CHEMPHYSCHEM COMMUNICATIONS DOI: 10.1002/cphc.201301118
Sterically Hindered Phthalocyanines for Dye-Sensitized Solar Cells: Influence of the Distance between the Aromatic Core and the Anchoring Group Maria-Eleni Ragoussi,[a] Jun-Ho Yum,[b] Aravind Kumar Chandiran,[b] Mine Ince,[b] Gema de la Torre,[a] Michael Grtzel,*[b] Mohammad K. Nazeeruddin,*[b] and Toms Torres*[a, c] A new phthalocyanine (Pc) bearing bulky peripheral substituents and a carboxylic anchoring group directly attached to the macrocycle has been prepared and used as a sensitizer in DSSCs, reaching 5.57 % power conversion efficiency. In addition, an enhanced performance for the TT40 dye, previously reported by us, was achieved in optimized devices, obtaining a new record efficiency with Pc-sensitized cells.
In the last few years, phthalocyanines (Pcs)[1] have attracted strong interest in the field of dye-sensitized solar cells (DSSCs)[2] and have proved to be realistic candidates for replacing the successful but costly ruthenium-based sensitizers.[3, 4] Pcs are characterized by their intense and tunable absorption in the red or near-infrared region, as well as their electrochemical, photochemical and thermal stability. However, due to their tendency to form molecular aggregates on the surface of titanium dioxide nanocrystals, the efficiency values of Pc-based DSSCs have not attained those of their less photostable porphyrin relatives.[5] In an effort to cope with this issue, the first breakthrough came with PCH001[6] and TT1,[7] which hold three tert-butyl groups in the periphery of the macrocycle in order to suppress the stacking of the dye on the TiO2 surface (Figure 1). The result was a 3.5 % overall photovoltaic power-conversion efficiency (PCE) in the case of TT1. Further improvement was achieved by the introduction of bulky 2,6-diphenylphenoxy substituents at the periphery of the Pc, with efficiencies of 4.6 % and 5.3 % for dyes PcS6[8] and PcS15[3b] , respectively, reported [a] Dr. M.-E. Ragoussi,+ Dr. G. de la Torre, Prof. T. Torres Universidad Autnoma de Madrid, Departamento de Qumica Orgnica Cantoblanco, 28049 Madrid (Spain) E-mail: [email protected] [b] Dr. J.-H. Yum,+ Dr. A. K. Chandiran, Dr. M. Ince, Prof. M. Grtzel, Prof. M. K. Nazeeruddin Laboratory for Photonics and Interfaces Institute of Chemical Sciences and Engineering School of Basic Sciences Swiss Federal Institute of Technology (EPFL) 1015 Lausanne (Switzerland) E-mail: [email protected] [email protected] [c] Prof. T. Torres IMDEA Nanociencia c/Faraday, 9, Cantoblanco, 28049 Madrid (Spain) [+] Both authors contributed equally to this study. Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201301118.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
by Kimura, Mori and co-workers (Figure 1). From our side, we reported on a Pc-sensitized solar cell based on the TT40 dye (Figure 1),[3a] which, in addition to the bulky peripheral substituents, possesses a rigid alkyne spacer between the dye and the anchoring group. Cells prepared with this dye yielded a PCE of 5.5 % under 100 mW cm 2 (1 sun irradiation) and 6.1 % under 9.5 m Wcm 2. In this case, it was demonstrated that the suppression of aggregation was successful and no addition of the co-adsorbent chenodeoxycholic acid (CDCA) was necessary. Additionally, studies on the relationship between the structure and the function of the spacer between the aromatic core and the anchoring carboxy group in Pcs have demonstrated that electronic communication between them, as well as directionality, are essential for obtaining high efficiencies.[9] In fact, the success of TT1 was the outcome not only of the minimization of the formation of molecular aggregates, but also the induction of directionality with the COOH anchoring group being directly connected to the Pc core. Moreover, when the ethynyl spacer, basis of the success of TT40, was used for the TT1 analogue, that is TT22 (Figure 1), the efficiency did not reach the same level.[3a] Taking into consideration all the above, the next rational step was the synthesis and study of the sensitizing capabilities of a new Pc bearing a carboxylic acid anchoring group connected to the Pc core without spacer, following the TT1 example, but keeping the thriving 2,6-diphenylphenoxy substitution pattern. Therefore, we envisaged the synthesis and study of TT58 (Figure 1). During the progress of our work, Mori et al. reported on a series of structurally related, sterically hindered Pc sensitizers,[10] two of which (i.e. compounds PcS17 and PcS18 in Figure 1) also bearing COOH groups directly linked to the Pc core, with the aim of improving the electronic connection with TiO2. They found that PcS17 performed worse (PCE = 4.6 %) than PcS15 (PCE = 5.3 %), and this fact was attributed to the poor dye density on the semiconductor surface as a consequence of the steric congestion around the COOH anchoring group. However, sensitizer PcS18 holding less bulky 2,6-diisopropylphenoxy substituents allowed the authors to reach efficiencies up to 5.9 % in optimized devices. We report, herein, the synthesis of Pc TT58 along with its use as a sensitizer in DSSCs, in an attempt to shed light on the actual influence of the steric congestion around the anchoring group. Moreover, we revisited the preparation of TT40-based devices in order to optimize their efficiencies. For the preparation of TT58, which is directly connected to the Pc macrocycle ChemPhysChem 2014, 15, 1033 – 1036
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Figure 1. Structures of Pc dyes TT1, TT22, PCH001, PcS6, PcS15, PcS17, PcS18, TT40, and the new Pc TT58.
by a carboxylic acid, we used a method recently developed by us for the preparation of carboxyPcs,[11] which consists in the direct carboxylation of the corresponding iodoPc. By this straightforward approach, the desired compound can be obtained in an efficient way, thus avoiding a multiple-step and lower-yielding synthesis that was up to recently employed for the preparation of carboxyphthalocyanines.[7, 12] The synthesis of TT58 was achieved in one step (see Scheme S1 in the Supporting Information), starting from iodoPc 1,[3a] in a reaction that takes place in a biphasic system. The desired Pc was obtained in 70 % yield and was fully characterized by spectroscopic techniques (see Figures S1–S3). In particular, UV/Vis data demonstrated the consistently low aggregation of the dye in THF solution (Figure 2). In addition, TT58 features almost identical wavelengths for the Soret and Q bands as the TT40 dye. The photovoltaic properties of TT58 were analyzed in a typical DSSC with a diodide/triiodide electrolyte under standard Air Mass 1.5 global (AM1.5G) solar irradiation. Using a photoanode consisting of 7 mm transparent TiO2 plus 5 mm scattering TiO2 film (7 + 5), the DSSC exhibited a photo-current density 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
(JSC) of 13.1 mA cm 2, an opencircuit potential (VOC) of 601 mV, and a fill factor 0.71, leading to a PCE of 5.57 % and 6.05 % at 1 sun and under 9.5 mW cm 2, respectively (Figure 3 and Table 1). Please note that the thickness of the TiO2 film is different from that used in the preparation of the TT40 device.[3] Remarkably, the TT58 solar cell shows an IPCE higher than 90 % at 700 nm (Figure 4), which is in agreement with the absorbance maximum found in the UV/ Vis spectrum of the molecule. At this point, it seemed rational to revisit the previous successful analogue of TT58, that is TT40, and study its performance under the new optimized conditions employed in the preparation of the TT58 device. Hence, under similar conditions, the reference TT40 dye displayed a PCE of 6.01 % and 6.49 % under 1 sun and 9.5 mW cm 2, respectively. It should be noted that the latter value is higher than the one previously reported by us for this particular Pc.[3a] This improvement is associated with the use of an optimized
Figure 2. UV/Vis spectra in THF (1 10 5 m) of TT58 (black line) and TT40 (grey line).
photoanode, for example, overnight dye sensitization and a thicker TiO2 film. As previously established for TT40 and other Pcs functionalized with bulky substituents, the high efficiencies of TT58- and TT40-based molecular photovoltaic devices obtained in this work, in the absence of any co-adsorbent, ChemPhysChem 2014, 15, 1033 – 1036
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Figure 3. Current–voltage (J–V) characteristics for DSSCs prepared with dye TT58 at various light intensities.
Figure 4. Incident photon-to-electron conversion efficiency of the photovoltaic device sensitized with TT58.
Table 1. J–V characteristics plotted as wavelengths of DSSCs sensitized with TT58 compared to TT40. Dye
I0 [mW cm 2]
Jsc [mA cm 2]
Voc [mV]
FF[a]
h [%][b]
TT58
9.5 50 100 9.5 50 100 9.5 100 9.5 100
1.40 6.94 13.1 1.32 6.68 12.8 1.45 13.9 1.06 10.7
547 587 601 566 604 618 568 621 540 605
0.75 0.73 0.71 0.76 0.72 0.69 0.75 0.70 0.77 0.72
6.05 5.94 5.57 6.02 5.84 5.46 6.49 6.01 4.61 4.69
TT58[c]
TT40 TT40[c]
[a] Fill factor. [b] Overall efficiency h is derived from Jsc Voc x FF/I0. [c] 0.05 mm CDCA is added in dye solution. All data reported here were measured 1 day after cell assembling.
are related to the lack of molecular aggregates on the surface of the TiO2 nanoparticles. This was reconfirmed upon addition of a disaggregating agent, namely chenodeoxycholic acid (CDCA), which brought about a decrease in the PV performance in both TT40- and TT58-sensitized devices (see Table 1), plausibly ascribed to a certain loss of dye loading (Figure S4). The effect of CDCA on dye loading is more pronounced for TT58 than for TT40, which was attributed to differences in dye adsorption kinetics. In order to speculate about the reasons for the different performances of TT40 and TT58, and, in particular, the differences in the VOC values, we performed photocurrent and photovoltage transient measurements.[13] The TT58-sensitized solar cell showed a nearly identical electron lifetime compared to the TT40 device (see Figure 5 top). This implies that the differences in VOC are not associated with injected charge loss by recombination events. Another important factor that influences the voltage of DSSCs is the shift of the TiO2 conduction-band (CB) 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 5. Plots of changes in interfacial electron lifetime as a function of the capacitance (top), and capacitance as a function of VOC (bottom) of TT40and TT58-sensitized solar cells.
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CHEMPHYSCHEM COMMUNICATIONS edge. From the transient measurements, the VOC of TT58 was estimated to be around 20 mV lower than that of the TT40 cell. This low VOC implies a downward shift of the band edge toward more positive potentials, which might be due to the influence of the dipole effect of the sensitizer (namely, the orientation of the molecule) on the TiO2 surface.[14] To elucidate the origin of this different dipole effect, more in-depth studies will be necessary which extend beyond the scope of this rapid communication. In brief, we report herein an optimized TT40-sensitized device, showing PCE values of 6.01 % and 6.49 % at 1 sun and under 9.5 mW cm 2, respectively, were attained. In addition, a newly designed Pc-dye, namely TT58, holding the same bulky peripheral substituents as TT40, but with the anchoring COOH group directly attached to the macrocycle, has been synthesized and tested, leading to a PCE value of 5.57 % and 6.05 % at 1 sun and under 9.5 mW cm 2, respectively. These results confirm that both directly linked carboxylic acid and carboxyethynyl anchoring groups are ideal for obtaining highly efficient Pc sensitizers. The excellent performance of TT58 suggests, contrary to what had previously been claimed,[10] that the bulkiness of the diphenylphenoxy moieties does not exert a critical influence on the adsorption capabilities of this family of compounds, regardless of the distance between the Pc core and the carboxylic function.
Acknowledgements Financial support is acknowledged from the European Union within the FP7-ENERGY-2012-1 framework, GLOBALSOL project, Proposal No 309194-2, the Spanish Ministerio de Educacin (MEC) and Ministerio de Ciencia e Innovacin (MICINN) (CTQ2011-24187/BQU and CONSOLIDER INGENIO 2010, CSD200700010 on Molecular Nanoscience, PRI-PIBUS-2011-1128), and the Comunidad de Madrid (MADRISOLAR-2, S2009/PPQ/1533). A.K.C., M.K.N., and M.G. acknowledge the financial contribution from the European FP7 project ORION (Grant agreement no. NMP229036). J.H.Y. acknowledges the joint development project funded by Dongjin Semichem Co., Ltd. (S. Korea). We thank Dr. Yongjoo Kim and Mr. Pascal Compte for their kind assistance.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org Keywords: dye-sensitized solar cells · energy conversion · phthalocyanines · substituent effects · steric effects [1] a) The Porphyrin Handbook, Vols. 15 – 20 (Eds.: K. M. Kadish, K. M. Smith, R. Guilard), Academic Press, San Diego, 2003; b) G. de La Torre, C. G. Claessens, T. Torres, Chem. Commun. 2007, 2000; c) G. de La Torre, G. Bottari, U. Hahn, T. Torres, Struct. Bonding (Berlin) 2010, 135, 1 – 44. [2] a) B. O’Regan, M. Grtzel, Nature 1991, 353, 737; b) M. Grtzel, Nature 2001, 414, 338. [3] a) M.-E. Ragoussi, J.-J. Cid, J.-H. Yum, G. de La Torre, D. Di Censo, M. Grtzel, M. K. Nazeeruddin, T. Torres, Angew. Chem. Int. Ed. 2012, 51, 4375; Angew. Chem. 2012, 124, 4451; b) M. Kimura, H. Nomoto, N. Masaki, S. Mori, Angew. Chem. Int. Ed. 2012, 51, 4371; Angew. Chem. 2012, 124, 4447; c) M. Garca-Iglesias, J.-J. Cid, J.-H. Yum, A. Forneli, P. Vzquez, M. K. Nazeeruddin, E. Palomares, M. Grtzel, T. Torres, Energy Environ. Sci. 2011, 4, 189. [4] a) M.-E. Ragoussi, M. Ince, T. Torres, Eur. J. Org. Chem. 2013, 6475 – 6489; b) M. V. Martnez-Daz, G. de La Torre, T. Torres, Chem. Commun. 2010, 46, 7090; c) M. V. Martnez-Daz, M. Ince, T. Torres, Monatsh. Chem. 2011, 142, 699. [5] a) L.-L. Li, E. W.-G. Diau, Chem. Soc. Rev. 2013, 42, 291; b) 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. Grtzel, Science 2011, 334, 629. [6] P. Y. Reddy, L. Giribabu, C. Lyness, H. J. Snaith, C. Vijaykumar, M. Chandrasekharam, M. Lakshmikantam, J.-H. Yum, K. Kalyanasundaram, M. Grtzel, M. K. Nazeeruddin, Angew. Chem. Int. Ed. 2007, 46, 373; Angew. Chem. 2007, 119, 377. [7] J.-J. Cid, J.-H. Yum, S.-R. Jang, M. K. Nazeeruddin, E. Martnez-Ferrero, E. Palomares, J. Ko, M. Grtzel, T. Torres, Angew. Chem. Int. Ed. 2007, 46, 8358; Angew. Chem. 2007, 119, 8510. [8] S. Mori, M. Nagata, Y. Nakahata, K. Yasuta, R. Goto, M. Kimura, M. Taya, J. Am. Chem. Soc. 2010, 132, 4054. [9] J.-J. Cid, M. Garca-Iglesias, J.-H. Yum, A. Forneli, J. Albero, E. MartnezFerrero, P. Vzquez, M. Grtzel, M. K. Nazeeruddin, E. Palomares, T. Torres, Chem. Eur. J. 2009, 15, 5130. [10] 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. [11] I. Aguirre de Carcer Garca, A. M. Sevim, A. de La Escosura, T. Torres, Org. Biomol. Chem. 2013, 11, 2237. [12] a) A. Gouloumis, S.-G. Liu, A. Sastre, P. Vazquez, L. Echegoyen, T. Torres, Chem. Eur. J. 2000, 6, 3600; b) D. M. Guldi, A. Gouloumis, P. Vazquez, T. Torres, V. Georgakilas, M. Prato, J. Am. Chem. Soc. 2005, 127, 5811. [13] B. C. O’Regan, F. Lenzmann, J. Phys. Chem. B 2004, 108, 4342. [14] a) N. Kopidakis, N. R. Neale, A. J. Frank, J. Phys. Chem. B 2006, 110, 12485; b) J.-H. Yum, R. Humphry-Baker, S. M. Zakeeruddin, M. K. Nazeeruddin, M. Grtzel, Nano Today 2010, 5, 91. Received: November 26, 2013 Published online on March 3, 2014
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Sterically Hindered Phthalocyanines for Dye-Sensitized Solar Cells: Influence of the Distance between the Aromatic Core and the Anchoring Group Maria-Eleni Ragoussi,[a] Jun-Ho Yum,[b] Aravind Kumar Chandiran,[b] Mine Ince,[b] Gema de la Torre,[a] Michael Grtzel,*[b] Mohammad K. Nazeeruddin,*[b] and Toms Torres*[a, c] A new phthalocyanine (Pc) bearing bulky peripheral substituents and a carboxylic anchoring group directly attached to the macrocycle has been prepared and used as a sensitizer in DSSCs, reaching 5.57 % power conversion efficiency. In addition, an enhanced performance for the TT40 dye, previously reported by us, was achieved in optimized devices, obtaining a new record efficiency with Pc-sensitized cells.
In the last few years, phthalocyanines (Pcs)[1] have attracted strong interest in the field of dye-sensitized solar cells (DSSCs)[2] and have proved to be realistic candidates for replacing the successful but costly ruthenium-based sensitizers.[3, 4] Pcs are characterized by their intense and tunable absorption in the red or near-infrared region, as well as their electrochemical, photochemical and thermal stability. However, due to their tendency to form molecular aggregates on the surface of titanium dioxide nanocrystals, the efficiency values of Pc-based DSSCs have not attained those of their less photostable porphyrin relatives.[5] In an effort to cope with this issue, the first breakthrough came with PCH001[6] and TT1,[7] which hold three tert-butyl groups in the periphery of the macrocycle in order to suppress the stacking of the dye on the TiO2 surface (Figure 1). The result was a 3.5 % overall photovoltaic power-conversion efficiency (PCE) in the case of TT1. Further improvement was achieved by the introduction of bulky 2,6-diphenylphenoxy substituents at the periphery of the Pc, with efficiencies of 4.6 % and 5.3 % for dyes PcS6[8] and PcS15[3b] , respectively, reported [a] Dr. M.-E. Ragoussi,+ Dr. G. de la Torre, Prof. T. Torres Universidad Autnoma de Madrid, Departamento de Qumica Orgnica Cantoblanco, 28049 Madrid (Spain) E-mail: [email protected] [b] Dr. J.-H. Yum,+ Dr. A. K. Chandiran, Dr. M. Ince, Prof. M. Grtzel, Prof. M. K. Nazeeruddin Laboratory for Photonics and Interfaces Institute of Chemical Sciences and Engineering School of Basic Sciences Swiss Federal Institute of Technology (EPFL) 1015 Lausanne (Switzerland) E-mail: [email protected] [email protected] [c] Prof. T. Torres IMDEA Nanociencia c/Faraday, 9, Cantoblanco, 28049 Madrid (Spain) [+] Both authors contributed equally to this study. Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201301118.
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by Kimura, Mori and co-workers (Figure 1). From our side, we reported on a Pc-sensitized solar cell based on the TT40 dye (Figure 1),[3a] which, in addition to the bulky peripheral substituents, possesses a rigid alkyne spacer between the dye and the anchoring group. Cells prepared with this dye yielded a PCE of 5.5 % under 100 mW cm 2 (1 sun irradiation) and 6.1 % under 9.5 m Wcm 2. In this case, it was demonstrated that the suppression of aggregation was successful and no addition of the co-adsorbent chenodeoxycholic acid (CDCA) was necessary. Additionally, studies on the relationship between the structure and the function of the spacer between the aromatic core and the anchoring carboxy group in Pcs have demonstrated that electronic communication between them, as well as directionality, are essential for obtaining high efficiencies.[9] In fact, the success of TT1 was the outcome not only of the minimization of the formation of molecular aggregates, but also the induction of directionality with the COOH anchoring group being directly connected to the Pc core. Moreover, when the ethynyl spacer, basis of the success of TT40, was used for the TT1 analogue, that is TT22 (Figure 1), the efficiency did not reach the same level.[3a] Taking into consideration all the above, the next rational step was the synthesis and study of the sensitizing capabilities of a new Pc bearing a carboxylic acid anchoring group connected to the Pc core without spacer, following the TT1 example, but keeping the thriving 2,6-diphenylphenoxy substitution pattern. Therefore, we envisaged the synthesis and study of TT58 (Figure 1). During the progress of our work, Mori et al. reported on a series of structurally related, sterically hindered Pc sensitizers,[10] two of which (i.e. compounds PcS17 and PcS18 in Figure 1) also bearing COOH groups directly linked to the Pc core, with the aim of improving the electronic connection with TiO2. They found that PcS17 performed worse (PCE = 4.6 %) than PcS15 (PCE = 5.3 %), and this fact was attributed to the poor dye density on the semiconductor surface as a consequence of the steric congestion around the COOH anchoring group. However, sensitizer PcS18 holding less bulky 2,6-diisopropylphenoxy substituents allowed the authors to reach efficiencies up to 5.9 % in optimized devices. We report, herein, the synthesis of Pc TT58 along with its use as a sensitizer in DSSCs, in an attempt to shed light on the actual influence of the steric congestion around the anchoring group. Moreover, we revisited the preparation of TT40-based devices in order to optimize their efficiencies. For the preparation of TT58, which is directly connected to the Pc macrocycle ChemPhysChem 2014, 15, 1033 – 1036
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Figure 1. Structures of Pc dyes TT1, TT22, PCH001, PcS6, PcS15, PcS17, PcS18, TT40, and the new Pc TT58.
by a carboxylic acid, we used a method recently developed by us for the preparation of carboxyPcs,[11] which consists in the direct carboxylation of the corresponding iodoPc. By this straightforward approach, the desired compound can be obtained in an efficient way, thus avoiding a multiple-step and lower-yielding synthesis that was up to recently employed for the preparation of carboxyphthalocyanines.[7, 12] The synthesis of TT58 was achieved in one step (see Scheme S1 in the Supporting Information), starting from iodoPc 1,[3a] in a reaction that takes place in a biphasic system. The desired Pc was obtained in 70 % yield and was fully characterized by spectroscopic techniques (see Figures S1–S3). In particular, UV/Vis data demonstrated the consistently low aggregation of the dye in THF solution (Figure 2). In addition, TT58 features almost identical wavelengths for the Soret and Q bands as the TT40 dye. The photovoltaic properties of TT58 were analyzed in a typical DSSC with a diodide/triiodide electrolyte under standard Air Mass 1.5 global (AM1.5G) solar irradiation. Using a photoanode consisting of 7 mm transparent TiO2 plus 5 mm scattering TiO2 film (7 + 5), the DSSC exhibited a photo-current density 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
(JSC) of 13.1 mA cm 2, an opencircuit potential (VOC) of 601 mV, and a fill factor 0.71, leading to a PCE of 5.57 % and 6.05 % at 1 sun and under 9.5 mW cm 2, respectively (Figure 3 and Table 1). Please note that the thickness of the TiO2 film is different from that used in the preparation of the TT40 device.[3] Remarkably, the TT58 solar cell shows an IPCE higher than 90 % at 700 nm (Figure 4), which is in agreement with the absorbance maximum found in the UV/ Vis spectrum of the molecule. At this point, it seemed rational to revisit the previous successful analogue of TT58, that is TT40, and study its performance under the new optimized conditions employed in the preparation of the TT58 device. Hence, under similar conditions, the reference TT40 dye displayed a PCE of 6.01 % and 6.49 % under 1 sun and 9.5 mW cm 2, respectively. It should be noted that the latter value is higher than the one previously reported by us for this particular Pc.[3a] This improvement is associated with the use of an optimized
Figure 2. UV/Vis spectra in THF (1 10 5 m) of TT58 (black line) and TT40 (grey line).
photoanode, for example, overnight dye sensitization and a thicker TiO2 film. As previously established for TT40 and other Pcs functionalized with bulky substituents, the high efficiencies of TT58- and TT40-based molecular photovoltaic devices obtained in this work, in the absence of any co-adsorbent, ChemPhysChem 2014, 15, 1033 – 1036
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Figure 3. Current–voltage (J–V) characteristics for DSSCs prepared with dye TT58 at various light intensities.
Figure 4. Incident photon-to-electron conversion efficiency of the photovoltaic device sensitized with TT58.
Table 1. J–V characteristics plotted as wavelengths of DSSCs sensitized with TT58 compared to TT40. Dye
I0 [mW cm 2]
Jsc [mA cm 2]
Voc [mV]
FF[a]
h [%][b]
TT58
9.5 50 100 9.5 50 100 9.5 100 9.5 100
1.40 6.94 13.1 1.32 6.68 12.8 1.45 13.9 1.06 10.7
547 587 601 566 604 618 568 621 540 605
0.75 0.73 0.71 0.76 0.72 0.69 0.75 0.70 0.77 0.72
6.05 5.94 5.57 6.02 5.84 5.46 6.49 6.01 4.61 4.69
TT58[c]
TT40 TT40[c]
[a] Fill factor. [b] Overall efficiency h is derived from Jsc Voc x FF/I0. [c] 0.05 mm CDCA is added in dye solution. All data reported here were measured 1 day after cell assembling.
are related to the lack of molecular aggregates on the surface of the TiO2 nanoparticles. This was reconfirmed upon addition of a disaggregating agent, namely chenodeoxycholic acid (CDCA), which brought about a decrease in the PV performance in both TT40- and TT58-sensitized devices (see Table 1), plausibly ascribed to a certain loss of dye loading (Figure S4). The effect of CDCA on dye loading is more pronounced for TT58 than for TT40, which was attributed to differences in dye adsorption kinetics. In order to speculate about the reasons for the different performances of TT40 and TT58, and, in particular, the differences in the VOC values, we performed photocurrent and photovoltage transient measurements.[13] The TT58-sensitized solar cell showed a nearly identical electron lifetime compared to the TT40 device (see Figure 5 top). This implies that the differences in VOC are not associated with injected charge loss by recombination events. Another important factor that influences the voltage of DSSCs is the shift of the TiO2 conduction-band (CB) 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 5. Plots of changes in interfacial electron lifetime as a function of the capacitance (top), and capacitance as a function of VOC (bottom) of TT40and TT58-sensitized solar cells.
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CHEMPHYSCHEM COMMUNICATIONS edge. From the transient measurements, the VOC of TT58 was estimated to be around 20 mV lower than that of the TT40 cell. This low VOC implies a downward shift of the band edge toward more positive potentials, which might be due to the influence of the dipole effect of the sensitizer (namely, the orientation of the molecule) on the TiO2 surface.[14] To elucidate the origin of this different dipole effect, more in-depth studies will be necessary which extend beyond the scope of this rapid communication. In brief, we report herein an optimized TT40-sensitized device, showing PCE values of 6.01 % and 6.49 % at 1 sun and under 9.5 mW cm 2, respectively, were attained. In addition, a newly designed Pc-dye, namely TT58, holding the same bulky peripheral substituents as TT40, but with the anchoring COOH group directly attached to the macrocycle, has been synthesized and tested, leading to a PCE value of 5.57 % and 6.05 % at 1 sun and under 9.5 mW cm 2, respectively. These results confirm that both directly linked carboxylic acid and carboxyethynyl anchoring groups are ideal for obtaining highly efficient Pc sensitizers. The excellent performance of TT58 suggests, contrary to what had previously been claimed,[10] that the bulkiness of the diphenylphenoxy moieties does not exert a critical influence on the adsorption capabilities of this family of compounds, regardless of the distance between the Pc core and the carboxylic function.
Acknowledgements Financial support is acknowledged from the European Union within the FP7-ENERGY-2012-1 framework, GLOBALSOL project, Proposal No 309194-2, the Spanish Ministerio de Educacin (MEC) and Ministerio de Ciencia e Innovacin (MICINN) (CTQ2011-24187/BQU and CONSOLIDER INGENIO 2010, CSD200700010 on Molecular Nanoscience, PRI-PIBUS-2011-1128), and the Comunidad de Madrid (MADRISOLAR-2, S2009/PPQ/1533). A.K.C., M.K.N., and M.G. acknowledge the financial contribution from the European FP7 project ORION (Grant agreement no. NMP229036). J.H.Y. acknowledges the joint development project funded by Dongjin Semichem Co., Ltd. (S. Korea). We thank Dr. Yongjoo Kim and Mr. Pascal Compte for their kind assistance.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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