Electrophoretic deposition of graphene nanosheets: A suitable method for fabrication of silver-graphene counter electrode for dye-sensitized solar cell

Colloids and Surfaces A: Physicochem. Eng. Aspects 520 (2017) 477–487

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Electrophoretic deposition of graphene nanosheets: A suitable method for fabrication of silver-graphene counter electrode for dye-sensitized solar cell Shahram Ghasemi ∗ , Sayed Reza Hosseini, Farimah Mousavi Nanochemistry Research Laboratory, Faculty of Chemistry, University of Mazandaran, 47416-95447, Babolsar, Iran

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• GO nanosheets were deposited at • • • •

the surface of FTO by electrophoretic technique. GO/FTO electrode were electrochemically reduced into ERGO/FTO. Silver nanoparticles were grown on ERGO/FTO electrode to produce Ag/ERGO/FTO. DSSC was fabricated out of TiO2 photoanode and Ag/ERGO/FTO counter electrode. The DSSC performed suitable parameters (Voc = 0.8 V, jsc = 29.04 mA cm−2 and ␩ = 4.24%).

a r t i c l e

i n f o

Article history: Received 21 September 2016 Received in revised form 26 January 2017 Accepted 3 February 2017 Available online 5 February 2017 Keywords: Dye-sensitized solar cell Graphene nanosheets Silver nanoparticles Electrophoretic deposition Iodine redox shuttle

a b s t r a c t Dye-sensitized solar cell (DSSC) was fabricated by a suitable counter electrode (CE) based on graphene nanosheets in order to facilitate electrochemical reduction of triiodide in organic medium. Graphene oxide (GO) nanosheets were deposited at fluorine doped tin oxide (FTO) glass substrate by electrophoretic deposition (EPD) technique as an easy short-time method. Then GO nanosheets were converted to electrochemically reduced graphene oxide (ERGO) nanosheets by chronoamperometery technique. Graphene nanosheet-based electrode was modified by silver nanoparticles and was characterized by field emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), X-ray elemental mapping (MAP) and atomic force microscopy (AFM) studies. Also, the electrochemical behaviors of as-prepared electrodes were investigated by cyclic voltammetry method where electrodes fabricated by graphene-silver nanohybride (Ag/ERGO/FTO) presented the highest current density and the lowest peak-to-peak separation in comparison to bare FTO, GO/FTO and ERGO/FTO electrodes in iodine electrolyte medium. Moreover, electrochemical impedance spectroscopy (EIS) was applied to investigate the electrocatalytic capability of the modified electrode both in I3 − /I− electrolyte and as a symmetrical dummy cell which represented relatively lower charge transfer resistance.

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (S. Ghasemi). http://dx.doi.org/10.1016/j.colsurfa.2017.02.004 0927-7757/© 2017 Elsevier B.V. All rights reserved.

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Finally, the fabricated DSSC containing Ag/ERGO/FTO CE represented suitable photovoltaic characteristic parameters (Voc = 0.8 V, jsc = 29.04 mA cm−2 and Á = 4.24%) under simulated light of AM1.5 G (1000 W m−2 ) compared to Pt-based DSSC (Voc = 0.66 V, jsc = 19.54 mA cm−2 and Á = 6.08%). Furthermore, the high conductivity of the cells was evaluated by EIS during the applying different biases. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Dye-sensitized solar cell (DSSC) is a liquid-junction photoelectrochemical cell which has attained scientist’s attention due to its cost-effective, low temperature fabrication and respectable power conversion efficiency, mostly after the first high (almost 7%) efficiency, world break up of O’Regan and Grätzel [1,2]. The key components of a DSSC are a photoanode which is composed of a mesoporous film of a wide band gap semiconductor usually TiO2 nanoparticles coated on a transparent conductive oxide (TCO) glass substrate and sensitized by dye molecules, a redox mediator (typically I3 − /I− in an aprotic electrolyte medium) and a counter electrode (CE). Among them, CE plays two important roles in the cell performance; it transfers electrons from the external circuit to the electrolyte solution and catalyzes the reduction of I3 − , thereby facilitating sensitizer regeneration. However, the electrochemical activity of iodine species at the surface of TCO (e.g. fluorine doped tin oxide (FTO)) is quiet sluggish and conventionally corrosionresistant platinized cathodes were utilized for this purpose. Since platinum is a precious metal with limited availability, many investigations have been achieved to increase the efficiency and the stability of the cell or diminish the overall cost of device fabrication especially in large scale by introducing new cathode materials. Conducting polymers have been widely used in order to catalyze the reduction of I3 − at the surface of CE in DSSCs. Polymers like polyaniline [3], polypyrrole [4] and poly (3,4ethylendioxythiophene) (PEDOT) [5] are highly desirable for scientists. Lately, carbon materials like graphene and carbon nanotube (CNT) have brought new area in the field of photovoltaic technology [6–8]. Graphene, an atom-thick two-dimensional (2D) carbon material is a good candidate among all carbon allotropes which can be used as transparent conducting electrodes due to its dominant electrical conductivity, high optical transparency and superb specific surface area. The remarkable thermal and electrical conductivity of graphene make it applicable as flexible conductor specially in organic photovoltaic cells [9]. Graphene, GO and composites based on them were widely used as different components in nanostructured solar cells [10–13]. Among various deposition techniques for the preparation of thin film of graphene like spin coating, chemical vapor deposition (CVD), drop evaporation and etc., electrophoretic deposition (EPD) has the applicability to prepare uniform and thickness-controlled layer film [14]. Although this zero-bandgap metal like material is highly conductive and stable, the enhanced performance of graphene in DSSC could be obtained by its nanocomposite with metal nanoparticles. Various strategies like vacuum deposition, evaporation and electrochemical deposition have been used to deposit different noble metal nanoparticles such as Pd [15] and Au [16] on graphene. Tjoa et al. proposed a facile photochemical method for synthesis of graphene-Pt nanoparticle composite for CE in order to catalyze the reduction of I3 − to I− in organic medium [17]. In order to enhance the electrical conductivity and the electrocatalytic properties of electrodes, silver nanoparticles have been found to be beneficial [18,19]. Also, the synergetic effect of Ag

nanoparticles and different conductive materials has been widely demonstrated [20,21]. Here, we introduces a nanohybride composed of silver nanoparticles uniformly distributed at the surface of ERGO nanosheet film as a promising cathode material to facilitate the electrochemical activity and decrease the resistance of FTO toward the iodine species in DSSC. 2. Experimental 2.1. Materials Commercial anatase titanium dioxide nanoparticles paste in two sizes (T/SP20 and R/SP300, Solaronix), cisbis(isothiocyanato)bis(2,20-bipyridyl-4,40-dicar-boxylato) ruthenium (II) bis-tetrabutyl ammonium (N719) (Dyesol), FTO transparent conductive glass (Solaronix), and Surlyn thermoplastic polymer (Solaronix) were purchased to construct solar cells. Also, graphite powder (Merck), sulfuric acid (98%, Merck), hydrochloric acid (37%, Merck), hydrogen peroxide (35%, Merck), potassium permanganate (99%, Merck), ethanol (99%, Fluka), acetone (99%, Fluka), silver nitrate (99%, Merck), lithium iodide (99%, Merck), iodine (I2 ) (99%, Merck), lithium perchlorate (99.99% Sigma), DSSC-specific commercial iodine electrolyte in acetonitrile (Iodolyte HI-30, Solaronix), sodium nitrate (NaNO3 ), potassium nitrate (KNO3 ), acetonitrile (99.99% Merck), tert-butanol (Aldrich), ammonia aqueous solution (25%, Merck) were used as received without further purification. Moreover, all aqueous solutions were prepared by double distillated water. 2.2. Apparatus Universal 320 centrifuge (Hettich, Germany) and 400 W tip sonicate (Ultrasonic Technology, Iran) were used during the preparation of GO nanosheets. DC power supply (Sanjesh, Iran) was used to deposit GO nanosheets film on FTO electrode. AUTOLAB 302N (Netherland) electrochemical analyzer and PalmSense potensiostate-galvanostate (Netherland) were applied for electrochemical deposition and investigation and electrochemical impedance spectroscopy (EIS), respectively. UV–vis spectrometer (2100 Beinjing, China), Raman spectrometer (Senterra-Bruker), Fourier transform infrared spectroscopy (FTIR) (Vector 22, Bruker, Germany), field emission scanning electron microscopy (FE-SEM, Mira 3 XMU FE-SEM/EDS, TESCAN) and SEM (EVO 18, Carl Zeiss, Germany) and atomic force microscopy (Easy scan 2 Flex AFM, Nanosurf, Swiss) were used to characterize GO, ERGO and ERGObased electrodes. The as-prepared DSSCs were tested under AM 1.5G solar simulator (Sharif Solar, Iran). 2.3. Methods 2.3.1. Preparation of graphene oxide nanosheets GO was prepared by modified Hummers method [22]. Briefly, 0.7 g graphite powder was stirred with 115 mL sulfuric acid. 15 g potassium permanganate was slowly added to the mixture during an hour and the temperature was kept bellow 7 ◦ C. Later, the mixture was diluted by adding 250 mL water and 15 mL H2 O2 (30%) to

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reduce the remained KMnO4 in the mixture. Finally, the GO precipitate was achieved by centrifugation and washing with 1.2 M HCl and then by water for several times. GO mixture was prepared by ultrasonication of 80 mg as-prepared GO powder in 20 mL water for 2 h. During this period, GO nanosheets were exfoliated from each other. 2.3.2. Preparation of GO nanosheets electrode Primarily, FTO glasses were well cleaned in an ultrasonic bath for an hour using in order: soap, water, 0.1 M HCl, water, acetone, methanol and 2-propanole at room temperature as an effective cleaning procedure [4]. In order to increase the hydrophilicity of the surface, they were further treated in RCA solution containing NH4 OH:H2 O2 :water (1:1:5) at 60 ◦ C for 30 min [23]. To deposit graphene on FTO, the EPD technique was used. A two-electrode glass cell was constructed based on cleaned FTO positive electrode and a stainless steel negative electrode. The distance was kept at 1 cm (Fig. 1). The Cell was filled by 4 mg mL−1 exfoliated GO solution and the voltage of 10 V was applied between two electrodes. In order to obtain a uniform thin layer of GO nanosheets on the surface of FTO, the deposition was took place in 1 min. After drying at room temperature, the electrode was kept in 60 ◦ C oven for an hour. The color of electrode was changed from light brown to dark brown which defines the pre-reduction of GO nanosheets. 2.3.3. Preparation of ERGO/FTO electrode A three-electrode system containing GO/FTO, Ag|AgCl|KCl (3 M) and platinum (Pt) foil as working, reference and CEs, respectively, was used to reduce electrochemically GO film into ERGO film. The cell was filled by 0.5 M sodium nitrate and the potential of −1.1 V was applied to GO/FTO for 1200 s. 2.3.4. Modification of ERGO nanosheets by silver nanoparticles In order to prepare ERGO/FTO electrode modified with silver nanoparticles, ERGO/FTO electrode was immersed into 4 mM AgNO3 solution for 10 min. Then, the electrode was fully rinsed with 0.1 M KNO3 solution to remove excess non-bonded silver ions from graphene nanosheets. In order to obtain silver nanoparticles, the potential of −0.5 V vs. Ag|AgCl|KCl (3 M) was applied to the electrode by chronoamperometry technique in 0.1 M KNO3 solution for 100 s. 2.3.5. Fabrication of dye-sensitized solar cell A photoanode was prepared via a doctor-Blading method. A transparent layer of 20-nm-sized anatase TiO2 particles followed by a refractive layer of 300-nm-sized ones were coated on the surface of FTO (active area 0.25 cm2 ). Then, the electrode was sintered in a furnace from 320 till 500 ◦ C for 3 h. Later, it was cooled down till 80 ◦ C and was immersed in 80 ◦ C warm 4 mM N719 dye solution in acetonitrile-tert-butanol (1:1) and was kept in room temperature for 22 h. After well raising, the photoanode and the Ag/ERGO/FTO CE were sandwiched together by a Surlyn thermoplastic polymer. The cell was filled by DSSC-specific commercial iodine electrolyte through the hole (d = 0.6 mm) in CE which was further sealed by a piece of glass and Surlyn. 3. Results and discussion As-prepared 0.01 mg mL−1 GO suspension was studied by Uv–vis spectroscopy (Fig. S1-a). The sharp 230 nm peak represents ␲-␲* transition of C C bond in an aromatic ring and the 310 nm broad shoulder can be related to n-␲* transition of C O bond in GO nanosheet structure [24]. Fig. S1-b shows SEM of the as-prepared GO powder with some degree of aggregation. Generally, defects and functional groups have critical effects on the electrocatalytic behavior of graphene and graphene oxide;

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especially by considering their effect on electrostatic interaction between the electrode’s surface and the redox couples in the electrolyte medium [25,26]. The hydrophilicity of exfoliated GO nanosheets which is due to its oxygen-rich functional groups like hydroxyl OH, carboxyl COOH, epoxide and carbonyl C O moieties, primarily facilitates film coating on the hydrophilic RCAtreated FTO, during the EPD procedure [23,26]. However, higher concentration of oxidized functionalities causes quiet sluggish electrochemical activity toward the iodine species [27]. Thus, the reduction of GO nanosheets is crucial for surface electrostatic modification, electrical conductivity improvement and electrochemical activities. Fig. S2 shows the current density (j) vs. time curve obtained during the irreversible electrochemical reduction of GO nanosheets to ERGO by applying chronoamperometry technique. This deoxygenation process leads to increase in the conductivity of the film coated on FTO and in its electrocatalytic activity toward the redox activity of iodine mediators. Also, graphene oxide was reduced by chemical method. For this aim, the as-prepared GO/FTO electrode was immersed in 10 mM NaBH4 solution according to the work of Shin et al. [28]. During this chemical pretreatment, oxygen containing functional group can be removed. However, this process leaded to complete deoxygenation of GO film and increase in hydrophobicity of graphene. The resulting film on the FTO surface with highly hydrophobic characteristic was detached (i.e. peeled off) from hydrophilic pretreated FTO, as can be seen in Fig. S3. With chemical reduction of GO by NaBH4 , reduced GO (RGO) has no tendency to aqueous solution and it is peeled off from FTO surface as separated film. In other hand, during the electrochemical reduction, deoxygenation of GO film is fully controlled by varying the potential and time which leaded to the preparation of film with high conductivity as well as high mechanical stability. According to this observation, we decided to apply electrochemical reduction to GO/FTO to prepare ERGO/FTO. The Fourier transform-infrared (FT-IR) spectroscopy displays partially removal of oxygen-containing functional groups (Fig. 2). In the spectrum, the absorption bands at 3427 cm−1 is known to be the O H stretching vibrations of water molecules intercalated in the graphene oxide structure. The peak presented at 2989 cm−1 is also known as C H stretching vibrations of carbons with sp3 hybridization. The wavenumbers of 1654 and 1407 cm−1 refer to carbonyl and carboxyl groups, respectively. Finally, the peaks at 1074 and 1290 cm−1 correspond to the stretching vibrations on alcoxy and epoxy functional groups [29–31]. Furthermore, Raman spectroscopy was used to investigate the conversion of GO into ERGO. Fig. S4 represents two bands at ∼1311 and ∼1585 cm−1 for graphene oxide and 1355 and 1597 cm−1 for graphene which are dedicated to the D-band and G-band, respectively. G-band is attributed to sp2 -hybridized carbon atoms in carbonaceous hexagonal lattice. However D-band is corresponded to the vibrations of sp3 -hybridized carbon atoms of defects, disorders and functional groups attached to the graphene sheet [32,33]. The D/G intensity ratio (ID /IG ) of GO nanosheets was obtained to be 1.88, however, it was diminished to 1.02 for ERGO nanosheets. This variation in the intensity ratio is due to the decrease the defect concentration in ERGO compared to that in GO. In order to reduce silver ions into silver particles at the surface of Ag+ /ERGO/FTO electrode, the chronoamperometry technique was used. Fig. S5 shows j-time diagram of Ag (I) reduction during applying the potential of −0.5 V vs. Ag|AgCl|KCl (3 M) to the electrode in 0.1 M KNO3 solution. As be seen in the chronoamperogram, the reduction current density reduced at high slope manner due to the reduction of silver ions. However, the current density became constant at approximately 100 s which refers to maximum silver ions reduction.

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Fig. 1. A schematic representation of EPD procedure.

Fig. 2. FT-IR spectrum of GO and ERGO.

The morphological study of both ERGO/FTO and Ag/ERGO/FTO electrodes were done by FE-SEM (Fig. 3(a) to (e)). In ERGO film (Fig. 3(a) and (b)), the wrinkles is formed probably due to the evolution of oxygen gas during the EPD process which arises from electrolysis of H2 O [34]. For comparison, FE-SEM image of GO/FTO was also taken which is relatively similar to that of ERGO/FTO (Fig. S6). Generally, when the electron probe hits an angular surface during FE-SEM imaging, more electrons can escape from the specimen in comparison to flat surfaces. Thus, the certain areas of the sample such as edges, spherical particles, wrinkles and cavities appear bright in FE-SEM images. Moreover, FE-SEM images determine the presence of well-distributed silver nanoparticles with sizes in the range of 20–25 nm at the surface of Ag/ERGO/FTO electrode (Fig. 3(c) and (d)). Also, the large particles in the range of 100–200 nm are agglomerated silver nanoparticles formed at the surface of wrinkled ERGO. Also, the cross-section image of Ag/ERGO/FTO electrode (Fig. 4) proves the presence of Ag/ERGO film with thickness of almost 420 nm on FTO. The low amount of material can be deposited during the EPD which makes this technique economically preferable in order to fabricate cheap and high efficient DSSCs.

Furthermore, the AFM studies were used to investigate the morphological structure of ERGO/FTO and Ag/ERGO/FTO electrodes in non-contact mode. Fig. 5 represents the 2D and 3D images of these two electrodes. Graphene wrinkles are also appeared in Fig. 5(a) and (b) and the average size of silver nanoparticles (Fig. 5(c) and (d)) is estimated to be 30 nm. The average surface roughness of ergo film on ERGO/FTO electrode was found to be 73.154 nm which was increased to 88.426 nm with formation of silver nanoparticles on the surface of Ag/ERGO/FTO electrode [35]. Energy dispersive spectroscopy (EDS) gives an elemental analysis of nanohybride Ag/ERGO/FTO electrode. The presence of AgL␣ , AgL␤ and CK␣ lines in the EDS spectrum of Ag/ERGO nanohybrid confirms the presence of Ag and graphene in the prepared film on FTO (Fig. 6(f)). Also, Table 1 shows the quantitative results of EDS study at the surface of Ag/ERGO/FTO electrode. The presence of intense Si and Sn lines in the spectrum arise from FTO and shows the interaction of X-ray with atoms in depth of the specimen. In order to determine the exact position of the detected atoms on Ag/ERGO/FTO by means of EDS, elemental mapping was used (Fig. 6). The different maps define the exact positions of (a) carbon, (b) silver, (c) silicon, (d) tin, (e) oxygen and (f) Nitrogen atoms on the

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Fig. 3. FE-SEM image of (a), (b) ERGO/FTO, (c), (d) Ag/ERGO/FTO electrodes at two different magnifications, (e) the cross-section of Ag/ERGO/FTO electrode.

sample. Nitrogen may be incorporated to the graphene during the GO reduction into ERGO in NaNO3 solution. Moreover, the oxygen sources correspond to the FTO and oxygen-containing functional groups remained in ERGO nanosheets. The electrocatalytic performance of as-prepared electrodes was investigated using cyclic voltammetry technique (Fig. 7 (a)). The

electrochemical reactions occurred in DSSC with iodine redox mediator are presented in equation 1 and 2. 3I− ⇔ I3 − + 2e

(1)

2I3 − ⇔ 3I2 + 2e

(2)

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Fig. 4. EDS results of Ag/ERGO/FTO electrode.

Table 1 Elemental quantity driven from EDS study on Ag/ERGO/FTO electrode. Element

Line

Int

Error

W%

A%

C N O Si Ag Sn

K␣ K␣ K␣ K␣ L␣ L␣

43.9 19.1 59.8 65.3 10.3 349.3

16.8123 16.8123 16.8123 13.9084 4.3551 4.3551

12.70 9.76 23.08 2.66 1.23 50.58

28.35 18.69 38.68 2.54 0.31 11.43

The peaks (I) and (III) in Fig. 7(a), refers to equation 1 and peaks (II) and (IV) also represent the equation 2 [36]. In DSSC, CE can catalyze the reduction of I3 − in the electrolyte and facilitate dye regeneration. Unfortunately, bare FTO is not capable to facilitate the electrochemical behavior of iodine species but modification of FTO with graphene or its nanohybride with silver nanoparticles provide higher electrocatalytic activity. The curve related to ERGO/FTO displays higher current densities for both two redox couples than GO/FTO which is due to improvement of the graphene-based film conductivity during the electrochemical reduction in NaNO3 . On other hand, the presence of Ag nanoparticles on the surface of the electrode enhances the current density of all redox processes. Ag nanoparticles can improve the conductivity of the film and provide active sites for catalyzing I− /I3 − redox reaction. Further electrochemical characterization of as-prepared electrodes was investigated by EIS technique in iodine electrolyte. Fig. 7(b) presents the Nyquist plots of electrodes in iodine medium by applying ac excitation signal of 10 mV in the frequency range of 2 × 104 Hz to 102 Hz. In Nyquist plot, Z’ and Z” are real and imaginary parts of the impedance, respectively. The semicircles diameters which correspond to the charge transfer resistance (RCT ) can be reduced by electrode modification. Nyquist plot proves that the conductivities of different electrodes are in the order of Ag/ERGO/FTO > ERGO/FTO > GO/FTO > bare FTO which are due to the electrocatalytic properties of silver nanoparticles, deoxygenation of graphene oxide nanosheets and

increasing the sp2 ␲-conjugated domains through the modification of FTO electrodes [37]. In addition, EIS technique was used to investigate the response of prepared cell to ac potential with different frequencies at various applied potentials. Fig. 7(c) shows Nyquist plot of Ag/ERGO/FTO electrode in a symmetric sandwich dummy cell configuration constructed from two similar Ag/ERGO/FTO electrodes at various applied biases in dark. As bias is applied, the high-frequency semicircle remains unchanged, while the lower-frequency semicircle shrinks until eventually three semicircles are visible. Charge-transfer resistance of the Ag/ERGO at the electrode surface causes the appearance of first semicircle in the Nyquist plot. Diffusion of electrolyte species which is affected by the separation between the electrodes is responsible for the second semicircle. This phenomena is known as Nernstian diffusion [38]. Moreover, the third semicircle appeared at high bias potential (0.8 V) and low frequency, is probably due to mass transport resistance of redox species from electrolyte to the surface of electrode [36]. The effect of GO concentration was investigated by cyclic voltammetry in iodine-based electrolyte using ERGO/FTO electrode. Lower peak-to-peak separation and higher peak current density are main parameters in choosing the optimum GO concentration. Fig. S7 represents the voltammograms of ERGO/FTO electrodes prepared by three different concentrations of GO suspensions in iodine-based electrolyte. As it is clearly seen in this figure, the ERGO/FTO electrode made of 4 mg mL−1 GO concentration represents the best electrocatalytic activity toward the redox activity of I3 − /I− couple in acetonitrile. This conclusion was driven from lower peak potential and higher current density of the mentioned electrode compared to the electrode prepared from 2 mg mL−1 GO. Although the electrode prepared from 6 mg mL−1 GO suspension presents the highest current density among all three electrodes, its higher peak-to-peak separation made it quite unfavorable for the electrocatalytic application. In other hand, the higher concentration of graphite oxide suspension may generally lead to lower exfolia-

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Fig. 5. The AFM images of (a) 2D and (b) 3D ERGO/FTO electrode and (c) 2D and (d) 3D of Ag/ERGO/FTO electrode.

tion of GO sheets during the sonication in constant condition. Also, increasing in GO concentration may cause the highest thickness of electrophoretically deposited film on the surface of FTO which negatively affects the electrocatalytic activity and mechanical stability of the final GO film. The performance of solar cells was qualified under AM1.5 G (1000 W m−2 ) simulated light connected to PalmSense potensiostate-galvanostate. The j-V curves were recorded and are presented in Fig. 8(a). S-shaped j−V curves of DSSCs equipped by carbonaceous CEs were reported elsewhere [26,39–41] which causes low amount of FF and almost low photo-to-current conversion efficiency despite of their high jsc and Voc . In order to compare the performance of prepared cell, a Pt-based CE DSSC was also fabricated and characterized under similar condition. Characteristic parameters like open circuit potential (Voc ), short circuit current density (jsc ), fill factor (FF) and power conversion efficiency (Á) are calculated and presented in Table 2. The modification of CE with silver nanoparticles and graphene enhanced the performance of DSSC. Both high open circuit potential and high short circuit current density of solar cells based on Ag/ERGO/FTO are of great features of prepared cells due to the highly electrocatalytic activity of silver nanoparticles at the surface of ERGO nanosheets toward the iodine redox shuttle in the electrolyte medium.

Table 2 Photovoltaic parameters of DSSCs. N

CE Type

Voc (V)

jsc (mA cm2− )

FF

␩%

1 2 3 4

Pt/FTO GO/FTO ERGO/FTO Ag/ERGO/FTO

0.66 0.75 0.78 0.80

19.54 17.37 25.80 29.04

0.47 0.14 0.17 0.18

6.08 1.85 3.36 4.24

The processes that take place in the cell are different in dark and under illumination. At Voc and under sunlight, the net current which flow through the cell is zero. However, in dark by applying forward bias, electrons react with I3 − after being transported through photoanode structure and meanwhile, I− is oxidized to I3 − at CE. Thus the net current density varies due to the applied bias voltage [42]. The Nyquist plots of DSSC equipped with Ag/ERGO nanohybride CE was recorded by applying different potentials a day after fabrication and is presented in Fig. 8(b). The Nyquist plots show one semicircle in all potenitials for measured frequency range of 0.01–20 kHz. The semicircle corresponds to the capacitance and charge-transfer resistance at the CE|I− /I3 − electrolyte interface which shrinks by increasing the bias potential. In high applied bias of 0.8 V, the impedance at low frequency (below 20 Hz) is appeared

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Fig. 6. Elemental mapping of Ag/ERGO/FTO electrode determining the exact position of (a) carbon, (b) silver, (c) silicon, (d) tin, (e) oxygen and (f) Nitrogen atoms.

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Fig. 7. (a) Cyclic voltammograms of FTO, GO/FTO, ERGO/FTO and Ag/ERGO/FTO electrodes at scan rate of 50 mV.s−1 , (b) Nyquist plots of as-prepared electrodes in electrolyte containing 0.01 M I2 , 0.05 M LiI and 0.5 M LiClO4 in acetonitrile and (c) Nyquist plots of symmetrical dummy cell in dark by applying forward bias. Table 3 The photovoltaic characteristic parameters of DSSCs fabricated by various carboneous CE materials. No.

CE Materials

Voc (V)

jsc (mA/cm−2 )

Á%

Ref.

1 2 3 4 5 6 7 8 9 10 11

Microcarbon Nanocarbon Graphene oxide Pt-reduced graphene oxide Carbon nanotube Functionalized multiwalled carbon nanotube TiO2 -black carbon nanocomposite Pt/reduced graphene oxide Single wall carbon nanohorns Graphene ink Ag/ERGO

0.74 0.74 0.69 0.72 0.59 0.68 0.71 0.73 0.68 0.64 0.80

13.3 14.6 0.59 14.1 10.95 9.5 15.5 16.3 14.48 7.44 29.4

1.87 6.73 0.27 6.77 4.18 3.52 7.4 7.7 4.09 3.00 4.24

[44] [44] [45] [46] [47] [48] [49] [50] [51] [52] This study

as a second semicircle corresponds to the Nernst diffusion of I− /I3 − within the electrolyte [14,43]. Generally, various compounds were used as counter electrode materials in DSSCs including metal or metal oxide nanoparticles, carbon nanomaterials, conducting polymers and etc. Table 3 compares the electrical parameters of DSSCs with different CEs. In

this study, the ability of silver-graphene nanohybrid was investigated by cyclic voltammetry, EIS and J-V experiments. The obtained results showed the ability of Ag/ERGO/FTO electrode to act as a reliable counter electrode in iodine-based DSSCs. The point that needs to be emphasized about this work is the high open circuit potential and extremely high short circuit current density compared to many

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Fig. 8. (a) j-V curves of DSSCs with different CEs under AM 1.5 G. illumination and (b) Nyquist plots of DSSC fabricated by the Ag/ERGO cathode at various applied biases (inset: same curves in higher frequency range).

DSSCs based on other counter electrodes especially carbonaceous ones. 4. Conclusions In summary, GO film was formed on FTO by EPD technique from dispersed GO mixture. Then, the prepared film was reduced electrochemically by applying negative voltage in NaNO3 aqueous solution. The silver nanoparticles were electrodeposited on graphene-modified FTO electrode from aqueous AgNO3 solution. Ag/ERGO/FTO was characterized by SEM, AFM, EDS and MAP analysis. Cyclic voltammetry and EIS studies demonstrate the high electrocatalytic activity, high current density and low charge transfer resistance toward iodine spices redox reactions in organic acetonitrile medium. Low cost, high open circuit potential and high

short circuit current density are of the great features of DSSC composed of Ag/ERGO/FTO. Also, the results deriven from DSSC j-V curve and Nyquist plot proves this claim. This study shows that the Ag/ERGO nanohybride can be a good candidate for catalyzing I3 − /I− redox reaction at the surface of FTO CE which leads to effective regeneration of dye molecules and suitable power conversion efficiency for DSSCs.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2017.02. 004.

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