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A New Ruthenium Sensitizer Containing Benzo[1,9]quinolizino(acridin-2-yl)vinyl-2,2′-bipyridine Ligand for Effective Nanocrystalline Dye-Sensitized Solar Cells
Novel ruthenium bipyridyl sensitizer incorporating conjugated benzo[1,9]quinolizino-(acridin-2-yl)vinyl-2,2′-bipyridine ligand [JJ-12] has been synthesized and demonstrated as efficient sensitizer in dye-sensitized solar cells. A mesoporous titania film stained with JJ-12 exhibits a remarkable incident monochromatic photon-to-current conversion efficiency of 82%. Under standard AM 1.5 sunlight, the solar cell using a liquid-based electrolyte consisting of 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.05 M I2, 0.1 M LiI, and 0.5 M tert-butylpyridine in acetonitrile exhibits a short-circuit photocurrent density of 16.47 mA/cm2, an open-circuit voltage of 0.71 V, and a fill factor of 0.71, corresponding to an overall conversion efficiency of 8.34 %.
Dye-sensitized solar cells (DSSCs) have been extensively studied as a novel sunlight-to-electricity conversion system [1–5]. The working principle of the dye-sensitized solar cell is the following. Upon photoexcitation, the dye molecules inject an electron into the conduction band of TiO2, leaving the dye in its oxidized state (D+, also referred to as dye cation). The dye is restored to its ground state by electron transfer from the redox pair. The regeneration of the sensitizer by iodide intercepts the recombination of the conduction band electron with the oxidized dye. The ions formed by oxidation of I- diffuse to the cathode where the regenerative cycle is completed by electron transfer to reduce to I-. DSSCs consist of a dye-sensitized TiO2 electrode, a Pt-sputtered counter electrode, and a redox couple. In these cells, the sensitizer is one of the key components for high-power conversion efficiencies. Polypyridyl ruthenium sensitizers such as cis-dithiocyanato-bis(4,4′-dicarboxy-2,2′-bipyridine)ruthenium (II) (N719) have shown to be excellent dyes in DSSCs, and their photoconversion efficiency is more than 10% under air mass 1.5 sunlight . The main drawback of this sensitizer is the lack of absorption in the red region of the visible spectrum. An important goal on ruthenium dyes has been the development of new ruthenium dyes having a wide and red-shifted MLCT band [7–9]. Numerous attempts have been made to molecularly engineer ruthenium sensitizers to broaden the absorption band and increase the molar absorption coefficient. One of the successful approaches is to replace one of the 4,4′-dicarboxylic acid-2,2′-bipyridine (dcbpy) anchoring ligands in N3 with a highly conjugated ancillary ligand. Several groups have successfully developed efficient ruthenium sensitizers by introducing extended conjugation units on the bipyridyl ligand such as thiophene [10–14] and alkoxybenzene [15–17] derivatives. Another approach is to synthesize efficient ruthenium sensitizers through a systematic tuning of the LUMO and HOMO energy levels of the ruthenium polypyridyl complexes by introducing a ligand with a low-lying π* molecular orbital or by destabilizing the metal orbital with a strong donor ligand . Thus, the systematic designs of efficient ruthenium sensitizers to enhance the absorption coefficient and red shift the MLCT band by increasing the conjugation length of the ancillary ligands such as a styryl-substituted bipyridine and using strong σ-donating power of the amine are well documented strategies. As even small structural modifications of sensitizers result in significant changes in redox energies and the threshold wavelength, the ruthenium sensitizer, JJ-12, is molecularly engineered in a way to have a red-shift and high absorption coefficient of MLCT band. Here, we report the synthesis of a novel ruthenium sensitizer containing benzo[1,9]-quinolizino(acridin-2-yl)vinyl-2,2′-bipyridine and its excellent photovoltaic performances in DSSCs (Figure 1).
2. Experimental Section
2.1. NMR Spectra, UV-Vis Spectra, and Emission Spectra Measurement
All of the reactions were carried out under an argon atmosphere. 1H and 13C NMR spectra were recorded on a Varian Mercury 300 spectrometer. UV-vis spectra were recorded in a 1 cm path length quartz cell on a Cary 5 spectrophotometer. Emission spectra were recorded on a Spex Fluorolog 112 using a 90° optical geometry. The emitted light was detected with a Hamamatsu R928 photomultiplier operated in single-photon counting mode.
2.2. Electrochemical Impedance Spectra (EIS) and I-V Curve Spectra Measurement
Electrochemical impedance spectra (EIS) of DSSCs were measured with an impedance analyzer (VersaSTAT 3, AMETEK) connected to a potentiostat under illumination conditions at room temperature. The spectra were scanned in a frequency range from 10-1 Hz to 105 Hz and AC amplitude 10 mV at room temperature.
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 an Si solar cell that was doubled-checked with an NREL-calibrated Si solar cell (PV Measurement Inc.). The applied potential and measured cell current were measured using a Keithley model 2400 digital source meter. The current-voltage 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.
2.3. Dye-Sensitized Solar Cell Fabrication
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 and ethanol. Then, the plates were immersed in 40 mM TiCl4 (aqueous) at 70°C for 30 min and washed with water and ethanol. 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 by using an Alpha-Step 250 surface profilometer (Tencor Instruments, San Jose, CA, USA). 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 and heated under same condition which was applied for TiO2 (Solaronix, Ti-Nanoxide T/SP) paste described above. The resulting film was composed of a 10 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. Then, they were immersed in JJ-12 (0.3 mM in ethanol) solutions and kept at room temperature for 24 h. 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 Pt in 1 mL ethanol) on an FTO plate and heating it (at 400°C) for 15 min. The dye adsorbed TiO2 electrodes, and the Pt 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 consisting of 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.05 M I2, 0.1 M LiI, and 0.5 M tert-butylpyridine in acetonitrile 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. Typical Synthesis Procedures and Analytical Data
4,4′-bis((E)-2-(4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-benzo[1,9]quinolizino[3,4,5,6,7-defg]acridin-2-yl)vinyl)-2,2′-bipyridine (7): 4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-benzo[1,9]quinolizino[3,4,5,6,7-defg]acridine-2-carbaldehyde (130 mg, 0.33 mmol), tetraethyl[2,2′-bipyridine]-4,4′-diylbis(methylene) bis(phosphonate) (80 mg, 0.17 mmol) and potassium tert-butoxide (55 mg, 0.50 mmol) were dissolved in tetrahydrofuran (50 mL), and the mixture was stirred for 0.5 h. After the solvent was removed under reduced pressure, H2O (50 mL) and methylene chloride (50 mL) were added. The organic layer was separated and dried in MgSO4. The solvent was removed under reduced pressure. The pure product 7 was obtained by column chromatography on silica gel (methylene chloride:hexane = 1 : 1, = 0.3). Yield: 90%. 1H NMR (CDCl3): 8.73 (d, 2H, Hz), 8.65 (s, 2H), 7.63 (s, 4H), 7.57 (d, 2H, Hz), 7.44 (m, 12H), 7.18 (m, 4H), 1.71 (s, 24H), 1.67 (s, 12H). 13C NMR (CDCl3): 156.5, 149.5, 146.3, 133.4, 132.8, 131.5, 130.6, 130.2, 130.1, 129.9, 123.9, 123.5, 123.3, 122.4, 120.9, 118.1, 35.6, 33.0. Anal. calcd. for C68H62N4: C, 87.33; H, 6.68. Found: C, 87.30; H, 6.71.
A mixture 4,4′-bis((E)-2-(4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-benzo[1,9]quinolizino[3,4,5,-6,7-defg]acridin-2-yl)vinyl)-2,2′-bipyridine (130 mg, 0.14 mmol) and a dichloro(p-cymene)ruthenium (II) dimer (43 mg, 0.07 mmol) in argon-degassed DMF (15 mL) was stirred at 70°C for 4 h under reduced light. Subsequently, 4,4′-dicarboxylic-2,2′-bipyridine (34 mg, 0.14 mmol) was added into the flask, and the reaction mixture was stirred at 160°C for 4 h. At last, an excess of NH4NCS (105 mg, 1.40 mmol) was added to the resulting dark solution, and the reaction continued for another 4 h at 140°C. Then the reaction mixture was cooled down to room temperature, and the solvent was removed under vacuum. Water was added to get the precipitate. The precipitate was filtered out, washed with water, and dried under vacuum. The resulting solid was dissolved in methanol containing 2,2 equivalent of tetrabutylammonium hydroxide to confer solubility by deprotonating the carboxylic group and purified on a Sephadex LH-20 column with methanol as eluent. The collected main band was concentrated, and the solution pH was lowered to 5.1 using 0.02 M nitric acid. The precipitate was collected on a sintered glass crucible by suction filtration and dried in air. Yield: 60%. 1H NMR (CD3OD): 9.35–8.90 (m, 6H), 8.23 (s, 1H), 8.08–7.18 (m, 24H), 6.86 (s, 1H), 1.61 (m, 24H), 1.33 (m, 12H), 0.87 (m, 9H). Anal. calcd. for C82H70N8O4RuS2: C, 70.52; H, 5.05. Found: C, 70.63; H, 5.01
3. Results and Discussion
The synthetic route for the preparation of JJ-12 is depicted in Scheme 1. First, trimethyl 2,2′,2′′-nitrilotribenzoate 3 was prepared by the Ullmann coupling reaction  of methyl 2-aminobenzoate 1 and methyl 2-iodobenzoate 2 and compound 6 was synthesized through methylation, cyclization and formylation from compound 3. 2-Bromo-4,4′-bis((E)-2-(4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-benzo[1,9]quinolizino[3,4,5,6,7-defg]acridin-2-yl)vinyl)-2,2′-bipyridine 7 was prepared from the reaction of 4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-benzo[1,9]quinolizino[3,4,5,6,7-defg]acridine-2-carbaldehyde 6 and tetraethyl[2,2′-bipyridine]-4,4′-diylbis(methylene))bis(phosphonate) using the Horner-Emmons-Wadsworth reaction . The one-pot synthetic procedure developed for heteroleptic polypyridyl ruthenium sensitizers was adapted for the preparation of new JJ-12 [21, 22]. The analytical and spectroscopic data of novel sensitizer are consistent with the formulated structure.
Figure 2 shows the ultraviolet-visible spectra of JJ-12, together with the N3 absorption spectrum as a reference. The polypyridyl complexes of JJ-12 show very broad and intense absorption peaks throughout the whole absorption region. The UV-vis spectrum of JJ-12 displays two absorption bands at 420 and 531 nm in the visible region, which are characteristic of the metal-to-ligand charge transfer (MLCT) bands [23, 24]. The low-energy MLCT band of JJ-12 at 531 nm is 11 nm red shifted relative to that of N3. The red shift of JJ-12 sensitizer is attributable to the increase of π-conjugation in an ancillary ligand and donor ability of benzo[1,9]quinolizinoacridine. The low-energy MLCT band at 531 nm of JJ-12 exhibits a molar extinction coefficient of 15.4 × 103 M-1 cm−1, which is higher than that of the N3 dye (14.4 × 103 M-1 cm−1). Adsorption of JJ-12 onto a TiO2 electrode was observed to broaden the absorption band and to red shift the absorption threshold up to 770 nm, ensuring a good light-harvesting efficiency (Figure 2(b)). Such broadening and red shift have been observed in many ruthenium sensitizers on TiO2 electrodes [25, 26].
We also observed that the JJ-12 exhibited strong luminescence maxima at 740–755 nm when they were excited with their MLCT absorption bands at 298 K in an air-equilibrated ethanol solution.
The electrochemical properties of the novel sensitizer JJ-12 were studied by cyclic voltammetry in CH3CN with 0.1 M tetrabutylammonium hexafluorophosphate using TiO2 film with adsorbed dyes as the working electrode. The oxidation potential of JJ-12 adsorbed on TiO2 film shows a quasi-reversible couple at 0.92 V versus NHE (Table 1). The value may be compared to 1.12 V versus NHE measured for N3. The 0.20 V cathodic shift of the JJ-12 oxidation potential compared to that of N3 is attributable to the influence of the electron-rich fused triphenylamine donor ring. The oxidation potential of JJ-12 sensitizer is energetically favorable for iodide oxidation . The reduction potentials of JJ-12 dye calculated from the oxidation potentials and the determined from the intersection of absorption and emission spectra are −0.96 V for JJ-12 versus NHE . The value is 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 [29, 30].
Figure 3 shows the incident monochromatic photon-to-current conversion efficiency (IPCE) using 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.05 M I2, 0.1 M LiI, and 0.5 M tert-butylpyridine in acetonitrile. The cell was fabricated using a double layer of 10 μm thick TiO2 (20 nm nanoparticles) and 4 μm thick scattering TiO2 (400 nm nanoparticles). The incident photon-to-current conversion efficiency (IPCE) of JJ-12 exceeds 70% in a broad spectral range from 440 to 630 nm, reaching a maximum 82% at 520 nm. The band tails off toward 800 nm, contributing to the broad spectral light harvesting that is characteristic of polypyridyl ruthenium dyes [31–33]. For reference, N3 dye affords a maximum IPCE of 75% at 520 nm under the same condition. The JJ-12 sensitizer’s IPCE spectrum is more enhanced in 420–740 nm range compared to that of N3 as a result of extended π-conjugation of fused triphenylamine ring, which is consistent with the absorption spectrum of JJ-12.
The - curve for the devices based on JJ-12 is shown and compared with that of N3 in Figure 4. Under standard global AM1.5 solar condition, the JJ-12 sensitized cell gave a short-circuit photocurrent density () of 16.47 mAcm−2, an open-circuit voltage () of 0.71 V, and a fill factor of 0.71, corresponding to an overall conversion efficiencies of 8.34%. The photovoltaic performance of the JJ-12 and N3 sensitized cells is presented in Table 1. Under the same condition, the N3 sensitized cell gave a of 14.52 mAcm−2, of 0.72 V, and a fill factor of 0.69, corresponding to of 7.23%.
The ac impedances of the cells were measured under the illumination conditions. Figure 5(a) shows the ac impedance spectra measured under open-circuit conditions and under illumination of 100 mWcm−2. The radius of the intermediate-frequency semicircle in the Nyquist plot decreased in the order of N3 (34.55 Ω) > JJ-12 (22.50 Ω), indicating the improved electron generation and transport. This result is in good agreement with that of short-circuit photocurrent trend shown in Table 1. Figure 5(b) shows the ac impedance spectrum measured under dark condition. In the dark under forward bias (−0.68 V), the semicircle in intermediate frequency regime demonstrates the dark reaction impedance caused by the electron transport from the conduction band of TiO2 to ions in electrolyte. The increased radius of the semicircle in intermediate frequency regime implies a reduced electron recombination rate at the dyed TiO2/electrolyte interface. In dark, the radius of the intermediate-frequency semicircle showed the increasing order of JJ-12 (214.70 Ω) < N3 (243.90 Ω), in accord with the trends of the value in Table 1.
The electron lifetime can be estimated from the following relationship. is the maximum frequency of the intermediate frequencies peak . The results are displayed in the form of a bode-phase plot, as shown in Figure 6. The values of JJ-12 and N3 are 22.63 Hz and 13.81 Hz, respectively. The values show a significant gap among the dyes, resulting in the increasing order of N3 > JJ-12. The different values might be caused by the different molecular structure of the dyes. The low value of in JJ-12 compared to those of N3 may be due to the defects of the JJ-12 monolayer on the TiO2 electrodes. The results of the electron lifetime are well consistent with those of the in Table 1.
In conclusion, a novel ruthenium bipyridyl sensitizer incorporating highly conjugated benzo[1,9]quinolizino(acridin-2-yl)vinyl units has been synthesized and characterized. A solar-cell device based on the sensitizer JJ-12 in conjunction with a volatile electrolyte yielded an overall conversion efficiency of 8.34%. The efficient performance of JJ-12 is attributed to its high absorption extinction coefficient of MLCT band and extended absorption in the visible region. We believe that the development of highly efficient ruthenium sensitizers is possible through meticulous molecular engineering.
This work was supported by the Electronics and Telecommunications Research Institute (ETRI).
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