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Organic-Ruthenium(II) Polypyridyl Complex Based Sensitizer for Dye-Sensitized Solar Cell Applications

Organic-Ruthenium(II) Polypyridyl Complex Based Sensitizer for Dye-Sensitized Solar Cell... Hindawi Publishing Corporation Advances in OptoElectronics Volume 2011, Article ID 294353, 8 pages doi:10.1155/2011/294353 Research Article Organic-Ruthenium(II) Polypyridyl Complex Based Sensitizer for Dye-Sensitized Solar Cell Applications 1 1 1 2 Lingamallu Giribabu, Varun Kumar Singh, Challuri Vijay Kumar, Yarasi Soujanya, 3 3 Veerannagari Gopal Reddy, and Paidi Yella Reddy Nanomaterials Laboratory, Inorganic & Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500607, India Molecular Modelling Group, Indian Institute of Chemical Technology, Hyderabad 500607, India Aisin Cosmos R&D Co. Ltd., HUDA Complex, Tarnaka, Hyderabad 500007, India Correspondence should be addressed to Lingamallu Giribabu, giribabu@iict.res.in Received 25 April 2011; Accepted 26 May 2011 Academic Editor: Surya Prakash Singh Copyright © 2011 Lingamallu Giribabu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A new high molar extinction coefficient organic-ruthenium(II) polypyridyl complex sensitizer (RD-Cou) that contains 2,2 ,6,6 - tetramethyl-9-thiophene-2-yl-2,3,5,6,6a,11c-hexahydro1H,4H-11oxa-3a-aza-benzoanthracene-10-one as extended π-conjugation of ancillary bipyridine ligand, 4,4 -dicaboxy-2,2 :6 ,2 -bipyridine, and a thiocyanate ligand in its molecular structure has been synthesized and completely characterized by CHN, Mass, H-NMR, UV-Vis, and fluorescence spectroscopies as well as cyclic voltammetry. The new sensitizer was tested in dye-sensitized solar cells using a durable redox electrolyte and compared its performance to that of standard sensitizer Z-907. 1. Introduction molecules, porphyrins, and phthalocyanines and so forth [6–9]. But only ruthenium-based sensitizers could have The increasing demand for power supply as well as marked their way towards commercialization because of environmental concern for the consumption of fossil fuel their high photoconversion efficiencies. The most successful have triggered a greater focus all over the world on renewable ruthenium charge transfer sensitizers employed in such cells energy sources over the past decades [1]. In this context, solar are bis(tetrabutylammonium)-cis-di(thiocyanato)-N,N -bis energy appears to be very attractive alternate: covering 0.16% (4-carboxylato-4 -carboxylic acid-2,2 -bipyridine)rutheni- of the earth with 10% efficient solar conversion systems um(II) (the N719 dye) and trithiocyanato 4,4 4 -trica- would provide power nearly twice the world’s consumption boxy-2,2 :6 ,2 -terpyridine ruthenium(II) (the black dye) rate of fossil energy [2]. For this reason, dye-sensitized solar produced solar-energy-to-electricity conversion efficiencies cells (DSSC) have emerged as one of the most promising (η)of >11% [10–13]. The high efficiency of these complexes candidates because of its cost-effective manufacturing, a are attributed to its suitable ground- and excited-state energy respectable high efficiency and a remarkable stability under levels with respect to the nanocrystalline TiO conduction − − the prolonged thermal and light soaking dual stress among band energy and matching redox properties with the I /I various photovoltaics [3–5]. A typical DSSC system consists redox couple. However, the durability of these devices is very of a nanocrystalline semiconductor that adsorbs a sensitizer low due to the leakage of volatile liquid redox electrolytes. on its surface, a Pt-counter electrode, and a redox mediator. In order to improve the device durability, one has to replace The photosensitizer plays a crucial role in achieving higher the liquid redox electrolyte with either quasisolid or solid photoconversion efficiency and has been actively studied redox electrolyte. Gratzel and coworkers have designed an globally. A wide variety of sensitizers have been studied alternative amphiphilic ruthenium(II) complex (Z-907)in for DSSC that includes various metal complexes, organic order to suit for quasisolid redox electrolytes [14, 15]. 2 Advances in OptoElectronics CO O H HOOC S S Ru Ru N N HOOC S COOH RD-Cou Z-907 Figure 1: Molecular structure of RD-Cou and Z-907. In order to further improve the efficiency of DSSC efficiency of the DSSC based on it [22–24]. The bipyridine devices based ruthenium(II) sensitizers one has to improve carboxylic acid has been used as the anchoring media onto its near-IR absorption because of its absorption maxima nanocrystalline TiO surface and the thiocyanate ligands to restricted at around 550 nm and more over the molar absorp- tune the redox properties of the ruthenium centre. Here tion coefficient of ruthenium(II) complexes are low causing in this paper, we report the synthesis, characterization, use of thicker TiO layers which further has disadvantage of and photovoltaic studies of new ruthenium complex-based achieving higher open circuit potential. Hence, research to supersensitizer RD-Cou as shown in Figure 1 and compare find optimum ruthenium-based sensitizers has been focused its efficiency with that of Z-907. primarily on enhancing the molar absorption coefficient as well as broadening of the metal-to-ligand charge transfer 2. Experimental band. For this reason, Gratzel and coworkers have increased the molar extension coefficient of ruthenium(II) complexes 2.1. Synthesis. 4,4 -dicarboxylic acid-2,2 -bipyridine (Bpy- by introducing extended π-conjugation concept in the acid), 4,4 -diethyl ester phosphonate-2,2 -bipyridine (Bpy- molecular structure [16, 17]. We have also adopted the phosphonate), and formyl coumarin (Cou-S-CHO)were same concept for increasing the molar extinction coefficient synthesized according to the procedures reported in the and reported a few ruthenium(II) polypyridyl complexes literature [25, 26]. [18–20]. Thiophene-derived units are good candidates for increasing the conjugation length of the ancillary ligand to 2.1.1. Synthesis of Bpy-Cou (L). The ligand Bpy-Cou (L) improve the light harvesting ability of a ruthenium complex was synthesized by using Wittig-Horner reaction [27]. NaH [21]. In surge, we have synthesized a new type of ruthenium (26 mg, 1.09 mmol) was added to a solution of Bpy-phos- sensitizer consisting of a donor (hole transport) Coumarin phonate (100 mg, 0.21 mmol) and Cou-S-CHO (196 mg, moiety bridged to the pyridyl groups by thiophene which 0.48 mmol) in 150 mL of dry tetrahydrofuran (THF). The resulted in extended π-conjugation and broadening in the resulting reaction mixture was refluxed overnight under metal-to-ligand charge transfer transition. The reason that nitrogen atmosphere. The reaction mixture was allowed to we have chosen Coumarin as an organic moiety is that it cool to room temperature and then filtered. The filtrate has absorption in 450 nm region where ruthenium(II) has is concentrated, and the obtained solid is washed with minimum absorption and more over this class sensitizers methanol and dried to get the desired product in pure have already shown good efficiency in DSSC devices. It form of 75% yield. Elemental analysis of Anal. Calcd. for is known in the literature that the introduction of donor C H N O S % (963): C, 74.81; H, 6.07; N, 5.82. Found: 60 58 4 4 2 organic moiety in ruthenium(II) polypyridyl complexes can C, 74.90; H, 6.05; N, 5.85. HNMR (CDCl ): δ, ppm 9.08 enhance the spectral response and, therefore, conversion (d, 2H), 8.6 (d, 2H), 8.4 (s, 2H), 7.9 (d, 2H), 7.7 (d, 2H), Advances in OptoElectronics 3 7.6 (s, 2H), 7.25 (d, 2H), 7.1 (d, 2H), 6.8 (s, 2H), 3.6 (m, (TBAP) as supporting electrolyte. The working electrode is 4H), 3.2 (m, 4H), 1.9–2.1 (m, 4H), 1.4 (m, 4H), 0.9–1.3 (s, glassy carbon, standard calomel electrode (SCE) is reference 24H). ESI-MS: m/z 962 [M] . UV/Vis., (ethanol): (λ , ε electrode, and platinum wire is an auxiliary electrode. max −1 −1 M cm ) = 473 (18,006). After a cyclic voltammogram (CV) had been recorded, ferrocene was added, and a second voltammogram was measured. Thermogravimetric measurements were carried 2.1.2. Ru(L)(p-cymene)(Cl) . A mixture of ligand Bpy- out on a Mettler Toledo TGA/SDTA 851e instrument heating Cou L (0.53 g, 1.25 mmol) and [Ru(Cl) -(p-cymene)] was 2 2 −1 rate at 10 Cmin with 10 mg of sample under nitrogen dissolved in ethanol: chloroform (8 : 2 v/v) mixture. The atmosphere. DFT calculations were done for the ground- resultant reaction mixture was refluxed for 4 hours under state optimization of RD-Cou at B3LYP/6-31g(d) using nitrogen atmosphere. Evaporation of the solvent under Gaussian 03 [28]. reduced pressure afforded the pure complex as an orange solid. 2.3. Device Fabrication. A screen-printed single- or double- layer film of interconnected TiO particles was used as 2.1.3. Synthesis of RD-Cou Dye. 4,4 -dicarboxylic acid-2,2 - mesoporous negative electrode. A 10 μm thick film of 20- bipyridine (75 mg, 0.308 m mol) was dissolved in dry DMF nm-sized TiO particles were first printed on the fluorine- at 80 C. To this DMF, solution of Ru(L)(p-cymene)(Cl) doped SnO (FTO) conducting glass electrode and further complex (350 mg, 0.154 mmol) was added. The reaction coated by a 5-μm-thick second layer of 400-nm-sized mixture was refluxed under nitrogen atmosphere for 4 h light scattering anatase particles. The detailed preparation and then cools to 80 C. To the reaction mixture was added procedures of TiO nanocrystals, pastes for screen printing, aqueous NH NCS (362 mg, 4.77 mmol in 10 mL of H O) 4 2 and nanostructured TiO film have been reported in the and then heated for further 2 h at 140 Cand cool to literature procedure [29–31]. A cycloidal TiO electrode room temperature. The solvent DMF was evaporated under 2 (∼0.74 cm ) was stained by immersing it into a dye solution reduced pressure, and water was added. The resulting purple containing RD-Cou or Z907 sensitizer (300 μM) in ethanol solid was filtered and washed with water. The crude complex solvent overnight. After washing with ethanol and drying was dissolved in basic methanol (with tetrabutylammonium by air flow, a sandwich cell was prepared using the dye- hydroxide (TBAOH)) and further purified on a Sephadex sensitized electrode and platinum-coated conducting glass LH-20 column with methanol as eluent. The main band was electrode as the counter electrode. The latter was prepared collected, concentrated, and precipitated with dilute acidic by chemical deposition of platinum from 0.05 M hexachloro- methanol to obtain pure desired complex. Elemental analysis platinic acid. The two electrodes were placed on the top of of Anal. Calcd. for C H N O RuS % (1424.29): C, 62.21; 74 66 8 8 4 each other using a thin polyethylene film (50 μm thick) as H, 4.94; N, 7.84. Found: C, 66.70; H, 5.03; N, 7.90. ESI- a spacer to form the electrolyte space. The empty cell was + −1 MS (C H N O RuS ): m/z 1670 [M-2H] IR (KBr) cm : ◦ 90 109 9 8 4 tightly held, and edges were heated to 130 C to seal the 3390, 2958, 2870, 2102, 1964, 1610, 1540, 1464, 1365, 1355, two electrodes together. The active surface area of the TiO 1231, 1056, 879, 784, 696. HNMR(CD OD): 9.08 (d, 4H), film electrode was ca. 0.74 cm . The redox electrolyte was 8.67 (d, 4H), 8.16 (s, 2H), 7.70 (d, 2H), 7.25 (m, 6H), 7.00 introduced into the cell through a predrilled hole of the (m, 4H), 6.96(d, 2H), 3.35 (s, 4H), 3.27 (s, 4H), 2.47 (s, counter electrode, which was later closed by a cover glass 4H), 1.77 (s, 4H), 1.25 (m, 24H). UV/Vis (ethanol): (λ , max to avoid the leakage of the electrolyte solution. The redox −1 −1 ε M cm ) = 498 (16,046), 384 (13,521). electrolyte is ionic liquid electrolyte, and the composition is 0.2 M I , 0.5 M guanidinium thiocyanate (GuSCN), and 2.2. Characterization Methods. UV-Vis spectra were mea- 0.5 M N-methyl benzimidazole (NMB) in a 65/35 v/v% sured in a 1 cm pathlength quartz cell using a Shimadzu mixture of 1-propyl-3-methylimidazolium iodide/1-Ethyl-3- model 1700 spectrophotometer. Steady state fluorescence methyl-imidazolium tetracyanoborate [PMII/EMIB(CN) ] spectra were recorded on a Spex model Fluoromax-3 spec- (Z580) [15]. trofluorometer using a 1 cm quartz cell. Solutions having optical density at the wavelength of excitation (λ )∼0.11. 2.4. Photoelectrochemical Measurements. The photovoltaic ex The H NMR spectra were recorded at 300 MHz on a Bruker performance of the dye-sensitized nanocrystalline TiO cells 300 Avance NMR spectrometer with X-WIN NMR software. was determined using the simulator SOLARONIX SA SR- The H spectra were referenced to tertramethylsilane. ESI IV unit Type 312. The spectral response was determined mass spectra were recorded on a Water Quattro Micro (Water by measuring the wavelength dependence of the incident Inc, USA). The infrared spectra were recorded on a Thermo photon-to-current conversion efficiency (IPCE) using light Nicolet Nexus 670 FT-IR spectrophotometer. The spectra of from a 100-W xenon lamp that was focused onto the the solid samples were recorded by dispersing the sample cell through a double monochromator. The current-voltage in NujolmullorasKBr wafers.Cyclicand differential characteristics were determined by applying an external pulse voltammetric measurements were performed on a potential bias to the cell and measuring the photocurrent PC-controlled CH instruments model CHI 620C electro- using a Keithley model 2420 digital source meter, and a chemical analyzer. Cyclic voltammetric experiments were 1000-W xenon lamp was used as the irradiation source. The performedon1mM dyesolutioninacetonitrile at scan rate spectral output of the lamp is set matched the AM 1.5 solar of 100 mV/s using 0.1 M tetrabutylammonium perchlorate spectrum in the region of 350–750 nm (mismatch 1.9%). 4 Advances in OptoElectronics COOH N N PO (C H ) PO (C H ) 3 2 5 2 3 2 5 2 HOOC O O p-cymene N O N O Ru DMF Bpy-phosphonate S N N Reflux 6 h NaH, THF + S Reflux 6 h NH NCS CHO N O N N Cou-S-CHO Bpy-Cou O RD-Coumarin Scheme 1 by the elemental analysis, ESI-MS, IR, UV-Visible, and fluorescence spectroscopies as well as cyclic voltammetry. ESI-MS spectrum consists of a molecular ion peak at 1670 (m/z) which corresponds to the presence of a one TBA molecule in its molecular structure. Figure 2 shows the absorption spectra of Cou-S-CHO, Bpy-Cou (L), RD-Cou,and Z-907 in ethanol, and the corresponding data are presented in Table 1. The absorp- tion maximum of coumarin in Cou-S-CHO is located at 470 nm. In Z-907, the absorption maximum at 540 nm belongs to the metal-to-ligand charge-transfer transition in singlet manifold ( MLCT). The absorption maximum of RD-Cou is centered at 498 nm with a molar extinction 300 400 500 600 700 800 −1 −1 coefficient of 16,046 M cm .From Figure 1, it is clear that Wavelength (nm) the absorption of RD-Cou is very broad not like typical ··· Figure 2: Electronic absorption spectra of ( ) Cou-S-CHO,(- - -) ruthenium(II) polypyridyl complexes. This is due to the . . Bpy-Cou (L),(—) RD-Cou,and (- - -) Z-907 in ethanol solvent. presence of coumarin moiety in its molecular structure, which absorbs at 470 nm. The absorption maxima of RD- Cou is bathochromic shift (540 nm in Z-907 to 498 nm in RD-Cou is hypsochromic) when compared to that of 3. Results and Discussion the standard sensitizer Z-907. The absorption maximum 3.1. Synthesis and Characterization. The details of the syn- at 387 nm belongs to the intraligand π-π transitions of thetic strategy adopted for the synthesis of RD-Cou complex bipyridine ligand. Figure 2 depicts the absorption spectra is shown in Scheme 1,and Z-907 was synthesized according of RD-Cou adsorbed onto 6-μm-thick TiO film. The to the literature method [14]. Both Bpy-phosphonate and absorption features of the ruthenium complex in solution as well as when anchored onto TiO surface are identical Cou-S-CHO were synthesized as per the reaction procedures reported in the literature [23, 24]. The C=Cdoublebond except for a slight red shift in the absorption maxima due was introduced at 4, 4 positions of the bipyridine ligand, to the interaction of anchoring groups with the surface as starting from Bpy-phosphonate with Cou-S-CHO using well as further broadening [32]. The emission spectra of RD- a Wittig-Horner reaction [27]. The ligand Bpy-Cou (L) Cou were measured in ethanol solvent at room temperature was completely characterized by using elemental analysis, and are shown in Figure 3. Excitation of lower energy Mass, IR, and H NMR spectroscopies. Finally, the RD-Cou MLCT transition of RD-Cou sensitizer produces an emission complex was synthesized by refluxing p-cymene complex centered at 690 nm. The excited singlet state energy (E ) 0-0 of RD-Cou was calculated from the onset of absorption and Bpy-Cou (L) in ethanol:chloroform mixture to get the corresponding chlorocomplex. The chlorocomplex with spectrum and was found 1.65 eV. However, the emission of Bpy-acid and aq. ammonium thiocyanate refluxed in DMF RD-Cou sensitizer was quenched when adsorbed onto the to get the desired complex after sephadex column purifi- TiO film as a consequence of electron injection from the cation. The complex RD-Cou was completely characterized excited state of Ru(II) into the conduction band of TiO . Absorbance Advances in OptoElectronics 5 Table 1: UV-visible emission and electrochemical data. λ ,nm, E VversusSCE max 1/2 b d ∗ Sensitizer λ ,nm E E ,eV −1 a em 0-0 ox −1 ε(mol cm ) Ox Red RD-Cou 498 (16.046) 690 0.72 −1.05 −1.47 1.65 −0.93 Z-907 540 (10,040) 720 0.65 −0.98 −1.64 1.62 −1.02 a b c Solvent: ethanol, Error limits: λ , ±1nm, ε ± 10%. Solvent: ethanol, λ , ±1nm. Solvent: DMF, Error limits: E ±0.03 V, 0.1 M TBAP. Error limits: max max 1/2 0.05 eV. 2 20 With a view to evaluate HOMO-LUMO levels of RD- Cou, we have carried out the electrochemistry by adopting the cyclic and differential pulse voltammetric techniques in acetonitrile solvent using 0.1 M tetrabutylammonium per- chlorate as supporting electrolyte and compared their data with that of the standard sensitizer Z-907 in Table 1. When 1 10 the potential is scanned between 0 and 1.0 V, chemically reversible redox waves with formal potentials at 0.65 and 0.72 V (versus SCE) were observed, which can be attributed to the one-electron oxidation of ruthenium center in both dyes. Compared to the standard Z-907 dye, the metal 0 0 300 400 500 600 700 800 center oxidation of RD-Cou is anodically shifted by 70 mV Wavelength (nm) indicating the electron-poor character of the new ligand as a result of the insertion of Coumarin moiety on extended Figure 3: Absorption (—) and emission spectra in ethanol and π-conjugation. It also undergoes two reductions at −1.05 absorption (- - -) spectra adsorbed onto a 6 μm thick TiO film of and 1.47 V, corresponding to the one electron reduction of RD-Cou. anchoring ligand. Furthermore, for a sensitizer in DSSC, the LUMO energy level should be compatible with the conduc- mixture of 1-propyl-3-methylimidazolium iodide/1-Ethyl-3- tion band edge energy of the TiO photoanode (0.80 V versus methyl-imidazolium tetracyanoborate [PMII/EMIB(CN) ] SCE), and its HOMO should be sufficiently low in energy to − − (Z580) and compared its performance with that of standard accept electrons from the I /I -based redox electrolyte (0.2 V sensitizer Z-907 under similar test cell conditions. The versus SCE). The excited oxidation potential of RD-Cou is additive guanidinium thiocyanate in redox electrolyte is −0.93 V and that of standard sensitizer Z-907 is −1.02 V, to improve the V by reducing the dark current [33]. oc which is above the conduction band of TiO .In Z-907, the We assume that in our case, also guanidinium thiocyanate excited state oxidation potential is sufficiently higher than in Z580 redox electrolyte is responsible for improve- that in RD-Cou, as a result electron injection into the TiO ment in V . 1-Ethyl-3-methylimidazolium tetracyanoborate oc conduction band is more efficient, and it should lead to a (EMIB(CN) ) is an ionic liquid of a low viscosity (19.8 cP better conversion efficiency than RD-Cou. at 20 C) and high chemical and thermal stability. By using To know the electronic distribution of RD-Cou sen- this redox electrolyte with this composition, Gratzel and sitizer, we performed DFT calculations of the electronic coworkers have observed an efficiency of 6.4% [15]. Figure 5 ground state of RD-Cou sensitizer using mPW1PW91 illustrates the photocurrent action spectra of RD-Cou method for the geometry optimization with LANL2DZ basis and Z-907, where the incident monochromatic photon-to- function on Ru and 6–31 g(d) basis function on C, H, N, O current conversion efficiencies (IPCE) values are plotted as a and S. As can be seen from the Figure 5, occupied orbitals function of excitation wavelength. The IPCE was calculated HOMO, HOMO-1 and HOMO-2, have the electron delocal- according to the following equation: ized over the Ru(II) metal and –NCS ligand. The LUMO, LUMO+1, and LUMO+2 are π orbitals delocalized over sc the bipyridine carboxylic acid ligand facilitating electron IPCE(%) = 1240 × 100, (1) injection from the excited state of RD-Cou sensitizer to φ the conduction band of TiO . These results are in good where λ is the wavelength (nm), J is the photocurrent agreement with other ruthenium(II) polypyridyl complexes SC density under short circuit conditions (mA/cm ), and φ is reported in the literature [16, 17]. the incident radiative flux (mW/cm ). We have observed IPCE values of 60 and 68% using RD-Cou and Z-907 3.2. Photovoltaic Measurements. The performance of newly sensitizers, respectively. From Figure 4, it is clear that the synthesized RD-Cou as a sensitizer with a sandwitch-type photocurrent action spectrum resembles the absorption nanocrystalline TiO was determined from measurements spectra except for a slight red shift by ca. 10 nm in both RD- on photovoltaic cells using an ionic liquid redox electrolyte, Cou and Z-907. The photoresponse of thin films displays a that is, 0.2 M I , 0.5 M guanidinium thiocyanate (GuSCN), broad spectral response covering the entire visible spectrum and 0.5 M N-methyl benzimidazole (NMB) in a 65/35 v/v% up to 800 nm in both the sensitizers. Absorbance Emission intensity (a.u.) 6 Advances in OptoElectronics RD-Coumarin HOMO HOMO−1 HOMO−2 HOMO−3 HOMO−4 LUMO LUMO+1 LUMO+2 LUMO+3 LUMO+4 Figure 4: Molecular orbital spatial orientation of RD-Cou. 80 12 400 500 600 700 800 0 0.1 0.2 0.3 0.4 0.5 0.6 Wavelength (nm) Voltage (V) Figure 5: Photocurrent action spectra of (brown line) RD-Cou and Figure 6: Current-voltage characteristics: (brown line) RD-Cou (blue line) Z-907 using Z-580 redox electrolyte. and (blue line) Z-907 using Z-580 redox electrolyte. Table 2: Photovoltaic performance of RD-Cum and Z-907 . b c c c Sensitizer Electrolyte J (mA/cm ) V (mV) ff η(%) sc oc V is the open-circuit voltage, and ff represents the fill oc RD-Cou Z580 8.80 650 0.68 4.24 factor.Wehaveobservedanoverall conversion efficiency of Z-907 Z580 11.97 650 0.68 5.20 4.24% under 1.0 sun irradiation (J = 8.80 mA/cm , V sc OC a 2 b Photoelectrode: TiO (10 + 5 μm and 0.74 cm ); electrolyte: 0.2 M 2 = 650 mV, ff = 0.68) using RD-Cou as sensitizer. Under I , 0.5 M guanidinium thiocyanate (GuSCN), and 0.5 M N-methyl ben- similar test cell conditions, the device based on Z-907 zimidazole (NMB) in a 65/35 v/v% mixture of 1-propyl-3-methylimid- −2 sensitizer [J = 11.97 mA cm , V = 650 mV, and ff = sc OC azolium iodide/1-Ethyl-3-methyl-imidazolium tetracyanoborate [PMII/ c 2 0.68] shows a photovoltaic conversion efficiency of 5.20%. EMIB(CN) ]. Error limits: J : ±0.20 mA/cm , V = ±30 mV, ff = ±0.03. 4 sc oc The low efficiency of RD-Cou when compared with that of standard sensitizer Z-907 is probably due to poor hole Figure 6 shows the photocurrent-voltage characteristics transport property from coumarin moiety to Ru(II). of RD-Cou and Z-907 using Z-580 as redox electrolyte under 1.0 sun irradiation (1000 W/m ), and corresponding data are 3.3. Thermal Studies. We have examined the thermal stability shown in Table 2. The solar-energy-to-electricity conversion of new ruthenium(II) polypyridyl sensitizer and compared efficiency (η), under white-light irradiation can be obtained their thermal stability with that of the standard sensitizer from the following equation: Z-907, using thermogravimetric analysis. Figure 7 shows the −2 J Am · V [V] × ff thermal behavior of RD-Cou. From the Figure, it is clear sc OC η [%] = × 100, (2) −2 [ ] that the sensitizer RD-Cou is stable up to 220 C. The initial I Wm weight loss between 200 to 250 C is attributed to the removal −2 where I is the photon flux (e.g., 1000 W m for 1.0 sun), J 0 sc of the carboxyl group. In contrast, the standard sensitizer Z- is the short-circuit photocurrent density under irradiation, 907 is stable up to 200 C. IPCE (%) Current (mA/cm ) Advances in OptoElectronics 7 [4] M. Gratz ¨ el, “Recent advances in sensitized mesoscopic solar 0.001 cells,” Accounts of Chemical Research, vol. 42, no. 11, pp. 1788– 0.0005 1798, 2009. [5] A. Jager ¨ -Waldau, “Photovoltaics and renewable energies in −0.0005 60 Europe,” Renewable and Sustainable Energy Reviews, vol. 11, −0.001 no. 7, pp. 1414–1437, 2007. 40 −0.0015 [6] G. Zhang, H. Bala, Y. Cheng et al., “High efficiency and stable −0.002 dye-sensitized solar cells with an organic chromophore featur- −0.0025 ing a binary π-conjugated spacer,” Chemical Communications, −0.003 no. 16, pp. 2198–2200, 2009. [7] T. Bessho, S. M. Zakeeruddin, C. Y. Yeh, E. W. G. Diau, 50 100 150 200 250 300 350 400 450 500 550 ◦ and M. Gratz ¨ el, “Highly efficient mesoscopic dye-sensitized ( C) solar cells based on donor-acceptor-substituted porphyrins,” Conversion Angewandte Chemie—International Edition, vol. 49, no. 37, pp. 6646–6649, 2010. ( C) (%) 28.56 0 [8] L.Giribabu, Ch.Vijaykumar,P.Y.Reddy,J.H.Yum,M. 91.74 1.07 Gratz ¨ el, and M. K. Nazeeruddin, “Unsymmetrical extended π- 154.92 1.79 conjugated zinc phthalocyanine for sensitization of nanocrys- 218.11 3.12 talline TiO films,” Journal of Chemical Sciences, vol. 121, no. 281.29 12.07 1, pp. 75–82, 2009. 344.47 25.83 407.66 41.61 [9] P.Y.Reddy,L.Giribabu, C. Lyness et al., “Efficient sensiti- 470.84 48.28 zation of nanocrystalline TiO films by a near-IR-absorbing 534.02 50.73 unsymmetrical zinc phthalocyanine,” Angewandte Chemie— 597.2 51.76 International Edition, vol. 46, no. 3, pp. 373–376, 2007. [10] M. Gratz ¨ el, “Photoelectrochemical cells,” Nature, vol. 414, no. Figure 7: TG/DTG curves of RD-Cou with heating rate of −1 6861, pp. 338–344, 2001. 10 Cmin under nitrogen. [11] M. K. Nazeeruddin, P. Pec ´ hy, T. Renouard et al., “Engineering of efficient panchromatic sensitizers for nanocrystalline TiO - based solar cells,” Journal of the American Chemical Society, vol. 4. Conclusions 123, no. 8, pp. 1613–1624, 2001. [12] M. K. Nazeeruddin, A. Kay, I. Rodicio et al., “Conver- In conclusion, we have designed and synthesized a new sion of light to electricity by cis-X bis(2,2 -bipyridyl-4,4 - Coumarin-Ruthenium(II) polypyridyl complex having an dicarboxylate)ruthenium(II) charge-transfer sensitizers (X = extended π-conjugation. The new complex was completely − − − − − Cl ,Br ,I ,CN , and SCN ) on nanocrystalline TiO characterized by elemental analysis, ESI-MS, IR, UV-Visible, electrodes,” Journal of the American Chemical Society, vol. 115, and fluorescence spectroscopies as well as cyclic voltamme- no. 14, pp. 6382–6390, 1993. try. The performance of new sensitizer was tested in dye- [13] B. O’Regan and M. Gratz ¨ el, “A low-cost, high-efficiency solar sensitized solar cells using a durable redox electrolyte and cell based on dye-sensitized colloidal TiO films,” Nature, vol. compared with that of standard sensitizer Z-907. The low 353, no. 6346, pp. 737–740, 1991. efficiency of device based on RD-Cou, when compared to [14] P. Wang, S. M. Zakeeruddin, J. E. Moser, M. K. Nazeeruddin, Z-907, is probably due to poor hole transport property from T. Sekiguchi, and M. Gratz ¨ el, “A stable quasi-solid-state dye- coumarin moiety to Ru(II). sensitized solar cell with an amphiphilic ruthenium sensitizer and polymer gel electrolyte,” Nature Materials,vol. 2, no.6, pp. 402–407, 2003. Acknowledgments [15] D. Kuang, P. Wang, S. Ito, S. M. Zakeeruddin, and M. The authors are thankful to the IICT-Aisin Cosmos col- Gratz ¨ el, “Stable mesoscopic dye-sensitized solar cells based on tetracyanoborate ionic liquid electrolyte,” Journal of the laborative project for financial support of this work. L. American Chemical Society, vol. 128, no. 24, pp. 7732–7733, Giribabu is thankful to the project SR/S1/IC21/2008 for partial financial support of this work. V. K. Singh and Ch. [16] P. Wang, C. Klein, R. Humphry-Baker, S. M. Zakeeruddin, V. Kumar are thankful to Council of Scientific and Industrial and M. Gratz ¨ el, “A high molar extinction coefficient sensitizer Research (CSIR) for a fellowship. for stable dye-sensitized solar cells,” Journal of the American Chemical Society, vol. 127, no. 3, pp. 808–809, 2005. References [17] P. Wang, S. M. Zakeeruddin, J. E. Moser et al., “Stable new sensitizer with improved light harvesting for nanocrystalline [1] N. Armaroli and V. Balzani, “The future of energy sup- dye-sensitized solar cells,” Advanced Materials, vol. 16, no. 20, ply: challenges and opportunities,” Angewandte Chemie— pp. 1806–1811, 2004. International Edition, vol. 46, no. 1-2, pp. 52–66, 2007. [18] L. Giribabu, T. Bessho, M. Srinivasu et al., “A new family of [2] R. F. Service, “Is it time to shoot for the sun?” Science, vol. 309, heteroleptic ruthenium polypyridyl complexes for sensitiza- no. 5734, pp. 548–551, 2005. tion of nanocrystalline TiO Flms,” Dalton Transactions, vol. [3] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, and H. Pettersson, 40, pp. 4497–4504, 2011. [19] L. Giribabu, V. K. Singh, M. Srinivasu et al., “Synthesis “Dye-sensitized solar cells,” Chemical Reviews, vol. 110, no. 11, pp. 6595–6663, 2010. and photoelectrochemical chacterization of a high molar (%) (1/ C) 8 Advances in OptoElectronics extinction coefficient heteroleptic ruthenium(II) complex,” Journal of Chemical Science. In press. [20] L. Giribabu, Ch. Vijaykumar, C. S. Rao et al., “High molar extinction coefficient amphiphilic ruthenium sensitizers for efficient and stable mesoscopic dye-sensitized solar cells,” Energy and Environmental Science, vol. 2, no. 7, pp. 770–773, [21] K. J. Jiang, N. Masaki, J. B. Xia, S. Noda, and S. Yanagida, “A novel ruthenium sensitizer with a hydrophobic 2-thiophen- 2-yl-vinyl- conjugated bipyridyl ligand for effective dye sen- sitized TiO solar cells,” Chemical Communications, no. 23, pp. 2460–2462, 2006. [22] C. Y. Chen, N. Pootrakulchote, S. J. Wu et al., “New ruthenium sensitizer with carbazole antennas for efficient and stable Thin-film Dye-sensitized solar cells,” Journal of Physical Chemistry C, vol. 113, no. 48, pp. 20752–20757, 2009. [23] K. Hara, M. Kurashige, Y. Dan-Oh et al., “Design of new coumarin dyes having thiophene moieties for highly efficient organic-dye-sensitized solar cells,” New Journal of Chemistry, vol. 27, no. 5, pp. 783–785, 2003. [24] K. Hara, K. Sayama, Y. Ohga, A. Shinpo, S. Suga, and H. Arakawa, “A coumarin-derivative dye sensitized nanocrys- talline TiO solar cell having a high solar-energy conversion efficiency up to 5.6%,” Chemical Communications,no. 6, pp. 569–570, 2001. [25] H. Zabri, I. Gillaizeau, C. A. Bignozzi et al., “Synthesis and comprehensive characterizations of new cis-RuL X (X = CI, 2 2 CN, and NCS) sensitizers for nanocrystalline TiO solar cell using bis-phosphonated bipyridine ligands (L),” Inorganic Chemistry, vol. 42, no. 21, pp. 6655–6666, 2003. [26] K. Hara, Z. S. Wang, T. 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Organic-Ruthenium(II) Polypyridyl Complex Based Sensitizer for Dye-Sensitized Solar Cell Applications

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Copyright © 2011 Lingamallu Giribabu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Hindawi Publishing Corporation Advances in OptoElectronics Volume 2011, Article ID 294353, 8 pages doi:10.1155/2011/294353 Research Article Organic-Ruthenium(II) Polypyridyl Complex Based Sensitizer for Dye-Sensitized Solar Cell Applications 1 1 1 2 Lingamallu Giribabu, Varun Kumar Singh, Challuri Vijay Kumar, Yarasi Soujanya, 3 3 Veerannagari Gopal Reddy, and Paidi Yella Reddy Nanomaterials Laboratory, Inorganic & Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500607, India Molecular Modelling Group, Indian Institute of Chemical Technology, Hyderabad 500607, India Aisin Cosmos R&D Co. Ltd., HUDA Complex, Tarnaka, Hyderabad 500007, India Correspondence should be addressed to Lingamallu Giribabu, giribabu@iict.res.in Received 25 April 2011; Accepted 26 May 2011 Academic Editor: Surya Prakash Singh Copyright © 2011 Lingamallu Giribabu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A new high molar extinction coefficient organic-ruthenium(II) polypyridyl complex sensitizer (RD-Cou) that contains 2,2 ,6,6 - tetramethyl-9-thiophene-2-yl-2,3,5,6,6a,11c-hexahydro1H,4H-11oxa-3a-aza-benzoanthracene-10-one as extended π-conjugation of ancillary bipyridine ligand, 4,4 -dicaboxy-2,2 :6 ,2 -bipyridine, and a thiocyanate ligand in its molecular structure has been synthesized and completely characterized by CHN, Mass, H-NMR, UV-Vis, and fluorescence spectroscopies as well as cyclic voltammetry. The new sensitizer was tested in dye-sensitized solar cells using a durable redox electrolyte and compared its performance to that of standard sensitizer Z-907. 1. Introduction molecules, porphyrins, and phthalocyanines and so forth [6–9]. But only ruthenium-based sensitizers could have The increasing demand for power supply as well as marked their way towards commercialization because of environmental concern for the consumption of fossil fuel their high photoconversion efficiencies. The most successful have triggered a greater focus all over the world on renewable ruthenium charge transfer sensitizers employed in such cells energy sources over the past decades [1]. In this context, solar are bis(tetrabutylammonium)-cis-di(thiocyanato)-N,N -bis energy appears to be very attractive alternate: covering 0.16% (4-carboxylato-4 -carboxylic acid-2,2 -bipyridine)rutheni- of the earth with 10% efficient solar conversion systems um(II) (the N719 dye) and trithiocyanato 4,4 4 -trica- would provide power nearly twice the world’s consumption boxy-2,2 :6 ,2 -terpyridine ruthenium(II) (the black dye) rate of fossil energy [2]. For this reason, dye-sensitized solar produced solar-energy-to-electricity conversion efficiencies cells (DSSC) have emerged as one of the most promising (η)of >11% [10–13]. The high efficiency of these complexes candidates because of its cost-effective manufacturing, a are attributed to its suitable ground- and excited-state energy respectable high efficiency and a remarkable stability under levels with respect to the nanocrystalline TiO conduction − − the prolonged thermal and light soaking dual stress among band energy and matching redox properties with the I /I various photovoltaics [3–5]. A typical DSSC system consists redox couple. However, the durability of these devices is very of a nanocrystalline semiconductor that adsorbs a sensitizer low due to the leakage of volatile liquid redox electrolytes. on its surface, a Pt-counter electrode, and a redox mediator. In order to improve the device durability, one has to replace The photosensitizer plays a crucial role in achieving higher the liquid redox electrolyte with either quasisolid or solid photoconversion efficiency and has been actively studied redox electrolyte. Gratzel and coworkers have designed an globally. A wide variety of sensitizers have been studied alternative amphiphilic ruthenium(II) complex (Z-907)in for DSSC that includes various metal complexes, organic order to suit for quasisolid redox electrolytes [14, 15]. 2 Advances in OptoElectronics CO O H HOOC S S Ru Ru N N HOOC S COOH RD-Cou Z-907 Figure 1: Molecular structure of RD-Cou and Z-907. In order to further improve the efficiency of DSSC efficiency of the DSSC based on it [22–24]. The bipyridine devices based ruthenium(II) sensitizers one has to improve carboxylic acid has been used as the anchoring media onto its near-IR absorption because of its absorption maxima nanocrystalline TiO surface and the thiocyanate ligands to restricted at around 550 nm and more over the molar absorp- tune the redox properties of the ruthenium centre. Here tion coefficient of ruthenium(II) complexes are low causing in this paper, we report the synthesis, characterization, use of thicker TiO layers which further has disadvantage of and photovoltaic studies of new ruthenium complex-based achieving higher open circuit potential. Hence, research to supersensitizer RD-Cou as shown in Figure 1 and compare find optimum ruthenium-based sensitizers has been focused its efficiency with that of Z-907. primarily on enhancing the molar absorption coefficient as well as broadening of the metal-to-ligand charge transfer 2. Experimental band. For this reason, Gratzel and coworkers have increased the molar extension coefficient of ruthenium(II) complexes 2.1. Synthesis. 4,4 -dicarboxylic acid-2,2 -bipyridine (Bpy- by introducing extended π-conjugation concept in the acid), 4,4 -diethyl ester phosphonate-2,2 -bipyridine (Bpy- molecular structure [16, 17]. We have also adopted the phosphonate), and formyl coumarin (Cou-S-CHO)were same concept for increasing the molar extinction coefficient synthesized according to the procedures reported in the and reported a few ruthenium(II) polypyridyl complexes literature [25, 26]. [18–20]. Thiophene-derived units are good candidates for increasing the conjugation length of the ancillary ligand to 2.1.1. Synthesis of Bpy-Cou (L). The ligand Bpy-Cou (L) improve the light harvesting ability of a ruthenium complex was synthesized by using Wittig-Horner reaction [27]. NaH [21]. In surge, we have synthesized a new type of ruthenium (26 mg, 1.09 mmol) was added to a solution of Bpy-phos- sensitizer consisting of a donor (hole transport) Coumarin phonate (100 mg, 0.21 mmol) and Cou-S-CHO (196 mg, moiety bridged to the pyridyl groups by thiophene which 0.48 mmol) in 150 mL of dry tetrahydrofuran (THF). The resulted in extended π-conjugation and broadening in the resulting reaction mixture was refluxed overnight under metal-to-ligand charge transfer transition. The reason that nitrogen atmosphere. The reaction mixture was allowed to we have chosen Coumarin as an organic moiety is that it cool to room temperature and then filtered. The filtrate has absorption in 450 nm region where ruthenium(II) has is concentrated, and the obtained solid is washed with minimum absorption and more over this class sensitizers methanol and dried to get the desired product in pure have already shown good efficiency in DSSC devices. It form of 75% yield. Elemental analysis of Anal. Calcd. for is known in the literature that the introduction of donor C H N O S % (963): C, 74.81; H, 6.07; N, 5.82. Found: 60 58 4 4 2 organic moiety in ruthenium(II) polypyridyl complexes can C, 74.90; H, 6.05; N, 5.85. HNMR (CDCl ): δ, ppm 9.08 enhance the spectral response and, therefore, conversion (d, 2H), 8.6 (d, 2H), 8.4 (s, 2H), 7.9 (d, 2H), 7.7 (d, 2H), Advances in OptoElectronics 3 7.6 (s, 2H), 7.25 (d, 2H), 7.1 (d, 2H), 6.8 (s, 2H), 3.6 (m, (TBAP) as supporting electrolyte. The working electrode is 4H), 3.2 (m, 4H), 1.9–2.1 (m, 4H), 1.4 (m, 4H), 0.9–1.3 (s, glassy carbon, standard calomel electrode (SCE) is reference 24H). ESI-MS: m/z 962 [M] . UV/Vis., (ethanol): (λ , ε electrode, and platinum wire is an auxiliary electrode. max −1 −1 M cm ) = 473 (18,006). After a cyclic voltammogram (CV) had been recorded, ferrocene was added, and a second voltammogram was measured. Thermogravimetric measurements were carried 2.1.2. Ru(L)(p-cymene)(Cl) . A mixture of ligand Bpy- out on a Mettler Toledo TGA/SDTA 851e instrument heating Cou L (0.53 g, 1.25 mmol) and [Ru(Cl) -(p-cymene)] was 2 2 −1 rate at 10 Cmin with 10 mg of sample under nitrogen dissolved in ethanol: chloroform (8 : 2 v/v) mixture. The atmosphere. DFT calculations were done for the ground- resultant reaction mixture was refluxed for 4 hours under state optimization of RD-Cou at B3LYP/6-31g(d) using nitrogen atmosphere. Evaporation of the solvent under Gaussian 03 [28]. reduced pressure afforded the pure complex as an orange solid. 2.3. Device Fabrication. A screen-printed single- or double- layer film of interconnected TiO particles was used as 2.1.3. Synthesis of RD-Cou Dye. 4,4 -dicarboxylic acid-2,2 - mesoporous negative electrode. A 10 μm thick film of 20- bipyridine (75 mg, 0.308 m mol) was dissolved in dry DMF nm-sized TiO particles were first printed on the fluorine- at 80 C. To this DMF, solution of Ru(L)(p-cymene)(Cl) doped SnO (FTO) conducting glass electrode and further complex (350 mg, 0.154 mmol) was added. The reaction coated by a 5-μm-thick second layer of 400-nm-sized mixture was refluxed under nitrogen atmosphere for 4 h light scattering anatase particles. The detailed preparation and then cools to 80 C. To the reaction mixture was added procedures of TiO nanocrystals, pastes for screen printing, aqueous NH NCS (362 mg, 4.77 mmol in 10 mL of H O) 4 2 and nanostructured TiO film have been reported in the and then heated for further 2 h at 140 Cand cool to literature procedure [29–31]. A cycloidal TiO electrode room temperature. The solvent DMF was evaporated under 2 (∼0.74 cm ) was stained by immersing it into a dye solution reduced pressure, and water was added. The resulting purple containing RD-Cou or Z907 sensitizer (300 μM) in ethanol solid was filtered and washed with water. The crude complex solvent overnight. After washing with ethanol and drying was dissolved in basic methanol (with tetrabutylammonium by air flow, a sandwich cell was prepared using the dye- hydroxide (TBAOH)) and further purified on a Sephadex sensitized electrode and platinum-coated conducting glass LH-20 column with methanol as eluent. The main band was electrode as the counter electrode. The latter was prepared collected, concentrated, and precipitated with dilute acidic by chemical deposition of platinum from 0.05 M hexachloro- methanol to obtain pure desired complex. Elemental analysis platinic acid. The two electrodes were placed on the top of of Anal. Calcd. for C H N O RuS % (1424.29): C, 62.21; 74 66 8 8 4 each other using a thin polyethylene film (50 μm thick) as H, 4.94; N, 7.84. Found: C, 66.70; H, 5.03; N, 7.90. ESI- a spacer to form the electrolyte space. The empty cell was + −1 MS (C H N O RuS ): m/z 1670 [M-2H] IR (KBr) cm : ◦ 90 109 9 8 4 tightly held, and edges were heated to 130 C to seal the 3390, 2958, 2870, 2102, 1964, 1610, 1540, 1464, 1365, 1355, two electrodes together. The active surface area of the TiO 1231, 1056, 879, 784, 696. HNMR(CD OD): 9.08 (d, 4H), film electrode was ca. 0.74 cm . The redox electrolyte was 8.67 (d, 4H), 8.16 (s, 2H), 7.70 (d, 2H), 7.25 (m, 6H), 7.00 introduced into the cell through a predrilled hole of the (m, 4H), 6.96(d, 2H), 3.35 (s, 4H), 3.27 (s, 4H), 2.47 (s, counter electrode, which was later closed by a cover glass 4H), 1.77 (s, 4H), 1.25 (m, 24H). UV/Vis (ethanol): (λ , max to avoid the leakage of the electrolyte solution. The redox −1 −1 ε M cm ) = 498 (16,046), 384 (13,521). electrolyte is ionic liquid electrolyte, and the composition is 0.2 M I , 0.5 M guanidinium thiocyanate (GuSCN), and 2.2. Characterization Methods. UV-Vis spectra were mea- 0.5 M N-methyl benzimidazole (NMB) in a 65/35 v/v% sured in a 1 cm pathlength quartz cell using a Shimadzu mixture of 1-propyl-3-methylimidazolium iodide/1-Ethyl-3- model 1700 spectrophotometer. Steady state fluorescence methyl-imidazolium tetracyanoborate [PMII/EMIB(CN) ] spectra were recorded on a Spex model Fluoromax-3 spec- (Z580) [15]. trofluorometer using a 1 cm quartz cell. Solutions having optical density at the wavelength of excitation (λ )∼0.11. 2.4. Photoelectrochemical Measurements. The photovoltaic ex The H NMR spectra were recorded at 300 MHz on a Bruker performance of the dye-sensitized nanocrystalline TiO cells 300 Avance NMR spectrometer with X-WIN NMR software. was determined using the simulator SOLARONIX SA SR- The H spectra were referenced to tertramethylsilane. ESI IV unit Type 312. The spectral response was determined mass spectra were recorded on a Water Quattro Micro (Water by measuring the wavelength dependence of the incident Inc, USA). The infrared spectra were recorded on a Thermo photon-to-current conversion efficiency (IPCE) using light Nicolet Nexus 670 FT-IR spectrophotometer. The spectra of from a 100-W xenon lamp that was focused onto the the solid samples were recorded by dispersing the sample cell through a double monochromator. The current-voltage in NujolmullorasKBr wafers.Cyclicand differential characteristics were determined by applying an external pulse voltammetric measurements were performed on a potential bias to the cell and measuring the photocurrent PC-controlled CH instruments model CHI 620C electro- using a Keithley model 2420 digital source meter, and a chemical analyzer. Cyclic voltammetric experiments were 1000-W xenon lamp was used as the irradiation source. The performedon1mM dyesolutioninacetonitrile at scan rate spectral output of the lamp is set matched the AM 1.5 solar of 100 mV/s using 0.1 M tetrabutylammonium perchlorate spectrum in the region of 350–750 nm (mismatch 1.9%). 4 Advances in OptoElectronics COOH N N PO (C H ) PO (C H ) 3 2 5 2 3 2 5 2 HOOC O O p-cymene N O N O Ru DMF Bpy-phosphonate S N N Reflux 6 h NaH, THF + S Reflux 6 h NH NCS CHO N O N N Cou-S-CHO Bpy-Cou O RD-Coumarin Scheme 1 by the elemental analysis, ESI-MS, IR, UV-Visible, and fluorescence spectroscopies as well as cyclic voltammetry. ESI-MS spectrum consists of a molecular ion peak at 1670 (m/z) which corresponds to the presence of a one TBA molecule in its molecular structure. Figure 2 shows the absorption spectra of Cou-S-CHO, Bpy-Cou (L), RD-Cou,and Z-907 in ethanol, and the corresponding data are presented in Table 1. The absorp- tion maximum of coumarin in Cou-S-CHO is located at 470 nm. In Z-907, the absorption maximum at 540 nm belongs to the metal-to-ligand charge-transfer transition in singlet manifold ( MLCT). The absorption maximum of RD-Cou is centered at 498 nm with a molar extinction 300 400 500 600 700 800 −1 −1 coefficient of 16,046 M cm .From Figure 1, it is clear that Wavelength (nm) the absorption of RD-Cou is very broad not like typical ··· Figure 2: Electronic absorption spectra of ( ) Cou-S-CHO,(- - -) ruthenium(II) polypyridyl complexes. This is due to the . . Bpy-Cou (L),(—) RD-Cou,and (- - -) Z-907 in ethanol solvent. presence of coumarin moiety in its molecular structure, which absorbs at 470 nm. The absorption maxima of RD- Cou is bathochromic shift (540 nm in Z-907 to 498 nm in RD-Cou is hypsochromic) when compared to that of 3. Results and Discussion the standard sensitizer Z-907. The absorption maximum 3.1. Synthesis and Characterization. The details of the syn- at 387 nm belongs to the intraligand π-π transitions of thetic strategy adopted for the synthesis of RD-Cou complex bipyridine ligand. Figure 2 depicts the absorption spectra is shown in Scheme 1,and Z-907 was synthesized according of RD-Cou adsorbed onto 6-μm-thick TiO film. The to the literature method [14]. Both Bpy-phosphonate and absorption features of the ruthenium complex in solution as well as when anchored onto TiO surface are identical Cou-S-CHO were synthesized as per the reaction procedures reported in the literature [23, 24]. The C=Cdoublebond except for a slight red shift in the absorption maxima due was introduced at 4, 4 positions of the bipyridine ligand, to the interaction of anchoring groups with the surface as starting from Bpy-phosphonate with Cou-S-CHO using well as further broadening [32]. The emission spectra of RD- a Wittig-Horner reaction [27]. The ligand Bpy-Cou (L) Cou were measured in ethanol solvent at room temperature was completely characterized by using elemental analysis, and are shown in Figure 3. Excitation of lower energy Mass, IR, and H NMR spectroscopies. Finally, the RD-Cou MLCT transition of RD-Cou sensitizer produces an emission complex was synthesized by refluxing p-cymene complex centered at 690 nm. The excited singlet state energy (E ) 0-0 of RD-Cou was calculated from the onset of absorption and Bpy-Cou (L) in ethanol:chloroform mixture to get the corresponding chlorocomplex. The chlorocomplex with spectrum and was found 1.65 eV. However, the emission of Bpy-acid and aq. ammonium thiocyanate refluxed in DMF RD-Cou sensitizer was quenched when adsorbed onto the to get the desired complex after sephadex column purifi- TiO film as a consequence of electron injection from the cation. The complex RD-Cou was completely characterized excited state of Ru(II) into the conduction band of TiO . Absorbance Advances in OptoElectronics 5 Table 1: UV-visible emission and electrochemical data. λ ,nm, E VversusSCE max 1/2 b d ∗ Sensitizer λ ,nm E E ,eV −1 a em 0-0 ox −1 ε(mol cm ) Ox Red RD-Cou 498 (16.046) 690 0.72 −1.05 −1.47 1.65 −0.93 Z-907 540 (10,040) 720 0.65 −0.98 −1.64 1.62 −1.02 a b c Solvent: ethanol, Error limits: λ , ±1nm, ε ± 10%. Solvent: ethanol, λ , ±1nm. Solvent: DMF, Error limits: E ±0.03 V, 0.1 M TBAP. Error limits: max max 1/2 0.05 eV. 2 20 With a view to evaluate HOMO-LUMO levels of RD- Cou, we have carried out the electrochemistry by adopting the cyclic and differential pulse voltammetric techniques in acetonitrile solvent using 0.1 M tetrabutylammonium per- chlorate as supporting electrolyte and compared their data with that of the standard sensitizer Z-907 in Table 1. When 1 10 the potential is scanned between 0 and 1.0 V, chemically reversible redox waves with formal potentials at 0.65 and 0.72 V (versus SCE) were observed, which can be attributed to the one-electron oxidation of ruthenium center in both dyes. Compared to the standard Z-907 dye, the metal 0 0 300 400 500 600 700 800 center oxidation of RD-Cou is anodically shifted by 70 mV Wavelength (nm) indicating the electron-poor character of the new ligand as a result of the insertion of Coumarin moiety on extended Figure 3: Absorption (—) and emission spectra in ethanol and π-conjugation. It also undergoes two reductions at −1.05 absorption (- - -) spectra adsorbed onto a 6 μm thick TiO film of and 1.47 V, corresponding to the one electron reduction of RD-Cou. anchoring ligand. Furthermore, for a sensitizer in DSSC, the LUMO energy level should be compatible with the conduc- mixture of 1-propyl-3-methylimidazolium iodide/1-Ethyl-3- tion band edge energy of the TiO photoanode (0.80 V versus methyl-imidazolium tetracyanoborate [PMII/EMIB(CN) ] SCE), and its HOMO should be sufficiently low in energy to − − (Z580) and compared its performance with that of standard accept electrons from the I /I -based redox electrolyte (0.2 V sensitizer Z-907 under similar test cell conditions. The versus SCE). The excited oxidation potential of RD-Cou is additive guanidinium thiocyanate in redox electrolyte is −0.93 V and that of standard sensitizer Z-907 is −1.02 V, to improve the V by reducing the dark current [33]. oc which is above the conduction band of TiO .In Z-907, the We assume that in our case, also guanidinium thiocyanate excited state oxidation potential is sufficiently higher than in Z580 redox electrolyte is responsible for improve- that in RD-Cou, as a result electron injection into the TiO ment in V . 1-Ethyl-3-methylimidazolium tetracyanoborate oc conduction band is more efficient, and it should lead to a (EMIB(CN) ) is an ionic liquid of a low viscosity (19.8 cP better conversion efficiency than RD-Cou. at 20 C) and high chemical and thermal stability. By using To know the electronic distribution of RD-Cou sen- this redox electrolyte with this composition, Gratzel and sitizer, we performed DFT calculations of the electronic coworkers have observed an efficiency of 6.4% [15]. Figure 5 ground state of RD-Cou sensitizer using mPW1PW91 illustrates the photocurrent action spectra of RD-Cou method for the geometry optimization with LANL2DZ basis and Z-907, where the incident monochromatic photon-to- function on Ru and 6–31 g(d) basis function on C, H, N, O current conversion efficiencies (IPCE) values are plotted as a and S. As can be seen from the Figure 5, occupied orbitals function of excitation wavelength. The IPCE was calculated HOMO, HOMO-1 and HOMO-2, have the electron delocal- according to the following equation: ized over the Ru(II) metal and –NCS ligand. The LUMO, LUMO+1, and LUMO+2 are π orbitals delocalized over sc the bipyridine carboxylic acid ligand facilitating electron IPCE(%) = 1240 × 100, (1) injection from the excited state of RD-Cou sensitizer to φ the conduction band of TiO . These results are in good where λ is the wavelength (nm), J is the photocurrent agreement with other ruthenium(II) polypyridyl complexes SC density under short circuit conditions (mA/cm ), and φ is reported in the literature [16, 17]. the incident radiative flux (mW/cm ). We have observed IPCE values of 60 and 68% using RD-Cou and Z-907 3.2. Photovoltaic Measurements. The performance of newly sensitizers, respectively. From Figure 4, it is clear that the synthesized RD-Cou as a sensitizer with a sandwitch-type photocurrent action spectrum resembles the absorption nanocrystalline TiO was determined from measurements spectra except for a slight red shift by ca. 10 nm in both RD- on photovoltaic cells using an ionic liquid redox electrolyte, Cou and Z-907. The photoresponse of thin films displays a that is, 0.2 M I , 0.5 M guanidinium thiocyanate (GuSCN), broad spectral response covering the entire visible spectrum and 0.5 M N-methyl benzimidazole (NMB) in a 65/35 v/v% up to 800 nm in both the sensitizers. Absorbance Emission intensity (a.u.) 6 Advances in OptoElectronics RD-Coumarin HOMO HOMO−1 HOMO−2 HOMO−3 HOMO−4 LUMO LUMO+1 LUMO+2 LUMO+3 LUMO+4 Figure 4: Molecular orbital spatial orientation of RD-Cou. 80 12 400 500 600 700 800 0 0.1 0.2 0.3 0.4 0.5 0.6 Wavelength (nm) Voltage (V) Figure 5: Photocurrent action spectra of (brown line) RD-Cou and Figure 6: Current-voltage characteristics: (brown line) RD-Cou (blue line) Z-907 using Z-580 redox electrolyte. and (blue line) Z-907 using Z-580 redox electrolyte. Table 2: Photovoltaic performance of RD-Cum and Z-907 . b c c c Sensitizer Electrolyte J (mA/cm ) V (mV) ff η(%) sc oc V is the open-circuit voltage, and ff represents the fill oc RD-Cou Z580 8.80 650 0.68 4.24 factor.Wehaveobservedanoverall conversion efficiency of Z-907 Z580 11.97 650 0.68 5.20 4.24% under 1.0 sun irradiation (J = 8.80 mA/cm , V sc OC a 2 b Photoelectrode: TiO (10 + 5 μm and 0.74 cm ); electrolyte: 0.2 M 2 = 650 mV, ff = 0.68) using RD-Cou as sensitizer. Under I , 0.5 M guanidinium thiocyanate (GuSCN), and 0.5 M N-methyl ben- similar test cell conditions, the device based on Z-907 zimidazole (NMB) in a 65/35 v/v% mixture of 1-propyl-3-methylimid- −2 sensitizer [J = 11.97 mA cm , V = 650 mV, and ff = sc OC azolium iodide/1-Ethyl-3-methyl-imidazolium tetracyanoborate [PMII/ c 2 0.68] shows a photovoltaic conversion efficiency of 5.20%. EMIB(CN) ]. Error limits: J : ±0.20 mA/cm , V = ±30 mV, ff = ±0.03. 4 sc oc The low efficiency of RD-Cou when compared with that of standard sensitizer Z-907 is probably due to poor hole Figure 6 shows the photocurrent-voltage characteristics transport property from coumarin moiety to Ru(II). of RD-Cou and Z-907 using Z-580 as redox electrolyte under 1.0 sun irradiation (1000 W/m ), and corresponding data are 3.3. Thermal Studies. We have examined the thermal stability shown in Table 2. The solar-energy-to-electricity conversion of new ruthenium(II) polypyridyl sensitizer and compared efficiency (η), under white-light irradiation can be obtained their thermal stability with that of the standard sensitizer from the following equation: Z-907, using thermogravimetric analysis. Figure 7 shows the −2 J Am · V [V] × ff thermal behavior of RD-Cou. From the Figure, it is clear sc OC η [%] = × 100, (2) −2 [ ] that the sensitizer RD-Cou is stable up to 220 C. The initial I Wm weight loss between 200 to 250 C is attributed to the removal −2 where I is the photon flux (e.g., 1000 W m for 1.0 sun), J 0 sc of the carboxyl group. In contrast, the standard sensitizer Z- is the short-circuit photocurrent density under irradiation, 907 is stable up to 200 C. IPCE (%) Current (mA/cm ) Advances in OptoElectronics 7 [4] M. Gratz ¨ el, “Recent advances in sensitized mesoscopic solar 0.001 cells,” Accounts of Chemical Research, vol. 42, no. 11, pp. 1788– 0.0005 1798, 2009. [5] A. Jager ¨ -Waldau, “Photovoltaics and renewable energies in −0.0005 60 Europe,” Renewable and Sustainable Energy Reviews, vol. 11, −0.001 no. 7, pp. 1414–1437, 2007. 40 −0.0015 [6] G. Zhang, H. Bala, Y. Cheng et al., “High efficiency and stable −0.002 dye-sensitized solar cells with an organic chromophore featur- −0.0025 ing a binary π-conjugated spacer,” Chemical Communications, −0.003 no. 16, pp. 2198–2200, 2009. [7] T. Bessho, S. M. Zakeeruddin, C. Y. Yeh, E. W. G. Diau, 50 100 150 200 250 300 350 400 450 500 550 ◦ and M. Gratz ¨ el, “Highly efficient mesoscopic dye-sensitized ( C) solar cells based on donor-acceptor-substituted porphyrins,” Conversion Angewandte Chemie—International Edition, vol. 49, no. 37, pp. 6646–6649, 2010. ( C) (%) 28.56 0 [8] L.Giribabu, Ch.Vijaykumar,P.Y.Reddy,J.H.Yum,M. 91.74 1.07 Gratz ¨ el, and M. K. Nazeeruddin, “Unsymmetrical extended π- 154.92 1.79 conjugated zinc phthalocyanine for sensitization of nanocrys- 218.11 3.12 talline TiO films,” Journal of Chemical Sciences, vol. 121, no. 281.29 12.07 1, pp. 75–82, 2009. 344.47 25.83 407.66 41.61 [9] P.Y.Reddy,L.Giribabu, C. Lyness et al., “Efficient sensiti- 470.84 48.28 zation of nanocrystalline TiO films by a near-IR-absorbing 534.02 50.73 unsymmetrical zinc phthalocyanine,” Angewandte Chemie— 597.2 51.76 International Edition, vol. 46, no. 3, pp. 373–376, 2007. [10] M. Gratz ¨ el, “Photoelectrochemical cells,” Nature, vol. 414, no. Figure 7: TG/DTG curves of RD-Cou with heating rate of −1 6861, pp. 338–344, 2001. 10 Cmin under nitrogen. [11] M. K. Nazeeruddin, P. Pec ´ hy, T. Renouard et al., “Engineering of efficient panchromatic sensitizers for nanocrystalline TiO - based solar cells,” Journal of the American Chemical Society, vol. 4. Conclusions 123, no. 8, pp. 1613–1624, 2001. [12] M. K. Nazeeruddin, A. Kay, I. Rodicio et al., “Conver- In conclusion, we have designed and synthesized a new sion of light to electricity by cis-X bis(2,2 -bipyridyl-4,4 - Coumarin-Ruthenium(II) polypyridyl complex having an dicarboxylate)ruthenium(II) charge-transfer sensitizers (X = extended π-conjugation. The new complex was completely − − − − − Cl ,Br ,I ,CN , and SCN ) on nanocrystalline TiO characterized by elemental analysis, ESI-MS, IR, UV-Visible, electrodes,” Journal of the American Chemical Society, vol. 115, and fluorescence spectroscopies as well as cyclic voltamme- no. 14, pp. 6382–6390, 1993. try. The performance of new sensitizer was tested in dye- [13] B. O’Regan and M. Gratz ¨ el, “A low-cost, high-efficiency solar sensitized solar cells using a durable redox electrolyte and cell based on dye-sensitized colloidal TiO films,” Nature, vol. compared with that of standard sensitizer Z-907. The low 353, no. 6346, pp. 737–740, 1991. efficiency of device based on RD-Cou, when compared to [14] P. Wang, S. M. Zakeeruddin, J. E. Moser, M. K. Nazeeruddin, Z-907, is probably due to poor hole transport property from T. Sekiguchi, and M. Gratz ¨ el, “A stable quasi-solid-state dye- coumarin moiety to Ru(II). sensitized solar cell with an amphiphilic ruthenium sensitizer and polymer gel electrolyte,” Nature Materials,vol. 2, no.6, pp. 402–407, 2003. Acknowledgments [15] D. Kuang, P. Wang, S. Ito, S. M. Zakeeruddin, and M. The authors are thankful to the IICT-Aisin Cosmos col- Gratz ¨ el, “Stable mesoscopic dye-sensitized solar cells based on tetracyanoborate ionic liquid electrolyte,” Journal of the laborative project for financial support of this work. L. American Chemical Society, vol. 128, no. 24, pp. 7732–7733, Giribabu is thankful to the project SR/S1/IC21/2008 for partial financial support of this work. V. K. Singh and Ch. [16] P. Wang, C. Klein, R. Humphry-Baker, S. M. Zakeeruddin, V. Kumar are thankful to Council of Scientific and Industrial and M. 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