Introduction

Dye-sensitized solar cells (DSSCs) are of great attention among solar cells due to low cost and high photovoltaic efficiency. The DSSC is composed of dye-sensitized TiO2 photo electrode, a counter electrode (CE), and \( {\text{I}}_{3}^{ - } /{\text{I}}^{ - } \) redox electrolyte solution. The CE is key component in DSSC, which is used to shift electrons from outer circuit to the electrolyte solution and to reduce the triiodide ions (\( {\text{I}}_{3}^{ - } \)) to Iodide ions (I) [1–4]. Generally, a thin platinum (Pt) film deposited on transparent conductive oxides (TCO) glass is widely adopted as CE in DSSC for the reduction of \( {\text{I}}_{3}^{ - } \). However, Pt CE is expensive and restricts its use at commercial scale DSSC [5, 6]. To reduce the fabrication cost of DSSC, it is required to adopt cost-effective alternative materials such as carbonous materials, conducting polymers, and their composites for CEs [7–17].

Among conducting polymers, polyaniline (PANI) is the most attractive material to replace Pt CE because of its easy synthesis, considerable catalytic activity, and good environmental stability [18, 19]. Li et al. [20], Qin et al. [21], and Xiao et al. [22] use electrochemically polymerized Polyaniline film on ITO glass as a counter electrode for DSSC and achieve a conversion efficiency up to 5.8 % with smaller charge transfer resistance and higher electrolyte catalytic activity of \( {\text{I}}_{3}^{ - } /{\text{I}}^{ - } \) redox reaction. Typically, PANI has conductivity of the order of 1 S cm−1. The coupling of metals with PANI is the most suitable choice to enhance its conductivity [23, 24].

The platinum nano-particle (PtNP)/PANI composite-based CEs have been investigated and exhibit unique properties of both PtNP and PANI. Higher conductivity is observed for the PtNP/PANI composite CE as compared to the PtNP and PANI CE. Consequently, PtNP/PANI composite-based DSSC showed better PV performance than the PANI and the Pt-based DSSCs [25, 26]. Many efforts have been made to synthesize other materials for the CEs using conducting polymer/metal composites. Since silver has high conductivity among all the metals, the incorporation of silver into PANI may enhance electrical and catalytic properties [27, 28]. A recent research by A.B.V. Kiran Kumar et al. has been focused on replacing ITO by Ag nanowire/PANI composite film for transparent electrodes because of their preferred conducting and optical properties [29]. In this study, Ag/PANI composite films are formed on stainless steel (SS) 304 substrate using electropolymerization technique. The electropolymerization method is simple and cost-effective to synthesize Ag/PANI composite films with good adhesion to the substrate and controllable surface morphology as compared to chemical polymerization [11]. The Ag/PANI composite as counter electrode for DSSC has not been studied earlier. The novelty of this work is the fabrication Ag/PANI composite CEs and the sequential investigation of morphology and conductivity of composite films along with the photoelectric performance of DSSC. This research describes a simple way to enhance the electrical and electrochemical properties of polyaniline doped with metal as a CE for DSSC.

Preparation of Silver/PANI composite CE

All electro-polymerization tests were performed in one compartment cell using three-electrode arrangement on Princeton 263A electrochemical work station. A graphite electrode (1 cm2), SS 304 sheet, and saturated calomel electrode (SCE) were used as counter electrode, working electrode, and reference electrode, respectively. The SS sheet is mechanically polished with fine emery paper and then rinsed with double-distilled water in an ultrasonic bath before using. The working area of the SS was 1 cm2.

Three electrodes were immersed in aqueous solution containing 0.2 M aniline, 0.01–0.1 M AgNO3, and 0.2 M H2SO4. Ag/PANI composite was deposited on SS sheet at 1.0 V versus SCE. The prepared samples were AgPANI-1, AgPANI-2, AgPANI-3, AgPANI-5, AgPANI-7, and AgPANI-10 having AgNO3 concentration of 0.01, 0.02, 0.03, 0.05, 0.07, and 0.1 M, respectively. A green film gradually formed on the SS sheet and becomes dark with the passage of time. The thickness of the film was measured and controlled by the amount of charge transfer between the electrode and the solution. In this work, the film thickness of Ag/PANI composite films was approximately 1 µm. After deposition, Ag/PANI Composite CE electrode was immersed in 0.5 M H2SO4 at 0.4 V versus SCE for 2 min to remove aniline monomer from the composite polymeric film. The Ag/PANI Composite CE was washed with distilled water and desiccated at 70 °C for 24 h. A pure PANI film was also deposited on SS using the same method without the addition of AgNO3. The thickness and morphology of Ag/PANI Composite CEs were adequately controlled by the precise control of the reaction conditions (time, concentration, temperature, and deposited charge).

Fabrication of DSSCs

The following procedure was used to fabricate photoanode of DSSC. Firstly, a thin TiO2-blocking layer was deposited on FTO substrate by immersing in 50 mM TiCl4 isopropanol solution at 70 °C for 30 min. Secondly, the TiO2 Colloid was prepared by mixing 2 g TiO2 (P25) powder, 4 mL DI (deionized water), 0.8 mL acetylacetone, and 0.1 mL Triton X. The mixture was stirred ultrasonically for two hours. The TiO2 colloid was coated on the TiO2-blocking layer using doctor blade method and sintered at 450 °C for 30 min. After cooling, the TiO2 anode was immersed in 0.5 mM solution of N719 dye in a mixture of acetonitrile/tert-butanol (volume ratio 1:1) for 24 h. Finally, a DSSC was fabricated by injecting a drop of redox electrolyte (0.1 M LiI, 0.05 M I2, 0.6 M tetra butylammonium Iodide, and 0.5 M tert-butyl pyridine in acetonitrile) in the aperture between photoanode and CE, made from PANI and Ag/PANI composite, and for comparison, a thermally decompose Pt on SS is also used as CE [30].

Characterizations and measurements

The electrical conductivity of PANI and Ag/PANI samples was measured by four-probe method using a current source SMU Keithley 237 and a Multimeter Keithley 2010. The X-ray diffractogram of the samples was obtained by 2001 Bruker-AXS diffractrometer using CuKα radiation. The surface morphologies of CEs were observed using a scanning electron microscopy (SEM; JSM-6480LV). Cyclic voltametry was performed on Princeton 263A electrochemical work station using three-electrode system. The graphite electrode, saturated calomel electrodes, and PANI and Ag/PANI were used as counter, reference, and working electrodes. The electrolyte was acetonitrile solution consisting of 10 mM LiI, 1 mM I2 as \( {\text{I}}_{3}^{ - } /{\text{I}}^{ - } \) redox couple, and 0.1 M LiClO4 as supporting electrolyte [31–33]. The Tafel polarization measurements were performed using a symmetric cell made of two identical CEs at a scan rate of 10 mV/s using the same electrolyte as that for DSSC. The photovoltaic efficiency of DSSC was measured under 100 mW/cm2 from a solar simulator (HAL-C100).

Results and discussion

Silver/PANI composite formation

In electrochemical synthesis, aniline monomer and silver nitrate in sulfuric acid solution are treated electrochemically to produce silver particles embedded in PANI film. Aniline is oxidized with silver nitrate to polyaniline while metallic silver and nitric acid are reaction by-products. A part of H2SO4 is neutralized by aniline to form PANI and reduce the acidity of the medium, which acts against the polymerization of aniline. This decrease in acidity is compensated by the nitric acid generated during the oxidation of aniline by silver nitrate. Nitric acid supports aniline oxidation and adjusts the acidity of the medium [34–36].

PANI produced during the oxidation of aniline is also able to reduce silver ions to metallic silver. Based upon the above prospective, two silver reductants, a monomer (aniline) and a polymer (PANI), are always present in the reaction mixture [37]. A simple redox reaction between PANI and the silver salt becomes complicated in the presence of two reductants of silver ions.

Structural characterization

The crystalline nature of the Ag/PANI composite is determined by XRD analysis as shown in Fig. 1. The diffraction peaks at 2θ = 38.54°, 44.9°, and 63.5° represent the cubic structure of Ag for the diffraction planes (111), (200), and (220), respectively. All the peaks are indexed to face-centered cubic structure [38]. The diffraction peaks confirm the incorporation of metallic silver particles in the composites and their crystalline nature. This result is well matched with standard spectrum of crystalline silver (JCPDS NO. 4-0783). XRD analysis confirms that the silver particles are embedded within the PANI matrix. The XRD patterns clearly indicate that intensity of peaks has been increased as the concentration of the silver is increased from 0.03 to 0.1 M.

Fig. 1
figure 1

XRD spectra of PANI and Ag/PANI composite

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The two peaks at low diffraction angle centered at 2θ = 20.3° and 24.7° show periodicity parallel and perpendicular to polymer chain, respectively [39]. The sharpness of these peaks denotes the degree of alignment of PANI chains, and the intensity shows the population of crystallites in that particular plane [40]. But in the present case, these two peaks are not sharp due to the poor orientation of PANI chains.

Morphological study

The surface morphology of PANI is shown in Fig. 2a. The surface of PANI film is quite rough because the electrochemical polymerization of PANI from aniline is a two-stage growth process. Firstly, a dense and compact layer of PANI appeared on the bare electrode and then PANI grows anisotropically to form a loosely bound structure [41, 42].

Fig. 2
figure 2

SEM micrographs of a PANI, b AgPANI-3, c AgPANI-5, d AgPANI-7, and e AgPANI-10. The a*, b*, c*, d*, and e* are the respective magnified images

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The content of metallic silver in Ag/PANI composite is found to be 18, 13, 9, and 7 wt % for AgPANI-10, AgPANI-7 AgPANI-5, and AgPANI-3, respectively, determined from energy-dispersive X-ray spectroscopy. Figure 2b–e is the SEM images of AgPANI-3, AgPANI-5, AgPANI-7, and AgPANI-10, respectively. In Fig. 2b, the white spots are the silver particles that bound to the polyaniline matrix due to firm affinity of silver for nitrogen. It is also observed that the spherical-shaped silver particles are uniformly dispersed in the PANI matrix. These particles are monodispersed at low concentration. The average diameter of silver particles is 200–250 nm; since the silver particles are created in the polyaniline solution, the particles are embedded in the PANI matrix [43]. The formation of relatively large Ag clusters is associated with the agglomeration of Ag particles for high silver concentration as shown in Fig. 2c. The agglomeration of particles is due to their high surface energy. Their agglomeration creates thermodynamically stable clusters [44, 45]. The porous surface with uniformly distributed silver particles is observed in Fig. 2d. This uniform porosity is an essential parameter for electrocatalytic activity. A honeycomb structure rather than distinct silver particles is observed in Fig. 2e for sample AgPANI-10.

Conductivity of silver/PANI composite

The conductivity of the Ag/PANI composite films is studied by four-probes measurement system and calculated by the following equation:

$$ \sigma = { \ln }2/\pi d*I/U, $$
(1)

where σ is the conductivity, d is the thickness of the film, which is 1 µm, and I and U are the current and potential difference, respectively [21].The sample code and the conductivity of the Ag/PANI composite films with different concentrations of AgNO3 are given in Table 1. The electrical conductivity of Ag/PANI composite is higher than pure PANI film and increases with the increase of silver content in the Ag/PANI composite. The lower conductivity of pure PANI is due to its amorphous nature as indicated in XRD pattern. The crystalline nature of silver particles forms ordered structure in the composite films, which permits improved electrical transport [46, 47].

Table 1 Conductivity of PANI and Ag/PANI films using different AgNO3 concentrations
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Electrochemical measurements

Figure 3 shows the cyclic voltametry curves of \( {\text{I}}_{3}^{ - } /{\text{I}}^{ - } \) redox couple on the CEs. The anodic current peak represents oxidation, and cathodic current peak is related to reduction [11, 48]. Table 2 shows cathodic peak current (I Pc), cathodic peak potential (E Pc), and peak to peak seperation (E pp) data for PANI, Ag/PANI composite, and Pt CEs. The reaction occurs on the CE of DSSC is the reduction of \( {\text{I}}_{3}^{ - } /{\text{I}}^{ - } \) redox couple, and thus only cathodic reaction is considered. The CV curves indicate that the cathodic current density of \( {\text{I}}_{3}^{ - } /{\text{I}}^{ - } \) redox couple for Ag/PANI composite CEs is higher than PANI CE which shows a rapid reaction rate and low charge transfer resistance of \( {\text{I}}_{3}^{ - } /{\text{I}}^{ - } \) redox couple on the Ag/PANI composite CEs. The cathodic current densities are gradually increased with Ag concentration, and the reductive peaks shift to more positive potentials. The Epp value (shown in Table 2) decreases in the order of PANI (556 mV) > AgPANI-3 (537mv) > AgPANI-5(524 mV) > AgPANI-10(518 mV) > Pt (480 mV) > AgPANI-7 (474 mV), whereas J sc increases in the order of Pt > AgPANI-7 > AgPANI-10 > AgPAN-5 > AgPANI-3, representing the same order of electrocatalytic activity. Moreover, the cathodic peak potential (EPc) of AgPANI-7 and Pt electrode is almost equal but more positive than other CEs fabricated in the present work. This indicates that the over potential for reduction of \( {\text{I}}_{3}^{ - } \) to \( {\text{I}}^{ - } \) for AgPANI-7 and Pt electrode is much smaller. A small E pp value and a large I pc are also observed when AgPANI-7 has been used as CE. Thus, AgPANI-7 has the highest electrocatalytic activity of \( {\text{I}}_{3}^{ - } /{\text{I}}^{ - } \) redox couple, which is equivalent to the Pt electrode, and it can be a substitute of Pt CE in DSSC [49].

Fig. 3
figure 3

Cyclic voltametry curves of various CEs at scan rate of 50 mV/sec

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Table 2 Cyclic voltametry and Tafel polarization measurements of different CEs
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To investigate the charge transfer properties of \( {\text{I}}_{3}^{ - } /{\text{I}}^{ - } \) redox couple on CE surface, Tafel polarization measurement is performed on symmetric cells using two similar electrodes with the same electrolyte as used in DSSC. Figure 4 shows the Tafel measurement with logarithmic current density (log J) as a function of voltage (V) at 25 C0. Theoretically, the Tafel polarization curves can be separated into three zones [50]. The curve at very high potential >0.6 V is assigned to the limiting diffusion zone which relates to the diffusion coefficient of \( {\text{I}}_{3}^{ - } /{\text{I}}^{ - } \) redox couple in DSSC, given by Eq. (2), where J lim is the limiting current density.

$$ D = 1/2nFc* J_{ \lim } $$
(2)
$$ V = 2.3RT({ \lg }j - { \lg }j_{\text{o}} )/\alpha nF $$
(3)
$$ J_{\text{o}} = RT/nFR_{\text{ct}} $$
(4)
Fig. 4
figure 4

Tafel curves of various symmetric cells

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The current at very low potential is assigned to polarization zone that originates from the electrochemical reaction, while curve at comparatively low potential >0.01 V is related to the Tafel zone, and voltage (V) is given by Eq. (3). In these equations, R is the universal gas constant, T is the temperature, j 0 is the exchange current density, F is the Faraday ‘s constant, α is the distribution coefficient, and n (n = 2) is the number of electrons involved in the reaction [51, 52].

The slopes of the curves in the Tafel zone for cathodic or anodic branches are in the order of pt > AgPANI-7 > AgPANI-10 > AgPANI-5 > AgPANI-3 > PANI. A steep slope indicates higher J0 that suggests large surface area and higher conductivity which is an important factor for large catalytic activity [53]. J 0 is related to R ct by Eq. (4) and calculated from the Tafel curves in Fig. 5. R ct is the charge transfer resistance for \( {\text{I}}_{3}^{ - } \) reduction at the electrolyte/CE interface. Apparently, the measured J 0 value also shows the order of Pt > AgPANI-7 > AgPANI-10 > AgPANI-5 > AgPANI-3 > PANI, and R ct shows the order of PANI > AgPANI-3 > AgPANI-5 > AgPANI-10 > AgPANI-7 > Pt. Thus, a CE with small Rct indicates less overpotential for an electron moving from CE surface to the electrolyte. A small R ct implies facile electron movement, while higher Rct indicates sluggish electron transfer.

Fig. 5
figure 5

Photocurrent-voltage curves of DSSCs using various CEs under standard illumination

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The R ct of the Ag/PANI composite CEs decreases with increasing concentration of silver. The lowest R ct value of 2.15 Ω cm2 for AgPANI-7 CE is obtained when silver concentration is 0.07 M, which is comparable to that of Pt CE (2.14 Ω cm2). Moreover, R ct increases by increasing the amount of silver due to poor electrocatalytic activity of Ag/PANI composite CE. A decrease in R ct value represents an increase in current density. AgPANI-7 CE with R ct value of 2.15 Ω cm2 has an exchange current density of 5.97 mA, whereas pure PANI CE has R ct value of 2.48 Ω cm2 corresponding to an exchange current density of 5.33 mA.cm2. The CE prepared with concentration of 0.07 M of silver shows the lowest R ct and the highest J 0, compared to other CEs. Thus, it shows highest electrocatalytic activity, which is in accordance with the CV result.

Photovoltaic characteristics of DSSC

Figure 5 shows the photovoltaic parameters of DSSCs using pure PANI and Ag/PANI composite CEs under light intensity of 100 mWcm−2. The related photovoltaic parameters are listed in Table 3, which shows that DSSCs employing Ag/PANI composite CEs exhibit enhanced photovoltaic efficiency as compared to pure PANI CE. In fact, the DSSC made from AgPANI-7CE reveals short-circuit current density (J sc), open-circuit voltage (V oc), and fill factor (FF) of 14.91 mA cm−2, 0.769 V, and 0.629, respectively, generating its photovoltaic efficiency 7.31 %, which is greater than 4.23 % for pure PANI electrode. The Ag/PANI composite CEs show much higher fill factor and photovoltaic efficiency, which display comparable short-circuit current density and open-circuit voltage than PANI CE. Such performance improvement is mainly due to higher electrocatalytic activity and large conductivity of Ag/PANI composite CEs [26].

Table 3 Photovoltaic parameters of DSSCs with different CEs
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All the DSSCs are made from the same TiO2 photoanode, dye, and liquid electrolyte; therefore, the measured V oc values are comparable. Generally, the V oc value was determined from the difference between the Fermi level of the electron in TiO2 and the formal potential of the redox couples on the CE [2, 54]. The J sc value of AgPANI-7 CE is higher among all CEs due to the high conductivity and good dispersion of silver particles into PANI matrix, which gives larger active surface areas for triiodides reduction, as mentioned in the CV tests. In addition, the value of R ct plays a major role in FF of the DSSCs which are made of the identical photoanodes and liquid electrolyte but with different CEs. [55]. The R ct of the Ag/PANI electrode is found to be smaller than PANI electrode as presented in Table 2. Therefore, the DSSC with Ag/PANI CE shows higher FF and photovoltaic efficiency. Furthermore, the FF of the DSSC fabricated with the AgPANI-7 CE (0.629) is higher than the PANI CE (0.547) but lower than that of Pt CE (0.703). This can be ascribed to its lower R ct values (2.15) than the PANI CE (2.41) but higher than Pt CE (2.14), still the DSSC with Ag/PANI-7 CE exhibits a higher cell efficiency of 7.31 % greater than Pt CE (6.84 %) due to its enhanced J Sc value. The conversion efficiency increases with increase in silver concentration and attains a maximum value when concentration is 0.07 M. As a result of these observations, the efficiency enhancement is regarded to the advancement of J sc and FF. For DSSCs using Ag/PANI CEs, the photovoltaic efficiency shows the order of AgPANI-7 > AgPANI-10 > AgPANI-5 > AgPANI-3 > PANI. The AgPANI-7 CE-based solar cell has the highest photovoltaic efficiency of 7.31 %. This photovoltaic efficiency is greater than other polyaniline composites-based CE reported by Lee et al. [26], Hsu [17], Peng [25], and Xiao [56], demonstrating that Ag/PANI composite is a favorable alternate as a CE for DSSCs.

Conclusions

In summary, PANI and Ag/PANI composite CEs are successfully fabricated on stainless steel 304 substrate by electropolymerization using different concentrations of silver. Main advantages of using Ag/PANI composite CEs are high electrical conductivity and large surface area. Electrochemical measurements reveal increased electrocatalytic activity and low charge transfer resistance of the Ag/PANI CEs that improve the photoelectric performance of the DSSCs. The DSSC based on Ag/PANI composite CE achieved a photovoltaic efficiency up to 7.31 %, which is greater than that of DSSC based on Pt CE (6.84 %). The above discussion demonstrates that Ag/PANI composite is a favorable candidate to replace Pt CE for commercialization of DSSCs.