New Triphenylamine-Based Organic Dyes with Different Numbers of

Article pubs.acs.org/JPCC

New Triphenylamine-Based Organic Dyes with Different Numbers of Anchoring Groups for Dye-Sensitized Solar Cells Surya Prakash Singh,*,† M. S. Roy,⊥ K. R. Justin Thomas,∥ S. Balaiah,† K. Bhanuprakash,† and G. D. Sharma*,‡,§ †

Inorganic & Physical Chemistry Division, Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500607, India Physics Department, Molecular Electronic and Optoelectronic Device Laboratory, JNV University, Jodhpur (Rajasthan) 342005, India § R&D Center for Science and Engineering, Jaipur Engineering College, Kukas, Jaipur (Rajasthan), India ⊥ Defence Laboratory, Jodhpur (Rajasthan), India ∥ Organic Materials Lab, Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee247 667, India ‡

S Supporting Information *

ABSTRACT: We synthesized two organic dyes (TPA−CN1−R2 and TPA−CN2−R1) based on the TPA core unit having structure A−D−A, which contain the triphenylamine moiety as an electron donor and both cyanovinylene 4-nitrophenyl and carboxylic (anchoring) units as electron acceptors. Nanocrystalline TiO2-based dye-sensitized solar cells (DSSCs) were fabricated using these dyes to investigate the effect of number of anchoring groups on their photovoltaic performance. The DSSCs based on TPA−CN1−R2 and TPA−CN2−R1 showed power conversion efficiency (PCE) of about 2.36% and 1.41%, respectively. The PCE has been significantly improved up to 4.37% and 2.8%, upon addition of 20 mM deoxycholic acid (DCA) to the dye solution for TiO2 sensitization. Coadsorption of DCA decreased dye coverage but significantly improved the value of the short-circuit photocurrent (Jsc). The breakup of π-stacked aggregates might improve the electron injection yield and thus Jsc. Electrochemical impedance spectra and current−voltage characteristics in the dark indicate that the electron lifetime was improved by coadsorption of DCA, accounting for the significant improvement of open-circuit voltage (Voc).

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The electrolyte, containing a redox couple I−/I3− mediator, is injected into the slit between the photoanode and the catalytic Pt counter electrode. A dye sensitizer needs to have efficient charge injection with photoexcitation. To obtain high power conversion efficiencies, the photogenerated electrons must flow into the oxide film with minimal losses against interfacial recombination. The typical Ru-complex sensitizers such as N3, N719, and black dye have a demonstrated power conversion efficiency (PCE) of up to 11% under AM1.5 irradiation,7 and these sensitizers are grafted onto the semiconductor through anchoring groups, such as carboxylate, which bind strongly to the oxide by coordinating the titanium ions on the surface. Owing to the limitation of rare metals and the difficulty in purifying ruthenium dyes, metal-free organic dyes have been the subject of intensive research.8 In addition, organic dyes have many advantages such as their low cost, easy purification, high molar absorption coefficients, and wide range of possibilities for

INTRODUCTION Dye-sensitized solar cells (DSSCs) have been intensively investigated during the past two decades as a promising alternative to conventional silicon-based solar cells due to their potentially low production cost and high power conversion efficiency.1 A typical DSSC consists of a dye-adsorbed photoanode and a Pt counter electrode filled with an iodide/ tri-iodide I−/I3− redox electrolyte. There are four main factors that affect the power conversion efficiency of DSSCs: the semiconductor anode, counter electrode, electrolyte with redox species, and photosensitive dyes. Much effort has been made toward improving the performance of DSSCs by means of increasing light harvest, improving charge transport, and reducing recombination, which can be implemented via optimizing photosensitizers,2 photoanodes,3 redox electrolytes,4 and counter electrodes.5 In a DSSC device, light is absorbed by the dye anchored on the TiO2 surface, and then electrons from the excited dye are injected into the conduction band of the TiO2, generating an electric current. At the same time, the ground state of the dye is regenerated by the electrolyte to give efficient charge separation.6 Thus, the dye is essential in DSSCs for efficient light harvesting and electron generation/transfer. © 2012 American Chemical Society

Received: November 15, 2011 Revised: January 31, 2012 Published: February 6, 2012 5941

dx.doi.org/10.1021/jp210971u | J. Phys. Chem. C 2012, 116, 5941−5950

The Journal of Physical Chemistry C

Article

Scheme 1. Chemical Structure of TPA-Based TPA−CN2−R1 and TPA−CN1−R2 Dyes

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EXPERIMENTAL DETAILS Characterization Method of Dyes. The IR spectra of dyes were recorded on a Perkin-Elmer 16PC FTIR spectrometer with KBr pellets. 1H NMR (400 MHz) spectra were obtained using a Bruker spectrometer. UV−visible spectra of the dyes in solution and adsorbed on TiO2 were recorded on a Perkin-Elmer UV−visible spectrophotometer. Regents and Solvents. 4-Nitrobenzyl cyanide was synthesized from nitration of benzyl cyanide with concentrated nitric acid and sulfuric acid. N,N-Dimethylformamide (DMF) and tetrahydrofuran (THF) were dried by distillation over CaH2. All other solvents and reagents were commercially purchased and used as supplied. Synthetic Procedure of Dyes. The starting triphenylamine (TPA) was synthesized from phenylamine and indobenzene in a nitrogen atmosphere (Cu catalyst, 115 °C, 3.5 h) according to the given procedure.14 The intermediate compounds 1a, 1b, 2a, 2b, 3a, and 3b (Scheme 2) were prepared as follows: The intermediate compounds 1a and 1b were prepared by treating TPA with different equivalents of POCl3 in DMF, according to given procedures.15 To a 60 mL THF solution including 0.230 g (1 mmol) of MePh3PI and 0.68 g (7.0 mmol) of t-BuoK, 0.78 mg (3.0 mmol) of compound 1a was added and stirred for 24 h at room temperature. After extraction using ether and evaporation of the solvent, the crude product was purified by column chromatography and compound 2 was obtained. Using the same manner of preparation as for compound 1, 0.30 g of yellow solid of compound 3 was obtained. TPA−CN2−R1 and TPA−CN1−R2 Dye. The solution of acetic acid was slowly added to the solution of 3a (0.57 g, 3.22 mmol) and pyridine (3 mL) in ethanol (30 mL) at 0−5 °C. The resultant mixture was stirred for 10 h and then concentrated under reduced pressure. The precipitate was filtered, washed, and dried to afford 4a. It was purified by column chromatography (dichloromethane:hexane 1:1). A flask was charged with a solution of 4a (0.30 g, 0.80 mmol) and 4-nitrobenzenyl cyanide (0.30 g, 1.78 mmol) in anhydrous ethanol (20 mL). Sodium hydroxide (0.20 g, 5.0 mmol) dissolved in ethanol (10 mL) was added to this solution. The reaction was stirred for 1 h at room temperature under N2 and then was concentrated under reduced pressure. The concentrate was cooled in a refrigerator to precipitate the solid. It was filtered, washed thoroughly with water, and dried to afford TPA−CN2−R1 and TPA-CN1-R2 dye. FTIR (KBr, cm−1): 2165 (cyano), 1580 and 1341 (nitro), 1312 (C−N stretching of TPA), 1584, 1504 (aromatic), 963

molecular design, and PCEs of about 10% have been achieved for DSSCs based on these dyes.9 In particular, the metal-free organic dyes having the electron donor −π-bridge−electron acceptor (D−π−A) structure are considered to be one of the most promising types of organic dye.10 Among donor groups, triphenylamine (TPA) and its derivatives have been widely used as sensitizers and displayed promising properties in the development of DSSCs, because the TPA unit suppresses the aggregation of the dye due to its nonplanar structure slightly but not completely as compared to planar structure.11 Also the carboxyl acid, cyanoacrylic acid, or rhodanine-3 acetic acid moiety is normally used as electron acceptor group for the attachment of the dye on the TiO2 surface12 because the carboxyl group can form an ester linkage with the TiO2 surface, thereby providing strong binding and good electron communication between the dye and the surface. Recently, a novel organic sensitizer, incorporating a hexyloxyl-disubstituted triphenylamine unit as the electron donor and dithiophene moiety as the conjugated spacer, yielded very high incident monochromatic photon-to-current conversion efficiency and remarkable stability.13 One of the strategies to obtain high PCE for organic DSSCs is to modify organic dyes in a systematic fashion by varying key components or substituents, because small structural changes of dyes may result in significant difference in the interfacial recombination and absorption behavior of the dyes on TiO2 surfaces. In this study we report the characterization of two new organic dyes which use one triphenylamine as donor, two cyanovinylene 4-nitrophenyls and one carboxylic acid as electron acceptor (TPA−CN2−R1) and one triphenylamine as donor, one cyanovinylene 4-nitrophenyl and two carboxylic acid units as electron acceptors (TPA−CN1−R2) and are used as sensitizers for the DSSCs. The chemical structure of these two dyes is shown in Scheme 1. The difference of the photovoltaic performance based on these dyes has been analyzed by UV−vis absorption, cyclic voltammogram, and electrochemical impedance spectroscopy (EIS). We showed that by incorporating two anchoring COOH groups into the triphenylamine-based dyes the efficiency can be improved significantly. The overall PCE of the DSSCs based on TPA− CN2−R1 and TPA−CN1−R2 dyes is 2.36% and 1.41%, respectively. The PCE was remarkably increased up to 4.37% (TPA−CN1−R2) and 2.89% (TPA−CN2−R1) when 20 mM DCA was added to the dye solution during the sensitization process. To understand the role of DCA in the significant enhancement of DSSC performance, the effect of coadsorption on the dye adsorption behavior, electron injection efficiency, and electron lifetime in DSSCs was studied. 5942



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Scheme 2. Synthesis Process of Triphenylamine-Based TPA−CN2−R1 and TPA−CN1−R2 Dyes

Fabrication of Dye-Sensitized Solar Cells. TiO2 paste was prepared by mixing 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 was obtained (TiO2). The whole procedure is slow under vigorous stirring. The mixture was stirred vigorously for 2−4 h

(trans vinylene bond), 1013 (cyanovinylene), 1694 (carboxylic CO), 1290 (C−O stretching and O−H deformation of carboxyl). 1H NMR (CdCl3, ppm): 8.19 (m, 8H, phenylene ortho to nitro), 7.83 (s, 4H, cyanovinylene), 7.10 (m, 4H, vinylene), 12.00 broad, (1H carboxyl), 7.65 (d, 2H, phenyl ortho to carboxyl). 5943



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at room temperature and then stirred for 4 h at 100 °C to form a transparent colloidal paste. The TiO2 paste was deposited on the F-doped tin oxide (FTO)-coated glass substrates by the doctor blade technique. The TiO2-coated FTO films were sintered at 450 °C for 30 min. After cooling to room temperature, the electrodes were impregnated in 0.05 M titanium tetrachloride aqueous solution, washed with distilled water, and again sintered at 450 °C for 30 min followed by cooling to room temperature. The thickness of the TiO2 layer is about 12 μm. The dyes were dissolved in THF solution (0.5 mM), and TiO2 electrodes were immersed into the dye solution and kept at room temperature for 24 h and then washed. We have varied the concentration of DCA coadsorbant from 10 to 30 mM. A thin Pt layer was deposited on a FTO conducting glass by thermal pyrolysis of H2PtCl4 in isopropyl alcohol solution and then heated at 450 °C for 30 min in air. One drop of electrolyte solution (0.6 M 1-propyl-2,3 dimethylimidazolium iodide (DMPImI), 0.1 M LiI, 0.05 M iodine, and 0.5 M tetrabutyl pyridine in acetonitrile) was deposited onto the surface of dyesensitized TiO2 electrode and penetrated inside the TiO2 via capillary action. The Pt-coated FTO electrode was then clipped onto the top of the TiO2 working electrode to form the complete DSSC. The active area of each DSSC thus prepared was about 0.4 cm2. Characterizations. The UV−vis absorption spectra of the dye-loaded TiO2 film was recorded on a Shimdzu UV−visible spectrophotometer. The cyclic voltammograms were measured with a three-electrode electrochemical cell on a potentiostat/ galvanostat PGSTAT30 electrochemical analyzer. The dyeloaded TiO2 film, platinum, and Ag/Ag+ (0.01 M AgNO3 + 0.1 MTBAP in acetonitrile) were employed as working, counter, and reference electrodes, respectively. The supporting electrolyte was 0.1 M LiClO4 in acetonitrile. The potential of the reference electrode is 0.49 V versus normal hydrogen electrode (NHE) and is calibrated with ferrocene immediately after cyclic voltammogram measurement. Electrochemical impedance spectra for DSSCs in dark and under illumination were measured with an electrochemical analyzer, equipped with FRA software. The spectra were scanned in the frequency range of 0.01−105 Hz at room temperature with applied potential set at open-circuit voltage. The magnitude of the modulation signal was set at 20 mV. Photovoltaic Measurements. The current−voltage characteristics of DSSCs were measured on a computer-controlled source meter (Keithley 2400). An AM 1.5 solar simulator with 150 W xenon lamp and an AM filter was used as the light source. Light intensity corresponding to AM1.5 (100 mW/ cm2) was calibrated using a standard silicon solar cell. Monochromatic incident photon-to-current conversion efficiency (IPCE) for the solar cell, plotted as a function of excitation wavelength, was recorded on a monochromator system. The output current under short-circuit condition at each wavelength was measured using the Keithley electrometer. Computational Method. The molecular geometries have been optimized using Gaussian 09 software at the RB3LYP/631G(d,p) level.16 The atomic positions of the molecules in all possible geometrical conformations were fully relaxed, and only the lowest-lying minima are reported. The minimization is carried using the density functional theory at the B3LYP level using the Berny optimization algorithm with a default integration grid. The B3LYP functional consists of Becke’s three-parameter hybrid exchange functional combined with the

Lee−Yang−Parr correlation functional. The theoretical equilibrium structures were obtained when the maximum internal forces acting on all the atoms and the stress were less than 4.5 × 10−4 eV/Å and 1.01 × 10−3 kBar, respectively. Frequency calculations are carried out to ensure that each optimized conformation has all positive frequencies and thus is a minimum on the potential energy surface. TD-DFT (timedependent density functional theory) studies have been carried out at the RB3LYP/6-311+G(d,p) level to estimate the lowest 25 singlet−singlet transitions in THF solvent media using the PCM solvation model. The absorption spectra were simulated by using the 25 lowest spin-allowed singlet transitions using GaussSum software.17

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RESULTS AND DISCUSSION Figure 1a shows the normalized UV−vis absorption spectra of the TPA−CN1−R2 and TPA−CN2−R1 dyes measured in 0.1

Figure 1. Optical absorption spectra of dyes (a) in THF solution and (b) adsorbed on TiO2 film.

mM THF solution. The absorption spectrum of both dyes displays two distinct absorption peaks in shorter wavelength (440−460) and longer wavelength regions (540−580 nm), respectively. The absorption peak in the shorter wavelength region corresponds to π−π* electron transition. The strong absorption peak in the longer wavelength region can be assigned to the intramolecular charge transfer (ICT) transition from the triphenylamine moiety (donor) to cyanovinylene 4nitrophenyl moiety (acceptor). Under the same conditions, the TPA−CN1−R2 dye exhibits an absorption peak at 576 nm that is red-shifted by 20 nm compared to that for TPA−CN2−R1 (absorption peak at 556 nm) dye, which may due to the conjugation length in the molecule or molecular strain. In the case of TPA−CN2−R1 having two (−CN and −NO2) functional groups, this is a strong electron-withdrawing group. However, TPA−CN1−R2 consists of only one (−CN and 5944



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NO2) functional group which has less electron-withdrawing capacity, and hence red shift is observed. Figure 1b shows the absorption spectrum of the two dyes adsorbed on a transparent TiO2 film. Compared to the spectrum in THF solution, a red shift and broadening of the absorption peak was observed in both the dyes on TiO2 surface, which can be attributed to the formation of J-aggregate.11b,18 Such spectral broadening allows the dye molecules to harvest visible light more efficiently. As shown in Figure 1b, the absorption spectra of TPA−CN1−R2 dye on the TiO2 surface is more broadened and red-shifted in comparison to that for TPA−CN2−R1 dye. To understand the nature of the optical transitions, results obtained by DFT and TDDFT methods are analyzed. Geometrical parameters of the molecules optimized at the RB3LYP/6-31 G(d,p) level are shown in Table 2. As expected

Figure 2. Simulated absorption spectra of TPA−CN1−R2 and TPA− CN2−R1 in THF phase obtained at the TD-B3LYP/6-311+G(d,p)// 6-31G(d,p) level of theory.

Table 1. Experimental Electrochemical Data of Dyes dye

Eo−o(eV)a

Eox (V) vs NHEb

Ered (V) vs NHEc

Egap(inj) (V)d

Egap(reg) (V)e

TPA− CN1−R2 TPA− CN2−R1

2.05

1.05

−1.00

0.50

0.65

1.94

0.99

−0.95

0.46

0.59

a

Eo−o was determined from the absorption on a set of the optical absorption. bRedox oxidation potential corresponds to HOMO energy level of the dyes in TiO2 measured from cyclic voltammetry. cRedox reduction potential corresponds to LUMO level of the dyes calculated from Eox − Eo−o. dEgap(inj) is the energy gap between the excited state of dyes and the conduction band of TiO2 (−0.5 V vs NHE). eEgap(reg) is the energy gap between the ground state of dye and the redox potential of I3−/I− in the electrolyte (0.4 V vs NHE).

the molecules are not planar but slightly twisted with respect to the central plane. This twisting could be dependent on the solvent, and thus charge transfer would be angle dependent. The stimulated absorption spectra of both the dyes are shown in Figure 2. The solvent phase (THF) TDDFT spectrum closely matches with the experimental observation. The frontier molecular orbitals of the molecules are shown in Figure 3. The computed vertical excitations, their oscillator strengths, and molecular orbital (MO) compositions of vertical transitions for these dyes are listed in Table 2. The singlet singlet transition is dominated in both molecules by charge transfer from the donor moieties to the acceptor moieties.

Figure 3. Frontier molecular orbitals (HOMO and LUMO) of TPA− CN1−R2 and TPA−CN2−R1 along with the orbital energies obtained at B3LYP/6-311+G(d,p)//6-31G(d,p).

Table 2. Computed Vertical Transitions and Their Orbital Contributions Along with the Oscillator Strength and Dipole Moments of TPA−CN1−R2 and TPA−CN2−R1 at the B3LYP/6-311+G(d,p)//6-31G(d,p) Level mol.

λmax (nm)

CI coefficients of major excitations

oscillator strength

dipole moment (D)

TPA−CN1−R2

574 447

HOMO → LUMO (0.70) HOMO → LUMO+1 (0.54) HOMO → LUMO+2 (0.45) HOMO → LUMO+2 (0.53) HOMO → LUMO+1 (−0.44) HOMO−1 → LUMO (0.62) HOMO → LUMO+3 (0.61) HOMO → LUMO (0.70) HOMO → LUMO+1 (0.70) HOMO → LUMO+2 (0.60) HOMO → LUMO+3 (0.60) HOMO−1→ LUMO (0.65)

0.8254 0.7351

8.0

438

TPA−CN2−R1

382 370 588 542 437 430 390

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0.4170 0.2733 0.0667 1.0984 0.4004 0.4861 0.2881 0.2104

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Based on the Tau relation,19 the optical energy band gap Eg can be obtained by plotting (αhν)2 vs hν and extrapolating the linear portion of (αhν)2 to zero. The lower optical energy band gap of TPA−CN1−R2 (1.68 eV) as compared to TPA− CN2−R1 (1.74 eV) indicates the red shift in the optical absorption band of TPA−CN1−R2. The TPA−CN1−R2 dye consists of one (−CN and NO2) functional group and two anchoring COOH groups, adsorbed on the TiO2 surface, resulting in more efficient excitation of electrons and a lower optical energy band gap. To evaluate the possibility of electron injection occurring from the excited state of dye molecules to the conduction band of TiO2, the electrochemical oxidation and reduction potentials of the dyes were measured by cyclic voltammetry (CV) using a three-electrode cell and an electrochemical impedance analyzer. The working electrode was a TiO2 film with the adsorbed dyes; the counter electrode was a Pt wire electrode. The supporting electrolyte was 0.1 M LiClO4 in dry acetonitrile, and an Ag/Ag+ electrode was used as reference electrode. The experimental electrochemical data are complied in Table 1. The oxidation potentials of the ground state of TPA−CN2−R1 and TPA− CN1−R2 dyes, which correspond to their HOMO levels, are 0.99 and 1.05 V vs NHE, respectively. These values are sufficiently more positive than the I−/I3− redox potential (0.4 V vs NHE). These results indicate that the oxidized dyes formed after the injection of electrons into the conduction band of TiO2 could accept the electrons from I− ions thermodynamically. The reduction potentials of TPA−CN2−R1 and TPA− CN1−R2 dyes, which correspond to their LUMO levels, are −0.90 and −1.05 V vs NHE, respectively, which are more negative that the TiO2 conduction band edge (−0.5 V vs NHE). The energy gaps between the LUMO level of dye and the conduction band level of TiO2 are 0. 40 and 0.46 V vs NHE, for TPA−CN2−R1 and TPA−CN1−R2 dyes, respectively. Since an energy gap of around 0.2 V is necessary for efficient electron injection,20 the energy gap values of the dyes being above this value indicates that the excited electrons of the dye molecule can be injected into the conduction band of TiO2 thermodynamically. Figure 4 shows the incident monochromatic photon-tocurrent conversion efficiency (IPCE) for the DSSCs based on TPA−CN2−R1 and TPA−CN1−R2. The IPCE for the TPA− CN2−R1 dye exhibits a maximum peak of about 42% at 630 nm, and that of the TPA−CN1−R2 dye is about 63% at 650 nm. The IPCE spectrum of the TPA−CN1−R2 dye is red-

shifted by 20 nm compared to that of TPA−CN2−R1 as a result of the increase in the number of COOH acceptor groups in the dye molecules, which is consistent with the absorption spectra of the two dyes. The decrease of the IPCE values above 700 nm toward the longer wavelength region is due to the decrease of light harvesting of the dyes. Figure 5a and 5b shows the current−voltage characteristics of DSSCs based on TPA−CN1−R2 and TPA−CN2−R1 dyes,

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Figure 5. Current voltage characteristics of DSSCs under illumination intensity 100 mW/cm2 with (a) TPA−CN2−R1 and (b) TPA− CN1−R2 with and without CDA coadsorbant.

Table 3. Photovoltaic Performance of DSSCs Based on TPA−CN1−R2 and TPA−CN2−R1 Dyes

a

dye

Jsc(mA/cm2)

Voc (V)

FF

PCE (%)

TPA−CN1−R2 TPA−CN1−R2a TPA−CN2−R1 TPA−CN2−R1a

6.9 9.2 5.1 7.4

0.61 0.66 0.53 0.61

0.56 0.72 0.52 0.64

2.36 4.37 1.41 2.89

With DCA as coadsorbent in the dye solution.

under simulated 100 mW/cm2 irradiation. Table 3 summaries the short-circuit photocurrent density (Jsc), open-circuit photovoltaic (Voc), fill factor (FF), and overall power conversion efficiency (PCE). The TPA−CN1−R2 based DSSC gave a Jsc of 6.9 mA/cm2, Voc of 0.61 V, and FF of 0.56, corresponding to a PCE of 2.36%. The values of both Jsc and Voc of the DSSCs-based TPA−CN2−R1 dye are lower than that for TPA−CN2−R1, resulting in an overall PCE of about 1.41%. To clarify the difference in the photovoltaic performance of the DSSCs, we have measured the amount of dye adsorbed on TiO2 film. The adsorbed dye amount of TPA−CN1−R2 and TPA−CN2−R1 on the TiO2 surface is

Figure 4. Spectra of incident photon-to-current conversion efficiency (IPCE) for DSSCs based on TPA−CN2−R1, TPA−CN1−R2 dyes and CDA coadsorbant TPA−CN1−R2. 5946



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2.8 × 10−7 mol/cm2 and 2.5 × 10−7 mol/cm2, respectively; this indicates that the influence of dye density on the photovoltaic performance of DSSCs is insignificant. The incorporation of a second anchoring group as an extra acceptor seems to improve the PCE of the DSSC, based on TPA−CN1−R2.8f Thus we assume that the extra anchoring group may enhance the injection electrons from excited dye molecules into the conduction band of TiO2, resulting in the suppression of the dark current and diminishing the approach that there are not only protons but also I3− ions (and maybe Li+) on the TiO2 surface.21 The adsorbed dye density is the same order in different DSSCs; therefore, dye aggregation alone cannot account for the low PCE of the TPA−CN2−R1-based DSSC. Sensitizer with two anchoring groups, upon adsorption, transfers more protons to the TiO2 surface leading to the positive shift of the conduction band edge of TiO2. Thus the sensitizer with two anchoring groups possesses a larger gap between the LUMO level of the sensitizer and the conduction band level of TiO2, leading to more efficient electron injection into TiO2 photoelectrode and thus increase in the Jsc.8c It is observed that the TPA−CN1−R2-based DSSC is more efficient than the TPA−CN2−R1-based DSSC, which is attributed to the higher values of Jsc, Voc, and FF. To clarify the above results, the kinetic parameters of photoinjected electrons within the oxidized dye in the DSSCs were measured by using electrochemical impedance spectroscopy (EIS).22 In these DSSCs, the electrons are transported through the mesoporous TiO2 network and are reacted with I3−. At the same time, I− is oxidized to I3− at the counter electrode. Figure 6a shows the Nyquist plots of DSSCs sensitized with different dyes. Three semicircles were observed in the Nyquist plots. The middle circle in the Nyquist plots indicates the charge transfer resistance at the TiO2/dye/electrolyte interface. These plots show that the radius of the middle frequency circle is higher for

the DSSC based on TPA−CN1−R2 as compared to that for the TPA−CN2−R1-based DSSC, indicating that the electron recombination resistance is high for the TPA−CN1−R2-based DSSC. The electron lifetime values derived from the curve fitting are 16.4 and 12.2 ms for TPA−CN1−R2- and TPA− CN2−R1-based DSSC, respectively. The Bode plots shown in Figure 6b also support the difference in the electron lifetime for TiO2 films, injected from the excited state of LUMO of the dyes. The middle frequency peaks of the DSSCs based on TPA−CN1−R2 shift to lower frequency relative to that of TPA−CN2−R1, indicating a shorter recombination lifetime for later dye-based DSSC. The peak in the intermediate frequency region is mainly attributed to the charge transfer at the TiO2/electrolyte interface.22 The increase in the recombination lifetime in the TiO2 film is associated with a pronounced rise in the charge transfer, implying that the additional anchoring group decreases the interfacial rate constant for electron capture by the I3− ions. In addition, at the high-frequency region, the peak maximum is at a slightly lower frequency for TPA−CN1−R2-based DSSC than TPA−CN2−R1, which corresponds to the slower charge transfer at the counter electrode. In order to gain more insight into the electron transport in the DSSCs based on these TPA-based dyes, the EIS of DSSCs were also measured under illumination. Modeling and fitting the EIS data of DSSCs measured at an open-circuit voltage under illumination makes it possible to deduce many electron transport parameters in the photoanode.23 Some of these parameters include the lifetime of electron in photoanode, the effective rate constant for recombination (keff), charge transport resistance in the photoanode (Rw), charge transfer resistance related to the recombination of an electron at interface (Rk), and the steady state electron density (ns). Moreover, the diffusion of an electron to the photoanode (Deff) of a DSSC can be estimated using the following equation20 Deff = (R k /R w )(L2 /τ)

where L is the thickness of the TiO2 photoanode and τ is the electron lifetime. In general, Deff is the key parameter that determines the PCE of DSSC.24

Figure 7. Nyquist plots of EIS for DSSCs sensitized with TPA−CN2− R1 and TPA−CN1−R2, under illumination under forward bias of 0.65 V.

The Nyquist plots for the DSSCs based on both the dyes under illumination are shown in Figure 7. As shown in Figure 7, the radius of the semicircle in the intermediate-frequency regime, according to the Nyquist plot, is in the order TPA− CN2−R1 > TPA−CN1−R2. These results indicates that the charge transport is in the order TPA−CN1−R2 > TPA−

Figure 6. EIS spectra (a) Nyquist plots and (b) Bode plots for DSSCs sensitized with TPA−CN2−R1, TPA−CN1−R2, and DCA coadsorbant TPA−CN1−R2, in the dark. 5947



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the dye and DCA compete for sites on the TiO2 surface in an equilibrium process.27 We conclude that LHE has been reduced slightly upon DCA coadsorption. Therefore, the IPCE improvement is attributed to the enhancements of electron injection yield and/or the charge collection yield. The above photovoltaic results indicate that the coadsorption of DCA is effective to improve the PCE of the DSSC. Because adsorption of DCA leaves protons on the TiO2 surface and hence charges the surface positively, the conduction band edge should be positively shifted by the coadsorption of DCA. The positive shift of CB enlarges the driving force for electron injection, which results in the enhancement in IPCE and Jsc.28 In addition, coadsorption can break up dye aggregation, and the nonaggregated dye molecules are favorable for electron injection. Since Voc is theoretically the difference between the Fermi energy level of TiO2 under illumination and the redox potential of the redox couple I3−/I−, the positive shift of conduction band edge upon adsorption of DCA will result in a decrease in Voc. However, we observed an increase in Voc after coadsorption of DCA in the present study. This enhancement can be attributed to the suppression of charge recombination between injected electron and I3− ions on the TiO2 surface by DCA coadsorption as evidenced by the dark current and EIS measurements. To understand the role of DCA in enhancing the Voc of DSSCs, the effect of DCA on dark current and EIS was studied. The J−V characteristics of DSSCs with and without DCA in TPA− CN1−R2 solution are shown in Figure 8. It can be seen from

CN2−R1, which coincides with the PCE of the DSSCs. The electron transport parameters, estimated form the EIS spectra, using a equivalent circuit based on a diffusion−recombination model,23,25 of the DSSCs with these two dyes were estimated. It is observed that in TPA−CN1−R2-based DSSC, the smaller values of keff and Rw collaborate with the higher values of Deff, ns, and τ which is consistent with the PCE value of TPA− CN1−R2-based DSSC. The value of PCE for the DSSCs based on these dyes is low, due to the dye aggregation at the TiO2 surface, resulting in low Jsc. When the dye is adsorbed onto the TiO2 surface, π−π stacking of organic dye molecules usually occurs because of the strong intermolecular interaction. Although π−π stacking is advantageous to the light harvesting because of its broad feature in the UV−visible absorption spectrum, π-stacked aggregate usually has insufficient injection of electrons from the excited state of dye into the conduction band of TiO 2 and recombination of electrons with oxidized dye molecules. Moreover, it is possible for the free electrons on the TiO2 surface to recombine with acceptors such as I3− ions. Such charge recombination leads to losses in both Jsc and Voc, resulting in the decrease in the PCE of DSSCs.26 Prohibition of π−π stacking with additive in the dye solution is a typical way to improve the PCE of DSSCs, suffering from the π−π stacking problem, and to improve the photovoltaic performance of DSSCs. We have investigated the effect of deoxycholic acid (DCA) coadsorption on the photovoltaic performance of DSSCs based on TPA-based dyes. We have varied the concentration of DCA in the dye solution and found that the optimum concentration is 20 mM. The current−voltage characteristics of the DSSCs based on these dyes with 20 mM DCA coadsorbent are also shown in Figure 5a and 5b, and the photovoltaic parameters are summarized in Table 3. DSSCs fabricated with DCA generally showed improved performance parameters (Jsc, Voc, and FF), resulting in enhanced overall PCE, i.e., 4.37% and 2.89% for TPA−CN1−R2 and TPA−CN2−R1 with DCA coadsorbent, respectively. This could be due to increased competition for adsorption on the TiO2 surface between dye and the DCA molecules when DCA is incorporated in dye solution. Incident photon-to-electron conversion efficiency (IPCE) as a function of wavelength is measured to evaluate the photoresponse of the photoelectrode. Figure 4 also compares the IPCE spectra for the DSSCs based on TPA−CN1−R2 and DCA coadsorbant TPA−CN1−R2. It can be seen from this figure that upon the addition of DCA in the solution IPCE was enhanced significantly. The maximum IPCE was improved from 64% to 75% when 20 mM DCA was included in the dye solution. Because dye can inject electrons to TiO2 more efficiently than aggregate,22a the dye/DCA layer obtained from the exposure of TiO2 to dye solution with DCA may contain a mixture of aggregate and dye. PCIPCE is a product of light-harvesting efficiency (LHE), the electron injection efficiency (ϕinj), and charge collection efficiency (ϕc) as illustrated in the following equation

Figure 8. Current −voltage characteristics of DSSCs based on TPA− CN1−R2 dye with and without DCA coadsorbant.

these characteristics that dark current onset potential shifted to a larger value, and dark current was also reduced upon coadsorption of DCA. This dark current change indicates that coadsorption of DCA leads to suppression of charge recombination between injection electrons and I3− ions in the electrolyte, which favors the increase in Voc. The enhancement of Voc is usually associated with the negative shift of conduction band edge or suppression of charge recombination. Consequently, suppression of charge recombination may compensate for the Voc loss because of proton exchange from DCA to the TiO2 surface, resulting in Voc improvement. To further understand the Voc enhancement upon the coadsorption with DCA, EIS spectra (Nyquist plots) were recorded in the dark and are shown in Figure 6a. We have observed that the radius of the semicircle in the intermediate frequency region, which represents the electron transfer at the TiO2/dye/electrolyte interface, increases upon the coadsorp-

IPCE(λ) = LHE(λ)x ϕinjx ϕc

We have recorded the UV−vis absorption spectra of TPA− CN1−R2 adsorbed on TiO 2 with and without DCA coadsorption. We found that when the DCA is added in dyeimmersing solution, the maximum absorbance decreases. The dye amount on the TiO2 surface reduces, which indicates that 5948



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tion of DCA. A larger radius of this semicircle in the intermediate frequency regime implies a lower rate of charge recombination between the injected electrons and I3− ions in the redox electrolyte. This indicates effective suppression of the back reaction of injected electrons with I3− ions. The EIS bode plot exhibits two Bode frequency peaks (Figure 6b) for electron transfer at the TiO2/dye/electrolyte interface and redox charge transfer at the counter electrode in increasing order. The peak frequency of the lower frequency region is related to the charge recombination rate, and the reciprocal is regarded as electron lifetime. It is observed that the middle-frequency peak shifts to a smaller value upon the addition of DCA in dye solution, indicating an increase in electron lifetime, upon DCA coadsorption. We assume that the addition of DCA, forming an insulting spacer, blocks the electron recombination to the I3− ions and thus enhances the electron lifetime.

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CONCLUSIONS In this paper, we have successfully designed two metal-free dyes (TPA−CN1−R2 and TPA−CN2−R1) that contain donors with triphenylamine and cyanovinylene 4-nitrophenyls and carboxylic acid acceptors and are used as sensitizers for nanocrystalline TiO2 DSSCs. We measured their photovoltaic properties to confirm the effects of number of anchoring units (single or double) on the performance of the DSSCs. The overall PCE of the DSSCs based on TPA−CN1−R2 and TPA−CN2−R1 is 2.36% and 1.41%. Although the amount of dyes adsorbed on the TiO2 surface is almost the same, the PCE of DSSC based on TPA−CN1−R2 showed higher Jsc, Voc, and PCE values due to its superior electron injection efficiency and reduced recombination rate. In addition, increasing the anchoring groups of the dye could lead to a longer blocking layer to avoid the charge recombination. We have observed that although the coadsorption of DCA reduced the dye loading on the TiO2 surface both the Jsc and Voc have been improved. The breakup of π-stacked aggregates might improve the electron injection yield and thus Jsc. The improvement of Voc is attributed to the suppressed charge recombination, revealed by the increase in electron lifetime. The overall PCE of the DSSCs based on DCA coadsorbent TPA−CN1−R2 and TPA−CN2− R1 is 4.47% and 2.89%, respectively.

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ASSOCIATED CONTENT

* Supporting Information S

Images of optimized geometries of TPA−CN1−R2 and TPA− CN2−R1 molecules at B3LYP/6-31 g(d,p.). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.P.S.); [email protected] (G.D.S.). Notes

The authors declare no competing financial interest.

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REFERENCES

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