Ionic liquid crystal as a hole transport layer of dye-sensitized solar cells

Abstract

Use of a new ionic liquid crystal, 1-dodecyl-3-methylimidazolium iodide, and iodine as an electrolyte of dye-sensitized solar cells leads to a high short circuit photocurrent density and a high light-to-electricity conversion efficiency, due to a self-assembled structure of the imidazolium cations, resulting in high conductivity of the electrolyte.

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Dye-sensitized solar cells (DSSC), constructed by using dye molecules, nanocrystalline metal oxides and liquid electrolytes, have attractive features in terms of the high light-to-electricity conversion efficiency, and the low production cost and energy.¹ The electrolytes, usually composed of an I⁻/I₃⁻ redox couple in organic solvents, are sealed between two electrodes. These organic solvents cause a serious problem of low durability due to evaporation.² It has been reported that DSSC using ionic liquids as a non-volatile solvent achieves high temperature stability.^3,4 However, the conversion efficiency of the cells using ionic liquids is lower than that using organic solvents, because the high viscosity of the ionic liquids retards the physical diffusion of I⁻ and I₃⁻. Many attempts to reduce the viscosity have not yet been successful.⁵ For enhancing the conductivity of ionic liquids leading to the high light-to-electricity conversion efficiency of DSSC, it seems to be necessary to arrange a pathway for fast charge transport.

Previously, we reported that the charge transport rate at high concentration of an I⁻/I₃⁻ redox couple in ionic liquids can be attributed to the exchange reaction of I⁻ + I₃⁻ → I₃⁻ + I⁻.^4,6 For enhancing the short circuit photocurrent density (J_SC) of DSSC using ionic liquids with a high concentration of an I⁻/I₃⁻ redox couple, the exchange reaction in the ionic liquids needs to be promoted. In this study, we report a new strategy for enhancing the conductivity of ionic liquid electrolytes; employing an ionic liquid crystal (ILC) as a constituent of an electrolyte, which forms a self-assembled structure and promotes the exchange reaction by the locally increased concentrations of I⁻ and I₃⁻. We have selected 1-dodecyl-3-methylimidazolium iodide (C₁₂MImI) as an ILC. This provides a self-assembled structure of the imidazolium cations like a solid, while maintaining the molecular dynamics like a liquid. The ILC with the smectic A phase (S_A) has a bilayer structure of interdigitated alkyl chains of the imidazolium cations, and I⁻ and I₃⁻ would be localized between the S_A layers. The locally high concentration would promote the exchange reaction. So, the ILC with the S_A phase would be suitable for the electrolyte of DSSC when aiming at the high light-to-electricity conversion efficiency.

A few examples of the ILC with the S_A phase, such as imidazolium salts consisting of cations with alkyl chains of C₁₂–C₁₈ and anions of hexafluorophosphate or bromide, have been reported.^7,8 Iodide is indispensable in a DSSC electrolyte, because the I⁻/I₃⁻ redox couple functions as a hole transport agent. However, an imidazolium salt with iodide as the counter anion has not been reported to be an ILC with a S_A phase. Here, we show for the first time that imidazolium iodides with alkyl chains longer than C₁₂ exhibit a S_A phase and that the liquid crystalline nature is preferable in terms of the hole transport layer in DSSC.

Imidazolium iodides with long alkyl chains were synthesized by the quaternization reaction of 1-methylimidazole with an equimolar amount of the corresponding alkyl iodide for 72 h under N₂ atmosphere at room temperature. The products were washed with n-hexane to remove the remaining starting materials and dried under vacuum at 40 °C for 4 h, and finally identified by ¹H NMR in CDCl₃ and differential scanning calorimetry (DSC). Imidazolium iodides with alkyl chains longer than C₁₂ showed a liquid crystalline phase. C₁₂MImI showed the lowest melting point and viscosity among them and these properties were suitable for the hole transport layer in DSSC. In this study, we selected C₁₂MImI and applied it with 0.65 M iodine (C₁₂MImI/I₂) as the hole transport layer in DSSC. We have compared the properties of C₁₂MImI/I₂ with an ionic liquid electrolyte; 1-undecyl-3-methylimidazolium iodide with 0.65 M iodine (C₁₁MImI/I₂).

C₁₂MImI showed a phase transition from a liquid to a liquid crystal at 80 °C, although C₁₁MImI did not show a liquid crystalline phase. The ionic liquid crystalline phase of C₁₂MImI was confirmed by polarized optical microscopy (POM). The characteristic focal conic domains observed during the cooling process suggested that the liquid crystalline phase of C₁₂MImI was a S_A phase.⁷

Photoelectrochemical cells were fabricated as previously described.⁹ C₁₁MImI/I₂ and C₁₂MImI/I₂ were used as the hole transport layer, respectively. C₁₂MImI/I₂ maintained the S_A phase ranging from 27 to 45 °C on heating. On the other hand, C₁₁MImI/I₂ showed a liquid phase above 37 °C.

The light-to-electricity conversion efficiencies of DSSC using C₁₂MImI/I₂ and C₁₁MImI/I₂ were evaluated at 40 °C under AM 1.5 irradiation from a solar simulator, adaptable for amorphous silicon solar cells according to the Japanese Industrial Standard.¹⁰ Each value for cell performance was taken as an average of at least 3 samples.

Fig. 1 shows photocurrent–voltage curves of the DSSC using C₁₂MImI/I₂ and C₁₁MImI/I₂. J_SC of the DSSC using C₁₂MImI/I₂ was higher than that using C₁₁MImI/I₂, while the open circuit voltage (V_OC) and fill factor (FF) were the almost same values as for C₁₁MImI/I_2. This result suggests that the higher conductivity of C₁₂MImI/I₂ than C₁₁MImI/I₂ could lead to a high J_SC.^11,12

Fig. 1 Photocurrent–voltage curves of the cells using C₁₂MImI/I₂ (bold solid curve) and C₁₁MImI/I₂ (solid curve) under AM 1.5 irradiation.

To examine the conductivities of C₁₂MImI/I₂ and C₁₁MImI/I₂, the diffusion-limited currents (I_lim) corresponding to the reaction of I₃⁻ + 2e⁻ → 3I⁻ were measured. The measurements were carried out at 40 °C by using a microelectrode and the diffusion coefficients (D) of I₃⁻ were calculated using the values of I_lim as previously described.⁶ According to this study, the D value calculated from I_lim intrinsically includes not only the simple physical diffusion coefficient but also the diffusion coefficient based on the exchange reaction.

The observed D value of C₁₂MImI/I₂ (4.2 × 10⁻⁸ cm² s⁻¹) was 1.3 times as large as that of C₁₁MImI/I₂ (3.2 × 10⁻⁸ cm² s⁻¹), while the viscosity of C₁₂MImI/I₂ was 2.5 times larger than that of C₁₁MImI/I₂. The simple physical diffusion coefficient of C₁₂MImI/I₂ should be smaller than C₁₁MImI/I₂. This result would lead to the idea that the diffusion coefficient based on the exchange reaction is increased in C₁₂MImI/I₂. It has been reported that anisotropic long-range conductive pathways between the S_A layers are formed in liquid crystals with a S_A phase.¹³ In C₁₂MImI/I₂, these pathways should be formed, and I⁻ and I₃⁻ should be located between the S_A layers consisting of the imidazolium cations. If a locally high concentration of I⁻ and I₃⁻ is achieved in the pathways, the exchange reaction should be promoted in C₁₂MImI/I₂, since the diffusion coefficient value based on the exchange reaction is proportional to the concentrations of I⁻ and I₃⁻. So, C₁₂MImI/I₂ could show a higher D value than C₁₁MImI/I₂ in which the cations should exist at random.

To prove that the exchange reaction was promoted in the pathways between the S_A layers of C₁₂MImI/I₂, we measured the ionic conductivities along the direction parallel (σ_i∥) and perpendicular (σ_i⊥) to the S_A layer plane¹³ at temperatures ranging from 32.5 to 57.5 °C. A glass plate with comb-shaped platinum electrodes (cell A) and a pair of indium tin oxide (ITO) electrodes (cell B) were employed for the measurements of σ_i∥ and σ_i⊥, respectively. The conoscopic image at 40 °C for C₁₂MImI/I₂ revealed that the self-assemblies of the cations formed a homeotropic alignment in the S_A phase on the glass surface.

Fig. 2 shows anisotropic ionic conductivities of C₁₂MImI/I₂ and isotropic ionic conductivities of C₁₁MImI/I₂ as a function of temperature. It was reported that the ionic conductivities of isotropic ionic liquids gradually increased with the increase in temperature as shown in the insert of Fig. 2.¹⁴ In contrast, for C₁₂MImI/I₂, the discontinuous changes of ionic conductivities were observed. The ionic conductivities parallel (σ_i∥) to the S_A layer of C₁₂MImI/I₂ decreased and those perpendicular (σ_i⊥) increased at 45 °C on heating. When the liquid crystalline phase-order disappeared above the phase transition temperature, the ionic conductivities measured in both cell A and cell B were on the same line within the limits of error. Taking into account that C₁₁MImI/I₂ did not show any discontinuous changes in σ_i∥ and σ_i⊥ over the whole temperature range, the discontinuous changes of C₁₂MImI/I₂ should be attributed to the conductive pathways formed between the S_A layers. The observed σ_i∥ values of C₁₂MImI/I₂ below 45 °C, which showed ionic conductivities along the direction of the conductive pathways, were enhanced because the exchange reaction would be promoted at the conductive pathways.

Fig. 2 Ionic conductivities for the samples of C₁₂MImI/I₂ and C₁₁MImI/I₂ (insert) along with the direction parallel σ_i∥ (filled circles) and perpendicular σ_i⊥ (open triangles) to the S_A layer plane of the homeotropically aligned ionic liquid crystal.

In the DSSC using C₁₂MImI/I₂, although the long-range order of ILC was not necessarily achieved in the mesoporous TiO₂ electrode, the localization of I⁻ and I₃⁻ between the S_A layers of each domain should result in the promotion of the exchange reaction in a hole transport layer and the enhancement in J_SC.

In conclusion, we have demonstrated a new strategy for enhancing the conductivity of the ionic liquid electrolytes containing an I⁻/I₃⁻ redox couple. This strategy is based on the promotion of the exchange reaction between I⁻ and I₃⁻ by the locally increased concentrations of I⁻ and I₃⁻. As a means of demonstrating this strategy, a new ionic liquid crystalline C₁₂MImI/I₂ with a self-assembled structure of the imidazolium cations was introduced. C₁₂MImI/I₂ showed a high ionic conductivity in spite of its high viscosity. The ionic liquid crystalline hole transport layer (C₁₂MImI/I₂) was applied to DSSC, which showed a higher J_SC and a higher light-to-electricity conversion efficiency than that using the ionic liquid hole transport layer (C₁₁MImI/I₂).

This research was supported in part by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry.

Notes and references

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