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Edalati_2017_J._Phys._D__Appl._Phys._50_065501.pdf

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Poly(3,4-ethylendioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was deposited on a fluoride-doped tin oxide glass substrate using a heuristic method to fabricate platinum-free counter electrodes for dye-sensitized solar cells (DSSCs). In this
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  Journal of Physics D: Applied Physics PAPER Heuristic method of fabricating counter electrodesin dye-sensitized solar cells based on aPEDOT:PSS layer as a catalytic material To cite this article: Sh Edalati et al   2017 J. Phys. D: Appl. Phys.   50  065501 View the article online for updates and enhancements. Related content Composite Films Based on Poly(3,4-ethylene dioxythiophene):Poly(styrenesulfonate) Conducting Polymer and TiCNanoparticles as the Counter Electrodesfor Flexible Dye-Sensitized Solar CellsMin-Hsin Yeh, Lu-Yin Lin, Yu-Yan Li et al.-Co-Electrophoretic Deposition MultiwallCarbon Nanotubes/Pt Counter Electrodesfor Dye-Sensitized Solar CellWasan Maiaugree, Samuk Pimanpang,Madsakorn Towannang et al.-Graphene-MoS2 nanosheet compositesas electrodes for dye sensitised solar cellsPeter Lynch, Umar Khan, Andrew Harveyet al.- Recent citations Nanoarchitectures in dye-sensitized solarcells: metal oxides, oxide perovskites andcarbon-based materialsJasmin S. Shaikh et al  -Cross Stacking of Nanopatterned PEDOTFilms for Use as Soft ElectrodesChihyun Park et al  - This content was downloaded from IP address 141.30.193.28 on 16/03/2018 at 15:33  1© 2017 IOP Publishing Ltd Printed in the UK 1. Introduction Dye-sensitized solar cells (DSSCs), introduced by O ’ Regan and Gratzel in 1991 [1], are of increasing importance owing to their low cost, ease of fabrication and potential for large-scale roll-to-roll processing. The function of completing the cycle is fulfilled by counter electrodes via the collection of electrons from external circuits and catalyzing the reduction of redox species [2]. Not only the high conductivity but also good cata-lyst activity with I 3 −  make platinum the best counter electrode; however, this rare expensive noble metal is considered as a serious barrier to progress in this field, attracting attention to Pt-free counter electrodes [2, 3]. Carbon nanomaterials [4, 5], conducting polymers [6, 7], metal oxides [8, 9], metal nitrides [10], metal carbides [11] and transition metal sulfides [12], directly or with suitable promoters, have been investigated as alternatives to Pt.Due to its good conductivity, high flexibility, remark-able stability, low cost as well as catalytic activity for I 3 −  reduction, poly(3,4-ethylendioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been widely used as a cata-lytic counter electrode in solar cells and organic LEDs [13]. Pioneered by Saito et al , PEDOT was chemically polymer-ized on a conductive glass and doped by P -toluenesulfonate (TsO) and PSS, which exhibited solar power efficiencies of 4.60% and 2.10%, respectively, compared with that of Journal of Physics D: Applied Physics Heuristic method of fabricating counter electrodes in dye-sensitized solar cells based on a PEDOT:PSS layer as a catalytic material Sh Edalati 1 , 2 , A Houshangi far 1 , 2 , N Torabi 1 , 2 , Z Baneshi 1 , 2  and A Behjat 1 , 2 1  Photonics Research Group, Engineering Research Centre, Yazd University, Yazd, Iran 2  Atomic and Molecular group, Faculty of Physics, Yazd University, Yazd, IranE-mail: abehjat@yazd.ac.ir (A Behjat)Received 27 June 2016, revised 4 November 2016Accepted for publication 8 December 2016Published 11 January 2017 Abstract Poly(3,4-ethylendioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was deposited on a fluoride-doped tin oxide glass substrate using a heuristic method to fabricate platinum-free counter electrodes for dye-sensitized solar cells (DSSCs). In this heuristic method a thin layer of PEDOT:PPS is obtained by spin coating the PEDOT:PSS on a Cu substrate and then removing the substrate with FeCl 3 . The characteristics of the deposited PEDOT:PSS were studied by energy dispersive x-ray analysis and scanning electron microscopy, which revealed the micro-electronic specifications of the cathode. The aforementioned DSSCs exhibited a solar conversion efficiency of 3.90%, which is far higher than that of DSSCs with pure PEDOT:PSS (1.89%). This enhancement is attributed not only to the micro-electronic specifications but also to the HNO 3  treatment through our heuristic method. The results of cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and Tafel polarization plots show the modified cathode has a dual function, including excellent conductivity and electrocatalytic activity for iodine reduction.Keywords: dye-sensitized solar cells, catalytic activity, platinum-free counter electrode, PEDOT: PSS(Some figures may appear in colour only in the online journal) 1361-6463/17/065501+7$33.00doi:10.1088/1361-6463/aa52a6J. Phys. D: Appl. Phys. 50  (2017) 065501 (7pp)  Sh Edalati et al  2 Pt (4.67%) [13]. The weak performance of PEDOT:PSS is attributed to its low fill factor (FF); that is, PEDOT:PSS has a lower conductivity than PEDOT:TsO. This is because PSS −  might prevent I 3 −  or I −  from contacting the active sites of the PEDOT chain, resulting in a significant decrease in the oxidation current and an increase in the overpotential [14]. The conductivity of PEDOT:PSS has been enhanced by dif-ferent methods [15], such as the use of organic solvents like dimethyl sulfoxide (DMSO) [16] and post-treatment with acid solutions [17, 18]. Furthermore, composite materials made of carbon [19, 20], metals [21, 22], metal oxides [23], metal nitrides [24] and metal sulfides [25] have been incor- porated in PEDOT:PSS to provide support for PEDOT:PSS chains and provide carrier transport sites and catalytic sites. It has been shown that thermal treatment can increase cell performance by increasing the surface area and crystallinity of PEDOT:PSS [26].Yeon et al  [18] have reported that 14 M HNO 3  at room temperature can increase the conductivity of PEDOT:PSS to 3964 (S cm − 1 ), because HNO 3  treatment removes a large amount of PSS and forms broad extended states around the Fermi level, leading to metal-like electrical properties and, subsequently, high conductivity. Moreover, HNO 3  influences the chemical bonding states of PSS and the conjugation length of PEDOT chains. As a result, a power conversion efficiency (PCE) of 8.61% was obtained for HNO 3 -treated PEDOT:PSS. However, the catalytic activity of HNO 3 -treated PEDOT:PSS needs more investigation. The pre-sent study addresses the effects of a heuristic technique of poly mer deposition which leads to treatment by HNO 3  while metallic particles are doped between PEDOT:PSS and FTO surfaces. Our technique prevents metallic particles from being in direct contact with electrolytes and produces a rela-tively high uniformity of particles all over the polymer layer. 2. Material and methods 2.1. Counter electrode preparation Pieces of copper foil (99.7% Cu, ~100 µ m thick, Merck Millipore, Germany) were cleaned in an ultrasonic bath with acetone for about 10 min. They were then kept in acetic acid for 10 min in order to remove any oxide or other metallic sur-face contamination [27]. The surface of the copper samples was then processed further using an electro-polishing tech-nique (5 V for 60 s). This was done according to a procedure in which macroscopic defects are removed and larger-scale surface roughness is reduced. The samples were immediately rinsed with deionized water and dried with Ar or a dry cold air flow to prevent them from oxidizing in air after contact with the acid. A thin layer of PEDOT:PSS (1.3 wt% disper-sion in H 2 O, Aldrich) was deposited on the surface of a Cu substrate by spin coating (1500 rpm for 60 s). The samples were left for about 8 h to dry naturally at room temperature. Then the copper foil was removed using an aqueous solution of 1.85 M FeCl 3  in deionized water as the etchant material. The etching process (i.e. chemical removal of the Cu layer) usually takes about 70 min and leaves the PEDOT:PSS layer floating on the etchant solution. The PEDOT:PSS layer was transferred to an acid dish and doped in 65% HNO 3  (14.4 M) for 10 min. Finally, the PEDOT:PSS layer floating on the acid dish was directly transferred onto an FTO-coated glass (Dyesol  R sq   =  15 Ω   □ – 1 ). The counter electrodes prepared by this method were left for 15 min. The polymer layer was relaxed and dried during this time. The electrodes were fur-ther dried in an oven at 75 ° C for 15 min. Two other counter electrodes were also prepared. PEDOT:PSS was spin-coated on a glass/FTO substrate at 1500 rpm for 60 s, and a layer of Pt paste was coated on the glass/FTO substrate using the doctor Figure 1.  EDX results for (a) pure PEDOT:PSS and (b) modified PEDOT:PSS. SEM images of (c) pure and (d) modified PEDOT:PSS surface of counter electrodes. J. Phys. D: Appl. Phys. 50  ( 2017 ) 065501  Sh Edalati et al  3 blade method. The Pt counter electrode was annealed at 450 ° C for 30 min. 2.2. Dye-sensitized solar cell assembly  The glass/FTO substrates were cleaned in ethanol, acetone and deionized water using an ultrasonic bath for 15 min in a sequence. Photo-anode components were prepared by depos-iting a thin layer of a TiO 2  paste (~20 nm in diameter) on FTO using the doctor blade technique. They were annealed at 120 ° C for 40 min. Then the TiO 2  paste with a particle size of about 400 nm was used as a scattering layer. It was applied on the first layer, followed by annealing at 450 ° C for 30 min. The photo-anode samples fabricated by this method were immersed in a dye solution (N719) for 24 h. The photo-anode and three different types of counter electrode (prepared as explained in section 2.1) were assembled into sandwich-type solar cells and, subsequently, an I −  /  I 3 −  electrolyte (10 mmol l − 1  LiI, 1 mmol l − 1  I 2 , 0.1 mol l − 1  LiClO 4 ) was injected between them. 2.3. Characterization A Keithley 2400 digital source meter accompanied by a solar light simulator (300 W, xenon lamp) calibrated by a reference Si solar cell was used to analyze the current – voltage characteristics of the fabricated DSSCs under AM1.5G (100 mW cm − 2 ) illumination. Using the same setup, Tafel curves were obtained for analyzing the catalytic activity of the counter electrodes. Cyclic voltammetry (CV) analysis was carried out in a three-electrode cell using an Autolab compact instrument (Metrohm Autolab PGSTAT101 series). An aceto-nitrile solution containing 10 mmol l − 1  LiI, 1 mmol l − 1  I 2  and 0.1 mol l − 1  LiClO 4  was used as an electrolyte. A standard Pt electrode was used as the working electrode at first to check for proper calibration of the instrument as well as a suitable dilution rate for the electrolyte, which was determined to be one-tenth of that used in the fabricated solar cells. A standard saturated calomel electrode was used as the reference electrode.Electrochemical impedance spectroscopy (EIS) was per-formed using an Autolab workstation. Data analysis, including fitting the data with equivalent circuits, was conducted using a ZView 3.4 software package. Furthermore, energy-dispersive x-ray (EDX) spectroscopy was performed on selected samples of the counter electrodes with PEDOT:PSS in order to investi-gate the extent of effects during doping and the transfer phases of the heuristic fabrication method. Also, using a spectro meter (Ocean Optics HR4000CG-UV – NIR), optical transmission and absorption spectra were obtained for the counter elec-trodes fabricated by both methods. 3. Results and discussion 3.1. Surface morphology and layer composition EDX spectroscopy was performed on pure PEDOT:PSS and modified PEDOT:PSS (figure 1) to determine whether any components were added to the PEDOT:PSS layer through mod-ification. A relatively high percentage of Cu and Fe particles were exhibited in the modified PEDOT:PSS layer — the metal particles were extracted from the Cu foil and the FeCl 3  solu-tion, and the bonds to the PEDOT:PSS molecules in the lower surface were coated by the PEDOT:PSS layer, protecting the metal particles from corrosion in the presence of the electrolyte. The other materials in the EDX result correspond to glass, the Figure 2.  Elemental mapping results of Fe and Cu particles doped in a modified PEDOT:PSS layer. Table 1.  Parameters of DSSCs fabricated with different counter electrodes.  J  sc  (mA cm − 2 ) V  oc  (V)FFPCE (%)Pure PEDOT:PSS13.4 ±  0.60.60 ±  0.030.23 ±  0.041.8 ±  0.3Modified PEDOT:PSS13.9 ±  0.70.67 ±  0.030.42 ±  0.053.9 ±  0.4Pt (ref.)14.8 ±  0.40.72 ±  0.020.48 ±  0.085.1 ±  0.7 Figure 3.  Current – voltage graphs for the fabricated DSSCs using PEDOT: PSS, modified PEDOT:PSS and Pt as counter electrodes. J. Phys. D: Appl. Phys. 50  ( 2017 ) 065501  Sh Edalati et al  4 FTO layer, carbon paste and the Au layer coated for analysis. The scanning electron microscope (SEM) images in figure 2 provide a deep insight into the homogeneous dispersion of the metallic particles. It can clearly be seen that pure PEDOT:PSS has a smooth surface, while the addition of both metal particles and HNO 3  treatment [18] makes it rough, which can provide either more catalysts or more carrier transport sites. 3.2. Current   – voltage and efficiency specifications The average results obtained from the photocurrent character-ization of DSSCs are summarized in table 1, and the photocur-rent density – voltage (  J  – V  ) diagrams for the fabricated DSSCs are depicted in figure 3. Although the modified PEDOT:PSS did not demonstrate an efficiency as high as that of Pt, the results show a significant improvement compared with the pure PEDOT:PSS.As can be seen, all the photovoltaic parameters were con-siderably improved, and the FF of the modified PEDOT:PSS was very close to that of the reference Pt DSSCs. Generally, the FF is related to series resistance, but it can vary with the catalyst activity when different counter electrodes are inves-tigated [9]. As we will discuss with regard to EIS and CV analysis, the increase in the FF corresponds to increase in the catalyst activity of the counter electrode via the heuristic method whereas the series resistance in the pure and modified PEDOT:PSS DSSCs remains almost constant.Enhancement of V  oc  and  J  sc  is attributed to the electronic specification of the PEDOT:PSS layer. Although V  oc  is defined as the difference between the conductive band of a semicon-ductor and the potential of a redox couple, investigation of different counter electrodes has shown variation in V  oc . Song et al  have shown that V  oc  and  J  sc  are strongly affected by the electronic properties of the counter electrode. This variation in V  oc  is associated with an over-potential needed to drive elec-tron transfer from PEDOT:PPS to tri-iodide, which induces a Figure 4.  Cyclic voltammograms for pure PEDOT:PSS, modified PEDOT:PSS and Pt DSSCs. Figure 5.  Tafel curves of symmetrical cells fabricated using three different types of counter electrodes. Figure 6.  Nyquist plots for the three types of counter electrode. J. Phys. D: Appl. Phys. 50  ( 2017 ) 065501
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