Understanding the Electrical Transport-Structure Relationship and Photovoltaic Properties of a [Succinonitrile-Ionic Liquid]-LiI-I2 Redox Electrolyte.

The properties of succinonitrile-based electrolytes can be enhanced by the addition of an ionic liquid (IL). Here, we have reported the relationship between the electrical transport properties and the structure of a new [(1 - x)succinonitrile:xIL]-LiI-I2 electrolyte, where the mole fraction (x) of the IL (1-butyl-3-methyl imidazolium iodide) was varied from 0 to 40%. Compositional variation revealed the optimum conducting electrolyte (OCE) at x = 10 mol %, possessing an electrical conductivity (σ25°C) of ∼7.5 mS cm-1 with an enhancement of ∼369%. The partial replacement of succinonitrile by the IL eliminated the abrupt change in the slope of the log σ vs T -1 plot at the melting temperature of the succinonitrile-LiI-I2 system, showing the Vogel-Tamman-Fulcher-type behavior owing to molecular chain disorder. Raman spectroscopy showed the I3 - concentration nearly twice the I5 - concentration for the OCE. Vibrational spectroscopy exhibited red shifts in the νC≡N, νCH2 , νa,CC, νa,N-CH3 , and νs,N-butyl modes, indicating an interaction between succinonitrile and the IL. The area ratio A CH2 /A C≡N increased slightly for x = 10 mol % (OCE) and largely for x > 10 mol %, indicating an increase in the C-H bond length. These observations indicated that the interaction between succinonitrile and the IL was enhanced at x > 10 mol %, which decreased the electrical conductivity of these electrolytes. Owing to fast ion transport, an OCE-based dye-sensitized solar cell showed a 40-55% decrease in the charge-transfer and Warburg resistances, resulting in ∼139 and ∼122% increases in J SC and η, respectively.

Introduction

Research on solar cells as renewable energy sources is of current importance because of several environmental issues.[1] The availability of large amounts of solar irradiance along the Tropic of Cancer, which passes through countries like Algeria, Libya, Egypt, Saudi Arabia, and India, is also advantageous.[1] As a third-generation solar cell, the dye-sensitized solar cell (DSSC)—also called the Gratzel cell—is striking in terms of its simple cell structure, cost-effective materials and manufacturing processes, short energy payback time (<1 year), ability to capture light from all incident angles, and enhanced performance under real outdoor conditions.[2] However, DSSCs require an electrolyte with fast ion conduction (i) to inhibit back electron transfer reactions, (ii) to regenerate dyes via the oxidation of I– into I3– at the working electrode, and (iii) to reduce I3– into I– at the counter electrode.[2,3] Using a liquid electrolyte consisting of 0.6 M 1-methyl-3-butyl imidazolium iodide, 0.03 M iodine, 0.1 M guanidinium thiocyanate, and 0.5 M tert-butylpyridine (TBP) in a 15:85 (v/v) mixture of valeronitrile and acetonitrile, Nazeeruddin et al.[4] reported a cell efficiency (η) of 11.2% at 1 sun. Chiba et al.[5] used a liquid electrolyte of 0.6 M dimethyl propyl imidazolium iodide, 0.1 M LiI, 0.05 M iodine, and 0.5 M TBP in acetonitrile to achieve a certified cell efficiency of 11.1% at 1 sun. The addition of the ionic liquid (IL) helped increase the electrical conductivity and decrease the solvent evaporation rate. However, as solvent-based cells are prone to leakage, especially at elevated operating temperatures (∼80 °C) owing to the increased internal pressure, strong hermetic sealing is required.

Wang et al.[6] synthesized a solvent-free electrolyte using the IL N-methyl-N-butylpyrrolidinium iodide and I2 in succinonitrile (SN; N ≡ C–CH2–CH2–C≡N). This electrolyte exhibited an electrical conductivity (σ25°C) of 3.3 mS cm–1 and apparent diffusion coefficients of 3.7 and 2.2 × 10–6 cm2 s–1 for I– and I3–, respectively. The properties of this electrolyte were better than those of a gel electrolyte containing an IL, iodine, a solvent, and a gelator.[7] This superior performance was attributed to the solid solvent/plasticizing properties of SN, which is a nonionic and low-molecular-weight plastic crystal with a low melting temperature (∼58 °C), high dielectric constant (∼55), and waxy nature.[8−10] The polar end of SN containing a nitrile group with negative partial charge coordinates with a cation for ion transport.[9−11] SN has a plastic crystal phase between the crystal-to-plastic crystal phase-transition temperature (−38 °C) and the melting temperature. The plastic crystal has a body-centered cubic structure with two SN molecules per unit cell, in which the central C–C bonds orient along the diagonals of the cube and N atoms are at the center of the faces. This structure allows a high molecular diffusivity through trans-gauche isomerization and molecular jump from one diagonal of the cube to another.[10−14] Since the invention of this first succinonitrile-based electrolyte, numerous SN–XI–I2 systems have been investigated, where X represents an IL cation.[15−21] A large size of cation, acting as a plasticizer, introduced amorphicity in the plastic crystal phase of the SN, which resulted in increased electrical conductivity of the electrolyte and thereby cell efficiency of the DSSC.[22] Instead of an IL, an organic salt such as tetrabutylammonium iodide or tetraethylammonium iodide has also been used with SN and I2, in which the tetrabutylammonium iodide exhibited better electrolytic and photovoltaic properties.[15,23,24]

In all cases, LiI has been the inorganic iodide salt used with SN and I2 because of the advantageous properties of Li+ ions.[21,25] In DSSCs, Li+ ions are adsorbed on the dye-sensitized TiO2 layer, thereby increasing the electron injection rate from the dye to the conduction band of TiO2. Byrne et al.[21] achieved a η value of 3.58% at 1 sun for the SN–LiI–I2 electrolyte (100:5:1, mol ratio) with the N719 dye. A thorough investigation of the electrical properties of this electrolyte by Gupta et al.[26] showed that increasing the LiI content to 5 mol % increased the electrical conductivity. The log σ vs T–1 plot showed two linear components, with a change in the slope at the melting temperature (∼37 °C for the electrolyte with 5 mol % LiI), and an increase in the activation energy with increasing LiI content. Armel et al.,[17] who used both an IL (0.5 M N-methyl-N-butylpyrrolidinium iodide) and LiI (0.1 M) with I2 (0.1 M) and N-methyl benzimidazole (0.2 M) in SN, reported a σ25°C value of ∼5 mS cm–1 and apparent diffusion coefficients of 3.8 and 2.5 × 10–6 cm2 s–1 for I– and I3–, respectively. Furthermore, using this electrolyte and a porphyrin dye, they obtained a η value of ∼4.2% at 1 sun.

To date, research in this area has focused mostly on electrolyte development and the photovoltaic properties of SN–XI–I2 redox mediators. In the present study, we thoroughly investigated the relationship between the electrical transport properties and structure of a new electrolyte: [(1 – x)SN:xIL]–LiI–I2, where SN was partially replaced by the IL, 1-butyl-3-methyl imidazolium iodide. Imidazolium is a planar five-membered aromatic ring that possesses two nitrogen atoms connected by alkyl chains.[27] We chose this commercially available IL, which has been used in one of the highest efficiency cells,[4] because it has a mid-range viscosity (∼1110 cP),[27] and a large imidazolium (C4H9–N13N+–CH3; IL+) cation that can act as a plasticizer to increase amorphicity.[22,25] Maintaining the LiI-to-SN and I2-to-LiI mole ratios of 5 and 10%, respectively,[26] we investigated the electrical transport properties in detail using impedance spectroscopy. Varying the IL mole ratio (x) from 0 to 40% allowed optimization of the electrical conductivity (σ25°C), whereas evaluation of the log σ vs T–1 trend enabled the determination of the molecular nature and the activation energy for ion transport. The structural properties of the electrolyte were studied in detail at room temperature using Raman spectroscopy and Fourier transform infrared (FT-IR) spectroscopy. Furthermore, we examined the photovoltaic properties of DSSCs with and without the optimized electrolyte using electrochemical impedance spectroscopy (EIS).

Results and Discussion

Electrical Properties

Figure shows the compositional variation of σ25°C for the [(1 – x)SN:xIL]–LiI–I2 electrolytes, where x = 0–40 mol %. The SN–LiI–I2 electrolyte (x = 0 mol %) had a σ25°C value of ∼1.6 mS cm–1. The partial replacement of SN by the IL up to x = 10 mol % led to an increase in the σ25°C value to ∼7.5 mS cm–1. However, further increases in x had a detrimental effect. Thus, the electrolyte with x = 10 mol %, which exhibited an ∼369% increase in the σ25°C value, is referred to as the optimum conducting electrolyte (OCE). We also estimated the relative conductivity enhancement factor, Δσ = [σ – σ]/σ = using the extrapolated σ25°C vs x data to understand the interaction phenomenon.[26,28] As shown by the Δσ vs x curve of the electrolytes in Figure , the Δσ value decreased from 2.46 (x = 5 mol %) to 0.35 (x = 10 mol %) and then constant negative values of approximately −0.1 were obtained for x = 15–40 mol %.

Figure 1

Variations of σ25°C and Δσ with x for the [(1 – x)SN:xIL]–LiI–I2 electrolytes (x = 0–40 mol %). The lines guide the eye.

The variations of σ25°C and Δσ with x can be explained using a well-known expression of electrical conductivity, σ = nqμ.[3,29] This expression indicates that a variation in σ25°C can be due to changes in the concentration (n) and/or mobility (μ) of the free ionic species. For the SN–LiI–I2 electrolyte, n corresponds to the sum of free Li+, I–, I3–, and I5– ionic species, obtained via dissociation of LiI by SN, with a negligible Li+–I– Coulombic interaction.[8,10,23,26,30] In this electrolyte, the mobility was controlled by the Li+–I–, NSN––Li+, and N′SN+–I3– ionic interactions. The notation NSN– is the nitrile nitrogen with a negative partial charge and N′SN+ is a complex, −CH2–C≡N–I+, formed between the cyano radical and I– with a positive partial charge.[9−11,26,31−33] The formation of the uncharged ion pair, e.g., Li+–I–, is a favorable factor for increasing the mobility in the electrolyte.[28] Furthermore, cyano radicals help Li+ ions to migrate in the electrolyte.[9−11] However, the N′SN+–I3– interaction affects the trans-gauche isomerization process of SN,[26,31,32] which leads to a higher diffusivity for the SN molecules than for the free cations and anions. Also, the molecular diffusivity was higher above the melting temperature of this electrolyte.[8] Kalaignan et al.[30] showed that the I5– ion helps to form the amorphous phase in the electrolyte for fast ion transport to a greater extent than I3– ion, and its increase in concentration increases the electrical conductivity. As shown in the Raman spectra study later, the electrolyte with x = 0 mol % has less concentration of the I5– ion and, therefore, less contribution to the electrical conductivity.

In contrast, the electrolyte with x = 5 mol % showed a large increase in the σ25°C value with the highest Δσ value owing to increases in both n and μ. The addition of a small amount of the IL into the electrolyte introduces imidazolium (IL+) and I– ions. Owing to its large size, the IL+ ion acts as a plasticizer to increase the amorphicity of the electrolyte, and thereby, the ionic mobility.[22] A high Δσ value indicates a low extent of interaction among the ionic species. However, the dominance of the NSN––IL+ interaction over the NSN––Li+ interaction is sufficient enough to liquefy the electrolyte. The electrolyte with x = 10 mol % (OCE) had the highest σ25°C value and a slightly reduced Δσ value. This behavior may be due to an increase in the amorphicity by and concentration of the IL+, I–, I3–, and I5– ions. The decrease in the Δσ value suggests possible interactions, such as NIL+ −I3–, N′SN+–I3–, and NIL+–I5–, along with the NSN––Li+ and NSN––IL+ interactions, where NSN+ is the IL’s nitrogen with a positive charge.[9−11,26,31−33] For the electrolytes with x = 15–40 mol %, we observed a gradual decrease in the σ25°C value and nearly constant negative Δσ values. This behavior is probably due to the dominance of the interactions among the ionic species, which reduce the free ionic species for ion transport, although the amorphicity increases with increasing x. These assertions are supported by the log σ vs T–1 and vibrational spectroscopy studies, as discussed below.

Figure a exhibits the log σ vs T–1 plots for the [(1 – x)SN:xIL]–LiI–I2 electrolytes, where x = 0–30 mol %. The SN–LiI–I2 electrolyte showed a linear decrease in the log σ value with increasing T–1 in temperature regions I and II, corresponding to below and above the melting temperature (∼37 °C) of the electrolyte.[8,9,26] This linear trend indicated that the SN–LiI–I2 electrolyte is a homogeneous system, in which Li+, I–, I3–, and I5– are mobile. The electrical transport properties of a homogeneous system can be described using an Arrhenius equation, σ = σo exp(−Ea/kBT), where σo is the preexponential factor, Ea is the activation energy, and kB is the Boltzmann constant.[3,29] We obtained Ea values of ∼0.87 and ∼0.15 eV from the slopes in temperature regions I and II, respectively. The latter value is low because the electrolyte is in a liquid form.[26]

Figure 2

(a) Log σ vs T–1 curves and (b) log σT1/2 vs (T – To)−1 plots of the [(1 – x)SN:xIL]–LiI–I2 electrolytes (x = 0–30 mol %). The inset in (b) shows the pseudoactivation energy (B) of the electrolytes.

The addition of the IL (x = 5–30 mol %) eliminated the sharp change in slope in the log σ vs T–1 plot. The curvature of the plots decreased up to x = 10 mol % and then increased, which indicates the Vogel–Tamman–Fulcher (VTF)-type behavior of the disordered chains.[26,28] The VTF-type behavior of the electrolytes can be described by an empirical relation, σ = AT–1/2 exp[−B/kB(T – To)], where B is the pseudoactivation energy and To is the temperature at which the free volume vanishes. As shown in Figure b, the log σT1/2 vs (T – To)−1 plots for the electrolytes with x = 5–30 mol % were linear. The B values calculated from the slopes are plotted in the inset of Figure b. The electrolytes with x = 5 and 10 mol % (OCE) had similar B values (∼0.034 eV), but further increasing x led to a rapid increase in the B value. This trend indicates that ion transport is the easiest in the OCE. As mentioned earlier, this is due to an increased concentration of free ionic species, high ionic mobility, and a low level of ionic interactions. The increase in the B value with the increase in x from 15 to 30 mol % indicated that ionic transport was hindered by the ionic interaction phenomena. Notably, the B values were quite low (0.034–0.043 eV) for the electrolytes with x = 5–30 mol %, as they are in a liquid state. These findings were also supported by the vibrational spectroscopy results, as discussed below.

Structural Properties

Figure shows a comparison of the Raman spectra of the [(1 – x)SN:xIL]–LiI–I2 electrolytes (x = 0–40 mol %) and the IL in three regions: (a) 80–220 cm–1, (b) 1000–1460 cm–1, and (c) 2235–3200 cm–1. The following notation is used for the modes: ν, stretching; δ, bending; ω, wagging; τ, twisting; ρ, rocking; s, symmetric; and a, asymmetric. The observed modes for the polyiodide ions (I3– and I5–), SN–LiI–I2, and IL are marked on the figure based on the literature.[26,30,34−37]

Figure 3

Raman spectra of the [(1 – x)SN:xIL]–LiI–I2 electrolytes (x = 0–40 mol %) and the IL: (a) 80–220 cm–1, (b) 1000–1460 cm–1, and (c) 2235–3200 cm–1.

Region (a) includes the stretching mode of I3– ions at ∼110 cm–1 and the bending mode of I5– ions at ∼147 cm–1. These ions originate from the reactions I2 + I– → I3– and I2 + I3– → I5–, where dissociated LiI and IL act as the sources of I– ions.[26,30,34,35] As mentioned earlier, an increase in the I5– ion concentration increases the electrical conductivity of the electrolyte; however, it is not useful for DSSC operation, which requires the I–/I3– redox couple only.[2,30] Therefore, we have determined the relative intensities I110/I2254, I147/I2254, and I147/I110 for all x values, as shown in Figure . An almost linear increase in the I147/I2254 value was observed with increasing x. In contrast, for the I110/I2254 values, the electrolytes with x = 0 and 10 mol % had similar values with a linear trend observed at higher x values, similar to that for the I147/I2254 value. The ratio I147/I110, however, showed an exponential-type increase with increasing x, a fast increase till x = 10 mol % (OCE) followed by a slow increase. This indicates a faster formation of I5– than I3– at x = 10 mol % with a trend of saturation for higher x values. In addition, for the OCE (x = 10 mol %), the I147/I2254 value was nearly half the I110/I2254 value, indicating that the I5– concentration is nearly half the I3– concentration, which made this electrolyte suitable for DSSC applications. Furthermore, the absence of peaks at ∼180 and ∼212 cm–1, which correspond to aggregated iodine and isolated iodine,[30,35] indicates the complete conversion of added I2 into I3– ions.

Figure 4

Relative intensities (I110/I2254, I147/I2254, and I147/I110) from the Raman spectra of the [(1 – x)SN:xIL]–LiI–I2 electrolytes (x = 0–40 mol %). The lines guide the eye.

Region (b), which is the fingerprint region, includes the weak modes of SN, as observed previously by Fengler and Ruoff.[36] This region also contains IL modes, such as vN-alkyl at 1024 cm–1 and δs,CC/δs,N-butyl, at ∼1420 cm–1. The IL addition slightly shifted the νCC mode of the electrolytes from ∼1028 cm–1 (x = 0 mol %) to ∼1025 cm–1 (40 mol %), whereas a large shift was observed for the δCH mode from ∼1430 cm–1 (x = 0 mol %) to 1424 (10 mol %; OCE) and 1422 cm–1 (x = 40 mol %). These shifts indicated that an interaction between SN and the IL weakens the C–C and C–H bonds.

Region (c) shows the stretching modes of SN and IL. The νs,CH mode of the electrolytes shifted from ∼2946 cm–1 (x = 0 mol %) to 2944 (10 mol %), 2941 (20 mol %), and 2925 cm–1 (40 mol %). In addition, the νs,CH mode appeared as a shoulder for the electrolytes with x ≥ 10 mol %. This band broadening with increasing x suggested that the C–H bond is lengthened.[26,38] We also observed a slight shift from 2254 cm–1 (x = 0 mol %) to 2252 cm–1 (40 mol %) and shortening for the νC≡N mode. We also noted a lengthening of the νs,N-butyl mode at ∼2872 cm–1 (shoulder) and the νs,CC ring mode at ∼3149 cm–1 with increasing x. These changes reveal that structural modifications occur at the N-sites of both SN and the IL.

The areas corresponding to the νC≡N and νCH modes changed with increasing x, as shown in Figure a. The area corresponding to the νC≡N mode (A2254) was similar up to x = 10 mol % (OCE) and then slightly decreased at x > 10 mol %. In contrast, the area corresponding to the νCH modes (A2946) increased slightly up to x = 10 mol % and then greatly at x > 10 mol %. Figure b shows the overall effect of IL addition on the relative areas of these bands (A2946/A2254) for the electrolytes. The observed two-step increase in the relative area, first slightly up to x = 10 mol % and then greatly at x > 10 mol %, indicates that the structural changes of the IL are dominant over those of SN at x > 10 mol %, resulting in more NSN––IL+ interaction and thereby decreasing the possibility of NSN––Li+ interactions for ion migration.[26,31,32] An increase in x may also aid the formation of I5– ions in alkyl radical domain, resulting in a decrease in the free redox species. These factors could be responsible for the decreased σ25°C values and increased B values of the electrolytes with x > 10 mol %. The observed lengthening of the C–C and C–H bonds indicates molecular chain disorder, yielding amorphicity in the electrolytes, which is more pronounced for the electrolytes with x > 10 mol %.[26,38] The amorphicity is further improved by the formation of Li+–I– pairs and the plasticizing property of the imidazolium cations. The assignment of these structural changes was also supported by the FT-IR analysis, as discussed in detail below.

Figure 5

(a) Areas and (b) relative areas of the bands corresponding to the νC≡N and νCH modes from the Raman and FT-IR spectra of the [(1 – x)SN:xIL]–LiI–I2 electrolytes (x = 0–40 mol %). The lines guide the eye.

Figure shows a comparison of the FT-IR spectra of the [(1 – x)SN:xIL]–LiI–I2 electrolytes (x = 0–40 mol %) and the IL in two regions: (a) 580–1590 cm–1 and (b) 2200–3200 cm–1. The relatively strong IR-active modes are marked on the figure based on the available literature.[26,36,37,39] Region (a), which is the fingerprint region, revealed a strong NSN––IL+ interaction based on a number of changes in the SN peaks, namely, δCC mode: 604 (x = 0 mol %), 604 cm–1 (10 mol %), shoulder (40 mol %); ρCH mode: 762 (x = 0 mol %), 755 (10 mol %), and 753 cm–1 (40 mol %); νC-CN mode: 818 (x = 0 mol %), 820 (10 mol %), and 829 cm–1 (40 mol %); τCH mode: 1232 (x = 0 mol %), 1229 (10 mol %), and 1227 cm–1 (40 mol %); and δCH mode: 1429 (x = 0 mol %), 1427 (10 mol %), and 1425 cm–1 (40 mol %). This interaction can also be visualized based on the IL peaks at 617, 648, 752, 825, 1023, 1168, 1378, 1461, and 1569 cm–1, which are generally prominent for the electrolytes with x > 10 mol %. These peaks belong to either the N-butyl group or the imidazolium ring.

Figure 6

FT-IR spectra of the [(1 – x)SN:xIL]–LiI–I2 electrolytes (x = 0–40 mol %) and the IL: (a) fingerprint region and (b) 2200–3200 cm–1.

Region (b) includes the stretching modes of SN and IL. An increase of x led to a slight red shift of the τC≡N mode: 2257 (x = 0 mol %), 2255 (10 mol %), and 2253 cm–1 (40 mol %), as well as a decrease in peak intensity. A corresponding large shift was observed for the νa,CH mode: 2988 (x = 0 mol %), 2981 (10 mol %), and 2963 cm–1 (40 mol %), whereas the νs,CH mode appeared as a shoulder. The addition of the IL also strengthened and shifted the distinct νa,CC ring mode from 3160 (x = 10 mol %) to 3150 cm–1 (40 mol %); the νa,N-CH mode from 3111 (x = 10 mol %) to 3092 cm–1 (40 mol %); and the νa,N-butyl mode from a shoulder (x = 10 mol %) to 2876 cm–1 (40 mol %). These structural changes inferred chain disorder via C–N, C–C, and C–H bond lengthening with increasing x, which we quantified based on the areas of the bands corresponding to the νCH mode at ∼2988 cm–1 (A2988) and the νC≡N mode at 2257 cm–1 (A2257).[26,38]

As shown in Figure a, the trends in these area vs x curves are similar to those estimated from the Raman spectra, with A2988 increasing slightly up to x = 10 mol % (OCE) and then greatly for x > 10 mol %, whereas A2257 remained unchanged up to x = 10 mol % and then slightly decreased for x > 10 mol %. The overall effect of IL addition on the relative area (A2988/A2257) is also similar to that obtained from the Raman spectra (Figure b), with a slight increase in the relative area up to x = 10 mol % followed by a large increase for x > 10 mol %. As mentioned earlier, these changes indicate that the IL has a dominant effect at x > 10 mol %, providing NIL+ sites that enhance NIL+–I3– interactions.[26,31,32] The decreasing contribution of SN with increasing x can be visualized at ∼2257 cm–1 (νC≡N mode), where the band broadening/splitting is reduced.[38] These changes indicate a decrease in the amount of NSN––Li+ interactions for ion migration and, hence, are consistent with the reduced σ25°C value.

Figure also indicates an increase in amorphicity with increasing x owing to lengthening of the C–H bonds.[26,38] This behavior is due to the large size of the imidazolium ions, the concentration of which increases linearly with increasing x. This change is also clearly visualized in Figure , which shows a linear increase in the relative intensities I1167/I2257 and I1572/I2257 with x, where the peaks at ∼1167 and ∼1572 cm–1 are the νN-alkyl modes of the IL. These data also confirm the minimal change in the νC≡N mode, as observed in Figure . Figure also shows that the relative intensities I1167/I2988 and I1572/I2988 both show an exponential dependence on x, increasing rapidly up to x = 10 mol % and then more gently at x > 10 mol %. This behavior suggests that there is a greater lengthening of the C–H bond of SN for IL concentrations with x > 10 mol %. It should be noted that a clear peak corresponding to isolated or aggregated LiI molecules was not observed at ∼433 cm–1, indicating the complete dissociation of the salt.[40]

Figure 7

Relative intensities (I1167/I2257, I1572/I2257, I1167/I2988, and I1572/I2988) from the FT-IR spectra of the [(1 – x)SN:xIL]–LiI–I2 electrolytes (x = 0–40 mol %). The lines guide the eye.

Photovoltaic Properties

Figure a shows the current density vs voltage curves of the DSSCs prepared using the [(1 – x)SN:xIL]–LiI–I2 electrolytes with x = 0 and 10 mol % (OCE). The curves exhibit typical solar cell behavior, and the evaluated photovoltaic parameters are listed in Table .[2] The OCE gave a short-circuit current density (JSC) of 9 mA cm–2, which is an ∼139% increase with respect to that of the electrolyte with x = 0 mol % (3.77 mA cm–2). The open-circuit voltage (VOC) values were ∼0.379 and ∼0.4 V for the electrolytes with x = 0 and 10 mol %, respectively. The introduction of the IL caused an ∼12% loss in the fill factor (FF), which decreased from ∼65% (x = 0 mol %) to ∼57% (OCE). These parameters resulted in an ∼122% increase in cell efficiency with the OCE to ∼2%.

Figure 8

(a) Current density–voltage and (b) Nyquist plots for the DSSCs prepared using the OCE (x = 10 mol %) and the SN–IL–I2 electrolyte (x = 0 mol %) at 25 °C.

Table 1

Photovoltaic Parameters and Electrochemical Resistances of the DSSCs Prepared Using the [(1 – x)SN:xIL]–LiI–I2 Electrolytes with x = 0 and 10 mol % (OCE)

x (mol %)	JSC (mA cm–2)	VOC (V)	FF (%)	η (%)	RPt (Ω cm2)	Rr (Ω cm2)	Rd (Ω cm2)
0	3.77	0.379	64.94	0.93	15.55	22.56	15.87
10	9.01	0.400	57.22	2.07	7.0	13.51	9.21

Figure b shows the Nyquist plots of the DSSCs prepared using the electrolytes with x = 0 and 10 mol % (OCE). The high-frequency (I), mid-frequency (II), and low-frequency domains (III) correspond to charge transfer at the Pt/electrolyte interface, electron recombination at the dye-sensitized TiO2/electrolyte interface, and ion transport.[25] Arcs I and II provide the charge-transfer resistances (RPt and Rr), whereas arc III yields the Warburg resistance (Rd). These resistances (Table ) were calculated by fitting the curves using the equivalent circuit: −[RS]I – [RPt ∥ CPE1]I – [(Rr – {Rd ∥ CPR3}III) ∥ CPE2]II—with the MEISP program.[25,41] The fitted curves are shown in Figure S1 of the Supporting Information. In this circuit, RS is the series resistance, which results from FTO and the leads, and CPE is a constant phase element. Compared to the DSSC with the SN–IL–I2 electrolyte (x = 0 mol %), the OCE-based DSSC exhibited decreased electrochemical resistances, with an ∼55% decrease in RPt, an ∼40% decrease in Rr, and an ∼42% decrease in Rd. This reduction in electrochemical resistance is due to the ∼369% increase in the σ25°C value when the OCE is used instead of the electrolyte with x = 0 mol % (∼7.5 and ∼1.6 mS cm–1, respectively). This fast ion transport environment allows the fast diffusion of ionic species for dye and I– regeneration, resulting in improved JSC and η values for the OCE-based DSSC.

Conclusions

A new [(1 – x)succinonitrile:xIL]–LiI–I2 electrolyte was synthesized through partially replacing succinonitrile with a commercially available ionic liquid (IL; 1-butyl-3-methyl imidazolium iodide). The electrolyte with an IL mole fraction (x) of 10 mol % exhibited the highest σ25°C value (∼7.5 mS cm–1, ∼369% increase relative to the electrolyte with x = 0 mol %) and was considered the optimum conducting electrolyte (OCE). This partial replacement resulted in molecular chain disorder, as indicated by the log σ vs T–1 plots, which showed a single downward curve, which gave a pseudoactivation energy of ∼0.034 eV for the OCE. Vibrational spectroscopy revealed a two-step trend, with slight changes in the intensity, area, and relative area of the I3–, C≡N, CH2, and N-alkyl modes for the OCE and then large changes for the electrolytes with x > 10 mol %. This behavior indicated that an interaction between succinonitrile and the IL was enhanced in the electrolytes with x > 10 mol %. Owing to the improved conductivity, the OCE-based DSSC exhibited a JSC value of ∼9 mA cm–2 and a η value of ∼2%, which were ∼139 and ∼122% greater than the corresponding values when the electrolyte with x = 0 mol % was used. These enhanced parameters were accompanied by a 40–55% decrease in the charge-transfer and Warburg resistances. These findings clearly related the electrical transport, structural, and photovoltaic properties of the electrolytes. It is also worth mentioning that the cell efficiency could be further improved using a working electrode with a hierarchical structure and TBP in the OCE.[2,3,42]

Experimental Section

Electrolyte Preparation

High-purity SN, 1-butyl-3-methyl imidazolium iodide, LiI, and I2 (Sigma-Aldrich) were used as received. Appropriate amounts of each ingredient were added to a glass vial, which was then maintained at ∼70 °C in an oven for a few minutes to melt SN. Then, the mixture was stirred at ∼60 °C for 24 h to obtain a homogeneous redox-couple electrolyte. It should be noted that the SN–LiI–I2 electrolyte was a solid at room temperature,[26] whereas the partial replacement of SN by the IL gave a liquid state, as visible in the log σ vs T–1 plot.

Electrolyte Characterization

The electrolyte characterization techniques used were similar to those described elsewhere.[26] First, a specially designed sample holder was kept at ∼70 °C in an oven. Then, the electrolyte solution of either melted SN–LiI–I2 or SN–IL–LiI–I2 was poured into space (thickness, t = 0.5 mm) between the platinum electrode plates (active area, a ∼16 mm2). After the sample holder was cooled to room temperature, it was connected to a HIOKI LCR meter (Japan; model IM 3533-01) and measurement were made using an ac amplitude of 20 mV and a frequency range of 10–105 Hz. The acquired complex impedance curve provided the resistance, R, which was used to obtain the electrical conductivity, σ. To obtain the log σ–T–1 data, we placed the sample holder, with an attached K-type thermocouple to measure the actual temperature, in a digital temperature-controlled oven.

A confocal Raman microscope (JASCO, Japan; model NRS-4500) with a laser excitation source at 532.07 nm, a range of 45–4000 cm–1, and a resolution of 1 cm–1 was used to collect the Raman spectra of the electrolytes. For FT-IR spectroscopy, each sample was prepared as an ∼0.25 mm thick pellet consisting of KBr powder and the electrolyte. An IRPrestige-21 spectrometer (Shimadzu, Japan) was used to record the spectra in the range of 400–4000 cm–1 with a resolution of 4 cm–1. The spectra were analyzed using the OMNIC software.

DSSC Fabrication and Characterization

Dyesol (Australia) products were used to fabricate the DSSCs. First, 18NR-T titania paste was coated on an FTO glass using the doctor-blade method and then sintered at 500 °C in several steps for 30 min to give mesoporous TiO2 with a thickness of nearly 12 μm as the working electrode. A 0.2 mM dye solution was prepared by dissolving the N719 dye in anhydrous absolute ethanol for nearly 24 h. Then, for dye sensitization, the TiO2-coated FTO glass was kept in the dye solution for 24 h. The TiO2 area was nearly 0.24 cm2. After rinsing with absolute ethanol, the area of the dye-sensitized TiO2 layer was barricaded using a 50 μm thick sealant film and covered using a platinized FTO glass with an ∼1 mm side hole. Subsequently, the cell was sealed by heating at ∼110 °C. The electrolyte solution maintained at ∼60 °C was injected into the vacant area of the cell through the hole, which was then closed using a sealant material and a square glass coverslip. The I–V data for the cell were collected using a PV measurement data acquisition system interfaced with a class BBA small area solar simulator. The solar irradiation intensity (100 mW cm–2; 1 sun; AM1.5 G) was standardized using a Si-based reference solar cell. The cell was characterized by EIS using a HIOKI LCR meter in the frequency range of 10–2–105 Hz with an ac amplitude of 20 mV and a biasing voltage equivalent to the open-circuit voltage (VOC) value under solar irradiation at 100 mW cm–2. The acquired Nyquist curve was fitted using the MEISP program.[25]