Photovoltage Improvement for Dye-Sensitized Solar Cells via Cone

May 13, 2009 - donor (S5, S6, and S7) were conveniently synthesized and used for dye-sensitized solar cells (DSSCs). A compact sensitizer layer is mol...
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J. Phys. Chem. C 2009, 113, 10307–10313

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Photovoltage Improvement for Dye-Sensitized Solar Cells via Cone-Shaped Structural Design Zhijun Ning,† Qiong Zhang,†,‡ Hongcui Pei,† Jiangfeng Luan,§ Changgui Lu,§ Yiping Cui,§ and He Tian*,† Lab for AdVanced Materials and Institute of Fine Chemicals, East China UniVersity of Science and Technology, 130 Meilong Road, Shanghai, 200237, People’s Republic of China, Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, S-10691 Stockholm, Sweden, and AdVanced Photonics Center, Southeast UniVersity Nanjing 210096, People’s Republic of China ReceiVed: March 18, 2009; ReVised Manuscript ReceiVed: April 25, 2009

In this article, three truexene-based dyes with 2-cyanoacrylic acid as acceptor and starburst triarylamine as donor (S5, S6, and S7) were conveniently synthesized and used for dye-sensitized solar cells (DSSCs). A compact sensitizer layer is molecular interfacially engineered on the TiO2 surface via cone-shaped sensitizers. As a result, the approach of the electrolyte to the TiO2 surface is blocked significantly by the compact sensitizer layer formed and the charge recombination in the DSSCs is proved to be retarded effectively. The monochromatic incident photon-to-current conversion efficiency of these sensitizers is near 90%. In addition, S7-sensitized solar cells yield an open-circuit voltage of 752 mV and a fill factor of 0.70, which are even higher than those of N719 under the same conditions. Introduction Increasing demand for energy will soon urge us to seek environmentally clean alternative energy resources. Dyesensitized solar cells (DSSCs) have attracted intense interest on account of their high performance in converting solar energy to electricity.1,2 The Ru-complex photosensitizers such as the black dye show record solar-energy-to-electricity conversion efficiency of 11%.3,4 Recently, organic dyes for DSSCs have been receiving enormous attention for their structure adjustability, high molar extinction coefficients, and facile preparation process with low cost.5,6 The conversion efficiency of organic dyes has been improving by leaps and bounds in recent years.7-11 However, the conversion efficiencies of organic dyes still lag behind those of the Ru complexes. The bottleneck in the further improvement of the conversion efficiency of organic DSSCs is their low open circuit photovoltage (Voc), which was mainly ascribed to the recombination of conduction-band electrons with triiodide.12,13 The Voc of the DSSCs depends, in theory, on the Femi energy edge of TiO2, FTO and the redox potential of the redox couple. However, charge recombination processes of injected electrons with I3- ions in the electrolyte and with dye cations lead to a measured Voc much lower than the theoretical value.14-16 The former is considered to be particularly critical for Voc and fill factor (ff) values. In order to block the approach of the triiodide electrolyte to the TiO2 surface, some attempts have been made at molecular structural modification of Ru dyes and organic sensitizers.3,12,13 However, up to now, the Voc of organic sensitizers still fell behind that of Ru complexes and there is lack of guidelines for the molecular design of organic sensitizers * Corresponding author: tel, +86-21-64252288; fax, +86-21-64252288; e-mail, [email protected]. † Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology. ‡ Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology. § Advanced Photonics Center, Southeast University.

Figure 1. The simple sketch map of the cone-shaped sensitizer and the TiO2 surface adsorbed with dye in the presence of redox electrolyte.

in order to retard the charge recombination. Most organic sensitizers consist of donor, linker, and acceptor moieties and usually have rigid rod-shaped configuration.17 After the rodshaped dyes are adsorbed on the surface of the spherical TiO2 particles, there will be a radial arrangement of the adsorbed dyes, the acceptor moiety with the anchoring segment pointing to the TiO2 surface and the donor part pointing away from the surface, as shown in Figure 1a.18 The rod-shaped molecules are susceptible to aggregation for the strong dipole-dipole interaction between the extended delocalized π-bonds.19,20 Close π-π aggregation between molecules can lead not only to selfquenching and reduction of electron injection into TiO2 but also to the instability of the organic dyes due to the formation of excited triplet states and unstable radicals under light irradiation.21 On the other hand, as can be seen in Figure 1a, the aggregation can aggravate charge recombination for the triiodide

10.1021/jp902408z CCC: $40.75  2009 American Chemical Society Published on Web 05/13/2009

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and can easily penetrate through the big interspaces between chromophores. Hara and co-workers found that the charge recombination can be reduced and Voc was improved by adding long alkyl chains on the π-conjugated segment of the rod-shape dyes.12 However, aggregation still persists and the Voc is lower than Ru dyes as usual.12,18b Recently, several research groups concurrently found that by transforming the rod-shaped molecule into a starburst type, usually employing the starburst triarylamine units as the donor, the Voc can also be improved.13 This enhancement should be ascribed to their aggregation-resistant nonplanar starburst configuration and multiphenyl units on the starburst triarylamine, which can block the approach of the I3ion to a certain degree. However, the aggregation is not eliminated and the redox couples can penetrate through the gaps in or between the rigid big benzene rings.13c Therefore, we deem that a more effective way might be adding long alkyl chains in the middle of the starburst-type dyes to form a cone-shaped sensitizer (as shown in Figure 1). In this case, the aggregation is anticipated to be prohibited further and it is prospective to form a compact sensitizer layer (as shown in Figure 1b) on the TiO2 surface to block the approach of the redox couple. In order to approve the above hypotheses, we designed and synthesized a series of starburst sensitizers bearing long alkyl chains in the π-conjugated segment. Truxene has received considerable attention in recent years for its optoelectronic applications, such as in organic light-emitting diodes (OLEDs), two-photon absorption, and fluorescence sensor.22,23 Besides the fantastic electro-optical properties, its starburst structure and convenience to be modified with some long alkyl substituents make it desirable for the molecular design.24 In this article, three truexene-based dyes with 2-cyanoacrylic acid as acceptor and starburst triarylamine as donor (S5, S6, and S7) have been synthesized. The cone-shaped configuration of sensitizers S6 and S7 enables them to form the compact sensitizer-layer on the TiO2 surface, and it was found that the Voc was improved effectively. Experimental Section Materials and Measurements. UV-vis spectra were recorded on a Varian Cary 500 spectrophotometer. Cyclic voltammetry was performed using a potentiostat/galvanostat model K0264 (Princeton Applied Research). Anhydrous CH2Cl2 was used as the solvent under inert atmosphere, and 0.1 M TBAHFP (tetra(n-butyl)ammonium hexafluorophosphate) was used as the supporting electrolyte. A platinum disk electrode was used as the working electrode, a platinum wire was the counter electrode, and saturated Ag/AgCl was the reference electrode. General Procedure for Preparation and Test of Solar Cells. The preparation of a dye-sensitized TiO2 electrode is as follows: A 150-200 g · dm-3 TiO2 colloidal dispersion, containing 40 wt % poly(ethylene glycol) (MW 20000), was prepared by following the procedure reported in the literature.1a Films of nanocrystalline TiO2 colloidal on FTO were prepared by sliding a glass rod over the conductive side of the FTO. Sintering was carried out at 450 °C for 30 min. Before immersion in the dye solution, these films were soaked in the 0.2 M aqueous TiCl4 solution overnight in a closed chamber, which has been proved to increase the short-circuit photocurrent significantly. The thickness of the TiO2 film was about 9 µm. After being washed with deionized water and fully rinsed with ethanol, the films were heated again at 450 °C followed by cooling to 80 °C and dipping into a 3 × 10-4 M solution of dyes in C2H5OH for 24 h at room temperature. The dye-coated TiO2 film, as

Ning et al. working electrode, was placed on top of an FTO glass as a counter electrode, on which Pt was sputtered. The redox electrolyte was introduced into the interelectrode space by capillary force. The photocurrent action spectra were measured with a model SR830 DSP lock-in amplifier and a model SR540 optical chopper and other optical system. Volt-current characteristic were recorded on model 2400 sourcemeter (Keithley Instruments, Inc., USA) and a AM-1.5G solar simulator (AM1.5G Newport-91160-1000) served as a white light source. Masks, which had 0.15 cm2 aperture in the middle, were attached on the outside of the glass having dye-adsorbed TiO2 film. The intensity of the illumination source was measured using a power meter. Measurement of Recombination Time Constant by Transient Photovoltage. The photovoltage transients of assembled devices were recorded with a digital oscilloscope (Tektronix TDS3052). The pulsed laser was produced by a Powerlite Precision II 8010 with 10 Hz repetition rate at 355 nm and a 5-7 ns pulse width at half-height and then was transformed into 500 nm pulse laser by Panther OPO. The beam size was slightly larger than 0.25 cm2 to cover the area of the device with incident power of 30 mW. The same redox electrolyte as in the I-V examination was employed. The average electron lifetime can be estimated approximately by fitting a decay of the open circuit voltage transient with exp (-t/τR), where t is time and τR is an average time constant before recombination. This lifetime is identical to that obtained by intensity-modulated photovoltage spectroscopy.24 Synthesis. 1H NMR spectra were obtained using a Bru¨ker AM 500 spectrometer (relative to TMS). Mass spectra were done with a Waters Micromass LCT mass spectrometer. Synthesis of 3. Diphenylamine (0.67 g, 4.0 mmol), copper powder (0.086 g, 1.43 mmol), potassium carbonate (1.38 g, 0.01 mol), and 18-crown-6 (0.066 g, 0.25 mmol) in 1,2-dichlorobenzene (10 mL) were heated to reflux. Compound 1 (1.78 g, 2.0 mmol) in 5 mL of 1,2-dichlorobenzene was added slowly, and the mixture was reacted at reflux for 24 h. The inorganic components were filtered off after cooling to room temperature. Then the product was precipitated into methanol and further purified by column chromatography with silica gel using hexane and CH2Cl2 as eluent to yield 0.88 g of a yellow solid (yield 45%). 1H NMR (500 MHz, CDCl3): 0.22-0.29 (m, 18H), 1.93-2.06 (m, 6H), 2.80-2.98(m, 6H), 7.03-7.58 (m, 26H) 8.05-8.18 (m, 3H). HRMS (m/z): [M]+ calcd for C63H59IN2, 970.3723; found, 970.3716. Synthesis of 4. The synthesis method resembles that of compound 3 (yield 43%). 1H NMR (500 MHz, CDCl3): 0.51-0.74 (m, 18H), 0.78-1.06 (m, 48H), 1.82-2.41 (m, 6H), 2.78-3.05(m, 6H), 7.06-7.65 (m, 26H), 8.03-8.24 (m, 3H). HRMS (m/z): [M]+ calcd for C87H107IN2, 1306.7479; found, 1306.7496. Synthesis of 5. The synthesis method resembles that of compound 3 (yield 48%). 1H NMR (500 MHz, CDCl3): 0.46-0.71 (m, 18H), 0.75-1.14 (m, 48H), 1.82-2.35 (m, 6H), 2.78-3.05(m, 6H), 3.88 (s, 12H), 7.03-7.69 (m, 22H), 8.03-8.24 (m, 3H). HRMS (m/z): [M]+ calcd for C91H115IN2O4, 1426.7902; found, 1426.7947. Synthesis of 6. Compound 3 (0.39 g, 0.4 mmol), 5-formylthiophen-2-ylboronic acid (69 mg, 0.44 mmol), Pd(PPh3)4 (18 mg, 0.016 mmol), and Na2CO3 (1.06 g, 0.01 mol) in 10 mL of THF and 10 mL of H2O were heated to reflux under a nitrogen atmosphere for 5 h. After cooling to room temperature, the reaction mixture pH was adjusted to 6 by adding 1 M HCl. The mixture was extracted with 20 mL of CH2Cl2 solution three

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Figure 2. Absorption spectra of dyes in CH2Cl2 (1 × 10-5 M) (a) and on TiO2 (b).

times. The organic portion was combined and removed by rotary evaporation. The residue was purified by column chromatography with silica gel using hexane and methylene chloride as eluent to yield 0.29 g of a yellow solid (yield 77%). 1H NMR (500 MHz, CDCl3): 0.12-0.24 (m, 18H), 1.82-2.01 (m, 4H), 2.16-2.28 (m, 2H), 2.73-2.94(m, 6H), 6.98 (d, 1H, J ) 10.0 Hz), 7.03 (d, 1H, J ) 8.5 Hz), 7.04-7.13 (m, 12H), 7.16 (d, 2H, J ) 12.4 Hz), 7.27-7.37 (m, 8H), 7.85 (d, 1H, J ) 8.0 Hz), 7.90 (d, 1H, J ) 3.9 Hz), 8.0 (s, 1H), 8.08 (d, 1H, J ) 3.9 Hz), 8.13 (d, 1H, J ) 8.7 Hz), 8.21 (d, 1H, J ) 8.4 Hz), 8.30 (d, 1H, J ) 8.4 Hz), 9.92 (s, 1H). 13C NMR (125 MHz, CDCl3): δ 182.7, 154.9, 154.2, 154.1, 154.0, 147.9, 146.7, 146.6, 144.0, 143.5, 142.7, 142.5, 142.0, 138.8, 138.7, 137.7, 137.5, 135.2, 130.9, 129.3, 125.4, 125.3, 124.8, 124.3, 123.8, 122.8, 122.0, 120.0, 117.6, 56.8, 29.5, 29.4, 29.2, 8.7, 8.6. HRMS (m/z): [M]+ calcd. for C68H62N2OS, 954.4583; found, 954.4581. Synthesis of 7. The synthesis method resembles that of compound 6 (yield 73%). 1H NMR (500 MHz, CDCl3): δ 0.52-0.71 (m, 18H), 0.72-1.09 (m, 48H), 1.84-2.38 (m, 6H), 2.74-3.02 (m, 6H), 7.01 (d, 1H, J ) 10.0 Hz), 7.05 (d, 1H, J ) 8.5 Hz), 7.08-7.17 (m, 12H), 7.19 (d, 2H, J ) 12.4 Hz), 7.33-7.39 (m, 8H), 7.91 (d, 1H, J ) 8.0 Hz), 7.95 (d, 1H, J ) 3.9 Hz), 8.05 (s, 1H), 8.12 (d, 1H, J ) 3.9 Hz), 8.16 (d, 1H, J ) 8.7 Hz), 8.24 (d, 1H, J ) 8.4 Hz), 8.36 (d, 1H, J ) 8.4 Hz), 9.96 (s, 1H). 13C NMR (125 MHz, CDCl3): δ 181.6, 154.0, 153.9, 153.8, 153.7, 146.9, 145.4, 145.3, 143.8, 143.4, 142.7, 141.1, 141.0, 137.3, 137.2, 136.5, 136.1, 134.0, 133.9, 129.7, 128.2, 124.4, 124.3, 124.0, 123.6, 123.0, 122.8, 122.7, 121.6, 121.1, 118.8, 116.7, 54.6, 54.5, 54.4, 35.7, 35.5, 30.5,30.4, 28.4, 28.3, 28.2, 23.0, 22.8, 22.7, 21.3, 21..2, 13.0, 12.9. HRMS (m/ z): [M]+ calcd for C92H110N2OS, 1290.8339; found, 1290.8352. Synthesis of 8. The synthesis method resembles that of compound 6 (yield 68%). 1H NMR (500 MHz, CDCl3): δ 0.50-0.73 (m, 18H), 0.71-1.12 (m, 48H), 1.85-2.39 (m, 6H), 2.74-3.02 (m, 6H), 3.85 (s, 12H), 6.87 (d, 8H, 8.8 Hz), 6.92 (m, 2H), 7.09 (m, 2H), 7.14 (d, 8H, J ) 8.8 Hz), 7.54 (d, 1H, J ) 4.0 Hz), 7.72 (m, 2H), 7.81 (d, 1H, J ) 4.0 Hz), 8.07 (d, 1H, J ) 8.8 Hz), 8.10 (d, 1H, J ) 8.4 Hz), 8.33 (d, 1H, J ) 8.8 Hz), 9.94 (s, 1H). 13C NMR (125 MHz, CDCl3): δ 183.4, 156.3, 155.9, 155.5, 155.4, 147.9, 145.0, 144.5, 143.6, 143.1, 142.6, 142.1, 139.1, 139.0, 138.2, 137.6, 134.1, 134.0, 131.2, 126.8, 125.9, 125.7, 125.2, 124.4, 120.5, 115.6, 115.3, 56.2, 56.1, 37.7, 37.4, 37.3, 32.2, 32.1, 30.3, 3.02, 30.1, 30.0, 24.7, 24.5, 23.4, 23.0, 22.9, 14.7, 14.6. HRMS (m/z): [M]+ calcd. for C96H118N2O5S, 1410.8761; found, 1410.8792. Synthesis of S5. Compound 6 (0.19 g, 0.2 mmol), 2-cyanoacetic acid (20 mg, 0.24 mmol), and ammonium acetate (30 mg, 0.4 mmol) in 10 mL of acetic acid were heated to reflux under a nitrogen atmosphere 8 h. After cooling to room temperature, the precipitate was filtered, and further recrystallization from methylene chloride and hexane solution to give 0.11 g of pure S5 as orange powder (yield 55%). 1H NMR (500

MHz, CDCl3): δ 0.10-0.31 (m, 18H), 1.81-2.21 (m, 6H), 2.68-3.03 (m, 6H), 7.01 (t, 6H, J ) 7.6 Hz), 7.15 (d, 8H, 8.0 Hz), 7.17-7.32 (m, 10H), 7.53-7.62 (m, 2H), 7.63-7.73 (m, 2H), 8.04-8.12 (m, 2H), 8.16-8.24 (m, 1H), 8.30-8.44 (m, 1H). 13C NMR (125 MHz, CDCl3): δ 167.8, 154.1, 153.9, 147.9, 146.6, 143.7, 143.4, 142.7, 142.2, 138.9, 138.7, 137.7, 135.1, 132.4, 131.0, 129.3, 128.9, 128.6, 125.3, 125.0, 124.8, 124.3, 124.2, 124.0, 122.8, 122.7, 122.0, 121.8, 119.7, 117.5, 117.4, 56.8, 56.7, 56.6, 29.7, 29.4, 29.2, 8.7, 8.6. HRMS (m/z): [M+1]+ calcd for C71H64N3O2S, 1022.4719; found, 1022.4720. Synthesis of S6. The synthesis method resembles that of compound S5 (yield 58%). 1H NMR (CDCl3 500 MHz,): 0.52-0.71 (m, 18H), 0.72-1.09 (m, 48H), 1.84-2.38 (m, 6H), 2.74-3.02(m, 6H), 7.02-7.09 (6H), 7.20 (d, 8H, J ) 8.4 Hz), 7.24 (d, 2H, J ) 5.2 Hz), 7.30 (t, 8H, J ) 8.4 Hz), 7.59 (d, 1H, J ) 4.0 Hz), 7.74 (s, 1H), 7.77 (d, 1H, 7.2 Hz), 7.86 (d, 1H, J ) 4.0 Hz), 8.14 (d, 1H, 8.8 Hz), 8.19 (d, 1H, 8.4 Hz), 8.36 (d, 1H, 8.4 Hz), 8.39 (s, 1H). 13C NMR (125 MHz, CDCl3): δ 159.4, 155.7, 155.0, 154.8, 148.0, 147.2, 146.5, 145.0, 144.5, 143.8, 142.5, 139.8, 138.4, 138.3, 137.2, 135.0, 134.7, 130.6, 129.2, 125.5, 125.4, 125.1, 124.9, 124.3, 124.1, 122.7, 122.2, 122.1, 119.8, 117.8, 116.5, 55.8, 55.7, 55.6, 36.8, 36.7, 31.6, 31.5, 31.4, 29.5, 29.4, 29.3, 24.1, 23.9, 22.3, 22.2, 14.0, 13.9. HRMS (m/ z): [M+1]+ calcd for C95H112N3O2S, 1358.8475; found, 1358.8429. Synthesis of S7. The synthesis method resembles that of compound S5 (yield 58%). 1H NMR (CDCl3 500 MHz,): 0.48-0.71 (m, 18H), 0.76-1.08 (m, 48H), 1.71-2.15 (m, 6H), 2.72-3.02(m, 6H), 3.85 (s, 12H), 6.87 (d, 8H, 8.8 Hz), 6.94 (m, 2H), 7.06 (m, 2H), 7.14 (d, 8H, J ) 8.8 Hz), 7.58 (d, 1H, J ) 3.6 Hz), 7.72 (s, 1H), 7.76 (d, 1H, J ) 8.4 Hz), 7.86 (d, 1H, J ) 3.6 Hz), 8.05 (d, 1H, J ) 8.8 Hz), 8.11 (d, 1H, J ) 8.4 Hz), 8.34 (d, 1H, J ) 8.8 Hz), 8.41 (s, 1H). 13C NMR (125 MHz, CDCl3): δ 157.9, 156.3, 155.6, 155.5, 148.5, 148.0, 145.2, 144.6, 143.7, 143.6, 142.1, 141.0, 139.1, 139.0, 137.6, 135.0, 134.0, 130.8, 126.8, 125.8, 125.7, 125.5, 125.0, 120.5, 120.1, 119.8, 115.6, 115.3, 56.4, 56.2, 37.8, 37.4, 37.3, 32.2, 32.1, 30.4, 30.2, 30.1, 30.0, 24.7, 24.5, 23.4, 23.0, 22.9, 14.7, 14.6. HRMS (m/z): [M+1]+ calcd for C99H120N3O6S, 1478.8898; found, 1478.8883. Results The synthetic route of all dyes is depicted in Scheme 1. Aryl amine substituted compounds were obtained via Cu-mediated Ullmann condensation of iodide truexene.25 The Suzuki coupling of aryl amine substituted truexene and 5-formylthiophen-2ylboronic acid led to the aldehyde precursors. The final products were obtained by Knoevenagel condensation of the corresponding aldehyde intermediate and 2-cyanoacetic acid. Absorption spectra of the three compounds in dilute solution of CH2Cl2 and TiO2 films are shown in Figure 2. These compounds exhibit two prominent bands in the solution,

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SCHEME 1: Synthesis of the Sensitizers

TABLE 1: Photophysical Parameters of the Dyes

TABLE 2: Performance Parameters of DSSCs

dye

S5

S6

S7

dye

S5

S6

S7

N719

λmaxa/nm (ε × 10-4 M-1 cm-1)

406(2.75) 361(5.42) 422 -5.22 -2.91

420(2.49) 360(5.44) 425 -5.14 -2.84

430(4.13) 365(6.65) 426 -4.98 -2.72

Jsc (mA/cm2) Voc (V) ff η (%) amounta (10-7mol cm-2)

7.75 0.689 0.73 3.90 1.117

7.89 0.731 0.74 4.27 1.021

6.86 0.752 0.70 3.61 0.911

11.78 0.728 0.66 5.66

λmaxb/nm HOMO (eV)c LUMO (eV)c

a Absorption maximum in CH2Cl2. b Absorption maximum on TiO2 film. c HOMO is derived by a comparison with the ionization potential of ferrocene, scan rate, 50 mV/s; LUMO is calculated from the oxidation potential and the energy at the maximum absorption.

appearing at 300-360 nm and 400-450 nm, respectively. The former is ascribed to a localized aromatic π-π* transition and the later is of charge transfer character. The extinction coefficient of S7 is distinctly larger than those of the other two dyes (Table 1), which is due to the stronger electron donating capability of S7 with methoxy groups. Figure 2b shows the absorption spectra of the photoelectrodes with adsorbed sensitizers. The amount of the dyes adsorbed on the TiO2 surface is shown in Table 2. The dye loading amounts of S6 and S7 are both lower than that of S5, which may be caused by their cone-shaped molecular configuration. For S7, the peripheral methoxy groups can expand the molecular size further and lead to the smallest amount of adsorbed dyes on the photoelectrode. To evaluate the possibility of electron transfer between the TiO2/dye/redox electrolyte systems, cyclic voltammograms were

a

Amount of the dyes adsorbed on TiO2 film.

performed in CH2Cl2 solution, using 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte. Two sets of measurements were carried out, one with the sensitizer adsorbed on the TiO2 films and the other dissolved in solution. In both tests, sensitizer demonstrates two reversible redox waves (Figure 3), indicative of the oxidations of the two triphenylamine units in the donor moiety. The examined highest occupied molecular orbital (HOMO) levels and the lowest unoccupied molecular orbital (LUMO) levels are listed in Table 1. The HOMO value of S7 is significantly higher than those of S5 and S6, which is due to the four methoxy electron donor groups. Judging from the LUMO value, the excited-state energy levels for the dyes are much higher than the bottom of the conduction band of TiO2 (-4.4 eV), indicating that the electron injection process from the excited dye molecule to TiO2 conduction band is energetically viable. To gain insight into the geometrical and electronic properties of these dyes, we mimicked the electronic structure of sensitizer

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Figure 3. (a) Oxidative cyclic voltammetry plots measured on the TiO2 films deposited on the conducting FTO glass. The photoelectrodes were used to replace the working electrode. (b) Oxidative cyclic voltammetry plots measured in CH2Cl2 solution.

Figure 4. Calculated HOMO (left) and LUMO (right) levels of S5.

Figure 5. Photocurrent action spectra (a) and photocurrent-voltage curves (b) of DSSCs.

Figure 6. Transient photovoltages (a) and current-potential curves obtained in the dark (b) of sensitized solar cells.

S5.26 The electron distribution of the HOMO and LUMO of S5 is shown in Figure 4. The HOMO of these compounds is a π orbital delocalized over the triarylamine moiety. The LUMO is a π* orbital delocalized across the 2-cyano-3-(thiophen-2yl)acrylic acid group. Distributions of HOMO and LUMO orbitals are completely separated, indicating that photoexcitation may lead to an efficient intramolecular charge separation with an effective photocurrent generation.5a

The IPCE data of S6 sensitizer plotted as a function of wavelength exhibit a strikingly high efficiency of 89% (Figure 5a). The IPCE spectra are consistent with the absorption spectra on the TiO2 film. The photovoltaic performance was measured at 100 mW cm-2 under AM 1.5 conditions, and the data are listed in Table 2. The corresponding photocurrent-voltage curves are shown in Figure 5b using 0.5 M 1-methyl-3-propyl imidiazolium iodide, 0.1 M LiI, 0.05 M I2, and 0.5 M tert-

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butylpyridine in acetonitrile as redox electrolyte. The Voc values of the cells based on S6 and S7 are dramatically higher than that of S5. Under the same conditions, the Voc and ff values of S6 and S7 are all higher than those of N719.

Ning et al. efficiency of DSSCs owing to original and versatile molecular design. However, the photocurrent intensity of these dyes is limited by their narrow IPCE conversion region. Our future work is focused on the molecular modification to broaden the absorption range.

Discussion The attachment of six alkyl groups on the truexene unit can be fulfilled in a one-step reaction with high yield.24 The common practice of introducing alkyl groups to sensitizers is alkylation of thiophene units.12 However, each thiophene unit bears only one alkyl chain, which imposes a burden to the syntheses since several thiophene units are needed to be connected one by one. Therefore, a π-conjugated unit which can be facilely modified with several alkyl chains is desirable. After the sensitizers were adsorbed on the TiO2 film, the maximum absorbance apex of S5 is shifted from 406 to 422 nm and the spectrum is bathochromically broadened, indicating that there is a certain degree of π-π aggregation.13 By contrast, there is no distinct shift or extension in the spectra of S6 and S7, which shows that the π-π aggregation is significantly reduced by the cone-shaped structure. Despite its lower absorbance, the IPCE value of S6 is slightly higher than that of S5, which may be ascribed to its higher electron injection efficiency because of the weak π-π aggregation. The lower IPCE and photocurrent values of S7 may be tentatively ascribed to the smaller dye adsorption amount and slower diffusion of I3- ions to the Pt electrode surface.15b The high Voc values of S6 and S7 indicate that the charge recombination process was effectively retarded. To verify the above viewpoint, the recombination lifetime (τR) of the photoinjected electron with the redox couples was measured by transient photovoltage at open circuit (Figure 5a).24 The measured τR values for the devices of S5, N719, S6, and S7 were 3.45, 4.78, 4.89, and 5.24 ms, respectively. The results are consistent with the Voc values of the devices. The longer recombination lifetimes of S6 and S7 in comparison to S5 suggests that the charge recombination process was prohibited more effectively for S6 and S7. In order to further confirm this judgment, a dark current test, a technique of testing the capability of the charge recombination, was conducted and the results are shown in Figure 5b.27 The onset potentials (potential value at the dark current value of -0.1 mA cm-2) are 0.54 and 0.53 V for S6 and S7, respectively, higher than 0.51 V of S5. At the dark current value of -2 mA cm-2, the potentials are 0.68, 0.72, 0.73, and 0.75 V for S5, N719, S6, and S7, respectively, which are in accordance with the Voc values obtained under light. All these results suggest that the charge recombination process was reduced further by the cone-shaped sensitizer, which could form a substantial compact sensitizer layer at the surface of the TiO2 to prevent the approach of the redox couple. Conclusions In this work, we designed and synthesized a new kind of cone-shaped sensitizers by linking several long alkyl chains in the middle of the triarylamine-truexene based dyes. Such coneshaped dyes proved to be valid in solving the intractable problem of the close π-π aggregation. More importantly, it can form a compact blocking layer on the TiO2 surface to suppress the approach of I3- ions in the electrolyte to the TiO2 particles and the Voc was improved greatly. Hence, we present here an effective strategy to retard the charge recombination processes in organic DSSCs by molecular structural modification of sensitizers. Our findings demonstrate that organic sensitizers are promising for the further improvement of the conversion

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