Two-dimensional X-shaped organic dyes bearing two anchoring groups to TiO2 photoanode for efficient dye-sensitized solar cells

Synthetic Metals 162 (2012) 2095–2101

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Two-dimensional X-shaped organic dyes bearing two anchoring groups to TiO2 photoanode for efficient dye-sensitized solar cells Sun Jae Kim a , Dong Uk Heo a , Beom Jin Yoo a , Boeun Kim b , Min Jae Ko b , Min Ju Cho a , Dong Hoon Choi a,∗ a b

Department of Chemistry, Research Institute for Natural Sciences, Korea University, Seoul 136-701, Republic of Korea Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea

a r t i c l e

i n f o

Article history: Received 24 August 2012 Received in revised form 28 September 2012 Accepted 3 October 2012 Available online 9 November 2012 Keywords: X-shaped molecule Synthesis Photosensitizer Solar cell Power conversion efficiency

a b s t r a c t Novel two-dimensional X-shaped donor–␲–acceptor (D–␲–A)-type dyes were designed and successfully synthesized for use in a dye-sensitized solar cell (DSSC). Two triphenylamine units in these dyes act as electron donor units, while two cyanoacrylic acid groups act as electron acceptor units and anchoring groups to the TiO2 photoanode. The photovoltaic properties of the newly synthesized dye-containing DSSCs were investigated to identify the effects of conjugation length between the electron donors and acceptors, and the molecular energy levels of the dyes. Among the three dyes we synthesized, (2E,2 E)-3,3 -(5 ,5 -(4,5-bis(4-(bis(4-tert-butylphenyl)amino)styryl)1,2-phenylene)bis(2,2 -bithiophene-5 ,5-diyl))bis(2-cyanoacrylic acid) (11) showed the highest power conversion efficiency of 3.14% (max = 4.06% with TiCl4 treatment) under AM 1.5G illumination (100 mW cm−2 ) in a photoactive area of 0.418 cm2 with short circuit current density of 7.27 mA cm−2 , open circuit photovoltage of 612 mV, and a fill factor of 70.6%. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Recently, dye-sensitized solar cells (DSSCs) have attracted much attention because they are undergoing intense development toward devices that are more efficient, cheap, and relatively stable compared to conventional p–n junction solar cells [1,2]. Improvement of the efficiency of conversion of solar energy to electricity has continued to be an important research goal for high-performance DSSCs. The standard structure of well-approved DSSCs is an electrochemical cell composed of a Ru-complex-based photosensitizing dye adsorbed onto a wide-band-gap electrode such as TiO2 or ZnO, an electrolyte containing I− /I3 − , and a Pt-coated counter electrode. The photon-to-current conversion mechanism of DSSCs is based on the injection of electrons from the excited photosensitizers into the conduction band of nanocrystalline TiO2 or ZnO. The oxidized photosensitizers are reduced by electron injection from the electrolyte. Thus, the photosensitizing dye plays an important role in capturing the photons and generating the electron/hole pair, transferring the charge to the interface of the electrode and electrolyte. At present, DSSC sensitizers based on Ru complexes have achieved power conversion efficiencies (PCEs) of ∼12% at standard global air mass 1.5 (AM 1.5G) sunlight [3–5]. Recently, sensitizers based on Zn-porphyrin dye were reported to show comparable

∗ Corresponding author. E-mail address: [email protected] (D.H. Choi). 0379-6779/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2012.10.003

PCEs of around 12% [6], whereas metal-free organic sensitizers merely showed PCEs in the range of 6.5–8.5% [7–11]. Organic dyes without metal atoms can exhibit great ␲–␲* charge transfer (CT) and intramolecular charge transfer (ICT) transitions relative to their Ru-based metal-ligand counterparts [12,13]. Although the PCEs and external quantum efficiencies (EQEs) in incident photon-toelectron conversion efficiency (IPCE) spectra achieved with the metal-free organic dyes used in DSSCs are considerably lower than those achieved with Ru-based dyes, they still showed promising potential for use as tunable photosensitizers. This is because metalfree organic dyes are environment friendly and have high molar extinction coefficients in a desired light wavelength range; further, flexibility of their molecular design and appropriate tuning of their molecular energy levels are advantageous to improvement of the PCEs in solar cells. The relatively low molar extinction coefficients of Ru-based dyes and their need for rare and expensive metals with tedious purification processes are major problems in terms of cost and environmental issues, hampering their large-scale application to DSSCs. A common strategy for the design of highly efficient donor–␲–acceptor (D–␲–A) systems is the use of conjugated linking groups which can be tethered to the TiO2 electrode as surface anchoring groups. The excitation light generates photoinduced intramolecular charge transfer (PICT) states, which can transfer photogenerated electrons to the conduction band of TiO2 . The preferential orientation of the dye on the TiO2 surface not only improves donor ability at a distance from the photoinjected electrons, but

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also diminishes the detrimental impact of backward transfer of the electrons. The objective of the work presented herein is to demonstrate the synthesis of novel X-shaped D–␲–A organic dyes for use in DSSCs. To study the role of the metal-free photosensitizer, we synthesized X-shaped organic dyes which contain two electron donating and two electron accepting moieties. In general, two tertbutyl-substituted triphenylamine units in these dyes act as electron donors, while two cyanoacrylic acid groups act as electron acceptors. The photovoltaic properties of solar cell devices with 7, 11, and 15 were measured to identify the effects of the conjugation length between the donor and acceptor moieties, and their molecular energy levels on the performance of the DSSC. The device performances were evaluated under standard AM 1.5G conditions yielding up to 3.14% efficiency in the DSSC based on dye 11 bearing a bithiophene block. Thus, it was found that extending the ␲-conjugated bridge led to an increase in the solar cell efficiency.

2. Experimental 2.1. Synthesis and materials All commercially available starting materials and solvents were purchased from Aldrich, TCI, Fluka, and Acros Organics, and used without further purification unless otherwise stated. HPLC-grade toluene and tetrahydrofuran (THF) were purchased from Samchun Chemical and distilled from CaH2 immediately before use. Intermediate compounds were synthesized according to literature methods [14,15]. 4,4 -(1E,1 E)-2,2 -(4,5-Dibromo-1,2-phenylene)bis(ethene-2,1diyl)bis(N,N-bis(4-tert-butylphenyl)aniline) (3): Compound 1 (3.95 g, 10.2 mmol) and compound 2 (2.5 g, 4.66 mmol) were dissolved in dry THF (40 mL). The THF solution was slowly added dropwise to a THF mother solution (20 mL) of potassium tertbutoxide (1.25 g, 11.2 mmol) at room temperature under an argon atmosphere and the mixture was stirred for 12 h. The reaction was quenched by the addition of HCl solution and the mixture was poured into water to give a yellow precipitate. The crude product was purified by silica gel column chromatography (eluent: 4:1 hexane/methylene chloride (MC)) to yield 3 (3.3 g, 71%) as a yellow powder. 1 H NMR (400 MHz, CDCl ) ı (ppm) 7.77 (s, 2H), 7.35 (d, 3 J = 8.60 Hz, 4H), 7.27 (d, J = 8.60 Hz, 8H), 7.14 (d, J = 16.04 Hz, 2H), 7.05–7.00 (t, 12H), 6.93 (d, J = 16.04 Hz, 2H), 1.31 (s, 36H). MS (MALDI-TOF) m/z: calcd. C66 H66 Br2 N2 [M+ ] 998.54; found 996.36. Anal. Calcd. for C66 H66 Br2 N2 : C, 74.59; H, 6.76; N, 2.74; found: C, 74.54; H, 6.66; N, 2.80. 4,4 -(1E,1 E)-2,2 -(4,5-Bis(5-(5,5-dimethyl-1,3-dioxan-2yl)thiophen-2-yl)-1,2-phenylene)bis(ethene-2,1-diyl)bis(N,N-bis(4tert-butylphenyl)aniline) (5): A mixture of 3 (0.4 g, 0.4 mmol), tributyl(5-(5,5-dimethyl-1,3-dioxan-2-yl)thiophen-2-yl)stannane (4) (0.68 g, 1.4 mmol), and Pd(PPh3 )4 (0.02 g, 0.02 mmol) was stirred in DMF (25 mL) at 80 ◦ C overnight. After cooling the reaction mixture, H2 O was added and the mixture was extracted with MC. The organic layer was separated and dried over MgSO4 . The solvent was removed under vacuum. The crude product was purified by silica gel column chromatography (eluent: 1:1.5 hexane/MC) to yield 5 (0.3 g, 61%) as a yellow powder. 1 H NMR (400 MHz, CDCl ) ı (ppm) 7.69 (s, 2H), 7.38 (d, 3 J = 8.60 Hz, 4H), 7.32–7.25 (m, 10H), 7.05–6.97 (m, 14H), 6.96 (d, J = 3.52 Hz, 2H), 6.68 (d, J = 3.52 Hz, 2H), 5.61 (s, 2H), 3.79 (d, J = 11.32 Hz, 4H), 3.66 (d, J = 10.96 Hz, 4H), 1.31 (s, 36H), 1.28 (s, 6H), 0.80 (s, 6H). MS (MALDI-TOF) m/z: calcd. C82 H92 N2 O4 S2 [M+ ] 1232.84; found 1232.65. Anal. Calcd. for C82 H92 N2 O4 S2 : C, 79.70; H, 7.16; N, 2.87; S, 5.15; found, C, 79.83; H, 7.52; N, 2.27; S, 5.20.

5,5 -(4,5-Bis(4-(bis(4-tert-butylphenyl)amino)styryl)-1,2phenylene)dithiophene-2-carbaldehyde (6): Compound 5 (0.33 g, 0.26 mmol) was dissolved in a mixture of dry THF (15 mL) and water (5 mL). Trifluoroacetic acid (15 mL) was then added and the resulting reaction mixture was stirred for 3 h at room temperature, carefully quenched with saturated NaHCO3 (aq), and extracted with MC. The organic layer was separated, dried over MgSO4 , and the solvent was removed under vacuum. The crude product was purified by silica gel column chromatography (eluent: MC) to yield 6 (0.22 g, 78%) as an orange powder. 1 H NMR (400 MHz, CDCl ) ı (ppm) 9.87 (s, 2H), 7.71 (s, 2H), 3 7.64 (d, J = 3.52 Hz, 2H), 7.39 (d, J = 8.60 Hz, 4H), 7.32–7.25 (m, 10H), 7.06–7.01 (m, 14H), 6.98 (d, J = 3.92 Hz, 2H), 1.31 (s, 36H). MS (MALDI-TOF) m/z: calcd. C72 H72 N2 O2 S2 [M+ ] 1061.76; found 1060.50. Anal. Calcd. for C72 H72 N2 O2 S2 : C, 81.48; H, 6.99; N, 2.64; S, 6.13; found, C, 81.47; H, 6.84; N, 2.64; S, 6.04. (2E,2 E)-3, -(5,5 -(4,5-Bis(4-(bis(4-tertbutylphenyl)amino)styryl)-1,2-phenylene)bis(thiophene-5,2diyl))bis(2-cyanoacrylic acid) (7): Compound 6 (210 mg, 0.188 mmol), cyanoacetic acid (200 mg, 1.88 mmol), and a catalytic amount of piperidine were added to ethanol/chloroform (28 mL, 1:3, v/v) and the mixture was refluxed for 12 h. The red solution was cooled to room temperature to yield a dark-colored precipitate that was filtered and thoroughly washed with acetic acid, water, and ethanol, successively. The precipitates were collected and dried under vacuum, yielding 7 (170 mg, 72%) as a brown powder. 1 H NMR (400 MHz, DMSO-d ) ı (ppm) 8.35 (s, 2H), 7.86–7.84 (t, 6 4H), 7.54 (d, J = 8.60 Hz, 4H), 7.49 (d, J = 16.44 Hz, 2H), 7.30–7.23 (m, 12H), 6.94 (d, J = 8.60 Hz, 8H), 6.83 (d, J = 8.60 Hz, 4H), 1.22 (s, 36H). MS (MALDI-TOF) m/z: calcd. C78 H74 N4 O4 S2 [M+ ] 1194.76; found 1194.52. Anal. Calcd. for C78 H74 N4 O4 S2 : C, 77.55; H, 6.42; N, 4.57; S, 5.32; found, C, 78.36; H, 6.24; N, 4.69; S, 5.36. 4,4 -(1E,1 E)-2,2 -(4,5-Bis(5 -(5,5-dimethyl-1,3-dioxan-2-yl)-2,2 bithiophen-5-yl)-1,2-phenylene)bis(ethene-2,1-diyl)bis(N,N-bis(4tert-butylphenyl)aniline) (9): A procedure similar to that described for compound 5 was followed using compound 8 instead of compound 4 to give 9 in 95% yield as a dark yellow powder. 1 H NMR (400 MHz, CDCl ) ı (ppm) 7.69 (s, 2H), 7.40 (d, 3 J = 8.24 Hz, 4H), 7.33 (d, J = 16.44 Hz, 2H), 7.27–7.22 (t, 10H), 7.05–7.00 (m, 18H), 6.85 (d, J = 3.92 Hz, 2H), 5.60 (s, 2H), 3.77 (d, J = 10.96 Hz, 4H), 3.65 (d, J = 10.92 Hz, 4H), 1.31 (s, 36H), 1.28 (s, 6H), 0.80 (s, 6H). MS (MALDI-TOF) m/z: calcd. C90 H96 N2 O4 S4 [M+ ] 1397.93; found 1396.63. Anal. Calcd. for C90 H96 N2 O4 S4 : C, 76.38; H, 6.04; N, 2.20; S, 9.01; found, C, 77.32; H, 6.92; N, 2.00; S, 9.17. 5 ,5 -(4,5-Bis(4-(bis(4-tert-butylphenyl)amino)styryl)-1,2phenylene)di-2,2 -bithiophene-5-carbaldehyde (10): A procedure similar to that described for compound 6 was followed using compound 9 instead of compound 5 to give 10 in 98% yield as an orange powder. 1 H NMR (400 MHz, CDCl ) ı (ppm) 9.84 (s, 2H), 7.70 (s, 2H), 7.65 3 (d, J = 3.92 Hz, 2H), 7.39 (d, J = 8.64 Hz, 4H), 7.32–7.25 (m, 12H), 7.21 (d, J = 3.88 Hz, 2H), 7.05–7.00 (m, 14H), 6.92 (d, J = 3.52 Hz, 2H), 1.31 (s, 36H). MS (MALDI-TOF) m/z: calcd. C80 H76 N2 O2 S4 [M+ ] 1224.48; found 1224.72. Anal. Calcd. for C80 H76 N2 O2 S4 : C, 76.39; H, 5.77; N, 2.05; S, 9.11; found, C, 78.39; H, 6.25; N, 2.29; S, 10.46. (2E,2 E)-3,3 -(5 ,5 -(4,5-Bis(4-(bis(4-tertbutylphenyl)amino)styryl)-1,2-phenylene)bis(2,2 -bithiophene5 ,5-diyl))bis(2-cyanoacrylic acid) (11): A procedure similar to that described for dye 7 using compound 10 instead of compound 6 to give 11 in 50% yield as a brown powder. 1 H NMR (300 MHz, DMSO-d ) ı (ppm) 8.43 (s, 2H), 7.92 (d, 6 J = 4.14 Hz, 2H), 7.82 (s, 2H), 7.54–7.44 (m, 8H), 7.32–7.21 (m, 12H), 7.13 (d, J = 3.54 Hz, 2H), 6.96 (d, J = 8.25 Hz, 8H), 6.85 (d, J = 7.95 Hz, 4H), 1.25 (s, 36H). MS (MALDI-TOF) m/z: calcd. C86 H78 N4 O4 S4 [M+ ] 1358.73; found 1358.49. Anal. Calcd. for C86 H78 N4 O4 S4 : C, 75.78; H, 5.66; N, 4.26; S, 9.33; found, C, 75.96; H, 5.78; N, 4.12; S, 9.43.

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4,4 -(1E,1 E)-2,2 -(4,5-Bis(5-(7-(5-(5,5-dimethyl-1,3-dioxan2-yl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen2-yl)-1,2-phenylene)bis(ethene-2,1-diyl)bis(N,N-bis(4-tertbutylphenyl)aniline) (13): To a THF solution of lithium diisopropylamide (LDA) (2.0 M, 0.66 mL) was added 4-(5-(5,5-dimethyl-1,3-dioxan-2-yl)thiophen-2-yl)-7-(thiophen2-yl)benzo[c][1,2,5]thiadiazole (0.5 g, 1.2 mmol) in dry THF at −78 ◦ C. After stirring the solution for 2 h at −78 ◦ C, trimethylstannyl chloride (0.265 g, 1.32 mmol) was added. After stirring for 5 h, the mixture was quenched with water and extracted with chloroform. The organic layer was separated and dried over MgSO4 . The solvent was removed under vacuum to give 4-(5-(5,5-dimethyl-1,3-dioxan-2-yl)thiophen-2-yl)-7-(5(trimethylstannyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (12) as a red powder. A procedure similar to that described for compound 5 was followed using compound 12 instead of compound 4 to give 13 in 47% yield as a red powder. 1 H NMR (400 MHz, CDCl ) ı (ppm) 8.05 (d, J = 3.92 Hz, 2H), 3 8.00 (d, J = 3.92 Hz, 2H), 7.83 (s, 4H), 7.43 (d, J = 8.00 Hz, 4H), 7.39 (d, J = 16.04 Hz, 2H), 7.30–7.23 (m, 14H), 7.22 (d, J = 3.92 Hz, 2H), 7.10–7.04 (m, 12H), 5.70 (s, 2H), 3.81 (d, J = 10.96 Hz, 4H), 3.70 (d, J = 11.36 Hz, 4H), 1.32 (s, 36H), 1.29 (s, 6H), 0.82 (s, 6H). MS (MALDITOF) m/z: calcd. C100 H100 N6 O4 S6 [M+ ] 1666.36; found 1666.61. Anal. Calcd. for C100 H100 N6 O4 S6 : C, 73.11; H, 6.53; N, 4.98; S, 11.08; found, C, 73.52; H, 6.05; N, 5.04; S,11.55. 5,5 -(7,7 -(5,5 -(4,5-Bis(4-(bis(4-tertbutylphenyl)amino)styryl)-1,2-phenylene)bis(thiophene-5,2diyl))bis(benzo[c][1,2,5]thiadiazole-7,4-diyl))dithiophene-2carbaldehyde (14): A procedure similar to that described for compound 6 was followed using compound 13 instead of compound 5 to give compound 14 in 80% yield as a red powder. 1 H NMR (400 MHz, CDCl3 ) ı (ppm) 9.94 (s, 2H), 8.15 (d, J = 3.92 Hz, 2H), 8.09 (d, J = 3.52 Hz, 2H), 7.91 (d, J = 7.44 Hz, 2H), 7.83 (d, J = 3.12 Hz, 2H), 7.79(s, 2H), 7.40 (d, J = 8.20 Hz, 4H), 7.35–7.23 (m, 14H), 7.11–7.03 (m, 14H), 1.32 (s, 36H). MS (MALDI-TOF) m/z: calcd. C92 H80 N6 O2 S6 [M+ ] 1493.38; found 1492.47. Anal. Calcd. for C92 H80 N6 O2 S6 : C, 73.13; H, 5.38; N, 4.93; S, 12.99; found, C, 73.96; H, 5.40; N, 5.62; S, 12.88. (2E,2 E)-3,3 -(5,5 -(7,7 -(5,5 -(4,5-Bis(4-(bis(4-tertbutylphenyl)amino)styryl)-1,2-phenylene)bis(thiophene-5,2diyl))bis(benzo[c][1,2,5]thiadiazole-7,4-diyl))bis(thiophene-5,2diyl))bis(2-cyanoacrylic acid) (15): A procedure similar to that described for compound 7 was followed using compound 14 instead of compound 6 to give dye 15 in 74% yield as a dark brown powder. 1 H NMR (300 MHz, DMSO-d ) ı (ppm) 8.55 (s, 2H), 8.42 (d, 6 J = 3.90 Hz, 2H), 8.29 (d, J = 3.66 Hz, 2H) 8.15 (d, J = 7.55 Hz, 2H), 7.98 (d, J = 3.20 Hz, 2H), 7.85 (s, 2H), 7.55 (d, J = 8.44 Hz, 4H), 7.46–7.30 (m, 14H), 7.16–7.00 (m, 14H), 1.30 (s, 36H). MS (MALDI-TOF) m/z: calcd. C98 H82 N8 O4 S6 [M+ ] 1626.98; found 1626.48. Anal. Calcd. for C98 H82 N8 O4 S6 : C, 72.85; H, 4.96; N, 6.74; S, 11.87; found, C, 72.29; H, 5.08; N, 6.88; S, 11.82. 2.2. Instrumental analysis The 1 H NMR spectra were recorded on a Varian Mercury 400 MHz NMR spectrometer using CDCl3 and DMSO-d6 purchased from Cambridge Isotope Laboratories, Inc. Elemental analyses were performed by the Center for Organic Reactions using an EA1112 elemental analyzer (Thermo Electron Corporation). MALDI-TOF mass analysis was performed on an LRF20 mass spectrometer (Bruker Daltonics) Absorption spectroscopy was performed using a UV–vis spectrophotometer (HP 8453, PDA type) in the wavelength range of 190–1100 nm. Photoluminescence (PL) spectra were recorded with a Hitachi F-7000 fluorescence spectrophotometer. The redox

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properties of the three dyes were examined by CV conducted using a potentiostat (EA161, eDAQ). A 0.10 M tetrabutylammonium hexafluorophosphate (Bu4 NPF6 ) solution in freshly dried DMF was employed as the electrolyte. The Ag/AgCl and Pt wire (0.5 mm in diameter) electrodes were utilized as reference and counter electrodes, respectively. The scan rate was 100 mV s−1 . 2.3. Assembly and characterization of DSSCs The DSSC was fabricated by the following method. The conducting glass substrate (FTO) was cleaned in ethanol by ultrasonication. The TiO2 pastes (TiO2 particle size: ca. 20 nm) were prepared using ethyl cellulose (Aldrich), lauric acid (Fluka), and terpineol (Aldrich). The prepared TiO2 paste was coated onto the pre-cleaned glass substrate using a doctor blade and sintered at 500 ◦ C for 30 min. The thickness of the sintered TiO2 layer was measured with an Alpha-Step IQ surface profiler (KLA-Tencor). For dye adsorption, the annealed TiO2 electrodes were immersed in dye solution (0.5 mM dye in DMF) at 50 ◦ C for 5 h. The Pt counter electrodes were prepared by thermal reduction of a thin film formed from 7 mM H2 PtCl6 in 2-propanol at 400 ◦ C for 20 min. The dye-adsorbed TiO2 electrode and Pt counter electrode were assembled using 20-␮m-thick Surlyn (Solaronix SA) as a bonding agent. A liquid electrolyte was introduced through a prepunctured hole on the counter electrode. The electrolyte was composed of 3-propyl-1methyl-imidazolium iodide (PMII, 0.7 M), LiI (0.2 M), I2 (0.05 M) and t-butylpyridine (TBP, 0.5 M) in acetonitrile/valeronitrile (85:15, v/v). The active areas of the dye-adsorbed TiO2 films were estimated using a digital microscope camera with image-analysis software (Moticam 1000, Motic Group Co., Ltd.). Electrochemical impedance spectra (EIS) were obtained in the dark using an SP-200 electrochemical workstation with frequencies ranging from 50 mHz to 100 kHz and potentials from −0.7 to −0.3 V. The obtained impedance spectra were fitted with Z-view software. 3. Results and discussion 3.1. Synthesis of new X-shaped D––A dyes We prepared three different organic photosensitizers containing two cyanoacrylic acids as electron acceptors and two 4-tert-butyl-N-(4-tert-butylphenyl)-N-phenylaniline units as electron donors. The synthetic procedures for 7, 11, and 15 are illustrated in Scheme 1. Compounds 1 and 2 were synthesized according to literature methods [16]. The Horner–Wadsworth–Emmons coupling reaction between aldehyde 1 and tetraethyl(4,5-dibromo-1,2phenylene)bis(methylene)diphosphonate afforded 3 in 71% yield. The three synthesized aldehyde compounds were protected with neopentyl glycol in the presence of p-toluenesulfonic acid. Next, protected compounds 4, 8, and 12 were treated with trimethyltin chloride after lithiation with lithium diisopropylamide. Then, compounds 5, 9, and 13 were prepared though the Stille coupling reaction in the presence of Pd(PPh3 )4 in DMF (25 mL) at 80 ◦ C. Compounds 5, 9, and 13 were treated with trifluoroacetic acid to yield corresponding aldehydes 6, 10, and 14, respectively. Knoevenagel condensation with cyanoacetic acid in the presence of piperidine provided the final desired products. 3.2. Absorption spectroscopy of the three dye molecules UV–vis absorption and PL spectra of the dye solutions in DMF and dye-adsorbed TiO2 films are illustrated in Fig. 1. The detailed optical and electrochemical properties of the organic dyes are summarized in Table 1. The absorption spectrum of 7 show a visible band at 400 nm (ε = 68,800 M−1 cm−1 ) corresponding to the

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Fig. 1. (A) Absorption (filled symbols) and PL spectra (open symbols) of solutions of the newly synthesized dyes in DMF. (B) Absorption spectra of dye-adsorbed TiO2 layers (conc. of dye solutions: 0.5 mM in DMF); triangle (7), circle (11), square (15).

␲–␲* transitions in the conjugated molecules. The spectral properties of 11 are similar to those of 7, although the former showed bathochromic shift of the absorption maximum wavelength (max ) and has a higher molar extinction coefficient (max = 411 nm, ε = 81 500 M−1 cm−1 ). Dye 15 (max = 485 nm, ε = 56,000 M−1 cm−1 ), which bears a 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole unit as a linker showed a significant spectral red-shift and a decrease in extinction coefficient in the lowest energy range when compared with 7 and 11. The insertion of a benzothiadiazole (BTDZ) unit clearly extends the ␲ conjugation of dye 15 and reduces the opt optical band gap energy (Eg ). However, the PL emission spectra of 7 and 15, in combination with UV–vis absorption spectra, do not exhibit significant differences in the Stokes shift between the dyes. However, the Stokes shift of 11 was found to be 5 nm larger than those of the other dyes, which might be due to slight aggregation between the dye molecules. The fluorescence maxima of 7, 11, and 15 appeared at 617, 605, and 619 nm, respectively. In particular, dye 15 showed pronounced fluorescence quenching behavior owing to donor–acceptor intramolecular charge transfer. Fig 1B presents the absorption spectra of the dyes adsorbed onto a TiO2 surface (thickness of TiO2 , t = 7.5–8.0 ␮m for 7 and 11; t = 10 ␮m for 15). The absorption spectra of the dyes adsorbed on the TiO2 layer were distinctly broadened relative to the absorption spectra of their DMF solutions. The broader absorption spectra of the dyes on the TiO2 film are expected to be favorable for light harvesting in the solar emission spectrum. In general, the formation of aggregates of organic dyes on the TiO2 surface induces the bathochromic shift of the absorption spectra in comparison with those of the solution state. In this comparison, no

Scheme 1. Synthesis of dyes 7, 11, and 15.

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c

a

7 11 15

b

Absorption and emission wavelengths measured in DMF solutions (0.2 mM). opt Optical band gaps calculated from absorption onsets Eg = 1240/cut off . EHOMO = −4.8 − (Eox − 0.48) eV where 0.48 eV is the onset value for the oxidation of ferrocene vs. Ag/Ag+ and LUMO levels of the dyes calculated by subtraction of the optical band gaps from the EHOMO values.

−2.59 −2.64 −3.08 −5.13 −5.14 −5.18 −1.73 −1.67 −1.24 2.54 2.50 2.10 306(4.9), 400(6.8) 304(3.5), 411(8.1) 307(6.4), 382(7.3), 485(5.6)

488 496 588

2.54 2.50 2.10

406 400 380, 485

521 524 618

617 605 619

0.81 0.83 0.86

HOMOc [eV] (Eox − E0–0 )/V max [nm] (TiO2 surface) Optical band gapb [eV] cut off [nm] max [nm]a (εmax [104 M−1 cm−1 ]) Dye

Table 1 Absorption, PL emission, and electrochemical properties of solutions of dyes 7, 11, and 15 in DMF.

cut off [nm] (TiO2 surface)

em [nm]

Eox /V (vs. NHE)

E0–0 /eV

LUMO [eV]

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Fig. 2. Energy-level diagram of the three dyes with a TiO2 semiconductor electrode and I− /I3 − redox couple in the electrolyte (units: eV).

significant spectral shift was observed for the solution and dyeadsorbed TiO2 film. In particular, dye 11 showed a shift of only 9 nm to the shorter-wavelength region, which might indicate that dissociation of ␲–␲-interaction and a small degree of H-type aggregation of dye molecules occurred on the TiO2 surface. 3.3. Molecular energy levels determined by CV and absorption spectroscopy The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are typically used to estimate the efficiency of electron injection from the dye in the excited state to the TiO2 , as well as the efficiency of dye regeneration in DSSCs [17]. The HOMO and LUMO energy levels of the dye molecules were estimated by CV and absorption spectroscopy, as shown in Fig. 2. Cyclic voltammograms were recorded for the dyes in their solution states and the potentials were measured with respect to an internal reference such as ferrocene (Fc/Fc+ ). The CV scans showed reversible oxidation peaks in the range of −2.0 to +2.0 V (vs. Ag/AgCl). To ensure regeneration of the dye molecules, the energy of the ionization potential in the dye molecules should be more positive than the redox potential of I− /I3 − . The results showed that all the synthesized organic dyes had more positive HOMO levels than the redox potential of I− /I3 − in the electrolyte. The oxidation potentials were +0.81, +0.83, and +0.86 V for 7, 11, and 15, respectively. As shown in Table 1, the HOMO energy levels of 7, 11, and 15 were −5.13, −5.14, and −5.18 eV, respectively. This implies that the oxidized dyes could be easily regenerated by the I− /I3 − redox couple. To determine the LUMO energy levels, we combined the oxidation potential obtained from CV analysis with the optical energy opt band gap (Eg ) obtained from the absorption edge in the absorption spectrum. In a highly efficient DSSC, the LUMO energy of the dye molecule must be shallower than that of the conduction band edge in TiO2 (−4.22 to −4.26 eV); this would ensure effective electron injection from the excited dye molecules to the conduction band of the TiO2 film. The LUMO levels of 7, 11, and 15 were determined to be −2.59, −2.64, and −3.08 eV, respectively. Thus, the three dyes satisfy the requirements for high DSSC performance.

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Fig. 3. (A) J–V characteristics of the devices I–III. (B) Incident photon-to-current conversion efficiencies in the devices I–III. Solid line (IPCE %), dotted lines (absorption spectra of dye-adsorbed TiO2 ) (a) device I, (b) device II, (c) device III.

Table 2 Photovoltaic performances and electron life-times of DSSCs fabricated with Xshaped photosensitizing dyes. Dye Device I 7 Device II 11 Device III 15

JSC (mA/cm2 )

VOC (mV)

FF (%)

 (%)

 e (ms)

6.90 7.27 8.12

617 612 555

72.6 70.6 65.0

3.09 3.14 2.93

4.01 2.52 0.38

3.4. DSSC devices made of the three synthesized dyes The newly synthesized organic dye sensitizers were used to fabricate DSSCs to study their current density–voltage (J–V) characteristics. The photovoltaic properties of DSSCs made of 7, 11, and 15, were investigated under optimized fabrication conditions and standard global AM 1.5 solar conditions. The devices I–III were fabricated with the dyes 7, 11, and 15, respectively. The properties were compared with that of a DSSC containing N719 dye, as shown in Table 2. The cell did not contain a scattering layer and was not treated with TiCl4 . A moderately high PCE (ave ) of 3.14 (max was ∼4.3% with TiCl4 treatment) was achieved in the DSSC made of 11 with a relatively large active area of 0.418 cm2 . For the device I, the conversion efficiency reached 3.09% with a short circuit current (Jsc ) of 6.90 mA cm−2 and an open circuit voltage (Voc ) of 617 mV. The lower PCE value is mainly due to the lower Jsc value (see Fig. 3A). Extension of the ␲-conjugated bridge by inserting a bithiophene unit into the structure of dye 11 led to an increase of the photocurrent by 0.37 mA cm−2 compared to the device I. Enhancement of the electron deficient property by adding a thiophenyl benzothiadiazole moiety to the linker/anchoring group affected the JSC value (∼8.12 mA cm−2 ) of the device III because of the smaller band gap energy and extended spectral response. The increment of JSC in the device can be ascribed to the

enhanced optical properties, namely a bathochromic shift of the absorption range. However, the relatively lower PCE value is mainly because of the lower VOC value (∼555 mV). The decrease in VOC of the device III can possibly be interpreted as the more facile charge recombination reaction. Even though there are more electrons harvested and injected into the mesoporous structure of TiO2 in the 15-sensitized cell, there are still some losses that influence the VOC . The IPCE plots for the above-mentioned devices are also presented in Fig. 3B. The photovoltaic responses of the devices I and II were quite similar to their absorption spectra; however, because of the higher molar extinction coefficient of 11, it exhibited slightly higher IPCE of around 73% at 510 nm as a result of extended ␲ conjugation. Although the absorption spectrum was shifted to a longer wavelength and induced lower band gap energy, the lower extinction coefficient of 15 at 600–650 nm resulted in less absorption so that its EQE became lower than those of 7 and 11. The IPCE spectrum of the device III is much broader and red shifted by about 45 nm compared to those of 7 and 11 as a result of narrow band gap energy and a relatively lower molecular extinction coefficient (ε) at 400–600 nm. The solar response of the device using 11 (300–650 nm) and its EQE were high, i.e., approximately 73% at 470 nm; however, the EQE of the device with 7 was 68% at 450 nm and that of 15 (300–730 nm) was 44% at 450 nm. In the solar response range of 300–650 nm, the IPCE values of the device II were markedly higher than those of the device I; thus, the slightly higher efficiency observed for the device II mainly originated from the relatively large short-circuit photocurrent density. The DSSC characteristics of the dyes are improved upon incorporation of bithiophene segments into the conjugation pathway, which altered the HOMO and LUMO levels and subsequently favored the electron-injection and recombination kinetics. The significant difference in the obtained open-circuit voltages (VOC = 555–617 mV) could be due to different degree of charge

Fig. 4. (A) Nyquist plots of the four DSSC cells at the light illumination. Empty circle: control device with N719, triangle: device I, filled circle: device II, diamond: device III. (B) The Bode plots for the four DSSC cells.

S.J. Kim et al. / Synthetic Metals 162 (2012) 2095–2101

recombination at the TiO2 /dye/electrolyte interface. The lower VOC in the DSSC made of the device III is partially because of the greater thickness of the TiO2 layer (t = 10 ␮m), which moves the conductin band shift in TiO2 . Among many factors, we investigated the effect of electron-life time on the VOC values in DSSCs. In order to investigate the electron life-time of the photosensitizing dyes in the mesoporous TiO2 , the dark electrochemical impedance spectroscopy (EIS) was carried out [18,19]. The variation of VOC of DSSCs is influenced by possible charge transport properties, which can be investigated by EIS. It was performed to study the electrode kinetics and interfacial charge transfer process (TiO2 /dye/electrolyte) in the DSSCs. The precise analysis of the impedance variations in DSSCs composed of four organic photosensitizers allowed us to compare the electron life-times in four different DSSCs [20]. We observed and analyzed the EIS under dark conditions, as shown as Nyquist plots in Fig. 4A. There are two well-defined semicircles in the high frequency (f ∼ 1000 Hz) and medium frequency (f ∼ 10 Hz) regions. The recombination rate caused by the backward electron transfer was estimated by the Bode spectra for the DSSCs, as shown in Fig. 4B. The electron life-times ( e s) were calculated and are illustrated in Table 2. The second peak in the sepctra was suggested to correspond to the electron transport and recombination in the TiO2 electrode. The peak shift from high frequency to low frequency reveals longer electron life-time because the frequency (fmed ) can be related to the inverse of  e in TiO2 films as ( e = 1/2fmed ) [21,22]. A shorter electron life-time ( e = 0.38 ms) at the interface of TiO2 /dye/electrolyte was obtained for the device III, compared with 4.01 ms and 2.52 ms for the devices I and II, respectively. The results supported the smallest VOC of the device III, which is well consistent with the results in the literatures [19,23,24]. 4. Conclusion Two-dimensional D–␲–A dyes containing two donor and two acceptor units were successfully synthesized and showed promising properties as photosensitizers for use in DSSCs. Extending the ␲-conjugated linker by introducing a bithiophene group has helped to significantly enhance the light-harvesting properties of the resulting compounds. Although the observed PCE values in the devices elaborated in this work were not very high, the X-shape of the conjugated dye was demonstrated to be a promising molecular frame for high-performance photosensitizing dyes. Most importantly, the cells sensitized with these dyes have greatly improved IPCE, which accounted for the much higher photocurrent. Acknowledgments This work was supported by the Korea Institute of Energy Technology Evaluation and planning (KETEP) grant funded

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by the Ministry of Knowledge Economy under contract No. 20103020010010-13-2-400. The authors also acknowledge the financial support from the Priority Research Centers Program (NRF20120005860) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST). References [1] B. O’Regan, M. Grätzel, Nature 353 (1991) 737. [2] N. Robertson, Angewandte Chemie International Edition 45 (2006) 2338. [3] T. Funaki, H. Funakoshi, O. Kitao, N. O-Komatsuzaki, K. Kasuga, K. Sayama, H. Sugihara, Angewandte Chemie International Edition 51 (2012) 1. [4] J.-H. Yum, I. Jung, C. Baik, J. Ko, M.K. Nazeeruddin, M. Grätzel, Energy and Environmental Science 2 (2009) 100. [5] F. Gao, Y. Wang, J. Zhang, D. Shi, M. Wang, R. Humphry-Baker, P. Wang, S.M. Zakeeruddin, M. Grätzel, Chemical Communications (2008) 2635. [6] A. Yella, H.-W. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, E.W.-G. Diau, C.-Y. Yeh, S.M. Zakeeruddin, M. Grätzel, Science 334 (2011) 629. [7] E. Kozma, I. Concina, A. Braga, L. Borgese, L.E. Depero, A. Vomiero, G. Sberveglieri, M. Catellani, Journal of Materials Chemistry 21 (2011) 13785. [8] Y.J. Changa, T.J. Chow, Journal of Materials Chemistry 21 (2011) 9523. [9] S. Haid, M. Marszalek, A. Mishra, M. Wielopolski, J. Teuscher, J.-E. Moser, R. Humphry-Baker, S.M. Zakeeruddin, Mi Grätzel, P. Bäuerle, Advanced Functional Materials 22 (2012) 1291. [10] J. He, F. Guo, X. Li, We Wu, J. Yang, J. Hua, Chemistry – A European Journal 18 (2012) 7903. [11] M.-E. Ragoussi, J.-J. Cid, J.-H. Yum, G. de la Torre, D. Di Censo, M. Grätzel, M.K. Nazeeruddin, T. Torres, Angewandte Chemie International Edition 51 (2012) 4375. [12] Z. Ning, Q. Zhang, W. Wu, H. Pei, B. Liu, H. Tian, The Journal of Organic Chemistry 73 (2008) 3791. [13] G. Zhou, N. Pschirer, J.C. Schöneboom, F. Eickemeyer, M. Baumgarten, K. Müllen, Chemistry of Materials 20 (2008) 1808. [14] C.-Y. Chen, S.-J. Wu, C.-G. Wu, J.-G. Chen, K.-C. Ho, Angewandte Chemie International Edition 45 (2006) 5822. [15] M.J. Cho, S.S. Park, Y.S. Yang, J.H. Kim, D.H. Choi, Synthetic Metals 160 (2010) 1754. [16] H. Kang, P. Zhu, Y. Yang, A. Facchetti, T.J. Marks, Journal of the American Chemical Society 126 (2004) 15974. [17] (a) K. Hara, Z.-S. Wang, T. Sato, A. Furube, R. Katoh, H. Sugihara, Y. Dan-Oh, C. Kasada, A. Shinpo, S. Suga, The Journal of Physical Chemistry B 109 (2005) 15476; (b) D.H. Kim, J.K. Lee, S.O. Kang, J.J. Ko, Tetrahedron Letters 63 (2007) 1913. [18] (a) D. Kuang, S. Ito, B. Wenger, C. Klein, J.E. Moser, R. Humphry-Baker, S.M. Zakeerudddin, M. Grätzel, Journal of the American Chemical Society 128 (2006) 4146; (b) D. Kuang, P. Wang, S. Ito, S.M. Zakeeruddin, M. Grätzel, Journal of the American Chemical Society 128 (2006) 7732; (c) J. Bisquert, The Journal of Physical Chemistry B 106 (2002) 325; (d) J. Bisquert, A. Zaban, M. Greenshtein, I. Mora-Sero, Journal of the American Chemical Society 126 (2004) 13550. [19] A. Anthonysamy, Y. Lee, B. Karunagaran, V. Ganapathy, S.-W. Rhee, S. Karthikeyan, K.S. Kim, M.J. Ko, N.-G. Park, M.-J. Ju, J.K. Kim, Journal of Materials Chemistry 21 (2011) 12389. [20] K.M. Lee, V. Suryanarayananb, K.C. Ho, Journal of Power Sources 188 (2009) 635. [21] S. Sun, L. Gao, Y. Liu, Applied Physics Letters 96 (2010) 083113. [22] R. Kern, R. Sastrawan, J. Ferber, R. Stangl, J. Luther, Electrochimica Acta 47 (2002) 4213. [23] Y. Yang, H. Hu, C.-H. Zhou, S. Xu, B. Sebo, X.-Z. Zhao, Journal of Power Sources 196 (2011) 2410. [24] K. Lee, S.W. Park, M.J. Ko, K. Kim, N.-G. Park, Nature Materials 8 (2009) 665.