Effects of Introducing Methoxy Groups into the Ancillary Ligands in Bis(diimine) Copper(I) Dyes for Dye-Sensitized Solar Cells

Abstract

A systematic investigation of four heteroleptic bis(diimine) copper(I) dyes in n-type Dye-Sensitized Solar Cells (DSSCs) is presented. The dyes are assembled using a stepwise, on-surface assembly. The dyes contain a phosphonic acid-functionalized 2,2′-bipyridine (bpy) anchoring domain (5) and ancillary bpy ligands that bear peripheral phenyl (1), 4-methoxyphenyl (2), 3,5-dimethoxyphenyl (3), or 3,4,5-trimethoxyphenyl (4) substituents. In masked DSSCs, the best overall photoconversion efficiency was obtained with the dye [Cu(5)(4)]⁺ (1.96% versus 5.79% for N719). Values of J_SC for both [Cu(5)(2)]⁺ (in which the 4-MeO group is electron releasing) and [Cu(5)(4)]⁺ (which combines electron-releasing and electron-withdrawing effects of the 4- and 3,5-substituents) and are enhanced with respect to [Cu(5)(1)]⁺. DSSCs with [Cu(5)(3)]⁺ show the lowest J_SC. Solid-state absorption spectra and external quantum efficiency spectra reveal that [Cu(5)(4)]⁺ benefits from an extended spectral range at higher energies. Values of V_OC are in the order [Cu(5)(4)]⁺ > [Cu(5)(1)]⁺ > [Cu(5)(2)]⁺ > [Cu(5)(3)]⁺. Density functional theory calculations suggest that methoxyphenyl character in MOs within the HOMO manifold in [Cu(5)(2)]⁺ and [Cu(5)(4)]⁺ may contribute to the enhanced performances of these dyes with respect to [Cu(5)(1)]⁺.

1. Introduction

Dye-sensitized solar cells (DSSCs) [1] convert solar to electrical energy using an optically transparent, wide-band gap semiconductor functionalized with a surface-bound dye which extends the absorption range into the visible spectrum [2,3,4]. The semiconductor is commonly mesoporous TiO₂ and the sensitizer is typically a ruthenium(II) complex such as the standard reference dye N719 (Scheme 1) or an organic dye in DSSCs based on n-type semiconductors. Photoconversion efficiencies (η) of up to 11–14% have been recorded using ruthenium-based, organic, or zinc(II) porphyrin-based sensitizers [5,6,7,8,9,10]. However, the scarcity of ruthenium in the Earth’s crust and its associated high cost have motivated us and others to investigate the use of inorganic dyes containing Earth abundant metals. Copper(I) [11,12,13] and iron(II) [14,15] complexes are of primary interest. For copper-sensitized DSSCs, reported values of η in the range 3–5% [16,17,18] confirm the potential of DSSCs containing copper(I) dyes. When comparing these lower photoconversion efficiencies with the values obtained for state-of-the-art ruthenium(II) dyes, it is important to recognize that the dye structures and dye/electrolyte combinations in the ruthenium-based systems have been optimized for over a quarter of a century. In contrast, copper-based DSSCs are still in their infancy and, with systematic tuning of dye and electrolyte components [16,19,20,21,22] and the use of co-sensitization [18], enhanced performances are gradually being achieved.

Most copper(I) sensitizers are bis(diimine) copper(I) complexes and “push–pull” dyes which facilitate electron injection necessarily require the use of heteroleptic complexes. To overcome the problems of rapid ligand redistribution in solution (Equation (1)), Odobel and coworkers have used the HETPHEN approach [17], in which sterically demanding groups in the 6,6′-positions of a 2,2′-bipyridine (bpy) or 2,9-positions in a 1,10-phenanthroline ligand stabilize the heteroleptic complex. Our own strategy is a “surfaces-as-ligands, surfaces as complexes” (SALSAC) approach in which the heteroleptic dye is assembled on the glass FTO/TiO₂ electrode in a stepwise manner [11]. For performance enhancement, heteroleptic dyes are essential and the SALSAC approach remains the most adaptable. It also has an advantage of allowing dye regeneration [23,24].

$$\mathbf{2}{\left[\mathrm{Cu}\right({\mathrm{L}}^{\mathrm{A}}\left)\right({\mathrm{L}}^{\mathrm{B}}\left)\right]}^{+}\leftrightarrows {\left[\mathrm{Cu}{\left({\mathrm{L}}^{\mathrm{A}}\right)}_2\right]}^{+}+{\left[\mathrm{Cu}{\left({\mathrm{L}}^{\mathrm{B}}\right)}_2\right]}^{+}$$

(1)

For the anchoring domain in a dye in DSSCs, carboxylic acid (or carboxylate) anchors are commonly employed [25]. However, for heteroleptic copper(I) dyes, we have found that phosphonic acids are superior to carboxylic acids [11]. This is consistent with the strong binding of phosphonic acids and phosphonates to TiO₂ [26] and the applications of phosphonic acid anchors in some ruthenium dyes [27].

We have investigated the use of ancillary ligands with hole-transporting dendrons [28], but such ligands are synthetically time-intensive, and we have generally found that dyes containing structurally simple ancillary ligands [16,18,29] perform as well, if not better, than those with more elaborate architecture. In accordance with the maxim “small is beautiful” [30], we now present an investigation of the effects on DSSC performance of introducing peripheral methoxy substituents into the ancillary ligand in bis(diimine) copper(I) dyes. The ancillary ligands 1–4 and anchoring ligand 5 (Scheme 2) each contain methyl substituents in the 6,6′-positions of the bpy unit to stabilize the copper(I) complexes. Copper(I) prefers a tetrahedral coordination geometry, and substituents close to the copper(I) centre prevent flattening to the square-planar geometry favoured by copper(II) [31]. The ancillary ligands 1, 2, 3, and 4 contain peripheral phenyl, 4-methoxyphenyl, 3,5-dimethoxyphenyl, and 3,4,5-trimethoxyphenyl substituents, respectively. We were interested to know how the overall electron-releasing behaviour of the para-OMe or electron-withdrawing nature of the meta-OMe substituents [32,33,34] would influence the DSSC performances of the dyes [Cu(5)(L_ancillary)]⁺ (L_ancillary = 1, 2, 3, or 4). Such substituent effects have been used by Wu et al. to tune properties of hole-transporting materials for perovskite solar cells [35].

2. Results and Discussion

2.1. Synthesis and Compound Characterization

Compounds 1–3 and 5 were prepared as previously reported [36,37,38,39]. Ligand 4 was synthesized using Kröhnke [40] methodology as shown in Scheme 3. The ¹H and ¹³C NMR spectra of 4 are displayed in Figures S1 and S2 and were assigned by COSY, NOESY, HMQC, and HMBC methods. A cross peak in the NOESY spectrum between H^Me and H^A5 distinguished the signals for H^A3 and H^A5. The methyl groups of the methoxy substituents give rise to ¹H NMR resonances at δ 3.97 and 3.92 ppm (relative integrals of 2:1) while the signal assigned to the protons of the 6,6′-dimethyl groups appears at δ 2.72 ppm.

The reaction of [Cu(MeCN)₄][PF₆] with ligands 1, 2, 3, or 4 yielded the orange-red homoleptic complexes [Cu(1)₂][PF₆], [Cu(2)₂][PF₆], [Cu(3)₂][PF₆], and [Cu(4)₂][PF₆], respectively. We have previously described [Cu(2)₂][PF₆], and the spectroscopic properties of the batch prepared in this study were fully in accord with the reported data [37]. Figures S3–S8 show ¹H and ¹³C NMR spectra of [Cu(1)₂][PF₆], [Cu(3)₂][PF₆], and [Cu(4)₂][PF₆]. Figure 1 shows the ¹H NMR spectrum of [Cu(4)₂][PF₆] as a representative example. The spectra were assigned by 2D methods. For [Cu(1)₂][PF₆], the ¹³C NMR signals for the phenyl ring C–H ¹³C nuclei appear in the range δ 130.9 to 128.4 ppm. On comparing [Cu(1)₂][PF₆] to [Cu(3)₂][PF₆], the signal for C^B3 shifts to δ 162.6 ppm, consistent with the OMe groups attached to C^B3. Signals for C^B2 and C^B4 appear at δ 106.7 and 102.2 ppm. In [Cu(4)₂][PF₆], nuclei C^B3 and C^B4 are characterized by ¹³C NMR signals at δ 154.2 and 139.9 ppm, respectively. In [Cu(4)₂][PF₆], the ¹³C NMR resonances for C^A2 and C^A6 could not be resolved in the 1D spectrum (Figure S8), and the HMBC spectrum was used to locate them at δ 157.4 and 152.1 ppm (Figure 2). In the respective electrospray mass spectrums of [Cu(1)₂][PF₆], [Cu(3)₂][PF₆], and [Cu(4)₂][PF₆], the base peak corresponded to the [M–PF₆]⁺ ion at major isotopomer (m/z) = 735.24, 975.31, and 1095.30, respectively. Each peak envelope showed the predicted isotope pattern (Figure S9).

2.2. Solution and Solid-State Absorption Spectra

The solution absorption spectra of the homoleptic complexes are presented in Figure 3 and the absorption maxima and intensity data are given in

Table 1. Absorption maxima for homoleptic copper(I) complexes (CH₂Cl₂, 5 × 10⁻⁵ mol dm⁻³) ^a.

Complex	λmax/nm (εmax/dm3 mol−1 cm−1)
	π*←π and π*←n	MLCT
[Cu(1)2][PF6]	255sh (56,600), 275 (68,300), 322 (37,400), 356sh (10,900)	483 (11,400)
[Cu(2)2][PF6]a	282 (62,500), 316sh (44,300), 329sh (39,000), 357sh (14,100)	484 (11,400)
[Cu(3)2][PF6]	278 (73,900), 318 (49,000), 354sh (11,400)	486 (13,600)
[Cu(4)2][PF6]	285 (69,400), 316 (51,600)	482 (13,100)

^a The absorption spectrum of a MeCN solution of [Cu(2)₂][PF₆] has previously been reported [37].

. We include the spectrum of [Cu(2)₂][PF₆] for comparison. The previously reported data was for an MeCN solution [37] whereas, here, the data is for CH₂Cl₂ solutions. Intense absorption bands below 390 nm arise from π*←π transitions in [Cu(1)₂][PF₆] and from π*←π and π*←n transitions in the compounds containing methoxy groups. Each compound exhibits a metal-to-ligand charge transfer (MLCT) band in the range 482–486 nm.

Heteroleptic dyes [Cu(5)(L_ancillary)]⁺ were assembled on transparent TiO₂ electrodes (i.e., no TiO₂ scattering layer) by sequential dipping of the glass FTO/TiO₂ electrodes in baths containing anchoring ligand 5 in DMSO (Scheme 2) and one of the homoleptic complexes in CH₂Cl₂ (Figure 4a). This process leads to the assembly of the surface-anchored heteroleptic dye, exemplified in Figure 4b by [Cu(5)(1)]⁺. The solid-state absorption spectra of the functionalized electrodes are shown in Figure 5, with the MLCT band appearing at 475 nm for [Cu(5)(1)]⁺ and [Cu(5)(2)]⁺, 476 nm for [Cu(5)(3)]⁺, and 473 nm for [Cu(5)(4)]⁺. A blue-shift of ca.10 nm is observed on going from the solution spectrum of [Cu(L_ancillary)₂]⁺ to the solid-state spectrum of the surface-bound [Cu(L_anchor)(L_ancillary)]⁺ complex. The protonation state of the phosphonic acid is not known.

2.3. DSSC Performances

Working electrodes with a scattering layer were functionalized with the heteroleptic dyes [Cu(5)(L_ancillary)]⁺ dyes (L_ancillary = 1, 2, 3, or 4) as summarized in Figure 4, and duplicate masked DSSCs for each dye were assembled (see Materials and Methods). Reference DSSCs containing N719 (Scheme 1) were also fabricated (see Materials and Methods), and the plots of current density (J) against potential (V) in Figure 6 confirm the reproducibility of their performances. We have observed [16] that copper-dye functionalized DSSCs may improve in performance after initial fabrication. The J–V measurements for the DSSCs were, therefore, made on the day of sealing the cells (day 0) and repeated seven days later. The performance parameters for these masked DSSCs are given in

Table 2. Performance parameters (under 1 sun illumination) of duplicate, masked DSSCs (mask aperture calibrated and ca. 0.06 cm²) with [Cu(5)(L_ancillary)]⁺ (L_ancillary = 1, 2, 3, or 4) on the day of sealing the cells, and after seven days. The data is compared to a DSSC containing N719 and relative η values are with respect to N719 set at 100% ^a.

Dye	On the Day of DSSC Fabrication
	JSC/mA cm–2	VOC/mV	ff/%	η/%	Relative η/%
[Cu(5)(1)]+	4.27	545	71	1.66	28.7
[Cu(5)(1)]+	4.20	536	70	1.58	27.3
N719 (cell 1) a	13.29	647	67	5.79	100
[Cu(5)(2)]+	4.87	528	71	1.82	30.1
[Cu(5)(2)]+	4.74	524	72	1.80	29.8
N719 (cell 2) a	13.91	635	68	6.04	100
[Cu(5)(3)]+	3.68	528	73	1.43	25.4
[Cu(5)(3)]+	3.90	516	72	1.46	24.2
N719 (cell 3) a	13.42	631	66	5.62	100
[Cu(5)(4)]+	4.79	567	72	1.96	33.9
[Cu(5)(4)]+	4.68	550	68	1.75	30.2
N719 (cell 1) a	13.29	647	67	5.79	100
Dye	7 Days after DSSC Fabrication
	JSC/mA cm–2	VOC/mV	ff/%	η/%	Relative η/%
[Cu(5)(1)]+	4.22	580	70	1.72	29.7
[Cu(5)(1)]+	4.19	573	70	1.68	29.0
N719 (cell 1) a	12.70	670	68	5.80	100
[Cu(5)(2)]+	4.33	569	71	1.74	28.0
[Cu(5)(2)]+	5.02	565	65	1.86	30.0
N719 (cell 2) a	13.23	690	68	6.21	100
[Cu(5)(3)]+	3.23	541	74	1.29	21.8
[Cu(5)(3)]+	4.30	562	70	1.70	28.8
N719 (cell 1) a	13.05	673	67	5.91	100
[Cu(5)(4)]+	4.75	580	67	1.86	32.1
[Cu(5)(4)]+	4.64	593	65	1.80	31.0
N719 (cell 3) a	12.70	670	68	5.80	100

^a Three different N719 reference cells were used: cell 1 for measurements with [Cu(5)(1)]⁺ and [Cu(5)(3)]⁺, cell 2 for measurements with [Cu(5)(3)]⁺, and cell 3 for measurements with [Cu(5)(4)]⁺.

. DSSCs with [Cu(5)(1)]⁺ and [Cu(5)(3)]⁺ were referenced to N719 cell 1, with [Cu(5)(2)]⁺ to N719 cell 2, and with [Cu(5)(4)]⁺ to N719 cell 3 (Table 2). The final column in Table 2 gives relative values of the efficiency η with respect to N719 set at 100%, and (on the day that the DSSCs were made) values range from 24.2% and 25.4% for [Cu(5)(3)]⁺ to 30.2% and 33.9% for [Cu(5)(4)]⁺. Comparisons of the DSSC parameters for days 0 and 7 indicate that the devices are stable over this period but that there are no ripening effects [41,42] leading to significantly enhanced performance.

Table 2 and the J–V curves in Figure 7 reveal that the highest short-circuit current densities (J_SC) are obtained for the dyes [Cu(5)(2)]⁺ and [Cu(5)(4)]⁺ with the 4-MeO and 3,4,5-(MeO)₃ substitution patterns in each phenyl ring (Scheme 2). While differences in J_SC are small for these two dyes, values of the open-circuit voltages (V_OC) are larger for [Cu(5)(4)]⁺ than [Cu(5)(2)]⁺. The replacement of the phenyl substituents in [Cu(5)(1)]⁺ by 4-methoxyphenyl or 3,4,5-trimethoxyphenyl groups in [Cu(5)(2)]⁺ and [Cu(5)(4)]⁺, respectively, enhances J_SC but leads to a fall in V_OC for [Cu(5)(2)]⁺, as opposed to a gain in V_OC for [Cu(5)(4)]⁺. While Figure 7 shows this for the best performing cells, the scatter plots in Figure 8 confirm that the trend is true for the duplicate cells. The presence of the 4-MeO group is critical. The dye [Cu(5)(3)]⁺ containing 3,5-dimethoxyphenyl groups exhibits the lowest values of V_OC and J_SC of all four dyes (Figure 7 and Figure 8). The trends are consistent with the competitive mesomeric (+M) and inductive (−I) effects of the methoxy substituents. Electrons are released by the +M effect but are withdrawn by the −I effect. On going from 1 to 2, the electron-releasing 4-methoxy groups [32,33,34] enhance the “push–pull” characteristics of the dye, resulting in higher J_SC. In contrast, introducing the electron-withdrawing 3,5-dimethoxy substituents [32,33,34] upon replacing ancillary ligand 1 by 3 has a detrimental effect on DSSC performance. Ancillary ligand 4 combines the effects of both ligands 2 and 3 and, interestingly, this results in the best-performing dye. DSSCs with [Cu(5)(4)]⁺ exhibit both the highest V_OC and J_SC (Figure 7 and Figure 8) and photoconversion efficiences up to 33.9% relative to N719 set at 100%. The trends in J_SC are reflected in the external quantum efficiency (EQE) spectra displayed in Figure 9.

Table 3. EQE maxima for duplicate DSSCs containing dyes [Cu(5)(L_ancillary)]⁺ (L_ancillary = 1–4).

Dye	λmax/nm	EQEmax/%
[Cu(5)(1)]+	470	42.7
[Cu(5)(1)]+	470	41.9
[Cu(5)(2)]+	480	46.5
[Cu(5)(2)]+	480	45.3
[Cu(5)(3)]+	470	39.1
[Cu(5)(3)]+	470	38.0
[Cu(5)(4)]+	470	45.3
[Cu(5)(4)]+	470	44.9

gives values of EQE_max and λ_max, and values of λ_max are similar to the absorption maxima in the solid-state spectra in Figure 5. The slightly lower EQE_max for [Cu(5)(4)]⁺ versus [Cu(5)(2)]⁺ is offset by the broader spectral range extending to higher energies; this is also observed in the solid-state absorption spectrum (Figure 5).

2.4. Characteristics of the HOMO and LUMO Manifolds

Ground state density functional theory (DFT) calculations were used to examine how methoxy-substitution affects the characteristics of the molecular orbitals in the HOMO and LUMO manifolds of the dyes. In an earlier study of bis(diimine) copper(I) complexes, we demonstrated that use of different atomic orbital basis sets (6-311++G** basis set on all atoms, 6-311++G** on Cu, 6-31G* on C, H and N, or 6-31G* on all atoms) strongly influences the calculated absorption spectra but had no significant effect upon the characteristics of the MOs lying in the HOMO and LUMO manifolds [43]. In the present investigation, we therefore employed a 6-31G* basis set on all atoms to reduce the computational burden, and we focus mainly on orbital composition.

The characters of the MOs from HOMO−3 to LUMO+1 and energy levels are shown in Figure 10 and Figure 11. For all sensitizers, the LUMO is localized on the anchoring ligand while the LUMO+1 is localized on the bpy unit of the ancillary ligand. The energy gap between the LUMO and LUMO+1 is similar in each complex (Figure 11). Inspection of the character of the HOMO in each complex is instructive. The HOMO is essentially centered on copper. Although, in each of the methoxy-functionalized dyes there are additional contributions from the ancillary ligand extending over the peripheral aryl groups. This is most apparent for [Cu(5)(2)]⁺. The appearance of dominant methoxyphenyl character in HOMO−3 in [Cu(5)(2)]⁺ and [Cu(5)(4)]⁺, as well in HOMO−2 and HOMO−3 in [Cu(5)(3)]⁺, may aid hole transport over the ancillary ligand. This is reminiscent of [Cu(5)(7)]⁺ (7 = 4,4′-bis(4-iodophenyl)-6,6-dimethyl-2,2′-bipyridine), for which we have suggested that the improved performance of DSSCs containing [Cu(5)(7)]⁺ with respect to other halo-analogues may be associated with better electron transfer from the electrolyte over the 4-IC₆H₄ substituent [16].

3. Materials and Methods

3.1. General

¹H and ¹³C NMR spectra were recorded on a Bruker Avance III-500 NMR spectrometer (Bruker BioSpin AG, Fällanden, Switzerland), and ¹H and ¹³C chemical shifts were referenced to residual solvent peaks with respect to δ(TMS) = 0 ppm. Solution and solid-state absorption spectra were recorded on a Cary 5000 spectrophotometer, (Agilent Technologies Inc., Santa Clara, CA, United States), respectively. Electrospray ionization (ESI) mass spectra were recorded on a Shimadzu LCMS-2020 instrument (Shimadzu Schweiz GmbH, Roemerstr, Switzerland), and high resolution ESI mass spectra were recorded on a Bruker maXis 4G QTOF instrument (Bruker Daltonics GmbH, Faellanden, Switzerland).

Compounds 1–3 and 5 were prepared as previously reported and spectroscopic data matching those reported [36,37,38,39] [Cu(MeCN)₄][PF₆] were prepared by the method in the literature [44], and [Cu(2)₂][PF₆] has previously been reported [37]. 3,4,5-Trimethoxybenzaldehyde was purchased from Sigma-Aldrich (Buchs, Switzerland).

3.2. Compound 4a

A suspension of 3,4,5-trimethoxybenzaldehyde (18.0 g, 89.9 mmol) and piperidine (0.897 mL, 8.99 mmol) in MeOH (150 mL) was heated to 50 °C and stirred until all solids had dissolved. A solution of 2,3-butanedione (3.97 mL, 45.0 mmol) in MeOH (30 mL) was added dropwise. The reaction mixture was heated to reflux for 4 h and slowly cooled to room temperature over 1 h. The precipitate that formed was separated by filtration and washed with Et₂O. Compound 4a was isolated as an orange solid (3.65 g, 11.3 mmol, 13.0%). ¹H NMR (500 MHz, CDCl₃) δ/ppm 7.81 (d, J = 16.1 Hz, 2H, H^alkene), 7.42 (d, J = 16.1 Hz, 2H, H^alkene), 6.89 (s, 4H, H^B2), 3.92 (s, 18 H, H^OMe).

3.3. Compound 4

Compound 4a (858 mg, 1.94 mmol), 1-(2-oxopropyl)pyridinium chloride (666 mg, 3.88 mmol), and NH₄OAc (4.49 g, 58.2 mmol) were dissolved in EtOH (100 mL) and heated to reflux overnight (~15 h). The solution was cooled to room temperature, and the precipitate that formed was separated by filtration and washed with Et₂O. Ligand 4 was isolated as an orange solid (230 mg, 0.445 mmol, 23.1%). ¹H NMR (500MHz, CDCl₃) δ/ppm 8.40 (d, J = 1.7 Hz, 2H, H^A3), 7.35 (d, J = 1.6 Hz, 2H, H^A5), 6.93 (s, 4H, H^B2), 3.97 (s, 12H, H^OMe3), 3.92 (s, 6H, H^OMe4), 2.72 (s, 6H, H^Me). ¹³C NMR (126 MHz, CDCl₃) δ/ppm 158.6 (C^A6), 156.7 (C^A2), 153.8 (C^B3), 150.0 (C^A4), 139.0 (C^B4), 134.9 (C^B1), 121.4 (C^A5), 116.9 (C^A3), 104.8 (C^B2), 61.2 (C^OMe3), 56.8 (C^OMe4), 24.9 (C^Me). ESI MS m/z 517.22 (calc. 517.23).

3.4. [Cu(1)₂][PF₆]

Ligand 1 (90.2 mg, 0.268 mmol) and [Cu(NCMe)₄][PF₆] (49.9 mg, 0.134 mmol) were dissolved in MeCN (25 mL) and stirred overnight at room temperature. The reaction mixture was then filtered, the solvent was removed under reduced pressure, and the resulting dark red solid was washed several times with Et₂O to yield [Cu(1)₂][PF₆] as dark red solid (95 mg, 0.108 mmol, 80.4%). ¹H NMR (500 MHz, CD₃CN) δ/ppm 8.68 (s, 4H, H^A3), 7.96 (d, J = 7.1 Hz, 8H, H^B2), 7.85 (s, 4H, H^A5), 7.60 (m, 12H, H^B3+B4), 2.42 (s, 12H, H^Me). ¹³C NMR (126 MHz, CD₃CN) δ/ppm 158.8 (C^A6), 153.3 (C^A2), 151.2 (C^A4), 138.1 (C^B1), 130.9 (C^B4), 130.3 (C^B3), 128.4 (C^B2), 124.6 (C^A5), 118.8 (C^A3), 25.5 (C^Me). ESI MS m/z 735.24 [M − PF₆]⁺ (calc. 735.25). HR ESI MS m/z 735.2542 [M − PF₆]⁺ (calc. 735.2543).

3.5. [Cu(3)₂][PF₆]

[Cu(3)₂][PF₆] was prepared in the same manner as [Cu(1)₂][PF₆], starting with 3 (150 mg, 0.329 mmol) and [Cu(NCMe)₄][PF₆] (61.3 mg, 0.165 mmol) in MeCN (25 mL). [Cu(3)₂][PF₆] was isolated as a dark red solid (135 mg, 0.121 mmol, 73.2%). ¹H NMR (500MHz, CD₃CN) δ/ppm 8.68 (s, 4H, H^A3), 7.83 (s, 4H, H^A5), 7.08 (d, J = 2.0 Hz, 8H, H^B2), 6.67 (s, 4H, H^B4), 3.89 (s, 24H, H^OMe), 2.34 (s, 12H, H^Me). ¹³C NMR (126 MHz, CD₃CN) δ/ppm 162.6 (C^B3), 158.6 (C^A6), 153.3 (C^A2), 151.0 (C^A4), 140.2 (C^B1), 124.9 (C^A5), 119.0 (C^A3), 106.7 (C^B2), 102.2 (C^B4), 56.4 (C^OMe), 25.4 (C^Me). ESI MS m/z 975.31 [M − PF₆]⁺ (calc. 975.34). HR ESI MS m/z 975.3399 [M − PF₆]⁺ (calc. 975.3389).

3.6. [Cu(4)₂][PF₆]

Ligand 4 (226 mg, 0.437 mmol) and [Cu(NCMe)₄][PF₆] (81.5 mg, 0.219 mmol) were dissolved in EtOH (25 mL) and the mixture was stirred for 4 h at room temperature. Water was added and the organic solvent removed under reduced pressure. The residue was filtered over Celite, washed with water and Et₂O, and re-dissolved in MeCN. The solvent was removed under reduced pressure yielding [Cu(4)₂][PF₆] as a dark red solid (189 mg, 0.152 mmol, 69.5%). ¹H NMR (500MHz, CDCl₃) δ/ppm 8.29 (s, 4H, H^A3), 7.63 (s, 4H, H^A5), 6.97 (s, 8H, H^B2+B6), 3.99 (s, 24H, H^OMe3), 3.93 (s, 12H, H^OMe4), 2.39 (s, 12H, H^Me). ¹³C NMR (126 MHz, CDCl₃) δ/ppm 157.4 (C^A6), 154.2 (C^B3), 152.1 (C^A2), 151.1 (C^A4), 139.9 (C^B4), 133.4 (C^B1), 124.3 (C^A5), 117.7 (C^A3), 105.0 (C^B2), 61.2 (C^OMe4), 56.7 (C^OMe3), 25.5 (C^Me). ESI MS m/z 1095.30 [M − PF₆]⁺ (calc. 1095.38).

3.7. DSSC Fabrication

TiO₂ electrodes (Solaronix Test Cell Titania Electrodes, Solaronix SA, Aubonne, Switzerland) were washed with milliQ water and HPLC grade EtOH, heated at 450 °C for 30 min, then cooled to ca. 80 °C. The thicknesses of the transparent and scattering layers were ~9 and 3 μm, respectively, by SEM [45]. The electrodes were then placed in a DMSO solution of 5 (1.0 mM) for 24 h at room temperature. The electrodes were removed from the solution, washed with DMSO and EtOH, and dried in an N₂ stream. Each electrode was then immersed in a CH₂Cl₂ solution of [Cu(1)₂][PF₆], [Cu(2)₂][PF₆], [Cu(3)₂][PF₆], or [Cu(4)₂][PF₆] (0.1 mM) for 3 days at room temperature. After removal from the dye-bath, the electrodes were washed with CH₂Cl₂ and dried in an N₂ stream. For N719 (Solaronix SA, Aubonne, Switzerland), TiO₂ electrodes were soaked in a solution of N719 (EtOH, 0.3 mM) for 3 days. The electrodes were taken out of the dye-bath, washed with EtOH, and dried in an N₂ stream. Commercial 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 organics.

Thermoplast hot-melt sealing foil (Solaronix Test Cell Gaskets, 60 µm, Aubonne, Switzerland) was used to join the working and counter-electrodes. The space between the electrodes was filled with electrolyte (LiI (0.1 M), I₂ (0.05 M), 1-methylbenzimidazole (0.5 M), 1-butyl-3-methylimidazolinium iodide (0.6 M) in 3-methoxypropionitrile) by vacuum backfilling through a hole in the counter electrode. Then, this hole was sealed (Solaronix Test Cell Sealings and Solaronix Test Cell Caps, Aubonne, Switzerland).

3.8. Electrodes for Solid-State Absorption Spectroscopy

Dye-functionalized electrodes were assembled using the immersion procedure above but using Solaronix Test Cell Titania Electrodes Transparent (Solaronix SA, Aubonne, Switzerland).

3.9. DSSC and External Quantum Efficiency (EQE) Measurements

The DSSCs were masked. The mask was made from a black-coloured copper sheet with an aperture (ca. 0.06 cm², each mask accurately calibrated) smaller than the surface area of TiO₂. Black tape was used to complete the top and side masking of each DSSC. Performance measurements were made by irradiating the DSSC 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 were made using a Spe Quest quantum efficiency setup (Rera Systems, Nijmegen, The Netherlands) operating with a 100 W halogen lamp (QTH) and a lambda 300 grating monochromator (L.O.T.-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).

3.10. DFT Calculations

Ground state density functional theory (DFT) calculations were carried out using Spartan 16 (v. 2.0.9) [46] at the B3LYP level with a 6-31G* basis set in vacuum. Initial energy optimization was carried out at a semi-empirical (PM3) level.

4. Conclusions

We have prepared and characterized the homoleptic copper(I) complexes [Cu(1)₂][PF₆], [Cu(3)₂][PF₆], and [Cu(4)₂][PF₆]. These and [Cu(2)₂][PF₆] have been used to assemble heteroleptic dyes on FTO/TiO₂ electrodes utilizing the SALSAC approach with the bis(phosphonic acid) 5 as the anchoring ligand. In masked n-type DSSCs, the highest efficiency η was obtained for [Cu(5)(4)]⁺ (1.96% versus 5.79% for N719, or a relative efficiency of 33.9% with respect to N719). Although values of J_SC are similar for DSSCs with [Cu(5)(2)]⁺ (electron releasing 4-MeO group in 2) and [Cu(5)(4)]⁺ (competitive electron-releasing and electron-withdrawing effects of 4-MeO and 3,5-(MeO)₂ groups in 4), DSSCs with [Cu(5)(2)]⁺ exhibit a lower V_OC than those with [Cu(5)(4)]⁺. DSSCs with [Cu(5)(3)]⁺ show the lowest J_SC and V_OC. Solid-state absorption spectra and EQE spectra demonstrate that [Cu(5)(4)]⁺ benefits from an extended spectral response at higher energies. Inspection of orbital characteristics from ground state DFT calculations demonstrates that orbital character in MOs within the HOMO manifold extends to the methoxyphenyl groups in [Cu(5)(2)]⁺ and [Cu(5)(4)]⁺, and we propose that this may contribute to the enhanced performances of these dyes with respect to [Cu(5)(1)]⁺.