ARTICLE pubs.acs.org/JPCC
Enhancing the Performance of Organic Dye-Sensitized Solar Cells via a Slight Structure Modification Kimin Lim,† Chulwoo Kim,‡ Juman Song,† Taejung Yu,† Woocherl Lim,§ Kihyung Song,§ Peng Wang,|| Ningning Zu,*,|| and Jaejung Ko*,† Photovoltaic Materials, Department of Material Chemistry, and ‡Department of Material Chemistry, Korea University, Jochiwon, Chungnam 339-700, Korea § Department of Chemistry, Korea National University of Education, Cheongwon, Chungbuk 363-791, Korea Changchun institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China
)
†
bS Supporting Information ABSTRACT: A novel organic sensitizer JK-225 incorporating a planar indeno[1,2-b]thiophene bridging group was synthesized and compared to its prototype sensitizer JK-2. JK-225 affords a short circuit photocurrent of 13.84 mA cm2, an open circuit voltage of 790 mV, and a fill factor of 0.75, corresponding to an overall conversion efficiency of 8.2% under standard AM 1.5 sunlight, which is much higher than that of 6.9% in JK-2.
’ INTRODUCTION Dye-sensitized solar cells (DSSCs) based on TiO2 nanocrystalline electrodes have attracted significant attention due to the low-cost fabrication and high power conversion efficiency.14 In these cells, the sensitizer is one of the key components. Several research groups have reported power conversion efficiencies over 11% by employing polypyridyl ruthenium photosensitizers.57 Recently, some organic dyes have been utilized as promising photosensitizers having impressive efficiencies in the range of 810%.811 However, the sharp and narrow absorption bands of organic dyes in the blue region impair their light harvesting capabilities. An important goal on organic sensitizers has been the structural variation of the prototype compounds possessing broad and intense spectral features in the red region.1217 Molecular engineering to obtain red-shift absorption spectra has been suggested for the extension of π-conjugation of spacers.18,19 Such a representative example was the structural modifications of the bridged bithiophene moiety due to the twisted nonplanar geometry.11,20,21 Recently, we reported the highly efficient and stable organic dye (JK-2) having bis-dimethylfluorenylamino group as a donor and bithiophene as a conjugated bridge.22 Because small structural changes of bridging unit result in significant changes in the absorption spectrum and the redox potential, affecting dramatically the performance of DSSCs,23 we introduced the 4,4-dimethyl-4H-indeno[1,2-b]thiophene in which a 2-phenylthiophene is bridged by a dimethylmethylene at the r 2011 American Chemical Society
20 ,3-position, showing enlargement of π-conjugation of the indeno[1,2-b]thiophene unit because the methylene bridge renders indeno[1,2-b]thiophene coplanarity (JK-225). This not only increases the extinction coefficient of the sensitizer by extending the π-conjugation of the ligand but also augments its hydrophobicity, preventing dye desorption by water. In this Article, as part of our efforts to investigate the structural modification that can enhance the efficiency,2426 we report a new organic sensitizer (JK-225) containing bis-dimethylfluorenyl amino group as electron donor and cyanoacrylic acid as electron acceptor bridged by indeno[1,2-b]thiophene unit and clarify the factors from where the difference comes by the spectroscopic tool. See Figure 1.
’ EXPERIMENTAL SECTION Materials. Acetonitrile, ethanol, and chlorobenzene were both distilled before use. Lithium iodide (LiI), iodine, and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) were purchased from Sigma-Aldrich. Guanidinium thiocyanate (GNCS), 4-tertbutylpyridine (TBP), and 3α,7α-dihyroxy-5β-cholic acid (cheno) were purchased from Fluka. 1,3-Dimethylimidazolium iodide Received: July 25, 2011 Revised: September 26, 2011 Published: October 07, 2011 22640
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Figure 1. Molecular structures of JK-2 and JK-225.
(DMII),27 dimethylimidazolium bis(trifluoromethanesulfonyl)imide (DMITFSI),27 and 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide (EMITFSI)27 were synthesized according to literature methods. The scattering TiO2 paste (WERO-2) was received as a gift from Dyesol. Synthesis. N,N-Bis(9,9-dimethyl-9H-fluoren-2-yl)-4,4-dimethyl4H-indeno[1,2-b]thiophen-6-amine (10). 6-Bromo-4,4-dimethyl4H-indeno[1,2-b]thiophene 9 (2.74 g, 9.8 mmol), bis(9,9-dimethyl-9H-fluoren-2-yl)amine 2 (4.74 g, 11.8 mmol), rac-BINAP (0.18 g, 0.3 mmol), sodium tert-butoxide (1.32 g, 13.7 mmol), and tris(dibenzylideneacetone)dipalladium(0) (0.09 g, 0.1 mmol) were dissolved in toluene (100 mL). The mixture was heated to 90 °C over a 24 h period. The reaction mixture was allowed to cool to room temperature. The organic layer was extracted with ether, dried over MgSO4, and concentrated. The crude product was then purified further by column chromatography (silica gel, dichloromethane/hexane, 1:6). A white-yellow solid was obtained. Yield: 61% (3.64 g). 1H NMR (300 MHz, CDCl3): 7.64 (d, 2H, J = 6.3 Hz), 7.59 (d, 2H, J = 8.7 Hz), 7.39 (d, 3H, J = 7.5 Hz), 7.347.27 (m, 5H), 7.267.23 (m, 3H), 7.12 (d, 2H, J = 8.4 Hz), 7.07 (d, 1H, J = 8.4 Hz), 7.01 (d, 1H, J = 5.4 Hz), 1.41 (s, 12H), 1.40 (s, 6H). 13C NMR (75 MHz, CDCl3): δ 157.9, 156.8, 155.1, 153.7, 147.4, 145.8, 139.8, 137.7, 136.8, 130.4, 127.3, 126.1, 125.4, 123.4, 122.9, 122.4, 121.0, 120.1, 119.9, 119.6, 118.4, 117.8, 46.3, 45.2, 27.1, 26.3. Anal. Calcd for C43H37NS: C, 85.99; H, 6.17. Found: C, 85.72; H, 6.11. N,N-Bis(9,9-dimethyl-9H-fluoren-2-yl)-2-iodo-4,4-dimethyl4H-indeno[1,2-b]thiophen-6-amine (11). n-BuLi solution in hexanes (2.5 M, 2.40 mL, 6.6 mmol) was added via a syringe to a stirred solution of N,N-bis(9,9-dimethyl-9H-fluoren-2-yl)4,4-dimethyl-4H-indeno[1,2-b]thiophen-6-amine 10 (3.31 g, 5.5 mmol) in THF (60 mL) at 0 °C. Stirring was continued for 1 h, and iodine THF solution was added via syringe at 0 °C. The reaction mixture was stirred at room temperature for 8 h. The reaction mixture was added to H2O, and then the layer was separated. The organic layer was dried over anhydride MgSO4. The solvent was distilled off under reduced pressure. The pure product was obtained by column chromatography on silica gel (dichloromethane/hexane, 1:6). Yield: 65% (2.61 g). 1H NMR (300 MHz, CDCl3): 7.65 (d, 2H, J = 6.9 Hz), 7.58 (d, 2H, J = 8.1 Hz), 7.39 (d, 3H, J = 8.1 Hz), 7.357.30 (m, 4H), 7.297.22 (m, 4H), 7.187.09 (m, 3H), 1.41 (s, 12H), 1.38 (s, 6H). 13C NMR (75 MHz, CDCl3): δ 158.1, 156.3, 154.9, 152.5, 145.1, 144.3, 137.7, 137.2, 136.1, 132.8, 129.4, 126.0, 124.2, 123.1, 122.9, 122.0, 116.2, 114.0, 113.1, 112.9, 112.6, 88.4, 46.2, 45.3, 27.2, 26.3. Anal. Calcd for C43H36INS: C, 71.11; H, 4.89. Found: C, 69.92; H, 4.85. N,N-Bis(9,9-dimethyl-9H-fluoren-2-yl)-4,4-dimethyl-2-(thiophen-2-yl)-4H-indeno[1,2-b]thiophen-6-amine (12). N,N-Bis(9,9-dimethyl-9H-fluoren-2-yl)-2-iodo-4,4-dimethyl-4H-indeno-
[1,2-b]thiophen-6-amine 11 (0.51 g, 0.7 mmol), 4,4,5,5-tetramethyl-2-(thiophen-2-yl)-1,3,2-dioxaborolane (0.18 g, 0.84 mmol), Pd(PPh3)4 (0.03 g, 0.02 mmol), and K2CO3 (0.48 g, 3.5 mmol) were dissolved in THF (40 mL)/H2O (5 mL), and the mixture was refluxed for 15 h. After evaporating the solvent under reduced pressure, H2O (10 mL) and dichloromethane (3 20 mL) were added. The organic layer was separated and dried in MgSO4. The solvent was removed under reduced pressure. The pure product was obtained by column chromatography on silica gel (dichloromethane/hexane, 1:4). Yield: 71% (0.34 g). 1H NMR (300 MHz, CDCl3): 7.66 (d, 2H, J = 6.6 Hz), 7.60 (d, 2H, J = 8.1 Hz), 7.40 (d, 3H, J = 7.8 Hz), 7.357.27 (m, 6H), 7.257.03 (m, 8H), 1.43 (s, 18H). 13C NMR (75 MHz, CDCl3): δ 158.3, 157.3, 155.0, 153.5, 147.6, 147.5, 145.9, 139.0, 138.6, 138.5, 136.7, 134.2, 134.0, 133.1, 131.3, 127.9, 127.1, 126.5, 123.9, 123.0, 122.6, 120.7, 119.5, 118.8, 118.5, 117.7, 47.0, 46.1, 27.2, 26.1. Anal. Calcd for C47H39NS2: C, 82.71; H, 5.73. Found: C, 82.42; H, 5.64. 5-(6-(Bis(9,9-dimethyl-9H-fluoren-2-yl)amino)-4,4-dimethyl4H-indeno[1,2-b]thiophen-2-yl)thiophene-2-carbaldehyde (13). To N,N-dimethylformamide (2 mL) was added POCl3 (0.7 mL) dropwise with ice-bath cooling and stirring. This solution was added to a stirred solution of N,N-bis(9,9-dimethyl-9H-fluoren-2-yl)-4,4-dimethyl-2-(thiophen-2-yl)-4H-indeno[1,2-b]thiophen-6-amine 12 (0.3 g, 0.5 mmol) in chloroform (15 mL), and the solution was stirred for 12 h at room temperature. The mixture was poured onto icewater, basified with NaOH (10 M), and extracted with dichloromethane. It was dried over MgSO4. The solvent was removed in vacuo. The pure product 13 was obtained by silica gel chromatography (ethyl acetate/hexane, 1:6). Yield: 68% (0.21 g). 1H NMR (300 MHz, CDCl3): δ 9.85 (s, 1H), 7.65 (d, 2H, J = 6.9 Hz), 7.61 (d, 2H, J = 8.1 Hz), 7.41 (d, 2H, J = 6.6 Hz), 7.357.23 (m, 11H), 7.157.07 (m, 3H), 1.42 (s, 18H). 13C NMR (75 MHz, CDCl3): δ 183.1, 159.0, 155.0, 154.5, 150.3, 149.2, 149.1, 148.8, 147.0, 146.2, 145.1, 137.6, 130.0, 129.2, 127.2, 126.9, 124.2, 123.9, 123.7, 122.8, 121.9, 121.0, 120.2, 119.8, 118.3, 117.6, 116.4, 47.0, 46.0, 27.2, 26.1. Anal. Calcd for C48H39NOS2: C, 81.13; H, 5.49. Found: C, 80.86; H, 5.41. (E)-3-(5-(6-(Bis(9,9-dimethyl-9H-fluoren-2-yl)amino)-4,4-dimethyl-4H-indeno[1,2-b]thiophen-2-yl)thiophen-2-yl)-2-cyanoacrylic acid (JK-225). 5-(6-(Bis(9,9-dimethyl-9H-fluoren-2yl)amino)-4,4-dimethyl-4H-indeno[1,2-b]thiophen-2-yl)thiophene-2-carbaldehyde 13 (0.06 g, 0.64 mmol) and cyanoacetic acid were vacuum-dried, and then dissolved in CHCl3 (5 mL) and acetonitrile (15 mL) containing piperidine (0.01 mL, 0.1 mmol). The solution was refluxed for 12 h. After being cooled to room temperature, the solution was extracted with dichloromethane and dried over MgSO4. The solvent was removed in vacuo. The pure product JK-225 was obtained by silica gel chromatography 22641
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Scheme 1. Schematic Diagram for the Synthesis of JK-225
(dichloromethane/methanol, 9:1). 1H NMR (300 MHz, DMSO): δ 8.08 (s, 1H), 7.74 (m, 4H), 7.517.42 (m, 5H), 7.317.29 (m, 8H), 7.046.95 (m, 3H), 1.37 (s, 18H). Anal. Calcd for C51H40N2O2S2: C, 78.71; H, 5.11. Found: C, 78.45; H, 5.03. Transient Absorption Measurements. Transient absorption measurements were performed on a LP920 laser flash spectrometer in conjunction with a nanosecond tunable OPOLett 355II laser. Transient absorption spectra were measured with an Andor iStar ICCD camera (gate width, 500 ns; detection time, 3 μs after laser pulse excitation) upon a laser excitation at 530 nm. Transient absorption kinetic traces were detected by a fast photomultiplier tube and recorded with a TDS 3012C digital signal analyzer. The sample was kept at a 45° angle with respect to the excitation beam. The monochromatic probe light was obtained from a xenon arc lamp through a narrow band-pass filter. Cell Fabrication. A bilayer titania film was employed as the negative electrode of DSCs. On a precleaned FTO conducting glass electrode (Nippon Sheet Glass, Solar, 4 mm thick), a 7 μm thick transparent layer consisting of 23 nm sized titania particles was first screen-printed and further coated by a 6 μm thick second layer of scattering titania particles (WERO-2, Dyesol). The film thickness was examined with a benchtop Ambios XP-1 stylus profilometer. The preparation procedures of TiO2 nanocrystals, paste for screen-printing, and nanostructured TiO2 film were reported in a previous paper.28 A cycloidal TiO2 electrode was stained by immersing it into a chlorobenzene solution containing 150 μM dye and 300 μM cheno coadsorbent for 9 h. After rinsing with chlorobenzene and drying by air flow, the dye-coated titania electrode was assembled with a thermally platinized FTO positive electrode possessing an electrolyteperfusion hole, which was beforehand produced with a sandblasting drill. The two electrodes were separated by a 30 μm thick Bynel (DuPont) hot-melt gasket. The internal space was
perfused with a liquid electrolyte with the aid of a vacuum backfilling system. The electrolyte was composed of 1 M DMII, 0.02 M I2, 0.05 M LiI, 1 M TBP, and 0.1 M GNCS in acetonitrile. Ultimately, the hole on the positive electrode was closed hermetically with a Bynel sheet and a thin glass cover by heating. Photovoltaic Characterization. A Keithley 2400 source meter and a Zolix Omni-λ300 monochromator equipped with a 500 W xenon lamp were used for photocurrent action spectrum measurements, with a wavelength sampling interval of 10 nm and a current sampling time of 2 s under the full computer control. A Hamamatsu S1337-1010BQ silicon diode used for IPCE measurements was calibrated in National Institute of Metrology, China. A model LS1000-4S-AM1.5G-1000W solar simulator (Solar Light Co., U.S.) in combination with a metal mesh was employed to give an irradiance of 100 mW cm2. The light intensity was tested with a PMA2144 pyranometer and a calibrated PMA 2100 dose control system. Currentvoltage (JV) characteristics were obtained by applying a bias potential to a testing cell and measuring photocurrent with a Keithley 2602 source meter under the full computer control. The measurements were fully automated using Labview 8.0. A metal mask with an aperture area of 0.158 cm2 was covered on a testing cell during all measurements. An antireflection film (λ < 380 nm, ARKTOP, ASAHI Glass) was adhered to the DSC photoanode during IPCE and JV measurements. The short-circuit photocurrent densities measured under this solar simulator are well consistent with the integral of IPCEs over the AM1.5G spectrum (ASTM G173-03), within a 5% error. Electrical Impedance Measurements. Electrical impedance experiments were carried out in the dark with an IM6ex electrochemical workstation, with a frequency range from 50 mHz to 100 kHz and a potential modulation of 10 mV. The obtained impedance spectra were fitted with the Z-view software (v2.80, Scribner Associates Inc.) in terms of appropriate equivalent 22642
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Table 1. Optical, Redox Parameters of Dyes λabsa/nm (ε/M1 cm1)
Eredoxb/V
E00c/V
ELUMOd/V
JK-2
364 (44 000), 452 (39 000)
1.04
2.23
1.19
JK-225
367 (35 000), 480 (55 000)
0.93
2.19
1.26
dye
a
Absorption spectra were measured in EtOH solution. b Redox potential of dyes on TiO2 were measured in CH3CN with 0.1 M (n-C4H9)4NPF6 with a scan rate of 50 mV s1 (vs NHE). c E00 was determined from intersection of absorption and emission spectra in ethanol. d ELUMO was calculated by EOX E00.
Figure 2. Absorption and emission spectra of JK-2 (red solid line) and JK-225 (blue solid line) in EtOH and absorption spectrum of JK-225 (blue dash line) anchored to TiO2 film. The emission spectrum of JK225 was obtained using the same solution at 480 nm at 298 K. The concentration for the solution was 105 M.
circuits. Some important device parameters, such as the chemical capacitance (Cμ) at the titania/dye/electrolyte interface and the charge-transfer resistance (Rct) at the titania/electrolyte interface, can be obtained by means of fitting the impedance spectra. On the other hand, the chemical capacitance and the chargetransfer resistance satisfy the following formulas through some simple derivations:29,30 e2 dð1 pÞ EF, redox Ec eV Cμ ¼ Nt exp exp ð1Þ kB Tc kB Tc kB Tc kB T βðEc EF, redox Þ βeV exp Rct ¼ exp kB T kB T k0 βNc β e2 dA ð2Þ where e is the elementary charge, d is the titania film thickness, p is the film porosity, kB is the Boltzmann constant, Tc is a characteristic temperature describing the distribution profile of interband states of a nanocrystalline titania film, Nt is the total density of surface states, V is the potential bias in electrical impedance measurements, which needs to be corrected through deducting the potential drop induced by the series resistance of the cell, T is the absolute temperature, A is the area of a titania film, Nc is the density of states in the conduction band, k0 is apparent recombination reaction rate constant, and β is the reaction order of titania electrons. Therefore, fitting Cμ and Rct in terms of eqs 1 and 2, respectively, we can attain the difference between the titania conduction band edge (Ec) and the electrolyte Fermi level (EF,redox), Ec EF,redox, k0, and β, which are key energetic and kinetic parameters affecting the device performance.
’ RESULTS AND DISCUSSION Synthesis of Sensitizer. Scheme 1 illustrates the synthetic protocol of organic dye JK-225. N-Phenylation of bromo-4,4dimethyl-4H-indeno[1,2-b] thiophene 931 was performed under Buchwald reaction,32 followed by n-buthyllithium and I2 addition to give 11. The fluorenylamine thiophene 12 was synthesized by the Suzuki coupling reaction3335 of 11 with 4,4,5,5-tetramethyl2-(thiophen-2-yl)-1,3,2-dioxaborolane. The thiophene derivative 12 was converted to their corresponding carbaldehyde 13
according to the VilsmeierHaack reaction.36,37 The aldehyde compound 13, upon reaction with cyanoacetic acid in the presence of piperidine, produced the sensitizer JK-225. Spectroscopic Studies. Figure 2 shows the absorption and emission spectrum of JK-225 in ethanol. The absorption spectrum of JK-225 exhibits two absorption maxima at 367 (ε = 35 000 dm3 mol1 cm1) and 480 nm (ε = 55 000 dm3 mol1 cm1), which is due to the ππ* transitions of the conjugated molecule. Interestingly, the absorption band at 480 nm in JK-225 is redshifted and broadened as compared to that of the prototype JK-2 sensitizer. A big and broad red-shift of JK-225 can be understood from molecular modeling study of the dye. The ground-state structure of JK-225 possesses a 1.6° twist between the indenothiophene and the thienyl unit, giving almost a planar configuration. For the JK-2 case, the dihedral angles between the aniline and the thienyl unit and two thienyl units are 22.7° and 2.2°, respectively, giving more twisting than those of JK-225. The thiophene unit in JK-225 has a small torsion angle with respect to its adjacent thiophenyl group in the indenothiophene bridged unit than that of JK-2, which should play a critical role for the enhanced absorption of JK-225 relative to JK-2. Accordingly, a high extinction coefficient and a significant red-shift of JK-225 relative to JK-2 derive from an increased delocalization over an entire conjugated system in JK-225. Similar distortions have been observed for other organic dyes.38,39 When the JK-225 sensitizer was adsorbed on TiO2 electrode, a broad and red-shift were found due to the J-aggregation. When the JK-225 sensitizer was excited at 480 nm in an air-equilibrated solution and at 298 K, it exhibits strong luminescence maximum at 656 nm. Electrochemical property of JK-225 sensitizer was scrutinized by cyclic voltammetry in acetonitrile containing tetrabutyl ammonium hexafluorophosphate. TiO2 film stained with sensitizer was used as working electrodes. The organic dye JK-225 adsorbed on TiO2 films show a quasi-reversible couple. The oxidation potential of JK-225 was measured to be 0.93 V versus NHE. Under the same conditions, the JK-2 dye showed the redox couple at 1.04 V versus NHE. The oxidation potential of JK-225 is more negative than that of JK-2, reflecting the increased electron-donating ability as compared to that of JK-2 due to the delocalization of the π-conjugation system on the indenothiophene unit. The reduction potential of JK-225 calculated from the oxidation potential and the E00 determined from the intersection of absorption and emission spectra are listed in Table 1. The excited-state oxidation potential (Eox) of JK-225 (1.26 V vs NHE) is more negative than the conduction band of TiO2 at approximately 0.5 V vs NHE. Therefore, the downhill energy offset of the LUMO ensures enough of a driving force for electron injection.40,41 Theoretical Calculation of Electronic Properties. To obtain the geometrical configuration and the electronic structure of JK225, molecular orbital calculation of JK-225 was performed 22643
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Figure 3. Frontier molecular orbitals of the HOMO1, HOMO, LUMO, and LUMO+1 with B3LYP/6-31G(d) of JK-225.
using the B3LYP/6-31G(d).42 The dihedral angle between the indenothiophene and the thienyl unit in JK-225 is calculated to be 1.6°, and the cyanoacrylic acid group is found to be almost coplanar with respect to the thienyl unit, reflecting the entire conjugation through the bridging unit. Figure 3 show the isodensity plots of the frontier molecular orbitals of JK-225. The calculation illustrates that the HOMO of JK-225 is delocalized over the π-conjugated system through the fluorenylamino unit and cyanoacrylic group and the LUMO is delocalized over the thienyl unit and cyanoacrylic unit. Examination of the HOMO and LUMO of JK-225 indicates that HOMOLUMO excitation moves the electron distribution from the fluorenylamino unit to the cyanoacrylic acid moiety, and the photoinduced electron transfer from the dyes to TiO2 electrode can occur efficiently at the HOMOLUMO transition. Transient Absorption. Laser-induced transient absorption technique was employed to probe the kinetic competition between the dye cations recombination with photoinjected electrons and their regeneration by electron-donating species in electrolytes, so as to evaluate the contribution of this dual-channel chargetransfer kinetics to the IPCE height of a DSC.43 We first measured the transient absorption spectra (Figure 4A) of dyecoated titania films using an ICCD camera, from which a strong absorption in the near-infrared region can be observed for both JK-2 and JK-225 dye cations with respect to their neutral counterparts. Moreover, taking account of the signal sensitivity of our PMT detector, a monochromatic probe light at 782 nm was chosen in the succedent kinetic measurements, where the excitation wavelength and fluence were carefully controlled to ensure that ∼7.6 1013 photons were absorbed by a dye-coated titania film (∼0.8 photon per nanoparticle). The half-reaction times (t1/2) of these charge transfer processes can be derived through fitting with a stretched exponential function. In the presence of an inert electrolyte with the inactive bis(trifluoromethanesulfonyl)amide (TFSI) anion, the absorption decays (traces a and b in Figure 4B and C) can be assigned to the charge recombination reaction between dye cations and titania electrons, producing half-reaction times of 5 and 3 ms for JK-2 and JK-225, respectively. Upon utilizing the realistic electrolyte for cell fabrication, the absorption decays (traces c and d) are significantly accelerated due to occurrence of dye regeneration
Figure 4. (A) Transient absorption spectra upon laser excitation at 530 nm of 2.3 μm thick, dye-coated titania films immersed in an inert electrolyte composed of 1 M dimethylimidazolium bis(trifluoromethane sulfonyl)imide, 0.05 M lithium bis(trifluoromethanesulfonyl)imide, and 1 M 4-tert-butylpyridine in acetonitrile. (B,C) Transient absorption kinetic traces of 11.0 μm thick, dye-coated titania films immersed in the inert (traces a and b) or iodine (traces c and d) electrolyte. Excitation wavelength and pulse fluence: 53 μJ cm2 at 602 nm (trace a); 52 μJ cm2 at 605 nm (trace c); 49 μJ cm2 at 654 nm (trace b); 49 μJ cm2 at 648 nm (trace d). The smooth lines are stretched exponential fittings over raw data obtained by averaging over 800 laser shots.
Table 2. Half-Reaction Times of Dye Cations Recombined DR with Titania Electrons (tCR 1/2) and Dye Regeneration (t1/2) and Kinetic Branch Ratios tCR 1/2/ms
tDR 1/2/μs
branch ratio
JK-2
5
1
5000
JK-225
3
7
429
dye
by iodide. The half-reaction times of regeneration kinetics are 1 and 7 μs for JK-2 and JK-225, respectively, and the longer halfreaction time for JK-225 can be interpreted by a less advantageous interfacial energetics for dye regeneration (formal redox potential: JK-2, 1.04 V and JK-225, 0.93 V vs NHE). We can see that the kinetic branch ratios of these two hole capture reactions by electrolytes and titania electrons exceed 400 (Table 2) for both photosensitizers, indicating that almost 100% of the dye cations can be intercepted by the iodide anions, hence a highyield net charge separation. Photovoltaic Performance. The incident phototo-collected electron conversion efficiency (IPCE) of the device on JK-225 is presented and compared to that on JK-2 in the inset of Figure 5. The IPCE of JK-225 cell exceeds 70% in a broad spectrum range from 420 to 620 nm, reaching its maximum of 86%. The onset of IPCE spectra employing JK-225 is 760 nm, about 45 nm redshifted as compared to that using JK-2. The IPCE spectrum for JK-225 is much broader and red-shifted due to its extension of the π-conjugation and planar configuration. Figure 5 shows the photocurrentvoltage (JV) curves of two devices fabricated with TiO2 films, which are anchored with JK-2 and JK-225. 22644
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Table 3. Photovoltaic Data Measured under the 100 mW cm2 AM1.5 Sunlight
Table 4. Parameters Derived from EIS dye
Jsc [mA cm2]
Voc [mV]
FF
ηa [%]
JK-2
12.34
750
0.75
6.9
JK-225
13.84
790
0.75
8.2
dye
Ec EF,redox [eV] K0 [cm3(1β) s1]
JK-2 JK-225
β
U0K [cm3 s1]
0.97
2.7 108
0.64
7.4 1021
1.00
4.1 10
0.68
6.7 1021
7
a
Performances of DSSCs were measured with 0.158 cm2 working area. Electrolyte: 1 M 1,3-dimethylimidazolium iodide, 0.02 M iodine, 0.05 mM lithium iodide, 1 M 4-tert-butylpyridine, and 0.1 M guanidinium thiocyanate in acetonitrile.
Figure 6. (A) Chemical capacitance (Cμ) and (B) interfacial charge transfer resistance (Rct) plotted as a function of potential bias (V). The solid lines in panels (A) and (B) are fittings in terms of eqs 1 and 2, respectively.
Figure 5. JV curve and IPCE spectra (inset) of JK-2 (9) and JK-225 (2). Cell area tested with a metal mask: 0.158 cm2. Dark currentbias potential relationship is shown as a solid line.
The photovoltaic performance of the devices is listed in Table 3. Under standard global AM 1.5 solar condition, the JK-225sensitized cell gave a short circuit photocurrent density (Jsc) of 13.84 mA cm2, an open circuit voltage (Voc) of 790 mV, and a fill factor of 0.75, corresponding to an overall conversion efficiency η of 8.2%. Under the same conditions, an overall conversion efficiency (η) of 6.9% for JK-2 sensitized cell (Jsc = 12.34 mA cm2; Voc = 750 mV; FF = 0.75) was obtained. A big Jsc enhancement of JK-225 relative to that of JK-2 stems from the high molar extinction coefficient and the red shift in the absorption spectrum of JK-225 relative to JK-2. Of particular importance is the 40 mV increase in Voc of the JK-225 cell. The increase of Voc in JK-225 can be interpreted as an upward shift of TiO2 band edge, as shown in the following section. It is known that the open-circuit photovoltage of a DSC is determined by titania quasi-Fermi level (EF,n) and electrolyte Fermi level (EF,redox) in terms of EF,n EF,redox = eVoc. Also, EF,n correlates the titania conduction band edge (Ec) and the free carrier density (n) through the following familiar expression: EF, n Ec n ¼ Nc exp ð3Þ kB T where Nc is the density of states in the conduction band, kB is the Boltzmann constant, and T is the absolute temperature. On the other hand, the interfacial charge recombination rate (Un), being equivalent to the photocurrent generation rate under opencircuit condition, which is proportional to the short-circuit current (Jsc), is given by44 Un ¼
Jsc ¼ k0 nβ ed
ð4Þ
where e is the elementary charge, d is the titania film thickness, k0 is apparent recombination reaction rate constant, and β is the reaction order of titania electrons. Integrating eq 3 with eq 4, the open-circuit photovoltage can be derived as Voc ¼
kB T Jsc Ec EF, redox ln þ eβ edU0K e
ð5Þ
where U0K = k0Ncβ is the effective recombination rate constant, which can be used to availably evaluate the recombination kinetics at the titania/electrolyte interface. Therefore, eq 5 reveals that for a given electrolyte, the Voc is dependent on the titania conduction band edge (Ec) as well as the photocurrent generation and interfacial charge recombination rates characterized by Jsc and U0K, respectively. We afforded the values of Ec EF,redox and U0k as listed in Table 4 through fitting (Figure 6) on the chemical capacitance (Cμ) at the titania/dye/electrolyte interface and the charge-transfer resistance (Rct) the titania/ electrolyte interface obtained from the electrical impedance spectroscopy in terms of eqs 1 and 2, respectively. It can be seen that for the JK-225 cell, the titania conduction band edge is higher than that of the JK-2 cell by 30 mV, and such an upward shift of Ec is just the dominant factor of 40 mV enhancement. Furthermore, the higher Jsc and slightly smaller U0k of JK-225 cell also contribute to its higher open-circuit photovaltage to some extent.
’ CONCLUSION We have synthesized an efficient organic sensitizer JK-225 containing a sterically hindered fluorenyl nit and a planar indenothiophene unit. A photovoltaic performance is quite sensitive to the structural modification of bridged unit. The organic sensitizer JK-225 having a rigid indenothiophene unit as a bridged group showed a good performance as compared to that of JK-2 incorporating the bridged bithiophene unit. The device based on the sensitizer JK-225 gave an overall conversion 22645
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The Journal of Physical Chemistry C efficiency of 8.2%. The efficiency is the highest one reported for DSSCs based on organic sensitizer. We believe that the development of efficient organic sensitizers with excellent stability is possible through the structural modifications, and work on these is now in progress.
’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: 82 41 860 1337. Fax: 82 41 867 5396. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the ERC (the Korean government (MEST)) program (no. R11-2009-088-02001-0), and the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (no. 2010T100100674). ’ REFERENCES (1) O’Regan, B.; Gr€atzel, M. Nature 1991, 353, 737–740. (2) Gr€atzel, M. Nature 2001, 414, 338–344. (3) Wang, P.; Klein, C.; Humphry-Baker, R.; Zakeeruddin, S. M.; Gr€atzel, M. J. Am. Chem. Soc. 2005, 127, 808–809. (4) Jang, S.-R.; Lee, C.; Ko, J.; Lee, J.; Vittal, R.; Kim, K.-J. Chem. Mater. 2006, 18, 5604–5608. (5) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gr€atzel, M. J. Am. Chem. Soc. 1993, 115, 6382–6390. (6) Gr€atzel, M. J. Photochem. Photobiol., A 2004, 164, 3–14. (7) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Y. Jpn. J. Appl. Phys., Part 2 2006, 45, L638–L640. (8) Wang, Z. S.; Cui, Y.; Hara, K.; Dan-oh, Y.; Kasada, C.; Shinpo, A. Adv. Mater. 2007, 19, 1138–1141. (9) Horiuchi, T.; Miura, H.; Sumioka, K.; Uchida, S. J. Am. Chem. Soc. 2004, 126, 12218–12219. (10) Koumura, N.; Wang, Z.-S.; Mori, S.; Miyashita, M.; Suzuki, E.; Hara, K. J. Am. Chem. Soc. 2006, 128, 14256–14257. (11) Zeng, W.; Cao, Y.; Bai, Y.; Wang, Y.; Shi, Y.; Zhang, M.; Wang, F.; Pan, C.; Wang, P. Chem. Mater. 2010, 22, 1915–1925. (12) Velusamy, M.; Justin Thomas, K. R.; Lin, J. T.; Hsu, Y.-C.; Ho, K.-C. Org. Lett. 2005, 10, 1899–1902. (13) Zhou, Q. M.; Hou, Q.; Zheng, L. P.; Deng, X. Y.; Yu, G.; Cao, Y. Appl. Phys. Lett. 2004, 84, 1653–1655. (14) Brabec, C. J.; Winder, C.; Sariciftci, N. S.; Hummelen, J. C.; Dhanabalan, A.; Van Hal, P. A.; Jamssem, R. A. Adv. Funct. Mater. 2002, 12, 709–712. (15) Winder, C.; Sariciftci, N. S. J. Mater. Chem. 2004, 12, 1077– 1086. (16) Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007, 91, 954–985. (17) Bundgaard, E.; Krebs, F. C. Macromolecules 2006, 39, 2823– 2831. (18) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Yoshihara, T.; Murai, M.; Kurashige, M.; Ito, S.; Shinpo, A.; Suga, S.; Arakawa, H. Adv. Funct. Mater. 2005, 15, 246–252. (19) Justin Thomas, K. R.; Hsu, Y.-C.; Lin, J.; Lee, K.-M.; Ho, K.-C.; Lai, C.-H.; Cheng, Y.-M.; Chou, P.-T. Chem. Mater. 2008, 20, 1830– 1840.
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