Natural Dye-Sensitized Solar Cells with Polyaniline Counter

Electrode

Garima Dwivedi , Guncha Munjal and Ashok N. Bhaskarwar

Department of Chemical Engineering, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi -110016, INDIA

Abstract. An inexpensive polyaniline was synthesized by electrochemical polymerizat ion on conducting

glass as a platinum substitute for tri-iodide reduction on the conducting glass substrate, and natural sensitizers

as a synthetic dyes replacement. Plant pigments such as chlorophyll, caroten oid, flavonoid, anthocyanin,

betalains are present in natural sensitizers are responsible for light absorption and the charge injection to the

conduction band of the semiconductor nanoparticles. The efficiencies of dye-sensitized solar cell (DSSC)

with natural sensitizers and synthetic dye i.e. N719 with polyaniline as a counter electrode catalyst was

compared.

Keywords: Dye-sensitized solar cells, polyaniline, natural sensitizers, counter electrode. betalains.

1. Introduction

In the 21st century, the biggest challenge is to replace fossil fuels with renewable-energy sources. With

increasing population, the demand for energy is also steeply increasing. Solar energy is one of the best

alternatives to meet this increasing energy demand. While fossil fuels emit CO2, thereby causing green-house

effect, solar energy has an advantage of being totally clean. The combination of nano-structured electrodes

and dyes efficient for charge-injection Professor Grätzel and his group developed a solar cell with photo-

conversion efficiency exceeding 7% in the year 1991 [1]and 10% in the 1993 [2]. This solar cell is called the

dye-sensitized nanocrystalline solar cell (DSSC) or simply the Grätzel cell named after its inventor. DSSC

have attracted attention of many researchers and companies because of their low costs and a reasonable

photochemical conversion efficiency (13%) for liquid state DSSC cells having a Co(II/III) tris(bipyridyl)–

based electrolyte in conjunction with a donor-p-bridge-acceptor zinc porphyrin dye [3]. Highest efficiency of

15% for perovskite structure, solid state DSSC, with inorganic-organic composite material (e.g.,

CH3NH3PbI3) as a dye sensitizer and in place of electrolyte an organic material as a hole transport material

(HTM). This DSSC has the structure of glass/FTO (conducting glass)/TiO2/CH3NH3PbI3/HTM/Au [4]. For

the first generation solar cells, the costs are high because of the mono-crystalline silicon being used in those;

for the second generation solar cells, the thinner active layer has low absorption of sunlight, but also

comparatively a low cost. The third generation solar cells offer a good performance and lower costs. The

third generation solar cells, i.e. DSSCs are of lower cost because of their simple assembly techniques. DSSC

also have advantages of flexibility, colour, and working under diffuse light conditions [5].

Conductive-polymer as a platinum replacement and vegetables dyes such as anthocyanins, betalains,

carotenoids, chlorophylls as natural sensitizers for N719 dye {cis-di(thiocyanato)-N-N′-bis (2,2′-bipyridyl-4-

carboxylic acid-4′-tetrabutylammonium carboxylate) ruthenium (II) [6]} replacement in DSSC, can further

reduce the cost of a DSSC. We have established in our studies, DSSC with low cost.

Corresponding author. Tel.: + 91 9811060433

E-mail address: garima.it.bhu@gmail.com, anbhaskarwar@gmail.com.

International Proceedings of Chemical, Biological and Environmental Engineering, V0l. 90 (2015)

DOI: 10.7763/IPCBEE. 2015. V90. 16

101

In plants anthocyanins are the one responsible for different colours of stems, leaves, roots, flowers, and

fruits. First DSSC with anthocyanins as a photo-sensitizer was reported by Tennokone et al. [7]. Dye

extracted from brinjal peel is anthocyanins based. The number of methoxy groups determines the type of

absorption wavelength, hydroxyl groups determines the intensity, and colour stability [8], [9]. Betalains dyes

are water soluble, have orange and red colour with strong absorption in the visible region of the

electromagnetic spectrum [7]. Dye extracted from beet root is betalains based. First DSSC with beet root

extract as photo-sensitizer was reported in the year 2002 with efficiency of 0.44%. In the year 2011 DSSC of

2.71% was reported with the beet root extracted dye only. Betalians are considered good due to their colour

strength, and the carboxyl functional groups present, which is responsible for strong binding with TiO2

nanoparticles [7]. There are several functional groups that are responsible for binding to the TiO2

nanoparticles such as phosphonic acids, carboxylic acids [10]. Carotenoids, organic compounds commonly

split in xanthophylls and carotenes. Dye extracted from capsicum is carotenoid based [9]. Carotenoid based

photo-sensitizer achieved efficiency up to 2.6%. Very interesting result was published by Wang et al.

carotenoids with chlorophyll derivatives as photo-sensitizer in DSSC with conversion efficiency of 4.2%

[11].

In this study, polyaniline electrode was prepared by cyclic voltammetry (CV) synthesis, which was used

as a counter electrode for a DSSC. Comparison study of different DSSC assembled using synthetic dye and

vegetable sensitizers were examined.

1.1. Working principle of a DSSC

DSSC consist of: (i) the working electrode, fluorine doped tin oxide (FTO) glass coated with TiO2

nanoparticles with dye adsorbed on it which act as a photo-anode. (ii) The counter electrode, FTO glass with

Polyaniline deposit on it which acts as a cathode and (iii) Electrolyte in between the electrodes.

When light falls from the working electrode (photo-anode) side, it passes through the transparent

conducting glass (TCO) sheet. The dye gets excited and transfer its electron to the conduction band of TiO2

nano-particles, with this removal of electron from the dye gets oxidized. This electron travels from TiO2

layer to the FTO glass and finally to the counter electrode thus completing the outer circuit.

The oxidized dye gets regenerated by electrons from the electrolyte containing redox (I-/I3

-) couple, by

converting iodide to tri-iodide, and the tri-iodide in the electrolyte is regenerated by the electron from

counter electrode. Thus the photon from the light is able to generate electricity completing the whole circuit

[12].

Fig. 1: Working principle of a DSSC

1

4

6 I-/I3

-

2 3

CB

VB

TiO2 Dye Electrolyte

D

D

*

Light

5 Load

Polyaniline

FTO

glass

102

2. Experimental Section

2.1. Preparation polyaniline counter electrode Polyaniline electrode was prepared by electro-polymerization (cyclic voltammetry). For cyclic

voltammetery (CV) three electrode assemblies were taken. Pre-cleaned FTO glass was used as a working

electrode, platinum wire as a counter electrode and Ag/AgCl as a reference electrode in an aqueous solution

containing 1.0M HClO4 and 0.2M aniline.

Fig. 2: Cyclic voltammogram (CV) curves of a polyaniline film with 27 sweep segment.

Fig. 3: SEM image of polyaniline deposit on FTO glass

The potential in range; Initial E =- 0.3V, high E = 1V, low E =- 0.3V and final E = 0.4V with sweep rate

50 mV/sec [13].

2.2. Preparation of sensitizers As per method reported in the literature natural dyes were extracted; beet root [14], green capsicum [15],

brinjal peel [16]. The Ruthenium N719 dye of 0.5mM solution in mixtures of 1:1 volume ratio of acetonitrile

and tert- butyl alcohol was used as synthetic dye [17].

2.3. Assembly of a DSSC Cleaning of transparent conducting oxide glass (FTO) glass in sonicator with bath for10 min each in

acetone, water + 1-2 drops of HCl, water, and ethanol. The pre-cleaned FTO glass plate 1 cm×1.5 cm were

immersed in aqueous 50 mM TiCl4 at 70 °C for duration of 30 min and rinsed with water and ethanol and

dried at 50 °C [18]. Compact layer paste of 20 nm TiO2 nanoparticles was coated on the FTO glass by screen

printing, of dimension 5 mm×5 mm, and kept for 5minutes inside desiccators, and then dried for 6 min at

-0.004

-0.003

-0.002

-0.001

0

0.001

0.002

0.003

0.004

-0.5 0 0.5 1 1.5

Cu

rre

nt(

A)

V vs Ag/AgCl

103

125 °C. This coating–drying procedure was repeated two times. After drying a nano-crystalline TiO2 layer at

125 °C, scattering layer paste containing 200 nm sized TiO2 nanoparticles was deposited by screen printing

on top of compact layer. The electrodes coated with the TiO2 nanoparticles, were gradually heated at 60°C

for 10 min, 190° for 10 min, 325 °C for 5 min, at 375 °C for 5 min, at 450 °C for 30 min [17]-[19]. After

cooling down slowly FTO glass with TiO2 layers to 80 °C, electrode were immersed into the dye solutions,

and kept at room temperature in tightly closed container under dark for 24 h, to ensure complete sensitizer

uptake. The DSSCs were assembled by sandwiching the photo-anode i.e. sensitized TiO2 and polyaniline

counter electrode by introducing the electrolyte containing solution of 0.03 M I2, 0.6 M Butyl imilidazolium

iodide(BMII), 0.5 M 4- tert-butylpyridine, 0.10 M guanidinium thiocyanate in mixtures of acetonitrile and

valeonitrile, in between the electrodes [6].

Fig. 4: vegetable dyes synthesized in our lab and synthetic N719 dye.

Fig. 5: Thickness of polyaniline coated on FTO glass measure by optical surface profilometer

2.4. Characterization of DSSC

To evaluate the performance of a DSSC, the photocurrent density-voltage curve was made using

potentiostat / galvanostat, under illumination of 100mW/cm2 using solar simulator. The equation for

efficiency calculation is described below [20].

η(%) =Voc × Jsc × FF

Pin

× 100 (1)

Here, η is the conversion efficiency, Jsc,Voc,FF are the short circuit current density, open-circuit voltage,

and the fill factor respectively. Pin is incident light energy.

104

3. Results and Discussion

The figure 5, shows thickness of polyaniline deposit on the FTO glass was approximately 950 nm

measured by optical surface profilometer (KLA tencor, microXam-100). Figure 6, shows the J-V plot of a

DSSC with polyaniline deposit on FTO as the counter electrode and N719 dye as sensitizer. Figure 7 shows

the comparison of J-V plots of DSSCs with polyaniline deposit on FTO as the counter electrode, and N719

or natural dye extracted from beet root, brinjal peel, and green capsicum as sensitizer. In the table 1, the

efficiencies of all the four DSSC were compared.

Fig. 6: J-V plot of a DSSC with N719 sensitizer and polyaniline as a counter electrode catalyst

Fig. 7: Comparison of J-V plots of DSSCs with N719, and natural sensitizers and polyaniline as a counter electrode

catalyst

It was observed that efficiency of a DSSC with N719 sensitizer was highest among all DSSCs assembled.

One of the reasons for lower efficiency for DSSCs with natural sensitizer may be due to the lower

concentration of dye. Among three vegetable dyes highest efficiency was obtained for beet root i.e. betalain

based dye, which may be due to the tinctorial strength of the dye, and strong binding of carboxyl group in

betalains with TiO2 nano-particles via ester type linkage [7]. In future, working on the synthesis of natural

sensitizer with optimal concentration, and mixing two or more vegetable dye as a photo-sensitizer in DSSC

may enhance the efficiency of DSSCs with lower cost.

Table 1: Photovoltaic performances of different DSSCs with polyaniline as a counter electrode catalyst.

Sensitizer Used Molecular

structure

Jsc (mA/cm2) Voc(V) FF Efficiency( η )%

N719 Ruthenium dye 8.50 0.72 0.39 2.38

Beet root Betalains 2.06 0.55 0.37 0.42

Brinjal peel Anthocyanin 1.40 0.41 0.26 0.14

Capsicum Carotenoid 0.44 0.37 0.22 0.03

-8

-6

-4

-2

0

2

4

6

8

10

0 0.5 1 1.5

Cu

rre

nt

de

nsi

ty (

mA

/cm

2)

Voltage (V)

Dark

Light

-25

-20

-15

-10

-5

0

5

10

0 0.5 1 1.5

Cu

rre

nt

de

nsi

ty (

mA

/cm

2)

Voltage (V)

N719

Beet root

Brinjal

Capsicum

Fig. 7: Comparison of J-V plots of DSSCs with N719, and natural sensitizers and polyaniline as a counter electrode

catalyst

105

4. Conclusions

This piece of research work shows good conversion efficiencies for DSSCs with poly-aniline as a

counter electrode catalyst. Even though, natural sensitizer based DSSCs have lower conversion efficiencies

as that of synthetic dye but they are non toxic, environment friendly, easily available and cheaper.

5. Acknowledgments

The authors thank Indian Institute of Technology- Delhi for financial assistance. The authors also thank

Ms. Maneesha Pande, Ms. Sudeshna for their constant support and advices.

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