An in situ polymerized PEDOT/Fe₃O₄ composite as a Pt-free counter electrode for highly efficient dye sensitized solar cells

Abstract

3,4-Ethylenedioxy thiophene (EDOT) precursor solution doped by Fe₃O₄ was spin-casted onto fluorine doped tin oxide (FTO) glass and formed poly(3,4-ethylenedioxy thiophene) (PEDOT)/Fe₃O₄ hybrid films by an in situ polyreaction. The films were utilized as the counter electrode in dye sensitized solar cells (DSSCs). Photoelectric conversion efficiency (PCE) for DSSCs based on PEDOT/Fe₃O₄ varied with the content of Fe₃O₄ in the precursor solution. When the content of Fe₃O₄ was 2 mg ml⁻¹ in the precursor solution (PEDOT/Fe₃O₄-2), the best performance (8.69%) was obtained. In comparison, that of a DSSC with a Pt counter electrode is 8.35%. According to the surface microtopography and electrochemical analysis, large active areas, consecutive electronic transmission channels and lower charge transfer resistance could be responsible for the high PCE.

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1. Introduction

Since M. Gratzel et al. made a breakthrough in 1991,¹ dye sensitized solar cells (DSSCs) have attracted much attention due to their low cost, easy preparation, good performance and environmental benignity. The task of the counter electrode (CE) is the reduction of the redox species used as a catalyst and collection of electrons from the external circuit. As we know, noble metal Pt reveals the optimal performance now and different kinds of CEs have been studied to replace Pt CEs, for example, carbon materials,^2–4 conductive polymer materials,^5–8 transition metal sulfides and diseleniums,^8–10 and alloy metals.^11,12 However, there remain some deficiencies to be overcome, such as complicated preparation, difficult large-area applications, instability and so on. Hence, developing a CE which has a low cost, flexible fabrication procedure and high catalytic activity is significant.^13,14

Poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most widely investigated conducting polymers because of its attractive properties such as a low band gap, remarkable environmental stability, high electrical conductivity and transparency.^15,16 Moreover, Yohannes and Inganas¹⁷ found that this material has excellent catalytic activity for I³⁻/I⁻ reduction and their study revealed that PEDOT could be used as a potential CE material for DSSCs. However, many researchers showed that a pure PEDOT CE cannot generate a satisfactory cell efficiency compared with Pt-based DSSCs. So modifications by incorporating PEDOT with nanomaterials have been suggested. It is reported that PEDOT/carbon,^18–20 PEDOT/metal,^21,22 PEDOT/metal oxide,^23,24 PEDOT/metal carbides or metal nitride composites^25,26 have been researched as the counter electrode. Among them, PEDOT/metal oxide showed a prominent performance. Maiaugree et al.²³ and Hu²⁴ and his coworkers explored PEDOT/TiO₂ and PEDOT/ZnO as DSSC counter electrodes and got photoelectric conversion efficiencies (PCE) of 8.49% and 8.17%, respectively. Meanwhile, little investigation has been conducted on PEDOT metal oxides as DSSC counter electrode catalysts. Fe₃O₄ has attracted attention because of its catalytic properties, high electronic conductivities, low cost and environmental benignity. Ma et al.²⁷ synthesized micron-sized Fe₃O₄ “flowers” and a rosin carbon/Fe₃O₄ composite as the counter electrode and achieved 7.65% and 8.11% as values for the PCEs, respectively.

Herein, a composite film consisting of PEDOT and Fe₃O₄ nanoparticles was prepared via a simple in situ polymerization and was used as the counter electrode for DSSCs. The concentration of Fe₃O₄ was varied from 0 mg ml⁻¹ (of precursor solution) to 3 mg ml⁻¹, leading to a difference in the catalytic performance of the composite. The best PCE achieved was 8.69%, when the concentration of Fe₃O₄ was 2 mg ml⁻¹ (of precursor solution), followed by DSSCs with Pt (8.38%).

2. Experimental

2.1 Preparation of Fe₃O₄ nanoparticles

Fe₃O₄ was synthesized using a wet chemical method based on the hydrolysis of Fe³⁺ and Fe²⁺ salts in the presence of NaOH as in a previous study.²⁸ FeCl₂·4H₂O (1 M) was dissolved in a NaOH·FeCl₃ (2 M) aqueous solution. After stirring for 30 min at room temperature, NaOH solution (2 M) was added dropwise until the pH of the reaction mixture solution was adjusted to 11–12. Finally, black Fe₃O₄ precipitate was obtained and collected using a strong magnet. These particles were washed several times with distilled water and alcohol and dried at 80 °C in vacuum oven for 12 h.

2.2 Fabrication of the PEDOT/Fe₃O₄ counter electrode

Fe₃O₄ (0.1 g ml⁻¹) was diffused in absolute ethyl alcohol using ultrasound equipment and mechanical agitation. EDOT (0.6 g), polyvinyl-pyrrolidone (PVP, 0.2 g) and pyridine (0.1 ml) were dissolved in 10 ml absolute ethyl alcohol and stirred for 10 min. The solution was added dropwise into 10 ml iron(III) p-toluene-sulfonate (2 g) ethanol solution with vigorous stirring. 0 ml, 1 ml, 2 ml and 3 ml of an Fe₃O₄ suspension were added into 5 ml of the mixed solution (corresponding to PEDOT, PEDOT/Fe₃O₄-1, PEDOT/Fe₃O₄-2 and PEDOT/Fe₃O₄-3 respectively) and all of the solutions were diluted to 10 ml. 100 μl of the diluted prepolymer was spin-coated onto 1.7 × 1.7 cm² FTO glass substrates (sheet resistance 14 Ω □⁻¹, Nippon Glass Co. JP) at 500 rpm for 2 s using a KW-4A spin processor, leaving a homogeneous prepolymer liquid film on the FTO glass, and these films were left to stand for 1 min. This was then repeated three times and the samples were left to stand for half an hour in the air. Lastly, the films were washed with ethanol until colourless and dried at 80 °C.

2.3 Preparation of TiO₂ photoanode and fabrication of DSSCs

Preparation of the TiO₂ blocking layer and the mesoporous TiO₂ electrode is described in the literature.^29,30 A dye was loaded on the film by immersing the TiO₂ film in a 0.3 mM dye N719 ethanol solution for 18–24 h. A dye-sensitized solar cell (DSSC) was assembled by the methods described previously.³⁰

2.4 Characterization

Powder X-ray diffraction (XRD) analysis was performed on a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å), operating at 40 kV/20 mA. The surface microtopography was observed by a field emission scanning electron microscope (FESEM, S-8000, HITACHI). The field emission scanning electron microscopy (FESEM) photos were taken on a JME-2100 transmission electron microscope operating at an accelerating voltage of 200 kV. Fourier transform infrared spectroscopy (FTIR) analysis was performed on a monocrystalline silicon piece. All of the electrochemical measurements were carried out using an electrochemical workstation (Zennium/IM6, Zahner, Germany). Photovoltaic parameters of the DSSCs were recorded with a KEITHLEY Model 2450 quick star guide under illumination by a Newport 91150V solar simulator (AM 1.5, 100 mW cm⁻²).

3. Results

3.1 Composition and morphology analysis

Fig. 1 shows the surface morphologies of the PEDOT and PEDOT/Fe₃O₄ composites. In Fig. 1(a), a relatively flat and smooth surface with little cracks was observed. Fig. 1(b)–(d) suggest that the composite films have a rougher surface morphology. As we can see, when the Fe₃O₄ content is 1 mg ml⁻¹ (Fig. 1(b)), the film is composed of 100–200 nm lumps and some short rodlike particles containing some tiny particles. Moreover, among these large particles, there are some micropore–mesopore structures. With an increase of Fe₃O₄, the number of lump structures increases (Fig. 1(c)). The particles in the film of PEDOT/Fe₃O₄-3 are the smallest, and the large particles almost disappear. Meanwhile a less porous structure can be observed (Fig. 1(d)). Fig. 1(e) shows a section view SEM image of PEDOT/Fe₃O₄-2 and one can see that the thickness of the film is about 200 nm.

Fig. 1 (a) SEM of PEDOT, (b) SEM of PEDOT/Fe₃O₄-1, (c) SEM of PEDOT/Fe₃O₄-2, (d) SEM of PEDOT/Fe₃O₄-3 (the bar is 100 nm in the photographs), (e) section view SEM image of PEDOT/Fe₃O₄-2 (f) FTIR of EDOT and PEDOT and (g) XRD of Fe₃O₄.

To clarify the elemental composition of the composites, FTIR experiments and XRD were carried out. Fig. 1(g) reveals the XRD pattern of the final Fe₃O₄, which is in good agreement with Fe₃O₄ (JCPDS #75-0449). In Fig. 1(g), the 2θ diffraction peaks at 30.36°, 35.8°, 43.5°, 57.5° and 63.15°, are assigned to the Fe₃O₄ planes of (220), (311), (400), (511), and (440), respectively.

Fig. 1(f) shows the FTIR spectroscopy of EDOT (A) and PEDOT (B). Characteristic peaks of EDOT are mainly observed at 1750–500 cm⁻¹ in the curve. Two strong and sharp absorption bands at 1522 cm⁻¹ and 1486 cm⁻¹ arise from the asymmetrical stretching vibration and symmetrical stretching vibration of C=C in the thiophene ring, respectively. The peaks at 1365 cm⁻¹, 930 cm⁻¹ and 892 cm⁻¹ respectively originate from the C–C stretching modes, C–S stretching vibration and unsaturated hydrocarbon keys in the thiophene ring. From curve (B), the intensity of the absorption characteristic bands from PEDOT decreases and the peak position shifts. In the thiophene ring, the bands due to the asymmetrical stretching vibration of C=C, the stretching modes of C–C and the stretching vibration of C–S shift towards the wavenumber values of 1515 cm⁻¹, 1332 cm⁻¹ and 981 cm⁻¹, respectively. Meanwhile, the peak of unsaturated hydrocarbon keys disappears, suggesting that the polymerization pattern of EDOT is α–α′.¹⁵

3.2 Photovoltaic performance of DSSCs

The performance for the DSSCs based on different counter electrodes was measured under one illumination (AM 1.5 G, 100 mW cm⁻²) and PCEs and fill factors are calculated according to the following equations:³¹where J_SC is the short-circuit current density, J_max is the current density at maximum power output, and V_OC is the open-circuit voltage and V_max is the voltage at the maximum power output. Here, P_in is the luminous flux which shone on the DSSCs.

In order to determine the optimal thickness of the counter electrode, films were prepared by spin-coating a PEDOT/Fe₃O₄-2 precursor solution from one to four times and the J–V curves are shown in Fig. 2(a) and the results are shown in Table 1. A thinner counter electrode film revealed a bad performance and as the thickness increased, the performance of the counter electrode became better. However, when the number of spin-coatings was over three, the PCE of the DSSC decreased. In our opinion, as the precursor solution placed on the FTO was sparse, the polymerization product (PEDOT) was also sparse. As a result, the film was imperfect and the catalytic properties were not satisfactory. However, too much precursor solution placed on the FTO afforded a thick electrode film and the electronic transmission distance in the counter electrode became longer. In summary, the spin-coating of the precursor solution is optimized as three times, which leads to a film thickness of about 200 nm.

Fig. 2 (a) Photocurrent–voltage curves of the DSSCs with different counter electrode thicknesses. (b) Photocurrent–voltage curves of the DSSCs based on different counter electrodes.

Table 1 Photovoltaic parameters of the DSSCs with different counter electrode thicknesses

Spin-casting times	VOC (V)	JSC (mA cm^−2)	FF	PCE (%)
One	0.727	15.6	0.470	5.33
Two	0.742	17.5	0.562	7.31
Three	0.740	18.6	0.630	8.69
Four	0.738	17.6	0.630	8.16

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The J–V curves of DSSCs based on different counter electrodes are shown in Fig. 2(b) and photovoltaic parameters are listed in Table 2. It is seen that the V_OC, J_SC, FF, and PCE are 0.740 V, 16.3 mA cm⁻², 0.615 and 7.40%, respectively, for DSSCs with in situ-polymerized PEDOT. When pure Fe₃O₄ was used as the CE, the DSSC showed a poor photovoltaic performance (PCE = 4.02%). Interestingly, when Fe₃O₄ was added to the CEs as an additive, the photovoltaic performance varied obviously. The V_OC of DSSCs based on PEDOT/Fe₃O₄ changed slightly, but the value of the J_SC and FF are dependent on the Fe₃O₄ content in the composite CEs. Maximum values for both the J_SC and FF were obtained in the DSSCs with PEDOT/Fe₃O₄-2, and as a result, the PCE of the DSSC with PEDOT/Fe₃O₄-2 is optimal (8.69%). Compared with Pt CEs (PCE = 8.35%), PEDOT/Fe₃O₄-2 also revealed good performance: an equal FF and a larger J_SC brought a slightly higher PCE value. Several reasons could be under consideration including active area, catalytic activity and film conductivity for CEs. To analyze these characters, Cyclic Voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Tafel polarization measurements were conducted.

Table 2 Photovoltaic parameters of the DSSCs with different counter electrodes

Electrodes	VOC (V)	JSC (mA cm^−2)	FF	PCE (%)
PEDOT	0.740	16.3	0.615	7.40
PEDOT/Fe3O4-1	0.744	17.7	0.611	8.04
PEDOT/Fe3O4-2	0.740	18.6	0.630	8.69
PEDOT/Fe3O4-3	0.743	14.9	0.606	6.73
Fe3O4	0.677	13.8	0.430	4.02
Pt	0.742	17.9	0.631	8.38

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3.3 Electrochemical measurements

To examine the reduction reaction of I₃⁻ on the CEs, cyclic voltammogram (CV) tests were carried out. The cyclic voltammograms (CVs) of the samples were measured in a three-electrode electrochemical cell with an electrochemical workstation (CHI 660C, Shanghai Chenhua Co., Ltd, China) by using a CE as the working electrode, a platinum wire electrode as the counter electrode and a saturated Ag/AgCl electrode as the reference electrode. Fig. 3(a) shows the CVs for the PEDOT, PEDOT/Fe₃O₄, Pt and Fe₃O₄ electrodes at a scan rate of 50 mV s⁻¹, and two pairs of redox peaks (OX₁ and RED₁, OX₂ and RED₂) can be observed in all of the spectrograms. This demonstrates that all kinds of electrodes can reduce I₃⁻ to I⁻ in order to ensure the dye’s renewability.

Fig. 3 (a) CVs for Pt, Fe₃O₄, PEDOT, PEDOT/Fe₃O₄-1, PEDOT/Fe₃O₄-2 and PEDOT/Fe₃O₄-3 at a scan rate of 50 mV s⁻¹, (b) CVs for PEDOT/Fe₃O₄-2 with different scan rates (from inner to outer: 0.025, 0.050, 0.075, 0.100, and 0.125 V s⁻¹), (c) the redox peak current versus the square root of scan rate. (d) EIS measurements of the dummy cell fabricated with two identical PEDOT, PEDOT/Fe₃O₄-1, PEDOT/Fe₃O₄-2, PEDOT/Fe₃O₄-3, Pt and Fe₃O₄ and equivalent circuit model for the I⁻/I₃⁻ reaction (the scatter plots are obtained from experimental measurements and the line is the simulation of the resultant spectra and the inset is the complete EIS of the dummy cell fabricated with two identical Fe₃O₄) (e) equivalent circuit model for the I⁻/I₃⁻ reaction.

The reduction reactions on the cathodic electrode and the oxidation reactions on the anode can be assigned as follows:³²

OX₁ at anode: 3I⁻ → I₃⁻ + 2e⁻ (3)

OX₂ at anode: 2I₃⁻ → 3I₂ + 2e⁻ (4)

RED₁ at cathode: I₃⁻ + 2e⁻ → 3I⁻ (5)

RED₂ at cathode: 3I₂ + 2e⁻ → 2I₃⁻ (6)

Usually, the first pair of redox peaks attracts more attention: the absolute value of the cathodic reduction peak current density (|I_RED1|) is associated with the catalytic activity velocity and peak-to-peak separation (E_PP) reflects the catalytic activity of the redox reaction.³ From Fig. 3(a), the PEDOT/Fe₃O₄-2 CE has the largest cathodic peak current (0.999 mA cm⁻²), compared with the PEDOT CE (0.906 mA cm⁻²), Pt CE (0.802 mA cm⁻²), PEDOT/Fe₃O₄-1 (0.936 mA cm⁻²) and PEDOT/Fe₃O₄-3 (0.804 mA cm⁻²). This means a faster redox reaction rate for I₃⁻ reduction on the PEDOT/Fe₃O₄-2 CE. Meanwhile, the E_PP values decrease in the order of PEDOT/Fe₃O₄-2 (0.361 V) < PEDOT/Fe₃O₄-3 (0.396 V) < PEDOT/Fe₃O₄-1 (0.415 V) < PEDOT (0.433 V) < Pt (0.558 V) < Fe₃O₄ (0.719 V). As mentioned above, the CE based on PEDOT/Fe₃O₄-2 is optimal and hybrid CEs have a larger |I_RED1| and smaller E_PP than the CE based on the pure material, illustrating that the moderate addition of Fe₃O₄ can improve the catalytic activity velocity and catalytic activity of the redox reaction.

Fig. 3(b) and (c) investigates the relationships between the peak current density and the square root of the scan rate for the PEDOT/Fe₃O₄-2 CE. According to the two CVs, with increasing scan rate, the cathodic peak and anodic peak current densities of the PEDOT/Fe₃O₄-2 CEs gradually and regularly shifted to the negative and positive directions, respectively. Meanwhile, the current density versus the (scan rate)^1/2 plots are almost linear, revealing that the redox reaction on the PEDOT/Fe₃O₄-2 CEs is a diffusion limitation reaction, and there is no specific interaction between the I⁻/I₃⁻ redox couple and PEDOT/Fe₃O₄-2 CE.³³

Electrochemical impedance spectroscopic (EIS) measurements were also exploited to analyze the symmetrical cell. An acetonitrile electrolyte containing 0.05 M I₂, 0.1 M LiI, 0.6 M tetrabutyl ammonium iodide and 0.5 M TBP was injected into cells and a 50 μm Surlyn film was used to separate the two films and to seal the cells. The Nyquist curves of the cells with PEDOT, PEDOT/Fe₃O₄ and Fe₃O₄ are presented in Fig. 3(d) and the detailed EIS fitting parameters, obtained from the equivalent circuit in Fig. 3(e), are displayed in Table 3.

Table 3 Electrochemical parameters for various counter electrodes

Electrodes	Test area (cm^2)	RS (Ω cm^2)	RCT (Ω cm^2)	1/2Cdl (μF cm^2)	ZN (Ω cm^2)
PEDOT	0.342	11.2	6.96	3.75	1.60
PEDOT/Fe3O4-1	0.350	12.6	5.27	11.8	0.960
PEDOT/Fe3O4-2	0.349	12.7	2.22	53.0	0.822
PEDOT/Fe3O4-3	0.359	10.7	10.8	3.64	1.63
Fe3O4	0.432	11.1	799	1.84	1668
Pt	0.349	12.14	1.16	1.20	1.13

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Typically, there are two semicircles in the frequency range of 0.1–100 kHz. The high frequency intercept of the left semicircle symbolizes the serial resistance (R_S) of symmetrical cells. One can see from Table 3, the variation of R_S is negligible (10.7–12.7 Ω) for all cells due to the same FTO substrate in cells. The charge transfer resistance (R_CT) at the CE/electrolyte interface and the electrical double-layer capacitor (CPE) are also represented in the left semicircle. In this study, the R_CT value for PEDOT/Fe₃O₄ decreased with adding just 2 mg ml⁻¹ of Fe₃O₄ in the precursor solution, but increased remarkably beyond that concentration. However, the change of the CPE has a reverse trend and is more strongly influenced by the Fe₃O₄ content. The Nernst diffusion impedance (Z_N) of the triiodide/iodide couple in the electrolyte can be observed from the right semicircle in the low-frequency range it arises from and it has a similar variation tendency with R_CT. Compared with the Pt CE, the R_CT and Z_N of PEDOT/Fe₃O₄-2 are 2.22 Ω cm² and 0.822 Ω cm², respectively and these values are comparable to the values for Pt (R_CT = 1.15 Ω cm² and Z_N = 1.13 Ω cm²). Furthermore, the values of 1/2CPE for PEDOT/Fe₃O₄-2 and Pt are 53.0 μF cm² and 1.20 μF cm², respectively.

Tafel polarization measurements, which are another means to aid the investigation of the interfacial charge-transfer properties of the redox couple in the electrolyte on the CEs, were carried out by using two identical electrodes in an acetonitrile electrolyte containing 0.05 M I₂, 0.1 M LiI, 0.6 M tetrabutyl ammonium iodide and 0.5 M TBP. Fig. 4 shows the Tafel polarization curves of PEDOT, PEDOT/Fe₃O₄-2, PEDOT/Fe₃O₄-1, PEDOT/Fe₃O₄-3, Pt, and Fe₃O₄ CEs.

Fig. 4 Tafel curves of the symmetrical dummy cells fabricated with two identical electrodes.

A typical Tafel curve covers three zones: the diffusion zone, Tafel zone and potential zone. J₀ and J_lim can be obtained as the intercept of the extrapolated linear region of the curve when the over-potential is zero and in the curve at high potential (horizontal part), respectively.³⁴ From Fig. 4, the slopes of the cathodic and anodic branches of the plots in the Tafel zone for the Pt CE are the highest followed by PEDOT/Fe₃O₄-2. In the corresponding slopes, the slopes of the Tafel curve for the composite had no evident changes. According to Fig. 4, the J₀ decreased in the order of PEDOT/Fe₃O₄-2 > Pt > PEDOT/Fe₃O₄-1 > PEDOT > PEDOT/Fe₃O₄-3 > Fe₃O₄. So the PEDOT/Fe₃O₄-2 electrode gives an excellent J_lim and Fe₃O₄ shows the lowest value of J_lim. PEDOT/Fe₃O₄-1, Pt, PDOT and PEDOT/Fe₃O₄-3 have medium values.

4. Discussion

SEM images in Fig. 1 show that a small Fe₃O₄ content results in large lumps and some micropore–mesopore structures. The lump size decreased as the Fe₃O₄ content increased. This is probably because the small amount Fe₃O₄ can be totally covered with PEDOT and bonded to lumps. The space between these large lumps can form in the micropore–mesopore structure. Meanwhile, plenty of PEDOT can prevent Fe₃O₄ particles from connecting to one another and the electronic transmission channel is more consecutive. However, when the Fe₃O₄ content increases, there is not enough PEDOT to bond them or cover them completely. As a result, more connections among Fe₃O₄ particles are produced and more surfaces of Fe₃O₄ are exposed, as shown in Fig. 5. More active areas and more consecutive electronic transmission channels are responsible for the large J_SC and PCE of DSSCs with PEDOT/Fe₃O₄-2. This is consistent with the results from the electrochemical testing. According to the CVs, the CE based on PEDOT/Fe₃O₄-2 is optimal with the largest |I_RED1| and smallest E_PP, which suggests a better catalytic activity velocity and catalytic activity of the redox reaction.²³

Fig. 5 Schematic proposing the PEDOT/Fe₃O₄ particle surfaces.

For the reduction of triiodide, R_CT is related to the catalytic activity of different CEs. The smaller the value of R_CT is, the better the catalytic activity of the CE is. Additionally, the CPE depends on the surface area of the CE: a larger value of CPE means a larger surface area of the CE. When the concentration of Fe₃O₄ in the precursor solution was 2 mg ml⁻¹, the CE had the smallest R_CT and the largest CPE. So, PEDOT/Fe₃O₄-2 also has the best performance and the parameter of PEDOT/Fe₃O₄-2 can be comparable to the parameter of Pt. What’s more, the FF is sensitive to the internal resistance in a DSSC, including both the R_S and the R_CT. The smaller the resistances are, the larger the value of the FF is. According to the EIS, the FF of the DSSCs based on PEDOT/Fe₃O₄-2 and the DSSCs based on Pt are larger than that of other CEs, which mainly depends on the lower value of R_CT.

In the Tafel curve, there are two important parameters related to the catalytic activity of the catalysts: the exchange current densities (J₀) and the limit diffusion current density (J_lim).³⁵

In the Tafel zone, larger slopes of the cathodic and anodic branches of the plots means that the electrode can trigger the reduction of I₃⁻ to I⁻ more effectively. J₀ is related to the electrocatalytic activity. R_CT can be estimated as eqn (7) and the resulting trend is in good accordance with the lower charge-transfer resistance R_CT measured by EIS. J_lim is determined by the diffusion of the I⁻/I₃⁻ redox couple in the electrolyte. Based on eqn (8), a large J_lim reveals the large diffusion coefficient in the diffusion zone. Where R is the gas constant, T is the temperature, F is the Faraday constant, and n is the number of electrons involved in the reaction at the electrode, here, n = 2, C is the concentration of I₃⁻, L is distance of electrodes. Thus, the highest diffusion coefficient (D) value is achieved by the PEDOT/Fe₃O₄-2 electrode, and this proves that PEDOT/Fe₃O₄-2 electrode holds a fast diffusion velocity of the redox couple in the electrolyte and the investigation from the Tafel polarization and EIS data are consistent.

5. Conclusion

The in situ polymerized PEDOT/Fe₃O₄ composite was explored as a Pt free CE for DSSCs. This hybrid material showed excellent performance and the highest power conversion efficiency (8.69%) was revealed by the DSSC with PEDOT/Fe₃O₄-2. Under the same conditions, the PCE of the Pt DSSC is 8.35%. As shown by CV, EIS and Tafel respectively, the improvement of the PCE should be due to the enhancement of the active area and the lower charge-transfer resistance of the film.

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