Di-branched di-anchoring organic dyes for dye-sensitized solar cells

Abstract

The first examples of di-branched di-anchoring organic sensitizers were synthesized and used in dye-sensitized solar cells leading to red-shifted IPCE maxima and increased photocurrent when compared to the corresponding mono-branched mono-anchoring dye, yielding power conversion efficiency of 5.7% (4.9% with ionic liquid electrolyte) with enhanced stability under 1 sun conditions from the di-anchoring groups.

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Broader context

Within 3rd generation photovoltaics, dye-sensitized solar cells (DSCs) show the best promise for high-conversion low-cost devices. In DSCs a dye-sensitizer captures photons and an electron/hole pair is generated and transferred to the electrodes. Ru(II)–polypyridyl complexes have best performed so far but more recently a number of organic chromophores have been tested due to their larger structural variety, tunable optical and electronic properties, and low-cost manufacturing. The most common investigated structure is a mono-branched donor–acceptor dipolar architecture, with the acceptor group anchored to the TiO₂ surface. Here we present the first examples of di-branched di-anchoring organic sensitizers having one donor and two acceptor groups, each ending with an anchoring functionality. The new organic sensitizers were synthesized and used in DSCs leading to red-shifted IPCE maxima and increased photocurrent when compared to the corresponding mono-branched mono-anchoring dye. DSCs yielded power conversion efficiency up to 5.7% (4.9% with ionic liquid electrolyte) with enhanced stability under 1 sun conditions caused by the di-anchoring groups. DFT/TDDFT calculations have been performed highlighting the factors affecting the measured photovoltaic efficiencies.

Introduction

In the field of organic and hybrid photovoltaics, dye-sensitized solar cells (DSCs) show the best promise for high-conversion low-cost devices.¹ In DSCs TiO₂nanoparticles are anchored with light-harvesting sensitizer dyes and are typically surrounded by a liquid-phase electrolyte. The dye-sensitizer captures photons and an electron/hole pair is generated and transferred at the interface with the inorganic semiconductor and the redox eletrolyte. Ru(II)–polypyridyl dyes,^2,3 yielding power conversion efficiency of 11.1% under standard AM 1.5 conditions, have performed best so far, the most representative example being bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II) (N3, N719).⁴ More recently a number of organic chromophores have been tested as metal-free alternatives to Ru(II) complexes.^5–7 Their advantage over conventional organometallic dyes consists in their larger structural variety which results in a tunable and more intense spectral absorption. They are readily available, easier to purify, and have low-cost manufacturing. Unfortunately, their efficiencies (2–9%) are still lower than that of ruthenium sensitizers.

By far the most common investigated structure of organic sensitizers is the D–π–A dipolar architecture, where a π electron-rich moiety (D) is linked to a π electron-poor (A) endgroup through a π spacer. The A group carries an anchoring group (typically carboxylic acid) to the TiO₂ anatase surface. During photoexcitation an intramolecular charge transfer takes place from D to A, from where the photoelectron is injected onto the conduction band of the semiconductor. In a very restricted number of cases quadrupolar^7a and branched⁷ geometries have been considered, where two D groups push towards a single A unit. However, in all of these cases only one anchoring functionality, corresponding to the single A group, was present. The low variety of organic frameworks is noteworthy in view of the vast literature of multibranched chromophores with variable structural symmetries in other fields than photovoltaics, such as nonlinear optics.⁸ Furthermore, the presence of only one anchoring functionality per molecule could represent a serious constraint with respect to Ru(II) sensitizers, where 1 to 4 anchoring groups are available for tunable interfacial electron transfer.⁹

Here we present the first di-branched di-anchoring organic sensitizers having one D and two A groups, each ending with an anchoring group. Our strategy (Fig. 1) is to extend to a multibranched geometry the most successful dipolar structural motifs for DSCs. Generally speaking, the branched multianchoring approach should carry a number of advantages over the unidimensional dipolar architectures: (1) larger structural variety to obtain panchromatic response; (2) enhanced optical density due to extended π-conjugated framework and; (3) multi-bonding to TiO₂nanoparticles. The most common building block for D–π–A organic senzitizers contains D = arylamino (typically a triarylamino derivative), π = aromatic or heteroaromatic conjugated spacer (typically a thiophene derivative), and A = 2-cyanoacrylic acid (acceptor and anchoring unit).^5–7 Thus, we have selected 3-[5-(4-(diphenylamino)styryl)thiophen-2-yl]-2-cyanoacrylic acid (D5, Fig. 1) as our uni-dimensional dipolar reference, for which a solar–to–electric conversion efficiency of 5.1% was reported.^6,7d We converted this structure into the di-branched analogue 4,4′-phenylamino-bis-[2-cyano-3-(5-styrylthiophen-2-yl)acrylic acid] (DB-1) by introducing two D–π–A arms, identical to that of D5, onto the same donor fragment. We have also investigated the di-branched chromophore 4,4′-phenylamino-bis-{[(5-styrylthiophen-2-yl)methylene]rhodanine-3-acetic acid} (DB-2), where the acceptor 2-cyanoacrylic acid was replaced by rhodanine-3-acetic acid.^5a Liquid and solvent-free DSCs have been fabricated with the novel sensitizers and their photoelectrochemical properties and stability data measured under various conditions. DFT/TDDFT calculations have been performed for the sensitizers in solution and for a TiO₂nanoparticle model, highlighting the factors affecting the measured photovoltaic efficiencies.

Fig. 1 Structures of di-branched di-anchoring DSC organic sensitizers and reference compound D5.

Experimental

General

NMR spectra were recorded on a Bruker AMX-500 instrument operating at 500.13 (¹H) and 125.77 MHz (¹³C). Coupling constants are given in Hz. High resolution mass spectra (HRMS) were recorded using a Bruker Daltonics ICR-FTMS APEX II spectrometer equipped with an electrospray ionization source. Flash chromatography was performed with Merck grade 9385 silica gel 230–400 mesh (60 Å). Reactions were performed under nitrogen in oven dried glassware and monitored by thin layer chromatography using UV light (254 nm and 365 nm) as visualizing agent. All reagents were obtained from commercial suppliers at the highest purity grade and used without further purification, except for POCl₃ that was distilled prior to use. Anhydrous solvent were purchased from Sigma Aldrich and used without further purification. Extracts were dried over Na₂SO₄ and filtered before removal of the solvent by evaporation. Melting points are uncorrected. Absorption spectra were recorded on a V-570 Jasco spectrophotomer. Emission spectra were recorded on a FP6200 Jasco spectrofluorimeter.

Synthesis

N,N-bis-(4-formylphenyl)aniline (1). Triphenylamine (9.82 g, 40 mmol) was dissolved in anhyd. DMF (40 mL, 37.9 g, 519 mmol). The solution was cooled to 0 °C and phosphorus oxychloride (37 mL, 60.9 g, 397 mmol) was added dropwise. The ice bath was removed and the orange solution was allowed to warm to room temperature and then heated to 105 °C for 17 h. The mixture was poured into ice-cold water and aq. NaOH (2 M) was added to neutrality, whereby a brown solid precipitated out. The solid was filtered, washed with water and EtOH. Flash column chromatography of the residue (silica gel, petroleum ether/dichloromethane 1 : 3) afforded 1 as a yellow solid (4.82 g, 16.0 mmol, 40%); δ_H (500 MHz; CDCl₃; Me₄Si) 9.92 (2 H, s, CHO), 7.70 (4 H, d, J 6.9, 3-H and 5-H of the 4-formylphenyl ring), 7.42 (2 H, t, J 8.0, 3-H and 5-H of the N-phenyl ring), 7.29 (1 H, t, J 7.5, 4-H of the N-phenyl ring), 7.22–7.18 (6 H, m, remaining aromatic protons).

N,N-bis-{4-[(thien-2-yl)vin-2-yl]phenyl}aniline (2). t-BuOK (780 mg, 6.85 mmol) was added to a solution of diethyl thiophen-2-yl-methylphosphonate¹⁰ (1.46 g, 6.23 mmol) in THF (15 mL) at 0 °C, and the resulting mixture was stirred for 30 min at the same temperature. A solution of dialdehyde 1 (931 mg, 3.09 mmol) in THF (10 mL) was added and the mixture stirred overnight at room temperature. The mixture was concentrated under reduced pressure and AcOEt (100 mL) and water (100 mL) were added. The layers were separated, the organic layer was washed with water (2 × 100 mL), dried, and the solvent was removed by rotary evaporation. The oily residue was dissolved in CH₂Cl₂ (3 mL), and cyclohexane (30 mL) was added under stirring. The precipitate was collected by filtration, washed with cyclohexane, and dried affording the product as a dark yellow powder (1.073 g, 2.32 mmol, 75%); δ_H (500 MHz; CDCl₃; Me₄Si) 7.37 (4 H, d, J 8.6, 3-H and 5-H of the phenyl ring adjacent to vinylene), 7.31 (2 H, t, J 8.4, 3-H and 5-H of the N-phenyl ring), 7.19 (2 H, d, J 5.0, 5-H of thiophene ring), 7.16 (2 H, d, J 16.0, α-H of vinylene to phenyl ring), 7.15 (2 H, d, J 8.8, 2-H and 6-H of the N-phenyl ring), 7.11–7.05 (7 H, m, remaining aromatic protons), 7.02 (2 H, dd, J 5.1, 3.5, 4-H of thiophene ring), 6.91 (2 H, d, J 16.0, β-H of vinylene to phenyl ring).

N,N-bis-{4-[(5-formylthien-2-yl)vin-2-yl]phenyl}aniline (3). POCl₃ (0.35 mL, 0.57 g, 3.75 mmol) was added dropwise to ice-cold DMF (0.29 mL, 0.27 g, 3.76 mmol) and the mixture was stirred at 0 °C for 30 min. The ice bath was removed and the glassy solid was dissolved in 1,2-dichloroethane (50 mL). A solution of 2 (290 mg, 0.63 mmol) in the same solvent (10 mL) was then added and the resulting mixture was stirred overnight. The solution was poured into saturated aq. Na₂CO₃ (150 mL) and the mixture was stirred for 1 h. CH₂Cl₂ (150 mL) was then added and the layers were separated. The organic layer was washed with water (3 × 150 mL) and dried. The solvent was evaporated under reduced pressure and the oily residue was dissolved in CH₂Cl₂ (3 mL). After adding cyclohexane (30 mL) the formed precipitate was separated, washed with the same solvent, and dried yielding the product as an orange solid (241 mg, 0.46 mmol, 73%); δ_H (500 MHz; CDCl₃; Me₄Si) 9.88 (2 H, s, CHO), 7.68 (2 H, d, J 4.0, 4-H of thiophene ring), 7.42 (4 H, d, J 8.7, 3-H and 5-H of the phenyl ring adjacent to vinylene), 7.34 (2 H, t, J 7.6, 3-H and 5-H of the N-phenyl ring), 7.18–7.09 (13 H, m, remaining aromatic protons).

DB-1. Cyanoacetic acid (97 mg, 1.14 mmol) and piperidine (cat.) were added to a stirred solution of dialdehyde 3 (100 mg, 0.19 mmol) in EtOH (10 mL), and the resulting mixture was refluxed for 9 h. After cooling to 0 °C a precipitate was obtained and collected upon filtration to afford DB-1 as a dark red-purple solid (100 mg, 0.15 mmol, 81%) which was submitted to double recrystallization from EtOH, mp > 200 °C (d); λ_max(EtOH)/nm 463 (c = 5.0 × 10⁻⁶ M), 470 (c = 1.0 × 10⁻⁵ M), 473 (c = 2.0 × 10⁻⁵ M) (ε/dm³ mol⁻¹ cm⁻¹ 44 500 ± 1 900); δ_H (500 MHz; DMSO-d6; Me₄Si) 8.42 (2 H, s, 3-H of 2-cyanoacrylic acid), 7.92 (2 H, d, J 4.0, 3-H of thiophene ring, ortho to cyanoacrylic acid), 7.62 (4 H, d, J 8.8, 3-H and 5-H of the phenyl ring adjacent to vinylene), 7.47 (2 H, d, J 16.1, α-H of vinylene to phenyl ring), 7.41 (2 H, d, J 4.0, 4-H of thiophene ring, meta to cyanoacrylic acid), 7.39 (2 H, t, J 8.0, 3-H and 5-H of the N-phenyl ring), 7.23 (2 H, d, J 16.1, β-H of vinylene to phenyl ring), 7.17 (1 H, t , J 7.4, 4-H of the N-phenyl ring), 7.13 (2 H, d, J 7.7, 2-H and 6-H of the N-phenyl ring), 7.02 (4 H, d, J 8.5, 2-H and 6-H of the phenyl ring adjacent to vinylene); δ_C (125.77 MHz; DMSO-d₆; Me₄Si) 165.84, 164.00, 151.85, 147.59, 146.68, 145.38, 140.52, 134.61, 132.14, 131.07, 130.31, 128.85, 127.34, 125.80, 124.88, 123.67, 120.14, 117.72; HRMSm/z 651.0817 (M⁺ requires 651.1286).

DB-2. Dialdehyde 3 (350 mg, 0.68 mmol) and rhodanine 3-acetic acid (780 mg, 4.45 mmol) were added to a stirred solution of AcONH₄ (130 mg, 1.70 mmol) in AcOH (20 mL) and the resulting mixture was refluxed for 5 h. Upon cooling to room temperature, the formed solid precipitate was collected by filtration and washed with cold AcOH to give the product as a dark purple solid (490 mg, 0.59 mmol, 87%) which was further purified by double recrystallization from EtOH, mp > 300 °C; λ_max(EtOH)/nm 510 (ε/dm³ mol⁻¹ cm⁻¹ 41 100 ± 1 000); δ_H (500 MHz; DMSO-d₆; Me₄Si) 8.15 (2 H, s, vinyl proton of methylenerhodanine 3-acetic acid), 7.76 (2 H, d, J 4.0, 3-H of thiophene ring, ortho to methylenerhodanine 3-acetic acid), 7.59 (4 H, d, J 8.6, 3-H and 5-H of the phenyl ring adjacent to vinylene), 7.45 (2 H, d, J 16.1, α-H of vinylene to phenyl ring), 7.39 (2 H, t, J 7.0, 3-H and 5-H of the N-phenyl ring), 7.38 (2 H, d, J 5.2, 4-H of thiophene ring, meta to methylenerhodanine 3-acetic acid), 7.25 (2 H, d, J 16.0, β-H of vinylene to phenyl ring), 7.17 (1 H, t, J 7.4, 4-H of the N-phenyl ring), 7.12 (2 H, d, J 7.9, 2-H and 6-H of the N-phenyl ring), 7.02 (4 H, d, J 8.5, 2-H and 6-H of the phenyl ring adjacent to vinylene), 4.73 (4 H, s, CH₂); δ_C (125.77 MHz; DMSO-d₆; Me₄Si) 192.28, 167.74, 166.42, 152.58, 147.51, 146.68, 138.48, 135.92, 131.73, 131.15, 130.31, 128.77, 128.61, 127.34, 125.78, 124.87, 123.69, 120.12, 118.50, 45.61; HRMSm/z 862.0296 (M-H requires 862.0303), 908.0111 (M-H + 2Na requires 908.0099).

Computational details

All the calculations have been performed with the Gaussian 03 program package.¹¹ A 6-31G* basis set was used throughout. The computational set up was calibrated in order to reproduce the absorption maxima of the reference D5 compound. In previous papers on D5 and similar push-pull systems^7d,12 we found a considerable red-shift of the lowest (intense) TDDFT excitation energy, calculated by the B3LYP functional, compared to the experimental absorption maxima. This shift was as large as 0.6–0.8 eV and was related to the inaccurate description of charge-transfer excited states in highly delocalized systems.^7d,12 Moreover, further red-shifts were calculated by introducing solvation effects. To overcome this limitation, we use here the B3LYP functional¹³ for geometry optimizations and employ the MPW1K functional,¹⁴ containing about 42% of Hartree–Fock exchange, for TDDFT excitation energies. The increased amount of Hartree–Fock exchanges ensures a correction of the Self Interaction Error typical of conventional functionals and also improves the long-range exchange behaviour. A comparison of B3LYP and MPW1K results for the protonated D5 system at the B3LYP geometry calculated in vacuo, followed by TDDFT calculations in vacuo and in ethanol solution, are reported in ESI Table S1†. The excitation energies calculated by MPW1K are in much better agreement with the experimental value than the B3LYP-calculated values (501 vs. 620 nm for MPW1K and B3LYP in EtOH solution, respectively, to be compared to the 471 nm experimental value, (Table 1). Such computational set-up was therefore adopted for all of the calculations reported in this work.

Table 1 Experimental and calculated λ_max/nm of D5, DB-1, and DB-2 in EtOH. Values in parentheses are calculated oscillator strengths

	Experimental			Computed
	EtOH	With acid ^a	With base^b	Protonated	Deprotonated
a AcOH or dil. HCl. b NaOH, triethylamine or tetrabutylammonium hydroxide. c Addition of AcOH to MeOH solution (Ref. 6a). d Concentration-dependent (see text). e Addition of triethylamine or tetrabutylammonium hydroxide; an irreversible transformation takes place with NaOH.
D5	441 ^7d	471^c		501 (1.67)	434 (1.73)
DB-1	463–473^d	494	460–462	482 (2.40)	458 (2.72)
DB-2	510	519	510^e	539 (2.89)	505 (3.13)

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Preparation and characterization of solar cells

DSCs have been prepared following an already reported procedure.¹⁵ Liquid and solvent-free (ionic liquid electrolyte) DSCs used photoanodes composed of 7.3 µm transparent layer (20 nm) and ca. 4 µm scattering layer (400 nm, CCIC, HPW-400) TiO₂ films.^15b The liquid electrolyte consisted of 0.6 M N-methyl-N-butyl imidazolium iodide, 0.04 M iodine, 0.02 M LiI, 0.05 M guanidinium thiocyanate, and 0.28 M tert-butylpyridine in 15/85 (v/v) mixture of valeronitrile and acetonitrile. The binary ionic liquid electrolyte consisted of 0.2 M iodine, 0.5 M N-butyl benzimidazole, and 0.1 M GuNCS in a mixture of 1-propyl-3-methylimidazolium iodide and 1-ethyl-3-methyl-imidazolium tetracyanoborate (volume ratio 65 : 35).

Results and discussion

Synthesis of sensitizers

DB-1 was obtained adapting a procedure reported for the synthesis of D5 (Scheme 1).⁶ Vilsmeier formylation of triphenylamine afforded a mixture of tris(4-formylphenyl)amine and N,N-bis-(4-formylphenyl)aniline 1, with the latter being the major product. The bis-formylation product was isolated by column chromatography in good yields and submitted to Horner–Wittig condensation with 2 equivalents of diethyl thiophen-2-ylmethylphosphonate to afford the bis-thiophene condensation product 2. Subsequent bis-formylation of the two terminal thiophene moieties to the bis-aldehyde 3 followed by double Knoevenagel condensation with a three-fold excess of cyanoacetic acid afforded DB-1 as a dark red-purple solid. The last three steps were accomplished in high yields. We found that running the final condensation step with a large excess of cyanoacetic acid avoids the formation of a mixture of mono- and bis-condensation adducts, difficult to separate because of the presence of the two carboxylic groups in DB-1. DB-2 was similarly obtained as a purple solid by final double condensation of rhodanine-3-acetic acid in refluxing AcONH₄/AcOH in high yields.

Scheme 1 Synthesis of DB-1 and DB-2. (i) DMF, POCl₃, 105 °C; (ii) diethyl thiophen-2-yl-methylphosphonate, t-BuOK, THF; (iii) DMF, POCl₃, 1,2-dichloroethane; (iv) cyanoacetic acid, piperidine, EtOH, reflux; (v) rhodanine 3-acetic acid, AcONH₄, AcOH, reflux.

Spectroscopic characterization

DB-1 shows an absorption peak in EtOH at 463–473 nm, attributed to the π–π* intramolecular charge-transfer transition from the amine donor to the 2-cyanoacrylic acid acceptor moiety (Fig. 2). This value should be compared with the 441 nm peak for D5 in the same solvent,^7d thus revealing a substantial bathochromic effect on going from the mono- to the di-branched structure (Table 1). We observed that such charge-transfer transition is concentration-dependent shifting in EtOH from 463 to 473 nm on going from 5.0 × 10⁻⁶ to 2.0 × 10⁻⁵ M solutions, which is due to association of proton at higher concentrations. The rhodanine-3-acetic acid derivative DB-2 shows a similar charge-transfer transition, which is further red-shifted with respect to both D5 and DB-1 (Fig. 2). Both di-branched dyes have a significant hyperchromic effect with respect to the reference system D5. The molar absorptivities ε of DB-1 and DB-2 are 44 500 and 41 100 dm³ mol⁻¹ cm⁻¹, respectively, to be compared with the value of 33 000 dm³ mol⁻¹ cm⁻¹ for D5.^7dDB-1 shows an emission peak at 605 nm in EtOH; no significant fluorescence was observed for DB-2.

Fig. 2 Normalized linear absorption spectra of DB-1 (2 × 10⁻⁵ M, solid line) and DB-2 (2 × 10⁻⁶ M, dashed line) in EtOH.

Edvinsson and Sun reported that the addition of acetic acid to D5 in protic solvents (MeOH) causes a red-shift of the absorption peak, concluding that the value in MeOH mainly originated from deprotonation.^6a Similar effects are reported elsewhere.^6b,16 These data suggest that the carboxylic acid functionality may be partially deprotonated in the ground state. In addition, the concentration dependence of the absorption peaks may be associated with the protonation/deprotonation equilibrium. It is important to determine the actual presence of the carboxylic group as a protonated or deprotonated form since this may significantly affect the anchoring mode on the TiO₂ surface.⁴ We therefore decided to better investigate the protonation/deprotonation effects by recording the UV-vis spectra in presence of different acids and bases. The absorption peak of DB-1 in EtOH is is red-shifted by 20–30 nm upon addition of a small amount of either AcOH or dil. HCl, similar to what was observed by addition of AcOH to a MeOH solution of D5 (Table 1). We attributed the bathochromically-shifted band to the fully protonated form of DB-1, in agreement with the computational investigation (vide infra). It should be noted that the recorded value in presence of acids (494 nm) is independent of the starting concentration, that is whether the absorption peak of the starting solution is 463 nm (lower concentration) or 473 nm (higher concentration). A similar effect, but to a smaller extent (9 nm shift), has been recorded for DB-2. As for DB-1, the value of absorption maximum (520 nm) is reached starting both from less (510 nm) or more concentrated (518 nm) solutions. We can therefore conclude that the blue-shift in absence of acids and dilute solutions indicates the presence of a partial dissociation of cyanoacrylic or rhodanine-3-acetic acid protons. This is easily explained on the basis that deprotonation to carboxylate –COO⁻ increases the LUMO energy of the ligand resulting an increased gap between the HOMO and the LUMO. The charge-transfer transition then occurs at higher energies compared to the protonated state. As anticipated, the different λ_max values obtained as a function of the concentration are likely to be associated with different degrees of deprotonation, as confirmed by the fact that in presence of an excess of weak or strong acids the same species (same absorption spectrum) is always obtained. Such species is thus identified as the fully protonated derivative. To confirm this hypothesis we recorded the electronic absorption spectra of DB-1 and DB-2 in EtOH in presence of different bases: NaOH, triethylamine, and tetrabutylammonium hydroxide (Table 1). Apart a few nm difference, likely to be due to different solvation effects, a single species (same spectrum) is always obtained regardless of the employed base and the starting concentration as well. As expected, such species, identified as the fully deprotonated form, are hypsochromically shifted with respect to the solution in absence of bases. It is very likely that similar effects are operating for other cyanoacrylic acid or rhodanine-3-acetic acid derivatives used as DSC sensitizers, as other authors have actually observed.^6,16

In order to investigate the anchoring mode of DB-1 and DB-2, ATR-FTIR spectra were performed on the TiO₂ surface and compared with those of the free dyes (ESI Fig. S8 and S9†). ATR-FTIR spectra of the DB-1 dye measured as a solid sample show a broad band at 1674 cm⁻¹ due to the carboxylic acid group. The intense peak in the region at 1170 cm⁻¹ is due to ν(CO) stretching. The band associated to ν(C≡N) of the cyanoacrylic acid group was observed at 2216 cm⁻¹. ATR-FTIR spectra of the DB-1 dye adsorbed on nanoporous TiO₂ films show the presence of carboxylate asymmetric and symmetric bands at 1587 and 1180 cm⁻¹, while the ν(C≡N) band remains unchanged, indicating that the cyano group of the cyanoacrylic moiety is not involved in anchoring the sensitizer. The presence of the carboxylate bands in the ATR-FTIR spectra of adsorbed sensitizer on TiO₂ testifies that the carboxylic acid group is dissociated and involved in adsorption on the TiO₂ surface. These data clearly show the absence of free carboxylic acid groups and thus confirm adsorption of the two branches onto the TiO₂ surface.

Computational investigation

DFT/TDDFT calculations were performed on DB-1, DB-2 and on the reference D5 (Table 1). Both protonated and deprotonated carboxylic groups were considered. The agreement between theory and experiment is good; also the calculated oscillator strengths follow the experimental ε trend. For DB-1 and D5 larger blue-shifts are calculated upon deprotonation than for DB-2, in line with the reduced conjugation involving the carboxylic group in the latter compound. The calculated transitions correspond to the lowest singlet–singlet excitations taking place between the HOMO and the LUMO (Fig. 3). The HOMO has a similar localization in DB-1 and DB-2, being mainly delocalized across the donor moieties. The LUMO of DB-1 resembles that of D5 (not shown) and extends through the cyanoacrylic groups, while that of DB-2 does not reach the non-conjugated carboxylic groups. The excited state of DB-1 responsible for the strong visible absorption is potentially strongly coupled to the TiO₂ surface by virtue of the cyanoacrylic contribution, whereas that of DB-2 is less spatially coupled to the TiO₂ conduction band.

Fig. 3 Isodensity plots of the HOMO and LUMO of DB-1 and DB-2.

Liquid and solvent-free DSCs

The new sensitizers have been used for fabricating DSCs to explore current–voltage characteristics. The incident monochromatic photon-to-current conversion efficiency (IPCE) of DB-1 plotted as a function of excitation wavelength exhibits a high value at plateau 78% (Fig. 4). The IPCE maxima is red-shifted by ca. 25 nm leading to higher photocurrent when compared to D5 sensitizer. The red-shifed response of DB-1 is consistent with calculated and experimental absorption behavior. The integrated current under the IPCE curve is 12.8 mA cm⁻², consistent with the photocurrent of solar cells under standard global AM 1.5 solar condition. Main photovolataic parameters are listed in Table 2. The DB-1 overall conversion efficiency η, derived from the equation: η = J_SC × V_OC × FF, is comparable to that of D5. However, under similar conditions, the DB-2 sensitized cell efficiency is more modest, likely because of the loss of charge by fast recombination process. Recently, Wiberg et al. found that the mono-branched rhodanine-3-acetic acid yielded 100% of injection efficiency despite the lack of conjugation extending out through the anchoring group.¹⁷ The efficiency-limiting reaction was explained on the basis of ultrafast charge recombination between the electrons injected into the TiO₂ and the oxidized dye.¹⁷ Our data are consistent with these findings since the electron lifetime (reciprocal recombination rate constant) in DB-1 was 2–3 times longer than DB-2 (Fig. 5).

Fig. 4 The photocurrent action spectra (IPCE) of D5, DB-1, and DB-2 DSCs.

Table 2 J/V characteristics of D5, DB-1, and DB-2 DSCs

Dye	J/mA cm^−2	V/mV	FF	Efficiency (%)
D5	11.56 ± 0.10	653 ± 5	0.73 ± 0.01	5.5 ± 0.1
DB-1	12.68 ± 0.15	612 ± 5	0.73 ± 0.01	5.7 ± 0.1
DB-2	3.65 ± 0.05	503 ± 5	0.69 ± 0.01	1.3 ± 0.1

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Fig. 5 The electron lifetime of DSC based on DB-1 (circle) and DB-2 dye (triangle).

Fig. 6 shows the initial photovoltaic performance of a D5 and DB-1 sensitized solar cell using a binary ionic liquid electrolyte (see Experimental). These cells were subjected to long-term accelerated ageing under light soaking conditions, at full intensity (100 mW cm⁻²) and 60 °C. The initial efficiency of DB-1 was 4.9%, which remained at 67% of the initial value after 1000 h of light soaking (Fig. 7). Under the same conditions, the overall efficiency of a D5 sensitized solar cell remained at 55% of its initial value (4.6%).

Fig. 6 J/V characteristics of DSCs based on DB-1 and D5 with ionic liquid electrolyte.

Fig. 7 Changed performance of DSCs based on D5 (left) and DB-1 (right) with ionic liquid electrolyte under full 1 sun at 60 °C.

Conclusions

In conclusion, we have designed and developed a model novel class of di-branched organic sensitizers constituted by one donor and two anchoring groups, which absorbs in the red region compared to its mono-branched analogue D5. The structural features of DB-1 sensitizer compared to D5 offer increased photocurrent (di-branched structure) and enhanced stability (presence of di-anchoring moiety) under 1 sun conditions. These dyes will provide critical clues about the synthetic choice of structural modifications that will boost further their panchromatic light harvesting ability for enhanced DSC performance.

Acknowledgements

We thank MIUR-FIRB RBNE033KMA for financial support. We are also grateful to Prof. Anders Hagfeldt's group and Prof. Licheng Sun's group (Royal Institute of Technology, Sweden) for a sample of D5 dye and to Mr. Alberto Jacopo Villa (University of Milano-Bicocca) for his contribution to the synthesis of DB-1 and DB-2.

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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C NMR, UV-vis, emission, calibration calculations, and ATR-FTIR spectra. See DOI: 10.1039/b910654e

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