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Axial anchoring designed silicon–porphyrin sensitizers for efficient dye-sensitized solar cells† Jing Liu,a Xichuan Yang*a and Licheng Sun*ab
DOI: 10.1039/c3cc46581k www.rsc.org/chemcomm
Using silicon as a central atom of porphyrin allows the introduction of axial ligands, which are not only employed to prevent the aggregation of the macrocycles but also anchor the dyes onto the TiO2 surface. A dye-sensitized solar cell with this porphyrin sensitizer achieved a broad IPCE of around 40–60% between 380 and 670 nm.
Dye-sensitized solar cells (DSSCs) have attracted global attention due to their low cost and environmentally friendly properties since the first report in 1991.1 During the past few years, the research mainly focused on the development of metal complexes as sensitizers, such as ruthenium complexes. Subsequently, metal-free organic dyes have been developed intensively due to their high molar extinction coefficients, easy synthesis, flexible structural modification, and ideal photovoltaic performance. However, metal-free organic sensitizers always show lower performance than the state of the art ruthenium complex dyes, due to their weak light response in the near infrared region. Therefore, it is significant to develop efficient panchromatic sensitizers for DSSCs. Porphyrins as sensitizers have gained more attention because of their intense Soret band at 400–450 nm and moderate Q-band at 500–700 nm.2 In fact, the zinc– porphyrin dye has achieved a record efficiency of over 12%,3 showing promising application in DSSCs. Usually, many porphyrin-based dyes exhibit a strong tendency to aggregate on the TiO2 surface due to p-interactions between the macrocycle discs,4,5 which affects the DSSCs performance (e.g., [5-(4-carboxyphenyl)-10,15,20-trisphenylporphyrin]-zinc(II),
ZnP in Fig. 1). For instance, the ZnP-sensitized DSSCs with chenodeoxycholic acid (CDCA) as a coadsorbent exhibited a much better performance than DSSCs without CDCA (see Fig. S2 in the ESI†). However, this problem could be resolved by introducing carboxylate groups into axial ligands6,7 and bulky substituents providing considerable steric hindrance, such as 3,5-di-tert-butylphenyl groups.8 In addition, the introduction of long alkyl chains in the dye structure would impair the charge recombination process, and improve solubility and DSSC performance.3,8 Here, we report two new silicon–porphyrin-based sensitizers, named LJ201 and LJ203 (see Fig. 1), whose novel structures may provide a promising way to surmount the problem related to the charge recombination. The choice of silicon (Si) as a central atom allows the introduction of axial ligands into the center, which can reduce the tendency of dye aggregation on the TiO2 surface, as phthalocyanines did.9,10 In this work, terephthalic acid units were introduced as axial ligands into the silicon core, which are not only employed to prevent the aggregation of the macrocycles, but also anchor the dyes onto the TiO2 surface. LJ201 was designed and synthesized with phenyl units at the porphyrin ring. Triphenylamine groups were also introduced into the dye LJ203 at the porphyrin ring, as their good electrondonating ability and non-planar structures might reduce the charge recombination on the TiO2 surface. In solar cell devices, it was found that LJ201 exhibited poor photovoltaic performance, which could be the result of insufficient electron injection from the excited state dye to the conduction band of TiO2.
a
State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), Linggong Rd. 2, 116024 Dalian, China. E-mail: [email protected] b School of Chemical Science and Engineering, Center of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, Teknikringen 30, 10044 Stockholm, Sweden. E-mail: [email protected] † Electronic supplementary information (ESI) available: General experimental procedures, DSSCs fabrication, the UV-Vis absorption spectra of dyes anchored onto TiO2 films with various CDCA concentrations, the ZnP-sensitized solar cell performance, the APCE of DSSCs sensitized with LJ203, and FTIR analytical data. See DOI: 10.1039/c3cc46581k
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Fig. 1
Molecular structures of ZnP, LJ201 and LJ203 dyes.
Chem. Commun., 2013, 49, 11785--11787
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Fig. 2 (a) Absorption spectra of LJ201 and LJ203 in THF; (b) absorption spectra of dyes on TiO2.
However, LJ203 exhibited higher photocurrent and better light response at wavelengths beyond 400 nm, performing better than LJ201 in DSSCs application. Compared to ZnP, the introduction of terephthalic acid units in LJ201 and LJ203 structures can suppress molecular aggregation (see Fig. S3 in the ESI†). Furthermore, the bulky triphenylamine units in LJ203 could also reduce the electron recombination. As a result, CDCA can be removed from the dye solutions. The UV-Vis absorption spectra of LJ201 and LJ203 are shown in Fig. 2. The molar extinction coefficients and absorption values of dyes are listed in Table 1. In solution, LJ201 showed an intense absorption peak for the Soret bands at 423 nm and small absorption peaks for the Q-bands at 558 and 600 nm. The Soret bands of LJ203 exhibited maximum peaks at 411 and 452 nm. The absorption peak at 411 nm should be generated from the triphenylamine units.11 The Q-band of LJ203 displayed a significant red shift of the absorption maximum to 621 nm compared to 600 nm for LJ201. On TiO2 films, the absorption spectra of the two dyes showed a slight red shift as
Table 1
Dye
Optical and electrochemical data for LJ201 and LJ203
e (at labs) labs on HOMOa LUMOc labs (nm) (M 1 cm 1) TiO2 (nm) (V) E0–0b (eV) (V)
LJ201 423 558 600
3.2 104 1.8 103 1.1 103
424 560 601
1.00
1.90
0.90
LJ203 411 452 572 621
1.8 2.9 4.3 4.8
104 104 103 103
415 458 570 625
0.96
1.80
0.84
a
The HOMO (vs. NHE) was measured using a photoelectrode (TiO2 adsorbed with dye) as the working electrode and the Ag/Ag+ electrode as a reference electrode. b E0–0 was estimated from UV-Vis absorption spectra. c LUMO = Eox E0–0 (vs. NHE). 11786
Chem. Commun., 2013, 49, 11785--11787
compared to those in solution. The introduction of axial ligands into the Si center can suppress the H-aggregation of dyes on the TiO2 surface,6,7 but it was found that their presence leads to the formation of J-aggregates. The HOMO (highest occupied molecular orbital) values of LJ201 and LJ203 were obtained according to the first oxidation potentials (Eox) measured by cyclic voltammetry (CV) in acetonitrile using a dye loaded-TiO2 electrode as the working electrode. The electrochemical properties are given in Table 1. Although the LUMO level of LJ203 shows a 60 mV positive shift compared to LJ201 due to increased electron-donating ability, it is still sufficient to carry out the electron injection into the conduction band of TiO2 ( 0.50 V vs. NHE).12 The HOMO level of LJ203 shows a 40 mV negative shift compared to LJ201 due to increased conjugation. Therefore, LJ203 has a slightly smaller energy loss between the HOMO and the I /I3 redox couple (0.40 V vs. NHE),12,13 which should result in better DSSC performance. 4-tert-Butylpyridine, as an electrolyte additive, can increase the Voc of DSSCs by shifting the conduction band to the negative direction.14 But this additive resulted in poor device performance for LJ201 and LJ203 since the lifted conduction band decreased the driving force for electron injection from excited dyes to TiO2. Therefore it was not employed in the electrolyte in this work. Under the optimized fabrication conditions, LJ203 achieves an Z of 3.0%, with a Jsc of 8.24 mA cm 2, a Voc of 486 mV, and a FF of 0.74. The Z value of the LJ203sensitized solar cell was two times that of the cell using LJ201 (Z = 1.0%, Jsc = 3.28 mA cm 2, Voc = 436 mV, FF = 0.72, shown in Fig. 3a), and improved by 43% compared to the reference cell using ZnP. From the incident photon to current conversion efficiencies (IPCEs) of DSSCs sensitized with LJ201 and LJ203 shown in Fig. 3b, the high photocurrent of LJ203-based DSSCs reflects efficient electron injection and ideal photoresponse in the near-IR region. LJ201 shows an IPCE between 300–670 nm, but its IPCE peak is just around 10–25%. In contrast, the LJ203 device exhibits a broad IPCE spectrum ending at 750 nm and its IPCE peak is approximately 40–60% between 380–670 nm, resulting in a high photocurrent. The absorbed photo-to-electric current generation efficiency (APCE) presents the true quantum yield for electric current generation from light, obtained by dividing the IPCE by the light-harvesting efficiency (LHE). The APCE values in the wavelength range from 520 to 660 nm are above 60% for LJ203 (Fig. S4 in the ESI†), indicating the efficient photocurrent generation. Electrochemical impedance spectroscopy (EIS) analysis was performed to gain an insight into the interfacial charge transfer processes in DSSCs and the Nyquist plots are shown in Fig. 4. Rrec and RCE represent the charge transfer resistance at the TiO2/dye/electrolyte interface and the counter electrode, respectively. The large semi-circle corresponds to Rrec, while the small semi-circle reflects RCE. A larger Rrec was obtained for the DSSCs sensitized with LJ203 than that measured for LJ201, indicating a larger charge transfer resistance at the TiO2/LJ203/electrolyte interface. This is caused by introducing triphenylamine units in LJ203. The bulky triphenylamine unit has a non-planar propeller-like structure; as a result, the electron recombination This journal is
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Fig. 3 (a) J–V curves of DSSCs sensitized with LJ201 and LJ203 measured under the illumination of sunlight AM 1.5 G; (b) IPCE spectra of DSSCs sensitized with LJ201 and LJ202.
and anchored dyes. Among binding modes, the bidentate mode achieves high quantum yields of electron injection into a semiconductor,16 as a result, LJ203 exhibited a more efficient electron injection into TiO2 compared to LJ201. In summary, we have reported two silicon–porphyrin sensitizers, LJ201 and LJ203, based on axial ligands. The absorption of LJ203 is red-shifted compared to the classic porphyrin sensitizer LJ201 while maintaining the energy levels necessary for efficient electron injection and dye regeneration. In addition, axial ligands and bulky triphenylamine units in LJ203 suppress the electron recombination at the TiO2/dye/electrolyte interface. LJ203-based DSSCs without CDCA as a co-absorbent exhibited good performance, showing potential application as a panchromatic sensitizer in solar-electrical conversion devices. Using axial ligands as anchoring groups to prevent dye aggregation is an effective strategy to prepare efficient porphyrin sensitizers for DSSC application in the future. We gratefully acknowledge the financial support of this work from the China Natural Science Foundation (Grant 21076039, Grant 21276044, Grant 21120102036 and 20923006), the National Basic Research Program of China (Grant No. 2009CB220009), the Swedish Energy Agency, K&A Wallenberg Foundation, and the State Key Laboratory of Fine Chemicals (KF0805), the Program for Innovative Research Team of Liaoning Province (Grant No. LS2010042).
Notes and references
Fig. 4 Nyquist plots of DSSCs sensitized with LJ201 and LJ203 measured at 0.5 V bias voltage in the dark.
at the TiO2/dye/electrolyte interface can be efficiently suppressed, leading to a higher Voc for DSSCs sensitized with LJ203. From the RCE aspect, the increase in radius when DSSCs were sensitized with LJ201 is responsible for the lower FF of the devices. By using the FTIR technique, the study of the interfacial binding mode of the dyes onto the TiO2 films was carried out to determine how the dyes attach to the TiO2 films. For LJ201 and LJ203 sodium salts, the frequency difference values (Dnas–s) between the antisymmetric (nas) and symmetric stretching (ns) modes of the COO group were 227 and 235 cm 1, respectively. When the dyes were anchored onto the TiO2 surface, different Dnas–s values were obtained, 249 cm 1 for LJ201 and 224 cm 1 for LJ203 (Fig. S5 and Table S1 in the ESI†). On the basis of the Deacon and Philips rule,15 the unidentate mode for the binding of LJ201 and the bidentate mode for LJ203 are suggested to be the major binding modes between the semiconductor surface
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1 G. W. Crabtree and N. S. Lewis, Phys. Today, 2007, 60, 37. ¨zel, 2 (a) T. Bessho, S. M. Zakeeruddin, C. Yeh, E. Diau and M. Grta Angew. Chem., Int. Ed., 2010, 49, 6646; (b) H. Mori, T. Tanaka and A. Osuka, J. Mater. Chem. C, 2013, 1, 2500; (c) H. Imahori, T. Umeyama and S. Ito, Acc. Chem. Res., 2009, 42, 1809. 3 A. Yella, H. Lee, H. Tsao, C. Yi, A. Chandiran, Md. Nazeeruddin, ¨zel, Science, 2011, E. Diau, C. Yeh, S. M. Zakeeruddin and M. Grta 334, 629. ¨zel, J. Phys. Chem., 1993, 97, 6272. 4 A. Kay and M. Grta 5 C. Wang, C. Lan, S. Hong, Y. Wang, T. Pan, C. Chang, H. Kuo, M. Kuo, E. Diau and C. Lin, Energy Environ. Sci., 2012, 5, 6933. 6 E. Palomares, M. V. Martı´nez-Dı´az, S. A. Haque, T. Torres and J. R. Durrant, Chem. Commun., 2004, 2112. ´pez-Duarte, M. Martinez-Diaz, A. Forneli, J. Albero, 7 B. O’Regan, I. Lo A. Morandeira, E. Palomares, T. Torres and J. Durrant, J. Am. Chem. Soc., 2008, 130, 2906. 8 H. Lu, C. Tsai, W. Yen, C. Hsieh, C. Lee, C. Yeh and E. Diau, J. Phys. Chem. C, 2009, 113, 20990. 9 M. Maree and T. Nyokong, J. Porphyrins Phthalocyanines, 2001, 5, 555. ¨tzel, 10 B. Lim, G. Y. Margulis, J. Yum, E. L. Unger, B. E. Hardin, M. Gra M. D. McGehee and A. Sellinger, Org. Lett., 2013, 15, 784. 11 (a) H. Tian, X. Yang and L. Sun, Adv. Funct. Mater., 2008, 18, 3461; (b) H. Tian, X. Yang, J. Cong, R. Chen, J. Liu, Y. Hao, A. Hagfeld and L. Sun, Chem. Commun., 2009, 6288. 12 Y. Hao, X. Yang, J. Cong, H. Tian, A. Hagfeldt and L. Sun, Chem. Commun., 2009, 4031. 13 Y. Liu, J. R. Jennings, M. Parameswaran and Q. Wang, Energy Environ. Sci., 2011, 4, 564. 14 K. Hara, T. Sato, R. Katoh, A. Furube, T. Yoshihara, M. Murai, M. Kurashige, S. Ite, A. Shinpo, S. Suga and H. Arakawa, Adv. Funct. Mater., 2005, 15, 246. 15 M. K. Nazeeruddin, R. Humphry-Baker, P. Liska and M. Gratzel, J. Phys. Chem. B, 2003, 107, 8981. 16 G. B. Deacon and R. Phillips, Coord. Chem. Rev., 1980, 33, 227.
Chem. Commun., 2013, 49, 11785--11787
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Published on 24 October 2013. Downloaded by St. Petersburg State University on 08/12/2013 23:50:40.
COMMUNICATION
Cite this: Chem. Commun., 2013, 49, 11785 Received 28th August 2013, Accepted 24th October 2013
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Axial anchoring designed silicon–porphyrin sensitizers for efficient dye-sensitized solar cells† Jing Liu,a Xichuan Yang*a and Licheng Sun*ab
DOI: 10.1039/c3cc46581k www.rsc.org/chemcomm
Using silicon as a central atom of porphyrin allows the introduction of axial ligands, which are not only employed to prevent the aggregation of the macrocycles but also anchor the dyes onto the TiO2 surface. A dye-sensitized solar cell with this porphyrin sensitizer achieved a broad IPCE of around 40–60% between 380 and 670 nm.
Dye-sensitized solar cells (DSSCs) have attracted global attention due to their low cost and environmentally friendly properties since the first report in 1991.1 During the past few years, the research mainly focused on the development of metal complexes as sensitizers, such as ruthenium complexes. Subsequently, metal-free organic dyes have been developed intensively due to their high molar extinction coefficients, easy synthesis, flexible structural modification, and ideal photovoltaic performance. However, metal-free organic sensitizers always show lower performance than the state of the art ruthenium complex dyes, due to their weak light response in the near infrared region. Therefore, it is significant to develop efficient panchromatic sensitizers for DSSCs. Porphyrins as sensitizers have gained more attention because of their intense Soret band at 400–450 nm and moderate Q-band at 500–700 nm.2 In fact, the zinc– porphyrin dye has achieved a record efficiency of over 12%,3 showing promising application in DSSCs. Usually, many porphyrin-based dyes exhibit a strong tendency to aggregate on the TiO2 surface due to p-interactions between the macrocycle discs,4,5 which affects the DSSCs performance (e.g., [5-(4-carboxyphenyl)-10,15,20-trisphenylporphyrin]-zinc(II),
ZnP in Fig. 1). For instance, the ZnP-sensitized DSSCs with chenodeoxycholic acid (CDCA) as a coadsorbent exhibited a much better performance than DSSCs without CDCA (see Fig. S2 in the ESI†). However, this problem could be resolved by introducing carboxylate groups into axial ligands6,7 and bulky substituents providing considerable steric hindrance, such as 3,5-di-tert-butylphenyl groups.8 In addition, the introduction of long alkyl chains in the dye structure would impair the charge recombination process, and improve solubility and DSSC performance.3,8 Here, we report two new silicon–porphyrin-based sensitizers, named LJ201 and LJ203 (see Fig. 1), whose novel structures may provide a promising way to surmount the problem related to the charge recombination. The choice of silicon (Si) as a central atom allows the introduction of axial ligands into the center, which can reduce the tendency of dye aggregation on the TiO2 surface, as phthalocyanines did.9,10 In this work, terephthalic acid units were introduced as axial ligands into the silicon core, which are not only employed to prevent the aggregation of the macrocycles, but also anchor the dyes onto the TiO2 surface. LJ201 was designed and synthesized with phenyl units at the porphyrin ring. Triphenylamine groups were also introduced into the dye LJ203 at the porphyrin ring, as their good electrondonating ability and non-planar structures might reduce the charge recombination on the TiO2 surface. In solar cell devices, it was found that LJ201 exhibited poor photovoltaic performance, which could be the result of insufficient electron injection from the excited state dye to the conduction band of TiO2.
a
State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), Linggong Rd. 2, 116024 Dalian, China. E-mail: [email protected] b School of Chemical Science and Engineering, Center of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, Teknikringen 30, 10044 Stockholm, Sweden. E-mail: [email protected] † Electronic supplementary information (ESI) available: General experimental procedures, DSSCs fabrication, the UV-Vis absorption spectra of dyes anchored onto TiO2 films with various CDCA concentrations, the ZnP-sensitized solar cell performance, the APCE of DSSCs sensitized with LJ203, and FTIR analytical data. See DOI: 10.1039/c3cc46581k
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The Royal Society of Chemistry 2013
Fig. 1
Molecular structures of ZnP, LJ201 and LJ203 dyes.
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Fig. 2 (a) Absorption spectra of LJ201 and LJ203 in THF; (b) absorption spectra of dyes on TiO2.
However, LJ203 exhibited higher photocurrent and better light response at wavelengths beyond 400 nm, performing better than LJ201 in DSSCs application. Compared to ZnP, the introduction of terephthalic acid units in LJ201 and LJ203 structures can suppress molecular aggregation (see Fig. S3 in the ESI†). Furthermore, the bulky triphenylamine units in LJ203 could also reduce the electron recombination. As a result, CDCA can be removed from the dye solutions. The UV-Vis absorption spectra of LJ201 and LJ203 are shown in Fig. 2. The molar extinction coefficients and absorption values of dyes are listed in Table 1. In solution, LJ201 showed an intense absorption peak for the Soret bands at 423 nm and small absorption peaks for the Q-bands at 558 and 600 nm. The Soret bands of LJ203 exhibited maximum peaks at 411 and 452 nm. The absorption peak at 411 nm should be generated from the triphenylamine units.11 The Q-band of LJ203 displayed a significant red shift of the absorption maximum to 621 nm compared to 600 nm for LJ201. On TiO2 films, the absorption spectra of the two dyes showed a slight red shift as
Table 1
Dye
Optical and electrochemical data for LJ201 and LJ203
e (at labs) labs on HOMOa LUMOc labs (nm) (M 1 cm 1) TiO2 (nm) (V) E0–0b (eV) (V)
LJ201 423 558 600
3.2 104 1.8 103 1.1 103
424 560 601
1.00
1.90
0.90
LJ203 411 452 572 621
1.8 2.9 4.3 4.8
104 104 103 103
415 458 570 625
0.96
1.80
0.84
a
The HOMO (vs. NHE) was measured using a photoelectrode (TiO2 adsorbed with dye) as the working electrode and the Ag/Ag+ electrode as a reference electrode. b E0–0 was estimated from UV-Vis absorption spectra. c LUMO = Eox E0–0 (vs. NHE). 11786
Chem. Commun., 2013, 49, 11785--11787
compared to those in solution. The introduction of axial ligands into the Si center can suppress the H-aggregation of dyes on the TiO2 surface,6,7 but it was found that their presence leads to the formation of J-aggregates. The HOMO (highest occupied molecular orbital) values of LJ201 and LJ203 were obtained according to the first oxidation potentials (Eox) measured by cyclic voltammetry (CV) in acetonitrile using a dye loaded-TiO2 electrode as the working electrode. The electrochemical properties are given in Table 1. Although the LUMO level of LJ203 shows a 60 mV positive shift compared to LJ201 due to increased electron-donating ability, it is still sufficient to carry out the electron injection into the conduction band of TiO2 ( 0.50 V vs. NHE).12 The HOMO level of LJ203 shows a 40 mV negative shift compared to LJ201 due to increased conjugation. Therefore, LJ203 has a slightly smaller energy loss between the HOMO and the I /I3 redox couple (0.40 V vs. NHE),12,13 which should result in better DSSC performance. 4-tert-Butylpyridine, as an electrolyte additive, can increase the Voc of DSSCs by shifting the conduction band to the negative direction.14 But this additive resulted in poor device performance for LJ201 and LJ203 since the lifted conduction band decreased the driving force for electron injection from excited dyes to TiO2. Therefore it was not employed in the electrolyte in this work. Under the optimized fabrication conditions, LJ203 achieves an Z of 3.0%, with a Jsc of 8.24 mA cm 2, a Voc of 486 mV, and a FF of 0.74. The Z value of the LJ203sensitized solar cell was two times that of the cell using LJ201 (Z = 1.0%, Jsc = 3.28 mA cm 2, Voc = 436 mV, FF = 0.72, shown in Fig. 3a), and improved by 43% compared to the reference cell using ZnP. From the incident photon to current conversion efficiencies (IPCEs) of DSSCs sensitized with LJ201 and LJ203 shown in Fig. 3b, the high photocurrent of LJ203-based DSSCs reflects efficient electron injection and ideal photoresponse in the near-IR region. LJ201 shows an IPCE between 300–670 nm, but its IPCE peak is just around 10–25%. In contrast, the LJ203 device exhibits a broad IPCE spectrum ending at 750 nm and its IPCE peak is approximately 40–60% between 380–670 nm, resulting in a high photocurrent. The absorbed photo-to-electric current generation efficiency (APCE) presents the true quantum yield for electric current generation from light, obtained by dividing the IPCE by the light-harvesting efficiency (LHE). The APCE values in the wavelength range from 520 to 660 nm are above 60% for LJ203 (Fig. S4 in the ESI†), indicating the efficient photocurrent generation. Electrochemical impedance spectroscopy (EIS) analysis was performed to gain an insight into the interfacial charge transfer processes in DSSCs and the Nyquist plots are shown in Fig. 4. Rrec and RCE represent the charge transfer resistance at the TiO2/dye/electrolyte interface and the counter electrode, respectively. The large semi-circle corresponds to Rrec, while the small semi-circle reflects RCE. A larger Rrec was obtained for the DSSCs sensitized with LJ203 than that measured for LJ201, indicating a larger charge transfer resistance at the TiO2/LJ203/electrolyte interface. This is caused by introducing triphenylamine units in LJ203. The bulky triphenylamine unit has a non-planar propeller-like structure; as a result, the electron recombination This journal is
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Fig. 3 (a) J–V curves of DSSCs sensitized with LJ201 and LJ203 measured under the illumination of sunlight AM 1.5 G; (b) IPCE spectra of DSSCs sensitized with LJ201 and LJ202.
and anchored dyes. Among binding modes, the bidentate mode achieves high quantum yields of electron injection into a semiconductor,16 as a result, LJ203 exhibited a more efficient electron injection into TiO2 compared to LJ201. In summary, we have reported two silicon–porphyrin sensitizers, LJ201 and LJ203, based on axial ligands. The absorption of LJ203 is red-shifted compared to the classic porphyrin sensitizer LJ201 while maintaining the energy levels necessary for efficient electron injection and dye regeneration. In addition, axial ligands and bulky triphenylamine units in LJ203 suppress the electron recombination at the TiO2/dye/electrolyte interface. LJ203-based DSSCs without CDCA as a co-absorbent exhibited good performance, showing potential application as a panchromatic sensitizer in solar-electrical conversion devices. Using axial ligands as anchoring groups to prevent dye aggregation is an effective strategy to prepare efficient porphyrin sensitizers for DSSC application in the future. We gratefully acknowledge the financial support of this work from the China Natural Science Foundation (Grant 21076039, Grant 21276044, Grant 21120102036 and 20923006), the National Basic Research Program of China (Grant No. 2009CB220009), the Swedish Energy Agency, K&A Wallenberg Foundation, and the State Key Laboratory of Fine Chemicals (KF0805), the Program for Innovative Research Team of Liaoning Province (Grant No. LS2010042).
Notes and references
Fig. 4 Nyquist plots of DSSCs sensitized with LJ201 and LJ203 measured at 0.5 V bias voltage in the dark.
at the TiO2/dye/electrolyte interface can be efficiently suppressed, leading to a higher Voc for DSSCs sensitized with LJ203. From the RCE aspect, the increase in radius when DSSCs were sensitized with LJ201 is responsible for the lower FF of the devices. By using the FTIR technique, the study of the interfacial binding mode of the dyes onto the TiO2 films was carried out to determine how the dyes attach to the TiO2 films. For LJ201 and LJ203 sodium salts, the frequency difference values (Dnas–s) between the antisymmetric (nas) and symmetric stretching (ns) modes of the COO group were 227 and 235 cm 1, respectively. When the dyes were anchored onto the TiO2 surface, different Dnas–s values were obtained, 249 cm 1 for LJ201 and 224 cm 1 for LJ203 (Fig. S5 and Table S1 in the ESI†). On the basis of the Deacon and Philips rule,15 the unidentate mode for the binding of LJ201 and the bidentate mode for LJ203 are suggested to be the major binding modes between the semiconductor surface
This journal is
c
The Royal Society of Chemistry 2013
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