Interplay between Dye Coverage and Photovoltaic Performances of

Article pubs.acs.org/JPCC

Interplay between Dye Coverage and Photovoltaic Performances of Dye-Sensitized Solar Cells Based on Organic Dyes Galhenage A. Sewvandi, Changdong Chen, Tomohiko Ishii, Takafumi Kusunose, Yasuhiro Tanaka, Shunsuke Nakanishi, and Qi Feng* Department of Advanced Materials Science, Faculty of Engineering, Kagawa University, 2217-20 Hayashi-cho, Takamatsu 761-0396, Japan S Supporting Information *

ABSTRACT: A high dye coverage on the TiO2 surface diminishes dye-sensitized solar cell (DSSC) performances because of the significant degree of excited electron and hole recombination that results from the strong dye−dye intermolecular interactions. In this study, interplay between dye coverage and photovoltaic performances of DSSCs was systematically investigated and discussed using adsorption isotherms, photovoltaic measurements, and impedance analyses. Commercially available P25 and laboratory synthesized {010}-faceted TiO2 nanoparticles were used in mesoporous electrodes, and MK-2 organic dye was used as a sensitizer. Estimated adsorption constant (Kad) and maximum adsorption density (Qm) were 1.03 × 105 dm3/mol and 1.39 × 10−6 mol/m2 for P25 and 1.50 × 105 dm3/mol and 8.62 × 10−7 mol/m2 for {010}faceted TiO2, respectively. The maximum TiO2 surface coverage of about 60% with {010}-faceted TiO2 and near 100% with P25 were observed in adsorption isotherms. I−V characteristics curves showed the continuous enhancement of open-circuit potential (Voc) with increasing coverage by confirming its high dependency on coverage. The P25 cell exhibited the maximum short-circuit photocurrent density (Jsc) at 84% of coverage which corresponded to the optimum coverage of MK-2 dye. At the optimum coverage the distance between dye molecules was estimated as 1.2 nm. Compared with P25, {010}-faceted TiO2 showed about 81% of Jsc and 75% of η enhancements although its maximum coverage (60%) was lower than the optimum coverage (84%). High performances of {010}-faceted TiO2 can be explained by the effective conversion of the irradiated light to photocurrent by strongly adsorbed dye molecules on the {010}-facet.

1. INTRODUCTION Dye-sensitized solar cells (DSSCs) have attracted both scientific (academic) and industrial interest since Grätzel and co-workers disclosed their low-cost energy generation potential.1 As a next generation of solar cells, DSSCs have several advantages over silicon-based solar cells such as the ability to maintain generation characteristics even in a weak light condition as well as at any incident angle and the ability to use a variety of designs and colors. DSSCs have a high potential to expand the range of use of solar cells including a variety of consumer-related applications in which conventional solar cells are unfit. A typical DSSC consists of a dye-sensitized TiO2 electrode, a Pt electrode, and an electrolyte. The sensitizer dye plays a crucial role in the DSSC system. A sensitizer dye which is anchored to the surface of TiO2 nanocrystals absorbs the solar radiation and transfers the photoexcited electron to the conduction band of the TiO2. Metal complexes2,3 and metalfree organic dyes4−16 have been extensively investigated for DSSCs. Compared with metal-free organic dyes, ruthenium complex dyes are delivering DSSCs with high photoenergy conversion efficiencies. But, it contains an expensive rare metal of ruthenium, and it requires tricky purification steps.6 On the other hand, metal-free organic dyes can be prepared rather © 2014 American Chemical Society

inexpensively by established synthesis strategies. The major advantages of the metal-free organic dyes are their large extinction coefficient, low cost, availability, and facile manufacturing. Those advantages lead to numerous studies on organic dyes for DSSCs. Molecular structures of organic dyes have been designed and modified to match the oxidation potential of the ground (highest occupied molecular orbital, HOMO) and excited states (lowest unoccupied molecular orbital, LUMO) with the redox potential of the redox couple and the conduction band of the semiconductor to make the fast regeneration of the dye and efficient electron transfer from LUMO to conduction band of TiO2.10,17 Generally, metal-free organic dye structure consists of a donor group, a π-conjugation linkage, and an acceptor group as shown in Figure 1. Upon solar radiation an electron is excited from the HOMO level, which is located on the electron rich donor part, to the LUMO level on acceptor group through the π-conjugation linkage. Dyes with different types of donor groups such as coumarins,15 indoline,18 carbazole,8,10 diphenyaneline,19 and ullazine17 have Received: June 12, 2014 Revised: August 13, 2014 Published: August 13, 2014 20184

dx.doi.org/10.1021/jp5058397 | J. Phys. Chem. C 2014, 118, 20184−20192

The Journal of Physical Chemistry C

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dye coverage on the photovoltaic performance, electronrecombination resistance (Rrc), and electron lifetime (τ) of DSSCs were also studied systematically.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. MK-2 (2-cyano-3-[5‴-(9ethyl-9H-carbazol-3-yl)-3′,3″,3‴,4-tetra-n-hexyl-[2,2′,5′,2″,5″,2‴]-quarterthiophen-5-yl] acrylic acid), purchased from Sigma-Aldrich, was used as the dye sensitizer. Other chemicals and reagents were analytical grade and used as received. The {010}-faceted TiO2 nanoparticles (PA) were synthesized by using the hydrothermal soft chemical method from layered titanate nanosheets as described in our previous study.25 An H+-form layered titanate H1.07Ti1.73O4 (10 g) was treated in a 0.1 M n-propylamine (1L) solution under stirring conditions at room temperature for 24 h to obtain its nanosheet colloidal solution. After adjusting the pH value to 3.5, the nanosheet colloidal solution was hydrothermally treated at 135 °C for 24 h to obtain the {010}-faceted TiO2 nanoparticles (PA). The synthesized PA nanoparticles have a particle size of about 20 nm and BET surface area of 63 m2/g. The commercially available P25 (Degussa, Germany) TiO2 nanoparticles with an average particle size of about 20 nm and BET surface area of 50 m2/g was also used in this study. 2.2. MK-2 Dye Adsorption on TiO2 Nanoparticles. The MK-2 dye adsorption experiments were performed in a concentration range of 0.01−0.3 mmol/L. Accurately weighed 10 mg TiO2 powder sample was added to 5 mL of MK-2 dye solution in a mixed solvent of THF/toluene (1:4). Then, it was allowed to adsorb the dye at room temperature for 24 h. After that, the liquid phase was centrifuged at 8000 rpm for 15 min. Then aliquot of the supernatant was analyzed using a JASCO V-530 UV−vis spectrophotometer. The amounts of dye adsorbed on per gram of TiO2 crystals (Q) were calculated by the difference between the initial (Ci) and the equilibrium (Ce) dye concentration using the equation

Figure 1. Molecular structure and dimension of MK-2 dye at its most stable conformation. Gray, cyan, yellow, blue, red, and pink represent C, H, S, N, O, and a lone pair, respectively.

been synthesized to obtain high extinction coefficient. The cyanoacetic moiety8,10,17 and the rhodanine moiety19,20 have been used as acceptor groups. The modification of either donor group or the anchor group has shown differences in coverage as well as in electronic and molecular surface properties at the TiO2 surface.19 Moieties with thiophene8,15 are widely used as a π-conjugated linkage because of their excellent electron transportation ability. Several research groups have attempted to enhance the cell performance by changing the length10,17 and the substitutes of the π-conjugation linkage.8,10,15,21 In these studies, special attention has been paid to the design principles of the dyes. Although molecular design is important, the efficiency of the DSSCs does not only depend on the molecular structure of the dye. The photovoltaic performance of DSSC is also highly dependent on the adsorption characteristics of the particular dye used as the sensitizer.22 Hara and co-workers synthesized carbazole dyes with alkyl-functionalized thiophenes with different alkyl chain lengths.9,12,14−16 They found that the increasing alkyl chain length decreased the dye adsorption density without lowering electron lifetime.9 Therefore, they suggest that in the terms of electron lifetime the important criterion is the dye coverage on TiO2 surface and not the adsorbed density. Core level photoelectron spectroscopy studies on the electronic and molecular properties of organic dye molecules suggest that difference in the dye coverage has a minor effect on the charge generation from the individual molecules.19 The high dye coverage on TiO2 surface will diminish the DSSC performance because of the significant degree of excited electron and hole recombination that results from strong dye−dye intermolecular interactions.23,24 To our knowledge, systematic studies on the influence of the adsorption properties and the TiO2 surface coverage by organic dye molecules on the photovoltaic performance of the DSSCs have not been reported. In this article, we report the adsorption isotherms for MK-2 organic dye (Figure 1) on commercially available TiO2 (P25) and laboratory synthesized {010}-faceted TiO2 nanoparticles. MK-2 dye was selected as the sensitizer because it does not need coadsorbate such as deoxycholic acid and its commercial availability. Effects of adsorption parameters (Qm and Kad) and

Q = (C i − Ce)V /W

(1)

where V is the volume of the dye solution (5 mL) and W is the mass of the TiO2 powder (10 mg). TiO2 powder samples were heated at 450 °C for 30 min before adsorption experiments were conducted. 2.3. Fabrication of DSSC. The PA-TiO2 nanoparticles and the commercially available P25-TiO2 nanoparticles were used in the TiO2 pastes preparation, and the TiO2 pastes were prepared as described elsewhere.26 First, FTO glass plates were cleaned using sonication with acetone followed by ethanol for 10 min. Then, the cleaned FTO glass plates were dipped into 0.1 M Ti[OCH(CH3)2]4 in ethanol solution for 20 s and dried at room temperature followed by calcination at 480 °C for 1 h to coat the FTO glass surface with a dense TiO2 thin film. Layers of TiO2 paste were coated on surface of FTO conducting glass plate by the screen printing technique and calcined as in our previous study.24 The calcined electrodes were again modified with Ti[OCH(CH3)2]4 solution as described above. Around 80 °C while cooling, the TiO2 electrodes were immersed into MK2 dye solution in a mixed solvent of THF/toluene and kept at room temperature for around 24 h to complete the dye adsorption. After that, the dye loaded electrodes were washed thoroughly by using the solvent of THF/toluene. The sandwich type solar cells were fabricated by introducing the redox electrolyte containing 0.1 M LiI, 0.05 M I2, 0.5 M tertbutylpyridine (TBP), and 0.6 M 1,2-dimethyl-3-propylimida20185



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the P25 were calculated as about 80% and 20%, respectively, from the intensity ratio of the XRD peaks corresponding to the (101) reflection of the anatase phase and the (110) reflection of the rutile phase. Field emission scanning electron microscopy (FE-SEM) images and high-resolution transmission electron microscopy (HR-TEM) images of P25 and PA nanocrystals are presented in Figure 3. The P25 sample has three kinds of

zolium iodide (DMPImI) in acetonitrile between a dye sensitized TiO2 electrode and a counter electrode of Pt coated FTO conducting glass. 2.4. Characterization. Powder X-ray diffraction (XRD) analyses of the samples were carried out on a Shimadzu XRD6100 X-ray diffractometer with Cu Kα (λ = 0.154 18). The morphology and the size of the particles were observed using field emission scanning electron microscopy (FE-SEM; Hitachi, Model S-900). Transmission electron microscopy (TEM) observations were performed on a JEOL Model JEM-3010 system at 300 kV. The specific surface (BET) areas were estimated from the nitrogen gas adsorption data obtained using a Quantachrome AUTOSORB-1-MP apparatus. TiO2 powder samples, heated at 450 °C for 30 min, were used in nitrogen gas adsorption. The concentrations of MK-2 dye were measured using a JASCO V-530 UV−vis spectrophotometer. The photocurrent−voltage (I−V) curves were obtained using a Hokuto-Denko BAS100B electrochemical analyzer with a YSSE40 Yamashita Denso solar simulator (AM 1.5; 100 mW/cm2). The cell active area for illumination was set to 0.25 cm2 by a mask. Electrochemical impedance measurements were performed with an impedance analyzer (Solartron SI 1287) under dark conditions. Impedance measurements were recorded in a frequency range from 0.1 Hz to 1 MHz with alternate current (ac) amplitude of 10 mV at an applied direct current (dc) bias of −0.8 V.

3. RESULTS AND DISCUSSION 3.1. Characterization of TiO2 Nanoparticles. The electron transfer in DSSC is strongly influenced by the electrostatic and chemical interactions between the TiO2 surface and the adsorbed dye molecules.27,28 Electrostatic and chemical natures of TiO2 nanoparticles are highly affected by the lattice facet on the crystal surface, crystal morphology, crystal size, and crystal phase.27,28 As shown in the powder Xray diffraction (XRD) patterns of the TiO2 nanoparticle samples (Figure 2), P25 consists of mixed phases of anatase and rutile, and laboratory synthesized TiO2 (PA) consists of single phase of anatase. The contents of anatase and rutile in

Figure 3. Field emission scanning electron microscopy (FE-SEM) images: (a) P25; (b) PA ({010}-faceted TiO2 nanocrystals). HR-TEM images: (c) P25; (d) PA.

particles morphologies: (1) irregular morphology with particle size of about 80 nm (20%) which corresponds to the rutile phase, (2) near-spherical (40%), and (3) tetragonal (40%) morphologies with particle size of about 20 nm which correspond to the anatase phase (Figure 3a). HR-TEM images of P25 revealed that near-spherical nanoparticles have not specific facet on the crystal surface, but tetragonal nanocrystals have the basal planes corresponding to a facet vertical to [111]direction and other four planes correspond to the {101}-facet (Figure 3c). The PA sample has two kinds of particle morphologies of rhombic (70%) and tetragonal (30%) morphologies with a size of about 20 nm (Figure 3b). As shown in Figure 3d, basal planes of PA nanoparticles correspond to (010) plane, namely {010}-faceted anatase nanoparticles.25 To understand the adsorption density of dye molecules on the crystal surfaces, the specific surface areas (SBET) of the TiO2 nanocrystals were measured using nitrogen gas adsorption data. The Brunauer−Emmett−Teller (BET) surface areas of P25 and PA samples were about 50 and 63 m2/ g, respectively. 3.2. MK-2 Dye Adsorption on TiO2 Nanocrystals. The relationship between the concentration of the MK-2 dye and the dye adsorbed density is described by the adsorption isotherms as shown in Figure 4a. The adsorption isotherms for MK-2 dye on P25 and PA nanocrystals show rapid uptake

Figure 2. Powder X-ray diffraction (XRD) patterns of P25 and PA nanoparticle samples. 20186



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Table 1. MK-2 Dye-Adsorption Parameters of the TiO2 Nanocrystals TiO2 P25 PA

Qm (mol/m2) −6

1.39 × 10 8.62 × 10−7

Kad (dm3/mol)

R2

max coverage (%)

1.03 × 10 1.50 × 105

0.987 0.998

100 60

5

kinds of lattice spaces have a different tendency to adsorb dye molecules.29 The different dye loadings on the different facets have also been reported for the N719 dye,30 but the reason behind it was not clearly explained. We speculate that the Ti atoms on the exposed surface dictate the possible adsorption sites because anchoring carboxyl groups of the dye molecules bond to the Ti atoms on the TiO2 surface. Therefore, the density of the Ti atoms on the exposed surface determines the maximum dye adsorption density. The adsorption equilibrium constant is a thermodynamic equilibrium constant which corresponds to the binding energy of the adsorption. The high Kad value for {010}-faceted TiO2 indicates the MK-2 dye is more strongly adsorbed on the {010}-faceted TiO2 than on the P25 surface. Different Kad values have been reported as a result of different binding configurations of the dye molecules on the nanocrystals with different morphologies.27 The TiO2 surface coverage by the dye molecules plays a major role in the terms of electron lifetime.9 Nonetheless, systematic studies on the influence of the TiO2 surface coverage by dye molecules on the photovoltaic performance of the DSSCs have not been reported. Organic dyes bearing a carboxylic acid as anchoring group stretch out almost perpendicular from the TiO2 surface.31,32 Therefore, TiO2 surface coverage can be calculated using the formula surface coverage = Q × NA × A

(3)

2

where Q (mol/m ) is the adsorption density as shown in Figure 4a, NA (= 6.022 × 1023 mol−1) is the Avogadro constant, and A (nm2) is cross-sectional area of the dye molecule. The MK-2 dye molecular dimension at the most stable conformation that is determined by means of minimize molecular mechanics (MM2) energy is shown in Figure 1; the A value (crosssectional area of the dye molecule) estimated from the dimension was 1.58 nm2. A 123% of maximum dye coverage for P25 nanoparticles was yielded from its Qm value. The adsorption isotherms indicated that a monomolecular layer of MK-2 dye was formed on the P25-TiO2 surface, suggesting the dye coverage over 100% is unreasonable. This result revealed that the adsorbed dye molecules were not at the most stable conformation as shown in Figure 1. The flexibility of hexyl chains in the dye molecule can easily switch to different conformations, which results a smaller cross-sectional area for the dye molecule. It has been reported that organic dyes having a cyanoacrylic anchoring group show the surface concentration of the dye (Γdye) less than 2 molecules/nm2 with the monomolecular layer coverage.33 The maximum Γdye values of P25 (0.8 molecules/ nm2) and PA (0.5 molecules/nm2) were accorded with the expected value for a monomolecular layer. To estimate the dye coverage on TiO2 surface, we assumed the dye coverage at the maximum adsorption density (Qm) of P25-TiO2 is 100%, and then the cross-sectional area of the dye molecule at the optimized conformation can be calculated as 1.28 nm2. Figure 4b shows the dye coverage on the TiO2 surfaces based on the optimized dye conformation. P25 and PA nanoparticles reach their maximum surface coverage of near 100 and 60% around

Figure 4. Isotherms of MK-2 dye on P25 and PA nanoparticles: (a) dye adsorption density variation with dye concentration; (b) dye coverage variation with dye concentration.

increase at highly dilute solution concentrations and then reach a plateau. This behavior shows typical chemisorption and indicates that all potential adsorption sites on the surface are occupied by adsorbate molecules. The adsorption isotherms data fitted with the Langmuir isotherm for both samples, indicating Langmuir type monomolecular layer adsorption on TiO2 nanocrystals. The Langmuir equation can be represented as follows: C /Q = 1/(Q mK ad) + C /Q m

(2)

3

where C (mol/dm ) is the equilibrium concentration of MK-2 dye in the solution, Q (mol/m2) is the dye adsorption density, Kad (dm3/mol) is the adsorption constant, and Qm (mol/m2) is the maximum adsorption density. The least-squares values (R2) of the experiment results fitting to formula 2 are 0.987 for P25 and 0.998 for PA nanocrystals (see Figure S1 in the Supporting Information). The adsorption constant and the maximum adsorption density were calculated by plotting C/Q against C, and the results are tabulated in Table 1. Compared with P25, PA nanocrystals show low Qm and high Kad for MK-2 dye. The maximum amount of adsorbed substance is primarily determined by the surface lattice of the solid on which the adsorption occurs. Furthermore, different 20187



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Table 2. Cell Parameters of DSSCs Fabricated Using Commercially Available P25 TiO2 at Different Coverage surface coverage (%)

Jsc (mA/cm2)

Voc (V)

η (%)

FF (%)

Rrc (Ω)

τ (ms)

Rs (Ω)

Cμ (μF)

17 42 62 84 93

2.82 3.01 3.50 5.44 4.99

0.59 0.65 0.73 0.64 0.68

1.18 1.35 1.79 2.40 2.37

0.70 0.68 0.70 0.69 0.70

17.8 18.3 22.6 30.2 39.4

2.82 3.76 6.05 11.2 10.3

12.5 13.2 12.2 12.8 12.1

151 204 266 368 260

Table 3. Cell Parameters of DSSCs Fabricated Using Laboratory Synthesized {010}-Faceted TiO2 at Different Coverage surface coverage (%)

Jsc (mA/cm2)

Voc (V)

η (%)

FF (%)

Rrc (Ω)

τ (ms)

Rs (Ω)

Cμ (μF)

14 27 40 43 54 60

1.80 4.62 5.01 6.60 7.69 9.79

0.52 0.56 0.57 0.58 0.60 0.65

0.64 1.81 2.01 2.65 3.20 4.24

0.68 0.69 0.69 0.69 0.69 0.66

13.5 13.5 15.6 16.1 16.4 17.2

7.56 7.66 13.9 16.9 19.5 23.0

11.0 10.5 10.8 10.8 10.4 11.5

544 558 872 1020 1200 1300

Figure 5. Photovoltaic-characteristics variation with surface coverage of P25 and PA-TiO2: (a) Jsc, (b) η, (c) Voc, and (d) fill factor.

the effect of coverage on photovoltaic performance, we measured the photovoltaic-characteristic variations with the TiO2 film thickness at the different MK-2 dye concentrations because the TiO2 film thickness affects the DSSCs performance.18,26,34 The optimum film thickness to produce highly efficient DSSCs is 6−8 μm for both kinds of TiO2 (see Figures S2 and S3 in the Supporting Information). The optimal film thickness depends on the electrolyte and TiO2 particle size.35 The film thickness for efficient DSSC based on N719 dye is 14−17 μm.26,34 Thin TiO2 films can be used with the metalfree organic dyes because of their high extinction coefficient

0.15 and 0.08 mmol/L equilibrium concentrations of MK-2 dye, respectively, and showed a large different coverage in a wide concentration range. This result supports the view that MK-2 dye molecules selectively interact with the {010}-faceted TiO2, namely less the adsorption sites on the {010}-facet surface than that on the P25 surface. 3.3. Current−Voltage Characteristic Curves. To study the effects of TiO2 surface coverage and adsorption constant on the DSSCs performance, DSSCs using these TiO2 nanocrystals were fabricated and allowed for dye adsorption in the 1.0 × 10−5−3.0 × 10−4 mol/L range of concentrations. To investigate 20188



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Figure 6. Graphical representation of the effect of dye molecular coverage on TiO2 surface.

(3.58 × 104 M−1 cm−1 for MK-2 dye17 and 1.39× 104 M−1 cm−1 for N719 dye36). The short-circuit photocurrent density (Jsc) and the efficiency (η) increase with the film thickness up to about 6−8 μm and then decrease (Figures S2 and S3). Thicker TiO2 film absorbs more dye and increases the Jsc. But increase of the back-transfer losses of injected electrons in TiO2 film and the increase in series resistance of DSSC can limit continuous increase of the Jsc.22 The open-circuit potential (Voc) and the fill factor (FF) were not significantly changed with the thickness. In the MK-2 dye, hexyl chains linked to the oligothiophene moiety (Figure 1) is effective to suppress charge recombination.10 These results suggest that the relatively negligible

variation in I−V parameters can be obtained in the thickness range of 6−8 μm. Therefore, the TiO2 film thickness is controlled in a range of 6−8 μm in next steps. In the DSSCs study, we assume that the adsorption isotherms of Figure 4b can be applied also to the TiO2 electrodes because FE-SEM results reveal that the morphologies of TiO2 nanocrystals almost not change after the TiO2 electrode fabrications (Figure S4 in the Supporting Information). The dye coverages on the TiO2 electrodes fabricated using different concentrations of the dye solutions were estimated from the adsorption isotherms, and the photovoltaic characteristics of the Jsc, Voc, η, and FF at different coverage are 20189



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tabulated in Tables 2 and 3, respectively. Furthermore, the Rrc and electron lifetime (τ) of an electron in TiO2 electrode variations with the coverage are shown in Figure 7. The

summarized in Tables 2 and 3. The photovoltaic characteristics variations with the surface coverage for P25 and PA cells are shown in Figure 5. The Jsc increases up to around 84% of the coverage and after that slightly decreases for P25 cells. This result suggests that there is optimum dye coverage for the effective conversion of the irradiated light to the photocurrent. The increase of the dye coverage results enhancement of the light harvesting, which enhances the Jsc. At the high coverage, however, the dye−dye intermolecular interactions cause nonradiative decay23,24 of the excited electrons to the ground state, which reduces the Jsc, as shown in Figure 6. The dye−dye intermolecular interactions become dominant when the dye coverage is above 84%, which offset the enhancement of the light harvesting. It has also reported that the addition of coadsorbate can diminish the dye−dye intermolecular interactions and then improve the IPCE performance of the DSSCs.15 On the other hand, the Jsc increases linearly with increasing the coverage up to the maximum coverage of 60% for the PA cells (Figure 5a). This is due to the maximum coverage of 60% is lower than the optimum dye coverage of 84%. We think if the coverage can be further increased, the further enhancements of Jsc are possible for the PA cells. The maximum Jsc value of the PA cell (9.8 mA/cm2) was 81% higher than that of the P25 cell (5.4 mA/cm2). The high Jsc values in PA cells might be attributed to the larger adsorption constant of MK-2 dye on the {010}-faceted TiO2 surface than that on the P25 surface, which gives strong electron coupling between the dye and TiO2 interface. We believe that the {010}-faceted TiO2 has distantsurface lattice to adsorb dye molecules with minimum dye−dye intermolecular interactions such that the excited electrons can be effectively injected into the conduction band of TiO2. In both cases, the Voc is increased with increasing the coverage (Figure 5c), but the fill factor is not greatly affected by variations in the coverage (Figure 5d). The behavior of the Voc can be attributed to suppress of the back-electron transfer between the injected electrons in the TiO2 conduction band and I3− ions in the electrolyte with increasing coverage (Figure 6). The η increases linearly with increasing the coverage for the PA cells, and the maximum value of 4.2% is shown at the maximum coverage of 60%, similar to the behavior of Jsc (Figure 5b). On the other hand, for the P25 cells, the η increases continuously with dye coverage and then reaches the plateau value of 2.4% at the coverage of around 84%. The η of the photovoltaic cell is calculated from Jsc, Voc, FF, and Iph (the intensity of the incident light):37 η = Jsc × Voc × FF/Iph

Figure 7. Variation of electron transfer parameters with surface coverage of P25 and PA TiO2: (a) electron recombination resistance; (b) electron lifetime.

electron lifetime (τ) and the chemical capacitance (Cμ) can be calculated using the formula

(4)

τ = R rcCμ = 1/ωrec

Therefore, even though Jsc is slightly decreased above 84% coverage, η is not decreased as a result of an increased Voc in the P25 cells. 3.4. Electrochemical Impedance Characteristic Curves. Electrochemical impedance spectroscopy (EIS) is a powerful, versatile technique to study the salient electronic and ionic processes in DSSC system. We performed EIS analyses to scrutinize the effect of electronic and ionic processes in the DSSCs on their performance variations. The electron-transfer resistance at the TiO2/electrolyte interface (Rrc), the electrontransfer resistance in the circuit (Rs), and the chemical capacitance (Cμ) values were estimated using the equivalent circuit of DSSC as shown in Figure S5 and Nyquist plot in Figure S6. The Rrc, Rs, and Cμ values for P25 and PA cells are

(5)

where ωrec is the angular frequency of the TiO2/electrolyte interface. The Rrc values increase with increasing coverage for both of P25 and PA cells, owing to the dye molecules adsorbed on TiO2 surface block out I3− from TiO2 surface as shown in Figure 6, which hinders the back transfer of photoelectrons from TiO2 to I3− in the electrolyte solution. It results in the improvements of Voc and τ in TiO2 electrode.38 Therefore, the higher Voc values (Figure 5c) of P25 cells than that of PA cells can be attributed to their larger Rrc values (Figure 7a). These Rrc values are well agreed with the dark-current response in the high-forward bias region (Figure S7 in the Supporting Information); namely, the larger the Rrc value, the lower the 20190



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4. CONCLUSION The {010}-faceted TiO2 nanoparticles show higher Kad and lower Qm for MK-2 dye adsorption than that of P25 nanoparticles. The maximum surface coverage of {010}-faceted TiO2 and P25 are about 60% and near 100%, respectively. The dye coverage strongly affects the performance parameters of DSSCs. The Voc values enhance continuously with increasing coverage owing to suppressing charge recombination on TiO2 surface. The P25 cell exhibits the maximum Jsc at 84% coverage, which corresponds to the efficient coverage of MK-2 dye. At the efficient coverage the distance between dye molecules was estimated as 1.2 nm. At the efficient coverage, high-light harvesting, low-dark current and minimum dye−dye intermolecular interactions lead to high performances. Compared with P25, {010}-faceted TiO2 yields significantly high efficiency due to its high Jsc, although its maximum coverage is lower than the efficient coverage. High performances of the PA cells can be attributed to the effective conversion of the irradiated light to photocurrent by strongly adsorbed dye molecules on the {010}-facet and high conductivity of the electrons in the {010}faceted TiO2 films. These findings suggest that the photovoltaic performances of the DSSCs are affected by the dye coverage as well as the properties of TiO2 nanoparticles.

dark-current density. However, at the same coverage, it is interesting that the P25 cells have larger Rrc values than PA cells. This result suggests that the Rrc value is affected not only by the coverage but also by other factors, such as the different conformations of the dye molecules adsorbed on the different TiO2 surfaces. The electron lifetime increases with increasing the coverage for the PA cells, while it reaches the maximum value at the coverage which yielded the maximum Jsc and afterward it declines for P25 (Figure 7b). The τ shows a similar trend to the Jsc variation for both types of TiO2 (Figure 5a). The τ in PA cell is longer than that in P25 cell (Figure 7b), although it has the smaller Rrc. With increasing the coverage, the Cμ variations for the PA and P25 cells correspond to their τ variations, respectively, and the Cμ values of PA cells are larger than which of P25 cells. As shown in formula 5, except the Rrc value, the Cμ value also affects the τ value that increases with increasing the Cμ value. The higher τ values in PA cells are due to their larger Cμ values than which in P25 cells. For P25 cells, after optimum coverage the τ is decreased due to decreased Cμ. On the basis of the results described above, we propose a surface model to explain the optimum dye coverage for the high performance DSSCs as shown in Figure 6. The model consists of three possible coverages: (1) low coverage, (2) high coverage, and (3) efficient coverage. At low coverage (1), large intermolecular distances prevent the dye−dye intermolecular interactions, low light harvesting, and the large dark current (low dye coverage provides additional back-electron transfer sites and enhances the dark current) lead to the low performances. At high dye coverage (2), high light harvesting can be achieved and I3− ions in the electrolyte solution are blocked out from TiO2 surface which reduces the dark current by suppressing back electron transfer. However, the dye−dye intermolecular interactions easily occur at high coverage, which cause nonradiative decay of the excited electrons to the ground state and diminution of the Jsc. At the efficient coverage (3), relatively high light harvesting, relatively low dark current, and the minimum dye−dye intermolecular interactions lead to optimum performances. Therefore, the highest performance can be achieved at the efficient coverage by controlling light harvesting, dark current, and dye−dye intermolecular interactions. The coverage of 84% at the maximum Jsc value for P25 cell could be realized as the efficient dye coverage for MK-2 dye. From this efficient dye coverage, we can calculate the efficient distance between dye molecules as 1.2 nm. Compared with P25, our laboratory synthesized PA nanoparticles showed about 81% of Jsc and 75% of η enhancements even though it had lower coverage (60%) than the efficient coverage. It implies that the dye molecules adsorbed on the {010}-faceted TiO2 nanoparticle surface can effectively convert the irradiated light to the photocurrent. It disclosed the high potential of {010}-faceted TiO2 nanoparticles to fabricate efficient DSSCs based on metal-free organic dyes. These findings suggest that the optimum performance of the DSSC depends on the coverage and properties of the TiO 2 nanoparticles. The interdependencies among the various components of the DSSCs dictate the overall device performance. It is imperative to control the TiO2 surface coverage to obtain efficient DSSCs by avoiding nonradiative decay of the excited electrons to the ground state.

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ASSOCIATED CONTENT

* Supporting Information S

Linearized absorption isotherms for MK-2 dye on P25 and PATiO2 nanoparticles, photovoltaic characteristics variation with P25-TiO2 film thickness, photovoltaic characteristics variation with PA-TiO 2 film thickness, FE-SEM images of TiO 2 nanoparticles and TiO2 film (TiO2 electrode before dye adsorption), equivalent circuit diagram and model used to fit the impedance data, Nyquist plots of DSSC fabricated using P25 and PA taitania electrodes with different dye coverage, and dark current density in log scale vs bias potential of the P25 and PA cells. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Q.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by Grants-in-Aid for Scientific Research (B) (No. 23350101 and No. 26289240) from Japan Society for the Promotion of Science.

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REFERENCES

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Interplay between Dye Coverage and Photovoltaic Performances of

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