity measurement The ionic conductivity () of the composite electrolytes was determined by complex impedance measurements. In this method, the bulk resistance (Rb ) is evaluated using complex impedance plots and the conductivity of the film is determined from the formula: = l/A Rb , where l is the thickness of the film and A is the area of sample. As shown in Fig. 4a, the ionic conductivity of E1 gradually enhances with increasing TiO2 nanoparticle contents up to 10%, and then the conductivity exhibits a decreased trend. The increase in ionic conductivity seems to be owing to the enlargement of the amorphous phase in the polymer matrix [34]. The decrease in ionic conductivity above the maximum point may result from the formation of TiO2 aggregation in polymer matrix [35]. The ionic conductivity of E2 increases linearly with increasing the concentrations of the oligo-PEG as shown in Fig. 4b. The addition of oligomer into PEO causes the increase of the interfacial contact between the electrolyte and the ZnO layer, resulting in an improved ionic conductivity [22]. The maximum conductivity (E3, 1.45 × 10−4 S cm−1 ) is obtained by adding TiO2 nanofillers and oligo-PEG into PEO electrolyte system as shown in Fig. 4c. On one hand, the introduction of
The photocurrent density-voltage curves for solid-state ZnO DSCs under the AM 1.5 sunlight illumination (100 mW/cm2 ) are presented in Fig. 5. Table 1 summarizes the measured and calculated values obtained from the typical I–V curves. As seen in Table 1, the Jsc and of device 1 are 5.4 mA/cm2 and 1.1%, respectively. For device 2 the of only 1.5% was obtained. When E3 electrolyte with high ionic conductivity and deep penetration was applied to device 3, the Jsc increased to 8.2 mA/cm2 and the enhanced up to 1.8%. Apparently, device 3 displays outstanding conversion efficiency, corresponding to 64% higher than that of device 1 and a 20% increment of device 2. In addition, as for device 3, we also study photovoltaic performances of the cells under low intensity of light (Fig. 6). The results show that the values of the cells
Fig. 6. I–V curves of the device 3 measured repeated under various light intensities.
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863 project (2007AA03Z343), and 973 program (grant no. 2009CB219903). References
Fig. 7. IPCE spectra of ZnO DSSCs based on three different polymer electrolytes.
have a slight enhancement reaching up to 2.0% under 60 mW/cm2 radiation. 3.6. IPCE measurements Fig. 7 shows that the photocurrent action spectra of solid-state ZnO DSCs. The device 3 shows a better photoelectrical response, and its absolute IPCE is obviously higher than that of the other two cells over the entire wavelength region from 400 to 800 nm. The maximum efficiency at the wavelength of 520 nm is in coincidence with the absorption maximum wavelength of the N719 dye. The IPCE peak height at 520 nm for device 3 is 57.3%, which is much higher than 39.1% for device 1, and 46.1% for device 2. This is also in good agreement with the observed higher Jsc and of device 3 as displayed in Fig. 5. 4. Conclusions In summary, we have successfully fabricated solid-state hierarchically structured ZnO dye-sensitized solar cells by introducing TiO2 particles as nanofillers into composite PEO/PEG electrolytes. The optimized composite electrolyte, PEO/oligo-PEG/TiO2 /LiI/I2 was evidenced to possess high ionic conductivity and excellent penetration. The solid-state ZnO cell fabricated with the optimal composite electrolyte achieved a high conversion efficiency of 1.8% at 100 mW/cm2 and 2.0% at 60 mW/cm2 . The work demonstrates the first realization of solid-state ZnO aggregates-based DSC prototype towards the practical application. Acknowledgements This work was supported financially by NSFC (nos. 50642042, 20821004, 20577002, 20776014), CPSF (no. 20080440303),
[1] B. O’Reagen, M. Gräzel, Nature 353 (1991) 373. [2] H. Xu, X. Tao, D.-T. Wang, Y.-Z. Zheng, J.-F. Chen, Electrochim. Acta 55 (2010) 2280. [3] F.-T. Kong, S.-Y. Dai, K.-J. Wang, Adv. Opto Elec. (2007) 75384. [4] M. Grätzel, Nature 414 (2001) 338. [5] E.M. Kaidashev, M. Lorenz, H. von Wenckstern, A. Rahm, H.C. Semmelhack, K.H. Han, G. Benndorf, C. Bundesmann, H. Hochmuth, M. Grundmann, Appl. Phys. Lett. 82 (2003) 3901. [6] K. Kakiuchi, E. Hosono, S. Fujihara, J. Photochem. Photobiol. A 179 (2006) 81. [7] T.R. Zhang, W.J. Dong, M.K. Brewer, S. Konar, R.N. Njabon, Z.R. Tian, J. Am. Chem. Soc. 128 (2006) 10960. [8] M. Guo, P. Diao, X. Wang, S.M. Cai, J. Solid State Chem. 178 (2005) 3210. [9] M. Guo, P. Diao, S.M. Cai, Chin. Chem. Lett. 15 (2004) 1113. [10] K. Keis, C. Bauer, G. Boschloo, A. Hagfeldt, K. Westermark, H. Rensmo, H. Siegbahn, J. Photochem. Photobiol. A 148 (2002) 57. [11] T.P. Chou, Q.F. Zhang, G.E. Fryxell, G.Z. Cao, Adv. Mater. 19 (2007) 2588. [12] U. Bach, D. Lupo, P. Comte, J.E. Moser, F. Weissörtel, J. Salbeck, H. Spreitzer, M. Grätzel, Nature 395 (1998) 583. [13] M. Biancardo, K. West, F.C. Krebs, J. Photochem. Photobiol. A 187 (2007) 395. [14] J. Reiter, J. Vondark, J. Michalek, Z. Micka, Electrochim. Acta 52 (2006) 1398. [15] T. Stergiopoulos, I.M. Arabatzis, G. Katsaros, P. Falaras, Nano Lett. 2 (2002) 1259. [16] V.C. Nogueira, C. Longo, A.F. Nogueira, M.A. Soto-Oviedo, M.-A. De Paoli, J. Photochem. Photobiol. A 181 (2006) 226. [17] D. Chen, Q. Zhang, G. Wang, H. Zhang, J.H. Li, Electrochem. Commun. 9 (2007) 2755. [18] N.O.V. Plank, I. Howard, A. Rao, M.W.B. Wilson, C. Ducati, R.S. Mane, J.S. Bendall, R.R.M. Louca, N.C. Greenham, H. Miura, R.H. Friend, H.J. Snaith, M.E. Welland, J. Phys. Chem. C 113 (2009) 18515. [19] Y.-Z. Zheng, X. Tao, L.-X. Wang, H. Xu, Q. Hou, W.-L. Zhou, J.-F. Chen, Chem. Mater. 22 (2010) 928. [20] G. Katsaros, T. Stergiopoulos, I.M. Arabatzis, K.G. Papadokostaki, P. Falaras, J. Photochem. Photobiol. A 149 (2002) 191. [21] J.Y. Lee, B. Bhattacharya, Y.H. Kim, H.-T. Jung, J.-K. Park, Solid State Commun. 149 (2009) 307. [22] J.H. Kim, M.-S. Kang, Y.J. Kim, J. Won, N.-G. Park, Y.S. Kang, Chem. Commun. (2004) 1662. [23] Y.J. Kim, M.H. Lee, H.J. Kim, G. Lim, Y.S. Chi, N.-G. Park, K. Kim, W.I. Lee, Adv. Mater. 21 (2009) 1. [24] J. van de Lagemaat, N.-G. Park, A.J. Frank, J. Phys. Chem. B 104 (2000) 2044. [25] C. Longo, A.F. Nogueira, M.-A. De Paoli, H. Cachet, J. Phys. Chem. B 106 (2002) 5925. [26] N. Papageorgiou, W.F. Maier, M. Grätzel, J. Electrochem. Soc. 144 (1997) 876. [27] Q. Wang, J.-E. Moser, M. Grätzel, J. Phys. Chem. B 109 (2005) 14945. [28] L. Han, N. Koide, Y. Chiba, T. Mitate, Appl. Phys. Lett. 84 (2004) 2433. [29] Y. Zhao, J. Zhai, J.L. He, X. Chen, L. Chen, L.B. Zhang, Y.X. Tian, L. Jiang, D.B. Zhu, Chem. Mater. 20 (2008) 6022. [30] Y.Z. Zheng, H.Y. Ding, M.L. Zhang, Thin Solid Films 516 (2008) 7381. [31] S.J. Lim, Y.S. Kang, D.-W. Kim, Electrochem. Commun. 12 (2010) 1037. [32] C.-P. Hsu, K.-M. Lee, J.-T.-W. Huang, C.-Y. Lin, C.-H. Lee, L.-P. Wang, S.-Y. Tsai, K.-C. Ho, Electrochim. Acta 53 (2008) 7514. [33] Y. Zhao, J. Zhai, J.L. He, X. Chen, L. Chen, L.B. Zhang, Y.X. Tain, L. Jiang, D.B. Zhu, Chem. Mater. 20 (2008) 6022. [34] Y.F. Zhou, W.C. Xiang, S. Chen, S.B. Fang, X.W. Zhou, J.B. Zhang, Y. Lin, Electrochim. Acta 54 (2009) 6645. [35] E. Stathatos, P. Lianos, U. Lavrencic-Stangar, B. Orel, Adv. Mater. 14 (2002) 354.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Solid-state dye-sensitized hierarchically structured ZnO solar cells Jie Deng a , Yan-zhen Zheng a,b , Qian Hou a , Jian-Feng Chen b,∗ , Weilie Zhou c , Xia Tao a,∗ a
Key Laboratory for Nanomaterials of the Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China Research Center of the Ministry of Education for High Gravity Engineering & Technology, Beijing University of Chemical Technology, Beijing 100029, China c Advanced Materials Research Institute University of New Orleans New Orleans, LA 70148, USA b
a r t i c l e
i n f o
Article history: Received 22 November 2010 Received in revised form 26 January 2011 Accepted 27 January 2011 Available online 3 February 2011 Keywords: Solid-state Dye-sensitized solar cell Hierarchically structured ZnO Composite electrolyte Photovoltaic performance
a b s t r a c t A novel solid-state hierarchically structured ZnO dye-sensitized solar cell (DSC) was assembled by using TiO2 as filler in polyethylene oxide (PEO)/polyethylene glycol (PEG) electrolytes and ZnO nanocrystalline aggregates as photoanode film. Under optimized composite polyelectrolyte containing PEO/oligoPEG/TiO2 /LiI/I2 the photovoltaic performance of the solid-state ZnO DSCs was significantly better, with an overall conversion efficiency () of 1.8% under irradiation of 100 mW/cm2 , which was higher than those of the cells with PEO/TiO2 /LiI/I2 ( = 1.1%) or PEO/oligo-PEG/LiI/I2 electrolyte ( = 1.5%). Further, the hierarchically structured ZnO-based cell showed a higher value of 2.0% under 60 mW/cm2 radiation. The morphologies, ionic conductivity of three different composite electrolytes and their performance to the DSCs were also studied by FESEM, I–V data, IPCE and EIS. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSCs) have been widely considered as potential inexpensive alternatives to conventional p–n junction photovoltaic devices over the past decades [1–3]. A typical DSC consists of a mesoporous nanocrystalline TiO2 film covered by a monolayer of dye molecules, redox electrolyte, and counter electrode. In TiO2 -based DSCs, the electron injection from a photoexcited dye in the conduction band of TiO2 is ultra-fast, on the order of femto seconds, but the electron recombination is high due to low electron mobility and transport properties. Thus, there has been a considerable interest in search for alternate materials and assemblies. ZnO is a promising wide band oxide material due to its large band gap (3.37 eV) and high electron mobility (115–155 cm2 V−1 s−1 ) [4–7]. An overall energy conversion efficiency based on ZnO DSCs has reached up to 2.4% for the nanowire/nanorod arrays [8], 2.3% for the nanotube arrays [9], 5.0% for nanocrystalline particles [10] and 3.5% for uniform nanocrystalline aggregates [11]. 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, and corrosion of the electrodes. Replacing the liquid electrolyte by a solid-state charge transport medium is a solution to these problems [12–17]. Very recently, Plank et al. reported a
∗ Corresponding authors. Tel.: +86 10 6445 3680; fax: +86 10 6443 4784. E-mail addresses: [email protected] (J.-F. Chen), [email protected] (X. Tao). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.01.099
solid-state ZnO nanowire DSC using organic dyes and core–shell nanostructures exhibiting a maximum efficiency of 0.71% [18]. But, to date, studies on solid-state ZnO DSCs are still rare and their conversion efficiencies also need to be further improved. In our work, three solid-state electrolytes including PEO/TiO2 /LiI/I2, PEO/oligo-PEG/LiI/I2 , and PEO/oligoPEG/TiO2 /LiI/I2 were first introduced into hierarchically structured ZnO DSCs. The hierarchically structured ZnO particles were composed of secondary ZnO colloids with ∼250 nm in diameter that were made up of 10–20 nm nanocrystallites exhibiting sufficient surface area for dye molecule adsorption and light scattering effect for efficient photon-capture. Based on three solid-state electrolyte ZnO DSCs, conversion efficiencies were achieved up to 1.8% and 2.0% under 100 and 60 mW/cm2 illuminations, respectively. Besides, the performance enhancement of the cells was also discussed. 2. Experimental 2.1. Materials Anhydrous lithium iodide (LiI), iodide (I2 ), poly(ethylene oxide) (PEO, Mw = 1,000,000) and H2 PtCl6 were obtained from Sigma. Zinc acetate dihydrate [(CH3 COO)2 Zn], diethylene glycol [(HOCH2 CH2 )2 O], NaOH, HCl, polyethylene glycol (PEG, Mw = 200), acetonitrile and ethanol (analytical grade purity) were purchased from Tianjin Chemical Reagents Co. and were used without further purification. Ru-based N719 dye (cis-bis (isothiocyanato) bis (2, 2-bipyridyl 4, 4-dicarboxylato) ruthenium (II)
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bis-tetrabutylammonium) was obtained from Solaronix (Aubonne, Switzerland). TiO2 nanoparticles (P25, mean particle size 21 nm) were obtained from Degussa (AG, Germany). All solutions used in this work were prepared with 18.2 M cm−1 water produced by a reagent water system (Easy pure II, Barnstead). 2.2. Preparation and characterization of ZnO aggregates The ZnO monodispersed aggregates were synthesized by hydrolyzing 0.02 mol [(CH3 CO2 )2 Zn] in 100 mL [(HOCH2 CH2 )2 O], as described in our previous report [19]. Under vigorous stirring, the mixture was heated to 160 ◦ C in an oil bath at a rate of 5 ◦ C min−1 and then refluxed for 20 h. Subsequently, the as-obtained colloid suspension was centrifuged at a rate of 5000 rpm for 10 min. After centrifugation, the obtained precipitates were redispersed in ethanol for further use. 2.3. Preparation of polymer electrolytes Three solid polymer electrolytes composed of PEO/TiO2 /LiI/I2 (denoted as E1), PEO/oligo-PEG/LiI/I2 (denoted as E2) and PEO/oligo-PEG/TiO2 /LiI/I2 (denoted as E3) were prepared as follows: E1 was obtained according to the literature [20]. Specially, 0.0383 g TiO2 was dispersed in acetonitrile, and then I− /I3 − redox couple (0.1 g LiI and 0.019 g I2 ) was added. Subsequently, 0.264 g PEO was slowly introduced and left under continuous stirring for 24 h. E2 was fabricated by dissolving 0.264 g PEO in acetonitrile and then a predetermined amount of oligo-PEG was added. The PEO:PEG ratio was fixed at 4:6 (w/w), whereas the monomer to cation ratio [EO]:[Li+ ] was fixed at 20:1 (LiI/I2 = 10/1, w/w) [21]. E3 was prepared by introducing 0.0383 g TiO2 powder as nanofiller into E2 electrolyte. 2.4. Fabrication of solid-state ZnO-based DSC Fluorine-doped tin oxide-coated glass (FTO, Hartford, 14 /sq, 80% transmittance) plates were used as the substrates of ZnO photoelectrode and counter electrodes. Prior to fabricating electrode, the FTO substrates was ultrasonically cleaned sequentially in HCl, acetone, ethanol and water each for 15 min. The ZnO photoelectrode films were deposited on FTO substrates using a drop-coat method. Thickness of films was estimated to be 6 ± 0.5 m by exactly controlling the added drops. After the films dried, they were sintered at 350 ◦ C for 60 min and then sensitized with N719 dye for 15 min. Counter electrodes were prepared by spin-coating 0.35 mM H2 PtCl6 solution onto FTO glass and annealed at 400 ◦ C for 15 min. A two step casting method was adopted for casting as-prepared polymer electrolytes on the ZnO layer [22], subsequently, covered by a platinized conducting glass to form a sandwich solar cell device. For convenience, DSCs assembled with E1, E2 and E3 were denoted as device 1, device 2 and device 3, respectively. During the assembly procedures, the active area of the resulting cell exposed in light was approximately 0.25 cm2 (0.5 cm × 0.5 cm). 2.5. Instruments and measurements The morphology of the samples were observed by field emission scanning electron microscope (FESEM, Hitachi S-4700 and JEOL, JSM 6701-F). The photocurrent–voltage characteristics of the DSCs were recorded by an electrochemical workstation (CHI 660C, ShangHai) under one sun condition using a solar light simulator (Newport 69911, AM 1.5, 100 mW/cm2 ). The incident photo to current conversion efficiency (IPCE) curves of DSCs were measured in the range of 400–800 nm by a Keithley model 2000 Source Meter under short circuit conditions using monochromatic light. The ionic conductivity of the solid-state electrolytes was evaluated
Fig. 1. (a) FESEM image of ZnO monodispersed aggregates. Inset showing a single aggregate at 350,000× with a length bar scale of 10 nm. (b) Cross-sectional FESEM images of the ZnO aggregate electrode.
using electrochemical impedance spectra (EIS) method at ambient temperature and humidity. The impedance measurement was carried out on the electrochemical workstation. The sample was sandwiched between two home-made stainless steel blocking electrodes (2.5 cm2 ). 3. Results and discussion 3.1. Morphological characterization of ZnO electrode film Fig. 1a shows that as-synthesized ZnO aggregates have a uniform spherical structure with diameter of ∼250 nm. A high magnification image as evident in the inset shows that a single submicrometer-sized aggregate is composed of ZnO nanosized crystallites with diameter of 10–20 nm. The average pore diameter of ZnO particles are approximately 10.8 nm and 123.9 nm as determined by the Brunauer-Emmett-Teller (BET) method from N2 desoprtion isotherm. The 10.8 nm-sized pores are regarded as the internal pores of the ZnO aggregates, whereas the 123.9 nmsized pores originated from the interstitial channels formed by the close-packed 250 nm-sized aggregates [23]. A cross-sectional FESEM image indicates that the film is well stacked with ZnO aggregates (see Fig. 1b). The thickness of ZnO film is determined to be about 6 m. 3.2. Interfacial contact between dye-adsorbed ZnO aggregates and electrolytes Since deeper penetration of polymer electrolytes into ZnO nanopores lies on the balance of the radius of gyration (Rg ) of
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Fig. 3. EIS analyses of ZnO-based DSSCs employing different polymer electrolytes (a) Nyquist plots (inset showing equivalent circuit model used for the DSC in this study). (b) Bode phase plots.
Fig. 2. Cross-sectional FESEM images of ZnO aggregate electrode incorporating electrolytes (a) E1, (b) E2 and (c) E3.
polymer chains and the pore diameter, it is understandable that oligo-PEG (Rg = 0.9 nm) can penetrate into the nanopores more easily compared to PEO (Rg = 63.0 nm) [21,22]. Fig. 2 shows that cross-sectional FESEM images of nanoporous ZnO aggregate electrode films after coating the composite electrolyte. Many particles can be clearly distinguished (Fig. 2a) and this means poor interfacial contact between E1 and the ZnO particles. Upon oligo-PEG being introduced into the PEO polymer electrolyte system, cross-sections of the particles became apparently smooth and homogeneous (Fig. 2b and c), suggesting better penetration of E2 and E3 into ZnO electrode films [22]. 3.3. EIS analyses The kinetics of electrochemical and photo-electrochemical process occurring in solid-state ZnO DSCs was investigated via EIS in
the frequency range of 0.01 Hz–10 KHz [24–30]. The data were collected under illumination of one sun at open-circuit potential and the Nyquist plots are shown in Fig. 3a. The equivalent circuit employed for the curve fitted the impedance spectra of the DSCs is also provided (see the inset of Fig. 3a). All the EIS spectra exhibit three arcs, which are attributed to electrochemical reaction at the Pt counter electrode (Rct1 ), the charge transfer at the ZnO/dye/electrode (Rct2 ) and the Warburg diffusion process of I− /I3 − (Rdiff ) [31–33]. The ohmic serial resistance (Rs ) can be estimated from the intercept on the real axis at high frequency range. Obviously, device 3 exhibits the lowest Rs among the three devices. This result indicates that the introduction of TiO2 and oligo-PEG improves the charge transfer reaction at the interface of Pt counter electrode and solid-state electrolyte. Fitting the medium-frequency range device 3 gives the smallest Rct2 and this means the lowest hole–electron recombination action, hence leading to high conversion efficiency. Fig. 3b shows the Bode phase plots of EIS spectra for the DSC employing with different polymer electrolytes. The characteristic frequency peaks of device 1, device 2 and device 3 shifted to lower frequency sequentially and the corresponding characteristic low-frequency peaks (fmax ) locate at 25.7 Hz for device 1, 14.4 Hz for device 2 and 11.9 Hz for device 3, respectively. The electron lifetime for recombination ( e ) in ZnO based DSCs is determined by fmax values, where e = 1/ωmin = 1/2fmax [24,26]. The recombination lifetime for device 3 is 13.4 ms, much longer than that of 6.2 ms for device 1, and 11.1 ms for device 2. This implies that device 3 has the longest electron lifetime, which could effectively reduce the electron recombination and consequently lead to a significant enhancement of the cell efficiency.
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Fig. 5. Photocurrent density–voltage curves of ZnO DSCs based on three different polymer electrolytes measured at 100 mW/cm2 illumination. Table 1 The photovoltaic characteristics of DSCs assembled with the composite electrolytes. All measurements were performed under AM 1.5 globe one sun light intensity of 100 mW/cm2 and the active areas were about 0.25 cm2 for all of the cells. Electrolytes
Jsc (mA/cm2 )
Voc (mV)
FF (%)
(a (%)
PEO/TiO2 PEO/oligo-PEG PEO/oligo-PEG/TiO2
5.4 5.9 8.2
532 515 528
39 51 42
1.1 1.5 1.8
a The conversion efficiency of the cell can be calculated from the equation = (Jsc ·Voc ·FF)/Pin , 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 density.
TiO2 nanofillers may decrease the crystallinity of the polymer and build a transfer channel for the redox couples [15,20]. On the other hand, the oligomer may also enhance the ion mobility as well as the penetration of electrolyte into the ZnO layer [21,22]. 3.5. Photovoltaic performance of ZnO-based DSSCs
Fig. 4. Ionic conductivities for solid-state polymer electrolytes: (a) PEO/TiO2 /Li/I2 (b) PEO/oligo-PEG/LiI/I2 and (c) PEO/oligo-PEG/TiO2 /LiI/I2 .
3.4. Ionic conductivity measurement The ionic conductivity () of the composite electrolytes was determined by complex impedance measurements. In this method, the bulk resistance (Rb ) is evaluated using complex impedance plots and the conductivity of the film is determined from the formula: = l/A Rb , where l is the thickness of the film and A is the area of sample. As shown in Fig. 4a, the ionic conductivity of E1 gradually enhances with increasing TiO2 nanoparticle contents up to 10%, and then the conductivity exhibits a decreased trend. The increase in ionic conductivity seems to be owing to the enlargement of the amorphous phase in the polymer matrix [34]. The decrease in ionic conductivity above the maximum point may result from the formation of TiO2 aggregation in polymer matrix [35]. The ionic conductivity of E2 increases linearly with increasing the concentrations of the oligo-PEG as shown in Fig. 4b. The addition of oligomer into PEO causes the increase of the interfacial contact between the electrolyte and the ZnO layer, resulting in an improved ionic conductivity [22]. The maximum conductivity (E3, 1.45 × 10−4 S cm−1 ) is obtained by adding TiO2 nanofillers and oligo-PEG into PEO electrolyte system as shown in Fig. 4c. On one hand, the introduction of
The photocurrent density-voltage curves for solid-state ZnO DSCs under the AM 1.5 sunlight illumination (100 mW/cm2 ) are presented in Fig. 5. Table 1 summarizes the measured and calculated values obtained from the typical I–V curves. As seen in Table 1, the Jsc and of device 1 are 5.4 mA/cm2 and 1.1%, respectively. For device 2 the of only 1.5% was obtained. When E3 electrolyte with high ionic conductivity and deep penetration was applied to device 3, the Jsc increased to 8.2 mA/cm2 and the enhanced up to 1.8%. Apparently, device 3 displays outstanding conversion efficiency, corresponding to 64% higher than that of device 1 and a 20% increment of device 2. In addition, as for device 3, we also study photovoltaic performances of the cells under low intensity of light (Fig. 6). The results show that the values of the cells
Fig. 6. I–V curves of the device 3 measured repeated under various light intensities.
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863 project (2007AA03Z343), and 973 program (grant no. 2009CB219903). References
Fig. 7. IPCE spectra of ZnO DSSCs based on three different polymer electrolytes.
have a slight enhancement reaching up to 2.0% under 60 mW/cm2 radiation. 3.6. IPCE measurements Fig. 7 shows that the photocurrent action spectra of solid-state ZnO DSCs. The device 3 shows a better photoelectrical response, and its absolute IPCE is obviously higher than that of the other two cells over the entire wavelength region from 400 to 800 nm. The maximum efficiency at the wavelength of 520 nm is in coincidence with the absorption maximum wavelength of the N719 dye. The IPCE peak height at 520 nm for device 3 is 57.3%, which is much higher than 39.1% for device 1, and 46.1% for device 2. This is also in good agreement with the observed higher Jsc and of device 3 as displayed in Fig. 5. 4. Conclusions In summary, we have successfully fabricated solid-state hierarchically structured ZnO dye-sensitized solar cells by introducing TiO2 particles as nanofillers into composite PEO/PEG electrolytes. The optimized composite electrolyte, PEO/oligo-PEG/TiO2 /LiI/I2 was evidenced to possess high ionic conductivity and excellent penetration. The solid-state ZnO cell fabricated with the optimal composite electrolyte achieved a high conversion efficiency of 1.8% at 100 mW/cm2 and 2.0% at 60 mW/cm2 . The work demonstrates the first realization of solid-state ZnO aggregates-based DSC prototype towards the practical application. Acknowledgements This work was supported financially by NSFC (nos. 50642042, 20821004, 20577002, 20776014), CPSF (no. 20080440303),
[1] B. O’Reagen, M. Gräzel, Nature 353 (1991) 373. [2] H. Xu, X. Tao, D.-T. Wang, Y.-Z. Zheng, J.-F. Chen, Electrochim. Acta 55 (2010) 2280. [3] F.-T. Kong, S.-Y. Dai, K.-J. Wang, Adv. Opto Elec. (2007) 75384. [4] M. Grätzel, Nature 414 (2001) 338. [5] E.M. Kaidashev, M. Lorenz, H. von Wenckstern, A. Rahm, H.C. Semmelhack, K.H. Han, G. Benndorf, C. Bundesmann, H. Hochmuth, M. Grundmann, Appl. Phys. Lett. 82 (2003) 3901. [6] K. Kakiuchi, E. Hosono, S. Fujihara, J. Photochem. Photobiol. A 179 (2006) 81. [7] T.R. Zhang, W.J. Dong, M.K. Brewer, S. Konar, R.N. Njabon, Z.R. Tian, J. Am. Chem. Soc. 128 (2006) 10960. [8] M. Guo, P. Diao, X. Wang, S.M. Cai, J. Solid State Chem. 178 (2005) 3210. [9] M. Guo, P. Diao, S.M. Cai, Chin. Chem. Lett. 15 (2004) 1113. [10] K. Keis, C. Bauer, G. Boschloo, A. Hagfeldt, K. Westermark, H. Rensmo, H. Siegbahn, J. Photochem. Photobiol. A 148 (2002) 57. [11] T.P. Chou, Q.F. Zhang, G.E. Fryxell, G.Z. Cao, Adv. Mater. 19 (2007) 2588. [12] U. Bach, D. Lupo, P. Comte, J.E. Moser, F. Weissörtel, J. Salbeck, H. Spreitzer, M. Grätzel, Nature 395 (1998) 583. [13] M. Biancardo, K. West, F.C. Krebs, J. Photochem. Photobiol. A 187 (2007) 395. [14] J. Reiter, J. Vondark, J. Michalek, Z. Micka, Electrochim. Acta 52 (2006) 1398. [15] T. Stergiopoulos, I.M. Arabatzis, G. Katsaros, P. Falaras, Nano Lett. 2 (2002) 1259. [16] V.C. Nogueira, C. Longo, A.F. Nogueira, M.A. Soto-Oviedo, M.-A. De Paoli, J. Photochem. Photobiol. A 181 (2006) 226. [17] D. Chen, Q. Zhang, G. Wang, H. Zhang, J.H. Li, Electrochem. Commun. 9 (2007) 2755. [18] N.O.V. Plank, I. Howard, A. Rao, M.W.B. Wilson, C. Ducati, R.S. Mane, J.S. Bendall, R.R.M. Louca, N.C. Greenham, H. Miura, R.H. Friend, H.J. Snaith, M.E. Welland, J. Phys. Chem. C 113 (2009) 18515. [19] Y.-Z. Zheng, X. Tao, L.-X. Wang, H. Xu, Q. Hou, W.-L. Zhou, J.-F. Chen, Chem. Mater. 22 (2010) 928. [20] G. Katsaros, T. Stergiopoulos, I.M. Arabatzis, K.G. Papadokostaki, P. Falaras, J. Photochem. Photobiol. A 149 (2002) 191. [21] J.Y. Lee, B. Bhattacharya, Y.H. Kim, H.-T. Jung, J.-K. Park, Solid State Commun. 149 (2009) 307. [22] J.H. Kim, M.-S. Kang, Y.J. Kim, J. Won, N.-G. Park, Y.S. Kang, Chem. Commun. (2004) 1662. [23] Y.J. Kim, M.H. Lee, H.J. Kim, G. Lim, Y.S. Chi, N.-G. Park, K. Kim, W.I. Lee, Adv. Mater. 21 (2009) 1. [24] J. van de Lagemaat, N.-G. Park, A.J. Frank, J. Phys. Chem. B 104 (2000) 2044. [25] C. Longo, A.F. Nogueira, M.-A. De Paoli, H. Cachet, J. Phys. Chem. B 106 (2002) 5925. [26] N. Papageorgiou, W.F. Maier, M. Grätzel, J. Electrochem. Soc. 144 (1997) 876. [27] Q. Wang, J.-E. Moser, M. Grätzel, J. Phys. Chem. B 109 (2005) 14945. [28] L. Han, N. Koide, Y. Chiba, T. Mitate, Appl. Phys. Lett. 84 (2004) 2433. [29] Y. Zhao, J. Zhai, J.L. He, X. Chen, L. Chen, L.B. Zhang, Y.X. Tian, L. Jiang, D.B. Zhu, Chem. Mater. 20 (2008) 6022. [30] Y.Z. Zheng, H.Y. Ding, M.L. Zhang, Thin Solid Films 516 (2008) 7381. [31] S.J. Lim, Y.S. Kang, D.-W. Kim, Electrochem. Commun. 12 (2010) 1037. [32] C.-P. Hsu, K.-M. Lee, J.-T.-W. Huang, C.-Y. Lin, C.-H. Lee, L.-P. Wang, S.-Y. Tsai, K.-C. Ho, Electrochim. Acta 53 (2008) 7514. [33] Y. Zhao, J. Zhai, J.L. He, X. Chen, L. Chen, L.B. Zhang, Y.X. Tain, L. Jiang, D.B. Zhu, Chem. Mater. 20 (2008) 6022. [34] Y.F. Zhou, W.C. Xiang, S. Chen, S.B. Fang, X.W. Zhou, J.B. Zhang, Y. Lin, Electrochim. Acta 54 (2009) 6645. [35] E. Stathatos, P. Lianos, U. Lavrencic-Stangar, B. Orel, Adv. Mater. 14 (2002) 354.