Photoelectric Conversion Properties of Dye-Sensitized Solar Cells

Article pubs.acs.org/JPCC

Photoelectric Conversion Properties of Dye-Sensitized Solar Cells Using Dye-Dispersing Titania Hiromasa Nishikiori,*,† Yohei Uesugi,† Rudi Agus Setiawan,† Tsuneo Fujii,† Wei Qian,‡ and Mostafa A. El-Sayed‡ †

Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan ‡ Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States ABSTRACT: The time-resolved fluorescence and photoelectrochemical properties of dyesensitized solar cells using crystalline titania electrodes coated with N3 dye-dispersing amorphous titania gel were investigated to clarify the influence of the dye−titania interaction and electron transfer on their photoelectric conversion performance. The photocurrent quantum efficiency of the electrodes was remarkably increased by a steam treatment due to the crystallization and densification of the amorphous titania layer compared to that of the untreated electrode. The electron injection from the dye to the crystalline titania foundation via the steam-treated titania dispersing the dye was confirmed to be more efficient than that in the conventional electrodes. The dye-dispersing titania layer prevented interaction between the dye molecules and back electron transfer from the titania to the electrolyte. The charge separation and photoelectric conversion performance of the dye-sensitized solar cells were improved by forming the specific dye-dispersing titania layer.

1. INTRODUCTION Solar cells have attracted considerable attention from many scientific fields as a sustainable device to solve energy and environmental problems. In the photochemistry field, one of the important processes, electron transfer, in photofunctional materials consisting of an organic−inorganic composite, such as the working electrodes of dye-sensitized solar cells (DSSCs), has been studied all over the world.1−3 Many scientists have focused on the electron donor−acceptor interaction between the dye molecules and the titania in the dye−titania systems used for the DSSCs.4−15 We have investigated the photoelectric conversion properties of the dye-doped titania gel, which is different from the conventional dye-adsorbed titania.16−20 The dye-doped amorphous gel films are prepared without heating from a titanium alkoxide sol containing the dye molecules by the sol−gel method. We postulated that the gel consists of amorphous, nanosized, and particle-like units, which have a semiconductorlike quasi-conduction band structure with a low density of states.16−18 The dye molecules exist in the nanopores of the gel. Furthermore, the effect of a hydrothermal treatment on their photoelectric conversion properties has also been investigated because it is an effective method for crystallizing the amorphous phase and improving the photoelectric conversion performance.16−20 It was reported that the crystallization of amorphous titania to anatase was achieved by a hydrothermal treatment at low temperature because water molecules catalyzed the rearrangement of the TiO6 octahedra.21 Such a wet process to prepare the titania has been widely investigated due to the advantage of preparing it at a low temperature.22−31 The © 2012 American Chemical Society

characteristics of the steam-treated dye−titania system are a high dispersion of the dye and a high contact area between the dye and titania. This is important because dye aggregation and interaction between the dye molecules suppress the electron injection.32−34 The dye-doped titania gel is an interesting material for the basic study of DSSCs because hydrothermal treatments cause its crystallization and changes in the titania structure and the dye−titania bond character. The investigation of the dye−titania interaction in this system will result in determining new information expected to be applicable to conventional solar systems from a different viewpoint. Our previous studies indicated that the hydrothermal treatment of a xanthene dye-doped amorphous titania film remarkably improved the photoelectric conversion efficiency due not only to its crystallization but also to the dye−titanium complex formation.18−20 The electron injection process from the dye excitation states to the titania conduction band is important for the photoelectric conversion. The complex formation induces an interaction between their orbitals. The photocurrent action spectrum was red-shifted, and the short circuit photocurrent and open circuit voltage values increased with the steam treatment time. This interaction caused the ligand-to-metal charge transfer (LMCT) interaction and a fast electron injection into the titania conduction band.8,9,14 The steam treatment promoted the dye−titania complex formation, a negative shift in the conduction band potential of the titania, Received: September 30, 2011 Revised: January 24, 2012 Published: January 25, 2012 4848

dx.doi.org/10.1021/jp2094388 | J. Phys. Chem. C 2012, 116, 4848−4854

The Journal of Physical Chemistry C

Article

2.3. Measurements. The surface morphology of the electrode samples was observed, and their layer thickness was estimated from their cross section using a field emission scanning electron microscope (Hitachi S-4100). The crystalline phase of the film samples was determined using an X-ray diffractometer (Rigaku RINT-2200 V). The UV−visible absorption spectra of the prepared electrode samples were obtained using a spectrophotometer (Shimadzu UV-2500). The dye-free titania films prepared on the glass plates were immersed in 3.0 × 10−4 mol dm−3 N3 dye ethanol solutions for 6 h and then dried at room temperature. The UV−visible absorption spectra of these films were measured to estimate the dye adsorption ability of the titania films. The iodine-based electrolyte was allowed to soak into the space between the electrode sample and the counter Pt electrode. Monochromatic light obtained from a fluorescence spectrophotometer (Shimazdu RF-5300) with a 150 W Xe short arc lamp (Ushio UXL-155) was irradiated on the electrodes for the spectroscopic measurements. During light irradiation, the short circuit currents of the electrodes were measured by an electrometer (Keithley model 617). The I−V curves of the electrodes were obtained by a potentiostat (Hokuto Denko HSV-100) during irradiation by visible light with a wavelength longer than 400 nm emitted by the 150 W Xe short arc lamp using a sharp cut filter. The intensity at each wavelength of the light source was obtained using a power meter (Molectron PM500A) to estimate the incident photonto-current conversion efficiency (IPCE), internal quantum efficiency, and internal energy conversion efficiency for the photocurrent generation from the excited dye in the electrode samples. The light intensity was confirmed to correlate with the results of the potassium ferrioxalate actinometry. The visible absorbance of the present electrode samples was lower than around 1.0 which was sufficient to measure the number of absorbed photons to calculate the quantum efficiency. The Ti:Sapphire femtosecond pulse laser and streak scope spectroscopic system were used for the time-resolved fluorescence measurements to obtain information about the fluorescence quenching due to the electron injection from the dye to the titania.35 The laser system (Clark MXR CPA 2001) generates laser pulses of 150 fs duration (fwhm) with an energy of 750 μJ at 750 nm at a repetition rate of 1 kHz. The second harmonics of the laser pulses (375 nm) was used for the excitation. The fluorescence signal was monitored using a streak scope system (Hamamatsu Photonics C4780). The fluorescence decay curves were obtained by integrating the fluorescence signals in the 600−800 nm region. The electrode samples for this measurement were washed with a 0.1 mol dm−3 sodium hydroxide aqueous solution and water to adjust to the same amount of N3 dye. The amount of the dye was confirmed by UV−visible absorption measurement.

and the electron injection from the dye to the titania. Therefore, our systems are applicable to the DSSC electrodes. In this study, the time-resolved fluorescence and photovoltaic measurements of the DSSCs using electrodes coated with the N3 dye-doped titania gel were obtained to clarify the influence of the dye−titania interaction and electron transfer process on their photoelectric conversion performance. Time-resolved fluorescence spectroscopy is more sensitive and accurate than transient absorption spectroscopy and, therefore, more suitable to obtain systematic experimental data.35 The internal quantum efficiency of the cells was examined to understand the electron injection process in the thin surface layers.

2. EXPERIMENTAL SECTION 2.1. Materials. Titanium tetraisopropoxide, ethanol, hydrochloric acid, nitric acid, sodium hydroxide, acetonitrile, iodine, and lithium iodide (Wako Pure Chemicals, S or reagent grade), cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato ruthenium(II) (N3 dye) (Peccell Technologies, PECD03), and titania paste (Catalysts & Chemicals Industries, PST-18NR) were used without further purification. Water was ionexchanged and distilled. Glass plates coated with an ITO transparent electrode (AGC Fabritech) were soaked in hydrochloric acid (1.0 mol dm−3) for 2 h and then rinsed with water. The electrolyte used for the electrical measurements consisted of an acetonitrile solution of iodine (5.0 × 10−2 mol dm−3) and lithium iodide (0.50 mol dm−3). 2.2. Preparation of Electrodes. To prepare the electrode sample coated with the crystalline titania foundation layer, the titania paste was spread on the glass plates with the ITO transparent electrode and heated at 500 °C for 30 min. The sol−gel reaction system was prepared by mixing 5.0 cm3 of titanium tetraisopropoxide, 25.0 cm3 of ethanol, 0.21 cm3 of water, and 0.21 cm3 of concentrated nitric acid as the catalyst for the sol−gel reaction. The N3 dye was dissolved in the sol− gel system in which the concentration was 1.0 × 10−2 mol dm−3. The electrode sample with the crystalline titania layer was dip-coated five times in the sol−gel system in which the reaction proceeded for 1 day to obtain a certain film thickness. This working electrode sample was labeled WE-s0. Glass plates without ITO were also coated with the sol−gel system to determine their XRD patterns and coated with the dye-free sol−gel system to estimate the dye adsorption ability of the titania films. The steam-treatment effects on the UV−visible absorption and photocurrent spectra of the WE-s0 electrode and on the XRD pattern of the film sample were investigated. Water was heated at 110 °C, and the electrode and XRD samples were exposed to the steam for 0−120 min. The pressure of the steam was about 140 kPa. The working electrode samples treated for 1, 3, and 6 h were labeled WE-s1, WE-s3, and WE-s6, respectively. The crystalline titania electrodes were coated with 1−10 layers of the N3 dye-dispersing titania steam-treated for 6 h to examine the influence of the number of layers on the photoelectric property. A conventional dye-sensitized electrode was also prepared to compare it with our original samples. The titania paste was spread on the glass plate with the ITO transparent electrode and heated at 500 °C for 30 min. This electrode was immersed in a 3.0 × 10−4 mol dm−3 N3 dye ethanol solution for 6 h and then dried at room temperature. This working electrode sample was labeled WE-c.

3. RESULTS AND DISCUSSION 3.1. Crystallinity of Titania. The untreated titania gel film had a structureless morphology and was amorphous. Figure 1 shows the SEM images of the N3 dye-dispersing titania films steam-treated for 1−6 h. The steam-treated films consisted of 10−20 nm particles as previously reported.16 The particle size and surface roughness slightly increased with the team treatment time. It was difficult to measure the specific surface area of the dye-dispersing titania layer, i.e., the adsorption isotherm of nitrogen gas, due to the small amount of the samples obtained from the films. The amount of the N3 dye 4849



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degree of crystallinity of the films was estimated from the ratio of the anatase peak area to the whole area including the amorphous halo peak. The crystalline properties of the films are summarized in Table 1. The crystallinity of the films increased Table 1. Crystalline Properties of Each Film steam treatment time/h

crystallite size/nm

degree of crystallinity/%

0 1 3 6 (titania paste)

4.4 ± 0.4 5.6 ± 0.6 6.7 ± 0.2 16 ± 1

0 5.9 7.1 9.6 100

with the steam treatment time, although it was much lower than that of the titania foundation. 3.2. Photoelectric Conversion Properties of the Electrodes Coated with the Dye-Dispersing Titania. Figure 3 shows the visible absorption spectrum of the electrode

Figure 1. SEM images of the N3 dye-dispersing titania films steamtreated for (1) 1, (2) 3, and (3) 6 h.

adsorbed on the titania films (10 cm2) untreated and steamtreated for 1, 3, and 6 h was 5.3, 4.6, 3.6, and 3.5 × 10−8 mol, respectively. The surface area of the films should decrease with the crystal growth. The XRD patterns of the N3 dye-dispersing titania films were obtained as a function of the treatment time as shown in Figure 2. No peak is found in the XRD pattern of the untreated

Figure 3. Visible absorption spectra of (1) WE-s0 and (2) WE-c.

with the untreated N3 dye-dispersing titania, WE-s0, compared to that of the conventional N3 dye-adsorbed titania electrode, WE-c. The thickness of the dye-free titania layer of WE-r0 and WE-c was 4.2 ± 1 μm. The thickness of the dye-containing layer of WE-r0 was ca. 350 nm. The spectrum of WE-c showed a peak at 530 nm due to the N3 dye, sharper than that of WEs0 and similar to that of the N3 dye in the solvents. The absorbance values of WE-c at 400−650 nm were higher than those of WE-s0 due to the higher dye content. The interaction between the dye molecules was weak based on the absorption spectrum, although the crystalline titania particles can adsorb the dye molecules forming the multilayers. The spectrum of WE-s0 ranged over a wide wavelength in the visible region. This spectrum was broader than that observed in the solvents due to the matrix effect of titania as observed in the fluoresceindoped titania.19,20 The dye molecules cannot be stacked in the dye-dispersing titania.36−38 As is well-known, the carboxylate group formed a chelate complex with the titanium species.4,5,8−10,14 The dye−titania interaction such as their complex formation made the spectra broaden. The UV−vis IPCE spectra and the I−V curves, observed during visible light irradiation, of WE-s0 and WE-c are shown in Figures 4 and 5, respectively. The IPCE values of WE-c were higher than those of WE-s0 in the visible region due to its higher absorbance. The spectral shape of WE-s0 in the visible region was similar to that in its absorption spectrum. On the other hand, the spectrum of WE-c exhibited a broader band than that of WE-s0 because an inner-filter effect occurred in the high absorbance region.33 The interaction between the dye

Figure 2. XRD patterns of the N3 dye-dispersing titania films steamtreated for (1) 0, (2) 1, (3) 3, and (4) 6 h.

amorphous gel film. A peak at around 25° was observed in the film steam-treated for 1 h, indicating that an anatase-type crystal was produced in the film. The peak appears sharper after the longer steam-treatment time. The size of the crystallites of these electrodes was estimated from their full-width at halfmaximum of the 25° peak using Sherrer’s equation, D = 0.9λ/β cosθ. They were 4−7 nm for the samples after the steam treatment for 1−6 h and much smaller than that of the titania foundation prepared from the titania paste, ca. 20 nm. The 4850



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Figure 6. Visible absorption spectra of (1) WE-s0, (2) WE-s1, (3) WE-s3, (4) WE-s6.

Figure 4. IPCE spectra of (1) WE-s0 and (2) WE-c.

Figure 5. I−V curves of (1) WE-s0 and (2) WE-c observed during visible light irradiation.

Figure 7. IPCE spectra of (1) WE-s0, (2) WE-s1, (3) WE-s3, and (4) WE-s6.

molecules, such as the intermolecular charge transfer or exciton migration, probably suppresses the electron injection.34 The spectral shape of WE-c resulted from the dye molecules densely adsorbed on the titania particle surface. The dye molecules can be weakly stacked and form multilayers. The UV IPCE values of WE-c were also higher than those of WE-s0. In WE-s0, the electrons must be transported through the amorphous titania layer to be injected into the conduction band of the titania foundation. The untreated amorphous titania layer had a lower electronic conductivity than that of the anatase titnaia. However, the dye molecules were dispersed in the amorphous titania layer and did not exhibit the inner-filter effect. The I−V curves of the electrodes indicated that the short circuit current density and open circuit voltage values of WE-c were higher than those of WE-s0. The former reflected the values of their IPCE spectra. The latter is presumed to result from the easy back electron transfer from the low electronically conductive amorphous titania to the electrolyte in WE-s0. 3.3. Influence of Steam Treatment on the Photoelectric Conversion Properties. Figure 6 shows the influence of the steam treatment time on the absorption spectrum of the titania electrodes coated with the N3 dyedispersing titania. The absorbance somewhat decreased with the steam treatment time because the dye molecules weakly interacting with the titania were desorbed into the water phase.16−20 The absorbance, however, did not decrease as much as that of the fluorescein-doped titania as previously reported because the N3 dye more strongly bonds to the titania by its four carboxylate groups.16,18−20 The change in the IPCE spectrum of the electrode with the stream treatment is shown in Figure 7. The IPCE values in the UV region increased with the treatment time and approached those of WE-c shown in Figure 4. This is due to the crystal growth of the anatase titania in the dye-containing layer based

on the XRD analysis. On the other hand, in spite of the visible absorbance decrease, the IPCE values in the visible region significantly increased with the treatment time. These results indicate that the steam treatment improved the electronic conductivity and electron transport in the dye-dispersing titania layer due to the crystallization and densification of the amorphous titania based on the XRD patterns as shown in Figure 2.16−20 The IPCE values of WE-s3 and WE-s6 in the visible region were higher than those of WE-c. The back electron transfer from the titania foundation to the electrolyte can cause a decrease in the photocurrent in the conventional electrodes because the titania easily contacts the electrolyte. The back electron transfer more easily occurs in WE-s0 due to the low electron injection efficiency from the amorphous titania. It is suggested that the efficiency of the electron transfer from the dye to the titania foundation was improved by the modified dye-dispersing titania layer. This is because the dyedispersing titania layer prevented the interaction between the dye molecules causing the inner-filter effect and controlled the back electron transfer from the titania to the electrolyte. The crystallization and densification of the titania layer induced not only the high electronic conductivity but also a certain blocking layer against the back electron transfer. The conduction band potential of the steam-treated titania is expected to be more negative than that of the well-crystallized titania because the former has a wider band gap than the latter.16,20 Therefore, the electron injection from the dye-dispersing titania layer to the titania foundation was promoted, and the back electron transfer was suppressed. Figure 8 shows the I−V curves of the above electrodes observed during the visible light irradiation. The short circuit current density values increased with the treatment time, reflecting the values of their IPCE spectra. The photoelectric 4851



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Figure 9. Fluorescence decay curves of N3 dye in (1) WE-s0, (2) WEs1, (3) WE-s3, (4) WE-s6, and (5) WE-C .

Figure 8. I−V curves of (1) WE-s0, (2) WE-s1, (3) WE-s3, and (4) WE-s6 observed during visible light irradiation.

relaxation of the triplet states. The longer lifetime component is assigned to the minor contribution of the electron injection and/or some other quenching processes of the triplet states. The stronger maximum fluorescence intensity was observed in the electrode treated for a shorter time. The yield of the electron injection from the dye to the titania can have an inverse relation to the fluorescence maximum intensity because the electron injection is reported to occur within 100 ps.35,39,40 Therefore, the relative electron injection yield in the dyedispersing titania increased with the steam treatment time as revealed by the transient absorption spectroscopy.18 3.5. Electron Transport in the Electrodes. The photoelectric conversion properties of the electrodes coated with several layers of the dye-dispersing titania were estimated to examine the effect of the dye-dispersing titania layer on preventing the dye interaction and back electron transfer. Figure 10 shows the influence of the number of N3 dyedispersing titania layers on the internal quantum efficiency for the photoelectric conversion of the titania electrodes. The efficiency proportionally increased with the number of layers up to five layers. The blocking effect against the back electron transfer can be regarded as being proportional to the layer thickness because the quantum efficiency of the photocurrent generation from the excited dye molecules is assumed to be independent of the dye content. If the dye molecules were aggregated with the increasing dye content, the quantum efficiency was not proportional to the layer number and will be saturated. Therefore, the blocking effect should strongly depend on the thickness of the dye-dispersing titania layer up to five layers. However, the efficiency decreased in the electrodes with more than seven layers. The electron transport in the dye-dispersing titania layer is lower than that in the titania foundation depending on its crystallinity. Therefore, because the dye-dispersing layer is thicker, the efficiency of the electron transport in the layer should be lower. The blocking effect remained in the electrodes with up to five layers of the dye-dispersing titania by overcoming its low electronic conductivity.

conversion properties of all the electrodes used in this study are summarized in Table 2. The photoelectric conversion performTable 2. Photoelectric Conversion Properties of Each Electrodea electrode

ISC/mA cm−2

VOC/V

FF

Pmax/mW cm−2

WE-s0 WE-s1 WE-s3 WE-s6 WE-c

5.58 9.29 14.0 13.4 6.74

0.44 0.47 0.45 0.46 0.48

0.40 0.64 0.49 0.57 0.40

0.98 2.8 3.1 3.5 1.3

QE (η)/% 29.6 43.9 73.5 75.7 48.2

(0.30) (0.85) (0.95) (1.11) (0.37)

a

ISC: short circuit photocurrent density. VOC: open circuit voltage. FF: fill factor. Pmax: maximum power. QE: internal quantum efficiency in the region of 400−900 nm. η: internal energy conversion efficiency under 550 mW cm−2 visible light source.

ance can be examined by the internal quantum efficiency, i.e., the absorbed photon to current conversion efficiency, because the present electrodes were transparent over the entire visible region. The internal quantum efficiencies of the present electrodes over the entire visible region were estimated. The energy conversion efficiencies relative to the total energy of the absorbed photons were also obtained as the internal energy conversion efficiencies under the visible light source, whose power was determined to be 550 mW cm−2. These efficiency values are also shown in Table 2. The current density and maximum power clearly increased with the treatment time. The higher efficiency was confirmed for the electrode coated with the steam-treated dye-dispersing titania. The internal quantum efficiency and internal energy conversion efficiency became 2.5 and 3.7 times higher due to the 6 h treatment. The charge separation was improved by forming the specific dye-dispersing titania layer. 3.4. Time-Resolved Fluorescence Measurements of the Electrodes Coated with the Dye-Dispersing Titania. The fluorescence quenching can provide information on the electron injection from the dye to the titania. Figure 9 shows the fluorescence decay curves of the N3 dye in the electrodes. The fluorescence signals were collected in the range of 600− 800 nm and fitted to the double exponential decay curves. The fluorescence intensities just after excitation, i.e., the maximum intensities, and fitting parameters are shown in Table 3. The decay curves consist of two components with the lifetimes of 0.14−0.17 and 1.1−1.6 ns. These values correspond to those reported for the dye-adsorbed titania systems.35 The shorter lifetime component was observed in a 50/50 (v/v) mixture of tert-butyl alcohol and acetonitrile and is assigned to some quick

4. CONCLUSIONS The time-resolved fluorescence and photoelectric measurements of the DSSCs using the crystalline titania electrodes coated with the N3 dye-dispersing amorphous titania gel were obtained to clarify the influence of the dye−titania interaction and electron transfer on their photoelectric conversion performance. The internal quantum efficiency of the cells was examined to understand the electron injection process in the thin surface layers. The photocurrent quantum efficiency of the electrode was remarkably increased by the steam treatment due 4852



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Table 3. Emission Intensity and Fitting Parameters of the Time-Resolved Fluorescence of Each Electrode electrode

τ1/ns

maximum intensity

WE-s0 WE-s1 WE-s3 WE-s6 WE-c

2800 2210 1530 1080 2340

± ± ± ± ±

90 50 40 30 50

0.16 0.15 0.15 0.15 0.16

± ± ± ± ±

τ2/ns

0.01 0.01 0.01 0.01 0.01

1.5 1.4 1.3 1.3 1.2

to its increasing electronic conductivity compared to that of the untreated electrode. The electron injection from the dye to the crystalline titania foundation via the steam-treated titania dispersing the dye was confirmed to be more efficient than that in the conventional electrodes. The dye-dispersing titania layer prevented the interaction between the dye molecules, causing an inner-filter effect or electron deactivation, and controlled the back electron transfer from the titania to the electrolyte. The crystallization and densification of the titania layer by the steam treatment induced not only a high electronic conductivity but also a definite blocking layer against the back electron transfer. This improved the charge separation and photoelectric conversion performance of the DSSC electrode.

AUTHOR INFORMATION

Corresponding Author

*Phone: +81-26-269-5536. Fax: +81-26-269-5550. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The preliminary work was supported by a Sasagawa Scientific Research Grant from The Japan Science Society, 1997−1998, and the Research Grant from The Thermal & Electric Energy Technology Foundation, 2001. This work was supported by the Ministry of Education, Science, Sports and Culture, Grant-inAid for Young Scientists (B), 2008−2009.

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0.1 0.1 0.1 0.1 0.1

A1 0.51 0.50 0.56 0.58 0.56

± ± ± ± ±

A2 0.01 0.01 0.01 0.01 0.01

0.49 0.50 0.44 0.42 0.44

± ± ± ± ±

0.01 0.01 0.01 0.01 0.01

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Figure 10. Internal quantum efficiency for the photoelectric conversion of the titania electrodes coated with 1−10 layers of the N3 dye-dispersing titania steam-treated for 6 h.

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