Photovoltaic performance of ruthenium complex dye

Materials Chemistry and Physics 142 (2013) 420e427

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Photovoltaic performance of ruthenium complex dye associated with number and position of carboxyl groups on bipyridine ligands King-Fu Lin a, b, *, Jen-Shyang Ni b, Chun-Hua Tseng a, Chun-Yi Hung a, Ken-Yen Liu a a b

Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

Ruthenium dye with different number of carboxyl groups adsorbed on the TiO2 with different tilt angle. Ruthenium dye injected the electrons to the TiO2 through the carboxyl groups. Photocurrent of DSSC, increased with the number of carboxyl groups on the ruthenium dye.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 March 2013 Received in revised form 28 June 2013 Accepted 28 July 2013

A series of ruthenium complex dyes with different number and position of carboxyl groups on bipyridine ligands, such as Ru(4-carboxyl-40 -methyl-2,20 -bipyridine)(4,40 -dimethyl-2,20 - bipyridine)(NCS)2 (denoted as Ru1A), Ru(4-carboxyl-40 -methyl-2,20 -bipyridine)2(NCS)2 (Ru11A), Ru(4,40 -dicarboxyl-2,20 -bipyridine)(4,40 dimethyl-2,20 -bipyridine)(NCS)2 (Ru2A), and Ru(4-carboxyl-40 -methyl-2,20 -bipyridine)(4,40 -dicarboxyl-2,20 bipyridine)(NCS)2 (Ru3A) were synthesized and compared with Ru(4,40 -dicarboxyl-2,20 -bipyridine)2 (NCS)2, commonly known as N3 dye for the adsorption behavior on the TiO2 surface and photovoltaic properties of dye-sensitized solar cells. The experimental results show that the tilt angle of ruthenium dyes on the TiO2 surface which is dependent on the number and position of their carboxyl groups strongly affected the photovoltaic performance. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Electronic materials Monolayers Interfaces Adsorption

1. Introduction Dye-sensitized solar cell (DSSC) invented by O’Regan and Grätzel in 1991 [1] has attracted intensive attention due to its low cost and facile fabrication [2e19]. Major components of the DSSC such as dyes [7e11], electrolyte systems [12e17] and counter electrodes [18,19] have been investigated considerably recently. The dye, being

* Corresponding author. Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan. Tel.: þ886 2 33661315; fax: þ886 2 23634562. E-mail address: kfl[email protected] (K.-F. Lin). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.07.038

chemisorbed on the mesoporous TiO2 surface, upon photo excitation will inject its excited electron into the conducting band of TiO2 performing like a human heart. The electrons after passing through the external circuit will reach the counter electrode to conduct the reduction reaction with triiodide in the electrolyte forming iodide ions, which then transport across the electrolyte region to regenerate the oxidized dye, thus resuming the photoelectric operation of dyes. The carboxyl groups of Ru(4,40 -dicarboxyl-2,20 -bipyridine)2(NCS)2 commonly known as N3 dye have been demonstrated as an anchor onto the mesoporous TiO2 surface via unidentate mode, bidentate chelating mode and bridging mode [20,21]. Therefore, only monolayer chemisorption of ruthenium dyes onto the mesoporous TiO2

K.-F. Lin et al. / Materials Chemistry and Physics 142 (2013) 420e427

surface for DSSC was attained [22]. The amphiphilic structure of ruthenium dye such as Ru(4,40 -dicarboxyl-2,20 -bipyridine)(4,40 binonyl-2,20 -bipyridine)(NCS)2 known as Z907 dye greatly affects the morphology of monolayer adsorption and the resulting hydrophobic surface has the advantage to reduce the charge recombination in the interface between the dye-sensitized TiO2 and electrolyte [10]. In this work, we synthesized the ruthenium dyes with different number and position of carboxyl groups on the bipyridine ligands, such as Ru(4-carboxyl-40 -methyl-2,20 - bipyridine)(4,40 -dimethyl2,20 -bipyridine)(NCS)2 (denoted as Ru1A), Ru(4-carboxyl-40 methyl-2,20 -bipyridine)2(NCS)2 (Ru11A), Ru(4,40 -dicarboxyl-2,20 bipyridine)(4,40 -dimethyl- 2,20 -bipyridine)(NCS)2 (Ru2A), and Ru(4-carboxyl-40 -methyl-2,20 -bipyridine)(4,40 -dicarboxyl-2,20 bipyridine)(NCS)2 (Ru3A), and compared their monolayer chemisorption behavior onto the TiO2 surface with the N3 dye. The photovoltaic properties of their fabricated DSSCs associated with the resulting monolayer chemisorption were also discussed. 2. Experimental 2.1. Materials All solvents and chemicals were obtained from Acores and Aldrich. The solvents were dried over sodium or CaH2 and distilled before use. N3 dye was obtained from Solaronix S. A., Aubonne, Switzerland. Ru1A, Ru11A, Ru2A and Ru3A with the chemical structures shown in Fig. 1a were synthesized by the typical onestep synthetic method developed for heteroleptic polypyridyl ruthenium complexes [23]. Their synthesis routes are schematically illustrated in Fig. 1b. 2.1.1. Synthesis of 4,40 -dimethyl-2,20 -bipyridine (dmbpy) ligand [24] 250 mL 4-picolin in flask was added with 10 g palladium (10% on charcoal). After refluxing for 72 h, it was added with 100 mL benzene. After refluxing for another 30 min, the reaction mixture was passed through the silica gel to remove the catalyst. The raw product, after removing solvent with rotary evaporator, was recrystallized from ethyl acetate to afford a powder form of dmbpy. (20 g, 32%) 1H NMR (500 MHz; CDCl3) d: 2.44 (6H, s), 7.13 (2H, d), 8.24 (2H, s), 8.53 (2H, d). 2.1.2. Synthesis of 4,40 -dicarboxy-2,20 -bipyridine (dcbpy) ligand [23] To a solution of 16.3 mmol Na2Cr2O7 in 25 mL concentrated sulfuric acid was slowly added 4 mmol of dmbpy with stirring. After 30 min, the reaction mixture was poured into 200 mL of cold water forming a light yellow precipitate. After filtration and drying, the obtained solid was dissolved in 10% NaOH aqueous solution followed by slow acidification to pH 2 with 10% aqueous HCl solution. After filtration, the compound was dried under vacuum to afford a white solid of dcbpy. (1.06 g, 80%) 1H NMR (500 MHz; CDCl3) d: 7.90 (1H, d), 8.84 (1H, s) and 8.90 (1H, d). 2.1.3. Synthesis of 4-methyl-40 -carboxy-2,20 -bipyridine (mcbpy) ligand [25] The synthesis was divided into two steps. In the first step, to a solution of 27.17 mmol dmbpy in 200 mL 1,4-dioxane was added 30.62 mmol SeO2 under Ar and refluxed for 24 h. After filtering, the solvent was removed by rotary evaporator. The remaining solid was added to 250 mL ethyl acetate. After refluxing for 1 h, the solution was filtered to remove the insoluble fraction and then extracted with 250 mL of 0.1 M Na2CO3(aq) and 250 mL of 0.3 M Na2S2O5(aq). The combined aqueous extracts were adjusted to pH 10 with NaHCO3. After further extracted with dichloromethane, the solvent was removed by rotary evaporator to afford a yellowish solid of 4-

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methyl-40 -carboxaldehyde-2,20 -bipyridine (mchobpy). (2.42 g, 90%) 1H NMR (500 MHz; CDCl3) d: 2.55 (3H, s), 7.33 (1H, s), 7.78 (1H, d), 8.39 (1H, s), 8.65 (1H, d), 8.93 (1H, d), 9.06 (1H, s), 10.22 (1H, s). In the second step, to a solution of 8.0 mmol mchobpy in 80 mL ethanol (95%) was added 20 mL of AgNO3 aqueous solution (0.42 M) with vigorously stirring. After fully dissolved, the solution was slowly added with 1 M NaOH(aq) until the precipitate appeared. After stirring for 24 h, the reaction mixture was evaporated under low pressure with a rotary evaporator to remove the ethanol. After washing with 250 mL of 2.0 M NaOH(aq) and 200 mL water, the solution was extracted with 500 mL dichloromethane. The solid product was precipitated by titrating the solution with 0.1 M HNO3, which was then filtered and dried to afford a white powder of mcbpy. (1.37 g, 76%) 1H NMR (500 MHz; d6-DMSO) d: 3.14 (3H, s), 7.21 (1H, d), 7.68 (1H, dd), 8.19 (1H, s), 8.49 (1H, d), 8.56 (1H, d), 8.70 (1H, s). 2.1.4. Synthesis of Ru1A [26] To a solution of [RuCl2(p-cymene)]2 (0.176 mmol) in 30 mL dry DMF was added dmbpy (0.352 mmol) with stirring. After reaction at 80 C under Ar for 4 h at dark, mcbpy (0.352 mmol) was added. After refluxing at 160 C for another 4 h, excess NH4NCS was added to the reaction mixture. The reaction proceeded at 130 C for 5 h and then the solvent was removed by rotary evaporator. 100 mL of water was added to the resulting mixture to remove excess NH4NCS. The waterinsoluble product was collected on a sintered glass crucible by suction filtration and washed with distilled water, followed by diethylether and dried in air. The crude product was dissolved in methanol containing tetrabutylammonium hydroxide and then passed through the Sephadex LH-20 column using methanol as the eluent. The main band was collected and concentrated. A few drops of 0.01 M HNO3 aqueous solution were added to precipitate the product. The yield was 31%. 1H NMR (500 MHz, d6-DMSO) d: 2.39 (6H,s), 2.42 (3H,s), 2.63e2.66 (10H, t), 7.04 (1H, d), 7.08 (1H, d), 7.12 (1H, d), 7.29 (1H, t), 7.35 (1H, d), 7.38 (1H, d), 7.57 (1H, dd), 7.78 (4H, m), 8.25 (1H, dd), 8.47 (1H, s), 8.48 (1H, s), 8.62 (1H, s), 8.63 (1H, s), 8.71 (1H, s), 8.82 (1H, s), 8.84 (1H, s), 8.98 (1H, s), 9.02 (1H, d), 9.06 (1H, d), 9.028 (1H, d), 9.46 (1H, d). The ratio of isomers (see Fig. 1a) was 5:6 according to the proton integrals at 7.57 and 8.25 ppm. 2.1.5. Synthesis of Ru11A The synthesis method is similar to Ru1A except that dmbpy was replaced by mcbpy. The yield was 33%. 1H NMR (500 MHz, d6DMSO) d: 2.41 (9H, s), 2.67 (9H, s), 7.09 (1H, d), 7.14 (1H, d), 7.35 (1H, d), 7.41 (1H, d), 7.52 (1H, d), 7.58 (1H, d), 7.73 (1H, d), 7.78 (1H, d), 7.81e7.84 (2H, m), 8.27e8.29 (2H, m), 8.72(2H, d), 8.84 (2H, d), 8.88 (2H, s), 9.00e9.02 (3H, m), 9.07 (1H, d), 9.41(1H, d), 9.45(1H, d). The ratio of isomers (see Fig. 1a) was 1:2:1 according to the proton integrals at 7.09, 7.14, and 7.58 ppm. 2.1.6. Synthesis of Ru2A The synthesis method is similar to Ru1A except that mcbpy was replaced by dcbpy. The yield was 37%. 1H NMR (500 MHz, d6-DMSO) d: 2.40 (3H, s), 2.67 (3H, s), 7.06 (1H, d), 7.30 (1H, d), 7.63 (1H, dd), 7.82 (1H, d),7.84 (1H, d), 8.30 (1H, dd), 8.50 (1H, s), 8.66 (1H, s), 8.95 (1H, s), 9.04 (1H, d), and 9.46 (1H, s). 2.1.7. Synthesis of Ru3A The synthesis method is similar to Ru1A except that dmbpy was replaced by dcpby. The yield was 38%. 1H NMR (500 MHz, d6DMSO) d: 2.40 (3H, s), 2.67 (3H, s) 7.11 (1H, d), 7.37 (1H, d), 7.54e 7.58 (2H, m), 7.66 (1H, d), 7.73 (1H, d), 7.8 (1H, d), 7.85e7.88 (2H, m), 8.33e8.36 (3H, m), 8.78 (1H, s), 8.89 (1H, s), 8.93 (1H, s), 9.00 (2H, d), 9.05 (2H, d), 9.15 (2H, s), 9.44 (2H, d), and 9.48 (1H, d). The ratio of isomers (see Fig. 1a) was 9:8 according to the proton integrals at 7.11 and 7.37 ppm.

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Fig. 1. (a) Chemical structure and (b) synthetic route of the dyes.

2.1.8. Preparation of dye samples for atomic force microscopy (AFM) [10,22] 0.1 M titanium tetraisopropoxide (TTIP) in ethanol (2.68 mL) which had been mixed with 0.24 mmol HCl was deposited on an automatically-flat mica disk by spin-coating and then heated to 450 C for 30 min to transform it into anatase TiO2 film. The dye solutions were prepared by dissolving the Ru1A, Ru11A, Ru2A, Ru3A and N3 dyes, respectively in a mixed solvent of acetonitrile (ACN)/t-butanol (1:1 by volume) at a concentration of 0.3 mM. The resulting TiO2 films on mica were immersed into individual dye solution for 10, 30, and 60 min. The dye-adsorbed TiO2 films at different adsorption time were dried at room temperature and investigated by AFM.

sputtering. For preparation of photo-electrode, a mesoporous film of anatase TiO2 coating on the FTO glass substrate was fabricated by solegel process according to the literature [27]. An active area of 0.25 cm2 TiO2 was selected from sintered electrode, immersed in a co-solvent of ACN/t-butanol (1:1 by volume) containing 0.3 mM Ru1A, Ru11A, Ru2A, Ru3A and N3 dyes, respectively for 24 h, rinsed with acetone and dried. The liquid electrolyte system containing proper amounts of 0.6 M propylmethylimidzolium iodide (PMII), 0.1 M LiI, 0.05 M I2, 0.1 M guanidine thiocyanate (GuNCS) and 0.5 M tert-butylpyridine (TBP) in methoxyproprionitrile (MPN) was prepared.

2.2. Fabrication of DSSC

The specific surface area of fabricated mesoporous TiO2 film was measured using BrunauereEmmetteTeuller (BET) method based on the nitrogen (N2) uptake at 77 K in a Micrometry Tristar-3000. The mesoporous TiO2 was removed from its sintered film on the FTO glass substrate by rubbing and then ground into a powder form, which was then weighed and loaded into a sample tube and

Dye-sensitized solar cells were fabricated using fluorine-doped tin oxide (FTO, 20e25 U/square) glass as substrate for both photo-electrode and counter-electrode. The counter-electrodes were fabricated by depositing a thin platinum layer with

2.3. Surface area measurement of mesoporous TiO2 films [22]


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vacuumed at 120 C for 24 h. The adsorptionedesorption isotherm curves were obtained from the BET measurements. 2.4. Measurements of dye adsorption amount on mesoporous TiO2 film The calibration curve was first established by measuring the UVevis spectra of dyes in 0.1 M NaOH solution of methanol at a concentration range of 106 to 104 M. The peak intensities of Ru1A, Ru11A, Ru2A, Ru3A and N3 dyes, respectively at the metalto-ligand charge transfer (MLCT) transitions were chosen for calibration. Several mesoporous TiO2 films in 0.5 0.5 cm2 square prepared for the photoelectrode were immersed into the 0.3 mM dye solutions (ACN/t-butanol, 1:1 by volume) at room temperature and removed sequentially after 3, 6, 9, 12, 24 and 48 h. After washed with ACN and dried, the dye adsorbed on the mesoporous TiO2 films was completely removed with 0.1 M NaOH in methanol ultrasonically and the amount of dyes desorbed was estimated by using UVevis spectroscopy. 2.5. Characterization Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance-500 MHz spectrometer by utilizing deuterated chloroform (CDCl3) and dimethyl sulphoxide (d6-DMSO) as solvents. Infrared spectra were recorded on a JASCO PLUS 480 FTIR spectrometer using KBr pellets and UV/vis spectra were recorded on a JASCO V-550 UV/vis spectrometer. AFM image was investigated with a Digital Instruments Dimension-3100 Multimode microscope in tapping mode. Photovoltaic characterization of the DSSCs was carried out by illuminating the cell with a 1000 W ozone-free Xenon lamp equipped with a water-based IR filter and AM 1.5 filter (Sciencetech). The photocurrentevoltage plots at 100 mW cm2 illumination and at dark were recorded with a potentiostat/galvanostat (PGSTAT 302N, Autolab, Eco-Chemie, the Netherlands). The monochromator was scanned through the UV/vis region to generate the IPCE plot as defined by IPCE ¼ 1240(Jsc/wl), where Jsc is the short-circuit current (mA cm2), w the incident irradiative flux (W cm2), and l the wavelength. The open-circuit potential decay transients and charge extraction measurements were carried out using the same potentiostat/galvanostat equipped with FRA2 module. Green light-emitting diode (LXHL-NM98, Luxeon, 530 nm) was used as the light source. 3. Results and discussion 3.1. Characterization of ruthenium dyes 1 H NMR spectra of Ru1A, Ru11A, Ru2A and Ru3A reveal that the pyridine rings in two different ligands are electronically in different environment (see Fig. S1 in supporting information). For example, each pyridyl ring protons in the dmbpy ligand of Ru2A have two different resonance peaks, indicating that the ligands were coordinated to ruthenium center in cis-heteroleptic complex form [22]. In addition, each pyridyl ring protons in the mcbpy ligand of Ru1A and Ru3A have four different resonance peaks, suggesting that they have two isomers with the chemical structure shown in Fig. 1a. On the same token, Ru11A has three isomers. The ratio of isomers for Ru1A is 5:6, for Ru3A is 8:9, and for Ru11A is 1:2:1. The FTIR spectra of Ru1A, Ru11A, Ru2A and Ru3A shown in Fig. 2 indicate that the intensity of absorption band at w1725 cm1 contributed by eCOOH groups increases with increasing the number of eCOOH groups on the bipyridine ligands as reference to the absorption peak at w2105 cm1 contributed by eNCS groups.

Fig. 2. FTIR spectra of the indicated dyes.

The UVevis absorption peak wavelength and molar extinction coefficients (3 ) of Ru1A, Ru11A, Ru2A, Ru3A and N3 dyes in the cosolvent of ACN/t-butanol (1:1 by volume) at various concentrations obtained from their UVevis absorption spectra (see Fig. S2 in supporting information) are summarized in Table 1. In general, the carboxyl group to substitute for the methyl group on the bipyridine ligand of ruthenium dyes shifts the MLCT transitions (dpep*) to longer wavelength, resulting from the electron withdrawing capability of the carboxylic acids. More substitutions shifted to red more. 3 is also notably increased. Moreover, double substitutions of carboxyl groups on a bipyridine ligand of Ruthenium dye such as Ru2A shifts to red more compared to two single substitutions on each bipyridine ligands of Ruthenium dye such as Ru11A. Notably, the high energy bands at the wavelength shorter than 320 nm in the UV/vis spectra of dyes are contributed by the pep* transitions of mcbpy, dcbpy and dmbpy ligands [23]. 3.2. Adsorption behavior of ruthenium dyes on TiO2 surface To monitor the adsorption behavior of Ru1A, Ru11A, Ru2A and Ru3A dyes on TiO2, we prepared a TiO2 thin film on mica by using TTIP as precursor and transformed it to an anatase form by heating at 450 C. Three pieces of TiO2 thin films on mica were dipped into individual dye solutions in ACN/t-butanol (1:1 by volume) at a concentration of 3 104 M and removed sequentially after 10, 30, and 60 min immersion. Their AFM images after dried were shown in Fig. 3. According to our previous studies [22], the N3 dyes in the ACN/t-butanol co-solvent tended to aggregate into vesicles. As the dye vesicles deposited on TiO2 thin film, only a few dye molecules on the vesicles bonded to the TiO2 surface via the carboxyl groups. The dye molecules which did not bond to the TiO2 can dissolve back to the solution. During dissolution, the dyes that participated in bonding presented a double layer first. The dyes on the top of double layer eventually dissolve leaving a monolayer of dye on the TiO2 surface. All of the Ru1A, Ru11A, Ru2A and Ru3A dyes showed the similar adsorption behavior as N3 dye on the surface of TiO2 thin film. For Table 1 UV/vis absorption properties of Ru1A, Ru11A, Ru2A, Ru3A and N3 dyes in the cosolvent of ACN and t-butanol (1:1 by volume). Dye

pep*, nm (L1*)

pep*, nm (L2*)

dpep*, nm (3 , 104 M1 cm1)

Ru1A Ru11A Ru2A Ru3A N3

213 214 213 214

293 302 292 307 315

356 369 425 392 419

*L1 and L2 are mcbpy, dcbpy or dmbpy ligands.

(0.79), (0.86), (0.94), (1.01), (1.34),

508 528 544 532 547

(0.79) (1.10) (0.90) (1.18) (1.41)

424


Fig. 3. AFM images of Ru1A, Ru11A, Ru2A and Ru3A dyes adsorbing on TiO2 thin films after immersion in ACN/t-butanol (1:1 by volume) solutions of dyes at a concentration of 3 104 M for (a) 10, (b) 30, and (c) 60 min. The insets in the figures show the section analysis of surface in vertical distance. The image size is 5 5 mm.

the specimens of 10 min dipping, the aggregated vesicles were observed evenly distributed on the TiO2 thin film (Fig. 3). Parts of them have dissolved slightly so that the center of the vesicles was dented. For the specimens of 30 min dipping, the vesicles became smaller by reducing the vertical height approximately to half. For the specimens of 60 min dipping, almost all the vesicles dissolved back to the solution leaving a monolayer of dye on the TiO2 surface with a surface roughness less than 1 nm. The gradual adsorption of Ru1A, Ru11A, Ru2A, Ru3A and N3 dyes on the mesoporous TiO2 for photoelectrode was investigated by UV/vis spectroscopy. Several pieces of mesoporous TiO2 sintering on the FTO substrate with the surface area of 0.5 0.5 cm2 were immersed into the dye solutions of 3 104 M in ACN/t-butanol (1:1 by volume) at room temperature for various allocated time. After they were removed, rinsed with ACN and dried, the adsorbed dyes were completely dissolved by 0.1 M NaOH aqueous solution ultrasonically and the adsorption amount was measured by UV/vis spectroscopy. Fig. 4 shows their adsorption amount versus time. The Ru1A and Ru2A dyes reached the maximum adsorption of 0.1137 and 0.1909 mmol, respectively after 6 h and then slightly decreased. However, N3 dye reached 0.062 mmol after 6 h and then slowly increased to 0.071 mmol after 48 h. If we assume the maximum adsorption is the amount to reach a monolayer adsorption, Ru3A had 0.094 mmol after 9 h of adsorption and Ru11A had 0.76 mmol after 12 h.

The surface area of the mesoporous TiO2 particles in the working section of 0.25 cm2 was measured as 0.0528 m2 by using the BET method (see Fig. S3 in the supporting information) according to our previous work [22]. Therefore, to reach a monolayer adsorption, each dye molecule would cover 0.46, 0.77, 0.93, 1.16 and 1.23 nm2 for Ru2A, Ru1A, Ru3A, Ru11A and N3 dye, respectively based on the

Fig. 4. Adsorption amount of the indicated dyes on the mesoporous TiO2 surface versus adsorption time by immersing the mesoporous TiO2 in the 3 104 M dye solutions of ACN/t-butanol (1:1 by volume) at room temperature.


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maximum adsorption data shown in Fig. 4. The N3 dye containing 4 carboxyl groups has been considered to be flat-lying on the TiO2 surface [22]. By assuming that all the dyes have the same flat-lying surface area as N3 dye, Ru2A, Ru1A, Ru3A, and Ru11A would lay on the TiO2 surface at the angle of 68 , 51, 40 , and 20 , respectively as schematically illustrated in Fig. 5. Notably, although Ru2A and Ru11A have equal number of carboxyl groups, the former has much higher tilt angle than the latter because of the position of the carboxyl groups. 3.3. Photovoltaic properties Fig. 6a shows the photocurrent densityevoltage (JeV) characteristics of the DSSCs with Ru1A, Ru11A, Ru2A, Ru3A and N3 dyes, respectively in liquid electrolyte under AM 1.5 and 100 mW cm2 illumination. Their IPCE spectra were shown in Fig. 6b. The comparison of open-circuit voltage (Voc), fill factor (FF), short-circuit photocurrent (Jsc), and power conversion efficiency (h) among the ruthenium dyes is shown in Fig. 7. Interestingly, Jsc increased with the number of carboxyl groups on the ruthenium dye, suggesting that the excited electrons injecting to the TiO2 mesoporous layer are through the carboxyl groups. Moreover, the Jsc for Ru2A higher than that for Ru11A is related to its higher adsorption amount (see Fig. 4) or higher molecular tilt angle (see Fig. 5). The increase of IPCE peak value with the number and position of carboxyl groups in the dye is also parallel to the increase of Jsc. Although the higher Jsc usually comes along with the lower FF, FF of the DSSC with Ru11A is the same as that with Ru2A. Besides, Voc of the DSSC with Ru11A is the lowest among all the dyes, which is obviously related to its highest dark current (see Fig. S3 in supporting information). All of the photovoltaic data are summarized in Table 2. The kinetics of charge transportation and possible charge recombination in the mesoporous TiO2 layers sensitized by Ru1A, Ru11A, Ru2A, Ru3A and N3 dyes in DSSCs were investigated next. Here, we used a green light-emitting diode as the light source to measure the voltage decay at open circuit condition and charge extraction at short circuit condition, the method of which has been developed by Duffy and Hagfeldt et al. [28e30]. Fig. 8a shows the voltage decay (Vd) of the DSSCs versus the duration time (td) in the

Fig. 5. Cartoons to illustrate the tilt angles of Ru1A, Ru11A, Ru2A, Ru3A and N3 dyes chemisorbing on the mesoporous TiO2 surface.

Fig. 6. (a) JeV curves and (b) IPCE spectra of the DSSCs sensitized by the indicated dyes with liquid electrolyte under AM 1.5 and 100 mW cm2 illumination.

dark after illuminated for 5 s at open circuit condition. The Vd of the DSSC with Ru11A has the highest decay rate reflecting to its highest charge recombination rate in the interface between the mesoporous TiO2 and liquid electrolyte, which accounts for its lowest Voc (see Fig. 7a) and highest dark current (Fig. S3 in supporting

Fig. 7. Comparison of (a) Voc and fill factor, and (b) h and Jsc among the indicated dyes to sensitize the DSSCs with liquid electrolyte under AM 1.5 and 100 mW cm2 illumination.

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Table 2 Photovoltaic properties of the DSSCs sensitized by Ru1A, Ru11A, Ru2A, Ru3A and N3 dyes, respectively under AM 1.5 and 100 mW cm2 illumination. Dye

h (%)


3.39 4.20 6.08 6.72 7.53

Jsc (mA cm2)

Voc (V)

0.05 0.12 0.09 0.10 0.09

0.68 0.65 0.68 0.67 0.68

0.01 0.01 0.01 0.01 0.01

6.59 10.87 14.96 17.92 19.63

0.08 0.20 0.24 0.13 0.15

FF 0.74 0.61 0.61 0.57 0.56

0.01 0.04 0.04 0.02 0.02

information). As the cells were set at short circuit condition in the dark immediately after illumination for 5 s, the charges (Q) collected at the photoelectrode versus time were shown in Fig. 8b. All of the collected charges increased rapidly in the beginning and leveled off at w25 s. Therefore, we regarded the collected charges of the cells at 25 s as the short circuit charges (Qsco) remaining in the mesoporous TiO2 layer. Qsco for the cells sensitized by the dyes listed in Table 3 shows N3 > Ru3A > Ru2A > Ru11A > Ru1A with the order similar to those of Jsc and IPCE peak values (see Fig. 6). The diameter of TiO2 nanoparticles in the mesoporous layer was w20 nm, by which the surface area of each TiO2 nanoparticle was estimated as w1257 nm2. Since the total surface area of mesoporous TiO2 layer in the 0.25 cm2 working area was 0.0528 m2, there would have 1.62 1014 TiO2 nanoparticles per centimeter square. Therefore, the number of charges per TiO2 nanoparticle remaining in the mesoporous layer after illumination for 5 s were estimated and included in Table 3. By the same token, the number of dye molecules adsorbed on each TiO2 nanoparticle were estimated according to the adsorption amount of dyes on the mesoporous TiO2 layer shown in Fig. 4. Therefore, the average charges

Table 3 Extracted charge data of DSSCs sensitized by Ru1A, Ru11A, Ru2A, Ru3A and N3 dyes, respectively after illumination for 5 s at open circuit condition. Dye

Qsco (mC cm2)

Electrons/TiO2 particle

No. of dye/TiO2 particle

Electrons/dye


63 74 104 118 151

2.43 2.85 4.01 4.55 5.82

1600 1000 2700 1300 1000

0.00152 0.00285 0.00148 0.00349 0.00581

injected by each dye after illumination for 5 s at open circuit condition were estimated and also included in Table 3. As the results, the number of charges injected by N3 dyes was 0.00581 per dye, the most amount, whereas those injected by Ru2A was 0.00148 per dye the least amount but close to Ru1A (0.00152 per dye). Therefore, it is noteworthy that the number of injecting charges to the TiO2 per dye by illumination not only related to its number of carboxyl groups but also related to the adsorption amount of the dye on TiO2. 4. Conclusions Ru2A, Ru1A, Ru3A and Ru11A have been synthesized and well characterized. As they were chemisorbed onto the TiO2 surface, their molecules laid on the TiO2 surface at the tilt angles of 68 , 51, 40 and 20 , respectively. As to their photovoltaic performance of DSSCs compared to N3 dye, both h and Jsc are increased with the number of carboxyl groups on the ruthenium dye, suggesting that the excited electrons injecting to the TiO2 mesoporous layer are through the carboxyl groups. Moreover, both h and Jsc of DSSC with Ru2A higher than those with Ru11A are owing to its higher adsorption amount or higher molecular tilt angle. Acknowledgment The authors thank for the financial support of the National Science Council in Taiwan, Republic of China, through Grants NSC98-2221-E-002-208-MY3. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2013.07.038. References

Fig. 8. (a) Voltage decay of DSSCs with the indicated dyes versus decay time after illumination for 5 s at open circuit condition, (b) extracted charges versus extraction time in short circuit condition at td ¼ 0.

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