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An efficient dye-sensitized solar cell based on a functionalized-triarylamine dye
Materials Letters 65 (2011) 1331–1333
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
An efficient dye-sensitized solar cell based on a functionalized-triarylamine dye Mao Liang, Xue-Ping Zong, Hong-Yu Han, Chao Chen, Zhe Sun, Song Xue ⁎ Department of Applied Chemistry, Tianjin University of Technology, Tianjin 300384, PR China
a r t i c l e
i n f o
Article history: Received 3 January 2011 Accepted 2 February 2011 Available online 17 February 2011 Keywords: Dye-sensitized solar cell Photovoltaic Functionalized-triarylamine N,N-Bis(4-(hept-1-enyl)phenyl)aniline
a b s t r a c t A new functionalized-triarylamine dye (MXD10) has been designed, synthesized, and characterized. Two CH3 (CH2)4CH=CH– units were introduced into triphenylamine for improvement of light harvesting, suppression of dye aggregation and retardation of charge recombination. Photophysical, electrochemical and photovoltaic measurements are in accord with the molecular design. Device based on MXD10 gave a maximum power conversion efficiency of 6.47% under simulated AM 1.5 irradiation (100 mW cm−2) with JSC = 15 mA/cm2, VOC = 635 mV, and ff = 0.68. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSSCs), developed by Grätzel and coworkers, have attracted considerable attention of many research groups in the past two decades owing to its high efficiency and low cost [1]. To date, two kinds of photosensitizers, ruthenium dyes and metal-free organic dyes, were developed for DSSCs. Metal-free organic dyes were regarded as an alternative to ruthenium dyes owing to their high molar absorption coefficient, and simple synthesis procedure with low cost [2–8]. However, the conversion efficiency of organic dyes has been still behind those of the ruthenium dyes. One of the major factors for the low conversion efficiency of many organic dyes in the DSSCs are the dye aggregates on the semiconductor surface and the recombination of injected electrons with the dye and dark current [8,9]. It is important to develop efficient organic dyes which suppress dye aggregation and retard charge recombination. Accordingly, here we reported on the design, synthesis, and application of a new functionalized-triarylamine sensitizer (MXD10), in which N,Nbis(4-(hept-1-enyl)phenyl)aniline unit was used as the donor, 3,4ethylenedioxythiophene (EDOT) unit as the bridge, and a cyanoacetic acid as the anchoring group. A triphenylamine dye, coded LJ1 [10], is also synthesized for a comparison. The optical, electrochemical, and photovoltaic properties are investigated in detail, which support the molecular design.
2. Experimental details The synthetic route of the MXD10 was shown in Scheme 1. The MXD10 was synthesized through the Witting reaction, the Vilsmeier ⁎ Corresponding author. Tel.: +86 22 60214250; fax: +86 22 60214252. E-mail address: [email protected] (S. Xue). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.02.002
reaction, the Suzuki coupling reaction and the Knoevenagel condensation reaction with the synthetic routes and the detailed procedures (synthesis and characterization, see Supporting Information). The starting material 1 was prepared according to literature procedures [11], which reacted with hexyltriphenylphosphonium bromide in the presence of t-BuOK to give intermediate 2. The compound 2 reacted with 2-(tributylstannyl)-3,4-(ethelenedioxy)thiophene to give 3. Aldehyde 4 was synthesized through the Vilsmeier–Haack reaction of 3 with POCl3 and DMF. Subsequently, the Knoevenagel condensation reaction of aldehyde 4 with cyanoacetic acid gave the target dye MXD10. 3. Results and discussion 3.1. UV–vis absorption/emission spectra The absorption and emission spectra of MXD10 and LJ1 in dichloromethane/methanol (2:1 v/v) are shown in Fig. 1. Two strong absorption bands at around 300–380 nm and 400–600 nm were observed for MXD10, which mainly stems from the intramolecular charge-transfer transition. The maximum absorption peak of MXD10 was red shifted ca. 30 nm in comparison with that of LJ1 together with increasing the molar extinction coefficient (ε = 45 000 M−1cm−1 at 487 nm for MXD10; ε = 25 300 M−1cm−1 at 457 nm for LJ1). The broader spectral response and higher molar absorption coefficient implied that introduction of two CH3(CH2)4CH=CH– units into triphenylamine was in favor of light harvesting. When MXD10 and LJ1 were excited with visible light, they exhibited strong luminescence maxima at 580 and 628 nm, respectively. The intersects of the normalized absorption and the emission spectra for MXD10 and LJ1 were at 525 nm and 549 nm, respectively. The amounts of the dyes adsorbed on the TiO2 surface were estimated spectroscopically by desorbing the dyes with a 0.01 M
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M. Liang et al. / Materials Letters 65 (2011) 1331–1333
Scheme 1. Synthetic route to the MXD10 and the structure of LJ1.
solution of KOH in methanol. The surface concentration was determined to be 1.3 × 10−7 mol cm−2 for MXD10. The amounts of LJ1 adsorbed on the TiO2 surface were also estimated to be 1.5 × 10−7 mol cm−2 under the same conditions, small size of molecular leading to more dye-uptake.
the iodine redox potential (0.4 V vs NHE). Thus, the oxidized MXD10 could be regenerated from the reduced species in the electrolyte to give an efficient charge separation.
3.2. Electrochemical properties The ground-state oxidation potential (E(S/S+)) of MXD10 and LJ1 in MeCN were determined from cyclic voltammogram (Fig. 2) under Ar atmosphere. The measurements were calibrated using ferrocene as standard. The redox potential of ferrocene internal reference was taken as 0.63 V vs NHE. The E(S/S+) corresponds to the highest occupied molecular orbital (HOMO). The excited-state reduction potential (E(S*/S+)), which corresponds to the lowest unoccupied molecular orbital (LUMO), could be calculated from E(S/S+) − E0–0 (E0–0 values were calculated from intersect of the normalized absorption and the emission spectra: E0–0 = 1240/λint). The HOMO levels for MXD10 and LJ1 were 0.99 V and 1.09 V, respectively. The LUMO levels for MXD10 and LJ1 were the same value of −1.27 V. It could be found that replacement of triphenylamine with N,N-bis(4-(hept-1-enyl)phenyl) aniline narrowed the HOMO–LUMO energy gaps and resulted in redshifted absorption spectra. The LUMO level for MXD10 was more negative than the conduction band of TiO2 (−0.5 V vs NHE), and the Egap between them provided sufficient driving forces for electron injection. On the other hand, the HOMO level for MXD10 was more positive than
Photovoltaic properties were tested based on cells with redox electrolyte consisting of 0.6 M DMPImI, 0.05 M I2 and 0.1 M LiI in acetonitrile. The incident photon-to-current conversion efficiency (IPCE) of DSSCs based on MXD10 and LJ1 is shown in the inset of Fig. 3. The IPCE of MXD10 was more than 70% in the spectral range from 410 to 560 nm, and reached its maximum of 80% at 485 nm. In contrast, LJ1 gave a relatively low integral value with a maximum of 73% at 470 nm. The J–V curves of DSSCs based on MXD10 and LJ1 measured under 100 mW cm−2 simulated sunlight (equivalent to AM1.5) are displayed in Fig. 3. MXD10 sensitized cell gave a high short-circuit photocurrent density (JSC) of 15 mA/cm2, an open-circuit voltage (VOC) of 635 mV, and a fill factor (ff) of 0.68, corresponding to an overall conversion efficiency (η) of 6.47%. In contrast, LJ1 sensitized cell gave a JSC of 12 mA/cm2, a VOC of 585 mV and a ff of 0.69, yielding an efficiency of 4.84%. MXD10 is superior to LJ1 in terms of higher JSC and higher VOC, leading to higher power conversion efficiency. In view of the amounts of dye on the TiO2 surface, the adsorption amount of LJ1 was higher than that of MXD10, and higher IPCE could be expected for LJ1.
Fig. 1. Absorption spectra and emission spectra of MXD10 and LJ1.
Fig. 2. Cyclic voltammogram of MXD10 and LJ1 in MeCN.
3.3. Photovoltaic properties
M. Liang et al. / Materials Letters 65 (2011) 1331–1333
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4. Conclusions In summary, we have designed and synthesized a new functionalizedtriarylamine dye MXD10. The introduction of two CH3(CH2)4CH=CH– units into triphenylamine broaded the spectral response and increased the molar absorption coefficient. N,N-Bis(4-(hept-1-enyl) phenyl)aniline moiety was superior to triphenylamine in terms of preventing dye aggregation and suppressing charge recombination. Device based on MXD10 yielded a maximum power conversion efficiency of 6.47% with JSC = 15 mA/cm2, VOC = 635 mV, and ff = 0.68. The maximum of IPCE is more than 80% in the visible region. Optimizing the triphenylamine dye by introduction of electronrichness unit with long alky chain is effective for the development of DSSCs. Fig. 3. J–V curve (top), IPCE spectra (inset) and dark current (bottom) of MXD10 and LJ1.
However, the maximum IPCE for LJ1 was lower than that of MXD10, which is mainly attributable to disadvantageous intermolecular energy transfer as a result of dye aggregate formation [12,13]. The results suggested that the presence of long alkyl chains on triphenylamine of MXD10 efficiently suppressed dye aggregation due to disturbance of the π–π stacking, which was in accord with previous reports [14,15]. In addition, the open circuit voltage of MXD10 with 635 mV was much higher than that of LJ1 with 585 mV under the same conditions. To give a plausible explanation for the improvement in VOC, measurement of the dark current–voltage characteristics was performed [16,17]. The dark current as a function of applied voltage is plotted in Fig. 3. The onset potentials of dark current (potential value at the dark current value of −0.1 mA cm−2) for MXD10 and LJ1 were 447 mV and 426 mV, respectively. The sequence is in accord with that of the VOC values. Compared with LJ1, the onset voltage for dark current of MXD10 was positively shifted, indicating the reduction of the interfacial electron recombination in the TiO2 with the I−/I− 3 redox couple in the electrolyte. Considering the similar adsorption geometry of the two dyes on TiO2 (see Supporting Information) [18], it is reasonable to conclude that the respectable increase of VOC was obtained by introduction of CH3(CH2)4CH=CH– unit which retarded the charge recombination process. Therefore, the improved power conversion efficiency was obtained for MXD10 due to the N,N-bis(4-(hept-1-enyl)phenyl)aniline unit, which prevented dye aggregation and suppressed charge recombination.
Acknowledgments We are grateful to the National 863 Program (2009AA05Z421), the National Natural Science Foundation of China (21003096) and the Tianjin Natural Science Foundation (09JCZDJC24400) for financial supports. References [1] O'Regan B, Gratzel M. Nature 1991;353:737–40. [2] Zeng WD, Cao YM, Bai Y, Wang YH, Shi YS, Zhang M, et al. Chem Mater 2010;22: 1915–25. [3] Tsao MH, Wu TY, Wang HP, Sun IW, Su SG, Lin YC, et al. Mater Lett 2011;65:583–6. [4] Chen GY, Zheng KB, Mo XL, Sun DL, Meng QH, Chen GR. Mater Lett 2010;64:1336–9. [5] Wang ZS, Cui Y, Hara K, Dan-oh Y, Kasada C, Shinpo A. Adv Mater 2007;19: 1138–41. [6] Hagberg DP, Yum JH, Lee H, De Angelis F, Marinado T, Karlsson KM, et al. J Am Chem Soc 2008;130:6259–66. [7] Kim S, Lee JK, Kang SO, Ko J, Yum JH, Fantacci S, et al. J Am Chem Soc 2006;128: 16701–7. [8] Mishra A, Fischer MKR, Bäuerle P. Angew Chem Int Ed 2009;48:2474–99. [9] Liu D, Fessenden RW, Hug GL, Kamat PV. J Phys Chem B 1997;101:2583–90. [10] Liu WH, Wu IC, Lai CH, Chou PT, Li YT, Chen CL, et al. Chem Commun 2008:5152–4. [11] Wang YJ, Sheu HS, Lai CK. Tetrahedron 2007;63:1695–705. [12] Liang YL, Peng B, Liang J, Tao ZL, Chen J. Org Lett 2010;12:1204–7. [13] Wang ZS, Hara K, Dan-oh Y, Kasada C, Shinpo A, Suga S, et al. J Phys Chem B 2005;109:3907–14. [14] Koumura N, Wang ZS, Mori S, Miyashita M, Suzuki E, Hara K. J Am Chem Soc 2006;128:14256–7. [15] Cao XY, Zhang WB, Wang JL, Zhou XH, Lu H, Pei J. J Am Chem Soc 2003;125: 12430–1. [16] Ning Z, Zhang Q, Pei H, Luan J, Lu C, Cui Y, et al. J Phys Chem C 2009;113:10307–13. [17] Liang M, Lu M, Wang QL, Chen WY, Han HY, Sun Z, et al. J Power Source 2011;196: 1657–64. [18] Liang Y, Peng B, Chen J. J Phys Chem C 2010;114:10992–8.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
An efficient dye-sensitized solar cell based on a functionalized-triarylamine dye Mao Liang, Xue-Ping Zong, Hong-Yu Han, Chao Chen, Zhe Sun, Song Xue ⁎ Department of Applied Chemistry, Tianjin University of Technology, Tianjin 300384, PR China
a r t i c l e
i n f o
Article history: Received 3 January 2011 Accepted 2 February 2011 Available online 17 February 2011 Keywords: Dye-sensitized solar cell Photovoltaic Functionalized-triarylamine N,N-Bis(4-(hept-1-enyl)phenyl)aniline
a b s t r a c t A new functionalized-triarylamine dye (MXD10) has been designed, synthesized, and characterized. Two CH3 (CH2)4CH=CH– units were introduced into triphenylamine for improvement of light harvesting, suppression of dye aggregation and retardation of charge recombination. Photophysical, electrochemical and photovoltaic measurements are in accord with the molecular design. Device based on MXD10 gave a maximum power conversion efficiency of 6.47% under simulated AM 1.5 irradiation (100 mW cm−2) with JSC = 15 mA/cm2, VOC = 635 mV, and ff = 0.68. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSSCs), developed by Grätzel and coworkers, have attracted considerable attention of many research groups in the past two decades owing to its high efficiency and low cost [1]. To date, two kinds of photosensitizers, ruthenium dyes and metal-free organic dyes, were developed for DSSCs. Metal-free organic dyes were regarded as an alternative to ruthenium dyes owing to their high molar absorption coefficient, and simple synthesis procedure with low cost [2–8]. However, the conversion efficiency of organic dyes has been still behind those of the ruthenium dyes. One of the major factors for the low conversion efficiency of many organic dyes in the DSSCs are the dye aggregates on the semiconductor surface and the recombination of injected electrons with the dye and dark current [8,9]. It is important to develop efficient organic dyes which suppress dye aggregation and retard charge recombination. Accordingly, here we reported on the design, synthesis, and application of a new functionalized-triarylamine sensitizer (MXD10), in which N,Nbis(4-(hept-1-enyl)phenyl)aniline unit was used as the donor, 3,4ethylenedioxythiophene (EDOT) unit as the bridge, and a cyanoacetic acid as the anchoring group. A triphenylamine dye, coded LJ1 [10], is also synthesized for a comparison. The optical, electrochemical, and photovoltaic properties are investigated in detail, which support the molecular design.
2. Experimental details The synthetic route of the MXD10 was shown in Scheme 1. The MXD10 was synthesized through the Witting reaction, the Vilsmeier ⁎ Corresponding author. Tel.: +86 22 60214250; fax: +86 22 60214252. E-mail address: [email protected] (S. Xue). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.02.002
reaction, the Suzuki coupling reaction and the Knoevenagel condensation reaction with the synthetic routes and the detailed procedures (synthesis and characterization, see Supporting Information). The starting material 1 was prepared according to literature procedures [11], which reacted with hexyltriphenylphosphonium bromide in the presence of t-BuOK to give intermediate 2. The compound 2 reacted with 2-(tributylstannyl)-3,4-(ethelenedioxy)thiophene to give 3. Aldehyde 4 was synthesized through the Vilsmeier–Haack reaction of 3 with POCl3 and DMF. Subsequently, the Knoevenagel condensation reaction of aldehyde 4 with cyanoacetic acid gave the target dye MXD10. 3. Results and discussion 3.1. UV–vis absorption/emission spectra The absorption and emission spectra of MXD10 and LJ1 in dichloromethane/methanol (2:1 v/v) are shown in Fig. 1. Two strong absorption bands at around 300–380 nm and 400–600 nm were observed for MXD10, which mainly stems from the intramolecular charge-transfer transition. The maximum absorption peak of MXD10 was red shifted ca. 30 nm in comparison with that of LJ1 together with increasing the molar extinction coefficient (ε = 45 000 M−1cm−1 at 487 nm for MXD10; ε = 25 300 M−1cm−1 at 457 nm for LJ1). The broader spectral response and higher molar absorption coefficient implied that introduction of two CH3(CH2)4CH=CH– units into triphenylamine was in favor of light harvesting. When MXD10 and LJ1 were excited with visible light, they exhibited strong luminescence maxima at 580 and 628 nm, respectively. The intersects of the normalized absorption and the emission spectra for MXD10 and LJ1 were at 525 nm and 549 nm, respectively. The amounts of the dyes adsorbed on the TiO2 surface were estimated spectroscopically by desorbing the dyes with a 0.01 M
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Scheme 1. Synthetic route to the MXD10 and the structure of LJ1.
solution of KOH in methanol. The surface concentration was determined to be 1.3 × 10−7 mol cm−2 for MXD10. The amounts of LJ1 adsorbed on the TiO2 surface were also estimated to be 1.5 × 10−7 mol cm−2 under the same conditions, small size of molecular leading to more dye-uptake.
the iodine redox potential (0.4 V vs NHE). Thus, the oxidized MXD10 could be regenerated from the reduced species in the electrolyte to give an efficient charge separation.
3.2. Electrochemical properties The ground-state oxidation potential (E(S/S+)) of MXD10 and LJ1 in MeCN were determined from cyclic voltammogram (Fig. 2) under Ar atmosphere. The measurements were calibrated using ferrocene as standard. The redox potential of ferrocene internal reference was taken as 0.63 V vs NHE. The E(S/S+) corresponds to the highest occupied molecular orbital (HOMO). The excited-state reduction potential (E(S*/S+)), which corresponds to the lowest unoccupied molecular orbital (LUMO), could be calculated from E(S/S+) − E0–0 (E0–0 values were calculated from intersect of the normalized absorption and the emission spectra: E0–0 = 1240/λint). The HOMO levels for MXD10 and LJ1 were 0.99 V and 1.09 V, respectively. The LUMO levels for MXD10 and LJ1 were the same value of −1.27 V. It could be found that replacement of triphenylamine with N,N-bis(4-(hept-1-enyl)phenyl) aniline narrowed the HOMO–LUMO energy gaps and resulted in redshifted absorption spectra. The LUMO level for MXD10 was more negative than the conduction band of TiO2 (−0.5 V vs NHE), and the Egap between them provided sufficient driving forces for electron injection. On the other hand, the HOMO level for MXD10 was more positive than
Photovoltaic properties were tested based on cells with redox electrolyte consisting of 0.6 M DMPImI, 0.05 M I2 and 0.1 M LiI in acetonitrile. The incident photon-to-current conversion efficiency (IPCE) of DSSCs based on MXD10 and LJ1 is shown in the inset of Fig. 3. The IPCE of MXD10 was more than 70% in the spectral range from 410 to 560 nm, and reached its maximum of 80% at 485 nm. In contrast, LJ1 gave a relatively low integral value with a maximum of 73% at 470 nm. The J–V curves of DSSCs based on MXD10 and LJ1 measured under 100 mW cm−2 simulated sunlight (equivalent to AM1.5) are displayed in Fig. 3. MXD10 sensitized cell gave a high short-circuit photocurrent density (JSC) of 15 mA/cm2, an open-circuit voltage (VOC) of 635 mV, and a fill factor (ff) of 0.68, corresponding to an overall conversion efficiency (η) of 6.47%. In contrast, LJ1 sensitized cell gave a JSC of 12 mA/cm2, a VOC of 585 mV and a ff of 0.69, yielding an efficiency of 4.84%. MXD10 is superior to LJ1 in terms of higher JSC and higher VOC, leading to higher power conversion efficiency. In view of the amounts of dye on the TiO2 surface, the adsorption amount of LJ1 was higher than that of MXD10, and higher IPCE could be expected for LJ1.
Fig. 1. Absorption spectra and emission spectra of MXD10 and LJ1.
Fig. 2. Cyclic voltammogram of MXD10 and LJ1 in MeCN.
3.3. Photovoltaic properties
M. Liang et al. / Materials Letters 65 (2011) 1331–1333
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4. Conclusions In summary, we have designed and synthesized a new functionalizedtriarylamine dye MXD10. The introduction of two CH3(CH2)4CH=CH– units into triphenylamine broaded the spectral response and increased the molar absorption coefficient. N,N-Bis(4-(hept-1-enyl) phenyl)aniline moiety was superior to triphenylamine in terms of preventing dye aggregation and suppressing charge recombination. Device based on MXD10 yielded a maximum power conversion efficiency of 6.47% with JSC = 15 mA/cm2, VOC = 635 mV, and ff = 0.68. The maximum of IPCE is more than 80% in the visible region. Optimizing the triphenylamine dye by introduction of electronrichness unit with long alky chain is effective for the development of DSSCs. Fig. 3. J–V curve (top), IPCE spectra (inset) and dark current (bottom) of MXD10 and LJ1.
However, the maximum IPCE for LJ1 was lower than that of MXD10, which is mainly attributable to disadvantageous intermolecular energy transfer as a result of dye aggregate formation [12,13]. The results suggested that the presence of long alkyl chains on triphenylamine of MXD10 efficiently suppressed dye aggregation due to disturbance of the π–π stacking, which was in accord with previous reports [14,15]. In addition, the open circuit voltage of MXD10 with 635 mV was much higher than that of LJ1 with 585 mV under the same conditions. To give a plausible explanation for the improvement in VOC, measurement of the dark current–voltage characteristics was performed [16,17]. The dark current as a function of applied voltage is plotted in Fig. 3. The onset potentials of dark current (potential value at the dark current value of −0.1 mA cm−2) for MXD10 and LJ1 were 447 mV and 426 mV, respectively. The sequence is in accord with that of the VOC values. Compared with LJ1, the onset voltage for dark current of MXD10 was positively shifted, indicating the reduction of the interfacial electron recombination in the TiO2 with the I−/I− 3 redox couple in the electrolyte. Considering the similar adsorption geometry of the two dyes on TiO2 (see Supporting Information) [18], it is reasonable to conclude that the respectable increase of VOC was obtained by introduction of CH3(CH2)4CH=CH– unit which retarded the charge recombination process. Therefore, the improved power conversion efficiency was obtained for MXD10 due to the N,N-bis(4-(hept-1-enyl)phenyl)aniline unit, which prevented dye aggregation and suppressed charge recombination.
Acknowledgments We are grateful to the National 863 Program (2009AA05Z421), the National Natural Science Foundation of China (21003096) and the Tianjin Natural Science Foundation (09JCZDJC24400) for financial supports. References [1] O'Regan B, Gratzel M. Nature 1991;353:737–40. [2] Zeng WD, Cao YM, Bai Y, Wang YH, Shi YS, Zhang M, et al. Chem Mater 2010;22: 1915–25. [3] Tsao MH, Wu TY, Wang HP, Sun IW, Su SG, Lin YC, et al. Mater Lett 2011;65:583–6. [4] Chen GY, Zheng KB, Mo XL, Sun DL, Meng QH, Chen GR. Mater Lett 2010;64:1336–9. [5] Wang ZS, Cui Y, Hara K, Dan-oh Y, Kasada C, Shinpo A. Adv Mater 2007;19: 1138–41. [6] Hagberg DP, Yum JH, Lee H, De Angelis F, Marinado T, Karlsson KM, et al. J Am Chem Soc 2008;130:6259–66. [7] Kim S, Lee JK, Kang SO, Ko J, Yum JH, Fantacci S, et al. J Am Chem Soc 2006;128: 16701–7. [8] Mishra A, Fischer MKR, Bäuerle P. Angew Chem Int Ed 2009;48:2474–99. [9] Liu D, Fessenden RW, Hug GL, Kamat PV. J Phys Chem B 1997;101:2583–90. [10] Liu WH, Wu IC, Lai CH, Chou PT, Li YT, Chen CL, et al. Chem Commun 2008:5152–4. [11] Wang YJ, Sheu HS, Lai CK. Tetrahedron 2007;63:1695–705. [12] Liang YL, Peng B, Liang J, Tao ZL, Chen J. Org Lett 2010;12:1204–7. [13] Wang ZS, Hara K, Dan-oh Y, Kasada C, Shinpo A, Suga S, et al. J Phys Chem B 2005;109:3907–14. [14] Koumura N, Wang ZS, Mori S, Miyashita M, Suzuki E, Hara K. J Am Chem Soc 2006;128:14256–7. [15] Cao XY, Zhang WB, Wang JL, Zhou XH, Lu H, Pei J. J Am Chem Soc 2003;125: 12430–1. [16] Ning Z, Zhang Q, Pei H, Luan J, Lu C, Cui Y, et al. J Phys Chem C 2009;113:10307–13. [17] Liang M, Lu M, Wang QL, Chen WY, Han HY, Sun Z, et al. J Power Source 2011;196: 1657–64. [18] Liang Y, Peng B, Chen J. J Phys Chem C 2010;114:10992–8.