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Takuro Ikeuchi, Hirotaka Nomoto, Naruhiko Masaki, Matthew J. Griffith, Shogo Mori, * and Mutsumi Kimura* 5
10
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Asymmetric zinc phthalocyanines with alkyl chain substituents formed highly efficient light-harvesting layers on TiO2 surface. Dye-sensitized solar cells using PcS20 exhibited a record efficiency of 6.4 % under one-sun irradiation. 40
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Dye-sensitized solar cells (DSSCs) have been expected as low1 cost, lightweight, and flexible energy conversion devices. To date, the highest conversion efficiency above 12 % has been achieved with π-conjugated macrocyclic zinc porphyrin sensitizer 2,3 YD2-o-C8 using cobalt polypyridine redox shuttles. Designed πconjugated macrocycles such as porphyrins and phthalocyanines have attracted special attention as versatile molecular platforms 4 of highly efficient dyes for DSSCs. While a lot of porphyrin-based sensitizers have been designed and synthesized to enhance the 5 conversion efficiency in DSSCs , the efficiencies of DSSCs employing red/near-IR absorbing phthalocyanine-based sensitizers had not been impressive due to their strong tendency to aggregate and the lack of directionality of electron transfer in the excited states. R3
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5.9% from solar cells employing PcS18 under one sun 7c condition. However, while the maximum value of IPCE was more than 80 %, the values around 520 nm was about 30 %. Since the absorption coefficient of the sensitizers at the wavelength is very low, in order to increase the efficiency, higher dye adsorption density is desired. This can be done by decreasing the molecular size. In addition to IPCE spectrum, open circuit voltage of the PcS18 cell was not high. This could be improved by modifying the size of peripheral substituents attached to the phthalocyanine core to cover the TiO2 surface by the dye molecules. In this communication, we examine the effect of the length and number of alkoxy groups around the phthalocyanine moiety on the photovoltaic properties. We now expect the alkoxy groups to have three functions, preventing aggregation, filling the space among dyes on TiO2 surface and increasing dye adsorption density. We found that an asymmetrical zinc phthalocyanine (ZnPc) PcS20 bearing propoxy groups in the 2 and 6 positions of peripheral phenoxy units (Figure 1) showed a record PCE value of 6.4 % under simulated air mass 1.5 global sunlight.
R1 R2
O
R2
COOH
O N
R3 R1
N
N
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N R2 R3
N N
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N O
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R2 R1
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PcS19: R1 = R2 = -O(CH2)7CH3, R3 = -H PcS20: R1 = R2 = -O(CH2)3CH3, R3 = -H PcS21: R1 = -O(CH2)7CH3, R2 = H, R3 = -OCH3
Figure 1. Structure of ZnPc sensitizers PcS19-21.
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To overcome these drawbacks of phthalocyanine-based sensitizers, several approaches such as steric suppression of aggregation, electronic push-pull structures through asymmetrical substitutions, and optimization of adsorption sites 6,7 have been reported. Recently, we showed that the structural modification of peripheral bulky substituents and adsorption site around the phthalocyanine core induced the improvement of incident photon-to-current efficiency (IPCE), and thereby achieved a solar-to-electric power-conversion efficiency (PCE) of
Figure 2. Absorption spectra of PcS20 (a) and PcS21 (b) in toluene (solid line) and adsorbed on the TiO2 film (dotted line). 60
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This journal is © The Royal Society of Chemistry [year]
Phthalocyanine precursors were synthesized from resorcinol or 2-hydroxy-4-methoxybenzaldehyde (Scheme S1 in ESI). The 8 aldehyde was converted to phenol using hydrogen peroxide. The phenols were reacted with 4,5-dichlorophthalonitirile in the 7 presence of K2CO3 to afford the phthalocyanine precursors. The asymmetrical ZnPcs PcS19-21 were prepared by a mixed [journal], [year], [vol], 00–00 | 1
Chemical Communications Accepted Manuscript
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Molecular Engineering of Zinc Phthalocyanine Sensitizers for Efficient DyeSensitized Solar Cell
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cyclotetramerisation of phthalocyanine precursors with methyl 9 3,4-dicyanobenzate in a 3:1 ratio by refluxing in 2(dimethylamino)ethanol in the presence of Zn(OAc)2, and subsequent hydrolysis with an aqueous alkaline solution. The alkoxy groups in the 2 and 6 positions of peripheral phenoxy units in PcS19 and PcS20 cover around the planar πconjugated ZnPc core (Fig. S2 in ESI). Figure 2 shows the absorption spectra of PcS20 in toluene and adsorbed onto a porous TiO2 films. The spectrum of PcS20 showed a split Q band at 675 and 695 nm in toluene. This is a typical characteristic of asymmetrical “push-pull” ZnPcs with donor and acceptor 10 substituents. While split Q bands observed in previously reported PcS18, the width of the split Q band was only 12 nm 7c (677 and 689 nm). The Q band of PcS20 was slightly red-shifted as compared with the PcS18 spectrum. These spectral differences between PcS18 and PcS20 are ascribed to the enhancing of electron pushing abilities of peripheral units through the introduction of electron donating alkoxy 8 substituents in the 2 and 6 positions of phenoxy units. The carboxylic acid in PcS20 can form an ester linkage with the surface of TiO2 to provide a strongly anchoring dye and electronic communication between the dye and TiO2. The absorption spectrum of PcS20 adsorbed on a 4 µm TiO2 film showed a sharp Q band at 695 nm, suggesting the prevention of aggregation among ZnPc cores in the adsorbed monolayer on TiO2 surface. The split Q bands of PcS20 in toluene changed into a monophasic Q band on the TiO2 film due to electronic 6 interaction of the adsorption site with the TiO2 surface. The PcS19 bearing long octyloxy groups exhibited a similar absorption spectra (Fig. S3 in ESI) to that of PcS20, but slightly sharper split in the Q bands. In contrast, the absorption spectrum of PcS21 having six octyloxy groups in only one orthoposition of peripheral phenoxy units showed a narrow split width (11nm) of the Q bands in toluene relative to PcS20, indicating a low pushing ability of the peripheral units. The Q band of PcS21 adsorbed on the TiO2 surface was broadened compared to that of PcS20 and a new absorption peak at 610 nm was observed, suggesting the formation of stacks among ZnPc cores of PcS21 in 11 the adsorbed monolayer on the TiO2 surface. The dye density adsorbed onto the porous TiO2 films were determined by measuring the absorbance of the phthalocyanines released from the dye-stained TiO2 films. The adsorption -11 densities on the TiO2 films were determined to be 2.7x10 mol -2 -11 -2 cm for PcS19 and 8.3x10 mol cm for PcS20. The adsorption density of PcS20 was about 1.5 and 3 times higher of that of PcS18 and PcS19, respectively. The area occupied by PcS20 on 2 the TiO2 surface was estimated to be 2.0 nm /molecule. This is close to the molecular dimension of PcS20 (width 2.8 nm x 2 thickness 0.8 nm = 2.2 nm /molecule) (Fig. S2 in ESI), suggesting the formation of a densely packed monolayer of PcS20 on the TiO2 surface with perpendicular orientation of the phthalocyanine plane. In contrast, the enlargement of molecular dimension in PcS19 having long octyl chains leads to a poor phthalocyanine density in the adsorbed monolayer on the TiO2. The DSSC performance of PcS19-21 was examined using double-layered TiO2 electrodes with electrolytes containing 0.6 7 M DMPImI, 0.1 M LiI, 0.05 M I2, 0.5 M tBP in acetonitrile. Figure 3a shows the photocurrent density-voltage curve of the DSSC
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using PcS20 adsorbed nanocrystalline TiO2 electrode under a standard AM 1.5 solar condition, and Table 1 shows the shortcircuit photocurrent density (Jsc), open-circuit photovoltage (Voc), fill factor (FF) and PCE obtained with PcS19-21. The PcS20 cell -2 exhibits a Jsc of 15.1 mA cm , a Voc of 600 mV, and a FF of 0.71, yielding a PCE of 6.4 % compared with 5.9 % for the previously -2 reported dye PcS18 (Voc = 613 mV, Jsc = 13.7 mA cm , FF = 7c 0.70). Whereas the Voc and FF values in PcS20 cell are almost the same as those in PcS18 cell, the Jsc is improved from 13.7 mA -2 -2 cm in PcS18 to 15.1 mA cm in PcS20. The IPCE spectrum of PcS20 cell followed the absorption feature of PcS20 adsorbed on the TiO2 electrode (Figure 3b). The onset of the IPCE spectrum was 820 nm and a maximum IPCE value was 86% at 600-720 nm corresponding to the Q band of ZnPc. The IPCE spectrum of PcS20 cell at 600-720 nm was wider than that of PcS18 cell, which is primarily attributed to the red-shifting of Q band by the introduction of electron donating alkoxy substituents. Furthermore, the high adsorption density of PcS20 on TiO2 resulted in the enhancement of IPCE values in the range of 450550 nm as compared with PcS18.
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Figure 3. a) Photocurrent voltage curves obtained with DSSCs based on PcS19 (blue), PcS20 (green), and PcS21 (red) under a standard global AM 1.5 solar condition. b) Incident photon-to-current conversion efficiency spectra for DSSCs based on PcS19 (blue), PcS20 (green), and PcS21 (red). 85
Table 1: Photovoltaic Performance of DSSCs based on PcS19-21 Sensitizers PcS19 PcS20 PcS21
Adsorption densitya Voc / mV Jsc / mA cm-2 x 10-11 / mol cm-2 2.7 620 14.3 8.3 600 15.1 8.3 618 9.8
FF
PCE / %
0.67 0.71 0.69
5.9 6.4 4.2
a
Adsorption densities were determined by measuring the absorbance of dyes released from the TiO2 films by immersing into THF containing tetrabutylammonium hydroxide methanoic solution.
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For the case of PcS19 having long octyloxy chains, PcS19 cell displayed a lower PCE value in comparison to PcS20 cell (Table 1). The lower PCE in PcS19 cell is mainly due to the lower Jsc value because of the lower adsorption density of PcS19 on the surface of TiO2. On the other hand, the Voc of PcS19 cell was 20 mV higher than that of PcS20 cell. In order to examine the origin of high Voc for PcS19, electron lifetime and density were measured (Fig. S4 in ESI). Little difference was observed on the Voc vs. electron density plot, suggesting that the difference in alkyl chain length in PcS19 and PcS20 has little influence on the shift of the conduction band edge position. Thus, the improvement of Voc for PcS19 cell should be attributed to the repression of charge recombination, which is related to electron lifetime. The electron lifetime value of the PcS19 cell was longer than that of PcS20 cell by around 3-fold at matched electron density. The electron lifetime of the PcS21 cell was also longer than that of the PcS20 This journal is © The Royal Society of Chemistry [year]
Chemical Communications Accepted Manuscript
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Faculty of Textile Science and Technology, Shinshu University, Ueda 3868567, Japan. Tel & Fax: +81-268-21-5499; E-mail: mkimura@ shinshuu.ac.jp & [email protected] † Electronic Supplementary Information (ESI) available: [details of synthetic procedures, DSSCs fabrication, oxidation potentials of dyes, and electron lifetime and density]. See DOI: 10.1039/b000000x/ 1 2
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B. O’Regan, M. Grätzel, Nature, 1991, 353, 737. A. Yella, H. –W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W. –G. Diau, C. –Y. Yeh, S. M. Zakeeruddin, M. Grätzel, Science, 2011, 334, 629. C. –L. Wang, C. –M. Lan, S. –H. Hong, Y. –F. Wang, T. –Y. Pan, C. –W. Chang, H. –H. Kuo, M. –Y. Kuo, E. W. –G. Diau, C. –Y. Lin, Energy Environ. Sci., 2012, 5, 6933. H. Imahori, T. Umeyama, S. Ito, Acc. Chem. Res., 2009, 42, 1809. a) C. W. Lee, H. –P. Lu, C. –M. Lan, Y. –L. Hung, Y. –R. Liang, W. –N. Yen, Y. –C. Liu, Y. –S. Lin, E. W. –G. Diau, C. –Y. Yeh, Chem. Eur. J., 2009, 15, 1403; b) A. J. Mozer, M. J. Griffith, G. Tsekouras, P. Wagner, G. G. Wallace, S. Mori, K. Sunahara, M. Miyashita, J. C. Earles, K. C. Gordon, L. Du, R. Kato, A. Furube, D. L. Officer, J. Am. Chem. Soc., 2009, 131, 15621; c) T. Bessho, S. M. Zakeeruddin, C. –Y. Yeh, E. W. –G. Diau, M. Grätzel, Angew. Chem. Int. Ed., 2010, 49, 6646; d) S. Mather, H. Iijima, Y. Toude, T. Ueyama, Y. Matano, S. Ito, N. V. Tkachenko, H. Lemmetyinen, H. Imahori, J. Phys. Chem. C, 2011, 115, 14415; e) Y. –C. Chang, C. –L. Wang, T. –Y. Pan, S. –H. Hong, C. –M. Lan, H. –H. Kuo, C. –F. Lo, H. –Y. Hsu, C. –Y. Lin, E. W. – G. Diau, Chem. Commun., 2011, 47, 8910; f) C. –L. Wang, Y. -C. Chang, C. –M. Lan, C. –F. Lo, E. W. –G. Diau, C. –Y. Lin, Energy Environ. Sci., 2011, 4, 1788. a) P. Y. Reddy, L. Giribabu, C. Lyness, H. J. Snaith, C. Vijaykumar, M. Chandrasekharam, M. Lakshmikantam, J. –H. Yum, K. Kalyanasundaram, M. Grätzel, M. K. Nazeeruddin, Angew. Chem. Int. Ed., 2007, 46, 373; b) J. –J. Cid, J. –H. Yum, S. –R. Jang, M. K. Nazeeruddin, E. Martínez-Ferrero, E. Palomares, J. Ko, M. Grätzel, T. Torres, Angew. Chem. Int. Ed., 2007, 46, 8358; c) J. –J. Cid, M. García-Iglesias, J. –H. Yum, A. Forneli, J. Albero, E. Martínez-Ferrero, P. Vázquez, M. Grätzel, M. K. Nazeerddin, E. Palomares, T. Torres, Chem. Eur. J., 2009, 15, 5130; d) I. López-Duarte, M. Wang, R. Humphry-Baker, M. Ince, M. V. Martínez-Díaz, M. K. Nazeeruddin, T. Torres, M. Grätzel, Angew. Chem. Int. Ed., 2012, 51, 1895; e) M. –E. Rogoussi, J. –J. Cid, J. –H. Yum, G. de la Torre, D. Di Censo, M. Grätzel, M. K. Nazeeruddin, T. Torres, Angew. Chem. Int. Ed., 2012, 51, 4375. a) S. Mori, M. Nagata, Y. Nakahata, K. Yasuta, R. Goto, M. Kimura, M. Taya, J. Am. Chem. Soc., 2010, 132, 4054; b) M. Kimura, H. Nomoto, N. Masaki, S. Mori, Angew. Chem. Int. Ed., 2012, 51, 4371, c) M. Kimura, H. Nomoto, H. Suzuki, T. Ikeuchi, H. Matsuzaki, T. N. Murakami, A. Furube, N. Masaki, M. J. Griffith, S. Mori, Chem. Eur. J., 2013, 19, 7496. C. F. van Nostrum, S. J. Picken, A. –J. Schouten, R. J. M. Nolte, J. Am. Chem. Soc., 1995, 117, 9957. G. Pozzi, S. Quici, M. C. Roffo, C. A. Bignozzi, S. Caramori, M. Orlandi, J. Phys. Chem. C, 2011, 115, 3777. J. Mack, N. Kobayashi, Chem. Rev., 2011, 111, 281. M. J. Stillman, T. Nyokong, Chapter 3 of Phthalocyanines Properties and Applications Vol. 1 (Eds.: C. C. Leznoff, A. B. P. Lever), VCH, New York, 1989, pp 139-247. a) N. Koumura, Z.-S. Wang, S. Mori, M. Miyashita, E. Suzuki, and K. Hara, J. Am. Chem. Soc., 2006, 128, 14256. (Addition and Correction 2008, 130, 4202), b) Z.-S. Wang, N. Koumura, Y. Cui, M. Takahashi, H. Sekiguchi, A. Mori, T. Kubo, A. Furube and K. Hara, Chem. Mater., 2008, 20, 3993. a) S. M. Feldt, E. A. Gibson, E. Gabrielsson, L. Sun, G. Boshloo, A. Hagfeldt, J. Am. Chem. Soc., 2010, 132, 16714; b) H. N. Tsao, C. Yi, T. Moehl, J. –H. Yum, S. M. Zakeeruddin, M. K. Nazeeruddin, M. Grätzel, ChemSusChem, 2011, 4, 591; c) D. Zhou, Q. Yu, N. Cai, Y. Bai, Y. Wang, P. Wang, Energy Environ. Sci., 2011, 4, 2030; d) J. –H. Yum, E. Baranoff, F. Kessler, T. Moehl, S. Ahmad, T. Bessho, A. Marchioro, E. Ghadiri, J. –E. Moser, C. Yi, M. K. Nazeeruddin, M. Grätzel, Nat. Commun., 2012, 3, 631.
Journal Name, [year], [vol], 00–00 | 3
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cell, implying that the covering the ZnPc with long alkyl chains could prevent I3 attraction around the positively charged Zn atom in the ZnPc. Considering the steric blocking effect for charge recombination, higher dye adsorption density may seem to be preferred. However, important criterion is the surface coverage by dyes. Longer electron lifetime with lower dye adsorption density was also seen for an organic dye consisting of carbazole and three thiophenes having alkyl chains (MK-1 dye) in comparison to the dyes having the same structure but no alkyl 12 chains attached. The IPCE values between 600 and 720 nm for PcS21 were about half of those for PcS20. The oxidation potential of PcS20 (0.90 V vs. NHE) is sufficient to be reduced by I as indicated by the high IPCE. Since this value is comparable to the other dyes (see ESI), the cationic state of PcS21 must also be efficiently reduced by I . Thus, the lower IPCE values of PcS21 are probably due to aggregation, which would cause energy transfer among dyes and/or sub-nanosecond recombination with dye cation. The IPCE values of the PcS20 cell are slightly lower than those of the PcS19 cell. From Fig. 1., it is not obvious if PcS20 is aggregated. However, the less sharply split Q band peaks of PcS20 implies that the dyes are slightly aggregated. Fig. S5 (In ESI) shows IPCE spectra of the cells prepared without scattering layers. For that case, except for Q band region, light harvesting efficiency is expected to be less than 1. Between 600 and 720 nm, the PcS20 cells showed a flat spectrum while the PcS19 cells showed two peaks. This result is also an indication of the aggregation for PcS20. Thus, the drawback of the shorter alkyl chains in not only the shorter electron lifetime, but also the slightly insufficient steric protection against aggregation. The DSSCs with PcS20 using Co(II/III) tris(bipyridyl) 13 tetracyanoborate complexes as redox shuttles were fabricated. The PcS20 cell using cobalt redox shuttles exhibits a Jsc of 4.2 mA -2 cm , a Voc of 614 mV, and a FF of 0.52, yielding a PCE of 1.4 %. The decoration of ZnPc with long alkoxy chains acted as a barrier against the approach of redox species onto the TiO2 surface. However, the PCEs for ZnPc-sensitized DSSCs based on cobalt redox shuttles were much lower than donor-π-bridge-acceptor 2,13 Investigations are underway to optimize (D-π-A) sensitizers. the structure of phthalocyanine-based D-π-A sensitizers to improve the PCEs in DSSCs based on cobalt redox shuttles. In summary, we have synthesized novel asymmetric zinc phthalocyanines PcS19-21 having alkoxy chains, and used them as a light-harvesting units in DSSCs The PcS20 cell gave a record efficiency for phthalocyanine-sensitized DSSCs of 6.4% under one-sun condition. Adsorption densities depended on the length and number of alkoxy chains, and the dense adsorption of PcS20 on TiO2 resulted in highly efficient performance of the DSSCs. According to the molecular design rules for ZnPc sensitizers, we are now developing near-IR harvesting DSSCs using ringexpanded naphthalocyanine sensitizers with spectral response up to 900 nm. This work has been partially supported by Grants-in-Aid for Scientific Research (B) (No. 22350086) from Japan Society for the Promotion of Science (JSPS) of Japan.
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Volume 46 | Number 32 | 2010
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This is an Accepted Manuscript, which has been through the RSC Publishing peer review process and has been accepted for publication. Chemical Communications www.rsc.org/chemcomm
Volume 46 | Number 32 | 28 August 2010 | Pages 5813–5976
ChemComm
the chemical
to therapeutic
etic and
Accepted Manuscripts are published online shortly after acceptance, which is prior to technical editing, formatting and proof reading. This free service from RSC Publishing allows authors to make their results available to the community, in citable form, before publication of the edited article. This Accepted Manuscript will be replaced by the edited and formatted Advance Article as soon as this is available. To cite this manuscript please use its permanent Digital Object Identifier (DOI®), which is identical for all formats of publication.
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Pages 5813–5976
org/journals
ISSN 1359-7345
COMMUNICATION J. Fraser Stoddart et al. Directed self-assembly of a ring-in-ring complex
FEATURE ARTICLE Wenbin Lin et al. Hybrid nanomaterials for biomedical applications
1359-7345(2010)46:32;1-H
Please note that technical editing may introduce minor changes to the text and/or graphics contained in the manuscript submitted by the author(s) which may alter content, and that the standard Terms & Conditions and the ethical guidelines that apply to the journal are still applicable. In no event shall the RSC be held responsible for any errors or omissions in these Accepted Manuscript manuscripts or any consequences arising from the use of any information contained in them.
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DOI: 10.1039/C3CC47714B
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ARTICLE TYPE
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Takuro Ikeuchi, Hirotaka Nomoto, Naruhiko Masaki, Matthew J. Griffith, Shogo Mori, * and Mutsumi Kimura* 5
10
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Asymmetric zinc phthalocyanines with alkyl chain substituents formed highly efficient light-harvesting layers on TiO2 surface. Dye-sensitized solar cells using PcS20 exhibited a record efficiency of 6.4 % under one-sun irradiation. 40
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Dye-sensitized solar cells (DSSCs) have been expected as low1 cost, lightweight, and flexible energy conversion devices. To date, the highest conversion efficiency above 12 % has been achieved with π-conjugated macrocyclic zinc porphyrin sensitizer 2,3 YD2-o-C8 using cobalt polypyridine redox shuttles. Designed πconjugated macrocycles such as porphyrins and phthalocyanines have attracted special attention as versatile molecular platforms 4 of highly efficient dyes for DSSCs. While a lot of porphyrin-based sensitizers have been designed and synthesized to enhance the 5 conversion efficiency in DSSCs , the efficiencies of DSSCs employing red/near-IR absorbing phthalocyanine-based sensitizers had not been impressive due to their strong tendency to aggregate and the lack of directionality of electron transfer in the excited states. R3
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5.9% from solar cells employing PcS18 under one sun 7c condition. However, while the maximum value of IPCE was more than 80 %, the values around 520 nm was about 30 %. Since the absorption coefficient of the sensitizers at the wavelength is very low, in order to increase the efficiency, higher dye adsorption density is desired. This can be done by decreasing the molecular size. In addition to IPCE spectrum, open circuit voltage of the PcS18 cell was not high. This could be improved by modifying the size of peripheral substituents attached to the phthalocyanine core to cover the TiO2 surface by the dye molecules. In this communication, we examine the effect of the length and number of alkoxy groups around the phthalocyanine moiety on the photovoltaic properties. We now expect the alkoxy groups to have three functions, preventing aggregation, filling the space among dyes on TiO2 surface and increasing dye adsorption density. We found that an asymmetrical zinc phthalocyanine (ZnPc) PcS20 bearing propoxy groups in the 2 and 6 positions of peripheral phenoxy units (Figure 1) showed a record PCE value of 6.4 % under simulated air mass 1.5 global sunlight.
R1 R2
O
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PcS19: R1 = R2 = -O(CH2)7CH3, R3 = -H PcS20: R1 = R2 = -O(CH2)3CH3, R3 = -H PcS21: R1 = -O(CH2)7CH3, R2 = H, R3 = -OCH3
Figure 1. Structure of ZnPc sensitizers PcS19-21.
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To overcome these drawbacks of phthalocyanine-based sensitizers, several approaches such as steric suppression of aggregation, electronic push-pull structures through asymmetrical substitutions, and optimization of adsorption sites 6,7 have been reported. Recently, we showed that the structural modification of peripheral bulky substituents and adsorption site around the phthalocyanine core induced the improvement of incident photon-to-current efficiency (IPCE), and thereby achieved a solar-to-electric power-conversion efficiency (PCE) of
Figure 2. Absorption spectra of PcS20 (a) and PcS21 (b) in toluene (solid line) and adsorbed on the TiO2 film (dotted line). 60
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This journal is © The Royal Society of Chemistry [year]
Phthalocyanine precursors were synthesized from resorcinol or 2-hydroxy-4-methoxybenzaldehyde (Scheme S1 in ESI). The 8 aldehyde was converted to phenol using hydrogen peroxide. The phenols were reacted with 4,5-dichlorophthalonitirile in the 7 presence of K2CO3 to afford the phthalocyanine precursors. The asymmetrical ZnPcs PcS19-21 were prepared by a mixed [journal], [year], [vol], 00–00 | 1
Chemical Communications Accepted Manuscript
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Molecular Engineering of Zinc Phthalocyanine Sensitizers for Efficient DyeSensitized Solar Cell
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cyclotetramerisation of phthalocyanine precursors with methyl 9 3,4-dicyanobenzate in a 3:1 ratio by refluxing in 2(dimethylamino)ethanol in the presence of Zn(OAc)2, and subsequent hydrolysis with an aqueous alkaline solution. The alkoxy groups in the 2 and 6 positions of peripheral phenoxy units in PcS19 and PcS20 cover around the planar πconjugated ZnPc core (Fig. S2 in ESI). Figure 2 shows the absorption spectra of PcS20 in toluene and adsorbed onto a porous TiO2 films. The spectrum of PcS20 showed a split Q band at 675 and 695 nm in toluene. This is a typical characteristic of asymmetrical “push-pull” ZnPcs with donor and acceptor 10 substituents. While split Q bands observed in previously reported PcS18, the width of the split Q band was only 12 nm 7c (677 and 689 nm). The Q band of PcS20 was slightly red-shifted as compared with the PcS18 spectrum. These spectral differences between PcS18 and PcS20 are ascribed to the enhancing of electron pushing abilities of peripheral units through the introduction of electron donating alkoxy 8 substituents in the 2 and 6 positions of phenoxy units. The carboxylic acid in PcS20 can form an ester linkage with the surface of TiO2 to provide a strongly anchoring dye and electronic communication between the dye and TiO2. The absorption spectrum of PcS20 adsorbed on a 4 µm TiO2 film showed a sharp Q band at 695 nm, suggesting the prevention of aggregation among ZnPc cores in the adsorbed monolayer on TiO2 surface. The split Q bands of PcS20 in toluene changed into a monophasic Q band on the TiO2 film due to electronic 6 interaction of the adsorption site with the TiO2 surface. The PcS19 bearing long octyloxy groups exhibited a similar absorption spectra (Fig. S3 in ESI) to that of PcS20, but slightly sharper split in the Q bands. In contrast, the absorption spectrum of PcS21 having six octyloxy groups in only one orthoposition of peripheral phenoxy units showed a narrow split width (11nm) of the Q bands in toluene relative to PcS20, indicating a low pushing ability of the peripheral units. The Q band of PcS21 adsorbed on the TiO2 surface was broadened compared to that of PcS20 and a new absorption peak at 610 nm was observed, suggesting the formation of stacks among ZnPc cores of PcS21 in 11 the adsorbed monolayer on the TiO2 surface. The dye density adsorbed onto the porous TiO2 films were determined by measuring the absorbance of the phthalocyanines released from the dye-stained TiO2 films. The adsorption -11 densities on the TiO2 films were determined to be 2.7x10 mol -2 -11 -2 cm for PcS19 and 8.3x10 mol cm for PcS20. The adsorption density of PcS20 was about 1.5 and 3 times higher of that of PcS18 and PcS19, respectively. The area occupied by PcS20 on 2 the TiO2 surface was estimated to be 2.0 nm /molecule. This is close to the molecular dimension of PcS20 (width 2.8 nm x 2 thickness 0.8 nm = 2.2 nm /molecule) (Fig. S2 in ESI), suggesting the formation of a densely packed monolayer of PcS20 on the TiO2 surface with perpendicular orientation of the phthalocyanine plane. In contrast, the enlargement of molecular dimension in PcS19 having long octyl chains leads to a poor phthalocyanine density in the adsorbed monolayer on the TiO2. The DSSC performance of PcS19-21 was examined using double-layered TiO2 electrodes with electrolytes containing 0.6 7 M DMPImI, 0.1 M LiI, 0.05 M I2, 0.5 M tBP in acetonitrile. Figure 3a shows the photocurrent density-voltage curve of the DSSC
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using PcS20 adsorbed nanocrystalline TiO2 electrode under a standard AM 1.5 solar condition, and Table 1 shows the shortcircuit photocurrent density (Jsc), open-circuit photovoltage (Voc), fill factor (FF) and PCE obtained with PcS19-21. The PcS20 cell -2 exhibits a Jsc of 15.1 mA cm , a Voc of 600 mV, and a FF of 0.71, yielding a PCE of 6.4 % compared with 5.9 % for the previously -2 reported dye PcS18 (Voc = 613 mV, Jsc = 13.7 mA cm , FF = 7c 0.70). Whereas the Voc and FF values in PcS20 cell are almost the same as those in PcS18 cell, the Jsc is improved from 13.7 mA -2 -2 cm in PcS18 to 15.1 mA cm in PcS20. The IPCE spectrum of PcS20 cell followed the absorption feature of PcS20 adsorbed on the TiO2 electrode (Figure 3b). The onset of the IPCE spectrum was 820 nm and a maximum IPCE value was 86% at 600-720 nm corresponding to the Q band of ZnPc. The IPCE spectrum of PcS20 cell at 600-720 nm was wider than that of PcS18 cell, which is primarily attributed to the red-shifting of Q band by the introduction of electron donating alkoxy substituents. Furthermore, the high adsorption density of PcS20 on TiO2 resulted in the enhancement of IPCE values in the range of 450550 nm as compared with PcS18.
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Figure 3. a) Photocurrent voltage curves obtained with DSSCs based on PcS19 (blue), PcS20 (green), and PcS21 (red) under a standard global AM 1.5 solar condition. b) Incident photon-to-current conversion efficiency spectra for DSSCs based on PcS19 (blue), PcS20 (green), and PcS21 (red). 85
Table 1: Photovoltaic Performance of DSSCs based on PcS19-21 Sensitizers PcS19 PcS20 PcS21
Adsorption densitya Voc / mV Jsc / mA cm-2 x 10-11 / mol cm-2 2.7 620 14.3 8.3 600 15.1 8.3 618 9.8
FF
PCE / %
0.67 0.71 0.69
5.9 6.4 4.2
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Adsorption densities were determined by measuring the absorbance of dyes released from the TiO2 films by immersing into THF containing tetrabutylammonium hydroxide methanoic solution.
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For the case of PcS19 having long octyloxy chains, PcS19 cell displayed a lower PCE value in comparison to PcS20 cell (Table 1). The lower PCE in PcS19 cell is mainly due to the lower Jsc value because of the lower adsorption density of PcS19 on the surface of TiO2. On the other hand, the Voc of PcS19 cell was 20 mV higher than that of PcS20 cell. In order to examine the origin of high Voc for PcS19, electron lifetime and density were measured (Fig. S4 in ESI). Little difference was observed on the Voc vs. electron density plot, suggesting that the difference in alkyl chain length in PcS19 and PcS20 has little influence on the shift of the conduction band edge position. Thus, the improvement of Voc for PcS19 cell should be attributed to the repression of charge recombination, which is related to electron lifetime. The electron lifetime value of the PcS19 cell was longer than that of PcS20 cell by around 3-fold at matched electron density. The electron lifetime of the PcS21 cell was also longer than that of the PcS20 This journal is © The Royal Society of Chemistry [year]
Chemical Communications Accepted Manuscript
DOI: 10.1039/C3CC47714B
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DOI: 10.1039/C3CC47714B
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Notes and references
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Faculty of Textile Science and Technology, Shinshu University, Ueda 3868567, Japan. Tel & Fax: +81-268-21-5499; E-mail: mkimura@ shinshuu.ac.jp & [email protected] † Electronic Supplementary Information (ESI) available: [details of synthetic procedures, DSSCs fabrication, oxidation potentials of dyes, and electron lifetime and density]. See DOI: 10.1039/b000000x/ 1 2
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B. O’Regan, M. Grätzel, Nature, 1991, 353, 737. A. Yella, H. –W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W. –G. Diau, C. –Y. Yeh, S. M. Zakeeruddin, M. Grätzel, Science, 2011, 334, 629. C. –L. Wang, C. –M. Lan, S. –H. Hong, Y. –F. Wang, T. –Y. Pan, C. –W. Chang, H. –H. Kuo, M. –Y. Kuo, E. W. –G. Diau, C. –Y. Lin, Energy Environ. Sci., 2012, 5, 6933. H. Imahori, T. Umeyama, S. Ito, Acc. Chem. Res., 2009, 42, 1809. a) C. W. Lee, H. –P. Lu, C. –M. Lan, Y. –L. Hung, Y. –R. Liang, W. –N. Yen, Y. –C. Liu, Y. –S. Lin, E. W. –G. Diau, C. –Y. Yeh, Chem. Eur. J., 2009, 15, 1403; b) A. J. Mozer, M. J. Griffith, G. Tsekouras, P. Wagner, G. G. Wallace, S. Mori, K. Sunahara, M. Miyashita, J. C. Earles, K. C. Gordon, L. Du, R. Kato, A. Furube, D. L. Officer, J. Am. Chem. Soc., 2009, 131, 15621; c) T. Bessho, S. M. Zakeeruddin, C. –Y. Yeh, E. W. –G. Diau, M. Grätzel, Angew. Chem. Int. Ed., 2010, 49, 6646; d) S. Mather, H. Iijima, Y. Toude, T. Ueyama, Y. Matano, S. Ito, N. V. Tkachenko, H. Lemmetyinen, H. Imahori, J. Phys. Chem. C, 2011, 115, 14415; e) Y. –C. Chang, C. –L. Wang, T. –Y. Pan, S. –H. Hong, C. –M. Lan, H. –H. Kuo, C. –F. Lo, H. –Y. Hsu, C. –Y. Lin, E. W. – G. Diau, Chem. Commun., 2011, 47, 8910; f) C. –L. Wang, Y. -C. Chang, C. –M. Lan, C. –F. Lo, E. W. –G. Diau, C. –Y. Lin, Energy Environ. Sci., 2011, 4, 1788. a) P. Y. Reddy, L. Giribabu, C. Lyness, H. J. Snaith, C. Vijaykumar, M. Chandrasekharam, M. Lakshmikantam, J. –H. Yum, K. Kalyanasundaram, M. Grätzel, M. K. Nazeeruddin, Angew. Chem. Int. Ed., 2007, 46, 373; b) J. –J. Cid, J. –H. Yum, S. –R. Jang, M. K. Nazeeruddin, E. Martínez-Ferrero, E. Palomares, J. Ko, M. Grätzel, T. Torres, Angew. Chem. Int. Ed., 2007, 46, 8358; c) J. –J. Cid, M. García-Iglesias, J. –H. Yum, A. Forneli, J. Albero, E. Martínez-Ferrero, P. Vázquez, M. Grätzel, M. K. Nazeerddin, E. Palomares, T. Torres, Chem. Eur. J., 2009, 15, 5130; d) I. López-Duarte, M. Wang, R. Humphry-Baker, M. Ince, M. V. Martínez-Díaz, M. K. Nazeeruddin, T. Torres, M. Grätzel, Angew. Chem. Int. Ed., 2012, 51, 1895; e) M. –E. Rogoussi, J. –J. Cid, J. –H. Yum, G. de la Torre, D. Di Censo, M. Grätzel, M. K. Nazeeruddin, T. Torres, Angew. Chem. Int. Ed., 2012, 51, 4375. a) S. Mori, M. Nagata, Y. Nakahata, K. Yasuta, R. Goto, M. Kimura, M. Taya, J. Am. Chem. Soc., 2010, 132, 4054; b) M. Kimura, H. Nomoto, N. Masaki, S. Mori, Angew. Chem. Int. Ed., 2012, 51, 4371, c) M. Kimura, H. Nomoto, H. Suzuki, T. Ikeuchi, H. Matsuzaki, T. N. Murakami, A. Furube, N. Masaki, M. J. Griffith, S. Mori, Chem. Eur. J., 2013, 19, 7496. C. F. van Nostrum, S. J. Picken, A. –J. Schouten, R. J. M. Nolte, J. Am. Chem. Soc., 1995, 117, 9957. G. Pozzi, S. Quici, M. C. Roffo, C. A. Bignozzi, S. Caramori, M. Orlandi, J. Phys. Chem. C, 2011, 115, 3777. J. Mack, N. Kobayashi, Chem. Rev., 2011, 111, 281. M. J. Stillman, T. Nyokong, Chapter 3 of Phthalocyanines Properties and Applications Vol. 1 (Eds.: C. C. Leznoff, A. B. P. Lever), VCH, New York, 1989, pp 139-247. a) N. Koumura, Z.-S. Wang, S. Mori, M. Miyashita, E. Suzuki, and K. Hara, J. Am. Chem. Soc., 2006, 128, 14256. (Addition and Correction 2008, 130, 4202), b) Z.-S. Wang, N. Koumura, Y. Cui, M. Takahashi, H. Sekiguchi, A. Mori, T. Kubo, A. Furube and K. Hara, Chem. Mater., 2008, 20, 3993. a) S. M. Feldt, E. A. Gibson, E. Gabrielsson, L. Sun, G. Boshloo, A. Hagfeldt, J. Am. Chem. Soc., 2010, 132, 16714; b) H. N. Tsao, C. Yi, T. Moehl, J. –H. Yum, S. M. Zakeeruddin, M. K. Nazeeruddin, M. Grätzel, ChemSusChem, 2011, 4, 591; c) D. Zhou, Q. Yu, N. Cai, Y. Bai, Y. Wang, P. Wang, Energy Environ. Sci., 2011, 4, 2030; d) J. –H. Yum, E. Baranoff, F. Kessler, T. Moehl, S. Ahmad, T. Bessho, A. Marchioro, E. Ghadiri, J. –E. Moser, C. Yi, M. K. Nazeeruddin, M. Grätzel, Nat. Commun., 2012, 3, 631.
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cell, implying that the covering the ZnPc with long alkyl chains could prevent I3 attraction around the positively charged Zn atom in the ZnPc. Considering the steric blocking effect for charge recombination, higher dye adsorption density may seem to be preferred. However, important criterion is the surface coverage by dyes. Longer electron lifetime with lower dye adsorption density was also seen for an organic dye consisting of carbazole and three thiophenes having alkyl chains (MK-1 dye) in comparison to the dyes having the same structure but no alkyl 12 chains attached. The IPCE values between 600 and 720 nm for PcS21 were about half of those for PcS20. The oxidation potential of PcS20 (0.90 V vs. NHE) is sufficient to be reduced by I as indicated by the high IPCE. Since this value is comparable to the other dyes (see ESI), the cationic state of PcS21 must also be efficiently reduced by I . Thus, the lower IPCE values of PcS21 are probably due to aggregation, which would cause energy transfer among dyes and/or sub-nanosecond recombination with dye cation. The IPCE values of the PcS20 cell are slightly lower than those of the PcS19 cell. From Fig. 1., it is not obvious if PcS20 is aggregated. However, the less sharply split Q band peaks of PcS20 implies that the dyes are slightly aggregated. Fig. S5 (In ESI) shows IPCE spectra of the cells prepared without scattering layers. For that case, except for Q band region, light harvesting efficiency is expected to be less than 1. Between 600 and 720 nm, the PcS20 cells showed a flat spectrum while the PcS19 cells showed two peaks. This result is also an indication of the aggregation for PcS20. Thus, the drawback of the shorter alkyl chains in not only the shorter electron lifetime, but also the slightly insufficient steric protection against aggregation. The DSSCs with PcS20 using Co(II/III) tris(bipyridyl) 13 tetracyanoborate complexes as redox shuttles were fabricated. The PcS20 cell using cobalt redox shuttles exhibits a Jsc of 4.2 mA -2 cm , a Voc of 614 mV, and a FF of 0.52, yielding a PCE of 1.4 %. The decoration of ZnPc with long alkoxy chains acted as a barrier against the approach of redox species onto the TiO2 surface. However, the PCEs for ZnPc-sensitized DSSCs based on cobalt redox shuttles were much lower than donor-π-bridge-acceptor 2,13 Investigations are underway to optimize (D-π-A) sensitizers. the structure of phthalocyanine-based D-π-A sensitizers to improve the PCEs in DSSCs based on cobalt redox shuttles. In summary, we have synthesized novel asymmetric zinc phthalocyanines PcS19-21 having alkoxy chains, and used them as a light-harvesting units in DSSCs The PcS20 cell gave a record efficiency for phthalocyanine-sensitized DSSCs of 6.4% under one-sun condition. Adsorption densities depended on the length and number of alkoxy chains, and the dense adsorption of PcS20 on TiO2 resulted in highly efficient performance of the DSSCs. According to the molecular design rules for ZnPc sensitizers, we are now developing near-IR harvesting DSSCs using ringexpanded naphthalocyanine sensitizers with spectral response up to 900 nm. This work has been partially supported by Grants-in-Aid for Scientific Research (B) (No. 22350086) from Japan Society for the Promotion of Science (JSPS) of Japan.
