Effect of Molecular Structure Perturbations on the Performance of the

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Effect of Molecular Structure Perturbations on the Performance of the D−A−π−A Dye Sensitized Solar Cells Xiongwu Kang,§,† Junxiang Zhang,§,‡ Daniel O’Neil,† Anthony J. Rojas,‡ Wayne Chen,‡ Paul Szymanski,† Seth R. Marder,*,‡ and Mostafa A. El-Sayed*,† †

Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States ‡ School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States S Supporting Information *

ABSTRACT: Three D−A−π−A organic dyes based on 5,6difluoro-2,1,3-benzothiadiazole (DFBTD) were synthesized by sequential direct arylation and characterized by spectroscopic and electrochemical techniques. Compared to 2,1,3-benzothiadiazole (BTD) analogue, the presence of two fluorine atoms on DFBTD results not only in a significant increase in the molar absorptivity but also in a blue shift of the onset of the absorption spectra. In the system of DFBTD-based sensitizers, replacing the thienyl unit bridge with disubstituted cyclopenta[1,2-b:5,4-b′]dithiophene (CPDT) further increases molar absorptivity and decreases the oxidation potential of the excited state E(s+/s*). Similarly, changing the indoline donor to 4-butoxy-N-(4-butoxyphenyl)-N-phenylaniline increases the optical band gap and decreases the oxidation potential of the excited state E(s+/s*) of the sensitizer. The correlation between the molecular structure of these sensitizers and the photovoltaic performance of the dye cells were examined using current− voltage scan, incident photon to electron conversion efficiency (IPCE), femtosecond transient absorption spectroscopy (TAS), and electrochemical impedance spectroscopy (EIS). The observed change of the photovoltage and photocurrent upon changing the molecular structure of the sensitizers are discussed in terms of the charge injection from the excited state to the interband gap states of TiO2 and the charge recombination between injected electrons into TiO2 film and electrolyte. In all, it is found that the most efficient D−A−π−A dye achieved an open circuit voltage of 0.717 V, short circuit current density of 18.8 mA/cm2, corresponding to an overall power conversion efficiency of 9.1%.

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bridge to construct YA422.7 This sensitizer demonstrates PCE of 7.28% and 9.18% with standard I3−/I− electrolyte and [Co(bpy)32+/3+ electrolyte. Inspired by previous work4,5 reported by Tian and coworkers, here, we report a series of novel D−A−π−A sensitizers using 5,6-difluoro-2,1,3-benzothiadiazole (DFBTD), as an effective electron withdrawing unit, which is analogous to, but a stronger acceptor than 2,1,3-benzothiadiazole, BTD, used in WS-25 and WS-9.6 The synthesis of these D−A−π−A sensitizers has been accomplished with moderate overall yield by sequential direct coupling8 of DFBTDs with different aryl bromides. We chose WS-2, a BTD-based high PCE sensitizer as a reference dye, made a DFBTD analogue (JZ117), to explore the impact of fluorines on photophysical and electronic properties. Moreover, we envisioned the incorporation of di-n-hexyl-substituted cyclopentadithiophene

INTRODUCTION Since the report of polypyridyl ruthenium(II) complex dyes in 1991,1 various types of dyes have been explored in the pursuit of advanced device performance.2,3 Among them, two broad types of dyes, metal-polypyridyl complexes and organic (metalfree) dyes received much interest. Organic dyes typically contain an electron-rich (donor) and electron-poor (acceptor) sections connected through a conjugated bridge. Recently, a variation on this design involving the use of a so-called “D−A−π−A” concept was proposed for designing organic sensitizers, in which an auxiliary electron-withdrawing unit was incorporated into the conjugated bridge to facilitate electron transfer from the donor to the acceptor.4 These type of D−A−π−A dyes are being actively explored and high solar to electric power conversion efficiencies (PCE) were obtained from several auxiliary π-bridge acceptors. For example, Tian and Zhu reported a series of 2,1,3-benzothiadiazole (BTD)based sensitizers, among which dyes WS-25 and WS-96 show 8.7% and 9.0% PCE, respectively. Very recently, it was reported that the substituted quinoxaline was used as a π-conjugating © 2014 American Chemical Society

Received: May 7, 2014 Revised: July 19, 2014 Published: July 21, 2014 4486

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Scheme 1. Molecular Structures of WS-2, JZ117, JZ145 and AR-II-13

Figure 1. UV−vis absorption spectra of WS-2, JZ117, JZ145, and AR-II-13 were measured in (a) CHCl3/methanol solution; the inset in panel (a) is zoom-in of absorption spectra from 650 to 740 nm; (b) on TiO2 film; (c) integrated area under the absorption curves of WS-2, JZ117, JZ145, and AR-II-13 in chloroform/methanol mixed solution (v/v = 4/1).

(CPDT) bridge in place of thiophene might disrupt undesirable dye aggregation, and the alkyl groups might decrease the interaction of the electrolyte and the titania interface and significantly contribute to the enhancement of photovoltage, photocurrent, and eventually the PCE. Upon this strategy, one

dye containing the CPDT group, JZ145 demonstrates the best photovoltaic performance among the four dyes, with PCE of 9.1% and VOC of 717 mV. We also, explore the use of 4-butoxyN-(4-butoxyphenyl)-N-phenylaniline the donor group for AR-II-13 (Scheme 1). The alkyl groups introduced on such 4487

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Table 1. Photophysical and Electrochemical Properties of Sensitizers WS-2, JZ117, JZ145, and AR-II-13 absorption

emission

dye

λmax/nma

ε [M−1 cm−1]

λonset/nmb

λmax/nma

Es+/s [V]c

E0−0 [eV]b

Es+/s* [V]d

ΔGei0(eV)e

θ1f (deg)

θ2g (deg)

WS-2 JZ117 JZ145 AR-II-13

551 538 549 540

16,400 24,100 55,800 40,300

661 639 635 611

774 760 741 721

0.98 1.07 1.03 0.99

1.88 1.94 1.95 1.99

−0.90 −0.87 −0.92 −1.00

0.40 0.37 0.42 0.50

33.27 33.97 35.94 35.09

1.78 0.07 1.50 0.28

a

In CHCl3/CH3OH = 4/1. bMeasured at the intersection of normalized absorption and emission. cMeasured in CH2Cl2; using ferrocene as internal standard (+0.69 V vs NHE). dCalculated according to the following formula: Es+/s* = Es+/s − E0−0. eΔGei0(eV) = Es+/s* − ECBE/TiO2 (conduction band edge of TiO2). fThe dihedral angle between the phenyl of donor and BT or DFBT core. gThe dihedral angle of BT or DFBT core with the other bridge. The angle was measured after DFT geometry optimization at B3LYP/6-31* level.

yields (42% and 54% in literature).5 For molecular structures, see Scheme 1. Optical Properties and Electrochemical Properties. Figure 1 shows the UV−vis absorption spectra of WS-2, JZ117, JZ145, and AR-II-13, in chloroform/methanol mixed solution (v/v = 4/1) and on TiO2 film. The wavelength of the absorption maxima (λmax) and absorption onset (λonset) of each sensitizer (estimated from the intersection of the absorption and emission spectra as shown in Supporting Information Figure S1) are summarized in Table 1. Both λmax and λonset of JZ117 are blue-shifted (13 and 22 nm) with respect to those of WS-2. The DFBTD auxiliary acceptor increases the molar extinction coefficient (ε) of JZ117 to 24 100 M−1 cm−1 from 16 400 M−1 cm−1 of its BTD counterpart, WS-2. Replacing the thiophene bridge with CPDT further enhances ε by a factor of 2.3. The slight redshift of λmax (11 nm) of JZ145 upon incorporation of CPDT is possibly due to the enhanced molecular conjugation by CPDT. Further structural modification on JZ145 by switching donor group from indoline to butoxy-substituted triphenylamine results in sensitizer AR-II13, with blue shift of the λmax by 9 nm and lower ε (40 300 M−1 cm−1). Overall, dye JZ145 exhibits the broadest absorption band and highest ε, thus leading to a larger integrated absorption area, as shown in Figure 1c. Upon adsorption on titania film, the dye deprotonation and aggregation5 might result in a shift and broadening of the absorption spectra. To investigate these effects, the absorption spectra of sensitizers on the TiO2 semiconductor were acquired (Figure 1b), which exhibit remarkable blue shift with respect to those acquired in solution. Such blue shifts are ascribed to the deprotonation of the dyes upon adsorption on TiO2, similar to the phenomena observed in Supporting Information Figure S2, where the λmax of sensitizers are strongly pH-dependent due to changes in the protonation state of the cyanoacrylic acceptor. Upon addition of triethylamine to the chloroform/methanol solution of the sensitizers leads to their deprotonation, all the dyes display blue-shifted absorption spectra with respect to that acquired in neutral solution because of the weaker electron withdrawing ability of the deprotonated acids. The energy offset of the dyes’ ground state oxidation potential (E(s+/s)) to the standard redox potential of the applied electrolyte controls the dye regeneration efficiency from its oxidized state. Measured by cyclic voltammetry, E(s+/s) of WS-2, JZ117, JZ145, and AR-II-13 are located at +0.98, +1.07, +1.03, and +0.99 V versus the normal hydrogen electrode (NHE), respectively (Table 1). These values suggest that these sensitizers can provide ample driving force (more than 600 mV) for ground state regeneration of sensitizers, in the case of I3−/I− redox electrolyte (0.35 V versus NHE).3 DFBTD in JZ117 increased its oxidation potential by 90 mV with respect

donor groups might further reduce the interaction between electrolyte and titania interface, which would further result in suppression of the charge recombination and improvement of the VOC. More importantly, it is essential to comprehensively understand the charge injection from the excited states of sensitizers to inter band gap states of the semiconductor and the charge recombination at the semiconductor/electrolyte interface, which influences the photocurrent and photovoltage respectively and forms the basis of the rational design for high efficient sensitizers. Therefore, to gain insight into the structure−performance correlation, transient absorption spectroscopy (TSA) and electrochemical impedance spectroscopy (EIS) were employed to investigate the impact of the molecular structure modification on the charge injection and charge recombination dynamics in fully assembled dye cells, respectively. It has been reported that the enhanced VOC of the dye cells upon incorporation of CPDT into sensitizers is due to suppression of charge recombination between injected electrons in the titania film and the oxidant species in the electrolyte.9 However, few detailed photophysical experiments for the improvement of the photocurrent upon the use of π bridge CPDT have been reported.9−12 Here, we show that incorporation of CPDT as a bridge instead of thiophene results in a remarkably long-lived excited state of the sensitizer, faster charge injection rate, and thus higher charge injection yield, contributing to the improved photocurrent of the resulting devices.

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RESULTS AND DISCUSSION Molecular Design and Synthesis by Sequential Direct Arylations. Recently, it was found that the C−H bonds on DFBTD8 can be functionalized sequentially by two different aryl bromides under palladium catalysis, without the use of boron or tin reagents. Utilizing this synthetic strategy, a series of D−A−π−A sensitizers were synthesized in an efficient manner. The aldehyde precursor to JZ117 was formed through direct coupling of DFBTD at 4- and 7-positions with indoline bromide and 5-bromothiophene-2-carbaldehyde in a yields of 71% and 51%, respectively (see Supporting Information). In the case of JZ145, the arylation with 6-bromo-4,4-dihexyl-4Hcyclopenta[1,2-b:5,4-b′]dithiophene-2-carbaldehyde produces the aldehyde intermediate with 61% yield. The preparation of AR-II-13 was described elsewhere.13 Knoevenagel condensation of these aldehydes with cyanoacetic acid afforded the desired sensitizers. In contrast, WS-2 was synthesized by following the literature procedure: a conventional crosscoupling reaction between the corresponding aryl boronic acids and dibromo-BTD at 4- and 7-positions in 38% and 49% 4488

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Table 2. Photovoltaic Parameters of JZ117 and WS-2-Based Dye Cells; TiO2 Films Were Sensitized in Different Dye Bath Solvents for 12 ha JSC (mA/cm2)

VOC (V) JZ117

WS-2

b

0.15 mM, No CDCA 0.15 mM, 20 mM CDCAb 0.30 mM, 20 mM CDCAb 0.30 mM, No CDCAc 0.3 mM, 20 mM CDCAc 0.3 mM, No CDCAb 0.3 mM, 20 mM CDCAb 0.3 mM, No CDCAc 0.3 mM, 20 mM CDCAc

0.643 0.641 0.642 0.642 0.643 0.651 0.650 0.638 0.643

± ± ± ± ± ± ±

15.4 16.8 17.1 13.6 15.5 19.7 19.7 17.7 18.6

0.001 0.000 0.001 0.002 0.002 0.001 0.000

± ± ± ± ± ± ±

FF (%) 71.0 67.6 67.5 70.9 70.4 66.3 66.8 66.8 67.3

0.1 0.5 0.1 0.2 0.2 0.2 0.5

± ± ± ± ± ± ±

PCE (%) 7.0 7.3 7.4 6.2 6.6 8.5 8.6 7.5 8.1

0.3 0.8 0.3 0.9 0.2 0.7 1.2

± ± ± ± ± ± ±

0.1 0.2 0.0 0.02 0.06 0.02 0.1

a

The data points that do not have standard deviations are averaged over two cells each. bDye bath solvent: ethanol/chloroform = 4/1. cDye bath solvent: ethanol/chloroform = 1/3.

from 0.15 to 0.3 mM, no apparent change of JSC and PCE was observed, even in the presence of 20 mM CDCA in dye bath. This suggests that dye aggregation may still limit the dye cell performance possibly due to the quenching of the dye excited state. Dye bath solvents effect on the performance of dye cells are also explored for WS-2. The resulting dye cells with ethanol/ chloroform (4:1) exhibit JSC of 19.7 mA/cm2 and PCE of 8.6%, which nicely matched the device performance in literature.5 However, the JSC and PCE of resulting dye cells prepared in bath solvent of ethanol/chloroform (1:3) are 18.6 mA/cm2 and 8.1% respectively. Interestingly, the use of deaggregating agent CDCA does not show any significant differentiation for WS-2 on the solar cell performance when ethanol/chloroform (4:1) used as the dye bath solvent, whereas in dye bath of ethanol/ chloroform (1:3), the JSC values increase from 17.7 to 18.6 mA/ cm2 and PCE from 7.5% to 8.1%. This might indicate less dye aggregation in ethanol/chloroform (4:1) than ethanol/chloroform (1:3). JZ145- and AR-II-13-based dye cells are prepared by using the optimized procedure, where the TiO2 films are soaked in a dye bath of ethanol/chloroform (4:1) with 0.3 mM dye and 20 mM CDCA. The photovoltaic parameters of the optimized dye cells based on the four dyes are summarized in Table 3. The J−

to WS-2. Replacement of thiophene by CPDT in JZ145 decreases E(s+/s) by 40 mV. AR-II-13 exhibits reduced reduction potential relative to that of JZ145, which might be ascribed to the stronger electron donating ability of the substituted triphenylamine than indoline.14 The energy difference between the dye’s excited state oxidation potential (E(s+/s*)) and the conduction band edge (CBE) of TiO2 (−0.5 V vs NHE)15,16 drives the electron injection from the photoexcited sensitizer to TiO2. E(S+/S*) can be estimated17 from E(S+/S) and the zero−zero excitation energy E0−0, according to E(S+/S*) = E(S+/S) − E0−0, neglecting any entropy change during the light absorption. The excited-state oxidation potential (E(s+/s*)) and charge-injection driving force (ΔGei0(eV)) for all the four sensitizers are listed in Table 1. The adoption of DFBTD in JZ117 results in less driving force (−30 mV) for electron injection from the photo excited state with respect to its BTD analogue WS-2. In contrast, E(s+/s*) of the CPDT bridge-containing sensitizers, JZ145 and AR-II-13, are more negative than that of JZ117 and WS-2, which might result in a greater charge-injection driving force and thus faster charge-injection rate from dye excited state to titania. Photovoltaic Performance. We explored the effect of dye bath solvents, deaggregating agent chenodeoxycholic acid (CDCA), and the dye concentration on the photovoltaic performance of dye cells based on WS-2 and JZ117. The corresponding photovoltaic parameters are listed in Table 2. The details for dye cell preparation are included in Supporting Information. The dye cells based on the dye bath solvent of ethanol/chloroform (4:1) demonstrate higher photocurrent density and PCE than those of the dye cells prepared with ethanol/chloroform (1:3). For JZ117, the dye cells prepared in the latter bath solvent with 0.3 mM dye and 20 mM CDCA, the JSC and PCE are found to be 15.5 mA/cm2 and 6.6%, respectively. However, dye cells prepared with bath solvent of ethanol/chloroform = 4/1, the JSC and PCE are shown to be 17.1 mA/cm2 and 7.4%, respectively. This may be ascribed to the different extent of dye aggregation in the two dye baths, as illustrated by the use of deaggregating agent CDCA. When we use ethanol/chloroform (4/1) as the bath solvent and the dye concentration of 0.15 mM, adding 20 mM CDCA in dye bath increases the JSC from 15.4 to 16.8 mA/cm2 and PCE from 7.0% to 7.3%, although the presence of the 20 mM CDCA in bath solvent significantly reduces the dye loading. Such effects are also observed when using ethanol/chloroform (1/3) as dye bath solvent with 0.3 mM dye. The addition of 20 mM CDCA in dye bath increases the JSC from 13.6 to 15.5 mA/cm2 and PCE from 6.2% to 6.6%. By changing the dye concentration

Table 3. Photovoltaic Parameters of WS-2-, JZ117-, JZ145-, and AR-II-13-Based Dye Cell; TiO2 Films Were Sensitized in Dye Solution with Molarity of 0.3 mM and 20 mM CDCA in Ethanol/Chloroform (4:1) for 12 h Dye WS-2 JZ117 JZ145 AR-II-13

VOC (V) 0.650 0.642 0.717 0.730

± ± ± ±

0.002 0.001 0.001 0.003

JSC (mA/cm2) 19.7 17.1 18.8 12.7

± ± ± ±

0.2 0.1 0.2 0.1

FF (%) 66.8 67.5 67.3 71.2

± ± ± ±

0.2 0.3 1.3 0.8

PCE (%) 8.6 7.4 9.1 6.6

± ± ± ±

0.1 0.1 0.2 0.1

V scans in dark and under one sun illumination used to determine the photovoltaic parameters are shown in Figure 2. Compared to BTD counterpart WS-2, JZ117 demonstrates slightly lower VOC, however, apparently reduced JSC and PCE, from 19.7 to 17.1 mA/cm2 and 8.6% to 7.4%, respectively. Upon the substitution of thienyl bridge by CPDT, the enhancement of cell performance from JZ117- to JZ145based dye cells is significant. The VOC, JSC, and PCE are enhanced by 75 mV, 1.7 mA/cm2, and 1.7%, respectively, achieving a PCE of 9.1% and VOC of 0.717 V.6 4489

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Figure 3. IPCE profiles for dye cells based on WS-2, JZ117, JZ145, and AR-II-13, with photoanodes soaked in dye bath with 0.3 mM dye and 20 mM CDCA for 12 h (ethanol:chloroform = 4:1).

Figure 2. J−V profiles for dye cells based on WS-2, JZ117, JZ145, and AR-II-13, with photoanodes soaked in dye bath with 0.3 mM dye and 20 mM CDCA for 12 h (ethanol:chloroform = 4:1). The samples were washed by ethanol only. The VOC and JSC for dye cells prepared with lower concentration AR-II-13 are slightly lower. The 0.15 mM dye bath sensitized solar cells have lower dark current than those prepared with 0.3 mM dye bath, indicating that the latter samples have less recombination reactions between electrons on the TiO2 film and the redox electrolyte. The dark current−voltage scan results are in agreement with measurements made under light.

still lower than that of WS-2. Compared to that of JZ145, IPCE of dye cells based on AR-II-13 dye with the butoxy-substituted triphenylamine donor has lower value over the whole spectrum by more than 20%, contributing to their lower photocurrent. Picosecond Dynamics of Charge Injection from Excited State of Dyes to TiO2. Modification of the molecular structure of the dye molecules could result in a change of the energy levels of dye molecules relative to the conduction band edge (CBE) of TiO2, the distribution of electron density of HOMO and LUMO over the molecular backbone and the electronic coupling between the dyes and TiO2. These changes could significantly affect the dynamics of the charge transfer from the photoexcited state of the dye molecules to the TiO2 sub-band gap states and affect the photocurrent. Thus, it is critical to study the charge injection dynamics from the excited states of the dyes to TiO2 sub-band gap states. Here, we studied this transfer process by femtosecond transient absorption spectroscopy for all four dyes grafted on both TiO2 and Al2O3 films in fully assembled cells with standard iodide electrolyte containing additives of tert-butylpyridine and Li+. The dye molecules are excited with a pump laser beam near their maximum absorption and the absorption of the excited state are monitored by a white light probe beam. The charge-injection dynamics are measured by monitoring the decay of the absorption of the excited state. It is assumed that the decay of the excited state of the dye molecules grafted on titania surfaces occurs through two pathways, decay to the ground state of the dye molecules and the charge-injection to the titania subbandgap states. However, the excited states of dyes on Al2O3 nanoparticle films decay mainly to the ground state. As shown in Figure 4, the decay dynamics of the dyes on both titania and Al2O3 in full cells are fitted with stretched exponential function,22,23 equation (I) f(t) = A × exp[(−t/τ)β] and plotted with solid lines. Here, f(t) represents the absorption intensity of excited state of dye molecules, A is the pre-exponential constant, t is the time, τ is the characteristic time constant of decay, and β is the the heterogeneity constant describing the distribution of the dye molecules on the substrate and electron accepting states in TiO2. Because it is not acceptable to directly compare characteristic time constant of decays with different β values, which could remarkably change the decay time constant, a weighted average time constant was proposed based on

Alkoxy-substituted triphenylamine groups have shown to be effective in improving the VOC.18−21 Here, in AR-II-13, butoxysubstituted triphenylamine was used as a donor group instead of indoline to explore the impact on VOC of dye cells based on these sensitizers. As anticipated, the dye cells based on AR-II13 exhibit VOC of 0.73 V, showing an enhancement of 13 mV compared to that of the resulting dye cells based on JZ145. As indicated by the dark J−V scans in Figure 2, the onset potentials of the dark current of the dye cells based on the four dyes are in the order of AR-II-13 > JZ145 > WS-2 ≈ JZ117, suggesting that the blocking effect of alkyl group in the molecular structures is prominent in suppressing the recombination of the injected electrons in TiO2 films with the oxidant in electrolyte and thus improving the VOC. However, JSC of the dye cells based on AR-II-13 is reduced by 6 mA/cm2, resulting in a low PCE of 6.6%. To examine the effect of the molecular structure on the current density of dye cells with each dye, the incident photonto-current conversion efficiency (IPCE) is acquired and given in Figure 3, as a function of the incident light wavelength. IPCE of dye cells with WS-2 is about 5% higher than that of dye cells based on JZ117 at the maximum absorption band. It is worth noting that the IPCE for devices with WS-2 at 740 nm is about 40%, whereas it is less than 22% for the DFBTD, because the former dye has better light harvesting ability for low energy photons, as confirmed in the absorption spectra in the inset of Figure 1a. Overall, higher and broader IPCE spectrum of the device based on WS-2 compared to that of JZ117 results in the higher JSC for former dye. Interestingly, the IPCE of dye cells sensitized with JZ145 is similar to that of WS-2 at the maximum absorption band but is still limited to the level of that of JZ117 from 690 to 830 nm. This is likely because that WS-2 exhibits the longest λonset among these dyes, as shown in inset of Figure 1a. Thus, the current density for dye cells based on JZ145, integrated over the whole spectral region of IPCE, is 4490

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Figure 4. Decay dynamics of photo excited state of WS-2, JZ117, JZ145, and AR-II-13 on (a) TiO2 and (b) Al2O3 nanoparticles films in assembled full cells in the presence of electrolyte. Pump and probe is set at 532 nm and around 750 nm (absorption maximum) respectively.

equation (II),24 τw = (τ)/(β)Γ(1/β), where Γ(1/β) is the generic gamma function. The experimental data (the scattered lines) overlap very well with the fitting results based on the stretched exponential function. By using the derived time constants from the fitting results and also the equation (II), the weighted average time constants were calculated. The electron injection rate constants and efficiencies were derived by substituting the weighted average time constants into equations kei = 1/τTiO2 − 1/τAl2O3 (III) and φei = 1 − τTiO2/τAl2O3 (IV). All the dynamic results are listed in Table 4.

The sensitizer JZ117 exhibits charge injection driving force of 370 mV, about 30 mV lower than that of its BTD analogue WS-2, resulting in a lowered charge injection rate constant (reduced from 1.76 × 10−10 s−1 to 0.78 × 10−10 s−1). In addition, the lifetime of the excited state of JZ117 was slightly shortened with regard to WS-2 (from 385 to 290 ps). Both the reduced charge injection rate constant and lifetime of the excited state contribute to the reduction of charge injection yield from 87.2% to 69.4%. Replacement of thiophene with CPDT in JZ145 results in enhancement of not only the charge injection driving force (0.42 mV) and the charge injection rate constant (1.14 × 10−10 s−1) but also the excited-state lifetime (612 ps), which is even longer than that of WS-2 (385 ps). The enhanced charge injection rate constant and the long-lived excited state lead to the enhancement of charge-injection yield to 87.5%, which might contribute to higher IPCE:25 at the maximum absorption band, the observed photocurrent is equivalent for WS-2 and JZ145, and 5% higher than that for JZ117. However, dye AR-II-13, with the donor group of butoxysubstituted triphenylamine, behaves very differently from the indoline counterpart. The only difference between JZ145 and

Table 4. Charge Injection Dynamics of Dye Cells Sensitized with WS-2, JZ117, JZ145, and AR-II-13, Respectivelya dye

τobs(ps)/TiO2

τobs(ps)/Al2O3

kei (10−10 s−1)

φei(%)

WS-2 JZ117 JZ145 AR-II-13

49.3 88.6 76.5 43

385 290 612 151

1.76 0.78 1.14 1.64

87.2 69.4 87.5 71.5

a

Pump and probe is set at 532 nm and around 750 nm (absorption maximum), respectively.

Figure 5. Electrochemical impedance spectroscopy: (a) Bode plots and (b) Nyquist plots for dye cells based on JZ117, JZ145, and AR-II-13 with dye bath of 0.3 mM, 20 mM CDCA, 12 h of soaking, in dark with bias of 0.7 V. 4491

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S4, the resulting devices based on WS-2 and JZ117 were also characterized by EIS in dark with bias of 0.64 V. The identical electrons lifetimes (5.0 ms) in titania film and charge transfer impedance for the resulting devices based on the two sensitizers suggest that the charge recombination dynamics for the two sensitizers on titania surfaces are almost the same and this is in agreement with the similar results of VOC.

AR-II-13 is the donor structure, the former using indoline and the latter adopting triphenylamine. The charge-injection driving force for AR-II-13 is 80 mV higher than JZ145 and, thus, a faster charge injection rate constant (1.14 × 10−10 s−1 for JZ145 and 1.64 × 10−10 s−1 for AR-II-13) was observed, as shown in Table 4. However, the remarkably shortened lifetime of the excited state of AR-II-13 over JZ145 (151 ps vs 612 ps) still results in lower charge injection efficiency for the former dye than the latter (71.5% vs 87.5%), which might at least partially account for the low IPCE of the resulting devices of AR-II-13. In this regard, it is worth noting that the amino functionality in WS-2 and JZ145 are in fused-ring structures, whereas in AR-II13 it is not, which can lead to additional relaxation pathways from the excited state of AR-II-13. From the dark current measurement and the VOC values, AR-II-13 is supposed to result in lower charge recombination rate at the TiO2/ electrolyte interface than JZ145 and the charge recombination is, therefore, not the cause of low IPCE for AR-II-13. Another possibility is the dye regeneration reaction of AR-II-13. E(s+/s) of AR-II-13 is 40 mV higher than that of JZ145, providing less driving force for the ground state regeneration. In addition, the bulky butoxy substituents on triphenylamine might prevent the tight coupling of the HOMO of AR-II-13 and I¯ and deteriorate the dye ground state regeneration efficiency. Both of them may contribute to the low photocurrent. Additionally, the chargeinjection driving force for AR-II-13 is 100 mV higher that of WS-2, but the charge injection rate constants are close to each other (1.64 × 10−10 s−1 vs 1.76 × 10−10 s−1). This might be due to either the difference of the coupling between the sensitizer and TiO2 film and the intramolecular charge transfer through the molecular backbone from the donor to acceptor upon photoexcitation or both.3,26 Electrochemical Impedance Spectroscopy (EIS) for Charge Recombination Reaction. EIS was used to study the charge recombination dynamics at the interface of titania/ electrolyte.23,27,28 Figure 5 displays (a) Bode plots and (b) Nyquist plots for dye cells based on JZ117, JZ145, and AR-II13 in dark with bias of 0.7 V. It has been known that charge recombination of injected electrons in TiO2 film with oxidant in electrolyte strongly depends on the Fermi level.29,30 Here, application of the same bias on the cells during EIS measurements ensures that the TiO2 films in the three cases have the same Fermi level, which is an important factor in governing charge recombination dynamics. From the frequency of the intermediate peak in Bode plots, which was ascribed to the recombination at the TiO2/electrolyte interface,31,32 the electron lifetime 1/(2πf) in the TiO2 film could be extracted, which is 3.4, 8.7, and 15.7 ms for JZ117, JZ145, and AR-II-13, respectively. The longer the electron lifetime in titania films, the higher the electron density and Fermi level will be ensured, which might further result in higher VOC. In addition, the radius of the intermediate semicircle in the Bode plots corresponds to the charge transfer impedance at the TiO2/electrolyte interface. The much larger semicircle for JZ145 than JZ117 indicates that the charge transfer in the former is more difficult than the latter and thus there is a higher electron density in the titania film, in agreement with the results derived from Nyquist plots above. Such difference might be caused by the screening effect of CPDT between the TiO2 film surface and electrolyte.10,18 The butoxy on indoline groups in AR-II-13 further suppress the charge recombination and the resulting devices exhibit the largest charge transfer impedance. To compare the difference between BTD and DFBTD, in Supporting Information Figure

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CONCLUSION In conclusion, the sequential direct arylation of 4-, 7-position of DFBTD with aryl bromides was demonstrated to be an efficient synthetic method for efficient D−A−π−A sensitizers. The correlation of the molecular structure of sensitizers with the photovoltaic performance of the resulting devices was examined. The substitution of BTD with DFBTD results in a blue shift of the resonance absorption, reduction of charge injection rate constant, charge injection yield of the dye cells, and photocurrent density of the resulting dye cells. Upon replacing the thiophene bridge by CPDT, the resulting dye JZ145 showed remarkably improved open circuit voltage due to the effective suppression of charge recombination by CPDT. In addition, the long-lived excited state and increased charge injection rate of JZ145 result in an improvement of high charge injection yield and photocurrent. Although butoxy-substituted triphenylamine donor in AR-II-13 slightly increases the open circuit voltage, it results in much lower photocurrent, and, thus power conversion efficiency than the indoline donor in JZ145, which is ascribed to the shorter lifetime of the excited state and lower charge injection yield. Thus, among the four D−A−π−A dyes, JZ145 with indoline donor and CPDT bridge is found to be the most efficient dye, which achieved a high open circuit voltage of 0.716 V and short circuit current density of 18.8 mA/ cm2, corresponding to an overall power conversion efficiency of 9.1% for the dye cells with standard I3¯/I¯ liquid electrolyte.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic details and dye characterizations, device fabrication protocols and characterizations, and electrochemical Impedance spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

These authors contributed equally to this paper.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS El-Sayed’s group would like to thank the financial support of the Office of Basic Energy Sciences of the U. S. Department of Energy under contract number DE-FG02-97ER14799, and Marder’s group would like to thank the National Science Foundation for support through the CCI Center for Selective C−H Functionalization (CHE-1205646).

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