A Journal of Accepted Article Title: Structural Effect of Pendant Unit in Thiocyanate-Free Ru(II) Sensitizers on Dye-Sensitized Solar Cell Performance Authors: Mutsumi Kimura, Shogo Mori, Takahiro Kono, and Rei Tamura This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700899 Link to VoR: http://dx.doi.org/10.1002/ejic.201700899 10.1002/ejic.201700899 European Journal of Inorganic Chemistry Structural Effect of Pendant Unit in Thiocyanate-Free Ru(II) Sensitizers on Dye-Sensitized Solar Cell Performance Rei Tamura, Takahiro Kono, Shogo Mori*, and Mutsumi Kimura* Department of Chemistry and Materials, Faculty of Textile Science and Technology, Shinshu University, Ueda 386-8567, Japan E-mail: email@example.com & firstname.lastname@example.org Corresponding Author: Prof. Dr. Mutsumi Kimura and Prof. Dr. Shogo Mori Mail Address: Faculty of Textile Science and Technology, Shinshu University, Ueda 386-8567, Japan TEL&FAX: +81-268-21-5499 E-mail: email@example.com & firstname.lastname@example.org Graphical Abstract Abstract: Combination of Ru complex sensitizers and Co complex redox couples for dye-sensitized solar cells (DSSCs) generally results in low power conversion efficiency. This has been interpreted with the facilitation of undesired electron transfer due to strong intermolecular interactions between the dye and the redox couple. To retard the interactions, two thiocyanate-free ruthenium (Ru) sensitizers are synthesized with a terpyridine attached with a triphenylamine (TPA) unit having branched alkoxy chains to the TPA unit. The main difference of the two dyes is the angle of the three phenyl rings in the TPA unit. The DSSCs using both the new dyes show higher short-circuit currents and open-circuit voltages in comparison to that using a Ru complex dye having non-branched alkyl chains. The Ru sensitizer having a more twisted TPA unit displays 1 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry relatively high power conversion efficiency without coadsorption of chenodeoxycholic acid, suggesting less intermolecular interaction among dyes and thus also with Co complex redox couples. Introduction Dye-sensitized solar cells (DSSCs) have been attracted attention as an alternative candidate for conventional solar cells.1 While thousands of sensitizers were synthesized, various design strategies for sensitizers have been proposed.2 One of strategies is an intramolecular arrangement of donor (D), light-harvesting chromophore as a p-linker, and acceptor (A) units.2 The addition of donor and acceptor units to chromophore can widen the absorption spectrum of the sensitizers. By the addition of a donor moiety to porphyrin chromophore, a power conversion efficiency (PCE) of 11% was obtained.3 Further improvement of PCE for DSSCs was achieved by introducing sterically protected structure to D-p-A porphyrin dye YD2-o-C8 for the use with cobalt2+/3+ tris(bipyridyl) complexes as a redox mediator.4 The cobalt complexes are able to increase open-circuit voltage (Voc) due to their more positive redox potential in comparison to that of I-/I3- redox couple.5 However, Co complexes generally cause problems of fast charge recombination in DSSCs. Thus, large efforts have been made to add a function to sensitizers to block the approach of Co complexes to the surface of TiO2 electrode. While the attempts were successful for organic and porphyrin dyes, Ru complex dyes had been suffering from fast recombination with Co complex redox couples.6 Initially designed Ru dyes for DSSCs contain SCN ligands. Even the total charge of Ru dyes is zero, such Ru complex dyes could even attract Co complex due to 2 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry electrostatic forces between localized negative charges at SCN ligands and positively charged Co complex redox couples.7 The Ru complex has relatively small HOMO-LUMO gap and thus large dispersion force, in other words, intermolecular force, is expected. The dispersion force is proportional to molecule’s polarizability, which scales with the HOMO-LUMO gap of the molecule. Thus, wide absorption sensitizers have an intrinsic issue for DSSCs.8 One of remedies is to add moieties which would not change the absorption spectrum but increase the distance between the framework of the dye and redox species.9 The two properties, localized charge and large polarizability, of the Ru complexs can thus increase the local concentration of Co complex in the vicinity of TiO2 surface. Recently, Wu et al. reported SCN free bipyridine based Ru complex giving 9.53 % energy conversion efficiency with Co complex redox couple.10 We have recently reported that a Ru complex dye, T7, which was designed by replacing thiocyanate ligands of a well-known Ru complex dye, called Black dye, with a tridentate ligand and by adding a bulky TPA moiety to the terpyridine ligand of the dye, increased charge separation efficiency and thus PCE values.11 Ru complexes with a terpyridine ligand can give wider absorption spectrum than those with a bipyridine ligand. Our result was interpreted with that the replacement of the SCNligands decreased the electrostatic interaction and the addition of bulky TPA moiety decreased the effect of dispersion force between the dyes and Co complexes. The addition of the TPA moiety did not widen the absorption spectrum, suggesting little change in polarizability. Thus, the bulky TPA moiety probably did not increase the intermolecular forces but worked to block the approach of Co complexes to the main 3 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry framework of the Ru complex dyes. The reduction of electrostatic and intermolecular forces resulted in less influence on the concentration of Co3+ near the Ru complex dye, retarding direct electron transfer from the excited dye to the Co complex and the charge recombination between injected electrons and Co complexes. In this study, we explored the structural modification of the SCN-free terpyridine based Ru sensitizers to reduce further the molecular interactions between Ru and Co complexes to increase the performance of DSSCs. Results and Discussion The chemical structures of new ligands (1 and 2) and dyes (T11 and T12) are shown in Figure 1. We expected that the introduction of branched alkoxy chains and the steric hindrance of free rotation in the TPA unit raise the blocking function of the Ru sensitizers, retarding charge transfers from the TiO2 electrode to the Co complex redox couples and from the dye to the redox couples. In addition, alkoxy was expected to increase the electron donating ability to enhance the electron injection yield from the dyes to TiO2. Fig. 1 The nitrogen center in TPA is linked to three electron-rich phenyl groups in a propeller-like geometry.12 The oxidation potential of TPA was negatively shifted when electron-donating groups were attached at the para-phenyl positions.13 Two terpyridine-based ligands 1 and 2 having different TPA structures were synthesized. 4 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry Two TPA units were prepared by the Buchwald-Hartwig amination between 4-iodo-1-(2-ethylhexyloxy)benzene and 4-bromoaniline or 4-bromo-2,6-dimethylaniline.11 After the conversion of bromide to boronic acid, the TPA units were coupled with the terpyridine ligand through the Suzuki coupling reaction6d. Fig. 2 Fig. 2a shows the absorption spectra of 1 and 2 in dimethylformamide (DMF). While both ligands displayed a broad absorption band centered at 390 nm, the molecular extinction coefficient (e) of 2 was lower than that of 1. The optimized geometry of TPA in 1 calculated at the DFT/B3LYP level of theory using 6-31G* basis set showed a propeller-like structure with D3 symmetry (Fig. 2b).14 The phenyl rings were symmetrically twisted from the central plain made of NCCC atoms.The phenyl rings attached with terpyridine ligand in 2 are more twisted from the central plain due to the steric hindrance of the methyl groups attached to the one of the phenyl ring. Cyclic voltammograms of 1 and 2 showed a reversible oxidation couple at +1.10 and +1.08 V vs. NHE. Fig. 3 Table 1 Two thiocyanate-free Ru dyes T11 and T12 were synthesized by the reaction of 1 or 2 with RuCl3·H2O and reacted with 2,6-bis(2’-(4’-trifluoromethyl)pyrazolyl) pyridine.15 The absorption spectra of T11 and T12 in DMF are shown in Fig. 3a, and the absorption maxima (lmax) and absorption coefficients (e) are collected in Table 1. Heteroleptic 5 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry bis-tridentate RuII dyes T11 and T12 exhibited a broad absorption band in the whole visible light region from 300 to 750 nm, and the spectral shapes of T11 and T12 were similar to those of TF dyes, which were reported by Chou et. al. and have similar structures,15 except for the absorption band at 420 nm corresponding to the TPA unit. The absorption band centered at 505 nm can be assigned to the metal-to-ligand charge-transfer (MLCT) transition to the dicarboxyterpyridine ligand.15 The Ru dyes exhibit a board absorption band at longer wavelength, and the onsets of the absorption spectra are close to 800 nm as shown in the inset of Fig. 3a. Differential pulse voltammogram (DPV) of a DMF solution containing T11 and tetrabutylammonium perchlorate as an electrolyte displayed bimodal oxidation waves at 0.90 and 0.98 V vs. NHE. T7 dye exhibited two oxidation potentials at 0.92 and 1.17 V vs. NHE in solution. The potential difference between the Ru2+/3+ center and the TPA in T11 was smaller than that of T7 by changing alkyl chains to alkoxy chains at the peripheral positions of TPA unit.11 T12 having the sterically crowding TPA unit showed a unimodal wave at 0.91 V vs. NHE, suggesting the overlapping of two oxidation potentials of the Ru center and the TPA. Figure 4 shows the optimized structure of T11 and T12 calculated at the DFT/B3LYP level of theory using 3-21G* basis set.16 The effect of solvation by acetonitrile was included by the conductor-like polarizable continuum mode.17 The 3-21G* basis set was chosen for the Ru complex dyes based on a paper by De Angelis et al.18 Figure 4 shows that the angle between the planes of the terpyridine and the dipyrazolylpyridine is 90 degree. The branched alkyl chains of T11 are close to on the plane of the dipyrazolylpyridine ligand while those of T12 are on the plane, which has 6 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry almost 45 degree from both the planes of the terpyridine and the dipyrazolylpyridine ligands. From the structure, the alkyl chains of T12 are expected to reduce the contact between the terpyridine and the dipyrazolylpyridine ligands among dyes when they are adsorbed on TiO2 surface. This is because the alkyl chains cover these ligands so that mostly only the alkyl chains would be contacting each other. Fig 4 Porous TiO2 films on the transparent conducting glass substrates were immersed into 0.2 mM dye solutions containing 20 mM chenodeoxycholic acid (CDCA) in toluene/ethanol (4:1 v/v).11 The co-adsorption of CDCA has been used to suppress undesired dye aggregation on TiO2 surface. The onsets of absorption spectra of T11 and T12 adsorbed onto TiO2 surface were blue-shifted by 30 nm relative to the spectra in DMF (Fig. S1). The adsorption of dyes on TiO2 surface could change the absorption spectral and electrochemical potential by the deprotonation of anchoring group and the overlapping of molecular orbitals. The dissociation of COOH groups into carboxylate anions affects the electron density for the coordination bonds between Ru ion and pyridines in the terpyridine ligand by reducing the electron accepting nature.19 The TiO2 film stained with T12 and CDCA showed a peak at +0.94 V vs. NHE with a shoulder peak at +0.85 V in a polarogram (Fig. S2). The oxidation potential for the Ru center in T12 was slightly negatively shifted by the adsorption onto TiO2 surface. The HOMO levels estimated from the first oxidation potentials were more positive than the redox potentials of I-/I3- (+0.40 V vs. NHE) and CoII/III tris(1,10-phenantroline) teteracyanoborate (Co(phen)3)5a (+0.61 V vs. NHE) (Fig. 3b). The LUMO energy levels 7 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry estimated from the absorption edge measured on TiO2 and the HOMO levels were more negative than the conduction-band edge potential of TiO2. These HOMO and LUMO levels for T11 and T12 fit the requirement for the operation of DSSCs. Fig. 5 Table 2 The performance of these sensitizers in DSSCs was examined by using two different redox electrolyte solutions. Fig. 5a and b show the photocurrent density-voltage (J-V) curves and incident-photon to current conversion efficiency (IPCE) spectra of DSSCs with I-/I3- redox couple using triple layered TiO2 electrodes (13 µm thick mesoporous layer (particle size, 20 nm), 7 µm scattering layer (particle size, 400 nm), and 7 µm reflection layer) stained with T11 and T12. The short-circuit photocurrent density (Jsc), open-circuit voltage (Voc), fill factors (FF), and overall cell efficiencies (PCE) for all the DSSCs are summarized in Table 2. The IPCE values of 400-550 nm for T11 and T12 cells reached above 85%. The PCEs of T11 and T12 cells were higher than that of T7.11 Comparison between T7 and T11 shows that the addition of branched alkoxy chains increased slightly the current density but resulted in little effect on the Voc value. Between T11 and T12, T12 showed higher Voc but lower Jsc value. These suggest that electron donating ability of alkoxy chain is effective for Jsc when the molecule has a structure having better p conjugation, that is, the flat structure of T11 has an advantage. The J-V characteristic of the DSSCs in the dark shows that the DSSC with T12 showed higher voltage to flow current. The difference in the dark current could be due to different TiO2 conduction band edge potential (Ecb) and/or 8 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry different charge recombination lifetimes and it is not obvious to discuss the recombination based on the J-V curves. Our previous study with Ru complex dyes showed that the Ecb did not depend on the structure of the dyes.11 Moreover we have also observed similar Ecb from various dyes.20 We are aware that some papers have shown different Ecb with different dyes. We suppose the apparent difference is due to the effect of electrolytes.21 Since we use the same electrolyte solution, we assume the values of Ecb for the DSSCs/T11 and T12 are the same. Then, the different in the dark current is attributed to the different in the recombination lifetime, that is, T12 probably retarded the charge recombination. Since the adsorbed dyes’ density in these DSSCs are low (Table 2), the retardation of the recombination was not due to the blocking function of the dye against the approach of Co3+ to TiO2 surface but probably due to the steric hindrance to be close to the main framework of the dye molecules. Since dispersion force is inversely proportional to the distance to the power of 6, the TPA unit and alkyl chains can reduce to the effect of the dispersion force between Co3+ and the dipyrazolylpyridine unit. Note that not only the length and number but also the location of alkyl chains has been shown as an important parameter for the recombination lifetime.6d Between T7 and T11, they showed almost the same Jsc and Voc values, suggesting they have similar charge recombination lifetime. Thus, to improve the function reducing intermolecular interactions, non-flat structure seems effective but the branched alkoxy chains did not add the blocking function for these dyes. T12 probably had less intermolecular forces with I3- than T11, resulting in less I3- concentration near TiO2 surface. 9 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry Fig. 5 Solar cells were then fabricated with an electrolyte solution containing CoII/III(phen)3 redox couple and double-layered TiO2 electrode (8 µm thick mesoporous layer and 4 µm scattering layer) having a larger pore size than that for I-/I3-. Higher porosity and thinner electrode were to avoid the mass transport limitation of larger-sized Co complexes in the electrolyte solution. The DSSCs/T12 gave a higher PCE value of 6.7% compared with T7 and T11 under the same electrode conditions (Figure 6a and Table 2). The Voc of T12 cell with Co complex redox couple was 30 mV higher than those of T7 and T11. This is probably due to the same reason for the Voc improvement in T12 cells with I-/I3- redox couple as described above. T12 also showed higher Jsc values than T7 and T11. This trend is different from the case when I-/I3- redox couple was employed. When Co complex redox couple is used with simple structure Ru complex dyes, they are attracted each other due to intermolecular forces and thus, electron transfer from excited dyes to Co complex is facilitated, decreasing Jsc value.11 If the steric structure of T12 reduces the intermolecular forces, it should result in less undesired electron transfer from the dye to Co3+ and thus less loss in Jsc. To check the effect of intermolecular interactions among the dyes, we also fabricated solar cells without co-adsorption of CDCA. Table 3, Fig. S3 and Fig. S4 show the results. The amounts of the adsorbed dyes were about five times higher when CDCA was not co-adsorbed. However, both Jsc and Voc values for the cells without CDCA were lower than those with CDCA. The lower Jsc with higher dye density could be due to that the electron/energy transfer among dyes becomes competitive with 10 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry electron injection process. Difference in Jsc values for DSSCs/T12 with and without CDCA was smaller than that for DSSCs/T11. This suggests that the structure of T12 was effective to reduce the intermolecular interactions not only with Co complex but also with Ru complex dyes. The lower values of Voc for DSSCs/T11 and T12 without CDCA are consistent with the interpretation that the dyes attract Co complexes, that is, more adsorbed dyes result in more attracted Co complexes, facilitating charge recombination between injected electrons in TiO2 and Co complexes in the electrolyte solution. In other words, the blocking function of T12 is still not ideal and there is a room to improve the structure further. For example, longer alkyl chains instead of methyl groups would help increasing the block functions. In conclusion, to retard intermolecular interactions between Ru complex dye and Co complex redox couples, two thiocyanate-free ruthenium (Ru) sensitizers were synthesized with a triphenylamine (TPA) unit having branched alkoxy chains (T11) and additional methyl groups to make more twisted structure in the TPA unit (T12). DSSCs using T11 showed higher short-circuit current in comparison to that using a Ru complex dye having non-branched alkyl chains but the branched alkoxy chains did not retard intermolecular interactions. DSSCs using T12 increased both open circuit voltage and short circuit current. T12 also showed relatively high power conversion efficiency without coadsorption of chenodeoxycholic acid, suggesting less intermolecular interaction among dyes and thus with Co complex redox couples. Less interaction resulted in lower local concentration of redox couples in the vicinity of TiO2 surface and 11 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry adsorbed dyes, reducing the electron transfer from TiO2 and excited dyes to Co complexes. Acknowledgements This work was partially supported by the New Energy and Industrial Technology Development Organization (NEDO) and JSPS KAKENHI Grant Number JP15H02172 and JP26288089. We thank Katsumi Kobayashi of Fujifilm Co. Ltd. for valuable discussion and Shingo Takano of Sumitomo Osaka Cement Co. Ltd. for the supply of TiO2 paste. References 1. Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Dye-Sensitized Solar Cells, Chem. Rev. 2010, 110, 6595-6663. 2. a) Mishra, A.; Fischer, M. K. R.; Bäuerle, P. Metal-Free Organic Dyes for Dye-Sensitized Solar Cells: From Structure: Property Relationships to Design Rules, Angew. Chem. Int. 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Cation Exchange at Semiconducting Oxide Surfaces: Origin of Light-Induced Performance Increases in Porphyrin Dye-Sensitized Solar Cells, J. Phys. Chem. C 2013, 117, 11885. 15 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry Figure Captions Fig. 1. Structure of examined ligands and Ru complex dyes, and previously published Ru complex dye. Fig. 2 a) Absorption spectra of 1 (solid line) and 2 (dotted line) in DMF. b) Optimized strcutre of TPA units in 1 and 2 by DFT calculations (TPy: terpyridine ligand). c) Molecular orbitals of 1 and 2 calcurated by the B3LYP level of theory using 6-31G(d) basis set. Fig. 3 a) Absorption spectra of T11 (solid line) and T12 (dotted line) in DMF. b) Energy level diagrams of TiO2, Ru complexes, and redox couple. Fig. 4 Calculated structures for T11 (a, c) and T12 (b, d). Top row shows side view (COOH are on left hand side and TPA on right hand side) and bottom row shows top view (view from TPA to COOH direction). Fig. 5 a) Photocurrent voltage curves obtained with DSSCs with I-/I3- redox couple based on T11 (black line) and T12 (red line) under a standard global AM 1.5 solar condition (solid line) and dark current (dotted line). b) Incident photon-to-current conversion efficiency spectrum for DSSC based on T11 (black line) and T12 (red line). Fig. 6 a) Photocurrent voltage curves obtained with DSSCs with Co(phen)3 redox couple based on T11 (black line) and T12 (red line) under a standard global AM 1.5 solar condition (solid line) and dark current (dotted line). b) Incident photon-to-current conversion efficiency spectrum for DSSC based on T11 (black line) and T12 (red line). 16 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry Table 1: Photophysical and electrochemical data for T11 and T12 Dyes lmax /nma (log e) E0-0 /eVb HOMO/Vc LUMO/Vd T11 715 (3.27),655 (3.28), 505 (4.11), 422 (5.50) 1.56 0.83 -0.73 T12 715 (3.27),655 (3.29), 505 (4.10), 420 (5.27) 1.56 0.84 -0.72 a Absorption peaks were measured in DMF. b Optical energy gap E0-0 were estimated from the onset of absorption spectra. c HOMO energy levels were estimated from the first peak of voltammogram measured for the dyes adsorbed onto TiO2 in electrolyte solutions by DPV (vs. NHE). d LUMO energy levels were estimated from LUMO = HOMO-E0-0 (vs. NHE). 17 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry Table 2. IV characteristics of DSSCs using T11, T12, and T7 with I-/I3- a or Co(Phen)3b redox couples under one sun condition. Dye Redox shuttle I-/I3- T11 Co(Phen)3 I-/I3- T12 Co(Phen)3 T7 a - e - I-/I3Co(Phen)3 Adsorption density x 10-5 / mol cm-3 1.7c 8.5d 1.7c 1.6c 10.5d 1.6c - Jsc / mAcm-2 Voc / V FF PCE /% 17.1 7.7 11.0 16.7 9.8 12.0 16.7 10.1 0.76 0.74 0.80 0.79 0.80 0.83 0.76 0.80 0.69 0.55 0.71 0.71 0.69 0.66 0.70 0.70 9.0c 3.2d 6.3c 9.3c 5.4d 6.7c 8.9c 5.7c I /I3 electrolyte system; [I2] = 0.05M, [LiI] = 0.10M, [DMPImI] = 0.60M, [tBP] = 0.50M in acetonitrile, b Co(phen)3 electrolyte system; [CoII(phen)3(B(CN)4)2] = 0.22M, [CoIII(phen)3(B(CN)4)3] = 0.02M, [LiClO4] = 0.20M, [tBP] = 0.50M in acetonitrile. DMPImI: dimethylpropylimidazolium iodide, tBP: 4-tert-butylpyridine. c Co-adsorbed with CDCA. d Without CDCA. e Data is from ref 7. 18 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry Fig. 1 Fig. 2 19 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry Fig. 3 20 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry Fig. 4. 21 This article is protected by copyright. All rights reserved. 10.1002/ejic.201700899 European Journal of Inorganic Chemistry Fig. 5 Fig. 6 22 This article is protected by copyright. All rights reserved.