Novel Broadly Absorbing Sensitizers with ... - ACS Publications

J. Phys. Chem. C 2010, 114, 12355–12363

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Novel Broadly Absorbing Sensitizers with Cyanovinylene 4-Nitrophenyl Segments and Various Anchoring Groups: Synthesis and Application for High-Efficiency Dye-Sensitized Solar Cells J. A. Mikroyannidis,*,† A. Kabanakis,† P. Balraju,‡,| and G. D. Sharma*,‡,§ Chemical Technology Laboratory, Department of Chemistry, UniVersity of Patras, GR-26500 Patras, Greece, Physics Department, Molecular Electronic and Optoelectronic DeVice Laboratory, JNV UniVersity, Jodhpur (Raj.) 342005, India, Jaipur Engineering College, Kukas, Jaipur (Raj.), India, and Molecular Electronics Laboratory, JNCASR, Bangalore, India ReceiVed: March 4, 2010; ReVised Manuscript ReceiVed: June 16, 2010

Three new organic sensitizers D1, D2, and D3 with carboxy, acid anhydride, and hydroxy anchoring groups, respectively, and terminal cyanovinylene 4-nitrophenyl segments were synthesized. Their absorption spectra were broad with long wavelength absorption maximum approximately at 620 nm and optical band gap of 1.64-1.73 eV. We have fabricated the quasi-solid-state dye-sensitized solar cells (DSSCs) using these dyes and found that the power conversion efficiency (PCE) is of the order D1 > D2 > D3. The maximum PCE is about 4.58% with D1, which is further improved up to 5.80% when chenodeoxyacholic acid (CDCA) coadsorbent is added in the dye solution. The superior PCE for the DSSC with D1 has been attributed to the higher energy gap between the LUMO level of D1 and the conduction band of the TiO2 as compared to the other two dyes. The further improvement in the PCE with the addition of the coadsorbent has been attributed to the longer electron lifetime, which leads to reduction in back recombination of injected electrons with electrolyte. The PCE of the DSSC based on D1 has been improved up to 6.9% with the incorporation of poly(vinylidene fluoride) (PVDF) in the polymer gel electrolyte. This has been correlated with the ionic conductivity of the polymer electrolyte blend. Introduction The need and desire to design cheap renewable energy sources have triggered intensive research work all over world for efficient, flexible, low-cost, and lightweight photovoltaic devices based on organic semiconducting materials.1 Dye-sensitized solar cells (DSSCs) have become one of the promising molecular photovoltaics due to their high photocurrent conversion efficiency as compared with amorphous silicon solar cells, as well as the potential of low-cost production.2,3 The DSSC is a molecular system that consists of a wide band gap nanocrystalline semiconductor photoanode, typically TiO2 or ZnO, an anchored molecular sensitizer, a redox electrolyte, and a platinized counter electrode. To increase the power conversion efficiency as well as the stability, all of the elements of the DSSC need to be optimized and there is still substantial potential for further improvement. Among these elements, the sensitizing dye plays the key role in light harvesting and energy conversion. Recently, organic dyes have been attracting much attention with respect to their application to molecular photovoltaics. While ruthenium complex dyes4 have kept the highest efficiency since the pioneering study,2 a large number of metal free organic dyes have been designed and synthesized in order to be used as molecular photovoltaics because of the potential of organic dyes.5 The highest energy conversion efficiency of DSSCs based * To whom correspondence should be addressed. J.A.M.: phone +30 2610997115,fax+302610997118,[email protected]. G.D.S.: phone 91-0291-2720857, fax 91-0291-2720856, and e-mail [email protected]. † University of Patras. ‡ JNV University. | JNCASR. § Jaipur Engineering College.

on organic dyes (∼9%), however, is still lower than that based on ruthenium dyes (∼11%). The reason for this lower efficiency is mainly the faster charge recombination of the injected electron in the nanoporous TiO2 electrode to a redox species in liquid electrolyte.6 Recent papers also show that DSSCs employing metal complex dyes such as porphyrin and phthalocyanine dyes also suffer from fast recombination.7 Thus, to improve the efficiency of DSSCs, a design guide for sensitizing dyes to retard the recombination is essential. Organic dyes for DSCs exhibiting high molar extinction coefficients can be prepared and easily modified and are also environmentally friendly. The high extinction coefficients are needed in solid-state devices where limitation of the photovoltaic performance by low hole mobility and insufficient pore filling requires thin TiO2 films.8 Many research groups focus on broadening the absorption spectra of the dyes in order to increase their efficiency. The spectral red-shift of organic sensitizers is frequently associated with parallel limitations. First, expanding the absorption by increasing the conjugation can result in sensitization problems often referred to as aggregation.9-11 Dye aggregation on the TiO2 surface, leading to low conversion efficiency of the DSSC, should be avoided. In particular, donor-acceptor π-conjugated dyes are liable to undergo π-stacked aggregation on the TiO2 surface, which leads to reduction in electron-injection yield from the dyes to the conduction band of the TiO2 owing to intermolecular energy transfer. Second, decreasing the HOMO-LUMO gap can result in driving force limitations for injection and regeneration.12 One principal difference between organic and Ru-based chromophores is that the main absorption band in the organic dye arises mainly due to a single, strong HOMO/LUMO excitation,

10.1021/jp101945z  2010 American Chemical Society Published on Web 06/29/2010

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SCHEME 1: Chemical Structures of Dyes D1, D2, and D3

Mikroyannidis et al. reduced back recombination of electrons with the electrolyte. We have also investigated the effect of the incorporation of PVDF in the polymer electrolyte and found that the DSSCs fabricated with the polymer blend with PEO/PVDF weight ratio 7:3 is about 6.9%. This has been related to the changes in the ionic conductivity of the polymer blend. Experimental Section

while the panchromatic absorbance of common Ru-dyes is attributed to numerous weakly allowed transitions. The HOMO energy level of organic dyes is generally too low to make any significant contribution except in the near-UV region. Moreover, it has been proven difficult to absorb the near-IR photons by extending the π-conjugation of the molecule, without negatively affecting the stability and overall efficiency.9 Given these limitations, it remains a challenge for organic sensitizers to compete with the high-incident photon-to-current conversion efficiency. Very recently we have synthesized a series of low band gap small molecules and polymers containing cyanovinylene 4-nitrophenyl segments.13 We have used these small molecules and polymers as electron donors for efficient bulk heterojunction solar cells. In the present investigation we extended this research line on the preparation of new metal free organic photosensitizers for DSSCs. In particular, we synthesized three new symmetrical dyes D1, D2, and D3 which contained terminal cyanovinylene 4-nitrophenyl segments (Scheme 1). These dyes were differentiated from the chemical structure of the central unit and the anchoring group. Specifically, the central units of D1 and D2 were similar and consisted of bis(thiophene)isobenzofurandione. D1 and D2 carried carboxy and acid anhydride anchoring group, respectively. Dye D3 contained bis(phenoxy)hexane as the central unit and hydroxy anchoring groups. The hexylene spacer of D3 is expected to enhance the solubility of this dye. Dyes D1 and D2 were synthesized by a five- and threestep reaction sequence, respectively. The synthesis of D3 was very simple since it was attained by two steps only. The wide variety of the structures of these dyes provides potential for molecular design and correlation of their properties. We have fabricated the quasi-solid-state DSSCs with these dyes, and found that the PCE of the DSSCs based D1 is higher than those based on the other two dyes. This has been attributed to the better electron injection efficiency from the excited state to dye into the conduction band of TiO2. The PCE has been further improved when the CDCA coadsorbent is added into the dye solution and this feature has been correlated with the

Characterization Methods. IR spectra were recorded on a Perkin-Elmer 16PC FT-IR spectrometer with KBr pellets. 1H NMR (400 MHz) spectra were obtained with a Bruker spectrometer. Chemical shifts (δ values) are given in parts per million with tetramethylsilane as an internal standard. UV-vis spectra were recorded on a Beckman DU-640 spectrometer with spectrograde THF. Elemental analyses were carried out with a Carlo Erba model EA1108 analyzer. Reagents and Solvents. 4-Aminobenzoic acid and 3,4dihydroxybenzaldehyde were recrystallized from water. Tributyl(thiophen-2-yl)stannane was prepared from the reaction of thiophene with n-BuLi in hexane and subsequently with tributylchlorostannane according to the literature.14 4-Nitrobenzyl cyanide was synthesized from the nitration of benzyl cyanide with concentrated nitric and sulfuric acid.15 N,N-Dimethylformamide (DMF) and tetrahydrofuran (THF) were dried by distillation over CaH2. All other reagents and solvents were commercially purchased and were used as supplied. Synthesis of Compounds. 4,7-Dibromoisobenzofuran-1,3dione (1). Compound 1 was synthesized according to a reported method.16 4-(4,7-Dibromo-1,3-dioxoisoindolin-2-yl)benzoic Acid (2). A flask was charged with a solution of 1 (1.00 g, 3.27 mmol) and 4-aminobenzoic acid (0.49 g, 3.57 mmol) in glacial acetic acid (25 mL). The solution was stirred and refluxed for 3 h under N2. Compound 2 precipitated during the reaction. It was then filtered off from the hot reaction mixture, washed with water, and dried. The crude product was purified by recrystallization from ethanol (0.91 g, 65%). FT-IR (KBr, cm-1): 3004 (O-H deformation of carboxyl); 1730 (imide CdO); 1694 (carboxylic CdO); 1608 (aromatic); 1290 (C-O stretching and O-H deformation of carboxyl). 1 H NMR (CDCl3, ppm): 12.00 (broad, 1H, carboxyl); 7.81 (s, 2H, benzofurandione); 7.65 (d, J ) 8.8 Hz, 2H, phenylene ortho to carboxyl); 6.58 (d, J ) 8.7 Hz, 2H, phenylene meta to carboxyl). Anal. Calcd for C15H7Br2NO4: C, 42.39; H, 1.66; N, 3.30. Found: C, 42.14; H, 1.75; N, 3.42. 4-(1,3-Dioxo-4,7-di(thiophen-2-yl)isoindolin-2-yl)benzoic Acid (3). A mixture of 2 (0.90 g, 2.12 mmol), tributyl(thiophene-2yl)stannane (1.90 g, 5.09 mmol), THF (20 mL), and PdCl2(PPh3)2 (0.045 g, 3% mol) was stirred and refluxed for 17 h under N2. It was subsequently filtered and the filtrate was concentrated under reduced pressure. The concentrate was treaturated with n-hexane and then purified by column chromatography, eluting with a mixture of dichloromethane and n-hexane (1:1) (0.58 g, 68%). FT-IR (KBr, cm-1): 3008 (O-H deformation of carboxyl); 1724 (imide CdO); 1694 (carboxylic CdO); 1608 (aromatic); 1296 (C-O stretching and O-H deformation of carboxyl). 1 H NMR (CDCl3, ppm): 12.03 (broad, 1H, carboxyl); 7.80 (s, 2H, benzofurandione); 7.63 (d, J ) 8.8 Hz, 2H, phenylene ortho to carboxyl); 7.32-7.12 (m, 6H, thiophene); 6.56 (d, J ) 8.7 Hz, 2H, phenylene meta to carboxyl). Anal. Calcd for C23H13NO4S2: C, 64.02; H, 3.04; N, 3.25. Found: C, 63.76; H, 2.88; N, 3.42.

Synthesis of Novel Broadly Absorbing Sensitizers 4-(4,7-Bis(5-formylthiophen-2-yl)-1,3-dioxoisoindolin-2-yl)benzoic Acid (4). A flask was charged with a solution of 3 (0.30 g, 0.69 mmol) in 1,2-dichloroethane (30 mL). DMF (0.17 g, 2.33 mmol) and POCl3 (0.36 g, 2.35 mmol) were added dropwise and the mixture was refluxed for 17 h. After cooling to room temperature and addition of dichloromethane (15 mL) and a saturated aqueous solution of CH3COONa (30 mL), the mixture was stirred for 2 h at room temperature. The organic phase was then washed with water and dried over Na2SO4. Solvent removal and column chromatography (silica gel/ dichloromethane) gave 4 (0.21 g, 62%). FT-IR (KBr, cm-1): 3008 (O-H deformation of carboxyl); 1722 (imide CdO); 1718 (carboxylic CdO); 1670 (aldehyde CdO); 1608 (aromatic); 1310 (C-O stretching and O-H deformation of carboxyl). 1 H NMR (CDCl3, ppm): 12.02 (broad, 1H, carboxyl); 9.90 (s, 2H, formyl); 7.80 (s, 2H, benzofurandione); 7.63 (d, J ) 8.8 Hz, 2H, phenylene ortho to carboxyl); 7.15 (m, 4H, thiophene); 6.57 (d, J ) 8.7 Hz, 2H, phenylene meta to carboxyl). Anal. Calcd for C25H13NO6S2: C, 61.59; H, 2.69; N, 2.87. Found: C, 61.24; H, 2.48; N, 3.03. Dye D1. A flask was charged with a solution of 4 (0.13 g, 0.26 mmol) and 4-nitrobenzyl cyanide (0.08 g, 0.52 mmol) in ethanol (20 mL). Sodium hydroxide (0.05 g, 1.25 mmol) dissolved in ethanol (10 mL) was added portionwise to the stirred solution. The mixture was stirred for 1 h at room temperature under N2 and then was concentrated under reduced pressure. The concentrate was cooled into a refrigerator to afford a dark-green solid. It was filtered and washed thoroughly with water. Then it was dissolved in ethanol and dilute hydrochloric acid was added to the solution to precipitate D1, which was filtered and washed with water (0.16 g, 76%). FT-IR (KBr, cm-1): 3000 (O-H deformation of carboxyl); 2161 (cyano); 1728 (imide CdO); 1694 (carboxylic CdO); 1590 (aromatic); 1528, 1346 (nitro). 1 H NMR (CDCl3, ppm): 12.00 (broad, 1H, carboxyl); 8.15 (m, 4H, phenylene ortho to nitro); 7.80 (s, 2H, benzofurandione); 7.70 (s, 2H, olefinic); 7.63 (d, J ) 8.8 Hz, 2H, phenylene ortho to carboxyl); 7.46 (m, 4H, phenylene meta to nitro); 7.12 (m, 4H, thiophene); 6.58 (d, J ) 8.7 Hz, 2H, phenylene meta to carboxyl). Anal. Calcd for C41H21N5O8S2: C, 63.48; H, 2.73; N, 9.03. Found: C, 62.76; H, 2.47; N, 8.92. 4,7-Di(thiophen-2-yl)isobenzofuran-1,3-dione (5). Compound 5 was synthesized in 73% yield from the reaction of 1 with tributyl(thiophen-2-yl)stannane in THF solution in the presence of PdCl2(PPh3)2 according to the procedure described for 3. FT-IR (KBr, cm-1): 1854, 1770, 1728 (acid anhydride); 1608 (aromatic). 1 H NMR (CDCl3, ppm): 7.80 (s, 2H, benzofurandione); 7.32-7.12 (m, 6H, thiophene). Anal. Calcd for C16H8O3S2: C, 61.52; H, 2.58. Found: C, 61.29; H, 2.41. 4,7-Di(4-formylthiophen-2-yl)isobenzofuran-1,3-dione (6). Compound 6 was synthesized in 65% yield from the reaction of 5 with DMF and POCl3 in 1,2-dichloroethane according to the procedure described for 4. FT-IR (KBr, cm-1): 1852, 1778, 1722 (acid anhydride); 1690 (aldehyde CdO); 1608 (aromatic). 1 H NMR (CDCl3, ppm): 9.90 (s, 2H, formyl); 7.80 (s, 2H, benzofurandione); 7.15 (m, 4H, thiophene).

J. Phys. Chem. C, Vol. 114, No. 28, 2010 12357 Anal. Calcd for C18H8O5S2: C, 58.69; H, 2.19. Found: C, 58.24; H, 2.28. Dye D2. A flask was charged with a solution of 6 (0.36 g, 0.98 mmol) and 4-nitrobenzyl cyanide (0.32 g, 1.96 mmol) in ethanol (25 mL). Sodium hydroxide (0.20 g, 5.00 mmol) dissolved in ethanol (15 mL) was added portionwise to the stirred solution. The mixture was stirred for 1 h at room temperature under N2 and then was concentrated under reduced pressure. The concentrate was cooled into a refrigerator to afford D2 as a dark-green solid. It was filtered, washed thoroughly with water, and dried (0.50 g, 78%). FT-IR (KBr, cm-1): 1852, 1778, 1722 (acid anhydride); 2158 (cyano); 1594 (aromatic); 1522, 1344 (nitro). 1 H NMR (CDCl3, ppm): 8.15 (m, 4H, phenylene ortho to nitro); 7.80 (s, 2H, benzofurandione); 7.70 (s, 2H, olefinic); 7.46 (m, 4H, phenylene meta to nitro); 7.12 (m, 4H, thiophene). Anal. Calcd for C34H16N4O7S2: C, 62.19; H, 2.46; N, 8.53. Found: C, 61.77; H, 2.35; N, 8.60. 1,6-Bis(2-hydroxy-4-formylphenoxy)hexane (7). A mixture of 3,4-dihydroxybenzaldehyde (0.82 g, 6.21 mmol), 1,6-dibromohexane (0.78 g, 3.10 mmol), and K2CO3 (1.07 g, 7.76 mmol) in acetonitrile (25 mL) was stirred and refluxed for 24 h under N2. The solvent was subsequently removed by distillation. Water was added to the residue and the precipitate was filtered, washed with water, and dried to afford 7 (0.69 g, 62%). FT-IR (KBr, cm-1): 3404 (O-H stretching); 2938, 2864 (C-H stretching of hexylene); 1600 (aromatic); 1268, 1130 (ether bond). 1 H NMR (CDCl3, ppm): 9.80 (s, 2H, formyl); 7.85-7.83 (m, 4H, phenylene ortho to formyl); 6.96 (m, 2H, phenylene meta to formyl); 5.35 (broad, 2H, hydroxyl); 4.07 (m, 4H, OCH2); 1.75 (m, 4H, OCH2CH2); 1.48 (m, 4H, O(CH2)2CH2). Anal. Calcd for C20H22O6: C, 67.03; H, 6.19. Found: C, 66.58; H, 6.37. Dye D3. Dye D3 was synthesized in 83% yield by reacting 7 with 4-nitrobenzyl cyanide in ethanol in the presence of sodium hydroxide according to the procedure described for D2. FT-IR (KBr, cm-1): 3430 (O-H stretching); 2932, 2864 (C-H stretching of hexylene); 1602 (aromatic); 1510, 1344 (nitro); 1276, 1184 (ether bond). 1 H NMR (CDCl3, ppm): 8.15 (m, 4H, phenylene ortho to nitro); 7.46 (m, 4H, phenylene meta to nitro); 7.70 (s, 2H, olefinic); 7.10-6.90 (m, 6H, other phenylene); 5.35 (broad, 2H, hydroxyl); 4.07 (m, 4H, OCH2); 1.75 (m, 4H, OCH2CH2); 1.48 (m, 4H, O(CH2)2CH2). Anal. Calcd for C36H30N4O8: C, 66.87; H, 4.68; N, 8.66. Found: C, 66.23; H, 4.56; N, 8.72. Fabrication of Dye-Sensitized TiO2 and Nitrogen-Doped TiO2 Electrode and DSSC Assembly. TiO2 paste was prepared by adding 1 g of TiO2 powder (P25, Degussa), 0.2 mL of acetic acid, and 1 mL of water, then 60 mL of ethanol was slowly added while sonicating the mixture for 3 h. Finally, Triton X-100 was added and a well-dispersed colloidal paste obtained (TiO2). The whole procedure is slow under vigorous stirring. The formed precipitate solution was stirred vigorously for 2-4 h and then stirred for 4 h at 80 °C to form a transparent colloidal paste. The TiO2 paste was deposited on the F-doped tin oxide (FTO) coated glass substrates by doctor blade technique. The TiO2-coated FTO films were sintered at 450 °C for 30 min. Before immersion in the dye solution, these films were soaked in a 0.2 M aqueous TiCl4 solution overnight in a closed chamber. After being washed with deionized water and fully rinsed with ethanol, the films were heated again at 450 °C for 30 min, followed by cooling to 60 °C.

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SCHEME 2: Device Structure of the Quasi-Solid-State DSSC

Mikroyannidis et al. surements were carried out by applying the bias of the open circuit voltage (Voc) and recorded over a frequency range of 0.2 to 105 Hz with ac amplitude of 10 mV with an electrochemical analyzer equipped with FRA. Results and Discussion

The dyes were dissolved in THF solution (0.5 mM) and the TiO2 electrode was immersed in the dye solution for 10 h and after sensitization the dye sensitized electrode was washed. For the composite electrolytes preparation, 0.0383 g of TiO2 powder (P25, Degussa) was dried for about 24 h at 250 °C. After the drying a quasi-solid-state polymer electrolyte containing 0.0383 g of P25 TiO2 powder, 0.1 g of LiI, 0.019 g of I2, 0.264 g of PEO, and 44 µL of 4-tert-butylpyridine in 1:1 acetone/propylene carbonate was prepared and the mixture was continuously stirred at 80 °C in a water bath for about 4-5 h. When the solution was homogeneous, the poly(vinylidene fluoride) (PVDF) was added under continuously stirring. The PVDF (β phase) was purchased from Aldrich (USA) and used without further purification. To improve the homogeneity, the mixture was stirred for another 10 h. The final product was placed in an oven at 120 °C to evaporate the solvents and achieve the slurry state. The polymer electrolyte was spread on the sensitized TiO2 film by spin coating to form a holeconducting layer. The counter electrode was platinized by spin coating the H2PtCl6 solution (2 mg in 1 mL of isopropanol) onto the FTO coated glass substrate and then heated at 450 °C for 30 min in air. The dye-sensitized photoelectrode containing the quasi-solid-state polymer electrolyte and the counter electrode were clamped together and separated by a 20 µm spacer. The device structure of the quasi-solid-state DSSC is shown in Scheme 2. The J-V characteristics of the devices in the dark and under illumination were measured by semiconductor parameter analyzer (Keithley 4200-SCS). A xenon light source (Oriel, USA) was used to give an irradiance of 100 mW/cm2 (the equivalent of one sun at AM 1.5) at the surface of the device. The AM 1.5 solar spectrum was obtained from the light source with use of air mass 1.5 filter. The J-V measurements of the devices under illumination were carried out in a dark chamber with a window of circular aperture of area 2 cm2 for light illumination. The photoaction spectrum of the devices was measured with a monochromator (Spex 500 M, USA) and the resulting photocurrent was measured with a Keithley electrometer (model 6514), which is interfaced to the computer by LABVIEW software. The electrochemical impedance spectra (EIS) mea-

Synthesis and Characterization. Schemes 3-5 outline the synthesis of the dyes D1, D2, and D3, respectively. In particular, for preparing D1 with carboxy anchoring group, isobenzofuran1,3-dione (phthalic anhydride) was brominated to afford the dibromo derivative 1.16 The reaction of the latter with 4-aminobenzoic acid gave the imide 2. This reacted subsequently with tributyl(thiophene-2-yl)stannane to yield 3. The formylation of 3 with DMF and POCl3 afforded the dialdehyde 4. Finally, the condensation of 4 with 4-nitrobenzyl cyanide in ethanol in the presence of NaOH yielded the target dye D1. An analogous synthetic route was applied for the synthesis of D2, which contains an acid anhydride anchoring group. Dye D3, with hydroxy anchoring groups, was synthesized starting from 3,4dihydroxybenzaldehyde. Specifically, this reacted with 1,6dibromohexane in the presence of K2CO3 to afford the dialdehyde 7. The condensation of 7 with 4-nitrobenzyl cyanide, as was described above, gave D3. All the dyes were soluble in common organic solvents such as THF, dichloromethane, chloroform, and acetone. D3 displayed higher solubility than the other dyes due to the presence of the central hexylene moiety. The dyes D1, D2, and D3 as well as their intermediate compounds 1-7 were characterized by FT-IR and 1H NMR spectroscopy (see the Experimental Section). All the dyes showed certain common absorption bands, which appeared approximately at 1530, 1350 (nitro), 2160 (cyano), and 1600 cm-1 (aromatic). Besides, D1 displayed characteristic absorptions at 1694 (carboxylic CdO), 3000 (OsH deformation of carboxyl), and 1728 cm-1 (imide CdO). Dye D2 exhibited additional characteristic absorptions at 1852, 1778, and 1722 cm-1 associated with the anhydride group. Finally, D3 showed additional absorptions at 3430 (OsH stretching), 2932, 2864 (CsH stretching of hexylene), and 1276, 1184 cm-1 (ether bond). The 1H NMR spectra of the dyes displayed common signals at δ 8.15 (phenylene ortho to nitro), 7.46 (phenylene meta to nitro), and 7.70 (olefinic). Since dyes D1 and D2 are differentiated from their anchoring groups only, their 1H NMR spectra were similar. However, D1 showed a characteristic broad peak at δ 12.00 assigned to the carboxyl. Finally, D3 displayed peaks at the range of δ 4.07-1.48 (hexylene) and 5.35 (hydroxyl). Photophysical and Electrochemical Properties. Figure 1 depicts the UV-vis absorption spectra of the dyes D1, D2, and D3 in both THF solution (10-5 M) and thin film. All the photophysical characteristics of the dyes are summarized in Table 1. The absorption spectra of the dyes were similar in THF solution and thin film with two maxima (λa,max). The short wavelength λa,max appeared approximately at 450 nm corresponding to n-π* transition. The long wavelength λa,max was located around 620 nm. For the dyes D1 and D2 this can be attributed to an intramolecular charge transfer (ICT) between the central electron-donating 2,2′-(1,4-phenylene)bisthiophene and the terminal electron-withdrawing cyanovinylene 4-nitrophenyls. For the dye D3 the long wavelength λa,max can be attributed to an ICT between the hydroxyphenyl and the cyanovinylene 4-nitrophenyls. Upon going from solution to thin film, a red shift of both long wavelength λa,max and absorption

Synthesis of Novel Broadly Absorbing Sensitizers

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SCHEME 3: Synthesis of Dye D1



onset was observed for D2 and D3 but not for D1. This feature of D1 could be associated with the carboxy anchoring group of this dye but it is not fully explicable. Generally, the absorption spectra of the dyes were broad and covered a wide range of the UV-vis and near-infrared spectrum up to ∼750 nm, which bodes well for their photovoltaic properties. The thin film absorption onset for D1, D2, and D3 was found at 718, 757, and 752 nm, which corresponds to an optical band gap of 1.73,

1.64, and 1.65 eV, respectively. Interestingly, all the dyes showed comparable photophysical characteristics which indicates that they are dominated by their common cyanovinylene 4-nitrophenyl terminal moieties and not by their different central unit. It has been well established in our previous publications that the presence of these segments affects the absorption spectra of the compounds by a significant red-shift and broadening.13 Moreover, it has been observed that all materials which contain

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Figure 2. UV-visible absorption spectra of dyes adsorbed on TiO2 films.

Figure 1. Normalized absorption spectra of dyes in THF solution (top) and thin film (bottom)

TABLE 1: Optical and Electrochemical Properties of Dyes dye

D1

D2

D3

λa,max in solution (nm) λa,maxa in thin film (nm) thin film absorption onset (nm) Egopt b (eV) HOMO (eV) LUMO(eV) Egel c (eV)

632 624 718 1.73 -5.1 -3.3 1.8

615 622 757 1.64 -5.4 -3.7 1.7

605 636 752 1.65 -5.4 -3.7 1.7

a

a λa,max: long wavelength absorption maxima from the UV-vis spectra in THF solution or in thin film. b Egopt: optical band gap determined from the absorption onset in thin film. c Egel: electrochemical band gap determined from cyclic voltammetry.

cyanovinylene 4-nitrophenyl moieties showed comparable optical band gaps regardless of their particular chemical structure.13 The UV-visible spectra of the sensitizers absorbed on TiO2 film are shown in Figure 2. It can be seen that the sensitizers absorbed on the TiO2 electrode broaden the spectra and redshifted both their maximum and threshold. Such broadening and red shifts have been observed in other organic sensitizers on TiO2 electrodes.17 Strong interactions between the adsorbed dye molecules and the oxide molecules in the TiO2 surface lead to the broadening of the absorption spectrum. This might be correlated with the dye-TiO2 and dye-dye interactions. The red shift in the absorption spectra of the dyes adsorbed onto TiO2 film is due to the interaction of the anchoring groups with the surface of the titanium ions. The broadened absorption of dyes loaded on the TiO2 film is advantageous for light harvesting of the solar spectrum. The amounts of adsorbed dyes on the TiO2 films were estimated by desorbing the dyes with basic solution, and the surface concentrations were determined to be

5.60 × 10-7, 5.45 × 10-7, and 5.78 × 10-7 mol cm-3 for D1, D2, and D3 sensitized films, respectively. The adsorbed dye density is similar in different TiO2 films and therefore we assume that the influence of dye density on the photovoltaic performance of the DSSCs is negligible. The energetic alignment of the HOMO and LUMO energy levels is crucial for an efficient operation of the sensitizer in a DSSC. To ensure efficient electron injection from the excited dye into the conduction band of the TiO2 films, the LUMO level must be higher in energy than the TiO2 conduction band edge. The HOMO level of the sensitizer must be lower in energy than the redox potential of the I-/I3- redox couple for efficient regeneration of the sensitizer cation after photoinduced electron injection into the TiO2 film. The electrochemical behavior of the sensitizers was investigated by cyclic voltammetry, and the data are listed in Table 1. The HOMO and LUMO levels of all sensitizers are suitable for DSSCs. The difference between the LUMO of dye and the conduction band of TiO2 should be higher than 0.2 eV for efficient electron injection. The LUMO energy levels (D1: -3.3 eV; D2 and D3: -3.7 eV) are sufficiently higher than the conduction band of TiO2 (-4.0 eV). Hence, an efficient electron transfer from the excited dye to the TiO2 is ensured. The HOMO energy levels of all dyes (D1: -5.1 eV; D2 and D3: -5.4 eV) are lower than the standard potential of the I-/I3- redox couple (-4.83 eV vs vacuum),18 and consequently a sufficient driving force for the regeneration of the oxidized sensitizer is available in DSSCs with use of these materials. Photovoltaic Performance of DSSCs. Photovoltaic tests were conducted to evaluate the potential of these organic sensitizers D1, D2, and D3 in quasi-solid-state DSSCs. The incident photon to current conversion efficiency (IPCE) of the DSSCs was measured by using following expression:

IPCE(%) )

1240Jsc(mA/cm2) λ(nm)Pin(mW/cm2)

where Jsc is the short circuit photocurrent density generated by the monochromatic light, and λ and Pin are the wavelength and intensity of the monochromatic light, respectively. The IPCE spectra of the DSSCs employing D1, D2, and D3 are shown in Figure 3. The IPCE spectrum of the DSSC follows the absorption spectra of the corresponding dye. The IPCE can also be expressed by the following expression:


J. Phys. Chem. C, Vol. 114, No. 28, 2010 12361

Figure 3. IPCE spectra of DSSCs fabricated in THF solution without CDCA.

IPCE(λ) ) LHE(λ)φinjηc

Figure 4. J-V characteristics under illumination of DSSCs based on D1, D2, and D3 dye without CDCA.

where LHE(λ) is the light harvesting efficiency of the dye absorbed into the TiO2 layer depending upon the amount of dye loading, φinj is the electron injection efficiency from the excited state of dye into the conduction band edge of TiO2, and ηc is the electron collection efficiency by the electrode. As shown in Figure 3, the IPCE value of D1 gives a relatively higher value than that of D2 and D3. We have measured the amount of dye loading into the surface of TiO2 and found that this is almost the same for all DSSCs, indicating that the LHE does not affect the IPCE value significantly. Therefore, the φinj and ηc are responsible for the increase in the IPCE value. The φinj depends upon the energy difference between the LUMO of the dye and the conduction band of the TiO2. The higher IPCE value of the DSSC based on D1 is probably due to a high energy gap between the LUMO level of D1 and the conduction band edge of TiO2, which leads to an increased electron injection efficiency relative to D2 and D3. This also may be due to the carboxy (COOH) anchoring group in the dye, while D2 and D3 have the acid anhydride and hydroxy anchoring group, respectively. The COOH group is more effectively attached to the nanocrystalline semiconductor as compared to other anchoring groups. The parameter ηc depends upon the electron lifetime in the conduction band of the TiO2. The electron lifetime for the DSSC based on D1 (as discussed later in the paper) is higher than that of other DSSCs and is also responsible for the enhancement in the PCE. The photovoltaic properties of the quasi-solid-state DSSCs fabricated with these dyes were measured under simulated 1.5AM irradiation (100 mW/cm2). The photocurrent-voltage (J-V) characteristics of the devices based on these dyes are shown in Figure 4. The photovoltaic parameters for these DSSCs are summarized in Table 2. The PCE for the three dyes without chenodeoxyacholic acid (CDCA) coadsorbent is of order D3 < D2 < D1 and specifically the corresponding PCE (η) value is 3.53%, 3.73%, and 4.58%. The higher PCE of D1 is mainly due to the increase in the Jsc, which may be attributed to the better electron injection efficiency from the LUMO of the D1 into the conduction band edge of the TiO2, and the better anchoring with the nanocrystalline semiconducting film. It is clear from the data of Table 2 that the efficiencies of the DSSCs are strongly affected by the chemical structure of the anchoring group.

For further optimization of the devices, CDCA was added into the dye bath giving much higher photovoltaic parameters which are listed in Table 2. The corresponding J-V characteristics of the DSSCs based on D1 and D2 are shown in Figure 5. The Jsc values for the DSSCs based on the dyes clearly increase after the addition of CDCA. This indicates that the strong aggregations were effectively suppressed by adding CDCA in the dye bath. The PCEs for the DSSCs based on the D1 and D2 with CDCA coadsorbent are 5.8% and 4.52%, respectively. Similar results have been reported for the DSSCs with other organic dyes recently.19 Electrochemical impedance spectroscopy (EIS) analysis was performed to study the interfacial charge transfer processes in the DSSCs based on different dyes, at forward bias almost equal to open circuit voltage (Voc) under illumination. The Nyquist plot and Bode phase plot are shown in Figure 6. The semicircle in the intermediate frequency region reflects mainly the recombination impedance caused by the electron loss from the conduction band of TiO2 to the I3- ions in electrolyte.20 The Nyquist plots (Figure 6a) show that the radius of the middle semicircle decreased in the order D3 > D2 > D1 indicating the improved charge generation and transport, which corresponds to the overall device efficiency. The electron lifetimes obtained by fitting the curves are 18.5, 12.7, and 9.6 ms for DSSCs based on D1, D2, and D3, respectively. The higher electron lifetime indicates suppression of back reaction of the electrons with the I3- in the electrolyte and this was reflected in the improvements of the photocurrent, yielding substantially enhanced device efficiency for the DSSCs based on D1 as compared to D2 and D3. When the DSSCs were fabricated with these dyes with CDCA as coadsorbent, the electron lifetimes of D1, D2, and D3 dye-based DSSCs are 25.6, 22.4, and 20.4 ms, respectively (estimated from the EIS data not shown). This indicates that the electron lifetime has been improved with the addition of the coadsorbent, which suppresses both the electron recombination and back reaction, thus enhancing the photocurrent. The Bode phase plots shown in Figure 6b support likewise the differences in the electron lifetime for the TiO2 films sensitized with the three dyes. In Bode plots, the high-frequency region corresponds to the charge transfer at counter electrode. There was no significant change in the position of the high-

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TABLE 2: Photovoltaic Performance of the Quasi-Solid-State DSSCs for the Three Dyes on Different Measured Conditions dye a

D1 D1b D2a D2b D3a a

short circuit current (Jsc) (mA/cm2)

open circuit voltage (Voc) (V)

fill factor (FF)

power conversion eff (η) (%)

12.15 13.70 10.50 11.24 9.85

0.65 0.68 0.63 0.67 0.64

0.58 0.62 0.56 0.60 0.56

4.58 5.80 3.70 4.52 3.53

Dye bath: THF solution (1 × 10-3 M). b Dye bath: THF solution (1 × 10-3 M) with addition of CDCA (1 × 10-3 M).

Figure 5. J-V characteristics under illumination of DSSCs based on D1 and D2 dye with CDCA.

frequency peaks for the three dyes studied. The middle frequency peak of the DSSCs based on D1 is shifted to lower frequency relative to D2 and D3, indicating longer lifetime for D1 as compared to D2 and D3. This result is in agreement with the observed increase in the Jsc for the DSSCs based on D1. It is observed that the middle frequency peak was further shifted to the lower frequency region by adding CDCA in the dye solution, indicating a longer electron lifetime. We concluded that the addition of CDCA causes the dye forming compact and monomolecular adsorption on the TiO2 surface due to the bulky CDCA molecule, and thus suppresses the back reaction. This effect further increases the PCE of the DSSCs. Effect of PVDF in the Polymer Electrolyte. It is well-known that fluorine,21 which is present in the PVDF, has the smallest ionic radius and the largest electronegativity value. The PVDF is a semicrystalline polymer that has a β phase. Thus PVDF could be expected to improve the ionic transportation and reduce the recombination rate at the semiconductor/solid state electrolytes or quasi-solid-state/electrolytes interface when applied in DSSCs. PVDF and TiO2 nanoparticles can reduce the semicrystalline area and increase the amorphous part of blended polymers, thus the ionic conductivity of the polymer will be increased.22 It was reported that when the PEO-based polymer electrolyte is treated above 60 °C, an amorphous state is observed23 and ions can transport throughout the PEO chains quickly. Therefore, we have treated the polymer electrolyte at 80 °C before using it for the fabrication of DSSCs utilizing D1 dye with CDCA coadsorbent. We have created the polymer electrolyte with different PEO/PVDF ratios and measured the conductivity of the polymer electrolyte using EIS data. The ionic conductivity of quasi-solid-state electrolyte was estimated at room temperature from the electrochemical impedance spectra data by using the following expression:24

Figure 6. (a) Nyquist plots of electrochemical impedance spectra (EIS) of DSSCs with D1, D2, and D3 dyes without CDCA. (b) Bode phase plots of electrochemical impedance spectra (EIS) of DSSCs with D1, D2, and D3 dyes.

σ)

l RbA

where Rb is the bulk resistance and l and A are the device dimensions (length and area, respectively). The value of Rb is calculated from the intersection of prolongation of the highfrequency line to the real impedance axis, in EIS measurement. The variation of ionic conductivity of the polymer electrolyte blend with the PEO/PVDF weight ratio is shown in Figure 7. It can be seen from this figure that the conductivity of the polymer electrolyte increases as the proportion of PVDF in the blend increases, and reaches a maximum value when the PEO/ PVDF weight ratio is 7:3 and decreases as the proportion of PVDF increases further. When the PEO/PVDF weight ratio is 7:3, the polymer electrolyte shows an ionic conductivity of 3.5 mS/cm. The increase in the conductivity of the polymer electrolyte with the increase in the proportion of PVDF originated from the changed dimensional structure of the polymer by blending two polymers of PEO and PVDF with TiO2 nanoparticles. We assume that the semicrystalline area was reduced and the amorphous part of the blended polymers was increased, so that the ionic conductivity has been improved. The PCE of the DSSCs follows the same trend with the increase in the PVDF component in the blend as the ionic conductivity


Figure 7. Variation of conductivity of polymer blend electrolyte and PCE of the DSSCs based on D1 with CDCA coadsorbent with the PEO/PVDF weight ratio in the polymer blend electrolyte.

shows. The PCE of the DSSCs is 6.9%, when the PEO/PVDF weight ratio is 7:3. When the PVDF ratio in the blend polymer electrolyte increases further, the efficiency decreases as shown in Figure 7. This decrease in ionic conductivity and PCE can be attributed to the breakage of the network structure and the decrease of the porosity.25 Conclusions New sensitizers D1, D2, and D3 with carboxy, acid anhydride, and hydroxy anchoring groups, respectively, were synthesized. They contained terminal cyanovinylene 4-nitrophenyl segments which red-shifted the absorption spectrum. The long wavelength absorption maximum of the dyes was located around 620 nm and their optical band gap was 1.64-1.73 eV. The photovoltaic performance of the quasi-solid-state DSSCs fabricated with these dyes as sensitizers was investigated. The PCE is affected considerably upon the chemical structure of the anchoring group in the sensitizer. We found that the PCE (4.58%) of the DSSCs based on D1 is higher than that for the other two dyes, which was attributed to the relatively higher electron injection efficiency from the excited state (LUMO) of the dye into the conduction band of TiO2. The PCE of the DSSCs has been further improved when the coadsorbent CDCA is added to the dye solution, which was attributed to the longer lifetime of injected electron and reduced back electron recombination. Finally, we have investigated the effect of the incorporation of PVDF into the polymer electrolyte blend on the PCE of the DSSCs, and found that the PCE is 6.9% when the PEO/PDVF weight ratio in the polymer electrolyte blend is 7:3. The incorporation of PVDF into the electrolyte leads to a significant increase in the ionic conductivity of the electrolyte, resulting in an improvement in the PCE of DSSCs. References and Notes (1) (a) Schilinsky, P.; Asawapirom, U.; Scherf, U.; Biele, M.; Brabec, C. J. Chem. Mater. 2005, 17, 2175–2177. (b) Wong, W.-Y. Macromol. Chem. Phys. 2008, 209, 14–24. (c) Krebs, F. C. Energy Mater. 2009, 93, 394–412. (d) Chen, L. M.; Hong, Z. R.; Li, G.; Yang, Y. AdV. Mater. 2009, 21, 1434–1449. (e) Winder, C.; Sariciftci, N. S. J. Mater. Chem. 2004, 14, 1077–1086. (f) Kroon, R.; Lenes, M.; Hummelen, J. C.; Blom, P.W. M.; de Boer, B. Polym. ReV. 2008, 48, 531–582. (g) Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Chem. ReV. 2009, 109, 5868–5923. (2) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737–740. (3) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269–277. (4) (a) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc.

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