Article

A novel route to 4,4’-disubstituted bipyridyl ligands in ruthenium complexes for dye-sensitized solar cells

Gylen Odlinga, Nina Chadwicka, Ewan Frost-Penningtona, D. Kishore Kumarb, Aruna Ivaturib, Hari M. Upadhyayab and Neil Robertsona,*

aSchool of Chemistry and EaStCHEM, University of Edinburgh, King’s Buildings, West Mains Road, Edinburgh EH93JJ, Scotland, UK

bEnergy Conversion Laboratory, Institute of Mechanical Process and Energy Engineering, School of Engineering and Physical Sciences, Heriot-Watt University, Riccarton, Edinburgh, EH14 4AS, Scotland, UK

A R T I C L E I N F O

Article history:

Received 00 December 00

Received in revised form 00 January 00

Accepted 00 February 00

Keywords:

Ruthenium(II) complex

Dye-sensitized solar cells

Nucleophilic aromatic substitution

A B S T R A C T

An in situ nucleophilic aromatic substitution reaction has been observed and applied as a simple route to introduce functionality to the 4,4’ positions of bipyridyl units coordinated to a Ru(II) centre. This has been used in the synthesis of a new Ru(II) sensitiser complex [RuII(4,4’-dicarboxy-2,2’-bipyridyl)(4,4’-dithiocyanate-2,2’-bipyridyl)dithiocyanate] 1 for use in dye-sensitised solar cells. 1 exhibits improved light harvesting in the 350-450 nm region in comparison with N719 and produced cells with power conversion efficiencies of 3.18 %.

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http://dx.doi.org/10.1016/j.rgo.2013.10.012

Meeting the global energy demand is one of the greatest challenges facing the world today. Solar energy represents the most abundant alternative to the traditional fossil fuel based energy economy and hence solar cells have garnered considerable interest in recent years. Relatively new to the field of solar technology are dye-sensitised solar cells (DSSCs) [1], which have the potential to fill a niche in the market for cheap and lightweight devices.

The operation of the DSSC relies in large part upon the dye. Ruthenium(II) bipyridyl dye complexes have been widely investigated since Grätzel & co-workers demonstrated the first cells with over 10% efficiency utilizing dyes such as [RuII(4,4’-dicarboxy-2,2’-bipyridyl)2(dithiocyanate] (N3) [2] or the doubly-deprotonated analogue (N719). Due to their success, many variations upon the N3 motif have been synthesised and characterised, often featuring mixed 4,4’-disubstituted bipyridyl ligands [3,4,5,6,7,8,9]. The preparation of these dye complexes is synthetically well-established [10], with the variety of dyes that can be prepared limited only to the number of bipyridyl ligands that can be conceived.

Here we report the unexpected observation in this context of an in situ nucleophilic aromatic substitution reaction and its application in ruthenium(II) dye complex synthesis. Our original intention was to synthesise ruthenium(II) dye complexes bearing 4,4’-dihalogenated ligands, full details of which will be published elsewhere [11]. However, synthesis of the 4,4’-difluorinated member of the series was not achieved, instead giving an alternate product 1 (Figure 1). Fluorine substituents in the 4,4’-positions on a bipyridyl ring are observed to be open to substitution by nucleophilic thiocyanate groups when the ring is bound to a ruthenium(II) centre. This reaction is closely related to known substitution reactions at fluoropyridines [12, 13] and while not originally intended here, represents a simple route to introduction of a novel functionality to the dye without the need for additional isolation and purification steps, or separate synthesis of a functionalized bipyridyl ligand. In addition to demonstrate this novel approach to dye synthesis, the light harvesting, electrochemical and cell performance properties of 1 are reported.

The ligands H2-dcbpy and 4,4’-F2-bpy were synthesised according to established literature procedures [14,15]. 1 was prepared using a typical procedure for Ru(II)-bipyridyl dyes with two dissimilar bipyridyl ligands (Figure 1). The reaction however, did not proceed as anticipated, with loss of the fluorine substituents conveniently observed in the 1H NMR spectrum of the dye, where coupling to 19F was not present in the final product (SI Figure 1). The groups replacing the fluorines were then identified as thiocyanate groups by ESI-MS and elemental analysis. This substitution was found to be activated by the presence of the ruthenium centre, as upon placing the free 4,4’-F2-bpy ligand in a solution of excess NH4NCS in DMF and heating for 24 hours no reaction occurred and the intact ligand was recovered. This forms the basis for a proposed mechanism for this substitution (Figure 2).

Figure 3. UV-Vis absorption spectra of 1 and a N719 standard

Figure 2. Proposed nucleophilic aromatic substitution mechanism

Figure 1. Synthetic route to 1

The presence of the positively charged metal centre stabilises the intermediate as illustrated by the resonance description on the centre right, and as such explains the activation of this process by the metal centre. Interestingly, this substitution has only been observed for the fluorine analogue in this series, the –chloro and –bromo analogues have produced no evidence of this reaction [11]. This suggests that breaking of the C-X bond is not rate-limiting in this case, leaving the attack of the nucleophile as a possible rate-limiting step. While so far we have only observed this substitution reaction occurring with the thiocyanate ligand, we propose that other nucleophiles could similarly be applied in this manner, to give a wide variety of dyes without the need to synthesise and isolate a large number of bipyridyl ligands. Additionally, this mechanism may work with other Lewis acidic centres, and could potentially be applied in this manner to metal complexes other than ruthenium dyes for dye sensitised solar cells.

The UV-Vis absorption spectra of 1 and an N719 standard were recorded in methanol (Figure 3). 1 exhibits the typical intense π-π* transitions in the UV region and charge transfer bands across the visible [16]. However, most notably, improved absorption was observed over the 350-450 nm region due to a shoulder peak not present in N719. Solar irradiance is high over this wavelength range, hence improved absorption in this region is encouraging. In addition to absorption, the emission spectrum of 1 was recorded in DMF (SI Figure 2). An emissive peak at 790 nm was observed and from the onset of this peak the ground state-excited state energy gap (Eo-o) was calculated as 1.88 eV. A summary of the UV-vis absorption and emission characteristics of 1 is given in Table 1.

Table 1

UV-Vis absorption and emission characteristics of 1.

λmax (nm) (ε/M-1 cm-1)

Emission λmax (nm)

Eo-o (eV)

308 (33200), 388 (10900), 450 (shoulder) 528 (7350)

790

1.88

Cyclic and square wave voltammograms of 1 (ESI Figures 3 & 4) showed a fully reversible redox couple in both the cyclic and square wave experiments at 0.06V vs Fc/Fc+ (0.69 V vs NHE). This wave is assigned as the ground state redox potential of 1 (Eox) and is sufficiently lower in energy than the iodide/triiodide electrolyte redox couple to allow dye regeneration within a DSSC [2]. In conjunction with the ground state – excited state energy gap found by the emission studies, the excited state reduction potential can be calculated as -1.19 V vs NHE [17], high enough above the TiO2 conduction band to allow injection [18].

DSSCs using 1 and, for comparison, N719 dyes were fabricated using typical method as outlined in the experimental section. Current-voltage curves are shown in Figure 4 and solar cell characteristics are presented in Table 2. We observed that cells containing 1 as a sensitiser have reduced open-circuit voltage (Voc) and short-circuit current (Jsc) in comparison to N719 and hence overall lower power conversion efficiency (PCE). It is possible that the reduced photocurrent can be attributed to the lower absorption coefficient of 1 around 500-550 nm where solar intensity is high, despite the improved absorption characteristics of 1 at shorter wavelengths (Table 2). An additional possibility is that the less positive reduction potential of 1 compared with N719 adversely affects the dye regeneration rate.

Figure 5. Bode plots of the impedance data for cells containing 1 and N719

Table 2

Photovoltaic properties of cells containing 1 and N719 reference under AM1.5G one sun illumination (1000 Wm-2) irradiation.

Figure 4. Current-voltage curves of cells containing 1

Dye

Voc (V)

Jsc (mA cm-2)

FF

PCE (%)

1

0.58

7.22

75.6

3.18

N719

0.69

12.72

79.8

7.00

To further investigate the cell performance, electrochemical impedance spectroscopy was carried out. A Bode plot of the results is shown (Figure 5) and the lifetimes of the impeding processes are calculated according to Eq. 1.

The lifetime of the high frequency peak corresponding to reduction of the electrolyte at the counter electrode remains unchanged as would be expected as the cells with 1 and N719 used the same counter electrode. However a shortening of the lifetime of the low frequency process in cells containing 1 was observed. This peak has been assigned in the literature as resulting from recombination between electrons in the TiO2 and the redox electrolyte (Eq. 2) [19].

The results of the lifetime calculations of the recombination processes for both cells containing 1 and N719 are shown in Table 3.

Table 3

Solar cell process lifetimes calculated from impedance spectra

Cell

Recombination Lifetime (ms)

1

3.90

N719

32.75

Accordingly, the lowered Voc observed for cells containing 1 can be attributed to the rate of this process being increased by a factor of around 10 compared to N719, suggesting that some feature of the dye 1 is causing this process to be catalysed. This is proposed to be due to the additional thiocyanate functionality in 1. Thiocyanate groups have been previously observed to form halogen bonds with the electrolyte species [20] and as such there are potentially two additional binding sites for the electrolyte on 1. The bipyridyl thiocyanate groups could be involved in binding the electrolyte species I3- and creating an area of high concentration in close proximity to the TiO2 surface, explaining the catalytic action of 1 upon the process described in Eq. 2. Recent work by Meyer and co-workers [21] showed that the position of such additional atoms or groups on a dye can be crucial. As described above heteroatoms such as sulfur on a peripheral part of the molecule can enhance e-(TiO2)-electrolyte recombination, however heteroatoms associated with positive charge density in the oxidized dye can have a beneficial effect on dye regeneration by interacting with reductant iodide ions [21]. In our case, with NCS groups appended to the peripheral bipyridyl group of 1, the observation of a shorter recombination lifetime is consistent with dominance of the former process.

In summary, we have demonstrated the use of an in situ nucleophilic aromatic substitution reaction in the synthesis of a ruthenium dye complex and applied this dye in a typical DSSC system. The dye 1 exhibited improved light absorption over the 350-450 nm range when compared with N719, but the fabricated cells exhibited a modest PCE of 3.18%. The reduced photocurrent is attributed to poorer absorption across 500-550 nm and/or poorer dye regeneration. Electrochemical impedance spectroscopy has indicated that the lower photovoltage and hence open circuit voltage in the DSSCs fabricated with 1 is caused by an increased rate of recombination of injected electrons with the redox species. This has been proposed to be due to interactions between the additional thiocyanate groups upon 1 and the redox electrolyte.

Experimental

All chemicals were purchased from Aldrich and used as received. UV/Vis spectra were recorded on a Jasco V670 spectrometer controlled using the SpectraManager software. Photoluminescence (PL) spectra were recorded with Fluoromax-3 fluorimeter controlled by the ISAMain software. Cyclic and square wave voltammograms were recorded using a three electrode system comprising of platinum counter, working electrodes and a Ag/AgCl reference electrode situated in a saturated Lithium Chloride solution in ethanol. The results were calibrated against the ferrocene redox potential as internal standard. Electrochemical experiments were conducted in a 0.1 M solution of supporting electrolyte (Tetrabutyl ammonium tetrafluoroborate - TBABF4) in N2 degassed DMF. All measurements were made at room temperature using a μAUTOLAB Type III potentiostat, driven by the electrochemical software GPES.

Synthesis of 1: A mixture of [Ru(p-cymene)Cl2]2 (102.7 mg, 0.16 mmol) and H2-dcbpy (81.3 mg, 0.33 mmol) were heated under N2 in dry DMF (20 ml) at 60 oC overnight. 4,4’-F2-bpy (63.7 mg, 0.33 mmol) was added and the temperature increased to 140 oC for 2 hours. Excess NH4NCS (1.0073 g, 13.23 mol, 40 times excess) was then added and heating continued at 140 oC for 3 hours. After allowing to cool, the solvent was then removed using reduced pressure and the resulting black solid stirred in water overnight to remove any unreacted NH4NCS. Upon filtering the crude product was formed as a dark red/brown solid. Purification was achieved by dissolving the crude product in NaOH solution (0.1 M, in methanol) and performing sephadex column chromatography using methanol as the eluent. This produced the pure product 1 (69.8 mg, 0.10 mmol, 60 % yield) as a dark red solid. 1H NMR (600 MHz, CD3OD) δ (ppm): 9.54 (d, J = 5.7 Hz, 1H), 8.93 (s, 1H), 8.85 (d, J = 6.7 Hz, 1H), 8.78 (s, 1H), 8.15 (dd, J = 5.6 Hz, 1.6 Hz, 1H), 7.76 (d, J = 6.0 Hz), 7.53 (dd, J = 5.9 Hz, 1.6 Hz, 1H), 7.52 (s, 1H), 7.40 (s, 1H), 7.01 (dd, J = 6.60 Hz, 2.47 Hz, 1H), 6.85 (d, J = 6.9Hz, 1H), 6.32 (dd, J = 6.7 Hz, 2.6 Hz, 1H). ESI-MS: m/z 730 [M-H]-. Anal. Calc.: C 44.10%, H 2.18%, N 12.91%. Found: C 44.08%, H 2.13%, N 12.80%.

DSSC Fabrication: Nanocrystalline TiO2 photoelectrodes were prepared using a screen printing method on fluorine doped tin oxide (TEC-15, Pilkington, UK) glass using standard literature procedures [22]. Sensitisation of these electrodes was achieved by immersion in a solution of 1 (0.5 mM in 4:1 ethanol:DMSO) for 24 hours at room temperature. Sandwich-type two-electrode DSSCs were then fabricated by sealing the electrodes together with surlyn (Solaronix,SA) and filling with an electrolyte consisting of 1-butyl-3-methylimidazolium iodide (0.6 M), I2 (0.05 M), 4-tert butylpyridine (0.5 M), LiI (0.1 M) in acetonitrile:valeronitrile (85:15). In addition, reference cells containing TiO2 photoelectrodes sensitized with a 0.5 mM solution of N719 (Dye-sol) in ethanol were also fabricated. The active area of the solar cells was 0.25 cm2. The current-voltage measurements were carried out using a computer-controlled digital source meter (Keithley 2400) under simulated AM1.5G irradiation from a solar simulator (92250 A, Newport, USA) under 1 sun equivalent (100 W/m2) condition..

ACKNOWLEDGEMENTS

We thank the EPRSC Supergen (EP/G031088/1) and Apex (EP/H040218/1) projects for financial support.

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[17] Equation used: E* = Eox – Eo-o

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