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Aggregation behavior of differently substituted Ru(II)-complex dyes as sensitizers for electrodeposited ZnO solar cells
Journal of Photochemistry and Photobiology A: Chemistry 242 (2012) 67–71
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Aggregation behavior of differently substituted Ru(II)-complex dyes as sensitizers for electrodeposited ZnO solar cells Asdim a , Keigo Ichinose a , Tomohiko Inomata b , Hideki Masuda b , Tsukasa Yoshida a,∗ a Center of Innovative Photovoltaic Systems (CIPS), Environmental and Renewable Energy Systems (ERES) Division, Graduate School of Engineering, Gifu University, Yanagido 1-1, Gifu 501-1193, Japan b Department of Material Science and Engineering, Graduate School of Engineering, Nagoya Institute of Technology, Showa-ku, Nagoya 466-8555, Japan
a r t i c l e
i n f o
Article history: Received 19 December 2011 Received in revised form 14 March 2012 Accepted 11 April 2012 Available online 7 June 2012 Keywords: Zinc oxide Electrodeposition Dye-sensitized solar cell Ru(II)-complex Aggregation
a b s t r a c t Properties of newly developed Ru(II)-complex dye, cis-bis(isothiocyanato) (2,2 -bipyridyl-4,4 dicarboxylato) (N-(4-butoxyphenyl)-N-2-pyridinyl-2-pyridinamine)-ruthenium(II) (J13), having a triarylamine ligand have been studied in comparison with cis-bis(isothiocyanato) bis(2,2 -bipyridyl-4,4 dicarboxylato)-ruthenium(II) (N3), as photosensitizers for solar cells employing electrodeposited porous ZnO thin films. Quantification of dye loading as well as change of the film morphology on extension of the soaking time in solutions of these dyes indicated spontaneous monolayer formation of the J13 dye, in contrast to the strongly aggregating N3 dye. Overloading of the N3 dye molecules not directly bound to ZnO caused decreased Jsc as well as worsening of F.F., so that the efficiency of the N3 cell created a peak with a moderate dipping time. On the other hand, the efficiency of the J13 cells simply saturated to a value of ca. 4% as the dye monolayer was formed, indicating its better chemical match with ZnO. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSSCs) are promising candidates of photovoltaic devices which can efficiently convert solar energy-toelectricity at substantially low cost [1,2]. Even though the highest conversion efficiency of 12.3% has recently been achieved on a DSSC employing titanium dioxide (TiO2 ) photoelectrode sensitized with zinc porphyrin dye having long alkyl substituent in combination with Co complex redox electrolyte [3], poly-pyridine Ru (II) complexes have traditionally been the best sensitizers to be combined with TiO2 [4,5]. The porous TiO2 electrode is usually prepared by sintering nanoparticles at a high temperature (450–500 ◦ C), so that the choice of the transparent substrate is limited to glass. In order to achieve versatility in application and also to reduce the production cost, use of plastic substrate is more preferable. Zinc oxide (ZnO) can often be synthesized at relatively low temperatures owing to its higher chemical reactivity than TiO2 . It has similar band positions but higher electron mobility than TiO2 [6], and thus be a promising alternative electrode material especially for realization of plastic DSSCs. We have developed a method to directly crystallize mesoporous ZnO thin films from water by use of
∗ Corresponding author. Present address: Department of Chemistry and Chemical Engineering, Faculty of Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan. Tel.: +81 58 293 2593; fax: +81 58 293 2594. E-mail addresses: [email protected], [email protected] (T. Yoshida). 1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jphotochem.2012.04.024
organic dye molecules such as eosin Y as a structure-directing agent (SDA) for cathodic electrodeposition of ZnO [7]. Because the entire process requires neither high temperature nor aggressive chemicals, soft electrode materials such as ITO coated PET films can be used as the substrate. Owing to its porous crystalline structure having both high crystallinity and high surface area, such ZnO thin films can perform as excellent photoelectrode materials to achieve high collection efficiency of photogenerated charge carriers [8]. When it was combined with an indoline dye, D149, a conversion efficiency of 5.6% could be achieved [7]. Achieving higher efficiency should then become possible by finding a dye that sensitizes ZnO as efficient as D149 and absorbs in a wider range. When Ru complexes known as the best sensitizer to date and has been the standard to TiO2 such as cis-bis(isothiocyanato) bis(2,2 bipyridyl-4,4 -dicarboxylato)-ruthenium(II), (hereafter called N3) [1] was combined to ZnO, only disappointingly low efficiencies were obtained [9–11]. Keis et al. revealed that the N3 dye acts as a too strong acid to the relatively soft ZnO and forms randomly structured Zn2+ /N3 aggregates that does not contribute to the photocurrent generation. Unmatched chemistry between ZnO and N3 is obviously the reason of the poor efficiency. That at the same time let us expect existence of other Ru complexes suited to ZnO to achieve high efficiencies, when formation of such aggregates is prevented. Recently, we have developed a series of new Ru complexes having triarylamine ligands as sensitizers for TiO2 . Triarylamine has a strong electron donating ability and has also been introduced to several organic dyes as sensitizers [12,13]. Among the dyes
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Asdim et al. / Journal of Photochemistry and Photobiology A: Chemistry 242 (2012) 67–71
Fig. 1. Structures of N3 (a) and J13 (b) molecules. The molar absorption coefficients (ε) of N3 and J13 are 14,000 M−1 cm−1 at 532 nm [14] and 7880 M−1 cm−1 at 537 nm in ethanol, respectively.
synthesized and tested, J13, shown in Fig. 1, has achieved a high efficiency () of 7.83% [15]. Since J13 has only two carboxylic acid groups as compared to four in N3, it could behave less aggressively than N3 on the ZnO surface. Such a motivation let us study the property of J13 dye as sensitizer for solar cells employing electrodeposited mesoporous ZnO in comparison with N3. We have given a focus especially to the formation of dye aggregates under controlled dipping periods and studied its influence to the solar cell performances. 2. Experimental F-doped SnO2 (FTO) coated glass (8 /, Asahi Glass) sheets were washed subsequently with detergent, pure water, acetone and 2-propanol. The FTO surface insulated with a photoresist masking except for a round hole with 6 mm (for DSSC electrodes) or 12 mm (for analysis of the films) diameter in the center. They were used in a configuration of a rotating disk electrode (RDE) to control mass transport for high homogeneity and reproducibility of the films. A home-made electrochemical system equipped with eight identical RDE in a single compartment cell was employed to produce 8 identical samples from one batch, so that precise comparison of dyes and their adsorption conditions was made possible. Porous ZnO thin films were prepared following the recipe reported in literature [16,17]. Briefly, after electrodepositing ZnO layer of 1 m thickness in a compact structure, porous ZnO was electrodeposited potentiostatically at −0.95 V (vs. Ag/AgCl) and ω = 500 rpm for 10 or 30 min (corresponding to 1.6 or 5.0 m thickness) from an aqueous solution (70 ◦ C) containing 5 mM ZnCl2 , 0.1 M KCl, 75 M eosin Y and saturated with O2 . Extraction of eosin Y by soaking the deposited films in a dilute KOH aqueous solution of pH 10.5 results in a porous structure [17]. ZnO thin films were sensitized by soaking them in 0.5 mM N3 (purity: 95%, Aldrich) and J13 dye solutions in tert-butyl alcohol/dimethylformamide (v/v = 1/1) mixture, at 60 ◦ C for controlled periods between 3 min and 8 h. The amount of adsorbed N3 and J13 was checked from the absorption spectrum of a solution dissolving the film in a 0.5 M NaOH solution in ethanol/water mixture (v/v = 1/1). The molar absorbance coefficient (ε) of N3 and J13 in the mixture solution was calculated to 12,300 M−1 cm−1 (at 510 nm) and 7300 M−1 cm−1 (at 520 nm), respectively. The change of the film morphology by dye adsorption was observed on a Hitachi S4800 field emission scanning electron microscope (FE-SEM). The dyed ZnO electrode and a Pt-sputtered FTO glass counter electrode were assembled into a sandwich cell using a Surlyn® film of 30 m thickness as a spacer. Electrolyte solution consisting of 0.1 M LiI (Wako), 0.05 M I2 (Wako), 0.6 M 1,2-dimethyl-3propylimidazolium iodide (Shikoku Chemicals Co.), and 0.5 M tert-butylpyridine (TBP, Nacalai tesque) in acetonitrile was filled by capillary action. I–V curves of the cells were measured by EKO MP-160 curve tracer under illumination with a simulated sun light (AM 1.5, 100 mW cm−2 ) generated by a Yamashita-Denso YSS150A. Photocurrent action spectra were measured on a Bunko-Keiki
CEP-2000 system under monochromatic light illumination with a constant photon flux of 5.0 × 1015 s−1 cm−2 . The active area of the cells was regulated to 0.2 cm2 using a photomask. 3. Results and discussion The cell properties were highly dependent on the soaking time of ZnO films in the dye solutions (Fig. 2). While the voltages were almost unchanged, short circuit current density (Jsc ) initially increased on extension of the soaking time due to increase of the adsorbed dye to improve the efficiency of light harvest. However, that of ZnO/N3 creates a maximum of ca. 8.5 mA cm−2 at 10 min and begin to decrease, while that of ZnO/J13 continues to increase and reaches a steady value of about 9 mA cm−2 . On the other hand, fill factor (F.F.) of the ZnO/N3 cell continues to worsen, while that of the ZnO/J13 cell is unchanged. As a consequence, the overall energy conversion efficiency () of the ZnO/J13 cell increases proportionally to the increase of Jsc and reaches a constant value of ca. 4%, whereas the highest efficiency of about 3% was reached for that soaked in N3 for 10 min and got worse for the longer dipping time. Worsening of the efficiency in the same trend, in fact even more seriously than the present example, has been reported in combination of nanoparticulate ZnO electrode with N3, and attributed to the overloading of the N3 dye to form aggregates [9]. Although such a problem of N3 could not be prevented by the use of electrodeposited ZnO, J13 appears to behave better than N3, being less sensitive to the soaking time. Fig. 3 compares the I–V curves of the ZnO/N3 and ZnO/J13 cells with the same soaking time of 1 h. The most prominent difference is the F.F., being much worse for the ZnO/N3 cell than ZnO/J13. The dark current of the ZnO/N3 cell is also much smaller than ZnO/J13, indicating a significantly suppressed electron transfer to I3 − ions in the electrolyte. The photocurrent action spectra of the ZnO/J13 and ZnO/N3 cells are shown in Fig. 4. The maximum IPCEs of about 58% and 70% are reached at 510 nm on ZnO/N3 and ZnO/J13 cells, respectively. IPCE is a product of the light harvesting efficiency
Fig. 2. Changes of conversion efficiency (circle, ), photocurrent density (triangle, Jsc ), and fill factor (square, F.F.) of the N3 (open symbols) and J13 (filled symbols) cells on soaking time of ZnO films in the dye solutions.
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of N3 abruptly increases and reaches about 4 times higher amount than that of J13 after 8 h of dipping. The saturation of J13 implies a spontaneous formation of a monolayer, while the behavior of N3 suggests formation of its aggregates. The apparent area occupied by a single dye molecule on the ZnO surface (Adye ) can be estimated from the amount of adsorbed dye (ndye ) as calculated by Eq. (2), Adye =
Fig. 3. I–V curves of the N3 (a) and J13 (b) cells employing ZnO films soaked in the dye solutions for 1 h, measured under illumination with an AM 1.5 stimulated sunlight (100 mW cm−2 ) (solid lines) and in the dark (dashed lines).
Fig. 4. Photocurrent action spectra of the N3 (a) and J13 (b) cells employing ZnO films soaked in the dye solutions for 1 h.
(LHE), the efficiency of charge separation on photoexcitation of dye bound to ZnO (inj ) and that of the collection of charge (coll ), IPCE = LHE × inj × coll
(1)
For the cells measured here, LHE practically reaches its saturation at the wavelength of light absorption maxima of these dyes. Since coll is expected to be the same for both cells as they employ the same electrode materials and electrolyte, the lower inj of N3 should be responsible to the lower IPCE. Dye molecules adsorbed as aggregates do absorb light but cannot inject electron to ZnO, leading to a decrease of Jsc despite of the increased LHE, so that inj of N3 becomes lower on higher dye loading. In order to study the differences of the aggregation behavior of N3 and J13, a thinner ZnO of 1.6 m was employed. The change of the amount of adsorbed dye for different soaking time is shown in Fig. 5. The amount of adsorbed N3 and J13 increase similarly up to 3 min. While that of J13 only slightly increase on extension of the dipping time and saturates at around 4 × 10−8 mol cm−2 , that
Fig. 5. The loading of the N3 and J13 dyes on soaking of the porous ZnO thin films (1.6 m thickness) in the dye solutions.
SZnO ndye NA
(2)
where NA is the Avogadro number (6.02 × 1023 mol−1 ) and SZnO is the total surface area of porous ZnO thin film used in the experiments, which is determined as 3.87 × 1016 nm2 cm−2 from the amount of adsorbed eosin Y and the ZnO surface area occupied by a single eosin Y molecule (3.32 nm2 [19]) since eosin Y is known to undergo monolayer adsorption on ZnO [18]. From nN3 and nJ13 after soaking for 8 h, AN3 and AJ13 are calculated as 0.423 and 1.42 nm2 , respectively. AN3 has been previously estimated as 1.6 nm2 for a monolayer of N3 on TiO2 [20]. Since the molecular structures of N3 and J13 are similar, similar values can be expected if monolayers of N3 and J13 are formed of ZnO. The clearly smaller value estimated for N3 indicates its aggregation adsorbed as multilayers and that of J13 close to 1.6 nm2 suggests a formation of a monolayer on ZnO. The morphology of the native porous ZnO thin film and those after dipping in dye solutions for 3 min and 8 h has been observed for their surface and cross section by SEM (Fig. 6). The porous nanostructure consisting of ZnO nanowire with ca. 10 nm thickness is well recognized for the native film (a and b). Dipping in the solution of J13 for 3 min does not cause a significant change in the overall structure (c and d), although slight reduction of the pore size can be recognized from a careful look of these pictures. Such a change of the morphology is probably associated with the surface adsorption of J13 molecules. On the contrary, dipping of the film in the N3 solution for 3 min causes a clear change of the morphology (e and f). The tips of ZnO nanowire can still be recognized but are obscured in the surface image. The cross section image clearly shows that the surface is covered with a thin layer of a shapeless solid, while inside of the film remains porous. This thin layer should be the aggregates of N3. Although similar amount of dyes was found for N3 and J13 after 3 min soaking, the way how they are adsorbed is totally different. The difference becomes significant after soaking for 8 h. While the structure of the film is unchanged, thus still being porous, for J13 (g and h), that for N3 is completely changed with a formation of a thick (ca. 0.5 m) layer of N3 aggregates on top of the porous ZnO layer (i and j). It is evident that N3 acts as an acid to continue to dissolve ZnO and the dissolved Zn2+ ion precipitates back as a randomly structured mixed aggregates with N3 onto the surface of the porous ZnO, rather than forming a dye multilayer [9,21]. On the other hand, the surface of ZnO is not attacked by J13 since it behaves as a milder acid than N3, and thus achieves a spontaneous formation of its monolayer on ZnO. The totally different adsorption behavior of N3 and J13 well explains the difference of the solar cell properties. The extra N3 molecules loaded as aggregates do not inject electrons but rather filter the light. Since the layer of N3 aggregates is formed on the outer surface of the porous ZnO, the opposite side of the light incidence, the filtering effect was not too severe. However, the presence of such a layer obviously blocks the transport of hole by the electrolyte as it clogs the pores. The severe worsening of F.F. can be understood as a consequence of the significant increase of hole transport resistance by prevention of electrolyte diffusion. None of such problems seem to occur in case of J13. As it spontaneously forms a dye monolayer on ZnO, Jsc simply increases on extension of the dipping time and F.F. remains constant. Unfortunately, the overall efficiency was not as high as that achieved by the same dye
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Fig. 6. Surface and cross section SEM images of ZnO thin films before dye adsorption (a and b), after dipping for 3 min in J13 (c and d) and N3 (e and f) solution, and for 8 h in J13 (g and h) and N3 (i and j) solution.
on TiO2 . The lower efficiency is attributed to the smaller current due to the limited IPCE as well as the limited light harvesting capability of J13 having a relatively small molar absorption coefficient, and also to the smaller voltage of the ZnO cells for which the effect of TBP added to the electrolyte to increase the voltage appear smaller than in the TiO2 cells. However, the present study evidently shows
the importance of “right chemistry” between the oxide electrode and dye molecules to allow spontaneous formation of dye monolayer for the efficient injection of electron from the dye molecule. It is important to pay attention to the chemical properties of dye molecules aside from their energy levels, when we want to develop new sensitizer dyes, especially in case of ZnO solar cells.
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4. Conclusion
References
The J13 Ru(II)-complex has been evaluated as a sensitizer for ZnO solar cell in comparison to the N3 dye. Analysis of the quantity of adsorbed dye as well as the change of the surface morphology clearly indicated strong aggregation of N3, while J13 underwent spontaneous formation of a monolayer. As a consequence, Jsc of the N3 cell creates a peak with a moderate time of dipping in the dye solution and it continues to decrease with excessive dye adsorption as it leads to a loading of dye molecules not directly bound to ZnO and thus cannot inject electron from its excited state. In addition, extended dipping of ZnO in N3 solution resulted in a formation of a thick outer layer of aggregates that hinder hole transport to significantly worsen F.F. None of such problems were observed for J13, for which Jsc continued to increase to a steady value and F.F. remained constant on extension of the dipping time. Although the efficiency of ca. 4% eventually achieved with the J13 dye is not satisfactorily high as it is still worse than the highest efficiency achieved for the ZnO solar cell in combination with organic dye sensitizer, the present study clearly indicates the importance of the right chemistry between ZnO and dye molecules to draw the best performance out of the system. Tuning of the chemical property in addition to the energy structure is therefore an important strategy to develop good photosensitizer for ZnO solar cells.
[1] B. O’Regan, M. Grätzel, Nature 353 (1991) 737. [2] M. Grätzel, Nature 414 (2001) 338. [3] 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 334 (2011) 629. [4] Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide, L. Han, Japan Journal of Applied Physics 45 (2006) 638. [5] M.K. Nazeeruddin, F.D. Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru, M. Grätzel, Journal of the American Chemical Society 127 (2005) 16835. [6] E. Bellingeri, D. Marre, L. Pellegrino, I. Pallecchi, G. Canu, M. Vignolo, C. Bernini, A.S. Siri, Superlattices and Microstructures 38 (2005) 446. [7] T. Yoshida, J. Zhang, D. Komatsu, S. Sawatani, H. Minoura, T. Pauporté, D. Lincot, T. Oekermann, D. Schlettwein, H. Tada, D. Wöhrle, K. Funabiki, M. Matsui, H. Miura, H. Yanagi, Advanced Functional Materials 19 (2009) 17. [8] T. Yoshida, M. Iwaya, H. Ando, T. Oekermann, K. Nonomura, D. Schlettwein, D. Wöhrle, H. Minoura, Chemical Communications (2004) 400. [9] K. Keis, J. Lindgren, S. Lindquist, A. Hagfeldt, Langmuir 16 (2000) 4688. [10] H. Horiuchi, R. Katoh, K. Hara, M. Yanagida, S. Murata, H. Arakawa, M. Tachiya, Journal of Physical Chemistry B 107 (2003) 2570. [11] K. Keis, E. Magnusson, H. Lindstrom, S. Lindquist, A. Hagfeldt, Solar Energy Materials and Solar Cells 73 (2002) 51. [12] D.P. Hagberg, T. Edvinsson, T. Marinado, G. Boschloo, A. Hagfeldt, L. Sun, Chemical Communications (2006) 2245. [13] S. Kim, J.K. Lee, S.O. Kang, J. Ko, J.H. Yum, S. Fantacci, F. De Angelis, D. Di Censo, M.K. Nazeeruddin, M. Grätzel, Journal of the American Chemical Society 128 (2006) 16701. [14] K.J. Jiang, J.B. Xia, N. Masaki, S. Noda, S. Yanagida, Inorganica Chimica Acta 361 (2008) 783. [15] Z. Jin, H. Masuda, N. Yamanaka, M. Minami, T. Nakamura, Y. Nishikitani, Journal of Physical Chemistry C 113 (2009) 2618. [16] K. Ichinose, Y. Kimikado, T. Yoshida, Electrochemistry 79 (2011) 146. [17] Y. Sakuragi, X.F. Wang, H. Miura, M. Matsui, T. Yoshida, Journal of Photochemistry and Photobiology A-Chemistry 216 (2010) 1. [18] T. Tani, S. Kikuchi, K. Hosoya, Bulletin of the Society of Scientific Photography of Japan 17 (1967) 24. [19] S. Hori, T. Yoshida, Materials Research Society Symposium Proceedings 1211 (2010) 40. [20] R. Zhang, J. Pan, E.P. Briggs, M. Thrash, L.L. Kerr, Solar Energy Materials and Solar Cells 92 (2008) 425. [21] T.P. Chou, Q. Zhang, G. Cao, Journal of Physical Chemistry C 111 (2007) 18804.
Acknowledgements This work was supported by Ministry of Education, Culture, Sports, Science and Technology, Program for Fostering Innovation (Global Type) “Tokai Region Nanotechnology Manufacturing Cluster” and 10002454-0 of New Energy and Industrial Technology Development Organization (NEDO) of Japan.
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Aggregation behavior of differently substituted Ru(II)-complex dyes as sensitizers for electrodeposited ZnO solar cells Asdim a , Keigo Ichinose a , Tomohiko Inomata b , Hideki Masuda b , Tsukasa Yoshida a,∗ a Center of Innovative Photovoltaic Systems (CIPS), Environmental and Renewable Energy Systems (ERES) Division, Graduate School of Engineering, Gifu University, Yanagido 1-1, Gifu 501-1193, Japan b Department of Material Science and Engineering, Graduate School of Engineering, Nagoya Institute of Technology, Showa-ku, Nagoya 466-8555, Japan
a r t i c l e
i n f o
Article history: Received 19 December 2011 Received in revised form 14 March 2012 Accepted 11 April 2012 Available online 7 June 2012 Keywords: Zinc oxide Electrodeposition Dye-sensitized solar cell Ru(II)-complex Aggregation
a b s t r a c t Properties of newly developed Ru(II)-complex dye, cis-bis(isothiocyanato) (2,2 -bipyridyl-4,4 dicarboxylato) (N-(4-butoxyphenyl)-N-2-pyridinyl-2-pyridinamine)-ruthenium(II) (J13), having a triarylamine ligand have been studied in comparison with cis-bis(isothiocyanato) bis(2,2 -bipyridyl-4,4 dicarboxylato)-ruthenium(II) (N3), as photosensitizers for solar cells employing electrodeposited porous ZnO thin films. Quantification of dye loading as well as change of the film morphology on extension of the soaking time in solutions of these dyes indicated spontaneous monolayer formation of the J13 dye, in contrast to the strongly aggregating N3 dye. Overloading of the N3 dye molecules not directly bound to ZnO caused decreased Jsc as well as worsening of F.F., so that the efficiency of the N3 cell created a peak with a moderate dipping time. On the other hand, the efficiency of the J13 cells simply saturated to a value of ca. 4% as the dye monolayer was formed, indicating its better chemical match with ZnO. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSSCs) are promising candidates of photovoltaic devices which can efficiently convert solar energy-toelectricity at substantially low cost [1,2]. Even though the highest conversion efficiency of 12.3% has recently been achieved on a DSSC employing titanium dioxide (TiO2 ) photoelectrode sensitized with zinc porphyrin dye having long alkyl substituent in combination with Co complex redox electrolyte [3], poly-pyridine Ru (II) complexes have traditionally been the best sensitizers to be combined with TiO2 [4,5]. The porous TiO2 electrode is usually prepared by sintering nanoparticles at a high temperature (450–500 ◦ C), so that the choice of the transparent substrate is limited to glass. In order to achieve versatility in application and also to reduce the production cost, use of plastic substrate is more preferable. Zinc oxide (ZnO) can often be synthesized at relatively low temperatures owing to its higher chemical reactivity than TiO2 . It has similar band positions but higher electron mobility than TiO2 [6], and thus be a promising alternative electrode material especially for realization of plastic DSSCs. We have developed a method to directly crystallize mesoporous ZnO thin films from water by use of
∗ Corresponding author. Present address: Department of Chemistry and Chemical Engineering, Faculty of Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan. Tel.: +81 58 293 2593; fax: +81 58 293 2594. E-mail addresses: [email protected], [email protected] (T. Yoshida). 1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jphotochem.2012.04.024
organic dye molecules such as eosin Y as a structure-directing agent (SDA) for cathodic electrodeposition of ZnO [7]. Because the entire process requires neither high temperature nor aggressive chemicals, soft electrode materials such as ITO coated PET films can be used as the substrate. Owing to its porous crystalline structure having both high crystallinity and high surface area, such ZnO thin films can perform as excellent photoelectrode materials to achieve high collection efficiency of photogenerated charge carriers [8]. When it was combined with an indoline dye, D149, a conversion efficiency of 5.6% could be achieved [7]. Achieving higher efficiency should then become possible by finding a dye that sensitizes ZnO as efficient as D149 and absorbs in a wider range. When Ru complexes known as the best sensitizer to date and has been the standard to TiO2 such as cis-bis(isothiocyanato) bis(2,2 bipyridyl-4,4 -dicarboxylato)-ruthenium(II), (hereafter called N3) [1] was combined to ZnO, only disappointingly low efficiencies were obtained [9–11]. Keis et al. revealed that the N3 dye acts as a too strong acid to the relatively soft ZnO and forms randomly structured Zn2+ /N3 aggregates that does not contribute to the photocurrent generation. Unmatched chemistry between ZnO and N3 is obviously the reason of the poor efficiency. That at the same time let us expect existence of other Ru complexes suited to ZnO to achieve high efficiencies, when formation of such aggregates is prevented. Recently, we have developed a series of new Ru complexes having triarylamine ligands as sensitizers for TiO2 . Triarylamine has a strong electron donating ability and has also been introduced to several organic dyes as sensitizers [12,13]. Among the dyes
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Asdim et al. / Journal of Photochemistry and Photobiology A: Chemistry 242 (2012) 67–71
Fig. 1. Structures of N3 (a) and J13 (b) molecules. The molar absorption coefficients (ε) of N3 and J13 are 14,000 M−1 cm−1 at 532 nm [14] and 7880 M−1 cm−1 at 537 nm in ethanol, respectively.
synthesized and tested, J13, shown in Fig. 1, has achieved a high efficiency () of 7.83% [15]. Since J13 has only two carboxylic acid groups as compared to four in N3, it could behave less aggressively than N3 on the ZnO surface. Such a motivation let us study the property of J13 dye as sensitizer for solar cells employing electrodeposited mesoporous ZnO in comparison with N3. We have given a focus especially to the formation of dye aggregates under controlled dipping periods and studied its influence to the solar cell performances. 2. Experimental F-doped SnO2 (FTO) coated glass (8 /, Asahi Glass) sheets were washed subsequently with detergent, pure water, acetone and 2-propanol. The FTO surface insulated with a photoresist masking except for a round hole with 6 mm (for DSSC electrodes) or 12 mm (for analysis of the films) diameter in the center. They were used in a configuration of a rotating disk electrode (RDE) to control mass transport for high homogeneity and reproducibility of the films. A home-made electrochemical system equipped with eight identical RDE in a single compartment cell was employed to produce 8 identical samples from one batch, so that precise comparison of dyes and their adsorption conditions was made possible. Porous ZnO thin films were prepared following the recipe reported in literature [16,17]. Briefly, after electrodepositing ZnO layer of 1 m thickness in a compact structure, porous ZnO was electrodeposited potentiostatically at −0.95 V (vs. Ag/AgCl) and ω = 500 rpm for 10 or 30 min (corresponding to 1.6 or 5.0 m thickness) from an aqueous solution (70 ◦ C) containing 5 mM ZnCl2 , 0.1 M KCl, 75 M eosin Y and saturated with O2 . Extraction of eosin Y by soaking the deposited films in a dilute KOH aqueous solution of pH 10.5 results in a porous structure [17]. ZnO thin films were sensitized by soaking them in 0.5 mM N3 (purity: 95%, Aldrich) and J13 dye solutions in tert-butyl alcohol/dimethylformamide (v/v = 1/1) mixture, at 60 ◦ C for controlled periods between 3 min and 8 h. The amount of adsorbed N3 and J13 was checked from the absorption spectrum of a solution dissolving the film in a 0.5 M NaOH solution in ethanol/water mixture (v/v = 1/1). The molar absorbance coefficient (ε) of N3 and J13 in the mixture solution was calculated to 12,300 M−1 cm−1 (at 510 nm) and 7300 M−1 cm−1 (at 520 nm), respectively. The change of the film morphology by dye adsorption was observed on a Hitachi S4800 field emission scanning electron microscope (FE-SEM). The dyed ZnO electrode and a Pt-sputtered FTO glass counter electrode were assembled into a sandwich cell using a Surlyn® film of 30 m thickness as a spacer. Electrolyte solution consisting of 0.1 M LiI (Wako), 0.05 M I2 (Wako), 0.6 M 1,2-dimethyl-3propylimidazolium iodide (Shikoku Chemicals Co.), and 0.5 M tert-butylpyridine (TBP, Nacalai tesque) in acetonitrile was filled by capillary action. I–V curves of the cells were measured by EKO MP-160 curve tracer under illumination with a simulated sun light (AM 1.5, 100 mW cm−2 ) generated by a Yamashita-Denso YSS150A. Photocurrent action spectra were measured on a Bunko-Keiki
CEP-2000 system under monochromatic light illumination with a constant photon flux of 5.0 × 1015 s−1 cm−2 . The active area of the cells was regulated to 0.2 cm2 using a photomask. 3. Results and discussion The cell properties were highly dependent on the soaking time of ZnO films in the dye solutions (Fig. 2). While the voltages were almost unchanged, short circuit current density (Jsc ) initially increased on extension of the soaking time due to increase of the adsorbed dye to improve the efficiency of light harvest. However, that of ZnO/N3 creates a maximum of ca. 8.5 mA cm−2 at 10 min and begin to decrease, while that of ZnO/J13 continues to increase and reaches a steady value of about 9 mA cm−2 . On the other hand, fill factor (F.F.) of the ZnO/N3 cell continues to worsen, while that of the ZnO/J13 cell is unchanged. As a consequence, the overall energy conversion efficiency () of the ZnO/J13 cell increases proportionally to the increase of Jsc and reaches a constant value of ca. 4%, whereas the highest efficiency of about 3% was reached for that soaked in N3 for 10 min and got worse for the longer dipping time. Worsening of the efficiency in the same trend, in fact even more seriously than the present example, has been reported in combination of nanoparticulate ZnO electrode with N3, and attributed to the overloading of the N3 dye to form aggregates [9]. Although such a problem of N3 could not be prevented by the use of electrodeposited ZnO, J13 appears to behave better than N3, being less sensitive to the soaking time. Fig. 3 compares the I–V curves of the ZnO/N3 and ZnO/J13 cells with the same soaking time of 1 h. The most prominent difference is the F.F., being much worse for the ZnO/N3 cell than ZnO/J13. The dark current of the ZnO/N3 cell is also much smaller than ZnO/J13, indicating a significantly suppressed electron transfer to I3 − ions in the electrolyte. The photocurrent action spectra of the ZnO/J13 and ZnO/N3 cells are shown in Fig. 4. The maximum IPCEs of about 58% and 70% are reached at 510 nm on ZnO/N3 and ZnO/J13 cells, respectively. IPCE is a product of the light harvesting efficiency
Fig. 2. Changes of conversion efficiency (circle, ), photocurrent density (triangle, Jsc ), and fill factor (square, F.F.) of the N3 (open symbols) and J13 (filled symbols) cells on soaking time of ZnO films in the dye solutions.
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of N3 abruptly increases and reaches about 4 times higher amount than that of J13 after 8 h of dipping. The saturation of J13 implies a spontaneous formation of a monolayer, while the behavior of N3 suggests formation of its aggregates. The apparent area occupied by a single dye molecule on the ZnO surface (Adye ) can be estimated from the amount of adsorbed dye (ndye ) as calculated by Eq. (2), Adye =
Fig. 3. I–V curves of the N3 (a) and J13 (b) cells employing ZnO films soaked in the dye solutions for 1 h, measured under illumination with an AM 1.5 stimulated sunlight (100 mW cm−2 ) (solid lines) and in the dark (dashed lines).
Fig. 4. Photocurrent action spectra of the N3 (a) and J13 (b) cells employing ZnO films soaked in the dye solutions for 1 h.
(LHE), the efficiency of charge separation on photoexcitation of dye bound to ZnO (inj ) and that of the collection of charge (coll ), IPCE = LHE × inj × coll
(1)
For the cells measured here, LHE practically reaches its saturation at the wavelength of light absorption maxima of these dyes. Since coll is expected to be the same for both cells as they employ the same electrode materials and electrolyte, the lower inj of N3 should be responsible to the lower IPCE. Dye molecules adsorbed as aggregates do absorb light but cannot inject electron to ZnO, leading to a decrease of Jsc despite of the increased LHE, so that inj of N3 becomes lower on higher dye loading. In order to study the differences of the aggregation behavior of N3 and J13, a thinner ZnO of 1.6 m was employed. The change of the amount of adsorbed dye for different soaking time is shown in Fig. 5. The amount of adsorbed N3 and J13 increase similarly up to 3 min. While that of J13 only slightly increase on extension of the dipping time and saturates at around 4 × 10−8 mol cm−2 , that
Fig. 5. The loading of the N3 and J13 dyes on soaking of the porous ZnO thin films (1.6 m thickness) in the dye solutions.
SZnO ndye NA
(2)
where NA is the Avogadro number (6.02 × 1023 mol−1 ) and SZnO is the total surface area of porous ZnO thin film used in the experiments, which is determined as 3.87 × 1016 nm2 cm−2 from the amount of adsorbed eosin Y and the ZnO surface area occupied by a single eosin Y molecule (3.32 nm2 [19]) since eosin Y is known to undergo monolayer adsorption on ZnO [18]. From nN3 and nJ13 after soaking for 8 h, AN3 and AJ13 are calculated as 0.423 and 1.42 nm2 , respectively. AN3 has been previously estimated as 1.6 nm2 for a monolayer of N3 on TiO2 [20]. Since the molecular structures of N3 and J13 are similar, similar values can be expected if monolayers of N3 and J13 are formed of ZnO. The clearly smaller value estimated for N3 indicates its aggregation adsorbed as multilayers and that of J13 close to 1.6 nm2 suggests a formation of a monolayer on ZnO. The morphology of the native porous ZnO thin film and those after dipping in dye solutions for 3 min and 8 h has been observed for their surface and cross section by SEM (Fig. 6). The porous nanostructure consisting of ZnO nanowire with ca. 10 nm thickness is well recognized for the native film (a and b). Dipping in the solution of J13 for 3 min does not cause a significant change in the overall structure (c and d), although slight reduction of the pore size can be recognized from a careful look of these pictures. Such a change of the morphology is probably associated with the surface adsorption of J13 molecules. On the contrary, dipping of the film in the N3 solution for 3 min causes a clear change of the morphology (e and f). The tips of ZnO nanowire can still be recognized but are obscured in the surface image. The cross section image clearly shows that the surface is covered with a thin layer of a shapeless solid, while inside of the film remains porous. This thin layer should be the aggregates of N3. Although similar amount of dyes was found for N3 and J13 after 3 min soaking, the way how they are adsorbed is totally different. The difference becomes significant after soaking for 8 h. While the structure of the film is unchanged, thus still being porous, for J13 (g and h), that for N3 is completely changed with a formation of a thick (ca. 0.5 m) layer of N3 aggregates on top of the porous ZnO layer (i and j). It is evident that N3 acts as an acid to continue to dissolve ZnO and the dissolved Zn2+ ion precipitates back as a randomly structured mixed aggregates with N3 onto the surface of the porous ZnO, rather than forming a dye multilayer [9,21]. On the other hand, the surface of ZnO is not attacked by J13 since it behaves as a milder acid than N3, and thus achieves a spontaneous formation of its monolayer on ZnO. The totally different adsorption behavior of N3 and J13 well explains the difference of the solar cell properties. The extra N3 molecules loaded as aggregates do not inject electrons but rather filter the light. Since the layer of N3 aggregates is formed on the outer surface of the porous ZnO, the opposite side of the light incidence, the filtering effect was not too severe. However, the presence of such a layer obviously blocks the transport of hole by the electrolyte as it clogs the pores. The severe worsening of F.F. can be understood as a consequence of the significant increase of hole transport resistance by prevention of electrolyte diffusion. None of such problems seem to occur in case of J13. As it spontaneously forms a dye monolayer on ZnO, Jsc simply increases on extension of the dipping time and F.F. remains constant. Unfortunately, the overall efficiency was not as high as that achieved by the same dye
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Fig. 6. Surface and cross section SEM images of ZnO thin films before dye adsorption (a and b), after dipping for 3 min in J13 (c and d) and N3 (e and f) solution, and for 8 h in J13 (g and h) and N3 (i and j) solution.
on TiO2 . The lower efficiency is attributed to the smaller current due to the limited IPCE as well as the limited light harvesting capability of J13 having a relatively small molar absorption coefficient, and also to the smaller voltage of the ZnO cells for which the effect of TBP added to the electrolyte to increase the voltage appear smaller than in the TiO2 cells. However, the present study evidently shows
the importance of “right chemistry” between the oxide electrode and dye molecules to allow spontaneous formation of dye monolayer for the efficient injection of electron from the dye molecule. It is important to pay attention to the chemical properties of dye molecules aside from their energy levels, when we want to develop new sensitizer dyes, especially in case of ZnO solar cells.
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4. Conclusion
References
The J13 Ru(II)-complex has been evaluated as a sensitizer for ZnO solar cell in comparison to the N3 dye. Analysis of the quantity of adsorbed dye as well as the change of the surface morphology clearly indicated strong aggregation of N3, while J13 underwent spontaneous formation of a monolayer. As a consequence, Jsc of the N3 cell creates a peak with a moderate time of dipping in the dye solution and it continues to decrease with excessive dye adsorption as it leads to a loading of dye molecules not directly bound to ZnO and thus cannot inject electron from its excited state. In addition, extended dipping of ZnO in N3 solution resulted in a formation of a thick outer layer of aggregates that hinder hole transport to significantly worsen F.F. None of such problems were observed for J13, for which Jsc continued to increase to a steady value and F.F. remained constant on extension of the dipping time. Although the efficiency of ca. 4% eventually achieved with the J13 dye is not satisfactorily high as it is still worse than the highest efficiency achieved for the ZnO solar cell in combination with organic dye sensitizer, the present study clearly indicates the importance of the right chemistry between ZnO and dye molecules to draw the best performance out of the system. Tuning of the chemical property in addition to the energy structure is therefore an important strategy to develop good photosensitizer for ZnO solar cells.
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Acknowledgements This work was supported by Ministry of Education, Culture, Sports, Science and Technology, Program for Fostering Innovation (Global Type) “Tokai Region Nanotechnology Manufacturing Cluster” and 10002454-0 of New Energy and Industrial Technology Development Organization (NEDO) of Japan.