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Probing mechanism of dye double layer formation from dye-cocktail solution for dye-sensitized solar cells
Thin Solid Films 519 (2010) 1087–1092
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Probing mechanism of dye double layer formation from dye-cocktail solution for dye-sensitized solar cells Yuhei Ogomi, Shyam S. Pandey ⁎, Shunta Kimura, Shuzi Hayase Graduate School of Life Science and System Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu, Kitakyushu 808-0196, Japan
a r t i c l e
i n f o
Available online 20 August 2010 Keywords: Dye-sensitized solar cells Nanocrystalline TiO2 Dye cocktail Dye double layer Rate of adsorption/desorption
a b s t r a c t Absorption of photon in wide wavelength region is an important requirement for the enhancement of photoconversion efficiency of dye-sensitized solar cells (DSSC). Lack of photon absorption from visible to NIR wavelength region by a single dye requires the use of plural dyes for the panchromatic sensitization of nanoporous TiO2. To our incredible surprise, when a dye cocktail of organic dye NK3705 and inorganic ruthenium based dye Z907 was implied for the dye adsorption, it led to the formation of dye double layer in spite of random arrangement of two dyes as evidenced from confocal laser microscopic investigations. Investigation pertaining to the evaluation of rate of dye adsorption and dye desorption for different organic and inorganic sensitizing dyes suggests that a combination of one dye with faster diffusion along with weak binding on TiO2 surface and another dye with slow diffusion along with strong binding leads to the formation of dye double layer from a dye mixture by a simple dipping process. © 2010 Elsevier B.V. All rights reserved.
1. Introduction More and more energy demand associated with ever increasing population and limited supply of energy from fossil fuels along with their harmful impact on our environment has made it necessary to pay a good deal of attention towards the search for safe and clean energy resources [1]. Conventional silicon based solar cells which are currently being used for harvesting the solar energy require costly and energy consuming processing and therefore, are unable to reach at common mass level implementation. In this context, dye-sensitized solar cells (DSSC) based on nanoporous TiO2 along with ruthenium/organic sensitizers are expected to play their pivotal role towards the development of less expensive and environment friendly photovoltaic devices. In spite of reproducible and respectable photoconversion efficiency by DSSC over 10% by ruthenium based inorganic sensitizers [2–4], their photon harvesting mainly in visible wavelength region are currently one of the major stumbling blocks towards the further enhancement in the photoconversion efficiency. One of the most important and challenging factors attributed to the lower efficiency of the DSSC is the lower optical absorption window (400–800 nm) of organic dyes, which is smaller than that of crystalline silicon (500–1100 nm) [5]. Amongst the key elements of DSSC, sensitizing dyes play the most important role in photon harvesting and an optimal sensitizer for such application should be ⁎ Corresponding author. Tel.: + 81 93 695 6044; fax: + 81 93 695 6005. E-mail address: [email protected] (S.S. Pandey). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.049
panchromatic and absorb all the light from visible to NIR wavelength region, which is presently difficult using a single dye. A probable solution for this problem is the use of two different dyes absorbing in the lower and higher wavelength regions and there are several reports about the enhancement of the DSSC performances using two are more dyes in a dye-cocktail solution [6–8]. It is widely believed that in DSSC based on dye-cocktail, dyes under investigation get adsorbed randomly [9–11]. In the random dye adsorption, unfavorable interactions between the neighboring dye molecules cannot be avoided which often lead to decrease in the photovoltaic performance. In this context, dye double layer structure has a significant advantage over the dye-cocktail to overcome the problem of unfavorable inter-dye interactions. Recently, Inakazu et al have proposed the extension of optical absorption window by selective adsorption of two dyes on nanoporous titania under pressurized CO2 atmosphere leading to the enhancement in photoconversion efficiency [12]. In another approach, Koo et al proposed a heterosensitizer junction DSSC using two dyes which led to the enhancement in the photon harvesting and increased photovoltaic performance [13]. It is worthwhile to mention that based on confocal laser microscopic investigation, Noma et al [14] have reported the fabrication of dye double layer structure from dye cocktail solution by judicious selection of two dyes NK3705 (organic sensitizer) and Z907 (inorganic sensitizer). This made us to investigate the underlying mechanism for this interesting observation which led the possibility of formation of dye double layer from a dye mixture on the nanoporous TiO2 by simple dipping process.
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2. Experimental 2.1. Materials and methods Different organic dyes viz. NK3705 (Hayashibara Biochemical Laboratories Inc, Japan), indolene dyes such as D77, D102, D131, D149 (Mitsubishi Paper Mills Ltd., Japan) and ruthenium based inorganic dyes such as N3, N719, N749 (black dye or BD) and Z907 (Solaronix SA) were purchased and used as received without further purification. Structures of these dyes along with the schematic structure of the DSSC have been shown in the Fig. 1. Adsorption of dyes in a solution of dye or dye mixture was conducted for different time intervals at room
temperature. DSSC efficiency was measured using simulated light irradiation of 100 mW/cm2 at AM 1.5. Dye adsorption on nanoporous TiO2 surface from cocktail solution was analyzed by digital imaging while the distribution of dyes in the case of dye double layer structure on TiO2 electrode after dye adsorption was investigated using confocal laser microscope (Nikon ECLIPSE TE 2000-E). In the measurement of dye distribution, focus was shifted by 1 μm step from top surface to the bottom of TiO2 layer with simultaneous monitoring of the fluorescence of adsorbed dye. For the investigation of speed of adsorption of different dyes on TiO2 surface and desorption of adsorbed dyes from the TiO2 surface, HT-paste of nanoporous TiO2 (Solaronix SA) having thickness of about
Fig. 1. (a) Molecular Structure of organic and inorganic dye sensitizers used in the present investigation and (b) Schematic of DSSC with single dye layer and dye double layer.
Y. Ogomi et al. / Thin Solid Films 519 (2010) 1087–1092
4 μm coated on glass by screen printing followed by its backing at 450 °C for 30 min was used. Final thickness of the TiO2 was later on accurately estimated using surface profiler for the calculation of rates of adsorption and desorption. Estimation of adsorption speed of the dye on titania surface was conducted by dipping the TiO2 coated glass plates in 0.3 mM of dye solution for different time intervals followed measurement of absorption spectrum using UV–visible spectrophotometer (JSACO model V550). Termination of dye adsorption was conducted when there was no further increase in the absorbance. Estimation of the dye desorption speed was conducted by detaching the dye from TiO2 surface in 50 mM solution of NaOH in t-butanol/ acetonitrile/ethanol/H2O (1:1:1:1 v/v) for different time intervals and was monitored spectrophotometrically. 2.2. DSSC fabrication and measurement of cell performance DSSC were fabricated using Ti-Nanoxide D paste (Solaronix SA) which was coated on a Low E glass (Nippon Sheet Glass Co., Ltd.) by a doctor-blade. The substrate was then baked at 450 °C to fabricate nanoporous TiO2 layer for dye adsorption. Adsorption of dyes on the nanoporous TiO2 was performed by dipping in the solution containing dye or dye mixture having a fixed dye concentration of 0.25 mM. A Pt sputtered SnO2/F layered glass substrate was employed as the counter electrode. Electrolyte (WWS30) containing LiI (500 mM), Iodine (50 mM), t-butylpyridine (580 mM), MeEtIm-DCA (ethylmethyl imidazolium dicyanoimide)(4:6 wt/wt) (600 mM) in acetonitrile was used to fabricate the DSSC. A Himilan film (Mitsui-DuPont Polychemical Co., Ltd.) of 25 μm thickness was used as a spacer. The cell area was 0.25 cm2. Performance of the Solar cells was evaluated using a photo-mask on the solar cell in order to remove the effect of the optical reflection under the irradiation of 100 mW/cm2 at AM 1.5. 3. Results and discussion 3.1. Verification of dye double layer formation from dye cocktail of Z907 and NK3705 Panchromatic sensitization is an important requirement for enhancing the photovoltaic performance by photon harvesting in the wide wavelength region. Lack of light absorption in the wide wavelength region by a single dye makes it necessary to use a mixture of two or more dyes for panchromatic sensitization of nanoporous TiO2. There are several reports pertaining to the utilization of this concept towards the enhancement of photoconversion efficiency of DSSC [15– 17]. It is a common belief that in the case of use of plural dyes for the dye sensitization, dyes are distributed randomly on the TiO2 surface. To our surprise, when a dye mixture of Z907 and NK3705 was used for the sensitization of nanoporous TiO2 during the optimization of dye cocktail towards the fabrication of efficient DSSC, there was clear contrast in the colour of dye adsorbed TiO2 electrode when it was observed from TiO2 side and glass facing side as shown in the Fig. 2 (a). This could be only possible when these two dyes under investigation are adsorbed in the bi-layer fashion rather than random adsorption since mixed random distribution of dyes on TiO2 would lead to the observation of same colour from both of the sides. To get further insight about the distribution of dyes on the nanoporous TiO2, we used confocal laser microscopy. We would like to mention here that the organic dye NK3705 shows a strong fluorescence between 485–540 nm when it was excited at its absorption maximum (408 nm) while inorganic ruthenium dye Z907 does not show any fluorescence. The fluorescence images of nanoporous TiO2 adsorbed from the dye mixture of NK3705 and Z907 has been shown in the Fig. 2 (b). As can be seen from this figure that thickness of NK3705 layer was 15 μm when dye adsorption was carried out for 2 days which decreases to the thickness of 12 μm after 6 days of dye adsorption out of 19 μm total thickness of nanoporous
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TiO2 used for the dye adsorption. Our confocal laser microscope is attached with a stage having X, Y and Z controller interfaced with personnel computer where the distribution of fluorescence of dye NK3705 on TiO2 surface was mapped by shifting the focus at the step of 1 μm. Fluorescence distribution of NK3705 on 19 μm thick TiO2 surface for adsorption of dyes from dye cocktail solution is shown in the Fig. 3. The excitation wavelength was 408 nm corresponding to the absorption maximum of NK3705. It was interesting to note that the fluorescence emission of NK3705 was observed in the almost all of the TiO2 thickness when dye adsorption from cocktail solution was conducted for 1–2 days. A perusal of the Fig. 3 clearly corroborates that with increasing the dye adsorption time, NK3705 fluorescence shifted further towards the bottom (glass) side and the fluorescence intensity significantly decreased after 10 days of dye adsorption. This can be explained by the fact that during initial phase of dye adsorption from the dye cocktail solution, NK 3705 get swiftly adsorbed on the entire TiO2 surface owing to its small molecular size and fast diffusion through the nano pores. With the passing adsorption time, it was gradually getting replaced with the Z907 from the top to bottom. It is, therefore, possible to control the distribution of bi-layer by managing the time interval of dye adsorption from the dye cocktail of Z907 and NK3705. 3.2. Photovoltaic performance of dye-sensitized solar cells Photovoltaic performance for DSSC based on single dye and dye adsorbed from dye cocktail having dye double layer structure has been shown in the Fig. 4 (a). It can be clearly seen from this figure and Table 1 exhibiting photovoltaic performance parameters that photoconversion efficiency for DSSC based on dye double layer architecture (7.1%) is better than that of photovoltaic performances shown by DSSC based on single dyes viz. Z907 (6.9%) and NK3705 (1.8%). From Table 1 exhibiting the DSSC parameters it can be seen that main factor responsible for the enhancement of the efficiency of DSSC based dye double layer is Jsc since all other cell parameters are not much altered. The increased performance of DSSC having dye double structure can be explained by widening of photon harvesting spectral range as compared to photon harvesting by the DSSC based on single dye which can be seen from the photo action spectra as shown in the Fig. 4 (b). IPCE (incident photon to current efficiency) also known as photo action spectrum exhibits that NK3705 shows photon harvesting mainly in the lower wavelength region of 300–500 nm while Z907 absorbs effectively from 400–800 nm and lacks the efficient photon harvesting in the lower wavelength region. DSSC based on dye double layer architecture having both of the dyes i.e. Z907 and NK3705 are able to harvest the photon efficiently in both of the lower as well as higher wavelength region and can be attributed to the enhancement in over all power conversion efficiency. It has been found that efficiency of the DSSC based on dye cocktail increases with dipping time and best efficiency was obtained when dye adsorption was carried out for the 6 days at room temperature. More longer dipping times leads to gradual removal of NK3705 by Z907 leading to a little decrease in the efficiency. The best photoconversion efficiency was obtained for the DSSC having dye double layer structure where the thickness of Z907 was more than 8 μm while the thickness of NK3705 was less than 12 μm. 3.3. Elucidation of mechanism for dye double layer formation The interesting observation of dye double layer formation from the dye cocktail of Z907 and NK3705 by simply dipping the nanoporous TiO2 in dye mixture without using any special dye application techniques fascinated us to explore the reason for this observation and propose a possible mechanism. As has been explained earlier that in the initial time frame of dye adsorption from the cocktail solution, mainly NK3705 get adsorbed on the TiO2 surface which is gradually replaced by dye Z907 and after about 10 days of dye adsorption nearly
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Fig. 2. (a) Digital photographic images of the dye adsorption on TiO2 (19 μm thick) from single dye and dye cocktail solutions and (b) cross-sectional view of dye adsorption from dye cocktail solution using confocal laser scanning microscope.
Fig. 3. Fluorescence intensity distribution mapping of NK3705 on 19 μm thick nanoporous TiO2 utilized for dye adsorption from dye cocktail of Z907 and NK3705.
Y. Ogomi et al. / Thin Solid Films 519 (2010) 1087–1092
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Fig. 5. Rate of dye adsorption on nanoporous TiO2 for different dyes (red colour) and rate of desorption of dyes from the surface (green colour) used in the present investigation. TiO2 film thickness was about 4 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web of this article.)
Fig. 4. (a) Photovoltaic characteristics of DSSC based on TiO2 sensitized with a solution of single dye and dye cocktail under 100 mA/cm2 simulated solar irradiation at AM 1.5 and (b) IPCE curves for DSSC based on TiO2 sensitized with single dye and dye cocktail solution.
all of NK3705 dye molecules get replaced. To obtain a further insight in detail about this observation, rate of dye adsorption on TiO2 surface along with its rate of desorption from the surface for the organic and inorganic dyes has been shown in the Fig. 5. It is clearly evident from the figure that rate of adsorption of NK3705 on the nanoporous TiO2 surface is faster than that of Z907 which could be attributed to its smaller molecular size leading to swift diffusion through the nanopores from top to the bottom. At the same time rate of desorption of NK3705 is more than 4 times faster than that of Z907 indicates the weaker binding of NK3705 with the TiO2 as compared to that of Z907. Therefore, in the initial time domain of dye adsorption first NK3705 get adsorbed first but owing to its relatively weak binding, get replaced by Z907 gradually with the increasing time of dye adsorption from the cocktail solution leading to dye double layer formation. It, therefore, implies that it is possible to adsorb two dyes in dye double layer architecture from a mixed dye solution provided that if we judiciously select one dye having faster adsorption and weak binding
Table 1 Photovoltaic parameters of DSSC based on TiO2 sensitized with the solution of respective single dye and dye cocktail. Dye
Jsc (mA/cm2)
Voc (V)
Fill factor (FF)
Efficiency
NK3705 Z907 Dye cocktail
3.55 15.05 15.68
0.69 0.69 0.70
0.71 0.67 0.64
1.8% 6.9% 7.1%
with TiO2 while another dye having relatively slower diffusion along with stronger binding. To further verify the condition for dye double layer formation from a dye cocktail solution, a new set of dye mixture i.e. a solution of organic dye D102 and an inorganic dye black dye (BD) was utilized for the dye adsorption on the TiO2 surface. Sensitizing dye D102 shows much faster adsorption (about 3 times) as compared BD but contrary to the first case (Z907 and NK3705 system), it exhibits more than four times stronger binding on TiO2 surface as compared to BD. Fig. 6 (a) shows the visual color changes from surfaces facing glass as well as TiO2 after dye adsorption from a dye cocktail solution of D102 and BD. Simple digital imaging of the dye adsorption reveal that it does not lead to the dye double layer formation owing to the observation of nearly the same color from both of the sides. This can be explained by the fact that fast adsorbing 1st dye having strong binding with TiO2 is unable to be replaced by the 2nd dye and therefore, such a dye cocktail system is unable to form dye double layer. In the another set of dye adsorption from cocktail solution a dye mixture containing organic dye D77 and inorganic ruthenium based dye BD was selected for the dye adsorption on the nanoporous TiO2 surface. As can be clearly seen from the Fig. 5 that dye D77 exhibits much faster adsorption as compared to that of BD but rate of desorption from the TiO2 surface is approximately similar. Thus both of dyes used in the present cocktail system have nearly same binding force with the TiO2 molecules on the titania surface. A perusal of the digital imaging of the dyes adsorbed from both of the TiO2 and glass facing sides shown in the Fig. 6 (b) again exhibits approximately similar color indicating that both of the dyes have been adsorbed randomly and there is no dye double layer formation from the dye cocktail solution containing D77 and BD. These experiment using such a dye cocktail systems reveal that in the case a dye cocktail system where the 1st dye having faster rate of adsorption with stronger or nearly similar binding strength with TiO2 as compared to the 2nd dye having slower rate of adsorption does not lead to the dye double layer formation. 4. Conclusion Dye double layer formation on nanoporous TiO2 surface from a dye cocktail solution of two dyes viz. NK3705 and Z907 has been successfully demonstrated as evidenced by visual colour observations and confocal laser microscopic investigations. A perusal of photo action spectra for DSSC based on dye double layer shows photon absorption in relatively wider wavelength region as compared to that of DSSC based on single dye leading to enhanced photovoltaic performance. Photovoltaic performance evaluation of DSSC consisted of dye double layer of NK3705 and Z907 adsorbed from dye cocktail solution gave a short circuit current density of 15.68 mA/cm2, an open
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Fig. 6. (a) Digital photographic images of the dye adsorption on TiO2 (4 μm thick) from single dye and dye cocktail solutions consisting of two dyes D102 and BD while (b) same with different dye cocktail system containing dye D77 and BD.
circuit voltage of 0.70 V and a fill factor of 0.64 corresponding to an overall power conversion efficiency of 7.1% which is higher than that of DSSC based on single dye. Investigation pertaining to the speed of dye adsorption on TiO2 surface and its rate of desorption from the titania surface reveal that a combination of two dyes having faster adsorption with weak binding force and slower adsorption with stronger binding leads to dye double layer formation from the dye cocktail. Acknowledgement Authors would like to acknowledge the financial support from the New Energy Industrial Technology Development Organization (NEDO), Japan. One of the authors Yuhei Ogomi would like to thank Fukuoka Industry Science and Technology Foundation under knowledge cluster project for financial support. References [1] Intergovernmental Panel on Climate Changes (IPCC), Fourth Assessment ReportClimate Change, 2007 http://www.ipcc-wg2.org (accessed Feb. 2010). [2] M. Gratzel, Inorg. Chem. 44 (2005) 6841.
[3] M.K. Nazeeruddin, P. Pechy, T. Renouard, S.M. Zakeeruddin, R. Humphry-Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia, G.B. Deacon, C.A. Bignozzi, M. Gratzel, J. Am. Chem. Soc. 123 (2001) 1613. [4] Z.-S. Wang, T. Yamaguchi, H. Sugihara, H. Arakawa, Langmuir 21 (2005) 4272. [5] G. Smestad, Sol. Energy Mater. Sol. Cells 32 (1994) 273. [6] R.Y. Ogura, S. Nakane, M. Morooka, M. Orihashi, Y. Suzuki, K. Noda, Appl. Phys. Lett. 94 (2009) 073308. [7] J.H. Yum, S.R. Jang, P. Walter, T. Geiger, F. Nuesch, S. Kim, J. Ko, M. Gratzel, M.K. Nazeeruddin, Chem. Commun. (2007) 4680. [8] M. Guo, P. Diao, Y.J. Ren, F. Meng, H. Tian, S.M. Kai, Sol. Energy Mater. Sol. Cells 88 (2005) 23. [9] A. Ehret, L. Stuhi, M.T. Spitler, J. Phys. Chem. B 105 (2001) 9960. [10] K. Sayama, S. Tsukagoshi, T. Mori, K. Hara, Y. Ohga, A. Shinpou, Y. Abe, S. Suga, H. Arakawa, Sol. Energy Mater. Sol. Cells 80 (2003) 47. [11] Y. Chen, Z. Zeng, C. Li, W. Wang, X. Wang, B. Zhang, New J. Chem. 29 (2005) 773. [12] F. Inakazu, Y. Noma, Y. Ogomi, S. Hayase, Appl. Phys. Lett. 93 (2008) 093304. [13] H.J. Koo, K. Kim, N.G. Park, S. Hwang, C. Park, C. Kim, Appl. Phys. Lett. 92 (2008) 142103. [14] Y. Noma, K. Iizuka, Y. Ogomi, S.S. Pandey, S. Hayase, Jpn. J. Appl. Phys. 48 (2009) 020213. [15] H. Choi, S. Kim, S.O. Kang, J. Ko, M.S. Kang, J.N. Clifford, A. Forneli, E. Palomares, M.K. Nazeeruddin, M. Gratzel, Angew. Chem. 120 (2008) 8383. [16] J.J. Cid, J.H. Yum, S.R. Jang, M.K. Nazeeruddin, E.M. Ferrero, E. Palomares, J. Ko, M. Gratzel, T. Torres, Angew. Chem. Int. Ed. 46 (2007) 8358. [17] V.P.S. Pereraa, P.K.D.D.P. Pitigalaa, M.K.I. Senevirathnea, K. Tennakone, Sol. Energy Mater. Sol. Cells 85 (2005) 91.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Probing mechanism of dye double layer formation from dye-cocktail solution for dye-sensitized solar cells Yuhei Ogomi, Shyam S. Pandey ⁎, Shunta Kimura, Shuzi Hayase Graduate School of Life Science and System Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu, Kitakyushu 808-0196, Japan
a r t i c l e
i n f o
Available online 20 August 2010 Keywords: Dye-sensitized solar cells Nanocrystalline TiO2 Dye cocktail Dye double layer Rate of adsorption/desorption
a b s t r a c t Absorption of photon in wide wavelength region is an important requirement for the enhancement of photoconversion efficiency of dye-sensitized solar cells (DSSC). Lack of photon absorption from visible to NIR wavelength region by a single dye requires the use of plural dyes for the panchromatic sensitization of nanoporous TiO2. To our incredible surprise, when a dye cocktail of organic dye NK3705 and inorganic ruthenium based dye Z907 was implied for the dye adsorption, it led to the formation of dye double layer in spite of random arrangement of two dyes as evidenced from confocal laser microscopic investigations. Investigation pertaining to the evaluation of rate of dye adsorption and dye desorption for different organic and inorganic sensitizing dyes suggests that a combination of one dye with faster diffusion along with weak binding on TiO2 surface and another dye with slow diffusion along with strong binding leads to the formation of dye double layer from a dye mixture by a simple dipping process. © 2010 Elsevier B.V. All rights reserved.
1. Introduction More and more energy demand associated with ever increasing population and limited supply of energy from fossil fuels along with their harmful impact on our environment has made it necessary to pay a good deal of attention towards the search for safe and clean energy resources [1]. Conventional silicon based solar cells which are currently being used for harvesting the solar energy require costly and energy consuming processing and therefore, are unable to reach at common mass level implementation. In this context, dye-sensitized solar cells (DSSC) based on nanoporous TiO2 along with ruthenium/organic sensitizers are expected to play their pivotal role towards the development of less expensive and environment friendly photovoltaic devices. In spite of reproducible and respectable photoconversion efficiency by DSSC over 10% by ruthenium based inorganic sensitizers [2–4], their photon harvesting mainly in visible wavelength region are currently one of the major stumbling blocks towards the further enhancement in the photoconversion efficiency. One of the most important and challenging factors attributed to the lower efficiency of the DSSC is the lower optical absorption window (400–800 nm) of organic dyes, which is smaller than that of crystalline silicon (500–1100 nm) [5]. Amongst the key elements of DSSC, sensitizing dyes play the most important role in photon harvesting and an optimal sensitizer for such application should be ⁎ Corresponding author. Tel.: + 81 93 695 6044; fax: + 81 93 695 6005. E-mail address: [email protected] (S.S. Pandey). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.049
panchromatic and absorb all the light from visible to NIR wavelength region, which is presently difficult using a single dye. A probable solution for this problem is the use of two different dyes absorbing in the lower and higher wavelength regions and there are several reports about the enhancement of the DSSC performances using two are more dyes in a dye-cocktail solution [6–8]. It is widely believed that in DSSC based on dye-cocktail, dyes under investigation get adsorbed randomly [9–11]. In the random dye adsorption, unfavorable interactions between the neighboring dye molecules cannot be avoided which often lead to decrease in the photovoltaic performance. In this context, dye double layer structure has a significant advantage over the dye-cocktail to overcome the problem of unfavorable inter-dye interactions. Recently, Inakazu et al have proposed the extension of optical absorption window by selective adsorption of two dyes on nanoporous titania under pressurized CO2 atmosphere leading to the enhancement in photoconversion efficiency [12]. In another approach, Koo et al proposed a heterosensitizer junction DSSC using two dyes which led to the enhancement in the photon harvesting and increased photovoltaic performance [13]. It is worthwhile to mention that based on confocal laser microscopic investigation, Noma et al [14] have reported the fabrication of dye double layer structure from dye cocktail solution by judicious selection of two dyes NK3705 (organic sensitizer) and Z907 (inorganic sensitizer). This made us to investigate the underlying mechanism for this interesting observation which led the possibility of formation of dye double layer from a dye mixture on the nanoporous TiO2 by simple dipping process.
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2. Experimental 2.1. Materials and methods Different organic dyes viz. NK3705 (Hayashibara Biochemical Laboratories Inc, Japan), indolene dyes such as D77, D102, D131, D149 (Mitsubishi Paper Mills Ltd., Japan) and ruthenium based inorganic dyes such as N3, N719, N749 (black dye or BD) and Z907 (Solaronix SA) were purchased and used as received without further purification. Structures of these dyes along with the schematic structure of the DSSC have been shown in the Fig. 1. Adsorption of dyes in a solution of dye or dye mixture was conducted for different time intervals at room
temperature. DSSC efficiency was measured using simulated light irradiation of 100 mW/cm2 at AM 1.5. Dye adsorption on nanoporous TiO2 surface from cocktail solution was analyzed by digital imaging while the distribution of dyes in the case of dye double layer structure on TiO2 electrode after dye adsorption was investigated using confocal laser microscope (Nikon ECLIPSE TE 2000-E). In the measurement of dye distribution, focus was shifted by 1 μm step from top surface to the bottom of TiO2 layer with simultaneous monitoring of the fluorescence of adsorbed dye. For the investigation of speed of adsorption of different dyes on TiO2 surface and desorption of adsorbed dyes from the TiO2 surface, HT-paste of nanoporous TiO2 (Solaronix SA) having thickness of about
Fig. 1. (a) Molecular Structure of organic and inorganic dye sensitizers used in the present investigation and (b) Schematic of DSSC with single dye layer and dye double layer.
Y. Ogomi et al. / Thin Solid Films 519 (2010) 1087–1092
4 μm coated on glass by screen printing followed by its backing at 450 °C for 30 min was used. Final thickness of the TiO2 was later on accurately estimated using surface profiler for the calculation of rates of adsorption and desorption. Estimation of adsorption speed of the dye on titania surface was conducted by dipping the TiO2 coated glass plates in 0.3 mM of dye solution for different time intervals followed measurement of absorption spectrum using UV–visible spectrophotometer (JSACO model V550). Termination of dye adsorption was conducted when there was no further increase in the absorbance. Estimation of the dye desorption speed was conducted by detaching the dye from TiO2 surface in 50 mM solution of NaOH in t-butanol/ acetonitrile/ethanol/H2O (1:1:1:1 v/v) for different time intervals and was monitored spectrophotometrically. 2.2. DSSC fabrication and measurement of cell performance DSSC were fabricated using Ti-Nanoxide D paste (Solaronix SA) which was coated on a Low E glass (Nippon Sheet Glass Co., Ltd.) by a doctor-blade. The substrate was then baked at 450 °C to fabricate nanoporous TiO2 layer for dye adsorption. Adsorption of dyes on the nanoporous TiO2 was performed by dipping in the solution containing dye or dye mixture having a fixed dye concentration of 0.25 mM. A Pt sputtered SnO2/F layered glass substrate was employed as the counter electrode. Electrolyte (WWS30) containing LiI (500 mM), Iodine (50 mM), t-butylpyridine (580 mM), MeEtIm-DCA (ethylmethyl imidazolium dicyanoimide)(4:6 wt/wt) (600 mM) in acetonitrile was used to fabricate the DSSC. A Himilan film (Mitsui-DuPont Polychemical Co., Ltd.) of 25 μm thickness was used as a spacer. The cell area was 0.25 cm2. Performance of the Solar cells was evaluated using a photo-mask on the solar cell in order to remove the effect of the optical reflection under the irradiation of 100 mW/cm2 at AM 1.5. 3. Results and discussion 3.1. Verification of dye double layer formation from dye cocktail of Z907 and NK3705 Panchromatic sensitization is an important requirement for enhancing the photovoltaic performance by photon harvesting in the wide wavelength region. Lack of light absorption in the wide wavelength region by a single dye makes it necessary to use a mixture of two or more dyes for panchromatic sensitization of nanoporous TiO2. There are several reports pertaining to the utilization of this concept towards the enhancement of photoconversion efficiency of DSSC [15– 17]. It is a common belief that in the case of use of plural dyes for the dye sensitization, dyes are distributed randomly on the TiO2 surface. To our surprise, when a dye mixture of Z907 and NK3705 was used for the sensitization of nanoporous TiO2 during the optimization of dye cocktail towards the fabrication of efficient DSSC, there was clear contrast in the colour of dye adsorbed TiO2 electrode when it was observed from TiO2 side and glass facing side as shown in the Fig. 2 (a). This could be only possible when these two dyes under investigation are adsorbed in the bi-layer fashion rather than random adsorption since mixed random distribution of dyes on TiO2 would lead to the observation of same colour from both of the sides. To get further insight about the distribution of dyes on the nanoporous TiO2, we used confocal laser microscopy. We would like to mention here that the organic dye NK3705 shows a strong fluorescence between 485–540 nm when it was excited at its absorption maximum (408 nm) while inorganic ruthenium dye Z907 does not show any fluorescence. The fluorescence images of nanoporous TiO2 adsorbed from the dye mixture of NK3705 and Z907 has been shown in the Fig. 2 (b). As can be seen from this figure that thickness of NK3705 layer was 15 μm when dye adsorption was carried out for 2 days which decreases to the thickness of 12 μm after 6 days of dye adsorption out of 19 μm total thickness of nanoporous
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TiO2 used for the dye adsorption. Our confocal laser microscope is attached with a stage having X, Y and Z controller interfaced with personnel computer where the distribution of fluorescence of dye NK3705 on TiO2 surface was mapped by shifting the focus at the step of 1 μm. Fluorescence distribution of NK3705 on 19 μm thick TiO2 surface for adsorption of dyes from dye cocktail solution is shown in the Fig. 3. The excitation wavelength was 408 nm corresponding to the absorption maximum of NK3705. It was interesting to note that the fluorescence emission of NK3705 was observed in the almost all of the TiO2 thickness when dye adsorption from cocktail solution was conducted for 1–2 days. A perusal of the Fig. 3 clearly corroborates that with increasing the dye adsorption time, NK3705 fluorescence shifted further towards the bottom (glass) side and the fluorescence intensity significantly decreased after 10 days of dye adsorption. This can be explained by the fact that during initial phase of dye adsorption from the dye cocktail solution, NK 3705 get swiftly adsorbed on the entire TiO2 surface owing to its small molecular size and fast diffusion through the nano pores. With the passing adsorption time, it was gradually getting replaced with the Z907 from the top to bottom. It is, therefore, possible to control the distribution of bi-layer by managing the time interval of dye adsorption from the dye cocktail of Z907 and NK3705. 3.2. Photovoltaic performance of dye-sensitized solar cells Photovoltaic performance for DSSC based on single dye and dye adsorbed from dye cocktail having dye double layer structure has been shown in the Fig. 4 (a). It can be clearly seen from this figure and Table 1 exhibiting photovoltaic performance parameters that photoconversion efficiency for DSSC based on dye double layer architecture (7.1%) is better than that of photovoltaic performances shown by DSSC based on single dyes viz. Z907 (6.9%) and NK3705 (1.8%). From Table 1 exhibiting the DSSC parameters it can be seen that main factor responsible for the enhancement of the efficiency of DSSC based dye double layer is Jsc since all other cell parameters are not much altered. The increased performance of DSSC having dye double structure can be explained by widening of photon harvesting spectral range as compared to photon harvesting by the DSSC based on single dye which can be seen from the photo action spectra as shown in the Fig. 4 (b). IPCE (incident photon to current efficiency) also known as photo action spectrum exhibits that NK3705 shows photon harvesting mainly in the lower wavelength region of 300–500 nm while Z907 absorbs effectively from 400–800 nm and lacks the efficient photon harvesting in the lower wavelength region. DSSC based on dye double layer architecture having both of the dyes i.e. Z907 and NK3705 are able to harvest the photon efficiently in both of the lower as well as higher wavelength region and can be attributed to the enhancement in over all power conversion efficiency. It has been found that efficiency of the DSSC based on dye cocktail increases with dipping time and best efficiency was obtained when dye adsorption was carried out for the 6 days at room temperature. More longer dipping times leads to gradual removal of NK3705 by Z907 leading to a little decrease in the efficiency. The best photoconversion efficiency was obtained for the DSSC having dye double layer structure where the thickness of Z907 was more than 8 μm while the thickness of NK3705 was less than 12 μm. 3.3. Elucidation of mechanism for dye double layer formation The interesting observation of dye double layer formation from the dye cocktail of Z907 and NK3705 by simply dipping the nanoporous TiO2 in dye mixture without using any special dye application techniques fascinated us to explore the reason for this observation and propose a possible mechanism. As has been explained earlier that in the initial time frame of dye adsorption from the cocktail solution, mainly NK3705 get adsorbed on the TiO2 surface which is gradually replaced by dye Z907 and after about 10 days of dye adsorption nearly
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Fig. 2. (a) Digital photographic images of the dye adsorption on TiO2 (19 μm thick) from single dye and dye cocktail solutions and (b) cross-sectional view of dye adsorption from dye cocktail solution using confocal laser scanning microscope.
Fig. 3. Fluorescence intensity distribution mapping of NK3705 on 19 μm thick nanoporous TiO2 utilized for dye adsorption from dye cocktail of Z907 and NK3705.
Y. Ogomi et al. / Thin Solid Films 519 (2010) 1087–1092
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Fig. 5. Rate of dye adsorption on nanoporous TiO2 for different dyes (red colour) and rate of desorption of dyes from the surface (green colour) used in the present investigation. TiO2 film thickness was about 4 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web of this article.)
Fig. 4. (a) Photovoltaic characteristics of DSSC based on TiO2 sensitized with a solution of single dye and dye cocktail under 100 mA/cm2 simulated solar irradiation at AM 1.5 and (b) IPCE curves for DSSC based on TiO2 sensitized with single dye and dye cocktail solution.
all of NK3705 dye molecules get replaced. To obtain a further insight in detail about this observation, rate of dye adsorption on TiO2 surface along with its rate of desorption from the surface for the organic and inorganic dyes has been shown in the Fig. 5. It is clearly evident from the figure that rate of adsorption of NK3705 on the nanoporous TiO2 surface is faster than that of Z907 which could be attributed to its smaller molecular size leading to swift diffusion through the nanopores from top to the bottom. At the same time rate of desorption of NK3705 is more than 4 times faster than that of Z907 indicates the weaker binding of NK3705 with the TiO2 as compared to that of Z907. Therefore, in the initial time domain of dye adsorption first NK3705 get adsorbed first but owing to its relatively weak binding, get replaced by Z907 gradually with the increasing time of dye adsorption from the cocktail solution leading to dye double layer formation. It, therefore, implies that it is possible to adsorb two dyes in dye double layer architecture from a mixed dye solution provided that if we judiciously select one dye having faster adsorption and weak binding
Table 1 Photovoltaic parameters of DSSC based on TiO2 sensitized with the solution of respective single dye and dye cocktail. Dye
Jsc (mA/cm2)
Voc (V)
Fill factor (FF)
Efficiency
NK3705 Z907 Dye cocktail
3.55 15.05 15.68
0.69 0.69 0.70
0.71 0.67 0.64
1.8% 6.9% 7.1%
with TiO2 while another dye having relatively slower diffusion along with stronger binding. To further verify the condition for dye double layer formation from a dye cocktail solution, a new set of dye mixture i.e. a solution of organic dye D102 and an inorganic dye black dye (BD) was utilized for the dye adsorption on the TiO2 surface. Sensitizing dye D102 shows much faster adsorption (about 3 times) as compared BD but contrary to the first case (Z907 and NK3705 system), it exhibits more than four times stronger binding on TiO2 surface as compared to BD. Fig. 6 (a) shows the visual color changes from surfaces facing glass as well as TiO2 after dye adsorption from a dye cocktail solution of D102 and BD. Simple digital imaging of the dye adsorption reveal that it does not lead to the dye double layer formation owing to the observation of nearly the same color from both of the sides. This can be explained by the fact that fast adsorbing 1st dye having strong binding with TiO2 is unable to be replaced by the 2nd dye and therefore, such a dye cocktail system is unable to form dye double layer. In the another set of dye adsorption from cocktail solution a dye mixture containing organic dye D77 and inorganic ruthenium based dye BD was selected for the dye adsorption on the nanoporous TiO2 surface. As can be clearly seen from the Fig. 5 that dye D77 exhibits much faster adsorption as compared to that of BD but rate of desorption from the TiO2 surface is approximately similar. Thus both of dyes used in the present cocktail system have nearly same binding force with the TiO2 molecules on the titania surface. A perusal of the digital imaging of the dyes adsorbed from both of the TiO2 and glass facing sides shown in the Fig. 6 (b) again exhibits approximately similar color indicating that both of the dyes have been adsorbed randomly and there is no dye double layer formation from the dye cocktail solution containing D77 and BD. These experiment using such a dye cocktail systems reveal that in the case a dye cocktail system where the 1st dye having faster rate of adsorption with stronger or nearly similar binding strength with TiO2 as compared to the 2nd dye having slower rate of adsorption does not lead to the dye double layer formation. 4. Conclusion Dye double layer formation on nanoporous TiO2 surface from a dye cocktail solution of two dyes viz. NK3705 and Z907 has been successfully demonstrated as evidenced by visual colour observations and confocal laser microscopic investigations. A perusal of photo action spectra for DSSC based on dye double layer shows photon absorption in relatively wider wavelength region as compared to that of DSSC based on single dye leading to enhanced photovoltaic performance. Photovoltaic performance evaluation of DSSC consisted of dye double layer of NK3705 and Z907 adsorbed from dye cocktail solution gave a short circuit current density of 15.68 mA/cm2, an open
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Fig. 6. (a) Digital photographic images of the dye adsorption on TiO2 (4 μm thick) from single dye and dye cocktail solutions consisting of two dyes D102 and BD while (b) same with different dye cocktail system containing dye D77 and BD.
circuit voltage of 0.70 V and a fill factor of 0.64 corresponding to an overall power conversion efficiency of 7.1% which is higher than that of DSSC based on single dye. Investigation pertaining to the speed of dye adsorption on TiO2 surface and its rate of desorption from the titania surface reveal that a combination of two dyes having faster adsorption with weak binding force and slower adsorption with stronger binding leads to dye double layer formation from the dye cocktail. Acknowledgement Authors would like to acknowledge the financial support from the New Energy Industrial Technology Development Organization (NEDO), Japan. One of the authors Yuhei Ogomi would like to thank Fukuoka Industry Science and Technology Foundation under knowledge cluster project for financial support. References [1] Intergovernmental Panel on Climate Changes (IPCC), Fourth Assessment ReportClimate Change, 2007 http://www.ipcc-wg2.org (accessed Feb. 2010). [2] M. Gratzel, Inorg. Chem. 44 (2005) 6841.
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