Are Alkynyl Spacers in Ancillary Ligands in Heteroleptic Bis(diimine)copper(I) Dyes Beneficial for Dye Performance in Dye-Sensitized Solar Cells?

The syntheses of 4,4'-bis(4-dimethylaminophenyl)-6,6'-dimethyl-2,2'-bipyridine (1), 4,4'-bis(4-dimethylaminophenylethynyl)-6,6'-dimethyl-2,2'-bipyridine (2), 4,4'-bis(4-diphenylaminophenyl)-6,6'-dimethyl-2,2'-bipyridine (3), and 4,4'-bis(4-diphenylaminophenylethynyl)-6,6'-dimethyl-2,2'-bipyridine (4) are reported along with the preparations and characterisations of their homoleptic copper(I) complexes [CuL2][PF6] (L = 1-4). The solution absorption spectra of the complexes exhibit ligand-centred absorptions in addition to absorptions in the visible region assigned to a combination of intra-ligand and metal-to-ligand charge-transfer. Heteroleptic [Cu(5)(Lancillary)]+ dyes in which 5 is the anchoring ligand ((6,6'-dimethyl-[2,2'-bipyridine]-4,4'-diyl)bis(4,1-phenylene))bis(phosphonic acid) and Lancillary = 1-4 have been assembled on fluorine-doped tin oxide (FTO)-TiO2 electrodes in dye-sensitized solar cells (DSCs). Performance parameters and external quantum efficiency (EQE) spectra of the DSCs (four fully-masked cells for each dye) reveal that the best performing dyes are [Cu(5)(1)]+ and [Cu(5)(3)]+. The alkynyl spacers are not beneficial, leading to a decrease in the short-circuit current density (JSC), confirmed by lower values of EQEmax. Addition of a co-absorbent (n-decylphosphonic acid) to [Cu(5)(1)]+ lead to no significant enhancement of performance for DSCs sensitized with [Cu(5)(1)]+. Electrochemical impedance spectroscopy (EIS) has been used to investigate the interfaces in DSCs; the analysis shows that more favourable electron injection into TiO2 is observed for sensitizers without the alkynyl spacer and confirms higher JSC values for [Cu(5)(1)]+.


Introduction
Future energy strategies will rely increasingly upon sustainable methods of energy generation in order to conform to the United Nations Sustainable Development Goals. Emerging energy economies identify solar-to-electricity conversion as a key technology. Currently, the most widely deployed technologies are based upon semiconductor photovoltaics, but a promising methodology is the so-called dye-sensitized solar cell (DSC). DSCs [1] convert solar to electrical energy using a wide-bandgap semiconductor such as nanoparticulate TiO 2 functionalized with a material that absorbs visible light [2][3][4][5]. The latter comprises the working electrode in an n-type DSC. Apart from the semiconductor and the dye, the other crucial components of the device are the electrolyte (incorporating a redox couple to facilitate electron and hole transport and to regenerate the ground-state of the dye after excitation), and the counter electrode (typically conductive glass coated with a thin layer of platinum to catalyse the regeneration of the reduced form of the redox couple). Some critical advances with DSCs using aqueous electrolytes have been made [6], but on the whole, electrolytes in the devices are A key factor in performance enhancement of copper(I)-based DSCs is systematic modification of the structures of 'push-pull' dyes (Scheme 1). The 'push-pull' characteristics are readily tuned in a heteroleptic copper(I) complex (in which the Cu atom is the linker) through structural modification of the anchoring and ancillary ligands, such that in a charge-separated state, the hole is located on the ancillary ligand and the electron on the anchoring ligand. Donor-π-acceptor (D-π-A) dyes fulfil the 'push-pull' characteristics and introducing alkynyl units to optimize electronic communication and conjugation has been found to be beneficial in some families of dyes [33]. The +I nature of alkynyl substituents [34] increases the electron density on the metal centre and enhance the population of the metal-to-ligand charge transfer (MLCT) state [35]. Within the context of bis(diimine)copper(I) complexes, Castellano and co-workers demonstrated that the incorporation of phenylethynyl groups into the 4,7-positions of 2,3,7,8-tetraalkylsubstituted-1,10-phenanthrolines resulted in a significant red-shift in the MLCT absorptions and shifts to more positive potentials for both the copper(I) oxidation and ligand-based reduction processes compared to the parent bis(2,3,7,8-tetramethyl-1,10phenanthroline)copper(I) complex. Most importantly, these beneficial effects were gained without loss in the long excited-state lifetimes, which result from the steric crowding imposed by the 2,3,7,8tetraalkyl groups [36]. 2,2'-Bipyridine ligands bearing phenylethynyl or 4-substituted-phenylethynyl substituents in the 4,4'-or 5,5'-positions have been the focus of some attention, see for example [37][38][39][40], although, to the best of our knowledge, the use of related bis(diimine)copper(I) complexes in Scheme 1. Schematic representation of a donor-acceptor ('push-pull') dye. In a heteroleptic bis(diimine)copper(I) dye, the anchoring ligand is the anchor/acceptor domain, the copper(I) centre is the linker, and the ancillary ligand is the donor.
While ruthenium dyes are still regarded as state-of-the-art, the abundance of ruthenium in the Earth's crust is only about 0.001 ppm [18], making explorations of sensitizers incorporating Earth-abundant first-row metals, such as copper [19][20][21][22] and iron [23][24][25][26] extremely attractive. Significant progress has been made in the last few years in improving the PCEs of DSCs sensitized with copper(I) dyes. Although the best values of η are below 5% [27][28][29], systematic efforts in modifying the sensitizer, redox couple and other electrolyte components, and the use of co-sensitizers have allowed us [28][29][30][31] and others [27,32] to show that there is a viable future for copper(I) sensitizers.
A key factor in performance enhancement of copper(I)-based DSCs is systematic modification of the structures of 'push-pull' dyes (Scheme 1). The 'push-pull' characteristics are readily tuned in a heteroleptic copper(I) complex (in which the Cu atom is the linker) through structural modification of the anchoring and ancillary ligands, such that in a charge-separated state, the hole is located on the ancillary ligand and the electron on the anchoring ligand. Donor-π-acceptor (D-π-A) dyes fulfil the 'push-pull' characteristics and introducing alkynyl units to optimize electronic communication and conjugation has been found to be beneficial in some families of dyes [33]. The +I nature of alkynyl substituents [34] increases the electron density on the metal centre and enhance the population of the metal-to-ligand charge transfer (MLCT) state [35]. Within the context of bis(diimine)copper(I) complexes, Castellano and co-workers demonstrated that the incorporation of phenylethynyl groups into the 4,7-positions of 2,3,7,8-tetraalkylsubstituted-1,10-phenanthrolines resulted in a significant red-shift in the MLCT absorptions and shifts to more positive potentials for both the copper(I) oxidation and ligand-based reduction processes compared to the parent bis(2,3,7,8-tetramethyl-1,10-phenanthroline)copper(I) complex. Most importantly, these beneficial effects were gained without loss in the long excited-state lifetimes, which result from the steric crowding imposed by the 2,3,7,8-tetraalkyl groups [36]. 2,2 -Bipyridine ligands bearing phenylethynyl or 4-substituted-phenylethynyl substituents in the 4,4or 5,5'-positions have been the focus of some attention, see for example [37][38][39][40], although, to the best of our knowledge, the use of related bis(diimine)copper(I) complexes in DSCs has not been explored. We now report the preparation and characterization of ligands 1-4 (Scheme 2), their homoleptic [CuL 2 ][PF 6 ] complexes, and the application of these compounds for the in situ assembly of heteroleptic copper(I) dyes on FTO/TiO 2 photoanodes (FTO = fluorine-doped tin oxide) in DSCs. Ligands 1-4 contain peripheral electron-donating NR 2 groups on the ancillary ligands, which are also known to be beneficial in stabilizing the hole remote from the semiconductor surface [21].
Molecules 2020, 25,  DSCs has not been explored. We now report the preparation and characterization of ligands 1-4 (Scheme 2), their homoleptic [CuL2][PF6] complexes, and the application of these compounds for the in situ assembly of heteroleptic copper(I) dyes on FTO/TiO2 photoanodes (FTO = fluorine-doped tin oxide) in DSCs. Ligands 1-4 contain peripheral electron-donating NR2 groups on the ancillary ligands, which are also known to be beneficial in stabilizing the hole remote from the semiconductor surface [21].

Ligand Syntheses and Characterization
Compound 1 was prepared using a Kröhnke approach [41] as shown in Scheme 3. For the diphenylamino-derivative 3, the most convenient route was a palladium-catalysed coupling of 4,4'bis(4-bromophenyl)-6,6'-dimethyl-2,2'-bipyridine and Ph2NH in the presence of base (Scheme 4) [42][43][44]. Scheme 5 summarizes the syntheses of ligands 2 and 4. The high-resolution mass spectra of compounds 1, 2, 3 and 4 exhibited base peaks at m/z 423.2544, 471.2548, 671.3166 and 719.3162 corresponding to the [M + H] + ions. The solid-state IR spectra of the ligands are shown in Figures S1-S4 (see Supporting Information). The presence of the alkynyl spacer gives rise to an absorption at 2198 cm −1 in 2 and 2207 cm −1 in 4, the stretching mode being IR active due to the asymmetrical substitution.

Ligand Syntheses and Characterization
Compound 1 was prepared using a Kröhnke approach [41] as shown in Scheme 3. For the diphenylamino-derivative 3, the most convenient route was a palladium-catalysed coupling of 4,4 -bis(4-bromophenyl)-6,6 -dimethyl-2,2 -bipyridine and Ph 2 NH in the presence of base (Scheme 4) [42][43][44]. Scheme 5 summarizes the syntheses of ligands 2 and 4. The high-resolution mass spectra of compounds 1, 2, 3 and 4 exhibited base peaks at m/z 423.2544, 471.2548, 671.3166 and 719.3162 corresponding to the [M + H] + ions. The solid-state IR spectra of the ligands are shown in Figures S1-S4 (see Supporting Information). The presence of the alkynyl spacer gives rise to an absorption at 2198 cm −1 in 2 and 2207 cm −1 in 4, the stretching mode being IR active due to the asymmetrical substitution.
The 1 H and 13 C{ 1 H} NMR spectra of ligands 1-4 were assigned using two-dimensional (2D) methods. Figures S5-S8 Figure S6). The NOESY cross-peak between the signals for H A5 and H A6-Me ( Figure S6) distinguished H A5 from H A3 . The resonances for H A5 from H A3 were assigned in a similar manner in compounds 2-4. In compounds 2 and 4, the alkyne unit was characterized by 13 Figure S6). The NOESY cross-peak between the signals for H A5 and H A6-Me ( Figure S6) distinguished H A5 from H A3 . The resonances for H A5 from H A3 were assigned in a similar manner in compounds 2-4. In compounds 2 and 4, the alkyne unit was characterized by 13  The solution absorption spectra of compounds 1 and 2 ( Figure 1) exhibit intense high-energy absorptions assigned to π*←π transitions [45]. Absorptions on the edge of the visible region are assigned to intra-ligand charge-transfer (ILCT) arising from transfer of charge from the electrondonating Me2N groups to the electron-poor bipyridine [21]. Introduction of NPh2 in place of NMe2 groups red-shifts the absorption spectra with a concomitant increase in values of the extinction The 1 H and 13 C{ 1 H} NMR spectra of ligands 1-4 were assigned using two-dimensional (2D) methods. Figures S5-S8 Figure S6). The NOESY cross-peak between the signals for H A5 and H A6-Me ( Figure S6) distinguished H A5 from H A3 . The resonances for H A5 from H A3 were assigned in a similar manner in compounds 2-4. In compounds 2 and 4, the alkyne unit was characterized by 13  The solution absorption spectra of compounds 1 and 2 ( Figure 1) exhibit intense high-energy absorptions assigned to π*←π transitions [45]. Absorptions on the edge of the visible region are assigned to intra-ligand charge-transfer (ILCT) arising from transfer of charge from the electrondonating Me2N groups to the electron-poor bipyridine [21]. Introduction of NPh2 in place of NMe2 groups red-shifts the absorption spectra with a concomitant increase in values of the extinction Scheme 5. Reaction scheme for the preparations of 2 (R = Me) and 4 (R = Ph). Conditions: (i) Pd(dppf)Cl 2 , CuI, PPh 3 , Me 3 SiC≡CH in Et 3 N, 80 • C overnight; (ii) K 2 CO 3 in MeOH, room temperature, 2 h; (iii) Pd(dppf)Cl 2 , CuI, PPh 3 in Et 3 N, 80 • C overnight. (dppf = 1,1 -bis(diphenylphosphano)ferrocene).
The solution absorption spectra of compounds 1 and 2 ( Figure 1) exhibit intense high-energy absorptions assigned to π*←π transitions [45]. Absorptions on the edge of the visible region are assigned to intra-ligand charge-transfer (ILCT) arising from transfer of charge from the electron-donating Me 2 N groups to the electron-poor bipyridine [21]. Introduction of NPh 2 in place of NMe 2 groups red-shifts the absorption spectra with a concomitant increase in values of the extinction coefficient ( Figure 1). A comparison of the spectra of 2 with 1, and of 4 with 3 shows the effect of extending the π-conjugation as the alkynyl unit is introduced, extending the absorption spectra towards longer wavelengths.

Syntheses and Characterization of Homoleptic Copper(I) Complexes
The  S36). Coordination to copper(I) results in a broadening of the methyl signal ( Figure 2), as well as shifting of the signal arising from proton H A5 .

Syntheses and Characterization of Homoleptic Copper(I) Complexes
The  6 ], absorptions at 2189 and 2200 cm −1 are consistent with the presence of the alkynyl unit ( Figures S21-S24). The 1 H and 13 C{ 1 H} NMR spectra were assigned using NOESY, COSY, HMQC and HMBC methods ( Figures S25-S36). Coordination to copper(I) results in a broadening of the methyl signal ( Figure 2), as well as shifting of the signal arising from proton H A5 .

Syntheses and Characterization of Homoleptic Copper(I) Complexes
The    The solution absorption spectra of the complexes were recorded in CHCl 3 ( Figure 3) and exhibit intense, high-energy absorptions arising from ligand-centred, spin-allowed transitions in addition to absorptions in the visible region. The lowest energy absorption around 500 nm is assigned to metal-to-ligand charge-transfer (MLCT). However, the band exhibits a higher extinction coefficient than is typical of bis(diimine)copper(I) complexes [19,46] due to overlap with the ligand-centred ILCT observed in the free ligands ( Figure 2). Inspection of Figure 3 reveals that the intensity of the absorption at ca. 500 nm is greatest for [Cu (3) 6 ], the absorption spectra extends further towards the red, consistent with extension of π-conjugation as the alkynyl unit is introduced. The solution absorption spectra of the complexes were recorded in CHCl3 ( Figure 3) and exhibit intense, high-energy absorptions arising from ligand-centred, spin-allowed transitions in addition to absorptions in the visible region. The lowest energy absorption around 500 nm is assigned to metalto-ligand charge-transfer (MLCT). However, the band exhibits a higher extinction coefficient than is typical of bis(diimine)copper(I) complexes [19,46] due to overlap with the ligand-centred ILCT observed in the free ligands ( Figure 2). Inspection of Figure 3 reveals that the intensity of the absorption at ca. 500 nm is greatest for [Cu (3) , the absorption spectra extends further towards the red, consistent with extension of πconjugation as the alkynyl unit is introduced. DFT calculations on [Cu(2)2] + as a representative complex confirmed that the orbital composition in the HOMO-manifold contains both metal and Me2N character, while the LUMO and LUMO+1 are predominantly bpy character (Figure 4a). A restricted hybrid HF-DFT SCF calculation was also performed on [Cu(2)2] + using a polarizable continuum solvation model (CHCl3) and revealed similar frontier molecular orbital ( MO) characteristics ( Figure S37). Note that the DFT-optimized geometries reveal that in ligand 1, the phenyl ring is twisted with respect to the pyridine ring to which it is bonded, as expected for minimizing inter-ring H...H contacts. In contrast, in 2, the aromatic rings on either side of the alkynyl unit are essentially coplanar. This is consistent with previously reported theoretical results [47] and with the distribution of dihedral angles between alkynyl-connected phenyl rings for compounds found in the Cambridge Structural Database (CSD v. 5.4.1 [48]) ( Figure  5). DFT calculations on [Cu(2) 2 ] + as a representative complex confirmed that the orbital composition in the HOMO-manifold contains both metal and Me 2 N character, while the LUMO and LUMO+1 are predominantly bpy character ( Figure 4a). A restricted hybrid HF-DFT SCF calculation was also performed on [Cu(2) 2 ] + using a polarizable continuum solvation model (CHCl 3 ) and revealed similar frontier molecular orbital ( MO) characteristics ( Figure S37). Note that the DFT-optimized geometries reveal that in ligand 1, the phenyl ring is twisted with respect to the pyridine ring to which it is bonded, as expected for minimizing inter-ring H...H contacts. In contrast, in 2, the aromatic rings on either side of the alkynyl unit are essentially coplanar. This is consistent with previously reported theoretical results [47] and with the distribution of dihedral angles between alkynyl-connected phenyl rings for compounds found in the Cambridge Structural Database (CSD v. 5.4.1 [48]) ( Figure 5).

Solid-State Absorption Spectra of the Surface-Bound Dyes and DSC Performances
The homoleptic compounds [CuL2][PF6] where L = 1-4, were used as a source of copper(I) and ancillary ligand 1-4 to assemble surface-bound heteroleptic dyes [Cu(5)(L)] + using the surfaces-asligands, surfaces-as-complex (SALSAC) ligand exchange methodology [50,51]. The phosphonic acid anchoring ligand 5 (Scheme 2) was selected because of its superior binding to TiO2 with respect to carboxylic acids [51], and because copper(I) dyes with phosphonic acids or phosphonates [52] with a phenyl spacer between the PO(OH)2 and bpy units are superior to carboxylic and cyanoacrylic acids in terms of DSC performance and/or ease of synthesis [53][54][55].
The solid-state absorption spectra of the heteroleptic dye-functionalized FTO/TiO2 electrodes were recorded. In order to better differentiate the absorption maxima in the visible region from the higher-energy tail arising from TiO2, the first-derivative spectra [56] were determined ( Figure 6). The maxima at around 510 nm in [Cu (5)(1)

Solid-State Absorption Spectra of the Surface-Bound Dyes and DSC Performances
The homoleptic compounds [CuL2][PF6] where L = 1-4, were used as a source of copper(I) and ancillary ligand 1-4 to assemble surface-bound heteroleptic dyes [Cu(5)(L)] + using the surfaces-asligands, surfaces-as-complex (SALSAC) ligand exchange methodology [50,51]. The phosphonic acid anchoring ligand 5 (Scheme 2) was selected because of its superior binding to TiO2 with respect to carboxylic acids [51], and because copper(I) dyes with phosphonic acids or phosphonates [52] with a phenyl spacer between the PO(OH)2 and bpy units are superior to carboxylic and cyanoacrylic acids in terms of DSC performance and/or ease of synthesis [53][54][55].
The solid-state absorption spectra of the heteroleptic dye-functionalized FTO/TiO2 electrodes were recorded. In order to better differentiate the absorption maxima in the visible region from the higher-energy tail arising from TiO2, the first-derivative spectra [56] were determined ( Figure 6). The maxima at around 510 nm in [Cu (5)(1)] + and [Cu(5)(3)] + , and 520 nm in [Cu(5)(2)] + and [Cu(5)(4)] + confirm that a red-shift in the absorptions accompanies the introduction of the alkyne spacer in the heteroleptic compounds. Absorption maxima for the surface-bound dyes compare with solution

Solid-State Absorption Spectra of the Surface-Bound Dyes and DSC Performances
The homoleptic compounds [CuL 2 ][PF 6 ] where L = 1-4, were used as a source of copper(I) and ancillary ligand 1-4 to assemble surface-bound heteroleptic dyes [Cu(5)(L)] + using the surfaces-asligands, surfaces-as-complex (SALSAC) ligand exchange methodology [50,51]. The phosphonic acid anchoring ligand 5 (Scheme 2) was selected because of its superior binding to TiO 2 with respect to carboxylic acids [51], and because copper(I) dyes with phosphonic acids or phosphonates [52] with a phenyl spacer between the PO(OH) 2 and bpy units are superior to carboxylic and cyanoacrylic acids in terms of DSC performance and/or ease of synthesis [53][54][55].
The solid-state absorption spectra of the heteroleptic dye-functionalized FTO/TiO 2 electrodes were recorded. In order to better differentiate the absorption maxima in the visible region from the higher-energy tail arising from TiO 2 , the first-derivative spectra [56] were determined ( Figure 6). The maxima at around 510 nm in [Cu (5)(1) (5)(2)] + show that the orbital compositions of the HOMO and HOMO-1 contain both metal and ancillary-ligand character, while the LUMO is localized on the anchoring ligand 5 (Figure 4b); the LUMO + 1 exhibits mainly ancillary ligand bpy character. Similar orbital characters are revealed for [Cu(5)(1)] + ( Figure S38 in the Supporting Information), indicating that the frontier orbital characteristics of the heteroleptic dyes are not significantly perturbed by the presence of the alkynyl spacers. Thus, the localization of LUMO character on the anchoring domain is consistent with what is desired for efficient electron injection from the anchoring ligand to the semiconductor in a DSC.   (5)(2)] + compared with the performance of a DSC containing the reference ruthenium(II) dye N719. In the final column of Table 1, we present relative efficiency values with N719 set at an arbitrary 100%. We [57] and others [58,59] find it useful to include relative values so as to permit valid comparisons of data recorded on different solar simulators or in different laboratories [57]. The first point to note in Table 1 is the reproducibility of the DSC performance parameters for a given sensitizer. This is also observed in the J-V (J = current density, V = voltage) plots in Figures 7 and S39. The most noticeable feature is that the DSCs containing [Cu(5)(2)] + have both lower JSC and VOC values than those with [Cu(5)(1)] + , indicating that the introduction of the alkynyl spacer in 2 is not beneficial. The PCEs for DSCs sensitized with [Cu(5)(1)] + are in the range 1.66-1.79%, which corresponds to 31.0-33.4% of the PCE of the reference DSC containing the ruthenium(II) dye N719. Figures 8 and S40 show the external quantum efficiency (EQE) spectra for the cells in Table 1, with values of λmax ≈ 490 nm and EQEmax ≈ 38% for [Cu(5)(1)] + , and λmax ≈ 470 nm and EQEmax ≈ 34% for [Cu(5)(2)] + . These data are consistent with the differences in the J-V curves for the two dyes.   (5)(2)] + compared with the performance of a DSC containing the reference ruthenium(II) dye N719. In the final column of Table 1, we present relative efficiency values with N719 set at an arbitrary 100%. We [57] and others [58,59] find it useful to include relative values so as to permit valid comparisons of data recorded on different solar simulators or in different laboratories [57]. The first point to note in Table 1 is the reproducibility of the DSC performance parameters for a given sensitizer. This is also observed in the J-V (J = current density, V = voltage) plots in Figure 7 and Figure S39. The most noticeable feature is that the DSCs containing [Cu (5)(2)] + have both lower J SC and V OC values than those with [Cu(5)(1)] + , indicating that the introduction of the alkynyl spacer in 2 is not beneficial. The PCEs for DSCs sensitized with [Cu(5)(1)] + are in the range 1.66-1.79%, which corresponds to 31.0-33.4% of the PCE of the reference DSC containing the ruthenium(II) dye N719. Figure 8 and Figure S40 show the external quantum efficiency (EQE) spectra for the cells in Table 1, with values of λ max ≈ 490 nm and EQE max ≈ 38% for [Cu(5)(1)] + , and λ max ≈ 470 nm and EQE max ≈ 34% for [Cu(5)(2)] + . These data are consistent with the differences in the J-V curves for the two dyes.      A critical question is why the introduction of the alkynyl spacer into the dye leads to a blueshifted EQE maximum (Figure 8) rather than the red-shift observed in the absorption spectra ( Figures  3 and 6). This reveals that the injection of electrons into the semiconductor is not benefitting from an extension of light absorption towards longer wavelengths. It is informative to compare the performance data and EQE spectra for DSCs with [Cu(5)(1)] + and [Cu(5)(2)] + with those sensitized with [Cu(5)(dmbpy)] + where dmbpy is 6,6'-dimethyl-2,2'-bipyridine (Scheme 6). Using a common A critical question is why the introduction of the alkynyl spacer into the dye leads to a blue-shifted EQE maximum (Figure 8) rather than the red-shift observed in the absorption spectra (Figures 3 and 6). This reveals that the injection of electrons into the semiconductor is not benefitting from an extension of light absorption towards longer wavelengths. It is informative to compare the performance data and EQE spectra for DSCs with [Cu(5)(1)] + and [Cu(5)(2)] + with those sensitized with [Cu(5)(dmbpy)] + where dmbpy is 6,6 -dimethyl-2,2 -bipyridine (Scheme 6). Using a common electrolyte, redox shuttle and sun simulator as employed in the present study (see Materials and Methods section), duplicate DSCs with [Cu(5)(dmbpy)] + exhibited values of J SC = 3.46 and 3.79 mA cm −2 , V OC = 522 and 527 mV, and η = 1.46% and 1.35% versus 5.91% for N719 (i.e., relative η = 24.7% and 22.8%). The EQE max values were 42% and 38% with a value of λ max = 470 nm [57]. These data are reminiscent of those for DSCs containing [Cu(5)(2)] + . We therefore see an enhancement of DSC performance on going from the dye [Cu(5)(dmbpy)] + to [Cu(5)(1)] + , but not from [Cu(5)(dmbpy)] + to [Cu(5)(2)] + . The comparison of EQE spectra shown in Figure 9 illustrates a gain in EQE arising from photon harvesting at longer wavelengths on going from [Cu(5)(dmbpy)] + to [Cu(5)(1)] + , but this enhancement is lost once the alkynyl spacer is introduced with the EQE spectrum of the DSC with [Cu (5)(2) (5)(7)] + to 480 nm [60], consistent with the red-shift seen on going from [Cu (5)(6) (5)(7)] + (peripheral OMe groups [60]); the electrolyte and redox shuttle are constant throughout. The comparison of this series of dyes reveals the beneficial effects of introducing the electron-donating, peripheral substituents, but the 'blocking' effect that the alkynyl group imparts on electron transfer and ultimate injection.  Figure 9 illustrates a gain in EQE arising from photon harvesting at longer wavelengths on going from [Cu (5) (5)(7)] + (peripheral OMe groups [60]); the electrolyte and redox shuttle are constant throughout. The comparison of this series of dyes reveals the beneficial effects of introducing the electron-donating, peripheral substituents, but the 'blocking' effect that the alkynyl group imparts on electron transfer and ultimate injection.    Figure 9 illustrates a gain in EQE arising from photon harvesting at longer wavelengths on going from [Cu (5) (5)(7)] + (peripheral OMe groups [60]); the electrolyte and redox shuttle are constant throughout. The comparison of this series of dyes reveals the beneficial effects of introducing the electron-donating, peripheral substituents, but the 'blocking' effect that the alkynyl group imparts on electron transfer and ultimate injection.   (5)(2)] + . Introduction of the alkynyl unit on going from 3 to 4 leads to lower J SC values although values of V OC are little affected ( Figure 10 and Table 2). The change from Me 2 N (ligands 1 and 2) to Ph 2 N (ligands 3 and 4) substituents results in a small improvement in DSC performance, and this is more noticeable on going from [Cu(5)(2)] + to [Cu(5)(4)] + than from [Cu(5)(1)] + to [Cu(5)(3)] + (Tables 1 and 2). This has its origins in a small increase in J SC , which, for four cells, lies in the range of 3.34-3.64 mA cm −2 for [Cu(5)(2)] + , and 3.96-4.24 mA cm −2 for [Cu(5)(4)] + . This trend is confirmed in the EQE spectra (Figure 10), which extend further to longer wavelengths for DSCs containing [Cu(5)(4)] + . As was observed for cells with dyes [Cu (5)(1) comparison of Figures 8 and 11 reveals an increase in EQEmax for [Cu(5)(4)] + (up to 42%) compared to [Cu(5)(2)] + (≈34%) as well as a significant extension of the spectrum for [Cu(5)(4)] + towards the red. These data are consistent with the improvement in JSC as the peripheral electron-donating NMe2 substituents are replaced by NPh2 groups.

Effects of Adding a Co-Adsorbent
It is well established that introducing co-adsorbents with dyes on the semiconductor surface increases values of VOC for ruthenium(II) dyes [61] and zinc(II) porphyrin dyes [62,63]. Similarly, the role of the co-adsorbent chenodeoxycholic acid was critical to achieving the high PCE of the [Cu(Lanchor)(Lancillary)] + dye in which Lanchor was 6,6'-dimesityl-2,2'-bipyridine-4,4'-dicarboxylic acid and Lancillary was a 2,2'-bipyridine containing electron-donating triphenylamino groups [21]. We were therefore interested to see whether use of a co-adsorbent could enhance the performance of the dyes in the present investigation, and we chose to investigate the dye [Cu(5)(1)] + . Chenodeoxycholic acid (cheno) is commonly employed as a co-adsorbent and contains a carboxylic acid anchoring unit. In contrast, anchoring ligand 5 bears a phosphonic acid group. Since we have shown that phosphonic anchoring ligands displace carboxylic anchors in TiO2 [51], we opted to use n-decylphosphonic acid (DPA, Scheme 6) instead of cheno as the co-adsorbent. Table 3 gives the DSC performance parameters for four DSCs sensitized with [Cu(5)(1)] + in the presence of the co-adsorbent DPA, and the J-V curves for the cells are shown in Figure 12. The ranges of JSC (4.42 to 4.75 mA cm −2 ) and JSC values (534 to 548 mV) are similar to those for cells with no coadsorbent (Table 1) leading to similar overall photoconversion efficiencies. The EQE spectra ( Figure  13) confirm reproducible behaviour for the four DSCs with values of EQEmax around 43% at a value of λmax = 490 nm. Again, these results reflect those of the devices in the absence of DPA. Evidence that the co-adsorbent is present on the semiconductor surface comes from the electrochemical impedance spectroscopic data presented below.

Effects of Adding a Co-Adsorbent
It is well established that introducing co-adsorbents with dyes on the semiconductor surface increases values of V OC for ruthenium(II) dyes [61] and zinc(II) porphyrin dyes [62,63]. Similarly, the role of the co-adsorbent chenodeoxycholic acid was critical to achieving the high PCE of the [Cu(L anchor )(L ancillary )] + dye in which L anchor was 6,6 -dimesityl-2,2 -bipyridine-4,4 -dicarboxylic acid and L ancillary was a 2,2 -bipyridine containing electron-donating triphenylamino groups [21]. We were therefore interested to see whether use of a co-adsorbent could enhance the performance of the dyes in the present investigation, and we chose to investigate the dye [Cu(5)(1)] + . Chenodeoxycholic acid (cheno) is commonly employed as a co-adsorbent and contains a carboxylic acid anchoring unit. In contrast, anchoring ligand 5 bears a phosphonic acid group. Since we have shown that phosphonic anchoring ligands displace carboxylic anchors in TiO 2 [51], we opted to use n-decylphosphonic acid (DPA, Scheme 6) instead of cheno as the co-adsorbent. Table 3 gives the DSC performance parameters for four DSCs sensitized with [Cu(5)(1)] + in the presence of the co-adsorbent DPA, and the J-V curves for the cells are shown in Figure 12. The ranges of J SC (4.42 to 4.75 mA cm −2 ) and J SC values (534 to 548 mV) are similar to those for cells with no co-adsorbent (Table 1) leading to similar overall photoconversion efficiencies. The EQE spectra ( Figure 13) confirm reproducible behaviour for the four DSCs with values of EQE max around 43% at a value of λ max = 490 nm. Again, these results reflect those of the devices in the absence of DPA. Evidence that the co-adsorbent is present on the semiconductor surface comes from the electrochemical impedance spectroscopic data presented below.

Electrochemical Impedance Spectroscopy (EIS)
As discussed above, the introduction of the alkynyl spacer into the dyes results in a red-shifted absorption (Figures 3 and 6), but a blue-shifted EQE maximum ( Figure 8) as well as a decrease of EQEmax. The lowering of the charge transfer energy results in more difficult electron injection into the semiconductor, and the values of JSC decrease. By using electrochemical impedance spectroscopy (EIS), it is possible to investigate the multiple interfacial electronic processes in a DSC [64,65] and we therefore decided to apply EIS in an attempt to gain insight into the origin of the poorer DSC performances for dyes containing the alkynyl spacers. EIS is a technique that can describe electronic processes in terms of electron and hole diffusion in the counter electrode/electrolyte/dyesemiconductor interfaces of DSCs. Parameters including the recombination resistance (Rrec), chemical capacitance (Cµ), electron/hole transport resistance (Rtr), electron lifetime (τ) and electron diffusion length (Ld) can be extracted from the fits of the experimental data typically presented as Nyquist and Bode plots. All measurements were performed at VOC conditions and a light intensity of 22 mW cm −2 . The equivalent circuit model used for fitting the measurements is shown in Figure 14.

Electrochemical Impedance Spectroscopy (EIS)
As discussed above, the introduction of the alkynyl spacer into the dyes results in a red-shifted absorption (Figures 3 and 6), but a blue-shifted EQE maximum ( Figure 8) as well as a decrease of EQEmax. The lowering of the charge transfer energy results in more difficult electron injection into the semiconductor, and the values of JSC decrease. By using electrochemical impedance spectroscopy (EIS), it is possible to investigate the multiple interfacial electronic processes in a DSC [64,65] and we therefore decided to apply EIS in an attempt to gain insight into the origin of the poorer DSC performances for dyes containing the alkynyl spacers. EIS is a technique that can describe electronic processes in terms of electron and hole diffusion in the counter electrode/electrolyte/dyesemiconductor interfaces of DSCs. Parameters including the recombination resistance (Rrec), chemical capacitance (Cµ), electron/hole transport resistance (Rtr), electron lifetime (τ) and electron diffusion length (Ld) can be extracted from the fits of the experimental data typically presented as Nyquist and Bode plots. All measurements were performed at VOC conditions and a light intensity of 22 mW cm −2 . The equivalent circuit model used for fitting the measurements is shown in Figure 14.

Electrochemical Impedance Spectroscopy (EIS)
As discussed above, the introduction of the alkynyl spacer into the dyes results in a red-shifted absorption (Figures 3 and 6), but a blue-shifted EQE maximum ( Figure 8) as well as a decrease of EQE max . The lowering of the charge transfer energy results in more difficult electron injection into the semiconductor, and the values of J SC decrease. By using electrochemical impedance spectroscopy (EIS), it is possible to investigate the multiple interfacial electronic processes in a DSC [64,65] and we therefore decided to apply EIS in an attempt to gain insight into the origin of the poorer DSC performances for dyes containing the alkynyl spacers. EIS is a technique that can describe electronic processes in terms of electron and hole diffusion in the counter electrode/electrolyte/dye-semiconductor interfaces of DSCs. Parameters including the recombination resistance (R rec ), chemical capacitance (C µ ), electron/hole transport resistance (R tr ), electron lifetime (τ) and electron diffusion length (L d ) can be extracted from the fits of the experimental data typically presented as Nyquist and Bode plots. All measurements were performed at V OC conditions and a light intensity of 22 mW cm −2 . The equivalent circuit model used for fitting the measurements is shown in Figure 14.
processes in terms of electron and hole diffusion in the counter electrode/electrolyte/dyesemiconductor interfaces of DSCs. Parameters including the recombination resistance (Rrec), chemical capacitance (Cµ), electron/hole transport resistance (Rtr), electron lifetime (τ) and electron diffusion length (Ld) can be extracted from the fits of the experimental data typically presented as Nyquist and Bode plots. All measurements were performed at VOC conditions and a light intensity of 22 mW cm −2 . The equivalent circuit model used for fitting the measurements is shown in Figure 14.  Figure 14. Equivalent circuit used to fit the electrochemical impedance spectroscopy (EIS) data, where R s is the series resistance, R Pt and CPE1 are the resistance and constant phase element of the platinum counter electrode, DX1 is an extended distributed element that represents the TiO 2 /electrolyte interface and W S is the Warburg element associated with diffusion of the electrolyte.
We first focused our attention on a comparison of DSCs sensitized with [Cu(5)(1)] + and [Cu(5)(2)] + , and parameters extracted from fitting of the experimental Nyquist plots (Figure 15) are given in Table S1 (all data) and Table 4 (the most representative DSC of a set of four). Going from the DSCs with [Cu(5)(1)] + to those [Cu(5)(2)] + , the values of R rec increase. At the same time, values of C µ are higher for [Cu(5)(1)] + than [Cu(5)(2)] + . This shows that more electron density is located in the conductive band of the semiconductor in the case of the dye [Cu(5)(1)] + , which contributes to greater J SC values. The short-circuit density depends not only on the charge injection, but also on the collection of photoinjected electrons at the anode [66], and the electron diffusion length has to be greater than thickness of TiO 2 for the efficient collection of electrons. The L d value depends on R rec and R tr and can be calculated from the Equation (1) where d (≈ 12 µm) is the thickness of the TiO 2 layer [67].

Ld = d Rrec/Rtr
(1) given in Table S1 (all data) and Table 4 (the most representative DSC of a set of four). Going from the DSCs with [Cu(5)(1)] + to those [Cu(5)(2)] + , the values of Rrec increase. At the same time, values of Cµ are higher for [Cu(5)(1)] + than [Cu(5)(2)] + . This shows that more electron density is located in the conductive band of the semiconductor in the case of the dye [Cu(5)(1)] + , which contributes to greater JSC values. The short-circuit density depends not only on the charge injection, but also on the collection of photoinjected electrons at the anode [66], and the electron diffusion length has to be greater than thickness of TiO2 for the efficient collection of electrons. The Ld value depends on Rrec and Rtr and can be calculated from the equation (1) where d (≈ 12 μm) is the thickness of the TiO2 layer [67].

/
(1)     (5)(1)] + with co-adsorbent increase with respect to [Cu (5)(1)] + with no co-adsorbent. However, this in accompanied by a decrease in C µ and a large increase in R rec and this results in similar overall performances for DSCs with and without co-adsorbent.
For a well-performing DSC, the electron lifetime (τ) needs to be longer than electron transport time (τ t ). This results in the efficient transport of the photoinjected electrons through the semiconductor, and this is the case for all the DSCs (Table 4 and Table S1). The extremely high τ for [Cu (5)(1) The diffusion length does not vary significantly between the dyes [Cu (5)(1)] + and [Cu(5)(2)] + and is about two times greater than d, despite the difference in EQEmax values and red-shifted enhancement in the range of the spectrum for [Cu (5)(1)] + . The addition of the co-adsorbent DPA in the DSCs with dyes [Cu(5)(1)] + results in a significant difference in EQEmax as well as in Ld. The diffusion length and EQEmax values for [Cu (5)(1)] + with co-adsorbent increase with respect to [Cu(5)(1)] + with no co-adsorbent. However, this in accompanied by a decrease in Cµ and a large increase in Rrec and this results in similar overall performances for DSCs with and without coadsorbent.
For a well-performing DSC, the electron lifetime (τ) needs to be longer than electron transport time (τt). This results in the efficient transport of the photoinjected electrons through the semiconductor, and this is the case for all the DSCs (Tables 4 and S1). The extremely high τ for [Cu (5)(1)  The most significant differences in EIS parameters are observed for values of the recombination resistance and chemical capacitance, and we conclude that [Cu(5)(1)] + has the most favourable electron injection into the conduction band of the semiconductor.
All reactions were carried out with chemicals used as received from Sigma Aldrich (Sigma Aldrich Chemie GmbH, 89555 Steinheim, Germany) or Fluorochem (Chemie Brunschwig AG, 4052 Basel, Switzerland) without further purification. Biotage Sfär silica HC D was purchased from Biotage (Biotage EU, 75103 Uppsala, Sweden). The most significant differences in EIS parameters are observed for values of the recombination resistance and chemical capacitance, and we conclude that [Cu(5)(1)] + has the most favourable electron injection into the conduction band of the semiconductor.
All reactions were carried out with chemicals used as received from Sigma Aldrich (Sigma Aldrich Chemie GmbH, 89555 Steinheim, Germany) or Fluorochem (Chemie Brunschwig AG, 4052 Basel, Switzerland) without further purification. Biotage Sfär silica HC D was purchased from Biotage (Biotage EU, 75103 Uppsala, Sweden).

Calculations
Ground state density functional theory DFT calculations were carried out using Spartan '18 (v. 1.3) [74] at the B3LYP level with a 6-31G* basis set in vacuum. We have previously demonstrated that for bis(diimine)copper(I) complexes, the choice of atomic orbital basis set (6-311++G** basis set on all atoms, 6-311++G** on Cu and 6-31G* basis set on C, H and N, or 6-31G* basis set on all atoms) has a negligible effect on the calculated MO compositions, while significantly influencing the calculated absorption spectra [75]. Hence, a 6-31G* basis set on all atoms was chosen to optimize computer time. Geometry optimization was also carried out at the DFT level after an initial geometry energy optimization had been completed at a semi-empirical (PM3) level. For the complex [Cu(2) 2 ] + , a restricted hybrid HF-DFT SCF calculation was also performed using Pulay DIIS plus Geometric Direct Minimization Polarizable Continuum solvation model in CHCl 3 using Spartan '18 (v. 1.3) [74].

DSC Fabrication
FTO/TiO 2 electrodes (Solaronix Test Cell Titania Electrodes, Solaronix SA, Aubonne, Switzerland) were washed with milliQ water and EtOH, heated at 450 • C for 30 min, and then cooled to ca. 80 • C. The electrodes were immediately immersed in a DMSO solution of 5 (1.0 mM) for 24 h, after which they were removed, washed with DMSO and EtOH, and finally dried in a stream of N 2 . To assemble the adsorbed heteroleptic dyes, each functionalized electrode was placed in a CH 2 Cl 2 solution (0.1 mM) of [CuL 2 ][PF 6 ] (L = 1, 2, 3 or 4) for 3 days at room temperature, and was then removed, washed with CH 2 Cl 2 and dried under an N 2 stream. For the reference dye N719 (Solaronix SA, Aubonne, Switzerland), FTO/TiO 2 electrodes (Solaronix Test Cell Titania Electrodes, Solaronix SA, Aubonne, Switzerland) were immersed in a solution of N719 (EtOH, 0.3 mM) for 1 day. Then the electrodes were removed from the solution, washed with EtOH and dried in a stream of N 2 . Counter electrodes (Solaronix Test Cell Platinum Electrodes, Solaronix SA, Aubonne, Switzerland) were washed with EtOH and then heated at 450 • C for 30 min to remove volatile organic impurities.

Electrodes for Solid-State Absorption Spectroscopy
The method in Section 3.13 was used to fabricate dye-functionalized electrodes, but starting with Solaronix Test Cell Titania Electrodes Transparent (Solaronix SA, Aubonne, Switzerland).

DSCs with Co-Adsorbent n-Decylphosphonic Acid
The method in Section 3.13 was used to fabricate dye-functionalized electrodes, but with one change to the dye bath procedure. In place of a DMSO solution of 5 (1.0 mM), a DMSO solution of containing both 5 (1.0 mM) and n-decylphosphonic acid (1.0 mM) was used. The dipping time was 24 h.

DSC, EQE and EIS Measurements
The DSCs were all masked before measurements. The mask was made from a black-coloured copper sheet with an accurately calibrated aperture smaller than the surface area of TiO 2 . Cells were also masked on the top and on the sides using black tape. Performance measurements were made by irradiating the DSC from behind with a LOT Quantum Design LS0811 instrument (LOT-QuantumDesign GmbH, Darmstadt, Germany, 100 mW cm −2 = 1 sun, AM1.5 G conditions) and the simulated light power was calibrated with a silicon reference cell.
EQE measurements used a Spe Quest quantum efficiency setup (ReRa Systems, Nijmegen, The Netherlands) with a 100 W halogen lamp (QTH) and a lambda 300 grating monochromator (LOT-Oriel GmbH & Co. KG, Darmstadt, Germany). The monochromatic light was modulated to 3 Hz using a chopper wheel (ThorLabs Inc., Newton, NJ, USA), and the cell response was amplified with a large dynamic range IV converter (Melles Griot B.V., Didam, the Netherlands) and measured with a SR830 DSP Lock-In amplifier (Stanford Research Systems Inc., Sunnyvale, CA, USA).
EIS measurements were performed using a ModuLab ® XM PhotoEchem photoelectrochemical measurement system (Solartron Metrology Ltd., Leicester, UK). The impedance was measured in galvanostatic mode at open-circuit potential of the cell at a light intensity of 22 mW cm −2 (590 nm) in the frequency range 0.05 Hz to 400 kHz using an amplitude of 10 mV. The impedance data were analysed using ZView ® software (Scribner Associates Inc., Southern Pines, NC, USA).

Conclusions
We have described the syntheses and characterisations of ligands 1-4 and the homoleptic copper(I) complexes [Cu (1)