Crown Ether-Substituted Carbazole Dye for Dye-Sensitized Solar Cells

Article pubs.acs.org/JPCC

Crown Ether-Substituted Carbazole Dye for Dye-Sensitized Solar Cells: Controlling the Local Ion Concentration at the TiO2/Dye/ Electrolyte Interface Yu Uemura,†,‡,§ Takurou N. Murakami,*,‡ and Nagatoshi Koumura*,†,‡ †

Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

‡

S Supporting Information *

ABSTRACT: The conduction band edge potentials (ECB) and electron lifetimes (τ) of the TiO2 electrodes in dye-sensitized solar cells (DSSCs) are affected by ion concentrations (e.g., Li+ and I−/I3−) at the TiO2/dye/electrolyte interface. To control the local concentrations of these ions in the vicinity of the TiO2 surface, a novel carbazole-based dye incorporating a 12-crown-4 ether on the carbazole donor (MK-70) was synthesized as a DSSC sensitizer. The interactions between Li+/I−/ I3− and MK-70 were compared with those between Li+/I−/I3− and MK-1, an analogue lacking the crown ether. The crown ether did not affect the ECB level of TiO2, but it did decrease τ at a high electrolytic Li+ concentration. Results suggest that localized Li+ ions associated with the crown ethers electrostatically attract surplus I3− from the bulk electrolyte even though the crown ethers are located far from the TiO2 surface. After the cells were aged, negative shifts in the ECB levels of the TiO2 electrode and blueshifts of the MK-70 absorption spectra were observed with electrolytes that included I−/I3− and Li+. The aging behavior may be determined by the balance between two attractive forces, K1 (I−/I3− and Li+ at the TiO2 surface) and K2 (dye interactions with I−/I3− and Li+).

1. INTRODUCTION Dye-sensitized solar cells (DSSCs)1−3 have attracted significant interest because of their high photoenergy conversion efficiencies and low manufacturing costs.4,5 In addition to ruthenium complex dyes, many metal-free organic dyes6−11 have been developed as sensitizers to date; their advantages include facile molecular structure modification and high absorption coefficients. In a DSSC, the open circuit voltage (VOC) is determined by the potential difference between the Fermi level of the TiO2 and the redox potential of the electrolyte. The Fermi level of the TiO2 changes with its electron density. At the same electron density, however, the Fermi level is determined by the conduction band edge potential (ECB) and electron lifetime (τ). Dyes, which affect both the photocurrent and photovoltage in the DSSC through their blocking and partial charge effects, influence the charge recombination reaction and therefore the electron lifetime.12 Moreover, as a DSSC ages, the ion concentration at the TiO2 surface may be changed by the partial charge effect of the dye, shifting the conduction band edge potential of the TiO2. Previously, we developed carbazole-based organic dyes (MK dyes) for DSSCs with a carbazole moiety as a donor, a hexylsubstituted oligothiophene as a π-conjugated linker, and a cyanoacrylic acid moiety as an acceptor. The alkyl side chains on the π-conjugated linker in the dye molecules had the © 2014 American Chemical Society

important role of suppressing charge recombination between the injected electrons in the nanoporous TiO2 electrode and I3− in the electrolyte by blocking the approach of I3− to the TiO2 surface.12−17 This resulted in improved VOC values in the DSSCs.12 On the other hand, the introduction of ether side chains on the π-conjugated linker in the dye molecules accelerated charge recombination; the increase in the local concentration of I3− occurred because the lone pairs of the oxygen atoms in the side chains attracted I3− through the Li+ in the electrolyte. The acceleration of the charge recombination due to the oxygen atoms in the ether side chains decreased the observed VOCs. Moreover, the dye/Li+/I3− interaction was strongly affected by the position of the oxygen atom in the side chain. In particular, the dyes in which side-chain ether oxygens were far from the π-conjugated backbone interacted with Li+/ I3− more strongly than those with oxygen atoms that were close to the π-conjugated backbone.17 Herein, to control the concentration of Li cations ([Li+]) in the vicinity of the TiO2 surface and to tune both the ECB level and τ in TiO2, we synthesized an MK dye as a DSSC sensitizer Special Issue: Michael Grätzel Festschrift Received: December 29, 2013 Revised: May 9, 2014 Published: May 14, 2014 16749

dx.doi.org/10.1021/jp412731z | J. Phys. Chem. C 2014, 118, 16749−16759

The Journal of Physical Chemistry C

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Figure 1. Molecular structures of (a) MK-70 and (b) MK-1.

in an acetonitrile (CH3CN) solution with 0.1 M tetrabutylammonium perchlorate (TBAP), and the three-electrode cell was purged with dry N2. The potential of the reference electrode was calibrated against a ferrocene redox couple (Fc/ Fc+). The amount of dye adsorbed on the TiO2 film (Γ) was estimated by the following procedure. After the preparation of a dye-loaded TiO2 film (6 μm thickness, 1 cm2 area), the dye molecules were desorbed by immersion in a 3% tetramethylammonium hydroxide solution in 20% THF−toluene. The ultraviolet−visible (UV−vis) spectrum of the solution was measured, and Γ was calculated from the absorbance of the solution and the absorption coefficient (ε). 2.2. Fabrication of DSSCs with Various Li+ Concentrations in the Electrolyte. Nanoporous TiO2 electrodes (4 μm thickness, 0.25 cm2 area) were prepared from TiO2 paste (DSL18NR-T, Dyesol) by annealing at 500 °C for 30 min on TiCl4-treated transparent conductive oxide (TCO) substrates (SnO2:F (fluorine-doped tin oxide, FTO); 9.5 Ω/square, Nippon Sheet Glass). The TiO2 electrodes were immersed in a 0.3 mM solution of each MK dye in CH3CN/t-BuOH/ toluene (1/1/1 by volume) for 18 h. After the electrodes were rinsed with CH3CN, they were placed onto a counter electrode of Pt-coated TCO and sealed with thermal adhesive film (Surlyn, DuPont). An electrolyte was injected through a drilled hole in the counter electrode, which was then covered with the film and a piece of slide glass. In this work, four electrolyte solutions with various Li+ concentrations were prepared (Table 1). As an example of the electrolyte preparation method, electrolyte A (0 M LiI) was prepared by mixing 0.70 M 1,2dimethyl-3-propylimidazolium iodide (DMPImI), 0.05 M I2, and 0.50 M 4-tert-butylpyridine (tBP) in CH3CN. The remaining electrolytes B−D were similarly prepared using the quantities listed in Table 1. All the electrolytes had the same

with a 12-crown-4 ether in the carbazole donor segment (MK70, Figure 1a). This was compared with an analogue MK dye lacking the attached crown ether (MK-1, Figure 1b). Crown ethers are well-known to bind alkali and alkaline earth metal ions selectively18,19 according to the number of oxygen atoms in the crown and the size of the cavity at the center of the molecule. Crown ethers have been used to impart new functions in various molecular devices.20−24 12-Crown-4, which has four oxygen atoms, was reported to specifically bind Li+.25,26 It is well-known that the ECB levels of TiO2 and the τ values in DSSCs change over time, from freshly prepared solar cells (within 1 h after fabrication) to solar cells aged 24 h27−30 because of the adsorption of Li+ and/or other electrolytic ions on the TiO2 surface. In this study, we elucidated the influence of Li+ binding in the crown ether portion of MK-70 and the resulting partial charge effect upon the aging behaviors of DSSCs using various electrolyte compositions in comparison to those of the unsubstituted MK-1. Then, we discussed the masstransfer mechanism and the local Li+ concentration profile at the interface between the electrolyte and TiO2 in the DSSCs.

2. EXPERIMENTAL METHODS 2.1. Materials and General Procedures. MK-70 was synthesized by a modification of a previously described procedure.13 The detailed synthetic procedures are provided in the Supporting Information. All starting materials and solvents for the synthesis were used without further purification after purchase from Wako Chemicals, Kanto Chemicals, Aldrich, Tokyo Chemical Industry Co. Ltd., and Merck. Column chromatography of all the products was performed on silica gel (Kanto, Silica Gel 60N, spherical, 40−50 μm), and some were further purified by preparative high-performance liquid chromatography (YRU-880 detector, Shimamura Tech.) on silica gel (TOSOH, TSKgel Silica-60). 1H and 13C NMR spectra were recorded on a Bruker Avance 400 (400 MHz) spectrometer in CDCl3 or THF-d8. high-resolution mass spectrometry (matrix-assisted laser desorption ionization time-of-flight mass spectrometry) results were analyzed by a Bruker autoflex speed TOF/TOF. Absorption spectra were measured on a Shimadzu UV-3101 PC spectrophotometer. Cyclic voltammograms were recorded using a HZ-3000 electrochemical analyzer (Hokuto Denko Co., Ltd.) via the three-electrode method at a scan rate of 100 mV s−1. A dyeloaded TiO2 thin film (ca. 1.5 μm thickness) was used as the working electrode. Pt wire (BAS Co. Ltd.) and Ag/Ag+ (BAS Co. Ltd.) were used as the counter and the reference electrodes, respectively. The three electrodes were immersed

Table 1. Various Electrolyte and Solution Compositions for DSSCs

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electrolyte

LiI (m/L)

LiClO4 (m/L)

DMPImI (m/L)

I2 (m/L)

tBP (m/L)

solvent

A B C D 1 2 3

0 0.02 0.05 0.50 − 0.05 −

− − − − − − 0.05

0.70 0.68 0.65 0.20 − − −

0.05 0.05 0.05 0.05 − − −

0.50 0.50 0.50 0.50 − − −

CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN



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Figure 2. Absorption spectra of MK-1 and MK-70 in (a) 20% THF−toluene and (b) in fresh dummy cells with sol-1 (CH3CN). (c) Absorption spectra of dyes in fresh dummy cells with sol-2 (0.05 M LiI) and sol-1 and (d) those with sol-3 (0.05 M LiClO4) and sol-1.

Table 2. Electrochemical Properties of MK-1 and MK-70 dye

λmaxa (nm) (ε (×104 M−1 cm−1))

λmaxb (nm)

λmaxc (nm)

λmaxd (nm)

EHOMOe (V vs NHE)

Egapf (eV)

ELUMO (V vs NHE)

Γg (×10−4 mol cm−3)

MK-1 MK-70

479 (3.50) 484 (3.57)

463 468

471 479

487 484

1.04 0.90

1.94 1.90

−0.90 −1.00

1.5 ± 0.1 1.3 ± 0.1

a UV spectrum of each dye was measured in 20% THF−toluene through a 1 mm cell. bUV spectrum of each dye was measured on TiO2 film (∼1.5 μm thickness) with sol-1 (CH3CN). cUV spectrum of each dye was measured on TiO2 film with sol-2 (0.05 M LiI in CH3CN). dUV spectrum of each dye was measured on TiO2 film with sol-3 (0.05 M LiClO4 in CH3CN). eEHOMO was measured by cyclic voltammetry on TiO2 film. fEgap was derived from the absorption onset wavelength on TiO2 film. gΓ is the adsorbed dye density on the TiO2 films with standard deviation.

concentration of I−/I3− ([LiI] + [DMPImI] = 0.70 M; [I2] = 0.05 M). 2.3. Photovoltaic Measurements. The incident-photonto-current conversion efficiency (IPCE) spectra of the DSSCs were measured using an IPCE measurement system (CEP99W, Bunkoukeiki Co., Ltd.) with a photomask (0.148 cm2). The current−voltage (I−V) characteristics were measured under AM 1.5G irradiation (100 mW cm−2) using a solar simulator (YSS-150A, Yamashita Denso) with a photomask (0.148 cm2). τ values were measured by the stepped lightinduced transient measurements of photocurrent and voltage (SLIM-PCV) method.31 Electron densities (n) at open circuit were measured by the charge extraction method.32 These methods are briefly explained in the Supporting Information. 2.4. UV−Vis Measurements of Dyes on TiO2 Electrodes. To prepare the dummy cells for spectral absorption measurements of the MK dyes, the dye-adsorbed TiO2 films (1.5 μm thickness, 0.25 cm2 area) on TCO substrates were sandwiched with a slide glass and sealed with thermal adhesive film in the same manner as that used for DSSC fabrication. A solution was injected via a drilled hole in the slide glass, which was then covered with the thermal adhesive film and a piece of slide glass. In this study, three different solutions (sol-1, sol-2, and sol-3) were mixed in the amounts as described in Table 1. Absorption spectra of the dummy cells were measured on a Shimadzu UV-3101 PC spectrophotometer using a photomask

(0.20 cm2 area). Absorbance spectra were calculated as the difference between the dummy cells with dye to that without dye.

3. RESULTS 3.1. Absorption and Electrochemical Properties of Dyes. Figure 2a shows the absorption spectra of MK-1 and MK-70 in 20% THF−toluene solution. The absorption maximum (484 nm) and the onset wavelength (600 nm) of MK-70 were slightly red-shifted in comparison to those of MK1 (479 and 594 nm, respectively). This is probably due to the presence of the crown ether as an electron-donating group. The molar extinction coefficients ε of the dyes were nearly the same, which suggests that the extent of π-conjugation in these dyes is similar in solution. On the other hand, the absorption spectrum of MK-70 on TiO2 with an electrolyte would likely be different from that of MK-1 because the crown ether in MK-70 may interact with Li+ in the electrolyte.33 To confirm that interaction, the absorption spectra of dummy cells with MK70 were measured in CH3CN (sol-1), LiI in CH3CN (sol-2), and LiClO4 in CH3CN (sol-3). The spectra were compared with the dummy cells of MK-1 under the same conditions. Figure 2b shows the absorption spectra of the dyes on TiO2 films in sol-1. In the absence of Li+, the differences in the absorption spectra were similar to those observed in 20% THF−toluene. The absorption spectra and peak wavelengths of 16751



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Figure 3. (a) IPCE spectra and (b) I−V curves of fresh DSSCs. (c) Open circuit voltages VOC and (d) electron lifetimes τ as functions of the electron density in fresh DSSCs. Four electrolyte compositions with different [Li+] were used for the fresh DSSCs: electrolyte A (0 M LiI, circles); electrolyte B (0.02 M LiI, triangles); electrolyte C (0.05 M LiI, diamonds); and electrolyte D (0.50 M LiI, squares). Black lines and red lines show results for MK-1 and MK-70, respectively. The I−V curves of the DSSCs were measured under AM 1.5G (100 mW cm−2) illumination conditions with a photomask (0.148 cm2) and without an antireflection film. Electron lifetimes and electron densities were measured by the SLIM-PCV and charge extraction methods, respectively.

increased in the electrolyte, for both MK dyes. The direction of the IPCE shift with [Li+] agreed with the shifts of the absorption spectra in the fresh dummy cells (Figure 2c,d). For the same [Li+] in the electrolytes, the onsets of the IPCE spectra for the DSSCs with MK-70 (MK-70-cells) were shifted to wavelengths slightly longer than those of the DSSCs with MK-1 (MK-1-cells) because of the absorption properties of these dyes. The I−V curves of fresh DSSCs with the four different electrolytes are depicted in Figure 3b, and their photovoltaic performance is summarized in Table 3. As expected from the corresponding IPCE spectra, the JSC values for DSSCs with both MK dyes increased with increasing [Li+] in the electrolyte. The JSC values for the MK-70-cells were similar to those of MK1-cells in the same electrolyte. The VOC values for the MK-70-

the dyes on TiO2 with sol-2 and sol-3 are shown in panels c and d of Figure 2 and Table 2, respectively. Both sets of absorption peaks and onset wavelengths were shifted to longer wavelengths compared to those in sol-1. There were no significant differences between MK-70 and MK-1, but electrostatic interactions in the Li+-containing solutions were clearly observed as redshifts of the absorption spectra for both dyes. The electrochemical properties of the dyes are summarized in Table 2. Although the potential levels (EHOMO and ELUMO) of MK-70 were somewhat negatively shifted compared with those of MK-1, they were sufficient for the reduction of dye cations from the I−/I3− redox couple in the electrolyte and electron injection from the excited dye molecules to the conduction band of TiO2, respectively. In addition, the dye amounts adsorbed on the TiO2 films (Γ) were nearly identical between MK-1 and MK-70. Thus, the light-harvesting abilities of the photoanode will be comparable between MK-1 and MK-70. 3.2. Effect of Li+ Concentration in Electrolyte on Performance of Fresh DSSCs. To investigate the aging behaviors of DSSCs, it is very important to measure their initial properties. Therefore, their I−V characteristics, IPCE spectra, and electron lifetimes were assessed within 1 h of fabrication. Figure 3a shows the IPCE spectra of fresh DSSCs in electrolytes A−D with differing [Li+]. In DSSCs for both MK dyes, the maximum IPCE values increased as the [Li+] increased from 0 M (electrolyte A) to 0.05 M (electrolyte C) and then were almost saturated at [Li+] concentrations above 0.5 M (electrolyte D). The maximum values of the IPCE spectra in the DSSCs based on either MK dye were nearly identical in the same electrolyte. The onset wavelengths of the IPCE spectra were shifted to longer wavelengths as [Li+]

Table 3. Photovoltaic Performance of Fresh DSSCsa with MK-70 and MK-1 at Four Electrolyte Li+ Concentrations [Li+] (M) 0 0.02 0.05 0.50

dye

wb (μm)

JSC (mA cm−2)

VOC (V)

FF

η (%)

MK-70 MK-1 MK-70 MK-1 MK-70 MK-1 MK-70 MK-1

4.0 4.1 4.0 4.1 3.9 3.9 4.4 4.0

5.56 5.63 9.44 8.74 10.5 10.3 11.2 11.0

0.75 0.74 0.68 0.69 0.70 0.73 0.63 0.66

0.80 0.81 0.76 0.74 0.74 0.74 0.67 0.67

3.4 3.4 4.8 4.5 5.5 5.6 4.8 4.9

Incident light: AM 1.5G (100 mW cm−2) with a photomask (0.148 cm2) and without an antireflection film. bTiO2 thickness. a

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ions localized near the crown ethers attract I3− and I− from the bulk electrolyte solution. Surplus I3− ions recombine with electrons at the TiO2 surface and induce shorter τ in the MK70-cell in the cases of electrolytes C and D. In other words, the partial charge effect may accelerate the recombination reaction in MK-70 even though the crown ether is located far from the TiO2 surface. In a previous report, a ruthenium complex dye having an ethylene oxide chain, K-51, showed JSC and VOC values higher than those of a ruthenium complex dye having an alkyl chain, Z-907.37 The higher VOC of the K-51-cell may be attributed to its higher electron density at 100 mW light because the higher JSC increased the electron density. Also, a possible partial charge on the isothiocyanate substituent of Z-907 may attract Li+ and I−/I3− to the TiO2 surface. Therefore, the partial charge of the ethylene oxide chain due to Li+ coordination in K-51 attracts these ions away from the TiO2 surface to the vicinity of the ethylene oxide chain, resulting in a reduced recombination reaction and increased VOC. The effects on DSSC performance upon introduction of an ethylene oxide or crown ether substituent to the dye structure might be different for the target dye having an intrinsic partial charge, such as Z-907, and the target dye without a significant partial charge, such as MK1. 3.3. Aged DSSCs with 0 M Li+ Electrolyte. Here, we discuss differences in the aging behaviors of DSSCs prepared with MK-1 and MK-70 dyes according to the concentration of Li+ in the electrolyte. When using Li+-free electrolyte A, the IPCE spectra of the DSSCs with both dyes aged for 24 h were nearly identical with those of fresh DSSCs (Figure 5a). The JSC values of the aged and fresh DSSCs were also similar (Figure 5b), as expected on the basis of IPCE results, whereas the VOC values of the aged DSSCs were slightly increased compared to those of the fresh (+20 mV) for both MK-1 and MK-70. The VOC and τ values as functions of the electron density in the aged and fresh DSSCs with electrolyte A are shown in Figure 5c,d. The ECB values in the aged DSSCs with both MK dyes were negatively shifted compared to those in the fresh DSSCs. As ΔECB is defined as the difference in the ECB levels between the aged and fresh DSSCs, the ΔECB values of the MK-70- and the MK-1-cells were nearly the same, i.e., ΔECB (MK-70) = ΔECB (MK-1) = +20 mV. τ values for the aged DSSCs were also identical with those in the fresh DSSCs. In addition, τ for the aged MK-70- and MK-1-cells were also comparable [τ(MK70) = τ(MK-1)]. Therefore, the difference in the VOC values between the aged and fresh DSSCs with both MK dyes can be simply attributed to the negative shift of the ECB levels. In other words, the molecular structure of the dye does not influence the degree of the aging behavior in DSSCs prepared without Li+ in the electrolyte. 3.4. Aged DSSCs with Low Li+ Concentration (0.02 M) in the Electrolyte. Figure 6a,b shows the IPCE spectra and photovoltaic performance of aged and fresh DSSCs based on both MK-70 and MK-1 with electrolyte B. The JSC values of the aged DSSCs with either MK-1 or MK-70 were nearly identical with those of the fresh DSSCs, based on the corresponding IPCE spectra. Although the VOC values of the aged DSSCs for each dye were also similar to those of the fresh DSSCs, the ECB levels in both aged DSSCs were negatively shifted relative to those of the fresh DSSCs (Figure 6c). The values of the negative shifts (ΔECB) for the MK-70- and MK-1-cells from the fresh to the aged condition were +20 mV and +50 mV, respectively [ΔECB (MK-70) < ΔECB (MK-1)]. The difference

and MK-1-cells also changed with the [Li+] in the electrolyte. With no Li+ (electrolyte A) and 0.02 M Li+ (electrolyte B), the VOC values were comparable between the two dyes. In contrast, for DSSCs prepared with 0.05 M Li+ (electrolyte C) and 0.50 M Li+ (electrolyte D), the VOC values of the MK-70-cells (0.70 V in electrolyte C, 0.63 V in electrolyte D) were lower than those of the MK-1-cells (0.73 V in electrolyte C, 0.66 V in electrolyte D). These results can be explained by the electron lifetime measurements of the DSSCs (vide infra). Figure 3c,d shows the VOC and τ values as functions of the electron density n in fresh DSSCs with the four electrolyte compositions. As the [Li+] increased from 0 M (electrolyte A) to 0.50 M (electrolyte D), the VOC at the same n decreased (Figure 3c). This means that the conduction band edge potentials ECB of TiO2 in the DSSCs with both MK dyes are positively shifted by the increasing amounts of Li+ adsorbed on the TiO2 surface. In general, the driving force for electron injection will be increased by positive shifts in the ECB, which results in increased maximum values in the IPCE spectra. Moreover, the positive shifts for the ECB decrease the driving force for electron recombination from TiO2 to the electrolyte, which results in increased τ values in the DSSCs. For the same [Li+], the ECB values for the DSSCs were not affected by the identity of the dyes. This result suggests that the amount of Li+ adsorbed on the TiO2 surface is comparable between the MK70- and MK-1-cells in each electrolyte. Thus, the difference in the VOC values from the I−V curves can simply be attributed to the difference in τ for the DSSCs. The τ values of the MK-70 and MK-1-cells were nearly comparable in electrolytes A (0 M LiI) and B (0.02 M LiI). By contrast, in electrolytes C (0.05 M LiI) and D (0.5 M LiI), the τ values for the MK-70-cells were half as long as those of the MK-1-cells. To explain the difference between fresh MK-70- and MK-1cells, we considered the conditions of their electrical double layers (Helmholtz compact layers and diffuse layers)34 formed by Li + and I − /I 3 −. DMPIm + was excluded from the consideration of the ECB shift; DMPIm+ penetration between TiO2 and the dye molecules is difficult because of its bulkiness, while Li+ can be easily adsorbed on the TiO2 surface because of its size.35,36 Figure 4 shows schematic models of the possible ion concentration profiles in the fresh MK-1- and MK-70-cells. The amounts of Li+ absorbed on the TiO2 surfaces were identical between the cells with the same electrolyte (Figure 3c). Besides the Li+ adsorbed on the TiO2 surface, some Li+ may have interacted with the crown ethers in MK-70. The Li+

Figure 4. Schematic models of possible ion concentration profiles in fresh DSSCs: MK-1 (black) and MK-70 (red). 16753



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Figure 5. (a) IPCE spectra and (b) I−V curves of aged and fresh DSSCs with electrolyte A. (c) Open circuit voltages VOC and (d) electron lifetimes τ as functions of electron density in aged and fresh DSSCs with electrolyte A. The I−V curves of the DSSCs were measured under AM 1.5G (100 mW cm−2) illumination conditions with a photomask (0.148 cm2) and without an antireflection film.

Figure 6. (a) IPCE spectra and (b) I−V curves of aged and fresh DSSCs with electrolyte B. (c) Open circuit voltages VOC and (d) electron lifetimes τ as functions of the electron density in aged and fresh DSSCs with electrolyte B. The I−V curves of the DSSCs were measured under AM 1.5G (100 mW cm−2) illumination conditions with a photomask (0.148 cm2) and without an antireflection film.

in the ECB levels in the aged MK-70- and MK-1-cells suggested that the amounts of ions adsorbed on the TiO2 surfaces were different between the two cells. As shown in Figure 6d, the electron lifetimes of the aged DSSCs were different from those of the fresh DSSCs. For the MK-70-cells, τ decreased 2-fold between the fresh and the aged conditions, whereas that for the corresponding MK-1-cells decreased 20-fold. Thus, the electron

lifetime in the aged MK-70-cell was more than ten times longer than that in the aged MK-1-cell [τ(MK-70) > τ(MK-1)]. The difference in τ between the aged MK-70- and MK-1-cells was affected by the difference in the ECB levels in the aged DSSCs because the driving force for the charge recombination was changed. Therefore, interestingly, the observation of nearly the same VOC values from the I−V curves between the aged and 16754



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Figure 7. (a) IPCE spectra and (b) I−V curves of fresh and aged DSSCs with electrolyte C. (c) Open circuit voltages VOC and (d) electron lifetimes τ as functions of the electron density in the fresh and aged DSSCs with electrolyte C. The I−V curves of the DSSCs were measured under AM 1.5G (100 mW cm−2) illumination conditions with a photomask (0.148 cm2) and without an antireflection film.

Figure 8. Absorption spectra of MK-1 and MK-70 in fresh and aged dummy cells with (a) sol-2 (0.05 M LiI) and (b) sol-3 (0.05 M LiClO4). Black lines and red lines show MK-1 and MK-70, respectively. Dotted lines show fresh dummy cells and solid lines show aged dummy cells. The aged dummy cells were measured after storage in the dark for 24 h.

IPCE spectra. On the other hand, the VOC of the aged MK-1cell was higher than that of fresh one, similar to the results observed for the MK-70-cell. Although the photovoltaic performance for both the MK-1and MK-70-based DSSCs during aging tended to change similarly toward lower JSC and higher VOC values in the I−V curves (Figure 7b), the mechanisms of aging for the two types of cells were totally different. The higher VOC of the aged MK1-cell was determined by balancing the negative shift of the ECB level (Figure 7c) and the decrease in the electron lifetime (Figure 7d). In contrast, the VOC increase of the aged MK-70cell was caused by the negative shift of the ECB level and a slight increment of the electron lifetime (Figure 7c). The ΔECB values of the MK-70- and MK-1-cells were +50 mV and +100 mV, respectively [ΔECB (MK-70) < ΔECB (MK-1)], because of the difference in the quantity of ions adsorbed on the TiO2 surface in the two aged cells. In the aged MK-70-cell, τ was more than five times longer than that in the aged MK-1-cell [τ(MK-70) > τ(MK-1)].

fresh DSSCs for both dyes can be attributed to balancing the effects of the negative shift of the ECB levels with the decrease of the electron lifetimes in the DSSCs. 3.5. Aged DSSCs with Moderate Li+ Concentration (0.05 M) in the Electrolyte. Figure 7a,b shows the IPCE spectra and the photovoltaic performance of fresh and aged DSSCs based on both MK-70 and MK-1 with electrolyte C. Both the JSC and the maximum value in the IPCE spectrum of the aged MK-70-cell decreased from those of the fresh cell. The VOC of the aged MK-70-cell was higher than that of the fresh MK-70-cell. The higher VOC of the aged cell was mainly caused by the negative shift of the ECB level (Figure 7c) because the τ values for the aged and fresh MK-70-cells were nearly identical (Figure 7d). In the case of MK-1, the onset wavelength of the IPCE spectrum of the aged MK-1-cell was obviously blueshifted in comparison to that of the fresh cell, and the aged material also revealed a decreased maximum value in the IPCE spectrum. The JSC of the aged MK-1-cell was much lower than that of the fresh MK-1-cell, reflecting the corresponding 16755



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Figure 9. (a) IPCE spectra and (b) I−V curves of aged and fresh DSSCs with electrolyte D. (c) Open circuit voltages VOC and (d) electron lifetimes τ as functions of the electron density in aged and fresh DSSCs with electrolyte D. The I−V curves of the DSSCs were measured under AM 1.5G (100 mW cm−2) illumination conditions with a photomask (0.148 cm2) and without an antireflection film.

Figure 10. Schematic models of possible ion concentration profiles for the aging behaviors in DSSCs based on (a) MK-1 and (b) MK-70.

To understand the remarkable differences in the blueshifts of the IPCE spectra between the MK-70- and MK-1-cells with electrolyte C, the absorption spectral changes for the MK dyes on TiO2 in the dummy cells with sol-2 (0.05 M LiI in CH3CN) during aging were considered (Figure 8a). The absorption maxima and onsets in the aged dummy cells were blueshifted compared with those of the fresh dummy cells for both MK

dyes. These blueshifts in the absorption spectra of the aged dummy cells with sol-2 would induce the blueshifts of the IPCE spectra in the DSSCs with electrolyte C. On the other hand, when sol-3 (0.05 M LiClO4 in CH3CN) was used instead of sol-2, shifts in the absorption spectra of both MK dyes in the dummy cells were not observed (Figure 8b). According to the UV−vis spectra of the fresh dummy cells (Figure 2), these 16756



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absorption spectra for MK-70 are scarcely visible because the effect of Li+ on the TiO2 surface remains. For the aged MK-70cell with electrolyte D, the Li+-trapping ability of MK-70 was saturated, and the concentration of Li+ on the TiO2 surface was dominated by the Li+ concentration of the electrolyte. The Li+ concentration on the TiO2 surface in the aged MK-1-cell with electrolyte D was also determined by the Li+ concentration in the electrolyte. In both cases (MK-1- and MK-70-cells), the I−/ I3− ions were attracted to the TiO2 surface by the positive charges of Li+ adsorbed there and these charges were partially canceled. Therefore, negative shifts in the ECB values in both MK-1 and MK-70-based DSSCs are similarly observed in electrolyte D.

results suggest that the dye molecules would be influenced by both the lithium cations and iodide anions in the electrolyte. 3.6. Aged DSSCs with High Li+ Concentration (0.5 M) in the Electrolyte. For DSSCs with high concentrations of lithium iodide (0.5 M LiI) in the electrolyte (electrolyte D), the aging behaviors for both MK dyes were similar. As shown in the IPCE spectra in Figure 9a, the onsets for the aged DSSCs based on either MK dye were blueshifted in comparison to those of the fresh DSSCs, and the maximum values of the IPCE spectra of the aged DSSCs were lower than those of the fresh DSSCs. The I−V curves of the DSSCs with electrolyte D are depicted in Figure 9b. Corresponding to the IPCE spectra, the JSC values of the aged DSSCs with both MK dyes decreased in comparison to those of the fresh DSSCs. On the other hand, the VOC values of the aged DSSCs with both MK dyes were obviously increased in comparison to those of the fresh DSSCs (+30 mV). Plots of VOC and τ as functions of the electron density in the aged and fresh DSSCs with electrolyte D are shown in panels c and d of Figure 9, respectively. The ECB values in the aged DSSCs with both MK dyes were negatively shifted compared to those in the fresh cells, and the ΔECB values were almost the same (+60 mV) between the MK-70and MK-1-cells [ΔECB (MK-70) = ΔECB (MK-1)]. In addition, the ECB levels in the aged MK-70- and MK-1-cells were similar. Therefore, the shorter electron lifetime in the aged MK-70-cell versus that in the aged MK-1-cell is simply caused by the partial charge effect in MK-70 (MK-70 < MK-1).

5. CONCLUSION To examine the partial charge effect on the donor moiety, a carbazole-based dye with a 12-crown-4 ether on the carbazole donor (MK-70) was newly designed and synthesized as a sensitizer for DSSCs. Its photovoltaic performance was compared with an that of analogue dye that lacked the crown ether (MK-1). Even though the crown ether was located far from the TiO2 surface, the electron lifetime in the MK-70-cell in the presence of a high Li+ concentration in the electrolyte was lower than that in the MK-1-cell because of the partial charge effect. That is, the Li+ localized near the crown ethers attracted surplus I3− by electrostatic interactions. In addition, we proposed that the 24 h aging behaviors in the DSSCs reflected in the negative shifts of their ECB levels and blueshifts in their absorption spectra could be caused by the cancellation of the positive charges of Li+ on the TiO2 surface by the approach of I−/I3− ions to that surface. The approach of I−/I3− to the TiO2 surface is mainly driven by two attractive forces: K1 (I−/I3− and Li+ on TiO2) and K2 (I−/I3− and Li+ with dye). The difference in the rates of aging behavior between MK-1- and MK-70-cells may be explained by the following mechanism. When K1 is larger than K2, such as for the MK-1-cell, the approach of I−/I3− ions to the TiO2 surface is accelerated because of the electrostatic interactions between I−/I3− and surface-bound Li+. On the other hand, when K1 is smaller than K2, such as for the MK-70-cell, the approach of I−/I3− to the TiO2 surface is suppressed because I−/I3− ions are trapped by Li+ coordination to the dye. This crown ether donor with its partial charge effect will be useful in designing ion concentration-controllable TiO2/dye/electrolyte interfaces for efficient and stable DSSCs.

4. DISCUSSION OF AGING BEHAVIORS We observed negative shifts in the ECB values and blueshifts in the absorption spectra for the aged DSSCs in the presence of both Li+ and I−. After aging 24 h, the photovoltaic performance of an MK-1-cell was more varied than that of an MK-70-cell when using electrolytes B and C. To explain these aging behaviors in the DSSCs, we considered suitable diffusion mechanisms for the cations and anions at the electrical double layer for the MK-1-cell (Figure 10a) and the MK-70-cell (Figure 10b). The models show that the aging behaviors are caused by the approach of I− and/or I3− toward the TiO2 surface. When these ions make this approach, two interactions may occur. One is the interaction between I−/I3− and Li+ adsorbed on the TiO2 surface (attractive force: K1), which accelerates the approach of I−/I3− to the TiO2 surface. Another is the interaction of I−/I3− and Li+ trapped by the donor segment of the dye molecule (attractive force: K2), which suppresses the approach of I−/I3− to the TiO2 surface. The ion concentration on the TiO2 surface will be determined by the balance between the two attractive forces, K1 and K2. For the aged MK-1-cells with electrolytes B and C, K2 can be neglected or is much smaller than K1 (K1 ≫ K2) because I−/I3− hardly interacts with the dye molecule in the absence of partial charges. The I−/I3− ions approach the TiO2 surface, where their negative charges balance the positive charges because of surfaced-adsorbed Li+ ions. This results in the negative shifts in the ECB values and the blueshifts of the absorption spectra. On the other hand, for the aged MK-70-cells with electrolytes B and C, K2 must be considered as well as K1 because the I−/I3− ions strongly interact with the dye through Li+ ions coordinated with the crown ethers. The approach of I−/I3− ions in the vicinity of the TiO2 surface may be suppressed, and the positive charges of the Li+ ions adsorbed on the TiO2 surface remain. Therefore, the negative shifts of the ECB values with MK-70 are smaller than those with MK-1, and the blueshifts of the

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic procedure for MK-70 preparation and measurement methods for τ and VOC versus electron density. 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]. Tel.: +81-29-861-3819. Fax: +81-29-861-4682. *E-mail: [email protected]. Tel.: +81-29-861-3819. Fax: +81-29-861-4682. Present Address §

Y.U.: Department of Nanoscience, Sojo University, 4-22-1 Ikeda, Nishiku, Kumamoto 860-0082, Japan 16757



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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the project “Research and development of high-efficiency and low-cost dye-sensitized solar cells and their mass production technologies based on the three phase-harmonized interface” of the New Energy and Industrial Technology Development Organization (NEDO), Japan.

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Crown Ether-Substituted Carbazole Dye for Dye-Sensitized Solar Cells

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