Anisotropic 1-D Aqueous Polymer Gel Electrolyte for Photo

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Anisotropic 1-D Aqueous Polymer Gel Electrolyte for Photo-electrochemical Devices: Improvement in hydrophobic TiO2-Dye/electrolyte interface Keval K Sonigara, HIren Machhi, Jyoti Prasad, Jayraj V Vaghasiya, Alain Gibaud, and Saurabh Sureshchandra Soni ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00444 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Applied Energy Materials

Anisotropic 1-D Aqueous Polymer Gel Electrolyte for Photo-electrochemical Devices: Improvement in hydrophobic TiO2-Dye/electrolyte interface Keval K. Sonigara,1 Jayraj V. Vaghasiya,1 Hiren K. Machhi,1 Jyoti Prasad,1 Alain Gibaud,**,2 Saurabh S. Soni*,1

1

Department of Chemistry, Sardar Patel University, Vallabh Vidhyanagar, 388120, Gujarat, INDIA

2

Institut des Molécules et Matériaux du Mans, Universite du Maine, Le Mans 72000, FRANCE

Key Words: Aqueous gel electrolytes, Liquid crystalline block copolymers, Micellar nanochannels, Anisotropic Ionic conductivity, Hydrophobic sensitizers, Water based dye solar cells.

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Abstract Aqueous photo-electrochemical devices have emerged recently as promising area due to their economic and ecological friendliness. In the present work, we have expedited surface active amphiphilic quasi-solid aqueous polymer gel electrolyte (PGE) with hydrophobic sensitizer SK3 in water based dye sensitized solar cell (DSSC). PGE was prepared from amphiphilic block copolymer (PEO)-(PPO)-(PEO) with iodide-triiodide couple in pure aqueous media without any organic solvent. This block copolymer,

with iodide-triiodide salt exhibits 1D-lamellar

microcrystalline phase which shows stability in the temperature range of 25 °C – 50 °C. Parallel ⊥ar) alignment of anisotropic lamellar microcrystalline phase pertaining (||al) and perpendicular (⊥ by PGE were characterized and applied in quasi solid DSSC. Temperature dependency of ionic conductivity, triiodide diffusion, differential scanning calorimetry, viscosity and 1-D lamellar anisotropic behavior were studied. Surface active effect of PGE at the hydrophobic dye sensitized photoanode was investigated and compared with liquid water based electrolyte. Due to the amphiphilic nature and thermoreversible sol-gel transition of PGE at a lower temperature (0 to -2 °C) allowing PGE to penetrate efficiently inside the hydrophobic surface of dye-TiO2 and resulted in a fused contact between dye-TiO2/PGE interface. This aqueous PGE successfully enhances the performance of DSSCs over liquid water based devices by improving their Voc and stability. Under 0.5 sun illumination, DSSC with 1-D lamellar perpendicularly align PGE shows an efficiency of 2.8% and stability up to 1000 h at 50 °C.


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1. Introduction As a green and sustainable energy generation device, dye sensitized solar cell (DSSC)1 makes a huge advancement in solar cell research. Recently, this low cost, easy fabrication and highly efficient technology showed a record power conversion efficiency (PCE) of 14.3% in a liquid electrolyte based DSSC under 1 sun illumination.2 These highly efficient devices are traditionally powered by redox couples in organic solvents which are costly, volatile, and hazardous. Rapidly growing DSSC technology still suffers from the problem of leakage and safety hazards, which pressurize the scientific community to shift the use of green, safe and sustainable components in the devices.3,4 Various classes of electrolytes like ionic liquids, quasisolid-state and polymer gels

with different additives have been investigated for possible

hazardous organic solvent free DSSCs.5-8 Moreover, water based DSSCs also received attention as a cost effective and green solution for the said disadvantages.9-11 Many research groups reported organic solvent free water based electrolytes for DSSC in the form of organic solventwater mixture,10 ionic liquid-water mixture,12 water based polymer gels8,13 and 100% water based cage for redox couples. F. Bella et. al reported several articles on water based DSSCs which covered possible various modification in photoanode, electrolyte as well as cathode for enhancement in PCE as well as stability and they also emphasized on bio-source materials for good environmental impact.14-17 Similarly, attempts with 100% water based liquid state DSSCs have been made by some groups where GuI/I2 couple in pure water shows efficiency as high 4.06%, but Jsc of the devices degrade to as low as 70% of their observed values over a period of three weeks under dark storage condition.18 Therefore, water based electrolytes are limited by their low efficiency compared to those organic solvent based electrolytes due to less wettability 3 ACS Paragon Plus Environment


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of photo-anode surface and dye desorption in presence of water. To solve these problems, various surfactants like polyethylene glycol,19 Tween-20,20 Triton-X100,21 anionic and cationic surfactants were introduced as additives in water based liquid electrolytes, which demonstrate the PCE of 3.96% and 4.66% under the simulated AM 1.5 solar spectrum illumination at 100 mW/cm2 and 50 mW/cm2 respectively, for liquid state aqueous DSSCs.22 Moreover, to avoid dye desorption in presence of water, hydrophobic sensitizers are preferred compared to hydrophilic moieties, and some special sensitizers have been developed for water based DSSCs.23 However, the above efforts violate the sustainable approach towards emphasizing and achieving 100% water based, stable and cost effective DSSCs. Water can solve the issues of hazards, cost and volatility, but the problem of sealing and leakage still need improvement for the liquid system. To make water based DSSCs superior, earlier we had prepared water based quasi-solid-state DSSC devices using amphiphilic block copolymers with ionic liquid, which possess 3D-network that facilitate the transfer of iodide/triiodide couple and device showed PCE of 2.1%.8 This polymer gel electrolyte (PGE) system contained 65% liquid media along with polymer and ionic liquid proportion which slow down the liquid evaporation and provide more stability. However, presence of high amount of water and low viscosity of aqueous PGEs hampered their use at elevated temperature for outdoor applications.8 Moreover, amphiphilic block copolymer at different polymer concentration in water (with and without salt), self-assembles in the different lyotropic microcrystalline phases like 3D cubic, 2D hexagonal and 1D lamellar and these show anisotropic conductivity.24 Recently, there are reports which cover the use of amphiphilic block-copolymer based aqueous gel electrolyte and 3Dnetwork polymeric hydrogels in smart electrochemical devices25,26 and lithium-ion batteries respectively.27-29 A microcrystalline network of F77 with aqueous iodide/triiodide in 3D cubic 4 ACS Paragon Plus Environment

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phase has highest conductivity with 30 %(w/w) polymer to water ratio, whereas 1D lamellar containing 65 %(w/w) polymer reflects a slightly lower but comparable conductivity with 3D cubic and which is higher than 2D hexagonal anisotropic liquid crystalline phase.24 The ideal benefit of 1D lamellar phase is, it contains a very low amount of water which provides good mechanical strength and least volatility because water molecules are bounded and trapped inside the cross-linked polymer matrix. Moreover, reports are also available for the anisotropic arrangement of 1D-lamellae in parallel and perpendicular fashion between two electrodes, among these two, the perpendicular alignment shows better conductivity against parallel geometry in organic solvents as ions can efficiently travel from self-assembled channels.30-32 Considering very few efforts on the development of water based quasi-solid state DSSCs (ssDSSCs), there is enough need to add advancement in this class through the water based quasisolid electrolytes. From the available large library of (PEO)-(PPO)-(PEO) (Pluronic) polymers and their tunable properties toward the liquid crystalline structure and capability to dissolve the salts, it encourages to explore them in quasi-solid DSSCs. Here, we have developed the pure water based quasi solid electrolyte using Pluronic F77 block-copolymer with 62% (w/w) polymer to solvent ratio containing LiI/I2, GuSCN and chenodeoxycholic acid (WG). The electrolyte possesses 1-D anisotropic lamellar and thermo-reversible characteristic, which allowed us to develop efficient and stable DSSC with metal-free carbazole based SK3 sensitizer in the ⊥ar arrangement (with respect to electrode surface) of the electrolyte. This quasi-solid DSSCs are superior to the liquid electrolyte based DSSCs with efficiency of 2.8% at 45 °C under 1 sun illumination and demonstrates long-term stability. 2. Experimental 2.1 Materials 5 ACS Paragon Plus Environment


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The amphiphilic block copolymer (EO52-PO35-EO52) (Pluronic F77, mol wt. 6600 g/mol) was obtained as gift sample from BASF, USA. Water used in this study was Milli – Q grade (18 MΩ). LiI, I2 and guanidinium thiocyanate, H2PtCl6 (99.9% metal basis), TiCl4, titanium isopropoxide, nano-crystalline TiO2 semiconductor (80% transmittance in the visible region, by spin coating at 2000 rpm. This thin layer of titania was annealed at 450 oC for 20 min. Titania paste consisting of TiO2 (P-25, anatase), ethyl cellulose and α-terpineol were deposited on above 6 ACS Paragon Plus Environment

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pretreated FTO glass by screen printing technique.34,35 The electrodes were fired into the tubular furnace at 500 oC for 30 min, and the net thickness of titania film was found 9 µm. Working electrodes were soaked into the TiCl4 solution for 20 min at 60 oC and sintered in air at 450 oC for 10 min. For sensitization, the electrodes were allowed to cool up to 70 oC and immersed into the solution of 1:5 SK3 sensitizer and chenodeoxycholic acid in anhydrous methylene dichloride: methanol solution (8:2) for 24 h. Electrodes were washed thoroughly with methylene dichloride and dried under the stream of nitrogen gas. Counter electrodes were prepared from H2PtCl6 solution by a spin coating method and rapidly fired into the furnace at 450 oC for 20 min DSSC devices were fabricated by sandwiching electrolyte between the photoanode and cathode. In WG based devices, a prior heating-cooling treatment between 0 °C - 10 °C was given for better penetration of gel electrolyte into porous titania photoanode due to thermo-reversible nature of PGE (Figure S2, Supporting Information). By keeping 60 µm spacer, cells were sealed using epoxy adhesive and stored in dark for 12 h prior to measurements. It should be noted that active area of the device was 0.18 cm2. Device was also prepared by using WL electrolyte for comparison. WG based devices allow to arrange into crystalline phase between photoanode and Pt cathode with parallel (||al) and perpendicular (⊥ ⊥ar) arrangements through the gravitational force during the sol-to-gel formation.

2.2.3 Characterization of Polymer Gel and DSSC SAXS measurements were carried out using the Rigaku SAXS diffractometer equipped with a Gabriel 2D wire detector (sample-to-detector distance 830 mm and beam wavelength, λ = 0.154 nm). Measurements were monitored as a function of the wave vector transfer, q (q = 4π sin θ/λ), after radial averaging with a typical time acquisition of 10,000 s. PGE was mounted in an 7 ACS Paragon Plus Environment


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aluminum cell having transparent windows. The cross section of samples was investigated by scanning electron microscopy (SEM) using the JEOL (JSM6010 LA) with Tungsten filament. DSC thermogram were carried out between 20 °C to 80 °C in Pyris, Perkin-Elmer. Viscosity measurements were carried out using rotary rheometer (MCR301, Anton Paar, Germany). The conductivity of the polymer gels were measured using Solartron 1260 (Impedance and Gain Phase analyzer) by sandwiching the sample between two stainless steel blocking electrodes. Electrochemical impedance spectroscopy (EIS) of the gels were recorded using same thin layer cell. AC frequency 1 MHz to 0.1 Hz was swept onto the test cell with 10 mV AC amplitude and DC potential keeping zero. Conductivity was calculated by the following equation:

=

(1)

where, σ is conductivity in S/cm, R is the ohmic resistance of the electrolyte, l is the distance between the two electrodes and A is the area of the electrodes.31 Steady-state linear sweep voltammetry (CHI600E, USA) of gels was recorded by sandwiching the gel between two platinum electrodes using 3M tape as a spacer in order to maintain the thickness constant. Potential from

-1 V to 1 V was applied onto the test gel with 5 mV/s scan rate.

I-V characterizations of DSSCs were carried out using Keithley 2400 source meter and solar simulator (PET, USA) with 100 W xenon lamp as light source equipped with a band pass filter and light intensity was set to 100 mW/cm2 (the light intensity was calibrated using standard Siphotodiode). Electrochemical impedance spectra (EIS) of DSSCs were obtained in the frequency range from 120 kHz to 50 Hz with 10 mV AC amplitude under dark conditions where applied


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DC potentials equal to the previously measured Voc values under 1 sun. It should be noted that all the measurements were carried out three times and their mean values are reported here. 3. Results and Discussion 3.1 1-D lamellar anisotropic microcrystalline behavior of WG electrolyte Amphiphilic block copolymer [(PEO)52(PPO)35(PEO)52] self-assembles into different anisotropic liquid crystalline phases and exhibit phase dependent conductivity in presence of salt.24 As documented earlier, out of all liquid crystalline phases, cubic and lamellar phase show comparable conductivity,24 based on that we have prepared 62 %(w/w) polymer gel which has higher viscosity along with thermoreversible characteristics. Figure 1b depicts small angle x-ray scattering pattern for WG electrolyte at 25 and 50 °C temperature. It shows clear Bragg reflections and its sequence confirms the 1-D lamellar structure arranged by the gravitational settlement.

Figure 1 (a) Digital image of electrolyte WL and WG at room temperature, and (b) SAXS pattern of PGE at 25 oC and 50 oC.



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At 25 oC, the SAXS pattern shows three Bragg peaks that correspond to 1 0 0, 2 0 0 and 3 0 0 planes appeared at equal intervals which is signature of the 1D-lamellar liquid crystalline structure. The respective inter planner distance calculated from the peak positions are 10.46 Å, 11.51 Å and 11.28 Å. In case of 50 °C, the SAXS pattern remain unchanged with distances found for planes

1 0 0, 2 0 0, 3 0 0 are 9.23 Å, 9.64°, and 10.52 Å, respectively. A slight

reduction in inter planner distance at elevated temperature indicates compression of lamellar phase due to slight dehydration of PEO layer. This behavior suggests that the WG possesses stable 1-D lamellar anisotropic structure in the temperature range from 25 – 50 oC. Since in this phase, polymer matrix contains less amount of solvent which provides lower volatility. This observation is confirmed from the differential scanning calorimetric (DSC) measurements (Figure S3a, Supporting information). There was no heat loss or gain observed in the temperature range of 20 to 60 °C and it supports the SAXS results that the observed lamellar phase is stable between 25 and 50 °C. However, a slight deformation in lamellar phase was noticed at 60 °C (Figure S4, Supporting Information). From the viscosity study (Figure S3b, Supporting Information), it was also confirmed that viscosity of this gel is independent of temperature (30 °C – 80 °C).

3.2 Temperature and anisotropy dependent conductivity in polymer gel The ionic conductivities of the polymer gels were recorded using AC impedance method by sandwiching them between two stainless steel disks. To evaluate the influence of parallel (||al) and perpendicular (⊥ ⊥ar) arrangement of 1-D anisotropic arrangement of PG on conductivity, the gels were introduced between two electrodes in different fashion. Figure S5 (Supporting Information) shows the detailed method for development of both arrangements between the 10 ACS Paragon Plus Environment

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blocking electrodes. Here, we found that polymer gels align in parallel (||al) and perpendicular (⊥ ⊥ar) arrangement is due to gravitation force only. Conductivity data was collected for parallel (||al) and perpendicularly (⊥ ⊥ar) aligned 1-D lamellar phases at different temperatures and fitted into the Arrhenius type equation (2). The plot of ln σ Vs 1/T shown in Figure 2a, which shows linear behavior of conductivity as a function of temperature.

( ) = (2) where Ea is the activation energy, R is the molar gas constant, A is a constant and T is the absolute temperature. Ea values for electrolytes were calculated by linear fitting and the extracted activation energies and conductivity at 25 oC and 50 °C are depicted in Table 1. -4.0 -4.2

2.5

(a)

WG (⊥) WG (II)

-4.6 -4.8 -5.0 -5.2 -5.4

WG (II) at 25°C WG (⊥ ) at 25°C WG (II) at 50°C WG (⊥ ) at 50°C

1.5 1.0 0.5 0.0

-5.6 -5.8 3.1

(b)

2.0

Current, mA

-4.4

lnσ , mS/cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.2

3.3

-1

3.4

3.5

-0.5 -1.0

1000/ T /K

-0.8

-0.6

-0.4

-0.2

0.0

Potential, V

Figure 2 (a) Arrhenius plot for temperature dependence conductivity and (b) steady-state linear sweep voltammetry (LSV) of PGEs.

Comparatively, at room temperature as well as higher temperature (50 °C), perpendicularly (⊥ ⊥ar) aligned WG has higher conductivity than parallel (||al). Figure 3 shows the schematic representation of ion transport in PGE matrix.



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Figure 3 Gravitation force induced 1-D lamellar anisotropic arrangement of aqueous PGE.

Two procedures were set to align the 1-D lamellae in different anisotropy by keeping device parallel and perpendicular to the gravity. In case of ||al (with respect to electrode) alignment of PPO-PEO blocks to the electrodes, the triiodides have to travel diagonal to the layer and it produces obstacle in the smooth diffusion. Whereas, in case of perpendicular (⊥ ⊥ar) arrangement of PEO-PPO blocks toward the electrodes, the ions get in plane facile transport which leads to higher conductivity and lower activation energy. To correlate the diffusion phenomena with conductivity behavior, we have employed diffusion of triiodide species at different temperatures for both the arrangement by steady state linear sweep voltammetry measurement.

Table 1 Conductivity (σ), activation energy (Ea) and diffusion coefficient ( ) of triiodide for PGEs.

PGE WG(‫)׀׀‬ WG(┴) a

T (°C)

σa (mS/cm)

25 50 25 50

4.84 9.65 6.43 10.82

Ea (Kcal/mol) 6.7 5.2

× 10-6 (cm2/s)b 0.90 4.22 1.22 11.4

measured from EIS study, bmeasured from linear sweep voltammetry

Figure 2b shows the steady-state linear sweep voltammogram for both arrangements of gels and the apparent diffusion coefficient of triiodide species was calculated using the following equation (3)36,37 : 12 ACS Paragon Plus Environment

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=

!"#$

(3)

where is the diffusion coefficient of triiodide ion, Jlim is the limiting current densities, L is the thickness of cell, n is number of electron transferred (n = 2), F is faraday constant (96480 C), % is the concentration of triiodide in gel. Last column of Table 1 reveals that the diffusion of triiodide ions follow same order as it observed in conductivity. These estimated diffusion coefficients for all the PGs are comparable with those reported (~1 × 10-6 cm2/s) by other group11 for pure water based PGEs.

3.3 Application of Polymer Gels as Electrolyte in DSSC Prior to apply WG electrolyte in DSSC, we have carried out adhesion study of a liquid electrolyte (WL) with conventional sensitizer N719 and metal free sensitizer SK3. The digital photograph of WL drop on dye loaded TiO2 (top view and side view) after 5 minutes are shown in Figure 4a. It clearly confirms the higher hydrophobic nature of SK3 dye compared to the N719. Here, the visual contact angle and de-penetration of WL in SK3 sensitized photoanode support its hydrophobicity and it will be expected to highly stable in a water environment.


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Current density, mA/cm2


7 6

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(b)

WL WG(II) WG(⊥)

5 4 3 2 1 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

Potential, V Figure 4 (a) Comparison of hydrophobicity of WL electrolyte on N719 and SK3 dyes sensitized TiO2 photoanode, and (b) I-V characteristic of DSSC containing differently arranged WG as electrolyte.

Figure 4b shows the I-V characteristic of DSSC fabricated from the pure water based electrolyte WL (liquid DSSC) and quasi-solid DSSC using WG(||al) and WG(⊥ ⊥ar) electrolyte at 25 °C under 1.5 AM, 100 mW/cm2 light intensity and relevant photovoltaic parameters, Jsc (short-circuit current density, mA/cm2), Voc (open circuit voltage, V), FF (fill factor, %) and PCE (η = power conversion efficiency, %) are tabulated in Table 2. WL based device reflects PCE of 1.17% in the pure aqueous liquid electrolyte and it is far lower than the organic solvent based electrolyte performance of SK3 dye.33 Noteworthy, this performance is quite comparable with the pure water based DSSC reports.20,21 The resultant poor efficiency in the liquid is due to lower Jsc, FF, and Voc values, 6.2 mA/cm2, 44%, and 0.431 V, respectively. Here, hydrophobic dye coated photoanode surface resists the liquid aqueous electrolyte media and it reduces the ionic diffusion at photoanode/electrolyte surface which is responsible for poor Voc. Pt-coated counter electrode surface also creates resistance due to the poor adhesion with an aqueous electrolyte. These combined defects increase the back electron transfer from the TiO2 conduction band to the I3¯ 14 ACS Paragon Plus Environment

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ions in the electrolyte resulting the dark current and decrease in Voc. Moreover, at counter electrode/electrolyte interface, I3¯ reduction reaction decreases due to weak adhesion, which leads to a poor dye regeneration. Due to this dual facts, the Voc, Jsc and FF decrease compared to organic electrolytes used with SK3 dye.33

Table 2 Photovoltaic performance parameters of dye solar cells from J-V curve at 25 °C.

Devices

Jsc (mA/cm2)

Voc (V)

FF (%)

η (%)

WG ( ||al )

2.0 ± 0.1

0.617 ± 0.005

73 ± 1

0.90 ± 0.05

WG( ⊥ar )

2.8 ± 0.1

0.620 ± 0.005

67 ± 1

1.16 ± 0.05

WL

6.2 ± 0.1

0.431 ± 0.005

44 ± 1

1.27 ± 0.05

The Voc depends on the dark current that is related to the charge recombination between electrons present at conduction band and the oxidized species of an electrolyte.38 The dark reaction is represented by eq (4): 3I‾

I3‾ + 2e‾ (CB)

dark reaction

(4)

Voc for DSSCs with an iodine redox electrolyte is represented by the following equation (5):39

&'# =

( )

!

*+

,- (./ 0 1

(5)

where, k and T are the Boltzmann constant and absolute temperature, respectively, Iinj is the injection current from dye to the semiconductor, ncb is the electron density on the conduction band of semiconductor, and ket represents the rate constant of reduction of I3¯ to I¯. According to



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eq. (5), Voc decreases with an increase of dark reaction. The decrease in Voc of the aqueous liquid electrolyte based device could be explained by the enhanced dark reaction. Quasi-solid DSSC show slight lower but comparable performance with higher PCE in WG(⊥ ⊥ar) (η = 1.16%) over WG(||al) (η = 0.92%) under similar conditions like in liquid DSSC. From the Figure 4b, the Voc values increased up to 0.62 V in both the anisotropic arrangement and Jsc value decreases tremendously from 6.3 mA/cm2 to 2.0 mA/cm2 and 2.8 mA/cm2 for WG(||al) and WG(⊥ ⊥ar), respectively. An increase in Voc can be governed by ; (i) thermoreversible behavior of aqueous WG that leads to lowering in viscosity during heating-cooling cycle and (ii) surface active nature of amphiphilic block copolymer which paves the way for effective penetration of WG in hydrophobic dye coated porous TiO2 surface. This is clearly seen in cross-section SEM image of dye-TiO2 surface with and without aqueous PG (Figure 5a and 5b). Figure 5a confirms highly porous morphology of TiO2 while Figure 5b describes the SEM image of WG coated photoanode after the cooling-heating treatment. The later SEM image confirms the deep penetration of amphiphilic WG electrolyte in the dye-TiO2 surface.

Figure 5 Cross-section SEM image of (a) bare dye-TiO2/electrolyte interface and (b) dye-TiO2 + WG of photoanode with electrolyte


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Moreover, WG also builds proper contact at the counter electrode which enhances the reduction of the I3¯to I¯. Both of these factors enhance the Voc due to reduction in the back electron transfer from the TiO2 CB to the I3¯ ions in the electrolyte, because I3¯ions have higher solubility in the hydrophilic water based polymer portions than at the interface with dye sensitized titania surface.40 A decrease in Jsc (from 6.2 mA/cm2 to 2.0 or 2.8 mA/cm2) in WG based devices compared to WL based device is might be due to slower I3¯/I¯ diffusion in microcrystalline lamellar selfassemblies compared to the pure liquid state at room temperature. Moreover, this gel electrolyte contains

62 %(w/w) polymer to liquid ratio, which indicates very less concentration of

redox species in gel media compare to the liquid media. Here, the WG(⊥ ⊥ar) based device show higher Jsc compared with WG(||a) based device, which might be due to the superior transport of iodide-triiodide species in the WG(⊥ ⊥ar) gel matrix. The potential advantages of this anisotropic PGE are non-volatility and thermal stability. Hence for outdoor utility, now-a-days, materials which are thermally stable and sustainable towards temperature variation have great importance for solar cell application.41 In outdoor environment, the light illumination and temperature vary based on seasons and geographical area. From conductivity and diffusion study of PGEs, the higher temperature performance is more superior and could be suitable for this application. We measured photovoltaic performance of devices at 50 °C temperature under similar conditions. Figure 6 shows the I-V characteristic at 50 °C for the WG (⊥ ⊥ar) and WG (||al) based DSSCs and corresponding photovoltaic parameters are tabulated in

Table 3.



10

Current density, mA/cm2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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WG(II) at 25°C WG(⊥ ) at 25°C WG(II) at 50°C WG(⊥ ) at 50°C

8 6 4 2 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Potential, V Figure 6 I-V characteristics of WG based DSSC devices at 50 °C under 1 sun illumination.

The trend of photovoltaic performance follows the order observed in conductivity and diffusion behavior. The PCE values are 2.0 % and 2.4 % for WG (||al) and WG (⊥ ⊥ar), respectively.

Table 3 Photovoltaic parameters at different illuminations at 50 °C.

Devices

WG (||al)

WG (⊥ar)

Pin

Jsc

Voc

FF

η

(mW/cm2)

(mA/cm2)

(V)

(%)

(%)

100

4.8 ± 0.1

0.600 ± 0.01

69 ± 1

2.0 ± 0.2

50

2.4 ± 0.1

0.605 ± 0.01

71 ± 1

2.1 ± 0.2

15

0.7 ± 0.1

0.603 ± 0.01

72 ± 1

2.0 ± 0.3

100

5.1 ± 0.1

0.600±0.01

78±1

2.4±0.2

50

2.6 ± 0.1

0.607±0.01

79±1

2.8±0.2

15

0.7 ± 0.1

0.603±0.01

79±1

2.4±0.3 18

ACS Paragon Plus Environment

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At higher temperature, the devices are performing well and WG (⊥ ⊥ar) is giving superior performance over WG (||al). The Jsc values were increased from 2.0 to 4.8 mA/cm2 for WG (||al) and 2.8 to 5.1 mA/cm2 for WG (⊥ ⊥ar) when temperature was raised from 25 oC to 50 °C, this is due to enhancement in diffusion of I3¯ in the polymer gel through lamellar structure. The performance also looks improved at 0.5 sun illumination compared to 1.0 sun and 0.15 sun illumination for both the devices at 50°C (Table 3). It is noticed that WG (⊥ ⊥ar) shows 2.8% PCE at 0.5 sun illumination as best performing device. To justify the dye coated TiO2/electrolyte interface characteristic in these DSSCs, Tafel polarization technique was used to magnify the redox reaction of I3¯/I¯ at photoanode. At the photoanode of DSSC, rate of cathodic and anodic reaction at the semiconductor-electrolyte interface can be explained by well-known Butler-Volmer eq. (6)42-44:

2 = −24 567

8, !"

9: − :); < − 67

8 !"

9: − :);

Anisotropic 1-D Aqueous Polymer Gel Electrolyte for Photo

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