Improved Efficiency of over 10% in Dye-Sensitized Solar Cells with a

ARTICLE pubs.acs.org/JPCC

Improved Efficiency of over 10% in Dye-Sensitized Solar Cells with a Ruthenium Complex and an Organic Dye Heterogeneously Positioning on a Single TiO2 Electrode Sheng-Qiang Fan,†,§ Chulwoo Kim,† Baizeng Fang,† Kai-Xing Liao,‡ Guan-Jun Yang,‡ Chang-Jiu Li,‡ Jeum-Jong Kim,† and Jaejung Ko*,† †

Department of Advanced Materials Chemistry, Korea University, Jochiwon, Chungnam 339-700, Republic of Korea State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049, PR China § Centre for Organic Photonics & Electronics, School of Chemistry & Molecular Biosciences, The University of Queensland, QLD 4072, Australia ‡

bS Supporting Information ABSTRACT: A ruthenium complex (JK-142) with an ancillary bipyridyl ligand substituted by a 3-carbazole-2-thiophenyl moiety was synthesized and explored as a sensitizer in cosensitized solar cells in combination with an organic dye (JK62). The extended π-conjugation in the ancillary ligand enables the JK-142 dye to have a red-shift light absorption band; however, the ineffective penetration of JK-142 molecules into the inner surface of TiO2 film results in low photovoltaic performance for the single dye sensitized solar cell due to its large molecular size of JK-142. Interestingly, when the deficient JK-142 electrode was employed to assemble a cosensitized solar cell by additionally adsorbing JK-62 dye, a considerably improved efficiency of up to 10.2% was achieved, which is favorably superior to that (ca. 8.68%) of N719 in the same device configurations. The results shown here not only provide new vision on how to produce highly efficient solar cells using dyes with extended molecular structure but also open up a new way to position different dyes on a single TiO2 film for cosensitization through controlling the molecule size.

1. INTRODUCTION In dye-sensitized solar cells (DSSCs), the development of sensitizing dyes to extend the optical threshold wavelength has been one of the key issues.1 In order to improve the efficiency of DSSCs, the sensitizer should be panchromatic, that is, absorb photons from the visible to near-infrared (NIR) region of the solar spectrum while maintaining sufficient thermodynamic driving force for the electron injection and dye regeneration process. Polypyridyl ruthenium complexes1,2 have been preferred choices of charge transfer sensitizers in view of their wider absorption band at the wavelength range of 400800 nm compared with metal-free organic sensitizers.3 Among ruthenium complexes, cis-bis(thiocyanato) bis(2,20 -bipyridyl-4,40 dicarboxylate) ruthenium bis(tetrabutylammonium) (N719) has maintained a clear leadship because of its excellent performance, particularly a high efficiency of over 11%.2 In order to further broaden the light absorption band toward the red region as well as improve the long-term stability of the DSSCs, a successful strategy involves replacing one of the 2,20 -bipyridyl4,40 -dicarboxylic acid (dcpbyH2) ligands in the mother compound of N719, [Ru(dcbpyH2)2(NCS)2] (N3), with an extended π-conjugated ancillary bipyridyl ligand containing r 2011 American Chemical Society

thiophene or alkoxybenzene derivatives.4 It is well established that a phenyl5 or thiophene6 group substituted to an ancillary polypyridyl ligand in ruthenium sensitizers causes a red shift and increases the absorption coefficient of the metal-to-ligand charge transfer (MLCT) band. Despite the many advantages possessed by these heteroleptic ruthenium sensitizers, however, few of them have surpassed N719 for use in DSSCs in terms of its cost and efficiency.2c Thus, an innovative systematic design of efficient ruthenium sensitizers, comparable to or better than N719, is strongly required. Because of the substantial challenge in the development of highly efficient sensitizers having both a broad absorption spectrum and matched energy levels simultaneously for DSSCs, extensive efforts have been made to use various cosensitizing methods which employed multiple dyes for attaining compensate absorption spectra.711 To realize cosensitization, a tandem structure was first developed by accommodating each dye to a separate subcell and then overlapping two or more subcells Received: January 23, 2011 Revised: March 7, 2011 Published: March 29, 2011 7747

dx.doi.org/10.1021/jp200700e | J. Phys. Chem. C 2011, 115, 7747–7754

The Journal of Physical Chemistry C

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Scheme 1. Molecular Structures of JK-142 and JK-62

together,8 but a very low percentage of solar light transmitted to the back subcells due to the light diffusion by the front Pt-loaded conducting glass counter-electrode, resulting in limited improvement in the photocurrent as well as the efficiency. To avoid the serious loss of incident light by the diffusion in tandem cells, a stepwise cosensitization methodology aiming at putting two kinds of dyes in a single TiO2 film was investigated. The process includes chemically depositing a layer of Al2O3 onto the first dye sensitized TiO2 film and then adsorbing the second dye on the surface of the Al2O3 overcoat.9 However, the stability between TiO2 films and Al2O3 layers need to be addressed in this case. In addition, the two dyes should be energetically aligned to achieve an effective interfacial charge-transfer cascade, which thus imposes an additional difficulty in molecular design. Therefore, other technologies which can incorporate several dyes onto different positions of a same TiO2 film not via the Al2O3 overcoat have been under active investigation. An impressive approach reported recently by Lee and Park et al.10 was to selectively position two or more dyes on a single TiO2 film by repeated adsorptiondesorption processes mimicking the concept of the stationary phase and the mobile phase in a column chromatograph. Nevertheless, the complexity of this method and the low efficiency (ca. 4.8%) obtained presently in those DSSCs pose a problem in practical applications. Apart from the tandem cell, the stepwise cosensitization via Al2O3, and the selectively positioning methodology, an approach called sequential adsorption might be a much simpler way to fulfill the cosensitization of multiple dyes on a single TiO2 film.11 The related electrodes were obtained by dipping TiO2 film into a first dye solution within a short time (ca. 3 h), followed by immersing in another dye solution and keeping for a longer time for full adsorption. Previous investigations have mainly focused on the selection of sensitizer combinations aiming at maximizing spectral compensation, for which metal-free organic dyes such as JK-2/TT111a or JK-2/SQ111b were employed in those reports. The reason for employing organic dyes in such cosensitized electrodes is probably the high molecular extinction coefficient (ε) of these dyes,3 which allows a sufficient light absorption even with a lower adsorption amount of each dye than that in the single dye-sensitized TiO2 electrode. In spite of the high ε of organic dyes, they generally have a narrow absorption band, thereby corresponding to lower efficiencies of the DSSCs compared to the ruthenium complex. In this study, a ruthenium complex (JK-142) (Scheme 1) with an ancillary ligand modified by a carbazole-thiophenyl group was synthesized for expanding the light absorption band. The unexpected low coverage of the new sensitizer on the TiO2 surface

and inaccessibility to the far side of the TiO2 film motivated us to use it in the cosensitized electrodes together with a relatively small-sized organic dye (JK-62) (Scheme 1) through the sequential adsorption process. The distribution of JK-142 on the TiO2 film and the additional adsorption amount of JK-62 on the electrode were explored and then correlated with the photovoltaic performance of the cosensitized solar cell. An efficiency of up to 10.2% has been demonstrated by the combination of cosensitizers, JK-142 and JK-62, which is much higher than that (ca. 8.68%) of a single dye (N719) sensitized solar cell with an identical device configuration. This study is expected to open up a new way to position different dyes onto a single TiO2 film by a simple sequential adsorption process through controlling the molecule size of the dyes.

2. EXPERIMENTAL SECTION 2.1. TiO2 Film. Fluorine-doped tin oxide (FTO) glass plates (Pilkington TEC Glass-TEC 8; solar 2.3 mm thickness) were cleaned in a detergent solution using an ultrasonic bath for 30 min and then rinsed with water (H2O) and ethanol (EtOH). Next, the plates were immersed in 40 mM TiCl4 (aq) at 70 °C for 30 min and washed with H2O and EtOH. A transparent nanocrystalline layer was prepared on the FTO glass plates by using a doctor blade printing TiO2 paste (Solaronix, Ti-Nanoxide T/SP), which was then dried for 2 h at 25 °C. The TiO2 electrodes were gradually heated under an air flow at 325 °C for 5 min, at 375 °C for 5 min, at 450 °C for 15 min, and at 500 °C for 15 min. The thickness of the transparent layer was measured using an Alpha-step 250 surface profilometer (Tencor Instruments, San Jose, CA). A paste containing 400-nm-sized anatase particles (CCIC PST-400C) was deposited by means of doctor blade printing to obtain the scattering layer and then dried for 2 h at 25 °C. The TiO2 electrodes were gradually heated under an air flow at 500 °C for 30 min. The resulting film was composed of an 8-μm-thick transparent layer and a 4-μm-thick scattering layer. The TiO2 electrodes were treated again with TiCl4 at 70 °C for 30 min and sintered at 500 °C for 30 min. The TiO2 film comprising a 14-μm-thick transparent layer and a 4-μm-thick scattering layer was fabricated by controlling the doctor blade printing process. In the following description, the TiO2 film thickness was referred to as the thickness of the transparent layer, namely, 8 and 14 μm, respectively. 2.2. Dye-Sensitized TiO2 Electrodes. The single dye sensitized TiO2 electrode was fabricated by immersing a TiO2 film in a corresponding JK-142 (0.3 mM in methanol), JK-62 (0.3 mM in EtOH), or N719 (0.3 mM in EtOH) solution and kept at room 7748

dx.doi.org/10.1021/jp200700e |J. Phys. Chem. C 2011, 115, 7747–7754


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Scheme 2. Schematic Diagram of the Synthesis of Ruthenium Sensitizer JK-142

temperature for 237 h. To prepare the cosensitized electrode, a TiO2 film was dipped in JK-142 solution for 24 h, rinsed with ethanol, and dipped in JK-62 solution for another 216 h. 2.3. DSSC Assembly. FTO plates for the counter electrodes were cleaned in an ultrasonic bath in H2O, acetone, and 0.1 M aqueous HCl, subsequently. The counter electrodes were prepared by placing a drop of an H2PtCl6 solution (2 mg of Pt in 1 mL of EtOH) on a FTO plate and heating at 400 °C for 15 min. The dye-adsorbed TiO2 electrodes and the platinum counter electrodes were assembled into a sealed sandwich-type cell by heating at 80 °C using a hot-melt ionomer film (Surlyn) as a spacer between the electrodes. A drop of the electrolyte solution3a was placed in the drilled hole of the counter electrode and was driven into the cell via vacuum backfilling. Finally, the hole was sealed using additional Surlyn and a cover glass (0.1 mm thickness). 2.4. Characterization. Photoelectrochemical data were measured using a 1000 W xenon light source (Oriel 91193) that was focused to give 1000 W m2, the equivalent of one sun at air mass (AM) 1.5, at the surface of the test cell. The light intensity was adjusted with a silicon solar cell that was double-checked with an NREL calibrated silicon solar cell (PV Measurement Inc.). The applied potential and measured cell current were recorded using a Keithley model 2400 digital source meter. The currentvoltage (J-V) characteristics of the cell under these conditions were determined by biasing the cell externally and measuring the generated photocurrent. This process was fully automated using Wavemetrics software. The electrochemical impedance spectra (EIS) measurements were carried out with an impedance analyzer (Parstat 2273, Princeton) in darkness at a bias potential of 0.5 to 0.7 V and 10 mV of amplitude over the frequency range of 0.1 Hz100 kHz. The incident photo to current conversion efficiency (IPCE) was measured using a system designed by PV measurements. A 75 W xenon lamp was applied as the light source for the monochromatic beam. For its calibration, a silicon photodiode (NIST-calibrated photodiode G425) was used. The adsorbed dye amounts were quantitatively

determined by dissolving the adsorbed dye with a 0.1 M tertbutylammonium hydroxide ethanol solution using a Cary 5 spectrophotometer. For analyzing the distribution of dye molecules on the TiO2 film, electron probe microanalysis (EPMA) was performed using JEOL JXA-8200 to test the relative content of elemental ruthenium.

3. RESULTS AND DISCUSSION The ruthenium sensitizer JK-142 has been synthesized by the stepwise synthetic protocol illustrated in Scheme 2. The key starting compound 5 was prepared in five steps starting from carbazole (see Supporting Information). The Suzuki coupling reaction12 of 4,40 -dibromobipyridine with 6 gave 7. JK-142 was synthesized in a one-pot reaction from the sequential reaction of [Ru(p-cymene)Cl2]2 with 7, followed by the reaction of the resulting complex with 4,40 -bicarboxylic acid-2,20 -bipyridine. The dichlororuthenium complex reacted with an excess of ammonium thiocyanate to afford the ruthenium sensitizer JK142. The metal-free organic dye JK-62 was synthesized in six steps as illustrated in Scheme 3. Carbaldehyde 8 was prepared according to the reported procedures13 followed by a reaction of protecting the aldehyde group with neophetylglycol to afford 9. The coupling reaction of 9 and a previously synthesized compound 1014 under HornerEmmonsWittig conditions15 led to an intermediate 11. The vinyl-benzene derivative 11 was converted into its corresponding benzyl aldehyde 12 by dedioxanylation with trifluoroacetic acid (TFA). An acetonitrile solution of 12 and cyanoacetic acid were refluxed in the presence of piperidine for 6 h. Solvent removal followed by purification using chromatography yielded JK-62. The UVvis and emission spectra of JK-142 and JK-62 in EtOH are shown in Figure 1. In order to get a clear comparison with N719, the optical parameters derived from Figure 1 are summarized in Table 1. The strong absorption band of JK-142 around 420 nm is due to an intraligand ππ* transition of the 7749



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Scheme 3. Schematic Diagram of the Synthesis of Organic Sensitizer JK-62

Figure 1. Absorption and emission spectra of (a) JK-142 and (b) JK-62 in solutions and on TiO2 films.

Table 1. Optical and Oxidation Parameters of Dyes dye

λabsa (nm) (ε (M1 cm1)) Eoxb (V) E00c (V) ELUMOd (V)

JK-142

421(29424), 540(13892)

0.92

1.84

0.92

JK-62

374(26413), 421(24522)

1.07

2.48

1.31

N719

380(14682), 520(14400)

a Absorption spectra were measured in ethanol solution. b Oxidation potentials of dyes on TiO2 were measured in CH3CN with 0.1 M (n-C4H9)4NPF6 with a scan rate of 50 mV s1. c E00 was determined from the intersection of absorption and emission spectra in ethanol. d ELUMO was calculated by Eox E00.

ancillary carbazole-substituted bipyridine, and the absorption spectra of above 500 nm are dominated by MLCT transitions. The low-energy MLCT absorption band of JK-142 at 540 nm is about 20 nm red-shift compared to that of N719. The extended

light harvesting of JK-142 is attributable to an electron donating ability in the ancillary ligand. The measured ε at 540 nm for JK142 is 13 892 M1 cm1, which is comparable to the value of N719 (14 400 M1 cm1). As for the organic dye JK62, it displays a high ε of 24522 M1 cm1 at 421 nm. However, JK-62 corresponds to a blue-shifted absorption band with a threshold wavelength only at 530 nm in comparison with that (ca. 710 nm) of JK-142. The absorption spectra of JK-142 and JK-62 on TiO2 films are red-shifted and broadened because of the J aggregation and interaction of the anchoring group with the surface titanium ion, ensuring a good light-harvesting efficiency. Excitation at the first absorption peak of JK-142 (540 nm) and JK-62 (421 nm) resulted in a strong emission centered at 778 and 610 nm, respectively. To evaluate the feasibility of electron transfer from the excited state of the sensitizers to the conduction band of the TiO2 7750



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Figure 2. Uptake of JK-142 and JK-62 on 14 μm-thick TiO2 films with different dipping times.

electrode, we carried out cyclic voltammetry experiments for JK142 and JK-62 electrodes using TBAPF6 as the supporting electrolyte and CH3CN as solvent according to a procedure depicted elsewhere.3a The oxidation potential (Eox) of JK-142 occurs at þ0.92 V versus NHE (Table 1), which is assigned to the RuII/III redox couple. This value is þ1.12 V versus NHE for N3. The 0.20 V cathodic shift of the JK-142 oxidation potential compared to that of N3 is attributable to the influence of the electron-rich carbazole-thiophene donor rings. As for the JK-62, Eox is ca. þ1.07 V versus NHE. The oxidation potential of both sensitizers is energetically favorable for iodide oxidation. The excited state reduction potentials of JK-142 and JK-62 calculated from Eox and E0016 are listed in Table 1. The ELUMO of the sensitizers (JK-142, 0.92 V vs NHE; JK-62, 1.31 V vs NHE) are much more negative than the conduction band level of TiO2 at approximately 0.5 V versus NHE, ensuring the thermodynamic driving force for charge injection.17 The improved absorption and the favorable electronic energy level structure of JK-142 dye make it very promising for use in DSSC. However, in order to achieve a high efficiency of the DSSC, sensitizer molecules are required to form an ordered and fully covered monolayer on the TiO2 surface in order to maximize the light harvesting and hinder the electron loss from the TiO2 surface voids. The adsorption process of JK-142 on 14 μm-thick TiO2 films was thus examined through dissolving the adsorbed dyes into an ethanol solution containing 0.1 M tertbutylammonium hydroxide followed by the measurement of the UVvis spectra, which are shown in Figure S2, Supporting Information. The adsorption amounts of JK-142 on the TiO2 films were calculated by fitting the absorbance to the LambertBeer law, and the results are listed in Figure 2. It demonstrates that a saturated adsorption amount of ca. 6.8 108 mol cm2 was attained for JK-142 after 24 h of dipping in the dye solution. This value was significantly lower than that of N719 (ca. 9.8 108 mol cm2), mainly due to the larger ancillary ligand of JK-142. Interestingly, the insufficient upload of JK-142 on TiO2 film was even visualized by naked eyes, that is, the back side of the TiO2 electrode (from FTO glass) remained nonstained even after 37 h of the dipping process. This observation suggests that the penetration of JK-142 into TiO2 film was highly restrained by its extended molecular size. In order to explore the penetration depth of JK-142 in the TiO2 films, EPMA was performed to investigate the distribution of the Ru element along the TiO2 thickness (Figure 3a). It is found that the relative Ru content decreased gradually to zero at ca. 8 μm, suggesting that JK-142 molecules were only adsorbed on the front half of the TiO2 film, while leaving the back half uncovered. Such a deficient

Figure 3. (a) EPMA analysis result showing the inhomogeneous ruthenium distribution along the TiO2 film and (b) a schematic diagram representing the distribution of JK-142 and JK-62 on the cosensitized electrode.

electrode is definitely not suitable to be employed in DSSC because most of the uncovered TiO2 surface functions as an electron recombination center.18 In spite of this, the result of JK-142 adsorbing on the 14 μmthick TiO2 film is very interesting because it may provide us a new way to selectively position dye molecules on the TiO2 film through controlling the molecular size and allow us to use a simple sequential adsorption process to produce a cosensitized electrode. To examine the possibility of making a cosensitized electrode, the small-sized JK-62 dye was employed for an improved adsorption inside the TiO2 film. The UVvis spectra of the desorbed dye in ethanol solutions (Figure S2 in the Supporting Information) and on the TiO2 electrodes (Figure S3, Supporting Information) clearly demonstrated the successful upload of JK-62. The adsorption amount of JK-62 was obtained by the difference of the UVvis spectra between the desorbed solutions of the JK-142 electrode and that of the cosensitized one, as shown in Figure 2. A saturated adsorption amount of JK62 on JK-142-sensitized TiO2 electrode was ca. 5.8 108 mol cm2, which is around 40% of the upload (ca. 14.7 108 mol cm2) of JK-62 on a blank TiO2 electrode. The additionally adsorbed JK62 molecules should be mainly located on the back part of the TiO2 electrode. Thus, the formed cosensitized electrode was used to propose a structure (Figure 3b) with the two dyes, JK-142 and JK-62, mainly positioning on each side of a single TiO2 film. The structure developed in this study is very similar to that established by Park et al.10 using selectively positioning methodology except that in the former it is difficult to control the clear distribution of each dye compared to that in the latter. However, it is worthy to note that the newly developed cosensitized electrode is only prepared by the simple sequential adsorption process, which is inexpensive and easily handled, and thereby deserves to be explored for the application in DSSC. Because of the great importance of the electron transport properties on the performance of the DSSC, they were investigated for the cosensitized electrodes using EIS. Figure 4a shows the typical Nyquist plots of both electrodes at an applied bias of 0.7 V. Three semicircles from left to right in the Nyquist plot represent the impedances of the charge transfer (Rct) on the Pt counterelectrode, the charge recombination (Rr) on the interface of the TiO2/dye/ electrolyte, and the electrolyte diffusion, respectively. The Rr of the cosensitized JK-142/JK-62 electrode (ca. 45 Ω cm2) was nearly double that of the JK-142 electrode (ca. 20 Ω cm2) and that of the 7751



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Figure 4. (a) Nyquist plots measured at 0.7 V in darkness and (b) electron lifetimes obtained from Nyquist plots measured at 0.5 V to 0.7 V in darkness for the cosensitized and single dye-sensitized electrodes based on 14 μm-thick TiO2 films. The cosensitized JK-142/JK-62 electrode was prepared by dipping in JK-142 solution for 24 h and then in JK-62 solution for 16 h.

Figure 5. IPCE (a) and J-V curves (b) of the cosensitized and singly dye-sensitized electrodes. The cosensitized JK-142/JK-62 electrode was prepared by dipping in JK-142 solution for 24 h and then in JK-62 solution for 16 h.

JK-62 electrode (ca. 23 Ω cm2), revealing a significant improvement on the retardation of charge recombination. The serious charge recombination of the JK-142 electrode should be attributed to the uncovered TiO2 surface area and that of the JK-62 electrode is probably related to the nature of the organic molecules whose HOMO-located moiety is closer to the TiO2 spatially than that of the ruthenium complex such as N719 or JK-142. Furthermore, the improved interface of the cosensitized JK-142/JK-62 electrode is also reflected from the increased electron lifetime, which was estimated from the characteristic frequency of the middle semicircle according to the procedures depicted by Adachi et al.19 As shown in Figure 4b, a much higher electron lifetime of the cosensitized JK142/JK-62 electrode was obtained compared with those of the singly dye-sensitized electrodes. Evidently, a more compact and ordered monolayer of the two sensitizers has been established on the TiO2 surface of the cosensitized electrode. The elevated electron lifetime is expected to increase the photovoltaic performance of the solar cells. Figure 5 shows typical IPCE (a) and J-V curves (b) of the cosensitized and single dye-sensitized electrodes. As shown in Figure 5a, the JK-62 DSSC reveals a relatively narrow IPCE response at 300650 nm and a high IPCE peak value of ca. 90% at 460 nm. As for the JK-142 DSSC, the IPCE red-shifts to 820 nm, while the peak value drops to ca. 70%. The lower IPCE of the JK-142 solar cell is mainly attributable to the deficient

Table 2. Photovoltaic Performance of the DSSCs on the Basis of Various Dyes and Their Combinations dye (TiO2 film thickness)

Jsca (mA 3 cm2)

Vocb (V)

FFc

ηd /%

JK-142 (14 μm)

15.44

0.64

0.70

7.28

JK-62 (14 μm)

12.17

0.64

0.69

5.36

JK-142/JK-62 (14 μm)e N719 (14 μm)

18.61 16.19

0.74 0.74

0.74 0.73

10.20 8.82

JK-142 (8 μm)

16.21

0.71

0.70

8.01

JK-142/JK-62 (8 μm)e

16.95

0.76

0.70

9.02

N719 (8 μm)

15.87

0.76

0.72

8.68

Jsc: short-circuit current density. b Voc: open-circuit voltage. c FF: fill factor. d η: power conversion efficiency. e The cosensitized JK-142/JK62 electrode was prepared by successively dipping in JK-142 solution for 24 h and then in JK-62 solution for another 16 h. a

covering of the dye molecules on the TiO2 surface. In contrast, the cosensitized JK-142/JK-62 DSSC not only demonstrates a broad IPCE response wavelength region of up to 820 nm because of the JK-142 absorption but also yields an improved IPCE peak value of ca. 85% at 510 nm owing to the full coverage of the TiO2 surface by additionally adsorbed JK-62 molecules. That is to say, the cosensitization of JK-142 and JK-62 by heterogeneously positioning on one TiO2 film has a significant synergistic effect on light harvesting and electron collection on TiO2. 7752


The Journal of Physical Chemistry C The photovoltaic parameters were also obtained through measuring J-V curves (as shown in Figure 5b) and are listed in Table 2. The JK-142 and JK-62 single dye sensitized solar cells present an energy conversion efficiency of 7.28% and 5.36%, respectively. These values are much lower than that (ca. 8.82%) of the N719 DSSC. As for the JK-142/JK-62 cosensitized electrode, the solar cell yields a short-circuit current density (Jsc) of 18.61 mA cm2, a photovoltage (Voc) of 0.737 V, a fill factor of 0.74, corresponding to a remarkably high efficiency (η) of 10.20%. Compared with the JK-142 and JK-62 single dye sensitized solar cells, the dramatic increase in the Jsc for the cosensitized DSSC is ascribed to the improved IPCE response, and the significant enhancement in the Voc is due to the increase of the electron lifetime on the TiO2 interface. Most importantly, the efficiency of the cosensitized DSSC is obviously higher than that of the N719 solar cells, which represents an optimized one in our laboratory conditions. To further understand the effect of the cosensitized electrode structure on the photovoltaic performance of the DSSC, thinner (8 μm) TiO2 films were also employed in the electrodes and DSSCs because JK-142 molecules can penetrate the entire TiO2 films of such thickness. With only JK-142 sensitizing on the thinner TiO2 film, the DSSC exhibits both higher Jsc and Voc (as shown in Table 2), and accordingly corresponded to a higher η (ca. 8.01%) than that (ca. 7.28%) with the thicker (14 μm) TiO2 film. It can be explained by the increased coverage ratio of sensitizers on the thinner TiO2 film because the dye molecules can penetrate the deepest region of such a film, as shown in Figure 3. However, this optimized efficiency of the DSSC with JK-142 on the thin TiO2 film is still lower than that of the N719 even though there is a broader absorption band for the former dye. Although an efficiency (6.50%) of the JK-142 DSSC comparable to that of N719 (6.27%) can be obtained by using more porous TiO2 films fabricated from commercially available P25 (Degussa) nanocrystalline powder (Figure S7, Supporting Information) for better penetration of JK-142 molecules into the TiO2 film, the result does not seem to make sense due to the much lower efficiency. More research work is planned in the future to pursue more proper TiO2 films for optimal sensitization by pure JK-142 molecules, as in the case of ruthenium complex sensitizers CYC-B6S/CYC-B6L with similar molecular strutures.4h The photovoltaic performance demonstrated by JK-142 and N719 in this study is consistent with that reported by An et al. for the sensitizers with similar extended structures.20b The disordered and noncompact monolayer of the JK-142 molecules on the TiO2 surface due to their large molecule diameter could be the main factors for its low efficiency. However, the deficient JK-142 dye monolayer has been evidently improved by inserting small-sized JK-62 molecules into the spaces of the JK-142 layer. As shown in Table 2, when JK-62 was additionally adsorbed on a 8-μm-thick JK-142 sensitized electrode, the DSSC presents a higher Jsc (from 16.21 mA cm2 to 16.95 mA cm2), an increased Voc (from 0.71 to 0.76 V), and a dramatically improved η (from 8.01% to 9.02%). Furthermore, a highest efficiency of up to 10.2% has been demonstrated upon using a thicker (14 μm) TiO2 film for the JK-142/JK-62 cosensitized DSSC, which is mainly owing to the increased JK-62 adsorption amount. Therefore, although large-sized dye molecules only position on the front part of the thick TiO2 film, the cosensitized electrodes made by additionally adsorbing another small sized dye which can penetrate into the inside of TiO2 can demonstrate complementary and synergistic

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photovoltaic performance. Evidently, in the cosensitized electrodes, the extended structured sensitizer presents broad lightharvesting for the DSSC, while the small-sized dye is responsible for filling the voids as well as covering on the far side of the TiO2 film that the large-sized dye molecules are unable to access. Furthermore, a relatively thicker TiO2 film is required for this cosensitized electrode because each dye is expected to have enough of a loading amount to play its maximal role in light harvesting. To inspect the wide applicability of this concept, we changed JK-62 with another small molecule sized organic dye (JK-2) which was synthesized according to a procedure reported before.3a As shown in Figure S6 in the Supporting Information, the JK-142/JK-2 cosensitized DSSCs using 14 μm-thick TiO2 films demonstrate an increased η of 9.44% compared with that of the singly JK-142 (η = 7.28%) or JK-2 (η = 8.01%) sensitized solar cell.3a To sum up, here we successfully display a high performance DSSC with cosensitizers unevenly distributing on the TiO2 electrode. Interestingly, the fabrication of this kind of cosensitized DSSC is simple and easily handled, which only requires one more dye dipping process. Compared with the tandem DSSCs, the one developed in this study is markedly cheaper because it only employs one single TiO2 film and one Pt counter-electrode. In fact, over 10% of the efficiency represents one of the best results for the cosensitized DSSCs. The high efficiency and simple production suggests very bright application prospects of this method in large-scale productions of DSSCs.

4. CONCLUSIONS We have developed a ruthenium complex featuring a red-shift light absorption band owing to an extended π-conjugation by a carbazole-thiophenyl moiety in an ancillary ligand. Meanwhile, the enlarged structure of the ruthenium complex molecules allows one to position them on the upper region of a nanoporous TiO2 film because of a permeation restriction. Then a cosensitized electrode was assembled by additionally adsorbing a smallsized metal-free organic dye which can penetrate into the inner side of the TiO2 film. The effectiveness of the DSSC on the basis of the heterogeneous electrode has been successfully demonstrated in the improved IPCE response and enhanced energy conversion efficiency. The advantages over the single dye sensitized solar cells have presented us valuable implications of a good way to coload multiple dyes on a single TiO2 electrode and a correct usage of the dyes with extended molecular structure. ’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed description for the synthesis of the compounds, as well as additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: þ82 41 8601337. Fax: þ82 41 8675396. E-mail: jko@ korea.ac.kr.

’ ACKNOWLEDGMENT This work was supported by the WCU (the Ministry of Education and Science) program (Grant R31-2008-000110035-0) , the New & Renewable Energy of the Korea Institute 7753


The Journal of Physical Chemistry C of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 2010T100100674), and Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (MEST) of Korea for the Center for Next Generation Dye-sensitized Solar Cells (No. 2010-0001842).

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Improved Efficiency of over 10% in Dye-Sensitized Solar Cells with a

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