Preparation of Nano-composite Gel Electrolytes with

Electrochimica Acta 212 (2016) 333–342

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Preparation of Nano-composite Gel Electrolytes with Metal Oxide Additives for Dye-sensitized Solar Cells Wei-Chen Changa,b , Sian-Yang Sieb , Wan-Chin Yub,** , Lu-Yin Linc,* , Ying-Jung Yub a b c

Institute of Nuclear Energy Research, Atomic Energy Council, 1000 Wenhua Rd., Chiaan Village, Lungtan, Taoyuan 325, Taiwan, ROC Institute of Organic and Polymeric Materials, National Taipei University of Technology, 1 Sec. 3, Zhongxiao E. Rd., Taipei 10608, Taiwan, ROC Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1 Sec. 3, Zhongxiao E. Rd., Taipei 10608, Taiwan, ROC

A R T I C L E I N F O

Article history: Received 5 May 2016 Received in revised form 6 June 2016 Accepted 3 July 2016 Available online 5 July 2016 Keywords: Dye-sensitized solar cell Electrolyte Long-term stability Quasi-solid-state Zinc oxide

A B S T R A C T

The dye-sensitized solar cell (DSSC) has been widely studied due to the low-cost and interesting efficiencies. The solar-to-electricity conversion efficiency (h) is an important factor governing the performance of DSSC, so is the long-term stability. In this study, three kinds of inorganic nanoparticles, i.e., SiO2, TiO2, and ZnO, are employed to solidify an acetonitrile-based liquid electrolyte for quasi-solidstate DSSCs based on ZnO photoanodes. The concentrations of I2 and LiI in the liquid electrolyte are first optimized with respect to the diffusion-limited current density of I3 ions and the photovoltaic performance of DSSC. Different amounts of the inorganic nanoparticles are then introduced into the optimized liquid electrolyte to accomplish gelation for quasi-solid-state DSSCs. All the nanoparticles tested can accomplish gelation while achieving a h higher than the reference liquid-state cell. The highest h of 4.17% was recorded for quasi-solid-state DSSCs under simulated full sunlight (AM1.5G, 100 mW/cm2) with 35.0 wt% ZnO nanoparticles as the gelator. This is due to an enhancement in short-circuit current density and open-circuit voltage as a result of efficient charge transfer channels and improved electron lifetime generated by the gelling nanoparticles. The electrochemical impedance spectroscopy was also applied to analyze the interfacial resistances in the devices. The optimized quasi-solid-state DSSC also shows excellent at-rest stability exhibiting 95% retention of the h value after 150 days of storage. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Dye-sensitized solar cells (DSSCs) have attracted much attention as one of the newly developed hybrid solar cells due to its easy fabrication and low-cost. A DSSC is composed of a photoanode, a counter electrode, and an electrolyte inserted between the two electrodes [1,2]. Usually, the electrolyte comprises a volatile organic solvent like acetonitrile, pentane nitrile, 3-methoxypropionitrile, and propylene carbonate, with the I/I3 redox couple participating to achieve a high ionic diffusion rate and therefore a high light-to-electricity conversion efficiency (h) [3]. However, the problems of sealing, corrosion and thus lack of stability limit the commercialization of DSSCs [4]. To solve these problems and improve the stability of DSSC, ionic liquid-based electrolytes have been developed. Bonhote et al. synthesized an ionic liquid and prepared the electrolyte for DSSCs

* Corresponding author. Tel: +886-2-27712171x2535; fax: +886-2-27317117. ** Corresponding author. Tel.: +886-2-27712171x2411; fax: +886-2-27317117. E-mail addresses: [email protected] (W.-C. Yu), [email protected] (L.-Y. Lin). http://dx.doi.org/10.1016/j.electacta.2016.07.009 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.

to attain improved stability [5]. Grätzel et al. prepared a quasisolid-state electrolyte using a sol-gel method with the addition of ionic liquids to achieve an h of 5.4% [6]. The same group also prepared an electrolyte with the ionic liquids 1-methyl-3propylimidazolium iodide (PMII) and 1-ethyl-3-methylimidazolium dicyanamide (EMIDCN) in the ratio of 13 to 7 to get an h of 5.7% [7]. Xiang et al. developed an aqueous gel electrolyte based on a cobalt-based redox mediator for DSSCs, and an h up to 4.1% as well as a higher device stability was observed for the gel electrolyte-based devices relative to those assembled with the corresponding liquid electrolyte [8]. Adriano et al. managed the electrode/electrolyte interfaces by using a smart design of the spatial composition of quasi-solid electrolytes. They prepared a siloxane-chain-enriched surface to increase the hydrophobicity and reduce water vapor permeation into the device, thereby enhancing the durability of the DSSCs [9]. Salvador et al. developed a DSSC gel electrolyte based on unmodified microcrystalline cellulose and ionic liquids to attain a maximum efficiency of 3.33% with long-term stability [10]. Bella et al. used a direct light-induced polymerization of cobalt-based redox shuttles for DSSCs, and a power conversion efficiency exceeding 6.5% (8.5% at low intensity)

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W.-C. Chang et al. / Electrochimica Acta 212 (2016) 333–342

was achieved for this system after 1800 h of mixed (light on/off, temperature high/low) accelerated aging tests [11]. Anantharaj et al. fabricated quasi-solid-state DSSCs with a stable gel electrolyte employing poly(ethylene oxide)-poly(ethylene glycol) for gelation, and the cell retained its initial value for 600 h while aging under ambient conditions [12]. Jayaweera et al. applied novel methods to solve the poor pore filling of quasi-solid polymer gel electrolyte and improve the performance of DSSCs [13]. Ri et al. prepared an iodine-free ionic liquid gel electrolyte using polyethylene oxide-polyethylene glycol as the gelator for Ti-foil-based DSSCs and overcame the visible light absorption and leakage problems [14]. Xia et al. reported a quasi-solid-state DSSC fabricated with the mixed-plasticizer modified polymer electrolyte to achieve better photovoltaic performance [15]. Many researchers have added gelators to form a physically or chemically cross-linked network in the electrolyte to increase the viscosity of the electrolyte and to suppress the leakage problem. Polymers and inorganic nanoparticles are the most commonly used gelators to accomplish the gelation of DSSC electrolyte. Polymer gel electrolytes are usually made by adding polymers, for example poly (vinylidene fluoride-co-hexafluoropropylene) PVDF-HFP, to the liquid electrolyte to increase the viscosity of the electrolyte, thereby suppressing solvent evaporation and enhancing the longterm stability of DSSC. However, this enhancement in stability usually comes at the expense of reduced device efficiencies, because the viscous polymer gel electrolyte also hinders the physical diffusion of redox couples within. In contrast, gelation by inorganic nanoparticles can reduce the fluidity of the electrolyte without sacrificing charge transfer rate or conversion efficiencies, because the charge transfer in the nanoparticle-based electrolyte can also proceed by the Grotthuss-type ion exchange mechanism, where physical transfer of ionic species is not necessary. In a nanoparticle-gelled electrolyte, the contiguous nanoparticles form a 3-D network and provide a suitable environment for the alignment of ions in the electrolyte, facilitating charge transfer via the Grotthuss-type ion exchange mechanism. Wang et al. used silica nanoparticles as the gelator to make the quasi-solid-state electrolyte, and a 6.6% h was obtained, which is competitive compared to the liquid electrolyte-based DSSC. This quasi-solidstate DSSC presented a high stability with 90% retention of h after a 30-day test at 80 C [16]. Usui et al. used several inorganic nanoparticles to make quasi-solid-state electrolytes based on the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIm-TFSI) to attain an h higher than 4.5% [17]. Yanagida et al. applied TiO2, SiO2, SnO2, carbon black, and multiwalled carbon nanotubes (MWCNTs) to accomplish the gelation of liquid electrolyte, and a 6.4% h was achieved by the TiO2-gelated DSSC under optimized conditions [18]. Lee et al. used TiO2 and SiO2 nanoparticles as the electrolyte gelator and the efficiencies of resulting cells were 7.47% and 6.58%, respectively. The superior performance of the TiO2-gelled DSSC is due to the more negatively charged surface of TiO2, which facilitates the alignment of I/I3 anions on the 3-D network, thus forming a path for charge transport via the Grotthuss-type ion exchange mechanism [19]. The DSSC with the TiO2-gelled electrolyte retained 80% of its initial h after 1200 h of storage [20]. Mohanty et al. used inorganic nanoparticles of different isoelectric points to gel the electrolyte. The DSSCs gelated with SiO2 or Al2O3 have better performance than the MgO-gelled ones, owing to more efficient Grotthuss-type transport in the former [18]. In fact, inorganic nanoparticles can also be used in conjunction with a polymer gelator. Adding a small amount of nanoparticles (less than what needed in a nanoparticlegelled electrolyte) is a viable mean of enhancing charge transfer in a polymer gel electrolyte. The nanoparticles in the resulting composite electrolyte can reduce the crystallinity of the polymer matrix and enhance the mobility of redox couple, thereby

improving the conversion efficiency of devices [21,22]. Inorganic nanoparticles have been demonstrated to have the ability to accomplish the gelation of liquid electrolytes and enhance the stability of DSSC. However, there is no study on the application of nanoparticle-gelled electrolyte to ZnO-based DSSCs and how different types of inorganic nanoparticles (as gelators) affect DSSC performance. In this study, quasi-solid-state electrolytes made by mixing SiO2, TiO2, or ZnO nanoparticles with the liquid electrolyte were applied to ZnO photoanode-based DSSCs. The LiI and I2 concentrations in the liquid electrolyte were optimized first as regard to the redox ability of the redox couple in the liquid electrolyte, which was evaluated using cyclic voltammetry (CV). Based on the optimized liquid electrolyte, various amounts of SiO2, TiO2, and ZnO nanoparticles were separately added to the liquid electrolyte to form quasi-solid-state electrolytes. The effect of different gelating nanoparticles on the photovoltaic performance of resulting quasi-solid-state DSSC was investigated. Of the nanoparticles studied, ZnO produced the best results. The DSSCs gelled with 35.0 wt% of ZnO reached a h of 4.17%, much higher than the h of 3.17% of the liquid-state counterparts. Furthermore, a 140-day at-rest stability test shows the best performing ZnO-gelled DSSC retained 95% of its initial h, whereas the liquid-state reference cell retained 85%. Using ZnO nanoparticles as the gelator enhances not only h but also the durability of DSSC. The main reasons are enhanced electron lifetime in the photoanode and reduced electrolyte leakage for the quasi-solid-state electrolyte. The results suggest that both the conversion efficiency and the durability of DSSC can be simultaneously enhanced through the incorporation of cheap and abundant inorganic nanoparticles into the liquid electrolyte using a simple blending method. Thus, the light-toelectricity conversion efficiency and the long-term stability are no longer trade-off factors in the field of DSSCs. 2. Experimental Section 2.1. Materials Lithium iodide (LiI) and 1,2-dimethyl-3-propylimidazolium iodide (PMII) were obtained from Merck. Acetonitrile was bought from J. T. Baker. Iodide (I2), tert-butylpyridine (TBP), ZnO nanoparticles, and SiO2 nanoparticles were acquired from SigmaAldrich. H2PtCl6 was purchased from Showa. Commercial TiO2 nanoparticle (P25) was obtained from Degussa. 2.2. Preparation of the liquid electrolyte and the quasi-solid-state electrolyte The liquid electrolyte was made by first adding 0.6 M PMII and various concentrations of I2 to acetonitrile under stirring, followed by the addition of 0.5 M TBP and various amounts of LiI to the acetonitrile solution. To prepare quasi-solid-state electrolytes, SiO2, TiO2, or ZnO nanoparticles were first dried in a vacuum oven overnight to remove water from the pores of the nanoparticles. Then different weight ratios of the nanoparticles were separately blended into the optimized liquid electrolyte that contained optimal amounts of I2 and LiI. 2.3. Dye-sensitized solar cell fabrication The ZnO paste was prepared by dispersing commercial ZnO nanoparticles (20 nm in size) in equal proportions of a-terpineol and ethylcellulose. A screen-printing technique was used to prepare ZnO film on fluorine-doped tin oxide (FTO) glass (Nippon Sheet Glass, 8–10 V/&, 2.2 mm-thick) as the photoanode of DSSCs. The FTO glass was cleaned as follows. First the FTO glass was


soaked in acetone under sonication for 30 min, followed by sonication in a neutral cleaner at 70 C for 30 min. To finish the cleaning process FTO glass was rinsed thoroughly with deionized water. A portion of 0.5 0.5 cm2 ZnO film was selected as the apparent cell area. The ZnO electrodes were then annealed at 150 C for 1 h to remove organic materials in the film and to increase the crystallinity of ZnO. After cooling to room temperature, the ZnO electrodes were immersed into a solution composed of 0.5 mM cis-bis (isothiocyanato) bis-(2,20 -bipyridyl-4,40 -dicarboxylato)-ruthenium (II) bis-tetrabutylammonium (N719, Solaronix) in a mixed solvent of acetonitrile and tert-butanol (volume ratio of 1:1) for about 2 h at the room temperature. The Pt counter electrode was prepared by decomposing H2PtCl6 on the FTO glasses at 400 C for 20 min. The ZnO photoanode and the Pt counter electrode were separated by a 60 mm hot-melting spacer and sealed by heating. The electrolyte was injected into the gap between the electrodes by capillarity through two holes on the FTO glass. 2.4. Characterization The morphologies of the SiO2, TiO2, and ZnO nanoparticles were investigated by using a field emission scanning electron microscope (FE-SEM, FEI Nova230). The current density-voltage (J-V) curves of DSSCs were measured under a white light source (Yamashita Denso, YSS-100A) with an irradiance of 100 mW cm2 (the equivalent of AM1.5G). The irradiance of simulated light was calibrated using a silicon photodiode (BS-520, Bunko Keiki). The evolution of the electron transport process in the cell was investigated using the electrochemical impedance spectroscopy (EIS), and the impedance measurements were performed under AM1.5 G illumination using an electrochemical analyzer (Autolab PGSTAT30, Eco-Chemie). The applied DC bias voltage and AC amplitude were set at VOC of the cell and 10 mV between the working and the counter electrodes, respectively. The frequency range extended from 102 to 105 Hz. 3. Results and Discussion 3.1. The effects of the concentration of I2 and LiI in the liquid electrolyte on the performance of DSSCs The well known redox reactions of the I/I3 redox couple in the electrolyte are as follows [23]. I2 + I $ I3

(1)

I3 + 2e ! 3I (at the counter electrode)

(2)

3I ! I3 + 2e (at the photoanode)

(3)

I3 + 2e (conduction band) ! 3I (side reaction at the photoanode) (4) The initial concentration of I3 varied with the amount of I2 in the electrolyte, as indicated in Eq. (1). The concentration of I3 has a great influence on the redox reaction rate, as shown by Eqs. (2)–(4), and therefore on the performance of DSSCs. Hence, the I2 concentration is important and worthy to be discussed and optimized. On the other hand, it has been reported that the small size of lithium ions make them easier to diffuse into the pores of the oxide film and form Li+-e dipoles with the electrons on ZnO. This dipole can migrate to the oxide surface and reduce the charge-

335

transfer resistance to improve the photocurrent density [24]. However, it has also been suggested that the incorporation of lithium ions in the electrolyte can induce a positive shift in the conduction band edge for TiO2, leading to the enlargement of the band gap between the conduction band of TiO2 and the lowest unoccupied molecular orbital (LUMO) of the sensitizer, as well as shortening the gap between the Fermi level of TiO2 and the redox potential of the electrolyte. These changes in the gaps between different levels result in reduced open-circuit voltage (VOC) [25– 28]. This phenomenon can also be found in the ZnO-based systems [29]. Hence, the concentrations of I2 and LiI in the electrolyte, which would respectively influence the amounts of I3 and I in the electrolyte, were examined in the first part of this study, with regard to the performance of DSSC and the diffusion resistance of the electrolyte. To gain an understanding of the ion transport performance in the electrolyte, cyclic voltammetric measurements were performed on Pt-Pt symmetric cells composed of two identical platinized FTO glasses with the electrolyte sandwiched inbetween. As shown in Fig. S1 in the electronic supplementary information (ESI), while the absolute value of the applied voltage increases, the current density increases as well and finally reaches a stable value, which is the so-called diffusion-limited current density. In this measurement, the I3 is the main ion influencing the mass transfer, so the diffusion-limited current density observed in the CV plots can be inferred to be the value for the I3 ions. The highest diffusion-limited current densities were obtained when there was 0.1 M I2 in the electrolyte, suggesting 0.1 M I2 was optimal. Among the cases containing 0.1 M I2, the one with 0.05 M LiI showed the largest diffusion-limited current density in the reduction region, suggesting 0.05 M being the optimal concentration of Li+ in the electrolyte. To evaluate the effects liquid electrolyte compositions on the photovoltaic performance of pertinent DSSCs, the J-V curves were measured, and the corresponding photovoltaic parameters are listed in Table S1 of ESI. As shown, the VOC values are smaller for the cells composed of higher concentrations of LiI in the electrolyte, probably due to the positive shift of the conduction band edge of ZnO and thus the reduction of the gap between the Fermi level of ZnO and the redox potential of the electrolyte. The short-circuit current density (JSC) value does not present any regular trend with the variations of I2 and LiI concentrations, probably due to excessive I ions and the multiple sources of I ions, i.e. from LiI and the ionic liquid PMII. The highest h value of 3.10% was obtained by DSSCs with 0.05 M I2 and 0.05 M LiI in the electrolyte. This electrolyte composition is inconsistent with the result obtained from the CV measurements, where the composition produced the largest diffusion-limited current density is 0.1 M I2 and 0.05 M LiI. Apparently, device efficiencies did not follow a linear relationship with the diffusion-limited current density, indicating that additional factors should be considered. Furthermore, the chargetransfer resistances in the DSSCs was evaluated by using the EIS technique, which is a powerful technique to investigate the electron transport kinetics and the interfacial charge transfer resistances of DSSCs [30]. Fig. S2(a) in the ESI shows Nyquist plots for the DSSCs with varying concentrations of I2 and LiI in the electrolyte along with the equivalent circuit in the inset. The enlarged plots at the low frequency region are also shown in Fig. S2 (b) for clearer observation. Generally, three semicircles can be observed in the impedance spectra of DSSCs. The first semicircle in the high-frequency range is assigned as the charge transfer resistance at interfaces of the Pt counter electrode (Rct1). The second semicircle in the mid-frequency range represents the charge transfer resistance at the interfaces between ZnO, the sensitizer, and the electrolyte (Rct2). The third semicircle in the low-frequency range corresponds to the Warburg diffusion process

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of I/I3 in the electrolyte (ZW) [31]. The first and third semicircles are similar for all the cases, indicating the charge transfer resistances at the corresponding interfaces are similar due to the liquid state of the electrolyte, even though the concentrations of the ions are varied. However, the DSSC with 0.05 M I2 and 0.05 M LiI in the liquid electrolyte presents the smallest second semicircle among all the cases, indicating that the charge transfer resistance at the interface between ZnO, the N719 dye, and the electrolyte is the smallest for this case. The result suggests that for the DSSC containing the optimized concentrations of I2 and LiI, the charge transfer resistance at the photoanode is the lowest, resulting in the largest JSC in this case. 3.2. Influences of the amount of the SiO2 gelator on the performance of quasi-solid-state dye-sensitized solar cells The inorganic nanoparticles SiO2 have the ability to form a cross-linked structure in the liquid electrolyte and therefore to increase the viscosity of the solution when they are incorporated into the liquid electrolyte. [31] However, their weight ratio to the liquid electrolyte need to be optimized to achieve high efficiency quasi-solid-state DSSCs, because the amount of SiO2 have significant effects on the viscosity and charge transport property of resulting gel electrolytes. The SEM image for the SiO2 nanoparticles is shown in Fig. S3 in the ESI. The individual SiO2 nanoparticles show a size of around 30 to 40 nm, but they tend to form aggregates of about 100 nm in size. The SiO2 nanoparticles were added to the liquid electrolyte at three different levels, i.e., 1.5 wt%, 3.0 wt%, and 5.0 wt%, to make quasi-solid-state electrolytes. The apparent diffusion coefficients (Dapp) for I3 were evaluated from the CV curves obtained from Pt-Pt symmetric cells that contained various SiO2-gelled electrolytes (Fig. 1(a)), and the corresponding values are summarized in Table 1. Compared to the liquid electrolyte, those gelated by the SiO2 nanoparticles had higher Dapp values. The Dapp value was enhanced to 1.33 105 cm2/s when 1.5 wt% SiO2 nanoparticles were added to the

electrolyte. When the amount of SiO2 nanoparticles was increased to 3.0 wt%, an even higher Dapp of 1.45 105 cm2/s was obtained. However, when 5.0 wt% SiO2 nanoparticles were used for the gelation, the Dapp value decreased to 1.38 105 cm2/s. It is thought that the cations (particularly the imidazolium cation of ionic liquid) in the electrolyte would be adsorbed on the negatively charged surface of the SiO2 nanoparticles. The I and I3 anions would then align around the SiO2 nanoparticles through electrostatic interaction with the adsorbed cations, leading to improved charge transfer by ion exchange. However, if an excessive amount of SiO2 nanoparticles is introduced into the electrolyte, the SiO2 nanoparticles are likely to aggregate, which may limit the ionic diffusion and reduce the diffusion coefficients. Further analyses on the photovoltaic performance were made for DSSCs gelled with 1.5 wt%, 3.0 wt%, and 5.0 wt% of the SiO2 nanoparticles and for those using the liquid electrolyte. The J-V curves are shown in Fig. 1(b), and the corresponding photovoltaic parameters are summarized in Table 1 for comparison. It can be observed that the JSC value increases significantly when the SiO2 nanoparticles are introduced into the electrolyte, compared with that of the liquid electrolyte cells. The results are probably due to enhanced Grotthuss-type ion exchange caused by the gelling nanoparticles, which form efficient charge transfer channels that allow charge transport by electron hopping [32]. The VOC value decreases slightly for the SiO2-gelled DSSCs. It is inferred that the SiO2 nanoparticles may cause the electrons in the conduction band (CB) of ZnO to transfer to the redox potential of the redox couple in the electrolyte, leading to the reduction of the electron lifetime and the enhancement of the possibility of charge recombination. Therefore, the cell gelated with 3.0 wt% SiO2 nanoparticles had the highest h value of 3.63%, and the corresponding JSC, VOC, and FF values were 8.8 mA/cm2, 0.62 V, and 0.67, respectively, which is a significant improvement over the h of 3.10% of the cell lacking the SiO2 nanoparticles. Furthermore, the Nyquist plots for the SiO2gelled quasi-solid-state DSSCs and the liquid-state counterpart are shown Fig. 1(c), and the enlarged plots at the low frequency region

Fig. 1. (a) The CV curves, (b) the J-V plots, (c) the Nyquist plots along with the equivalent circuit, and (d) the enlarged Nyquist plots of (c) at the low frequency region for the DSSCs fabricated using the electrolyte with different amounts of the SiO2 additive.


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Table 1 The diffusion-limited current density (J), ion apparent diffusion coefficient (Dapp), and the photovoltaic parameters for the DSSCs fabricated using the electrolyte with different amounts of the SiO2 additive. Amount of the SiO2 additive (wt%) 0 1.5 3.0 5.0

J (mA/cm2) 36.55 42.85 46.94 44.36

Dapp (cm2/s) 5

1.14 10 1.33 105 1.45 105 1.38 105

are presented in Fig. 1(d) for clearer observation. The DSSC with 3.0 wt% SiO2 nanoparticles added in the electrolyte presents the smallest semicircle in the mid frequency region, suggesting it has the smallest Rct2 value among all the cases. The largest semicircle in the middle frequency region and the largest Rct2 value were obtained for the cell employing the liquid electrolyte. The results verify that by adding SiO2 nanoparticles into the electrolyte, the interfacial resistance between the ZnO layer, the sensitizer, and the electrolyte is reduced. The results are due to the formation of effective charge transfer channels in the quasi-solid-state electrolyte system loaded with inorganic nanoparticles, which favors the Grotthus-type ion exchange mechanism. The diffusion resistances in the electrolyte are similar for all the cases. The result suggests that the enhanced performance of the SiO2-gelled DSSC is not due to the improved diffusion ability of the electrolyte, but the reduction of the resistance in the photoanode. This is supported by the similarity in the ZW values for all cases and the smallest Rct2 value of the 3.0 wt% SiO2 device. This is consistent with the observation that the 3.0 wt% SiO2 device has the highest JSC and h values, as shown Fig. 1(b) and Table 1. 3.3. Influences of the amount of the TiO2 gelator on the performance of quasi-solid-state electrolyte dye-sensitized solar cells After examining the SiO2 nanoparticles as a gelling agent, we turned our attention to another gelator, TiO2 nanoparticles, which

JSC (mA/cm2)

VOC (V)

FF

h (%)

6.72 8.59 8.80 8.51

0.65 0.62 0.62 0.61

0.71 0.67 0.67 0.67

3.17 3.56 3.63 3.53

have the same ability as the SiO2 nanoparticles to form a crosslinked structure in the liquid electrolyte, allowing the generation of quasi-solid-state electrolytes [31]. The amount of TiO2 nanoparticles introduced into the liquid electrolyte was optimized. The SEM image for the TiO2 nanoparticles is shown in Fig. S4 in the ESI. The TiO2 nanoparticles exhibit irregular shapes with several small protuberances on the surface, and the average size for the nanoparticle is around 40 to 50 nm. Quasi-solid-state electrolytes were made by adding 10.0 wt%, 12.5 wt%, 15.0 wt%, 17.5 wt% and 20.0 wt% of the TiO2 nanoparticles into the liquid electrolyte. To analyze the ion transport performances of the electrolytes, the CV curves for the Pt-Pt symmetric cells were measured, as shown in Fig. 2(a), and the corresponding Dapp values for I3 were then evaluated as given in Table 2. The Dapp values of 1.56 105, 1.64 105, 1.74 105, 1.55 105, and 1.54 105 cm2/s were obtained for the quasi-solid-state electrolytes gelated by using 10.0 wt%, 12.5 wt%, 15.0 wt%, 17.5 wt% and 20.0 wt% TiO2 nanoparticles, respectively, which are much higher than the Dapp value of 1.14 105 cm2/s for the liquid electrolyte. The trend is similar to that of the SiO2-based gel electrolytes. There exists an optimal weight ratio of the nanoparticle gelator, too low or too high a weight percentage leads to reduced Dapp. Based on the Dapp value, the optimal weight percentage of the TiO2 nanoparticles was determined to be 15.0 wt%, much higher than the optimal percentage of the SiO2 nanoparticles. This is probably caused by their differences in size, density, and surface charge.

Fig. 2. (a) The CV curves, (b) the J-V plots, (c) the Nyquist plots along with the equivalent circuit, and (d) the enlarged Nyquist plots of (c) at the low frequency region for the DSSCs fabricated using the electrolyte with different amounts of the TiO2 additive.

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Table 2 The diffusion-limited current density (J), ion apparent diffusion coefficient (Dapp), and the photovoltaic parameters for the DSSCs fabricated using the electrolyte with different amounts of the TiO2 additive. Amount of the TiO2 additive (wt%) 0 10.0 12.5 15.0 17.5 20.0

J (mA/cm2) 36.55 50.06 52.90 56.16 50.06 49.55

Dapp (cm2/s) 5

1.14 10 1.56 105 1.64 105 1.74 105 1.55 105 1.54 105

Further analyses on the photovoltaic performance of resulting TiO2-gelled quasi-solid-state DSSCs were performed. The J-V curves of these DSSCs and the liquid electrolyte reference cell are shown in Fig. 2(b), with the corresponding photovoltaic parameters given in Table 2. Higher JSC values were obtained for the cells with the TiO2 nanoparticles incorporated in the electrolyte, compared to the DSSC employing the pure liquid electrolyte. This is due to the enhanced charge transportation caused by the Grotthus-type ion exchange mechanism when nanoparticles are incorporated into the electrolyte [32]. On the other hand, the VOC values show small reductions for the cells using the TiO2 gelator, as compared with the liquid-state reference cell. The result is due to the decrease in the electron lifetime and the enhancement in the charge recombination in the former cases. Hence, the cell gelated with 15.0 wt% TiO2 nanoparticles had the highest h value of 3.74%, and its JSC, VOC, and FF values were 9.0 mA/cm2, 0.61 V, and 0.67, respectively. The charge transfer resistances at the interfaces in the DSSCs were further examined by using the EIS technique. The Nyquist plots for the TiO2-gelled quasi-solid-state DSSCs and the cell lacking the TiO2 gelator are shown in Fig. 2(c). The liquid-state cell has a larger semicircle in the mid frequency region than the quasi-solid-state cells. The result suggests a larger charge transfer resistance (Rct2) exists at the ZnO/dye/electrolyte interface for the liquid-state DSSC, owing to the lack of efficient charge transportation channels, which only occur when inorganic nanoparticles are incorporated into the electrolyte. Also, Fig. 2(d) presents the enlarged Nyquist plots in the low frequency region, which indicate similar diffusion resistances in the electrolyte (ZW) for all the cases. The results are similar to what obtained when the SiO2 nanoparticles are used as the gelator. The result suggests that nanoparticle gelators may have similar effects regardless of their types when they are applied to accomplish the gelation of the electrolyte, as long as they are negatively charged. The smallest Rct2 value was obtained by the 15.0 wt% TiO2-gelled DSSC, which is consistent with the high JSC and h values of this cell (see Fig. 2(b) and Table 2). Also similar ZW values were observed for all the DSSCs. 3.4. Influences of the amount of the ZnO gelator on the performance of the quasi-solid-state electrolyte dye-sensitized solar cells The gelators, SiO2 and TiO2 nanoparticles, have been applied to prepare quasi-solid-state electrolytes, and the corresponding electrochemical parameters for the resulting DSSCs have been presented in previous sections. Here, ZnO nanoparticles were used as the gelator for preparing quasi-solid-state electrolytes. Like the SiO2 and TiO2 nanoparticles, the ZnO nanoparticles can also increase the viscosity of the electrolyte by forming a crosslinked structure in the liquid electrolyte [31]. Therefore, the amount of ZnO nanoparticles added to the liquid electrolyte to form quasi-solid-state electrolytes was discussed. The morphology of the ZnO nanoparticles was examined by SEM, and the

JSC (mA/cm2)

VOC (V)

FF

h (%)

7.2 8.7 8.8 9.0 8.6 8.7

0.65 0.61 0.60 0.61 0.61 0.61

0.65 0.66 0.66 0.67 0.67 0.67

3.10 3.53 3.54 3.74 3.55 3.56

image is shown in Fig. S5 in the ESI. The ZnO nanoparticles exhibit a rod-like structure with a size ranging from 50 to 130 nm, which is much larger than those of the SiO2 and TiO2 nanoparticles. The ZnO weight ratios of 30.0, 35.0, and 40.0 wt% were applied to the preparation of quasi-solid-state electrolytes. The CV curves of the Pt-Pt symmetric cells were measured to analyze the electrochemical performances of the electrolytes, as shown in Fig. 3(a), and the corresponding Dapp values for I3 are estimated as presented in Table 3. The Dapp value for I3 is estimated to be 1.14 105 cm2/s for the liquid electrolyte, and the value increases to 1.22 105 cm2/s when 30.0 wt% ZnO nanoparticles is introduced to the liquid electrolyte for gelation. Increasing the weight ratio of ZnO further to 35.0 wt% leads to a higher Dapp value of 1.39 105 cm2/s, but when 40.0 wt% ZnO nanoparticles are added to the liquid electrolyte, the Dapp value decreases to 1.22 105 cm2/s. The initial enhancement in the Dapp value with the increasing weight ratio of ZnO nanoparticles is probably due to the formation of a more complete 3D network for charge transport via the Grotthusstype ion exchange mechanism. The reduction of the Dapp value by increasing the weight ratio of ZnO nanoparticles further to 40.0 wt % is probably due to the aggregation of the ZnO nanoparticles in the electrolyte. The Dapp value vs. weight ratio trend is similar to what observed when the SiO2 and TiO2 nanoparticles are used as the gelator. The J-V curves, as presented in Fig. 3(b), were obtained for quasi-solid-state DSSCs that contained various amounts of the ZnO gelator and for the liquid electrolyte cell to evaluate their photovoltaic performances, and the corresponding photovoltaic parameters are listed in Table 3. The JSC values of the quasi-solidstate DSSCs are larger than the liquid-state reference cell. The result is similar to those obtained with SiO2 and TiO2 as the gelator, as a result of enhanced charge transportation due to the Grotthustype mechanism [32]. Introducing 35.0 wt% ZnO nanoparticles into the electrolyte leads to the largest Dapp value for I3 ions and the best performing cell with the following photovoltaic parameters: h = 4.17%, JSC = 9.0 mA/cm2, VOC = 0.70 V, and FF = 0.66, indicating the optimal weight ratio of ZnO gelator is 35.0 wt%. The EIS technique was then applied to analyze the charge transfer resistance at the interfaces for quasi-solid-state DSSCs with different weight ratios of the ZnO gelator, as shown by the Nyquist plots in Fig. 3(c). Again the liquid electrolyte DSSC exhibit a larger semicircle in the mid frequency region than the quasisolid-state devices, just like what have been observed when SiO2 and TiO2 nanoparticles are used as the gelator. This indicates that charge transfer resistance (Rct2) at the ZnO/dye/electrolyte interface is reduced when ZnO nanoparticles are introduced into the electrolyte, because the ZnO nanoparticles can form efficient channels in the electrolyte to help charge transportation. Similar diffusion resistances in the electrolyte (ZW) were again found for all the cases, as what have been observed for the SiO2 and TiO2 gelator cases. The 35.0 wt% ZnO-gelled DSSC has the smallest Rct2, which is consistent with its highest JSC and h values, as shown in both Fig. 3(b) and Table 3.


339

Fig. 3. (a) The CV curves, (b) the J-V plots, and (c) the Nyquist plots along with the equivalent circuit for the DSSCs fabricated using the electrolyte with different amounts of the ZnO additive.

3.5. Comparison of quasi-solid-state dye-sensitized solar cells with different inorganic nanoparticles as the gelator Quasi-solid-state DSSCs are usually fabricated with TiO2 photoanodes [33–35]. In this study, quasi-solid-state DSSCs were fabricated with a ZnO photoanode and three different nanoparticle gelators, SiO2, TiO2, and ZnO. The optimized weight percentages of SiO2, TiO2, and ZnO are 3.0 wt%, 15.0 wt%, and 35.0 wt%, respectively. A main reason for the large difference in the optimized weight percentage is probably the size of the nanoparticles. As observed in Figs. S3, S4, and S5 of the ESI, the sizes of the SiO2, TiO2, and ZnO particles are around 30–40 nm, 40–50 nm, and 50– 130 nm, respectively. The inorganic nanoparticles form a 3-D network in the electrolyte through particle-particle and particleelectrolyte interactions. The specific surface area of nanoparticles is expected to decrease with an increase in their size, so at the same weight percentage the ZnO nanoparticles are expected to have the lowest surface area through which to interact with the electrolyte. Consequently a higher weight percentage is needed to gelate the electrolyte when the ZnO nanoparticles are used as the gelator. Also, when too many nanoparticles are present in the electrolyte agglomeration is likely to occur, which can interfere with the alignment of ions and thus limit charge transfer. The aggregation is

expected to form more easily when the size of the nanoparticles is small. Therefore, the optimized weight percentage of the nanoparticles is lower for smaller nanoparticles. Here the electrochemical performances for quasi-solid-state DSSCs prepared with the optimized weight ratios of SiO2, TiO2, and ZnO nanoparticles are compared. Fig. 4(a) shows the CV curves of Pt-Pt symmetric cells that contained various electrolytes. The symmetric cell with the liquid electrolyte shows the smallest diffusion-limited current density, whereas the one containing the 35.0 wt% ZnO-gelled electrolyte presents the largest diffusion-limited current density. The results suggest the liquid electrolyte has the smallest Dapp value for I3 while the ZnO-gelated electrolyte has the largest Dapp value for I3. The cases using SiO2 and TiO2 as the gelator present similar current densities, and therefore their apparent diffusion coefficient for I3 are inferred to be similar. The overall comparison on the photovoltaic performance of the quasi-solid-state and the liquid-state DSSCs is presented in the form of J-V curves shown in Fig. 4(b), and the corresponding photovoltaic parameters are listed in Table 4. It is clear that the JSC values are greatly enhanced for the quasi-solid-state DSSCs as compared with the value for the liquidstate one. As mentioned earlier in the text, the inorganic nanoparticles have the ability to form efficient charge transport channels in the electrolyte to enhance the transportation of

Table 3 The diffusion-limited current density (J), ion apparent diffusion coefficient (Dapp), and the photovoltaic parameters for the DSSCs fabricated using the electrolyte with different amounts of the ZnO additive. Amount of the ZnO additive (wt%)

J (mA/cm2)

Dapp (cm2/s)

JSC (mA/cm2)

VOC (V)

FF

h (%)

0 30.0 35.0 40.0

36.55 39.37 44.69 39.35

1.14 105 1.22 105 1.39 105 1.22 105

7.2 7.8 9.0 8.2

0.63 0.66 0.70 0.67

0.66 0.65 0.66 0.65

3.12 3.43 4.17 3.72

340


Fig. 4. (a) The CV curves, (b) the J-V plots, (c) the Nyquist plots along with the equivalent circuit, and (d) the Bode plots for the DSSCs fabricated using the electrolyte with SiO2, TiO2, and ZnO additives in the optimized amounts and for the cell fabricated using the electrolyte without adding any additive (labeled as liquid electrolyte).

Table 4 The electron lifetime (te) and the photovoltaic parameters for the DSSCs fabricated using the electrolyte with SiO2, TiO2, and ZnO additives in the optimized amounts and those of the cell fabricated using the electrolyte without adding any additive. Additive

te (ms)

JSC (mA/cm2)

VOC (V)

FF

h (%)

w/o 3.0 wt% SiO2 15.0 wt% TiO2 35.0 wt% ZnO

1.63 1.19 1.19 2.25

6.7 8.8 9.0 9.0

0.65 0.62 0.61 0.70

0.71 0.67 0.67 0.66

3.17 3.63 3.74 4.17

charges, and therefore the corresponding JSC values are enhanced significantly. Compared with the VOC value for the liquid electrolyte-based cell, the VOC values are reduced for SiO2-gelled and TiO2-gelled DSSCs, but the opposite happens for ZnO-gelated cells, where VOC value increases. It is inferred that the SiO2 and TiO2 nanoparticles can induce the electrons in the CB of ZnO in the photoanode transferring to the redox potential of the redox couple in the electrolyte, leading to the electron lifetime reduction and recombination enhancement. However, the addition of the ZnO nanoparticles in the electrolyte may not have this effect since the gelator ZnO in the electrolyte is the same material as the ZnO in the photoanode. The addition of ZnO in the electrolyte even improves the Voc value for the pertinent DSSC. Therefore, the highest h value of 4.17% was obtained for the DSSC using ZnO nanoparticles as the gelator. This value represents 33% enhancement as compared with the h of the liquid-state reference DSSC. Fig. 4(c) shows the Nyquist plots for DSSCs composed of the liquid electrolyte and the quasi-solid-state electrolytes that separately use 3.0 wt% SiO2, 15.0 wt% TiO2, and 35.0 wt% ZnO nanoparticles as the gelator. The largest semicircle in the mid frequency region is clearly observed for the liquid-electrolyte DSSC, indicating the largest Rct2 value for this case. As mentioned previously in the text, adding inorganic nanoparticles in the electrolyte can reduce the interfacial resistance between the ZnO layer, the sensitizer, and the

electrolyte, owing to enhanced charge transportation caused by the formation of the effective channels in the electrolyte. Also, the diffusion resistance and the ZW values are similar for all the cases. Furthermore, the Bode plots were made to investigate the electron lifetime (te) of the DSSCs using different electrolytes, as shown in Fig. 4(d). The longest electron lifetime of 2.25 ms evaluated by using the smallest frequency was obtained for the ZnO-gelated DSSC, while the smallest electron lifetime of 1.19 ms was observed for the cells using SiO2 and TiO2 as the gelator. The liquid electrolyte-based DSSC shows an electron lifetime of 1.63 ms, which is smaller than the value for the ZnO-gelled DSSC but larger than those for the quasi-solid-state DSSCs using the SiO2 and TiO2 nanoparticles as the gelator. This result displays the same trend as the VOC value, suggesting the VOC value is mainly influenced by the electron lifetime of the system. Lastly, the liquid-electrolyte reference cell and the quasisolid-state DSSC that achieves the highest h, i.e., the one using 35.0 wt% ZnO as the gelator, were subjected to an at-rest long-term stability test for 170 days. The samples were kept in the dark at the room temperature with their h measured periodically. The normalized h values for these DSSCs as a function of postassembly time are shown in Fig. 5. The h values show slight increases as compared to the initial value for the quasi-solid-state DSSC, but this phenomenon is observed for the DSSC using the liquid electrolyte. It is inferred that the quasi-solid-state electrolyte is composed of numerous inorganic nanoparticles, so the quasi-solid-state electrolyte does not diffuse as efficient as the liquid electrolyte. Hence the increase in the h value for the quasisolid-state DSSC at the beginning is probably due to the completion of the electrolyte diffusion. As the aging process proceeded even further, the h values for both cases started to decrease. After 150 days of aging, the quasi-solid-state DSSC retains 95% of the h value and presents a much higher stability than the liquid-electrolyte DSSC, which shows 15% decay of its initial h after 140 days of aging.


Fig. 5. The normalized efficiency as a function of the time for testing the long-term stability for the DSSCs composed of the quasi-solid-state electrolyte gelated by using 35.0 wt% ZnO and of the liquid electrolyte for over than 130 days.

4. Conclusions Quasi-solid-state electrolytes gelated by inorganic nanoparticles, which are usually paired with TiO2-based photoanodes, are adopted here to fabricate DSSCs based on a ZnO photoanode. Three different nanoparticles, i.e., SiO2, TiO2, and ZnO were used as the gelator to prepare quasi-solid-state electrolytes, and the nanoparticle-to-liquid-electrolyte weight ratio was optimized in terms of photovoltaic performances of resulting DSSCs. Using the nanoparticles as the gelator significantly improves the JSC values for resulting quasi-solid-state DSSCs, as compared with the cell using the liquid electrolyte. This is mainly due to the effective channels generated by the nanoparticle gelator for charge transportation via the Grotthuss-type ion exchange mechanism. The VOC values are smaller for those quasi-solid-state DSSCs using SiO2 or TiO2 nanoparticles as the gelator, but the value is larger for the cell using ZnO nanoparticles as the gelator, as compared with that for the liquid-electrolyte DSSC. This is probably due to the difference in the relative potential of the CB for the ZnO layer in the photoanode and the redox potential of the redox couple in the electrolyte, leading to the smaller electron lifetime for the cells gelled by SiO2 or TiO2 and longer electron lifetime for those gelled by ZnO. The largest h value of 4.17% was achieved by the quasi-solid-state DSSC that gelated by 35.0 wt% ZnO nanoparticles. This h value represents a 33% enhancement compared to the h value for the liquidelectrolyte DSSC. The ZnO-gelated quasi-solid-state DSSC also exhibits better long-term at-rest stability than the liquid-state cells by retaining 95% of the initial h value for 150 days. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.07.009. References [1] L.Y. Lin, C.P. Lee, R. Vittal, K.C. Ho, Selective conditions for the fabrication of a flexible dye-sensitized solar cell with Ti/TiO2 photoanode, J. Power Sources 195 (2010) 4344–4349. [2] C.P. Lee, L.Y. Lin, K.W. Tsai, R. Vittal, K.C. Ho, Enhanced performance of dyesensitized solar cell with thermally-treated TiN in its TiO2 film prepared at low temperature, J. Power Sources 196 (2011) 1632–1638. [3] B. O'Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (1991) 737–740. [4] G. Wolfbauer, A.M. Bond, J.C. Eklund, D.R. MacFarlane, A channel flow cell system specifically designed to test the e$ciency of redox shuttles in dye sensitized solar cells, Sol. Energ. Mat. Sol. C 70 (2001) 85–101.

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