Substitution of Ethynyl-Thiophene Chromophores on Ruthenium Sensitizers : Influence on Thermal and Photovoltaic Performance of Dye-Sensitized Solar Cells

A new high molar extinction coefficient ruthenium(II) bipyridyl complex, “Ru(2,2-bipyridine-4,4′-dicarboxylic acid)(4,4′-bis((3hexylthiophen-2-yl)ethynyl)-2,2′-bipyridine)(NCS)2 (N(C4H9)4), MC101” was synthesized and fully characterized by 1H-NMR, ESI-MASS, FT-IR, UV-Vis., and fluorescence spectroscopes. The dye showed relatively high molar extinction coefficient of 25.0 × 103 M−1 cm−1 at λ maximum of 544 nm, while the reference C101 has shown 15.8 × 103 M−1cm−1 at λ maximum 528 nm. The monochromatic incident photon-to-collected electron conversion efficiency of 44.1% was obtained for MC101 over the entire visible range, while the C101 sensitized solar cell fabricated and evaluated under identical conditions exhibited 40.1%. The DSSCs fabricated with 0.54 cm2 active area TiO2 electrodes and high efficient electrolyte (E01), from the sensitizers MC101 and C101 exhibited energy conversion efficiencies of 3.25% (short-circuit current density (JSC) = 7.32 mA/cm2, VOC = 610 mV, ff = 0.725) and 2.94% (JSC = 6.60 mA/cm2; VOC = 630 mV; ff = 0.709), respectively, under air mass of 1.5 sunlight.


Introduction
Photovoltaic (PV) cells generating clean electricity are now getting ready for significant market expansion in this new millennium, as the solar energy is the major renewable energy source and the major alternative to the fast depleting and polluting fossil fuels [1].In the past decades, low-cost excitonic solar cells attracted worldwide attention among academic and industrial players as potential candidates for the future PV market [2].Among this class of organic photovoltaics, the mesoscopic dye-sensitized solar cell [3][4][5] (DSSC) has achieved a respectable high-efficiency [6,7] and a remarkable stability under the prolonged thermal and light-soaking dual stress [8][9][10][11][12].The record efficiency of ∼11% [6,7] in DSSCs measured under the air mass 1.5 global (AM 1.5 G) illumination is achieved with the wellknown N719 sensitizer employing a volatile acetonitrilebased electrolyte.However, stability under prolonged heating at 80 • C has proved too hard to reach with the high-efficiency N719-based cells.In 2003, a thermally stable, ∼7% efficiency DSSC [9] was disclosed, employing the amphiphilic Z907 sensitizer [8] and a 3-methoxypropionitrile-(MPN-) based electrolyte avoiding lithium salts as additives.However, the molar extinction coefficient of this sensitizer is somewhat lower than that of the standard N719 dye.Meanwhile, a compromise between efficiency and high temperature stability has been noted for the Z907 sensitizer [13].For commercial applications of DSSCs, it is necessary to employ nonvolatile or even solvent-free electrolytes.However, with a lowfluidity electrolyte, the charge collection yield becomes low due to the shortened electron diffusion length.Enhancing the optical absorptivity of a stained mesoporous film can counter this effect.Thus, Wang et al. initiated the concept of developing high molar extinction coefficient, amphiphilic ruthenium sensitizer, [14] followed by other groups, [15][16][17][18][19][20] with a motivation to enhance device efficiency of DSSCs.In this context, sensitizers such as K19 [10,11] and its analogue K77 [12] have been successfully developed to fabricate thermally stable DSSCs showing ∼8 and ∼8.5% initial efficiencies, respectively.The sensitizer C101 with a molar extinction coefficient of 15.7 × 10 3 M −1 cm −1 demonstrated benchmarks under the illumination of AM 1.5 G full sunlight, such as a 11.0% efficiency along with an acetonitrile-based electrolyte, a longterm stable >9% device using a low volatility electrolyte, and a long-term stable 7.4% device employing an ionic liquid electrolyte [20].Chen et al. [19] communicated a similar sensitizer like C101 with a molar extinction coefficient of 15.7 × 103 M −1 cm −1 , showing an efficiency of 7.39% with a highly volatile acetonitrile-based electrolyte.

Advances in OptoElectronics
As a part of our ongoing programme on the development of various metal free organic, phthalocyanine, and ruthenium sensitizers for applications in DSSCs [21][22][23][24][25][26], we recently reported a new record high efficiency DSSC using novel coadsorbents [27].In this paper, we present a new promising sensitizer, coded MC101 (Figure 1) wherein the ancillary ligand, 4,4 -bis(5-hexylthiophen-2-yl)-2,2 -bipyridine (L2), of C101 is replaced with 4,4 -bis((3-hexylthiophen-2-yl)ethynyl)-2,2 -bipyridine (L1).The alkynyl thiophene group incorporates increased conjugation in the ancillary ligand, as the triple bond conjugated to double bonds (in the thiophene moiety) in order to achieve a higher molar extinction coefficient.The electron transfer is expected to be faster in this ligand due to the linear nature of alkynyl group and hence more stable than the isomerization (cis-trans) prone double bonds.We describe the synthesis and complete characterization of MC101 sensitizer and the photovoltaic performance of DSSC of MC101 in combination with high efficiency (E01) electrolyte.The photovoltaic characteristics were compared with that of C101.

Synthesis and Characterization.
All solvents and reagents, unless otherwise stated, were of Laboratory Reagent Grade and used as received.Brucker 300 Avance 1 H-NMR spectrometer run at 500 MHz was employed to record the 1 H NMR spectrum.Shimadzu LCMS-2010EV model with ESI probe was employed for MASS analysis.Shimadzu UV-Vis spectrometer (model 1700) and Fluorolog 3, J.Y.Horiba Fluorescence spectrometer were employed to record the electronic absorption and emission spectra.The 4,4 -dibromo-2,2 -bipyrdine, procured from Heterocycles and Catalysts, Gundeldingerstrasse 174, CH-4053 Basel, Switzerland, was used as received.The 3-bromo-thiophene, procured from Sigma-Aldrich.

Synthesis of 3-hexylthiophene (3)
. Mg (0.208 g, 8.58 mmol) was suspended in dry diethyl ether (15 mL) at 0 • C under nitrogen atmosphere.To this suspension, a solution of 1-bromohexane (1.03 mL, 7.35 mmol) in diethyl ether (5 mL) was added drop wise and the mixture stirred at room temperature for 2 hours then Grignard reagent thus obtained was added in portions to a solution of 3bromothiophene (1.000 g, 6.13 mmol) and [NiCl 2 (dppp) 2 ] (0.07 mg, 0.12 mmol) in diethyl ether and the mixture stirred for another 3 hours at room temperature.The reaction mixture was then quenched with water, the organic layer separated, and the aqueous layer extracted with diethyl ether.The combined organic layers were dried over sodium sulfate and the solvent removed under reduced pressure.The crude compound was purified by column chromatography (silica gel) eluting with hexane to give yellow oil (0.926 g, 90%). 1 H NMR (CDCl 3 , 25

Synthesis of Ruthenium (II) Complex MC101. 4,4bis [(3-hexylthiophene
)ethynyl]-2,2 -bipyridine (0.085 g, 0.163 mmol) and dichloro(p-cymene)ruthenium(II) dimer (0.050 g, 0.082 mmol) in dry DMF were heated at 60 • C for a period of 4 hours under argon in the dark.Subsequently, 4,4 -dicarboxylic acid-2,2 -bipyridine (0.040 g, 0.163 mmol) was added and the reaction mixture was heated to 140 • C for another 4 hours.To the resulting dark green solution was added NH 4 NCS (0.193 g, 2.53 mmol) in water (2 mL), and the reaction mixture was further heated for 3 hours at 150 • C. DMF was removed on rotoevaporatour under high vacuum, and water (250 mL) was added to get the precipitate.The solid was filtered off, washed with water and diethyl ether, and dried under vacuum.The crude compound was dissolved in methanol and dichloromethane and further purified on the sephadex LH-20 with methanol/dichloromethane (60/40 v/v) as eluent about three times to get pure MC101 as dark brown solid (0.090 g, 60%). 1 H NMR (CDCl 3 , 25

Absorption, Emission, and Electrochemical Properties.
The electronic absorption spectrum of MC101 was recorded in ethanol and was compared with that of reference C101 (Figure 2(a)).The new sensitizer showed one intense absorption band at 544 nm, which is assigned to the metal toligand charge transition, and a relatively high intensity shoulder-type absorption band was observed at 410 nm.MC101 sensitizer showed molar extinction coefficient of 25,000 M −1 cm −1 at λ maximum of 544 nm, while the reference C101 has shown 15,800 M −1 cm −1 at λ maximum of 528 nm under comparable conditions.As compared to C101, the new sensitizer showed increased molar extinction coefficient with bathochromic shift in the absorption spectrum by 16 nm.The introduction of -C≡C-bonds through 4,4 -bis [(3-hexyl thiophene)ethynyl]-2,2 -bipyridine (L1) in MC101 complex exhibits high light harvesting ability as compared to 4,4 -bis(5-hexylthiophen-2-yl)-2,2 -bipyridine (L2) in C101 sensitizer by enhancing their molar extinction coefficient and by shifting the absorption spectrum towards IR region, which could be the contribution of extended -C≡C-of L1 relative to L2 [21][22][23][24][25][26].Incorporation of alkyl thiophenes conjugated with bipyridine through -C≡C-into the ruthenium(II) pyridyl complex act as electron donor and consequently on excitation of the sensitizer, a faster charge transfer from HOMO to LUMO could be expected.
Besides the molar extinction coefficient and the absorption range, the size and geometrical structure of the sensitizer also influence the photovoltaic performance of DSSC.In order to see the quantity of the new sensitizer absorbed on TiO 2 films relative to C101 sensitizer, the absorption measurements over TiO 2 films were carried out by staining 7 μm thick TiO 2 electrodes in 0.3 mM dye solutions prepared in ethanol (Figure 2(b)).Prior to cooling the TiO 2 electrodes, the dye solutions are sonicated for 10-15 minutes, and then TiO 2 electrodes, at around 100 • C, are immersed into the dye solutions and allowed to soak for 16 hours under the dark.After that the electrodes were washed with ethanol to remove unadsorbed dye molecules and dried under nitrogen purging.As compared to C101, the new sensitizer is expected to show little lower film absorption over TiO 2 films due to its relatively larger size of the dye molecule, and the absorption measurements over TiO 2 films showed relatively increased film absorption for MC101 as compared to C101.To compare the anchoring pattern and surface morphology of the new sensitizer, the absorbance maxima of low energy absorption band of MC101 sensitizer are normalized by the corresponding absorbance maxima of C101, while their molar extinction coefficients are normalized by the molar extinction coefficient of C101.The film absorbance ratio of MC101 sensitizer calculated is lower than that of C101 indicating its lower packing density of the dye molecules on TiO 2 surface relative to C101.As expected, the lowest packing density of these sensitizers could be probably due to relatively little larger molecular diameter.
The cyclic voltammetry measurements of the new sensitizer was conducted to ensure that the LUMO of the new complex is suitable for injecting electrons into the conduction band of TiO 2 and also whether their HOMOs match the energy level of the I 2 /I − 3 redox couple.The cyclic voltammogram of the dye was measured 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 shows the voltammogram of MC101 dye in acetonitrile.Reversible redox was observed for the sensitizer, similar to other ruthenium(II) pyridyl complexes [28].The measured oxidation and reduction potentials are 0.820 and −0.80 V, respectively.The more positive potential of the sensitizer, relative to I − /I − 3 redox couple (0.24 V versus SCE) in the electrolyte, provides a large thermodynamic driving force for the regeneration of the dye by iodide.Based on the absorption and emission spectra of the complex, the excitation transition energy (E 0−0 ) is estimated to be 1.957 (obtained by converting the wavelengths of 634 nm, at which their individual absorption and emission spectra intersect, into electron volts).The standard potential (φ 0 (S + /S * )) calculated from the relation of [φ 0 (S + /S) = φ 0 (S + /S * ) −E 0−0 ] is −1.130V versus SCE.So, φ 0 (S + /S * ) value is more negative (or higher in energy) than the conduction band edge of TiO 2 (−0.8 V versus SCE) providing ample thermodynamic driving force to inject electrons from the dye to TiO 2 .

Computational Studies.
The electronic ground-states of fully protonated MC101 was optimized in gaseous phase by using the Density Functional Theory (DFT) with MPW1PW91 method and lanl2dz basis set for ruthenium and 6-31G(d) for H, C, N, O, S atoms as implemented in Gaussian 09W and Gaussian View 5 interface software Supporting electrolyte is 0.1 M tetrabutylammonium perchlorate in acetonitrile.[29,30].Under identical conditions, the reference C101 was also optimized to compare electronic distribution in unoccupied and occupied frontier molecular orbitals.In order to see the influence of the ancillary bipyridine ligands 4,4bis((3-hexylthiophen-2-yl)ethynyl)-2,2 -bipyridine (L1) relative to 4,4 -bis(5-hexylthiophen-2-yl)-2,2 -bipyridine (L2) on optoelectronic properties of the corresponding ruthenium(II) complexes, the geometrical optimization of the electronic ground-states of L1 and L2 was also studied.Isodensity surface values are fixed at 0.04 for ruthenium(II) complexes, while for the ancillary ligands, the isodensity surface values are fixed at 0.02.The occupied (HOMO to HOMO-4) and unoccupied (LUMO to LUMO+4) frontier orbitals of L1 are shown in Figure 4.The HOMO and HOMO-1 orbitals of L1 are degenerate and have π-orbitals over bipyridine moiety and thiophen-2-yl, while in HOMO-2 orbital, the π-orbitals are almost localized on bipyridine.The HOMO-3 and HOMO-4 orbitals are again degenerate and the π-orbitals are localized over thiophene moieties.In case of LUMO and LUMO+1 orbitals of L1, the π * -orbitals are delocalized over π-system with maximum components on the bipyridine, which depresses their energy levels relative to those of L2.In case of LUMO+3 and LUMO+4, π *orbitals are localized on bipyridine and thiophen-2-yl.
The electronic distributions in unoccupied and occupied frontier molecular orbitals of MC101 are shown in Figure 5, while the corresponding orbitals of C101 calculated under similar conditions are shown in Figure 6.As expected, the first highest occupied molecular orbital (HOMO) of MC101 exhibits ruthenium t2g character with size mixing from thiocyanate ligand, while π-clouds for other HOMO-1 and HOMO-2 orbitals are considered with size mixing from one of thiocyanate ligands.In case of C101, the first three occupied (HOMO to HOMO-2) orbitals follow t2g character with size mixing from thiocyanate ligand.The π-clouds for HOMO-3 orbital for both the dyes are nonbonding combination localized on the NCS ligand.HOMO-4 and HOMO-5 orbitals of MC101 resemble those of corresponding orbitals of C101, in which π-clouds have Similar to LUMO of C101, the electron distribution in LUMO of MC101 moves toward the anchoring groups; while assuming similar molecular orbital geometry when adsorbed on TiO 2 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.In case of higher unoccupied molecular orbitals of the ancillary bipyridyl ligand, L1, the π * -orbitals move from π-system to bipyridine moieties, and these transfers in MC101 significantly depress the LUMO+3 and LUMO+4 orbitals and a moderate extending in case of LUMO+1 orbital relative to those of C101.The calculated dipole moment for MC101 is 28.7.Debye is larger as compared to that of C101 (18.2 Debye).The increased dipole moment in gaseous phase signifies the formation of nearly stable geometry and facilitates better charge transfer for MC101 as compared to C101.

Thermal Stability.
Although a very good conversion efficiency was obtained, the introduction of -C≡C-acetylene groups through 4,4 -bis(5-hexylthiophen-2-yl)-2,2bipyridine in MC101 complex may severely influence the thermal stability relative to C101 sensitizer.Hence the TGA analysis of L1, L2, and the corresponding ruthenium(II) complexes was performed using a TGA/SDTA 851 e thermal system (Mettler Toledo, Switzerland) at heating rate of 10 • C/min in the temperature range of 25-600 • C under N 2 atmosphere (flow rate of 30 mL/min).Film samples ranging from 8 to 10 mg were placed in the sample pan and heated, while weight losses are recorded against temperature difference.The thermograms of MC101 and C101 dyes are shown in (Figure 7), in which the derivative of % conversion plotted against temperature.The thermograms show gradual decomposition of MC101 complex as compared to C101, while the ligands are thermally stable.

Conclusions
The new MC101 sensitizer with a new ancillary bipyridine ligand has several advantages.(1) Thiophene is a  more electron-rich moiety; incorporation of thiophene onto bipyridine ligands can raise the energy levels of the metal center and the LUMO of the ligands.(2) Facile functionalization of thiophene groups also offers relatively efficient synthetic solutions to solubility, polarity, and band-gap tuning.(3) The alkynyl thiophene can be considered as bisacetylene moiety, where one acetylene is bridged with sulfur atom effectively providing aromatic stability while other free acetylene helps in the extension of conjugation for red shifting the absorption as well as increasing the molar extinction coefficient.Thus MC101 showed higher molar extinction coefficient combined with red shift in absorption and comparable performance with C101 sensitized solar cells, fabricated and evaluated under similar conditions.The TG analysis showed relatively decreased thermal stability for MC101 as compared to C101.
The incident photon-to-current conversion efficiency (IPCEs) of the DSSC constructed based on this dye is shown in Fig-ure 8(a).The monochromatic incident photonto-collected electron conversion efficiency of 44.1% was obtained for over the entire visible range, which is less than unity suggesting a low charge collection yields.Under comparable conditions, C101 exhibited IPCE value of 40.1%.The photo current-voltage measurements were executed under Air Mass (AM) of 1.5 sunlight, and the the typical current density versus voltage curve is shown in Figure 8(b).The energy conversion efficiency of 3.25% (shortcircuit current density (J SC ) = 7.32 mA/cm 2 , V OC = 610 mV, ff = 0.725) was obtained for MC101 sensitizer, while under comparable conditions, C101 exhibited conversion efficiency of 2.94%(J SC ) = 6.60 mA/cm 2 , V OC = 630 mV, and ff = 0.709) under Air Mass of 1.5 sunlight.