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Influence of electropolymerized polypyrrole optical properties on bifacial Dye-Sensitized Solar Cells
Accepted Manuscript Influence of electropolymerized polypyrrole optical properties on bifacial DyeSensitized Solar Cells Nicola Sangiorgi, Alessandra Sanson PII:
S0032-3861(17)30779-6
DOI:
10.1016/j.polymer.2017.08.014
Reference:
JPOL 19916
To appear in:
Polymer
Received Date: 1 August 2017 Revised Date:
0032-3861 0032-3861
Accepted Date: 7 August 2017
Please cite this article as: Sangiorgi N, Sanson A, Influence of electropolymerized polypyrrole optical properties on bifacial Dye-Sensitized Solar Cells, Polymer (2017), doi: 10.1016/j.polymer.2017.08.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Title Influence of electropolymerized polypyrrole optical properties on bifacial Dye-Sensitized Solar Cells. Author names and affiliations. Nicola Sangiorgia,b*, Alessandra Sansona a
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ISTEC-CNR, Institute of Science and Technology for Ceramics, National Research Council of Italy, Via Granarolo 64, 48018 Faenza, RA, Italy. b
Department of Chemical Science and Technologies, University of Rome “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Rome, Italy.
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*Corresponding author: Nicola Sangiorgi
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CNR-ISTEC, Institute of Science and Technology for Ceramics, National Research Council of Italy, Via Granarolo 64, 48018 Faenza, RA, Italy. e-mail address: [email protected] phone: +39 0546699743 fax: +39 0546 46381
Highlights:
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Influence of anions concentration on the transparency of polypyrrole is studied. Bifacial DSCs based on electrochemically deposited polypyrrole are developed. Influence of the optical interface on the performance of bifacial DSC is studied. Small doping anions produce the highest efficiency ratio on bifacial DSC.
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In this work the effect of different doping anions on transparency, photovoltaic efficiency and interface properties of electropolymerized polypyrrole (PPy) films as counter electrode in bifacial DSSCs was study. The transparency of the counter electrode becomes relevant in order to enhance the final device efficiency and for alternative application area like of Building-Integrated Photovoltaic. PPy was prepared by electrochemical process with doping anions: chloride, perchlorate, sulfate and dodecylbenzenesulfonate. PPy doped with high concentration (0.5 M) of small anions (chloride) produced the highest transparency film (65% at 525 nm), the DSSCs with highest efficiency ratio (close to 70%) and lowest device reflectance (11%). Considering all of these results, not only the transparency of counter electrode was found to influence the efficiency of the bifacial DSSCs, but also the optical properties of PPy/electrolyte interface. This interface is probably strictly affected to the PPy morphology produced by different doping anions and the electropolymerization process used.
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ACCEPTED MANUSCRIPT 1. Introduction
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Recently, much attention has been paid on Dye-Sensitized Solar Cells (DSSCs) as a promising photovoltaic technology formed by eco-friendly components [1-6]. Building-Integrated Photovoltaic (BIPV) in particular, can boost the transition to market for the DSSCs. One of the more desirable characteristics of DSSCs for BIPV in fact is to be bifacial, and therefore convert light coming from both side of the cell, i.e. from outdoor and indoor. For this purpose however, both the photoanode and the counter electrode must be highly transparent [7-8]. This property is also required in order to increase the incident light harvesting and therefore enhance the overall conversion efficiency [9-11]. In fact, if the sunlight irradiation comes simultaneously from the front and the rear side of the cell more dye molecules are excited enhancing the short-circuit current density and the efficiency of the entire cell. Finally, bifacial DSSCs thanks to their advantage of higher light-harvesting and their capabilities of utilizing the incident light from both side, could help to further bring down the cost of energy production [12]. Traditionally, Platinum is used as transparent DSSCs counter electrode but alternative materials must be nowadays considered. Platinum is expensive, scarce in nature and shows stability problem with the iodide/triiodide electrolyte in organic solvent used in the most common DSSC architecture. Moreover is not so effective when other redox couples such cobalt-complex, T2/T- (transparent electrolyte) are considered [13-15]. Literature reports a lot of possible alternative materials, such as carbon materials (carbon black, nanotube, graphene), polymer-carbon composite, transition metal compounds and conducting polymers (polyaniline, polythiophene, polypyrrole) [16-19]. As described before, efficient bifacial DSSCs requires an highest transparent counter electrode. Zhao et al. [20] achieved a conversion efficiency of 5.04% and 6.07% in rear and front side illumination condition respectively using a transparent carbon counter electrode prepared by carbonization. On the other hand, metal selenide alloys of Cobalt, Nickel and Iron were able to increase the final device efficiency of carbon based one [21-23]. Also conducting polymer like poly (3,4ethylenedioxythiophene) (PEDOT) and polyaniline (PANI) and hybrid compounds with graphene were successfully used to obtained an high efficient bifacial DSSCs [9,10,24,25]. PEDOT film in particular was able to produce bifacial DSSCs with efficiency of 6.35% and 4.98% (front and rear side illumination) while comparable results were obtained for PANI counter electrodes (6.54% and 4.26%) [24,9]. Recently, polypyrrole (PPy) has received a lot of attention as a promising alternative to Platinum counter electrode for traditional and bifacial DSSCs due to its high conductivity, facile synthesis, low cost, high catalytic activity for I3- reduction [26-32]. PPy can be easily prepared by chemical oxidation or electrochemical oxidation of pyrrole monomer. The second one is usually preferred due to controllable initiation and termination of polymerization, easy and in situ polymerization at low temperature, controllable film thickness (controlling the amount of charge that flow inside the electrode) and amount of doping anions that is possible to include in the polymeric chain [27,33,34]. Zhang et al. [27] reports a study about the influence of doping anions on the properties of electropolymerized PPy as counter electrode in traditional DSSCs where the film doped with dodecylbenzenesulfonate anions shows the photovoltaic conversion efficiency up to 5.40%. To best of author knowledge, there are no reports regarding the influence of other doping anions on the properties of electropolymerized PPy film used as counter electrode in bifacial DSSCs and how these anions can influence the final efficiency. So, the aim of this work was to investigate the influence of doping anions concentration and chemical properties on transmittance, catalytic activity and photovoltaic efficiency of PPy based DSSCs. In particular, these properties were evaluated in bifacial devices in order to understand which parameters control the working mechanisms of these cells and therefore their efficiencies. Four different doping anions were considered: potassium chloride, lithium perchlorate, sulphuric acid and sodium dodecylbenzenesulfonate.
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ACCEPTED MANUSCRIPT 2. Experimental 2.1 Preparation of electropolymerized PPy and Platinum electrodes
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Electropolymerization of pyrrole (98% Sigma Aldrich) was performed in potentiostatic condition of 1 V vs SCE for different times with an Autolab PGSTAT302N (Metrohm, the Nederland). The Fluorine doped Tin Oxide coated glasses (FTO, sheet resistance 7 Ω/sq. Sigma-Aldrich) were precleaned before deposition using isopropyl alcohol in an ultrasonic bath for 15 minutes. The electropolymerization was carried out in a conventional three-electrodes electrochemical cell with FTO (6.25 cm2 substrate with deposition area of 0.25 cm2) as working electrode, a Platinum plate as auxiliary electrode and SCE (AMEL electrochemistry, Italy) as a reference. The polymerization was performed under stirring condition in an aqueous solution (Milli-Q water, >18 MΩ cm) containing 0.05 M pyrrole with different concentrations of doping salts of potassium chloride (Merck), lithium perchlorate (Sigma Aldrich), sulphuric acid (96% Sigma Aldrich) and sodium dodecylbenzenesulfonate (Sigma Aldrich) respectively. Doping anions concentration were changed between 0.2 M (low concentration) and 0.5 M (high concentration). After polymerization, the obtained PPy films named (Cl-, ClO4-, SO42- and DBS-) were rinsed with Milli-Q water in order to remove the unreacted pyrrole monomer and dried in air for one night. As a reference, a Platinum electrode were also prepared by sputtering process (Q150T S/E/ES Quorum Technologies Ltd., U.K.) and thermal treatment at 450°C for 30 minutes. 2.2. Depositions of TiO2 photoanodes and DSSCs assembly
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TiO2 screen printing inks were deposited onto FTO glass substrates using a semi-automatic screenprinter (AUR’EL 900, AUR’EL Automation s.p.a., Italy), and treated at 450°C for 30 minutes. The thickness of the films was adjusted to reach 8 µm. In the samples used for the final devices the substrates were coated with a TiO2 blocking layer (BL), according to the procedure reported by Sangiorgi et al. [35]. After sintering, the films were immersed in a 50mm TiCl4 aqueous solution at 70°C, and then fired at 450°C for 30 minutes. The as-obtained photoanodes were then dipped for 16 hours in a 0.3 mM ethanolic solution of (cis-diisothiocyanato-bis (2,2’-bipyridyl-4,4’dicarboxylato) ruthenium (II) bis (tetrabutylammonium) (N719 dye, Sigma-Aldrich). A pre-drilled FTO coated glass covered with a sputtered Pt layer or electropolymerized PPy film was used as counter electrode. After the photoanode sensitization the electrodes were assembled into a sandwich type cell and sealed with a hot melt gasket of Meltonix (thickness 25 µm, Solaronix, Switzerland). Electrolyte (Iodolyte Z100, Solaronix, Switzerland), was introduced in the cell via vacuum back filling through the hole in the counter electrode. Finally the hole was sealed using a Meltonix film and a small cover glass. The active area of the solar cells was fixed at 0.25 cm2. 2.3 Films and DSSCs characterizations Transmittance spectra of the PPy films in the UV-Vis range were recorded on PVE300 system (Bentham, United Kingdom). The morphologies of the PPy films were observed by field emission gun–scanning electron microscope (FE – SEM, ΣIGMA, Zeiss, Germany). The catalytic activity of PPy film for the 3I-/I3- redox reaction was evaluated by cyclic voltammetry (CV) in an acetonitrile solution containing 0.010 M LiI (Sigma Aldrich), 0.001 M I2 (Sigma Aldrich) and 0.1 M LiClO4 (Sigma Aldrich). The working electrode was an FTO/PPy or FTO/Pt, the reference was SCE and auxiliary a Platinum plate. The CV analysis were performed in the potential range between ±1 V vs SCE at different scan rate and prior to each analyses the solutions were purged with N2 for 10 minutes. Moreover, the Tafel polarization tests on 3I-/I3- redox reaction were done in the potential range between ±1 at scan rate of 50 mV/s. For this analysis, a symmetric cells with two identical PPy or Platinum electrodes with an active area of 0.25 cm2 were assembled with the same 3
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electrolyte used in the DSSCs. In order to evaluate the DSSCs performance, J-V curves in frontand rear-illumination conditions were done under AM 1.5 simulated illumination with power density of 1000 W m-2 (calibrated with a standard silicon cell). The J-V curves were measured using an Abet Technologies solar simulator (SUN 2000, USA) and a Keithley 2400 source/meter (Keithley, USA). In order to understand the influence of the PPy optical properties on the efficiency ratio of bifacial DSSCs, the reflectance spectra of complete cells were acquired by PVE300 system with integrating sphere. Electrochemical Impedance Spectroscopy (EIS) of Pt and PPy based DSSCs was also performed in the frequency range from 105 to 0.05 Hz, with amplitude of 10 mV, in the dark and open circuit conditions. For the fitting procedure Z-View (Scribner Associates Inc.) software was used. All of these electrochemical characterizations were done by Autolab PGSTAT302N and Nova 1.10 (Eco Chemie, the Nederland) was used as software to analyse the obtained data. The graphical elaborations of the experimental values were made using OriginLab Pro 2015 (USA).
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3. Results and discussion
3.1 Influence of doping anions concentration on morphology and transmittance
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In order to study the influence of the anions on the PPy transparency, it is essential to compare films with the same thickness. During the electropolymerization process, the charge that flow though the FTO electrode was determined and used to determine the amount of PPy deposited on the substrate. The mean film thickness was therefore estimated from the electrical charge associated to the pyrrole oxidation considering the Fadaray’s law, and assuming 100% current efficiency of PPy formation [29]. So, the PPy films doped with different anions were electropolymerized with the same amount of electrical charge flowing on each electrode in order to obtained the same thickness. Even if with comparable thicknesses, each film was produced using electrolyte solution with different conductivity. So, the corresponding charges were found to be 0.008 C/cm2 for Cl- anions, 0.040 C/cm2 for ClO4-, 0.300 C/cm2 for SO42- and finally 0.200 C/cm2 for DBS-. Two different concentrations (0.2 M and 0.5 M) were considered for each anions. The influence of doping anions concentration on the morphology of PPy film is shown in Figure 1. All the films presented a porous structures and the presence of small grains and bigger aggregates.
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Figure 1. FE-SEM micrographs of bare FTO and PPy films prepared with different concentration of doping anions.
Figure 1 shows that higher doping anions concentration produced compact PPy films characterized by globular shape grains. On the other hand, low anions concentration lead to grains with shape and dimensions not comparable. The particles and aggregates are smaller and more homogeneous in the samples with higher amount of anions. It is interesting to note that the morphology of PPy doped with Cl- anions is very similar to the one of bare FTO. Figure 2 shows the transmittance spectra of PPy electrodes prepared with the different concentration of doping anions. These spectra were acquired using bare FTO as a reference. 5
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Figure 2. Transmittance spectra of PPy films doped with 0.2M and 0.5 M anions concentration.
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The spectral region used for transmittance determination was chosen around 525 nm, on the basis of highest absorbance characteristic of the common Ruthenium-based dyes (like N719 and N3) [36]. Figure 2 shows highest transmittance values for the PPy doped with high concentration of doping anions. In particular the main difference in transmittance values were found for ClO4- (T0.5M – T0.2M = 33%) and SO42- (39.5%) in respect to DBS- (10%) and Cl- (2%). These differences can be ascribed mainly to the different morphology produced by the high concentration of doping anions (Figure 1) where more compact and homogeneous films were obtained. Based on these results, the films obtained with 0.5M of anions were considered to determine the influence of the nature of these ions on transmittance, catalytic activity for I3- reduction and photovoltaic properties in bifacial DSSCs based on PPy counter electrode. 3.2 Influence of doping anions on PPy morphology, transmittance and catalytic activity
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A new set of PPy films (with 0.5 M of doping anions) were electropolymerized with the same amount of charge (0.04 C/cm2). The as-obtained PPy films obtained showed the morphologies and transmittance spectra reported in Figures 3 and 4.
Figure 3. FE-SEM micrographs of PPy films prepared with different doping anions.
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Figure 4. Transmittance spectra of PPy films.
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The transmittance increase following the order of ClO4- < SO42- < DBS- < Cl- indicating that the very small doping anions (Cl-) produce the highest transparency films. The ClO4- shows low transmittance probably due to a non-homogeneous morphology (Figure 3) while Cl- and DBSshows compact and homogenous nano-sized particles and aggregates. In order to determine their catalytic properties the different PPy films were analysed using cyclic voltammetry (CV). Figure 5 shows the CV analyses of Platinum electrode (as reference) and PPy films at 20 mV/s.
Figure 5. CV curves of Pt and PPy films doped with different anions.
Each sample is characterized by two pairs of redox peaks: the left one represents the oxidation and reduction of 3I-/I3- while the right one indicates the oxidation and reduction of 3I2/2I3-. The presence of these oxidation and reduction peaks confirms that PPy films can be used as DSSCs counter electrode. For this application, the left pair are much more important because described the real reaction at the counter electrode side, and the correspondent catalytic activity increase in order of Cl- < ClO4- < SO42- < DBS-. Moreover, the figure shows that the 3I-/I3- redox reaction appears for DBS-, Cl- and ClO4- at the same potential of Pt electrode, while for SO42- these potentials are shifted of some hundreds millivolts. The CV results are clearly influenced by the electrode morphology and by the conductivity of PPy films (doping anions influence the electrical conductivity of the polymer). In fact, DBS- and SO42- anions produced PPy porous films able to provide the highest catalytic activity. 7
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In order to evaluate the kinetic of charge transfer at the interface electrode/electrolyte, the Tafel polarization were measured on symmetrical cells. Current exchange density (j0) and charge transfer resistance (RCT) were determined following the equation RCT = R T / F j0 where R is the gas constant, T is the temperature (Kelvin) and F is the Faraday constant [37]. Figure 6 shows the Tafel plot for the different samples while Table 1 reports the extrapolated parameters.
Figure 6. Tafel plot of Pt and PPy electrodes in a symmetrical cells configuration. Table 1. Exchange current density (j0) extrapolated from Figure 6 and calculated RCT.
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sample j0 (mA/cm2) RCT (Ω/cm2) 21.5 1.2 Pt 15.8 1.6 DBS 3.52 7.3 SO421.33 19 ClO40.43 59 Cl-
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The exchange current density (j0) was calculated from the intersection of the linear anodic and cathodic part of each curve. These results confirmed the trend obtained for the CV analyses, with RCT increasing in the order Cl- < ClO4- < SO42- < DBS- indicating the best electrochemical and catalytic properties for the PPy film doped with large dimension anions. These results can be explained by the porous structure and high electrical conductivity produced by the PPy doped with DBS- than the polymeric films doped with the other anions. The value obtained for DBS- are in good agreement with previous report produced by Zhang et al.[27]. 3.3 Photovoltaic performance, electrochemical and optical properties of the final DSSCs The photovoltaic analyses of the different DSSCs with PPy and Platinum films (as reference) counter electrodes were acquired in front and rear illumination conditions. Three different cells for each PPy systems were prepared and tested and the best J-V curves are shown in Figure 7. The derived photovoltaic parameters are summarized in Table 2.
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Figure 7. J-V curves of DSSCs based on Pt and PPy counter electrodes in front- and rear-illumination conditions.
Table 2. Photovoltaic parameters extrapolated from Figure 7.
Rear-illumination Jsc Voc FF η 2 (mA/cm ) (mV) (%) (%) 6.2±0.7 704±4 73±2 3.20±0.51 3.2±0.2 640±3 43±2 0.90±0.06 2.4±0.3 627±3 35±3 0.50±0.06 2.1±0.2 621±2 47±1 0.65±0.05 1.5±0.3 607±3 50±2 0.41±0.02
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Front-illumination Jsc Voc FF sample 2 (mA/cm ) (mV) (%) 9.9±0.8 708±5 71±2 Pt 6.7±0.6 660±2 30±4 Cl7.8±0.6 679±1 31±1 ClO4 8.1±0.4 671±2 38±2 SO426.7±0.5 672±1 53±2 DBS-
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DSSCs based on PPy counter electrode are less efficient than those based on Platinum thanks to the highest catalytic activity of the latter. In front-illumination conditions the overall efficiency (η) of the DSSCs based on PPy increases following the trend Cl-
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Figure 8. Nyquist plots of DSSCs based on different PPy and Pt counter electrodes. Inset shows the equivalent circuit employed to fit the experimental points.
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The fitting curves were obtained applying the traditional diffusion-recombination model used to describe the working mechanism of DSSCs [40]. Electronic parameters that describe each components and interfaces of the dye cell were extrapolated using the equivalent circuit reported in Figure 8, where Rs is the contact resistance which also includes the sheet resistance of the electrodes themselves, RCT1 and CPE1 describe the charge transfer resistance and capacitance of the counter electrode/electrolyte interface respectively and finally RCT2 and CPE2 are related to the charge transfer resistance and electrode capacitance of the photoanode/dye/electrolyte interface [40]. In order to investigate the electrochemical properties of the counter electrode/electrolyte interface, only the RS, RCT1 elements were therefore considered. These extrapolated parameters are reported in Table 3. Table 3. Fitting parameters extrapolated from Figure 8.
RS (Ω) 7 23 19 18 18
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sample Pt ClClO4SO42DBS-
RCT1 (Ω) 4 38 25 23 21
All the experimental data were interpolated using the equivalent circuit described above and was able to fit all the spectra giving χ2 values of the order of 10-4. A low χ2 value indicates a good fitting procedure. The data reported in Table 3 confirm the previous catalytic results. In particular, PPy doped with ClO4- SO42- and DBS- shows low RS values than Cl- sample as an indication of a better films adhesion on the FTO substrate and good electronic conductivities. The RCT1 values follow the same trend obtained for the photovoltaic efficiency in front illumination condition (Table 2). In fact, decreasing the charge transfer resistance at the counter electrode side the final conversion efficiency increases. The lowest value of RCT1 obtained for PPy doped with DBS- is a clear justification of a highest FF value obtained in the final DSSCs. Reducing the charge-transfer resistance for triiodide reduction at the counter electrode leads to the reduction of a series resistance inside the DSSCs increasing the FF and finally the photovoltaic efficiency [26,32]. Figure 9 shows a graphical relationship between PPy film transmittance, charge transfer resistance at the counter electrode/electrolyte interface (RCT1) and efficiency ratio. 10
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Figure 9. Graphical relationship between PPy films transmittance, charge transfer resistance (RCT1) and efficiency ratio.
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These results indicate that it is not only the transparency of the counter electrode to influence the efficiency in bifacial DSSCs because no linear correlation was found between each parameter considered (in particular for SO42- and DBS- films). Moreover, no correlation was found between charge-transfer resistance and efficiency ratio indicating the limited impact of the catalytic properties of PPy on the efficiency ratio of bifacial DSSCs (for example DBS- possesses the lowest RCT1 value but also the lowest efficiency ratio). One possible solution could be linked to the optical properties of the interfaces inside the DSSCs. Therefore, in order to investigate this aspect in more details, the reflectance spectra of these devices were measured [41,42]. Figure 10 and 11 show the reflectance spectra in the UV-Vis range and the graphical correlation between the obtained results.
Figure 10. Reflectance spectra of DSSCs based on different PPy counter electrodes.
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Figure 11. Graphical relationship between PPy films transmittance, efficiency ratio and DSSCs reflectance.
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Figures 9 and 10 show different trend than the other obtained before: when the efficiency ratio is high (Cl-) the DSSCs reflectance is low (around 11%) while the efficiency ratio is low (DBS-) the reflectance is high (15.5%). The same trend was obtained also for the DSSCs based on PPy doped with the other anions (ClO4- and SO42-) as an indication of the correlation between these two properties. These results clearly show that not only the transparency and the catalytic properties of counter electrode influence the final efficiency in bifacial DSSCs but also the optical properties of counter electrode/electrolyte interface. The properties of this interface are affected by the PPy morphologies and therefore by the doping anions that are used during electropolymerization process: small doping anion like Cl- produces suitable morphology able to maximize the efficiency ratio on bifacial DSSCs.
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In summary, in this work the influence of the counter electrode morphology and relative optical properties of the counter electrode/electrolyte interface was evaluated in order to control the efficiency of bifacial DSSCs. Two different concentration of doping anions where considered, with the higher one (0.5 M) producing higher transparent PPy films thanks to the morphologies obtained. Among the four different anions considered, the transmittance was found to increase following the trend ClO4- < SO42- < DBS- < Cl- whereas the catalytic activity and photovoltaic performance increase differently (Cl- < ClO4- < SO42- < DBS-). Cl- anions produce the highest transparent film while the DBS- improved the catalytic properties of PPy film due to a morphology characterized by the presence of small particles on the surface of a porous films. The efficiency of a bifacial DSSCs was found to strongly depend on the optical properties of the interfaces inside the cells and not only by the transmittance of the counter electrode. Therefore it is important to have a deep analysis of all the catalytic, transparency and optical properties of the interfaces in order to obtain a clear indication of the most suitable material for bifacial DSSCs. The work showed that electropolymerization is an easy and low cost procedure that can be used to produce PPy film as possible alternative as Platinum-free counter electrode for DSSCs application. Moreover, of particular importance is the possibility of easily control the synthesis conditions and doping anions used in the electropolymerization process in order to control the morphology of the film and therefore the cell efficiency.
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ACCEPTED MANUSCRIPT Acknowledgments The authors wish to thanks Riccardo Bendoni and Alex Sangiorgi for the TiO2 depositions and for the useful discussions. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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[19] L. Kavan, JJ.-H. Yum, M. Grätzel, Graphene-based cathodes for liquid-junction dye sensitized solar cells: electrocatalytic and mass transport effects, Electrochim. Acta 128 (2014) 349-359. [20] C. Bu, Y. Liu, Z. Yu, S. You, N. Huang, L. Liang, X.-Z. Zhao, Highly transparent carbon counter electrode prepared via an in situ carbonization method for bifacial dye-sensitized solar cells, ACS Appl. Mater. Interfaces 5 (2013) 7432-7438. [21] Y. Duan, Q. Tang, J. Liu, B. He, L. Yu, Transparent metal selenide alloy counter electrodes for high-efficiency bifacial dye-sensitized solar cells, Angew. Chem. Int. Ed. 53 (2014) 14569-14574. [22] J. Liu, Q. Tang, B. He, L. Yu, Cost-effective, transparent iron selenide nanoporous alloy counter electrode for bifacial dye-sensitized solar cell, J. Power Sources 282, 2015, 79-86. [23] Y. Duan, Q. Tang, B. He, R. Li, L. Yu, Transparent nickel selenide alloy counter electrodes for bifacial dye-sensitized solar cells exceeding 10% efficiency, Nanoscale 6 (2014) 12601-12608. [24] Y. Rong, Z. Ku, X. Li, H. Han, Transparent bifacial dye-sensitized solar cells based on an electrochemically polymerized organic counter electrode and an iodine-free polymer gel electrolyte, J. Mater. Sci. 50 (2015) 3803-3811. [25] W. Hong, Y. Xu, G. Lu, C. Li, G. Shi, Transparent graphene/PEDOT-PSS composite films as counter electrodes of dye-sensitized solar cells, Electrochem. Commun. 10 (2008) 1555-1558. [26] C. Bu, Q. Tai, Y. Liu, S. Guo, X. Zhao, A transparent and stable polypyrrole counter electrode for dye-sensitized solar cell, J. Power Source 221 (2013) 78-83. [27] X. Zhang, S. Wang, S. Lu, J. Su, T. He, Influence of doping anions on structure and properties of electro-polymerized polypyrrole counter electrodes for use in dye-sensitized solar cells, J. Power Source 246 (2014) 491-498. [28] J. Wu, Q. Li, L. Fan, Z. Lan, P. Li, J. Lin, S. Hao, High-performance polypyrrole nanoparticles counter electrode for dye-sensitized solar cells, J. Power Source 181 (2008) 172-176. [29] T. Patois, B. Lakard, S. Monney, X. Roizard, P. Fievet, Characterization of the surface properties of polypyrrole films: influence of electrodeposition parameters, Synthetic Metals 161 (2011) 2498-2505. [30] L.Y. Chang, C.-T. Li, Y.-Y. Li, C.-P. Lee, M.-H. Yeh, K.-C. Ho, J.-J. Lin, Morphological influence of polypyrrole nanoparticles on the performance of dye-sensitized solar cells, Electrochim. Acta 155 (2015) 263-271. [31] D.K. Hwang, D. Song, S.S. Jeon, T.H. Han, Y.S. Kang, S.S. Im, Ultrathin polypyrrole nanosheets doped with HCl as counter electrodes in dye-sensitized solar cells, J. Mater. Chem. A 2 (2014) 859-865. [32] S. Lu, S. Wang, R. Han, T. Feng, L. Guo, X. Zhang, D. Liu, T. He, The working mechanism and performance of polypyrrole as a counter electrode for dye-sensitized solar cells, J. Mater. Chem. A 2 (2014) 12805-12811. [33] M. Atobe, H. Tsuji, R. Asami, T. Fuchigami, A study on doping-undoping properties of polypyrrole films electropolymerized under ultrasonication, J. of the Electrochemical Society 153 (2006) D10-D13. [34] S. Sadki, P. Schottland, N. Brodie, G. Sabouraud, The mechanisms of pyrrole electropolymerization, Chem. Soc. Rev. 29 (2000) 283-293. [35] A. Sangiorgi, R. Bendoni, N. Sangiorgi, A. Sanson, B. Ballarin, Optimized TiO2 blocking layer for dye-sensitized solar cells, Ceramics International 40 (2014) 10727-10735. [36] M.K.Nazeeruddin, A. Kay, I. Rodicio, R. H.- Baker, E. Müller, P. Liska, N. Vlachopoulos, M. Grätzel, Conversion of light to electricity by cis-Ruthenium on nanocristalline TiO2 electrodes, J. Am. Chem. Soc. 115 (1993) 6382-6390. [37] A.J.Bard and L.R.Faulkner, Electrochemical Methods Fundamental and Applications (Second Edition), John Wiley & Sons Inc. United States of America, 2001, pp. 92-115. [38] J. Halme, P. Vahermaa, K. Miettunen, P. Lund, Device Physics of Dye Solar Cells, Adv. Mater. 22 (2010) E210-E234.
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[39] H. Li, Y. Xiao, G. Han, M. Li, Honeycomb-like polypyrrole/multi-wall carbon nanotube film as an effective counter electrode in bifacial dye-sensitized solar cells, J. Mater. Sci. 52 (2017) 84218431. [40] J. Bisquert, G. G.-Belmonte, F. F.-Santiago, N. S. Ferriols, P. Bogdanoff, E. C. Pereira, Doubling exponent models for the analysis of porous film electrodes by Impedance. Relaxation of TiO2 nanoporous in aqueous solution, J. Phys.Chem. B 104 (2000) 2287-2298. [41] O. Bouvard, S. Vanzo, A. Schüler, Experimental determination of optical and thermal properties of semi-transparent photovoltaic modules based on dye-sensitized solar cells, Energy Procedia 78 (2015) 453-458. [42] S. Wenger, M. Schmid, G. Rothenberg, A. Gentsch, M. Grätzel, J.O. Schumacher, Coupled optical and electronic modelling of dye-sensitized solar cells for steady-state parameter extraction, J. Phys. Chem. C 115 (2011) 10218-10229.
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S0032-3861(17)30779-6
DOI:
10.1016/j.polymer.2017.08.014
Reference:
JPOL 19916
To appear in:
Polymer
Received Date: 1 August 2017 Revised Date:
0032-3861 0032-3861
Accepted Date: 7 August 2017
Please cite this article as: Sangiorgi N, Sanson A, Influence of electropolymerized polypyrrole optical properties on bifacial Dye-Sensitized Solar Cells, Polymer (2017), doi: 10.1016/j.polymer.2017.08.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Title Influence of electropolymerized polypyrrole optical properties on bifacial Dye-Sensitized Solar Cells. Author names and affiliations. Nicola Sangiorgia,b*, Alessandra Sansona a
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ISTEC-CNR, Institute of Science and Technology for Ceramics, National Research Council of Italy, Via Granarolo 64, 48018 Faenza, RA, Italy. b
Department of Chemical Science and Technologies, University of Rome “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Rome, Italy.
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*Corresponding author: Nicola Sangiorgi
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CNR-ISTEC, Institute of Science and Technology for Ceramics, National Research Council of Italy, Via Granarolo 64, 48018 Faenza, RA, Italy. e-mail address: [email protected] phone: +39 0546699743 fax: +39 0546 46381
Highlights:
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Influence of anions concentration on the transparency of polypyrrole is studied. Bifacial DSCs based on electrochemically deposited polypyrrole are developed. Influence of the optical interface on the performance of bifacial DSC is studied. Small doping anions produce the highest efficiency ratio on bifacial DSC.
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In this work the effect of different doping anions on transparency, photovoltaic efficiency and interface properties of electropolymerized polypyrrole (PPy) films as counter electrode in bifacial DSSCs was study. The transparency of the counter electrode becomes relevant in order to enhance the final device efficiency and for alternative application area like of Building-Integrated Photovoltaic. PPy was prepared by electrochemical process with doping anions: chloride, perchlorate, sulfate and dodecylbenzenesulfonate. PPy doped with high concentration (0.5 M) of small anions (chloride) produced the highest transparency film (65% at 525 nm), the DSSCs with highest efficiency ratio (close to 70%) and lowest device reflectance (11%). Considering all of these results, not only the transparency of counter electrode was found to influence the efficiency of the bifacial DSSCs, but also the optical properties of PPy/electrolyte interface. This interface is probably strictly affected to the PPy morphology produced by different doping anions and the electropolymerization process used.
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Recently, much attention has been paid on Dye-Sensitized Solar Cells (DSSCs) as a promising photovoltaic technology formed by eco-friendly components [1-6]. Building-Integrated Photovoltaic (BIPV) in particular, can boost the transition to market for the DSSCs. One of the more desirable characteristics of DSSCs for BIPV in fact is to be bifacial, and therefore convert light coming from both side of the cell, i.e. from outdoor and indoor. For this purpose however, both the photoanode and the counter electrode must be highly transparent [7-8]. This property is also required in order to increase the incident light harvesting and therefore enhance the overall conversion efficiency [9-11]. In fact, if the sunlight irradiation comes simultaneously from the front and the rear side of the cell more dye molecules are excited enhancing the short-circuit current density and the efficiency of the entire cell. Finally, bifacial DSSCs thanks to their advantage of higher light-harvesting and their capabilities of utilizing the incident light from both side, could help to further bring down the cost of energy production [12]. Traditionally, Platinum is used as transparent DSSCs counter electrode but alternative materials must be nowadays considered. Platinum is expensive, scarce in nature and shows stability problem with the iodide/triiodide electrolyte in organic solvent used in the most common DSSC architecture. Moreover is not so effective when other redox couples such cobalt-complex, T2/T- (transparent electrolyte) are considered [13-15]. Literature reports a lot of possible alternative materials, such as carbon materials (carbon black, nanotube, graphene), polymer-carbon composite, transition metal compounds and conducting polymers (polyaniline, polythiophene, polypyrrole) [16-19]. As described before, efficient bifacial DSSCs requires an highest transparent counter electrode. Zhao et al. [20] achieved a conversion efficiency of 5.04% and 6.07% in rear and front side illumination condition respectively using a transparent carbon counter electrode prepared by carbonization. On the other hand, metal selenide alloys of Cobalt, Nickel and Iron were able to increase the final device efficiency of carbon based one [21-23]. Also conducting polymer like poly (3,4ethylenedioxythiophene) (PEDOT) and polyaniline (PANI) and hybrid compounds with graphene were successfully used to obtained an high efficient bifacial DSSCs [9,10,24,25]. PEDOT film in particular was able to produce bifacial DSSCs with efficiency of 6.35% and 4.98% (front and rear side illumination) while comparable results were obtained for PANI counter electrodes (6.54% and 4.26%) [24,9]. Recently, polypyrrole (PPy) has received a lot of attention as a promising alternative to Platinum counter electrode for traditional and bifacial DSSCs due to its high conductivity, facile synthesis, low cost, high catalytic activity for I3- reduction [26-32]. PPy can be easily prepared by chemical oxidation or electrochemical oxidation of pyrrole monomer. The second one is usually preferred due to controllable initiation and termination of polymerization, easy and in situ polymerization at low temperature, controllable film thickness (controlling the amount of charge that flow inside the electrode) and amount of doping anions that is possible to include in the polymeric chain [27,33,34]. Zhang et al. [27] reports a study about the influence of doping anions on the properties of electropolymerized PPy as counter electrode in traditional DSSCs where the film doped with dodecylbenzenesulfonate anions shows the photovoltaic conversion efficiency up to 5.40%. To best of author knowledge, there are no reports regarding the influence of other doping anions on the properties of electropolymerized PPy film used as counter electrode in bifacial DSSCs and how these anions can influence the final efficiency. So, the aim of this work was to investigate the influence of doping anions concentration and chemical properties on transmittance, catalytic activity and photovoltaic efficiency of PPy based DSSCs. In particular, these properties were evaluated in bifacial devices in order to understand which parameters control the working mechanisms of these cells and therefore their efficiencies. Four different doping anions were considered: potassium chloride, lithium perchlorate, sulphuric acid and sodium dodecylbenzenesulfonate.
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Electropolymerization of pyrrole (98% Sigma Aldrich) was performed in potentiostatic condition of 1 V vs SCE for different times with an Autolab PGSTAT302N (Metrohm, the Nederland). The Fluorine doped Tin Oxide coated glasses (FTO, sheet resistance 7 Ω/sq. Sigma-Aldrich) were precleaned before deposition using isopropyl alcohol in an ultrasonic bath for 15 minutes. The electropolymerization was carried out in a conventional three-electrodes electrochemical cell with FTO (6.25 cm2 substrate with deposition area of 0.25 cm2) as working electrode, a Platinum plate as auxiliary electrode and SCE (AMEL electrochemistry, Italy) as a reference. The polymerization was performed under stirring condition in an aqueous solution (Milli-Q water, >18 MΩ cm) containing 0.05 M pyrrole with different concentrations of doping salts of potassium chloride (Merck), lithium perchlorate (Sigma Aldrich), sulphuric acid (96% Sigma Aldrich) and sodium dodecylbenzenesulfonate (Sigma Aldrich) respectively. Doping anions concentration were changed between 0.2 M (low concentration) and 0.5 M (high concentration). After polymerization, the obtained PPy films named (Cl-, ClO4-, SO42- and DBS-) were rinsed with Milli-Q water in order to remove the unreacted pyrrole monomer and dried in air for one night. As a reference, a Platinum electrode were also prepared by sputtering process (Q150T S/E/ES Quorum Technologies Ltd., U.K.) and thermal treatment at 450°C for 30 minutes. 2.2. Depositions of TiO2 photoanodes and DSSCs assembly
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TiO2 screen printing inks were deposited onto FTO glass substrates using a semi-automatic screenprinter (AUR’EL 900, AUR’EL Automation s.p.a., Italy), and treated at 450°C for 30 minutes. The thickness of the films was adjusted to reach 8 µm. In the samples used for the final devices the substrates were coated with a TiO2 blocking layer (BL), according to the procedure reported by Sangiorgi et al. [35]. After sintering, the films were immersed in a 50mm TiCl4 aqueous solution at 70°C, and then fired at 450°C for 30 minutes. The as-obtained photoanodes were then dipped for 16 hours in a 0.3 mM ethanolic solution of (cis-diisothiocyanato-bis (2,2’-bipyridyl-4,4’dicarboxylato) ruthenium (II) bis (tetrabutylammonium) (N719 dye, Sigma-Aldrich). A pre-drilled FTO coated glass covered with a sputtered Pt layer or electropolymerized PPy film was used as counter electrode. After the photoanode sensitization the electrodes were assembled into a sandwich type cell and sealed with a hot melt gasket of Meltonix (thickness 25 µm, Solaronix, Switzerland). Electrolyte (Iodolyte Z100, Solaronix, Switzerland), was introduced in the cell via vacuum back filling through the hole in the counter electrode. Finally the hole was sealed using a Meltonix film and a small cover glass. The active area of the solar cells was fixed at 0.25 cm2. 2.3 Films and DSSCs characterizations Transmittance spectra of the PPy films in the UV-Vis range were recorded on PVE300 system (Bentham, United Kingdom). The morphologies of the PPy films were observed by field emission gun–scanning electron microscope (FE – SEM, ΣIGMA, Zeiss, Germany). The catalytic activity of PPy film for the 3I-/I3- redox reaction was evaluated by cyclic voltammetry (CV) in an acetonitrile solution containing 0.010 M LiI (Sigma Aldrich), 0.001 M I2 (Sigma Aldrich) and 0.1 M LiClO4 (Sigma Aldrich). The working electrode was an FTO/PPy or FTO/Pt, the reference was SCE and auxiliary a Platinum plate. The CV analysis were performed in the potential range between ±1 V vs SCE at different scan rate and prior to each analyses the solutions were purged with N2 for 10 minutes. Moreover, the Tafel polarization tests on 3I-/I3- redox reaction were done in the potential range between ±1 at scan rate of 50 mV/s. For this analysis, a symmetric cells with two identical PPy or Platinum electrodes with an active area of 0.25 cm2 were assembled with the same 3
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electrolyte used in the DSSCs. In order to evaluate the DSSCs performance, J-V curves in frontand rear-illumination conditions were done under AM 1.5 simulated illumination with power density of 1000 W m-2 (calibrated with a standard silicon cell). The J-V curves were measured using an Abet Technologies solar simulator (SUN 2000, USA) and a Keithley 2400 source/meter (Keithley, USA). In order to understand the influence of the PPy optical properties on the efficiency ratio of bifacial DSSCs, the reflectance spectra of complete cells were acquired by PVE300 system with integrating sphere. Electrochemical Impedance Spectroscopy (EIS) of Pt and PPy based DSSCs was also performed in the frequency range from 105 to 0.05 Hz, with amplitude of 10 mV, in the dark and open circuit conditions. For the fitting procedure Z-View (Scribner Associates Inc.) software was used. All of these electrochemical characterizations were done by Autolab PGSTAT302N and Nova 1.10 (Eco Chemie, the Nederland) was used as software to analyse the obtained data. The graphical elaborations of the experimental values were made using OriginLab Pro 2015 (USA).
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3. Results and discussion
3.1 Influence of doping anions concentration on morphology and transmittance
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In order to study the influence of the anions on the PPy transparency, it is essential to compare films with the same thickness. During the electropolymerization process, the charge that flow though the FTO electrode was determined and used to determine the amount of PPy deposited on the substrate. The mean film thickness was therefore estimated from the electrical charge associated to the pyrrole oxidation considering the Fadaray’s law, and assuming 100% current efficiency of PPy formation [29]. So, the PPy films doped with different anions were electropolymerized with the same amount of electrical charge flowing on each electrode in order to obtained the same thickness. Even if with comparable thicknesses, each film was produced using electrolyte solution with different conductivity. So, the corresponding charges were found to be 0.008 C/cm2 for Cl- anions, 0.040 C/cm2 for ClO4-, 0.300 C/cm2 for SO42- and finally 0.200 C/cm2 for DBS-. Two different concentrations (0.2 M and 0.5 M) were considered for each anions. The influence of doping anions concentration on the morphology of PPy film is shown in Figure 1. All the films presented a porous structures and the presence of small grains and bigger aggregates.
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Figure 1. FE-SEM micrographs of bare FTO and PPy films prepared with different concentration of doping anions.
Figure 1 shows that higher doping anions concentration produced compact PPy films characterized by globular shape grains. On the other hand, low anions concentration lead to grains with shape and dimensions not comparable. The particles and aggregates are smaller and more homogeneous in the samples with higher amount of anions. It is interesting to note that the morphology of PPy doped with Cl- anions is very similar to the one of bare FTO. Figure 2 shows the transmittance spectra of PPy electrodes prepared with the different concentration of doping anions. These spectra were acquired using bare FTO as a reference. 5
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Figure 2. Transmittance spectra of PPy films doped with 0.2M and 0.5 M anions concentration.
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The spectral region used for transmittance determination was chosen around 525 nm, on the basis of highest absorbance characteristic of the common Ruthenium-based dyes (like N719 and N3) [36]. Figure 2 shows highest transmittance values for the PPy doped with high concentration of doping anions. In particular the main difference in transmittance values were found for ClO4- (T0.5M – T0.2M = 33%) and SO42- (39.5%) in respect to DBS- (10%) and Cl- (2%). These differences can be ascribed mainly to the different morphology produced by the high concentration of doping anions (Figure 1) where more compact and homogeneous films were obtained. Based on these results, the films obtained with 0.5M of anions were considered to determine the influence of the nature of these ions on transmittance, catalytic activity for I3- reduction and photovoltaic properties in bifacial DSSCs based on PPy counter electrode. 3.2 Influence of doping anions on PPy morphology, transmittance and catalytic activity
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A new set of PPy films (with 0.5 M of doping anions) were electropolymerized with the same amount of charge (0.04 C/cm2). The as-obtained PPy films obtained showed the morphologies and transmittance spectra reported in Figures 3 and 4.
Figure 3. FE-SEM micrographs of PPy films prepared with different doping anions.
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Figure 4. Transmittance spectra of PPy films.
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The transmittance increase following the order of ClO4- < SO42- < DBS- < Cl- indicating that the very small doping anions (Cl-) produce the highest transparency films. The ClO4- shows low transmittance probably due to a non-homogeneous morphology (Figure 3) while Cl- and DBSshows compact and homogenous nano-sized particles and aggregates. In order to determine their catalytic properties the different PPy films were analysed using cyclic voltammetry (CV). Figure 5 shows the CV analyses of Platinum electrode (as reference) and PPy films at 20 mV/s.
Figure 5. CV curves of Pt and PPy films doped with different anions.
Each sample is characterized by two pairs of redox peaks: the left one represents the oxidation and reduction of 3I-/I3- while the right one indicates the oxidation and reduction of 3I2/2I3-. The presence of these oxidation and reduction peaks confirms that PPy films can be used as DSSCs counter electrode. For this application, the left pair are much more important because described the real reaction at the counter electrode side, and the correspondent catalytic activity increase in order of Cl- < ClO4- < SO42- < DBS-. Moreover, the figure shows that the 3I-/I3- redox reaction appears for DBS-, Cl- and ClO4- at the same potential of Pt electrode, while for SO42- these potentials are shifted of some hundreds millivolts. The CV results are clearly influenced by the electrode morphology and by the conductivity of PPy films (doping anions influence the electrical conductivity of the polymer). In fact, DBS- and SO42- anions produced PPy porous films able to provide the highest catalytic activity. 7
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In order to evaluate the kinetic of charge transfer at the interface electrode/electrolyte, the Tafel polarization were measured on symmetrical cells. Current exchange density (j0) and charge transfer resistance (RCT) were determined following the equation RCT = R T / F j0 where R is the gas constant, T is the temperature (Kelvin) and F is the Faraday constant [37]. Figure 6 shows the Tafel plot for the different samples while Table 1 reports the extrapolated parameters.
Figure 6. Tafel plot of Pt and PPy electrodes in a symmetrical cells configuration. Table 1. Exchange current density (j0) extrapolated from Figure 6 and calculated RCT.
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sample j0 (mA/cm2) RCT (Ω/cm2) 21.5 1.2 Pt 15.8 1.6 DBS 3.52 7.3 SO421.33 19 ClO40.43 59 Cl-
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The exchange current density (j0) was calculated from the intersection of the linear anodic and cathodic part of each curve. These results confirmed the trend obtained for the CV analyses, with RCT increasing in the order Cl- < ClO4- < SO42- < DBS- indicating the best electrochemical and catalytic properties for the PPy film doped with large dimension anions. These results can be explained by the porous structure and high electrical conductivity produced by the PPy doped with DBS- than the polymeric films doped with the other anions. The value obtained for DBS- are in good agreement with previous report produced by Zhang et al.[27]. 3.3 Photovoltaic performance, electrochemical and optical properties of the final DSSCs The photovoltaic analyses of the different DSSCs with PPy and Platinum films (as reference) counter electrodes were acquired in front and rear illumination conditions. Three different cells for each PPy systems were prepared and tested and the best J-V curves are shown in Figure 7. The derived photovoltaic parameters are summarized in Table 2.
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Figure 7. J-V curves of DSSCs based on Pt and PPy counter electrodes in front- and rear-illumination conditions.
Table 2. Photovoltaic parameters extrapolated from Figure 7.
Rear-illumination Jsc Voc FF η 2 (mA/cm ) (mV) (%) (%) 6.2±0.7 704±4 73±2 3.20±0.51 3.2±0.2 640±3 43±2 0.90±0.06 2.4±0.3 627±3 35±3 0.50±0.06 2.1±0.2 621±2 47±1 0.65±0.05 1.5±0.3 607±3 50±2 0.41±0.02
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Front-illumination Jsc Voc FF sample 2 (mA/cm ) (mV) (%) 9.9±0.8 708±5 71±2 Pt 6.7±0.6 660±2 30±4 Cl7.8±0.6 679±1 31±1 ClO4 8.1±0.4 671±2 38±2 SO426.7±0.5 672±1 53±2 DBS-
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DSSCs based on PPy counter electrode are less efficient than those based on Platinum thanks to the highest catalytic activity of the latter. In front-illumination conditions the overall efficiency (η) of the DSSCs based on PPy increases following the trend Cl-
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Figure 8. Nyquist plots of DSSCs based on different PPy and Pt counter electrodes. Inset shows the equivalent circuit employed to fit the experimental points.
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The fitting curves were obtained applying the traditional diffusion-recombination model used to describe the working mechanism of DSSCs [40]. Electronic parameters that describe each components and interfaces of the dye cell were extrapolated using the equivalent circuit reported in Figure 8, where Rs is the contact resistance which also includes the sheet resistance of the electrodes themselves, RCT1 and CPE1 describe the charge transfer resistance and capacitance of the counter electrode/electrolyte interface respectively and finally RCT2 and CPE2 are related to the charge transfer resistance and electrode capacitance of the photoanode/dye/electrolyte interface [40]. In order to investigate the electrochemical properties of the counter electrode/electrolyte interface, only the RS, RCT1 elements were therefore considered. These extrapolated parameters are reported in Table 3. Table 3. Fitting parameters extrapolated from Figure 8.
RS (Ω) 7 23 19 18 18
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sample Pt ClClO4SO42DBS-
RCT1 (Ω) 4 38 25 23 21
All the experimental data were interpolated using the equivalent circuit described above and was able to fit all the spectra giving χ2 values of the order of 10-4. A low χ2 value indicates a good fitting procedure. The data reported in Table 3 confirm the previous catalytic results. In particular, PPy doped with ClO4- SO42- and DBS- shows low RS values than Cl- sample as an indication of a better films adhesion on the FTO substrate and good electronic conductivities. The RCT1 values follow the same trend obtained for the photovoltaic efficiency in front illumination condition (Table 2). In fact, decreasing the charge transfer resistance at the counter electrode side the final conversion efficiency increases. The lowest value of RCT1 obtained for PPy doped with DBS- is a clear justification of a highest FF value obtained in the final DSSCs. Reducing the charge-transfer resistance for triiodide reduction at the counter electrode leads to the reduction of a series resistance inside the DSSCs increasing the FF and finally the photovoltaic efficiency [26,32]. Figure 9 shows a graphical relationship between PPy film transmittance, charge transfer resistance at the counter electrode/electrolyte interface (RCT1) and efficiency ratio. 10
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Figure 9. Graphical relationship between PPy films transmittance, charge transfer resistance (RCT1) and efficiency ratio.
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These results indicate that it is not only the transparency of the counter electrode to influence the efficiency in bifacial DSSCs because no linear correlation was found between each parameter considered (in particular for SO42- and DBS- films). Moreover, no correlation was found between charge-transfer resistance and efficiency ratio indicating the limited impact of the catalytic properties of PPy on the efficiency ratio of bifacial DSSCs (for example DBS- possesses the lowest RCT1 value but also the lowest efficiency ratio). One possible solution could be linked to the optical properties of the interfaces inside the DSSCs. Therefore, in order to investigate this aspect in more details, the reflectance spectra of these devices were measured [41,42]. Figure 10 and 11 show the reflectance spectra in the UV-Vis range and the graphical correlation between the obtained results.
Figure 10. Reflectance spectra of DSSCs based on different PPy counter electrodes.
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Figure 11. Graphical relationship between PPy films transmittance, efficiency ratio and DSSCs reflectance.
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Figures 9 and 10 show different trend than the other obtained before: when the efficiency ratio is high (Cl-) the DSSCs reflectance is low (around 11%) while the efficiency ratio is low (DBS-) the reflectance is high (15.5%). The same trend was obtained also for the DSSCs based on PPy doped with the other anions (ClO4- and SO42-) as an indication of the correlation between these two properties. These results clearly show that not only the transparency and the catalytic properties of counter electrode influence the final efficiency in bifacial DSSCs but also the optical properties of counter electrode/electrolyte interface. The properties of this interface are affected by the PPy morphologies and therefore by the doping anions that are used during electropolymerization process: small doping anion like Cl- produces suitable morphology able to maximize the efficiency ratio on bifacial DSSCs.
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In summary, in this work the influence of the counter electrode morphology and relative optical properties of the counter electrode/electrolyte interface was evaluated in order to control the efficiency of bifacial DSSCs. Two different concentration of doping anions where considered, with the higher one (0.5 M) producing higher transparent PPy films thanks to the morphologies obtained. Among the four different anions considered, the transmittance was found to increase following the trend ClO4- < SO42- < DBS- < Cl- whereas the catalytic activity and photovoltaic performance increase differently (Cl- < ClO4- < SO42- < DBS-). Cl- anions produce the highest transparent film while the DBS- improved the catalytic properties of PPy film due to a morphology characterized by the presence of small particles on the surface of a porous films. The efficiency of a bifacial DSSCs was found to strongly depend on the optical properties of the interfaces inside the cells and not only by the transmittance of the counter electrode. Therefore it is important to have a deep analysis of all the catalytic, transparency and optical properties of the interfaces in order to obtain a clear indication of the most suitable material for bifacial DSSCs. The work showed that electropolymerization is an easy and low cost procedure that can be used to produce PPy film as possible alternative as Platinum-free counter electrode for DSSCs application. Moreover, of particular importance is the possibility of easily control the synthesis conditions and doping anions used in the electropolymerization process in order to control the morphology of the film and therefore the cell efficiency.
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ACCEPTED MANUSCRIPT Acknowledgments The authors wish to thanks Riccardo Bendoni and Alex Sangiorgi for the TiO2 depositions and for the useful discussions. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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