Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
High-Efficiency Cycloruthenated Sensitizers for Dye-Sensitized Solar Cells The-Duy Nguyen, Yen-Po Lan, and Chun-Guey Wu* Department of Chemistry and Research Center for New Generation Photovoltaics, National Central University, Jhong-Li 32001, Taiwan, ROC S Supporting Information *
ABSTRACT: Four thiocyanate-free ruthenium sensitizers (DUY24−DUY27) containing 2-thienylpyridine moiety as cyclometalating core were synthesized for dye-sensitized solar cell (DSC) application. To the best of our knowledge, DUY24−DUY27 are four best-efficiency sensitizers having 42%−65% higher efficiencies compared to those of the published sensitizers comprising the same type of the cyclometalating ancillary ligands. The significant characteristic of DUY24−DUY27 is their β-lowest unoccupied spin orbital (β-LUSO) distributes remarkably on the cyclometalating ligands, especially on the soft sulfur atom, which strengthens the interaction between the oxidized dye and iodide ion for efficient dye regeneration. The photovoltaic performance of DUY24−DUY27-based DSCs supports that the dye regeneration (therefore the short-circuit photocurrent density (JSC) of the cell) can be improved by not only lowering the highest occupied molecular orbital energy level of the dye molecule but also distributing the β-LUSO properly on the soft atoms. The study provides an important new guide for designing high-efficiency ruthenium-based dyes for DSC application.
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INTRODUCTION Dye-sensitized solar cell (DSC) was developed by O’Regan and Grätzel in 1991.1 Over the past two decades, the redox mediator I3−/I− is still the most popular choice, since other redox couples have not much success in the sense of the stability.2,3 During the course of DSC operation, the dye regeneration is expected to occur as rapidly as possible to minimize the undesirable charge recombination between the injected electron and the oxidized dye as well as to increase the JSC. Additionally, a rapid dye regeneration may also preclude the stability problem of the oxidized dyes.4 In DSC working condition, the oxidation of a dye would involve the excitation of one electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), and then the excited electron on the LUMO injects to the TiO2 conduction band. After the electron injection, the HOMO of the dye molecule becomes a single occupied molecular orbital (SOMO), which involves an α-highest occupied spin orbital (α-HOSO) and a β-lowest unoccupied spin orbital (β-LUSO).5 The dye regeneration corresponds to the transfer of one electron from the reductant in the electrolyte (i.e., iodide ion) to the β-LUSO of the oxidized dye.6 In reality, most of highly efficient dyes (η > 10%)7 contain thiocyanate ligands (on which the β-LUSO locates)6 to support the dye regeneration through sulfur−iodide interaction. Therefore, in addition to the dye HOMO energy level should be lower than the redox potential of the electrolyte,8 the β-LUSO distribution is also a key parameter to determine the dyeregeneration efficiency and therefore the photovoltaic perform© XXXX American Chemical Society
ance of the cell. In the dye-regeneration process, a direct interaction between the ruthenium atom and iodide ion is unfavorable due to the nonexistence of seven-coordinated ruthenium center.9 Hence, increasing the β-LUSO distribution on ancillary ligands is a way to promote the dye +−I− interaction, especially on the thiocyanate-free ruthenium sensitizers. In this study, four thiocyanate-free ruthenium sensitizers (DUY24−DUY27, the structures were displayed in Figure 1) containing 2-thienylpyridine moiety as the core of the cyclometalating ligand were synthesized and applied in DSC. 2-Thienylpyridine moiety was chosen because the low resonance energy of the thienyl five-member ring can induce its π-electrons sharing noticeably to the ruthenium center, which may increase the β-LUSO distribution on the thiophene ring, facilitating the dye-regeneration process. DSCs based on these 2-thienylpyridine cyclometalating ligand containing ruthenium sensitizers will probably have high JSC due to the strong metal-to-ligand charge transfer (MLCT) band (more electron density on the metal center) and fast dye regeneration (strong interaction between β-LUSO (on S atom in thiophene ring) and iodide ion).
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EXPERIMENTAL SECTION
Materials and Measurements. All reagents were obtained from the commercial sources and used as received unless specified. The
Received: November 9, 2017
A
DOI: 10.1021/acs.inorgchem.7b02862 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 1. Structures of DUY24−DUY27 sensitizers.
Figure 2. 1H NMR spectra (500 MHz, CD3OD, δ ppm) of DUY24−DUY27. Only the aromatic region is displayed. The signals in red belong to the protons on the cyclometalating thiophene ligand. detail preparation and structural characterization of DUY24−DUY27 complexes can be found in the Supporting Information. Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker 300 or 500 MHz NMR spectrometer. High-resolution mass spectra (HRMS) were obtained by using JMS-700 HRMS. Elemental analyses were performed with a PerkinElmer 2400 CHNS/O analyzer. UV−Vis spectra of the sensitizers dissolved in ethanol or absorbed on TiO2 film were measured by using a Cary 300 Bio spectrometer. Voltammetric measurements were performed in a single-compartment, threeelectrode cell with a platinum wire counter electrode and a platinum disk working electrode. The reference electrode is Ag+/Ag, and the supporting electrolyte is 0.1 M tetrabutylammonium (TBA) perchlorate in ethanol. The square-wave voltammograms (potential step increment: 5 mV; frequency: 25 Hz) were recorded by using a potentiostat/galvanostat (PGSTAT 30, Autolab, Eco-Chemie), and ferrocene was used as an internal standard. Electrochemical impedance spectroscopy (EIS) was performed in an Eco Chemie Autolab PGSTAT30 Potentiostat/Galvanostat at 50 mV voltage steps with a sinusoidal potential perturbation of 10 mV in dark. Intensitymodulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) were recorded with a Zahner Zennium controlled intensity modulated photospectroscopy, CIMPS-
1 potentiostat installed with a light intensity modulated function. All the measurements (EIS, IMPS, IMVS) and relevant calculations can be found in our previous papers.10,11 Density Functional Theory Calculation. All computational calculations were performed by using the Gaussian 09 program similar to our previous report.12 Device Fabrication and Photovoltaic Performance Characterization. The preparation of TiO2 pastes, fabrication of TiO2 anode, dye loading, assembly of the cell, and photovoltaic parameter measurements all are similar to what we reported previously10 unless specified.
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RESULTS AND DISCUSSION Synthesis and Structural Characterization. The complexes DUY24−DUY27 were synthesized by initially activating the C−H bond of the ancillary ligand to establish a cycloruthenation bond with subsequent coordination of Et2dcbpy anchoring ligands (Et2dcbpy = diethyl 4,4′-dicarboxylate-2,2′-bipyridine) to the ruthenium center followed by basic hydrolysis of the ethyl ester portions in Et2dcbpy as the detail B
DOI: 10.1021/acs.inorgchem.7b02862 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry synthesis provided in the Supporting Information. It was reported13 that the base (solid NaOH, which is almost insoluble in the reaction solvent (acetonitrile)) plays a role in the initial activation of the C−H bond, which affects significantly the reaction yields. In this study, we used KOH/ MeOH solution, instead of solid NaOH, as the base in the cyclometalation reaction. In this way, the contact between the reagents increased, and the reactions were promoted. For the purification of the final complexes, we found that the C18 reversed-phase chromatography was very efficient to delicately purify the sensitizers in a short time, especially for removing subtle side products and excess amounts of TBAOH. The structures of the sensitizers were elucidated by 1H NMR, HRMS, and elemental analysis (see the data listed in Supporting Information). These analytical methods confirmed that DUY24−DUY27 all contain three TBA+ cations. The presence of TBA+ ions makes the sensitizers have high solubility in many common organic solvents such as CH2Cl2, CHCl3, ethyl acetate (EtOAc), dioxane, MeCN, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetone, EtOH, MeOH, and H2O, which facilitates the preparation of dye solutions for spectroscopic measurements and cell fabrication. The aromatic region of the 1H NMR spectra for the sensitizers is illustrated in Figure 2, in which the high-intensity and sharp singlets (marked in red) were assigned to the protons on the anionic thiophene systems (thiophene or thieno[3,2-b]thiophene coordinating directly to the ruthenium center). For cycloruthenated complexes, more upfield chemical shift of the proton on an anionic moiety of a cyclometalated ligand indicates higher electron density around the proton as well as higher electron-donating ability of the anionic moiety. As a consequence, the HOMO energy level of the cycloruthenated complex is higher due to more electron density around the ruthenium center. Therefore, the chemical shifts of the protons on the anionic thiophene systems are 6.05 ppm (DUY24), 6.17 ppm (DUY25), 6.93 ppm (DUY26), and 7.25 ppm (DUY27), indicating that thiophene ring has a higher electron-donating ability compared to fused thiophene (thieno[3,2-b]thiophenes) moiety. In addition, the chemical shifts also show that the electron-donating ability of the thiophene systems is reduced by the presence of an adjacent electronegative sulfur atom. On the basis of the chemical shifts of the protons on the thiophene system, the HOMO energy levels of DUY24−DUY27 can also be predicted as DUY24 < DUY25 < DUY26 < DUY27 (vs the normal hydrogen electrode (NHE)), which was consistent with the values obtained from square-wave voltammograms, which be discussed in the later paragraph. Optical Properties. The absorption spectra of DUY24− DUY27 and N719 dissolved in EtOH are depicted in Figure 3A. The absorption coefficients of DUY24−DUY27 are all significantly higher than that of N719 in the whole absorption range, which originates from an increase of the π-conjugation systems when two thiocyanate ligands (in N719) were replaced with a cyclometalating ligand (in DUY24−DUY27 sensitizers). The red-shifted absorption maxima (λmax) of the lowest-energy bands of DUY24−DUY27 compared to that of N719 implies that the electron-donating ability of a cyclometalating ligand is stronger than that of two thiocyanate ligands. The absorption profiles of DUY24−DUY27 sensitizers are very broad and complicated due to the low symmetry of the molecular structures which result in relatively complicated orbital distribution and increase in the number of allowed electronic transitions.13 All sensitizers showed strong absorption bands at
Figure 3. UV/Vis absorption spectra of (A) DUY24−27 and N719 dissolved in EtOH and (B) DUY24−27 anchored on transparent TiO2 thin films.
the wavelength below 400 nm with the molar absorption coefficient (ε) in the range of (2.5−8.5) × 104 M−1 cm−1 assigned to the π−π* transitions of the ancillary ligands and the ligand-to-ligand charge transfer (LLCT).12,13 The remarkable difference in the absorption intensity at ∼350 nm for DUY24, DUY25 compared to DUY26, DUY27 arises from the more πconjugated length of the thieno[3,2-b]thiophene than that of the thiophene. Furthermore, adding a sulfur atom in the alkyl chain, such as DUY24/DUY25 or DUY26/DUY27 pair, also increases the absorption intensity in this wavelength range. Broad absorption bands at longer wavelengths (400−600 nm) are primarily from the mixed-metal/ligand to ligand (mainly anchoring ligand) charge-transfer (M/LLCT) transitions.12 As mentioned above, compared to the thiophene ring, the thieno[3,2-b]thiophene moiety has a lower electrondonating ability. Moreover, the electron-withdrawing sulfur adjacent to the thiophene and thieno[3,2-b]thiophene (in cases of DUY25 and DUY27, respectively) lowers the electrondonating ability of these moieties. As a consequence, the λmax values of the lowest-energy bands are in the order of DUY27 < DUY26 < DUY25 < DUY24 (see Figure 3A). The absorption C
DOI: 10.1021/acs.inorgchem.7b02862 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry coefficients of DUY26 and DUY27 are higher than those of DUY24 and DUY25, respectively, due to the thieno[3,2b]thiophene moiety having higher π-conjugation than the thiophene ring. By adding the exterior sulfur atom adjacent to the thiophene or thieno[3,2-b]thiophene moiety, the absorption coefficients of M/LLCT bands increase only slightly (DUY25 and DUY27 vs DUY24 and DUY26, respectively). This is because the 2p orbitals of the heteroaromatic ring and 3p orbitals of the exterior sulfur atom do not overlap significantly to extend the π-conjugation length. The strength of the M/LLCT bands means that more photons can be absorbed and possibly translated to electric current. DUY26 and DUY27 have stronger absorption in M/LLCT bands compared to DUY24 and DUY25; one may expect that DUY26- and DUY27-based DSCs will have higher J SC compared to the cells sensitized by DUY24 and DUY25. Nevertheless, to predict the trend in JSC of DUY24−DUY27 sensitized cells, the change in the optical property of the dye upon being adsorbed onto TiO2 film and dye loading also need to be considered besides the dye absorption coefficient. The absorption spectra of DUY24−DUY27 anchored on thin TiO2 films are shown in Figure 3B. Interestingly, the absorption maxima of DUY24−DUY27 adsorbed on TiO2 films red-shifted compared to those of the dyes in EtOH. In general, the carboxylic acid anchoring group of the sensitizer adsorbed on TiO2 will deprotonate, resulting in the blue shift of the absoption maximum.14,15 Ruthenium-based sensitizer has a spherical-like geometry; therefore, J-aggregation occurs unlikely. Therefore, the red shift of the absorption maxima when DUY24−DUY27 adsorbed on TiO2 films suggests that the anchoring group of the complexes is carboxylate and not carboxylic acid (no deprotonation reaction occurs). TiO2 as an electron acceptor will lower the LUMO level and therefore will reduce the energy gap (and red-shift the absorption maximum) of the dye molecule. The absoption data support the correct sensitizers’ structures presented in Figure 1. The trend of the absorption intensity for DUY24−DUY27 adsorbed TiO2 film is not consistent with the absorption coefficient of the dyes dissolved in EtOH, due to the difference in the dye-loading, which will be discussed in more detail in the later paragraphs. Electrochemical Properties. Square-wave voltammograms (SWV), HOMO energy levels (EHOMO, obtained from the SWV), energy gaps (Egap, determined from the absorption onset), and LUMO energy levels (ELUMO = EHOMO − Egap) of DUY24−DUY27 dyes are displayed in Figure 4. DUY26 and DUY27 both contain an anionic thieno[3,2-b]thiophene segment have the HOMO energy levels more positive than DUY24 and DUY25, which contain an anionic thiophene moiety. This implies that the thieno[3,2-b]thiophene segment owns higher resonance energy and therefore has lower electron-donating ability compared to the thiophene ring. By replacing the hexyl chain of DUY24 and DUY26 with the hexylthio chain to form DUY25 and DUY27, the HOMO levels lower by ∼60 and 20 mV, respectively. The electronwithdrawing inductive effect of the sulfur atom adjacent to the anionic thiophene or thieno[3,2-b]thiophene moieties reduces the electron-donating ability of the anionic portions, resulting in lower HOMO levels. The distance between the hexylthio chain and the cyclometalating site of DUY25 is shorter than that of DUY27. As a consequence, the inductive effect of the hexylthio chain on the cyclometalating site for DUY25 is stronger than that for DUY27, and therefore, the lowering in the HOMO level is more significant (60 vs 20 mV).
Figure 4. (A) Square-wave voltammograms and (B) frontier orbital energy level diagram of DUY24−DUY27 sensitizers. (HOMO energy levels (EHOMO) were obtained from the square-wave voltammograms. LUMO energy levels (ELUMO) were calculated by the formula of ELUMO = EHOMO − Egap, where Egap (energy gap) values are calculated from the onsets of the absorption spectra illustrated in (A)).
In contrast to the HOMO energy levels, the similarity in the LUMO energy levels (see Figure 4B) indicates that the structure of the cyclometalating ancillary ligands have a negligible effect on the LUMO energy, since the LUMO contributed mostly from the anchoring ligand. As a consequence, the energy gaps (Egap) of DUY24−DUY27 are directly related to the HOMO energy levels. The HOMO energy levels of DUY24−DUY27 sensitizers are in the range from 0.80 to 0.91 V (vs NHE), which is more positive than the threshold (∼0.79 V vs NHE)8 for efficient dye regeneration by iodide electrolyte system. The LUMO energy levels of all sensitizers are at least 670 mV higher than the conduction band edge of TiO2, providing a sufficient driving force for electron injection. It was known16 that the electron-injection efficiency can reach up to 99% when the energy difference between dyeLUMO level and TiO2 conduction band edge is larger than 200 mV. The driving force of more than 670 mV of DUY24− DUY27 sensitizers leaves a possibility to modify the dye structure to lower the LUMO level, reduce the band gap, and increase the JSC of the corresponding cell. Theoretical Calculation of the Frontier Orbital Distributions and Electronic Transitions. B3LYP/ LanL2DZ density functional theory (DFT) calculations were performed on DUY24−DUY27 to qualitatively assess their frontier molecular orbital distribution. The first 10 calculated frontier orbital distribution of each sensitizer was illustrated in Figures S1−S4, Supporting Information. The data show that the HOMO orbitals of all DUY24−DUY27 molecules D
DOI: 10.1021/acs.inorgchem.7b02862 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
difference maps of DUY24 dye. Computed transition assignments (see Table S1, Supporting Information) reveal that these transitions mainly belong to the M/LLCT. The significant excitation states in the low-energy absorption are S4 ( f = 0.0748, at 605.68 nm), S6 (f = 0.1860, at 556.50 nm), S7 ( f = 0.0896, at 515.72 nm), S11 (f = 0.0913, at 479.81 nm), S14 (f = 0.1713, at 453.85 nm), S15 (f = 0.2080, at 441.25 nm), and S21 (f = 0.2170, at 410.49 nm). The transitions S4, S6, S7, S14, and S15 are contributing significantly to the photocurrent production when applied in DSC, because they come from the excitations of the electrons from metal/ligand to anchoring ligand. Oppositely, the transitions S11 and S21 may not contribute significantly to the photocurrent of the cell, because 48% of S11 and 40% of S21 are from the excitation of metal/ ligand to the LUMO+4 orbital locating mainly on the cyclometalated ancillary ligand. Photovoltaic Performance. The J−V and IPCE curves of DUY24−DUY27 sensitized DSCs are displayed in Figure 6. The photovoltaic parameters and dye loading of the cells are summarized in Table 1. The trend in the dye loading is DUY26 < DUY27 < DUY24 < DUY25, arising from the molecular size: DUY24 and DUY25 are smaller than DUY26 and DUY27. Furthermore, the interaction between the dyes and TiO2 surface is probably increased in the presence of the exterior sulfur atom, which has nonbonded 3p electrons to contribute the electron donating ability of the dye molecules; therefore, DUY25 and DUY27 have larger dye loading compared to DUY24 and DUY26, respectively. Data listed in Table 1 also show that, under the illumination of AM 1.5 G simulated sunlight, DUY24-, DUY25-, DUY26-, and DUY27-based cells exhibit the efficiencies of 6.00%, 6.54%, 6.49%, and 6.97%, respectively, while that of the N719-based cell is 7.26% (under the same cell fabrication procedures). Interstingly, compared to the published sensitizers containing 2-thienylpyridine cyclometalating ligands,17−19 DUY24−DUY27-based cells showed 42%−65% higher efficiencies than the reported highest one (Dye NC103, λmax (560 nm) = 0.63 × 104 M−1 cm−1, JSC = 9.45 mA/cm2, VOC = 630 mV, FF = 0.71, η = 4.22%, EHOMO = 0.87 V, ELUMO = −0.72 V vs NHE). The better photovoltaic performance of DUY24−DUY27-based cells is attributed to their higher absorption coefficient as well as better electroninjection efficiency due to their higher LUMO energy levels (see Figure 4B) compared to that of NC103. Therefore, DUY24−DUY27-based cells have much larger Jsc values.
distribute primarily on both the ruthenium center and the cyclometalating ligands, while the LUMO orbitals virtually locate on the anchoring ligands. The orbital distribution indicates that the lowest-energy absorption bands originate from the M/LLCT transitions. The HOMOs distributed more on the anionic moieties of the cyclometalating ligands implies that the HOMO levels are very sensitive to the structure of the cyclometalating ligand, which can be easily modified by adding suitable substituent(s) on it. The LUMOs located on the anchoring groups provide an efficient pathway for the electron injection from the excited dye into TiO2 conduction band. To gain more insight into the low-energy absorption bands of DUY24−DUY27 sensitizers, time-dependent (TD) DFT calculations of the singlet electronic transitions were performed on the geometry-optimized structures of the sensitizers using a B3LYP/LanL2DZ level of theory. The absorption spectra of all sensitizers are remarkably similar at the range of 400−750 nm; therefore, only the TD-DFT calculation result of DUY24 was used here as a representative to assign transition states in the absorption spectrum of the DUY-series dyes. Figure 5 displays
Figure 5. UV/Vis spectrum in EtOH and calculated absorption spectrum with the oscillator strengths of DUY24 sensitizer. (The occupied and unoccupied orbitals are represented in pink and blue, respectively).
the experimental and calculated absorption spectra along with calculated oscillator strengths (f) as well as the electron density
Figure 6. (A) J−V and (B) IPCE curves of DUY24−DUY27 sensitized devices. E
DOI: 10.1021/acs.inorgchem.7b02862 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 1. Photovoltaic Parameters and EIS Data of DUY24−DUY27 Sensitizer-Based Devices
a
dye
JSC (mA/cm2)
VOC (V)
FF
η (%)
DUY24 DUY25 DUY26 DUY27 N719
14.9 15.4 15.6 16.5 14.8
0.57 0.60 0.60 0.61 0.67
0.71 0.71 0.69 0.69 0.73
6.00 6.54 6.49 6.97 7.26
dye loading (1 × 10−8 mol/cm2)/absorbancea at λmax (cm−2) 8.76/1.5 14.5/2.6 6.40/1.3 7.45/1.6
× × × ×
R2 (Ω), 1 sun
R2 (Ω), in dark
12.7 12.2 9.38 8.90
63.6 65.3 80.6 91.6
10−3 10−3 10−3 10−3
Absorbance at λmax (cm−2): Dye loading (mol·cm−2) times absorption coefficiency (abs mol−1) of the dye molecule.
Figure 7. Calculated β-LUSO distributions of DUY24−DUY27 (isovalue = 0.04).
Figure 8. EIS spectra (Nyquist plots) of DUY24−DUY27 sensitized cells. (A) under AM 1.5 G illumination (at 100 mW/cm2) and (B) in the dark.
The open-circuit photovoltage (VOC) and fill factor (FF) of DUY24−DUY27 based cells are quite close to each other. The differences in the efficiencies of the cells based on our series dyes are mainly from the JSC values. The tendency of the IPCE values displayed in Figure 6B is DUY24 < DUY25 < DUY26 < DUY27, which is consistent with the trend of HOMO energy levels (lower HOMO level has higher IPCE value; see also Figure 4B) but different from the trends of the absorption (dyeloading times absorption coefficient of the dye molecule) of dye-loaded TiO2 films (see Table 1). This phenomenon indicates that the JSC strongly depended on the HOMO level, which determines the dye-regeneration efficiency. Nevertheless, on the one hand, after looking at the data listed in Table 1 and the HOMO levels of the dyes displayed in Figure 4B carefully, we can find that the difference in the HOMO level for DUY26 and DUY27 is only 0.02 eV, but DUY27 sensitized cell has the JSC 0.9 mA/cm2 higher than the cell based DUY26. On the other hand, the difference in the HOMO level for DUY26 and DUY25 is 0.03 eV, but the JSC values of both dyes are very close. There must be some other factors, besides the HOMO level of the sensitizer, that affect the JSC of a DSC. Many studies12,20−23 have also shown the mismatch between the dye-regeneration efficiency and the energy level of the HOMO, indicating that the kinetics of the dye regeneration
depends also on some other sophisticated factors, not only on the driving force. In the dye-regeneration process, the interaction between a dye cation and reductant must take place before one electron can transfer from the reductant to the dye cation no matter what the driving force is. Therefore, DFT calculations were also performed on DUY24−DUY27 cations to reveal the β-LUSO distributions (Figure 7), which determine the sites on the dye cations that directly interact with the redox mediators.6 The β-LUSO of all oxidized DUY24−DUY27 mainly delocalizes on the ruthenium center and the cyclometalating ligands. A direct interaction between the ruthenium center and the soft iodide ion (used in our cell) is unfavorable due to the nonexistence of seven-coordinated ruthenium center;9 hence, the β-LUSO distributions on the ligands, especially on the soft sulfur atoms, play a role in determining the dye+−I− interaction and therefore the dye-regeneration efficiency. Both [DUY24]+ and [DUY26]+ have the β-LUSO distributed on one sulfur atom, while the β-LUSO distribution of both [DUY25]+ and [DUY27]+ locates on two sulfur atoms. These types of distributions indicate that the interactions between the iodide ion with [DUY25]+ and [DUY27]+ are more favorable than with [DUY24] + and [DUY26] + , respectively. Combining the HOMO energy levels of the sensitizers, the efficiency in the dye regeneration is in the order F
DOI: 10.1021/acs.inorgchem.7b02862 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry of DUY27 > DUY26, DUY25 > DUY24, which is exactly consistent with the JSC of the corresponding cells (see Table 1). Therefore, if the dye-regeneration efficiency is also determined by the β-LUSO distribution, an efficient dye-regeneration can be obtained by designing dye molecules having the β-LUSO distribution on soft atoms when I−/I3− redox couple is used as an electrolyte in a DSC. Charge-Transfer Kinetics Inside the Cells. To obtain the dynamics of the interfacial charge-transfer processes within the devices, EIS, IMPS, and IMVS measurements were performed to estimate the resistance at the TiO2/dye/electrolyte interface, the electron transport time (τtr), and the electron lifetime (τn) of the electron on TiO2 film. EIS spectra (Nyquist plots) of the devices obtained under illumination and in the dark are displayed in Figure 8. The resistance (R2) values calculated from the second semicircle of the EIS Nyquist plot typically representing the TiO2/dye/electrolyte interfacial resistance are also listed in Table 1.24 Under illumination, smaller R2 implies more facile electron transport and dye regeneration. Hence DUY24−DUY27-based cells exhibit the R2 trend as DUY24 > DUY25 > DUY26 > DUY27, consistent with the trend of JSC. Additionally, the trends of the R2 and HOMO energy level (see Table 1 and Figure 4B) indicate that the lower HOMO level probably is the reason for the smaller R2 value under illumination. Meanwhile, in dark condition, the R2 values for the cells are arranged in the order as DUY24 < DUY25 < DUY26 < DUY27, which is consistent with the order of the VOC values for the cells (see Table 1), due to the larger R2 in dark condition that suggests less charge recombination in the cell. Electron-transport time (τtr, the time that the electrons on TiO2 transiting across the photoanode) and electron lifetime (τn, also called recombination time, the time that the electrons on TiO2 recombined with the oxidant in electrolyte) as a function of the light intensity for DSCs obtained from IMPS and IMVS measurements are displayed in Figure 9. It was known that for a good performance cell, the electron transport time should be shorter than the electron lifetime for the electron to transport to anode before recombining with the electrolyte.25 The τtr values for the cells based on DUY24− DUY27 dyes are much smaller than the τn values, suggesting that all cells could have high efficiencies. The ascending order in τtr values is DUY24 < DUY25 = DUY26 < DUY27 (see Figure 9A), which represented the trend of the efficiency for the electron injection from the excited dye to the TiO2 conduction band, supposing that TiO2 films used in all devices are basically identical. Therefore, the trend of the τtr values is consistent with the order of the LUMO energy levels (see Figure 4B). On the other hand, the τtr order is inconsistent with the JSC trend (DUY24 < DUY25 < DUY26 < DUY27) of the cells (see Table 1). This is due to the JSC being simultaneously affected by the electron injection and the dye regeneration. The inconsistency between the τtr and JSC trends indicates that the difference in JSC values of all devices are primarily determined by the dye regeneration efficiency, since all dyes have sufficient high driving force (>670 mV) for electron injection.
Figure 9. (A) Electron-transport time (τtr) and (B) electron lifetime (τn) as a function of the light intensity for DSCs sensitized by DUY24−DUY27.
cies. A significant advantage of these sensitizers is that the βLUSO of the oxidized dye distributes mainly on the cyclometalating ligands, especially on the soft sulfur atom(s). Therefore, the strong interaction between the soft sulfur atom and soft iodide ion favors the efficient dye regeneration. The photovoltaic performance of DUY24−DUY27 sensitizers supports that the dye-regeneration efficiency (therefore the JSC) of the cell is determined by not only the HOMO energy level but also the β-LUSO distribution of the oxidized dye. Therefore, the dye regeneration can be improved by designing dye molecules that have not only the HOMO energy level lower than the dye-regeneration threshold but also the β-LUSO distributed on the soft atoms. These results provide an important guide for designing high-efficiency dyes for dyesensitized solar cells.
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ASSOCIATED CONTENT
* Supporting Information S
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02862. Schematic illustrations and written descriptions of syntheses of ligands and complexes, including analytical data; tabulated selective transition states; illustrated frontier orbitals of DUY24−DUY27 (PDF)
CONCLUSION Four thiocyanate-free sensitizers (DUY24−DUY27) containing 2-thienylpyridine as a cyclometalating ligand were prepared and applied in DSC. Compared to the cell based on the published sensitizers (NC103) comprising the same typed ligands, DUY24−DUY27-based cells exhibit 42%−65% higher efficienG
DOI: 10.1021/acs.inorgchem.7b02862 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Chun-Guey Wu: 0000-0001-8540-5602 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support (Grant NSC 104-2113-M-008-002-MY3) from the Ministry of Science and Technology (MOST), Taiwan, is gratefully acknowledged. The device fabrications and photovoltaic parameter measurements were performed at the Advanced Laboratory of Accommodation and Research for Organic Photovoltaics, MOST, Taiwan, Republic of China.
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REFERENCES
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DOI: 10.1021/acs.inorgchem.7b02862 Inorg. Chem. XXXX, XXX, XXX−XXX
