Ionic-Liquid-like Polysiloxane Electrolytes for Highly Stable Solid-State

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Ionic Liquid-like Polysiloxane Electrolytes for Highly Stable Solid-State Dye-Sensitized Solar Cells Anil Kumar Bharwal, Laura Manceriu, Cristina Iojoiu, Jennifer Dewalque, Thierry Toupance, Lionel Hirsch, Catherine Henrist, and Fannie Alloin ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00769 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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ACS Applied Energy Materials

Ionic Liquid-like Polysiloxane Electrolytes for Highly Stable Solid-State Dye-Sensitized Solar Cells Anil Kumar Bharwal,a, b Laura Manceriu, b Cristina Iojoiu, a Jennifer Dewalque,b Thierry Toupance,cLionel Hirsch, d Catherine Henrist, b ,*Fannie Alloin a, *

a

Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, Grenoble INP (Institute of Engineering Univ. Grenoble Alpes), LEPMI, 38000 Grenoble, France b

University of Liège, CESAM-GREEnMAT, Allée du Six-Aout 13, 4000 Liège (Sart Tilman), Belgium

c

Université de Bordeaux, Institut des Sciences Moléculaires (ISM), UMR 5255 CNRS, 351 Cours de la Libération, 33405 Talence Cedex, France d

IMS, University of Bordeaux, UMR 5218, F-33405 Talence, France and IMS, CNRS, UMR 5218, F-33405 Talence, France

*

Corresponding authors:

E-mail addresses: [email protected] (F. Alloin), [email protected] (C. Henrist),

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Abstract: With the purpose of achieving highly stable and efficient polymer electrolytes for dye-sensitized solar cell (DSSC) applications, a series of new poly(ionic liquids) (PILs), namely poly(1-Nmethylimidazolium-pentylpolydimethylsiloxane)iodide,

with

different

ratios

of

pentylimidazolium iodide moieties, have been studied and employed to prepare solid-state electrolytes for DSSCs. PILs were further mixed with two types of ionic liquids (ILs), 1-methyl3-propylimidazolium

iodide

(MPII)

and

1-methyl-3-propylimidazolium

bis(trifluoromethylsulfonyl)imide (MPITFSI), and a plasticizer, such as ethylene carbonate (EC), in order to lower their viscosity and to increase the diffusion coefficient and ionic conductivity. The assembled devices prepared, using the quasi solid-state electrolytes showed, light-toelectricity conversion efficiencies up to 6% for an active area of 0.2064 cm2. After 250 days, PILs-based cells retained 84% of their initial efficiency. These new findings encourage worldwide practical applications of DSSCs.

Keywords: PILs, ILs, electrolytes, blends, viscosity, ionic conductivity, diffusion coefficient, dye-sensitized solar cells, stability


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1. Introduction

In the era of new technologies for harvesting renewable energy resources, mesoscopic dyesensitized solar cells (DSSCs) show many attractive features such as aesthetics, transparency, environmental compatibility, eco-friendly, easy to process, cost effective and high power conversion efficiencies (PCE).1 These DSSCs consist of a dye sensitized-TiO2 photoelectrode, which acts as an electron acceptor and transport layer. They are surface-modified with a dye monolayer for light harvesting and electron injection into the conduction band of TiO2. In order to regenerate the system after light absorption, the resulting dye-sensitized photoanode then comes into contact with an electron donor material, either a redox electrolyte or a holetransporter. In liquid-based devices, the electrolyte plays a key role because it provides the required ionic conductivity and diffusion coefficient of the redox mediators and allows fast dye regeneration through charge transfer reactions with the dye molecules. The I-/I3- redox couple in a high boiling point organic solvent is the most used electrolyte in liquid DSSCs, which has led to a certified record PCE of 11.9%.2,3 To further enhance the PCEs, Yella et al. reported the use of cobalt-based redox electrolyte as an alternative to the iodine based electrolyte, associated with organic dye co-sensitization, which yielded a PCE of 12.3%.4 Further optimization of cobaltbased liquid electrolyte and organic dyes led to a record PCE of 14.3%.5 Unfortunately, such electrolytes demonstrate much lower long-term stability trends compared to the traditional I-/I3redox couple. More generally, the main drawbacks of liquid electrolytes are corrosion, leakage and poor long-term stability, which reduce the performance of DSSCs over prolonged operation.6 To overcome these problems, solid and gel materials such as p-type inorganic semiconductors7,8, inorganic and organic hole transport materials9–13, and polymer-based gel electrolytes14–17 have 3 ACS Paragon Plus Environment


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been developed to replace these liquid electrolytes. However, diffusion coefficients in solid electrolytes are usually less (~10-7cm2 s-1) than in liquid systems (~10-4 - 10-5 cm2 s-1) and therefore, solid- and gel-state DSSCs have lower PCEs than their liquid counterparts.18 One of the methods to solve these problems is to use quasi-solid-state electrolytes (QSSEs). In recent years, ILs or IL/nano-components based QSSEs as a replacement for traditional liquid electrolytes have attracted much attention.19–21 QSSEs have long-term stability, like solid electrolytes with high ionic conductivity and an excellent interfacial contact property, like liquid electrolytes.21–24 More recently, growing attention has been paid to poly(ionic liquids) (PILs) because this new class of polymers combines the unique properties of ionic liquid (IL), improved mechanical durability and dimensional control after polymerization.25 PILs have been successfully applied for quasi-solid-state DSSCs which yielded high PCE and excellent stability.26–29 Polysiloxane-based PILs are very promising materials because polysiloxanes have highly flexible backbone and very low glass transition temperature (Tg). Therefore, polysiloxanes-based electrolytes have received much attention.30 However, polysiloxane-based polymer electrolytes are gums at room temperature rather than solids, and recent efforts have been focused on the design and synthesis of polymers with both high ion transport and good dimensional stability. Hooper et al. showed that the lithium salt-doped polysiloxane polymer, prepared from the condensation of bis-[oligo(ethylene glycol) ether propyl] dichlorosilane, exhibits an anionic conductivity of ~10-4 S cm-1 at 25 oC.31 Walkowiak et al. have reported the highest ionic conductivity obtained with polysiloxane-based polymer electrolytes at room temperature, i.e. 103

S cm-1, which is close to that of a liquid electrolyte and fulfills the requirements for industrial

application.32 In 2001, a DSSC, assembled with a gel network polymer electrolyte based on 4 ACS Paragon Plus Environment

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polysiloxane and PEO, containing 20 wt. % of LiI, 5 wt. % of I2 and 150 wt. % of the mixture ethylene carbonate (EC)/propylene carbonate (PC) (3:1 v/v), was reported which led to a PCE of 2.9%.33 Recently, Manca et al. have successfully employed an ion conductive polysiloxanebased polymer electrolyte for DSSCs application. The DSSC assembled with 40 wt. % of polysiloxane electrolytes dissolved in MPN-based ionic liquid solution has shown an efficiency of 6.37%.34 In

this

work,

we

have

studied

a

series

of

new

poly(1-N-methylimidazolium-

pentylpolydimethylsiloxane)iodide as PILs, having different ratios of pentyl-imidazolium iodide moieties, for photovoltaic applications in DSSCs. Generally, the major shortcomings associated with pure PILs as electrolytes in DSSCs are the low values of ionic conductivity and diffusion coefficient, which causes poor photovoltaic performance in DSSCs. Incorporating ILs into PILs is believed to enhance the ionic conductivity and diffusion coefficient.35,36 In our previous work, we exploited the possibility of improving conductivity of the synthesized PILs with ILs, such as 1-methyl-3-propylimidazolium

iodide

(MPII)

and

1-methyl-3-propylimidazolium

bis(trifluoromethylsulfonyl)imide (MPITFSI), and non-ionic solvents, such as ethylene carbonate (EC), and systematically studied their electrochemical properties.37 Solid-state DSSCs based on PILs/ILs blends were thus fabricated and showed good stability.26,38 However, the effects of plasticizer, such as ethylene carbonate (EC), on polymer electrolytes, such as polysiloxane, and its influence on photovoltaic properties have rarely been reported. It is well known that EC is a high-viscosity solvent with high dielectric constant, which makes it a suitable choice for ionization of salts to reach high ionic conductivities.39 We herein describe a series of PILs-ILs and PILs-EC based electrolytes with improved physicochemical and electrochemical properties, which were employed successfully in DSSCs. 5 ACS Paragon Plus Environment


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The influence of the properties of the polysiloxane based-PILs on the photovoltaic performance when assembled as quasi-solid-state DSSCs has been studied. Such fabricated quasi-solid-state DSSCs have shown high efficiency with long term stability revealing that the polysiloxane based electrolytes are preferred for highly stable solid-state DSSCs. 2. Experimental section 2.1. Materials Fluorine Tin Oxide (FTO) coated glass substrates (TEC15:15 ohm/sq) and TiO2 colloidal pastes (18 NRT and T-Nanoxide R/SP) were purchased from Dyesol. N-719 dye and Surlyn (60 µm) spacer were purchased from Solaronix. Acetonitrile was purchased from ABCR GmbH and Co. Iodine, 1-butyl-3-methylimidazolium iodide, guanidine thiocyanate, 4-tert-butylpyridine, valeronitrile, acetylacetone (AcAc), titanium isopropoxide (TTIP) and ethylene carbonate were all obtained from Sigma-Aldrich and used without any further purification. 2.2. Fabrication of PILs-based electrolytes PILs and ILs were prepared according to our previously reported procedures.37 The PILs, poly(1-N-methylimidazolium-pentylpolydimethylsiloxane)iodide

having

different

ionic

functionalities (further referred to as P1, P2 and P3 in this work), were synthesized by functionalization and quaternization of the poly(methylhydrosiloxane)-co-methylhydrosiloxane copolymer (with increasing Si-H functionality) as reported in our previous work.37 The ILs, 1methyl-3

propylimidazolium-iodide

(MPII)

and

1-methyl-3-propyl-imidazolium-

Bis(trifluoromethane)sulfonimide (MPITFSI) were synthesized as reported elsewhere.37,40 Polymer electrolytes were prepared from three synthesized PILs having different ionic functionalities. The polymer mixtures and iodine (as shown in Table S1) were dissolved in a very


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small amount of acetonitrile/valeronitrile (1/1, v/v). Nine different types of polymer electrolytes were prepared as illustrated in the Table S1. Appropriate weights of the components (PILs, IL, I2, and solvent) were mixed in a closed bottle and then continuously stirred at 50 oC until complete dissolution. The resulting blends are further designated as "electrolyte". The "reference" redox liquid electrolyte was composed of 0.6 M 1-butyl-3-methylimidazolium iodide, 0.03 M iodine, 0.1 M guanidine thiocyanate and 0.5 M 4-tert-butylpyridine in acetonitrile/valeronitrile (85/15, v/v).41

2.3. Fabrication of DSSC To fabricate the devices, a TiO2 blocking layer (BL) was first deposited on FTO substrates. The property of BL is known to have significant influence on the overall device performance.42 The TiO2 BL of approximately 70 nm was prepared by the spin coating of TTIP solution in TTIPAcAc solution at 2000 RPM for 60 s and thermally stabilized at 150 oC for 15 min. Subsequently, a porous layer of TiO2 was deposited by spin-coating from TiO2-nanoparticles based paste (Dyesol 18 NRT dispersed in ethanol) at 1500 RPM for 60 s and then stabilized at 180 °C on a hot-plate for 20 min. This spin-coating procedure was repeated several times till the desired film thickness was obtained. Finally an additional scattering layer was deposited by the use of Dyesol T-Nanoxide R/SP colloidal paste (dispersed in ethanol) by spin-coating at 1500 RPM for 60 s. Subsequently, the photoanodes were sintered at 500 °C for 30 min to remove organic additives and to crystallize anatase phase in the TiO2 BL. The overall thickness of the TiO2-mesoporous electrodes studied in this work were kept at approximately 11 µm (8 µm of photoanode + 3 µm of scattering layer). To ensure the connectivity between grains, the porous films were treated by soaking in a 0.04 M TiCl4 aqueous solution at 60 °C for 30 min. After



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drying, the photoanodes were sintered again in air for 30 min at 450 °C. The freshly sintered photoanodes were sensitized with dye molecules by immersion overnight in 2.7×10-4 M N719 alcoholic solution at room temperature. The dye uptake was estimated to be 1.98 ×10-9 mol mm-2. Finally, the dyed photoanodes were washed with ethanol and dried. They were then assembled in a sandwich configuration with a counter-electrode and Surlyn (60 µm) as a spacer. The counterelectrode consisted of a platinum-coated FTO with a hole drilled from the uncoated side. The polymer electrolytes were introduced into the cell by vacuum back filling through the hole. Prior to sealing, the volatile solvent was removed by heating the cell at 100 oC for 15 min on the hot plate, followed by vacuum drying overnight. The electrolyte injection hole was firmly sealed with Surlyn and a microscope cover glass. A mask of black plastic tape was applied on the cells with an active area of 0.2064 cm2. 2.4. Characterization techniques Thickness of the TiO2 photoanodes, as well as electrolyte infiltration, was estimated by scanning electron microscopy (FEG-SEM XL30, FEI). Energy-dispersive X-ray spectroscopy (EDX) elemental mapping was used on cross sections of assembled cells to assert the pore filling. Photocurrent-voltage (J–V) characteristics of the DSSCs were measured by a solar simulator (from Newport Spectra Physics) coupled with a Keithley 2400 Source Meter under a simulated Air Mass 1.5G solar spectrum (irradiance: 70 mW cm-1, i.e. 0.7 sun) with an aperture mask of 0.2064 cm2. Dye uptake was estimated by dye desorption from dye-sensitized electrodes in a 10-3 M KOH solution in water. The absorption spectra of the resulting dye solution were recorded using a UV-visible spectrophotometer (Perkin Elmer UV-Vis Spectrometer Lambda 14P). Incident Photon to Charge Carrier Efficiency (IPCE) data were collected in the 300 - 800 nm range using a Xe lamp associated with a monochromator (Triax 180, JobinYvon). No bias 8 ACS Paragon Plus Environment

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light was employed to illuminate the cell. The current produced was measured in steps of 2 nm after 2 s of radiation exposure with a Keithley 6487 picoammeter, to ensure steady state conditions. The incident photon flux was measured with a 6-in. diameter calibrated integrated sphere (Labsphere) and a silicon detector. Electrochemical impedance spectroscopy was performed using a BioLogic SP-200 potentiostat (Science Instrument), and data were analyzed with the EC-lab software. EIS spectra were recorded over a frequency range of 10 mHz to 100 kHz under dark conditions at room temperature by applying a bias equal to the Voc value determined from the J-V curves.

3. Results and discussion 3.1. Photovoltaic Performance of DSSCs Figure S1 shows images of PILs. All three samples possess different viscosity. P1 and P2 are quasi-solid at room temperature, whereas P3 is solid due to large ionic interactions. As PILsbased electrolytes have been thoroughly characterized in a previous study37, the more relevant data for applications in DSSCs are summarized in Table 1. Imidazolium based ILs are selected in order to have the same cationic species than the one grafted on the polysiloxane backbone as shown in the Figure S2. This similarity allows a strong affinity between the IL and the ionic function of the PIL and thus promotes the phase separation between the hydrophobic structure of the polymer and the ionic functions. Phase separation induces the creation of ion channels, improving the ionic conduction of the mixture and thus the performance of the device. Cole-Cole plots of the PILs and PILs blends with ILs and EC are shown in the Figure S3.



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Table 1- Thermal and physiochemical properties of PILs-based electrolytes at 25 oC.37 Electrolyte

Tg (oC)

µ (Pa s)

σ (S cm-1)

D(I3-)(cm2 s-1)

Reference41

-

-

-

7.0×10-6

P1

-123

13

9×10-6

3.0×10-9

P2

-71

106

8×10-6

2.5×10-9

P3

-47

1200

1.6×10-5

8.0×10-9

P2:MPII(2:1)

-74

-

1.3×10-4

-

P2:MPII(1:1)

-83

3.6

1.5×10-4

7.2×10-8

P2:MPITFSI(2:1)

-71

-

4.0×10-4

-

-4

1.4×10-7

P2:MPITFSI(1:1)

-75

0.4

6.9×10

P3:EC(8:1)

-70

-

1.8×10-4

1.1×10-7

P3:EC(3:1)

-75

-

1.1×10-3

2.8×10-7

The conductivity and diffusion coefficient of the PILs blended with ILs and EC is higher than the pure PILs due to lower viscosity. Since the diffusion of ions strongly depends on the viscosity of the electrolyte, lower viscosity leads to a higher diffusion coefficient and thus higher conductivity. In the present work, 11 µm thick TiO2 photoanodes were used in DSSCs in combination with different polymer electrolytes. Photocurrent density-voltage characteristics (three cells replica for each electrolyte) under ~ 0.7 sun illumination are depicted in Figure 1A and Table 2. The device fabricated with the liquid electrolyte (reference) showed a PCE of 7.7 % with Jsc of 10.0 mA cm-2, Voc of 0.75 V and FF of 68%. Among the three pristine PILs, the highest energy conversion efficiency was obtained for pure P2 (3.0%) followed by pure P1 (2.5%), which was one order of magnitude higher than the value found for pure P3 (0.2 %). The higher conversion efficiency obtained for pure P2 can be correlated to its high ionic conductivity (8×10-6 S cm-1), better diffusion coefficient (2.5×10-9 cm2 s-1) and best compromise in terms of viscosity (106 Pa s). In contrast, P3 shows the lowest efficiency due to the poor pore infiltration 10 ACS Paragon Plus Environment

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caused by the highest viscosity (1200 Pa s) at room temperature. The short-circuit photocurrent density of the device including the liquid electrolyte is higher than that of those containing pure PILs electrolytes (P1, P2, and P3). The limitation in Jsc for DSSCs involving pure PILs as compared to liquid-based systems is mainly attributed to the limited ion conduction, which is restrained by the polymer viscosity. Besides, the Voc of the polysiloxane-based PILs devices is significantly lower (0.65 V to 0.11 V) in comparison with Voc of the reference liquid-based device (0.75 V). The decrease in Voc is due to a shift in the conduction band caused by the accumulation of oxidized species near the TiO2 surface. The low I3- mobility of the polysiloxanebased electrolyte causes a build-up near the TiO2 and increases the probability of electron recombination. In solid-state or quasi-solid-state electrolyte systems, the main drawback of PILs is their high viscosity, which makes the ion diffusion rather slow. Therefore, transport of I3– ions to the counter-electrode in a polysiloxane-based matrix can constitute a rate-limiting step in DSSCs.43 As a consequence, depletion of I3- at the counter-electrode takes place, and an overpotential that lowers the voltage output of the solar cell is observed.



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Figure 1. J-V curves of DSSCs based on the Dyesol photoelectrode in combination with liquid and PILs-based electrolytes under ~ 0.7 sun illumination (A) and in the dark (B). Table 2. Photovoltaic parameters of the DSSCs containing PILs and their blends under ~ 0.7 sun illumination. Electrolyte

Jsc (mA cm-2) 10.0

FF (%)

Reference (liquid)

Voc (V) 0.75

P1

0.65

P2

68

Average ɳ (%) 7.7

Best (ɳ %) 8.0

4.2

62

2.5

2.7

0.56

5.7

63

3.0

3.0

P3

0.11

5.9

25

0.2

0.2

P2:MPII(2:1)

0.61

8.2

66

4.9

5.0

P2:MPII(1:1)

0.62

9.9

62

5.6

5.9

P2:MPITFSI(2:1)

0.57

6.9

66

3.8

4.0

P2:MPITFSI(1:1)

0.61

9.0

66

5.3

5.6

P3:EC(8:1)

0.55

3.8

63

1.9

2.1

P3:EC(3:1)

0.65

9.8

64

6.1

6.3


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Imidazolium iodide-based ILs have been widely and effectively used for DSSC applications.41,44,45 In this work, MPII and MPITFSI were selected to study the influence of ILs on the polymer electrolyte P2 and the consequences on their photovoltaic performances. Furthermore, it has been reported that MPII provides excellent efficiency and good stability in DSSCs.45 Energy conversion efficiencies of 4.9% and 5.6% were achieved with the incorporation of 33 and 50 wt. % of MPII respectively, which is comparable to the efficiency reported by Manca et al.34,46 It can be noted that a 33 wt. % amount of MPII is sufficient to reach a 1.6 fold efficiency increment. This can be related to a highly improved ionic conductivity, drastically reduced viscosity and an improved diffusion rate with the addition of MPII. The raise found for the value of Voc in P2/ILs blends-based electrolytes could be ascribed to the remarkably higher diffusion coefficient and conductivity of the electrolyte than those of the pure polymer. The Voc value for DSSCs with I−/I3− redox electrolyte can be represented by the following equation 1:47

=

ln

(1)

where, k and T are the Boltzmann constant and absolute temperature, respectively. Iinj is the injection current from dye to semiconductor, ncb is the electron density on the conduction band of the semiconductor and ket is the rate constant for I3− reduction. According to equation (1), the higher Voc found for the P2/ILs blends-based electrolytes is related to the suppression of the dark current at the TiO2 electrode/electrolyte interface (Figure 1B).48 It has been demonstrated that the dark current originates from the reduction of I3− by conduction band electrons from TiO2.49 The decrease of the I3− reduction rate should lead to an increase of Voc (according to equation 1).



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PILs/ILs with TFSI ions have already been used effectively in DSSCs and showed higher inherent ionic conductivity.26,50 An addition of about 50 wt. % MPITFSI to P2 causes around 1.8 fold improvement in efficiency. The improved performance of P2:MPITFSI-based cells can be attributed to the improved diffusion coefficient, higher ionic conductivity and lower viscosity. Slight improvement in the Voc of P2:MPITFSI-based devices can be assigned to the suppression of the dark current at the TiO2/electrolyte interface as shown in Figure 1B.51,52 Energy conversion efficiencies of 3.8% and 5.3% were achieved with the incorporation of 33 and 50 wt. % of MPITFSI respectively. Moreover, both ILs (MPII and MPITFSI) led to efficient PILs/ILs infiltration into the working TiO2 electrode. The P2/MPII blends showed higher Jsc due to extra release of I- from MPII, despite having inferior conductivity and diffusion coefficient than P2/MPITFSI counterparts, resulting in a higher efficiency of the DSSC. We presume that extra MPII can act as both a source of redox electrolytes and lone charge transfer intermediate.53,54 The increase of ionic conductivity observed with the addition of EC is caused by decrease of viscosity and increase of ionic dissociation due to the higher dielectric constant and solvating ability of EC.37,55 The increase of both Voc and Jsc, and as a consequence the PCEs of the DSSCs, with the addition of 11 and 25 wt. % of EC in P3 is related to the reduction of back electron transfer reaction and suppression of dark current due to improved ionic conductivity as shown in Figure 1B. The dependence of Jsc on EC concentration in the electrolyte can be explained on the basis of the shift of flat band potential of TiO2. As shown in Figure 1A, the P3:EC(3:1) based cells show highly improved Jsc, largely improved Voc and FF owing to its high ionic conductivity (1.1×10-3 S cm-1), high diffusion coefficient (2.8×10-7cm2 s-1) and improved pore infiltration. The device based on the P3:EC (3:1) combination showed a Jsc of 9.8 mA cm-2, a Voc of 0.65 V and a FF of 64%. The resulting PCE of 6.1% is, to the best of our knowledge, one of the best 14 ACS Paragon Plus Environment

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performance values ever reported for quasi-solid-polysiloxane based electrolytes. Normally, the charge transport mechanism in the quasi-solid electrolytes is hindered compared to that in liquid electrolytes. Kubo et al. discovered comparable efficiencies for organic liquid free solidified pure ionic liquids using organic gelators, where the high efficiencies are ascribed to a Grotthus-type mechanism for ionic conduction taking place through the iodide ions.56 The higher concentration of iodine in P3/EC electrolyte system can generate polyiodides, which can facilitate the a Grotthus-type mechanism as illustrated in Figure S4.57,58 As proven in our previous study, we associate the aggregation of ionic functions in the highly viscous polymer P3 due to the high non-polar character of siloxanes backbone and high ionicity of side chains.37 This process leads to the formation of non-polar/ionic phase separated domains that favors a favorable pathway for I- diffusion as schematized in Figure 2. When EC molecules penetrate and solvate the ionic liquid part, the high polar EC interacts preferentially with the ionic function, leading to their dissociation and percolation of ionic domains. This microstructure might promote the Grotthus mechanism leading to a high conductivity of P3/EC system.



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Figure 2. Schematic illustration of a nanoscale phase-separated morphology inside hydrophilic domains, showing the regular distribution of ionic channels containing side-chains, ions and EC. 3.2. Incident photon-to-current conversion efficiency (IPCE) IPCE curves of the DSSCs are shown in Figure 3. The maximum IPCE values at 530 nm are 23%, 39%, 37%, 42% for pure P2, P2:MPII(1:1), P2:MPITFSI(1:1) and P3:EC(3:1)-based DSSCs respectively. IPCE values of these devices are consistent with the photovoltaic conversion efficiency (ɳ) results. The improvement of IPCE is expected to enhance chargegeneration efficiency and inhibit electron recombination.59


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Figure 3. IPCE curves of DSSCs containing different electrolytes.

3.3. Electrochemical impedance spectroscopy (EIS) Electrochemical impedance spectroscopy (EIS) was used to evaluate the charge transfer resistance at the TiO2/dye/electrolyte interfaces and the diffusion resistance.60 The Nyquist and Bode phase plots of the DSSCs comprising pure P2, P2:MPITFSI(1:1), P2:MPII(1:1) and P3:EC(3:1) electrolytes are shown in Figure 4A and 4B respectively. As observed in literature, the EIS spectrum of a DSSC comprises of three responses in the studied frequency range (10 mHz to 100 kHz).61 The resistance obtained at very high frequencies (VHF) are not logically associated with the electrolyte resistance, and are significantly modified by the erratically and poor contact between the cell and the cable. The high frequency (HF) and medium frequency (MF) semicircles correspond to the charge-transfer reaction at the counter electrode and TiO2 electrode respectively, while the response at low frequency (LF) is associated with the diffusion process of the I3– in the I–/I3– redox electrolyte.62 In our study, the circuit Rs + Rct1//Qct1 + 17 ACS Paragon Plus Environment


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Rct2//Qct2 + Wdiff, (// symbolizes a connection in parallel, and + symbolizes a connection in series) was used to fit the EIS spectra. The charge transfer resistance at the counter electrode (Rct1) and the charge transfer resistance at the TiO2/dye electrode (Rct2) are given in Table 3. Furthermore, Bode plots confirm the clear separation of the three phenomena according to their relaxation times (Figure 4B).

Figure 4. The EIS Nyquist plots (A) and Bode phase plots (B) measured in dark for DSSCs with different electrolytes. The dots in Figure A represent raw data and the continuous line the fit.

It can be seen that both Rct1 and Rct2 decrease with the addition of ILs due to the enhanced I– /I3– mobility in the polymer, as confirmed by the I3- diffusion coefficient values.63 The decreased Rct2 values, upon addition of ILs and EC, ensure low recombination resistance at the photoanode/electrolyte interface and are in agreement with literature reports on DSSCs incorporating PILs and polysiloxane-based electrolytes.34,46 The DSSC including P3:EC(3:1) electrolyte shows the lowest Rdiff owing to its high ionic conductivity and diffusion coefficient,


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resulting in a very high Voc of 0.65 V. The Rdiff values of the DSSCs are consistent with the dark current, FF and Jsc variations shown in Figure 1. Table 3. Parameters obtained by fitting the EIS spectra of the DSSCs incorporating different electrolytes. Device

Rs(Ω)

Rct1 (Ω)

Rct2 (Ω)

Rdiff (Ω)

fmax (Hz)

τe (ms)

P2

55

19

40

21

11

14

P2:MPII(1:1)

48

6

21

21

8

19

P2:MPITFSI(1:1)

43

8

16

10

21

7.5

P3:EC(3:1)

51

4

12

7

17

9

The effective lifetime of electrons (τe) before recombination in TiO2 photoelectrodes can be determined from equation 2:64

=

1 2л

(2)

where fmax is the maximum frequency in the MF response associated with the transfer reaction at the TiO2 electrode. The fmax and electron lifetime values are summarized in Table 3. We assume that with pure P2, the electron will be crossing the TiO2 from grain to grain rather than through the TiO2/electrolyte interfaces, as opposed to the case with less viscous electrolyte, when charge transfer and recombination at the TiO2/electrolyte interface are more likely to take place due to the electrolyte facile percolation between the TiO2 particles. The addition of MPII to P2 shifts the middle frequency peak to lower frequencies, indicating reduced charge-recombination rate and therefore, prolonged electron lifetime (τe = 19 ms). However, the electron lifetime for the P2 19 ACS Paragon Plus Environment


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based cells, comprising the most viscous electrolyte, is higher (14 ms) than the P2:MPITFSI(1:1) based cells (7.5 ms), despite having higher viscosity and lower diffusion coefficients. It can be seen from Figure 4B, that upon the addition of MPITFSI, fmax shifts to a higher value indicating that the electron lifetime has been shortened, in agreement with the dark current results. MPITFSI positively shifts the conduction band of the TiO2 making the excited electrons easy to recombine with the I3− in the polymer electrolyte.65 The P3:EC(3:1) based DSSC shows low electron lifetime of 9 ms because electrons traveling through the porous TiO2 are mainly encountering TiO2/electrolyte interfaces, which is favored by the facile electrolyte percolation.

3.4. Pore infiltration The good interfacial contact between the photoanode and the electrolyte is essential for efficient regeneration of the dye after electron injection into TiO2 conduction band and is one of the key factors affecting the photovoltaic performances of quasi-solid-state solar cells. Therefore, cross-sectional SEM images were taken first to evaluate the infiltration of P3:EC(3:1) electrolyte into mesoporous TiO2 photoanode. Figure 5A shows a porous and rough structure for the TiO2 film. After filling with the electrolyte, the TiO2 cross-section is covered thoroughly by a layer of electrolyte (Figure 5B). Individual nanoparticles can not be distinguished and the porous character has disappeared. This indicated the complete infiltration of polymer into the TiO2 film. This result is further confirmed by EDX elemental analysis. The element mapping, as shown in Figure 5C, demonstrates a homogenous distribution of silicon and iodine atoms across the 11 µm thick TiO2 film, thus explaining the excellent photovoltaic performance.


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Figure 5. Cross-section SEM images of TiO2 photoanode showing the scattering layer (large grains, upper layer) and mesoporous under layer, before (A) and after (B) filling with P3:EC(3:1) electrolyte. (C) shows EDX mapping of P3:EC(3:1) electrolyte filled TiO2 film, taken from a disassembled DSSC. 3.5. Stability of DSSCs To investigate the long-term stability of the DSSCs, the cells fabricated with the liquid and polymer electrolytes were stored in ambient conditions for an extended duration (several months). The photovoltaic parameters were measured approximately every 30 days as shown in Figure 6A. It can be seen that the DSSCs based on pure PILs and PILs blends maintained nearly 80-90% or more of their initial performance after 250 days. It can be observed that the efficiency of reference liquid electrolyte-based DSSC was 13% of its initial efficiency due to possible electrolyte leakage, whereas pure P1 and P2 polymers retained about 93% and 60% of their initial efficiency respectively (Figure 6B). Interestingly, the highly viscous pure P3 polymer based DSSC showed an approximate 400% raise due to improved penetration of the electrolyte into TiO2 pores over an extended period of time. The ILs and EC possess low volatility, high chemical and thermal stability. Therefore, the addition of ILs and EC into polymers does not degrade the cell stability. It can be pointed out that intrinsic 21 ACS Paragon Plus Environment


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hydrophobicity of polysiloxane side chain effectively prevents moisture adsorption, thus further contributing to improve the long term stability. The cells containing P2-ILs and P3-EC still retained most of their efficiency after 250 days of measurement (Figure 6B). The evolution of cells parameters (Voc, Jsc, FF and η) over 250 days is shown in Figures S5-S7.

Figure 6. Evaluation of the long term stability of the cells containing liquid and different polymer electrolytes over a period of 250 days (A) and efficiency loss values of the DSSCs after 250 days (B).

As shown in Table S2 and Figure S7, the champion cell, P3:EC(3:1)-based DSSC, retained about 84% of its initial efficiency measured after 250 days and similar device performance of 5.1% can be obtained at both high (1 sun) and moderate (0.7 sun) simulated sunlight irradiance. 4. Conclusion A

series

of

polymer

electrolytes

based

on

new

poly(1-N-methylimidazolium-

pentylpolydimethylsiloxane)iodide (PILs) having different viscosities, as well as their blends


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with ILs and EC, were successfully implemented as efficient polymer gel electrolytes for DSSCs. The synthesized electrolytes showed high stability and a greater solvent retention capability. The addition of ionic liquids to P2 caused an increase in Voc and Jsc values, and resulted in improved photovoltaic performance. Efficiencies of 5.6% and 5.3% were achieved with the incorporation of 50 wt. % of MPII and 50 wt. % of MPITFSI. It has been demonstrated that MPII provides extra I- ions and a longer electron lifetime compared to TFSI- ions. Besides, addition of EC to P3, used as a plasticizing solvent to enhance conductivity and I3- diffusion, causes an increase in Voc, Jsc and FF values, resulting in an improved photovoltaic performance. The highest efficiency of 6.1% was achieved with incorporation of 25 wt. % of EC in P3, which is the highest ever reported efficiency for DSSC incorporating polysiloxane-based PILs with EC. The quasi-solid-state DSSCs exhibited a prolonged stability maintaining up to ~ 85% of their initial efficiencies after 250 days. These results demonstrate the high potential of PILs/ILs and PILs/EC blends, and pave the way towards the optimization of novel polymer electrolyte blends for solar energy conversion devices and other energy related applications.

Acknowledgements The authors wish to acknowledge the financial support by the ARC 4 - Énergies - Région RhôneAlpes and IDS FunMAT (International Doctorate School in Functional Materials) for PhD fellowship. This work was performed within the framework of the Centre of Excellence of Multifunctional Architectured Materials "CEMAM" n° AN-10-LABX-44-01.



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GRAPHICAL ABSTRACT:



166x154mm (150 x 150 DPI)

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Ionic-Liquid-like Polysiloxane Electrolytes for Highly Stable Solid-State

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