Dye-Sensitized Solar Cell Using a TiO2 Nanocrystalline Film

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Langmuir 2003, 19, 8872-8875

Dye-Sensitized Solar Cell Using a TiO2 Nanocrystalline Film Electrode Modified by an Aluminum Phthalocyanine and Myristic Acid Coadsorption Layer Yutaka Amao* and Tasuku Komori Department of Applied Chemistry, Oita University, Dannoharu 700, Oita 870-1192, Japan Received June 7, 2003. In Final Form: July 29, 2003 A dye-sensitized solar cell using a nanocrystalline TiO2 film electrode modified by aluminum phthalocyanine, aluminum 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine hydroxide (AlTPPc), and a myristic acid adsorbed layer was developed. The UV-vis absorption spectrum of the TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer shows that the aggregation of AlTPPc molecules was suppressed by myristic acid. For a cell using AlTPPc adsorbed on a nanocrystalline TiO2 film electrode, ISC, VOC, and FF were 0.026 mA cm-2, 186 mV, and 40.4%, respectively. The maximum power output was estimated to be 2.25 µW cm-2. In contrast, ISC, VOC, and FF values were 0.061 mA cm-2, 225 mV, and 40.0%, respectively, and the maximum power output was estimated to be 5.40 µW cm-2 using a TiO2 film electrode modified by an AlTPPc and myristic acid layer. The ISC and VOC values increased with the concentration ratio of myristic acid to AlTPPc until 0.1 and then decreased, and the IPCE value at 700 nm of the solar cell based on the nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer is 1.7 times larger than that of the cell using AlTPPc adsorbed on a nanocrystalline TiO2 film electrode.

1. Introduction Dye-sensitized solar cells have attracted much attention as low cost conventional solid-state photovoltaic cells.1 Many studies on the dye-sensitized solar cells have been reported.2-5 The most successful photoinduced electrontransfer sensitizers employed so far in these cells are ruthenium(II) polypyridyl complexes. The photovoltaic conversion efficiency was 10% in the dye-sensitized nanocrystalline solar cells using ruthenium(II) polypyridyl complexes.2-15 To improve further the performance of the dye-sensitized nanocrystalline solar cells using ruthenium(II) polypyridyl complexes, it is imperative to enhance their near-infrared response, which is weak, owing to the * To whom correspondence should be addressed. Phone: +8197-554-7972. Fax: +81-97-554-7972. E-mail: [email protected]. (1) O’Regann, B.; Gra¨tzel, M. 1991 Nature 1991, 353, 737. (2) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Vlachpoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (3) Burnside, S. D.; Shklover, V.; Barbe, C.; Comte, P.; Arendse, F.; Brooks, K.; Gra¨tzel, M. Chem. Mater. 1998, 10, 2419. (4) Nakade, S.; Kambe, S.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2001, 105, 9150. (5) Kitamura, T.; Maitani, M.; Matsuda, M.; Wada, Y.; Yanagida, S. Chem. Lett. 2001, 1054. (6) Argazzi, R.; Bignozzi, C.; Hcimer, T.; Castellano, F.; Meyer, G. Inorg. Chem. 1994, 33, 5741. (7) Huang, S. Y.; Schlichtho¨rl, G.; Gra¨tzel, M.; Frank, A. J. J. Phys. Chem. B 1997, 101, 2576. (8) Zhang, D.; Ito, S.; Wada, Y.; Kitamura, T.; Yanagida, S. Chem. Lett., 2001, 1042. (9) Cao, F.; Oskam, G.; Searson, P.; Stipkala, J. M.; Heimer, T.; Farzad, F.; Meyer, G. J. J. Phys. Chem. 1995, 99, 17071. (10) Smestad, G.; Bignozzi, C.; Argazzi, R. Sol. Energy Mater. Sol. Cells 1994, 32, 259. (11) Bechinger, C.; Ferrere, S.; Zaban, A.; Sprague, J.; Gregg, B. Nature 1996, 383, 608. (12) Murakoshi, K.; Kogure, R.; Wada, Y.; Yanagida, S. Chem. Lett. 1997, 471. (13) Papageorigiou, N.; Athanassov, Y.; Bonhote, P.; Pettersson, H.; Azam, A.; Gra¨tzel, M. J. Electrochem. Soc. 1996, 143, 3099. (14) Pechy, P.; Rotzinger, F. P.; Nazeeruddin, M. K.; Kohle, O.; Zakeeruddin, S. M.; Humphry-Baker, R.; Gra¨tzel, M. J. Chem. Soc., Chem. Commun. 1995, 65. (15) Wada, Y.; Tomita, K.; Murakoshi, K.; Yanagida, S. J. Chem. Res. (S) 1996, 320.

small extinction coefficient of such ruthenium(II) complexes above 600 nm. Metallophthalocyanines possess an intense absorption band in the far-red and near-infrared regions and excellent photostability and chemical and thermal stability.5 Effective near-infrared sensitized solar cells using the zinc(II), aluminum(III), and metal-free phthalocyanines with carboxylic acid or sulfonic acid groups and without a carboxylic acid group have been reported.16-20 As metallophthalocyanine molecules usually aggregate on the solid surface, in general, the sensitization activity decreases in the solid state. Thus, the suppression of aggregation of metallophthalocyanine molecules is desired. To suppress the aggregation of metallophthalocyanine molecules, cholic acid was adsorbed with metallophthalocyanine onto the nanocrystalline TiO2 film electrode. The cell performance was improved by using the cholic acid and metallophthalocyanine adsorbed onto the nanocrystalline TiO2 film electrode.18 However, the effects of the concentration ratio of metallophthalocyanine to cholic acid on the properties of solar cells have not been clarified. In contrast, the aggregation of metallophthalocyanines also will be suppressed by a coadsorbed simple hydrocarbon with a long alkyl chain such as myristic acid. Thus, the cell performance was improved using the nanocrystalline TiO2 film electrode modified by a hydrocarbon with a long alkyl chain and metallophthalocyanine coadsorbed layers. In this work aluminum phthalocyanine with phenoxy groups, aluminum 2,9,16,23-tetraphenoxy-29H,31Hphthalocyanine hydroxide (AlTPPc; see Figure 1), and a myristic acid adsorbed nanocrystalline TiO2 film electrode (16) Leznoff, C. C.; Lever, A. B. P. Phthalocyanines: Properties and Applications; VCH: New York, 1993; Vol. 4. (17) Jianjun, H.; Hagfeldt, A.; Lindquist, S. E.; Grennberg, H.; Kordi. F.; Sun, L.; Åkermark, B. Langmuir 2001, 17, 2743. (18) Nazeeruddin, M. K.; Humphry-Baker, R.; Gra¨tzel, M.; Wo¨hrle, D.; Schnurpfeir, G.; Schneider, G.; Hirth, A.; Trombach, N. J. Porphyrins Phthalocyanines 1999, 3, 230. (19) Komori, T.; Amao, Y. J. Porphyrins Phthalocyanines 2002, 6, 211. (20) Komori, T.; Amao, Y. J. Porphyrins Phthalocyanines 2003, 7, 131.

10.1021/la035001u CCC: $25.00 © 2003 American Chemical Society Published on Web 09/11/2003

Dye-Sensitized Solar Cell

Langmuir, Vol. 19, No. 21, 2003 8873 of AlTPPc. The amount of AlTPPc adsorbed onto the nanocrystalline TiO2 electrode was determined using a spectrophotometer (Multispec-1500 Shimadzu) according to a previously reported method.21 2.5. Photoelectric Characterization of a Solar Cell Using a Nanocrystalline TiO2 Film Electrode Modified by an AlTPPc and Myristic Acid Adsorbed Layer. The photocurrent-photovoltage characteristics of the nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer were measured with a sandwich type cell. The working electrode with a nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer was gently squeezed together with a carbon-coated OTE glass electrode using a spring and irradiated from the working electrode side. The 0.05 mol dm-3 I2/0.5 mol dm-3 KI in ethylene glycol/acetonitrile solution was used as the redox electrolyte. A 200 W tungsten lamp was used as light source for the photocurrent and photovoltage characteristics with the two digital multimeters model 2000-J (Keithley) and model 34401A (Agilent). The distance between the lamp and a test cell was 4.0 cm. The active electrode area was typically 4.0 cm2. The light intensity on the surface of a test cell was 80 mW cm-2, measured with a laser power meter model AN/2 (Ophir Optronics, Inc). The fill factor (FF) is defined by

Figure 1. Chemical structures of 2,9,16,23-tetraphenoxy29H,31H-phthalocyanine hydroxide (AlTPPc) and myristic acid.

were prepared and the effect of myristic acid on the photoelectrical properties of the solar cell using visible light sensitization of a nanocrystalline TiO2 film by AlTPPc was investigated. 2. Experimental Section 2.1. Materials. Aluminum 2,9,16,23-tetraphenoxy-29H,31Hphthalocyanine hydroxide (AlTPPc) was purchased from Aldrich Co. Ltd. Myristic acid was obtained from Kishida Chemical Co. Ltd. Titanium dioxide powder (P25) was purchased from Degussa. The 0.05 mol dm-3 I2/0.5 mol dm-3 KI in ethylene glycol/ acetonitrile solution was obtained from Sol Ideas Technology Development. The other chemicals were analytical grade or the highest grade available. A conductive glass plate (10-15 Ω/square SnO2: fluorine coated) was obtained from Nihon Sheet Glass Co. Ltd. 2.2. Spectroscopic Measurements. Absorption spectra of an AlTPPc and myristic acid layer adsorbed onto a TiO2 nanocrystalline film electrode were recorded using a spectrophotometer (Multispec-1500 Shimadzu). The emission spectra of an AlTPPc and myristic acid layer adsorbed onto a TiO2 nanocrystalline film electrode were measured using a spectrofluorophotometer with a 150 W xenon lamp as a visible excitation light source (RF-5300PC Shimadzu). The excitation and emission band-passes were 5.0 nm, respectively. 2.3. Preparation of a Nanocrystalline TiO2 Film Electrode. The nanocrystalline TiO2 film was prepared by a similar procedure to that described in the literature.2,4,5,8 TiO2 powder was dispersed by grinding in concentrated nitric acid aqueous solution (pH ) 1.0). The viscous suspension was spread onto a transparent conductive glass plate (OTE) (5 cm × 5 cm) at room temperature using Scotch tape as a spacer. A thin film was obtained by raking off the excess suspension with a glass rod. After the tape was removed and the plate was dried using a hot plate at 80 °C for 30 min, this plate was annealed at 450 °C for 30 min under ambient conditions to form a nanocrystalline TiO2 film electrode. The thickness of the film, determined by using a micron-sensitive caliper, was about 10 µm. 2.4. Preparation of a Nanocrystalline TiO2 Film Electrode Modified by an AlTPPc and Myristic Acid Adsorbed Layer. A nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer was prepared as follows. An OTE glass plate with a nanocrystalline TiO2 film was dipped into a dichloromethane solution of AlTPPc and myristic acid at room temperature for 24 h. After dipping, the plate was washed by ethanol and then dried under vacuum overnight. The AlTPPc concentration was fixed to 0.1 mmol dm-3. The molar composition ratios of AlTPPc to myristic acid were changed from 0 to 0.5. The presence of myristic acid is necessary to avoid surface aggregation

FF ) IPh(max)VPh(max)/ISCVOC

(1)

where IPh(max) and VPh(max) are the photocurrent density and photovoltage for the maximum power output and ISC and VOC are the short-circuit photocurrent density and the open-circuit photovoltage. 2.6. Incident Photon-to-Current Conversion Efficiency (IPCE) of the Solar Cell Using a Nanocrystalline TiO2 Film Electrode Modified by an AlTPPc and Myristic Acid Adsorbed Layer. A 400 W xenon lamp with a monochromator was used as light source for photocurrent action spectra measurements. The cell was operated in the short-circuit mode. The incident photon-to-current conversion efficiency (IPCE) values were determined to be between 400 and 760 nm. The IPCE was then calculated according to the following equation:

IPCE ) 1240iph (µΑ)/[P (µW)]λ (nm)

(2)

where iph and P are the photocurrent and power of the incident radiation per unit area and λ is the wavelength of the monochromatic light.

3. Results and Discussion 3.1. Absorption Spectra of an AlTPPc and Myristic Acid Layer Adsorbed onto a Nanocrystalline TiO2 Film. UV-vis absorption spectra of AlTPPc in dichloromethane solution (a), AlTPPc adsorbed on a nanocrystalline TiO2 film (b), and AlTPPc and myristic acid adsorbed on a nanocrystalline TiO2 film (c) in the visible region are shown in Figure 2, respectively. The intense absorption band of AlTPPc in the visible region has a maximum at 690 nm ( ) 30 000 dm3 mol-1 cm-1) in dichloromethane solution. On the other hand, the intense reflectance band of AlTPPc adsorbed on a nanocrystalline TiO2 film in the visible region has a maximum at 700 nm and the broaden band appeared in the visible region between 500 and 600 nm, indicating that the aggregation of AlTPPc molecules on the TiO2 film occurred. In contrast, the UV-vis absorption spectra of AlTPPc and myristic acid adsorbed on a nanocrystalline TiO2 film are similar to that of AlTPPc in dichloromethane solution. In all cases of a nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer, the amount of AlTPPc adsorbed onto the TiO2 film was estimated to be ∼10-9 mol cm-2 according to the previously reported method.21 The intense absorption band of AlTPPc in the (21) Saeki, A.; Sakai, H.; Kamogawa, K.; Kondo, Y.; Yoshino, N.; Uchiyama, H.; Harwell, J. H.; Abe, M. Langmuir 2000, 16, 9991.

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Figure 2. UV-vis absorption spectra of AlTPPc in dichloromethane solution (a), AlTPPc on a TiO2 film (b), and a TiO2 film modified by an AlTPPc and myristic acid adsorption layer (c).

Figure 3. Fluorescence spectra of AlTPPc in dichloromethane solution (a), AlTPPc and myristic acid adsorbed on a nanocrystalline Al2O3 film (b), and a TiO2 film (c). The excitation wavelength was 600 nm.

visible region has a maximum at 690 nm. These results indicate that the aggregation of AlTPPc molecules on the TiO2 film was suppressed by myristic acid coadsorption. As myristic acid has a carboxylate group, myristic acid has a large ability of adsorption onto the TiO2 film compared with that of AlTPPc. At first, myristic acid adsorbed onto the TiO2 film and the hydrophobic domain of the myristic acid molecules layer formed onto the TiO2 film, and then, the AlTPPc molecules dispersed in the hydrophobic domain site. Thus, the aggregation of AlTPPc molecules on the TiO2 film will be suppressed by myristic acid. 3.2. Fluorescence Spectra of a Nanocrystalline TiO2 Film Electrode Modified by an AlTPPc and Myristic Acid Adsorbed Layer. The emission spectra of AlTPPc in dichloromethane solution (a) and of AlTPPc and myristic acid adsorbed on a nanocrystalline Al2O3 film (b) and on a nanocrystalline TiO2 film (c) with 600 nm excitation are shown in Figure 3. In all cases, the maximum of the luminescence is located at 704 nm. In the case of AlTPPc and myristic acid adsorbed on a nanocrystalline Al2O3 film, the fluorescence maximum at 704 nm remained the same as that seen in dichloromethane. On the other hand, the fluorescence intensity of AlTPPc and myristic acid adsorbed on a nanocrystalline TiO2 film was decreased, indicating that the emission of AlTPPc was effectively quenched by nanocrystalline TiO2. In the previously report, the emission quenching processes of aluminum phthalocyanine adsorbed on nanocrystalline TiO2 and Al2O3 films were studied using time-resolved fluorescence spectroscopy.18 The fluorescence lifetime of aluminum phthalocyanine adsorbed on nanocrystalline TiO2 was shorter than that of aluminum phthalocyanine

Amao and Komori

Figure 4. Photocurrent-photovoltage curves of the cell using an AlTPPc adsorbed TiO2 film electrode (a) and a TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer (b and c): [myristic acid]/[AlTPPc] ) 0.1 (b) and 0.5 (c).

on nanocrystalline Al2O3, indicating that the emission of aluminum phthalocyanine was quenched by the electron injection from the excited singlet state of aluminum phthalocyanine into the conduction band of TiO2 particles.18 In our work, the quenching of the emission of AlTPPc was also found to be due to electron injection from the excited singlet state of AlTPPc into the conduction band of TiO2 particles. 3.3. Photocurrent-Photovoltage Characterization of a Solar Cell Using a Nanocrystalline TiO2 Film Electrode Modified by an AlTPPc and Myristic Acid Adsorbed Layer. The effect of the alkyl chain length of the coadsorbed carboxylic acid on the cell performance was investigated. The VOC and ISC values increased with the number of carbons in the alkyl chain (n) up to 14 and then were constant values. Thus, myristic acid (n ) 14) is selected as a carboxylic acid with a long alkyl chain. Figure 4 shows the photocurrent-photovoltage characteristics of a sandwich solar cell based on AlTPPc adsorbed on a nanocrystalline TiO2 film electrode (a) and a nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer (b and c) irradiated with a 200 W tungsten lamp with the light intensity 80 mW cm-2 as a light source. The concentration ratios of AlTPPc to myristic acid of parts b and c were 1:0.1 and 1:0.5, respectively. For a cell using AlTPPc adsorbed on a nanocrystalline TiO2 film electrode, the ISC, VOC, and FF values were 0.026 mA cm-2, 186 mV, and 40.4%, respectively. The maximum power output was estimated to be 2.25 µW cm-2. For a solar cell using the layer with the ratio of AlTPPc to myristic acid 1:0.1, on the other hand, the ISC, VOC, and FF values were 0.061 mA cm-2, 225 mV, and 40.0%, respectively. The maximum power output was estimated to be 5.40 µW cm-2. These results show that the cell performance was improved using a nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer. In contrast, the ISC, VOC, and FF values of the solar cell using the layer with the ratio of AlTPPc to myristic acid 1:0.5 were 0.020 mA cm-2, 138 mV, and 40.1%, respectively. The maximum power output was estimated to be 1.10 µW cm-2. The effect of the concentration ratio of AlTPPc to myristic acid on the properties of the solar cell was studied. Figure 5 shows the relationship among ISC, VOC, and the concentration ratio of AlTPPc to myristic acid adsorbed onto a nanocrystalline TiO2 film electrode. The ISC and VOC values increased with the concentration ratio of myristic acid to AlTPPc until 0.1 and then decreased. As the myristic acid layer is an insulator, in general, it is difficult to create contact between the nanocrystalline TiO2 film electrode modified by an

Dye-Sensitized Solar Cell

Figure 5. Relationship among short-circuit photocurrent, opencircuit voltage, and the ratio of myristic acid to AlTPPc.

Figure 6. Photocurrent action spectra of the cell using an AlTPPc adsorbed TiO2 film electrode (open circle) and a TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer (closed circle): [myristic acid]/[AlTPPc] ) 0.1.

AlTPPc and myristic acid adsorbed layer and the redox electrolyte at a large concentration ratio of myristic acid to AlTPPc ([myristic acid]/[AlTPPc] > 0.2). Thus, the photovoltage and photocurrent decreased at a large concentration ratio of myristic acid to AlTPPc. Next let us focus on the stability of the solar cell based on the nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer against continuous irradiation. For all cells using a cell based on the nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer, only little photocurrent and photovoltage changes were observed after 24 h of irradiation with a 200 W tungsten lamp. These results indicate that AlTPPc has a good photostability and that a solar cell based on the nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer is stable under continuous irradiation. 3.4. Photocurrent Action Spectrum of a Solar Cell Using a Nanocrystalline TiO2 Film Electrode Modified by an AlTPPc and Myristic Acid Adsorbed Layer. Figure 6 shows the photocurrent action spectra of a sandwich solar cell based on AlTPPc adsorbed on a nanocrystalline TiO2 film electrode (open circle) and a nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer (closed circle), where the incident photon-to-current conversion efficiency (IPCE) is plotted as a function of wavelength (500-860 nm). The

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concentration ratio of AlTPPc to myristic acid was 1:0.1. For a solar cell based on AlTPPc adsorbed on a nanocrystalline TiO2 film electrode, 0.83% of the IPCE value was obtained around the wavelength of the absorption maximum at 700 nm. For a solar cell based on the nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer, on the other hand, 1.31% of the IPCE value was obtained around the wavelength of absorption maximum at 680 nm. The IPCE value at 700 nm of a solar cell based on the nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer is larger than that of the cell AlTPPc adsorbed on a nanocrystalline TiO2 film electrode. Moreover, the IPCE values of the cell AlTPPc adsorbed on a nanocrystalline TiO2 film electrode are lower compared with those of a solar cell based on the nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer in the 450-600 nm region. This result shows that the coadsorbed myristic acid layer suppresses the aggregation of AlTPPc onto the TiO2 film. Thus, the photon-to-current conversion efficiency was improved using a nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer compared with that in the absence of myristic acid. IPCE values at 700 nm also increased with the concentration ratio of myristic acid to AlTPPc until 0.1 and then decreased. These result also indicated that contact between the nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer and the redox electrolyte is suppressed at a large concentration ratio of myristic acid to AlTPPc ([myristic acid]/[AlTPPc] > 0.2). However, the IPCE values at 450 nm are larger than that of the cell AlTPPc adsorbed on a nanocrystalline TiO2 film electrode. 4. Conclusion In this work aluminum phthalocyanine with phenoxy groups, aluminum 2,9,16,23-tetraphenoxy-29H,31Hphthalocyanine hydroxide (AlTPPc), and myristic acid adsorbed on a nanocrystalline TiO2 film electrode were prepared and the effect of myristic acid on the photoelectrical properties of the solar cell using visible light sensitization of a nanocrystalline TiO2 film by AlTPPc was investigated. From the result of the UV-vis absorption spectra measurement, the aggregation of AlTPPc molecules on the TiO2 film was suppressed by myristic acid coadsorption. The effect of the alkyl chain length of the coadsorbed hydrocarbon with an alkyl chain on the cell performance was investigated, and the VOC and ISC values increased with the number of carbons in the alkyl chain up to 13 and then were constant values. The ISC and VOC values increased with the concentration ratio of myristic acid to AlTPPc until 0.1 and then decreased, and the IPCE value at 700 nm of a solar cell based on the nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer is larger than that of the cell using AlTPPc adsorbed on a nanocrystalline TiO2 film electrode. These results indicate that the cell performance was improved using a nanocrystalline TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer compared with that in the absence of myristic acid. Acknowledgment. This work was partially supported by a Special Fund from the Venture Business Laboratory of Oita University. LA035001U