- Email: [email protected]
Inorganic Oxide Electrolyte for High Efficiency Quasi-solid-state Dye-sensitized Solar Cell
Available online at www.sciencedirect.com
Procedia Engineering 36 (2012) 13 – 18
IUMRS-ICA 2011
A Novel Composite Polysaccharide/Inorganic Oxide Electrolyte for High Efficiency Quasi-solid-state Dyesensitized Solar Cell Ying Yang a,*, Xue-yi Guoa, Xing-zhong Zhaob a
School of Metallurgical Science & Engineering, Central South University, Changsha 410083, China b Department of Physics and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Wuhan University, Wuhan 430072, China
Abstract A new composite polymer electrolyte consisting of low-molecular mass polysaccharide (agarose) filled with titanium oxide and containing LiI and I2 was developed. The introduction of the inorganic filler (TiO2) into the agarose polymer matrix changes the thermal and ionic conductive properties of host polymer electrolyte. The polysaccharide electrolyte with different inorganic filler TiO2 concentrations (0-30 wt %) is studied systematically by differential scanning calorimetry (DSC) and the AC impedance spectra. Upon addition of the inorganic oxide, the glass transition temperature (Tg) of the electrolyte decreases within the range of 0-2.5 wt % TiO2 changed electrolytes, which results in high conductivity in these electrolytes. Then the Tg increases with the TiO2 content until 30 wt % and the ionic conductivity of the electrolyte decreases in this concentration range. From the electrochemical analysis, it is observed that the electron lifetime in TiO2 of DSSC decreases with inorganic filler contents, which results in lower Voc in the high TiO2 content added DSSC. Based on these results, the optimized efficiency of titania added DSSC is 4.74% at 2.5 wt % titania concentration.
© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of MRS-Taiwan Keywords: Composite agarose/NMP/titania/I-/I3- electrolyte; polymer electrolyte; inorganic nanoparticle filler; quasi-solid state; dye sensitized solar cell
* Corresponding author. Tel.: +86-731-88877863; Fax: +86-737-88836207. E-mail address: [email protected].
1877-7058 © 2012 Published by Elsevier Ltd. doi:10.1016/j.proeng.2012.03.004
14
Ying Yan et al. / Procedia Engineering 36 (2012) 13 – 18
1. Introduction Dye-sensitized solar cells have attracted great scientific and technological interest as potential alternatives to classical photovoltaic devices due to their high conversion efficiency and low production cost [1, 2]. However, the stability and long-term operation of the cell are affected by solvent evaporation or leakage of liquid electrolyte. Thus, the commercial exploitation of these devices needs the replacement of the liquid electrolyte by a polymer gel or solid electrolyte. To improve the performance and stability of the DSSC, much effort has been directed towards development of non- volatile electrolytes meeting the stability requirements for outdoor applications in the last decade [3-5]. Impressive improvements have been achieved on the photovoltaic performance and the stability of devices [6-8]. Recent researchers on polymer electrolyte proved that the incorporation of inorganic nanoparticle filler in polymer electrolyte yields higher ionic conductivity [9, 10]. The filler provides a solid like support matrix, allowing the amorphous polymer to maintain its liquid-like characteristics in terms of fast ionic mobility at the microscopic level. This could help in enhancing the cell performance, and further could increase the stability and lifetime of these devices [11]. Polysaccharide (agarose and ț-carrageenan), can form an elastic solid containing excess of water [12], the electrochemical and photochemical reactions can take place in the solid. The molecular diffusion [13] and ionic conductivity in this solid are almost the same as in liquid water [14]. Therefore, this interesting solid containing a large excess liquid can be used as an electrolyte medium instead of water for conventional electrochemical measurement and offer a new heterogeneous medium for photoenergy conversion. On the other hand, polysaccharide is considered to be a good polymer matrix to form crosslinking networks in the polymer electrolyte with other components, due to rich hydroxyl groups in its molecule structure. The formation of continuous networks in the electrolyte is beneficial for the ion transportation, thus the ionic conductivity and the performance of the corresponding DSSC will be improved. In this paper, we report on the preparation of a homogeneous agarose based gel electrolyte simply by directly dissolving agarose into 1-methyl-2-pyrrolidinone (NMP). The agarose polymer electrolytes with different inorganic filler TiO2 concentrations (0-30 wt %) were prepared and the photoelectric performances of the corresponding DSSCs as a function of TiO2 content are studied systematically. The thermal (DSC) and electrochemical impedance studies reveal the pivotal influence exerted by the agarose and TiO2 in the polymer electrolyte on the ionic conductivity and photovoltaic response of the DSSC. 2. Experiments 2.1. Sample preparation Agarose (Mw=5000g mol-1) was purchased from SCRC, China and the molecular structure of the agarose in this study is presented in Fig. 1. Lithium iodide (LiI, 99%) was purchased from Acros. TiO2 (P25, 20–30 nm) was purchased from Degussa AG, Germany. Iodine (I2) and other common reagents and organic solvents were purchased from SCRC, China. For the composite electrolyte preparation, 0.2g agarose was added into 10g NMP and stirred at 80 oC in the hermetic brown flask in silicone bath for 4 h. Then the inorganic nanoparticels (TiO2, P25) with the concentration range 0~30 wt % of the polymer were slowly introduced and stirred for another 2 h. After that, solid LiI, and I2 with LiI/I2=10:1 (molar ratio, LiI: 0.08143g; I2: 0.01542g) were added to the above polymer solutions and stirred continuously in the hermetic brown flask until a homogeneous viscous liquid was formed.
Ying Yan et al. / Procedia Engineering 36 (2012) 13 – 18
Nanoporous TiO2 electrodes were prepared according to previous report [15]. The assembly of the dye-sensitized solar cell was according to the following steps. The counter-electrode is platinum plate. The electrolyte was sandwiched between the photoelectrode and platinized counter electrode and was heated under 73 ƕC to contraol the evaporation of the solvent in oven until forming very viscous gel. 2.2. Sample characterization Differential scanning calorimetry (DSC) of the polymer electrolytes with different inorganic filler TiO2 concentrations were carried out with Netzsch DSC 200PC (Germany). The samples were heated at a rate of 10 ƕC miní1 from í100 to 200 ƕC under nitrogen flow. The ionic conductivity of polymer electrolytes was obtained using an electrochemical cell consisting of the electrolyte film sandwiched between two platinum blocking electroldes. The impedance measurements of the electrolyte films were carried out on AC impedance analyzer (Agilent 4294A, USA) from 40 Hz to 1 MHz with signal amplitude of 10mV. The electrochemical impedance measurements of the cells with different TiO2 concentrations were carried out on the CHI 660C electrochemical system (Chenghua, Shanghai), over the frequency range of 0.001–105 Hz and the perturbation voltage of 10 mV. These DSSCs were kept under the light illumination and the forward bias of í0.6 V was applied on all the samples. Photocurrent–voltage (I–V) measurements were recorded using a computer-controlled Keithley 2400 digital source meter unit (USA) by illuminating the cell through the active photoelectrode under Oriel solar simulator (Oriel, 91192). The intensity of the incident light was calibrated by a Si-1787 photodiode (spectral response range: 320 to 730 nm). The power of the incident white light from the solar simulator was 73 mWcmí2. The active DSSC area was controlled at 0.25 cm2 by a mask. All of the measurements mentioned above were taken at ambient environment. 3. Experiments Results and Discussion 3.1. DSC Characteristics of electrolytes containing different TiO2 nanoparticle concentrations Figure 1 shows the relationship between the Tg and the filler TiO2 nanoparticle contents (0, 2.5, 5, 7.5, 10, 15, 30 wt %) in the agarose electrolytes. Tg values of the electrolytes with different TiO2 contents are variable in the range of -87~-61 oC and they follow a clear trend with filler concentration. The Tg values start from -77.3 oC, gradually decrease to -86.3 oC at 2.5 wt % TiO2 content and then increase continuously to -60.8 oC as the filler concentration increasing to 30 wt %.
Fig. 1. Glass transition temperature, Tg, of agarose based polymer electrolytes depending on the TiO2 nano-particle concentration.
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Ying Yan et al. / Procedia Engineering 36 (2012) 13 – 18
As Tg is much lower in the lower TiO2 concentrations (0-5 wt %) than that in the higher TiO2 concentrations (>5 wt %), the amorphous phase becomes more flexible and the ionic conductivity should be enhanced in this TiO2 concentration range (0-5 wt %). The observed lower values of Tg in the 0-5 wt % TiO2 concentration range is due to the interaction of the polymer with the filler particles, which hinders the inter/intra-molecule cross-linking interactions within/between polymers [16]. Meanwhile, the Tg increases rapidly in the higher TiO2 nanoparticle concentrations (exceeding 5 wt %), which may be due to the possible formation of crystalline phase if the abundant filler particle acts as a nucleation center of the crystalline polymer phase [17]. 3.2. Conductivity characteristics of electrolytes containing different TiO2 nanoparticle concentrations The dependence of ionic conductivity of the agarose electrolytes on TiO2 concentration at 30 oC is represented in Fig. 2. With an increase in TiO2 contents in agarose electrolyte systems, the ionic conductivities increase, reach a maximum value of 5.12×10-4 S cm-1at a filler content of 5 wt %, and then decrease as the filler increasing to 30 wt %.
Fig. 2. Conductivity isotherms determined at 30 oC for agarose based polymer electrolytes depending on TiO2 nano-particle concentration.
At low TiO2 contents (5 wt %), the interaction between polymer and nanoparticles form an interface between these two components, which acts as bridge for ion transport in the electrolytes. This would contribute mainly to enhancement of total conductivity. However, at high TiO2 contents (>5 wt %), continuous non-conductive phase built up by the ceramic fillers would block ion transport in the electrolytes, leading to low ionic conductivity [18]. 3.3. Photovoltaic performance and electrochemical characterization of the agarose electrolyte based DSSC with different TiO2 nanoparticle concentrations The photovoltaic performance of the DSSCs is optimized for different concentrations of TiO2 nanoparticle. The dependence of short-circuit current (Jsc), open-circuit voltage (Voc), the fill factor (FF), and the photo-to-current conversion efficiency (Ș) on the TiO2 concentration is shown in Fig. 3.
Ying Yan et al. / Procedia Engineering 36 (2012) 13 – 18
Fig. 3. Variation of (a) photo-to-current conversion efficiency (Ș), (b) fill factor (FF), (c) open-circuit voltage Voc, and (d) short-circuit photocurrent density Jsc with TiO2 nano-particle concentrations in the agarose polymer electrolytes.
The Jsc of the DSSC increases appreciably from 7.76 to 11 mA cm-2 with the increase of TiO2 nanoparticles from 0 to 2.5 wt % and decreases with the further increase. While the Voc decreases with increasing TiO2 concentration particularly in the range of 0- 2.5 wt %. The best efficiency (Ș) in the TiO2 nanoparticle concentration studied is 4.74% at 2.5 wt %. This shows that 2.5 wt % of nano- TiO2 is enough for getting good performance of the DSSC. Meanwhile, to further understand the electron and ionic transport in the DSSCs with different nanoTiO2 addition, the electrochemical impedance spectroscopy was studied, and the results are shown in Fig. 4. The figure is given in the form of Bode (Fig. 4a) plots. Figure 4b exhibits the relationship between the TiO2 filler concentration and the electron lifetime. There are three peaks in Bode phase plots (Fig. 4a) which are attributed to the charge-transfer processes (I3í +e=3Ií) at the Pt/electrolyte interface (frequency regions 103-105 Hz); the charge recombination reaction process (I3í +2ecbí(TiO2) =3Ií) occurred at nanocrystalline TiO2/electrolyte interface (frequency regions 1-103 Hz); and the Ií/I3- diffusion process occurred in the electrolyte (frequency regions 10-1-1 Hz) [19, 20]. In the Bode phase plots, the characteristic middle frequencies are inversely proportional to the electron lifetime (Ȧ = IJí1) in TiO2 according to the EIS theory [21, 22]. As seen in Fig. 4a the characteristic middle frequency peaks of the DSSCs with different content of TiO2 shifts to higher frequency as TiO2 amounts increasing. The middle frequency peak, which is located at 3.98 Hz of the 0 wt% TiO2 added DSSC, moves gradually to 11.9 Hz of the 30 wt % TiO2 added DSSC. In Fig. 4b, the electron lifetime decreases with the increase in TiO2 content, this indicated a shorter electron lifetime and more back-reaction in the higher TiO2 content based DSSC. The increased electron back-recombination implies lower Voc of the DSSC. 4. Conclusions Agarose based polymer electrolytes for DSSC with different p inorganic filler TiO2 concentrations were studied. The initial increase in TiO2 concentrations improves the conductivity of the polymer electrolytes, while further increase in TiO2 concentrations makes the conductivity decrease. The maximum energy conversion efficiency of 4.74% was achieved at 2.5 wt % TiO2 based DSSC. The Voc of the cells with these polymer electrolytes decrease with TiO2 concentration in electrolyte. This result is closely related to the electron lifetime in the cells according to electrochemical impedance studies.
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Ying Yan et al. / Procedia Engineering 36 (2012) 13 – 18
Acknowledgements We acknowledge the financial support of the National Nature Science Foundation of China (61006047 and 50125309) and the financial support from the Ministry of Science and Technology of China through Hi-Tech plan (funding No. 2006AA03Z347).
References [1] O’Regan B., M. Grätzel. Nature, 1991, 353: 737-740. [2] Grätzel M. Nature, 2001, 414: 338-344. [3] Noguei A. F., Longo C., De Paoli M.-A., Coord. Chem. Rev., 2004, 248:1455-1468. [4] Bai Y., Cao Y-M., Zhang J., Wang M-K., Li R-Z., . Wang P, Zakeeruddin S. M., Grätzel M. Nat. Mater. 2008, 7:626-630.. [5] Cerneaux S., Zakeeruddin S. M., Pringle J. M., Cheng Y-B., Grätzel M., Spiccia L. Adv. Funct. Mater. 2007, 17: 3200-3206. [6] Wu J-H., Hao S-C., Lan Z., Lin J-M., Huang M-L., Huang Y-F., Fang L-Q., Yin S., Sato T. Adv. Funct. Mater., 2007, 17:2645-2652. [7] Wu J-H., Hao S-C., Lan Z., Lin J-M., Huang M-L., Huang Y-F. Li P-J., Yin S., Sato T. J. Am. Chem. Soc. 2008, 130:1156811569. [8] Wang P., Zakeeruddin S. M., Moser J. E., Nazeeruddin M. K., Sejugychi T., Grätzel M. Nat. Mater. 2003, 2: 402-407. [9] Khan S. A., Baker G. L., Colson S. Chem. Mater. 1994, 6: 2359-2363. [10] Croce F., Appetecchi G. B., Persi L., Scrosati B. Nature, 1998, 394: 6-458. [11] Han H. W., Liu W., Zhang J., Zhao X. Z. Adv. Funct. Mater. 2005, 15: 1940-1944. [12] Kaneko M., Mochizuki N., Suzuki K., Shiroishi H., Ishikawa K. Chem. Lett. 2002, 31: 530-534. [13] Ueno H., Kaneko M. J. Electroanal. Chem. 2004, 568: 87-92. [14] Ueno H., Endo Y., Kaburagi Y., Kaneko M. J. Electroanal. Chem. 2004, 570: 95-100. [15] Yang Y., Zhou C-H., Xu S., Hu H., Chen B-L., Zhang J., Wu S-J., Liu W., Zhao X-Z. J. Power Sources. 2008, 185: 14921498. [16] Ash B. J., Schadler L. S., Siegel R. W. Mater. Lett. 2002, 55: 83-87. [17] Choi B. K., Kim Y. W., Shin K. H. J. Power Sources. 1997, 68: 357-360. [18] Wen Z-Y., Itoh T., Ikeda M., Hirata N., Kubo M., Yamamoto O. J. Power Sources. 2000, 90: 20-26. [19] Fuke N., Fukui A., Komiya R., Islam A., Chiba Y., Yanagida M., Yamanaka R., Han L. Chem. Mater. 2008, 20: 4974-4979. [20] Wang M., Lin Y., Zhou X.W., Xiao X.R., Yang L., Feng S.J., Li X.P. Mater. Chem. Phys. 2008, 107: 61-66. [21] Wang Q., Moser J.E., Grätzel M. J. Phys. Chem. B. 2005, 104: 2044-2052. [22] Kern R., Sastrawan R., Ferber J., Stangl R., Luther. J. Electrochim. Acta. 2002, 47: 4213-4225.
Procedia Engineering 36 (2012) 13 – 18
IUMRS-ICA 2011
A Novel Composite Polysaccharide/Inorganic Oxide Electrolyte for High Efficiency Quasi-solid-state Dyesensitized Solar Cell Ying Yang a,*, Xue-yi Guoa, Xing-zhong Zhaob a
School of Metallurgical Science & Engineering, Central South University, Changsha 410083, China b Department of Physics and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Wuhan University, Wuhan 430072, China
Abstract A new composite polymer electrolyte consisting of low-molecular mass polysaccharide (agarose) filled with titanium oxide and containing LiI and I2 was developed. The introduction of the inorganic filler (TiO2) into the agarose polymer matrix changes the thermal and ionic conductive properties of host polymer electrolyte. The polysaccharide electrolyte with different inorganic filler TiO2 concentrations (0-30 wt %) is studied systematically by differential scanning calorimetry (DSC) and the AC impedance spectra. Upon addition of the inorganic oxide, the glass transition temperature (Tg) of the electrolyte decreases within the range of 0-2.5 wt % TiO2 changed electrolytes, which results in high conductivity in these electrolytes. Then the Tg increases with the TiO2 content until 30 wt % and the ionic conductivity of the electrolyte decreases in this concentration range. From the electrochemical analysis, it is observed that the electron lifetime in TiO2 of DSSC decreases with inorganic filler contents, which results in lower Voc in the high TiO2 content added DSSC. Based on these results, the optimized efficiency of titania added DSSC is 4.74% at 2.5 wt % titania concentration.
© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of MRS-Taiwan Keywords: Composite agarose/NMP/titania/I-/I3- electrolyte; polymer electrolyte; inorganic nanoparticle filler; quasi-solid state; dye sensitized solar cell
* Corresponding author. Tel.: +86-731-88877863; Fax: +86-737-88836207. E-mail address: [email protected].
1877-7058 © 2012 Published by Elsevier Ltd. doi:10.1016/j.proeng.2012.03.004
14
Ying Yan et al. / Procedia Engineering 36 (2012) 13 – 18
1. Introduction Dye-sensitized solar cells have attracted great scientific and technological interest as potential alternatives to classical photovoltaic devices due to their high conversion efficiency and low production cost [1, 2]. However, the stability and long-term operation of the cell are affected by solvent evaporation or leakage of liquid electrolyte. Thus, the commercial exploitation of these devices needs the replacement of the liquid electrolyte by a polymer gel or solid electrolyte. To improve the performance and stability of the DSSC, much effort has been directed towards development of non- volatile electrolytes meeting the stability requirements for outdoor applications in the last decade [3-5]. Impressive improvements have been achieved on the photovoltaic performance and the stability of devices [6-8]. Recent researchers on polymer electrolyte proved that the incorporation of inorganic nanoparticle filler in polymer electrolyte yields higher ionic conductivity [9, 10]. The filler provides a solid like support matrix, allowing the amorphous polymer to maintain its liquid-like characteristics in terms of fast ionic mobility at the microscopic level. This could help in enhancing the cell performance, and further could increase the stability and lifetime of these devices [11]. Polysaccharide (agarose and ț-carrageenan), can form an elastic solid containing excess of water [12], the electrochemical and photochemical reactions can take place in the solid. The molecular diffusion [13] and ionic conductivity in this solid are almost the same as in liquid water [14]. Therefore, this interesting solid containing a large excess liquid can be used as an electrolyte medium instead of water for conventional electrochemical measurement and offer a new heterogeneous medium for photoenergy conversion. On the other hand, polysaccharide is considered to be a good polymer matrix to form crosslinking networks in the polymer electrolyte with other components, due to rich hydroxyl groups in its molecule structure. The formation of continuous networks in the electrolyte is beneficial for the ion transportation, thus the ionic conductivity and the performance of the corresponding DSSC will be improved. In this paper, we report on the preparation of a homogeneous agarose based gel electrolyte simply by directly dissolving agarose into 1-methyl-2-pyrrolidinone (NMP). The agarose polymer electrolytes with different inorganic filler TiO2 concentrations (0-30 wt %) were prepared and the photoelectric performances of the corresponding DSSCs as a function of TiO2 content are studied systematically. The thermal (DSC) and electrochemical impedance studies reveal the pivotal influence exerted by the agarose and TiO2 in the polymer electrolyte on the ionic conductivity and photovoltaic response of the DSSC. 2. Experiments 2.1. Sample preparation Agarose (Mw=5000g mol-1) was purchased from SCRC, China and the molecular structure of the agarose in this study is presented in Fig. 1. Lithium iodide (LiI, 99%) was purchased from Acros. TiO2 (P25, 20–30 nm) was purchased from Degussa AG, Germany. Iodine (I2) and other common reagents and organic solvents were purchased from SCRC, China. For the composite electrolyte preparation, 0.2g agarose was added into 10g NMP and stirred at 80 oC in the hermetic brown flask in silicone bath for 4 h. Then the inorganic nanoparticels (TiO2, P25) with the concentration range 0~30 wt % of the polymer were slowly introduced and stirred for another 2 h. After that, solid LiI, and I2 with LiI/I2=10:1 (molar ratio, LiI: 0.08143g; I2: 0.01542g) were added to the above polymer solutions and stirred continuously in the hermetic brown flask until a homogeneous viscous liquid was formed.
Ying Yan et al. / Procedia Engineering 36 (2012) 13 – 18
Nanoporous TiO2 electrodes were prepared according to previous report [15]. The assembly of the dye-sensitized solar cell was according to the following steps. The counter-electrode is platinum plate. The electrolyte was sandwiched between the photoelectrode and platinized counter electrode and was heated under 73 ƕC to contraol the evaporation of the solvent in oven until forming very viscous gel. 2.2. Sample characterization Differential scanning calorimetry (DSC) of the polymer electrolytes with different inorganic filler TiO2 concentrations were carried out with Netzsch DSC 200PC (Germany). The samples were heated at a rate of 10 ƕC miní1 from í100 to 200 ƕC under nitrogen flow. The ionic conductivity of polymer electrolytes was obtained using an electrochemical cell consisting of the electrolyte film sandwiched between two platinum blocking electroldes. The impedance measurements of the electrolyte films were carried out on AC impedance analyzer (Agilent 4294A, USA) from 40 Hz to 1 MHz with signal amplitude of 10mV. The electrochemical impedance measurements of the cells with different TiO2 concentrations were carried out on the CHI 660C electrochemical system (Chenghua, Shanghai), over the frequency range of 0.001–105 Hz and the perturbation voltage of 10 mV. These DSSCs were kept under the light illumination and the forward bias of í0.6 V was applied on all the samples. Photocurrent–voltage (I–V) measurements were recorded using a computer-controlled Keithley 2400 digital source meter unit (USA) by illuminating the cell through the active photoelectrode under Oriel solar simulator (Oriel, 91192). The intensity of the incident light was calibrated by a Si-1787 photodiode (spectral response range: 320 to 730 nm). The power of the incident white light from the solar simulator was 73 mWcmí2. The active DSSC area was controlled at 0.25 cm2 by a mask. All of the measurements mentioned above were taken at ambient environment. 3. Experiments Results and Discussion 3.1. DSC Characteristics of electrolytes containing different TiO2 nanoparticle concentrations Figure 1 shows the relationship between the Tg and the filler TiO2 nanoparticle contents (0, 2.5, 5, 7.5, 10, 15, 30 wt %) in the agarose electrolytes. Tg values of the electrolytes with different TiO2 contents are variable in the range of -87~-61 oC and they follow a clear trend with filler concentration. The Tg values start from -77.3 oC, gradually decrease to -86.3 oC at 2.5 wt % TiO2 content and then increase continuously to -60.8 oC as the filler concentration increasing to 30 wt %.
Fig. 1. Glass transition temperature, Tg, of agarose based polymer electrolytes depending on the TiO2 nano-particle concentration.
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Ying Yan et al. / Procedia Engineering 36 (2012) 13 – 18
As Tg is much lower in the lower TiO2 concentrations (0-5 wt %) than that in the higher TiO2 concentrations (>5 wt %), the amorphous phase becomes more flexible and the ionic conductivity should be enhanced in this TiO2 concentration range (0-5 wt %). The observed lower values of Tg in the 0-5 wt % TiO2 concentration range is due to the interaction of the polymer with the filler particles, which hinders the inter/intra-molecule cross-linking interactions within/between polymers [16]. Meanwhile, the Tg increases rapidly in the higher TiO2 nanoparticle concentrations (exceeding 5 wt %), which may be due to the possible formation of crystalline phase if the abundant filler particle acts as a nucleation center of the crystalline polymer phase [17]. 3.2. Conductivity characteristics of electrolytes containing different TiO2 nanoparticle concentrations The dependence of ionic conductivity of the agarose electrolytes on TiO2 concentration at 30 oC is represented in Fig. 2. With an increase in TiO2 contents in agarose electrolyte systems, the ionic conductivities increase, reach a maximum value of 5.12×10-4 S cm-1at a filler content of 5 wt %, and then decrease as the filler increasing to 30 wt %.
Fig. 2. Conductivity isotherms determined at 30 oC for agarose based polymer electrolytes depending on TiO2 nano-particle concentration.
At low TiO2 contents (5 wt %), the interaction between polymer and nanoparticles form an interface between these two components, which acts as bridge for ion transport in the electrolytes. This would contribute mainly to enhancement of total conductivity. However, at high TiO2 contents (>5 wt %), continuous non-conductive phase built up by the ceramic fillers would block ion transport in the electrolytes, leading to low ionic conductivity [18]. 3.3. Photovoltaic performance and electrochemical characterization of the agarose electrolyte based DSSC with different TiO2 nanoparticle concentrations The photovoltaic performance of the DSSCs is optimized for different concentrations of TiO2 nanoparticle. The dependence of short-circuit current (Jsc), open-circuit voltage (Voc), the fill factor (FF), and the photo-to-current conversion efficiency (Ș) on the TiO2 concentration is shown in Fig. 3.
Ying Yan et al. / Procedia Engineering 36 (2012) 13 – 18
Fig. 3. Variation of (a) photo-to-current conversion efficiency (Ș), (b) fill factor (FF), (c) open-circuit voltage Voc, and (d) short-circuit photocurrent density Jsc with TiO2 nano-particle concentrations in the agarose polymer electrolytes.
The Jsc of the DSSC increases appreciably from 7.76 to 11 mA cm-2 with the increase of TiO2 nanoparticles from 0 to 2.5 wt % and decreases with the further increase. While the Voc decreases with increasing TiO2 concentration particularly in the range of 0- 2.5 wt %. The best efficiency (Ș) in the TiO2 nanoparticle concentration studied is 4.74% at 2.5 wt %. This shows that 2.5 wt % of nano- TiO2 is enough for getting good performance of the DSSC. Meanwhile, to further understand the electron and ionic transport in the DSSCs with different nanoTiO2 addition, the electrochemical impedance spectroscopy was studied, and the results are shown in Fig. 4. The figure is given in the form of Bode (Fig. 4a) plots. Figure 4b exhibits the relationship between the TiO2 filler concentration and the electron lifetime. There are three peaks in Bode phase plots (Fig. 4a) which are attributed to the charge-transfer processes (I3í +e=3Ií) at the Pt/electrolyte interface (frequency regions 103-105 Hz); the charge recombination reaction process (I3í +2ecbí(TiO2) =3Ií) occurred at nanocrystalline TiO2/electrolyte interface (frequency regions 1-103 Hz); and the Ií/I3- diffusion process occurred in the electrolyte (frequency regions 10-1-1 Hz) [19, 20]. In the Bode phase plots, the characteristic middle frequencies are inversely proportional to the electron lifetime (Ȧ = IJí1) in TiO2 according to the EIS theory [21, 22]. As seen in Fig. 4a the characteristic middle frequency peaks of the DSSCs with different content of TiO2 shifts to higher frequency as TiO2 amounts increasing. The middle frequency peak, which is located at 3.98 Hz of the 0 wt% TiO2 added DSSC, moves gradually to 11.9 Hz of the 30 wt % TiO2 added DSSC. In Fig. 4b, the electron lifetime decreases with the increase in TiO2 content, this indicated a shorter electron lifetime and more back-reaction in the higher TiO2 content based DSSC. The increased electron back-recombination implies lower Voc of the DSSC. 4. Conclusions Agarose based polymer electrolytes for DSSC with different p inorganic filler TiO2 concentrations were studied. The initial increase in TiO2 concentrations improves the conductivity of the polymer electrolytes, while further increase in TiO2 concentrations makes the conductivity decrease. The maximum energy conversion efficiency of 4.74% was achieved at 2.5 wt % TiO2 based DSSC. The Voc of the cells with these polymer electrolytes decrease with TiO2 concentration in electrolyte. This result is closely related to the electron lifetime in the cells according to electrochemical impedance studies.
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Ying Yan et al. / Procedia Engineering 36 (2012) 13 – 18
Acknowledgements We acknowledge the financial support of the National Nature Science Foundation of China (61006047 and 50125309) and the financial support from the Ministry of Science and Technology of China through Hi-Tech plan (funding No. 2006AA03Z347).
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