ARTICLE pubs.acs.org/JPCC
Enhanced Electron Transport through Template-Derived Pore Channels in Dye-Sensitized Solar Cells Sarika Phadke,*,† Aurelien Du Pasquier,‡ and Dunbar P. Birnie, III§ †
National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune, India 411008 Cabot Superior MicroPowders, 5401 Venice Avenue, Albuquerque, New Mexico 87113, United States § Department of Materials Science and Engineering, Rutgers University, 607 Taylor Road, Piscataway, New Jersey 08854, United States ‡
ABSTRACT: Dye-sensitized solar cells use porous nanoparticle TiO2 coating as their photoanode. To obtain effective dye adsorption and better electrolyte permeation in the TiO2 coating, the porosity structure must be fully, threedimensionally interconnected. Templated processing offers the advantage of preparing TiO2 photoanodes in controlled shape and interconnection to the porosity structures that result, including the creation of dual size scale porosity in some cases. In the present work, an emulsion templating method was employed to obtain the dual porosity in the titania coating. We have studied the effect of the enhanced permeation of the electrolyte provided by these interconnected pore channels using electrochemical impedance spectroscopy. The change in internal resistances of the dye-sensitized solar cell during operation is correlated with the microstructural features using scanning electron microscopy, mercury porosimetry and IV characterization.
’ INTRODUCTION Dye-sensitized solar cells (DSSC) using nanocrystalline TiO2 coatings as the photoanode are technically and economically appealing, as a candidate for low-cost photovoltaic devices.14 In a dye-sensitized solar cell, the dye molecules, adsorbed on the titania surface, get photoexcited upon irradiation and inject electrons into the conduction band of the TiO2 nanoparticles. This electron injection process occurs much faster than electron relaxation back to the dye sensitizer, allowing charge separation to take place with high efficiency. The nanometer-sized, sintered TiO2 particles allow electronic conduction toward the transparent conductive oxide (TCO). The dye molecule recovers its initial electronic state by oxidizing nearby electrolyte ions, usually an organic solvent with an iodide/triiodide redox couple where the iodide recovers its initial electronic state by reduction of the triiodide at the back counter electrode. Thus, the circuit is completed by electron transport through the external load. The TiO2 photoanode is supposed to perform the following four major functions during the operation of the DSSC: (1) provide high surface area for plentiful dye adsorption sites; (2) accept electrons from the excited dye; (3) conduct electrons to the transparent conducting oxide front electrode; (4) help regeneration of the dye by allowing electrolyte conduction through the porous structure. In order to perform the first and the last function as stated above, the TiO2 film should possess meso- and macroporosity with interconnecting pore channels.59 Several methods have been tried to obtain a porous structure of TiO2 nanoparticles including combination of solgel and templating techniques.1019 Jiu13 used copolymer F-127 (poly(ethylene oxide)106-poly(propylene r 2011 American Chemical Society
oxide)70-poly(ethylene oxide)106) and surfactant CTAB (cetyltrimethylammonium bromide) to obtain templated growth of porous TiO2 films. A similar structure was obtained by Zukalova18 using Pluronic P-123. Although a high surface area was achieved in these works, the pore diameters obtained were quite small, in the range of 48 nm which affects the diffusion of redox species and penetration of the organic hole conductor adversely. In the earlier work of our research group, we demonstrated simple templating methods using aqueous latex polystyrene dispersion8,19 and oil-in-water emulsion14 to obtain TiO2 coatings with meso- and macroporosity, which showed improved film quality as well as solar cell performance. Very interesting microstructures of TiO2 coatings were obtained especially with the emulsion templating method,14 which utilized a stable emulsion of paraffin oil in water, dispersed with hydrophilic titania nanoparticles. The dispersion was blade-coated onto FTO-coated glass and heat treated slowly to remove the oil droplets which acted as soft templates to create pores in the TiO2 film. This method provided a very cost-effective and simple way to obtain uniform titania coatings with interpenetrating pore channels. Electrochemical impedance spectroscopy (EIS) is a steadystate method used to obtain current response of an electrochemical cell to an applied AC potential over a range of frequencies.20 Electrochemical impedance is usually measured with a very small input signal, which does not perturb the system Received: May 28, 2011 Revised: June 29, 2011 Published: July 15, 2011 18342
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Figure 1. Scanning electron micrographs of (a) nontemplated TiO2 coating, (b) 20 wt % emulsion-templated coating, (c) 40 wt % emulsion-templated coating (scale bar: 2 μm).
much. The data is represented as the Nyquist plot or Bode plot and analyzed by fitting to an equivalent electrical circuit model, based on physical electrochemistry of the system. Recently EIS has been widely used to analyze internal impedances of the dyesensitized solar cells and study various electronic and ionic processes that occur during the operation of the cell.2131 In the present work, EIS was used as a diagnostic tool to study the effect of emulsion templating on the electron transport energetics and kinetics of the photoelectrochemical processes in the dye-sensitized solar cells, especially in relation to the increased pore-channel openness and interconnection. EIS data for emulsion-templated and nontemplated cells were compared by using an appropriate equivalent circuit and analyzing the circuit parameters extracted from the experimental data.
’ EXPERIMENTAL METHODS Stable oil-in-water emulsions were prepared using paraffin oil and Tween 80/Span 80 emulsifier blend obtained from SigmaAldrich.14 Two samples with 20 and 40 wt % oil in water were used to perform the analysis. P25 Titanium dioxide nanoparticles were obtained from Evonik/Degussa. The P25 titania nanoparticles were dispersed in the emulsion using a Hamilton Beach Scovill 936 homogenizer. A control sample with only P25 TiO2 paste was prepared with no emulsion (referred to as nontemplated sample). Acetyl acetone and Triton X-100 additives were also used to prepare the dispersion to improve the coating quality and adhesion. A single layer of coating was deposited onto FTOcoated glasss by the doctor blade technique, using scotch tape to control the thickness. The coatings were dried in air for 15 min and then annealed at 450 °C for 30 min. At least four samples of each kind were prepared, and the average results have been reported. The microstructure of the coatings was studied using a Zeiss DSM 982 scanning electron microscope. Micromeritics AutoPore 9400 mercury porosimetry was used to characterize the pore structure. To make the dye-sensitized solar cells, first the blade-coated titania films were dipped in 0.3 mM N719 dye solution overnight and then assembled with a 25 μm thick microporous polyolefin separator and Pt-sputtered FTO glass counter electrode. The electrolyte was based on 3-methoxypropionitrile solvent with a recipe of 0.3 M 1-hexyl-3-methylimidazolium iodide, 0.5 M 4-tert-butylpyridine, 0.05 M iodine, and 0.01 M lithium iodide. An Oriel solar simulator operating at AM1.5 was used to obtain the currentvoltage characteristics of the solar cells. The electrochemical impedance spectroscopy was performed using an impedance analyzer (Solartron analytical, 1255B) connected to a potentiostat (Solartron Analytical, 1287).
Figure 2. Pore structure analysis of nontemplated TiO2 coating, 20 wt % emulsion-templated coating, and 40 wt % emulsion-templated coating using mercury porosimetry.
EIS spectra were measured over a frequency range of 101 to 106 Hz at 298 K under open circuit condition with 10 mV AC amplitude.
’ RESULTS AND DISCUSSION Microstructure. Figure 1 shows the scanning electron micrographs comparing the microstructure of the three samples: a nontemplated TiO2 coating, a TiO2 coating templated with 20 wt % oil in water emulsion, and a TiO2 coating templated with 40 wt % oil in water emulsion. Figure 2 depicts the pore distribution of the same three samples obtained using mercury porosimetry. According to Figure 1a, the nontemplated TiO2 coating had a very dense microstructure. The pores were mainly arising from the nanocrystalline TiO2 particles connected together at the necks due to the sintering process. These pores were extremely small, in the range of ∼30 nm as per the mercury porosimetry data. On the other hand, the 20 wt % emulsion-templated coating (Figure 1b) had dual or hierarchical porosity where the mesoporosity, with pores in the range of ∼30 nm arising from clusters of the TiO2 nanoparticles and macroporosity (∼300 nm pores), was due to the pores left behind by the oil droplets dispersed in the emulsion. These macropores should provide better access to the dye molecules to adsorb on titania nanoparticles as well as faster pathways for the electrolyte to permeate into the entire TiO2 coating. The particle size analysis data taken for the 20 wt % oil in water emulsion gave a mean oil droplet diameter of ∼200300 nm which matched well with the SEM and mercury porosimetry data. For the 40 wt % emulsion, the oil droplets were larger, in the range of ∼12 μm as per the particle size analysis 18343
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data. Figure 1c shows the pores corresponding to this size. According to the mercury porosimetry data, this sample was more polydisperse and therefore gave a broader peak in between 200 and 800 nm. The total porosity of the nontemplated, 20 wt % emulsion-templated, and 40 wt % emulsion-templated samples were 53%, 69%, and 82%, respectively. CurrentVoltage Characteristics. The IV characteristics of the dye-sensitized solar cells made with the emulsion-templated and nontemplated TiO2 coating are plotted in Figure 3. It was observed that the efficiency of the 20 wt % emulsiontemplated solar cell was highest among all, almost twice that of the other cells. The photocurrent as well as the photovoltage was increased in the emulsion-templated cell. The 40 wt % emulsiontemplated cell showed lower photocurrent, photovoltage, and fill factor values and hence showed lower efficiency. Ni. et al.32 have studied the effect of porosity of the photoelectrode on the photocurrent and photovoltage of the DSSC. They observed that the photovoltage of DSSC plateaus at higher porosities but that the photocurrent goes through an optimum value. This can be understood through the correlation of porosity with the pore size distribution, with the surface area, with the light absorption of the electrode, and with the charge transport through the electrode. If the pores are smaller (around 100 nm), the increased porosity will not affect the surface area much and would in fact facilitate dye and electrolyte permeation, which would enhance the photocurrent. But the porosity beyond an optimum value (>70%) decreases the surface area and increases the fraction of terminated TiO2 particles or dead ends inside the coating, which affect the photocurrent adversely. Table 1 shows the solar cell parameters of the three samples along with the thickness values of the photoelectrodes. Although all three TiO2 coatings were only one layer thick, the actual thickness of the emulsiontemplated coatings was higher than that of the nontemplated coating. This can be mainly attributed to the higher viscosity of the TiO2 emulsion paste, resulting in a thicker wet coating and higher porosity of the calcined coating. The thickness of the 20%
emulsion-templated coating was comparable to the nontemplated coating, being 20 and 17 μm, respectively. But the 40% emulsion-templated titania coating was much thicker, around 29 μm, which was higher than the usual desired thickness (∼15 μm) of TiO2 coating for DSSC.13 Therefore, the lower efficiency of the 40% emulsion-templated solar cell was also correlated to the higher thickness which caused the loss of many photogenerated electrons before they were transported to the FTO electrode. The higher fraction of terminated/dead end TiO2 nanoparticles in the photoelectrode with high porosity and high thickness leads to a large number of trapping surface states, which reduces the photovoltage,33 as seen in Table 1. The decrease in the fill factor of the 40% templated cell can also be attributed to the increased resistance due to higher thickness. More detail discussion about correlating the IV results with the electron transport kinetics in the DSSC is presented in the next section in context with the impedance analysis. Electrochemical Impedance Spectroscopy. In order to study the effect of emulsion templating, on the electron transport and recombination kinetics of the dye-sensitized solar cell, electrochemical impedance spectroscopy was employed. Electrochemical impedance data is usually represented in the form of either a Nyquist plot or a Bode plot. In the present work, Nyquist plot representations are presented. Typically the Nyquist plot of a dye-sensitized solar cell shows three semicircles, corresponding to the three main photoelectrochemical processes occurring during the cell operation, as follows: (1) electron transfer at the Pt counter electrode; (2) electron transfer at the TiO2/dye/ electrolyte interface; (3) Nernst diffusion of iodine/iodide species through the electrolyte and TiO2 coating.21,23,24,2731 This is shown as the inset in Figure 4a. The semicircle associated with the Nernst diffusion process is usually seen at frequencies between 0.01 to 0.1 Hz and therefore requires a long recording time. The cells tested in the present research were not completely sealed; therefore, the EIS data acquisition time was limited to a couple of minutes per cell. Otherwise, the gradual evaporation of the volatile electrolyte could have influenced the results. Also, it was more important to observe the semicircle associated with the TiO2/dye/electrolyte interface and study the changes in it with respect to the added porosity in the titania electrode through emulsion templating. Therefore, the frequency range selected for the EIS data acquisition was from 106 to 0.1 Hz, which means only two semicircles associated with the (1) Pt electrode interface and (2) TiO2/dye/ electrolyte interface were observed. Figure 4a shows the Nyquist plot obtained for the nontemplated dye-sensitized solar cell, which clearly shows the two semicircles. The smaller semicircle is seen from 63 kHz to 316 Hz, and the larger semicircle is seen in the frequency range of 316 Hz down to 0.1 Hz. In order to extract parameters related to the charge transport and recombination in the emulsion-templated and nontemplated dye-sensitized solar cells, a model as depicted in Figure 4b was used.34 This model, along with the unified equation deduced by Adachi et al., was used to describe the electron transport and
Figure 3. IV plot of dye-sensitized solar cells with templated and nontemplated TiO2 coatings under 100 mW/cm2 illumination.
Table 1. Solar Cell Parameters sample
photocurrent density (mA 3 cm2)
photovoltage (V)
fill factor
efficiency (%)
thickness (micrometer)
nontemplated TiO2
3.84
0.68
0.74
2.17
17
20% templated TiO2
7.72
0.70
0.69
4.17
20
40% templated TiO2
5.96
0.64
0.67
2.86
29
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(3) The lifetime of electron in the working electrode, τeff, is obtained from inverse of keff. (4) The diameter of the second semicircle, Rk, is correlated to the charge transfer resistance of the TiO2/dye/electrolyte interface. (5) The thickness of the electrode (L), Rk, and keff are related to the steady-state electron density, ns, in the TiO2 electrode through a parameter ‘Con’ in the following way:
charge recombination behavior in nanocrystalline TiO2 electrodes in DSSC, as follows: (1) The resistance element Rs is correlated to the sheet resistance of the transparent conducting substrate (FTO in this case) and the contact resistance of the FTO/TiO2. (2) The peak frequency of the second semicircle, ωmax, is correlated to the rate of recombination of electrons, keff, at the TiO2/dye/electrolyte interface.
Con ¼ R k LK eff ¼ ðK B T=q2 Ans Þ
ð1Þ
where KB is Boltzmann constant, T is absolute temperature, q is the charge of a proton, and A is the electrode area. (KBT/q2A) = 0.641 is used in the following analysis. Thus, ns is inversely proportional to parameter Con. (6) The shape of the second semicircle, especially in the high frequency region, is correlated to the ratio Rk/Rw, Rw being the charge transport resistance of TiO2 electrode. (7) The electron diffusion coefficient Deff can be calculated from Deff ¼ ðR k =R w ÞL2 keff
Figure 4. (a) Impedance spectra of nontemplated DSSC. The inset shows a typical experimental data with three semicircles recorded from 0.01 Hz to 106 Hz. (b) Schematic of DSSC showing the three interfaces and the equivalent circuit used for EIS data analysis.
Figure 5. Impedance spectra of nontemplated, 20 wt % emulsiontemplated, and 40 wt % emulsion-templated DSSC.
ð2Þ
Also, the first and the third semicircle in the Nyquist plot can give information about the resistance at the Pt electrode and impedance of the diffusion of the triiodide species in the electrolyte. This is not discussed in detail in the present analysis. Figure 5 shows the comparison of EIS data of the nontemplated and emulsion-templated dye-sensitized solar cells along with the identification of the parameters as described above. The parameters are given in more detail in Table 2. It is very clearly visible that the impedance of the second arc was decreased with increasing porosity of the TiO2 electrode. It is important to note that despite the increasing thickness of the TiO2 coating from nontemplated to 40 wt % emulsion-templated coating, which was higher than the optimum thickness, this kind of trend was observed.33 The high frequency arc associated with the Pt counter electrode interface was almost the same for all three samples. Also, the Rs component, correlated to the FTO sheet resistance and TiO2/FTO contact resistance, was almost the same, showing slightly lower resistances for the emulsiontemplated coating, which could be due to the better adhesion of these coatings to the substrate. The characteristic frequency of the second arc, ωmax, was almost same for all three samples. It only varied from 1.25 to 1.99 Hz from nontemplated to 40% emulsion-templated cell. Since ωmax is associated with the recombination rate constant, keff, at the TiO2/dye/electrolyte interface and the lifetime τeff is inversely proportional to keff, τeff varies only slightly from 0.8 to 0.5 s from nontemplated to 40% emulsion-templated cell. However, the diameter of second arc, Rk, was decreased considerably for the emulsion-templated samples, which also indicated that the charge transfer resistance at the TiO2/dye/electrolyte is lower in the coatings with interpenetrating pore networks. Interestingly, when parameter
Table 2. Charge Transport Parameters Extracted from Electrochemical Impedance Analysis sample name
Keff (s1)
τeff (s)
Rk (Ω)
Con (Ω cms1)
ns (cm3) 1018
Deff (cm2 s1) 104
nontemplated TiO2 20% templated TiO2
1.25 1.58
0.8 0.6
54.97 34.25
0.11 0.10
5.82 6.41
3.61 4.42
40% templated TiO2
1.99
0.5
24.13
0.13
4.93
1.67
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The Journal of Physical Chemistry C Con was calculated from eq 1, using L, Rk, and keff, it was observed that the 20% emulsion-templated solar cell had the lowest value among the three samples, which also demonstrated that the steady-state electron density in this sample was highest. This data is in agreement with the IV results shown in the earlier section, showing the highest photocurrent and hence efficiency of the solar cell. The electron diffusion coefficient Deff, calculated from eq 2, also showed that the 20% emulsiontemplated DSSC had the highest Deff, clearly proving that the increased pore network had improved the charge transport kinetics of the DSSC. This also proved that there is an optimum value of porosity and pore size for which the solar cell performance is highest. During calculation of the Deff parameter, the shape of the second arc was taken into consideration. For the 40% emulsion-templated sample, the shape deviated from a true circle near the high frequency region; hence, Rk was almost equal to Rw. In the other two cases, the ratio Rk/Rw was closer to 10. From the results mentioned above, the 20 wt % emulsiontemplated solar cell had the highest Deff, a low Keff, the lowest Con, and hence the highest ns and low Rk, therefore showing optimum performance of the DSSC. The dual size scale porosity provided faster conduction pathways for electrolyte ions and therefore improved solar cell performance.
’ CONCLUSION A low cost and scalable material such as oil-in-water emulsion was used as the templating material to obtain interpenetrating pore networks in TiO2 photoanodes for dye-sensitized solar cells. Both 20 wt % and 40 wt % oil in water emulsion-templated TiO2 coatings were prepared, and their microstructure and pore structure were compared with the nontemplated TiO2 coating. SEM micrographs and the mercury porosimetry results clearly exhibited the meso- and macroporous structure of the emulsiontemplated coatings. Under 100 mW/cm2 illumination, the 20 wt % emulsion-templated DSSC demonstrated twice as much efficiency as compared to the nontemplated cells. Depending on the particular electrolyte viscosity, then we expect that the emulsion ratio and oil droplet size could be tuned to help construct templated structures with improved device performance. Electrochemical impedance spectroscopy was used to study the effect of templating on the electron transport and recombination behavior of the TiO2/dye/electrolyte interface. Although the recombination rate at the TiO2 electrode remained unaffected upon introduction of pore structures, the steady-state electron density and electron diffusion coefficient of the templated cells was much higher as compared to the nontemplated cell. This proved the effectiveness of the emulsion template method to incorporate hierarchical, interconnected network of pores into the TiO2 electrode in order to improve the charge transport kinetics. ’ AUTHOR INFORMATION Corresponding Author
*Tel: +91-9545596322. E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank the NSF Ceramic and Composite Materials Center for supporting this project.
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