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Quasi-solid-state dye-sensitized solar cells based on gel electrolytes containing different alkali metal iodide salts
Solid State Ionics 179 (2008) 2027–2030
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Quasi-solid-state dye-sensitized solar cells based on gel electrolytes containing different alkali metal iodide salts Xiaolin Shen a, Weilin Xu b,⁎, Jie Xu b, Guijie Liang b, Hongjun Yang b, Mu Yao c a b c
School of Materials Science & Engineering, Xi'an Jiaotong University, Xi'an 710049, China Wuhan University of Science & Engineering, Wuhan 430073, China Xi'an Polytechnic University, Xi'an 710049, China
A R T I C L E
I N F O
Article history: Received 19 April 2008 Received in revised form 20 June 2008 Accepted 30 June 2008 Keywords: Quasi-solid-state dye-sensitized solar cell Alkali metal iodide salts Open current voltage Solubility PEO gel electrolyte
A B S T R A C T Gel electrolytes were prepared by adding different alkali metal iodide salts RI (R+ = Li+, Na+, K+, Rb+ or Cs+) and I2 into acetonitrile gelated with poly(ethylene-oxide) (PEO). KI, RbI and CsI, poorly soluble in liquid electrolyte, can dissolve completely in PEO gel electrolyte due to a strong interaction of cation and PEO chains. All gel electrolytes exhibit high conductivity in the range of 10− 3 S/cm and contain I− with concentration of 0.3 M. The effect of R+ in PEO gel electrolyte on the performance of quasi-solid-state dyesensitized solar cells (DSCs) was investigated. The results showed that the open circuit voltage (Voc) increases with the increased radius of alkali metal cation. It is explained by the rise of electron Fermi level (EF) of TiO2 caused by a decrease in I−3 diffusion with the increase of radius of R+. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSCs) based on TiO2 porous films [1,2] are one of the potential low-cost alternatives to traditional silicon solar cell and a maximal 1ight-electric power conversion efficiency of 11% [3] was already achieved with liquid electrolyte containing a (I−/I−3) redox couple in organic solution, such as acetonitrile, propylene carbonate and ethylene carbonate. However, the volatility of organic solvent leads to a decrease in performance of DSCs. This drawback limits practical application of DSCs. Great efforts have been devoted to overcome this problem. Substituting the gel liquid electrolyte for liquid electrolyte [4] is an efficient way to prevent the leakage of electrolyte [5–10]. Recently, several types of polymers [11–13] as gelator have been used in gel electrolyte for DSCs. For these electrolytes, the concentration of I− ions is sufficient for reducing the oxidated sensitizer of DSC during operations, if the I− ions are supplied by only adding the LiI or NaI in the gel electrolyte. However, it is interesting that the organic iodide salts containing large cation, such as N-methyl pyridine iodide [11] or 1, 3-dimethyl-3-imidazolinium iodine [7] are also added into gel electrolyte to supply parts of I− ions. The same strategy was adopted for DSCs based on liquid electrolytes [14,15]. It implies that large cation in gel electrolytes contribute to improve the performance of ⁎ Corresponding author. Tel./fax: +86 27 87426559. E-mail address: [email protected] (W. Xu). 0167-2738/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.06.027
quasi-solid-state DSCs. In addition, LiI commonly used for DSCs is deliquescent and more expensive than other alkali metal iodide salts, such as NaI, KI. In this paper, a series of poly (ethylene-oxide) (PEO) gel electrolytes containing different alkali metal iodide salts RI (R+ = Li+, Na+, K+, Rb+, Cs+) were employed in DSC to investigate the effect of large inorganic cation on the performance of quasi-solid-state DSCs. PEO was selected as gelator of the gel electrolytes in our experiment due to a high fill factor (FF) for DSCs based on this polymer [7,8,12,13]. It is well known that KI, RbI and CsI with large cation with respect to radius of Li+ were hardly employed in a liquid electrolyte for DSCs because of their poor solubility in organic solution. PEO has a great deal of polar ether oxygen in its molecular chains, which could interact with the cation of the alkali metal iodide salts [5]. It is expected that the solubility of RI with large cation would be improved in PEO gel electrolyte by the interaction of the R+ and ether oxygen and thus photovoltaic performance of DSCs based on PEO gel electrolytes could be obtained by using alkali metal iodide salts with large cation. 2. Experimental 2.1. Materials Alkali metal iodide salts RI (R+ = Li+, Na+, K+, Rb+, Cs+) (A.R. grade) and poly(ethylene-oxide) (PEO, Mη = 1.5 × 106, 99%) were used as received. Conducting glass substrate (FTO glass, fluorine doped tin
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X. Shen et al. / Solid State Ionics 179 (2008) 2027–2030
Fig. 1. XRD spectra of KI, RbI, CsI crystals and the PEO gel electrolytes containing the three salts with the concentration of 0.3 M.
oxide over-layer, sheet resistance 15 Ω cm− 2) was purchased from Libbey Owens Ford Industries, USA. Sensitizing dye cis-bis(isothiocyanate)-bis(2,2′-bipyri-dine-4,4′-dicarboxylate) ruthenium (N3) was purchased from SOLARO-NIX. Other solvents and reagents were used as received.
Fig. 2. IR spectra of PEO gel electrolytes with alkali metal iodide RI (R+ = Li+, Na+, K+, Rb+, Cs+) salts and the pure PEO gel.
electrodes is 40 μm controlled by a spacer. The electrodes had an active area of 1 cm2. FTIR spectra of PEO gel electrolytes were measured on a Tensor 27 FTIR spectrometer (Bruker). 3. Results and discussion
2.2. Preparation of gel electrolytes 3.1. Solubility of KI, RbI and CsI in PEO gel electrolyte A series of mixtures were prepared by adding 1.2 × 10− 3 mol RI (R+ = Li+, Na+, K+, Rb+, Cs+), 1.2 × 10− 4 mol I2 and 0.8 g PEO into 4 ml acetonitrile. The gel electrolytes were obtained until the mixtures became homogenous and unflowable. A series of liquid electrolytes with RI without adding PEO were also prepared according to the above ratio as control. 2.3. Assembly of the DSCs The TiO2 electrodes were prepared by spreading TiO2 (P25, Degussa) colloidal paste on a FTO glass with a glass rod as squeegee and a spacing tape to make uniform films. After drying, the coated substrate was sintered at 450 °C in air for 0.5 h and cooled to 80 °C, and then immersed in dye sensitization solution of 0.3 mM N3 in ethanol for 12 h. The film thickness is about 10 μm. The prepared PEO gel electrolytes was casted onto the TiO2 electrode sensitized by N3 and then a platinum (Pt) counter electrode was pressed on top of the TiO2 electrode to form a DSC. Effective area of the cell was 0.5 cm2.
In general, LiI is used to supply I− for DSCs and the I− concentration in liquid electrolytes is not lower than 0.3 M [16–18]. Other alkali metal iodide salts, such as KI, RbI and CsI, dissolved poorly in acetonitrile and the I− concentration in liquid electrolytes is lower than 0.3 M, if one of these salts was added in acetonitrile according to this concentration. In this case, the photocurrent density for DSCs would decrease because part of the N3 dye cation could not be regenerated by I− at a lower concentration. However, the study on LiI-doped PEO shows that there is a stronger interaction between Li+ and ether oxygen of PEO [19], which could enhance the solubility of salts in PEO. Thus, it is thought that KI, RbI and CsI would dissolve well in PEO gel electrolytes. In our experiment, a significant improvement in solubility of the three salts was observed in
2.4. Measurements The photocurrent and photovoltage of the cells were measured in an electrochemical workstation analyzer (model LK9805) by a twoelectrode arrangement. A 500-W Xenon lamp served as light source. The light density was 100 mW/cm2, monitored by an irradiatometer. X-ray diffraction (XRD) spectra of KI, RbI, CsI crystals and of PEO gel with these three salts were tested by a X-ray diffractometer (Rigaku Ultrima Ш, Japan). The ionic conductivities of these gel electrolytes at room temperature were tested with a conductivity meter (model DDS11A; LIDA), which was calibrated with KCl solution (0.1 M) before the experiments. The limiting current of I−3 in each resulting PEO gel electrolytes was determined by linear sweep voltammetry using a cell sandwiching the PEO gel electrolyte. The cell is a symmetric configuration of FTO/Pt/gel electrolyte/Pt/FTO. The gap between two
Fig. 3. Conductivity for liquid electrolytes and PEO gel electrolytes containing different alkali metal iodide salts RI (R+ = Li+, Na+, K+, Rb+, Cs+).
X. Shen et al. / Solid State Ionics 179 (2008) 2027–2030
PEO gel electrolyte compared with liquid electrolytes. Fig. 1 shows the XRD spectra of KI, RbI, CsI crystals and PEO gel electrolytes containing the three salts with the concentration of 0.3 M. It was seen that the diffraction peaks for KI, RbI and CsI crystals have disappeared in the XRD spectra of these gel electrolytes, which indicates that the three salts are dissolved completely in PEO gel electrolytes at this concentration.
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Table 1 Influence of the cation size of the alkali metal iodide salts on triiodide ion diffusion coefficients in PEO gel electrolytes (I−: 0.3 M, I2: 0.03 M) Cation in electrolytes
Li+
Na+
K+
Rb+
Cs+
D (I−3) × 10− 6 (cm2/s)
1.98
2.14
1.89
1.69
1.57
3.2. Interaction between ether oxygen in PEO molecule and R+ ions In order to explain the complete dissolution of KI, RbI and CsI in PEO gel electrolytes, the interaction between alkali metal iodide RI (R+ = Li+, Na+, K+, Rb+, Cs+) salts and PEO molecule was investigated by FTIR spectroscopy. Fig. 2 shows the IR spectra of PEO gel electrolytes with alkali metal iodide RI (R+ = Li+, Na+, K+, Rb+, Cs+) salts and pure PEO gel. The peaks at 1100 cm− 1 and 962 cm− 1 [19] are due to the stretching of ether bond (C–O) of PEO and those at 918 cm− 1and 1040 cm− 1 are attributed to the stretching of C`N of acetonitrile. Obviously, the intensities of the C–O peak for PEO in the presence of R+ are higher than those of pure PEO gel, whereas the intensities of the C ≡ N peak are almost unchanged. The increase in intensity of the C–O peak is assigned to the formation of R–O bond between R+ and chains of PEO [19]. In this case, ionic bond of KI, RbI or CsI could be broken in PEO gel electrolyte. Consequently, these RI could dissolve in PEO gel to form a gel electrolyte suitable to be used in a quasi-solid-state DSCs. 3.3. Ionic conductivity of PEO gel electrolyte and liquid electrolyte with different RI The conductivities for PEO electrolytes with RI salts are investigated compared with those for the corresponding liquid electrolytes. Fig. 3 is the conductivity for liquid electrolytes and PEO gel electrolytes containing different alkali metal iodide salts RI (R+ = Li+, Na+, K+, Rb+, Cs+). As shown in Fig. 3, for LiI and NaI which can dissolve completely in acetonitrile, the conductivity ratio of gel electrolyte to liquid electrolyte remains constant at about 0.50. The variation in conductivity is attributed to the viscosity of the PEO gel electrolyte of LiI or NaI that is much higher than that of the corresponding liquid electrolyte. For KI, RbI and CsI which are poorly soluble in acetonitrile, although their conductivity behavior is also affected negatively by the viscosity factor, the value of the conductivity ratio increases significantly (0.85 for KI, 1.11 for RbI, 1.27 for CsI). The increase in the conductivity ratio is due to the significantly increased concentra-
Fig. 4. Voltammograms for the symmetrical electrochemical cell sandwiching PEO gel electrolyte with RI (R+ = Li+, Na+, K+, Rb+, Cs+) salts. Cell gap: 40 μm. Active surface area: 1 cm2. Scanning rate: 10 mV s− 1.
tion of free cation and I− ions in KI, RbI or CsI gel electrolyte caused by the complete solution of these three salts in PEO gel electrolytes, confirmed by the XRD spectra of their gel electrolytes (Fig. 1). 3.4. Behavior of I−3 diffusion in the gel electrolyte with different RI A cell with a symmetric configuration of FTO/Pt/gel electrolyte/Pt/ FTO was used to investigate the diffusion coefficients of I− or I−3 in gel electrolyte. Fig. 4 presents the characteristic linear sweep voltammetry curves of the PEO gel electrolyte with RI salts. When the applied voltage increased from 0 to 1 V, the current went up and reached a constant value corresponding to the limiting current. Apparent diffusion coefficients of the current-limiting species in gel electrolyte could be obtained according to the following equation [20]. Dapp ¼
Ilim d 2nFC
where n is the electron number per molecule, Ilim is the limiting current, d is the cell gap, F is the Faraday constant and C is the bulk concentration of electroactive species. Taking into account that the diffusion coefficient ratio of I− to I−3 species was 1:1–1.3 [21], the limiting current was mainly determined by species (I− or I−3) with a relatively low concentration and its value is proportional to the concentration of the species. For each PEO gel electrolyte containing RI, the concentration of I− is higher than that of I−3 (I2) as described in the Experimental section since RI dissolved completely in PEO gel electrolytes. Therefore the I−3 ions should be the current-limiting species. The apparent diffusion coefficient of I−3 ions for PEO gel electrolyte of RI (R+ = Li+, Na+, K+, Rb+, Cs+) is calculated and the value is listed in Table 1. It can be seen that I−3 diffusion in PEO gel electrolytes decreases with the increase of cationic radius of alkali metal iodide salts, except that of Li+. An explanation of the abnormal diffusion behavior of I−3 for Li+ is not given since our purpose is to investigate the effect of large cation on performance of DSCs.
Fig. 5. Photocurrent–voltage curves for dye-sensitized solar cell based on the PEO gel electrolyte containing RI (R+ = Li+, Na+, K+, Rb+, Cs+).
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experiment are useful for the selection of inorganic iodide salt for DSCs. Further work on improving the performance of DSCs is under way. 4. Conclusions In this work, the effect of cation of alkali metal iodide salts RI (R+ = Li+, Na , K+, Rb+or Cs+) in PEO gel electrolyte on performance of quasi-solidstate DSCs was investigated. The FF for DSCs is almost the same and Jsc changes irregularly and indistinctively, while the Voc increases with the increased radius of alkali metal cation (R+). The enhancement in Voc may be correlated with the decrease in recombination reaction between the conduction band electron and I−3 on the TiO2 surface. Relatively lower cost and better chemical stability make the KI more suitable as iodide salt for DSCs based on PEO gel electrolytes. +
Acknowledgements
Fig. 6. Open circuit voltage for dye-sensitized solar cell based on the PEO gel electrolyte containing RI (R+ = Li+, Na+, K+, Rb+, Cs+).
3.5. Photovoltaic performance The effect of cation of iodide organic salts on the photovoltaic performance of DSCs based on liquid electrolyte was first reported by Zaban et al. [22]. It was concluded that the large organic cation improved the open circuit voltage (Voc). In this study, we investigated the effect of cation of alkali metal iodide salts of 0.3 M in PEO gel electrolytes. Fig. 5 shows the photocurrent–voltage curves for cells fabricated with PEO gel electrolytes containing RI (R+ = Li+, Na+, K+, Rb+, Cs+) respectively under AM 1.5 irradiation. For these DSCs, the values of fill factor (FF) are almost the same and the short circuit photocurrent density (Jsc) changes irregularly and indistinctively. In contrast to the Jsc, the Voc increases from 590 mV to 670 mV with the increased ionic radius of R+ (from Li+ to Cs+), as shown in Fig. 6. The variation in Voc can be attributed to a decrease of the charge recombination rate on the TiO2/electrolyte interface. The injected electron in TiO2 can recombine with I−3 in the PEO gel electrolyte. The I−3 diffusion in PEO gel electrolytes decreases with the increase of the cationic radius of alkali metal iodide salts, except that of Li+, according to previous results (see Table 1). Consequently, the recombination reaction speed will decrease and the electron Fermi level (EF) of TiO2 will rise with the increase of cationic radius, leading to an increase in Voc since the Voc represents the difference between EF and EI−/I3−. The increase in EF of TiO2 with the decreased speed of the recombination reaction between the conduction band electrons and I−3 has been reported in the literature [11,17]. Although I−3 diffusion in PEO gel electrolyte with Li+ is slower than that of Na+ (see Table 1), it can be seen that Voc for Li+ is lower that of Na+. This is due to the fact that Li+ with the smallest radius among R+ can penetrate across the N3 dye layer and be absorbed directly on the surface of TiO2 particles [22], resulting in the EF of TiO2 in the presence of Li+ that is lower than that of Na+. The conversion efficiency for these DSCs is lower than that reported in Refs. [9,18]. However, the results obtained in our
The authors gratefully acknowledge the financial support of the Key Laboratory of Advanced Textile Materials and Manufacturing Technology (No. 2006005) (Zhejiang Sci-tech University), the Key Project of Science and Technology Research of Ministry of Education (No. 208089), and the Natural Science Foundation of Hubei Province (No. 2007ABA075). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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B. O'Regan, B.M. Grätzel, Nature 353 (1991) 737. M. Grätzel, Nature 414 (2001) 338. M. Grätzel, J. Photochem. Photobiol.C: Chem. 164 (2004) 3–14. Y.J. Kim, J.H. Kim, M. -S. Kang, M.J. Lee, Y. Won, J.C. Lee, Y.S. Kang, Adv. Mater. 16 (2004) 1753. A.F. Nogueira, C. Longo, M. -A. De Paoli, Coordin. Chem. Rev. 248 (2004) 1455. P. Wang, S.M. Zakeeruddin, P. Comte, I. Exnar, M. Grätzel, J. Am. Chem. Soc. 125 (2003) 1166. J. -B. Xia, F. -Y. Li, C. -H. Huang, J. Zhai, L. Jiang, Sol. Energ. Mat. Sol. C. 90 (2006) 944. Y. -J. Ren, Z. -C. Zhang, S. -B. Fang, M. -Z. Yang, S. -M. Cai, Sol. Energ. Mat. Sol. C. 71 (2002) 253. X. Zhang, H. Yang, H. -M. Xiong, F.-Y. Li, Y. -Y. Xia, J. Power Sources 160 (2006) 1451–1455. Z. Lan, J.-H. Wu, D.-B. Wang, S.-C. Hao, J.-M. Lin, Y.-F. Huang, Sol. Energy 80 (2006) 1483. Z. Lan, J.-H. Wu, D. -B. Wang, S.-C. Hao, J.-M. Lin, Y.-F. Huang, Sol. Energy 81 (2007) 117. Z. Lan, J.-H. Wu, J.-M. Lin, M.-L. Huang, J. Power Sources 164 (2007) 921. F.-J. Li, F.-Y. Cheng, J.-F. Shi, F.-S. Cai, M. Liang, J. Chen, J. Power Sources 165 (2007) 911. S. Ito, S.M. Zakeeruddin, R. Humphry-Baker, P. Liska, R. Charvet, P. Comte, M.K. Nazeeruddin, P. Pechy, M. Takata, H. Miura, S. Uchida, M. Grätzel, Adv. Mater. 18 (2006) 1202. T. Horiuchi, H. Miura, K. Sumioka, S. Uchida, J. Am. Chem. Soc. 126 (2004) 12218. S.Y. Huang, G. Schichthörl, A.J. Nozik, M. Grätzel, A.J. Frank, J. Phys. Chem. B. 101 (1997) 2576. M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, P. Liska, N. Vlachopoulos, M. Grätzel, J. Phys. Chem. B. 115 (1993) 6382. W. Kubo, K. Murakoshi, T. Kitamura, S. Yoshida, M. Haruki, K. Hanabusa, H. Shirai, Y. Wada, S. Yanagida, J. Phys. Chem. B. 105 (2001) 12809. X.M. Sui, C.L. Shao, Y.C. Liu, Polymer. 48 (2007) 1459. S. Sakaguchi, H. Ueki, T. Kato, T. Kado, R. Shiratuchi, W. Takashima, K. Kaneto, S. Hayase, J. Photochem. Photobiol. A: Chem. 164 (2004) 117. A. Usami, Chem. Phys. Lett. 292 (1998) 223. A. Zaban, S. Ferrere, B.A. Gregg, J. Phys. Chem. B. 102 (1998) 452.
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Quasi-solid-state dye-sensitized solar cells based on gel electrolytes containing different alkali metal iodide salts Xiaolin Shen a, Weilin Xu b,⁎, Jie Xu b, Guijie Liang b, Hongjun Yang b, Mu Yao c a b c
School of Materials Science & Engineering, Xi'an Jiaotong University, Xi'an 710049, China Wuhan University of Science & Engineering, Wuhan 430073, China Xi'an Polytechnic University, Xi'an 710049, China
A R T I C L E
I N F O
Article history: Received 19 April 2008 Received in revised form 20 June 2008 Accepted 30 June 2008 Keywords: Quasi-solid-state dye-sensitized solar cell Alkali metal iodide salts Open current voltage Solubility PEO gel electrolyte
A B S T R A C T Gel electrolytes were prepared by adding different alkali metal iodide salts RI (R+ = Li+, Na+, K+, Rb+ or Cs+) and I2 into acetonitrile gelated with poly(ethylene-oxide) (PEO). KI, RbI and CsI, poorly soluble in liquid electrolyte, can dissolve completely in PEO gel electrolyte due to a strong interaction of cation and PEO chains. All gel electrolytes exhibit high conductivity in the range of 10− 3 S/cm and contain I− with concentration of 0.3 M. The effect of R+ in PEO gel electrolyte on the performance of quasi-solid-state dyesensitized solar cells (DSCs) was investigated. The results showed that the open circuit voltage (Voc) increases with the increased radius of alkali metal cation. It is explained by the rise of electron Fermi level (EF) of TiO2 caused by a decrease in I−3 diffusion with the increase of radius of R+. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSCs) based on TiO2 porous films [1,2] are one of the potential low-cost alternatives to traditional silicon solar cell and a maximal 1ight-electric power conversion efficiency of 11% [3] was already achieved with liquid electrolyte containing a (I−/I−3) redox couple in organic solution, such as acetonitrile, propylene carbonate and ethylene carbonate. However, the volatility of organic solvent leads to a decrease in performance of DSCs. This drawback limits practical application of DSCs. Great efforts have been devoted to overcome this problem. Substituting the gel liquid electrolyte for liquid electrolyte [4] is an efficient way to prevent the leakage of electrolyte [5–10]. Recently, several types of polymers [11–13] as gelator have been used in gel electrolyte for DSCs. For these electrolytes, the concentration of I− ions is sufficient for reducing the oxidated sensitizer of DSC during operations, if the I− ions are supplied by only adding the LiI or NaI in the gel electrolyte. However, it is interesting that the organic iodide salts containing large cation, such as N-methyl pyridine iodide [11] or 1, 3-dimethyl-3-imidazolinium iodine [7] are also added into gel electrolyte to supply parts of I− ions. The same strategy was adopted for DSCs based on liquid electrolytes [14,15]. It implies that large cation in gel electrolytes contribute to improve the performance of ⁎ Corresponding author. Tel./fax: +86 27 87426559. E-mail address: [email protected] (W. Xu). 0167-2738/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.06.027
quasi-solid-state DSCs. In addition, LiI commonly used for DSCs is deliquescent and more expensive than other alkali metal iodide salts, such as NaI, KI. In this paper, a series of poly (ethylene-oxide) (PEO) gel electrolytes containing different alkali metal iodide salts RI (R+ = Li+, Na+, K+, Rb+, Cs+) were employed in DSC to investigate the effect of large inorganic cation on the performance of quasi-solid-state DSCs. PEO was selected as gelator of the gel electrolytes in our experiment due to a high fill factor (FF) for DSCs based on this polymer [7,8,12,13]. It is well known that KI, RbI and CsI with large cation with respect to radius of Li+ were hardly employed in a liquid electrolyte for DSCs because of their poor solubility in organic solution. PEO has a great deal of polar ether oxygen in its molecular chains, which could interact with the cation of the alkali metal iodide salts [5]. It is expected that the solubility of RI with large cation would be improved in PEO gel electrolyte by the interaction of the R+ and ether oxygen and thus photovoltaic performance of DSCs based on PEO gel electrolytes could be obtained by using alkali metal iodide salts with large cation. 2. Experimental 2.1. Materials Alkali metal iodide salts RI (R+ = Li+, Na+, K+, Rb+, Cs+) (A.R. grade) and poly(ethylene-oxide) (PEO, Mη = 1.5 × 106, 99%) were used as received. Conducting glass substrate (FTO glass, fluorine doped tin
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X. Shen et al. / Solid State Ionics 179 (2008) 2027–2030
Fig. 1. XRD spectra of KI, RbI, CsI crystals and the PEO gel electrolytes containing the three salts with the concentration of 0.3 M.
oxide over-layer, sheet resistance 15 Ω cm− 2) was purchased from Libbey Owens Ford Industries, USA. Sensitizing dye cis-bis(isothiocyanate)-bis(2,2′-bipyri-dine-4,4′-dicarboxylate) ruthenium (N3) was purchased from SOLARO-NIX. Other solvents and reagents were used as received.
Fig. 2. IR spectra of PEO gel electrolytes with alkali metal iodide RI (R+ = Li+, Na+, K+, Rb+, Cs+) salts and the pure PEO gel.
electrodes is 40 μm controlled by a spacer. The electrodes had an active area of 1 cm2. FTIR spectra of PEO gel electrolytes were measured on a Tensor 27 FTIR spectrometer (Bruker). 3. Results and discussion
2.2. Preparation of gel electrolytes 3.1. Solubility of KI, RbI and CsI in PEO gel electrolyte A series of mixtures were prepared by adding 1.2 × 10− 3 mol RI (R+ = Li+, Na+, K+, Rb+, Cs+), 1.2 × 10− 4 mol I2 and 0.8 g PEO into 4 ml acetonitrile. The gel electrolytes were obtained until the mixtures became homogenous and unflowable. A series of liquid electrolytes with RI without adding PEO were also prepared according to the above ratio as control. 2.3. Assembly of the DSCs The TiO2 electrodes were prepared by spreading TiO2 (P25, Degussa) colloidal paste on a FTO glass with a glass rod as squeegee and a spacing tape to make uniform films. After drying, the coated substrate was sintered at 450 °C in air for 0.5 h and cooled to 80 °C, and then immersed in dye sensitization solution of 0.3 mM N3 in ethanol for 12 h. The film thickness is about 10 μm. The prepared PEO gel electrolytes was casted onto the TiO2 electrode sensitized by N3 and then a platinum (Pt) counter electrode was pressed on top of the TiO2 electrode to form a DSC. Effective area of the cell was 0.5 cm2.
In general, LiI is used to supply I− for DSCs and the I− concentration in liquid electrolytes is not lower than 0.3 M [16–18]. Other alkali metal iodide salts, such as KI, RbI and CsI, dissolved poorly in acetonitrile and the I− concentration in liquid electrolytes is lower than 0.3 M, if one of these salts was added in acetonitrile according to this concentration. In this case, the photocurrent density for DSCs would decrease because part of the N3 dye cation could not be regenerated by I− at a lower concentration. However, the study on LiI-doped PEO shows that there is a stronger interaction between Li+ and ether oxygen of PEO [19], which could enhance the solubility of salts in PEO. Thus, it is thought that KI, RbI and CsI would dissolve well in PEO gel electrolytes. In our experiment, a significant improvement in solubility of the three salts was observed in
2.4. Measurements The photocurrent and photovoltage of the cells were measured in an electrochemical workstation analyzer (model LK9805) by a twoelectrode arrangement. A 500-W Xenon lamp served as light source. The light density was 100 mW/cm2, monitored by an irradiatometer. X-ray diffraction (XRD) spectra of KI, RbI, CsI crystals and of PEO gel with these three salts were tested by a X-ray diffractometer (Rigaku Ultrima Ш, Japan). The ionic conductivities of these gel electrolytes at room temperature were tested with a conductivity meter (model DDS11A; LIDA), which was calibrated with KCl solution (0.1 M) before the experiments. The limiting current of I−3 in each resulting PEO gel electrolytes was determined by linear sweep voltammetry using a cell sandwiching the PEO gel electrolyte. The cell is a symmetric configuration of FTO/Pt/gel electrolyte/Pt/FTO. The gap between two
Fig. 3. Conductivity for liquid electrolytes and PEO gel electrolytes containing different alkali metal iodide salts RI (R+ = Li+, Na+, K+, Rb+, Cs+).
X. Shen et al. / Solid State Ionics 179 (2008) 2027–2030
PEO gel electrolyte compared with liquid electrolytes. Fig. 1 shows the XRD spectra of KI, RbI, CsI crystals and PEO gel electrolytes containing the three salts with the concentration of 0.3 M. It was seen that the diffraction peaks for KI, RbI and CsI crystals have disappeared in the XRD spectra of these gel electrolytes, which indicates that the three salts are dissolved completely in PEO gel electrolytes at this concentration.
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Table 1 Influence of the cation size of the alkali metal iodide salts on triiodide ion diffusion coefficients in PEO gel electrolytes (I−: 0.3 M, I2: 0.03 M) Cation in electrolytes
Li+
Na+
K+
Rb+
Cs+
D (I−3) × 10− 6 (cm2/s)
1.98
2.14
1.89
1.69
1.57
3.2. Interaction between ether oxygen in PEO molecule and R+ ions In order to explain the complete dissolution of KI, RbI and CsI in PEO gel electrolytes, the interaction between alkali metal iodide RI (R+ = Li+, Na+, K+, Rb+, Cs+) salts and PEO molecule was investigated by FTIR spectroscopy. Fig. 2 shows the IR spectra of PEO gel electrolytes with alkali metal iodide RI (R+ = Li+, Na+, K+, Rb+, Cs+) salts and pure PEO gel. The peaks at 1100 cm− 1 and 962 cm− 1 [19] are due to the stretching of ether bond (C–O) of PEO and those at 918 cm− 1and 1040 cm− 1 are attributed to the stretching of C`N of acetonitrile. Obviously, the intensities of the C–O peak for PEO in the presence of R+ are higher than those of pure PEO gel, whereas the intensities of the C ≡ N peak are almost unchanged. The increase in intensity of the C–O peak is assigned to the formation of R–O bond between R+ and chains of PEO [19]. In this case, ionic bond of KI, RbI or CsI could be broken in PEO gel electrolyte. Consequently, these RI could dissolve in PEO gel to form a gel electrolyte suitable to be used in a quasi-solid-state DSCs. 3.3. Ionic conductivity of PEO gel electrolyte and liquid electrolyte with different RI The conductivities for PEO electrolytes with RI salts are investigated compared with those for the corresponding liquid electrolytes. Fig. 3 is the conductivity for liquid electrolytes and PEO gel electrolytes containing different alkali metal iodide salts RI (R+ = Li+, Na+, K+, Rb+, Cs+). As shown in Fig. 3, for LiI and NaI which can dissolve completely in acetonitrile, the conductivity ratio of gel electrolyte to liquid electrolyte remains constant at about 0.50. The variation in conductivity is attributed to the viscosity of the PEO gel electrolyte of LiI or NaI that is much higher than that of the corresponding liquid electrolyte. For KI, RbI and CsI which are poorly soluble in acetonitrile, although their conductivity behavior is also affected negatively by the viscosity factor, the value of the conductivity ratio increases significantly (0.85 for KI, 1.11 for RbI, 1.27 for CsI). The increase in the conductivity ratio is due to the significantly increased concentra-
Fig. 4. Voltammograms for the symmetrical electrochemical cell sandwiching PEO gel electrolyte with RI (R+ = Li+, Na+, K+, Rb+, Cs+) salts. Cell gap: 40 μm. Active surface area: 1 cm2. Scanning rate: 10 mV s− 1.
tion of free cation and I− ions in KI, RbI or CsI gel electrolyte caused by the complete solution of these three salts in PEO gel electrolytes, confirmed by the XRD spectra of their gel electrolytes (Fig. 1). 3.4. Behavior of I−3 diffusion in the gel electrolyte with different RI A cell with a symmetric configuration of FTO/Pt/gel electrolyte/Pt/ FTO was used to investigate the diffusion coefficients of I− or I−3 in gel electrolyte. Fig. 4 presents the characteristic linear sweep voltammetry curves of the PEO gel electrolyte with RI salts. When the applied voltage increased from 0 to 1 V, the current went up and reached a constant value corresponding to the limiting current. Apparent diffusion coefficients of the current-limiting species in gel electrolyte could be obtained according to the following equation [20]. Dapp ¼
Ilim d 2nFC
where n is the electron number per molecule, Ilim is the limiting current, d is the cell gap, F is the Faraday constant and C is the bulk concentration of electroactive species. Taking into account that the diffusion coefficient ratio of I− to I−3 species was 1:1–1.3 [21], the limiting current was mainly determined by species (I− or I−3) with a relatively low concentration and its value is proportional to the concentration of the species. For each PEO gel electrolyte containing RI, the concentration of I− is higher than that of I−3 (I2) as described in the Experimental section since RI dissolved completely in PEO gel electrolytes. Therefore the I−3 ions should be the current-limiting species. The apparent diffusion coefficient of I−3 ions for PEO gel electrolyte of RI (R+ = Li+, Na+, K+, Rb+, Cs+) is calculated and the value is listed in Table 1. It can be seen that I−3 diffusion in PEO gel electrolytes decreases with the increase of cationic radius of alkali metal iodide salts, except that of Li+. An explanation of the abnormal diffusion behavior of I−3 for Li+ is not given since our purpose is to investigate the effect of large cation on performance of DSCs.
Fig. 5. Photocurrent–voltage curves for dye-sensitized solar cell based on the PEO gel electrolyte containing RI (R+ = Li+, Na+, K+, Rb+, Cs+).
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experiment are useful for the selection of inorganic iodide salt for DSCs. Further work on improving the performance of DSCs is under way. 4. Conclusions In this work, the effect of cation of alkali metal iodide salts RI (R+ = Li+, Na , K+, Rb+or Cs+) in PEO gel electrolyte on performance of quasi-solidstate DSCs was investigated. The FF for DSCs is almost the same and Jsc changes irregularly and indistinctively, while the Voc increases with the increased radius of alkali metal cation (R+). The enhancement in Voc may be correlated with the decrease in recombination reaction between the conduction band electron and I−3 on the TiO2 surface. Relatively lower cost and better chemical stability make the KI more suitable as iodide salt for DSCs based on PEO gel electrolytes. +
Acknowledgements
Fig. 6. Open circuit voltage for dye-sensitized solar cell based on the PEO gel electrolyte containing RI (R+ = Li+, Na+, K+, Rb+, Cs+).
3.5. Photovoltaic performance The effect of cation of iodide organic salts on the photovoltaic performance of DSCs based on liquid electrolyte was first reported by Zaban et al. [22]. It was concluded that the large organic cation improved the open circuit voltage (Voc). In this study, we investigated the effect of cation of alkali metal iodide salts of 0.3 M in PEO gel electrolytes. Fig. 5 shows the photocurrent–voltage curves for cells fabricated with PEO gel electrolytes containing RI (R+ = Li+, Na+, K+, Rb+, Cs+) respectively under AM 1.5 irradiation. For these DSCs, the values of fill factor (FF) are almost the same and the short circuit photocurrent density (Jsc) changes irregularly and indistinctively. In contrast to the Jsc, the Voc increases from 590 mV to 670 mV with the increased ionic radius of R+ (from Li+ to Cs+), as shown in Fig. 6. The variation in Voc can be attributed to a decrease of the charge recombination rate on the TiO2/electrolyte interface. The injected electron in TiO2 can recombine with I−3 in the PEO gel electrolyte. The I−3 diffusion in PEO gel electrolytes decreases with the increase of the cationic radius of alkali metal iodide salts, except that of Li+, according to previous results (see Table 1). Consequently, the recombination reaction speed will decrease and the electron Fermi level (EF) of TiO2 will rise with the increase of cationic radius, leading to an increase in Voc since the Voc represents the difference between EF and EI−/I3−. The increase in EF of TiO2 with the decreased speed of the recombination reaction between the conduction band electrons and I−3 has been reported in the literature [11,17]. Although I−3 diffusion in PEO gel electrolyte with Li+ is slower than that of Na+ (see Table 1), it can be seen that Voc for Li+ is lower that of Na+. This is due to the fact that Li+ with the smallest radius among R+ can penetrate across the N3 dye layer and be absorbed directly on the surface of TiO2 particles [22], resulting in the EF of TiO2 in the presence of Li+ that is lower than that of Na+. The conversion efficiency for these DSCs is lower than that reported in Refs. [9,18]. However, the results obtained in our
The authors gratefully acknowledge the financial support of the Key Laboratory of Advanced Textile Materials and Manufacturing Technology (No. 2006005) (Zhejiang Sci-tech University), the Key Project of Science and Technology Research of Ministry of Education (No. 208089), and the Natural Science Foundation of Hubei Province (No. 2007ABA075). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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