Positional isomerism makes a di ff erence : phosphonic acid anchoring ligands with thienyl spacers in copper ( I )-based dye-sensitized solar cells †

With the aim of improving the photoconversion efficiencies of heteroleptic [Cu(Lanchor)(Lancillary)] + dyes in n-type dye-sensitized solar cells (DSCs), the previously favoured anchor ((6,6’-dimethyl-[2,2’-bipyridine]4,4’-diyl)bis(4,1-phenylene))bis(phosphonic acid) (1) has been replaced by analogues 2 and 3 containing 2-thienyl spacers between the 2,2’-bipyridine metal-binding domain and the phosphonic acid anchoring groups. The synthesis and characterization of 2 and 3 (2-thienyl spacer with phosphonic acid in the 5and 4-positions, respectively) are reported. A stepwise, on-surface method was used to assemble [Cu (Lanchor)(Lancillary)] + dyes onto FTO/TiO2 electrodes with Lanchor = 1, 2 or 3, and Lancillary = 6,6’-bis(trifluoromethyl)-2,2’-bipyridine (4), 6-trifluoromethyl-2,2’-bipyridine (5), 6,6’-dimethyl-2,2’-bipyridine (6), and 6-methyl-2,2’-bipyridine (7). Changing the solvent in the dye-bath from CH2Cl2 to acetone had only a small effect on the photoconversion efficiencies of [Cu(1)(4)], [Cu(1)(5)] and [Cu(1)(6)]; the optimal dye in this series was [Cu(1)(5)]. Comparable DSC performances were achieved by using either anchor 1 or 2, but there is improved electron injection if the phosphonic acid group is in the 4rather than 5-position of the thienyl ring (i.e. anchor 3 is superior to 2). Similar open-circuit voltages (VOC) are achieved on going from 1 to 3 with a given Lancillary; although there is typically a gain in short-circuit current denisty (JSC) on going from 1 or 3 to 2, there is an ≈50–60 mV drop in VOC on introducing 2 as the anchor. The best photoconversion efficiencies are obtained for the dye [Cu(3)(5)] (η = 2.40% relative to an N719 reference of 5.76%). The conclusions reached from plots of current-density (J) against potential (V), and external quantum efficiency spectra are supported by electrochemical impedance spectroscopic measurements.


Introduction
Recent improvements in the performance of copper-based dyesensitized solar cells (DSCs) have highlighted the realistic prospect of future DSCs utilizing Earth-abundant metals such as copper 1 and iron 2 rather than rare and expensive metals such as ruthenium.Breaking the 3% 3 then 4% 4 barriers of photoconversion efficiencies of DSCs containing copper(I) sensitizers has provided significant impetus to this research field.In order to tune the properties of copper(I) sensitizers and build into their design electronic 'push-pull' features, heteroleptic complexes of the type shown in Scheme 1 are desirable.However, isolating heteroleptic [Cu(L anchor )(L ancillary )] + dyes is hampered by their tendency in solution to form equilibrium mixtures with the homoleptic [Cu(L anchor ) 2 ] + and [Cu(L ancillary ) 2 ] + complexes.To overcome this problem, Odobel and coworkers have focused attention on the advantages of the HETPHEN approach (HETPHEN = heteroleptic bisphenanthroline), [4][5][6] and we have developed a 'surfaces-asligands, surfaces-as-complexes' strategy in which the heteroleptic complex (Scheme 1 and eqn (1)) is assembled directly on the semiconductor surface. 1,7The latter provides a flexible means of combining a wide range of ancillary ligands with a given anchoring domain, allowing the benefits of different ligand combinations in a [Cu(L anchor )(L ancillary )] + dye (Scheme 1) to be rapidly recognized.

ð1Þ
The use of phosphonic acid anchors for copper(I) sensitizers leads to enhanced DSC performance with respect to analogues with carboxylic acid anchors. 8Grätzel has also described the advantages of phosphonic acid anchors in ruthenium sensitizers, 9 and Meyer and coworkers have shown that in [Ru(bpy) 3 ] 2+ -based sensitizers, use of 4,4′-{(HO) 2 OP} 2 -bpy (bpy = 2,2′-bipyridine) anchors provides an optimal balance between electron injection efficiency and dye stability. 10In most recent investigations, we have favoured the use of anchoring ligand 1 (Scheme 2); incorporation of an arene spacer between the bpy copper-binding unit and the phosphonic acid functionality improves the photoconversion efficiency of the dye. 11A 6,6′-substitution pattern is beneficial in stabilizing the copper(I) oxidation state by preventing flattening distortions, and the effects on going from 2,9-dimethyl-to 2,9-bis(trifluoromethyl)-to 2,9-diphenyl-substituted 1,10-phenanthroline in [Cu( phen) 2 ] + complexes has been discussed by Armaroli et al. 12 Long excited-state lifetimes have been observed as a result of the combined effects of placing bulky substituents in the 2,9-and 3,8-positions of the phen domains in a [Cu( phen) 2 ] + photosensitizer. 13Although Odobel has shown that 6,6′-dimesityl-2,2′-bipyridine-4,4′-dicarboxylic acid is an effective anchor, 4 we have demonstrated that a change from 6,6′-dimethyl substitution pattern in 1 to 6,6′-diphenyl substituents is detrimental to DSC performance. 14,15We report here the synthesis and application of two new phosphonic acid anchoring ligands (2 and 3, Scheme 2) which incorporate 2-thienyl spacers, the advantages of which in dyes are well recognized. 16Since a combination of 1 with simple halofunctionalized bpy ancillary ligands leads to surprisingly efficient [Cu(1)(L ancillary )] + dyes, 3,17 we chose to use ancillary ligands 4 and 5 (Scheme 3) in a comparative investigation of heteroleptic dyes [Cu(L anchor )(L ancillary )] + in which L anchor = 1, 2 or 3, and L ancillary = 4-7 (Scheme 3).

Experimental
General 1 H and 13 C NMR spectra were recorded on Bruker DRX-500 or 600 NMR spectrometers with chemical shifts referenced to residual solvent peaks (TMS = δ 0 ppm).Electrospray mass spectra and high resolution ESI-MS were measured on Bruker Esquire 3000 plus and Bruker maXis 4G instruments, respectively.Solution electronic absorption spectra were recorded on a Agilent 8453 spectrophotometer, and solid-state spectra on a Cary 5000 spectrophotometer.A Biotage Initiator 8 microwave reactor was used for reactions carried out under microwave conditions.
Esters 2a and 3a were converted to phosphonic acids 2 and 3 by treatment with Me 3 SiBr, 24 and the products were isolated as white solids in 61 and 81% yields, respectively.The transformation was confirmed in the 1 H and 13 C NMR spectra by the disappearance of the signals for the ethyl groups in 2a and 3a.Compounds 2a, 3a, 2 and 3 were characterized spectroscopically and by mass spectrometry.Elemental analysis for 3a and 2 were acceptable; acid 3 analysed as the hydrate 3•3.5H 2 O.This is not unreasonable given the presence of two phosphonic acid functionalities, and a singlet at δ 3.43 ppm in the 1 H NMR spectrum of a DMSO-d 6 solution of 2 was observed, consistent with H 2 O/HOD.Satisfactory elemental analytical data for 2a could not be obtained, but composition was confirmed by high resolution electrospray mass spectrometry (m/z: 620.1407 corresponding to [M + H] + ).For each compound, the highest mass peak in the ESI mass spectrum was assigned to [M + H] + (m/z 621.1 for 2a and 3a, and 509.1 for 2 and 3).Differing solubilities of 2 and 3 compared to 2a and 3a in common solvents meant that the NMR spectra of the acids and esters were recorded in DMSO-d 6 and CDCl 3 , respectively.On going from 2a to 2, and 3a to 3, the singlet in the 31 P{ 1 H} NMR spectrum shifted from δ +10.8 to +4.0 ppm, and from δ +12.0 to +6.1 ppm, respectively.The 1 H and 13 C NMR spectra of the compounds were assigned using DEPT, COSY, NOESY, HMQC and HMBC methods (see Experimental section and Scheme 4 for atom labelling).For 3a, the 13 C NMR resonance for C A3 (δ 115.2 ppm) could only be located using the HMBC spectrum from the C A3 /H A5 cross-peak.On going from the bromo-precursor to 2a (Scheme 4), the singlet in the 13  Single crystals of 2a were grown by slowly evaporating an acetone/cyclohexane solution of 2a.The compound crystallizes in the monoclinic space group P2 1 /c, and the structure of the centrosymmetric molecule is shown in Fig. 2; selected bond metrics are given in the figure caption and are as expected.The bpy domain is necessarily planar, and the plane of the thienyl unit is twisted 22.2°with respect to the pyridine ring to which it is bonded.The PO(OEt) 2 unit is positioned so that the ethoxy groups are above and below the thiophene ring plane, as expected on steric grounds; the O10-P6-C5-C4 torsion angle is 17.3(3)°.The pyridine atom N17 acts as a bifurcated acceptor 25 in C-H⋯N interactions to a thiophene CH and a methyl CH in adjacent molecules (Fig. 3 These, along with C-H ethyl ⋯O PO close contacts, are dominant packing forces are the crystal.

DSC reproducibility and N719 reference DSCs
For the DSC measurements detailed below, duplicate cells for each sensitizer were made and all data are presented to illustrate the reproducibility of the results.All DSCs were fully masked. 26,27Photoconversion efficiencies are compared with the performance of reference DSCs containing the ruthenium dye N719.Using such relative η values allows valid comparisons between measurements made in different investigations, in different research laboratories, and using different commer-cial sun simulators (which, in our experience, vary 28 ).To validate the reference values, six DSCs containing N719 1 were assembled (see Experimental section).The reproducibility of plots of current density ( J) against potential (V) are confirmed in Fig. S6; † Table S1 † gives the performance parameters of the N719 DSCs which show only small variation between devices.In the discussion that follows, average values (Table S1 †) of short-circuit current density ( J SC ), open-circuit voltage (V OC ), fill-factor (ff ) and photoconversion efficiency (η) for N719 are used.

Performances of DSCs with dyes with anchor 1: solvent effects
We recently showed that DSCs containing the copper(I) dyes [Cu(1)(4)] + and [Cu(1)(5)] + with fluorinated ancillary ligands (Scheme 3) exhibit photoconversion efficiencies of ≈30-34% relative to N719 set at 100%, and outperform DSCs containing [Cu(1)(6)] + in which 6 contains methyl substituents in the 6-and 6′-positions (Scheme 3). 17In this study, 17 the exchange reaction (eqn (1)) used to assemble the copper(I) dyes on the TiO 2 electrode was carried out in CH 2 Cl 2 .Dye-bath solvents are known to influence the performance of DSCs, 29,30 and we attempted to enhance the photoconversion efficiency of dyes [Cu(1)(4)] + and [Cu(1)(5)] + by replacing CH 2 Cl 2 in the dye-bath by acetone. 29DSCs were made using commercial TiO 2 electrodes that were sequentially dipped in a dye-bath of a DMSO solution of anchor 1, and then in a bath containing an acetone solutions of either 4, 5 or 6 (see Experimental section).The performances and external quantum efficiencies (EQEs) of duplicate DSCs were measured one, three and seven days after fabrication; an I − /I 3 − electrolyte was used in these solar cells.
Values of J SC , V OC , ff and η are given in Table 1.The last column in Table 1 gives values of η relative to N719, for which the relative efficiency is set at 100%.J-V curves and EQE spectra for the DSCs were recorded one day after sealing the cells, and are shown in Fig. 4 and 5.
Although the sealed DSCs are stable, both acetone and CH 2 Cl 2 solutions of [Cu( 4    [Cu(1)(5)] + (460 nm) to [Cu(2)(5)] + and [Cu(3)(5)] + (470 nm), and the absorption band is broadened towards the red-end of the spectrum.Performance data for the DSCs over a period of a week are given in Table 2, and there is satisfactory reproducibility between the performances of pairs of cells containing the same dye.Fig. 7 shows the J-V curves for the DSCs on the day of cell assembly.Replacement of the phenyl spacer in anchor 1 by a 2-thienyl spacer in 2 and 3 results in an increase in the short-circuit current density, and this is most noticeable for the sensitizer [Cu(3)(5)] + .Fig. 7 and Table 2 illustrate that values of V OC for DSCs with [Cu(1)(5)] + and [Cu(3)(5)] + are essentially the same, whereas devices with [Cu(2)(5)] + consistently exhibit lower values of V OC .Within experimental error, the global efficiencies of the DSCs are stable over time, and Table 2 shows that the best performing dye is that containing anchor 3, i.e. the 2-thienyl spacer with phosphonic acid in the 4-position (Scheme 2).The EQE spectra in Fig. 8 (see also Table 3) are consistent with the enhanced J SC values for [Cu(2)(5)] + and [Cu(3)(5)] + .The values of EQE max of 52.9 and 51.8% (Table 3) for the duplicate DSCs with [Cu(3)(5)] + coupled with the enhanced quantum efficiences towards the red-end of the spectrum compared to the EQE spectra for the DSCs containing [Cu(1)(5)] + (Fig. 8) are noteworthy.The EQE spectra of DSCs with [Cu(2)(5)] + (Fig. 8) also show a broad spectral response.
To further validate the improvement in V OC on going from 2 to 3, we repeated the DSC performance measurements using in-house, screen-printed FTO/TiO 2 electrodes.The dyes [Cu(2)(5)] + and [Cu(3)(5)] + were assembled on the electrodes by the step-wise approach shown in eqn (1); the photoanodes were incorporated into DSCs along with I − /I 3 − electrolyte.DSC performance parameters are given in Table S2, † and Fig. 9 shows the respective J-V curves.The ∼55-60 mV gain in V OC mirrors that seen when commercial electrodes were used (Fig. 7 vs.Fig.9), and values of J SC are similar.EQE spectra (Fig. S7 †) all show EQE max > 50% (Table 4) and the same extension of the spectral response to higher wavelengths.Thus, data    for DSCs made using self-printed FTO/TiO 2 electrodes confirm that higher V OC values are achieved if the phosphonic acid anchor-groups are located in the 4-rather than 5-positions of the thienyl functionality.
The longer-term stability of DSCs containing [Cu(2)(5)] + and [Cu(3)(5)] + (both with commercial and in-house screen-printed electrodes) was assessed by measurements made after 41 or 46 days.A comparison of the data in Table 5 with the characteristics of 7-day old DSCs containing [Cu(2)(5)] + and [Cu(3)(5)] + (Tables 2 and S2 †) confirm little change in J SC , V OC and η values for a given DSC over a 6-week period.

Combining thienyl-spaced anchors with different ancillary ligands
We have now shown that although comparable DSC performances are achieved using either a 2-thienyl or phenyl spacer in the phosphonic acid anchoring ligand, higher V OC values result if the phosphonic acid group is in the 4-rather than 5-position of the thienyl ring.This conclusion was reached using a single ancillary ligand, 5. To validate the conclusions, we extended the study to dyes containing combinations of 1, 2 and 3 with ancillary ligands 6 and 7 (Scheme 3).
Tables 6 and S3 † present performance parameters for pairs of DSCs containing [Cu(L anchor )(L ancillary )] + with L anchor = 1-3 and L ancillary = 6, 7. J-V curves for one set of DSCs with each ancillary ligand are shown in Fig. 10; Fig. S8 † gives a compari-     ] + (Fig. 10b).Both the anchors with the thienyl spacer perform well, but a significant gain in V OC (∼40 to 50 mV) is achieved on going from anchor 2 to 3.
One versus two methyl groups, and a methyl versus trifluoromethyl group in L ancillary In this section, we comment briefly on the comparison of one or two methyl substituents in the ancillary ligand, and compare the performances of DSCs with [Cu(L anchor )( 7)] + and [Cu(L anchor )(5)] + (5 and 7 differ in bearing either a CF 3 or CH 3 group in the 6-position of the bpy unit).
In previous investigations, we have shown that the incorporation of methyl substituents in the 6-and 6′-positions of a bpybased ancillary ligand improves DSC performance. 7,14The data in Table 6 for pairs of sensitizers [Cu(L anchor )( 6)] + and [Cu (L anchor )(7)] + illustrate enhanced DSC performance on going from an ancillary ligand with a 6,6′-dimethyl substitution to one with a single 6-methyl group (an observation that is supported by the EIS measurments discussed later).In all dyes, the anchoring ligand contains a 6,6′-dimethyl-2,2′-bipyridine core.The improvement is most noticable for dyes with anchors 2 and 3, and has its origins in increased values of both J SC and V OC .
The data in Table 2, Fig. 7 and 8 illustrate how the performances of DSCs with the dyes [Cu(L anchor )(5)] + vary as a function of L anchor .Analogous information for the series of [Cu(L anchor ) (7)] + dyes is presented in Table 6, Fig. 10 and 11.A comparison of the two sets of results confirms that the replacement of a 6-methyl substituent in 7 by a trifluoromethyl group in 5 leads to an enhancement in photoconversion efficiency.On going from [Cu(1)(7)] + to [Cu(1)(5)] + , J SC increases from 4.58 to 5.45 mA cm −2 (best values are selected from duplicate DSCs) on the day the DSCs were sealed, and the difference is maintained after a week (3.79 versus 4.74 mA cm −2 ).Enhanced V OC is also observed on the initial day (551 to 582 mV), with this differentiation retained as the DSCs age.For DSCs containing [Cu(3)(7)] + or [Cu(3)(5)] + , there is a notable improvement in J SC on introducing the fluoro-substituents.For 7-day old DSCs, values of J SC = 4.13 and 4.44 mA cm −2 for [Cu(3)(7)] + (Table 6) increase to 5.10 and 5.42 mA cm −2 for [Cu(3)(5)] + (Table 2), and the global efficiencies increase from 1.77 and 1.96% for duplicate DSCs to 2.10 and 2.25% (compared to 5.90% for N719).The trends substantiate those previously reported, 17 and in order to gain a better understanding of the contributing factors, we have applied the method of electrochemical impedance spectroscopy.Electrochemical impedance spectroscopy: introductory comments EIS is a powerful tool for the analysis and evaluation of the performance and properties of electrochemical systems and is highly suited to investigations of charge transfer processes in DSCs. 31,32A small alternating current (AC) of varying frequency is applied close to the open-circuit potential of the DSC which may be placed under a bias-light of different intensities. 33The internal impedances at the various frequencies correspond to different processes in the cell.The results are commonly plotted in a Nyquist plot (Fig. 13) where each semi-circle in the plot provides information about the charge-transfer resistances and capacitances of the corresponding interface in a DSC. 34ecombination reactions or 'back reactions' are the main limiting factors affecting the photoconversion efficiency of a DSC. 35rom the parameters extracted from EIS data, the length of the electron diffusion, L d , and the electron lifetime (τ) in the semiconductor (SC), can be obtained. 36These are given by eqn ( 2) and ( 3), respectively, where R t is the transport resistance in the SC, C µ is the chemical capacitance of the TiO 2 /electrolyte interface (with the capacitance of the transparent conductive oxide layer/electrolyte neglected at higher bias voltages when the SC is conducting) and L is the thickness of the porous active layer.
Studies of R t are best carried out at lower bias-light intensities, since at higher open-circuit voltages, the SC exhibits electron-transport properties closer to that of a conductor; thus, R t becomes lower and more difficult to observe.At a given illumination intensity (given that the geometry of the electrode is constant), it can be assumed that the Fermi level (E F ) and the resistance of TiO 2 (R 0 ) are constant.From eqn (4) it then follows that a change of the lower level of the conduction band (E cb ) is the one main parameter that will affect R t on the surface, close to the dye/electrolyte. 37 The electrolyte composition and the dye are factors that influence the values of E cb and therefore V OC . 37In a well-performing DSC, the recombination resistance should be significantly higher than the transport resistance.In this case, electrons injected into the TiO 2 are readily collected, and the tendency to recombine with the oxidized dye or the oxidized form of the electrolyte is suppressed.It follows from eqn (2) that the normalized diffusion length L d /L will be greater than unity. 36The physical meaning is that L d for the injected electron to reach the p-n junction and combine with the generated hole should be at least longer than L.

EIS measurements: DSCs with dyes with anchor 1
We initially focus on EIS data for 3-day old DSCs containing the dyes [Cu(1)(4)] + , [Cu(1)(5)] + , [Cu(1)(6)] + and [Cu(1)(7)] + with the phenyl-spacer in the anchoring ligand.As discussed earlier, DSC performance parameters showed that dyes containing CF 3 groups in the ancillary ligand exhibit higher V OC values than the CH 3 analogues.This trend is confirmed by the V OC values determined at different light intensities from EIS measurements.Table S4 † presents all data, and Tables 8 and 9 give data at the lowest and highest light intensities measured; Nyquist and Bode plots are shown in Table S5.† A higher V OC implies that more electrons are present in the conduction band of the SC, which indicates a more negative conduction band level (E cb ).This is supported by the values of R t given in Table 9 (where EIS data are from measurements at lower biaslight intensities).The values of R t are significantly lower for dyes containing 4 and 5 than 6 and 7. On the other hand, the DSCs containing [Cu(1)(6)] + or [Cu(1)(7)] + exhibit higher R rec    8 and  9 reveals that the DSC with [Cu(1)(5)] + exhibits both a high capacitance and a relatively high recombination resistance, but also has the lowest R t and the highest L d /L of all the [Cu(1)(L ancillary )] + dyes.This is reflected in the overall performance of this DSC.
The DSC with [Cu(1)( 6)] + has the highest R rec , but also the lowest capacitance which in combination with the highest R t value (Table 9) will counteract the transport of the injected electrons, resulting in a relatively poor performance.In Table 8, it is interesting that the monosubstituted ligands 5 and 7 give rise to a higher capacitance, indicating that the asymmetric structure enhances electron injection into the TiO 2 leading to an increase in DSC efficiency.This is consistent with the photoconversion efficiences presented in Tables 2 and 6 and discussed earlier.

Dalton Transactions Paper
This journal is © The Royal Society of Chemistry 2016 EIS measurements: DSCs with different anchoring ligands Fig. 17 shows values of R rec and τ for DSCs with dyes containing anchors 2 and 3 (with thienyl-spacers) and ancillary ligands 5 and 7.The value of V OC (Tables 8 and 9) is highest in measurements for dyes with anchor 3, consistent with conclusions based on J-V curves (see earlier discussion).Both the recombination resistance and the capacitance (Fig. 18) are higher for these dyes, and the transport resistance is lower.These factors all contribute to the higher photoconversion efficiencies of DSCs with dyes [Cu(3)(5)] + and [Cu(3)(7)] + .As noted above, a higher R t combined with a low R rec limits the extent to which electrons are injected and at the same time, the resistance for the electron to recombine with the oxidized dye is low.This is the case for the measurements of DSCs with dyes containing anchor 2, where L d /L is lower than in its counterpart with 3.
As discussed earlier and shown in eqn (4), a higher R t may be accompanied by a higher E cb which can reduce the charge injection efficiency.This will consequently decrease the V OC due to a lower density of electrons in the conduction band.This is demonstrated in Table 9 by comparing data for [Cu(2)(7)] + and [Cu(3)(7)] + ; anchor 3 lowers R t by ∼60% and improves the V OC by more than 20 mV.When the conduction band level is very high in a SC, resulting in a large R t , the phenomenon is observed in an EIS measurement as Gerischer resistance in the mid-range frequency region.This is effectively displayed as a distorted semi-circle in the plot of [Cu(2)(5)] + in Fig. 19, where the EIS spectra from [Cu(2)(5)] + and [Cu(3)(5)] + are compared.

Conclusions
The current work extends our previous investigation of the favourable effects of introducing CF 3 groups into the ancillary ligands in [Cu(L anchor )(L ancillary )] + dyes in DSCs. 17 6)] + has only a small effect on DSC performance.The optimal dye in this series in terms of photoconversion efficiency and dye stability is [Cu(1)(5)] + .Achieving enhancement of DSC performance by replacing the phenyl spacer in anchor 1 by a 2-thienyl unit is dependent upon the position of substitution of the phosphonic acid group in the thienyl ring.Improved V OC is observed when the PO(OH) 2 anchor is in the 4-rather than 5-position (i.e. 3 is better than 2).Similar values of V OC are obtained on going from 1 to 3 with a given L ancillary ,   but on going from 1 or 3 to 2, there is an ≈50-60 mV drop in V OC ; countering this, there is typically a gain in J SC when 2 is used as L anchor.The best photoconversion efficienies are obtained for the dye [Cu(3)(5)] + (η = 2.40% relative to a N719 reference of 5.76%).
The conclusions reached from plots of current-density ( J) against potential (V), and external quantum efficiency spectra are supported by electrochemical impedance spectroscopic measurements.The EIS data showed that introducing CF 3 in place of Me substituents into L ancillary dramatically lowers the transport resistance, R t , and increases C µ ; R t decreases and C µ increases when L ancillary is 6-substituted rather than 6,6′-disubstituted.In agreement with J-V curves, EIS data for DSCs with dyes containing 5 or 7 combined 2 or 3 reveal that use of 3 results in a significant enhancement in V OC ( particularly noteworthy when L ancillary = 7).Furthermore, on going from [Cu(3) (L ancillary ] + to [Cu(2)(L ancillary ] + , R t is dramatically increased (L ancillary = 5 or 7).Of the four dyes with thienyl spacers in the anchor that were tested, [Cu(3)(5)] + (which has the highest photoconversion efficiency) has the second highest capacitance as well as a rather high R rec together and the next lowest R t , indicating that electrons are readily injected into the conduction band; the recombination rate is such that a high V OC results and, therefore, DSC performance improves.
C NMR spectrum for C B5 at δ 114.1 ppm is replaced by a doublet ( J PC = 201 Hz) at δ 129.0 ppm.Similarly, 3a exhibited a doublet ( J PC = 197 Hz) at δ 131.2 ppm for C B4 , in contrast to a singlet at δ 111.2 ppm for C B4 in the bromo-precursor.The aromatic regions of the 1 H NMR spectra of phosphonic acids 2 and 3 are shown in Fig. 1.Broadening of the signals is most likely a solvent effect.

Fig. 3
Fig. 3 Bifurcated N⋯H-C interactions between molecules of 2a.Hydrogen atoms except for those in the functional groups involved in hydrogen bonding, are omitted for clarity.

Fig. 13
Fig.13Schematic representation of a Nyquist plot.R s is the electrical resistance in cables, contacts and the resistance of the semi-conductor/ FTO glass.R Pt is the charge transfer resistance of the interface cathode/ electrolyte.R rec is the electron recombination resistance of the active layer/electrolyte and W s is the Warburg short resistance of the ion diffusion in the electrolyte close to the active layer surface.

Fig. 19
Fig. 19 Nyquist plots for [Cu(2)(5)] + and [Cu(3)(5)] + measured under a light intensity of 0.44 mW cm −2 .The data points shown in red were measured at 0.3 Hz.The inset shows an expansion of the data measured in the high-to-mid frequency region.

Table 7
EQE maxima for duplicate DSCs containing [Cu(L anchor )(7)] + with L anchor = 1-3 and using commercial electrodes; the spectra were recorded on the day of cell assembly

Table 8
Impedance data obtained during measurements at 22 mW cm −2 .EIS measurements are carried out 3 days after DSC assembly a The cell efficiency is that of the particular cell measured with EIS.

Table 9
Impedance data obtained during measurements at 2.4 mW cm −2 .EIS measurements are carried out 3 days after DSC assembly