A Novel Poly ( 3 , 4-ethylenedioxythiophene )-graphene Oxide / Titanium Dioxide Composites Counter Electrode for Dye-Sensitized Solar Cell

1Department of Chemistry, Faculty of Science, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia 2Department of Electrical and Electronics Engineering, Faculty of Engineering, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia 3Functional Device Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia 4Materials Synthesis and Characterization Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia


Introduction
Since the most recent decades, dye-sensitized solar cells (DSSCs) have received great interest, as a substitute toward renewable energies due to the severe energy crisis [1].A typical DSSC counter electrode (CE) which is platinum (Pt) metal is an expensive metal and thus limits its practical application [2].Thus, it is critical to find new materials, which are well-appointed for replacing Pt that possesses good electroconductivity and electrocatalytic activity (ECA) toward the reduction of tri-iodide ions and cost-effective [1,3].Recently, researchers are interested in the utilization of Pt-free materials such as conducting polymers (CPs) [4], carbon materials [5], and transition metals based inorganic materials [6] as new CE materials in DSSCs due to their remarkable properties that can enhance the efficiency of the solar cell [7].Poly(3,4-ethylenedioxythiophene) (PEDOT) is a conducting polymer suitable to be utilized as a CE due to its good stability, high electrochemical activity, and excellent transparency [8][9][10].Reference [11] reported that PEDOT CE displayed the highest power conversion efficiency (PCE) of 1.35% compared to polypyrrole CE (0.41%) and polythiophene CE (0.49%) due to the fact that PEDOT CE has high ECA and low charge transfer resistance ( ct ).In addition, the high surface roughness of PEDOT observed from the FESEM images compared to other polymers also contributed to higher ECA and hence increases the PCE of PEDOT CE [11].

Fabrication of TiO 2 Thin Film
Photoanode. 2 g of commercial TiO 2 powder (Degussa P25) was mixed with 8 ml of ethanol.The mixture was then stirred until all the TiO 2 powder was dissolved.0.16 ml TTIP was added to intensify the viscosity of the TiO 2 paste.The mixture was stirred for 30 minutes and proceeded by sonication in an ultrasonic bath for 30 minutes to remove any impurity present.The prepared pastes were coated onto a conductive ITO glass using doctor blade technique.The film was then heated for 2 hours at 100 ∘ C using a hot plate.The as-prepared TiO 2 thin film was then cooled.After the cooling process, the photoanode was immersed in the 0.2 mM dye solution of N719 dissolved in the equivalent ratio of acetonitrile: tert-butyl alcohol for 24 hours in order for the dye molecules to be fully absorbed.The active area was 0.25 cm 2 .

Fabrication of Platinum, GO, PEDOT, PEDOT-GO, and
PEDOT-GO/TiO 2 Counter Electrode.Pt electrode was fabricated by spin coated 50 L of 0.2 mM chloroplatinic acid hexahydrate on ITO at 1500 rpm for 15 s and was baked at 400 ∘ C for 4 hours.The GO CE was prepared by depositing 1 mL of GO (1 mg/mL) on ITO by drop casting whereas PEDOT and PEDOT-GO CEs were prepared potentiostatically at 1.2 V for 100 s with a three-electrode system.The electropolymerization of PEDOT/GO was conducted in an aqueous solution containing 10 mM EDOT and 1 mg/mL GO.For comparison, PEDOT CE was also prepared in 0.1 M LiClO 4 without the presence of GO.The silver/silver chloride (Ag/AgCl), TiO 2 coated ITO, and Pt wire were used as a reference, working, and CEs, respectively.

Characterization Techniques.
The electrodeposited CEs were analyzed using Shimadzu XRD Diffractometer with Cu K radiation ( = 1.54 Å) to obtain the crystalline phase of the materials.The scan range was set from 8 ∘ to 60 ∘ .Each diffractogram was compared to the reference pattern from the Joint Committee on Powder Diffraction Standards (JCPDS).Perkin-Elmer FTIR spectrophotometer coupled with attenuated total reflectance (UATR) accessory was used to identify the functional groups.The surface morphology of the CEs was evaluated using field emission scanning electron microscopy (FESEM, JEOL JSM-7600F).Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to study the ECA and  ct of the CEs toward the reduction of tri-iodide to iodide.CV measurements were performed using a three-electrode system in a solution containing 1 mM tri-iodide and 0.1 M LiClO 4 in acetonitrile.The reference and CE were Ag/Ag + and platinum wire, respectively.The potential range applied was −1.0 V to 2.0 V with a scan rate of 0.05 Vs −1 .EIS measurements were performed in a dark condition using a two-electrode system at open circuit potential (OCP) with AC amplitude of 0.8 V between the frequency range of 100 kHz and 1 Hz in the presence of Iodolyte Z-100.Tafel polarization measurements were conducted at a scan rate of 10 mVs −1 .A symmetric cell containing two identical CEs and an electrolyte Iodolyte Z-100 was used.All electrochemical measurements were carried out using a potentiostat (Autolab PGSTAT204) equipped with NOVA software.Current-voltage measurements were performed using SCS 4200, Keithley Instruments, USA, under 1.5 A.M. (100 mWcm −2 ) irradiation.

Results and Discussion
3.1.Fourier Transformed Infrared.Figure 1 shows the FTIR spectra of TiO 2 , GO, PEDOT-GO, and PEDOT-GO/TiO 2 CEs.The TiO 2 spectrum shows a band at 545 cm −1 which corresponds to the Ti-O-Ti stretching [21].The peak centered at 936 cm −1 belongs to O-Ti-O bonding in anatase TiO 2 [22].The GO spectrum displays a broad O-H absorption band centered at 3393 cm −1 [23].The peaks at 1717, 1424, 1169, and 1063 cm −1 are attributed to the C=O carboxylic group, C-OH deformation, C-O-C, and C-O epoxy, respectively [24].The FTIR spectrum of PEDOT shows main characteristic 4000 3500 3000 2500 2000 1500 1000 500 bands at 1510, 1310, and 1155 cm −1 which are attributed to the asymmetric stretching mode of C=C, C-C and the stretching mode of C-O-C, respectively [25].The C-S bond stretching vibrations in the thiophene ring can be observed at 762 cm −1 [26].The FTIR spectrum of PEDOT-GO shows a combination of GO and PEDOT spectra.All the peaks are seen in PEDOT and GO spectra also present in PEDOT-GO/TiO 2 ; however, the peaks of TiO 2 (Ti-O-Ti and O-Ti-O bending at 545 and 936 cm −1 , resp.) are overlapped with the peak of PEDOT (C-S vibration at 634 cm −1 ) and GO (C-O epoxy at 838 cm −1 ).

Morphological Study.
The morphology of PEDOT, GO, TiO 2 , PEDOT-GO, and PEDOT-GO/TiO 2 CEs were examined using FESEM.Figure 2(a) shows the FESEM images of electropolymerized PEDOT on ITO glass substrate.The electropolymerized PEDOT shows similar morphology as reported by [27] which is a rough and dense granular shaped morphology was observed.The FESEM image of PEDOT/GO (Figure 2(c)) reveals a uniform wrinkled surface in which resembles paper-like sheet, contributing to an increase in the surface area [28].This FESEM image is similar to the morphology of GO (Figure 2 3.3.XRD Analysis.XRD analysis was performed to examine the crystal structure of GO, TiO 2 , PEDOT, PEDOT-GO, and PEDOT-GO/TiO 2 and the XRD patterns are shown in Figure 3.The characteristic peaks at 25.4 ∘ (101), 37.9 ∘ (004), 48.0 ∘ (200), 55.0 ∘ (211), 64.5 ∘ (204), and 77.7 ∘ (215) are associated with the crystal planes of anatase TiO 2 (JCPDS 01-073-1764) [30].The (001) peak of GO at 10.6 ∘ is originated from the stacked graphene sheets in the GO [31].All conducting polymers show a characteristic peak assigned to the interchain planar ring stacking at approximately 2∼26 ∘ [32][33][34].In this work, PEDOT also shows a peak at 25.2 ∘ indexed by (107) which is due to interchange planar ring stacking (JCPDS 00-048-1449).PEDOT-GO composite possesses peaks at 10.6 ∘ and 24.8 ∘ , indicating that PEDOT-GO was successfully deposited on ITO glass whereas PEDOT-GO/TiO 2 which possesses all the peak belongs to anatase TiO 2 , stacked graphene sheets of GO, and interchain planar ring stacking of carbon in PEDOT that indicates that all three materials were present in the PEDOT-GO/TiO 2 composite.

Cyclic Voltammetry.
In order to study the ECA properties, CV measurements were carried out and the cyclic voltammograms of platinum, TiO 2 , PEDOT, PEDOT-GO, and PEDOT-GO/TiO 2 CEs are shown in Figure 4.A pair of redox peaks was noticed for platinum, PEDOT-GO, and PEDOT-GO/TiO 2 films.The peak in the range 0.75-1.5 V indicates the oxidation of iodide to tri-iodide (see (1)) and the peak in the range of −0.25 to 0.5 V indicates the reduction of tri-iodide to iodide (see (2)).
The current densities of two pairs of oxidation and reduction peaks for PEDOT-GO/TiO 2 are higher than PEDOT-GO and platinum, demonstrating excellent electrocatalytic behavior [35].The reaction rate of the catalyst for the reduction of I − 3 ions in the electrolyte can be influenced by the cathodic peak current density  CP in a CV curve.An enhanced ECA for the catalytic material can be obtained by a higher  CP absolute value.The  CP value of PEDOT-GO/TiO 2 (−5.49mA⋅cm −2 ) is higher than the platinum (−3.57mA⋅cm −2 ) and PEDOT-GO (−3.54 mA⋅cm −2 ), suggesting that the ECA toward the I − 3 /I − redox reaction is greater for PEDOT-GO/TiO 2 compared with PEDOT-GO/TiO 2 and PEDOT-GO.These results are in agreement with  ct obtained from EIS measurements and DSSC with higher efficiency.

Electrochemical Impedance Spectroscopy.
In the DSSCs, the electrolyte of I − 3 /I − plays a vital role to preserve the regeneration cycle of the electron.The TiO 2 nanoparticles absorb dye molecules and the photon from the sunlight excites the electron of the dye molecules to the CE through an oxidation process.This oxidation process of dye produces a hole in TiO 2 ; hence, the electrolyte must provide electrons to be transferred back to occupy the holes of TiO 2 , which allows the electron of the dye molecule to be generated [36].Thus, the I − 3 ions should continuously undergo reduction at the CE, as shown in reaction (2).A symmetric cell composed of two identical electrodes was used to perform the EIS analysis in order to study the ohmic series resistance (  ) of the substrate and its catalytic layer and the charge transfer resistance ( ct ) for the reduction of I − 3 at the electrode/electrolyte interface.Figure 5 shows the Nyquist plots of platinum, TiO 2 , PEDOT, PEDOT-GO, and PEDOT-GO/TiO 2 CE.The   value of PEDOT-GO/TiO 2 (22.2 Ω) is lower than PEDOT-GO (30.6 Ω) and platinum (175.8Ω), implying better electric conductivity of the former.A smaller value of  ct (9 Ω) is also observed for PEDOT-GO/TiO 2 compared to platinum (19 Ω), indicating good ECA toward the reduction of I − 3 for PEDOT-GO/TiO 2 CE.This result is in agreement with the CV analysis.Figure 6 shows the equivalent circuit used for fitting the Nyquist plots of all CEs.The circuits include the ohmic series resistance (  ), constant phase element (CPE) that represents the irregularity of sample, and  ct .It was observed that GO has the highest  ct (Table 1) due to the low conductivity followed by TiO 2 and PEDOT.However, after PEDOT is incorporated with GO, the  ct value of PEDOT-GO drops to 21 Ω, indicating faster electron transfer rate due to good charge propagation behavior and low contact resistance [37].3.6.Photovoltaic Performances of DSSCs. Figure 7 shows the current-voltage (-) curves of the DSSC prepared using platinum, TiO 2 , PEDOT, PEDOT-GO, and PEDOT-GO/TiO 2 as CEs under AM 1.5 illumination (100 mW/cm 2 ).The open circuit voltage ( oc ), short-circuit current ( sc ), maximum power ( max ), fill factor (FF), and overall conversion efficiency () are summarized in Table 2. Apparently,  TiO 2 CE yielded the lowest power conversion efficiency (PCE) with 0.001% due to slow electron transport and low electron mobility causing recombination process to occur in the electrolyte, hence generating low efficiency [38].
Although PEDOT has excellent conductivity and chemical stability, it also produces a low efficiency (0.015%) that could be due to its small surface area [39] and subsequently resulted in low active site for reduction of iodide to triiodide.In order to overcome this problem a high surface area carbon-based material which is graphene oxide (GO)  is incorporated with PEDOT to form a synergistic effect to obtain a higher efficiency of DSSC [40].As a result, the PCE of PEDOT-GO CE is enhanced to 0.68%, which is 45.5 times higher than PEDOT CE.In addition, TiO 2 as a source of metal oxide was introduced to PEDOT-GO to increase the performance of DSSC.Even though TiO 2 itself has low efficiency (0.001%), the PCE of TiO 2 incorporated with PEDOT-GO composite gives a promising result with the PCE of 1.166%.The PCE of PEDOT-GO/TiO 2 CE is higher than platinum CE (0.727%) and comparable with conducting polymer based CEs reported in the literature [41,42].The novel PEDOT-GO/TiO 2 CE produced in this work is a promising CE due to an enhance PCE observed in PEDOT-GO/TiO 2 CE compared to PEDOT-GO, PEDOT, and TiO 2 CEs.It gives a clear proof that PEDOT-GO/TiO 2 is capable of producing a synergistic effect that utilizes the advantages of each material.The maximum power densities of CEs are displayed in Figure 8.The  max value can be calculated from the photocurrent-voltage graph based on  =  ⋅ .
(3) From the graph, PEDOT-GO-TiO 2 clearly shows the domination on the ability to produce maximum power compared to other CEs which is 1.166 mW/cm 2 due to excellent ECA of PEDOT-GO/TiO 2 toward the reduction of iodide to tri-iodide.The maximum power produced by platinum, PEDOT-GO, PEDOT, and TiO 2 CEs is 0.727, 0.683, 0.015, and 0.001 mW/cm 2 , respectively.Due to the high conductivity of PEDOT and high surface area of GO, the maximum power yielded by PEDOT-GO composite is higher than a single compound PEDOT and GO.It is obvious that the maximum power and  of the PEDOT-GO/TiO 2 CE are higher than those of PEDOT-GO CE, which can be corroborated by the data from Tafel polarization curves.polarization curve and the extension of the linear segment to the zero bias using [43] where  and  are constant,  is the room temperature, and  is the number of electrons involved in the reaction.The limiting diffusion exchange current density ( lim ) can also be analyzed from Tafel plot using [44] where  is the coefficient of tri-iodide,  is the spacer thickness,  is the number of electrons involved in the reduction of tri-iodide at the electrode,  is the Faraday constant, and  is the tri-iodide concentration. lim depends on the diffusion coefficient of I − /I − 3 redox couple.From the Tafel plot, PEDOT-GO/TiO 2 exhibits a higher  lim (1.625 mA/cm 2 ) compared to PEDOT-GO (1.023 mA/cm 2 ) and platinum (0.4627 mA/cm 2 ).A lower  lim value of PEDOT-GO and platinum compared to PEDOT-GO/TiO 2 reveals that PEDOT-GO and platinum have a marginal edge over PEDOT-GO/TiO 2 in terms of improving ECA; nonetheless, PEDOT-GO/TiO 2 CE provides sufficient ECA.  values also show a similar trend as of the  lim values where PEDOT-GO/TiO 2 , PEDOT-GO, and platinum yielded 0.332, −0.280, and −1.015 mA/cm 2 , respectively.A higher   reveals that PEDOT-GO/TiO 2 possesses high ECA toward the reduction of tri-iodide to iodide, whereas a higher  lim indicates good contact and better diffusion of electrolyte within the CE [43].A comparison of the Tafel curves between PEDOT-GO/TiO 2 and PEDOT-GO CEs indicates that the former has a higher catalytic activity compared to the latter.
Note that the I − /I − 3 diffusion capability in the PEDOT-GO/TiO 2 CE is more prominent than that in the PEDOT-GO CE.

Conclusion
In summary, a novel PEDOT-GO/TiO 2 CE for DSSC was successfully fabricated using a facile technique.TiO 2 incorporated with PEDOT-GO showed that the electrochemical properties of the final device were affected by catalytic activity layer due to the synergistic effect produced by the combination of three different materials.By applying the PEDOT-GO/TiO 2 composite film as the CE, an efficiency of 1.166% was achieved for the DSSC, which is 58.6% higher than that of a cell based on the PEDOT-GO CE.An excellent enhancement in the conductivity and good electrochemical conductivity for the reduction of I − 3 to I − were obtained for PEDOT-GO/TiO 2 CE due to the high conductivity of PEDOT, high surface area of GO, and high active area of TiO 2 nanoparticles.Hence, PEDOT-GO/TiO 2 CE is a promising material as a CE for DSSC.
(b)); however, the wrinkled morphology of PEDOT/GO (Figure2(c)) is more pronounced than GO (Figure2(b)) indicating an increase in surface area of the material.As shown in Figure2(d) the FESEM image of TiO 2 reveals similar morphology as observed by[29] which is porous spherical nanoparticles that is important in the adsorption of the tri-iodide ion.The incorporation of electropolymerized PEDOT-GO onto TiO 2 coated ITO to produce PEDOT-GO/TiO 2 CE (Figure2(e)) shows a mixture of porous spherical nanoparticles and paper-like sheet morphology.However, the porous spherical nanoparticles morphology of TiO 2 (Figure2(d)) is more pronounced than PEDOT-GO (Figure2(c)).This result indicates a synergistic effect of both materials resulting in more active sites being exposed for tri-iodide reduction and subsequently achieving enhanced ECA to yield high PCE.