Fluorous Molecules for Dye-Sensitized Solar Cells - American

ARTICLE pubs.acs.org/JPCC

Fluorous Molecules for Dye-Sensitized Solar Cells: Synthesis and Photoelectrochemistry of Unsymmetrical Zinc Phthalocyanine Sensitizers with Bulky Fluorophilic Donor Groups Gianluca Pozzi,*,† Silvio Quici,† Maria Concetta Raffo,† Carlo Alberto Bignozzi,§ Stefano Caramori,*,§ and Michele Orlandi§ †

Istituto di Scienze e Tecnologie Molecolari del Consiglio Nazionale delle Ricerche, ISTM-CNR, via Golgi 19, 20133 Milano, Italy Dipartimento di Chimica, Universita di Ferrara, Via Luigi Borsari 46, 44121 Ferrara, Italy

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bS Supporting Information ABSTRACT: Two sterically hindered zinc phthalocyanines bearing fluorous alkoxy substituents and carboxylic acid groups acting respectively as electron donor and electron acceptor/anchoring units, were synthesized and their ability to photosensitize nanocrystalline TiO2 films was investigated. The electronic properties of these new dyes were characterized by combining UV-vis spectroscopy, electrochemical techniques and DFT computation. Nanocrystalline TiO2 electrodes treated with the fluorous dyes were assembled in dye-sensitized solar cells (DSCs) and further characterized by photocurrent action spectroscopy, photocurrent-photovoltage measurements and electrochemical impedance spectroscopy. The maximum photon-to-current conversion efficiencies (IPCE) in the near IR region compared favorably with those achieved with some of the best phthalocyanine dyes investigated so far, thus highlighting the potential of fluorous molecules as DSC components. A maximum power conversion efficiency (η) of 1.3% was attained in the absence of any additive such as antiaggregating chenodeoxycholic acid or conduction band edge modifier tert-butylpyridine. The basic information gathered in this study will serve as guidelines for the design of new fluorous dyes for highly efficient DSCs.

’ INTRODUCTION Over recent years, there has been growing interest in organic-, hybrid organic-inorganic and all-inorganic photovoltaic systems capable of converting sun light directly to electric energy, as viable alternatives to current silicon-based technologies.1 Among the various strategies adopted, those based on the sensitization of a nanostructured semiconductor metal oxide interface with a light-absorbing dye are particularly promising.2 At the present stage, the highest power conversion efficiency values (η) in this field have been achieved with mesoscopic dye-sensitized solar cells (DSCs) featuring Ru(II)-polypyridyl complexes with a wide light absorption range as sensitizers, in combination with liquid electrolytes containing the I-/I3- couple as a redox shuttle for the regeneration of the oxidized dye,3 but the scarcity of ruthenium might be a bottleneck for large-scale DSC production, as well as the high cost of Ru(II)-polypyridyl complexes. The moderate molar extinction coefficients of these sensitizers also hinders the realization of efficient thin-film devices such as DSCs incorporating solid-state hole-transporters.2a,4 Readily accessible substitute sensitizers, embracing complexes of abundant transition metals5 and purely organic dyes with improved lightharvesting properties,6 are thus actively pursued. As a consequence of these efforts, efficiencies obtained with organic dyes are approaching those observed with Ru(II)-polypyridyl complexes, although much work is still needed in order to enhance their long-term stability in DSCs and also to expand their r 2011 American Chemical Society

light-harvesting capability in the red/near-infrared (NIR) spectral regions of sun light, which account for a consistent portion of the solar energy reaching the Earth’s surface. In the quest for ideal ruthenium-free sensitizers, dyes with large π-conjugated systems suitable for efficient electron-transfer processes, such as phthalocyanine (Pc) derivatives, are receiving considerable attention.6b,7 The most prominent feature of Pcs is their intense and tunable absorption in the red/NIR region coupled with transparency over a large portion of the visible spectrum. This makes them potentially useful sensitizers for both liquid- and solid-state DSCs. In addition, the pleasant blue-green color of photovoltaic devices containing Pcs is especially attractive for building integrated photovoltaics (BIPV) applications such as photovoltaic windows.8 Unfortunately, the power efficiencies currently achieved with Pcs-based DSCs, in most cases η < 1%, are still lower than those obtained with the best organic dyes (η up to 9%). This poor performance have been mainly ascribed to the general tendency of Pcs to form molecular aggregates on the surface of the metal oxide nanocrystals,9,10 and to the lack of directionality of the electronic orbitals of symmetrically substituted Pcs in the excited state.11 Aggregation of metal complexes of Pcs can be suppressed by combining peripheral substitution of the flat macrocycle with bulky tert-butyl groups and axial Received: October 14, 2010 Revised: January 21, 2011 Published: February 16, 2011 3777

dx.doi.org/10.1021/jp109846k | J. Phys. Chem. C 2011, 115, 3777–3788

The Journal of Physical Chemistry C coordination of the metal center with carboxylated ligands that also serve as anchoring groups to the surface of nanocrystalline TiO2.10b Nevertheless, this innovative molecular setup failed to produce viable DSCs since the power conversion efficiency reached a modest 0.2% value and, more importantly, photocurrent generation occurred mostly in the UV/blue spectral region. It was later discovered that the concurrent introduction of electron donating tert-butyl substituents and electron withdrawing carboxylic acid groups at selected positions of the Pc ring provides an efficient electron transfer from the excited dye to the TiO2 conduction band via good electronic coupling between the LUMO of the dye and the Ti 3d orbital.11 Such improved molecular setup also relieved aggregation problems. As a result, η ranging from 0.4% to 3.5% were attained with unsymmetrical zinc phthalocyanine complexes (ZnPcs), although the coadsorption of antiaggregation additives such as chenodeoxycholic acid (Cheno) was required to boost the cell performance.11d Additional examples of unsymmetrical ZnPcs designed to incorporate favorable push-pull and steric effects have been recently reported, where bulky, electron rich aryl substituents rotated with respect to the macrocycle plane were shown to effectively prevent aggregation.12 Contrastive results have been obtained by applying this concept. A highly sterically hindered ZnPc having six 4-tert-butylphenyl groups displayed moderate cell performance (η = 0.57%) that was not modified by addition of Cheno,12a whereas a promising η = 4.6% has been achieved with DSCs based on a structurally related ZnPc having the same carboxylated aryl group as the anchoring element and six electron-donating 2,6-diphenylphenoxy groups.12b There is thus an obvious need for new designed Pcs to fully take advantage of their potential as photosensitizers in DSCs, and we were attracted by the possible benefits offered by unsymmetrical fluorous ZnPcs. Highly fluorinated organic compounds, including fluorous molecules which are characterized by the presence of extended saturated fluorocarbon domains, have been already successfully explored in novel functional materials,13 in electronic and optoelectronic applications,14 and in energy storage and conversion devices.15 In the photovoltaic field, highly fluorinated molecules and polymers with improved stability and tailored HOMO-LUMO energy states resulting from the electronwithdrawing character of fluorine substituents have been recently proposed as active components of p-n heterojunctions.16 Morphology control at different scales in electron acceptordonor blends can be attained thanks to the unique phase properties of fluorous molecules, leading to improved photovoltaic performance in bulk heterojunction polymer solar cells.17 In the case of DSCs, highly fluorinated ionic additives have been used in combination with solid hole-transporters in order to facilitate charge compensation,2b whereas the long-term thermal and light soaking stability of the devices have been increased by using a gel electrolyte based on a stable polyfluorinated copolymer.18 However, the crucial issues of dye stability, lightharvesting, and electron-injection properties have never been addressed from this perspective. Some of us have been interested for a long time in the design and synthesis of catalytically active metal complexes of fluorous Pcs and related nitrogen ligands.19 In that event, it was recognized that, besides their positive impact on the chemical stability, fluorocarbon moieties such as perfluoroalkyl chains CnF2nþ1 can have a great influence on the catalyst behavior through energetically favorable fluorophilic interactions leading to

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Scheme 1. Synthesis of ZnPc Precursors

Scheme 2. Statistical Condensation of Phthalonitriles and Hydrolysis of Ester Groups

self-assembled nanoarchitectures where a fluid hydro- and organophobic fluorous barrier surrounds nonfluorinated molecular moieties.19 A similar effect could be possibly exploited to reduce the parasite electron/electrolyte recombination process on the semiconductor surface, which has been identified as another critical point in Pc-sensitized DSCs with I-/I3- as a redox mediator due to the tendency of large π-conjugated systems to bind I2.20 On these premises we started a research program aiming at the design and synthesis of fluorous photosensitizers for DSCs. We here report on the effects of the introduction of highly branched fluorophilic substituents on the push-pull character, steric bulk and electron-injection properties of ZnPcs.

’ EXPERIMENTAL METHODS Synthesis of Fluorous Dyes. General Remarks. An overview of the synthesis is depicted in Schemes 1 and 2. All commercially available chemicals were used as received without further purification. Solvents were purified by standard methods and dried if necessary, except spectroscopic grade CFCl2CF2Cl (CFC-113, Aldrich) and R,R,R -trifluorotoluene (BTF, Fluka) that were 3778

dx.doi.org/10.1021/jp109846k |J. Phys. Chem. C 2011, 115, 3777–3788

The Journal of Physical Chemistry C used as received. The highly branched fluorinated alcohol 5,21 and dipentyl 4,5-dicyanophthalate 1022 were prepared as described in the literature. Methyl 3,4-diodobenzoate 7,23 and methyl 3,4-dicyanobenzoate 824 had been previously obtained following less convenient synthetic procedures. Reactions were monitored by thin layer chromatography (TLC) that was conducted on plates precoated with silica gel Si 60-F254 (Merck, Germany). Column chromatography was carried out on silica gel SI 60 (Merck, Germany), mesh size 0.040-0.063 mm. Melting points (uncorrected) were determined with a B€uchi Melting Point B-540 capillary melting point apparatus. 1H NMR and 13C NMR were recorded on a Bruker Avance 400 spectrometer (400 and 100.6 MHz, respectively); 19F NMR spectra were recorded on a Bruker AC 300 spectrometer (282 MHz). UV-vis measurements in solution were performed on a Nicolet Evolution 500 (Thermo Electron Corporation) spectrophotometer. FT-IR spectra were recorded on a Perkin-Elmer 1725X instrument. MALDI mass spectra were obtained with a TOF-TOF mass spectrometer AUTOFLEX III (Bruker Daltonics) at the Centro Interdipartimentale Grandi Apparecchiature (C.I.G.A.) of the University of Milano: 2,5-dihydroxybenzoic acid (DHB) was used as the matrix. Elemental analyses were carried out by the Departmental Service of Microanalysis (University of Milano). Fluorous Phthalonitrile (6). The branched fluorous alcohol 5 (0.79 g, 1.00 mmol) was dissolved under nitrogen in a mixture of dry DMF (2.5 mL) and BTF (CF3C6H5, 1.0 mL) in a flame-dried Schlenk flask. Anhydrous K2CO3 (0.28 g, 2.03 mmol) and 4-nitrophthalonitrile (0.17 g, 1.00 mmol) were added and the mixture was vigorously stirred at room temperature for 24 h. After addition of H2O and Et2O the aqueous layer was separated and extracted twice with Et2O. The combined organic extracts were washed with H2O, dried over MgSO4 and concentrated in vacuum. The residue was purified by column chromatography (silica gel, petroleum ether, then petroleum ether/Et2O 4/1 to 7/ 3) affording the title compound as a white solid (0.74 g, 80% yield). Mp: 108.5-109.5 °C. 1H NMR (400 MHz, CDCl3): δ [ppm] = 7.78 (d, JHH = 8.7 Hz, 1H), 7.24 (d, JHH = 2.56 Hz, 1H), 7.17 (dd, JHH = 2.6, 8.7 Hz, 1H), 4.16 (s, 6H), 4.01 (s, 2H). 13C NMR (100.6 MHz, CDCl3): δ [ppm] = 160.9, 135.9, 120.2 (q, JCF = 292 Hz, 1H), 118.4, 119.6, 119.1, 115.5, 115.1, 109.5, 65.1, 64.5, 46.6. 19F NMR (282 MHz, CDCl3): δ [ppm] = -70.8 (s, 27F). Anal. Calcd for C24H9F27N2O4: C, 31.95; H, 1.01; N, 3.10. Found: C, 31.90; H, 0.96; N, 3.03. Methyl 3,4-Diodobenzoate (7). To a suspension of 4-iodobenzoic acid methyl ester (1.31 g, 5.00 mmol) in 96% H2SO4 (7 mL), N-iodosuccinimide (1.13 g, 5.00 mmol) was added. The black mixture was stirred at room temperature for 8 h, then it was poured into ice-water. The dark oil formed on the bottom was dissolved in AcOEt and the solution was washed with H2O, 5% aqueous NaHSO3, brine and dried over MgSO4. After removal of the solvent at reduced pressure, the crude product was purified by column chromatography (silica gel, petroleum ether/AcOEt 95/5) affording the title compound as a white solid (1.64 g, 84.5% yield). Physical and spectroscopic data were in agreement with those reported in the literature.23 Methyl 3,4-Dicyanobenzoate (8). Methyl 3,4-diodobenzoate 7 (0.50 g, 1.29 mmol) Pd2(dba)3 (47 mg, 0.05 mmol), dppf (60 mg, 0.08 mmol), Zn powder (16 mg, 0.24 mmol), and Zn(CN)2 (0.18 g, 1.54 mmol) were transferred under nitrogen into a flame-dried Schlenk flask. Dry N,N-dimethylacetamide (10 mL) was added and the resulting mixture was heated at 120 °C and stirred for 2 h, then cooled to room temperature, and partitioned

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between H2O and AcOEt. The resulting biphasic system was filtered through Celite, the organic layer was separated, and the aqueous layer was extracted with AcOEt. The combined organic extracts were washed with H2O and brine, and dried over MgSO4. After removal of the solvent at reduced pressure, the crude product was purified by column chromatography (silica gel, hexane/AcOEt 7/3) affording the title compound as a white solid (0.18 g, 74.5% yield). Physical and spectroscopic data were in agreement with those reported in the literature.24 Pentyl 3,4-Dicyanobenzoate (9). To a solution of methyl 3,4dicyanobenzoate 8 (0.37 g, 1.98 mmol) in 1-pentanol (30 mL) 96% H2SO4 (0.5 mL) was added. The mixture was warmed to 150 °C and stirred overnight, after which pentanol in excess was removed under reduced pressure. The residue was taken up in CH2Cl2 and the solution was washed with H2O, 5% aq. NaHCO3, brine and dried over MgSO4. After removal of the solvent at reduced pressure, the crude product was purified by column chromatography (silica gel, petroleum ether/AcOEt 9/1) affording the title compound as a pale yellow solid (0.30 g, 62.5% yield). Mp: 47-48 °C. 1H NMR (400 MHz, CDCl3): δ [ppm] = 8.45 (d, JHH = 1.5 Hz, 1H), 8.37 (dd, JHH = 1.5, 8.1 Hz, 1H), 7.92 (d, JHH = 8.1 Hz, 1H), 4.39 (t, JHH = 6.7 Hz, 2H), 1.80 (m, 2H), 1.41 (m, 4H), 0.95 (t, JHH = 6.9 Hz, 3H). 13C NMR (100.6 MHz, CDCl3): δ [ppm] = 163.1, 135.1, 134.3, 133.8, 133.7, 119.2, 116.5, 114.7, 114.6, 66.8, 28.2, 28.1, 22.3, 13.9. Anal. Calcd for C14H14N2O2: C, 69.41; H, 5.82; N, 11.56. Found: C, 69.47; H, 5.79; N, 11.31. Fluorous Zn-Phthalocyanine (3). In a Schlenk flask, fluorous phthalonitrile 1 (274 mg, 0.30 mmol), pentyl 3,4-dicyanobenzoate 9 (24 mg, 0.10 mmol), and Zn(OAc)2 (26 mg, 0.14 mmol) were suspended in 1-pentanol (5 mL). The mixture was warmed to 145 °C under a nitrogen atmosphere and stirred until it became homogeneous. DBU (30 μL, 0.20 mmol) was added and the mixture was heated at 145 °C for further 24 h. After cooling to room temperature the intense blue reaction mixture was poured into MeOH and the dark-blue solid was collected by filtration on a B€uchner funnel. The solid was thoroughly washed with MeOH, dried under reduced pressure and subjected to column chromatography (neutral Al2O3, hexane/AcOEt/Et2O 8/1/1) to give the unsymmetrical ZnPc 3 as a mixture of regioisomers (46 mg, 15% yield). MS (MALDI-TOF, DHB): m/z 3056.13 [M þ H]þ. UV-vis (Et2O, 1.6 10-5 M): λmax (log ε) = 349 (4.70), 605 (4.35), 664 (5.07) 678 (5.07) nm. FTIR (KBr): 2960 (sh), 2925 (m), 2854 (w), 1720 (w), 1664 (sh), 1614 (m), 1471 (br, m), 1384 (sh), 1255 (s, br), 1153 (s), 1016 (s), 973 (s), 728 (m), 538 (w) cm -1. Anal. Calcd for C89H47F81N8O14Zn: C, 34.97; H, 1.55; N, 3.67. Found: C, 34.51; H, 1.62; N, 3.01. Fluorous Zn-Phthalocyanine (4). The title compound (76 mg, 12% yield) was obtained as a mixture of regioisomers by reacting fluorous phthalonitrile 1 (550 mg, 0.60 mmol), dipentyl 4,5-dicyanophthalate 10 (71 mg, 0.20 mmol) and Zn(OAc)2 (51 mg, 0.28 mmol) in 1-pentanol (8 mL), in the presence of DBU (30 μL, 0.20 mmol) as a base. Reaction conditions and work-up procedure were identical to those reported for ZnPc 3. MS (MALDI-TOF, DHB): m/z 3170.07 [M þ H]þ. UV-vis (Et2O, 1.6 10-5 M): λmax (log ε) = 349 (4.72), 603 (4.25), 630 (4.32), 661 (4.90), 685 (4.93) nm. FT-IR (KBr): 2964 (m), 2925 (sh), 2854 (sh), 1725 (m), 1679 (sh), 1614 (m), 1471 (br, m), 1384 (sh), 1257 (s, br) 1153 (s), 1089 (w), 1016 (m), 973 (s), 802 (m), 728 (m), 538 (w) cm -1. Anal. Calcd for C95H57F81N8O16Zn: C, 35.99; H, 1.81; N, 3.53. Found: C, 36.63; H, 2.16; N, 3.13. 3779


The Journal of Physical Chemistry C Fluorous Zn-Phthalocyanine (1). To a suspension of ZnPc 3 (31 mg, 0.01 mmol) in 1-pentanol (2 mL) was added powdered KOH (217 mg, 3.87 mmol). The mixture was stirred at room temperature for 4 days, and then Et2O was added to dissolve any blue solid residue. The ethereal solution was treated with 10% aqueous HCl to neutralize unreacted KOH and the final pH of the aqueous layer was set at 2. The aqueous layer was separated the blue organic layer was washed with H2O and brine and dried over MgSO4. After removal of the solvent at reduced pressure, the solid residue was subjected to column chromatography (silica gel, Et2O/hexane 7/3) to give the unsymmetrical ZnPc 1 as a mixture of regioisomers (17 mg, 57% yield). MS (MALDITOF, DHB): m/z 2986.20 [M þ H]þ, UV-vis (Et2O, 3.0 10-6 M): λmax (log ε) = 349 (4.75), 605 (4.36), 664 (5.02) 675 (5.04) nm. FT-IR (KBr): 3440 (br), 2956 (sh), 2925 (s), 2854 (m), 1735 (sh), 1700 (m), 1614 (m), 1467 (br, m), 1384 (sh), 1255 (s, br) 1153 (s), 1097 (w), 1016 (m), 973 (s), 808 (w), 729 (m), 538 (w) cm -1. Anal. Calcd for C84H37F81N8O14Zn: C, 33.78; H, 1.25; N, 3.75. Found: C, 33.18; H, 1.43; N, 3.24. Fluorous Zn-Phthalocyanine (2). The title compound (24 mg, 39% yield) was obtained as a mixture of regioisomers by hydrolysis of ZnPc 4 (63 mg, 0.02 mmol) with powdered KOH (420 mg, 7.50 mmol) in 1-pentanol (3 mL). Reaction conditions and work-up procedure were identical to those reported for ZnPc 1, except for column chromatography (silica gel, Et2O then Et2O/DMF 9/1). MS (MALDI-TOF, DHB): m/z 3030.12 [M þ H]þ. UV-vis (Et2O/CFCl2CF2Cl 3/1 v/v 3.0 10-6 M): λmax (log ε) = 344 (4.72), 633 (4.36), 664 (4.85) 681 (4.85) nm. FT-IR (KBr): 3430 (br), 2960 (sh), 2927 (m), 2856 (sh), 1738 (sh), 1701 (sh), 1612 (m), 1469 (br, m), 1384 (m), 1255 (s, br) 1153 (s), 1093 (w), 1016 (m), 973 (s), 815 (w), 728 (m), 538 (w) cm -1. Anal. Calcd for C95H57F81N8O16Zn: C, 35.99; H, 1.81; N, 3.53. Found: C, 35.42; H, 2.08; N, 3.27. Dye-Sensitized Solar Cells. General Remarks. Transparent titanium dioxide was either from Dyesol (DSL NR18 NT) or prepared according to literature procedures by acidic hydrolysis of titanium tetraisopropoxide.25 Opaque TiO2 paste was purchased from Dyesol (DSL NR18AO). Conductive glass (FTO TEC 8, 8 Ω/square) were from Hartford Glass, Indiana. Reagent grade propylmethylimidazolium iodide (PMII), iodine, lithium iodide, chenodeoxycholic acid (Cheno), 4-tert-butylpyridine (Tbpy) electrochemical grade tetrabutylammonium hexafluorophosphate ((TBA)PF6) were from Aldrich and used as received. Methoxypropionitrile (MPN) was from Aldrich and distilled under reduced pressure prior to use. All other solvents (acetonitrile, ethanol, acetone, CFCl2CF2Cl) were of spectroscopic grade and used without further purification. Electrode Fabrication and Solar Cell Assembly. TiO2 gel was cast onto well-cleaned FTO substrates by doctor blading. The resulting film were gently dried in warm air and fired at 450 °C for 45 min. The TiO2 films (∼6 μm thick) were then treated with TiCl4 according to literature directions,26 and annealed at 450 °C before being immersed, still warm, in the dye solution. Because of the very poor solubility of the fluorous dyes in most of the solvents commonly employed for TiO2 sensitization, dye absorption was carried out for 24 h in the dark at room temperature from a mixed 1/9 CFCl2CF2Cl/EtOH solvent mixture which assured both a reasonable solubility of the molecular sensitizers (∼10-5 M) and the appropriate polarity for an effective permeation of the polar TiO2 surface. Immediately prior to cell assembly, the resulting photoanodes were

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extracted from the adsorption bath, rinsed thoroughly with ethanol and dried under a gentle warm air stream. PEDOT counter electrodes were obtained from a 10-2 M EDOT (ethylene-dioxy-tiophene) solution in 0.1 M LiClO4 by potentiostatic deposition onto well cleaned FTO substrates at 1.6 V vs SCE for 30 s. The resulting deep blue coated electrodes were rinsed with acetonitrile and ethanol, dried and stored in the dark. Parafilm sealed dye-sensitized cells (DSC) were built by pressing the sensitized photoanode against a PEDOT counter electrode equipped with a parafilm frame used to confine the liquid electrolyte (PMII 0.6 M/0.1 M LiI/0.02 M I2 in methoxypropionitrile) inside the cell. The thickness of liquid layer corresponded roughly to the thickness of the frame borders (=120 μm). Emission Spectroscopy. Emission spectra (Supporting Information) were recorded on a Jobin Yvon Spex Fluoromax 2 spectrofluorimeter equipped with a Hamamatsu R3896 phototube by using a 2 nm band-pass, an integration time of 1 s and an excitation wavelength of 630 nm. Emission lifetimes were measured using a TCSPC (Time Correlated Single Photon Counting) apparatus (PicoQuant Picoharp 300) equipped with subnanosecond LED sources (280, 380, 460, and 600 nm; 500700 ps pulsewidth) powered with a PicoQuant PDL 800-B variable (2.5-40 MHz) pulsed power supply. Fluorescence was observed at 700 nm. The resulting decays (Supporting Information) were analyzed by means of PicoQuant FluoFit global fluorescence decay analysis software and satisfactorily fitted with monoexponential functions. Photoelectrochemistry. Photon to current action spectra were collected by focusing the monochromatic light generated by an Applied Photophysics monochromator (spectral bandwidth =10 nm) onto the photoanode of sandwich type photoelectrochemical cell. The illuminated area was 0.25 cm2. Short circuit photocurrents were measured with an Agilent 34401A multimeter. IPCE was calculated according to IPCE % ¼ 1:24 103 ðV nmÞ

J ðμA cm-2 Þ λ ðnmÞ P ðW m-2 Þ

where J is the monochromatic photocurrent density, P is the radiant power density, and λ is the wavelength of the incident light. J-V curves were obtained under the simulated solar illumination (AM 1.5 G) generated by an ABET sun simulator by using an EcoChemie PGSTAT 30/2 electrochemical workstation controlled by the Ecochemie GPES 4.9 software. The linear potential sweep rate was 10 mV/s. Incident white light irradiance was measured with a Molectron Powermax 5200 Power-meter. Electrochemical impedance spectroscopy (EIS) was performed under illumination at open circuit conditions and in the dark at various constant potentials by applying a single sine perturbation of 10 mV amplitude in the 105-10-1 Hz frequency range with an EcoChemie FRA2 frequency response analyzer controlled by the NOVA 1.5 software. The resulting impedance plots were satisfactorily fitted in terms of equivalent electric circuits27 with the Zsimp Win 3.22 software. The maximum relative standard error was e10%. The photovoltage decays of sandwich DSCs under open circuit conditions were acquired by switching off the irradiation after a stable photovoltage was attained and maintained for several seconds under AM 1.5 G illumination. 3780


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Figure 1. Molecular structures of fluorous ZnPcs.

Computational Study. Ground state DFT calculations coupled to direct equilibrium geometry optimization were carried out on multiple core computers by using both Spartan 08 and Gaussian 03 programs with equivalent results. The input electronic density for building the hybrid functional was obtained from a preliminary restricted Hartree-Fock (6-31G*) procedure. TD DFT (B3LYP-6-31G*) calculations were carried out on the geometry optimized structure, by considering the first three singlet excited states. All calculations were performed in vacuum.

’ RESULTS AND DISCUSSION Design and Synthesis of Fluorous Dyes. Two unsymmetrical ZnPcs (1 and 2, Figure 1) have been synthesized in this first attempt to incorporate fluorous moieties in the molecular structure of photosensitizers for DSCs. They contain one or two electron-withdrawing carboxylic functions, which also serve to graft the complexes to nanocrystalline TiO2, directly linked to one of the benzene units of the Pc ring. The presence of connecting elements was deemed to be unnecessary in the light of the results reported by Nazeeruddin, Palomares, Torres, and co-workers.11d In both complexes, the remaining three benzene units bear the same highly branched fluorous alkoxy substituent in which the fluorine atoms reside on the outer spherical surface. This arrangement of substituents was chosen in order to minimize aggregation and to bestow correct directionality to the electronic orbitals in the excited state of ZnPc. In this context, the choice of a suitable fluorous moiety was crucial. Indeed, some small- and medium-sized fluorocarbon peripheral substituents are known to effectively suppress the aggregation tendency of Pc derivatives,28 but their electron withdrawing capacity would actually hinder the pull effect of the carboxylic groups and disfavor the electron transfer from the photoexcited dye to the TiO2 conduction band. Therefore, the insertion of appropriate insulating spacers in-between the Pc ring and highly electronegative CnF2nþ1 groups had to be considered. Preliminary DFT calculations carried out on simplified model molecules showed that uncomplicated fluorinated alkoxy groups such as -OCHF3 were suitable substituents in order to obtain LUMO with sufficient electron densities on and near the anchoring carboxylic functions, as required for good electronic coupling between the

Figure 2. Frontier molecular orbitals of model unsymmetrical ZnPcs bearing -OCF3 substituents calculated at the DFT-B3LYP-6-31G* level.

excited state of the dye and the 3d orbital of TiO2 (vide infra). In the case of comparable perfluoroalkoxy substituents (e.g., 2,2,2trifluoroethoxy groups), experimental data suggest that the CH2O- linker can actually shield the Pc ring from the electronic effect of fluorine atoms, although not completely.28d,f We thus reasoned that the introduction of additional ether bonds could eliminate this residual influence, and this idea was put into effect by selecting a fluorous substituent where three perfluoro-tertbutyl groups are arranged around a pentaerythritol hub. Unsymmetrical Pcs with identical substituents on three of the benzene units and a different substitution pattern on the forth one, here referred to as A3B products, can be prepared by several strategies.29 Although chemoselective routes based on the expansion of subphthalocyanine rings,29b or solid-phase synthesis29c offer several advantages, in particular in terms of ease of separation of the desired A3B products, statistical condensation of two different Pc precursors in solution remains the most common approach to the synthesis of push-pull metal complexes of Pcs, including those specifically designed as photosensitizers for DSCs.11,12,22 The A3B products ZnPcs 1 and 2 were similarly prepared, starting from fluorous phthalonitrile 6 and substituted phthalonitriles 7 and 9 (Scheme 1), respectively. The key precursor 6 was readily obtained by displacement reaction of the nitro group of 4-nitrophthalonitrile with the known fluorous alcohol 5,21 which behaved as a good nucleophile despite the presence of three perfluoro-tert-butyl groups. This peculiar reactivity supported our initial hypothesis that the electron withdrawing effect of the fluorous substituent might have been efficiently shielded by the introduction of suitable insulating units. Phthalonitrile 9 bearing a single carboxylic functionality protected as a pentyl ester was synthesized according to the three-step procedure depicted in Scheme 1, whereas the third required precursor, phthalonitrile 10 (Scheme 2), was prepared as previously described by Tylleman et al.22 3781



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Figure 3. Relevant electronic transitions and calculated visible absorption spectrum at the TDDFT B3LYP 6-31G* level for the model complex ZnPc(COOH)2.

Statistical condensation of 6 and 9 (3/1 molar ratio, Scheme 2) was performed in 1-pentanol, in the presence of 1,8-diaza-bicyclo[5.4.0]undec-7-ene (DBU) as a base and Zn(OAc)2 as a metal source, to give the intermediate A3B ZnPc carboxylic ester 3 that was separated chromatographically as a mixture of regioisomers from the reaction mixture containing A4, A2B2, and AB3 products. The desired ZnPc carboxylic acid 1 was finally obtained, again as a mixture of regioisomers, by basic hydrolysis of 3. The same method was applied in the synthesis of 2 from precursors 6 and 10. The overall yields of the cyclotetramerization/hydrolysis procedure were 8.6% and 4.7% in the case of 1 and 2, respectively. Although ZnPcs 1-4 were soluble enough in ethers or in mixtures of organic and fluorinated solvents to give solutions suitable for characterization by analysis techniques such as HRMS, their very low solubility in most usual organic solvents, in particular halogenated ones, precluded a detailed NMR analysis. This solubility profile (resembling that of other metal complexes of fluorous Pcs)19a,30 is in line with the presence of nine perfluoro-tert-butyl groups ranking very high on Rabai’s scale of fluorophilicity.31 Computational Study. DFT calculations were employed to evaluate the possibility to integrate fluorinated alkoxy substituents in the structure of unsymmetrical ZnPc with -COOH anchoring groups without triggering undesired distortions of the electronic density of the frontier orbitals. The computational study was performed, for sake of simplicity, on ZnPcs bearing uncomplicated -OCF3 groups, but otherwise identical to 1 and 2. The geometry optimized structures show that both model mono- and dicarboxy ZnPc derivatives (ZnPcCOOH and ZnPc(COOH)2, respectively) display a perfectly planar geometry of

the aromatic macrocycle. The -OCF3 groups bend out of the plane of the aromatic ring, with a dihedral angle of ca. 17°. As expected, in both cases the spatial distribution of the frontier orbitals is nearly the same: the HOMO is symmetrically distributed over the macrocycle with a relevant density on the fused benzene rings, while the LUMO is essentially distributed along the x axis of the molecule with a non-negligible density on the carbon adjacent to the carboxylic binding groups, a position which should guarantee a good electronic coupling with the acceptor states of TiO2 (Figure 2). The calculation also showed that when a strong electron acceptor, like a Ti(IV) cation, is bound to the carboxylate, the LUMO undergoes a considerable spatial shift on the carboxy group with an enhancement of the charge transfer character of the HOMO-LUMO transition. Although the binding to a simple Ti(IV) cation by no means reflects the true interaction of the dye with the semiconductor, it provides a semiquantitative indication that the presence of the carboxylic functions conjugated with the macrocycle allows for an adequate directionality of the excited state and a good electronic coupling for the photoinduced charge transfer. Interestingly, in good agreement with the experimental DPV results obtained with 1 and 2 (Figure 4, vide infra), the HOMO of ZnPc(COOH)2 (-5.47 eV) is about 110 meV lower than that of ZnPc(COOH) (-5.36 eV) probably due to the increased electron withdrawing effect of the two carboxylic moieties, while, compared to the corresponding -OCH3 substituted Zn phthalocyanine, the electronic effect of the -OCF3 groups over the spatial distribution of the frontier orbitals is minor, suggesting that the O-C group insulates quite effectively the aromatic core from the inductive effect of fluorine atoms. However, due to electronic repulsions, compared to OCH3 groups which 3782



Figure 4. UV-vis absorption spectra of 1 in Et2O at different concentrations.

essentially lye in the phthalocyanine plane, -OCF3 substituents are bent out of the plane of the aromatic ring with a dihedral angle variable between 16° and 19°. The negligible influence of fluorinated alkoxy substituents on the relevant electronic properties (i.e., frontier orbital energies, relevant electronic transitions) of the macrocycle was further confirmed by DFT calculations performed on a -OC(CF3)3 substituted phthalocyanine (see Supporting Information) where, however, a larger dihedral angle of ca. 80° between the sterically hindered pendants and the aromatic ring plane was obtained. The HOMO-LUMO gap and the results of the time dependent density functional theory (TDDFT) calculations on the model molecules (Figure 3) are in reasonable agreement with the experimentally measured absorption spectrum for 3 and 4, showing an intense absorption band in the red part of the visible spectrum, with a maximum around 600 nm, resulting essentially from the HOMO f LUMO (602 nm) and HOMO f LUMO þ 1 (592 nm) π-π* transitions. The LUMO þ1 orbital shows a marked distribution along the y direction, and lies energetically very close to the LUMO, as it would be expected from symmetry considerations. Optical and Electrochemical Properties. UV/vis absorption spectra of 1 and 2 in solution (Figures 4 and 5) are characterized by intense B- and Q-bands centered around 350 and 670 nm, respectively, typical of Pc derivatives. Strong absorbance in the far red region of the UV/vis spectrum was thus retained upon introduction of the fluorous peripheral substituents. For both compounds, the electronic absorption spectra in solution are characterized by splitting of their Q bands, which is particularly evident in the case of 2 (Figure 5). This splitting had been previously observed in zinc complexes of push-pull A3B Pcs,32 and attributed to the perturbation of the excited state energy levels arising from the interaction between the electron donatingand the electron withdrawing substituents. The position of the B and Q bands and corresponding molar absorption coefficients are unaffected by dilution in the concentration range 10-5-10-7 M, thus showing that at concentrations useful for the fabrication of sensitized TiO2 electrodes aggregation of 1 and 2 in solution is prevented by the bulky fluorous substituents which efficiently hinders π-π interactions between the flat phthalocyanine cores.

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Figure 5. UV-vis absorption spectra of 2 in Et2O/CFCl2CF2Cl 3/1 at different concentrations.

Figure 6. Differential pulse voltametry (DPV) of 1 (blue) and 2 (red) in CFCl2CF2Cl/CH3CN 1/10 þ 0.1 M (TBA)PF6 at a glassy carbon working electrode. Modulation amplitude 10 mV.

The unsymmetrical fluorous ZnPcs dyes 1 and 2 exhibit a nearly reversible oxidative behavior, testified by a peak separation (observed in cyclic voltammetry mode at 20 mV/s) in the 70-90 mV range without compensation of the cell resistance. As can be readily appreciated from the DPV (Figure 6) the oxidation of 2 is anodically shifted of about 110 mV with respect to 1. In both cases the ground state oxidation potential is adequate for the exoergonic oxidation of iodide in the operational cell. However, this process should be thermodynamically more favored in the case of the doubly substituted dye 2 for which a driving force of ∼-0.6 eV can be calculated. Both ZnPc species are emitting in fluid solution at room temperature (Figure 7), showing a structured band corresponding to the S1 f S0 radiative deactivation with a limited Stokes shift indicating a small distortion of the excited state, as would be expected from transitions involving highly delocalized π systems. In both cases the excited state oxidation potential (Eox*) calculated from E*ox = E1/2 - E0-0, where E0-0 is the optical excitation energy calculated from the maximum of the first vibronic band, results > -1 V vs SCE allowing for an effective charge transfer to the empty band of TiO2. The excited state 3783



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lifetimes of the two ZnPcs are very similar, being 3.7 ns for 1 and 2.9 ns for 2 (Supporting Information). A shorter lifetime for 2 was qualitatively expected on the basis of the energy gap law, given the slightly lower energy of its excited state.

Figure 7. Emission spectra of 1 (blue) and 2 (red) in CFCl2CF2Cl/ EtOH 1/10. λexc = 630 nm. Bandwidth: 2 nm.

Relevant spectroscopic and electrochemical data showing the feasibility of the electron injection from excited singlet state of the fluorous dyes to the conduction band of TiO2 and the reduction of the radical cations thus formed by I-/I3-redox couple are summarized in Table 1. The incident photon-to-current conversion efficiencies (IPCE) spectra for 1 and 2 adsorbed onto transparent and opaque TiO2 (Figure 8) are in good agreement with the corresponding absorption spectrum on TiO2 film (Figure 9), which maintains the characteristic intense absorption in the red part of the visible spectrum observed in solution, consistent with the effective injection from the excited state of the molecular sensitizers. Unsurprisingly, the best performances are found on opaque substrates, where an IPCEmax of up to 70% at 670 nm is observed for 2. On transparent TiO2 films IPCEs are lower, with best values of about 50%, but the general trend is preserved, with 2 nearly doubling the photoresponse of 1, despite its less negative excited state oxidation potential. Considering that on transparent TiO2 the maximum optical density is >2 for both sensitizers, it follows that the maximum APCE = IPCEmax/LHEmax = Φinjηcol (APCE = absorbed photon-to-current conversion efficiency, LHE(λ) = light harvesting efficiency = 1-10-A(λ)) is practically coincident with the maximum IPCE and is of the order

Table 1. Optical and Electrochemical Properties of Fluorous Zn-Phthalocyanine Complexes ZnPc 1

λabsa/nm

λemb/nm (τ/ns)

E0-0c/ eV

E1/2d/V

E*oxe/V

ΔGinjf/eV

ΔGregg/eV

349

687 (3.7)

1.80

0.59

-1.21

-0.51

-0.49

694 (2.9)

1.79

0.70

-1.09

-0.39

-0.60

605 664 675 2

344 633 664 681

a

Wavelengths for B and Q bands maxima in solution. b Wavelengths for emission maxima in solution by exciting at 630 nm. c Estimated from the intersection of the normalized absorption and emission spectra. d Determined by using differential pulse voltammetry (vs SCE). e Excited-state oxidation potential estimated from E1/2 and E0-0 (vs SCE). f Driving force for the electron injection from the ZnPc excited singlet state to the conduction band of TiO2 (-0.7 V vs SCE). g Driving force for the regeneration of the ZnPc radical cation by the redox couple I-/I3- (þ0.1 V vs SCE).

Figure 8. (a) Photocurrent action spectra of 1 (blue) and 2 (red) in sandwich DSCs made with opaque TiO2; (b) Photocurrent action spectra of 1 (blue) and 2 (red) in sandwich DSCs made with transparent TiO2. 3784



Figure 9. Diffuse reflectance spectra (transformed by the Kubelka and Munk method) of 1 (blue) and 2 (red) adsorbed onto opaque TiO2.

Figure 10. IPCE spectra of 2 in the absence (red) and in the presence (black) of 20 mM Cheno.

of 50% and 25% for 2 and 1 respectively. The quantum yield for electron injection from the ZnPc excited singlet state to the conduction band of TiO2 (Φinj) essentially depends on the excited state lifetime, on its energy and on the electronic coupling between the donor (excited dye) and the acceptor (TiO2) wave functions. Given the similarity of the calculated electronic properties for the two model fluorous ZnPc structures (Computational Study), and the close excited state lifetimes measured for 1 and 2, the nature of their excited states, their deactivation pathways and the electronic coupling with TiO2 are expected to be nearly the same. At the same time, the energetic is advantageous to 1, which, exhibiting a more negative excited state potential compared to the dicarboxylated analogue, would display a more thermodynamically favored charge injection (Table 1). Thus, the observed behavior cannot be explained by differences in Φinj, on whose basis the trend would be opposite to that experimentally observed. Consequently, different conversions probably arise from differences in the electron collection efficiency ηcol, which takes into account losses due to electron recombination with the electrolyte and/or with the oxidized sensitizer. In our case, the excited state quenching due to dye π stacking, is believed to be of minor importance: in fact no benefits in

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Figure 11. J-V curves under AM1.5 G illumination. Photosensitizers = 1 (blue) and 2 (red). Cells equipped with PEDOT counter electrodes.

monochromatic photon to electron conversion are observed by coadsorption in the presence of 20 mM chenodeoxycholic acid, which, on the contrary, tends to limit the IPCE (Figure 10) by competing with dye adsorption, reducing the coverage of the titania surface by photoactive species. Thus, it can be inferred that, even in the absence of coadsorbates, the bulky fluorous chains introduced on the methoxy substituents are effectively capable to act, on the surface, as spacers, preventing aggregation of the adsorbed dye. The J-V curves (Figure 11) under AM 1.5 G illumination are in qualitative agreement with the photoaction spectra, pointing out, even more markedly than under monochromatic irradiation, a more effective power conversion by 2, which reaches 1.3% efficiency compared to only 0.35% for 1. The major difference between the two dyes is found in the photocurrent, which is about three times lower for the monocarboxylated than for dicarboxylated derivative, whereas a slight difference (ca. 30 mV) is found in photovoltage. Electrochemical impedance spectroscopy (EIS) was employed to gain insight into the interfacial charge transfer processes. EIS experiments involving DSCs under illumination at open circuit show strong differences between the two dyes under investigation. The Nyquist plot of the cell sensitized by 2 (Figure 12a) shows the documented features of a good quality cell:27 two kinetic loops are found at high and mid frequency, originated by redox reactions at the PEDOT counter electrode and at the TiO2/ electrolyte interface, while, at low frequencies, the straight line that curves toward the real axis at very low frequencies has been ascribed to the mass transport of the redox couple. The charge transfer resistance at the PEDOT counter electrode is of the order of 6.5 Ω/cm2, testifying the good catalytic activity of the PEDOT substrate, while the charge transfer resistance at the TiO2/electrolyte interface is 76 Ω/cm2, a value comparable to that reported for interfaces sensitized by effective ruthenium dyes. On the other hand, the Nyquist plot of the cell sensitized by 1 shows a plot largely dominated by the mid frequency loop, which overlaps with both features originated by the charge transfer at the counter electrode and by the mass transport of the electrolyte. To an increased charge transfer resistance (2900 Ω/cm2) it corresponds a drastic reduction of the photoanode capacitance (from 3.1 10-4 F in the case of 2 to 6.3 10-6 F 3785



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Figure 12. (a) Nyquist plot for 2 sensitized DSC under 1 sun illumination at Voc. Red circles: experimental values. Yellow rhombs: calculated values. (b) Nyquist plot for 1 sensitized DSC under 1 sun illumination at Voc. Blue triangles: experimental values. Pale green rhombs: calculated values.

Figure 13. Normalized imaginary -Z vs frequency for 1 (blue) and 2 (red) sensitized DSCs. Note the slight low frequency shift of the mid frequency peak for 2.

Figure 14. Nyquist plot for 1 (blue) and 2 (red) sensitized DSCs in the dark under forward bias -300 or -350 mV.

for 1), indicating a decreased steady state electronic density into the TiO2 surface under illumination. The characteristic time (τ =

Figure 15. Electron lifetime from photovoltage decay measurements in sandwich DSCs: 1 (blue) and 2 (red).

RC) of the RQ (Q = constant phase element) circuit modeling the TiO2/electrolyte interface, which can be appreciated also from the intermediate frequency peak (∼10-100 Hz interval) of the -(Im Z) vs frequency plot (Figure.13), is of 10-2 s for 2 and of 8.5 10-3 s for 1, where the large increase in charge transfer resistance is accompanied by the consistent drop in capacitance. This characteristic lifetime has been shown to correspond to the lifetime of the electrons in the solid under open circuit conditions.33 In the dark, at negative potential, the behavior of the cell is in both cases dominated by the electron transfer from TiO2 to the electrolyte (dark current) and the order is essentially reversed (Figure 14). In this case it is the surface sensitized by 1 to show a decreased electron transfer resistance, indicating a more facile electron recapture by triiodide species contained in the electrolyte, as a probable result of an incomplete surface coverage and passivation by the monocarboxylated dye. In fact, the estimated surface concentration of 2 (9.4 10-8 mol/cm2) is nearly twice that of 1 (5.8 10-8 mol/cm2). The electron lifetimes obtained from photovoltage decay measurements34 (Figure 15) are in reasonable agreement with the evidence gained from EIS spectroscopy. Although at early times, in correspondence of the maximum Voc, a reliable acquisition is limited by the relatively slow sampling time (0.1 s) of our 3786


The Journal of Physical Chemistry C apparatus, it can be seen that the electron lifetime is close to 0.01 s, consistent with the EIS results. Most importantly, it can be appreciated on a logarithmic scale that essentially at every photovoltage the electron lifetime measured in the presence of sensitizer 2 is longer than that observed for 1. According to EIS, this is the result of an improved hole transfer kinetic to I-, in agreement with the more positive ground state oxidation potential, and to a reduced recombination with I3-, resulting for a more complete TiO2 surface passivation by the dye which, thanks to the double carboxylic functions, displays an increased affinity for the TiO2 surface.

’ CONCLUSIONS A bulky, electronically reversed, fluorous substituent in which the correct pairing of perfluoroalkyl groups and electron-rich spacers allows to shield the electron withdrawing effect exerted by the former has been employed as the electron releasing substituent in the unsymmetrical push-pull ZnPcs 1 and 2 featuring electron withdrawing carboxylic groups. This unprecedented molecular setup retains some advantages typical of perfluoroalkyl-substituted Pcs, such as their reduced aggregation tendency, without hindering the electron injection from the photoexcited dye to the TiO2 conduction band. Indeed, the peak incident photon-to-current conversion efficiencies (IPCE) in the near IR region reached almost 70% with ZnPc 2 having two COOH anchoring groups, in the absence of any additive such as antiaggregating Cheno or conduction band edge modifier tert-butylpyridine. In this regard, this prototype fluorous sensitizer compares favorably with most phthalocyanine dyes investigated so far. In the case of ZnPc 1 with just one COOH anchoring group an IPCEmax value of 35% was observed. Radiant-toelectric power conversion of DSCs based on the new fluorous dyes were in qualitative agreement with the photoaction spectra and were promising, if consideration is made for the relatively thin (∼6 μm) TiO2 layer used in our experiments. An higher power conversion was achieved with ZnPc 2, which gave 1.3% efficiency with respect to the 0.35% efficiency observed for ZnPc 1. If compared to ZnPc 1, the improved interfacial charge transfer kinetics of ZnPc 2 arises from a larger driving force for iodide oxidation, resulting from its higher ground state oxidation potential and from a more complete passivation of the titania surface against recombination with the oxidized electrolyte. Since both features are a consequence of the presence of two carboxylic functions on ZnPc 2, the insertion of multiple electron withdrawing anchoring groups should be generally considered advantageous for this type of sensitizers. In summary, we have shown for the first time that tailor-made fluorous moieties can be successfully used to tune the electronic distribution and the antiaggregation properties of photosensitizing dyes for DSCs. Although the sterically hindered fluorous chains may help in shielding the titania surface and the dye itself against the back recombination involving I2 or I3-, the existence of specific fluorophilic interactions preventing electron recombination is not yet confirmed by the present investigations, which, on the opposite, show clear recombination processes taking place at relatively small (-300 mV) direct voltages. Nevertheless, the basic information obtained will be useful as guidelines for the development of new fluorous sensitizers for DSCs with improved photovoltaic properties. ’ ASSOCIATED CONTENT

bS

Supporting Information. Optimized ground state structure and isodensity surface plots of the frontier molecular orbitals

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of a model unsymmetrical A3B ZnPc(COOH)2 bearing -OC(CF3)3 substituents, and emission spectroscopy data for ZnPcs 1 and 2, and cyclic voltammetry of the diluted (ca. 200) iodide/ triiodide electrolyte. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*To whom correspondence should be addressed. G.P.: telephone þ39-2-50314163, fax þ39-2-50314159, e-mail: [email protected]. S.C.: telephone þ39 0532 455146, fax þ39 0532 240709, e-mail: [email protected].

’ ACKNOWLEDGMENT The authors gratefully acknowledge the CNR-Regione Lombardia “Mind in Italy” project, Polo Solare Organico Regione Lazio and PRIN 2008 (Design and Development of New Components for Highly Efficient Dye Sensitized Solar Cells) for financial support. Mr. Pietro Colombo (ISTM-CNR) and Dr. Anna Daghetti (University of Milano) for characterization of fluorous zinc phthalocyanines. Dr. Rita Boaretto (University of Ferrara) for technical assistance. ’ REFERENCES (1) (a) Pagliaro, M.; Palmisano, G.; Ciriminna, R. Flexible Solar Cells; Wiley-VCH: Weinheim, Germany, 2008. (b) J. Phys. Chem. C 2009, Virtual Issue Nanotechnology for Next Generation Solar Cells. (2) (a) O’Regan, B.; Gr€atzel, M. Nature 1991, 335, 737–740. (b) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weiss€ortel, F.; Salbeck, J.; Spreitzert, H.; Gr€atzel, M. Nature 1998, 395, 583–585. (c) He, J.; Lindstr€ om, H.; Hagfeldt, A.; Lindquist, S. E. Sol. Energy Mater. Sol. Cells 2000, 62, 265–273. (d) Shankar, K.; Feng, X.; Grimes, C. A. ACS Nano 2009, 3, 788–794. (3) Gr€atzel, M. Acc. Chem. Res. 2009, 42, 1781–1798. (4) (a) Snaith, H. J.; Schmidt-Mende, L. Adv. Mater. 2007, 19, 3187– 3200. (b) Yum, J.-H.; Chen, P.; Gr€atzel, M.; Nazeruddin, M. K. ChemSusChem 2008, 1, 699–707. (5) Recent examples:(a) Bessho, T.; Constable, E. C.; Gr€atzel, M.; Redondo, A. H.; Housecroft, C. E.; Kylberg, W.; Nazeeruddin, M. K.; Neuburger, M.; Schaffner, S. Chem. Commun. 2008, 3717–3719. (b) Bernhardt, P. V.; Boschloo, G. K.; Bozoglian, F.; Hagfeldt, A.; Martinez, M.; Sienra, B. New J. Chem. 2008, 32, 705–711. (c) Kumar, A.; Chauhan, R.; Molloy, K. C.; Kociok-K€ohn, G.; Bahadur, L.; Singh, N. Chem.—Eur. J. 2010, 16, 4307–4314. (d) Xiao, L.; Liu, Y.; Xiu, Q.; Zhang, L.; Guo, L.; Zhang, H.; Zhong, C. Tetrahedron 2010, 66, 2835–2842. (6) Comprehensive reviews:(a) Mishra, A.; Fischer, M. K. R.; B€auerle, P. Angew. Chem., Int. Ed. 2009, 48, 2474–2499. (b) Ooyama, Y.; Harima, Y. Eur. J. Org. Chem. 2009, 2903–2934. (7) (a) Imahori, H.; Umemaya, T.; Ito, S. Acc. Chem. Res. 2009, 42, 1809–1818. (b) Martínez-Díaz, M. V.; de la Torre, G.; Torres, T. Chem. Commun. 2010, 7090–7108. (8) (a) Hinsch, A.; Brandt, H.; Veurman, W.; Hemming, S.; Nittel, M.; Wuerfel, U.; Putyra, P.; Lang-Koetz, C.; Stabe, M.; Beucker, S.; Fichter, K. Sol. Energy Mater. Sol. Cells 2009, 93, 820–824. (b) Koeppe, R.; Hoeglinger, D.; Troshin, P. A.; Lyubovskaya, R. N.; Razumov, V. F.; Sariciftci, N. S. ChemSusChem 2009, 2, 309–313. (9) (a) Nazeeruddin, M. K.; Humphry-Baker, R.; Gr€atzel, M.; W€ohrle, D.; Schnurpfeil, G.; Schneider, G.; Hirth, A.; Trombach, N. J. Porphyrins Phthalocyanines 1999, 3, 230–237. (b) He, J.; Benk€ o, G.; Korodi, F.; Polívka, T.; Lomoth, R.; Åkermark, B.; Sun, L.; Hagfeldt, A.; Sundstr€om, V. J. Am. Chem. Soc. 2002, 124, 4922–4932. (10) (a) Yanagisawa, M.; Korodi, F.; He, J.; Sun, L.; Sundstr€om, V.; Åkermark, B. J. Porphyrins Phthalocyanines 2002, 6, 217–224. 3787


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