Substitution of Carbazole Modified Fluorenes as π-Extension in Ru ( II ) Complex-Influence on Performance of Dye-Sensitized Solar Cells

A new high molar extinction coefficient ruthenium(II) bipyridyl complex “cis-Ru(4,4′-bis(9,9-dibutyl-7-(3,6-di-tert-butyl-9Hcarbazol-9-yl)-9H-fluoren-2-yl)-2,2′-bipyridine)(2,2′-bipyridine-4,4′-dicarboxylic acid)(NCS)2, BPFC” has been synthesized and characterized by FT-IR, 1H-NMR, and ESI-MASS spectroscopes. The sensitizer showed molar extinction coefficient of 18.5 × 10 M−1cm−1, larger as compared to the reference N719, which showed 14.4 × 10 M−1cm−1. The test cells fabricated using BPFC sensitizer employing high performance volatile electrolyte, (E01) containing 0.05 M I2, 0.1 M LiI, 0.6 M 1,2-dimethyl3-n-propylimidazolium iodide, 0.5 M 4-tert-butylpyridine in acetonitrile solvent, exhibited solar-to-electric energy conversion efficiency (η) of 4.65% (short-circuit current density (JSC) = 11.52 mA/cm2, open-circuit voltage (VOC) = 566 mV, fill factor = 0.72) under Air Mass 1.5 sunlight, lower as compared to the reference N719 sensitized solar cell, fabricated under similar conditions, which exhibited η-value of 6.5% (JSC = 14.3 mA/cm2, VOC = 640 mV, fill factor = 0.71). UV-Vis measurements conducted on TiO2 films showed decreased film absorption ratios for BPFC as compared to those of reference N719. Staining TiO2 electrodes immediately after sonication of dye solutions enhanced film absorption ratios of BPFC relative to those of N719. Timedependent density functional theory (TD-DFT) calculations show higher oscillation strengths for 4,4′-bis(9,9-dibutyl-7-(3,6di-tert-butyl-9H-carbazol-9-yl)-9H-fluoren-2-yl)-2,2′-bipyridine relative to 2,2′-bipyridine-4,4′-dicarboxylic acid and increased spectral response for the corresponding BPFC complex.


Introduction
Dye sensitized solar cells (DSSCs) attracted intense attention among scientific as well as industrial organizations because of their high photon-to-electricity conversion efficiency and low cost compared to traditional photoelectrochemical cells [1][2][3][4][5].Since Graetzel introduced the first highly efficient nanocrystalline TiO 2 sensitized solar cell based on ruthenium(II) bipyridyl complex, N3 as sensitizer, there have been several modifications to improve the overall performance of the test cell devices [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22].Among all the components employed in DSSC, sensitizer plays a key role in photovoltaic performance in respect of efficiency and long-term durability.The important tunable properties of sensitizers for high efficient DSSCs are broad absorption (400 to 900 nm) and high molar extinction coefficient (thin films and solid state DSSCs), thermal and photochemical stability (long durable), compatibility with TiO 2 semiconductor conduction band (efficient electron injection) and redox electrolyte (efficient dye regeneration), nonplanar molecular structure, and so forth [6,8,[22][23][24].Thiophene containing oligomers have been extensively explored as the active organic materials for OFET applications due to the ease in chemical modification of the structures, allowing fine-tuning their optical and electronic properties [25].They exhibit high fieldeffect mobility, which have been related to both, the close packing through π-interactions and the high degree of local order of molecules [26].Thiphene oligomers display poor stability especially in the solid state, limiting their practical applications, where as fluorine-based oligomers showed both improved stability and lower HOMO level as compared to thiophene oligomers.Endcaping of oligofluorenes with diphenylamino group has been shown to offer advantages in terms of lowering their first ionization potentials, enhancing thermal stability and inducing good amorphous morphological stability [27].Thus, fluorene unit as the core is known to display interesting chemical and electronic characteristics.We have been engaged in our laboratory to synthesize durable and high efficient new organic, phthalocyanine as well as ruthenium(II) bipyridyl dyes, [22][23][24][28][29][30][31][32] and came across a report of super sensitizer, where carbazole was incorporated on ancillary bipyridyl through conjugation of thiophene moiety [33].And as a part of our continued efforts in this area of research, we became interested to synthesize a new ruthenium(II) bipyridyl complex BPFC by introducing 4,4 -bis(9,9-dibutyl-7-(3,6-di-tert-butyl-9Hcarbazol-9-yl)-9H-fluoren-2-yl)-2,2 -bipyridine as an ancillary ligand, and the influence of increased conjugation length on photovoltaic performance and thermal stability was evaluated relative to N719 sensitizer.Multiple performance increasing features such as alkyl groups (n-butyl and tbutyl), triarylamine equivalent (carbazole), biphenyl group in fluorene as extended conjugation have been incorporated in the new ruthenium(II) complex BPFC for achieving better overall performance.

Synthesis of 2-Bromofluorene (1).
To a solution of fluorene (0.500 g, 1.8 mmoL) in dry acetone was added N-bromosuccinimide (0.320 g, 1.8 mmol) under nitrogen atmosphere.After maintaining at 80 • C for 3 hours, cool to room temperature and ice water was added and then extracted with dichloromethane.The crude compound was purified on silica gel column chromatography using hexane/ethyl acetate mixture in 9/1 as eluent.Yield: 90% 1

Synthesis of 9,9-Dibutyl-9H-Fluoren-2-Ylboronic Acid (3).
To a 100 mL two neck glass flask containing 2 (0.500 g, 1.4 mmol) in dry THF (20 mL) and a magnetic stirrer bar at -78 • C, n-BuLi (1.05 mL, 1.05 mmol) was added under nitrogen atmosphere while maintaining good stirring.After stirring for 1 hour, triisopropylborate (0.484 mL, 2.1 mmol) was added.After stirring for further 2 hours, the reaction mixture was first quenched with water and then aqueous HCl (6 M, 20 mL) was added drop wise fashion until the solution turned acidic and then extracted with dichloromethane.The combined organic layers were dried over anhydrous sodium sulphate and concentrated with rotavapour.Purification was carried out by column chromatography on silica gel using hexane/ethyl acetate mixture (4/6 v/v) as eluent.Yield: 50%. 1

Synthesis of 4,4-Bis(9,9-Dibutyl
In a 25 mL one-necked round bottom flask equipped with a condenser were placed 3 (0.440 g, 1.375 mmol), barium hydroxide octahydrate (1.355 g, 4.297 mmol), and palladium tetrakis triphenyl phosphine (0.106 g, 0.091 mmol).The reaction flask was evacuated and filled with nitrogen gas, and is charged with 1.4-dioxane (4 mL), water (1.35 mL) and 4,4 -dibromo-2,2 -bipyridine (0.180 g, 0.573 mmol).The reaction mixture was refluxed for 24 hours under nitrogen gas and then cooled to room temperature.The dioxane was removed and the contents were poured into dichloromethane, the precipitate was removed through filter paper, and the organic layer was washed with 1 M-NaOH aqueous solution, NaCl (100 mL) and dried over sodium sulphate.After concentration on rotavapour, small quantity of methanol was added.The precipitate formed was filtered and purified on column chromatography with silica gel using mixture dichlorometh -ane/methanol, (9/1 v/v) to obtain the pure product as pale yellow solid Yield 50%.

Synthesis of 4,4-Bis(9,9-Dibutyl-7-Iodo-9H-Fluorene-2-yl)-[2, 2]-Bipyridine (5).
A mixture of 4 (0.200 g, 0.282 mmol), iodine (0.172 g, 0.677 mmol), conc H 2 SO 4 (0.036 mL) and water (0.013 mL) in glacial acetic acid (10 mL) were taken into 1-neck 100 mL round bottom flask.Then, the mixture was stirred at 80 • C under nitrogen gas for 4 hours, and then, the reaction mixture was cooled to room temperature.The solution was poured into large amount of ice cool water.The resulting mixture was extracted with dichloromethane and then washed with water and then dried over Na 2 SO 4 and concentrated on rotavapour, small quantity of methanol was added.The precipitate formed was separated and purified on column chromatography with silica gel using mixture (DCM/methanol, 9/1) to obtain the pure product as pale yellow solid Yield 45%. 1    between the nanocrystalline TiO 2 and the conducting FTO matrix, was coated over the cleaned plates using a 40 mM TiCl 4 aqueous solution, and then, the plates were heated at 70

Absorption and Emission
Properties.The electronic absorption spectrum of BPFC sensitizer recorded in ethanol is shown in Figure 2(a), and the spectrum was compared with that of the reference N719 sensitizer.The complex exhibited one absorption band at longer wavelength region and a shoulder type band in short wavelength region.The molar extinction coefficient of low-energy absorption band of BPFC is 18.5 × 10 3 M −1 cm −1 , which is larger as compared to that of reference N719 sensitizer that showed molar extinction coefficient of 14.4 × 10 3 M −1 cm −1 .Compared to N719, the increase in π-conjugation length by introduction of 4, 4 -bis(9,9-dibutyl-7-(3,6-di-tert-butyl-9H-carbazol-9-yl)-9H-fluoren-2-yl)-2,2 -bipyridine in BPFC complex increases the molar extinction coefficient and the spectral response of the new complex.The high-energy shoulder type absorption band of the complex with increased molar extinction coefficient could be attributed to the π-π * transitions of fluorene segments which exhibit strong absorption at around 400 nm.The high-energy absorption band of ruthenium(II)bipyridyl complexes is contributed by two components: one is metal to ligand charge transfer transition, while the other one is π-π * transitions of the ancillary bipyridyl ligand, L1.The 4,4 -bis(9,9-dibutyl-7-(3,6-di-tert-butyl-9H-carbazol-9-yl)-9H-fluoren-2-yl)-2,2 -bipyridine (L1) exhibits two absorption bands, one at around 303 nm and the other one at 322 nm and these characteristic absorption bands exists even in BPFC complex but slightly at higher wavelengths.This indicates that the π-π * transitions in the ancillary bipyridyl ligand, L1 are strong and significantly contributing to the high-energy absorption band of BPFC complex.The emission spectra of BPFC sensitizer was recorded in ethanol by exciting the complex with its absorption maxima of low-energy absorption band.The emission spectrum was analyzed by Gaussian reconvolution method to integrate emission peak to estimate the emission maxima.
Besides the molar extinction coefficient, the other important property, that is, quantity of dye absorped on TiO 2 films and the pattern of dye absorpsion, also influences the light-harvesting capability and the photovoltaic performance of DSSC devices.The molecular diagonal diameter and geometrical structure of any dye vary with respect to π-system substituted on one of the bipyridine moieties of N719.As compared to simple 2,2 -bipyridine-4,4 -dicarboxylic acid in N3, the 4,4 -bis(9,9-dibutyl-7-(3,6-di-tert-butyl-9H-carbazol-9-yl)-9H-fluoren-2-yl)-2,2bipyridine in BPFC complex largely increases the diagonal molecular size, and hence, it is expected to show significant effect on film absorptions of BPFC.Therefore, the film absorption measurements over TiO 2 surface were carried out by staining 7.0 μm thick TiO 2 (18 NRT layered) films Advances in OptoElectronics in 0.3 mM dye solutions prepared in ethanol for a period of 16 hours under dark.The absorption spectrum of BPFC-sensitized TiO 2 film was recorded and was compared with that of N719 sensitized TiO 2 film (Figure 2(b)).The measurements indicate that the dye molecules are anchored on TiO 2 surface, and the film absorbencies for both the dyes are similar.To compare the anchoring pattern and surface morphology of the new sensitizer, the absorbance maxima of low energy absorption band of BPFC sensitizer is normalized by the corresponding absorbance maxima of N719, while their molar extinction coefficients are normalized by the molar extinction coefficient of N719.The film absorbance ratio of BPFC calculated is lower than that of N719 indicating its lower packing density of the dye molecules on TiO 2 surface.Staining TiO 2 electrodes immediately after sonication of BPFC dye solution showed much higher film absorption ratios as compared to those TiO 2 films stained for prolonged soaking times in the BPFC dye solutions, and this indicates the necessity of sonication before staining the TiO 2 electrodes, particularly when sensitizers with larger diagonal diameter are employed.

Electrochemical Properties.
In dye-sensitized solar cells, favorite energy offset between dye and titania is a basic requirement for any high-efficiency solar cell, in which the sensitizer's immediate charge generation yield from the excited state has a direct influence on the performance of DSSC device.To measure the electrochemical properties of BPFC dye, cyclic voltammetry was employed using tetrabutyl ammonium perchlorate (0.1 M in acetonitrile) as an electrolyte and ferrocene as an internal standard at 0.42 V versus SCE (Figure 3).The oxidation and reduction potentials of BPFC are 0.79 V and −0.80 V, respectively.The more positive potential of the sensitizer, relative to I − /I − 3 redox couple in the electrolyte provide a large thermodynamic driving force for the regeneration of the dye by iodide.Based on absorption and emission spectra, the excitation transition energy (E 0−0 ) of BPFC was estimated to be 1.87 eV and the standard potential (ϕ 0 (S+/S * )) calculated from the relation of [ϕ 0 (S+/S) = ϕ 0 (S+/S * )-E 0−0 ] for the sensitizer was −1.08 V versus SCE.So, ϕ 0 (S+/S * ) value is more negative (or higher in energy) than the conduction band edge of TiO 2 providing a thermodynamic driving force to inject electron from the dye to TiO 2 .

Computational Studies.
In order to augment the molar extinction coefficient with the π-conjugation extension through 4, 4 -bis(9,9-dibutyl-7-(3,6-di-tert-butyl-9H-carbazol-9-yl)-9H-fluoren-2-yl)-2,2 -bipyridine (L1) in BPFC relative to 2,2 -bipyridine -4,4 -dicarboxylic acid in N719 complex, the electronic ground state of fully protonated complex is optimized.To see the influence of the ancillary ligand, L1 on the corresponding ruthenium(II) complex, the electronic ground state of the ligand was optimized using B3LYP/6-31G(d) method.Based on the optimized structures, TD-DFT calculations were performed to see the optical properties of the ligand and the corresponding ruthenium(II) complex.The unoccupied (LUMO+4 to LUMO) Potential (V) surface, the close position of the LUMO to the anchoring moieties is expected to enhance the overlap with the 3d orbitals of TiO 2 leading to favored electron injection.
2.6.Thermal Stability.One of the parameters desired to sustain the initial photovoltaic performance of the DSSC over a long period is the high thermal stability of the ruthenium(II) sensitizer.In order to evaluate the thermal stability of the new sensitizer relative to N719, TGA analysis were performed using a TGA/SDTA 851 e thermal system (Mettler Toledo, Switzerland) at heating rate of 10 • C/min in the temperature range of 50-600 • C under N 2 atmosphere (flow rate of 30 mL/min) and the influence of 4,4 -bis(9,9-dibutyl-7-(3,6-di-tert-butyl-9H-carbazol-9yl)-9H-fluoren-2-yl)-2,2 -bipyridine on thermal stability of BPFC complex was studied.Film samples ranging from 8 to 10 mg were placed in the sample pan and heated, while weight loss and temperature difference were recorded as a function of temperature.The thermogram of BPFC obtained under identical conditions with that of N719 was compared.Figure 6 shows the derivative of % conversion with respect to temperature, in which both the thermograms of BPFC and N719 initially follow similar trend up to around 200 • C and after this, BPFC loses its mass quickly and hence a decrease in thermal stability by around 50 • C was observed as compared to that of N719.This indicates that the substitution of 4,4-bis(9,9-dibutyl-9H-fluorene-2-yl)- [2,2] bipyridine reduces the thermal stability of the ruthenium(II) sensitizer.conversion efficiency due to lower J SC and lower IPCE of BPFC relative to that of N719 Table 1.

Conclusions
The new sensitizer BPFC was carefully designed considering the following (1) The alkylgroups (two n-butyl and two tbutyl on each pyridyl of ancillary ligand) not only increase the solubility of the sensitizer in organic solvents but also serve as electron donating apart from inhibiting water induced desorption of the sensitizer from the TiO 2 .(2) Carbazole is a triarylamine equivalent which is known to improve the efficiency of the sensitizer.(3) The biphenyl group in Fluorene moiety serves as extended conjugation for enhancing the molar extinction coefficient and provides aromatic stability to the molecule.(4) The bipyridine dicarboxylic acid provides excellent anchoring of the complex on to the TiO 2 surface facilitating easy electron injection.Thus, the multiple performance increasing features of the BPFC sensitizer makes the ruthenium(II) complex unique for DSSC application and showed solar-to-electric energy conversion efficiency (η) 4.6%, while under similar fabrication and measurement conditions, standard N719 showed 6.5% efficiency.The new ruthenium(II) bipyridyl sensitizer showed relatively high molar extinction coefficient.Upon  sensitization with nano-crystalline TiO 2 , the dye showed decreased film absorption ratios, whereas staining the TiO 2 electrodes with freshly sonicated dye solutions showed relatively increased film absorption ratios on TiO 2 surface.The lower solar-to-electrical energy conversion efficiency could be result of lower IPCE value and lower film absorption ratios.Design and development of super sensitizers with similar high performance features but with less bulky nature are under progress.
• C for 30 minutes.The working electrodes are composed with 9 μm thickness of 18 nm TiO 2 particles (D18T) as transparent layer over which 4.8 μm thickness of 400 nm anatase TiO 2 particles (CCIC, HPW-400) as scattering layer.The TiO 2 -coated films were gradually heated under an air flow at 325 • C for 5 minutes, at 375 • C for 5 minutes, at 450 • C for 15 minutes, and at 500 • C for 15 minutes.While cooling, when the temperature attained to around 90 • C, the electrodes were immersed in 0.3 mM dye solutions of ethanol and soaked for 16 hours under the dark.The dye-sensitized