Near-Infrared Asymmetrical Squaraine Sensitizers for Highly Efficient

Article pubs.acs.org/cm

Near-Infrared Asymmetrical Squaraine Sensitizers for Highly Efficient Dye Sensitized Solar Cells: The Effect of π‑Bridges and Anchoring Groups on Solar Cell Performance Fadi M. Jradi,†,§ Xiongwu Kang,‡,§ Daniel O’Neil,‡ Gabriel Pajares,† Yulia A. Getmanenko,† Paul Szymanski,‡ Timothy C. Parker,† Mostafa A. El-Sayed,*,‡ and Seth R. Marder*,† †

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

ABSTRACT: Conventional squaraine dyes exhibit an intense absorption band in the red region of the solar spectrum and with appropriate design can also have high energy absorption as well, making them interesting building blocks toward achieving panchromatic dyes for dye sensitized solar cell (DSSC) applications. In this report, eight squaraine dyes with thiophene, 4-hexyl-4Hdithieno[3,2-b:2′,3′-d]pyrrole, dithieno[3,2-b:2′,3′-d]thiophene, and 4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene (DTS) πbridges with cyanoacetic acid (CA) and cyanophosphonic acid (PA) acceptor/anchoring groups are synthesized to extend the squaraine absorption into the 450−550 nm region and to provide different spatial arrangements of solubilizing groups. Squaraines with CA anchoring groups have higher power conversion efficiencies compared to their PA analogs, with the highest being 8.9% for the DTS-based dye, which is among the highest reported in the literature for squaraine dyes. This is due to high short circuit currents (JSC) and increased open circuit voltages (VOC). Dyes with PA anchoring groups exhibited lower JSC resulting from decreased charge injection efficiency, as determined by femtosecond transient absorption spectroscopy. This study suggests that out-ofplane bulky substituents may increase DSSC performance not only by increasing JSC through decreased aggregation but also by increasing VOC through decreased TiO2/electrolyte recombination.

1. INTRODUCTION In 1991, Grätzel and O’Regan introduced the concept of low cost, high efficiency dye sensitized solar cells (DSSCs) based on the mesoporous titanium oxide films.1 Since then, an impressive library of organic and organometallic dyes have been designed2−4 and tested in an attempt to fabricate solar cells with ever increasing power conversion efficiencies (PCEs). Currently, state of the art DSSCs are porphyrin-based with a PCE of 13% and incident photon-to-current efficiency (IPCE) up to 90% over the entire visible spectrum.5 Considering that losses of light due to reflection and absorption by the fluorine doped tin oxide (FTO) glass can amount up to 10%,6 the IPCE of devices in the visible region is effectively saturated; however, their performance could be improved by harvesting red to nearinfrared (NIR) light, which may make them more competitive with traditional photovoltaic devices.7 Squaraine dyes8 exhibit high molar extinction coefficients, on the order of 105 M−1 cm−1 at 650 nm, and when compared, for example, to porphyrin dyes whose λmax is around 450 nm, they have potential to harvest NIR light and thus act as a promising © XXXX American Chemical Society

building block for panchromatic dyes if high energy absorption can be maintained.9−11 However, the performance of squarainebased DSSCs has been limited by a relatively low open circuit voltage VOC,12 significant dye aggregation that has been suggested to reduce the short circuit current JSC,13,14 and a relatively low absorption between 400 and 600 nm, which resulted in low IPCEs in this absorption region. Asymmetrical squaraine-based DSSCs with PCEs ranging from 4 to 5% have been reported,15,16 and structural modifications have pushed PCEs to 6.7% with “YR6”13 (Figure 1) by introducing high energy absorption via the use of a thiophene (T) bridge and then to 7.3% with “JD10”14 (Figure 1), by replacing thiophene with 4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) bridge. With CPDT, JSC increased to 16.4 mA/cm2 in JD10 in comparison with 14.8 mA/cm2 in YR6, which was ascribed to a combination of enhanced high energy absorption Received: December 15, 2014 Revised: March 17, 2015

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DOI: 10.1021/cm5045946 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

In this report, we further studied the effect of various πbridges that have high energy absorption bands complementary to the NIR absorption of the squaraine to increase panchromaticity and IPCE.14 To achieve this, different bridges, 4-hexyl-4H-dithieno[3,2-b:2′,3′-d]pyrrole (DTP), dithieno[3,2b:2′,3′-d]thiophene (DTT), and 4,4-bis(2-ethylhexyl)-4Hsilolo[3,2-b:4,5-b′]dithiophene (DTS), have been covalently linked to a squaraine donor yielding asymmetrical push−pull donor−π−acceptor (D−π−A) structures (Figure 1). Furthermore, by having two branched out-of-plane 2-ethylhexyl chains on DTS such as in DTS-CA and DTS-PA (Figure 1), we demonstrated that reducing dye aggregation in squaraines has a considerable positive effect on their short circuit currents (JSC), IPCEs, and the overall dye cell performance. The majority of photosensitizers employ carboxylic acid (CA) anchoring groups due to ease of synthesis and relative stability. Moreover, DSSCs fabricated with CA-based sensitizers have been shown to outperform analogs with different anchoring groups.19−21 This enhanced performance is usually manifested as higher JSC; however, the reasons behind this are still under debate and have been attributed to many factors. For example, Brennan et al. compared three different porphyrinbased dyes that differ only by their anchoring groups and found that substituting the CA anchoring group with either a silatrane or a phosphonic acid (PA) anchoring group caused a 55% drop in JSC and hence PCE.19 This phenomenon was attributed to the higher surface coverage of CA dyes due to an upright orientation on the surface, which maximizes the number of dye

Figure 1. Molecular structures of the squaraine dyes. YR613 and JD1014 have been reported previously and used here for comparison.

by the π bridge and disruption of aggregation by the hexyl chains on CPDT.14 More recently, symmetrical and asymmetrical cis-configured squaraines17,18 with strong electronwithdrawing functionality on the squaric acid core have been reported, resulting in a red-shift of the main absorption band, which enhanced the overall efficiency of the corresponding DSSCs. Scheme 1. Synthetic Routes Towards the Squaraine Dyesa

(a, for 6) 2, PdCl2(dppf)·CH2Cl2, K2CO3, toluene, methanol, 70 °C, overnight. (b, for 7 and 8, respectively) 3 or 4, Pd(PPh3)2Cl2, toluene, 70 °C, overnight. (c, for 9) (i) 5, Pd(PPh3)2Cl2, toluene, 70 °C, overnight; (ii) DMF, POCl3, 0 to 70 °C, 3 h. (d) 6 or 7 or 8, CNCH2CO2H, piperidine, toluene, 70 °C, 4−6 h. (e) 9, CNCH2CO2H, (NH4)2CO3, propanoic acid, toluene, 100 °C, overnight. (f) CNCH2PO(OCH2CH3)2, (NH4)2CO3, propanoic acid, toluene. (g) (i) TMSBr, DCE, 70 °C; 2−4 h; (ii) methanol, water, room temperature. (i) (i) CH3ONHCH3·HCl, THF, −78 °C, 0.5 h, followed by LDA (2.0 equiv), −78 °C, 1 h; (ii) 3a or 4a, −78 °C, 2 h; (iii) LDA for 3a, n-butyllithium for 4a, −78 °C, 1 h; (iv) R3SnCl, −78 °C to room temperature, overnight; (v) H2O, room temperature, 0.5 h. a

B


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Chemistry of Materials molecules bound per surface area.22 In another study Wiberg et al.20 substituted the CA group in a polyene triphenylamine dye with a rhodanine anchoring group, which resulted in a decrease in PCE from 6.0% to 1.7% due to a diminishing IPCE. This was attributed to increased electron recombination from TiO2 to the oxidized dye due to more electrons being present in TiO2 surface trap states since the nonconjugated rhodanine anchoring group does not possess “deep” electron injection.20 Mulhern et al.23 and Murakami et al.21 have independently demonstrated that devices fabricated with PA dyes are much more stable under prolonged operating time compared to their CA counterparts and responded better to stress tests; however, dyes having multiple CA anchoring groups have shown prolonged device stability after 1000 h of light soaking.18 Given this enhanced device stability, along with promising performance (8.0%24 and 6.4%25), we investigated squarainebased dyes with PA anchoring groups in addition to their CAcontaining analogues (Figure 1).

Table 1. Optical and Electrochemical Properties of YR6, TS3, and CA and PA Series dyes

λmax [nm]a

ε [M−1 cm−1]a

fb

E0−0opt [eV]c

E(S+/S) [V]d

E(S+/S*) [V]e

YR6 T-PA TS3 DTP-PA DTT-CA DTT-PA DTS-CA DTS-PA

659 655 670 670 662 661 667 666

279 000 311 000 160 000 204 000 231 000 259 000 257 000 214 000

1.39 1.49 1.34 1.43 1.28 1.43 1.58 1.32

1.76 1.85 1.74 1.82 1.76 1.84 1.82 1.83

+0.80 +0.86 +0.74 +0.82 +0.80 +0.85 +0.84 +0.88

−0.96 −0.99 −1.00 −1.00 −0.99 −0.99 −0.98 −0.95

a

Derived from absorption spectra in ethanol. bOscillator strength f = 4.32 × 10−9 ∫ ε(υ) dυ. cDetermined from the intersection of the normalized absorption and emission spectra. dHalfwave ground state oxidation potentials determined via cyclic voltammetry. eCalculated according to the following equation E(S+/S*) = E(S+/S) − E0−0.

2. RESULTS AND DISCUSSION 2.1. Materials. The synthesis of the squaraine dyes utilized the common intermediate squaraine 1 and the functional πbridges DTT 3, DTP 4, and DTS 5 as illustrated in Scheme 1. The synthesis of bridges 3 and 4 were accomplished from aldehydes 3a and 4a, respectively, by utilizing an in situ aldehyde protection26 step before lithiation and stannylation, which is described in full detail in the Supporting Information. Stille coupling27 of squaraine 1 with functionalized π-bridges 3, 4, and 5 gave the corresponding aldehydes 7−9 in ∼50% yields. Knoevenagel condensation13 of compounds 7−9 with cyanoacetic acid provided the CA dyes DTT-CA, TS3, and DTS-CA in 34%, 60%, and 70% yields, respectively. Knoevenagel condensations of compounds 7−9 with diethyl cyanomethylphosphonate yielded compounds 10−13 in 50−80% yields (Scheme 1), which were followed by hydrolysis with bromotrimethylsilane, to get the PA dyes T-PA and DTP-PA in 85% and 93% yields, respectively. DTS-PA, obtained in ∼85% yield, contained ∼10% of what we assign as the DTS-PA monoethyl ester by HPLC analysis (Figures S5 and S6 in the Supporting Information). DTT-PA was obtained in 50% yield, possibly to lower solubility (Scheme 1). The synthesis of YR6, reported previously,13 has been included for comparison. 2.2. Optoelectronic Properties and Aggregation Effects. Upon light absorption in push−pull systems, charge is transferred from the donor to the acceptor through the conjugated bridge. Hence, modifying the bridge affects the optical properties of the dyes.2,3 Table 1 summarizes the optical and electronic properties of the dyes YR6, TS3, and CA and PA dyes presented in Figure 1. In each of the two series (for example, the dyes with CA anchoring group) replacing the T bridge by the more extended DTP, DTT, and DTS bridges resulted in a slight bathochromic shift of the low energy squaraine-based band (λmax). The extent of this shift, though, was minimal with the largest shift being 11 nm (Figure 2). However, as expected, these conjugated bridges introduced high-energy bands between 400 and 550 nm, depending on the nature of the bridge, with molar extinction coefficients ranging from 26 000 M−1 cm−1 for YR6 with a T bridge up to 42 000 M−1cm−1 for TS3 with a DTP bridge (Figure 2). Molar extinction coefficients of the main squaraine band (εmax) decreased with the extension of the conjugation up to 43% in the case of the DTP bridge in comparison with YR6. However, this reduction in the extinction coefficient at the λmax was

Figure 2. UV−vis absorption spectra of YR6, TS3, and CA and PA dyes series in ethanol. Axis labels for inset are the same as the figure.

compensated to some extent by the band broadening possibly attributed to a higher extent of intramolecular charge transfer in the less aromatic pyrrole-containing DTP, which rendered the integrated area under the absorption peaks similar to or even higher than in the case of the dye with the T bridge, as indicated in the oscillator strength values f in Table 1. The CA and PA groups had little effect on the squaraine absorption band, but their effect on the high energy bands was more pronounced and led to hypsochromic shift by as high as 22 nm in case of the DTP bridge (TS3 and DTP-PA, in Figure 2 inset). The ground-state oxidation potential (ES+/S) and the excitedstate oxidation potential (ES+/S*) of a dye with respect to the electrochemical potential of the electrolyte and the conduction band edge (CBE) of TiO2, respectively, are critical for solar cell operation. To ensure optimal solar cell performance, sufficient driving force is required for both charge injection from the excited dye into TiO2 CBE and the dye regeneration by the triiodide/iodide redox couple (I3−/I−). The energy difference between ES+/S* and the CBE of TiO2 (−0.5 V vs NHE)28,29 dictates this driving force, and for efficient charge injection in squaraine based cells, this driving force is required to be at least 150 meV.30 To ensure efficient dye regeneration by the triiodide/iodide redox couple (0.35 V vs NHE),31 a ES+/S higher than 0.65 V vs NHE is recommended.31 Based on C


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significant dye aggregation, which is likely associated with planarity of the dye. As for DTS-CA and DTS-PA, this peak is reduced into a shoulder as expected, since the 2-ethylhexyl chains on the bridge project out of the plane of the squaraine dyes and thus may reduce π−π stacking and dye aggregation. The effects of dye aggregation on the performance of the assembled dye cells with standard I3−/I− electrolyte were also explored (Table S1). The presence of CDCA generally increased PCE of the devices, indicating a certain amount of aggregation in all the dyes;34−36 however, dyes with PA anchoring groups demonstrated a stronger dependence on the CDCA/dye ratio compared to their CA counterparts. Belonging to the bile acid family, CDCA is an amphiphilic molecule34 that has been shown to form elongated micelles in aqueous media35,36 and has been extensively used in dye sensitized solar cells to assist disaggregation of dyes. Hydrogen bonding of (RO)2P(O)OH, ROP(O)(OH)2, and PA groups have been reported to be significantly stronger than corresponding carboxylic acids,37,38 and as a result they might form hydrogen bonded clusters.39 This increased hydrogen bonding may result in the formation of aggregates of PA dyes that are more difficult to disaggregate than their CA counterparts, resulting in the stronger dependence of PA on CDCA/dye ratio. 2.3. Photovoltaic Measurements. The performance parameters for devices based on each dye using the optimized CDCA/dye ratios are listed in Table 3, and the corresponding J−V curves are presented in Figure 4. The JSC and VOC of the CA dyes TS3 and DTT-CA are consistent with that of the previously reported squaraine DSSCs,13,14,16 especially with regard to their VOC. This consistently low VOC in squaraine based DSSCs has limited their performance despite their relatively high JSC, which reached as high as 16.4 mA/cm2 in the case of JD10,14 putting it slightly lower than that of porphyrinbased SM315, whose JSC is 18.1 mA/cm2, and whose PCE of 13% is the highest yet reported.5 DSSCs fabricated with DTSCA, on the other hand, saw an increase in VOC, which to our knowledge is the highest for a squaraine-based cell at 682 mV, a 47 mV increase over JD10. Combined with a relatively high JSC equal to 19.1 mA/cm2, the DTS-CA DSSC exhibited a PCE equal to 8.9%, an 18% increase over JD10. On the basis of the absorption spectra of DTS-CA on TiO2 surfaces (Figure 3), the increased JSC may be a result from decreased aggregation due to the out-of-plane 2-ethylhexyl groups, as discussed earlier. Dyes in the PA series exhibited much lower PCEs compared to their CA counterparts, which is due in large part to lower JSC, which dropped by 72% going from DTT-CA to DTT-PA (Figure 4). To gain insight into the observed JSC, IPCE spectra were recorded for DSSCs made with the respective dyes, and the results are shown in Figure 5. In general, the IPCE of the dyes in the CA series had an onset around 800 nm, with the exception of DTS-CA, whose onset was around 850 nm. Moreover, DTS-CA demonstrated the highest IPCE values among all other dyes in the CA series, reaching 90% at low energies and 82% at higher energies, while all the other dyes plateaued at ∼70%. The high IPCE values reported for these dyes at lower wavelength, despite their relatively low molar absorptivities in this region, likely results from the presence of the 4 μm light scattering layer formed of anatase particles.42,43 Dyes in the PA series, on the other hand, showed narrower features and overall lower IPCEs reaching a maximum at ∼60%. DTT-PA and DTP-PA achieved an IPCE of only ∼30%. The

these criteria, all of the dyes should exhibit sufficient driving force ΔGei0 for charge injection into TiO2 (Table 2) of at least Table 2. Charge Injection Dynamics of YR6, TS3, and Other CA and PA Seriesa dye

τobs[ps]/ TiO2

τobs[ps]/ Al2O3

Kei [10‑10 s‑1]

ΔGei0 [V]

ΔGreg0 [V]

φei [%]

YR6 T-PA TS3 DTP-PA DTT-CA DTT-PA DTS-CA DTS-PA

1.8 8.3 1.0 16.0 19.8 19.1 1.2 4.0

123.9 80.3 14.1 127.3 3942 32.3 22.6 18.5

52.4 10.8 97.6 5.5 5.0 2.2 78.9 19.6

0.46 0.49 0.50 0.50 0.49 0.49 0.48 0.43

0.50 0.56 0.44 0.52 0.50 0.55 0.54 0.58

98.5 89.6 97.6 87.4 99.0 41.0 94.6 78.6

a

The assembled dye cells were pumped near the ground state absorption for each dye and probed near their excited state absorption maxima. All data were fit with stretched exponentials.

450 mV, which is significantly higher than what is required and indicates that charge injection should be thermodynamically favorable. As for dye regeneration, the ground state potentials ES+/S of all dyes ranged between 0.74 and 0.88 V vs NHE, higher than the 0.65 V required. In order to gain additional insight into the optical behavior of dyes in a functioning cell, namely upon aggregation, the absorption spectra on TiO2 films were collected (Figure 3).

Figure 3. UV−vis absorption spectra of YR6, TS3, and other CA and PA dyes series on TiO2 film with 0.05 mM dye (0.1 mM for TS3) and 10 mM CDCA after 1 h of soaking.

TiO2 films were immersed in a dye solution dissolved in ethanol/CHCl3 (4:1) containing 3α,7α-dihydroxy-5β-cholic acid (chenodeoxycholic acid, CDCA) for 1 h at room temperature. The films were then washed with ethanol and dried under nitrogen before the UV−vis absorption spectra were collected. In addition to the general broadening of the spectra expected for the dyes adsorbed on surfaces, a peak blueshifted from the main squaraine absorption band was observed. This peak has been observed before for squaraine dyes and is usually attributed to H-aggregation.32,33 In all dyes except for those containing the DTS bridges, i.e. DTS-CA and DTS-PA, this peak is well resolved at 630 nm, suggesting the presence of D


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Chemistry of Materials Table 3. Photovoltaic Performancea,b of the Optimized Dye Cells YR6 T-PA TS3 DTP-PA DTT-CA DTT-PA DTS-CA DTS-PA

dye conc. [mM]

CDCA conc. [mM]

0.1 0.05 0.1 0.05 0.1 0.1 0.05 0.05

10 10 10 10 10 50 10 10

JSC [mA/cm2]

VOC [V] 0.647 0.644 0.61 0.642 0.644 0.621 0.682 0.676

± 0.006 ± 0.000 ± ± ± ± ±

14.2 9.6 13.5 5.9 13.1 3.7 19.1 10.4

0.002 0.004 0.002 0.000 0.002

± 0.3 ± 0.3 ± ± ± ± ±

0.4 0.3 0.2 0.2 0.2

FF[%] 70.4 72.2 68.3 73.5 71.6 76.3 68.3 70.5

± 0.2 ± 0.6 ± ± ± ± ±

0.1 1.1 0.2 0.7 0.4

PCE [%] 6.5 4.6 5.6 2.8 6.0 1.8 8.9 5.0

± 0.1 ± 0.2 ± ± ± ± ±

0.3 0.1 0.1 0.2 0.1

a

Based on measurements of three devices for all dyes with the exception of TS3 with the standard deviation shown. bThe photovoltaic measurements were conducted in the absence of a mask with a cell active area of 0.36 cm2; when a mask40,41 was utilized, a decrease of up to 10% in PCE was observed. Dye soaking time is 4 h.

efficiency, and dye regeneration efficiency.2 Unlike traditional ruthenium dyes, squaraines tend to have much shorter lived excited states, and relaxation from excited state to ground state potentially competes with electron injection to the conduction band of TiO2. To determine the effect of the excited state lifetime and charge injection on the dye cell performance, we measured the excited state lifetimes and decay dynamics of the dyes adsorbed on TiO2 and Al2O3 films in fully assembled dye cells in the presence of electrolytes by femtosecond transientabsorption spectroscopy (details of the setup/experiment are available in the Supporting Information). It is assumed that the excited state of the dye on Al2O3 film decays to the ground state, and there is no charge injection, while excited states of dyes on TiO2 films may quench through charge transfer to the conduction band of TiO2. The decay dynamics of the excited state on both Al2O3 and TiO2 films were determined by following the excited state absorption peak (typically 475 to 550 nm) and were fitted using the stretched exponential function:30

Figure 4. Photovoltaic performance of the optimized dye cells with TiO2 films being soaked in dye bath with 10 mM CDCA and different concentration of dye for 4 h.

⎡⎛ t ⎞ β ⎤ f (t ) = A × exp⎢⎜ − ⎟ ⎥ ⎣⎝ τ ⎠ ⎦

(I)

where A is the pre-exponential constant, τ is the average characteristic time, and β is the heterogeneity constant, accounting for the distribution of the local variations of the dye molecules on the substrate. Since there is variation of β for different dyes, the weighted average lifetime of the excited state was calculated by using eq II:44 τobs =

τ ⎛1⎞ Γ⎜ ⎟ β ⎝β⎠

(II)

where Γ is the generic gamma function. The charge injection rate constant (kei) and charge injection efficiency (φei) were derived using the eqs III and IV: kei = Figure 5. IPCE of the optimized dye cells with TiO2 films being soaked in a dye bath with the same dye/CDCA ratio in Table 3.

1 τTiO2

−

1 τAl 2O3

(III)

And

lower IPCE going from CA to PA based molecules relates well to the lower JSC and PCEs discussed earlier. 2.4. Charge Injection and Recombination Dynamics. In an attempt to fully understand the observed IPCE, a more detailed study of the charge-injection and recombination dynamics was carried out. IPCE is mainly determined by light harvesting, charge injection efficiency, charge collection

φei =

1 τTiO2

− 1 τTiO2

1 τAl2O3

(IV)

and summarized in Table 2 along with the charge injection and dye regeneration driving force (ΔGei0 and ΔGreg0). Charge injection efficiencies for the CA-containing dyes (95%−99%) are higher than those for the PA-containing dyes E


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Figure 6. Impedance measurements of the dye cells with TiO2 films being soaked in a dye bath with 10 mM CDCA and different concentrations of dye for 4 h: YR6 (0.1 mM), T-PA (0.05 mM), TS3 (0.1 mM), DTT-PA (0.05 mM), DTT-CA (0.1 mM), DTP-PA (0.1 mM, 50 mM CDCA), DTS-CA (0.05 mM), DTS-PA (0.05 mM). Nyquist plots (left) and Bode plots (right).

(41%−90%). For all the PA-containing dyes, κei was significantly smaller than that of their CA counterparts (kei(CA)/kei(PA) = 2.3−17.7), which is consistent with the lower IPCE seen for PA dyes (23−63%) in comparison with the CA dyes (56−88%). This may result from several factors such as a higher extent of dye aggregation leading to enhanced quenching, decreased electron density near the TiO2 surface for the PA anchoring group in comparison with the CA counterpart, smaller orbital overlap between the PA and the TiO2 surface compared to the CA anchoring groups, a shift in the CBE of the TiO2 upon binding of the PA anchoring group that results in less ΔGei0 for PA than for CA, or combinations of these factors. Also it is important to note that the lifetime of the excited state of the CA dyes with alkyl groups on the π-bridge, e.g. TS3 (14.1 ps) and DTS-CA (22.6 ps), are significantly smaller than those of the nonalkylated π-bridge CA dyes YR6 (123.9 ps) and DTT-CA (3942 ps), possibly due to an increased number of vibrational pathways for deactivation of the excited state in the alkylated π-bridge dyes. Thus, there may be some degree of trade-off between using alkyl groups to decrease aggregation and increase JSC, although increased deactivation of the excited state from more vibrational pathways may not affect JSC as significantly as aggregation. Most importantly, the higher IPCE seen in the DTS-CA devices (88%) compared to the other CA dyes (55−71%) is not consistent with the charge injection efficiency (94.6% for DTSCA and 97.6−99.0% for CA dyes). The higher IPCE of DTSCA at the maximum absorption band might suggest slower charge recombination for DTS-CA and thus higher charge collection efficiency. In addition, DTS-CA also demonstrates a 40 meV higher dye regeneration driving force, possibly resulting in more efficient dye regeneration45 and higher IPCE. The charge recombination dynamics between the injected electrons in TiO2 film and I3− in electrolyte competes with the charge collection process and limits both the VOC and JSC.46−48 Electrochemical impedance spectroscopy has been used to study this effect in the frequency domain, and the results are displayed in Figure 6. The Bode plots and Nyquist plots in Figure 6 for the dye cells were acquired in the dark with a bias of 0.64 V, where the bias voltage is applied in the opposite direction of the photovoltage generated under illumination. The largest semicircle and the highest frequency peak observed in Nyquist plots and Bode plots in Figure 6 are ascribed to

charge transfer at the TiO2/electrolyte interface,46,49 which is the charge recombination between injected electrons and electrolyte. The radius of the intermediate semicircle represents the charge-transfer impedance from TiO2 film to electrolyte oxidant I3−, while the lifetime of electrons in TiO2 film were derived from equation τ = (2πf)−1, where f is the frequency of the peak in Bode plots. The most striking feature of these plots is that the charge-recombination impedance and electron lifetime of the dye cells based on PA dyes are larger than their CA counterparts, as shown in Table 4. For instance, the Table 4. Frequency of the Intermediate Peak in the Bode Plots in Figure 5b and the Lifetime of Electrons in TiO2 Film, Which Was Derived by τ = (2πf)−1 dyes

YR6

T-PA

TS3

DTPPA

DTTCA

DTTPA

DTSCA

DTSPA

f [Hz] τ (ms)

21.3 7.5

16.7 9.5

27.7 5.7

11.0 14.4

25.6 6.2

11.2 14.2

17.5 9.1

9.9 16.1

charge recombination dynamics for the dyes with DTP and DTT bridges showed the largest dependence on the anchoring group CA/PA, which changed the τ from 5.7 to 14.4 ms and 6.2 to 14.2 ms, respectively, while the electron lifetime in TiO2 film for dyes with thiophene bridges are slightly changed by anchoring group (7.5 and 9.5 ms for CA and PA, respectively). The lifetime of the electrons in TiO2 films also showed dependence on the bridges for each set of dyes with CA and PA anchoring groups. The DTP and DTT bridges resulted in the fastest charge-recombination dynamics among CA dyes, while the thiophene bridge resulted in the highest recombination reaction rate for PA dyes. Most importantly, dyes with the DTS bridge demonstrated the slowest charge-recombination impedance and longest lifetime of injected electrons in TiO2 film within their respective series. The slowest charge recombination rate for DTS dyes resulted in the highest VOC for corresponding dye cells among all these dyes with either CA or PA anchoring groups, with a VOC of 0.682 and 0.676 V, respectively.

3. CONCLUSION In summary, the effect of four different π bridges and two anchoring groups on the optoelectronic properties and DSSC F


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performance of squaraine-based sensitizers were investigated. Dyes with DTS bridges achieved the highest efficiencies in their respective series (8.9% in the case of CA anchoring group and 5.0% in the case of PA anchoring group), which is attributed to higher JSC and VOC due to lower dye aggregation and slower recombination rates, respectively. Femtosecond transient absorption spectroscopy showed that going from CA to PA based dyes resulted in reduction in charge injection efficiency, which coupled with higher affinity toward dye aggregation yielded a lower and narrower IPCE and hence PCEs. On the other hand, electrical impedance measurements showed that PA based dyes may exhibit a slower charge recombination rate between injected electrons and electrolyte than their CA counterpart. The study suggests that aggregation may be alleviated in squaraine-based dyes by including out-of-plane alkyl groups on the π bridge, as is evident by the effectiveness of the DTS bridge in increasing the JSC, and ultimately the PCE. Even though these alkyl groups seem to decrease the excited state lifetimes possibly due to increased vibrational deactivation pathways, this study also suggests that the out-of-plane alkyl groups may increase VOC by reducing the interaction of the electrolyte with the TiO2 surface.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic details and dye characterizations, device fabrication protocols and characterizations, as well as all relevant 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 §

The authors wish to declare that F.M.J. and X.K. contributed equally to this body of work. 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. Marder’s group would like to thank the Center for Interface Science: Solar Electric Materials (CISSEM) an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001084 (F.M.J. and S.R.M. were funded by CISSEM), and the Air Force Office of Scientific Research through the BIONIC program (FA9550-091-0162).

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Near-Infrared Asymmetrical Squaraine Sensitizers for Highly Efficient

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