Organic Dyes Incorporating Bis-hexapropyltruxeneamino Moiety for

Dec 13, 2010 - We report here on the synthesis and photophysical/electrochemical properties of three functional triarylamine organic dyes (MXD5−7) a...
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J. Phys. Chem. C 2011, 115, 274–281

Organic Dyes Incorporating Bis-hexapropyltruxeneamino Moiety for Efficient Dye-Sensitized Solar Cells Meng Lu, Mao Liang,* Hong-Yu Han, Zhe Sun, and Song Xue* Department of Applied Chemistry, Tianjin UniVersity of Technology, Tianjin, 300384, People’s Republic of China ReceiVed: August 07, 2010; ReVised Manuscript ReceiVed: NoVember 11, 2010

We report here on the synthesis and photophysical/electrochemical properties of three functional triarylamine organic dyes (MXD5-7) as well as their application in dye-sensitized nanocrystalline TiO2 solar cells (DSSCs). For the designed dyes, the nonplanar structures of bis-hexapropyltruxeneamino take the role of electron donor. The introduction of bis-hexapropyltruxeneamino units brought about superior performance over the simple triphenylamine dye, in terms of light-capturing abilities and suppressing dye aggregation. Among three dyes, the DSSCs based on the dye MXD7 showed the best photovoltaic performance: a short-circuit photocurrent density (JSC) of 11.8 mA cm-2, an open-circuit photovoltage (VOC) of 772 mV, and a fill factor (ff) of 0.68, corresponding to an overall conversion efficiency of 6.18% under 100 mW cm-2 irradiation. These dyes exhibited high VOC values, possible origin for which was investigated regarding the TiO2 surface blocking, conduction band movement, and electrolyte-dye interaction. Introduction

electron donating and transporting capability, as well as its special propeller starburst molecular structure.41 Furthermore, it was found that functional triarylamines (e.g., bis-dimethylfluorenylamino and alkoxy-substituted triphenylamine) prevented surface aggregation, retarded interfacial charge recombination, and increased the stability of dye when exposed to light and high temperature, thereby generating high-power conversion efficiencies and stability. Despite the significant improvement of triarylamine-based organic dyes, much work is still desired to further promote the power conversion efficiency. Starburst truxene, recognized as a potential starting material for organic semiconductors, liquid crystalline compounds, and fullerene,42 has attracted our interest for design of photosenzitizers due to its following special properties: (1) bulky rigid conjugation structure; (2) facile introduction of alkyl chains; (3) outstanding thermal stability.43,44 Organic dyes using bis-hexaalkyltruxeneamino in place of triphenylamine as electron donor are reasonably expected to improve optical properties, suppress dye aggregation, and hopefully retarded charge recombination. To the best of our knowledge, no bis-hexaalkyltruxeneamino-based dipolar sensitizers have been reported. Herein we report on the synthesis, characterization, and photovoltaic properties of these bis-hexapropyltruxeneamino-based organic dyes, namely, the MXD5-7 (Scheme 1) dyes. These dyes contain bis-hexapropyltruxeneamino as electron donor and cyanoacrylic acid as electron acceptor (anchoring groups). In particular, the π spacers are varied to show the role of bishexapropyltruxeneamino on the photovoltaic performance. A triphenylamine dye employed one thiophene as linker, coded L1,45,46 is also synthesized for a comparison. Photovoltaic performance measurements, electrochemical impedance, and theoretical calculation are in support of the molecular design, giving a clear view of the influence coming from bis-hexapropyltruxeneamino and π spacer.

Dye sensitized solar cells (DSSCs) have been explored for photovoltaic applications because of their low cost and high conversion efficiency since reported by O’Regan and Gra¨tzel.1 Conventional DSSCs typically contain four components: a mesoporous semiconductor metal oxide film; a sensitizer (dye); an electrolyte/hole transporter; and a counterelectrode.2,3 It has been observed that the dye sensitizer plays a decisive role in device performance. Actually, much effort has been devoted to the synthesis and optimizing the dye component. To date, two kinds of dye sensitizers have been developed, ruthenium complexes and metal-free organic dyes. The former class of compounds contains expensive ruthenium metal and requires careful synthesis and tricky purification steps. On the other hand, the latter class can be prepared rather inexpensively by following established design strategies.4 In recent years, metal-free organic dyes were developed as an alternative to noble metal complexes for light harvesting owing to their high molar absorption coefficient, high efficiency and low cost in comparison to ruthenium complexes. Various kinds of metal-free organic dyes suchascoumarin,5,6 indoline,7,8 triphenylamine,9-19 dialkylaniline,20-22 bis(dimethylfluorenyl) aminophenyl,23-29 merocyanine,30,31 hemicyanine,32,33 and carbazole134,35 have been investigated as sensitizers for DSSCs. The power conversion efficiencies of 9.7-10.3% have been achieved based on organic dyes.36-38 By far the most common investigated organic sensitizers are usually composed by an electron donor (D), a π spacer, and an electron acceptor (A). This D-π-A dipolar architecture produces an effective intramolecular charge transfer from D to A during photoexcitation. The properties of D-π-A dyes can be easily tuned by molecular design based on donor and spacer. We are interested in the design and synthesis of a suitable donor for constructing effective organic sensitizers for DSSCs.39,40 In recent years, a substantial number of dyes with triarylamine as electron donor were developed for DSSCs because of its good

Experimental Section

* To whom correspondence should be addressed. Tel.: +86 22 60214250. Fax: +86 22 60214252. E-mail: (M.L.) [email protected]; (S.X.) [email protected].

Materials and Methods. The bis-hexapropyltruxeneaminobased organic dyes were synthesized under Witting reaction, Vilsmeier reaction, Suzuki coupling, and Knoevenagel conden-

10.1021/jp107439d  2011 American Chemical Society Published on Web 12/13/2010

Organic Dyes Incorporating Bis-Hexapropyltruxeneamino

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SCHEME 1: Molecular Structures of MXD5-7 and L1 (R ) Propyl)

sation reaction with the synthetic routes and the detailed procedures (synthesis and characterization, see Supporting Information). The starting material iodide-substituted truxene was prepared by adopting literature procedures.47 5-Formylthiophen-2-ylboronic acid was purchased from Alfa. N,N-Dimethylformamide was dried over and distilled from CaH2 under an atmosphere of nitrogen. Phosphorus oxychloride was freshly distilled before use. Dichloromethane and chloroform were distilled from calcium hydride under nitrogen atmosphere. Tertbutylpyridine and lithium iodide were purchased from Aldrich. All other solvents and chemicals used in this work were analytical grade and used without further purification. Melting points of the samples were taken on an RY-1 melting point apparatus (Tianfen, China). 1H NMR and 13C NMR spectra were recorded on a Bruker AM-300 or AM-400 spectrometer using CDCl3 as solvent in all cases. The reported chemical shifts were against TMS. High-resolution mass spectra were obtained with a Micromass GCT-TOF mass spectrometer. Optical and Electrochemical Measurements. The absorption spectra of the dyes in CH2Cl2 (DCM) solution and on sensitized TiO2 film were measured with a Jasco V-550 UV/ vis spectrophotometer. Adsorption of the dye on the TiO2 surface was done by soaking the TiO2 electrode in a dye bath (ethanol/ dichloromethane ) 3:1, standard concentration 3 × 10-4 M) at room temperature for 48 h. Fluorescence measurement was carried with a HITACHI F-4500 fluorescence spectrophotometer. FT-IR spectra of dyes were recorded with a Bio-Rad FTS 135 FT-IR instrument. Electrochemical measurements were performed at room temperature under Ar atmosphere on a Voltammetric Analyzer (Metrohm, µAutolab III) with polymer-coated ITO glass as the working electrode, and platinum (Pt) plate as the counter electrode, using Ag/Ag+ (nonaqueous) electrode as reference electrode with a scan rate of 50 mV/s. Tetrabutylammonium perchlorate (TBAP, 0.1 mol/L) and acetonitrile were used as supporting electrolyte and solvent, respectively. The measurements were calibrated using ferrocene as standard. The redox potential of ferrocene internal reference was taken as 0.63 V versus NHE.48 The oxidation potential (Eox) of as-synthesized dyes was determined from square-wave voltammograms. The Eox corresponds to the highest occupied molecular orbital (HOMO). The reduction potential (Ered), which corresponds to the lowest unoccupied molecular orbital (LUMO), can be calculated from Eox - E0-0 (E0-0 values were calculated from λint: E0-0 ) 1240/λint). Electrochemical impedance spectroscopy (EIS) in the frequency range of 100 mHz to 100 kHz was performed with a PARSTAT 2273 potentiostat/galvanostat/FRA in the dark with the alternate current amplitude set at 10 mV. Forward biases of 650-850 mV were applied to the dyesensitized TiO2 electrode during the measurement. The resulting curves were fitted to the appropriate equivalent circuit.

Theoretical Calculation Methods. The geometrical structures of the three dyes were optimized by performed density functional theory (DFT) calculations and time-dependent DFT (TDDFT) calculations of the excited states at the B3LYP/631+G(d) level with the Gaussian 03W program package.49 Polarizabilities and dipole moments were calculated at the B3LYP/6-31+G(d) level of theory. We performed the calculations on a model in which the propyl substituents have been replaced by methyls and not taking into consideration any rearrangements occurring upon binding the molecules to TiO2. Optimized 3D structures were generated with the ChemBio3D ultra 11.0 program. Fabrication and Photovoltaic Measurement of DSSCs. TiO2 colloid was prepared according to the literature method,50 which was used for the preparation of the nanocrystalline films. The TiO2 paste consisting of 18 wt % TiO2, 9 wt % ethyl cellulose, and 73 wt % terpineol was first prepared, which was printed on a conducting glass (Nippon Sheet Glass, Hyogo, Japan, fluorine-doped SnO2 over layer, sheet resistance of 10 Ω/sq) using a screen printing technique. The thickness of the TiO2 film was controlled by selection of screen mesh size and repetition of printing. The film was dried in air at 120 °C for 30 min and calcined at 500 °C for 30 min under flowing oxygen before cooling to room temperature. The heated electrodes were impregnated with a 0.05 M titanium tetrachloride solution in a water-saturated desiccator at 70 °C for 30 min and fired again to give a ca. 10 Ωm thick mesoscopic TiO2 film. The TiO2 electrode was stained by immersing it into a dye solution containing 300 µM dye sensitizers (ethanol-dichloromethane 3:1 for MXD5-7 and L1) for 48 h at room temperature. Then, the sensitized-electrode was rinsed with dry ethanol and dried by a dry air flow. Pt catalyst was deposited on the FTO glass by coating with a drop of H2PtCl6 solution (40 mM in ethanol) with the heat treatment at 395 °C for 15 min to give photoanode. The dye-covered TiO2 electrode and Pt-counter electrode were assembled into a sandwich type cell according to the literature method.50 The DSSCs had an active area of 0.16 cm2 and electrolyte composed of 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide (DMPImI), 0.1 M LiI, 0.05 M I2, and 0.5 M tertbutylpyridine in acetonitrile. The photocurrent-voltage (I-V) characteristics of the solar cells were carried out using a Keithley 2400 digital source meter controlled by a computer and a standard AM1.5 solar simulatorOriel 91160-1000 (300W) SOLAR SIMULATOR 2 × 2 BEAM. The light intensity was calibrated by an Oriel reference solar cell. The action spectra of monochromatic incident photonto-current conversion efficiency (IPCE) for solar cell were performed by using a commercial setup (QTest Station 2000 IPCE Measurement System, CROWNTECH, U.S.A.).

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Figure 1. Absorption spectra of MXD5-7 and L1 in dichloromethane.

Results and Discussion Photophysical Properties. The absorption spectra of the MXD5-7 in CH2Cl2 solution are shown in Figure 1. The characteristic data are summarized in Table 1. In CH2Cl2 solution, all the dyes display two strong absorption bands at around 300-380 nm and 450-650 nm. The absorption spectrum of MXD6 (peak at 540 nm with molar extinction coefficient (ε) of 62 000 M-1 · cm-1) is red-shifted in comparison with that of MXD5 (peak at 498 nm with ε of 52 000 M-1 · cm-1), owing to the expansion of conjugation systems by inserting a double bond into π-spacer. Under similar conditions, the MXD7 sensitizer containing EDOT unit exhibits absorption bands at 519 nm (ε ) 67 000 M-1 · cm-1) that is red shifted as compared to the MXD5 sensitizer, which may rise from the 3,4ethylenedioxyl group enhancing the conjugated efficiency of EDOT and phenyl ring. Also, this can be understood from molecular modeling studies of MXD5 and MXD7 (Figure S1 in the Supporting Information). Theoretical computation shows that the ground state structure of MXD5 possesses a 20° twist between the bis-hexapropyltruxeneamino and thiophene, and the corresponding dihedral angle in MXD7 is 14.0°, thus resulting in a more planar conjugating system and therefore red shift of absorption band. As for MXD6, the dihedral angles around the double bond are 0°, giving a planar configuration and broadslight harvesting. For comparison, absorption spectrum of L1 in CH2Cl2 was also shown in Figure 1. MXD5-7 show two strong absorptions while only one absorption band exhibits for L1 (peak at 474 nm, ε ) 28 000 M-1 · cm-1). Compared to L1, ca. 24 nm red shift in the absorption peak together with significant increases in molar extinction coefficient (about 1.86 fold) was found for MXD5. The broader spectral response and higher molar absorption coefficient of MXD5 mainly resulted from a stronger electron-donating ability of bis-hexapropyltruxeneamino donor than that of triphenylamine donor, which is desirable for harvesting more solar light.

Figure 2. Absorption spectra of MXD5-7 and L1 adsorbed on TiO2 films.

Figure 2 shows absorption spectra of MXD5-7 adsorbed on the surface of transparent mesoporous TiO2 films (3 µm). Absorption of the blank TiO2 film was subtracted from the curve. When the dyes were attached on TiO2 films, the absorption spectra may blue shift or red shift as compared to that in solutions because of strong interactions between the dyes and the semiconductor surface. Generally, H-aggregation resulted in the blue shift and J-aggregation for the red shift. Absorption spectra of MXD5-7 were about 30-50 nm blue shifted in the region of 450-650 nm when anchored at the TiO2 surface compared to solution spectra. Such a phenomenon was commonly observed in the spectral response of other organic dyes, which may be ascribed to the formation of dye H-aggregates on the TiO2 surface and/or the interaction between the dyes and TiO2.51 The amounts of the dyes adsorbed on the TiO2 surface were estimated spec troscopically by desorbing the dyes with a 0.01 M solution of KOH in methanol, and the surface concentrations were determined to be 0.91 × 10-7 mol cm-2 for MXD5, 1.2 × 10-7 mol cm-2 for MXD6, and 1.0 × 10-7 mol cm-2 for MXD7. The amounts of L1 adsorbed on the TiO2 surface was also estimated to be 1.8 × 10-7 mol cm-2 in the same conditions, small size of molecular leading to more dyeuptake. When the four as-synthesized dyes are excited with visible light, they exhibit strong luminescence maxima at 619 to 628 nm (Figure S2 in the Supporting Information). The corresponding data are also summarized in Table 1. To gain insight into the geometrical, electronic, and optical properties of MXD5-7, DFT calculations and time-dependent DFT (TDDFT) calculations of the excited states were performed. We optimized the molecular structure of MXD5-7 in the vacuo, and the isodensity surface plots of HOMO and LUMO are presented in Figure 3. For the three dyes, the HOMO is delocalized throughout the entire molecule, with maximum components on the bis-hexapropyltruxeneamino; the LUMO is, on the other hand, a single π* orbital delocalized across the phenyl, spacers, and cyanoacrylic acid groups with sizable

TABLE 1: Optical Properties and Electrochemical Properties of MXD5-7 and L1 dye

λmax/nm (ε/103 M-1cm-1)a

λmax/nmb

λmax/nmc

λint/nmd

E0-0/eVe

Eox/V vs NHEf

Ered/V vs NHEg

MXD5 MXD6 MXD7 L1

369 (110), 498 (52) 379 (95), 540 (62) 373 (97), 519 (67) 474 (28)

619 628 622 610

361, 466 376, 488 367, 477 422

566 584 580 552

2.19 2.12 2.14 2.25

1.05 1.0 0.85 1.18

-1.14 -1.12 -1.29 -1.07

a Absorption spectra of dyes measured in CH2Cl2 with the concentrate of solution 1.0 × 10-5 M, ε is the extinction coefficient at λmax of absorption b Emission spectra of dyes measured in CH2Cl2 with the concentrate of solution 1.0 × 10-5 M, ε is the extinction coefficient at λmax of absorption. c Absorption spectra of dyes adsorbed on TiO2. d The intersect of the normalized absorption and the emission spectra. e E0-0 values were estimated from the intersections of normalized absorption and emission spectra in CH2Cl2, λint: E0-0 ) 1240/λint. f Eox was recoded by peak values of square-wave voltammograms. g Ered was calculated from Eox - E0-0.

Organic Dyes Incorporating Bis-Hexapropyltruxeneamino

Figure 3. Isodensity surface plots of the HOMO and LUMO of MXD5-7.

Figure 4. Schematic energy levels of MXD5-7 and L1 based on absorption and electrochemical data.

contributions from the latter; the LUMO+1/LUMO+2 is a π* orbital of the bis-hexapropyl truxeneamino. The low-energy absorptions stem from the π-π* transitions from HOMO to LUMO while the high-energy absorptions are mainly due to the electron transitions from HOMO to LUMO+1 and HOMO to LUMO+2 (Figure S3 and Table S1 in the Supporting Information). Redox Properties. The oxidation potential (Eox) of assynthesized dyes was determined from square-wave voltammograms under Ar atmosphere to estimate the HOMO and the LUMO of corresponding dyes. The Eox and reduction potential (Ered) correspond to the HOMO and LUMO level, respectively. The energetic alignment of HOMO and LUMO energy levels are crucial for an efficient operation of the sensitizer in a DSSC. In principle, the LUMO level should higher in energy than the TiO2 conduction band edge for efficient electron injection; the HOMO should lie below the energy level of I-/I3- redox couple for efficient regeneration of oxidized dyes.

J. Phys. Chem. C, Vol. 115, No. 1, 2011 277 As depicted in Figure 4, the HOMO levels of the dyes MXD5-7 are 1.05, 1.0, and 0.85 V versus NHE, respectively. Inserting a CdC double bond or replacement of EDOT moiety shifts the HOMO level negatively. Noticeable, EDOT moiety has a stronger influence on the HOMO and LUMO level than that of CdC double bond, reflecting the strong electron-donating character of EDOT moiety. Compared to MXD5, narrower gaps between HOMO and LUMO were observed for both MXD6 and MXD7. The HOMO and LUMO levels of L1 are 1.18 and -1.07 V versus NHE, respectively. It can be found that replacement of triphenylamine with bis-hexapropyltruxeneamino narrows the HOMO-LUMO energy gaps and results in red shift of absorption spectra. The HOMO-LUMO levels of MXD5-7 dyes are suitable for DSSCs. The LUMO levels for these dyes (-1.12 to -1.29 eV) are more negative than the conduction band of TiO2 (-0.5 V vs NHE), which provided sufficient driving forces for electron injection. On the other hand, the HOMO levels for these dyes (0.85 to 1.05 eV) are more positive than the iodine redox potential (0.4 V vs NHE).52 Thus, these oxidized dyes can be regenerated from the reduced species in the electrolyte to give an efficient charge separation. Photovoltaic Performance. The incident photon-to-current conversion efficiency (IPCE) of DSSCs based on MXD5-7 and L1 are measured in the visible region (400-800 nm), as shown in Figure 5a. Solar cells based on MXD6 and MXD7 show broad IPCEs in accordance to the broad absorption spectrum achieved by increasing the π-linker conjugation or introduction of EDOT, respectively. The IPCE spectrum for MXD7 exhibits a high plateau from 470 to 550 nm where the incident photon to current conversion efficiency reaches 83% at 500 nm. The IPCE value for MXD5 is as high as 76% but its spectrum is not broad due to spectral limitation. Photovoltaic tests were conducted to evaluate the potential of the MXD5-7 dyes in dye-sensitized solar cells. The J-V curves of the DSSCs sensitized by MXD5-7 and L1 dyes are shown in Figure 5b. The detailed photovoltaic parameters are summarized in Table 2. Under standard global AM 1.5 solar conditions, the MXD7-sensitized cell exhibited better photovoltaic performances than those of MXD5-6; a short-circuit photocurrent density (JSC) of 11.5 mA cm-2, an open-circuit voltage (VOC) of 765 mV, and a fill factor (ff) of 0.68, corresponding to an overall conversion efficiency (η) of 5.98%. The power conversion efficiencies for the MXD5 and MXD6 dyes are 5.23 and 5.02%, respectively. Under the same measuring conditions, the L1-sensitized cell showed an efficiency of 4.55%. The introduction of bis-hexapropyltruxeneamino brought about superior performance over the simple triphenylamine dye

Figure 5. IPCE spectra (a) and current-potential (J-V) curves (b) for DSSCs based on the MXD5-7 and L1 dyes.

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TABLE 2: Photovoltaic Performance and Electron Lifetime of DSSCs Sensitized with MXD5-7 and L1 dye a

MXD5 MXD5b MXD6a MXD6b MXD7a MXD7b L1a

JSC (mA cm-2)

VOC (V)

ff

η (%)

τ (ms)

10.2 10.4 10.3 11.6 11.5 11.8 9.7

754 763 707 710 765 771 690

0.68 0.67 0.69 0.70 0.68 0.68 0.68

5.23 5.32 5.02 5.76 5.98 6.18 4.55

46 19 63 11

a

Photovoltaic performances of DSSCs based on MXD5-7 and L1 with no CDCA. b Photovoltaic performances of DSSCs based on MXD5-7 with 3 mM CDCA.

Influence of TiO2 Surface Blocking, Conduction Band Movement, and Electrolyte-Dye Interaction on the OpenCircuit Voltage for DSSCs based on MXD5-7 and L1. Recently, correlation between the molecular structures of organic dyes on the open-circuit voltage has been actively studied since the open-circuit voltage for DSSCs based on organic dyes is still on the lower side compared to that for ruthenium dyes.54 Generally, influence of dye on VOC has mostly been attributed to electron lifetime, which is related to factors such as molecular size, and dye adsorption behavior. The value of VOC is determined by the potential difference between the Fermi level of TiO2 (EFn) and the chemical potential of the redox species (Ered) in the electrolyte, which could be described as eq 155

VOC ) Ered - ECB - γ

Figure 6. J-V curves of DSSCs sensitized by MXD5-7 containing different concentration of CDCA as coadsorbent under AM 1.5 irradiation (100 mW cm-2).

in terms of not only photocurrent but also open-circuit photovoltage. The somewhat higher JSC value of the DSSCs sensitized by MXD5-7 than that of L1 is attributed to the broader and higher IPCE spectra. The open circuit voltages of the MXD5-7 dyes are much higher than that that of L1 (i.e., VOC of MXD5 reaches 754 mV compared to that of L1, 690 mV) under similar conditions, consequently leading to an increase of power conversion efficiency. Many organic dyes are aggregating on the semiconductor surface via molecular stacking, leading to self-quenching and reducing electron injection into TiO2. Generally, chenodeoxycholic acid (CDCA) is used as coadsorbent to dissociate the π-stacked dye aggregates and improve the electron-injection yield, thus affording higher JSC value.4 The corresponding J-V curves of the DSSCs sensitized by MXD5-7 with and without the addition of CDCA are shown in Figure 6, and photovoltaic data are summarized in Table 2. With CDCA addition (3 mM), the DSSCs based on MXD5-7 afforded an efficiency of 5.32, 5.76, and 6.18%, respectively. The slight increase in power conversion efficiency for MXD5 and MXD7 by the addition of CDCA might indicate that the aggregation of these two dyes on the TiO2 surface is not obvious (the absorption spectra of TiO2 films exposed to the solutions of the dyes containing different concentration of CDCA are shown in Supporting Information, Figure S5, which also suggests that the possible π-stacked aggregation is not obvious). These results are consistent with our expectation. In the case of MXD6, the JSC value increased from 10.3 to 11.6 mA cm-2, leading to a significant enhancement on η value when CDCA was added. One of the possible reasons for the increased JSC of MXD6 in the presence of CDCA is that the charge recombination rate is reduced by the coadsortption of CDCA (see Figure S6 in Supporting Information). The similar results were also observed in triphenylamine dyes containing CdC bond in π spacer.53

( )

kBT Ne ln e n

(1)

where γ is a characteristic constant of TiO2 tailing states, kB is the Boltzmann constant, T is temperature (293 K in this work), e is elementary charge, and Ne is the effective density of states at the TiO2 conduction band edge. Considering that Ered would not change strongly in DSSCs fabricated under similar conditions, VOC is determined by the potential of the conduction band edge (ECB) and the electron density (n) in TiO2. These two parameters are closely related to the surface charge and charge recombination, respectively. Possible factors influencing VOC for DSSCs based on MXD5-7 and L1 dyes involve the TiO2 surface blocking, conduction band movement, and electrolyte-dye interaction, which will be discussed in detail in the following sections. TiO2 Surface Blocking and Interfacial Charge Recombination. Electrochemical impedance spectroscopy (EIS) is a powerful tool for identifying electronic and ionic transport processes in DSSCs, which provides valuable information for the understanding of photovoltaic parameters (JSC, VOC, ff and η).56 Typical EIS Nyquist plots and Bode phase plots for DSSCs based on the MXD5-7 and L1 dyes measured in the dark under a forward bias of -0.7 V are shown in Figure 7a. The equivalent circuit57 presented in Figure 7a was used to fit the experimental data of all of the samples. Rs is the series resistance accounting for the transport resistance of the TCO and the electrolyte. Cµ and RCT are the chemical capacitance and the charge recombination resistance at the TiO2/electrolyte interface, respectively. CPt and RPt are the interfacial capacitance and charge transport resistance at the Pt/electrolyte interface, respectively. The larger semicircle at lower frequencies represents the interfacial charge transfer resistances (RCT) at the TiO2/dye/electrolyte interface. The fitted RCT increases in the order of L1 (52 Ω) < MXD6 (80 Ω) < MXD5 (109 Ω) < MXD7 (276 Ω), which is consistent with the sequence of VOC values in the devices. The smaller RCT value means the electron recombination from the conduction band to the electrolyte occurring more easily, and thus results in lower VOC. Clearly, electron recombination in devices based on L1 and MXD6 is faster than that of MXD5 and MXD7. The results can be cross-checked by measuring the J-V characteristics of the DSSCs in the dark (Figure 7b). The onset potentials of dark current (potential value at the dark current value of -0.1 mA cm-2) for the MXD5-7 and L1 dyes are in the order of L1 (489 mV) < MXD6 (492 mV) < MXD5 (556 mV) < MXD7 (600 mV), the sequence of which is in accordance with that of RCT values. By fitting the EIS curves, another important parameters for DSSCs, electron lifetime (τ), could be extracted from the Cµ and RCT using τ ) CµRCT. The fitted τ increases in the order of L1 (11 ms) < MXD6 (19 ms)
MXD7 > MXD6 > L1, indicating a sequential negative shift of the conduction band edge. Generally, negative shifts of ECB lead to improvement of open-circuit photovoltage of DSSCs. Compared to the results of photovoltaic measurements, the sequence of conduction band edge shifts for MXD5 versus MXD6 versus L1 is in accordance with that of VOC but not for MXD5 versus MXD7. The increased VOC is mainly ascribed to the major increase in RCT (Figure 9b) for MXD7 compared to MXD5. At the forward bias of -0.7 V, the device based on MXD7 shows a decrease of Cµ (from 422 to 227 µF) and an increase of RCT (from 109 to 276 Ω) compared to that of L1, yielding a higher τ of 63 ms. In the range of potentials studied, the order of τ is in good accordance with that for VOC, implying that it is charge recombination, rather than the position of CB, that governs VOC. Possible Interactions between Dye Molecules and Acceptor Species in the Electrolyte. Organic molecules with different dipole moments and polarizability (these parameters were listed in Table 3) can modify the electronic properties of the semiconductors and affect the nature of interaction between dye and acceptor species.65 Furthermore, a better correlation was found between electron lifetimes and polarizabilities than with dipole moments.54 In general, organic dyes with high polarizabilities show strong interaction with surrounding species and induce an increase in the local concentrations of acceptor species. The electron lifetime for MXD6 was significantly

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Figure 9. Chemical capacitance Cµ (a), interfacial charge transfer resistance RCT (b), and electron lifetime τ (c) fitted from impedance spectra under a series of applied potentials.

TABLE 3: Calculated Dipole Moments and Polarizabilities for MXD5-7 and L1 Using B3LYP/6-31G(d) via Gaussian 03 dye

MXD5

MXD6

MXD7

L1

dipole (D) polarizability (Å3)

12.44 165.60

10.78 181.49

8.37 169.64

11.83 61.55

shorter than that for MXD5 and MXD7 (Figure 9c). MXD6 at the TiO2 surface induces an increase in the local concentrations of acceptor species due to its high polarizability (Table 3). The enhancing concentration of acceptor species on the TiO2 surface increases the possibility of acceptor species penetrating the adsorbed dye layer, leading to electron recombination, which results in short electron lifetime. The electron lifetimes observed for MXD5 and MXD7 DSSCs are significantly longer than that of L1 although the two dyes have larger polarizability than L1. These results suggest that dependence of electron lifetime on the concentration of acceptor species will be weak when the surface blocking of dye is compact enough to block the acceptor species approaching the TiO2 surface. Conclusions We have synthesized three functional triarylamine organic dyes, MXD5-7, incorporating the bis-hexapropyltruxeneamino moiety for application in dye-sensitized solar cells. The nonplanar structure of bis-hexapropyltruxeneamino is introduced for suppression dye aggregation and retarded charge recombination. Photovoltaic performance measurements, electrochemical impedance, and theoretical calculation are in support of the molecular design. Among the three dyes studied, a maximum power conversion efficiency of 6.18% was obtained under simulated AM 1.5 solar irradiation (100 mW cm-2) with a DSSCs based on MXD7 (JSC ) 11.8 mA cm-2, VOC ) 772 mV, ff ) 0.68) with the addition of 3 mM CDCA as coadsorbent. The test of stability of DSSCs based on the as-

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