UDK 678.7:544.6.018.47-036.5
Original scientific article/lzvirni znanstveni članek
ISSN 1580-2949 MTAEC9, 49(1)123(2015)
NEW SOLlD-POLYMER-ELECTROLYTE MATERIAL FOR DYE-SENSlTlZED SOLAR CELLS
NOVl ELEKTROLlTNl MATERIAL NA OSNOVl TRDNEGA POLIMERA ZA SONČNE CELlCE, OBČUTLJlVE ZA SVETLOBO
Vivek K. Singh, Bhaskar Bhattacharya, Shashank Shukla, Pramod K. Singh
Material research laboratory, School of Basic Sciences & Research, Sharda University, G. Noida 201310, lndia vivekv445@gmail.com; singhpk71@gmail.com
Prejem rokopisa - received: 2014-02-08; sprejem za objavo - accepted for publication: 2014-02-26
A solid-polymer electrolyte consisting of polyvinylpyrrolidone (PVP) doped with ammonium iodide (NH4l) was developed and characterized for a possible application in a dye-sensitized solar cell. Complex impedance spectroscopy revealed an increase in the conductivity and the maximum conductivity was obtained at the w = 50 % NH4l mass concentration. Light photographs confirmed an enhancement in the amorphous nature of the host which was affirmed by XRD measurement. The composite nature of the polymer-electrolyte film was also confirmed with the FTlR spectrum. A dye-sensitized solar cell (DSSC) was fabricated using the most conductive film that showed an efficiency of 0.025 % at the 1 sun condition. Keywords: polymer electrolyte, conductivity, FTlR, XRD, dye-sensitized solar cell
Razvit in karakteriziran je bil trdni polimerni elektrolit, ki ga sestavlja polivinil pirolidon (PVP), dopiran z amonijevim iodidom (NH4l), za morebitno uporabo za sončne celice, občutljive za svetlobo. Kompleksna impedančna spektroskopija je odkrila povečano prevodnost z maksimumom pri masni koncentraciji w = 50 % NH4l. Posnetki s svetlobno mikroskopijo so odkrili povečanje deleža amorfne osnove, kar so potrdile tudi XRD-meritve. Kompozitno naravo polimerne elektrolitne plasti je potrdil tudi FTlR-spekter. Sončne celice, občutljive za svetlobo (DSSC), so bile izdelane z uporabo najbolj prevodne plasti, ki je pokazala učinkovitost 0,025 % v razmerah 1 sun.
Ključne besede: polimerni elektrolit, prevodnost, FTlR, XRD, za svetlobo občutljive sončne celice
1 INTRODUCTION
Polymer electrolytes are promising candidates for electromechanical-device applications chiefly because they mechanically behave like solids but their internal
structure and, consequently, the conductivity behavior closely resemble the liquid state.1 The main advantages of polymeric electrolytes are satisfactory mechanical properties, easy fabrication of thin films and an ability to form a good electrode/electrolyte contact.2-5
Polyvinylpyrrolidone (PVP), also commonly called polyvidone or povidone, is a polymer made of N-vinyl-pyrrolidone monomer. PVP was first synthesized by Prof. Walter Reppe and a patent was filed in 1939. When dry it is a light flaky powder, which readily absorbs up to 40 % of its weight in atmospheric water. PVP is soluble in water and other polar solvents. Since it has excellent wetting properties and readily forms films, it makes a good coating or an additive to coatings. PVP is used in a wide variety of applications in medicine, pharmacy, cosmetics and industrial production. When added to iodine, PVP forms a complex called povidone-iodine exhibiting disinfectant properties and being beneficial for dye-sen-
sitized solar-cell applications where iodide/polyiodide redox couple is frequently added to the electrolyte. Dye-sensitized solar cells (DSSCs) were first reported by O'Regan and Grätzel in 1991.6
Over the past decade, DSSCs have been intensely investigated as potential alternatives to the conventional
inorganic photovoltaic devices due to their low production cost and good efficiency for a conversion of solar energy into electricity. A typical cell consists of a nano-crystalline mesoporous titanium dioxide film sensitized with a monolayer dye, an electrolyte containing iodide/triiodide as the redox couple and a platinum counter electrode. Liquid electrolytes were replaced with solid-polymer electrolytes because the former lead to corrosion, evaporation and leakage. Thus, the solid-polymer electrolytes improved the long-term stability of DSSCs.
ln the present paper, we report on new solid-polymer electrolyte films of a polyvinylpyrrolidone (PVP) complex with ammonium iodide (NH4l) and a DSSC that was fabricated using the film with the maximum electrical conductivity.
2 MATERIALS AND METHOD
Polyvinylpyrrolidone (PVP, Mw = 130,000), ammonium iodide (NH4l) and iodine (l2) were purchased from Sigma-Aldrich, USA while methanol was purchased from Qualikems Fine Chem. Pvt. Ltd., Vadodara, lndia.
The following approach was taken to prepare the electrolytes. PVP (500 mg) was dissolved in about 4 mL of methanol under continuous magnetic stirring (~ 30 min) or until complete dissolution at room temperature. Then an appropriate quantity of NH4l salts was added to
the PVP methanolic solution and stirred continuously. After the solvent evaporation the polymer-salt complex was poured into polypropylene Petri dishes. Free-standing films of different PVP compositions - w(NH4I)/% (where w = (10, 20, 30, 40, 50, 60, 70) %) - were obtained and further characterized using various characterization tools.
A dye-sensitized solar cell with an active area of 0.72 cm2 was fabricated with the procedure reported else-where.7 With the common procedure the TiO2 paste was applied on the fluorine-doped SnO2 substrate (FTO) using the doctor-blade method. The adhesive scotch tape was used to control the thickness of the as-coated TiO2 film with a thickness of « 50 ^m, followed by a heat treatment at 500 °C for 30 min. The porous TiO2 film formed on the FTO substrate was 10 ^m thick and a pore had a diameter of 10-15 nm.8-13 The porous TiO2 film on the FTO substrate was then immersed in a ruthenium sensitizer dye solution (0.5 mmol N-719, Solaronix, in ethanol) and left overnight to allow a sufficient dye adsorption. This TiO2 working electrode with the dye was then rinsed off with distilled water and ethanol solution. A Pt-thin-film-coated counter electrode was prepared separately by spin-coating the H2PtCl6 solution onto the FTO substrate. The viscous polymer-electrolyte solution (- 400 ^L) containing PVP:NH4I + I2 (the maximum a) was finally cast on the working electrode (a two-step casting) and sandwiched between the platinized counter electrode and the TiO2 working electrode.
We used steel electrodes as contacts to measure the ionic conductivity (a) and we calculated ionic-conductivity values using the following formula:
a = G • l/A	(1)
where a is the ionic conductivity, G is the conductance (in the case of 1/^b, Rb is the bulk resistance where the Nyquist plot intercepts with the real axis), l is the thickness of the sample and A is the area of the given sample.
The cole-cole plot (complex impedance plot) of a typical sample of the PVP + w(NH4I) 50 % polymer electrolyte is shown in Figure 1. The room-temperature ionic conductivity (deduced from different cole-cole plots) of polymer electrolytes as a function of the NH4I concentration is shown in Figure 2 and its values are listed in Table 1.
Table 1: Room-temperature ionic conductivity of the PVP:NH4I polymer electrolyte system
Tabela 1: Ionska prevodnost PVP:NH4I polimernega elektrolitskega sistema pri sobni temperaturi
Composition (w(NH4I)/%)	Conductivity (S cm-1)
10	2.24 X 10-5
20	2.57 X 10-5
30	2.63 X 10-5
40	1.09 X 10-4
50	7.55 X 10-4
60	4.85 X 10-4
70	7.45 X 10-5
3 RESULTS AND DISCUSSIONS 3.1 Conductivitty measurement
Ionic conductivity of the polyvinylpyrrolidone-based
polymer-electrolyte film was measured using a CH
Instruments workstation (model 604D, USA) over a fre-
quency range of 100-105 Hz.
As observed in Figure 2 and Table 1, the ionic conductivity (a) increases with the increase in the NH4I concentration and reaches its maximum at the w(NH4I) = 50 % (a = 7.55 X 10-4 S/cm) concentration and then it decreases. The increase in the ionic conductivity with the increasing NH4I concentration can be related to the increase in the number of mobile charge carriers, while the possible decrease in the ionic conductivity at a NH4I mass concentration greater than 50 % can be attributed to the formation of ion multiples.
Figure 1: Cole-cole plot of the PVP + w(NH4I) 50 % polymer-electrolyte system
Slika 1: Cole-cole-diagram polimernega elektrolitnega sistema PVP + w(NH4I) 50 %
Figure 2: Effect of the NH4I amount on the conductivity of the polymer electrolyte (PVP:NH4I) measured at room temperature Slika 2: Vpliv vsebnosti NH4I na prevodnost polimernega elektrolita (PVP:NH4I), izmerjeno pri sobni temperaturi
The ionic conductivity (a) in the case of an electrolyte system is given as:
a = n q ^	(2)
where n is the charge carrier density, q is the charge of the carrier and ^ is the mobility of the carriers. Therefore, any increase in either n or q will certainly affect the value of ionic conductivity.
3.2	X-ray diffraction
The crystallinity of the polymer electrolyte was further affirmed by X-ray diffraction patterns (XRD) using a Rigaku D/max-2500 XRD diffractometer at a scan rate of 5° min. The recorded X-ray diffraction patterns of pure PVP and NH4I doped PVP polymer electrolytes are shown in Figure 3. It is clear that pure PVP shows well-known amorphous peaks around 20 values of 23°. The incorporation of NH4I into the PVP matrix decreases the intensity of the peaks (the suppression in crystallinity). It also appears that the XRD data relating to the NH4I doped PVP polymer electrolyte shows only the peaks related to either PVP or NH4I, which clearly affirms the composite nature of the polymer-electrolyte system. Additionally, the PVP-NH4I data does not contain any other peaks related to the NH4I sample, affirming a complete dissolution of NH4I in the Sago Palm matrix.
3.3	FTIR spectroscopy
The FTIR spectra of pristine PVP, NH4I and the PVP doped with NH4I were recorded between 4000 cm-1 and 400 cm-1 on a PerkinElmer Spectrophotometer 883 as shown in Figure 4. Pure NH4I shows well defined peaks at (3131, 1622 and 1398) cm-1, where the first peak corresponds to the N-H stretch, while the other two correspond to the N-H bending. In the spectrum of pure PVP, the peaks at (847, 895 and 934) cm-1 correspond to para-, di-substituted and mono-substituted C-H bend-
Figure 4: FTIR spectra of pure PVP, NH4I and PVP + NH4I polymer electrolyte
Slika 4: FTIR-spektri čistega PVP, NH4I in PVP + NH4I polimernega elektrolita
ings. The bands between 1450 cm-1 and 1600 cm-1 correspond to the C=C stretching. The peaks at 1075 cm-1 and 2135 cm-1 correspond to the C-N stretching, while the ones at 1018 cm-1 and 1172 cm-1 correspond to the C-C stretching. The CH3 bending is shown at 1375 cm-1. The peak at 1835 cm-1 corresponds to the presence of C=O bonds. In the spectrum of PVP + NH4I the peaks at 843 cm-1 and 934 cm-1 correspond to the C-H bending. The peaks at 1100-1300 cm-1 correspond to the C-C stretching. The C-N stretching is shown at 1074 cm-1. The CH2 bending is given by the peak at 1439 cm-1. The bands between 1550 cm-1 and 1640 cm-1 correspond to the N-H bending. The peak at 1848 cm-1 corresponds to the C=O stretching, while the C=C peaks are indicated by the peaks between 1450 cm-1 and 1600 cm-1.
It is also clear from the figure that almost all the peaks related to the host materials (PVP and NH4I) are present in the NH4I doped PVP polymer-electrolyte sample. The disappearance of any new peaks other than those of the host materials clearly affirms the composite nature of the samples, also supported by our XRD data.
Figure 3: XRD pattern of pure PVP and PVP + NH4I polymer electrolyte
Slika 3: XRD-posnetek čistega PVP in PVP + NH4I polimernega elektrolita
3.4 Light microscopy
Light microscopy (LM) of a polymer-electrolyte sample with the dimensions of 1 cm x 1 cm was carried out using a Leica Leitz DMRX light microscope. The obtained photographs are shown in Figure 5. It is noted that the pure PVP film (Figure 5a) shows well-ordered patches, confirming its semicrystalline nature. This pattern is a bit different from the micrographs of the PEO matrix. Due to an addition of NH4I to the PVP matrix (Figure 5b) the patch size becomes random and the crystallinity seems to be disturbed. The decrease in the crystallinity (an ordered pattern) showed a further increase in the amorphicity (a non-ordered pattern) where different sizes of rough patches are distributed randomly within the polymer matrix. It is believed that the amorphous regions (the non-ordered pattern) are conductivity-rich regions and, hence, our light micro-
^ initial 1 final
(3)
Figure 6: Current-versus-time plot (?ion measurement) of a typical PVP:NH4I polymer-electrolyte matrix measured at room temperature Slika 6: Odvisnost toka od ~asa (?ion meritev) zna~ilne PVP:NH4I osnove polimernega elektrolita, izmerjena pri sobni temperaturi
Figure 5: Light microscope photographs of: a) pure PVP, b) PVP + 40 % NH4I polymer-electrolyte matrix
Slika 5: Posnetka s svetlobnim mikroskopom: a) ~isti PVP, b) PVP + 40 % NH4I osnova polimernega elektrolita
graphs showed good agreement with the ionic-conductivity data.
To further specify the nature of the charge carriers (ionic or electronic) we carried out the ionic-transference-number measurement. Figure 6 shows the current-versus-time measurement for a typical sample of the arrowroot-60 % NH4I polymer-electrolyte matrix. In this study we applied a fixed DC voltage and the current was recorded with respect to time following a well-established formula:
Figure 7: Mechanism of ion transport in the PVP:NH4I polymer-electrolyte matrix
Slika 7: Mehanizem potovanja ionov v osnovi polimernega elektrolita PVP:NH4I
where /initial is the initial current and /final is the final residual current.
The observed ionic-transference number is 0.93 showing that our biopolymer electrolyte is essentially an ion-conducting system.10
3.5	Ion-transport mechanism
The ion-transport mechanism in the PVP polymer-electrolyte matrix can be easily understood using Figure 7. According to the literature, in most of the polyethers incorporated with alkali halides the anion contribution is more dominant.14 However, in the case of the polyethers doped with the other salts like NH4ClO4 the cationic part is more dominating1516. The NH4+ ions of NH4I are coordinated with the ether oxygen of PVP and I- anions hang outside. The weakly bonded H in NH4+ can be easily dissociated under the influence of a DC electric field forming H+ ions. These H+ or NH4+ ions can jump via each coordinating site as shown in Figure 7.
3.6	DSSC performance
A dye-sensitized solar cell (DSSC) was prepared with the PVP:NH4I/I2 polymer electrolyte with the maximum ionic conductivity (w(NH4I) = 50 %). Iodine was also added to prepare the redox couple (10 % with respect to the iodide salt). The recorded J-V characteristic is shown in Figure 8. The fabricated DSSC shows
Figure 8: Current density versus voltage characteristic of the PVP:50 % NH4I/I2 polymer-electrolyte film at the 1 sun condition Slika 8: Gostota toka v odvisnosti od zna~ilnosti napetosti v plasti polimernega elektrolita PVP:50 % NH4I/I2 v razmerah 1 sun
/
initial
an open-circuit voltage (Voc) of 0.35 V and a short-circuit-current density (/sc) of 0.1 mA/cm2 with the overall efficiency of 0.025 %.
The observed efficiency was much lower when compared to the other polymeric systems reported in11-13. This was expected since the observed conductivity in the present case was much lower (by ~ 1-2 order of magnitude).
4 CONCLUSION
A solid polymer-electrolyte film consisting of PVP doped with the NH4I salt was successfully prepared and characterized. It was observed that NH4I doping enhances the ionic conductivity and the conductivity maximum was obtained at the 50 % NH4I salt mass concentration with the conductivity value of 7.55 x 10-4 S/cm. XRD and IR affirmed the composite nature of the polymer electrolyte, while optical micrograph and XRD affirmed the reduction in the crystallinity due to the NH4I doping. Using a solid polymer electrolyte with the maximum ionic conductivity, we fabricated a DSSC that shows a reasonable photo response at the 1 sun condition.
Acknowledgments
This work was supported by a DST project (DST/ TSG/PT/2012/51) of the Government of India.
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