POSS-Based Electrolyte for Efficient Solid-State Dye-Sensitized Solar

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POSS-Based Electrolyte for Efficient Solid-State Dye-Sensitized Solar Cells at Sub-Zero Temperatures Kai Lv, Wei Zhang, Lu Zhang, and Zhong-Sheng Wang* Department of Chemistry, Lab of Advanced Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, 2205 Songhu Road, Shanghai 200438, P. R. China S Supporting Information *

ABSTRACT: To expand the application of solid-state dye-sensitized solar cells (ssDSSCs) to low temperatures, it is necessary to develop new solid electrolytes with low glass transition temperature (Tg). The Tg is regulated by varying the length of alkyl chain that is connected with the nitrogen atom in the imidazolium ring linked to the polyhedral oligomeric silsesquioxane (POSS). The Tg as low as −8.8 °C is achieved with the POSS grafted with methyl-substituted imidazolium. The effect of alkyl group on the conductivity, Tg, and photovoltaic performance has also been investigated. The conductivity and power conversion efficiency increase with the alkyl length, while the Tg first increases and then decreases with the alkyl length. Among the synthesized POSS-based ionic conductors, the POSS grafted with the methyl-substituted imidazolium yields the highest power conversion efficiency of 6.98% at RT due to its highest conductivity, and the efficiency (6.52%) is still good at −4 °C, as its Tg (−8.8 °C) is lower than the working temperature (−4 °C). This finding suggests that the POSS-based solid electrolyte is promising for subzero-temperature applications of ssDSSCs. KEYWORDS: dye-sensitized solar cells, POSS, solid-state electrolyte, alkyl length effect, low temperature operation

1. INTRODUCTION Dye-sensitized solar cells (DSSCs) have good industrialization prospects because of their potential low cost and environmental friendliness.1 As one of the key components in DSSCs, the electrolyte can be liquid or solid-state. Although the liquid electrolyte exhibits state-of-the-art efficiency, it suffers from leakage and can cause the dye desorbed from the TiO2 surface. By comparison, the solid electrolyte is easy to seal up and suitable for practical applications, but the efficiency obtained so far is much lower than that with the liquid electrolyte.2−44 The main causes of the low efficiency for the solid-state dyesensitized solar cells (ssDSSCs) are the low conductivity of the solid electrolyte as well as the interfacial contact between the photoanode and the solid electrolyte. Imidazolium iodides are important additives for liquid electrolytes45 and also used as ionic liquid solvents replacing volatile organic solvents.46−48 Recently, we successfully improved the conductivity of solid imidazolium iodides by attaching functional groups to the imidazolium ring, and employed them as solid electrolytes for ssDSSC. 49,50 Imidazolium iodides are small molecules, which tend to crystallize during electrolyte injection followed by evaporation of the solvent. This drawback hampers the pore filling of the TiO2 film with the solid-state electrolyte and thus leads to poor contacts between dye-sensitized titanium dioxide and electrolyte. A crystal growth inhibitor is usually added to these solid electrolytes to improve the pore filling and interfacial contact. In addition, linking polyhedral oligomeric silsesquioxane © 2016 American Chemical Society

(POSS) to the imidazolium ring gives rise to an amorphous nature of the material, which improves the filling of solid electrolytes into the pores of TiO2 without adding a crystal growth inhibitor.51 As the elastomeric state of materials is advantageous to good interfacial contact, it is necessary to prepare solid electrolytes with low glass transition temperatures.52 We previously developed POSS-based ionic conductors with glass transition temperature of 5−6 °C,52 which ensures good interfacial contact between solid electrolyte and the TiO2 film during the operation of ssDSSCs at temperatures around or above RT. To make the ssDSSC work efficiently at low temperatures below 0 °C, it is necessary to synthesize new POSS-based ionic conductors with glass transition temperature below 0 °C. In this work, we prepared a series of POSS-based imidazolium iodides linked with alkyl groups having different chain lengths (Figure 1). The effect of alkyl length on the glass transition temperature, ionic conductivity and the corresponding solar cell performance has been investigated. The POSS-based imidazolium iodide with a methyl group processes glass transition temperature as low as −8.8 °C, which will be a favorable factor for operations of ssDSSCs below 0 °C. Received: December 17, 2015 Accepted: February 10, 2016 Published: February 10, 2016 5343

DOI: 10.1021/acsami.5b12353 ACS Appl. Mater. Interfaces 2016, 8, 5343−5350

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ACS Applied Materials & Interfaces

Figure 1. Structures of POSS-based imidazolium iodides with various alkyl groups of different lengths.

Scheme 1. Synthetic Route for POSS-Based Ionic Conductors: POSS-8Im-M, POSS-8Im-E, POSS-8Im-B, and POSS-8Im-O

1141.5568; found 1141.5476. Decomposition onset temperature: 252 °C. POSS-8Im-B: 1H NMR (DMSO-d6, 400 Hz, δ ppm): 9.37(s, 8H); 7.87(d, 8H); 7.84(d, 8H); 4.19(t, 32H); 1.79(m, 32H), 1.22(m, 16H); 0.88(t, 24H); 0.56(t, 16H). 13C NMR (DMSO-d6, 100 MHz, δ ppm): 136.7, 122.9, 51.5, 49.3, 31.9, 27.5, 19.5, 14.0. 29Si NMR (DMSO-d6, 100 MHz, δ ppm): − 66.83. ESI-HRMS (m/z): calcd for for Si8O12C80N16I6H1452+, 1253.6820; found 1253.6742. Decomposition onset temperature: 278 °C. POSS-8Im-O: 1H NMR (DMSO-d6, 400 Hz, δ ppm): 9.32(s, 8H); 7.86(d, 8H); 7.83(d, 8H); 4.18(t, 32H); 1.80(m, 32H), 1.22(m, 80H); 0.82(t, 24H); 0.58(t, 16H). 13C NMR (DMSO-d6, 100 MHz, δ ppm): 136.7, 122.9, 51.6, 49.6, 31.9, 30.0, 29.2, 26.3, 22.7, 14.6. 29Si NMR (DMSO-d6, 100 MHz, δ ppm): − 66.85. ESI-HRMS (m/z): calcd for Si8O12C112N16I6H2092+, 1478.4340; found 1478.4295. Decomposition onset temperature: 289 °C 2.3. Characterizations. 1H NMR and 13C NMR spectra were recorded on a Varian 400 MHz NMR spectrometer with tetramethylsilane (TMS) as an internal standard. 29Si NMR spectra were recorded on a DMX-500 instrument with tetramethylsilane (TMS) as an internal standard. Fourier transform infrared (FT-IR) measurements were performed on Shimadzu IRAffinity-1 FT-IR spectrometer. Thermogravimetric (TG) analysis was performed on a TG-DTA 2000S system (Mac Sciences Co. Ltd., Yokohama, Japan) at a heating rate of 10 °C min−1. Differential scanning calorimetry (DSC) was studied on a Shimadzu DSC-60A at a heating rate of 5 °C min−1. Solid-state electrolytes were sandwiched between two identical and parallel Pt electrodes and were then sealed up. The distance between the two electrodes was 30 μm. The ionic conductivity was estimated by recording the electrochemical impedance spectrum (EIS) at applied bias of 0 V and ac amplitude of 10 mV on an electrochemical workstation (Zahner XPOT, Germany). The current density−voltage (J−V) curves of ssDSSCs was recorded on a Keithley 2420 source meter under illumination of simulated AM1.5G solar light coming from a solar simulator (Sol3A equipped with a 450 W Xe lamp and an

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. LiI, 1-methylbenziimidazole (NMBI), and I2 were obtained from Acros. NaI, 3-chloropropyltrimethoxysilane, tetrabutylammonium iodide, and iodopropane were purchased from Aladdin. Organic solvents used in this work were purified using standard processes. Other chemical reagents were used as received from commercial sources without further purification. Transparent conductive glass (F-doped SnO2, FTO, 14 Ω/square, transmittance of 85%, Nippon Sheet Glass Co., Japan) was used as the substrate for the fabrication of TiO2 thin film electrodes. 2.2. Synthesis of POSS-Based Imidazolium Iodides. The synthetic route for ionic conductors is shown in Scheme 1. A and B (Scheme 1) were synthesized according to the literature.53,54 1.3 equiv of 1-alkyl imidazole, where alkyl = methyl, ethyl, butyl, or octyl, and B (0.5 g, 0.28 mmol) were dissolved in THF (15 mL), and then the mixture was stirred at RT for 48 h. After washing with THF, the precipitate was dissolved in methanol and dropped into THF for reprecipitation to produce the designed ionic conductors. The prepared ionic conductors are respectively expressed with POSS-8Im-M, POSS-8Im-E, POSS-8Im-B, and POSS-8Im-O, where M, E, B, and O stand for methyl, ethyl, butyl, and octyl, respectively. The characterization data for these ionic conductors are listed below. POSS-8Im-M: 1H NMR (DMSO-d6, 400 Hz, δ ppm): 9.18(s, 8H); 7.81(d, 8H); 7.72(d, 8H); 4.17(t, 16H); 3.86(t, 24H); 1.73(m, 16H); 0.59(t, 16H). 13C NMR (DMSO-d6, 100 MHz, δ ppm): 137.2, 122.7, 51.4, 36.6, 25.8 29Si NMR (DMSO-d6, 100 MHz, δ ppm): − 66.76. ESI-HRMS (m/z): calcd for Si8O12C56N16I6H972+, 1085.4942; found 1085.4953. Decomposition onset temperature: 233 °C. POSS-8Im-E: 1H NMR (DMSO-d6, 400 Hz, δ ppm): 9.26(s, 8H); 7.82(d, 8H); 7.79(d, 8H); 4.20(t, 16H); 4.16(t, 16H); 1.81(m, 16H); 1.44(t, 24H); 0.59(t, 16H). 13C NMR (DMSO-d6, 100 MHz, δ ppm): 136.4, 122.9, 51.6, 45.0, 24.1, 15.8. 29Si NMR (DMSO-d6, 100 MHz, δ ppm): − 66.79. ESI-HRMS (m/z): calcd for Si8O12C64N16I6H1132+, 5344


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ACS Applied Materials & Interfaces AM1.5G filter, Newport). The light intensity was calibrated using a standard Si solar cell (Newport 91150). Action spectra of the incident monochromatic photon-to-electron conversion efficiency (IPCE) for the solar cells were recorded using an SM-250 system (Bunkoh-Keiki, Japan). The intensity of monochromatic light was measured with a Si detector (S1337-1010BQ). 2.4. Fabrication and Evaluation of ssDSSCs. TiO2 films (12 μm) composed of nanoparticles (25 nm) were fabricated with a screen printing method and sintered at 500 °C for 2 h. The film thickness was measured with a surface profiler (Veeco Dektak 150, USA). The sintered films were then treated with 0.05 M TiCl4 aqueous solution at 70 °C for 30 min followed by calcinations at 450 °C for 30 min. When TiO2 electrodes were cooled down to 120 °C, they were dipped in dye (FNE29, Figure S1) solutions (0.3 mM in toluene) for 24 h at RT. The Pt-coated FTO as the counter electrode and the dye-loaded film as the working electrode were separated by a hot-melt Surlyn film (30 μm) and sealed together by pressing them under heat. The methanol solution of the solid electrolyte was injected repeatedly into the interspace between the working and counter electrodes from the two holes predrilled on the back of the counter electrode and dried on a hot plate with the temperature of 50 °C until the TiO2 porous film was filled with solid-state electrolyte. The cell was further dried at 50 °C under vacuum for 1 h to remove the residual methanol. Finally, the two holes were sealed with a Surlyn film covered with a thin glass slide under heat.

Figure 3. (a) EIS spectra of dummy cells with electrolytes containing ionic conductor POSS-8Im-M and different additives, and (b) equivalent circuit.

8Im-M with various additives. Two semicircles were observed in the spectra, the left one in the high-frequency region resulting from the electrode/electrolyte interface and the right one on the low-frequency region stemming from the bulk electrolyte.56 The left semicircle reflects the charge-transfer resistance (2Rct) at the electrode/electrolyte interface and the corresponding constant phase element (CPE). The intercept of the left arc on the real axis is the series resistance (Rs). The right semicircle in the low-frequency region is due to the Nernst diffusion impedance (ZN) of the electrolyte. By fitting the EIS spectra with the equivalent circuit, shown in Figure 3b, the Rs, Rct, and ZN are determined, and the conductivity (σ) can be calculated from the ZN following σ = d/(ZN × A), where d is the thickness and A is the area of the solid electrolyte. The Rs, Rct, and conductivity are summarized in Table 1 for comparison.

3. RESULTS AND DISCUSSION 3.1. Thermal Analysis. Figure 2 shows the DSC curves for the ionic conductors, from which the glass-transition temper-

Table 1. Conductivity for POSS-8Im-M with Different Additives at RT σ/mS cm−1 Rct/Ω

Electrolyte (molar ratio) POSS-8Im-M POSS-8Im-M + I2 (1.5:1) POSS-8Im-M + I2 + LiI (1.5:1:6) POSS-8Im-M + I2 + LiI + NMBI (1.5:1:6:10)

Figure 2. DSC curves of ionic conductors.

0.23 0.40 1.25 1.68

3.1 1.8 1.6 1.2

Rs/Ω 6.8 6.6 6.4 6.0

The conductivity at RT for POSS-8Im-M was 0.23 mS cm−1. Upon doping of I2 (POSS-8Im-M:I2 = 1.5:1), the conductivity increased (or the diameter of the right arc decreased) significantly due to the Grotthus charge exchange.57 Further doping of LiI (POSS-8Im-M:I2:LiI = 1.5:1:6) further increased the conductivity due to the incremented number of ions. As NMBI (solid) is frequently included in the electrolyte to improve the photovoltage,58 we also measured the conductivity of the POSS-based solid electrolyte containing NMBI (molar ratio, POSS-8Im-M:I2:LiI:NMBI = 1.5:1:6:10). The ratio of components in the mixture solid electrolyte was determined according to our previous report.51 The conductivity at RT for POSS-8Im-M with different additives is summarized in Table 1. It is seen from Table 1 that the Rs changes slightly with additives. The Rs is the series resistance contributed from wires, collection electrodes and contacts. The slight changes of Rs among these dummy cells are attributed to the slight differences of electrodes and contacts. However, the Rct decreases

ature (Tg) is determined. The Tg for POSS-8Im-M, POSS-8ImE, POSS-8Im-B, and POSS-8Im-O are −8.8, 22.4, 54.4, and 3.8 °C, respectively, as indicated in Figure 2. Evidently, the alkyl chain has a big effect on the Tg, which is determined by molecular weight, flexibility of molecular chains, and so on. When the alkyl chain is short, Tg increases with the elongation of the alkyl chain up to butyl because of the increased molecular weight. When the alkyl length reaches a certain extent, the effect of molecular weight diminishes while the flexibility becomes a dominant factor. From POSS-8Im-B to POSS-8Im-O, the decrease of Tg is attributed to the higher flexibility of the octyl group.55 3.2. Electrochemical Characterizations. The conductivity of the solid electrolyte is crucial to the performance of ssDSSCs. As the photovoltaic performance depends on the doping of various additives in the ionic conductor, POSS-8ImM was chosen to investigate the effect of additives on the conductivity. Figure 3a shows the EIS Nyquist spectra of POSS5345


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ACS Applied Materials & Interfaces significantly with progressive addition of additives. The Rct is related to the reduction of triiodides at the electrode/ electrolyte interface. The higher the conductivity, the greater the interfacial concentration of triiodides, making the reduction of triiodides easier. Therefore, the Rct decreases with the increase in conductivity. At the same ratio of components, the EIS spectra of these ionic conductors with different alkyl chains are compared in Figure 4, and the conductivities, Rct and Rs are summarized in

Figure 5. EIS spectra of dummy cells with the electrolyte containing POSS-8Im-M and additives (POSS-8Im-M:I2:LiI:NMBI = 1.5:1:6:10) measured at a series of temperatures.

where σo is a constant, Ea is the activation energy, T is the absolute temperature, and k is the Boltzmann’s constant.59 A linear relationship between the logarithm of conductivity and the reciprocal of absolute temperature observed in Figure 6

Figure 4. EIS spectra of POSS-based ionic conductors doped with the same additives (POSS:I2:LiI:NMBI = 1.5:1:6:10).

Table 2. Conductivity at RT for POSS-Based Ionic Conductors Containing Additives (POSS:I2:LiI:NMBI = 1.5:1:6:10), and the Charge Transfer Resistance (Rct) at the Electrode/Electrolyte Interface POSS:I2:LiI:NMBI = 1.5:1:6:10

σ/mS cm−1

Rct/Ω

Rs/Ω

POSS-8Im-M POSS-8Im-E POSS-8Im-B POSS-8Im-O

1.68 1.04 0.54 0.38

1.2 3.5 4.0 6.9

6.0 6.5 6.7 6.7

Figure 6. Temperature dependence of ionic conductivity for the electrolyte with POSS-8Im-M and additives (POSS-8ImM:I2:LiI:NMBI = 1.5:1:6:10).

Table 2. It is seen from Table 2 that the conductivity decreases with increasing the alkyl length. This is likely because the longer alkyl chain has a larger blocking effect on the ion movement caused by the charge transfer along the polyiodide chain. It is concluded that a shorter alkyl chain linked to the imdazolium ring is advantageous to achieve a higher conductivity. These results indicate that the conductivity of POSS-based ionic conductors can be tuned by varying the alkyl length. Similarly, the Rs changes slightly for these different ionic conductors while the Rct decreases with the increase in conductivity. To investigate the mechanism of ion transport for these POSS-based ionic conductors, the electrolyte containing POSS8Im-M and additives (POSS-8Im-M:I2:LiI:NMBI = 1.5:1:6:10) was chosen to conduct EIS analysis at a series of temperatures. Figure 5 displays the temperature dependence of EIS spectra. As the temperature increases, ZN gradually decreases (Figure 5). The condcuctivity (σ) increases with the temperature following the Arrhenius equation:

reflects ionic conduction of the electrolyte. The charge transfer along the polyiodide chain obeying the relay-type Grotthus mechanism57 contributes to the ionic conduction. The mechanism of ion transport is a thermally activated process, in which Ea is the energy barrier that must be overcome for ionic conduction to take place. The Ea is determined to be 12.7 kJ mol−1 from the slope of ln(σ/S cm−1) ∼ 1/(T/K) shown in Figure 6. 3.3. Effect of Electrolyte Composition on Photovoltaic Performance. As the conductivity of these POSS-based ionic conductors was not sufficiently high to operate the ssDSSC, iodine was doped into the ionic conductors so that the conductivity reached a level, at which the device could work. POSS-8Im-M was chosen to investigate the effect of electrolyte composition on photovoltaic performance. Five parallel devices were fabricated for each composition. Figure 7 shows the J−V curves of the ssDSSCs containing POSS-8Im-M with various additives, and the performance parameters are summarized in Table 3. For the binary solid electrolyte (POSS:I2 = 1.5:1), the average power conversion efficiency (η) was 4.30% with short-

σ = σo exp( −Ea /kT ) 5346


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alkyl length as a result of decreased Jsc. As a consequence, the η gradually decreased with the alkyl length, and the best performance was achieved with POSS-8Im-M containing a methyl group. After optimization of the TiO2 anode with a scattering layer, the η for POSS-8Im-M was improved to 6.98% with Jsc of 13.58 mA cm−2, Voc of 724 mV, and FF of 0.71 (Table S1). Figure 8a shows the J−V curve of the optimized ssDSSC, and the corresponding IPCE spectrum is shown in Figure 8b. The maximum IPCE reaches ∼80%, suggesting that the conductivity is sufficiently high so that the dye can be regenerated in a timely manner after photoexcited electron injection. Compared with the solid-state electrolytes reported in the literature (Table S2, references listed therein), this solid electrolyte is promising due to its good performance. As POSS-8Im-M has low Tg of −8.8 °C, it is expected for POSS-8Im-M to exhibit good photovoltaic performance at lower temperatures than RT because a lower Tg value can ensure good interfacial contact with the photoanode at lower temperatures.61 To confirm this prediction, the performance of the best ssDSSC was also tested at −4 °C, as shown in Figure 8a for comparison. At −4 °C, the device exhibited η of 6.52% with Jsc of 12.79 mA cm−2, Voc of 739 mV, and FF of 0.69 (Table S1). As compared to the performance at RT, the device at −4 °C showed higher Voc due to the upward shift of the conduction band edge with decreasing temperature62 but lower Jsc due to the decreased conductivity with decreasing temperature and the upward shifted conduction band as well. The slightly decreased FF is attributed to the decreased conductivity with temperature. The drop of efficiency was only 7% when the test temperature was decreased from RT to −4 °C. By contrast, the performance became very poor for POSS8Im-E when the temperature decreased from RT to −4 °C due to the higher Tg of 22.4 °C. Furthermore, the device exhibited good long-term stability at −4 °C up to 30 days, as shown in Figure S2. Since the working temperature (−4 °C) is higher than the Tg (−8.8 °C) for POSS-8Im-M, the solid electrolyte should be in the elastomeric state at −4 °C. Therefore, crystallization does not occur at this temperature. As the elastomeric state can ensure good interfacial contact between solid electrolyte and TiO2, dye regeneration at −4 °C should be efficient for POSS8Im-M with a low Tg (−8.8 °C). However, POSS-8Im-E is in the glass state at −4 °C because of its high Tg (22.4 °C), and therefore, dye regeneration at −4 °C should be less efficient for POSS-8Im-E. For this reason, photocurrent decreased remarkably for POSS-8Im-E when the temperature decreased from RT to −4 °C, resulting in a big efficiency drop by more than 30%. As the stability of an electrolyte is more relevant at higher temperatures, we tested device performance (Table S1) at higher temperatures. The Jsc increased with the temperature due to the enhanced conductivity while Voc decreased with the temperature due to the downward shift of the conduction band edge with temperature.62 The power conversion efficiency did not change significantly in the temperature range of −4 to 55 °C. Further increasing temperature to 65 °C, the Jsc remained similar while Voc decreased significantly, leading to a significant drop of power conversion efficiency. Finally, the ssDSSC with POSS-8Im-M based solid electrolyte was soaked under one sun for 30 days, and the current− voltage curve was periodically measured after light soaking for several days. Figure 9 displays the evolution of performance parameters with time. Fluctuations of Voc and FF with time

Figure 7. J−V curves for ssDSSCs based on POSS-8Im-M with different additives.

Table 3. Effect of Electrolyte Composition on Photovoltaic Performance POSS-8Im-M

Jsc/mA cm−2

Voc/mV

FF

η/%

+ I2 + I2 + LiI + I2 + LiI + NMBI

8.10 9.47 9.65

747 701 754

0.71 0.69 0.71

4.30 ± 0.26 4.58 ± 0.42 5.17 ± 0.11

circuit photocurrent (Jsc) of 8.10 mA cm−2, open-circuit photovoltage (Voc) of 747 mV and fill factor (FF) of 0.71. When LiI was further doped into the solid electrolyte (POSS:I2:LiI = 1.5:1:6), the Jsc increased to 9.47 mA cm−2 while the Voc decreased to 701 mV because of the downward shift of conduction band edge of TiO2 due to the surface adsorption of Li+.60 The increased conductivity upon addition of LiI is another cause for the improved photocurrent. The overall result was that the η increased to 4.58%. Upon introduction of NMBI (solid), which is an often-used additive to improve Voc,58 to the electrolyte (POSS: I2:LiI:NMBI = 1.5:1:6:10), the η increased to 5.17% mainly due to the enhanced Voc. The four-component solid electrolyte was used to investigate the effect of alkyl chain. 3.4. Effect of Alkyl Chain Length on Photovoltaic Performance. We have investigated the effect of alkyl chain on the photovoltaic performance using the four-component solid electrolyte. Table 4 gives the average data for four parallel Table 4. Photovoltaic Performance Parameters for ssDSSCs Containing Different POSS-Based Ionic Conductors with the Same Additives (POSS:I2:LiI:NMBI = 1.5:1:6:10) POSS-8Im-M POSS-8Im-E POSS-8Im-B POSS-8Im-O

Jsc/mA cm−2

Voc/mV

FF/%

9.65 8.32 7.89 6.18

754 726 711 698

71 72 73 73

η/% 5.17 4.35 4.10 3.15

± ± ± ±

0.11 0.15 0.17 0.17

devices. In general, the Jsc decreased with the alkyl length. With increasing the alkyl length, the blocking effect of the alkyl group on the charge transport becomes more prominent, resulting in decreased conductivity. The decrease in Jsc with increasing the alkyl length is the result of decreased conductivity. The Fermilevel of the TiO2 photoanode under illumination shifts positively with the decrease in Jsc, leading to decreased Voc with the alkyl length. Therefore, the Voc also decreased with the 5347


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Figure 8. (a) J−V curve and (b) IPCE spectrum of the best ssDSSC based on solid POSS-8Im-M electrolyte (POSS:I2:LiI:NMBI = 1.5:1:6:10).

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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.5b12353. The structure of the organic dye used in this work, the long-term stability at −4 °C, the photovoltaic performance data for the device at different temperatures, and the table comparing our electrolyte and the solid-state electrolytes reported in the literature. (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: (+86)21-5163-0345. Notes

Figure 9. Evolution of Voc, Jsc, FF, and η of the ssDSSC with POSS8Im-M based solid electrolyte under one sun soaking.

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was financially supported by STCSM (12JC1401500).

were observed, while Jsc increased gradually up to day 15, remained hardly changed from day 15 to day 25, followed by a slight decrease from day 25 onward. The increase in Jsc from the start to the 15th day is attributed to the aging effect, which favors interfacial contact between the photoanode and solid electrolytes. As a consequence, the power conversion efficiency first increased, reached the highest from the tenth to the twenty-fifth day, followed by a slight decrease thereafter. The data shown in Figure 9 demonstrate that these solid devices have good long-term stability.

REFERENCES

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4. CONCLUSIONS In summary, POSS-based imidazolium iodide ionic conductors with alkyl chains of different lengths have been designed and synthesized for use in ssDSSCs. The alkyl length exerts a significant effect on the glass transition temperature, conductivity, and solar cell performance. The glass transition temperature first increases and then decreases with the alkyl length. The conductivity increases with decreasing the alkyl length, resulting in increased short-circuit photocurrent and power conversion efficiency following the same trend. Among the four POSS-based ionic conductors mixed with I2, LiI, and NMBI (POSS:I2:LiI:NMBI = 1.5:1:6:10), POSS-8Im-M produces the highest efficiency of 6.98% at RT, and the efficiency remains 93% at −4 °C due to its low glass transition temperature. This work will help to design new solid-state electrolytes for highly efficient ssDSSCs at both room temperature and low temperatures. 5348


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