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Sulfamic Acid-Doped Polyaniline Nanofibers Thin Film-Based Counter Electrode: Application in Dye-Sensitized Solar Cells Sadia Ameen,† M. Shaheer Akhtar,‡ Young Soon Kim,† O-Bong Yang,‡ and Hyung-Shik Shin*,† Energy Materials and Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National UniVersity, Jeonju-561756, Republic of Korea ReceiVed: December 21, 2009; ReVised Manuscript ReceiVed: January 29, 2010
Uniform polyaniline nanofibers (PANI NFs) and chemically doped sulfamic acid (SFA) PANI NFs, synthesized via template free interfacial polymerization process, were used as new counter electrodes materials for the fabrication of the highly efficient dye-sensitized solar cells (DSSCs). The PANI NFs-based fabricated DSSCs exhibited a solar-to-electricity conversion efficiency of ∼4.0%, while the SFA-doped PANI NFs-based DSSC demonstrated ∼27% improvement in the solar-to-electricity conversion efficiency. The obtained solar-toelectricity conversion efficiency for SFA-doped PANI NFs-based DSSC was 5.5% under 100 mW/cm2 (AM1.5). The enhancement in the conversion efficiency was due to the incorporation of SFA into the PANI NFs, which resulted in the higher electrocatalytic activity for the I3-/I- redox reaction. 1. Introduction Dye-sensitized solar cells (DSSCs) have been widely used due to their easier fabrication process,1 high efficiency, low cost,2,3 etc. Generally, a platinum-conducting glass is employed as a counter electrode for the DSSCs to catalyze the reduction (I3- to I-) of redox electrolyte to keep the low overvoltage at reasonable photocurrent density, where the Pt layer acts as an electrocatalyst.4-6 However, conventional conductive glasses such as indium tin oxide (ITO) or fluorine-doped tin oxide (FTO) without a catalyst present a low rate of reduction for counter electrodes, and thus the counter electrode must be coated with a catalytic material to accelerate the reaction.7 Platinum (Pt) is one of the most expensive rare metals on the earth.8 The increased cost of Pt has conjured the replacement of Pt, as the counter electrodes, with the other cheaper materials to reduce the production cost of the cells.9,10 The catalytic activity of inexpensive conducting polymers in DSSCs such as poly (3,4ethylenedioxythiophene) (PEDOT),11-14 polypyrrole (PPy), and polyaniline (PANI) has been investigated to find a replacement for the expensive Pt-based counter electrode.15 These conducting polymers are found to be the promising candidates for the counter electrode materials due to their unique properties including the high conductivity, good stability, catalytic activity, and low cost for I3- reduction.16,17 Among various conducting polymers, PANI is one of the most extensively studied conducting polymers due to its easy synthesis, high conductivity, good environmental stability, and interesting redox catalytic properties.18,19 Therefore, considering these interesting properties, researchers are inclined to use nanostructures of PANI (nanoparticles, nanorods, nanofibers, etc.) for the fabrication of efficient electronic nanodevices as reported in the literature.20-24 Even though literature is available on the use of PANI NFs for the * Corresponding author. Fax: +82-63-270-2306. E-mail: hsshin@ chonbuk.ac.kr. † Energy Materials and Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju-561756, Republic of Korea. ‡ School of Semiconductor and Chemical Engineering & Solar Energy Research Center, Chonbuk National University, Jeonju-561756, Republic of Korea.
fabrication of nanodevices, still there is a need to work on PANI NFs to increase its applicability. Inspired by the latest findings based on the alternatives of Pt with conducting polymers,25-27 we have attempted to use the undoped and doped PANI NFs as efficient counter electrodes for the fabrication of DSSCs. In this Article, a simple method has been developed to increase the conductivity of PANI NFs using sulfamic acid (SFA) as a dopant. The SFA is one of the best materials for PANI doping in terms of its important properties such as high solubility, easy handling, nonvolatile stable solid acid, and low corrosiveness. The PANI NFs counter electrode-based fabricated DSSC exhibits a solar-to-electricity conversion efficiency of ∼4.0%, while the SFA-doped PANI NFs counter electrodebased DSSC demonstrates ∼27% improvement in the solar-toelectricity conversion efficiency. The obtained solar-to-electricity conversion efficiency for SFA-doped PANI NFs counter electrodebased DSSC is 5.5% under 100 mW/cm2 (AM1.5). 2. Experimental Section The typical interfacial reaction for the synthesis of PANI NFs was performed by taking a 5 mmol amount of aniline monomer, dissolved in 10 mL of chloroform (organic phase), into a glass vial. Ammonium peroxydisulphate (APS) of 1 mmol, dissolved in 10 mL of 1.2 M HCl solution, was later added to the glass vial. The solution was kept for 24 h for complete polymerization. The synthesis of SFA-doped PANI NFs was carried out in a similar way with the addition of the aqueous 2% SFA (0.01 g) dopant solution to the PANI NFs. Finally, SFA-doped PANI NFs were obtained through filtering and washing with deionized water and dried at 60 °C for 18 h. Dried SFA-doped PANI NFs as well PANI NFs were separately added into 10 mL of chloroform solvent, and the mixture was subjected to ultrasonic irradiation for 15 min to achieve the uniform dispersion of NFs. Undoped and SFA-doped PANI NFs were deposited on the surface of the cleaned fluorinated tin oxide (FTO, 8Ω/sq, 80% transmittance in the visible light, Hartford Glass Co.) glass by spin-casting method at room temperature. Finally, the deposited PANI NFs FTO glass substrates were annealed at 150 °C for 20 min to achieve PANI
10.1021/jp912037w 2010 American Chemical Society Published on Web 02/22/2010
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Figure 1. Possible doping mechanism of PANI NFs by SFA.
NFs thin film electrode. The nanoporous TiO2 thin film was prepared by forming a slurry of titania (TiO2, Degussa P25) nanoparticles powder (500 mg) with 4% aqueous polyethylene glycol (PEG) solution. The slurry was then spread by doctor blade technique on FTO glass substrate by preparing an active area of 0.25 cm2 with the film thickness of ∼12 µm. The TiO2coated substrate was sintered at 450 °C for 30 min. Thereafter, the TiO2 thin film substrates were immersed in the 0.3 mM ruthenium(II) 535 bis-TBA (N-719, Solaronix) dye solution for 24 h. The dye-adsorbed TiO2 electrode was then rinsed with ethanol and dried under a nitrogen stream. The prepared PANI NFs counter electrode was placed over the dye-adsorbed TiO2 working electrode, and the edges of the cell were sealed with 60 µm thick Surlyn sheet (SX 1170-60, Solaronix). Sealing was accomplished by hot-pressing of the two electrodes together at 80 °C. Finally, the specified composition of the electrolyte (0.5 M LiI, 0.05 mM I2, and 0.2 M tert-butyl pyridine in acetonitrile) was introduced through holes in the counter electrode using a syringe on the dye-immobilized TiO2 thin film electrode, and finally was sandwiched together using cell holders. The morphological observation was done by using a field emission scanning electron microscope (FESEM, Hitachi S-4700). The morphological characterization was obtained via a transmission electron microscope (TEM, H-7650, Hitachi, Japan). UV-visible was used for the optical properties of the synthesized PANI NFs and SFA-doped PANI NFs using UV-vis spectroscopy (UV-2550, Shimadzu, Japan). Electrical conductivity of samples was measured by two-probe DC method at room temperature using potentiostat (VersaSTAT 4). Cyclic voltammetry (CV) was carried out in a three-electrode one compartment cell with PANI NFs and SFA-doped PANI NFs working electrode, Pt foil counter electrode, and an Ag/AgCl reference electrode dipped in an acetonitrile solution of 10 mM LiI, 1 mM I2, and 0.1 M LiClO4 and was performed by using the VersaSTAT4 electrochemical measurement system. A current density (J)-voltage (V) curve was measured using a computerized digital miltimeter (model 2000, Keithley) with a variable load. A 1000 W metal halide lamp served as a light source, and its light intensity was adjusted to simulated AM1.5 at 100 mW/cm2 with a Si photo detector fitted with a Ka-5 filter as a reference, calibrated at NREL, USA. A black tape mask was placed on top of the cell during the J-V measurement. The operating temperature in the range of 20-30 °C was maintained by the small cooling fan. With this setup, the spectral mismatch parameter for dye cells is within 3% of unity,28 allowing for measurements of actual solar efficiencies without
Figure 2. FESEM images of (a) SFA-doped PANI NFs and (b) PANI NFs.
the need for correction. The incident photon-to-current conversion efficiency (IPCE) as a function of wavelength was measured with a 150 W Xe lamp in combination with a 520 nm monochromator, and a Keithley 236 source measure unit was controlled by computer software. 3. Results and Discussion The proposed doping mechanism for PANI with SFA is shown in Figure 1. First, aniline monomer is polymerized to PANI in the presence of APS as catalyst. It is reported that the acid doping of PANI NFs forms the hydrogen bonding with PANI.29 Generally, SFA exhibits a good pKa value of 1.04 in water and exits as various radicals and ions like NH2SO3 · , NH2 · , NH3+SO3 · NH3+, and SO3-.30 Thus, these free radicals and ions of SFA form the hydrogen bonding with the -NH2 group of the PANI NFs backbone during the doping procedure. Figure 2 shows FESEM images of PANI NFs and SFA-doped PANI NFs. PANI NFs (Figure 2b) exhibit well-defined fibrous morphology with the diameter of 30 nm. The diameter of PANI NFs has considerably increased to ∼40 nm after doping with SFA, as shown in Figure 2a. The doping of SFA by chemical method causes some aggregation of PANI NFs, which might result in the formation of voids into the fibrous network of PANI NFs.31 Additionally, Figure 3 shows the TEM images of PANI NFs and SFA-doped PANI NFs to justify the doping effect on the morphology of PANI NFs. From Figure 3a, the entrapping of SFA into the fibers of PANI results in the increase of average diameter by ∼40 nm as compared to undoped PANI NFs (Figure 3b). The counter electrode made from these doped fibrous structures might facilitate the redox reaction of the electrolyte into the network of PANI NFs. The UV-vis spectra of PANI NFs and SFA-doped PANI NFs are displayed in Figure 4. In case of PANI NFs, the spectra
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Figure 3. TEM images of (a) SFA-doped PANI NFs and (b) PANI NFs.
Figure 5. Cyclic voltammetry (CV) of iodide species on PANI NFs and SFA-doped PANI NFs electrodes in acetonitrile solution with 10 mM LiI, 1 mM I2, and 0.1 M LiClO4.
Figure 4. UV-vis spectra of PANI NFs and SFA-doped PANI NFs.
exhibit a prominent peak at 298 nm. However, due to doping, the splitting and the appearance of two new peaks at 288 and 296 nm are observed. Additionally, the hump at 358 nm is observed for pristine PANI NF, which corresponds to π-π* transitions centered on the benzenoid rings. Whereas, SFAdoped PANI NFs exhibit the peak shifting to the higher wavelength from 358 to 380 nm, indicating the formation of a doping level due to the “exciton” transition, which may be caused by the interband charge transfer from benzenoid to quinoid moieties of the protonated PANI NFs. Moreover, in the UV-visible spectra of SFA-doped PANI NFs, peak (i) retains its position and a slight blue shift in the peak at 296 nm from 298 nm is noticed; and (ii) a considerably large red shift in peak at 380 nm from 358 nm is also observed. The observed red shift in the peaks may be assigned to the selective site for the interactions between SFA dopants and the quinoid ring of emeraldine salt (ES), facilitating the charge transfer between the quinoid unit of ES and the dopant via highly reactive imine groups. To understand the ion diffusivity and reaction kinetics of the I-/I3- redox couple, cyclic voltammetry (CV) has been carried out with an electrochemical system of different electrodes. Figure 5 shows the CV curves of PANI NFs and SFA-doped PANI NFs electrodes. Generally, in DSSC, electrons are injected into photo-oxidized dye from I- ions in the electrolyte, and the produced I3- ions are reduced at the counter electrode; the redox reactions are explained by eqs 1 and 2.32
3I- / I3- + 2e-
(1)
3I3- / 3I3 + 2e-
(2)
It could be seen that two pairs of redox waves are observed in the CV curves of the electrodes (Figure 5). The relative positive and negative pairs are associated with the redox reaction of I2/
Figure 6. J-V curve of a fabricated solar cell of PANI NFs and SFAdoped PANI NFs as counter electrodes in (a) light illumination of 100 mW/cm2 and (b) dark conditions. Inset of (b) shows the stability curve of DSSCs fabricated with SFA-doped PANI NFs counter electrodes under light illumination.
I3- and with the reaction of I3-/I-, respectively. The SFA-doped PANI NFs electrode exhibits a redox current density higher than that of the PANI NFs electrode. By the analysis of CV curves, SFA-doped PANI NFs electrode attains a reasonably high anodic peak current (Ia) of 0.24 mA/cm2 and cathodic peak current (Ic) of -0.17 mA/cm2 with a considerably high value of switching point (0.22 mA/cm2). However, the undoped PANI NFs electrode exhibits a low Ia of 0.21 mA/cm2 and Ic of -0.2 mA/ cm2 with a low switching point (0.17 mA/cm2). These results suggest that the high peak current might increase the redox reaction rate at SFA-doped PANI NFs counter electrode, which may attribute to its high electrical conductivity and surface area.33 Moreover, the higher redox current density describes the stronger electrocatalytic activity toward the reduction of I2 to I- ions and increases the reduction of the I3- ions to I- ions in the redox couple at the counter electrode.34 Therefore, the higher
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TABLE 1: Electrical Conductivity and Performance of Solar Cells Fabricated with PANI NFs and SFA-Doped PANI NF as Counter Electrodes photovoltaic performance counter electrode PANI NFs SFA-doped PANI NFs
electrical conductivity (S/cm) -8
10.89 × 10 2.06 × 10-4
redox current density of SFA-doped PANI NFs counter electrode clearly shows the superior electrocatalytic activity. Figure 6 shows the J-V curve of the DSSCs fabricated with the counter electrodes made of PANI NFs and SFA-doped PANI NFs under dark and light intensity of 100 mW/cm2 (1.5AM). The J-V curves of all cells are shown in Figure 6a and summarized in Table 1. DSSCs fabricated with SFA-doped PANI NFs counter electrode achieve a high conversion efficiency (η) of 5.5% with a high short circuit current (JSC) of 13.6 mA/cm2, open circuit voltage (VOC) of 0.74 V, and fill factor (FF) of 0.53. It can be seen that the conversion efficiency increases from 4.0% to 5.5% after SFA doping into the PANI NFs. DSSC fabricated with SFA-doped PANI NFs counter electrode has appreciably enhanced the conversion efficiency by ∼27% more than that of DSSC fabricated with PANI NFs counter electrode. Further, the SFA-doped PANI NFs counter electrode has significantly increased the JSC and VOC of ∼20% and ∼10%, respectively, as compared to the DSSC fabricated with PANI NFs counter electrode. It might have resulted from the higher electrocatalytic activity of SFA-doped PANI NFs, which serves a good path for the charge transport of I-/I3- redox (as described in Figure 5). It indicates that the SFA doping has increased the fast reaction of I-/I3- species at counter electrode. Therefore, the superior photovoltaic properties such as η, JSC, and VOC of the cell are attributed to the sufficiently high conductivity and electrocatalytic activity of doped PANI NFs, which alleviates the reduction of I3- at the thin SFA-doped PANI NFs layers. This result is consistent with CV results. Furthermore, Figure 6b shows the dark current-voltage characteristics of PANI NFs and SFA-doped PANI NFs counter electrode-based DSSCs. Both devices show low dark current at the same forward bias voltage of ∼0.500 V. After SFA doping on PANI NFs, the dark current has slightly shifted to higher voltage due to the higher resistance of the reaction between electrons in the TiO2 layer with I3- in the electrolyte layer. It shows the increased VOC with less variation in FF. The VOC of DSSC is generally defined by the difference between I-/I3redox potential and the Fermi energy level in TiO2. The increase in VOC is related to the negatively shifted conduction band of TiO2 and positively shifted I-/I3- redox energy level, which originated from the low dark current of the device.35 Moreover, the electrons enter into the cell through the conduction band of the TiO2 electrode and reduce the I3- ions followed by the oxidation of resulted I- ions at counter electrode in the dark under forward bias.36 In our case, the high VOC is attributed to the low dark current at high forward voltage and high electrocatalytic activity of SFA-doped PANI NFs. They may change the energy band levels of the TiO2 conduction band and I-/I3redox potential because the SFA-doped PANI NFs oxidized Iions to produce the high amount of I3- ions. This result is in excellent agreement with CV curves of counter electrode (Figure 5). However, good attachment of PANI NFs on FTO glass confirms the device stability to the long-term exposure of the electrolyte, which results in low dark current in the device. DSSCs fabricated with SFA-doped PANI NFs counter electrode exhibit less variation in conversion efficiency after 15 days
IPCE (%)
VOC (V)
JSC (mA/cm2)
FF
η (%)
54 70
0.68 0.74
10.94 13.62
0.54 0.53
4.0 5.5
(Figure 6b, inset), indicating that the prepared counter electrode is stable to the corrosive electrolyte. The incident photon-to-current conversion efficiency (IPCE) measurement is carried out to explain the photocurrent of fabricated DSSCs. The IPCE is plotted as a function of excitation wavelength and explained by the following equation:
IPCE (%) ) 1240JSC/λPin
(3)
where JSC is the short circuit current, λ is the wavelength of the incident light, and Pin is the power of the incident light. Figure 7 presents the IPCE curves of DSSCs fabricated with PANI NFs and SFA-doped PANI NFs counter electrodes. DSSCs fabricated with PANI NFs counter electrode exhibit the low IPCE of ∼54% in the absorption range of 400-650 nm. The IPCE value is prominently increased by ∼70% with the SFAdoped PANI NFs counter electrode-based DSSCs. It is noteworthy that the IPCE of the device is considerably enhanced by ∼24% upon SFA doping on PANI NFs-based counter electrodes. The enhanced IPCE results are consistent with high electrical conductivity37 and the electrocatalytic activity of the SFA-doped PANI NFs electrode. The enhanced IPCE in DSSCs with SFA-doped PANI NFs electrode results in the high JSC and photovoltaic performance, which are related to its high electrical conductivity and the higher reduction of I3- to I- in the electrolyte at the interface of PANI NFs layer and electrolyte. Thus, the high JSC, VOC, and the photovoltaic performance of the DSSC, fabricated by SFA-doped PANI NFs, impute the influence of SFA doping on PANI NFs. The doped PANI NFs as counter electrode could be accounted for as the potential counter electrode materials to substitute the traditional Pt counter electrode. 4. Conclusions Undoped PANI NFs and SFA-doped PANI NFs are prepared and deposited on a conducting FTO glass to construct a PANI counter electrode for the fabrication of efficient DSSCs. FESEM and TEM images show the enhancement in the average diameter
Figure 7. IPCE curves of the DSSCs fabricated with PANI NFs and SFA-doped PANI NFs counter electrodes.
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of PANI NFs to 40 nm due to entrapping of dopant into the PANI NFs. The UV-vis spectra support the existence of SFA dopant into the nanofibrous structure. The SFA-doped PANI NFs counter electrode increases the JSC and VOC up to ∼20% and ∼10%, respectively, as compared to the DSSC, fabricated with the PANI NFs counter electrode. The superior photovoltaic properties such as η, JSC, VOC, and IPCE of the cell are credible to the sufficiently high conductivity and electrocatalytic activity of SFA-doped PANI NFs, which alleviate the reduction of I3at the SFA-doped PANI NFs thin layer. The ease of preparation and cheap cost allow undoped and doped PANI NFs electrode to be a plausible alternative counter electrode with improved electrocatalytic and photovoltaic properties of DSSCs. Acknowledgment. The grant of Post Doc program, Chonbuk National University (2009) and NRF research grant no. R012007-000-20810-0 are fully acknowledged. We would also like to thank Mr. Kang Jong-Gyun, Center for University-Wide Research Facilities, Chonbuk National University for his cooperation in TEM images. We also acknowledge the Korea Basic Science Institute, Jeonju branch, for use of their FESEM facility. References and Notes (1) Regan, B. O.; Gratzel, M. Nature 1991, 353, 737–740. (2) Gratzel, M. Nature 2001, 414, 338–344. (3) Wu, J. H.; Lan, Z.; Lin, J. M.; Huang, M. L.; Hao, S. C.; Sato, T.; Yin, S. AdV. Mater. 2007, 19, 4006–4011. (4) Kay, A.; Gra¨tzel, M. Sol. Energy Mater. Sol. Cells 1996, 44, 99– 117. (5) Papageorgiou, N.; Athanassov, Y.; Armand, M.; Bonhote, P.; Pettersson, H.; Azam, A.; Gra¨tzel, M. J. Electrochem. Soc. 1996, 143, 3099– 3108. (6) Papageorgiou, N.; Maier, W. F.; Gratzel, M. J. Electrochem. Soc. 1997, 144, 876–84. (7) Hauch, A.; Georg, A. Electrochim. Acta 2001, 46, 3457–3466. (8) Smestad, G.; Bignozzi, C.; Argazzi, R. Sol. Energy Mater. Sol. Cells 1994, 32, 259–272. (9) Papageorgiou, N.; Liska, P.; Kay, A.; Gra¨tzel, M. J. Electrochem. Soc. 1999, 146, 898–907. (10) Imoto, K.; Takahashi, K.; Yamaguchi, T.; Komura, T.; Nakamura, J.; Murata, K. Sol. Energy Mater. Sol. Cells 2003, 79, 459–469. (11) Yohannes, T.; Inganas, O. Sol. Energy Mater. Solar Cells 1998, 51, 193–202. (12) Saito, Y.; Kitamura, T.; Wada, Y.; Yanagida, S. Chem. Lett. 2002, 1060–1061.
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