Efficient gel-state dye-sensitized solar cells adopting polymer gel electrolyte based on poly(methyl methacrylate)

Organic Electronics xxx (2013) xxx–xxx

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Efficient gel-state dye-sensitized solar cells adopting polymer gel electrolyte based on poly(methyl methacrylate) Chih-Hung Tsai a,⇑, Chun-Yang Lu b,c, Ming-Che Chen b,c, Tsung-Wei Huang b,c, Chung-Chih Wu b,c,⇑, Yi-Wen Chung d,⇑ a

Department of Opto-Electronic Engineering, National Dong Hwa University, Hualien 97401, Taiwan, ROC Department of Electrical Engineering, Graduate Institute of Photonic and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan, ROC c Graduate Institute of Electronics Engineering, Innovative Photonics Advanced Research Center (i-PARC), National Taiwan University, Taipei 10617, Taiwan, ROC d Tintable Smart Material Co., Ltd., Tainan 71145, Taiwan, ROC b

a r t i c l e

i n f o

Article history: Received 12 July 2013 Received in revised form 20 July 2013 Accepted 20 July 2013 Available online xxxx Keywords: DSSCs Polymer electrolyte Gel-state PMMA Gelator

a b s t r a c t We report an effective method to fabricate gel-state dye-sensitized solar cells (DSSCs) based on the PMMA polymer gel electrolyte. In this approach, the liquid-state polymer electrolyte solution was prepared by simply mixing the traditional liquid-state electrolyte with the polymer gelator solution and was injected into the DSSC in its liquid state. The liquid-state polymer electrolyte was then converted to the gel state (i.e., in situ gelation) simply by evaporating a portion of solvents at elevated temperatures. With this approach, liquid electrolytes already well developed and optimized for DSSCs can be readily adopted. Filling in the liquid/solution state ensures effective penetration of the electrolyte into pores of TiO2 nanoparticle electrodes for attaining good contact and interface properties. The in situ gelation by heating (solvent evaporation) much simplifies the process. The polymer gel electrolyte thus prepared exhibited high ion conductivity and diffusivity comparable to those of traditional liquid electrolytes. The gel-state DSSCs thus fabricated exhibited a high power-conversion efficiency of 8.03% and much improved stability compared to the traditional liquid DSSC. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Increasing energy demands and concerns have encouraged scientists to develop low-cost and easily accessible renewable energy sources in recent years. In comparison to the traditional silicon-based solar cells, dye-sensitized solar cells (DSSCs) are promising nextgeneration photovoltaic cells because they are versatile, energy saving, and environmentally friendly [1–3]. Therefore, DSSCs have attracted much attention since the ⇑ Corresponding authors. Address: Department of Electrical Engineering, Graduate Institute of Photonic and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan, ROC (C.-C. Wu). Tel.: +886 2 33663636; fax: +886 2 33669404. E-mail addresses: [email protected] (C.-H. Tsai), chungwu@cc. ee.ntu.edu.tw (C.-C. Wu), [email protected] (Y.-W. Chung).

breakthrough made by O’Regan and Grätzel [4]. To date, DSSCs with power conversion efficiencies exceeding 11% have been realized using ruthenium (Ru)-based sensitizers [5]. Recently, DSSCs sensitized with complementary porphyrin-based and organic dyes have achieved a power conversion efficiency as high as 12.3% [6], making contemporary DSSCs highly competitive with other thin-film photovoltaic technologies. Typically, a DSSC consists of a transparent conductive substrate, a porous thin-film photoelectrode composed of TiO2 nanoparticles, dyes, liquid electrolytes, and a counter electrode [7–9]. However, issues associated with liquid electrolytes, such as leakage and volatilization of the liquid, possible desorption and photodegradation of adsorbed dyes, and corrosion of the Pt counter electrode, could limit the long-term use of DSSCs [10]. To solve these problems, extensive studies have been attempted to

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Please cite this article in press as: C.-H. Tsai et al., Efficient gel-state dye-sensitized solar cells adopting polymer gel electrolyte based on poly(methyl methacrylate), Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.07.026

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substitute liquid electrolytes with solid-state or gel-state electrolytes, such as organic or inorganic hole conductors [11], gel electrolytes incorporating redox couples [12], and polymer electrolytes [13]. Although solid-state electrolytes may be ideal, they in general give lower power conversion efficiency than the liquid electrolytes because of poorer contact of the solid-state charge transport materials with the dye-coated TiO2 surface [14]. Thus, quasi-solid-state electrolytes, like polymer gel electrolytes [15,16], that may provide better contacting and filling properties with nanostructured photoelectrodes and counter electrodes, are also actively pursued. In addition, polymer gel electrolytes could provide high ionic conductivities with trapping liquid electrolytes in cages formed by the polymer host matrices [15,16]. Up to date, various polymers or co-polymers have been used as the gelators in gel-state DSSCs, such as poly(acrylonitrile-co-vinyl acetate) (PAN–VA) [17], poly (methyl methacrylate) (PMMA) [18], poly(ethylene glycol) (PEG) [19], poly(methyl methacrylate-co-methacrylate acid)/poly(ethylene glycol) [P(MMA-co-MAA)/PEG] [20], and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDFHFP) [21]. To fill these polymer gel electrolytes into nanostructured electrodes in DSSCs, usually they need to be processed in their melting states at elevated temperatures or by in situ polymerization/co-polymerization after filling [15–21]. These methods, however, may still suffer poorer filling/contacting properties or more complicated control of reactions and compositions. In this study, we report an effective method to fabricate gel-state DSSCs based on the PMMA polymer gel electrolyte. In this approach, the liquid-state polymer electrolyte solution was first prepared and injected into the DSSC. After injection, the liquid-state polymer electrolyte was then converted to the gel state (i.e., in situ gelation) simply by driving away a portion of solvents in the electrolyte at elevated temperatures. Filling in the liquid/solution state ensures effective penetration of the electrolyte into pores of TiO2 nanoparticle electrodes for attaining good contact and interface properties. The in situ gelation by heating (solvent evaporation) much simplifies the process. The gel-state DSSCs thus fabricated exhibited a high powerconversion efficiency of 8.03% and improved stability. 2. Experiments 2.1. Preparation of the gelator and polymer electrolyte The liquid-state polymer electrolyte was prepared by mixing the traditional liquid-state electrolyte and the polymer gelator solution. The gelator solution was composed of poly(methyl methacrylate) (PMMA), tetrahydrofuran (THF, boiling point = 66 °C), propylene carbonate (PC, boiling point = 242 °C), and ethylene carbonate (EC, boiling point = 261 °C). THF was used to dissolve PMMA, PC was used to increase the ionic conductivity of electrolytes [22], and EC was used to increase the ionic conductivity and plasticity of the electrolytes [23]. The relative weight ratios of PMMA:PC:EC in THF was 25:35:40. In preparation of the gelator solution, PMMA was first added into the mixture of THF/PC. The mixture solution was stirred at 70 °C

until homogeneous, and then EC was added into the solution. The liquid electrolyte that is efficient for DSSCs based on ruthenium dye N719, [cis-di(thiocyanato)-N-N0 bis(2,20 -bipyridyl-4-carboxylic acid-40 -tetrabutyl-ammonium carboxylate) ruthenium (II)] [24] was used. The liquid electrolyte contained 0.6 M 1-butyl-3-methylimidazolium iodide (BMII), 0.03 M I2, 0.5 M 4-tert-butylpyridine, and 0.1 M guanidinium thiocyanate in a mixture of acetonitrile (boiling point = 82 °C) and valeronitrile (boiling point = 140 °C) with a volume ratio of 85:15, v/v [25]. The polymer electrolyte solution was then prepared by mixing 20 wt.% of the gelater solution with 80 wt.% of the liquid electrolyte solution at room temperature. The polymer electrolyte was initially in the liquid state, permitting effective filling and penetration of the electrolyte into pores of TiO2 nanoparticle electrodes for attaining good contact and interface properties. The liquid-state polymer electrolyte solution can be converted to the (high viscosity) gel state simply by heating the sample (at 70 °C for 20 min) to drive away (evaporate) a portion of the THF solvent. The heating temperature of 70 °C, higher than the boiling point of the PMMA solvent THF (66 °C) but lower than the boiling point of the electrolyte solvent acetonitrile (82 °C), was chosen to selectively evaporate THF but not affect the electrolyte solvent acetonitrile significantly. With this approach, liquid electrolytes already optimized for DSSCs can be easily applied to gel-state electrolytes by simply mixing with the gelator solution and subsequent facile in situ gelation. 2.2. Characterization of electrolytes The physical, thermal, and electrochemical properties of the electrolytes were characterized. The viscosities of liquid and polymer electrolytes were measured with a viscosity meter (Brookfield Viscometer, USA) to estimate the degree of gelation of the electrolytes. The viscosities of the traditional liquid electrolyte, the polymer electrolyte in the solution state (before heat treatment) and in the gel state (after heat treatment) were measured for comparison. Differential scanning calorimetry (DSC, LT-Modulate DSC 2920, TA Instrument, USA) was used to characterize thermal properties of the gel-state polymer electrolyte, with a scan rate of 5 °C/min. The ionic conductivities of liquid and polymer electrolytes were measured with a conductivity meter (HANNA Instrument, HI9033). The conductivity meter was calibrated with a standard solution (Myron L Company, 442–1000) at 25 °C prior to the experiments. The ionic diffusivities of liquid and polymer electrolytes were obtained from the electrochemical impedance spectroscopy (EIS) of symmetrical sandwich device structures. Two identical Pt electrodes (40 nm-thick Pt films deposited on glass substrates by e-beam evaporation) were assembled with a sealant. The electrolyte was injected between two Pt electrodes through a drilled hole, followed by the heat treatment (at 70 °C for 20 min) if necessary. The EIS was then conducted using an impedance analyzer, in a frequency range of 0.1 Hz to 1 MHz, under a 0 V bias and an ac amplitude of 10 mV. The ionic diffusivities of the electrolytes were then extracted from fitting the EIS results.

Please cite this article in press as: C.-H. Tsai et al., Efficient gel-state dye-sensitized solar cells adopting polymer gel electrolyte based on poly(methyl methacrylate), Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.07.026

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2.3. Fabrication of DSSCs Fig. 1 schematically illustrates the device structure and gelation process of the gel-state DSSC. The FTO substrates, the 12 lm-thick transparent TiO2 nanoparticle layer (average particle size: 20 nm), the 4 lm-thick TiO2 scattering layer (average particle size: 400 nm), and Pt counter electrodes were prepared following methods previously reported [26]. The nanoporous TiO2 electrodes were then immersed into a dye solution at room temperature for 24 h for dye adsorption. The dye solution was composed of 0.5 mM ruthenium dye N719 and 0.5 mM chenodeoxycholic acid (CDCA, as a co-adsorbent) in the acetonitrile/ tert-butanol mixture (1:1) [27]. The dye-adsorbed TiO2 working electrode and a counter electrode were then assembled into a sealed DSSC cell with a sealant spacer between the two electrode plates. For the fabrication of the liquid DSSC, a drop of the liquid electrolyte (as described in Section 2.1) was injected into the cell through a drilled hole, then the drilled hole was sealed using the sealant and a cover glass. For the gel-state DSSC, the liquid-state

polymer electrolyte was first injected into the cell through a drilled hole at room temperature. Then, the device was heated at 70 °C for 20 min to convert the polymer electrolyte from the liquid state to the gel state. Finally, the drilled hole was sealed using the sealant and a cover glass.

2.4. DSSC characterization A mask with an aperture area of 0.125 cm2 was covered on a testing cell during measurements. The photocurrent density–voltage (J–V) characteristics and the incident photon-to-current conversion efficiency (IPCE) spectra of the DSSCs were measured following methods previously reported [28–30]. The J–V characteristics were used to extract the short-circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF), and power conversion efficiency (Eff.) of the DSSCs. To test the stability of the device upon storage at room temperature in air, J–V characteristics of the device was repeatedly taken after different storage times. In addition, the electrochemical impedance spectroscopy (EIS) was also used to analyze the internal impedance properties of DSSCs, following the methods previously reported [31,32].

3. Results and discussions 3.1. Properties of the electrolytes The viscosities of the traditional liquid electrolyte, the polymer electrolyte in the solution state (before heat treatment) and in the gel state (after heat treatment) were measured and are listed in Table 1 for comparison. The traditional liquid electrolyte exhibited a low viscosity of less than 1.5 cp. With addition of the gelator solution into the traditional electrolyte solution, the viscosity of the polymer electrolyte solution before heat treatment increased a bit to 4.8 cp, indicating still a rather fluidic liquid state. After heat treatment (heating at 70 °C for 20 min), the viscosity of the polymer electrolyte greatly increased to 5820 cp, indicating that the polymer electrolyte was converted from the liquid/solution state into the gel state. A photo of the gel-state polymer electrolyte is shown in the inset of Fig. 2, which reveals its highly viscous property. The gelation mainly resulted from evaporation of the THF solvent and the increase of the PMMA concentration. Differential scanning calorimetry (DSC) of the gel-state polymer electrolyte (obtained after heating at 70 °C for 20 min), as shown in Fig. 2, revealed no significant phase

Table 1 Viscosity, ion conductivity, and ion diffusivity of various electrolytes.

Fig. 1. The schematic fabrication process of the gel-state polymer electrolyte DSSC.

Electrolyte

Viscosity (cp)

Conductivity (mS/cm)

Diffusivity (cm2/s)

Liquid electrolyte Liquid-state polymer electrolyte Gel-state polymer electrolyte

<1.5 4.8

9.63 8.31

2.24  106 2.06  106

5820

7.19

1.93  106

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of the THF solvent. The heating temperature of 70 °C, higher than the boiling point of the PMMA solvent THF (66 °C) but lower than the boiling point of the electrolyte solvent acetonitrile (82 °C), was chosen to selectively evaporate THF but not affect the electrolyte solvent acetonitrile significantly. We found that heating at temperatures higher than 82 °C led to significant evaporation of the electrolyte solvent acetonitrile and more drastic degradation of the electrolyte characteristics (ion conductivity, diffusivity). Thus, to keep as much DSSC performance as possible, such higher temperatures were not used for preparing the gel electrolytes and gel-state DSSCs.

Fig. 3(a) and (b) shows the J–V characteristics and the IPCE spectra of the DSSCs based on liquid and gel-state electrolytes. The photovoltaic characteristics of the two devices, including the peak IPCE values, short-circuit current density (JSC), the open-circuit voltage (VOC), the fill factor (FF), and the conversion efficiency (Eff.), are summarized in Table 2. The peak IPCE, JSC, VOC, and FF of the DSSC based on the liquid electrolyte are 91.6%, 18.28 mA/cm2, 0.76 V, and 0.72, respectively, yielding an overall conversion efficiency of 10.01%. The peak IPCE, JSC, VOC and FF of the DSSC based on the gel-state polymer electrolyte are 78.2%,

(a)

20

Current Density (mA/cm2)

transition between 40 and 200 °C, indicating that the gelstate polymer electrolyte was rather thermally stable. The ionic conductivity and ionic diffusivities of the traditional liquid electrolyte, the polymer electrolyte in the solution state (before heat treatment) and in the gel state (after heat treatment) were also measured and are listed in Table 1 for comparison. These properties provide information on the mobility of the ions and their interaction with the solvent. In general, the power conversion efficiency of DSSCs is dependent on the mobility of the redox couple (I/I3) and the ionic conductivity of the electrolyte. The ion conductivities of the liquid electrolyte, liquid-state polymer electrolyte, and gel-state polymer electrolyte are 9.63 mS/cm, 8.31 mS/cm, and 7.19 mS/cm, respectively. Similarly, the ion diffusivities of the liquid electrolyte, liquid-state polymer electrolyte, and gel-state polymer electrolyte are 2.24  106 cm2/s, 2.06  106 cm2/s, and 1.93  106 cm2/s, respectively. Again, with addition of the gelator solution into the traditional electrolyte solution, both the ionic conductivity and diffusivity of the liquid-state polymer electrolyte before heat treatment dropped a bit. After heat treatment (heating at 70 °C for 20 min), both the ionic conductivity and diffusivity of the gel-state polymer electrolyte further dropped a bit. Yet, both the ionic conductivity and diffusivity of the gel-state polymer electrolyte maintained rather high values comparable to those of the traditional liquid electrolyte, even though the viscosity had increased by over three orders of magnitude. The capability to keep high conductivity in the highly viscous gel state is the particular advantage of the polymer gel electrolyte. During the gelation process, there is still significant amount of liquid electrolytes trapped in cages formed by three-dimensional polymer networks, which are highly beneficial for ion conduction and yet also enhance thermal stability of the system [15,16]. The gel electrolyte approach here also has the particular advantage that liquid electrolytes already optimized for DSSCs can be readily applied to gel-state electrolytes by simply mixing with the gelator solution and subsequent facile gelation. In this work, the liquid-state polymer electrolyte solution was converted to the (high viscosity) gel state by heating the sample at 70 °C to drive away (evaporate) a portion

3.2. Device characteristics of DSSCs using liquid and gel electrolytes

18 16 14 12 10

Liquid electrolyte Gel-state eletrolyte

8 6 4 2 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Voltage (V)

(b)

100 90 80 70

IPCE (%)

Fig. 2. Differential scanning calorimetric (DSC) trace of the gel-state polymer electrolyte.

60 50 40 30

Liquid electrolyte Gel-state electrolyte

20 10 0 300 350 400 450 500 550 600 650 700 750

Wavelength (nm) Fig. 3. (a) J–V and (b) IPCE characteristics of the DSSCs based on liquid and gel-state electrolytes.

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Device

Peak IPCE (%)

Jsc (mA/cm2)

VOC (V)

Fill factor

Eff. (%)

Liquid DSSC Gel-state DSSC

91.6 78.2

18.28 15.53

0.76 0.75

0.72 0.69

10.01 8.03

15.53 mA/cm2, 0.75 V, and 0.69, respectively, yielding an overall conversion efficiency of 8.03%. Although the gelstate DSSC adopted a much more viscous electrolyte (three orders of magnitude higher), it however still exhibited well-behaved photovoltaic characteristics and gave rather high power conversion efficiency of 8.03%, reaching 80% of the liquid-state DSSC. Such a power conversion efficiency is among the highest reported for gel-state DSSC [15–21], and perhaps is the highest ever reported for DSSCs adopting polymer gel electrolytes based on the common and quite readily available polymer PMMA [15–21]. In addition, the preparation of materials, electrolyte injection and gelation processes here are particularly straightforward and feasible. Both gel-state and liquid DSSCs show similar VOC. The lower conversion efficiency of the gel-state DSSC is mainly due to its slightly lower FF and lower JSC. The lower JSC of the gel-state DSSC is consistent with its lower IPCE. The lower FF, JSC, and IPCE of the gel-state device perhaps are associated with the lower ionic conductivity and diffusivity of the PMMA-based polymer gel electrolyte and the interface properties of the gel electrolyte with the photoelectrode and counter electrode (as will be discussed in the next section). To test the stability of the devices upon storage at room temperature in air, J–V characteristics of the devices were repeatedly taken after different storage times. Fig. 4 shows the normalized conversion efficiencies of gel-state and liquid DSSCs as a function of the storage time. It can be seen that the efficiency of the liquid DSSC decreased constantly with time. The efficiency of the liquid DSSC decreased by 21% after 1200 h (50 days) and dropped by 50% after 3000 h (125 days). In contrast, the gel-state DSSC exhibited much improved stability. The gel-state DSSC retained

nearly the same efficiency as the initial state (decreased by only 2%) after 1200 h and kept 90% of the initial efficiency even after 3000 h. The results indicate that in the gel-state electrolyte, trapping of electrolyte liquid in cages formed by polymer networks is beneficial for mitigating issues associated with liquid electrolytes, such as leakage and volatilization of the electrolyte liquid, possible desorption and degradation of adsorbed dyes, and corrosion of the Pt counter electrode [10]. 3.3. Electrochemical impedance spectroscopy of devices The effects of liquid and gel-state electrolytes on photovoltaic characteristics of DSSCs were further investigated by electrochemical impedance spectroscopy (EIS). EIS is a useful tool for characterizing interfacial charge-transfer processes in DSSCs, such as the charge recombination at the TiO2/dye/electrolyte interface, electron transport in the TiO2 electrode, electron transfer at the counter electrode, and ion transport in the electrolyte [33,34]. In this study, EIS was conducted by subjecting the cell to the constant AM 1.5G 100 mW/cm2 illumination and to the bias at the open-circuit voltage Voc of the cell (namely, the condition of no DC current). Fig. 5(a) shows the EIS Nyquist plots (i.e., minus imaginary part of the impedance Z00 vs. the real part of the impedance Z0 when sweeping the frequency) for DSSCs using liquid and gel-state electrolytes. In the frequency range investigated (0.1 Hz to 1 MHz), a smaller semicircle (to the right) occurs in the lowest frequency range of 0.1–1 Hz; a larger semicircle (at the center) occurs in the frequency range of 1 Hz to 1 kHz; another smaller semicircle (to the left) occurs in the frequency range above 1 kHz. With the bias illumination and voltage applied, the

(a)

40

Liquid electrolyte (measured) Liquid electrolyte (fitted) Gel-state electrolyte (measured) Gel-state electrolyte (fitted)

30

- Z" (ohm)

Table 2 Characteristics of DSSCs based on liquid and gel-state electrolytes.

20

10

Normalized Efficiency

1.0 0

0.8

0

20

40

60

80

100

120

Z' (ohm)

0.6 Liquid electrolyte Gel-state electrolyte

0.4

(b)

0.2 0.0

0

500

1000

1500

2000

2500

3000

Time (hour) Fig. 4. Normalized conversion efficiencies of liquid and gel-state DSSCs as a function of the storage time.

Fig. 5. (a) Measured and fitted EIS Nyquist plots for the DSSCs based on liquid and gel-state electrolytes and (b) the equivalent circuit used to reproduce measured Nyquist plots.

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semicircle at the lowest frequencies corresponds to the charge Nernstian diffusion in the electrolyte; the semicircle for 1 Hz to 1 kHz corresponds to the charge transfer processes at the TiO2/dye/electrolyte interface [35]; the semicircle at even higher frequencies corresponds to the charge transfer processes at the Pt/electrolyte interface [36]. To extract quantitative impedance characteristics of the DSSCs, the equivalent circuit model presented in Fig. 5(b) was used to analyze the internal impedance of a DSSC [37]. The series resistance RS is associated with the contribution from the FTO and counter electrodes. The resistance RPt in parallel with the constant-phase element CPEPt are associated with the impedance at the Pt/electrolyte interface [38]. The impedance of a constant-phase element (CPE) has the general form of Z(x) = Q(jx)a, where x is the angular frequency, Q the CPE parameter and a the CPE exponent. Q is a constant parameter and the parameter a can assume values between 0 and 1 [39]. The parameter a describes the degree of the capacitive character; when a = 1, the CPE is like a regular capacitance. The CPE distributed element is introduced to account for dispersive characteristics of the interfacial capacitance, which are induced by spatial inhomogeneities (e.g. roughness) of the Pt/electrolyte interface [40]. The resistance Rct in parallel with the capacitance Cl are associated with the impedance at the TiO2/dye/electrolyte interface [41], in which Rct and Cl represent the charge-transfer resistance and the chemical capacitance at the TiO2/dye/electrolyte interface, respectively. WD (Warburg impedance) corresponds to the ion diffusion resistance of the electrolyte in DSSCs. The Warburg impedance has the form of:



 qffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffi W D ¼ ð1=Y 0 Þ= ðjxÞ tanh B ðjxÞ

ð1Þ

pffiffiffi Y 0 ¼ 1=ð 2  Aw Þ

ð2Þ

pffiffiffiffi B ¼ d= D

ð3Þ

where x is the angular frequency, Y0 is the magnitude of the admittance, Aw is the Warburg coefficient, d is the Nernst diffusion layer thickness and D is the diffusion coefficient of triiodide [42]. As shown in Fig. 5(a), by carefully adjusting the device parameters, each curve in the Nyquist plot can be accurately reproduced using the equivalent circuit of Fig. 5(b). The parameters used to fit the curves for various devices are shown in Table 3. In Table 3, one sees that the properties associated with the series resistance (RS) remain similar for the two devices. The extracted RPt increased from 12.66 X for the liquid DSSC to 15.32 X for the gel-state DSSC. The increase of RPt indicates that the charge transfer processes at the Pt/electrolyte interface is less effective with the gel-state polymer electrolyte. The extracted charge-transfer resistance Rct (for the TiO2/dye/electrolyte interface) of the liquid device and gel-state electrolyte device are 36.65 X and 48.14 X, respectively. The increase of Rct in the gel-state DSSC indicates that electron generation and transfer at the TiO2/dye/electrolyte interface are less efficient with the use of the gel-state polymer electrolyte,

Table 3 The parameters used to fit the EIS Nyquist plots of the DSSCs in Fig. 5(a). Device

Liquid DSSC

Gel-state DSSC

RS (ohm) RPt (ohm) CPEPt (Q/a) RCt (ohm) Cl (F) Y0 (S s1/2) B (s1/2)

15.01 12.66 3.43  104/0.85 36.65 1.15  104 5.64  102 0.748

15.24 15.32 3.70  104/0.85 48.14 1.39  104 3.21  102 0.757

which is consistent with the photovoltaic characteristics of devices. Furthermore, the extracted Y0 value (magnitude of the admittance associated with ion diffusion in electrolytes) decreases from 5.64  102 S s1/2 for the liquid electrolyte to 3.21  102 S s1/2 for the gel electrolyte. It corresponds to an increase in ionic diffusion impedance in the gel-state electrolyte and is also consistent with previous results of ion conductivities and diffusivities in different electrolytes. In general, the EIS results are in good agreement with performances of different DSSCs and suggest that the lower efficiency of the current gel-state DSSC (vs. liquid DSSC) is associated with the properties of the gel electrolyte itself as well as its interface properties with TiO2/dye and the counter electrode. In future, it may be worth further varying the compositions of the current gel electrolyte system to see if these properties and DSSC performance can be further enhanced.

4. Conclusions In summary, we report an effective method to fabricate gel-state DSSCs based on the PMMA polymer gel electrolyte. In this approach, the liquid-state polymer electrolyte solution was prepared by simply mixing the traditional liquid-state electrolyte and the polymer gelator solution and was injected into the DSSC in its liquid state. After injection, the liquid-state polymer electrolyte was then converted to the gel state (i.e., in situ gelation) simply by driving away (evaporating) a portion of solvents in the electrolyte at elevated temperatures. The gel electrolyte approach here has the particular advantage that liquid electrolytes already optimized for DSSCs can be readily applied to gel-state electrolytes. Filling in the liquid/solution state ensures effective penetration of the electrolyte into pores of TiO2 nanoparticle electrodes for attaining good contact and interface properties. The in situ gelation by heating (solvent evaporation) much simplifies the process. The polymer gel electrolyte thus prepared exhibited high ion conductivity and diffusivity comparable to those of the traditional liquid electrolyte. The gel-state DSSCs thus fabricated exhibited a high power-conversion efficiency of 8.03% and much improved stability compared to the traditional liquid DSSC. Acknowledgement The authors gratefully acknowledge the financial support from National Science Council of Taiwan (Project Nos. NSC 102-2221-E-002-203-MY3, NSC 99-2221-E-002-

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