Alternation of Charge Injection and Recombination in Dye-Sensitized

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Alternation of Charge Injection and Recombination in Dye-Sensitized Solar Cells by the Addition of Nonconjugated Bridge to Organic Dyes Xue-Hua Zhang,† Junichi Ogawa,∥ Kenji Sunahara,‡ Yan Cui,† Yu Uemura,‡ Tsutomu Miyasaka,§ Akihiro Furube,†,‡ Nagatoshi Koumura,†,‡ Kohjiro Hara,*,† and Shogo Mori*,∥ †

National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan § Graduate School of Engineering, Toin University of Yokohama, Yokohama, Kanagawa 225-8502, Japan ∥ Division of Chemistry and Materials, Faculty of Textile Science and Technology, Shinshu University, Tokida, Ueda 386-8567, Japan ‡

S Supporting Information *

ABSTRACT: Most metal-free organic dyes for dye-sensitized solar cells are designed by following a donor conjugated-bridge acceptor structure with a carboxyl acid as an anchoring unit. In this work, we examined the influence of a nonconjugated methylene unit between the cyano group and carboxyl acid by applying it to a previously reported carbazole dye, MK-2. Two dyes, MKZ-35 and -36, were synthesized with glycine and β-alanine, respectively. Dye-sensitized TiO2 solar cells (DSSCs) with MKZ-35 and -36 showed smaller values in the short-circuit current (Jsc) and higher values in opencircuit voltage (Voc) compared with the values with MK-2. The lower Jsc was due to less injection efficiency and fast geminate recombination while the higher Voc was attributed to longer lifetime of the injected electrons in the DSSCs. DFT calculations showed that MKZ-35 dyes interact with each other. One possible explanation for the longer electron lifetime is that the interacted molecules may act as a 3D enlarged dimer molecule or form an induced quadrupole, reducing the interaction between the dyes and acceptor species. On the other hand, the longer electron lifetime with MKZ-36 than that with MK-2 seems to be due to the longer distance between the TiO2 surface and conjugated framework of the dye.

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INTRODUCTION Dye-sensitized solar cells (DSSCs) have attracted extensive interest over the past two decades.1 This is because the expected ratio of the energy conversion efficiency to the cost of materials and fabrication processes could be lower than that for the conventional solar cells. In the DSSCs, light is absorbed by sensitizers anchored on a porous semiconductor electrode immersed in an electrolyte. Excited electrons in the sensitizers are injected into the conduction band of the semiconductor, while the ground state of the sensitizer is regenerated by the electrolyte, resulting in charge separation. There are many factors determining the photovoltaic performance of the DSSCs. Sensitizer is one of the most important components, and as a result, huge attempts have been made to develop the sensitizers. While ruthenium polypyridyl complexes have been actively investigated, metal-free organic dyes have also been attracting intensive research efforts by virtue of their high molar extinction coefficients (ε), facile modification, and inexpensive material costs. Many kinds of metal-free organic dyes have been investigated for DSSCs, and encouraging energy conversion efficiencies have been achieved. The high efficiency is mainly based on the increase of the short-circuit current (Jsc) by © 2013 American Chemical Society

extending the conjugated linker length, which can increase the absorption spectrum.2 On the other hand, organic dyes often showed lower open-circuit voltage (Voc) in comparison to the case of Ru complex dyes. The Voc is determined by the difference between the Fermi level of TiO2 (EF) and the redox potential of the electrolyte. The EF scales logarithmically with the electron density in the TiO2. The electron density is roughly proportional to the product of charge injection rate and electron lifetime, and thus, the lifetime is one of the important parameters. We have reported that the addition of alkyl chains to organic dyes was effective to increase the Voc3 and revealed that this was due to the blocking effect of the dye layer, which increased the electron lifetime.4 Recently, various organic dyes have succeeded to gain open-circuit voltage mostly by extending the blocking effect by attaching bulky moieties to the framework of organic dyes.5 The low Voc observed with many organic dyes have been attributed to the shorter lifetime of the injected electrons in the Received: October 22, 2012 Revised: December 21, 2012 Published: January 25, 2013 2024

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between the induced dipole in the dye and the TiO2 surface. This is to locate the I3− attraction source, that is, the induced dipole, further from the TiO2 surface. In view of energy conversion efficiency, anchoring through a nonconjugated linker may result in the decrease of injection yield.11,12 Thus, this approach is usually not taken to design the dyes for DSSCs, and no systematic work has been done in view of the electron lifetime. Here, we have synthesized dyes having different methylene lengths. Indeed, the newly synthesized dye with nonconjugated unit showed long electron lifetime as expected. However, the reason was not as expected.

TiO2 but not due to the positive shift of the TiO2 conduction band edge potential.4,6 The shorter lifetime was probably caused by the combination of three factors: (1) less blocking effect against the approach of I3−, (2) indirect attraction of I3− due to the partial charge of the dye molecules, and (3) attraction of I3− due to dispersion forces.6 The addition of alkyl chains not always7,8 but often enhances the blocking effect. To reduce the effect of the partial charges and induced dipole in sensitizers, one way is to increase the distance from the charges and the induced dipole in the sensitizers to the acceptor species. For example, the additions of obstacle unit, which is not to add functions for light absorption properties but is to increase the distance, to the dyes can be employed. Alkyl chains can be used not only to add the blocking effect but also as the obstacle unit.9 The addition of bulky moieties to the dye framework without conjugation is also effective.10 Here, nonconjugation is a key point because an increase of conjugation length results in the increase of the polarizability, causing the increase of the dispersion force. In this work, we examined the effect of the nonconjugated linker between the dye framework and anchoring unit. Figure 1

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EXPERIMENTAL SECTION Materials and General Procedures. All starting materials and solvents for synthesis, measurements, and solar cell fabrication were purchased from Wako Chemicals, Kanto Chemicals, Tomiyama Pure Chemical Industries Ltd., Aldrich, Tokyo Chemical Industry Co., Ltd., and/or Merck and used without further purification. 1H NMR and 13C NMR spectra were recorded on a BrukerAvance 400 (400 MHz) spectrometer in CDCl3 or THF-d8, and chemical shifts were reported as δ values (ppm) relative to internal tetramethylsilane (TMS). Fourier transform infrared (FT-IR) spectra were measured with a Perkin-Elmer Spectrum One spectrophotometer with an attenuated total reflection (ATR) system equipped with a ZnSe prism. Mass spectra were measured on a JEOL MS600H mass spectrometer, and elemental analyses were taken on a CE Instruments EA1110 automatic element analyzer. Absorption spectra were measured on a SHIMADZU UV3101 PC spectrophotometer, and solvents used for spectroscopy experiments were spectrophotometric grade. Cyclic voltammetry measurements were carried out on a CHI610B electrochemical analyzer, and dye-loaded TiO2 film, platinum, and Ag/Ag+ (0.01 M AgNO3 + 0.1 M tetrabutylammonium perchlorate in acetonitrile) were employed as working, counter, and reference electrodes, respectively. The supporting electrolyte was 0.1 M tetrabutylammonium perchlorate in acetonitrile, which was degassed with N2 for 15 min before scanning, and the scanning rate was 100 mV s−1. The potential of the reference electrode was calibrated with ferrocene immediately after CV measurements.

Figure 1. Structures of MK-2, MKZ-35, and MKZ-36.

shows the previously reported dye and newly synthesized dyes. A motivation is to extend the distance between the dye’s HOMO and TiO2 surface to retard the charge transfer from TiO2 to the dye. This recombination process had not been a problem for most DSSCs but is becoming an important issue when redox couples having a more positive redox potential are employed. Another motivation is to extend the distance Scheme 1. Synthesis of MKZ-35, MKZ-36, and MK-2

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Transient Absorption Measurement. Transient absorption was measured by a diffuse reflectance femtosecond transient absorption system reported previously.16 In short, an amplified Ti:sapphire laser was combined with an optical parametric amplifier to obtain a pump beam at 532 nm and with white-light continuum generation optics to obtain a probe beam. Pump and probe beams were focused on a fully assembled DSSC under open-circuit conditions, and diffusely reflected light was collected by an achromatic lens and detected by an InGaAs photodetector through a monochromator. The pump light intensity was 0.1 μJ/pulse, and the focal size on the DSSC electrode was ∼0.3 mm in diameter. The transient absorption intensity is displayed as % absorption = (1 − R/R0) × 100, where R and R0 are the probe pulse intensities with and without excitation, respectively. Transient Photovoltage Measurements. Electron lifetimes in the DSSCs were obtained by stepped light-induced transient measurements of photovoltage. Experimental procedures were prepared using the reported methods.17 In short, DSSCs were irradiated by a diode laser (Coherent, Lablaser, 635 nm, 10 mW), and the decay of open circuit voltage, caused by a stepwise decrease of a small fraction of the laser intensity, was measured. A lens was used to expand the laser spot so that the whole cell was irradiated by the laser. The measurement was repeated with various initial laser intensities, giving different electron density in the DSSCs. τ was obtained by fitting an exponential function, exp(−t/τ), to the voltage decay. The electron density was estimated by the charge extraction method introduced by Peter and co-workers.18 Irradiated laser light was turned off simultaneously as the circuit condition was switched from the open circuit to the short circuit condition. Then, the induced transient current was recorded, and the amount of the charges in the DSSC at open circuit conditions was obtained by numerical integration of the transient current. The control of the laser intensity and circuit conditions was performed by LabVIEW with a DAQ card (National Instruments). Electron lifetime in the dark was measured by darkSCIM method using a galvanostat controlled by the DAQ card.19 Fluorescence Measurements. Fluorescence spectra of 0.01 mM MK-2 or MKZ-35 in a mixed solvent (t-BuOH/AN/ toluene = 1:1:1 vol. ratio) were measured with and without I3−. Electrolytes were 1:1 ratio of TBAI and I2 and their concentrations in the dye solutions were varied between 0 and 0.2 mM. Fixed 0.2 mM of TBAClO4 was also dissolved in the solution. MK-2 and MKZ-35 were excited at wavelengths of 440 and 410 nm, respectively. DFT Calculations. Theoretical calculations were performed on Gaussion 09 program package (revision A.02) using density functional theory (DFT) method.20 Becke’s three-parameter hybrid functional with the LYP correlation functional (B3LYP)21 was employed together with 6-31G(d) basis set. Geometry optimizations and electronic properties of the dyes were carried out without any symmetry constraint in the gas phase. Vibrational spectra of the optimized structures were calculated to confirm no imaginary frequency.

Synthesis of the Dyes. Carbaldehyde 6 and MK-2 were synthesized according to our previous methods,3 and N(cyanoacetyl)glycine tert-butyl ester 1, N-(cyanoacetyl)alanine tert-butyl ester 2, N-(cyanoacetyl)glycine 3, and N(cyanoacetyl)alanine 4 were synthesized using the reported procedures.13 Finally, the condensation of aldehyde 6 with N(cyanoacetyl)glycine 3 or N-(cyanoacetyl)alanine 4 in acetonitrile and toluene in the presence of piperidine gave dyes MKZ35 and -36, respectively. The synthetic route is illustrated in Scheme 1. The dyes were characterized by 1H NMR, 13C NMR, IR, mass spectra, and elemental analyses. The detailed characterizations of all of the intermediates and the dyes are illustrated in the Supporting Information. Fabrication of DSSCs and Photovoltaic Measurements. F-SnO2 (FTO) coated glass substrates (Asahi glass, 10 ohm/square) were cleaned in a detergent solution by an ultrasonic bath, rinsed with water and ethanol, and then dried using N2 current. A set of TiO2 electrodes was prepared from commercially available TiO2 paste (Nanoxide-T, Solaronix). Other sets of TiO2 films, 1.5 and 5.5 μm electrode, consisting of only ∼20 nm nanoparticles or 15 μm electrode consisting of a 10 μm transparent layer and a 5 μm scattering layer, were prepared with homemade TiO2 nanoparticles using a screen printing technique, following TiCl4 treatment.14 The TiCl4 treatment often results in the increase of Jsc without decreasing Voc. The TiO2 films were immersed in the toluene solution of the dyes (0.3 mM) for about 12 h. The dye adsorbed TiO2 film electrode and Pt-counter electrode were assembled in a sealed sandwich solar cell with a hot-melt Surlyn film (30 μm in thickness) as spacer between the electrodes. A drop of the electrolyte solution was driven into the cell through the hole in the counter electrode via the suction through another drilled hole. Finally, the two holes were sealed using additional hotmelt Surlyn film covered with a thin glass slide. In this work, two kinds of electrolytes were employed: (A) 0.6 M 1,2dimethyl-3-n-propylimidazolium iodide (DMPImI) + 0.1 M LiI + 0.05 M I2 + 0.5 M TBP in acetonitrile (AN) and (B) 0.20 M instead of 0.05 M I2 in electrolyte A. For the case of MK-2, electrolyte B gave a higher efficiency, because the bulky MK-2 molecule increased the electrolyte diffusion resistance and thus required higher I3− concentration.15 Several identical cells were prepared for each of the preparation conditions. Comparisons were also done several times to confirm the trends observed from experiments. The prepared dye-sensitized solar cells were illuminated through the conducting glass support with a black mask with an aperture area of 0.144 or 0.2354 cm2 in order to avoid the penetrating of diffuse light into the active dye-loaded film (the cell area of the TiO2 film electrode was ca. 0.19 or 0.25 cm2). The performance of the dye sensitized solar cells was characterized by incident photon-to-current conversion efficiency (IPCE) and photocurrent−voltage (J−V) measurements. External IPCE were measured with a CEP-99W system (Bunkoh-Keiki Co., Ltd.). J−V curves were obtained from a computer controlled source meter (Advantest, R6243) under illumination of simulated AM 1.5 G solar light from an AM 1.5 solar simulator (Yamashita Denso, YSS-100A, and Wacom Co., Japan, WXS-80C-3 with a 300 W Xe lamp and an AM 1.5 filter). The incident light intensity was calibrated by a standard crystalline silicon solar cell with an IR-cutoff filter (Schott, KG5), giving the photo response range of amorphous silicon solar cell produced and calibrated by Japan Quality Assurance Organization.

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RESULTS Absorption and Electrochemical Properties. The normalized absorption spectra of the amino acid substituted MKZ dyes (MKZ-35 and -36) and unsubstituted MK dye (MK-2) in toluene solutions and adsorbed on the transparent TiO2 films (1.5 μm in thickness) are shown in Figure 2. The 2026

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Figure 2. Absorption spectra of the MKZ-35, MKZ-36, and MK-2 in toluene solution (a) and on 1.5 μm thick homemade TiO2 particle based film in air (b). For panel b, a bare TiO2 film was used as a reference.

Table 1. Photophysical and Electrochemical Properties for Dyesa

MK-2 MKZ-35 MKZ-36

λmax/nm (ε/104 M−1 cm−1)

λmax/nm, on TiO2 film

495 (4.26) 485 (4.39) 480 (4.15)

460 470 473

amount of adosorbed dye/ −8 10 mol cm−2 μm−1 2.4b 2.2b 2.3b

2.5c 2.4c 2.0c

HOMO/V (vs NHE)

E0−0/eV

LUMO/V (vs NHE)

0.96 0.93 0.94

1.85 1.87 1.88

−0.89 −0.94 −0.94

a λmax and the corresponding ε were obtained in toluene solution. The formal oxidation potential of the dye-loaded TiO2 film is taken as the HOMO, and the LUMO is calculated with the expression of LUMO = HOMO − E0−0, where E0−0 is derived from the absorption onset wavelength of the dye-loaded film. bOn homemade TiO2 particles based electrode. cOn commercial TiO2 particles based electrode.

Table 2. Photovoltaic Performance of the DSSCsa

photophysical and electrochemical data are summarized in Table 1. Similar to MK-2, the MKZ dyes exhibit their major electronic absorption in the range of 400−600 nm in solution. Due to the introduction of the amino acid group, the absorption maxima of the MKZ dyes were shifted to shorter wavelengths in comparison to that of MK-2. This is probably because the amino acid group weakened the electronwithdrawing ability of the acceptor part. The ε at the maximum absorption in toluene solution was 43 900 and 41 500 M−1 cm−1 for MKZ-35 and -36, respectively. When these dyes are adsorbed on transparent TiO2 films, the peaks of the absorption spectra shift toward shorter wavelength region, as shown in Figure 2b and Table 1. This is probably caused by specific interactions between the dyes and TiO2 or deprotonation of the dyes. However, due to the substitution of the amino acid group, the degree of the MKZ dyes’ blue shift on TiO2 film was a little smaller than that of MK-2. This means that the strength of the specific interactions/deprotonation of the MKZ dyes on TiO2 films became smaller than that of MK-2. Among the carbazole dyes, the amounts of adsorbed dyes on the homemade TiO2 based samples were similar. On the commercial TiO2 based electrodes, MK-2 and MKZ-35, showed similar amounts of dyes while MKZ-36 showed a bit smaller values (Table 2). These differences between the TiO2 electrodes and dyes could be due the relative size of the dye molecules to the pore size of the TiO2 electrodes. Cyclic voltammetry measurements were carried out to determine the ground state oxidation potentials (taking as HOMO) of the dyes. The excited state oxidation potential (taking as LUMO) was estimated by subtracting the 0−0 transition energy (E0−0) from HOMO level energy, while E0−0 is derived from the absorption onset wavelength of the dyeloaded film. As shown in Table 1, both HOMO and LUMO

dye b,d

MK-2 MKZ-35b,d MKZ-36b,d MK-2c,d MKZ-35c,d MK-2c,e MKZ-35c,e

thickness/μm

Jsc/mA cm−2

Voc/V

FF

η (%)

3.2 3.5 3.4 5.5 5.5 15 15

9.9 8.7 7.3 12.7 11.7 15.5 14.7

0.70 0.75 0.72 0.74 0.77 0.70 0.73

0.61 0.65 0.69 0.64 0.70 0.71 0.75

4.2 4.3 3.6 6.0 6.3 7.7 8.0

a Incident light: AM 1.5G (100 mW cm−2). bPrepared with nanoxide-T with a 0.19 cm2 cell area and measured with 0.144 cm2 mask. c Prepared with homemade TiO2 electrode with a 0.25 cm2 cell area and measured with0.2354 cm2 mask. dElectrolyte A. eElectrolyte B.

energies of the MKZ dyes decreased slightly, and E0−0 became a little larger compared with that of MK-2. The LUMO energy of the MKZ dyes were much more negative than the conduction band edge of TiO2, which is located at ca. −0.5 V (vs NHE), and the HOMO energy of the dyes were more positive than the iodine redox potential value (∼0.4 V, vs NHE), thus providing a thermodynamic driving force for the electron injection to TiO2 and the dye regeneration reaction from iodine redox. Photovoltaic Performance. Figure 3 shows the action spectra of IPCE for DSSCs based on these dyes using 5.5 μm TiO2 (transparent layer) electrode and the electrolyte A. The DSSCs based on the new MKZ dyes produced maximum IPCE values below 70%, whereas the DSSCs based on MK-2 produced IPCE values of higher than 70%. With the extension of the nonconjugated methylene group, the IPCE value decreased. The IPCE is a product of three efficiencies, that is, IPCE = LHE × Φin × Φc, in which LHE is the light harvesting efficiency of the dye-loaded film, Φin is the electron injection 2027

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DSSCs with MKZ-35 and -36 as well as MK-2, the electron injection occurred within 500 fs. The subnano second decays are ascribed to the recombination between the dye cation and the injected electron. Since there is I− in the electrolyte, the decay signal could be due to the reduction of dye cation by I−. However we also observed subnanosecond decay without electrolytes and similar decay dynamics for the dye cation and injected electron (Figure S3, Supporting Information), ruling out the possibility of the reduction by I−. Thus, Figure 4 suggests that the low IPCE values of DSSCs with MKZ dyes are due to insufficient electron injection and subnanosecond recombination. We have also fitted the decays with double exponential function; y = y0 + y1 exp(−t/τ1) + y2 exp(−t/τ2). The amplitude and time constant of the fast component were similar among the dyes and the values were between 0.15 and 0.18 and 5 and 7 ps, respectively. On the other hand the amplitudes and time constants of the slow component were 0.18, 0.33, and 0.40 and 109, 142, and 211 ps, for MK-2, MKZ35, and MKZ-36, respectively. Among the dyes, the amplitude of the slower component was increased with the length of the nonconjugated bridge, suggesting more recombination occurred with the increase of the length. The same trend was seen for dye adsorbed TiO2 films without electrolytes (Figure S4. S.I.). This trend is not expected based on a simple distance related electron tunneling model, while the more recombination agreed to lower IPCE values for MKZ-36. Similar fast recombination has been reported from different dyes,23 but the reason of the fast recombination is not yet clear. I−V Characteristics. The photovoltaic performances of the DSSCs based on these dyes are summarized in Table 2. Jsc decreased in the order of MK-2 > MKZ-35 > MKZ-36, which is consistent with the decrease of the IPCE values. The Voc (0.72−0.75 V) for the DSSCs based on these MKZ dyes were higher than those for MK-2. After optimization using 15 μm TiO2 films and the electrolyte B, the maximum η value of 8.0% for MKZ-35 sensitized solar cell was obtained under AM 1.5G irradiation (100 mW cm−2), with Jsc of 14.7 mA cm−2, Voc of 0.73 V, and FF of 0.75 (Figure 5). Electron Lifetime Measurements. Figure 6 shows the electron lifetime and Voc values as a function of electron density in the DSSCs with the three dyes. Among the DSSCs with the dyes, the Voc values at the matched electron density were comparable, suggesting the conduction band edge potentials

Figure 3. IPCE of DSSCs based on MKZ dyes compared with that of MK-2 dyes using 5.5 μm TiO2 electrode and electrolyte A.

efficiency, and Φc is the collection efficiency of the injected electron at the back contact of the photoanode. Since the absorbance was in the range of 1.7−2.3 even for the dye-loaded ∼1.5 μm TiO2 film, the LHE should be very close to unity for the dye-loaded TiO2 films in ∼5.5 μm thickness. Transient Absorption Study. A possible reason of the lower IPCE values with MKZ-35 and -36 is low injection yield due to the nonconjugated bridge. The transient absorption spectra from 700 to 1000 nm were measured. Spectra of MKZ35 and -36 showed strong S−S absorption around 900 nm at 2 ps and quickly decayed within 50 ps (see the Supporting Information, Figure S2). In this time scale, no rise component was observed, indicating that this quick S−S absorption decay is due to the quenching without competing with electron injection.22 This means that the MKZ-35 and -36 has lower initial injection yields than that of MK-2. Figure 4 shows the

Figure 4. Normalized diffuse reflectance transient absorption decay of DSSCs, probed at 750 nm, pumped at 532 nm.

normalized transient absorption kinetics probed at 750 nm. At this probe wavelength, the absorption was due to the dye cation, and the transient absorption is mainly caused by the change in the dye cation concentration, which was caused by charge injection and recombination. Therefore, we can evaluate the electron injection and recombination dynamics from the observed transient absorption. All of the transient absorption decay profiles showed very fast rise (within the time resolution of our setup, ∼500 fs) and subnano second decay. Thus, for the

Figure 5. Optimized I−V and IPCE (inset) curves of a DSSC based on MKZ-35 using 15 μm TiO2 film and electrolyte B. 2028

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Figure 6. Electron lifetime (a) and open circuit voltage (b) of DSSCs with various amounts of dyes as a function of electron density. “Bare” denotes the cell without dye adsorption. “Full, middle, low” denotes the adsorbed amount of dyes, and the numbers in the parentheses are the percentage of the dye amount of “full” cells. The lifetimes were measured under laser irradiation except for “bare” and “dark”.

were similar among the DSSCs. On the other hand, the DSSCs with MKZ-35 and -36 showed longer lifetimes than the DSSCs with MK-2, and the DSSCs with MKZ-35 showed the longest lifetimes. The order of the electron lifetimes agreed with the order of Voc values among the dyes. The electron lifetimes of the DSSCs prepared with a reduced amount of adsorbed dyes and without dyes were also plotted in Figure 6. The fully loaded cells showed longer lifetimes than the bare TiO2 cells, suggesting that the carbazole dyes examined here have a blocking effect. When the amount of these dyes was reduced, the electron lifetimes decreased. For the cases of MK-2 and MKZ-36, the lifetimes became much shorter than the lifetimes of the bare cells. This means that these dyes facilitated the charge recombination. The facilitation can be interpreted as being that the dyes themselves attract I3− by dispersive force, and the effect emerged when the density of the adsorbed dyes on the TiO2 surface decreased; that is, there are spaces for I3− to be around the dyes. On the other hand, when the amount of MKZ-35 was reduced, the lifetime became slightly shorter than that in the bare cells but not as short as the case of MKZ-36 and MK-2. This suggests that MKZ-35 could also attract I3− but the strength of the attraction was not as strong as the cases of MK-2 and MKZ-36. To check the possibility of recombination with dye cation, the electron lifetime in the DSSCs with MK-2, MKZ-35 and MKZ-36 was also measured in the dark (data only for MKZ-36 is shown in Figure 6). For the dyes, the lifetime was the same with the electron lifetime measured under light illumination. Since there is no dye-cation in the dark, the similar values of the electron lifetime suggest that the dye cation under illumination is not the major source of the recombination. Fluorescence Quenching Measurements. To check if the dyes attract I3− in dye solutions in the absence of TiO2, fluorescence spectra for MK-2 and MKZ-35 were measured with various I3− concentrations. Figure 7 shows the intensity ratio of fluorescence with various amounts of I3−. Excited states of both dyes were efficiently quenched by a low concentration of I3−. Following the discussion by Splan et al.,24 one possible explanation for such efficient quenching for short excited state dyes is the formation of a complex between the dyes and I3−,

Figure 7. Peak intensity ratio of fluorescence from 0.01 mM MK-2 and MKZ-35 in solutions with and without I3−. F0 and F stand for the fluorescence without and with I3−, respectively. Peak wavelengths were around 579 and 537 nm for MK-2 and MKZ-35, respectively.

since there is no sufficient time for collision by diffusion. We also measured excited state lifetimes for MK-2 and MKZ-35 without I3− using fluorescence decay measurements (data not shown). Their lifetimes were comparable (between 0.7 to 0.8 ns). Therefore, in Figure 7, MK-2 showed more efficient quenching than MKZ-35, implying a stronger attraction between MK-2 and I3− than that between MKZ-35 and I3−. DFT Calculations. DFT calculations were performed to examine the possibility of interactions between dyes. First, the structures of MK-2 and MKZ-35 molecules were optimized, and then further optimizations were done by placing two dye molecules close each other with various initial positions of the two molecules. Figure 8 shows the results. The energy of the two molecules was compared with the case of two molecules located apart. For the case of MK-2, the decreased energy was 5.2 kcal/mol by locating close to each other, whereas that of MKZ-35 was 4.7 and 13.5 kcal/mol depending on the location of interactions. The value of 13.5 kcal/mol was obtained when the carbonyl group of the acrylamide and the proton of carboxyl acid become close each other (a magnified figure is in the Supporting Information). 2029

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Figure 8. Optimized structures of two MK-2 (left) ,two MKZ-35 (middle), and two MKZ-35 (right). The cases of MKZ-35 were obtained by starting from different initial positions of the two dyes.

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the attraction to the I3−, the longer distance would help to reduce the surface concentration of I3−, resulting in the longer lifetime than the values in the DSSCs with MK-2. In view of electron injection, the longer nonconjugated linker decreased the injection yield and facilitated subnanosecond recombination. To exploit the long electron lifetime with the long nonconjugated linker, another function needs to be included for the sensitizer, for example, attaching a moiety increasing excited state lifetime and reducing the tunneling barrier, to overcome the disadvantage in the electron injection.

DISCUSSIONS Introduction of the nonconjugated bridge to a carbazole dye resulted in the decrease of Jsc and the increase of Voc. The increased Voc for the case of MKZ-35 was caused by the increased electron lifetime. When the amount of adsorbed MKZ-35 was decreased, the lifetime did not decrease as large as the case of MK-2 and MKZ-36, implying that the molecular interaction between the dye and I3− was retarded for MKZ-35. The observed lifetimes in the DSSCs with the carbazole dyes were probably due to the results of the combination of the blocking effect of the dye layer and the dispersion force between the dyes and I3−. The longest lifetime with MKZ-35 can be interpreted as less dispersion force with similar blocking effect in comparison to the case of MK-2 and MKZ-36. The dispersion force is proportional to the polarizability of the molecules and inversely proportional to the distance to the power of the sixth. Polarizability scales roughly with the HOMO−LUMO gap energy, and thus, it is not the reason of the longest lifetime observed with MKZ-35 among the carbazole dyes. DFT calculations suggest that MKZ-35 tends to interact with each other in the dye solutions. Thus, when these paired dyes are adsorbed on the TiO2 surface, they still may be located close each other because the paired dye would hit the TiO2 surface at the same time. Then the closely located dyes could form an induced quadrupole, which reduce the electrostatic force in comparison to that by a dipole. This would be the reason for the apparent smaller interaction between MKZ-35 and I3−. Fluorescent quenching measurements of MK2 and MKZ-35 by I3− also suggest less interaction for MKZ-35, consistent with the measured electron lifetime. Another reason would be that simply the two dyes close each other form a kind of 3D enlarged molecule, and the alkyl chains surrounding the conjugated framework block the approach of the I3− to the framework. Since the dispersion force rapidly decreases with distance, I3− molecules feel little force from the dipole formed in the dye framework. Similar trends have been observed with other dyes having 3D structures.9,10 For the case of MKZ-36, the effect of dispersion force seems larger than the case of MKZ-35 but similar with that of MK-2, as the similar trend in the electron lifetime was seen with MK-2 when the amounts of the dyes were varied. The shorter lifetime in the DSSCs with MKZ-36 than that with MKZ-35 could be due to the longer nonconjugated bridge providing more freedom of the bending of the molecules. Thus the space between the two dye frameworks could become large enough for I3− to enter between the frameworks. On the other hand, the longer nonconjugated linker could locate the conjugated framework of the MKZ-36 further from the surface of TiO2. Since the dipole formed in the framework can be the source of

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CONCLUSIONS The effects of a nonconjugated bridge between the framework of a dye and TiO2 on the injection efficiency and recombination lifetime were investigated. With increasing the bridge length, the injection efficiency was decreased, whereas the injection occurred within 500 fs for the three dyes examined here. Recombination between the injected dyes and I3− was retarded by the addition of the nonconjugated bridge. The fluorescence of dyes in solutions was also not efficiently quenched by I3− for the nonconjugated dye. With the results of DFT calculations, the retardation of recombination and quenching could be due to the interaction between the nonconjugated bridge dyes. If the fast recombination and efficient quenching were due to interaction/weak complex formation between dye and I3− and if it were due to dispersion force, locating two dyes close each other would induce the force between the dyes and reduce the dispersion force between the dyes and I3−. Such a condition could be obtained by designing the dyes to attach to each other. However, the interaction should be differentiated from aggregation, which typically reduces the injection yield. In other words, the intermolecular interactions should occur at the moiety beside the conjugated framework.

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ASSOCIATED CONTENT

S Supporting Information *

Characterization of intermediates and MKZ-35 and -36, transient absorption spectra of dyes, transient absorption of dyes at two different probe wavelengths, magnified image of optimized two MKZ-35 dyes, and the complete author list for ref 20. 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] (K.H.); [email protected] (S.M.). Notes

The authors declare no competing financial interest. 2030

dx.doi.org/10.1021/jp310425z | J. Phys. Chem. C 2013, 117, 2024−2031

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ACKNOWLEDGMENTS This work was partially supported by Kanto Bureau of Economy, Trade and Industry (METI-Kanto). REFERENCES

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