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Plastic–polymer composite electrolytes for solid state dye-sensitized solar cells
Electrochimica Acta 55 (2010) 6415–6419
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Plastic–polymer composite electrolytes for solid state dye-sensitized solar cells Y. Jiang, Y.L. Cao, P. Liu, J.F. Qian, H.X. Yang ∗ Department of Chemistry, Wuhan University, Wuhan 430072, Hubei, China
a r t i c l e
i n f o
Article history: Received 17 February 2010 Received in revised form 10 May 2010 Accepted 16 June 2010 Available online 23 June 2010 Keywords: Dye-sensitized solar cells Plastic–polymer composite electrolyte Soft solid electrolyte Poly(N-alkyl-1-vinyl-imidazolium) iodide Succinonitrile
a b s t r a c t Three types of alkyl-substituted poly(N-alkyl-1-vinyl-imidazolium) iodides were synthesized and plasticized using succinonitrile as a solid plasticizer to develop a series of novel solvent-free plastic–polymer composite electrolytes. All these electrolytes appeared as a soft solid at room temperature and became sticky gel state at high temperature of 100 ◦ C. Among the as-prepared plastic–polymer electrolytes, the SCN–PMVII (succinonitrile–poly(1-vinyl-3-methylimidazolium) iodide) electrolytes with a SCN content of 40–60 wt.% showed a room temperature conductivity of 1.0–1.6 mS cm−1 and a photoconversion efficiency of >4.1%, which are comparable to those observed from liquid organic carbonate electrolyte and the DSSCs using the liquid electrolyte at the same experimental conditions. Also, the DSSCs assembled with the SCN–PMVII electrolytes maintained their photoconversion efficiency very steadily during aging test of 50 days despite of being placed at 40 ◦ C under 1 sun illumination or stored at 60 ◦ C in an oven. Since these plastic–polymer electrolytes are solvent-free, highly conductive and electrochemically compatible, it is possible to use this type electrolyte for development of practical DSSCs. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSSCs) have attracted significant attentions in past decade as a new generation of photovoltaic cells because of their high photoconversion efficiency and low cost [1,2]. Though the DSSCs based on organic liquid electrolytes have demonstrated a high photoelectric conversion efficiency of >10% [3,4], this type of the cells has encountered a severe problem of the leakage and evaporation of the organic solvent, which lead to technological difficulties for the long-term stability and reliable encapsulation of these cells. Thus, it is highly desirable to replace the volatile organic electrolytes in view of practicable applications of DSSCs. In the pursuit of solvent-free electrolytes for solid state DSSCs, soft matter electrolytes, such as polymers [5–7], ionic liquids [8,9] and plastic crystal electrolytes [10–12], seem to be a good choice of the candidate electrolytes for DSSC applications because of their appropriate ionic conductivity and electrochemical compatibility. However, the pure polymer electrolytes reported so far have usually an insufficient ambient ionic conductivity and poorer contact with the nanocrystalline photoelectrodes, the DSSCs with such electrolytes have yet achieved a photoconversion efficiency of ∼5%, about a half of the photoconversion efficiency of the DSSCs using organic liquid electrolytes. The plastic crystal electrolytes used now for DSSCs are mostly based on succinonitrile (SCN), which has a quite low melting point of 58 ◦ C in pure crystalline state and melts at room temperature with the addition of commonly used elec-
∗ Corresponding author. Tel.: +86 27 68754526; fax: +86 27 87884476. E-mail address: [email protected] (H.X. Yang). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.06.046
trolyte salts. Such plastic crystal electrolytes, similarly like ionic liquid electrolytes, may still cause the problem of long-term stability, particularly at elevated temperature under sunlight in outdoor applications. Recently, plastic–polymer or ionic liquid–polymer composite electrolytes are actively developed for rechargeable lithium batteries [13–16] and electrochromic devices [17,18] by incorporating small molecules of plastic crystal electrolytes or ionic liquids into the matrixes of ionic conducting polymers and polymeric ionic liquids so as to form soft solid state electrolyte membranes. These composite electrolytes should be adaptable for solid state DSSCs because of their combined advantages of mechanical stability of polymers and high ionic conductivity of plastic crystal or ionic liquid electrolyte. In this work, we synthesized a series of plastic–polymer composite electrolytes using poly(alkyllimidazolium iodide) as iodide-conducting polymer matrixes and adding SCN as a solid plasticizer to decrease the affinity of the polymeric cations with the dissociable iodide anions so as to develop a new type of solvent-free electrolytes for DSSCs. Herein, the electrochemical and photovoltaic performances of the plastic–polymer composite electrolytes are described. 2. Experimental 2.1. Preparation of the plastic–polymer composite electrolytes Three kinds of poly(1-vinyl-3-alkylimidazolium) iodide with different lengths of alkyl chain (poly(1-vinyl3-methylimidazolium) iodide (PMVII), poly(1-vinyl-3-
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Y. Jiang et al. / Electrochimica Acta 55 (2010) 6415–6419
ethylimidazolium) iodide (PEVII), and poly(1-vinyl-3butylimidazolium) iodide (PBVII)) were synthesized by solution polymerization of corresponding 1-vinyl-3-alkyllimidazolium iodide monomers at 80 ◦ C in ethanol using 0.5% 2,2 azobisisobutyronitrile (AIBN) as initiator. The alkyllimidazolium iodides were first synthesized by adding 0.33 mol of alkyl iodide (RI, R = methyl, ethyl, n-butyl) dropwise into 150 mL of ethanol containing 0.3 mol of 1-vinylimidazole at 80 ◦ C under dry nitrogen atmosphere. The reaction solution was kept stirring for approximately 12 h and then poured into ether and put aside until two liquid phases formed clearly. After then, the underlayer liquid was taken out, washed twice with ether and then dried in a vacuum oven at 40 ◦ C to constant weight as described previously in Refs. [19,20,8]. All the alkyllimidazolium iodides thus synthesized appeared to be deep yellow liquids at room temperature. The synthetic chemical reactions for the poly(alkyllimidazolium) iodides may be described as Fig. 1. TGA curves of the SCN–PMVII composite electrolytes with different weight percentage of SCN: a. 0%; b. 60 wt.%; c. 40 wt.% and d. 20 wt.%. The I2 content in the electrolytes was kept at a molar ratio of I2 /PMVII = 0.2.
The plastic–polymer composite electrolytes were prepared by mixing poly(1-vinyl-3-alkylimidazolium) iodide (PMVII, or PEVII, PBVII) and SCN at a given molar ratio, then heating the mixture to form a melted blend at 85 ◦ C, and finally, adding a required percent of I2 into the melt under vigorous stirring for 1 h. When the molten mixture was cooled naturally down to room temperature, it solidified into a rubber-like solid state electrolyte. 2.2. Fabrication of DSSC cells A photoelectrode consisted of a TiO2 blocking layer and a nanocrystalline TiO2 on transparent fluorine-doped tin oxide glass (FTO, 15 cm−2 , Nippon Sheet Glass). To prepare the TiO2 blocking layer, an edge of the FTO glass was capped with adhesive tape to provide a noncoated area for electrical contact, and then a 0.02 mol L−1 TiCl4 aqueous solution was spread on the FTO glass. After drying the solution and removing the tape, the electrode was sintered at 500 ◦ C for 30 min. To control the thickness of the nanocrystalline TiO2 , the substrate was covered with adhesive tape again. TiO2 paste (0.3 g TiO2 powder (P25, Degussa), 0.06 g ethyl cellulose, 5 mL ethanol and 1 mL terpineol grounded 40 min) was then cast with a glass sheet sliding over the tape-covered edges and annealed at 500 ◦ C for 30 min in air. The photoelectrode thus prepared was dipped into a 0.2 mol L−1 TiCl4 aqueous solution for 30 min at 80 ◦ C until TiCl4 was hydrolyzed and then washed with distilled water to remove residual TiCl4 and again sintered at 500 ◦ C in air for 30 min. The TiO2 film electrode was immersed overnight in a 5 × 10−4 mol L−1 ethanol solution of Ru(dcbpy)2 (NCS)2 (535-bis TBA or N719, Solaronix) at room temperature and then rinsed with anhydrous ethanol and dried again. The counter electrode was prepared by electrodeposition of platinum onto the same FTO glass. In assembling DSSC cells, the plastic–polymer composite electrolyte was firstly preheated at 85 ◦ C to convert into a viscous liquid, and injected into the TiO2 electrode. The DSSCs were assembled by clamping the TiO2 photoanode/Pt-FTO counter electrode together with an 80-m-thick thermal adhesive film as a separating layer. After cooling down to room temperature, a uniform solid state electrolyte was formed in the cells. 2.3. Characterization and instruments The thermal stability of the composite electrolytes was examined by differential scanning calorimetric analysis (DSC) using a TA
instrument (Q200). Thermogravimetric analysis (TG) was carried out on a TA instrument (Q500). The crystalline structures of the composite electrolytes were characterized by XRD measurements on a Shimadzu XRD-6000 diffractometer with a Cu K␣ source. Ionic conductivities of the electrolytes were evaluated by a conductivity Meter (DDS-307, Leici, Shanghai) equipped with a platinum black electrode (DJS-1, Shanghai). The electrochemical redox behaviors of I− /I3 − in the composite electrolytes were examined by cyclic voltammetry (CV) using a Pt microdisk electrode (0.1 mm in diameter) as working electrode and a larger platinum sheet as both counter electrode and reference electrode. The data acquisitions for CV and photocurrent–photovoltage measurements were carried out by a CHI600C electrochemical station (Shanghai, China). A solar light simulator (Oriel, 91160) was used as the white light source to give AM 1.5 and 100 mW cm−2 illumination on the surface of the DSSC cells with a mask of 0.162 cm2 aperture. The intensity of incident light was calibrated with a radiant power/energy meter (Oriel, 70260) before each experiment. 3. Results and discussion 3.1. Physical appearances and thermal properties Three samples of pure poly(1-vinyl-3-alkylimidazolium) iodides (PMVII, PEVII and PBVII) synthesized in this work are all white-yellowish solid powders at room temperature. All the XRD patterns of these powders displayed a featureless broad band at lower angles of 2 = 15–25◦ , suggesting an amorphous structure of the iodide polymers. TG analysis of the iodide polymers showed a rapid weight loss of ≥80% in the temperature region of 260–350 ◦ C, indicative of the thermal decomposition of the polymers, and also, the thermal stability of these alkyl-substituted polymers was observed to be in the order: PMVII > PEVII > PBVII, which are in good agreement with the data reported recently by Mecerreyes and co-workers [19]. Similar to the results reported in literature [19], all the as-prepared poly(1-vinyl-3-alkylimidazolium) iodides showed a quite poor ionic conductivity of 10−6 to 10−7 S cm−1 , possibly due to a strong affinity of the polymeric cations for the iodide anions. To increase the ion dissociation and thus to enhance the ionic conduction in the polymer matrixes, we plasticized the iodide polymers using plastic crystalline SCN as a plasticizer. As an example, Fig. 1 displays the TG curves of the SCN–PMVII composite electrolyte with different percentage of SCN. As shown in Fig. 1, pure
Y. Jiang et al. / Electrochimica Acta 55 (2010) 6415–6419
Fig. 2. DSC traces of the SCN–PMVII composite electrolytes with different weight percentage of SCN: a. 0%; b. 60 wt.%; c. 40 wt.% and d. 20 wt.%. The I2 content in the electrolytes was kept at a molar ratio of I2 /PMVII = 0.2.
PMVII exhibits a single step of weight loss at ∼320 ◦ C, whereas the SCN–PMVII composite electrolytes displays two-stage weight losses at ∼90 and ∼320 ◦ C, respectively. Obviously, these two steps of weight losses for the SCN–PMVII composites arose from the evaporation of SCN and thermal decomposition of PMVII. Fig. 2 shows the DSC curves of the SCN–PMVII composite electrolytes compared with pure PMVII. As shown in Fig. 2, the PMVII alone shows a small endothermic peak at ∼100 ◦ C, which represents a configurational transformation of PMVII polymer and remains unchanged in the SCN–PMVII composite electrolytes. Nevertheless, the melting point (Tm ) of SCN varies considerably with the SCN content in the SCN–PMVII electrolytes. For the SCN–PMVII electrolytes with SCN content higher than 60 wt.%, the Tm value of SCN appeared at 52 ◦ C, which is very close to the Tm (58 ◦ C) of pure SCN crystal [13–16]. Once the SCN content was lowered down below 40 wt.%, the Tm of SCN shifted from 22 ◦ C (40 wt.% SCN) to 17 ◦ C (20 wt.% SCN), as shown in Fig. 2. These DSC features suggest that SCN may exist as a crystalline phase in the composite electrolytes at high SCN content but disperse uniformly in the host matrix of the PMVII polymer at low SCN content. XRD analysis of the composite electrolytes supports this inference. As shown in Fig. 3,
Fig. 3. XRD patterns of the SCN–PMVII composite electrolytes at different SCN content of a. 0%; b. 60 wt.%; c. 40 wt.% and d. 20 wt.%. The I2 content in the electrolytes was kept at a molar ratio of I2 /PMVII = 0.2.
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Fig. 4. Temperature dependence of the ionic conductivity for the SCN–PMVII composite electrolytes with different SCN content of a. 60 wt.%; b. 40 wt.%; c. 20 wt.%; and d. for the SCN/PEVII composite electrolyte with 40 wt.% SCN; e. for the SCN/PBVII composite electrolyte with 40 wt.% SCN. Molar ratio of I2 /polymer was fixed at 0.2.
two XRD lines at 2 = 19.7◦ and 28.25◦ , characteristic of the diffractions from the (0 1 1) and (0 0 2) planes of SCN, are only observable for the SCN–PMVII composite electrolytes with the SCN content of 60 wt.% and disappear from the other samples with decreased SCN content. 3.2. Ionic conductivity and electrochemical compatibility Ionic conductivities and their temperature dependences of the three iodide polymers plasticized with different percentages of SCN were measured and are depicted in Fig. 4. Curves a, b, c in Fig. 4 correspond to the SCN–PMVII composite electrolyte at different SCN content of 60, 40 and 20 wt.%, and d and e in Fig. 4 represent the SCN–PEVII and SCN–PBVII composites at a SCN content of 40 wt.%, respectively. In general, the plastic–polymer electrolytes displayed an ambiguous conductivity–temperature relationship, which obeys neither the classical Arrhenius law for liquid electrolytes nor the free-volume law (Vogel–Tammann–Fulcher model) for polymer electrolytes. The log − T−1 profiles for all the SCNplasticized iodide polymers appeared most likely in a similar fashion as plastic crystal electrolytes [11,16]. A most striking feature of the SCN-plasticized polymers electrolytes is the strong influence of the alkyl substituent on the ionic conductivity. As reflected in Fig. 4, the ionic conductivity decreases remarkably from 1.2 × 10−3 S cm−1 for the SCN–PMVII electrolyte to 1.6 × 10−4 S cm−1 for the SCN–PEVII electrolyte, and finally down to 3 × 10−5 S cm−1 for the SCN–PBVII composite at 25 ◦ C and at the same SCN content of 40 wt.%. In addition, the ionic conductivity of all the synthesized composite polymers decreases slowly with decreased SCN content. For instance, the SCN–PMVII electrolytes showed a gradual decrease in the ionic conductivity from 1.6 mS cm−1 at 60 wt.% SCN to ∼0.7 mS cm−1 at 20 wt.% SCN, behaving typically a plasticized polymer electrolytes composed of polymer and plasticizers such as organic electrolytes and ionic liquids. Nevertheless, this room temperature conductivity observed from the SCN–PMVII composites is comparable to that of liquid organic electrolytes and sufficiently usable for DSSCs. In DSSCs, the dye regeneration and hole transport are carried out by I− /I3 − redox couple. Therefore, the electrochemical compatibility of these plastic–polymer electrolytes for DSSCs could be evaluated by cyclic voltammetric behaviors of I− /I3 − couple. Fig. 5 gives a typical CV curve of the I− /I3 − couple in the SCN–PMVII electrolyte at a SCN content of 40 wt.%. In a wide potential region of
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Y. Jiang et al. / Electrochimica Acta 55 (2010) 6415–6419 Table 1 The cell performances of the DSSCs based on the SCN–PMVII composite electrolytes with various molar ratios of SCN:PMVII in comparison with that of the DSSCs using PC electrolyte. SCN/PMVII
I2 /PMVII
Isc (mA cm−2 )
Voc (V)
FF
(%)
60/40 wt.% 40/60 wt.% 20/80 wt.% PC electrolyte
0.2 0.2 0.2 –
9.92 9.86 9.14 10.74
0.63 0.62 0.62 0.70
0.66 0.69 0.58 0.64
4.11 4.21 3.27 4.81
Fig. 5. Typical CV curve of I− /I3 − couple in the SCN–PMVII electrolyte with the SCN content of 40 wt.%. The molar ratio of I2 /PMVII = 0.2. Scan rate: 1 mV s−1 .
−0.6 to +0.6 V (vs I3 − /I− ), there were only two pairs of reversible redox peaks observed at the right potentials corresponding to the electrochemical redox reactions of the iodine/triiodide couple and no any other CV signals from the electrolyte components were detected, suggesting that the composite electrolyte is electrochemically stable and workable in the DSSCs condition. In comparison, the two CV peaks of the I− /I3 − couple in the composite electrolyte appeared very similarly in the shape and in the potential position to those reported from liquid organic electrolytes [21,22], implying a rapid electrochemical redox and transport of I− /I3 − couple in the composite electrolyte. 3.3. Photovoltaic performance characteristics Fig. 6 compares the photocurrent–photovoltage curves of the DSSCs assembled using the SCN–PMVII electrolytes with different SCN content. The molar ratio of I2 in these electrolytes was fixed at an optimized ratio of I2 : PMVII = 0.2. The open circuit voltage (Voc ), short circuit photocurrent (Isc ), filling factor (FF) and overall photoconversion efficiency () of the DSSCs are listed in Table 1. From Fig. 6 and Table 1, it can be seen that at a SCN content ≥40 wt.%, the DSSCs based on the SCN–PMVII electrolytes show very similar photovoltaic response with almost the same photoconversion
Fig. 7. I–V curves of the DSSCs assembled with the SCN–polymer electrolyte with different polymer matrixes: a. PMVII; b. PEVII and c. PBVII. The weight ratio of SCN/polymer was fixed at 0.4 and the molar ratio of I2 /polymer = 0.2.
efficiency of = 4.1–4.2%. This photoconversion efficiency is comparable to those obtained from the DSSCs with organic carbonate electrolytes ( = 4.81% [23]) at our parallel experiments. Once the SCN content was down to ≤20 wt.%, the DSSCs with this SCN–PMVII electrolyte gave markedly declined photocurrent with the conversion efficiency down to 3.2%. This phenomenon is obviously resulted from a rapid decrease in the ionic conductivity of the electrolyte systems with decreased SCN content. The I–V curves of the DSSCs assembled with three types of the SCN-plasticized iodide polymer electrolytes are compared in Fig. 7 and their photovoltaic parameters derived from the I–V curves are summarized in Table 2. Though the DSSCs using the SCN–PEVII electrolyte displays similar values of Isc and Voc as the DSSCs with the SCN–PMVII electrolyte, their FF value (0.54) is much lower, leading to a poor conversion efficiency of 3.2%. In comparison, the DSSCs with the PBVII-based electrolyte gives the lowest Isc , Voc and FF values and therefore shows the lowest value of only 2.6%, because of its poorest conductivity in the three electrolyte families. 3.4. Long-term operating stability To evaluate the long-term stability of the plastic–polymer electrolytes, we assembled the DSSCs with optimized electrolyte composition and stored the DSSCs separately at different conditions and took them out once every few days for photovoltaic performance measurements. Fig. 8 shows the time-dependent change in Table 2 The electrolyte compositions and the cell performances of the DSSCs based on the SCN–polymer composite electrolytes with different polymer matrixes.
Fig. 6. I–V curves of the DSSCs assembled with the SCN–PMVII electrolyte with different SCN content of a. 60 wt.%; b, 40 wt.%; c. 20 wt.%. The molar ratio of I2 /PMVII = 0.2.
Polymer
SCN/polymer
I2 /polymer
Isc (mA cm−2 )
Voc (V)
FF
(%)
PMVII PEVII PBVII
40/60 wt.% 40/60 wt.% 40/60 wt.%
0.2 0.2 0.2
9.86 9.85 9.20
0.62 0.62 0.57
0.69 0.54 0.58
4.21 3.31 2.60
Y. Jiang et al. / Electrochimica Acta 55 (2010) 6415–6419
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temperature and the best photovoltaic performances used for DSSCs. The ionic conductivity of the SCN–PMVII electrolytes with the SCN content of 40–60 wt.% reached a quite high level of 1.0–1.6 mS cm−1 , comparable to that of commonly used organic liquid electrolytes. The DSSCs with these electrolytes demonstrated a photoconversion efficiency of >4.1%, almost similar to those observed from the DSSCs using organic liquid carbonate or acetonitrile electrolyte at the same experimental conditions. In addition, the DSSCs using the SCN–PMVII electrolytes exhibited an excellent durability at 60 ◦ C storage with the photoconversion efficiency remained steadily during aging test of 50 days. These results may suggest a promising application of these electrolytes for development of practicable DSSCs. Acknowledgements We acknowledge financial support by National Basic Research Program of China (2009CB220100). Fig. 8. Changes in the photoconversion efficiency of the DSSCs with the SCN–PMVII (40:60 wt.%) electrolyte stored: a. at 40 ◦ C under 1 sun illumination; b. at 60 ◦ C in an oven.
the photoconversion efficiency of the DSSCs placed at 40 ◦ C under 1 sun and separately stored at 60 ◦ C in an oven. In the fist 2 weeks of aging test, the photoconversion efficiencies of both the cells increased slightly due possibly to a better infiltration of the electrolyte into the mesoporous TiO2 electrode so as to build up a better contacted electrode/electrolyte interface. After 2 weeks, both the DSSCs attained to a stable state and kept their efficiencies steadily at the initial values in the aging duration of 50 days. Such an excellent durability of the DSSCs is no doubt resulted from the plastic–polymer electrolytes, which possess not only the sufficient electrochemical activity but also avoid the leakage and evaporation of electrolyte components commonly encountered in the DSSCs using organic liquid electrolytes. 4. Conclusions Three alkyl-substituted poly(N-alkyl-1-vinyl-imidazolium) iodides were synthesized and plasticized using succinonitrile as a crystal plasticizer to develop a series of novel solvent-free plastic–polymer electrolytes. These electrolytes all appeared as a soft solid state at room temperature and maintained sticky gel state at high temperature of 100 ◦ C. Among the plastic–polymer electrolytes thus prepared, the SCN–PMVII (succinonitrile–poly(1-vinyl-3-methylimidazolium) iodide) electrolyte showed the highest conductivity at room
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Plastic–polymer composite electrolytes for solid state dye-sensitized solar cells Y. Jiang, Y.L. Cao, P. Liu, J.F. Qian, H.X. Yang ∗ Department of Chemistry, Wuhan University, Wuhan 430072, Hubei, China
a r t i c l e
i n f o
Article history: Received 17 February 2010 Received in revised form 10 May 2010 Accepted 16 June 2010 Available online 23 June 2010 Keywords: Dye-sensitized solar cells Plastic–polymer composite electrolyte Soft solid electrolyte Poly(N-alkyl-1-vinyl-imidazolium) iodide Succinonitrile
a b s t r a c t Three types of alkyl-substituted poly(N-alkyl-1-vinyl-imidazolium) iodides were synthesized and plasticized using succinonitrile as a solid plasticizer to develop a series of novel solvent-free plastic–polymer composite electrolytes. All these electrolytes appeared as a soft solid at room temperature and became sticky gel state at high temperature of 100 ◦ C. Among the as-prepared plastic–polymer electrolytes, the SCN–PMVII (succinonitrile–poly(1-vinyl-3-methylimidazolium) iodide) electrolytes with a SCN content of 40–60 wt.% showed a room temperature conductivity of 1.0–1.6 mS cm−1 and a photoconversion efficiency of >4.1%, which are comparable to those observed from liquid organic carbonate electrolyte and the DSSCs using the liquid electrolyte at the same experimental conditions. Also, the DSSCs assembled with the SCN–PMVII electrolytes maintained their photoconversion efficiency very steadily during aging test of 50 days despite of being placed at 40 ◦ C under 1 sun illumination or stored at 60 ◦ C in an oven. Since these plastic–polymer electrolytes are solvent-free, highly conductive and electrochemically compatible, it is possible to use this type electrolyte for development of practical DSSCs. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSSCs) have attracted significant attentions in past decade as a new generation of photovoltaic cells because of their high photoconversion efficiency and low cost [1,2]. Though the DSSCs based on organic liquid electrolytes have demonstrated a high photoelectric conversion efficiency of >10% [3,4], this type of the cells has encountered a severe problem of the leakage and evaporation of the organic solvent, which lead to technological difficulties for the long-term stability and reliable encapsulation of these cells. Thus, it is highly desirable to replace the volatile organic electrolytes in view of practicable applications of DSSCs. In the pursuit of solvent-free electrolytes for solid state DSSCs, soft matter electrolytes, such as polymers [5–7], ionic liquids [8,9] and plastic crystal electrolytes [10–12], seem to be a good choice of the candidate electrolytes for DSSC applications because of their appropriate ionic conductivity and electrochemical compatibility. However, the pure polymer electrolytes reported so far have usually an insufficient ambient ionic conductivity and poorer contact with the nanocrystalline photoelectrodes, the DSSCs with such electrolytes have yet achieved a photoconversion efficiency of ∼5%, about a half of the photoconversion efficiency of the DSSCs using organic liquid electrolytes. The plastic crystal electrolytes used now for DSSCs are mostly based on succinonitrile (SCN), which has a quite low melting point of 58 ◦ C in pure crystalline state and melts at room temperature with the addition of commonly used elec-
∗ Corresponding author. Tel.: +86 27 68754526; fax: +86 27 87884476. E-mail address: [email protected] (H.X. Yang). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.06.046
trolyte salts. Such plastic crystal electrolytes, similarly like ionic liquid electrolytes, may still cause the problem of long-term stability, particularly at elevated temperature under sunlight in outdoor applications. Recently, plastic–polymer or ionic liquid–polymer composite electrolytes are actively developed for rechargeable lithium batteries [13–16] and electrochromic devices [17,18] by incorporating small molecules of plastic crystal electrolytes or ionic liquids into the matrixes of ionic conducting polymers and polymeric ionic liquids so as to form soft solid state electrolyte membranes. These composite electrolytes should be adaptable for solid state DSSCs because of their combined advantages of mechanical stability of polymers and high ionic conductivity of plastic crystal or ionic liquid electrolyte. In this work, we synthesized a series of plastic–polymer composite electrolytes using poly(alkyllimidazolium iodide) as iodide-conducting polymer matrixes and adding SCN as a solid plasticizer to decrease the affinity of the polymeric cations with the dissociable iodide anions so as to develop a new type of solvent-free electrolytes for DSSCs. Herein, the electrochemical and photovoltaic performances of the plastic–polymer composite electrolytes are described. 2. Experimental 2.1. Preparation of the plastic–polymer composite electrolytes Three kinds of poly(1-vinyl-3-alkylimidazolium) iodide with different lengths of alkyl chain (poly(1-vinyl3-methylimidazolium) iodide (PMVII), poly(1-vinyl-3-
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ethylimidazolium) iodide (PEVII), and poly(1-vinyl-3butylimidazolium) iodide (PBVII)) were synthesized by solution polymerization of corresponding 1-vinyl-3-alkyllimidazolium iodide monomers at 80 ◦ C in ethanol using 0.5% 2,2 azobisisobutyronitrile (AIBN) as initiator. The alkyllimidazolium iodides were first synthesized by adding 0.33 mol of alkyl iodide (RI, R = methyl, ethyl, n-butyl) dropwise into 150 mL of ethanol containing 0.3 mol of 1-vinylimidazole at 80 ◦ C under dry nitrogen atmosphere. The reaction solution was kept stirring for approximately 12 h and then poured into ether and put aside until two liquid phases formed clearly. After then, the underlayer liquid was taken out, washed twice with ether and then dried in a vacuum oven at 40 ◦ C to constant weight as described previously in Refs. [19,20,8]. All the alkyllimidazolium iodides thus synthesized appeared to be deep yellow liquids at room temperature. The synthetic chemical reactions for the poly(alkyllimidazolium) iodides may be described as Fig. 1. TGA curves of the SCN–PMVII composite electrolytes with different weight percentage of SCN: a. 0%; b. 60 wt.%; c. 40 wt.% and d. 20 wt.%. The I2 content in the electrolytes was kept at a molar ratio of I2 /PMVII = 0.2.
The plastic–polymer composite electrolytes were prepared by mixing poly(1-vinyl-3-alkylimidazolium) iodide (PMVII, or PEVII, PBVII) and SCN at a given molar ratio, then heating the mixture to form a melted blend at 85 ◦ C, and finally, adding a required percent of I2 into the melt under vigorous stirring for 1 h. When the molten mixture was cooled naturally down to room temperature, it solidified into a rubber-like solid state electrolyte. 2.2. Fabrication of DSSC cells A photoelectrode consisted of a TiO2 blocking layer and a nanocrystalline TiO2 on transparent fluorine-doped tin oxide glass (FTO, 15 cm−2 , Nippon Sheet Glass). To prepare the TiO2 blocking layer, an edge of the FTO glass was capped with adhesive tape to provide a noncoated area for electrical contact, and then a 0.02 mol L−1 TiCl4 aqueous solution was spread on the FTO glass. After drying the solution and removing the tape, the electrode was sintered at 500 ◦ C for 30 min. To control the thickness of the nanocrystalline TiO2 , the substrate was covered with adhesive tape again. TiO2 paste (0.3 g TiO2 powder (P25, Degussa), 0.06 g ethyl cellulose, 5 mL ethanol and 1 mL terpineol grounded 40 min) was then cast with a glass sheet sliding over the tape-covered edges and annealed at 500 ◦ C for 30 min in air. The photoelectrode thus prepared was dipped into a 0.2 mol L−1 TiCl4 aqueous solution for 30 min at 80 ◦ C until TiCl4 was hydrolyzed and then washed with distilled water to remove residual TiCl4 and again sintered at 500 ◦ C in air for 30 min. The TiO2 film electrode was immersed overnight in a 5 × 10−4 mol L−1 ethanol solution of Ru(dcbpy)2 (NCS)2 (535-bis TBA or N719, Solaronix) at room temperature and then rinsed with anhydrous ethanol and dried again. The counter electrode was prepared by electrodeposition of platinum onto the same FTO glass. In assembling DSSC cells, the plastic–polymer composite electrolyte was firstly preheated at 85 ◦ C to convert into a viscous liquid, and injected into the TiO2 electrode. The DSSCs were assembled by clamping the TiO2 photoanode/Pt-FTO counter electrode together with an 80-m-thick thermal adhesive film as a separating layer. After cooling down to room temperature, a uniform solid state electrolyte was formed in the cells. 2.3. Characterization and instruments The thermal stability of the composite electrolytes was examined by differential scanning calorimetric analysis (DSC) using a TA
instrument (Q200). Thermogravimetric analysis (TG) was carried out on a TA instrument (Q500). The crystalline structures of the composite electrolytes were characterized by XRD measurements on a Shimadzu XRD-6000 diffractometer with a Cu K␣ source. Ionic conductivities of the electrolytes were evaluated by a conductivity Meter (DDS-307, Leici, Shanghai) equipped with a platinum black electrode (DJS-1, Shanghai). The electrochemical redox behaviors of I− /I3 − in the composite electrolytes were examined by cyclic voltammetry (CV) using a Pt microdisk electrode (0.1 mm in diameter) as working electrode and a larger platinum sheet as both counter electrode and reference electrode. The data acquisitions for CV and photocurrent–photovoltage measurements were carried out by a CHI600C electrochemical station (Shanghai, China). A solar light simulator (Oriel, 91160) was used as the white light source to give AM 1.5 and 100 mW cm−2 illumination on the surface of the DSSC cells with a mask of 0.162 cm2 aperture. The intensity of incident light was calibrated with a radiant power/energy meter (Oriel, 70260) before each experiment. 3. Results and discussion 3.1. Physical appearances and thermal properties Three samples of pure poly(1-vinyl-3-alkylimidazolium) iodides (PMVII, PEVII and PBVII) synthesized in this work are all white-yellowish solid powders at room temperature. All the XRD patterns of these powders displayed a featureless broad band at lower angles of 2 = 15–25◦ , suggesting an amorphous structure of the iodide polymers. TG analysis of the iodide polymers showed a rapid weight loss of ≥80% in the temperature region of 260–350 ◦ C, indicative of the thermal decomposition of the polymers, and also, the thermal stability of these alkyl-substituted polymers was observed to be in the order: PMVII > PEVII > PBVII, which are in good agreement with the data reported recently by Mecerreyes and co-workers [19]. Similar to the results reported in literature [19], all the as-prepared poly(1-vinyl-3-alkylimidazolium) iodides showed a quite poor ionic conductivity of 10−6 to 10−7 S cm−1 , possibly due to a strong affinity of the polymeric cations for the iodide anions. To increase the ion dissociation and thus to enhance the ionic conduction in the polymer matrixes, we plasticized the iodide polymers using plastic crystalline SCN as a plasticizer. As an example, Fig. 1 displays the TG curves of the SCN–PMVII composite electrolyte with different percentage of SCN. As shown in Fig. 1, pure
Y. Jiang et al. / Electrochimica Acta 55 (2010) 6415–6419
Fig. 2. DSC traces of the SCN–PMVII composite electrolytes with different weight percentage of SCN: a. 0%; b. 60 wt.%; c. 40 wt.% and d. 20 wt.%. The I2 content in the electrolytes was kept at a molar ratio of I2 /PMVII = 0.2.
PMVII exhibits a single step of weight loss at ∼320 ◦ C, whereas the SCN–PMVII composite electrolytes displays two-stage weight losses at ∼90 and ∼320 ◦ C, respectively. Obviously, these two steps of weight losses for the SCN–PMVII composites arose from the evaporation of SCN and thermal decomposition of PMVII. Fig. 2 shows the DSC curves of the SCN–PMVII composite electrolytes compared with pure PMVII. As shown in Fig. 2, the PMVII alone shows a small endothermic peak at ∼100 ◦ C, which represents a configurational transformation of PMVII polymer and remains unchanged in the SCN–PMVII composite electrolytes. Nevertheless, the melting point (Tm ) of SCN varies considerably with the SCN content in the SCN–PMVII electrolytes. For the SCN–PMVII electrolytes with SCN content higher than 60 wt.%, the Tm value of SCN appeared at 52 ◦ C, which is very close to the Tm (58 ◦ C) of pure SCN crystal [13–16]. Once the SCN content was lowered down below 40 wt.%, the Tm of SCN shifted from 22 ◦ C (40 wt.% SCN) to 17 ◦ C (20 wt.% SCN), as shown in Fig. 2. These DSC features suggest that SCN may exist as a crystalline phase in the composite electrolytes at high SCN content but disperse uniformly in the host matrix of the PMVII polymer at low SCN content. XRD analysis of the composite electrolytes supports this inference. As shown in Fig. 3,
Fig. 3. XRD patterns of the SCN–PMVII composite electrolytes at different SCN content of a. 0%; b. 60 wt.%; c. 40 wt.% and d. 20 wt.%. The I2 content in the electrolytes was kept at a molar ratio of I2 /PMVII = 0.2.
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Fig. 4. Temperature dependence of the ionic conductivity for the SCN–PMVII composite electrolytes with different SCN content of a. 60 wt.%; b. 40 wt.%; c. 20 wt.%; and d. for the SCN/PEVII composite electrolyte with 40 wt.% SCN; e. for the SCN/PBVII composite electrolyte with 40 wt.% SCN. Molar ratio of I2 /polymer was fixed at 0.2.
two XRD lines at 2 = 19.7◦ and 28.25◦ , characteristic of the diffractions from the (0 1 1) and (0 0 2) planes of SCN, are only observable for the SCN–PMVII composite electrolytes with the SCN content of 60 wt.% and disappear from the other samples with decreased SCN content. 3.2. Ionic conductivity and electrochemical compatibility Ionic conductivities and their temperature dependences of the three iodide polymers plasticized with different percentages of SCN were measured and are depicted in Fig. 4. Curves a, b, c in Fig. 4 correspond to the SCN–PMVII composite electrolyte at different SCN content of 60, 40 and 20 wt.%, and d and e in Fig. 4 represent the SCN–PEVII and SCN–PBVII composites at a SCN content of 40 wt.%, respectively. In general, the plastic–polymer electrolytes displayed an ambiguous conductivity–temperature relationship, which obeys neither the classical Arrhenius law for liquid electrolytes nor the free-volume law (Vogel–Tammann–Fulcher model) for polymer electrolytes. The log − T−1 profiles for all the SCNplasticized iodide polymers appeared most likely in a similar fashion as plastic crystal electrolytes [11,16]. A most striking feature of the SCN-plasticized polymers electrolytes is the strong influence of the alkyl substituent on the ionic conductivity. As reflected in Fig. 4, the ionic conductivity decreases remarkably from 1.2 × 10−3 S cm−1 for the SCN–PMVII electrolyte to 1.6 × 10−4 S cm−1 for the SCN–PEVII electrolyte, and finally down to 3 × 10−5 S cm−1 for the SCN–PBVII composite at 25 ◦ C and at the same SCN content of 40 wt.%. In addition, the ionic conductivity of all the synthesized composite polymers decreases slowly with decreased SCN content. For instance, the SCN–PMVII electrolytes showed a gradual decrease in the ionic conductivity from 1.6 mS cm−1 at 60 wt.% SCN to ∼0.7 mS cm−1 at 20 wt.% SCN, behaving typically a plasticized polymer electrolytes composed of polymer and plasticizers such as organic electrolytes and ionic liquids. Nevertheless, this room temperature conductivity observed from the SCN–PMVII composites is comparable to that of liquid organic electrolytes and sufficiently usable for DSSCs. In DSSCs, the dye regeneration and hole transport are carried out by I− /I3 − redox couple. Therefore, the electrochemical compatibility of these plastic–polymer electrolytes for DSSCs could be evaluated by cyclic voltammetric behaviors of I− /I3 − couple. Fig. 5 gives a typical CV curve of the I− /I3 − couple in the SCN–PMVII electrolyte at a SCN content of 40 wt.%. In a wide potential region of
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Y. Jiang et al. / Electrochimica Acta 55 (2010) 6415–6419 Table 1 The cell performances of the DSSCs based on the SCN–PMVII composite electrolytes with various molar ratios of SCN:PMVII in comparison with that of the DSSCs using PC electrolyte. SCN/PMVII
I2 /PMVII
Isc (mA cm−2 )
Voc (V)
FF
(%)
60/40 wt.% 40/60 wt.% 20/80 wt.% PC electrolyte
0.2 0.2 0.2 –
9.92 9.86 9.14 10.74
0.63 0.62 0.62 0.70
0.66 0.69 0.58 0.64
4.11 4.21 3.27 4.81
Fig. 5. Typical CV curve of I− /I3 − couple in the SCN–PMVII electrolyte with the SCN content of 40 wt.%. The molar ratio of I2 /PMVII = 0.2. Scan rate: 1 mV s−1 .
−0.6 to +0.6 V (vs I3 − /I− ), there were only two pairs of reversible redox peaks observed at the right potentials corresponding to the electrochemical redox reactions of the iodine/triiodide couple and no any other CV signals from the electrolyte components were detected, suggesting that the composite electrolyte is electrochemically stable and workable in the DSSCs condition. In comparison, the two CV peaks of the I− /I3 − couple in the composite electrolyte appeared very similarly in the shape and in the potential position to those reported from liquid organic electrolytes [21,22], implying a rapid electrochemical redox and transport of I− /I3 − couple in the composite electrolyte. 3.3. Photovoltaic performance characteristics Fig. 6 compares the photocurrent–photovoltage curves of the DSSCs assembled using the SCN–PMVII electrolytes with different SCN content. The molar ratio of I2 in these electrolytes was fixed at an optimized ratio of I2 : PMVII = 0.2. The open circuit voltage (Voc ), short circuit photocurrent (Isc ), filling factor (FF) and overall photoconversion efficiency () of the DSSCs are listed in Table 1. From Fig. 6 and Table 1, it can be seen that at a SCN content ≥40 wt.%, the DSSCs based on the SCN–PMVII electrolytes show very similar photovoltaic response with almost the same photoconversion
Fig. 7. I–V curves of the DSSCs assembled with the SCN–polymer electrolyte with different polymer matrixes: a. PMVII; b. PEVII and c. PBVII. The weight ratio of SCN/polymer was fixed at 0.4 and the molar ratio of I2 /polymer = 0.2.
efficiency of = 4.1–4.2%. This photoconversion efficiency is comparable to those obtained from the DSSCs with organic carbonate electrolytes ( = 4.81% [23]) at our parallel experiments. Once the SCN content was down to ≤20 wt.%, the DSSCs with this SCN–PMVII electrolyte gave markedly declined photocurrent with the conversion efficiency down to 3.2%. This phenomenon is obviously resulted from a rapid decrease in the ionic conductivity of the electrolyte systems with decreased SCN content. The I–V curves of the DSSCs assembled with three types of the SCN-plasticized iodide polymer electrolytes are compared in Fig. 7 and their photovoltaic parameters derived from the I–V curves are summarized in Table 2. Though the DSSCs using the SCN–PEVII electrolyte displays similar values of Isc and Voc as the DSSCs with the SCN–PMVII electrolyte, their FF value (0.54) is much lower, leading to a poor conversion efficiency of 3.2%. In comparison, the DSSCs with the PBVII-based electrolyte gives the lowest Isc , Voc and FF values and therefore shows the lowest value of only 2.6%, because of its poorest conductivity in the three electrolyte families. 3.4. Long-term operating stability To evaluate the long-term stability of the plastic–polymer electrolytes, we assembled the DSSCs with optimized electrolyte composition and stored the DSSCs separately at different conditions and took them out once every few days for photovoltaic performance measurements. Fig. 8 shows the time-dependent change in Table 2 The electrolyte compositions and the cell performances of the DSSCs based on the SCN–polymer composite electrolytes with different polymer matrixes.
Fig. 6. I–V curves of the DSSCs assembled with the SCN–PMVII electrolyte with different SCN content of a. 60 wt.%; b, 40 wt.%; c. 20 wt.%. The molar ratio of I2 /PMVII = 0.2.
Polymer
SCN/polymer
I2 /polymer
Isc (mA cm−2 )
Voc (V)
FF
(%)
PMVII PEVII PBVII
40/60 wt.% 40/60 wt.% 40/60 wt.%
0.2 0.2 0.2
9.86 9.85 9.20
0.62 0.62 0.57
0.69 0.54 0.58
4.21 3.31 2.60
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temperature and the best photovoltaic performances used for DSSCs. The ionic conductivity of the SCN–PMVII electrolytes with the SCN content of 40–60 wt.% reached a quite high level of 1.0–1.6 mS cm−1 , comparable to that of commonly used organic liquid electrolytes. The DSSCs with these electrolytes demonstrated a photoconversion efficiency of >4.1%, almost similar to those observed from the DSSCs using organic liquid carbonate or acetonitrile electrolyte at the same experimental conditions. In addition, the DSSCs using the SCN–PMVII electrolytes exhibited an excellent durability at 60 ◦ C storage with the photoconversion efficiency remained steadily during aging test of 50 days. These results may suggest a promising application of these electrolytes for development of practicable DSSCs. Acknowledgements We acknowledge financial support by National Basic Research Program of China (2009CB220100). Fig. 8. Changes in the photoconversion efficiency of the DSSCs with the SCN–PMVII (40:60 wt.%) electrolyte stored: a. at 40 ◦ C under 1 sun illumination; b. at 60 ◦ C in an oven.
the photoconversion efficiency of the DSSCs placed at 40 ◦ C under 1 sun and separately stored at 60 ◦ C in an oven. In the fist 2 weeks of aging test, the photoconversion efficiencies of both the cells increased slightly due possibly to a better infiltration of the electrolyte into the mesoporous TiO2 electrode so as to build up a better contacted electrode/electrolyte interface. After 2 weeks, both the DSSCs attained to a stable state and kept their efficiencies steadily at the initial values in the aging duration of 50 days. Such an excellent durability of the DSSCs is no doubt resulted from the plastic–polymer electrolytes, which possess not only the sufficient electrochemical activity but also avoid the leakage and evaporation of electrolyte components commonly encountered in the DSSCs using organic liquid electrolytes. 4. Conclusions Three alkyl-substituted poly(N-alkyl-1-vinyl-imidazolium) iodides were synthesized and plasticized using succinonitrile as a crystal plasticizer to develop a series of novel solvent-free plastic–polymer electrolytes. These electrolytes all appeared as a soft solid state at room temperature and maintained sticky gel state at high temperature of 100 ◦ C. Among the plastic–polymer electrolytes thus prepared, the SCN–PMVII (succinonitrile–poly(1-vinyl-3-methylimidazolium) iodide) electrolyte showed the highest conductivity at room
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