Electrical, optical and electrophotochemical studies

Measurement 102 (2017) 214–219

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Electrical, optical and electrophotochemical studies on agarose based biopolymer electrolyte towards dye sensitized solar cell application Rahul Singh a,⇑, B. Bhattacharya a, S.K. Tomar b, Vijay Singh c, Pramod K. Singh a a

Material Research Laboratory, Department of Physics, School of Basic Sciences & Research, Sharda University, Greater Noida 201 310, India Department of Chemistry, J. K. Lakshmipat University, Jaipur 302026, India c Department of Chemical Engineering, Konkuk University, Seoul 143-701, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 28 September 2016 Received in revised form 14 November 2016 Accepted 10 February 2017 Available online 14 February 2017 Keywords: Electrolyte Ionic liquid Biopolymer DSSC XRD FTIR

a b s t r a c t This paper reports the latest work in our laboratory on biopolymer electrolyte for dye sensitized solar cell (DSSC) application. Biopolymer electrolyte comprises agarose, potassium iodide (KI) were dissolved in two different solvents dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). To further achieve the reasonable conductivity for DSSC application we have added a low viscosity ionic liquid (IL) in the biopolymer electrolytes. The room temperature conductivity maximum was obtained near 60:40 compositions. Structural, electrical and photo electrochemical studies have been carried out in details and explained. These electrolytes were further used in the fabrication of DSSC, and comparative measured values of the fill factor (FF), Open circuit voltage (Voc), current density (Jsc) and efficiency at 100 mW/ cm were tabulated. The results obtained from data evaluation affirm that biopolymer – IL gel electrolytes appears promising candidates for energy devices. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, biopolymer materials, such as chitosan, agarose, and phytagel, have been frequently used in electrochemical devices such as batteries, fuel cells, electrochromic display devices and DSSC [1–5]. These biopolymer electrolytes falls in the category of marine algae biopolymers extracted from red seaweed polysaccharides which are hydrophilic in nature [3]. Agarose based biopolymer electrolyte forms a stable gel and thick film which is mechanically strong, showing liquid like conductivity which make them suitable candidate for device application. Further agarose forms a good crosslink biopolymer matrix which is considered to be an effective polymer matrix and can benefit the ionic transport properties. Carefully evaluation of biopolymer electrolytes already indicates that agarose based electrolyte shows liquid like conductivity (102–103 S cm1) which is suitable for an efficient DSSC [2,3]. A nanostructured DSSC comprises wide band gap semiconductors electrodes and electroactive polymers have been extensively studied [6–8]. In a common DSSC, porous electrodes operate in

⇑ Corresponding author. E-mail addresses: [email protected] (R. Singh), pramodkumar.singh@ sharda.ac.in (P.K. Singh). http://dx.doi.org/10.1016/j.measurement.2017.02.014 0263-2241/Ó 2017 Elsevier Ltd. All rights reserved.

contact with an electrolyte by simultaneous transport of electronic and ionic species in the solid and semi-liquid phase, respectively. The absorption of light in the DSSC occurs due to dye molecules and the charge separation by electron injection from the dye to the TiO2 at the semiconductor electrolyte interface. The concept of DSSC is believed to reduce the production costs and energy payback time significantly as compared to standard silicon. The conversion efficiency varies between 6 and 13% depending on the module size and technology [9–12]. The highest conversion efficiency till date for DSSC was achieved by using liquid electrolyte albeit liquid electrolytes already have well known problems like leakage, corrosion, evaporation etc. which force us to find other alternatives. Solid polymer electrolytes (SPE) are solid ion conductors formed by dissolving salts in polymers having high molecular weight. They can be prepared by an economical and reliable process in semisolid or solid form, [13–22]. In present paper we focused our attention towards developing a SPE based on biopolymer electrolyte. We have developed new biopolymer electrolytes using potassium iodide (KI) salt as iodide source. To further clarify the role of solvents we have developed biopolymer electrolytes in two different solvents i.e. Agarose biopolymer with KI and IL in DMF and Agarose, KI and IL in DMSO. The electrical, optical and structural studies along with photovoltaic performances are presented in detail.

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2. Experimental procedure 2.1. Preparation of gel biopolymer electrolyte The chemicals used in present study have been purified before use. Agarose with average molecular weight (Mw = 5000 g/mol) was purchased from HI-MEDIA, Mumbai, India. Other chemicals like potassium iodide (KI) with molecular wt. 166 g/mol, ionic liquid: 1-ethyl-3-methylimidazolium dicyanamide (molecular wt. 177.21) were obtained from Sigma Aldrich, USA., The solvents used in present study i.e. dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) were obtained from Fisher Scientific, India. As in the common preparation method of gel polymer electrolyte (GPE), fixed amounts of agarose powder (0.175 g) was dissolved in solvents (DMSO, DMF 20 ml) in a beaker with continuous stirring at constant heating of 70 °C. The same ratio as stock solution was used for overall experiment (written here as B1). Stoichiometric ratios of potassium iodide (KI) was then dissolved in DMSO and DMF (5 ml.) respectively in another beaker (written here as B2). By adding B2 in B1 drop wise resulted in a clear transparent GPE. In next step, drop wise ionic liquid (IL) was added in B1 using pipette and left for continuous stirring (30 mins) and used as electrolyte in DSSC. 2.2. Dye sensitized solar cell fabrication For laboratory-scale DSSC fabrication, we cut fluorine-doped tin oxide (FTO) glass plate (1x1 cm2 with resistivity 8 X/sq from Sigma Aldrich, USA) and cleaned thoroughly in an ultrasonic bath with ethanol and methanol, followed by distilled water and acetone. Few drops Ti(IV) bis(ethyl acetoacetato)-diisopropoxide solution (2 wt.% in 1–butanol) as a blocking layer of titanium paste (purchased from Sigma Aldrich, USA) was then coated on FTO glass plate with the help of a spin coater and heated at 450 °C for 30 min in a programmable muffle furnace. To prepare porous titanium dioxide (TiO2) electrode, TiO2 paste (supplied from Solaronix, Switzerland) was spread over the conducting surface of FTO and heated at 450 °C for 30 min providing nice porous (10–15 nm pore diameter) TiO2 film of approximately 10 mm thickness. The counter electrode was prepared by coating a layer of platinum (spin coating using chloroplatinic acid (H2PtCl6) with molecular wt. 409.81, purchased from Sigma Aldrich) on another piece of FTOglass and then calcined at 400 °C for 30 min. The working electrode (stated above) was dipped into 0.5 mM 535-bisTBA (N3 dye purchased from Sigma Aldrich) in distilled ethanol solution for overnight. The polymer electrolyte (with maximum ionic conductivity) was then sandwiched between the working (WE) and the counter electrodes (CE).

A is the area of given sample [13–15]. The calculated values of ionic conductivity are shown in Fig. 1. The conductivity initially increases attain maxima at 40 wt.% KI concentration in both samples where conductivity value approaches around 102 S cm1 and then starts to decrease. The nature of conductivity behavior is reported in literature [13–15]. The ionic conductivity (r) in case of electrolyte system is given as

r¼nql

ð2Þ

where n the is charge carrier density, q is the charge of the carrier, m is the mobility of the carriers. The doping of KI provided more charge carriers which will certainly enhances the value of ionic conductivity. Therefore any change in either of the parameters n or m will certainly affected the value of ionic conductivity. This is due to the fact that KI gets dissociated in the matrix and K+ and I ions start contributing to the conductivity. However, the rate of increase of conductivity is found to be different for different matrix. It is clear that addition of KI and IL in Agarose matrix enhances the ionic conductivity of the film. A comparative study of the two different samples shows that DMF based GPE is more stable and reliable than the other. It has the highest ionic conductivity 102 S/cm at room temperature as shown in Fig. 1. Conductivity attains maxima at 40 wt.% KI concentration in both sets samples. The conductivity value approaches around 102 S cm1 and then gradually decreases. The increase in the ionic conductivity with increasing KI concentration can be due to the increment in the number of mobile charge carriers while the possible decrease in the ionic conductivity at a KI concentration greater than 40 wt.% can be attributed to the formation of ion multiples [14–16]. With knowing the fact that high conductivity always assists in boosting efficiency of a DSSC, we have added a low viscosity IL (30 wt.%) 1-ethyl-3-methylimidazolium dicyanamide (EMImDCA) within the biopolymer matrix having highest conductivity (i.e. 60:40 compositions) [23]. The achieved conductivity after dispersion of IL was 7.41 103 and 4.23 102 S/cm in DMSO and DMF respectively. These high conducting IL doped solid biopolymer electrolytes were used in fabricating DSSCs. 3.2. Polarized optical microscopy Polarized microscopy (POM) has been used to further clarify the role of doping of KI salt and ionic liquid (1-ethyl-3methylimidazolium dicyanamide) in agarose based GPE matrix. We have recorded POM micrographs using a POM instrument (BA310POL, Motic). Pure Agarose gel in de ionized water as a solvent shows a rough matrix with interconnected gel grains while adding KI enhances amorphous regions in Fig. 2a [22]. POM images

-1

10

3.1. Ionic conductivity The ionic conductivity measurement of the agarose-KI based biopolymer electrolyte films with varying solvent such as DMSO and DMF were carried out using CH instrument workstation (model 604D, USA) over frequency range 100 Hz–1 MHz. To measure ionic conductivity we have sandwiched free standing biopolymer electrolyte films between steel electrodes and the electrical conductivity was evaluated using the formula

1 l r¼ Rb A

ð1Þ

where r is ionic conductivity, Rb is the bulk resistance where the Nyquist plot intercepts with the real axis, l is thickness of sample,

Ion conductivity (S/cm)

3. Results and discussion

DMF DMSO -2

10

-3

10

-4

10

0

20

40

60

80

100

Composition of KI in wt. % Fig. 1. Ionic conductivity of agarose-KI biopolymer electrolyte with solvents DMSO and DMF.

216


(a)

Pure Agarose

(b)

(c)

DMF + KI

(d)

DMF +KI +IL

(e)

DMSO+ KI

DMSO +KI +IL

Fig. 2. Polarized optical micrographs at 100 zoom with corresponding scale (a) pure agarose (7 mm), (b) agarose (36mm), KI in DMF, (c) agarose, KI and IL in DMF (51 mm), (d) agarose, KI in DMSO (36 mm), (e) agarose, KI and IL in DMSO (57 mm).

of Agarose and KI dissolve in DMF (Fig. 2b) shows rough patterns with some portions contains brown and yellowish which is possibly due to grain boundaries [24]. Adding IL assists in enhancing amorphous regions (blackish portions) which is clearly observed in Fig. 2c. A similar report has been observed in the case where DMSO was used as solvent (Fig. 2d, e).

current shoots up sharply attaining the maxima then decreases due to charge polarization and get saturated [5,26]. By monitoring initial current and final current we can calculate the ionic transference number using Eq. (3). The ionic transference number of Agarose based electrolyte with and without IL using two different solvents DMF and DMSO are lies in between 0.88 and 0.92 which confirms the ionic nature of the biopolymer electrolyte system.

3.3. Ionic transference number measurement using dc polarization method

tion ¼

The ionic transference number for biopolymer electrolyte films with varying solvent have been determined using simple dc polarization method. We have applied a fixed DC potential of 0.25 V for 6 h to the sandwiched structured electrode (steel plates)// biopolymer electrolytes//steel plates system. The DC current is monitored with respect to time using Keithley source measure unit 2400. Initially the current increases due ionic movement results

Iinitial Ifinal Iinitial

ð3Þ

3.4. FTIR studies Fourier Transform Infrared Spectroscopy (FTIR) spectra of pure biopolymer and biopolymer doped with KI in DMF, DMSO, and DMF with IL and DMSO with IL were recorded between 4000 and 450 cm1 resolution using Perkin Elmer 882. The different bands

217


assigned with various functional groups are listed in Table 1. It is clear from the figure (Fig. 3) and Table that almost all the peaks related to host materials (agarose) are present in biopolymer doped KI sample. It is also clear that (Table 1) that there is some shifting in peak portion in biopolymer-salt complex which indicates complex formation as we observed in common polyetherssalt complex polymer [26,27]. It is clear from Fig. 3 that all the peaks related to host material agarose are present in KI doped agarose sample. Peak shifts is observed in the KI doped samples such as Agarose + KI + DMSO and Agarose + KI + DMSO + IL from 3445 cm1 to 3434, 3382 and 3387 cm1 respectively of OAH stretch and as shown in Fig. 3. There are no traces of peak position of OAH stretch in the sample Agarose + KI + DMF and Agarose + KI + DMF + IL Fig. 3. From IR spectra it is also observed that the peak shifts of standard peak position agarose from 1121 cm1 to 1109, 1010 and 1011 cm1 in the sample Agarose + KI + DMSO and Agarose + KI + DMSO + IL respectively of CAO stretch shown in Fig. 3 and also some extra peak is also observed in the sample Agarose + KI + DMSO and Agarose + KI + DMSO + IL as tabulated in the table. But heavy shifting is observed in the sample Agarose + KI + DMF and Agarose + KI + DMF + IL from 1121 cm1 to 1228 and 1216 cm1 in both sample of bond CAO stretch Fig. 3. These shifting of peak position are due to the addition of solvent and dispersoid which can be easily demonstrated by hydrogen bonding and also this indicates that there is formation of high amorphous region in the material which helps in enhancing the electrical properties of the system. The peak position of the CAC stretch is observed in all samples same but there is slight change in the Agarose + KI + DMSO, Agarose + KI + DMSO + IL from 1400 cm1 to 1437 and 1406 cm1 in both sample, whereas heavy changes in the sample Agarose + KI + DMF and Agarose + KI + DMF + IL from 1400 cm1 to 1434 cm1 in both sample. It is also clear from Table 1 that there is some shifting in peak portion in biopolymer-salt complex which indicates complex formation, observed in common polyethers-salt complex [15,16]. A comparison of the spectra for the pure Agarose gel film and the gel polymer electrolyte doped with IL (IL-KI in varying solvent with Agarose matrix) further shows the changes in the spectral features of biopolymer matrix in terms of shifts in peak positions or peak intensities variations, which is due to ion–polymer interaction as stated above [15–28]. These significant changes in the spectral features in terms of the appearance of new peaks are due to the doping of KI in three different solvent with and without IL [20]. This could be easily understood by using Agarose biopolymer: KI

Pure Agarose

Agarose+ KI +DMSO +Ionic Liquid

Agarose+ KI +DMSO

Agarose+ KI +DMF +Ionic Liquid

Agarose+ KI +DMF

Fig. 3. Fourier Transform Infrared Spectroscopy (FTIR) spectra’s of pure agarose, agarose with KI in DMF, DMSO, DMF with IL and DMSO with IL.

polymer electrolyte in which cation (K+) and anion (I) charges play dominant roles in pair formation. K+ of KI, which is weakly bounded and can easily be dissociated under the influence of an electric field. These free and sufficient K+ ions can hop together with I consistently at the coordinating side of the biopolymer structure, this hopping mechanism inside the agarose based electrolyte can be seen [26,27]. These coordinating side increases in the case of Agarose + KI + DMF based electrolyte due to ionic motion and bio polymer segmental motion also get increased when compared with Agarose + KI + DMSO based biopolymer (agarose) electrolyte. When the concentrations of KI was increased, the charge carrier number of (K+ and I) also increased, increasing the overall conductivity of the system [27]. 3.5. XRD studies X-ray diffraction patterns of different samples of biopolymerssalt electrolyte and IL doped biopolymer-salt electrolyte are recorded using Rigaku D/max-2500 with scan rate 2° min. It is clear that all salts are well dissolved in biopolymer matrix which is affirmed by disappearance of XRD peaks related to salts in biopolymer-salt complex or and IL doped biopolymer-salt elec-

Table 1 Fourier Transform Infrared Spectroscopy (FTIR) spectra of pure agarose, agarose with KI in DMF, DMSO, DMF with IL and DMSO with IL. Absorption bands in cm1 Pure agarose

Agarose + KI + DMSO

Agarose + KI + DMSO + IL

Agarose + KI + DMF

Agarose + KI + DMF + IL

3445.86

3382

3387

–

–

– 2124.66 – – 1636.32 – 1400.34 – –

– – – 1644 – – 1437, 1405 – 1315

– 2141

2970

1316, 1170

3006, 2970 2361 1738 – – 1505 1434 1366 1228, 1216

1738 – – – 1434 1366 1228,1216

1121.00 – 687.28 666.77

1010 950 704 –

1011 950 897, 704 –

– – – –

– – – –

1645 – – 1436, 1406

Frequency ranges

Groups (bonds)

Functional groups

3500–3200 3640–3610 3000–2850 2260–2100

(s,b) (s,sh) (m) (w)

OAH stretch, free hydroxyl, H–bonded CAH (stretch) ACCA stretch

Alcohols, phenols

1680–1640 1650–1580 1550–1475 1500–1400 1370–1350 1300–1150 1300–1150

(m) (m) (s) (m) (m) (m) (m)

AC@CA stretch NAH bend NAO asymmetric stretch CAC stretch (in–ring) CAH rock CAO stretch CAH wag (ACH2X) CAO stretch OAH bend CAH ‘‘oop” CABr stretch

1320–1000 (s) 950–910 (m) 900–675 (s) 690–515 (m)

Alkane Alkynes Alkenes 1° amines Nitro compounds Aromatics Alkanes Alcohols, carboxylic acids, esters, ethers alkyl halides Alcohols, carboxylic acids, esters, ethers Carboxylic acid Aromatics Alkyl halides

218


Current in Ampere

-3

1.0x10

-4

8.0x10

-4

6.0x10

-4

4.0x10

-4

2.0x10

Voltage in Volts 0.0 Agarose+ DMSO -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6KI+0.8 1.0 -4

-2.0x10

Agarose+ KI+ DMSO+ IL Agarose+ KI+ DMF Agarose+ KI+ DMF+ IL

-4

-4.0x10 Fig. 4. XRD pattern of the biopolymer based electrolyte. (a) pure agarose, (b) agarose with KI in DMSO, (c) agarose with KI in DMSO and IL, (d) agarose with KI in DMF, (e) agarose with KI in DMF and IL.

trolyte XRD spectra [Fig. 4]. Pure Agarose shows two broad peaks observed at 15° and 30° (Fig. 4a). These peaks indicate the semiamorphous nature of Agarose film [15,16]. These peaks are getting broader after insertion of either KI or IL which affirms that addition of KI or IL in biopolymer matrix enhances the amorphicity [Fig. 4b–e]. This confirms the complexation of biopolymer with salt and IL that disorders the crystallinity and oriented arrangement of biopolymer matrix system. The XRD patterns further shows that addition of IL increases the amorphous behavior of biopolymer matrix/KI system enhancing the overall ionic conductivity of the system and increasing the ion mobility and flexibility of biopolymer matrix. It may be noted that for better conductivity and device performance, it is always expected that the crystallinity should be low and matrix shows high amorphicity [16]. This observation indicates better conductivity of AG based systems as discussed earlier in Section 3.1. 3.6. Dye sensitized solar cell performance (I–V Curve) DSSCs have been fabricated using sandwiched structure of working electrode (WE)//biopolymer electrolytes//counter electrode (CE) structure using standard procedure used in our laboratory [3–22]. The photovoltaic performances (I–V curve) were examined with Keithley 2400 source meter at 1 sun condition. As far as electrolyte is concerned we have taken maximum conducting biopolymer electrolytes with and without IL in two different solvents i.e. DMF and DMSO. For developing redox couple in Agarose-KI matrix we have added extra iodine in biopolymer: KI matrix [3–15]. The recorded I-V curves are shown in Fig. 5 and photovoltaic parameters are listed in Table 2. The overall solar conversion efficiency (g) is a product of the short-circuit current density (Jsc) the open-circuit photovoltage (Voc) and the fill factor (FF) following Eq. (4):

g¼

Jsc Voc FF Pin

ð4Þ

Table 2 Photovoltaic parameters of Agarose based biopolymer electrolytes with and without IL in DMF and DMSO solvents at 1 sun condition. Composition

Jsc (A cm2)

Voc (V)

FF (%)

Area (cm2)

ƞ (%)

Agarose + KI + DMSO + I2 Agarose + KI + DMF + I2 Agarose + KI + DMSO + IL + I2 Agarose + KI + DMF + IL + I2

0.0010 0.0010 0.0019 0.0033

0.57 0.75 0.63 0.62

0.56 0.77 0.60 0.66

0.25 0.25 0.25 0.25

0.33 0.47 0.72 1.36

Fig. 5. The comparative study of current-voltage characteristic of DSSCs using agarose based biopolymer electrolytes with and without IL in solvents DMF and DMSO.

where Pin is the total solar power incident on the cell, 100 mW cm2 for air mass (AM) 1.5. Therefore the only way to improve the power efficiency is to increase current density (Jsc), open circuit voltage (Voc), and/or the fill factor (FF). From Fig. 5 and Table 2 it is clear that DSSCs based on biopolymer electrolytes with both solvent DMSO and DMF and with Agarose:KI:I2 + IL based electrolyte achieve highest conductivity with highest overall efficiency (g) of 1.36% at 1 sun condition. 4. Conclusion The biopolymer electrolytes are synthesized in two different solvents i.e. DMF and DMSO and tested for DSSC application. Complex impedance spectroscopy reveals the enhancement of ionic conductivity by doping salts (KI). The IL doped biopolymer-KI matrix shows further enhancement in conductivity value by evaluating the facts that dispersion of IL provides more ions and also assisted in reducing crystallinity. The reduction of crystallinity was affirmed by optical microscopy (POM) and XRD studies. The biopolymer-salt-IL complex formation confirms by FTIR. The enhancement in number of charge carriers as well as reduction in crystallinity of biopolymer matrix by IL doping clearly indicates the importance of low viscosity of IL. The ion transport number measurement confirmed the ionic nature of these biopolymer electrolytes. The fabricated DSSCs using two different solvents DMSO and DMF and with maximum conductivity biopolymer electrolytes films further confirm that the DMF is best solvent for the fabrication of DSSC for Agarose based biopolymer systems. Acknowledgement This work was supported by Department of Science & Technology project (DST/TSG/PT/2012/51-C) government of India. References [1] R.W. Lenz, Biodegradable polymers, Adv. Polym. Sci. 107 (1993) 1–40. [2] V.L. Finkenstadt, Natural polysaccharides as electroactive polymers, Appl. Microbiol. Biotechnol. 67 (2005) 735–745. [3] R. Singh, N.A. Jadhav, S. Majumder, B. Bhattacharya, P.K. Singh, Novel biopolymer gel electrolyte for dye-sensitized solar cell application, Carbohydr. Polym. 91 (2013) 682–685. [4] R.C. Agrawal, G.P. Pandey, Solid polymer electrolytes: materials designing and all-solid-state battery applications: an overview, J. Phys. D Appl. Phys. 41 (2008) 223001. [5] F.M. Gray, Solid Polymer Electrolytes: Fundamentals and Technological Applications, VCH Publishers, New York, 1991. [6] M. Grätzel, Dye-sensitized solar cells, Photochem. Photobiol. C: Photochem. Rev. 4 (2003) 145. [7] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (1991) 737–740.

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