Iodine-Pseudohalogen Ionic Liquid-Based Electrolytes for Quasi-Solid

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Iodine-Pseudohalogen Ionic Liquid-Based Electrolytes for QuasiSolid-State Dye-Sensitized Solar Cells Annkatrin Lennert,† Michelle Sternberg,‡ Karsten Meyer,‡ Rubén D. Costa,*,†,§ and Dirk M. Guldi*,† †

Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany ‡ Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germany § IMDEA Materials Institute Eric Kandel 2, 28906 Getafe, Madrid, Spain S Supporting Information *

ABSTRACT: In the current work, novel symmetrically alkylsubstituted imidazolium-based ionic liquids have been synthesized featuring either iodide (I−) or selenocyanate (SeCN−) as counteranions. Physicochemical assays based on spectroscopy and electrochemistry techniques have been performed to identify the best ionic liquid for application as electrolytes in quasi-solidstate dye-sensitized solar cells (qssDSSC). The latter were mixed with additives such as 4-tert-butylpyridine (4tbpy) and guanidinium thiocyanate (GuSCN) to optimize electrode surface coverage, ionic diffusion, and dye regeneration. In addition, we demonstrate that electrolytes containing a mixture of I2 and (SeCN)2 enhance the open-circuit voltage of the final quasi-solidstate device by up to 70 mV. As such, iodine-pseudohalogen electrolytes reveal in qssDSSCs a good balance between dye regeneration and hole transport and, in turn, enhance the overall solar energy conversion efficiency by 70% with respect to reference qssDSSCs with iodine-based electrolytes. Finally, devices with the iodine-pseudohalogen electrolyte show a 1000 h stable efficiency of 7−8% under outdoor temperature operation conditions and 1 sun illumination. KEYWORDS: ionic liquids, quasi-solid-state electrolyte, quasi-solid-state dye-sensitized solar cells, outdoor conditions, iodine-pseudohalogen redox couple

1. INTRODUCTION Silicon solar cells are an environmentally sustainable source of energy. Their lifetime and efficiency are clear assets. However, the high temperatures, which are needed for the production of highly pure silicon, constitute major drawbacks. Likewise, processing with materials such as, for example, HF is a concern and imposes additional problems. Recycling silicon solar cells again involves processes based on toxic chemicals and high temperatures. However, because cheap, easy-to-handle alternatives are highly desired, dye-sensitized solar cells (DSSC) come as viable and cost-effective sources of renewable energy into the game.1 In recent years, concerted efforts into optimizing the semiconductor and the sensitizer turned into a significant progress,2−5 whereas both liquid- and quasi-solidstate electrolytes still feature numerous challenges. The most commonly used electrolytes in liquid-state DSSCs (lsDSSC) are based on I−/I3− and CoII/CoIII complexes as redox couples. For example, Yella et al. reported on an overall efficiency exceeding 13% when combining a porphyrin sensitizer and a CoII/III(tris)bipyridine electrolyte and Hanaya et al. achieved 14.3% cosensitizing two organic dyes and a CoII/III(tris)phenantroline electrolyte.2,3 Although these efficiencies are remarkable, liquid-state electrolytes bear the problem of short © XXXX American Chemical Society

lifetimes due to solvent evaporation leading to leakages. Another major drawback of iodine-based electrolytes is their corrosiveness.6 One strategy to circumvent such setbacks is to investigate quasi-solid-state DSSCs (qssDSSC). Leading examples are gelelectrolytes,7−9 hole-conducting materials like the PEDOTpolymer,10,11 or ionic liquids (ILs).12−14 Among these, ILs give rise to low volatility, thermal and chemical stability, as well as wide electrochemical windows.15 State-of-the-art qssDSSCs show efficiencies of up to 8% with an iodine redox couple based on ionic liquids.16,17 Nevertheless, IL-based electrolytes show rather low ionic conductivities and poor penetration into the electrode network due to their high viscosity. This affects, however, both, the open-circuit voltage (VOC) and the fill factor (FF) in qssDSSCs. On one hand, increasing the ionic diffusion and coverage of the electrode through, for example, additives and crystal growth inhibitors,18,19 and, on the other hand, tailor-made chemical structures featuring triiodide channels, Special Issue: Hupp 60th Birthday Forum Received: January 31, 2017 Accepted: April 17, 2017

A

DOI: 10.1021/acsami.7b01522 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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v/v solution of N719 (Sigma-Aldrich) (5 × 10−4 M) at room temperature for 16 h. For counter-electrode fabrication, FTO plates with holes of 1 mm diameter at the edge of the active area were used. Preceding the fabrication of the counter-electrodes, the FTO slides were cleaned following the aforementioned procedure. A thin film of chloroplatinic acid, namely H2PtCl6 (0.5 mM) in ethanol, prepared from chloroplatinic acid hydrate ∼38% Pt basis (Sigma-Aldrich), was spread over the FTO plates, and dried in air prior to baking at 400 °C for 30 min. Subsequently, both electrodes were sealed together with a transparent film of Surlyn 1125−25 (Solaronix) cut as a frame around the nanocrystalline TiO2 film. Iodine double sublimed (Merck) was used as received. (SeCN)2 was prepared according to literature.25 All electrolytes contained guanidinium thiocyanate ≥99.0% (Fluka) (0.1 M) and 4-tert-butylpyridine 96% (Sigma-Aldrich) (0.5 M) in MeCN (Sigma-Aldrich). The electrolyte was introduced into the device through the above-mentioned holes in the counter-electrode by vacuum-filling. The results presented here are the best devices of 5 and show a deviation of ±15%. For quasi-solid-state measurements the unsealed device was then left at 90 °C for 15 min to evaporate the MeCN from the electrolyte. All devices were sealed with Surlyn and a glass plate on top. Following, the measurements were performed from 30 up to 100 °C and recooling the samples down to their initial temperature. Photocurrent measurements were carried out under 1 sun and AM 1.5 conditions using a custom-made solar simulator adjustable 150 W Xe-lamp source (LOT) combined with appropriate filters. Current−voltage measurements and electrochemical impedance spectroscopy (EIS) were conducted by using a potentiostat (PGSTAT 30 Autolab Metrohm) in the range of 0.1−0.9 V with a scan rate of 0.244 V/s. The temperature control was fabricated by using a hot plate with a thermostat, directly contacted to the device. The stability was measured by using the aforementioned setup fixing it at 70 °C.

assist in achieving higher VOC and FF values.20 We have shown, in the context of the latter, that double-alkyl-substituted ionic liquid-based electrolytes are suitable candidates. Molecular pathways are created in their lattice structure, which facilitate the ionic diffusion under device operation conditions.20 Although the aforementioned concepts have emerged as effective measures, the low VOC still remains as a major bottleneck. This limits the overall power conversion efficiency of the device. Several attempts have been made to increase the V OC by substituting the iodine redox couple with a pseudohalogen redox couple with moderate success.21,22 Here, we focused on addressing this problem by developing quasi-solid-state electrolytes featuring a iodine-pseudohalogen redox couple together with a new series of symmetrically alkylsubstituted ionic liquids with two different counteranions, namely iodide and selenocyanate, as shown in Scheme 1.23 Scheme 1. Synthesis of the 1,3-Dialkylimidazolium Selenocyanate ([CnCnIm]+[SeCN]−) and Iodide ([CnCnIm]+[I]−) Salts

3. RESULTS AND DISCUSSION 3.1. Synthesis of the Ionic Liquids. A series of 1,3dialkylimidazolium-based ionic liquids, namely [CnCnIm]+[A]− (n = 6, 8, 10, and 12; the length of the alkyl chain) with A = I¯, SeCN¯ and Im = imidazole, were prepared by ion exchange from [CnCnIm]+[Cl]− using NaI and KSeCN salts − Scheme 1. [CnCnIm]+[Cl]− (n = 6, 8, 10, and 12) were prepared according to a previously reported procedure.26 Under inert gas atmosphere (N2), N-trimethylsilylimidazole is reacted with the corresponding 1-chloroalkene, followed by recrystallization of the resulting 1,3-dialkylimidazolium chlorides [CnCnIm]+[Cl]− at −30 °C. The synthesis of [CnCnIm]+[I]− (n = 6, 8, 10, and 12) were performed as reported elsewhere and are from here on referred to as CnI.27 After anion exchange with KSeCN, the [CnCnIm]+[SeCN]− (n = 6, 8, 10, and 12), later referred to as CnSeCN, was carried out in MeCN, which was later evaporated to dryness. After evaporation, KCl was precipitated from CH2Cl2, and the organic solution was washed with water, in which the selenocyanate ILs are insoluble, removing the trace salt impurities. All ILs [CnCnIm]+[A]− (A = Cl¯, I¯, SeCN¯, n = 6, 8, 10, and 12) were recrystallized from CH2Cl2/Et2O at −30 °C and dried in vacuo at 50 °C for 48 h. 3.2. Characterization of the Ionic Liquids. Absorptions of the pure ionic liquids were recorded in MeCN with a concentration of 1 × 10−4 M, see Figure S1. Iodide-containing ILs (C6I, C8I, C10I, and C12I) show characteristic absorption spectra with maxima at 290 and 360 nm,28 whereas the absorption spectra of selenocyanate featuring ILs (C6SeCN, C8SeCN, C10SeCN, and C12SeCN) are featureless through much of the UV and visible regions. Importantly, the length of the alkyl side chains imposes no appreciable changes on the absorption features. Next, to determine the impact of the side chains on the thermal properties, DSC measurements were

Notably, an iodine-pseudohalogen redox couple in quasi-solidstate devices has, to the best of our knowledge, never been tested under outdoor conditions, that is, 50−70 °C and 1 sun illumination of AM 1.5. The most remarkable result is the optimization of the electrolytes with additives, as it leads to an increase of the VOC of up to 70 mV and to an overall efficiency enhancement of around 70% with respect to devices with traditional iodine electrolytes. Finally, the best qssDSSCs feature a 1000 h stable efficiency of 7−8% under outdoor temperature operation conditions and 1 sun illumination.

2. EXPERIMENTAL SECTION 2.1. Electrolyte Characterization. Steady-state absorption measurements of the electrolytes were recorded with a Lambda 2 spectrometer in acetonitrile. Redox potentials of the electrolyte were measured using cyclic voltammetry and square wave voltammetry with TBAPF6 (0.1 M) in acetonitrile (METROHM FRA 2 μAutolab Type III). Differential scanning calorimetry (DSC) was measured on a Netzsch DSC 204 with a heating rate of 5 K min−1. Linear sweep voltammetry (METROHM FRA 2 μAutolab Type III) was performed using the anodic and cathodic steady-state polarization diffusion current densities (JSS) of the electrolyte sandwiched between two fluorine-doped tin oxide (FTO) electrodes coated with platinum, i.e., FTO−Pt/electrolyte/Pt−FTO to calculate the diffusion coefficient of the ionic liquid electrolytes. 2.2. Device Fabrication and Characterization. The preparation of the devices was done by using standard procedure explained as follows.24 FTO TEC 8 plates (Xop Glass Company) were extensively cleaned by using ultrasonication in subsequent baths of acetone, detergent, and 2-propanol for 15 min each, just before the deposition of a blocking layer of TiCl4 at 70 °C for 30 min which was calcinated at 450 °C.24 TiO2 (TISP, Solaronix) was doctorbladed using a circular template with a diameter of 5 mm and a thickness of 50 μm onto an FTO slide, dried at 125 °C for 6 min, and subsequently baked at 500 °C for 30 min. The electrode was immersed in tert-butanol: MeCN 1:1 B


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Table 1. Phase Transition Temperatures Determined by DSC, Oxidative Potentials (Eox) Measured against Ag/Ag+, and Ionic Diffusion Coefficients (D) of the Pure Ionic Liquids onset temperature a

compd

liquid-crystalline phase (°C)

isotropic phasea (°C)

isotropic phaseb (°C)

57.9

C6I C8I C10I C12I C6SeCN C8SeCN C10SeCN C12SeCN

17.5 40.4

30.5 88.5

21.1 84.6

40.0

5.8 70.2

68.0

liquid-crystalline phaseb (°C)

Eoxc (V)

−4.13

0.121 0.123 0.122 0.121 0.181 0.179 0.185 0.194

−3.2 17.1 98.8 8.5 13.9

Dd (m2s−1) 2.09 1.17 2.1 5.29 1.40 1.19 1.16 4.82

× × × × × × × ×

10−14 10−14 10−15 10−16 10−15 10−15 10−15 10−16

Heating rate 5 K min−1. bCooling rate 5 K min−1. cPotentials measured with respective redox partners at a molar ratio of 1:0.05 and an IL concentration of 1 × 10−3 M in acetonitrile. dD measured via LSV assays at 30 °C.

a

selenium- and iodide-based ILs shows that the latter features better D. For instance, the selenium-based IL with the shortest alkyl chains, C6SeCN, has a D of 1.40 × 10−15 m2 s−1, that is 1 order of magnitude lower than the value of 2.09 × 10−14 m2 s−1 of the corresponding iodide-based IL, C6I. This difference is not unexpected, given the fact that (SeCN)3 is appreciably larger than I3−, resulting in slower ionic diffusion.31 3.3. Characterization of the Electrolytes for qssDSSCs. All electrolytes were prepared by mixing additives 0.1 M GuSCN and 0.5 M 4tbpy, 0.6 M ILs, and 0.1 M of the redox partner(s) in MeCN, see Experimental Section for more details. The amount of additives was optimized beforehand, Figure S4. In the case of quasi-solid-state devices, the solvent was removed by heating at 90 °C for 15 min. Electrolytes based on ILs are referred to as CnI/I2 or CnSeCN/(SeCN)2, in which n is the length of the alkyl chain. In case of implementation of the iodine-pseudohalogen couple electrolytes are referred to as CnI/(I2)x((SeCN)2)1−x or CnSeCN/((SeCN)2)x(I2)1−x, where x refers to the mole fraction. Initially, we determined the absorption features of the electrolytes in MeCN, inset in Figure S1. For both iodine- and selenium-based electrolytes, the absorption features are identical to those described for pure ILs. Next, we focused on the impact of the additives on the thermal features of the electrolytes. Addition of 4-tbpy, whose boiling point is higher than the working temperatures of the qssDSSCs, assists in eliminating the liquid crystalline phase previously observed in C10I, C12I, C10SeCN, and C12SeCN, as well as shifting the onset of the liquid phase to lower temperatures, Figure S5 and Table S1. For example, the onsets of the liquid crystalline and the liquid phases of C12SeCN/(SeCN)2 are shifted from 40 to 31 °C and from 70.2 to 42.8 °C, respectively. Under device working conditions, the ILs will be in their liquid phase and will demonstrate their highest ion diffusivity. We also performed CV measurements to probe the influence of the additives on the oxidation features of the different electrolytes. We conclude from Table S2 that the influence is marginal with oxidations that shift from 0.181 to 0.174 V for C6SeCN and C6SeCN/(SeCN)2, respectively. This is attributed to the presence of the additive 4-tbpy in the electrolyte.32 Considering that in the electrolytes the onset of the liquid phase was significantly reduced, larger ionic diffusion coefficients are likely to be seen under both liquid and quasisolid-state conditions. In the liquid-state electrolytes, JSS values were not clearly discernible from the limiting diffusion current. In the quasi-solid-state, however, JSS values were noted and, as expected, they were 1 order of magnitude higher than for the pure ILs, Table S2. As a matter of fact, both sets of ILs show

performed with the pure ionic liquids. Short-alkyl-chain ILs, C6I, C8I, C6SeCN, and C8SeCN, are viscous at room temperature. Only a single phase change, namely from the liquid crystalline phase to the liquid phase, is noted in temperature regime ranging from room temperature to 100 °C. C10I, C12I, C10SeCN, and C12SeCN are solids at room temperature and exhibit two phase changes in the same temperature regime, that is, from the solid phase to the liquid crystalline phase and from the liquid crystalline phase to the liquid phase, Figure S2 and Table 1. For example, C12SeCN transforms from solid to liquid crystalline and to isotropic at 40 and 70.2 °C, respectively, whereas C6SeCN or C8SeCN feature no real phase change, but glass transitions in the temperature region between 40 and 80 °C. A parameter that governs the VOC in devices is the redox potential of the ILs. These were determined by means of cyclic voltammetric measurements in MeCN with ILs at a concentration of 1 × 10−3 M versus Ag/ Ag+ electrode. Figure S3 indicates that for iodide-based ILs in the presence of I2 as a redox partner the first oxidation sets in at around 0.12 V. For selenium-based ILs with (SeCN)2 as redox partner the first oxidation is shifted to 0.19 V. Both values are in sound agreement with those reported in the literature.23,29 It is noteworthy that the length of the alkylchains on the IL does not influence the oxidation. Both the positively shifted oxidation and the featureless absorption of selenium-based ILs, render them promising alternatives to electrolytes featuring iodide-based ILs. Finally, the ionic diffusion coefficients (D), which influence the device performance in terms of FF, were determined by means of linear sweep voltammetry (LSV) assays applying eq 1.30

D=

JSS l F 2nc(I−3 )

(1)

Here, l is the separation length between the two electrodes, n the number of electrons, F the Faraday constant, and c(X3−) the concentration of the redox active species. Our experiments involve measuring the anodic and cathodic steady-state polarization diffusion current densities (JSS) in a setup, in which the electrolyte is sandwiched between two fluorinedoped tin oxide (FTO) electrodes coated with platinum, FTO−Pt/electrolyte/Pt−FTO. Measurements with pure ILs show a correlation between the diffusion coefficient and the length of the alkyl chain, Table 1. ILs with longer side chains form more rigid networks and feature lower ionic diffusion coefficients caused by the steric hindrance. More importantly, the direct comparison between C


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ACS Applied Materials & Interfaces the same trends. First, the Ds reduce upon increasing the alkyl chain length. Second, the iodine-based electrolytes diffuse notably faster than the selenium-based electrolytes: 1.2 × 10−13 m2s−1 for C6I/I2 versus 4.2 × 10−15 m2s−1 for C6SeCN/ (SeCN)2. Taking all of the aforementioned into account, we postulate that electrolytes featuring iodine should be superior in terms of D values, leading to enhanced FFs values, whereas electrolytes featuring (SeCN)2 should provide higher VOC values. As such, the concept of a mixed redox couple, namely a mixture of I2 and (SeCN)2, was investigated. In a first step, we looked for the optimum I2 to (SeCN)2 molar ratio mixing their solutions in different molar ratios. In terms of absorption features, an increase of (SeCN)2 in the solutions leads to a slight attenuation of the 360 nm absorption, Figure 1. In terms of redox features, Table 2, they shift notably toward more positive values with increasing (SeCN)2 content.

and C6I/(I2), respectively, Table 3. For the iodine/iodide redox couple, a Grotthuss-type mechanism has been established as means of charge transport33 and Oskam et al. have not observed any (SeCN)3− electropolymerization, suggesting a similar behavior for (SeCN)− to I−.25 Moreover, ILs are also able to interact with the redox couple via a Grotthuss mechanism.34 On this basis, we postulate that the excellent D values most likely stem from a combination of forming I−(SeCN)2 complexes and of favorable interactions of the redox active species with the cationic moiety of the ILs. 3.4. Device Fabrication and Analysis. DSSCs were fabricated with TiO2 as n-type electrode, N719 as dye, and platinum as counter electrodes; see the Experimental Section for more details. Representative photocurrent density versus applied voltage dependencies are shown in Figure S6. Table S3 summarizes the corresponding figures-of-merit. Devices with C6I/I2 and C6SeCN/(SeCN)2 perform equally well in the liquid state, reaching efficiencies of 5.6 and 5.0%, respectively. Subtle differences are seen in short circuit current densities (JSC) and VOC, which are higher for iodine-based devices and higher for selenium-based devices, respectively. Slightly different VOC values are caused by differences in the redox potentials, while variations in the JSC are due to slower ionic diffusion of (SeCN)3− relative to I3− and slower regeneration of the SeCNbased redox couple.12 For devices with C6I/I2 a loss of VOC of 120 mV is noted in a comparison with lsDSSCs, but JSC and FF are similar. Overall, the device efficiencies are reduced up to 2.64% under room temperature conditions. To evaluate the device performance under outdoor conditions, we monitored the device figures-of-merit under 1 sun illumination at AM 1.5 conditions and during heating−cooling cycles, Figure 2. In detail, the temperature was increased from room temperature to 100 °C and back to room temperature in increments of 10 °C and 5 min. As shown in Figure 3, the efficiency of iodinebased devices increases exponentially up to a value of 4.74% under outdoor temperature conditions. Responsible for this trend is a steadily increasing JSC, whereas VOC and FF are invariable. However, the same trend is not present for devices with the electrolyte C6SeCN/(SeCN)2, see Figure 4 and Table 4. Here, qssDSSCs feature low FF and, in addition, the device is no longer operative after one heating and cooling cycle due to the instability of the C6SeCN/(SeCN)2 electrolyte. Next, we turned to the above-mentioned iodine-pseudohalogen electrolytes. Here, we focused on the optimum combination in terms of oxidation potential and ionic diffusivity: C6I/(I2)0.75((SeCN)2)0.25. A direct comparison of devices running on C6I/I2 and C6I/(I2)0.75((SeCN)2)0.25 electrolytes supports the notion that the latter outperforms the electrolytes based on I−/I3− redox couple, see Figure 5 and Table 4. For instance, significant increases in VOC from 0.65 to 0.75 V and in JSC from 9.63 to 17.98 mA/cm2, but unchanged FF values emerge for devices with iodine-pseudohalogen electrolyte when compared to C6I/I2. As a matter of fact, the efficiency of C6I/(I2)0.75((SeCN)2)0.25 qssDSSCs is at room temperature nearly double that of C6I/I2 devices under the same conditions, Figure 3. More striking, the efficiency of the former remains at 8% under heating and declines slowly under cooling, Figure 3. Encouraged by these results, we prepared iodine-pseudohalogen electrolytes built on C6SeCN, 0.075 I2, and 0.025 M (SeCN)2, C6SeCN/((SeCN2))0.25(I2)0.75. In this particular case, we noted a 1.7-fold improvement in the overall efficiency at 70 °C upon comparison to devices filled with C6SeCN/(SeCN)2 due to changes in FF, Figure 4. Although

Figure 1. Normalized UV/vis absorption spectra of different molar ratios of the hybrid-redox couple, i.e., I2/(SeCN)2 ratios of 1:0 (black), 4:1 (red), 1:1 (blue), 1:4 (orange), and 0:1 (green).

Table 2. Redox Potentials of the Hybrid Redox Couple in Relation to the I2/(SeCN)2 Ratio Measured against Ag/Ag+ redox potential (V) I2/(SeCN)2

0.164 0:1

0.150 1:4

0.127 1:1

0.123 4:1

0.100 1:0

As shown in Figure 1, the absorption features C6I/ (I2)0.75((SeCN)2)0.25 are slightly affected relative to C6I/I2. Concerning the redox potentials, an increase in (SeCN)2 resulted in a shift of the oxidation to more positive values. In turn, the oxidation seems tunable in the range between the values of pure I2 and pure (SeCN)2 without being largely influenced by the addition of 4-tbpy, Table 3. In addition, the LSV assays show that D increases up to values of 3.21 × 10−13 compared to 1.88 × 10−13 m2s−1 for C6I/(I2)0.75((SeCN)2)0.25 Table 3. Ionic Diffusion Coefficients (D) for Quasi-SolidState Electrolytes Based On C6I With Different Ratios of the Hybrid Redox Couple at 30 °C and Oxidative Potentials (Eox) of the Hybrid Electrolytes in Relation to the I2/ (SeCN)2 Ratio Measured against Ag/Ag+ in MeCN electrolyte

Eox (V)

C6I/I2 C6I/(I2)0.75((SeCN)2)0.25 C6I/(I2)0.5((SeCN)2)0.5 C6I/(I2)0.25((SeCN)2)0.75 C6I/SeCN2

0.100 0.109 0.137 0.149 0.179

D (m2 s−1) 1.88 3.21 2.01 1.59 1.35

× × × × ×

10−13 10−13 10−13 10−13 10−13 D


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Figure 2. J−V curves of qssDSSCs using C6I/I2 (left) and C6SeCN/(SeCN)2 (right). The color code for the temperatures is violet, 30 °C; blue, 40 °C; light blue, 50 °C; dark green, 60 °C; green, 70 °C; yellow, 80 °C; orange, 90 °C; and red, 100 °C.

Figure 3. Figures-of-merit for qssDSSCs based on C6I without (left) and with (right) iodine-pseudohalogen redox couple under working conditions (solid line, heating up; dashed line: cooling down). The color code is black: C6I/I2, and red: C6I/(I2)0.75((SeCN)2)0.25.

the aforementioned trend in the figures-of-merit, all devices have in common their excellent stability over the temperature cycle from 30 to 100 °C. Compared to C6I/I2 and C6SeCN/ (SeCN)2, the iodine-pseudohalogen ones feature only a weak hysteresis, Figures 3 and 4. A likely rationale is found in the EIS analysis. Here, we evaluated EIS measurements between 30 and 100 °C for devices containing C6I/I2, C6I/ (I2)0.75((SeCN)2)0.25, C6SeCN/(SeCN)2, and C6SeCN/ ((SeCN2))0.25(I2)0.75 electrolytes. In Figure 6, the Nyquist plots of C6I/I2 and C6I/ (I2)0.75((SeCN)2)0.25 electrolytes under light and dark conditions at 30 °C are contrasted. Please note that the equivalent circuit, which is used for the fitting, is illustrated in Figure S7. RS is the series resistance inherent to the measurement setup. The first semicircle, at frequencies between 1000 and 10 000 Hz relates to the charge transfer resistance (Rc) across the interface of the Pt counter electrode.35 The second semicircle in the range from 100 to 1000 Hz is ascribed to charge transfer processes across the TiO2/electrolyte interface. It describes the

we noted a reasonable stability for C6SeCN/ ((SeCN2))0.25(I2)0.75 in the heating/cooling assays, the VOC of iodine-pseudohalogen devices was on average around 50 mV lower than for non-iodine-pseudohalogen devices, Table 4. In summary, the only benefit stemming from the use of these iodine-pseudohalogen devices is a better balance between regeneration and recombination, as reflected in the FF values. To further improve VOC, we applied an iodine-pseudohalogen redox couple consisting of 0.075 M (SeCN)2, and 0.025 M I2. At this ratio the oxidation potential is close to that of pure (SeCN)2, but D is much lower compared to the aforementioned C6SeCN/((SeCN2))0.25(I2)0.75 as shown in Table 3. At 70 °C, VOC values increased by around 10 and 60 mV for C6I/(I2)0.25((SeCN)2)0.75 and C6SeCN/(SeCN)2)0.75(I2)0.25 devices, respectively. The main trade-off is, however, a lower JSC caused by a hindered regeneration of the (SeCN)2 redox pair. The higher (SeCN)2 content in C6I/(I2)0.25((SeCN)2)0.75 and C6SeCN/(SeCN)2)0.75(I2)0.25 unbalances regeneration as well recombination and causes a loss in current density. Besides E


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Figure 4. Figures-of-merit for qssDSSCs based on C6SeCN without (left) and with (right) iodine-pseudohalogen redox couple under working conditions (solid line, heating up; dashed line, cooling down). The color code is blue, C6SeCN/(SeCN)2; and orange, C6SeCN/ ((SeCN)2)0.25(I2)0.75.

Table 4. Figures of Merit of Devices with quasi-Solid-State C6-Electrolytes with and without Hybrid-Redox Couple at RT and 70°C ionic liquid at RT C6I/I2 C6I/(I2)0.25((SeCN)2)0.75 C6I/(I2)0.75((SeCN)2)0.25 C6SeCN/(SeCN)2 C6SeCN/(SeCN)2)0.75(I2)0.25 C6SeCN/(SeCN)2)0.25(I2)0.75 at 70 °C C6I/I2 C6I/(I2)0.25((SeCN)2)0.75 C6I/(I2)0.75((SeCN)2)0.25 C6SeCN/(SeCN)2 C6SeCN/(SeCN)2)0.75(I2)0.25 C6SeCN/(SeCN)2)0.25(I2)0.75

VOC (V)

JSC (mA/cm2)

FF

η (%)

0.65 0.71 0.74 0.80 0.83 0.73

9.73 4.57 17.98 6.99 1.80 5.29

0.55 0.78 0.59 0.22 0.23 0.44

3.50 2.53 7.88 1.24 0.35 1.70

0.64 0.70 0.73 0.73 0.75 0.69

12.16 5.74 19.05 8.33 1.87 8.45

0.61 0.79 0.58 0.21 0.17 0.36

4.74 3.17 8.07 1.22 0.24 2.11

Figure 5. Comparison of J−V curves of qssDSSCs using electrolytes C6I/I2 (black) and C6I/(I2)0.75((SeCN)2)0.25 (red) at 70 °C.

where kB is the Boltzmann constant, T is the temperature, n is the number of exchanged electrons, q is the elemental charge, AV is Avogadro’s constant, l is the separation length between the two electrodes, and Rw is the resistance taken from the Nyquist plot. An overview over the parameters obtained from EIS is provided in Table 5. The detection of the third semicircle under dark conditions is caused by presence of leakage current as reported by other groups.36 Under illumination, similar Rl values were derived for C6I/I2 and C6I/(I2)0.75((SeCN)2)0.25 devices. However, it was impossible to estimate an exact value for selenium-based devices, supported by one broad peak in the Bode phase instead of two distinguishable ones in the frequency range between 10 000 and 100 Hz, Figure S8. In the dark, Rd values offer a rationale for the higher e ffic i e n c y o f C 6 I / I 2 d e v i c e s co m p a r e d t o C 6 I/ (I2)0.75((SeCN)2)0.25 devices. Rd values of 56.1 and 64.1 Ω signify a lower rate of recombination and, in turn, a slightly

transport resistance (Rl) under light and the resistance against recombination (Rd) in the dark. From both values, the collection efficiency ηcoll is typically calculated as R ηcoll = 1 − l Rd (2) The third semicircle is found between 0.1 and 100 Hz and is attributed to the Warburg diffusion (Rw) of the redox couple. From the Warburg resistance under illumination, D is determined as35

D=

kBT 2 2

n q AV lR w

(3) F


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Figure 6. Nyquist plots measured at 30 °C under light (left) and dark (right) conditions of qssDSSCs based on C6I. The color code is black, C6I/I2; red, C6I/(I2)0.75((SeCN)2)0.25.

Table 5. EIS Parameters for qssDSSCs with and without Iodine-Pseudohalogen Redox Couple Measured at 30 °C device

Rl (Ω)

Rd (Ω)

ηcoll

D (m2 s−1)

C6I/I2 C6I/(I2)0.75((SeCN)2)0.25 C6SeCN/(SeCN)2 C6SeCN/(SeCN)2)0.25(I2)0.75

11.1 8.74

56.1 64.1 986 157

0.80 0.86

6.77 6.92 5.20 1.18

× × × ×

10−7 10−7 10−9 10−7

higher ηcoll for the iodine-pseudohalogen devices. When going from C6SeCN/(SeCN)2 to C6SeCN/((SeCN)2)0.25(I2)0.75 with smaller Rd values of 986 versus 157 Ω, respectively, evolved, see Table 5. At 100 °C, the values for the different resistances were generally lower, as the applied temperature accelerated injection and diffusion processes. Nevertheless, the overall trends remained the same as at 30 °C, see Figure S9 and Table S4. Lastly, we compared the Warburg diffusion for devices with the four electrolytes and found trends that are similar to those seen in the LSV measurements vide supra. Here, the iodinepseudohalogen devices feature an enhanced D compared to devices with only one redox partner, Table 5 and Table S4. Both trends, that is, high D for the electrolyte and low Rl /high Rd of the devices, are desirable properties for efficient qssDSSCs.36 EIS findings corroborate the sovereign efficiencies of iodine-pseudohalogen qssDSSCs. In the final assay, we monitored the long-term stability of qssDSSCs by monitoring the figures-of-merit at 70 °C over the course of 1000 h, Figure 7. Devices with C6I/I2 and C6I/ (I2)0.75((SeCN)2)0.25 electrolytes are long-term stable with an efficiency loss of less than 20%. A closer look at the figures-ofmerit suggests that the efficiency loss is reasonably attributed to a lowering of the current density, whereas VOC and FF remain constant during the course of the investigations, Figure S10. Dye degradation seems to be mainly responsible for the device degradation, whereas electrode and electrolyte appear stable.

Figure 7. Relative efficiency loss at 70 °C over 1000 h for qssDSSCs featuring C6I/I2 (black) and C6I/(I2)0.75((SeCN)2)0.25 (red) electrolytes.

mixtures of different mole fractions of I2 and (SeCN)2 were prepared, affording iodine-pseudohalogen redox couples with a lack of visible absorption, optimum oxidation potentials, and best D values. All ILs were turned into electrolytes and tested in lsDSSCs. Assays resulted in well performing devices with ∼5− 6% efficiencies. However, in quasi-solid-state devices those containing iodine-based electrolytes (C6I/I2: ∼ 4%) performed markedly better than those having SeCN-based electrolytes (C6SeCN/(SeCN)2: ∼1%). Quasi-solid-state performance is increased by implementing the iodine-pseudohalogen redox couple, which increases D as well as the redox potential values, leading to higher JSC and VOC. At this point, changes in the regeneration of the dye when using the new electrolyte cannot be ruled out. This aspect will be investigated in future studies by means of transient absorption spectroscopy. Overall, efficiencies of 7−8% were realized in devices based on C6I/(I2)0.75((SeCN)2)0.25 electrolytes under outdoor conditions. 7−8% surpass the efficiencies found in solid- and quasi-solid-state devices with SeCN−/(SeCN)3− redox couples by up to 5%,12,16,21 as well as those reported by Song. et al. with an iodine-pseudohalogen redox couple.23 As such, the device efficiencies achieved with C6I/(I2)0.75((SeCN)2)0.25 are among the highest known for state-of-the-art quasi-solid-state qssDSSC8,22 and show a loss of less than 20% over 1000 h operating under outdoor conditions.

4. CONCLUSION A set of novel, symmetrically alkyl-substituted ILs with either iodide or selenocyanate as counteranions has been synthesized. Spectroscopic, electrochemical, and DSC measurements have been conducted to characterize the ILs. Those ILs with selenocyanate ([CnCnIm]+[SeCN]−) lack appreciable absorptions in the visible region and redox potentials in the range of 0.17−0.19 V, whereas ILs with iodide ([CnCnIm]+[I]−) have better D values. To combine the advantages of both materials, G


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01522. Figures S1−S10 and Tables S1−S4 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Karsten Meyer: 0000-0002-7844-2998 Dirk M. Guldi: 0000-0002-3960-1765 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the EAM cluster in the frame of the DFG excellence programs for their support. D.M.G. acknowledges the DFG, ECRC, and the ZMP for financial and intellectual support.

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REFERENCES

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Iodine-Pseudohalogen Ionic Liquid-Based Electrolytes for Quasi-Solid

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