Optimizing the Performance of a Plastic Dye-Sensitized Solar Cell

ARTICLE pubs.acs.org/JPCC

Optimizing the Performance of a Plastic Dye-Sensitized Solar Cell Byunghong Lee, D. B. Buchholz, Peijun Guo, Dae-Kue Hwang, and R. P. H. Chang* Materials Research Institute and Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States ABSTRACT: This article describes that a fluorine plasma treatment can increase the nanopore filling of a plastic electrolyte in a dye-sensitized solar cell to improve its performance. The one-step fluorine treatment can be used in a controlled way to increase the size of nanopores and nanochannels in the TiO2 nanoparticle electrode and, at the same time, passivate the TiO2 nanoparticle surfaces. In combination with the fluorine treatment, a sequential electrolyte filling process has been developed that allows the overall cell conversion efficiency to be increased by as much as 25%. The plastic-based electrolyte cells are found to be much more stable compared with their counterpart, the liquid electrolyte cells. Using this new process, and in combination with a photon confinement scheme, the overall cell efficiency can reach to about 9% using a masked frame measurement technique.

’ INTRODUCTION Research and development on dye-sensitized solar cells (DSSCs)1,2 has demonstrated that it is a very promising and cost-effective technology. With further development and improvement in conversion efficiency, it should be competitive to conventional silicon technology. One of the manufacturing challenges for DSSCs has been the need for a robust sealing process that would prevent the liquid electrolyte in the cells from leakage and evaporation. At the same time, the electrolyte, the iodide/triiodide redox couple, is quite corrosive to the surroundings. The design of the conventional DSSC, also known as the Gratzel cell, is based on the nano concept of surface-to-volume ratio. Compared to a thin film p
n junction solar cell, the DSSC can be modeled as an unipolar-junction solar cell where n-type TiO2 nanoparticle (NP) film serves as the anode electrode. At the same time, the NPs are coated with absorbing dye molecules and they are in contact with an electrolyte, which serves to transport the charges to the cathode of the cell. In this configuration, the surface area for a given volume used to convert solar energy to electricity is orders of magnitude large than the conventional thin film structure. The big challenge for the case of plastic DSSCs is to ensure good contact between the plastic electrolyte and the dye molecule coated TiO2 nanoparticles in order to efficiently extract charges from the cell. Unlike the case of the liquid electrolyte, the plastic electrolyte has much higher viscosity (in most cases, by a factor of 10) and thus is much harder to infiltrate into the nanopores and nanochannels of the TiO2 nanoparticle film. This is a major drawback in fabricating “all-solid-state” DSSCs. In the case of liquid electrolyte cells, by optimizing the size of TiO2 nanoparticles, efficient infiltration of the liquid electrolyte r 2011 American Chemical Society

into the nanostructure has been achieved. The optimal liquid electrolyte recipe used in the literature was as follows: The liquid electrolyte was prepared by dissolving 0.6 M 1-butyl-3-methylimidazolium iodide (BMII), 0.03 M iodine, 0.1 M guanidinium thiocyanate (GSCN), and 0.5 M 4-tert-butylpyridine (tBP) in acetonitrile and valeronitrile (85:15 v/v). The ionic dopants have conductivities as high as 9.6 mS/cm.3,3b The solvent has a relatively low viscosity, and it can enhance ion conductivity for rapid diffusion of mediators (I
/I3
).4 The diffusion coefficient of I3
is estimated to be (4
6.2)  10
6 cm2 s
1.5,6 With this liquid electrolyte recipe, cells with efficiencies as high as 12% have been reported.3b Numerous scientific and engineering research studies on “allsolid-state” DSSCs have emerged in an attempt to replace the liquid electrolytes with organic or inorganic hole conductors, such as p-type semiconductors,7
9 organic hole-transport materials,10
12 and solvent-free polymer electrolytes.13
15 However, the photovoltaic performance of these types of “all-solidstate” DSSCs still lag behind those of the liquid type. The solid electrolyte replacements tend to have a lower ionic conductivity and difficulty in making good contact with TiO2 nanoparticles in films with thicknesses in the range of 12 μm, where the best cell efficiency was found for the case of the liquid electrolyte in our earlier study.16 Beyond this film thickness, the cell efficiency starts to decrease. What are some of the major challenges facing the fabrication of an “all-solid-state” plastic DSSC then? They include the following: Received: December 13, 2010 Revised: April 1, 2011 Published: April 27, 2011 9787

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The Journal of Physical Chemistry C (1) the need for a plastic crystal with a matrix that possesses unique properties that, when doped with ions, will provide high diffusion and mobility; (2) the plastic electrolyte needs to have a low enough viscosity to allow ease of infiltration into the TiO2 NP films during device fabrication and be in the solid state at room temperature; and (3) an improved interfacial junction among TiO2 nanoparticles (NPs), dye molecules, and the plastic electrolyte. Appropriate band bending due to charge accumulation at the TiO2 NP and the electrolyte interface is important to help increase the values of the open-circuit voltage, Voc, and short-circuit current, Jsc. Thus, there are three related concepts and challenges for this work: (1) a need to develop a process to enlarge the TiO2 NP nanopores and nanochannels in the film so that the more viscous plastic crystal electrolyte can easily infiltrate into the film; (2) it is well-known that etching of materials usually starts from defect sites on the surface; thus, as a result of etching, the surfaces of the TiO2 NPs need to be well passivated to minimize charge recombination; and (3) the need to ensure that interfacial junctions are well structured to increase the Voc and Jsc of the cell. This article reports how a fluorine plasma etching process can be used to increase the sizes of the nanopores and nanochannels in the TiO2 NP film electrodes and thereby improve the infiltration of a viscous plastic electrolyte used in the experiments. In addition, this unique one-step etching process allows the excess fluorine atoms to bond to the surface atoms of the TiO2 NP of the film and thus help to reduce surface states and minimize charge particle recombination. It is also shown how a two-step sequential infiltration process of the plastic crystal electrolyte and additives can improve cell performance. By combining these processes, we show that an overall 25% cell efficiency can be achieved. It is described in the Experimental Section how the electrodes were prepared, the fluorine plasma etching and surface passivation process, and the preparation and infiltration of the plastic electrolyte into the TiO2 NP films. Following this section, a discussion is given on the DSSC performance based on results from both dc and ac cell measurements. Using a simple transport model, the transport coefficients were extracted from experimental curves with a multiparameter fitting software. A comparison of results between the cases of the liquid electrolyte (our standard) and that of the plastic-based electrolyte is then carried out. From this study, it is demonstrated that the concept of using a fluorine plasma etching process, followed by a two-step sequential infiltration of the plastic electrolyte and additives, can greatly improve the performance of “all-solid-state” DSSCs. In addition, photon confinement and cell stability measurements are also reported.

’ EXPERIMENTAL SECTION In this section, the cell fabrication process is described in three major sections: electrode preparation, fluorine etching of the TiO2 NP film, and plastic electrolyte infiltration. Electrode Preparation. For the fabrication of the plastic DSSC, a recipe that was developed earlier was used for the case of the DSSC with the liquid electrolyte.3b,17 A two-step autoclaving technique was applied to obtain the high-purity anatase TiO2 NPs. A 30 wt % commercially available TiO2 powder (P25, Degussa) that consisted of ca. 30% rutile and 70% anatase in the crystalline phase was hydrothermally treated with 10 N NaOH in an autoclave at 130 °C for 20 h, followed by repeated washing with 0.1 N HNO3 to reach a pH value of ca. 1.5, as described in the literature.18 The pure anatase colloidal TiO2 NPs were obtained

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Figure 1. Schematic diagram of the plasma etching system.

by autoclaving the low-pH titanate suspension at 240 °C for 12 h. Anatase TiO2 NPs were investigated by using a field-emission scanning electron microscope (SEM, S4800, Hitachi) equipped with an energy-dispersive spectrometer (EDS) to investigate the TiO2 NPs and determine the compositions of the samples at 20 kV. The TiO2 NPs were also characterized by X-ray diffraction (D/Max-A, Rigaku) measurements. These measurements indicated that the TiO2 NPs were pure anatase and of high quality. A paste of anatase TiO2 powder was made by stirring with the mixture of 0.5 g of anatase
TiO2 NPs, 100 μL of Triton X-100, and 0.2 g of polyethylene glycol (PEG, Fluka, Mw = 20 000) into 3 mL of acetic acid (0.1 M). The TiO2 paste was spread on a SnO2/F-coated glass substrate (Pilkington, TEC 8 glass, 8 Ω/sq, 2.3 mm thick) by the doctor-blade technique to give a flat and smooth surface using an adhesive tape spacer. The film thickness was governed by the height of the adhesive tape. The exact thickness of the TiO2 film was determined by a surface profiler (Tencor P-10). Finally, the TiO2-coated electrode was gradually calcined to remove the polymer under an air flow at 150 °C for 15 min, at 320 °C for 10 min, and at 500 °C for 30 min, leaving a pure anatase TiO2 NP film. The counter electrode was produced by coating the F-doped SnO2 (FTO) glass with a thin layer of a 5 mM solution of H2PtCl6 in isopropanol and was heated at 400 °C for 20 min. The two electrodes were sealed together with thermal melt polymer film (24 μm thick, DuPont). The typical active area of the cell was about 0.3 cm2. The exact area of the photoanode was calibrated by an optical scanner under a resolution of 600 dpi (dots per inch). Fluorine Plasma Etching. Before soaking the TiO2 NP film in the dye solution, the TiO2 NP film was first etched to increase the nanopore and channel spaces to allow better penetration of both the dye solution and, especially, the plastic electrolyte. As shown below, this is a very intricate process. Figure 1 shows a schematic diagram of the plasma etching system used in our experiment. The TiO2 NP films were placed in a custom-built cylindrical microwave plasma reactor. Microwaves at 2.45 GHz and 600 W, were launched into an 11.5 cm diameter cylindrical waveguide from a rectangular waveguide via a mode converter designed to maximize the TE01 and exclude the TE11 transmission modes. The waveguide was coupled to a horizontal 11.5 cm diameter cylindrical vacuum chamber via a quartz window. The reaction gas was introduced at a position close to the quartz window and travels down the axial direction to the turbomolecular mechanical pump stack. The samples to be treated were placed between 9788

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Figure 2. SEM images of top and side views of (a, c) pre-etch film and (b, d) CF4-etched TiO2 film.

the gas inlet and the pump stack, near the radial center of the reactor, at 135° from the vertical such that the surface to be treated faces the quartz window. The etching process was performed under the following conditions: the flow rates of the reaction gases used were 45 sccm for CF4 and 10 sccm for O2 at a total chamber pressure of 2 mTorr. This gas mixture is known in the silicon processing technology to generate the most active fluorine atoms in the plasma for etching purposes.19,20 From surface roughness measurements using the profiler, it was observed that, after each etching step, not only was the film thickness being reduced but also the surface became rough as well. The chemistry of the CF4/O2 plasma is rather complex with many reaction paths, as discussed in refs 21
23. Although TiF4 is a solid at room temperature with a sublimation point of 284 °C,24 both metallic Ti and TiO2 have been etched by plasma processes using CF4.25 For the purpose of explaining the main etching that was taking place on the TiO2 NP surfaces, we adopt the follow set of reaction equations. CF4 þ O f COF2 þ 2F TiO2 ðsÞ þ FðgÞ f TiOx Fy ðsÞ þ TiF4 ðs; gÞ þ O2 ðgÞ The free fluorine reacts with the TiO2 in a stepwise fashion to form a number of titanium fluoroxy compounds and, ultimately, TiF4,22 some of which have sufficient energy to be volatilized from the substrate surface during the etching process and some of which are volatilized during subsequent processing. For the plasma etching process, there was no ion bombardment on the surface of the TiO2. Thus, the etching proceeded in an isotropic manner, that is, no preferred direction. Using this process, it was expected that nanosized channels would be enlarged to allow better penetration of both the dye molecules and the plastic electrolyte, as seen in a series of SEM images: Figure 2a,c shows the top and side views of the TiO2 NP film before etching and Figure 2b,d after 30 min of etching. The sketches at the bottom of

Figure 3. (a) Film thickness and weight as a function of etching time. Each data point represents a different sample. (b) Relative content of oxygen and fluorine in TiO2 NPs as a function of plasma etch time.

Figure 2 illustrate the result of etching where it was expected more materials were being etched near the surface. Along with the etching process, it was expected that the surface of the NP would be populated with fluorine atoms and bonded to Ti sites, as discussed below. Figure 3a shows a typical film removal rate along with the weight reduction of the film due to plasma etching. The presence of an initial period of a very slow etching rate should be noted (about 10 min) before material removal starts to take place. This is consistent with the SEM observation that, during the first period of etching, the plasma enlarges the pores of the surface of the TiO2 NP film. As the pore structure opened up, the active fluorine species could more easily permeate the rest of the TiO2 NP film and thus have access to a greater surface area to react with. This increased ease of permeation through the TiO2 pore structure is believed to extend to the infiltration of the plasticbased electrolyte. A second effect of the plasma etching is to incorporate fluorine into the film. It is difficult to determine an absolute assay of both very light (atomic number less than 14) and heavier elements by energy-dispersive X-ray spectroscopy (EDS). However, EDS can quite accurately determine the relative proportions of two very light elements. Figure 3b displays the relative proportions of oxygen and fluorine in the TiO2 NP film as a function of plasma etch time. After an initial period of little compositional change, which mirrors the initial period observed in the weight reduction data, the amount of oxygen observed decreases monotonically with time and a proportional increase in fluorine is observed. This indicates that the etching process not only removed TiO2 but also incorporated fluorine into the film. To determine the nature of the fluorine incorporation, the TiO2 NP films were 9789

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Figure 5. Scheme of an N-methyl-N-butylpyrrolidium iodide (P1,4)doped succinonitrile plastic crystal electrolyte.

Figure 4. (a) Pre- and post-etch Ti 2p XPS spectra, (b) post-etch F 1s XPS spectrum, and (c) pre- and post-etch XPS O 1s spectra.

examined by X-ray photon spectroscopy (XPS) both prior to and after plasma etching. XPS was performed using an Omicron ESCA probe. An Al-kR X-ray source and a takeoff angle of 45° were used. Charging was neutralized with an electron gun, and any residual charging shift was calibrated using the binding energy of the carbon 1s peak (284.5 eV). The peak locations and areas were determined by fitting the data to a Gaussian
Lorentzian (75
90% Gaussian) line shape with 3% asymmetry. Base lines were generated using the Shirley method.26 The relative atomic concentrations (Cx) were calculated using27 the determined areas (Ax) and atomic sensitivity factors (Sx) according to the formula Cx = (Ax/Sx)/Σi (Ai/Si). Prior to plasma treatment, the Ti 2p3/2 and Ti 2p1/2 peaks can be well fitted with a single Gaussian
Lorentzian curve (Figure 4a). Their respective locations of 458.5 and 464.2 eV, and energy separation of 5.7 eV, are indicative of Ti4þ bonded to oxygen.27 There is no indication of Ti3þ, which would present itself as a low-energy shoulder.28 After plasma treatment, the peak locations shifted to the slightly higher binding energies of 458.7 and 464.4 eV. The 5.7 eV separation between the Ti 2p3/2 and 2p1/2 peaks and the absence of a low-energy shoulder indicate that the titanium retains in a tetravalent state. The shift to higher binding energy would be expected if the fluorine, being more electronegative than oxygen, is bonded directly to the titanium. The post-plasma etching F 1s peak is also indicative of the fluorine being bonded directly to the titanium (Figure 4b). The majority of the F 1s signal (99%) is observed at 684.8 eV, which has been attributed to TiOF2. The minor (1%) high-energy shoulder at 686.8 eV has been attributed to fluorine substitutionally occupying an oxygen lattice site in TiO2.29 The fluorine is present at ∼14% of the oxygen level. Two O 1s peaks are observed prior to plasma treatment (Figure 4c). The major peak, representing ∼90% of the total oxygen, is observed at a binding energy of 529.8 eV and is associated with oxygen bonded as TiO2. The balance of the oxygen appears as a high-energy shoulder at 531.4 eV and is indicative of oxygen bonded as Ti
OH.30 After plasma treatment, the peak associated with TiO2 is shifted to a slightly higher binding energy of 529.9 eV. Once the etching process was completed, the next step in the DSSC fabrication was to infiltrate the dye and then the plastic electrolyte materials into the porous TiO2 NP film. Prior to

electrolyte infiltration, for photosensitization, the calcined and plasma-etched TiO2 NP electrode was immersed in the ethanol solution containing purified 3  10
4 M cis-di(thiocynato)-N, N0 -bis (2,20 -bipyridyl-4-caboxylic acid-40 -tetrabutylammonium carboxylate) ruthenium(II) (N719, Solaronix) for 18 h at room temperature. The dye-adsorbed TiO2 electrode was rinsed with ethanol and dried under a nitrogen flow. Plastic-Based Electrolyte. For cell fabrication, a (P1,4I)doped succinonitrile plastic crystal electrolyte was used.31,32 This electrolyte has the best performance for solid-state DSSCs, as reported in the literature.33 Doping of ions into the plastic crystal phase of succinonitrile leads to enhanced diffusivity and high ionic conduction. Recently, Alarco et al and others have dissolved a large variety of salts in the highly polar medium based plastic crystals, such as succinonitrile, to fabricate solid-state ionic conductors.34
37 The plastic crystal electrolyte (Figure 5) used in our experiment was prepared by mixing synthesized N-methylN-butylpyrrolidinium iodide (P1,4I),31,33 I2, and succinonitrile for the doped ions as the solid solvent in a mole ratio of 5:1:100, and the mixture was then heated to 70 °C. At room temperature, this compound electrolyte has a fast ionic conductivity of 3.0 ( 0.2 mS/cm. The observed fast ion transport in this solid material can be seen as a decoupling of diffusion and shear relaxation times, which probably originates from local defect rotations in the succinonitrile plastic crystal.38
40 Although this plastic electrolyte-based DSSC with high ion conductivity showed the best cell efficiency among other competing electrolyte materials, the photovoltaic characteristics still lag behind those of liquidtype DSSCs. From the published literature and the authors’ earlier work with liquid electrolytes, it was learned that, in addition to a redox mediator in the liquid system, two kinds of additives, such as 4-tertbutylpyridine (tBP) and guanidinium thiocyanate (GSCN), can be used to improve the liquid cell performance.17,3b,16 The addition of tBP into the electrolyte leads to a significant improvement of the open-circuit voltage (Voc), ascribed to either the suppression of dark current due to the blocking effect of tBP at the TiO2/ electrolyte interface41 or a shift of the titania conduction band.42,43 GSCN is another important and frequently used additive in electrolytes for DSSCs.44,45 It was reported that a combination of GSCN and tBP additives in the electrolyte results in a remarkable improvement of Voc and photocurrent because of the collective effect of a slower recombination reaction and a positive shift of the conduction band.45,46 However, for this solid-state system, when these additives were added to the iodide-doped succinonitrile, it had a solubility problem that tended to inhibit the formation of the plastic phase. It is believed that the additives were leading to a variation in the trans
gauche isomerization that accompanies the phase change.34,35 To avoid this problem, a two-step infiltration process was developed. We first injected the liquid-state electrolyte along with the additives tBP and GSCN into the TiO2 NP film and dried this electrolyte by putting the NP film in an oven at 80 °C for 12 h. By drying the liquid electrolyte, additives with a high boiling point remained entrapped in the TiO2 pores while most 9790

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of the solvent was evaporated. After that, the plastic electrolyte was injected into the cell at 80 °C. The cell was then cooled to room temperature to obtain a waxy solid electrolyte with the presence of additives. Using this two-step process, a room temperature ionic conductivity of 3.2 ( 0.2 mS/cm was obtained through the weak interaction between cations from additives and the nitriles of succinonitrile. In addition, the interfacial structure at the TiO2 NP
dye molecule
plastic electrolyte was also improved. From current
voltage measurements, it is found that the ionic additives have greatly improved the cell efficiency by as much as 18% through the increase of the values of the Voc and Jsc, along with the decrease in the internal cell resistance. Thus, we adopted this process.

(Solartron 1260), and a potentiostat (Solartron 1287) when the device was applied at its Voc. An additional low-amplitude modulation sinusoidal voltage of 10 mVrms was also applied between an anode and a cathode of a device over the frequency range of 0.05
150k Hz. Figure 6 shows the J
V characteristics of the two typical plastic electrolyte cells, one fabricated with fluorine plasma etching of the TiO2 NP films (Figure 6, curve d) and the other without (Figure 6, curve c). For comparison purposes, plots are presented for two typical liquid electrolyte cells that were also fabricated with and without fluorine plasma etching (Figure 6, curves b and a). The fabrication of the standard liquid electrolyte cell has been described in our earlier work.16 Below, a discussion and comparison is carried out for the electrical properties of these cells, as tabulated in Table 1. For the liquid electrolyte cells, both cells have the same Jsc, whereas the plasma etched cells showed a higher Voc and fill factor (FF). As a result, the plasma etched cell has a slightly higher efficiency by about 8%. On the other hand, for the case of the plastic electrolyte, there is a much larger difference in conversion efficiency between cells that were etched and not etched. As seen in Table 1, the etched cell has its Voc increased from 0.711 to 0.739 V, the Jsc increased from 14.2 to 15.4 mA/cm2, and the FF from 64.7% to 71.6%. These results give a large increase in the overall cell efficiency from 6.54% to 8.14%, and this demonstrates the importance of plasma etching of the TiO2 NP films for the plastic DSSCs. The electrochemical impedance spectroscopy (EIS) analysis can help one to understand the kinetics of electrochemical processing occurring in the DSSCs. Studies were carried out on the effects of the plasma etching on the interfacial kinetics in the TiO2 surface and network by adopting the equation of Adachi et al.,47 who consolidated the models developed by Kern et al48 and Bisquert et al.49,50 The complex impedance of a typical DSSC is the sum of each of the components, given as Z0, Z1, Z2, and Z3. They are, respectively, the contact impedance (which is usually real, Z0 = R0); Pt-catalyzed counter electrode impedance, Z1; the complex impedance, Z2, which represents the interface among the semiconductor (TiO2), dye molecule, and the electrolyte; and the diffusion of triiodide ions related (Warburg) impedance, Z3. Thus, the total Z(Z0 þ iZ00 ) is equal to the sum of Z0 þ Z1 þ Z2 þ Z3, (see Figure 7a,b, the ideal EIS circles). The ac impedance measurements provide the information on the internal properties of the cell device. The measured impedance values (discussed below) are intimately connected to material quality and interfacial properties, as well as the charge transport kinetics of the cell47,51

’ RESULTS AND ANALYSIS Electrical Measurements and Studies. For this study, tens of plastic DSSCs were fabricated, measured, and analyzed. The devices were evaluated under 100 mW/cm2 AM 1.5G simulated sunlight with a class A solar cell analyzer (Spectra Nova Tech.). A silicon solar cell fitted with a KG3 filter tested and certified by the National Renewable Energy Laboratory (NREL) was used for calibration. The KG3 filter accounts for the different light absorptions of the dye-sensitized solar cell and silicon and ensures that the spectral mismatch correction factor approaches unity. The electrochemical impedance results were measured under the same light illumination with an impedance analyzer

Figure 6. J
V characteristics of the two plastic electrolyte cells and two liquid electrolyte cells for comparison purposes: one with and the other without plasma etching of the TiO2 NP films.

Table 1. EIS Analysis and J
V Characteristics of Liquid and Solid Electrolytes at an Initial Thickness of 20 ( 0.5 μm and, after Etching, a Thickness of 15 ( 0.5 μm J
V characteristics

electrochemical impedance measurements Deff electrolyte (10
5 cm2 keff type

s
1)

τeff

Rk

Rw Rk/

(Hz) (ms) (Ω) (Ω) Rw

ns

Con

D1

Rtotal VOC JSC (mA/ FF EFF

(Ω cm s
1) Rd (Ω) (1018 cm
3) (10
7 cm2 s
1) (Ω)

(V)

cm2)

(%) (%)

(a) TiO2 NPs

liquid

7.76

12.6

79.4 5.7 2.4 2.38

0.116

3.5

4.41

4.20

39.1 0.784

18.5

69.1 10.0

(b) CF4
TiO2 NPs

liquid

9.47

15.9

62.9 5.7 2.3 2.48

0.140

3.5

5.12

5.70

35.6 0.807

18.3

73.1 10.8

15.9

62.9 9.2 4.2 2.20

(c) TiO2 NPs

solid

3.35

(d) CF4
TiO2 NPs

solid

3.43

9.46 105

9.6 4.2 2.30

0.126

7.1

2.19

1.9

73.4 0.711

14.2

64.7 6.54

0.114

7.6

4.81

2.2

56.1 0.739

15.4

71.6 8.14

9791

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Figure 7. (a) A representative electrical equivalent circuit of a typical DSSC. (b) An ideal ESI plot of a DSSC with R0, R1, R2, and R3, along the real part of the impedance and each peak frequency ωZ1, ωZ2, and ωZ3, respectively.

It is correct to set Z0 = R0 because it represents the resistance from the FTO and the metal Ohmic contact, which only has a real part of the impedance. In general, three semicircles are observed clearly in the measured frequency range of 0.05
150 kHz. The Ohmic serial resistance (R0) in the high-frequency region conforms to the electrolyte/FTO glass resistance, whereas the resistances R1, R2, and R3 relate to the charge-transfer resistance of the electrolyte at the counter electrode surface in the highfrequency region (R1), resistance reflecting the photoinjection electrons within TiO2 film in the middle-frequency region (R2), and the semicircle in the low-frequency region (R3) corresponding to the Nernst diffusion with the electrolyte, respectively. Figure 8a shows the Nyquist plots with the best-fit model of the DSSCs having the different types of electrolytes, and TiO2 NP films with and without CF4/O2 plasma etching. The best-fit parameters are tabulated in Table 1, which includes the parameters for electron kinetics in TiO2/dye/electrolyte. The direct current resistance at ω = 0 is given by a simple function of both the electron transport resistance in TiO2, Rw = (kBT/q2Ans)  (L/Deff) = Con  (L/Deff), and the charge-transfer resistance in recombination of electrons at the TiO2/electrolyte interface, Rk = Con  (1/Lkeff), where kB, T, q, A, and ns represent the Boltzmann constant, absolute temperature, charge of an electron, the electrode area, and the steady-state electron density in the conduction band, respectively. The first-order reaction rate constant for the loss of electrons, keff, which is estimated to be equal to the peak frequency of the central arc, ωmax, electron lifetime (τeff = 1/keff), and the effective electron diffusion coefficient, Deff = (Rk/Rw)  L2keff. The diffusion coefficient of I3
is determined from the diffusion-limited current and those determined from the finite Warburg impedance of triiodide in electrolyte: D1 = (1/2.5)δ2ωmax, where δ and ωmax represent the thickness of the electrolyte between TiO2 film and counter electrode and the peak frequency of the low frequency, respectively. The model calculation and data fitting provide some physical insight into the differences in the transport properties between the liquid and the plastic electrolytes and effects due to plasma etching

of the TiO2 NP films of the cells. From Table 1, it is seen that, for the case of the liquid electrolyte cell, the CF4/O2 etched device shows that the charge density value, ns, in the TiO2 conduction band increased by 16%, the interfacial recombination rate keff was increased by 26%, and the value of Deff was also increased at about 22% compared with that of the unetched sample. These changes show that the etching has some effect on the transport properties of the cell, but the overall effect is not pronounced because the cell efficiency only increased by 8% with etching. On the other hand, for the case of the plastic electrolyte, the changes in transport coefficient values are more pronounced. From Table 1, it is seen for the case of the etched sample that the value for the charge density, ns, in the TiO2 conduction band increased by nearly 120%, the interfacial recombination rate keff was decreased by 40%, and the value of Deff was nearly unchanged. These changes are quite significant, and as a result, the cell efficiency was increased nearly 25%! It can be concluded here that plasma etching has greatly improved the cell efficiency of our “solid” device by improving the infiltration of the plastic electrolyte and reducing the recombination rate as a result of the surface passivation of the TiO2 NPs' surface by our process. From J
V measurements, we see that this etching process has increased the value of the FF by about 11%. Furthermore, the electron diffusion coefficient rate of triiodide D1 for CF4-etched TiO2 films with the plastic electrolyte is 2.2  10
7 cm2 s
1, 15% higher than that obtained from unetched TiO2 film, at 1.9  10
7 cm2 s
1. Hence, the CF4-etched TiO2 film combined with a two-step infiltrations process plays a key role in attaining the higher cell efficiency with the plastic electrolyte. Figure 8b shows plots of the magnitude and the Bode phase of the impedance of the cell as a function of the modulation frequency. By comparison with the liquid electrolyte cell, the plastic cell has 50% more internal resistance, thus, a lower cell efficiency. In the case of the plastic electrolyte, it is seen that the fluorine-etched sample has a lower overall internal resistance compared with the nonetched sample. In addition, there is a crossover of the magnitude of impedance between the etched 9792

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Figure 8. (a) EIS plots and (b) magnitude of the impedance and Bode phase plots with the best-fit model having different electrolyte types.

Figure 10. (a) J
V characteristics and (b) EIS plots for each of the J
V curves with 3-D PhC.

Table 2. J
V Characteristics on 3D PhC: An Initial Thickness of 25( 0.5 μm and, after Etching, a Thickness of 15( 0.5 μm J
V characteristics

liquid

JSC

FF

EFF

(V)

(mA/cm2)

(%)

(%)

0.312 ( 0.15

0.827

18.9

74.6

11.7

0.827 0.829

21.6 17.4

73.5 74.3

13.1 10.7

(b) w/ PhC (masked)

Figure 9. Configuration of the cell with the 3-D photonic crystals.

and the unetched samples and an obvious shift in the phase in the high-frequency regime near 40 KHz. The exact explanation for this is not available at this time. Photon Confinement. In earlier work by the authors,16 it was shown that, by using 3-D photonic crystals, the overall cell efficiency can be increased by as much as 12%. In this work, the same technique was also adopted as a confinement procedure to increase the efficiency of etched cells for both liquid- and

(a) non

VOC area (cm2)

solid

0.304 ( 0.21

0.763

17.1

71.4

(b) w/ PhC

(a) non

0.763

19.2

71.3

(masked)

0.769

15.8

72.4

9.37 10.5 8.79

plastic-based electrolytes. As discussed earlier, it was found that a large portion of the photon flux was not fully absorbed by the dye molecules as the light traverses the cells. To remedy this situation, 3-D photonic crystals were attached behind the 9793

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Figure 11. Photo of (a) a plastic crystal electrolyte at room temperature and (b) liquid- and (c) plastic-based DSSCs after 3 weeks.

infiltration of the plastic electrolyte into the porous TiO2 NP film with a fluorine etching process to enlarge the porous channels in the film. In addition to etching, a two-step process has also been developed whereby additives have been incorporated into the plastic electrolyte to increase the cell efficiency. On the basis of a simple transport model, it is noted that the processing scheme has greatly lowered the internal resistance by improving the interface properties and lowering the losses throughout the cell. As a result of the fluorine treatment and processing steps, the conversion efficiencies of the solid-state cells were improved by as much as 25%. Another added benefit to a solid-state device is its long-term stability.

’ AUTHOR INFORMATION Figure 12. Parameters Jsc, Voc, FF, and cell efficiency as a function of time.

counter electrode to reflect the light back into the cells for further conversion. Figure 9 illustrates the configuration of the cell with the 3-D photonic crystals. For the case of N719 dye, it was found that the optimum photon confinement was to use a stack of two 3-D inverse photonic crystals, one with hole diameters of 375 nm and another with hole diameters of 410 nm. Figure 10a gives the J
V plots for both the liquid and the plastic electrolytes, and the case of etched and unetched samples. The corresponding impedance measurements and the best-fit curves from modeling are given in Figure 10b. From these plots and the best-fit transport values, it is seen that the main effect is an increase in the values of Jsc. For this set of cells, an increase of the conversion efficiency of 12% for both liquid and plastic electrolyte cells has been obtained. From Table 2, it is seen that the efficiencies for the liquid electrolyte and the plastic electrolyte are 13% and 10.5%, respectively. By measuring the same cells using the procedure (with a masked frame) of Ito et al., the efficiencies of the cells were decreased to 10.7% and 8.79%, respectively.3b Stability Studies. To illustrate the stability of the plastic cells, a comparison is carried out in Figure 11: two cells prepared and sealed in the same way, except one was filled with liquid electrolyte and the other with our plastic electrolyte. It is noted that, after 3 weeks, the left (liquid) cell showed that most of its electrolyte in the device was evaporated, whereas the right (“solid”) cell retained its electrolyte and the conversion efficiency was pretty much unchanged. For this work, we have tested tens of cells. Figure 12 displays the parameters Jsc, Voc, FF, and cell efficiency as a function of time. For a period of 60 days, it is found that the cell parameters are more or less unchanged.

’ CONCLUSION This paper provides a fabrication process on how it is possible to improve the efficiency of a plastic DSSC by improving the

Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors acknowledge the following support for this collaborative research: DOE-DE-FG02-08ER46536 for B.L. and D.B.B.; DOE-Energy Frontier Research Center, ANSER, and DE-SC0001059 for B.L. and P.G.; the NSF ECCS-0823345 for D.-K.H.; the NSF DMR 0843962 for R.P.H.C., and the NSFMRSEC DMR-0520513 for use of the facilities. ’ REFERENCES (1) Gratzel, M. Photoelectrochemical cells. Nature 2001, 414, 338–344. (2) Gr€atzel, M. Solar energy conversion by dye-sensitized photovoltaic cells. Inorg. Chem. 2005, 44, 6841–6851. (3) (a) Bonhote, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Gratzel, M. Hydrophobic, highly conductive ambient-temperature molten salts. Inorg. Chem. 1996, 35, 1168–1178. (b) Ito, S.; Nazeeruddin, M. K.; Liska, P.; Comte, P.; Charvet, R.; Pechy, P.; Jirousek, M.; Kay, A.; Zakeeruddin, S. M.; Gr€atzel, M. Photovoltaic characterization of dye-sensitized solar cells: Effect of device masking on conversion efficiency. Prog. Photovoltaics 2006, 14, 589–601. (4) Fukui, A.; Komiya, R.; Yamanaka, R.; Islam, A.; Han, L. Effect of a redox electrolyte in mixed solvents on the photovoltaic performance of a dye-sensitized solar cell. Sol. Energy Mater. Sol. Cells 2006, 90, 649–658. (5) Liu, Y.; Hagfeldt, A.; Xiao, X.-R.; Lindquist, S.-E. Investigation of influence of redox species on the interfacial energetics of a dye-sensitized nanoporous TiO2 solar cell. Sol. Energy Mater. Sol. Cells 1998, 55, 267–281. (6) Paulsson, H.; Kloo, L.; Hagfeldt, A.; Boschloo, G. Electron transport and recombination in dye-sensitized solar cells with ionic liquid electrolytes. J. Electroanal. Chem. 2006, 586, 56–61. (7) Morandeira, A.; Fortage, J.; Edvinsson, T.; Le Pleux, L.; Blart, E.; Boschloo, G.; Hagfeldt, A.; Hammarstrom, L.; Odobel, F. Improved photon-to-current conversion efficiency with a nanoporous p-type NiO 9794

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