Abstract
Ruthenium(II) complex containing cis-[Ru(H2dcbiq)(L)(NCS)2], where H2dcbiq = 4,4′-dicarboxy-2,2′-biquinoline and L = 4,4′-di-tert-butyl-2-2′-dipyridyl coded as SPS-02, was synthesized and fully characterized. This complex showed appreciably broad absorption range. The new complex was used as photosensitizer in nanocrystalline TiO2 dye-sensitized solar cell application. cis-[Ru(H2dcbiq)(L)(NCS)2] (SPS-02) achieved efficient sensitization of nanocrystalline TiO2 over the whole visible range, extending into the near-IR region (ca. 1000 nm) with superior short-circuit photocurrent density ( = 9.13 mA cm−2) and conversion efficiency (η = 2.09%) compared with complex cis-[Ru(H2dcbiq)2(NCS)2] (1) under an irradiation of full sunlight (100 mW cm−2).
1. Introduction
Dye-sensitized solar cells (DSSCs) have been developed as an efficient, low-cost alternatives to the silicon technology during the last years [1–6]. Based on dye-sensitized nanocrystalline titanium dioxide as photoelectrode, power conversion efficiencies of over 11% have been reached [7]. In order to improve the efficiency of DSSCs, the sensitizer should absorb photons in the near-IR region as well as over the entire visible region of the solar spectrum. Considerable efforts have been made on 2,2′-bipyridine- and 2,2′:6′,2′′-terpyridine-based ruthenium complexes to improve the light-harvesting properties through introduction of extended π-conjugated system [6]. 2,2′-biquinoline has low π-energy level than both 2,2′-bipyridine and 2,2′:6′,2′′-terpyridine and has potential to improve IR absorbance with its analogues Ru complexes. Arakawa and coworkers studied solar cells based on TiO2 sensitized with homoleptic ruthenium complexes containing 4,4′-dicarboxy-2,2′-biquinoline cis-[Ru(H2dcbiq)2(NCS)2] (1) and reported that these solar cells have an enhanced red response in the near-IR region, but the solar cell performance was decreased due to the poor electron injection efficiency on to TiO2 [8]. As a part of our research program in the development of new photosensitizers [9–14], we have strategically designed a heteroleptic dye SPS-02 (Figure 1) having both 4,4′-di-tert-butyl-2-2′-dipyridyl and 4,4′-dicarboxy-2,2′-biquinoline in the same molecule, which exhibits better performance in the IR region.
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2. Experimental
2.1. Synthesis of SPS-02
[RuCl2(p-cymene)]2] (0.228 g, 0.37 mmol) was dissolved in DMF (30 mL), and 4,4′-di-tert-butyl-2-2′-dipyridyl (0.20 g, 0.74 mmol) was added. The reaction mixture was heated at 80°C under nitrogen for 2 h, and then 4,4′-dicarboxy-2,2′-biquinoline (0.313 g, 0.74 mmol) was added. The reaction mixture was refluxed at 160°C for another 4 h under reduced-light conditions to avoid light-induced cis to trans isomerization. An excess of NH4NCS (1.78 g, 23.4 mmol) was then added to the reaction mixture, which was then heated at 130°C for a further 5 h. The solvent was removed with a rotary evaporator. Water was added to the resulting semisolid to remove excess NH4NCS. The water insoluble product was collected, washed first with distilled water and then with diethyl ether, and dried. The crude complex was dissolved in a solution of sodium hydroxide (0.4 g) in water (10 mL). The concentrated solution was charged onto a Sephadex LH-20 column and eluted with water. The main brown band was collected and concentrated to 3 mL. The required complex was isolated upon addition of a few drops of 0.01 M aqueous HNO3. Complex SPS-02 was obtained after membrane filtration as brownish solid 78 mg. Anal. calcd for C40H36N6O4RuS2: C, 57.89; H, 4.37; N, 10.13. Found: C, 57.43; H, 4.52; N, 9.93.
2.2. Preparation of TiO2 Electrode
Nanocrystalline TiO2 photoelectrodes of about 20 μm thickness (area: 0.25 cm2) were prepared using a variation of a method reported by Nazeeruddin et al. [15]. Fluorine-doped tin oxide-coated glass electrodes (Nippon Sheet Glass Co., Japan) with a sheet resistance of 8–10 ohm−2 and an optical transmission of >80% in the visible range were used. Anatase TiO2 colloids (particle size ~13 nm) were obtained from commercial sources (Ti-Nanoxide D/SP, Solaronix). The nanocrystalline TiO2 thin films of approximately 20 μm thickness were deposited onto the conducting glass by screen printing. The film was then sintered at 500°C for 1 h. The film thickness was measured with a Surfcom 1400A surface profiler (Tokyo Seimitsu Co. Ltd.). The electrodes were impregnated with a 50 mM titanium tetrachloride solution and sintered at 500°C. The dye solutions ( M) were prepared in 1 : 1 acetonitrile and tert-butyl alcohol solvents. Deoxycholic acid as a coadsorbent was added to the dye solution at a concentration of 20 mM. The electrodes were immersed in the dye solutions and then kept at 25°C for 20 h to adsorb the dye onto the TiO2 surface.
2.3. Fabrication of Dye-Sensitized Solar Cell
Photovoltaic measurements were performed in a two-electrode sandwich cell configuration. The dye-deposited TiO2 film was used as the working electrode and a platinum-coated conducting glass as the counter electrode. The two electrodes were separated by a surlyn spacer (40 μm thick) and sealed up by heating the polymer frame. The electrolyte was composed of 0.6 M dimethylpropyl-imidazolium iodide (DMPII), 0.05 M I2, and 0.1 M LiI in acetonitrile.
2.4. Photovoltaic Characterization
The working electrode was illuminated through a conducting glass. The current-voltage characteristics were measured using the previously reported method [16] with a solar simulator (AM-1.5, 100 mW/cm2, WXS-155S-10: Wacom Denso Co. Japan). Monochromatic incident photon-to-current conversion efficiency (IPCE) for the solar cell, plotted as a function of excitation wavelength, was recorded on a CEP-2000 system (Bunkoh-Keiki Co. Ltd.).
3. Results and Discussion
Heteroleptic rythenium(II) complex SPS-02 was prepared by direct treatment of [RuCl2(p-cymene)]2, 4,4′-Di-tert-butyl-2-2′-dipyridyl and 4,4′-dicarboxy-2,2′-biquinoline in refluxing DMF. Figure 2 shows the absorption spectrum of complex SPS-02 in ethanol.
The strong absorption band of the complex SPS-02 in the region between 250 and 350 nm are due to π-π* transitions. An intense and broad MLCT band at 619 nm with a shoulder at about 500 nm was observed for SPS-02. This enhanced red absorption of complex SPS-02 with MLCT band at 619 nm renders it attractive candidate as a panchromatic charge-transfer sensitizer for DSC application.
Figure 3 shows the photocurrent action spectra for complex SPS-02 where the incident photon-to-current conversion efficiency (IPCE) values are plotted as a function of wavelength. The photoresponse of thin films extends upto 1000 nm. We observed an IPCE of 45% in SPS-02 at about 610 nm, while in the case of complex 1, the IPCE was 20%. The DSC sensitized with complex SPS-02 exhibits superior light-harvesting properties in the near-IR region. The superior performance of SPS-02 was attributed to the superior light-harvesting efficiency of high- and low-energy photons.
Figure 4 shows a photocurrent density-voltage curve of a sealed solar cell based on complex SPS-02 and 1 under AM 1.5 G simulated solar light at a light intensity of 100 mW cm−2 with a metal mask of 0.25 cm2. The photovoltaic performance of complex SPS-02 and 1 on nanocrystalline TiO2 electrode was studied using an electrolyte with a composition of 0.6 M dimethylpropyl-imidazolium iodide (DMPII), 0.05 M I2, and 0.5 M LiI in acetonitrile. The short-circuit photocurrent density (), open-circuit voltage (), fill factor (FF), and overall cell efficiencies () for test cells constructed from each dye are summarized in Table 1. The test cell sensitized with complex SPS-02 showed a photocurrent density of 9.13 mA cm−2, an open circuit potential of 0.35 V, and a fill factor of 0.65, corresponding to an overall conversion efficiency (η) of 2.09%. Under similar fabrication and evaluation conditions, complex 1 gives mA cm−2, V, and FF = 0.50, corresponding to an overall conversion efficiency (η) 1.04%. Though the new Ru complex has spectral response in a wide energy range (400–1000) the low value of short-circuit photocurrent () and cell efficiency (η) are attributed to its low LUMO level which inhibits efficient electron injection on to the conduction band of TiO2.
4. Conclusions
A new ruthenium(II) complex containing cis-[Ru(H2dcbiq)(L)(NCS)2], where H2dcbiq = 4,4′-dicarboxy-2,2′-biquinoline and L = 4,4′-Di-tert-butyl-2-2′-dipyridyl coded as SPS-02, was synthesized and characterized. This new dye exhibited efficient light-harvesting efficiency in a wide energy range (400–1000) which makes it as a potential photosensitizer for nanocrystalline TiO2 dye-sensitized solar cells. The performances of the dye-sensitized solar cells exhibited superior in terms of short-circuit photocurrent density ( = 9.13 mA cm−2) and conversion efficiency (η = 2.09%) compared with complex 1.