Dye-sensitized solar cells based on porous conjugated polymer counter electrodes

Thin Solid Films 573 (2014) 112–116

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Dye-sensitized solar cells based on porous conjugated polymer counter electrodes Naeimeh Torabi, Abbas Behjat ⁎, Fatemeh Jafari Photonics Research Group, Engineering Research Centre, Yazd University, Yazd, Iran Atomic and Molecular Groups, Faculty of Physics, Yazd University, Yazd, Iran

a r t i c l e

i n f o

Article history: Received 23 March 2014 Received in revised form 7 November 2014 Accepted 10 November 2014 Available online 15 November 2014 Keywords: Dye-sensitized solar cells Electrochemical deposition Poly-3-methyl-thiophene Counter electrode Porous structure Nanostructure

a b s t r a c t In this paper, we report platinum-free dye-sensitized solar cells that were fabricated using a grown porous poly3-methyl-thiophene (P3MT) counter electrode. The growing of the porous P3MT was performed by an electrochemical deposition method. This method is easy and affordable unlike the common expensive deposition methods. The morphology of P3MT films was studied by scanning electron microscopy images. It was observed that polymer layers grown with a current density of 2 mA/cm2 have a clear porous and rough structure as compared to layers grown with a lower current density. To understand the reaction kinetics and the catalytic activities of the counter electrodes with P3MT for 3I−/I− 3 redox reaction, cyclic voltammetry (CV) was performed. Based on the analysis of CV, it was shown that this layer can be used as a counter electrode for dye-sensitized solar cells. The electro deposition conditions during the growth of polymer layers such as current density, the morphology of polymer films and the duration of polymerization have a significant role in the current–voltage characterization of the fabricated solar cells. The performance of the fabricated solar cells was improved by optimization of these parameters. The highest efficiency of 2.76% was obtained by using porous P3MT in the counter electrode. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Since dye-sensitized solar cells (DSSCs) were introduced by Grätzel [1], they have attracted much attention as cheap and next-generation solar cells. Their maximum conversion efficiency of over 10% [2] suggests that they are a promising type of solar cells. For commercial usages, it is important to develop low-cost materials and to achieve maximum efficiency for the cells. A typical DSSC consists of a transparent conductive substrate, a porous thin-film photoelectrode composed of titanium dioxide (TiO2) nanoparticles, dyes, an electrolyte, and a counter electrode [3]. The requirements for the counter electrode in a DSSC are effective reduction of oxidized species and good chemical stability in the electrolyte systems used in DSSCs [4]. So far, various materials have been used as counter electrodes of DSSCs [5–7]. Platinum (Pt) is a cathode most commonly used in DSSCs. Recently, several attempts have been made to replace high-cost Pt counter electrodes with conjugated polymers because of their high electrochemical activity. Poly(3,4ethylenedioxythiophene) (PEDOT) is the most preferred conducting polymer which is considered for use as a counter electrode in DSSCs [8–16]. In many studies, researchers have observed that pure polymer counter electrodes have a lower efficiency than Pt-based DSSCs [8,10, ⁎ Corresponding author at: Atomic and Molecular Groups, Faculty of Physics, Yazd University, Yazd, Iran. Tel.: +98 353 8122773; fax: +98 353 8200132. E-mail address: [email protected] (A. Behjat).

http://dx.doi.org/10.1016/j.tsf.2014.11.034 0040-6090/© 2014 Elsevier B.V. All rights reserved.

11,16]. In order to enhance the performance of polymer-based DSSCs, other materials such as graphene or nanoparticles are incorporated with polymer to increase the film surface area, conductivity and catalytic activity. For example, Hong et al. [11] incorporated graphene with PEDOT. They obtained an increased efficiency of ~ 4.5% compared to ~2.3% for pure PEDOT-based cells. Saito et al. [8] used polymerized ptoluenesulfonate doped poly(3,4-ethylenedioxythiophene) (PEDOTTsO) as counter electrodes and obtained a conversion efficiency of ∼ 3.6% under 100 mW/cm2. Electrochemical polymerization of PEDOT has been carried out in the presence of different anions and polymerization potentials with the reported efficiency of 4.2% [14]. Improving the performance of dye-sensitized solar cells has also been reported. According to Nazeeruddin et al. [15], the improvement was done based on a PEDOT counter electrode and using synthesized highly porous PEDOT films. Other polymers such as polypyrrole (PPy) and polyaniline (PANI) can be used as a counter electrode in dye-sensitized solar cells [17–21]. Recently, Tang et al. [22] synthesized PPy and PANI nanostructures by chemical and electrodeposition techniques. They used double-layered PANI consisting of a nanoparticle and a nanofiber layer as a counter electrode and reported an efficiency of 6.58%. In this study, we use poly-3-methyl-thiophene (P3MT) as a counter electrode. The advantages of P3MT, compared with those of other polymers in the polythiophene derivations, include less susceptibility to oxidation during electrochemical processes [23] and possession of the highest electric conductivity within the polythiophene family [24]. In addition,

N. Torabi et al. / Thin Solid Films 573 (2014) 112–116

P3MT is not soluble in most solvents, which can result in better chemical stability and is compatible with liquid electrolytes in dyesensitized solar cells [25]. Researchers have investigated the photovoltaic properties and application of P3MT in various structures [26–30]. In this paper, we report an effective way of producing porous P3MT layers that can be used as counter electrodes in DSSCs due to their relatively high catalytic properties. P3MT films can be deposited on conductive substrates using inexpensive low-temperature deposition techniques [31,32], which is one of the advantages of polymer-based counter electrodes. In this study, we demonstrate that, by controlling the growth conditions, porous P3MT structures can be obtained, and that they are potentially appropriate for the construction of dye-sensitized solar cells.

2. Experimental procedure 2.1. TiO2 paste TiO2 nanoparticles with diameters of 20 nm and 400 nm were synthesized by acetic acid-catalyzed hydrolysis of titanium isopropoxide (97%, Aldrich), followed by autoclaving at 220 °C for 12 h. To prepare a screen-printable paste, the water in the autoclaved TiO2 colloid solution was replaced by ethanol. Ethyl cellulose (Aldrich), lauric acid (98%, Fluka) and terpineol (97%, Fluka) were added to the ethanol solution of the TiO2 particles, and then ethanol was removed from the solution using a rotary evaporator to obtain viscous pastes.

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2.4. Solar cell characterization The current–voltage (J–V) characteristics of DSSCs were measured using a Keithley 2400 digital source meter under AM1.5G (100 mW/cm 2) illumination with a solar light simulator (300 W, Xenon lamp) calibrated by a reference Si solar cell. The current–voltage characteristics of the DSSCs were used to determine the short-circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF), and power conversion efficiency (Eff.) of those solar cells. The active area of the DSS cells was 0.25 cm2. The catalytic activity of the counter electrode was measured by cyclic voltammetry (CV) in a three-compartment cell using an Autolab potentiostat/galvanostat (PGSTAT-302 N, Eco Chemie, Netherlands). While the prepared polymeric counter electrodes were used as the working electrode, Platinum (Pt) foil and Ag/Ag+ served as the counter electrode and the reference electrode respectively. The scan rate used was 100 mV/s, while the electrolyte was an acetonitrile solution containing 10 mM LiI, 1 mM I2, and 100 mM tetrabutylammonium tetrafluoroborate. The morphology of the polymer films was also monitored using scanning electron microscope (SEM) images. The images were recorded using the TESCAN SEM system. The operating voltage of the system was 15 kV. The thickness of the polymer films was determined by cross-sectional SEM images. An Ocean Optics spectrometer, model HR 4000, was used for absorption measurements. 3. Results and discussion 3.1. Properties of polymer counter electrodes

The P3MT electro deposition was carried out in an acetonitrile solution containing 0.1 M 3-methylthiophene (3MT) (98%, Aldrich) and 0.1 M tetrabutylammonium tetrafluoroborate (Merck) at room temperature. Polymerization was carried out by applying different constant current densities (Galvanostatic method). The applied current densities varied between 1.0 and 2.5 mA/cm2. Fluorine-doped tin oxide glass (FTO) substrates were used as the anode in the electrochemical polymerization process, and a platinum plate of a similar surface area was used as the cathode. The separation between the electrodes was kept around 2 cm. The electrochemically deposited P3MT layer was subsequently reduced in 1 M hydrazine (NH2-NH2) in a tetrahydrofuran solution for 1 min.

2.3. Solar cell fabrication To prepare the devices, the fluorine-doped tin oxide glass (FTO) substrates were used with a sheet resistance of 8 Ω/cm2. These substrates were washed with deionized water, acetone, ethanol, and 2-propanol, and finally rinsed with water. A layer of TiO2 (20 nm, anatase) paste was deposited by a doctor-blading method on the top of the FTO layers. After the film was dried at 120 °C, another layer of TiO2 (400 nm, anatase) paste was coated as a light-scattering layer, and the deposited film was annealed at 550 °C for 1 h to produce a mesoporous TiO2 film. When the temperature decreased to 80 °C, the prepared samples were soaked in a 0.3 mM dye solution (a solvent mixture of acetonitrile and tert-butyl alcohol in the volume ratio of 1:1) for 24 h at room temperature. Di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′bipyridyl-4,4′-dicarboxylate)ruthenium(II)(N719) was used as a sensitizer. Various P3MT counter electrodes were prepared for the DSSCs as described in Section 2.2. The DSSC samples were assembled using a dye-adsorbed TiO2 working electrode and P3MT films as a counter electrode. Then, a liquid electrolyte (0.5 M lithium iodide and 0.05 M iodine in acetonitrile) was injected into the midst of the sandwiched electrodes.

The as-grown polymer layers were incorporated with electrolyte ions through an electrochemical approach. By reducing the polymers, the ions could be removed from the surface [25]. The photovoltaic study reported here is based on P3MT reduced chemically with hydrazine. The as-grown P3MT films appeared in a deep blue color, but the reduced P3MT films displayed a purplish red color. The absorption spectrum of the P3MT layer is shown in Fig. 1. In this spectrum, two local maxima are observed; one in the region near 1.5 eV and the other, which is more intense, in the region close to 2.4 eV. According to Sun and Frank [33] and Nicho et al. [34], these peaks correspond to a bipolaron band and the π–π* transition respectively. Using the method described by dos Reis et al. [32], the energy gap was calculated around 2.14 eV. The absorption spectrum and the energy gap were found consistent with the reports of P3MT synthesis by the electrochemical deposition method [32,35–37]. Since the morphology of electrochemically grown P3MT is critical to the final solar cell structure, different growth conditions were applied in order to obtain more favorable porous polymer layers. It was found that,

Absorbance (arb.unit)

2.2. Polymerization of 3-methylthiophene

1.34

1.84

2.34

Energy (eV) Fig. 1. The absorption spectrum of electrochemically deposited layer of P3MT (chemically reduced by hydrazine).

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1 mA/cm2

2 mA/cm2

Fig. 2. SEM images of P3MT layer deposited with 1 mA/cm2 and 2 mA/cm2 for 360 s. Scale bars are 2 μm.

electrode under the same conditions [38]. However, the reduction potential to yield from triiodide to iodide on the P3MT counter electrodes shifted negatively compared to that on the Pt electrode, which means that P3MT counter electrodes have a larger resistance. 3.2. Characteristics of DSSCs using porous polymer counter electrodes Fig. 4 shows the J–V characteristics of the fabricated DSSCs using the prepared P3MT counter electrodes. The polymeric layers were deposited with various current densities. Table 1 summarizes the photovoltaic characteristics of these devices and the DSSCs based on the Pt counter electrode, including the short-circuit current (JSC), the open-circuit voltage (VOC), the fill factor (FF), and the conversion efficiency (Eff.). Under the illumination of simulated solar light (AM 1.5G, 100 mW/cm2), the device with a counter electrode polymerized at 2 mA/cm2 exhibited VOC = 0.63 V, JSC = 8.88 mA/cm2, and FF = 50%, while the device with a counter electrode polymerized at 1 mA/cm2 presented VOC = 0.63 V, JSC = 4.91 mA/cm2, and FF = 52%. The PCEs of these two devices were 2.76% and 1.25% respectively. As it can be seen, the morphology of the counter electrode has an effective role in the performance of the fabricated cells. For a current density of 2.5 mA/cm2, the cells do not work efficiently, which can be caused by electrolyte diffusion through the highly porous cathode and reaching FTO. The J–V curves presented in Fig. 4 suggest that the applied current density during the deposition of the counter electrode plays an essential role in the performance of the

5

12

4

10

3

Current Density (mA/cm 2)

Current Density (mA/cm2)

among other parameters such as solvent type, monomer and electrolyte concentrations, the oxidation rate determined by the current density also plays a significant role in the morphology of grown P3MT. This effect is clearly illustrated in Fig. 2, where SEM images taken from the P3MT films grown with two different current densities are presented. It was observed that favorable porous P3MT could be grown on the FTO substrate at the current density of 2 mA/cm2. On the contrary, a lower oxidation current density tended to produce a rather smooth deposition of P3MT. Fig. 3 shows the cyclic voltammograms obtained from CV measurements for the Pt electrode and P3MT counter electrodes deposited with different current densities. In these cyclic voltammograms, the cathodic current peaks (IPC) correspond to the reduction of I−3 ions through an interaction with the polymer electrode. In general, the magnitude of IPC represents the catalytic capability of a counter electrode for reduction of I−3 in DSSCs. These CV results clearly indicate that deposition of P3MT layers by controlling the current density during polymerization can be an effective way to increase the active surface areas of counter electrodes and, thus, to enhance their electro-catalytic ability. Comparing the cyclic voltammograms of the P3MT electrodes and the Pt electrode shows a much larger current density of the I− 3 reduction peak for the P3MT electrodes than that for the Pt electrode. The porous structure of the polymer layer may be a cause for this large current density. This suggests a faster reaction rate on the P3MT electrode than on the Pt electrode. In other words, the charge-transfer resistance for the − redox reaction is lower on the P3MT electrode than on the Pt I− 3 /I

Pt

2 1 0 -1 -2 -0.70

P3MT diposited with 1mA/cm2

8

P3MT deposited with 2.5 mA/cm2

6 4

P3MT deposited with 2 mA/cm2

2 0 -2 -4 -6

-0.20 0.30 Voltage vs. Ag/Ag+

0.80

-2

-1.5

-1

-0.5 0 Voltage vs. Ag/Ag+

0.5

1

Fig. 3. Cyclic voltammogram curves of the polymer layers that were polymerized with different current densities at a scan rate of 100 mV/s in 10 mM LiI, 1 mM I2, and 0.1 M tetrabutylammonium tetrafluoroborate in an acetonitrile solution (right) compared with Pt electrode (left).

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115

Current Density (mA/cm2)

10 9

(a) 2 mA/cm2 (360 s)

8

(b) 2 mA/cm2 (600 s)

7

(c)1 mA/cm2 (600 s) (d) 1 mA/cm2 (360 s)

6

(e)2.5 mA/cm2 (360 s)

5 4 3 2 1 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Voltage (V) Fig. 4. J–V curves of DSSCs based on P3MT counter electrodes deposited with a current density of 1 mA/cm2, 2 mA/cm2 and 2.5 mA/cm2 for 360 and 600 s.

Table 1 Summary of open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF) and solar cell efficiency of the fabricated DSSCs using P3MT and Pt as counter electrodes. Counter electrode

VOC (V)

JSC (mA/cm2)

FF

Eff. (%)

P3MT deposited with 1 mA/cm2 P3MT deposited with 2 mA/cm2 P3MT deposited with 2.5 mA/cm2 Pt

0.63 0.63 0.63 0.7

4.91 8.88 4.79 15.29

0.52 0.50 0.25 0.47

1.25 2.76 0.76 5.06

devices. As demonstrated in Fig. 2, polymerization with various current densities causes various morphologies for the deposited polymer layers. The current–voltage characteristics indicate that the enhanced electrocatalytic activity of the porous P3MT counter electrodes leads to more efficient DSSCs. Therefore, the effect of the applied current density is considerable on the J–V characteristics of the fabricated solar cells. However, the thickness of the polymer layer can also affect the cells performance. Therefore, interpretation of this result requires extra caution. Polymer thickness at each current density can be controlled by duration of polymerization. So, to understand how thickness affects cells performance, the thicknesses of the counter electrodes were changed by varying the polymerization time at each applied current density. The crosssectional SEM images of the P3MT films prepared by an electrochemical deposition method indicated that the thickness varied between 250 nm to 800 nm depending on the deposition conditions. These P3MT layers were used to fabricate solar cells, and an optimum result was obtained for each case by comparison. For example, the current–voltage characteristics of the polymer-based cathode cells that were deposited during two different polymerization times are shown in Fig. 4. Curves (a) and (b) are related to polymerization with a current density of 2 mA/cm2 in 360 and 600 s respectively. Curves (c) and (d) are related to polymerization with a current density of 1 mA/cm2 in 600 and 360 s respectively. Polymerization was done for other different time durations (i.e. 120, 300, and 420 s). By applying a current density of 2 mA/cm2 J–V curves occurred between curves (a) and (b), and, with a current density of 1 mA/cm2, J–V curves occurred between curves (c) and (d). As these results suggest, time duration of polymerization can affect J–V curves; however, variations of current density in polymerization results in

obvious changes in solar cells' performance. Therefore, current density plays a more significant role as a parameter than polymerization time does in determining J–V characteristics. The characteristics of the cells which were fabricated using polymer layers deposited within various polymerization times are summarized in Table 2. It should be noted that the P3MT counter electrodes all show lower conversion efficiencies, as compared to the Pt electrode (Tables 1 and 2), although they show higher redox reactivity. It is known that the Pt electrode itself, at the same time, acts as a light reflector in the cell and, thus, can remarkably improve the efficiency of the solar cell [39]. Another point of consideration is the finding of some earlier studies in that P3MT materials grown at different current densities can have quite different conjugation lengths. This leads to a difference in carrier mobility and electric conductivity when they are in a doped state [24]. Therefore, in addition to surface morphology, these parameters too result in better performance of devices with counter electrodes polymerized at higher current densities. 4. Conclusion We have reported an effective way of producing porous P3MT counter electrodes of dye-sensitized solar cells using an electrochemical deposition method. By controlling the current density during the growth of polymer layers, porous films were produced. The polymerized P3MT layers with different current densities and various porous structures were characterized for their morphological and electrochemical properties. These P3MT layers were used as counter electrodes to fabricate DSSCs. The results suggest that porous P3MT electrodes can effectively enhance the active surface areas and the catalytic activity at the P3MT/electrolyte interface of a DSSC. As a result, the short-circuit current and the power conversion efficiency of DSSCs can be enhanced by using porous P3MT counter electrodes. Acknowledgments The support of the photonics research group of the Physics Department, Yazd University for laboratory is gratefully acknowledged. The

Table 2 Summary of open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF) and solar cell efficiency of the fabricated DSSCs using polymeric cathodes polymerized with various current densities during various times. Counter electrode P3MT deposited with 1 P3MT deposited with 1 P3MT deposited with 2 P3MT deposited with 2

2

mA/cm mA/cm2 mA/cm2 mA/cm2

Duration of polymerization

VOC (V)

JSC (mA/cm2)

FF

Eff. (%)

360 600 360 600

0.63 0.63 0.63 0.63

3.97 4.91 8.88 6.62

0.25 0.52 0.50 0.49

0.64 1.25 2.76 2.06

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authors also would like to thank Prof. B. F. Mirjalili, from the Chemistry Dept. (Polymer group) of Yazd University for interpretation of some results.

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