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Structural alternation of tandem dye-sensitized solar cells based on mesh-type of counter electrode
G Model EA 24859 No. of Pages 5
Electrochimica Acta xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Structural alternation of tandem dye-sensitized solar cells based on mesh-type of counter electrode Hyunwoong Seo * , Shinji Hashimoto, Daiki Ichida, Naho Itagaki, Kazunori Koga, Masaharu Shiratani Graduate School of Information Science and Electrical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 December 2014 Received in revised form 17 April 2015 Accepted 17 April 2015 Available online xxx
Tandem dye-sensitized solar cells (DSCs) are very effective to improve light absorption characteristics and overall performance. The structure of conventional tandem DSC is an assembly of two independent DSCs. Therefore, additional TiO2 layer, Pt film, and transparent conductive oxide (TCO) electrodes weaken incident light to the bottom cell and complicate the fabrication as compared with standard DSCs. Here, this work proposed the structural alternation of tandem DSC as a solution. Mesh type of counter electrode was inserted between top and bottom cells instead of TCO electrodes. Two photo electrodes shared electrolyte and counter electrode in this structure. High aperture ratio of mesh increased light penetration into bottom cell and led to the performance improvement. Structural alternation also simplified the fabrication. It could be fabricated like standard DSCs. After dye arrangement and TiO2 layer of bottom cell were controlled, the photovoltaic performance of proposed tandem DSC was enhanced and it was higher than conventional tandem DSC. Finally, the long-term stability of proposed tandem DSC was secured by the control of sealing walls. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Tandem dye-sensitized solar cell Mesh counter electrode Structural alternation Long-term stability
1. Introduction Dye-sensitized solar cells (DSCs) have attracted lots of attention in the photovoltaic research field since first development in the early 1990s [1,2]. Its simple fabrication process, low manufacturing cost and short energy payback time strengthened the cost-competitiveness. Its unique characteristics such as transparency, various colors, and flexibility made DSCs specialized in the portable, wearable, and building integrated photovoltaics. Although highly efficient perovskite solar cells were recently appeared [3–5], more stable DSC is still an attractive and competitive photovoltaic device. To enhance DSC performance, highly transparent electrodes with low resistivity, conductive semiconductor oxide materials, electrolyte for better charge transportation, and sensitizers with strong absorption were researched so far [6–14]. Within a range of related researches, the improvement on the absorption of sensitizer is very important for large current and high efficiency because it is directly connected to the photo-generation. Many dyes were developed so far but only a few dyes such as N719, N749, and
* Corresponding author. Tel: +81-92-802-3723; Fax: +81-92-802-3723 E-mail address: [email protected] (H. Seo).
N3 were widely used [15–17]. There is still the demand of the strong absorption in the wide wavelength range although dye absorption was much enhanced so far. Tandem DSCs were developed to supplement insufficient absorption of single dye [18–22]. Multiple dyes of different absorption characteristics were stacked and overall absorption was enhanced and widened. The structure of conventional tandem DSC is shown in Fig. 1(a). It is an assembly of two independent DSCs. Tandem DSCs definitely harvest more photons and have higher performance than standard DSCs. However, TiO2/dye layer, Pt film, and two TCO layers are additionally necessary. Especially, expensive Pt and TCOs are one of crucial causes of the increase in manufacturing cost. In addition, its fabrication becomes complicated. As a solution, this work proposed the structural alternation of tandem DSC shown in Fig. 1(b). In this structure, two photo electrodes share mesh type of counter electrode and redox electrolyte. Therefore, its fabrication is similar to a standard DSC. TCO-less counter electrode leads to manufacturing cost saving and the increase in incident light intensity to the bottom cell. N719 and N749 dyes were combined in this tandem DSC. In order to verify the effect of structural alternation, photovoltaic properties, cyclic voltammetry (CV), incident photon to current conversion efficiency (IPCE), and long-term stability were examined.
http://dx.doi.org/10.1016/j.electacta.2015.04.105 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: H. Seo, et al., Structural alternation of tandem dye-sensitized solar cells based on mesh-type of counter electrode, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.04.105
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Fig. 1. Structural diagrams of (a) conventional and (b) proposed tandem DSCs.
2. Experimental Proposed tandem DSCs were fabricated as follows. Fluorinedoped tin oxide (FTO) substrates (15 V/sq., Pilkington) were used for the photo electrodes. 16 13 mm2 sized FTO substrates were sequentially cleaned by sonicating in acetone, ethyl alcohol, and distilled water. They were dried using a stream of nitrogen. TiO2 paste was prepared by general procedure [23]. TiO2 paste was uniformly coated on FTO substrate by doctor-blade method. TiO2 layer changed to nano-porous structure after heat treatment at 450 C for 30 min. The thickness of TiO2 layer was about 7 mm. Nano-porous TiO2/FTO electrodes were soaked into N719 (RuC58H86N8O8S2) or N749 (C69H117O6N9S3Ru) dye solution. Dye adsorption time was 16 h because our previous work verified that dye adsorption was saturated after 12 h [24,25]. Ti mesh (Unique Wire Weaving Co., Inc.) was used for the counter electrode. Mesh was uniformly weaved by Ti strings with a diameter of 76.2 mm. Its aperture ratio was 88.4% and the transmittance was enough secured. 16 13 mm2 sized Ti mesh was cleaned by sonicating in ethyl alcohol. Dried Ti mesh was dipped into a 100 mM H2PtCl6 in isopropanol and dried at 120 C. After that, it was sintered at 400 C for 30 min. N719 and N749 dye-sensitized photo electrodes and Pt deposited mesh counter electrode were sealed using thermoplastic sealants (SX 1170-25, Solaronix). Sealants were put on two photo electrodes and mesh counter electrode was inserted between photo electrodes. Assembly was sealed at 100 C by pressing. In order to find stable sealing condition, the thickness of sealants was varied from 25 to 75 mm. Redox electrolyte which consisted of 0.5 M LiI, 0.05 M I2 and 0.5 M 4-tertbutylpyridine in acetonitrile was injected through a pre-drilled hole into the bottom electrode. The fabrication was completed after holes were sealed. In order to compare photovoltaic properties, conventional tandem DSC was also prepared as a reference. As shown in Fig. 1(a), one-sided FTO substrates were prepared for top and bottom electrodes and double-sided FTO substrate was prepared for intermediate electrodes. Nano-porous TiO2 layer was deposited on top and intermediate FTO substrates. 10 mM H2PtCl6 in isopropanol was spin-coated on bottom FTO and another side of intermediate FTO substrates with a rotating speed of 3000 rpm for 60 s and sintered at 400 C for 30 min. Three electrodes were sequentially sealed. Dye adsorption and electrolyte injection were identically conducted as described above. All completed tandem DSCs were kept under the dark and open-circuit conditions for 24 h to allow electrolyte to penetrate into the nano-pores. Their photovoltaic properties were measured under dark and irradiated conditions. 1 sun (air mass 1.5, 100 mW/cm2) light was irradiated and I-V (current-voltage) characteristic curves were measured by a source meter (Model 2400, Keithley Instrument, Inc.). IPCE (SM-250-P1, Bunkoukeiki)
was measured in the wavelength range from 250 to 1100 nm. During irradiance and characterization, a black mask was fitted by the active area of 0.20 cm2. CV was investigated using the electrochemical analysis instrument (SP-150, Biologic SAS). It was performed at a scan rate of 100 mV/s in the potential range from 2 to 2 V. For stability measurement, conventional and proposed tandem DSCs were continuously exposed to irradiation. Their photovoltaic properties were measured up to 1000 h. 3. Results and Discussion In conventional tandem DSCs, short and long wavelength absorptive dyes were arranged on top and bottom cells, respectively [18–22]. However, optical path to bottom cell was changed by structural alternation of proposed tandem DSCs. Accordingly, some photons were lost by absorption, scattering, and reflection of electrolyte although its rate was not large. In addition, the performance of N719 DSC is generally higher than that of N749 DSC despite its relatively narrow absorption range [26,27]. Therefore, it was necessary to compare photovoltaic characteristics according to dye arrangement. Fig. 2 shows (a) photovoltaic properties and (b) IPCE of N749 top/N719 bottom and N719 top/N749 bottom tandem DSCs. Two tandem DSCs showed different open-circuit voltage (VOC). Tandem DSC of N719 top/N749 bottom had slightly higher VOC than that of N749 top/N719 bottom. N719 DSC generally has higher VOC than N749 DSC. Our standard N719 and N749 DSCs had VOC of 0.75 V and 0.71 V, respectively. Other researches also reported same tendency [28–30]. Consequently, VOC of N719 top/N749 bottom tandem DSC had relatively high VOC because its N719 top cell was fully photo-generated by 1 sun illumination. On the other hand, short-circuit current density (JSC) of N749 top/N719 bottom tandem DSC was higher than that of N719 top/N749 bottom tandem DSC although standard N719 DSC had larger photocurrent than N749 DSC. It meant that N719 bottom cell had more photo-generation than N749 bottom cell. These results were verified by IPCE results in Fig. 2(b). N749 top/N719 bottom tandem DSC had high photo-generation in whole wavelength range. Especially, it had stronger absorption in long wavelength region. In other words, photons in the wavelength range over 700 nm were not enough for the photo-generation of N749 bottom cells while the photo-generation was conducted by incident photons in short wavelength. This obviously came from the change of optical path and the disturbance of electrolyte. As a result, N749 top/N719 bottom tandem DSC had larger JSC and higher performance than N719 top/N749 bottom tandem DSC in spite of relatively low VOC. N749 top/N719 bottom tandem DSC had 0.69 V of VOC, 15.85 mA/cm2 of JSC, 0.62 of fill factor (FF), and 6.78% of efficiency while N719 top/N749 bottom tandem DSC had 0.73 V
Please cite this article in press as: H. Seo, et al., Structural alternation of tandem dye-sensitized solar cells based on mesh-type of counter electrode, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.04.105
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Fig. 2. (a) I-V characteristic curves and (b) Incident photon to current conversion efficiencies of N749 top/N719 bottom and N719 top/N749 bottom tandem DSCs based on mesh type of counter electrodes.
of VOC, 14.50 mA/cm2 of JSC, 0.53 of fill factor (FF), and 5.56% of efficiency. Therefore, dye arrangement of N749 top/N719 bottom was fixed in subsequent experiments. Above results confirmed that quite weakened incident light arrived at bottom cell. Therefore, bottom cell required very effective light harvesting for better performance. Accordingly, TiO2 thickness of bottom cell was controlled. 7 and 10 mm thick TiO2 layers were prepared by repetition of doctor blading. The increase of TiO2 thickness led to the enhancement on light confinement and absorption with increased dye amount [28]. Fig. 3 shows (a) photovoltaic properties and (b) IPCE of N749 top/N719 bottom tandem DSCs with different TiO2 thickness of bottom cell. There was no improvement with thicker TiO2 against our expectation. The performance of both tandem DSCs was almost same regardless of TiO2 thickness. It meant that all incident photons to bottom cell were fully harvested by 7 mm thick TiO2/dye layer. Accordingly, 10 mm thick TiO2/dye layer had longer path of electron transfer and some dyes were not photo-excited due to insufficient photons. Long electron path increased the recombination rate and dyes of ground state disturbed charge transfer in nano-porous TiO2 network. Their similar IPCE verified insufficient photon energy of bottom cell and the disturbance of charge transfer was verified by slightly decreased FF. Therefore, TiO2 thickness of bottom cell was fixed as 7 mm in subsequent experiments. After proposed tandem DSC was structurally optimized, its performance was compared with conventional tandem DSC. Conventional tandem DSC had N719 top/N749 bottom structure as other researches reported. Fig. 4 shows their (a) photovoltaic properties and (b) IPCE. The performance of proposed tandem DSC was above-defined and performance of conventional tandem DSC was defined by 0.72 V of VOC, 13.55 mA/cm2 of JSC, 0.68 of fill factor (FF), and 6.66% of efficiency. Structural alternation had slightly low VOC. As mentioned in Fig. 2, it came from original characteristics of dyes. However, JSC was much increased. Photo-generation in long wavelength range was much enhanced by structural alternation
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Fig. 3. (a) I-V characteristic curves and (b) Incident photon to current conversion efficiencies of tandem DSCs based on mesh type of counter electrodes according to TiO2 thickness of bottom cell.
although IPCE peak value of proposed tandem DSC was lower than that of conventional tandem DSC. Both top cells were fully photo-generated by 1 sun irradiation. Accordingly, the performance of bottom cells was dominant in overall performance of tandem DSCs. In proposed tandem DSC, the amount of incident photons to bottom cell was increased by the removal of two TCO layers and glass substrate. The increase of incident photons made
Fig. 4. The comparison of (a) I-V characteristics and (b) Incident photon to current conversion efficiencies of conventional and proposed tandem DSCs.
Please cite this article in press as: H. Seo, et al., Structural alternation of tandem dye-sensitized solar cells based on mesh-type of counter electrode, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.04.105
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Fig. 5. Cyclic voltammetry of conventional and proposed tandem DSCs.
more electrons photo-generated from bottom cell. It finally led to the increase of JSC. However, the increase of overall performance was not large due to the decrease of FF. CV analysis verified the decrease of FF. Fig. 5 shows CV of conventional and proposed tandem DSCs. In general CV of a DSC, there are two peaks in positive and negative domains. Positive domain is concerned in the regeneration of dye and its peak indicates the activity of redox reaction between dye and electrolyte. Negative domain is related to the catalytic reaction of counter electrode and its peak indicates the redox activity between electrolyte and counter electrode [31–34]. We focused on negative domain because counter electrode was mainly changed in this work. The mesh counter electrode of proposed tandem DSC had lower negative peak than conventional tandem DSC. In other words, proposed tandem DSC had relatively slow catalytic reaction on counter electrode. Slow electron supply from counter electrode to electrolyte caused slow dye recovery. Consequently, FF was decreased and the improvement on the photocurrent waslimited by slow electron cycle. This deterioration resulted from structural change because catalytic material of counter electrode was identical. Proposed tandem DSC had some factors of FF degradation. Amount of Pt catalysis was much decreased because two Pt thin films of 0.20 cm2 were substituted by Pt adsorbed on a mesh electrode. Mesh counter electrode was shared by two photo electrodes. Nevertheless, the decrement in FF was only 0.06 thanks to the increase of catalytic interface by structural change from flat to mesh counter electrodes. Therefore, the performance of proposed tandem DSC is expected to be more improved with structural optimization of mesh counter electrode and catalytic activation. At initial stage, a 25 mm thick sealant was inserted between photo and counter electrodes and its performance was rapidly decreased. Mesh counter electrode had an uneven structure that Ti strings were weaved while conventional counter electrode was flat. Therefore, thin sealant was easy to be destroyed by mesh electrode. It meant that proposed tandem DSC had a risk of volatile electrolyte leakage and contact with photo electrodes. They deteriorated the overall performance. Therefore, solid sealing was very important for the long-term stability. Thick sealants are effective for solid sealing but wide gap between photo and counter electrodes is not good for higher performance. The stability of proposed tandem DSC was examined according to the thickness of sealing walls. The thickness was controlled as 25, 50, and 75 mm. Fig. 6 shows the change of photovoltaic performance according to irradiation time. The photovoltaic performance was continuously deteriorated with 25 mm thick sealant since first measurement. It was not enough for sealing due to relatively thick Ti mesh. 50 mm thick sealant was solider than 25 mm thick sealant and tandem DSC did not contact with photo electrodes. Proposed tandem DSC with
Fig. 6. The performance change of conventional and proposed tandem DSCs according to irradiation time
50 mm thick sealant maintained 90% of initial performance during 500 h of irradiation. However, its performance was also deteriorated below 70% of initial performance after irradiation for 1000 h. On the other hand, 75 mm thick sealant was enough to seal proposed tandem DSC. Its performance was not deteriorated up to 1000 h of irradiation. It was relatively thick as compared with conventional tandem DSC using 25 mm thick sealants. More electrolyte was necessary and it was also one of reasons for low FF because path of charge transfer in redox electrolyte was lengthened. Thicker sealant was not applied any longer after long-term stability was secured by 75 mm thick sealant. 4. Conclusions In this work, we proposed structural alternation of tandem DSC based on Ti mesh counter electrode. It was different from conventional tandem DSC which was the assembly of two independent DSCs. The fabrication was simplified like single DSC and manufacturing cost was down with the reduction of TCO and Pt. Above all, the performance was expected to be enhanced by the increase of incident photons to bottom cell. Proposed tandem DSC was optimized by dye arrangement and TiO2 thickness variation of bottom cell. As a result, proposed tandem DSC had higher performance than conventional tandem DSC although FF was decreased by relatively low catalytic activity and long charge transfer path. Finally, the stability test according to the thickness of sealing wall was examined and tandem DSC based on mesh type of counter electrode had good long-term stability using 75 mm thick sealants. Higher performance is expected by subsequent research to improve catalytic activity of mesh counter electrode. Acknowledgment This work was supported by New Energy and Industrial Technology Development Organization (NEDO). References [1] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films, Nature 353 (1991) 737. [2] B. O’Regan, M. Grätzel, D. Fitzmaurice, Optical electrochemistry I: steady-state spectroscopy of conduction-band electrons in a metal oxide semiconductor electrode, Chem. Phys. Lett. 183 (1991) 89. [3] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells, J. Am. Chem. Soc. 131 (2009) 6050. [4] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites, Science 338 (2012) 643. [5] J. Burschka, N. Pellet, S. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin, and Michael Grätzel Sequential deposition as a route to high-performance perovskite-sensitized solar cells, Nature 499 (2013) 316.
Please cite this article in press as: H. Seo, et al., Structural alternation of tandem dye-sensitized solar cells based on mesh-type of counter electrode, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.04.105
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Please cite this article in press as: H. Seo, et al., Structural alternation of tandem dye-sensitized solar cells based on mesh-type of counter electrode, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.04.105
Electrochimica Acta xxx (2015) xxx–xxx
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Structural alternation of tandem dye-sensitized solar cells based on mesh-type of counter electrode Hyunwoong Seo * , Shinji Hashimoto, Daiki Ichida, Naho Itagaki, Kazunori Koga, Masaharu Shiratani Graduate School of Information Science and Electrical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 December 2014 Received in revised form 17 April 2015 Accepted 17 April 2015 Available online xxx
Tandem dye-sensitized solar cells (DSCs) are very effective to improve light absorption characteristics and overall performance. The structure of conventional tandem DSC is an assembly of two independent DSCs. Therefore, additional TiO2 layer, Pt film, and transparent conductive oxide (TCO) electrodes weaken incident light to the bottom cell and complicate the fabrication as compared with standard DSCs. Here, this work proposed the structural alternation of tandem DSC as a solution. Mesh type of counter electrode was inserted between top and bottom cells instead of TCO electrodes. Two photo electrodes shared electrolyte and counter electrode in this structure. High aperture ratio of mesh increased light penetration into bottom cell and led to the performance improvement. Structural alternation also simplified the fabrication. It could be fabricated like standard DSCs. After dye arrangement and TiO2 layer of bottom cell were controlled, the photovoltaic performance of proposed tandem DSC was enhanced and it was higher than conventional tandem DSC. Finally, the long-term stability of proposed tandem DSC was secured by the control of sealing walls. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Tandem dye-sensitized solar cell Mesh counter electrode Structural alternation Long-term stability
1. Introduction Dye-sensitized solar cells (DSCs) have attracted lots of attention in the photovoltaic research field since first development in the early 1990s [1,2]. Its simple fabrication process, low manufacturing cost and short energy payback time strengthened the cost-competitiveness. Its unique characteristics such as transparency, various colors, and flexibility made DSCs specialized in the portable, wearable, and building integrated photovoltaics. Although highly efficient perovskite solar cells were recently appeared [3–5], more stable DSC is still an attractive and competitive photovoltaic device. To enhance DSC performance, highly transparent electrodes with low resistivity, conductive semiconductor oxide materials, electrolyte for better charge transportation, and sensitizers with strong absorption were researched so far [6–14]. Within a range of related researches, the improvement on the absorption of sensitizer is very important for large current and high efficiency because it is directly connected to the photo-generation. Many dyes were developed so far but only a few dyes such as N719, N749, and
* Corresponding author. Tel: +81-92-802-3723; Fax: +81-92-802-3723 E-mail address: [email protected] (H. Seo).
N3 were widely used [15–17]. There is still the demand of the strong absorption in the wide wavelength range although dye absorption was much enhanced so far. Tandem DSCs were developed to supplement insufficient absorption of single dye [18–22]. Multiple dyes of different absorption characteristics were stacked and overall absorption was enhanced and widened. The structure of conventional tandem DSC is shown in Fig. 1(a). It is an assembly of two independent DSCs. Tandem DSCs definitely harvest more photons and have higher performance than standard DSCs. However, TiO2/dye layer, Pt film, and two TCO layers are additionally necessary. Especially, expensive Pt and TCOs are one of crucial causes of the increase in manufacturing cost. In addition, its fabrication becomes complicated. As a solution, this work proposed the structural alternation of tandem DSC shown in Fig. 1(b). In this structure, two photo electrodes share mesh type of counter electrode and redox electrolyte. Therefore, its fabrication is similar to a standard DSC. TCO-less counter electrode leads to manufacturing cost saving and the increase in incident light intensity to the bottom cell. N719 and N749 dyes were combined in this tandem DSC. In order to verify the effect of structural alternation, photovoltaic properties, cyclic voltammetry (CV), incident photon to current conversion efficiency (IPCE), and long-term stability were examined.
http://dx.doi.org/10.1016/j.electacta.2015.04.105 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: H. Seo, et al., Structural alternation of tandem dye-sensitized solar cells based on mesh-type of counter electrode, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.04.105
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Fig. 1. Structural diagrams of (a) conventional and (b) proposed tandem DSCs.
2. Experimental Proposed tandem DSCs were fabricated as follows. Fluorinedoped tin oxide (FTO) substrates (15 V/sq., Pilkington) were used for the photo electrodes. 16 13 mm2 sized FTO substrates were sequentially cleaned by sonicating in acetone, ethyl alcohol, and distilled water. They were dried using a stream of nitrogen. TiO2 paste was prepared by general procedure [23]. TiO2 paste was uniformly coated on FTO substrate by doctor-blade method. TiO2 layer changed to nano-porous structure after heat treatment at 450 C for 30 min. The thickness of TiO2 layer was about 7 mm. Nano-porous TiO2/FTO electrodes were soaked into N719 (RuC58H86N8O8S2) or N749 (C69H117O6N9S3Ru) dye solution. Dye adsorption time was 16 h because our previous work verified that dye adsorption was saturated after 12 h [24,25]. Ti mesh (Unique Wire Weaving Co., Inc.) was used for the counter electrode. Mesh was uniformly weaved by Ti strings with a diameter of 76.2 mm. Its aperture ratio was 88.4% and the transmittance was enough secured. 16 13 mm2 sized Ti mesh was cleaned by sonicating in ethyl alcohol. Dried Ti mesh was dipped into a 100 mM H2PtCl6 in isopropanol and dried at 120 C. After that, it was sintered at 400 C for 30 min. N719 and N749 dye-sensitized photo electrodes and Pt deposited mesh counter electrode were sealed using thermoplastic sealants (SX 1170-25, Solaronix). Sealants were put on two photo electrodes and mesh counter electrode was inserted between photo electrodes. Assembly was sealed at 100 C by pressing. In order to find stable sealing condition, the thickness of sealants was varied from 25 to 75 mm. Redox electrolyte which consisted of 0.5 M LiI, 0.05 M I2 and 0.5 M 4-tertbutylpyridine in acetonitrile was injected through a pre-drilled hole into the bottom electrode. The fabrication was completed after holes were sealed. In order to compare photovoltaic properties, conventional tandem DSC was also prepared as a reference. As shown in Fig. 1(a), one-sided FTO substrates were prepared for top and bottom electrodes and double-sided FTO substrate was prepared for intermediate electrodes. Nano-porous TiO2 layer was deposited on top and intermediate FTO substrates. 10 mM H2PtCl6 in isopropanol was spin-coated on bottom FTO and another side of intermediate FTO substrates with a rotating speed of 3000 rpm for 60 s and sintered at 400 C for 30 min. Three electrodes were sequentially sealed. Dye adsorption and electrolyte injection were identically conducted as described above. All completed tandem DSCs were kept under the dark and open-circuit conditions for 24 h to allow electrolyte to penetrate into the nano-pores. Their photovoltaic properties were measured under dark and irradiated conditions. 1 sun (air mass 1.5, 100 mW/cm2) light was irradiated and I-V (current-voltage) characteristic curves were measured by a source meter (Model 2400, Keithley Instrument, Inc.). IPCE (SM-250-P1, Bunkoukeiki)
was measured in the wavelength range from 250 to 1100 nm. During irradiance and characterization, a black mask was fitted by the active area of 0.20 cm2. CV was investigated using the electrochemical analysis instrument (SP-150, Biologic SAS). It was performed at a scan rate of 100 mV/s in the potential range from 2 to 2 V. For stability measurement, conventional and proposed tandem DSCs were continuously exposed to irradiation. Their photovoltaic properties were measured up to 1000 h. 3. Results and Discussion In conventional tandem DSCs, short and long wavelength absorptive dyes were arranged on top and bottom cells, respectively [18–22]. However, optical path to bottom cell was changed by structural alternation of proposed tandem DSCs. Accordingly, some photons were lost by absorption, scattering, and reflection of electrolyte although its rate was not large. In addition, the performance of N719 DSC is generally higher than that of N749 DSC despite its relatively narrow absorption range [26,27]. Therefore, it was necessary to compare photovoltaic characteristics according to dye arrangement. Fig. 2 shows (a) photovoltaic properties and (b) IPCE of N749 top/N719 bottom and N719 top/N749 bottom tandem DSCs. Two tandem DSCs showed different open-circuit voltage (VOC). Tandem DSC of N719 top/N749 bottom had slightly higher VOC than that of N749 top/N719 bottom. N719 DSC generally has higher VOC than N749 DSC. Our standard N719 and N749 DSCs had VOC of 0.75 V and 0.71 V, respectively. Other researches also reported same tendency [28–30]. Consequently, VOC of N719 top/N749 bottom tandem DSC had relatively high VOC because its N719 top cell was fully photo-generated by 1 sun illumination. On the other hand, short-circuit current density (JSC) of N749 top/N719 bottom tandem DSC was higher than that of N719 top/N749 bottom tandem DSC although standard N719 DSC had larger photocurrent than N749 DSC. It meant that N719 bottom cell had more photo-generation than N749 bottom cell. These results were verified by IPCE results in Fig. 2(b). N749 top/N719 bottom tandem DSC had high photo-generation in whole wavelength range. Especially, it had stronger absorption in long wavelength region. In other words, photons in the wavelength range over 700 nm were not enough for the photo-generation of N749 bottom cells while the photo-generation was conducted by incident photons in short wavelength. This obviously came from the change of optical path and the disturbance of electrolyte. As a result, N749 top/N719 bottom tandem DSC had larger JSC and higher performance than N719 top/N749 bottom tandem DSC in spite of relatively low VOC. N749 top/N719 bottom tandem DSC had 0.69 V of VOC, 15.85 mA/cm2 of JSC, 0.62 of fill factor (FF), and 6.78% of efficiency while N719 top/N749 bottom tandem DSC had 0.73 V
Please cite this article in press as: H. Seo, et al., Structural alternation of tandem dye-sensitized solar cells based on mesh-type of counter electrode, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.04.105
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Fig. 2. (a) I-V characteristic curves and (b) Incident photon to current conversion efficiencies of N749 top/N719 bottom and N719 top/N749 bottom tandem DSCs based on mesh type of counter electrodes.
of VOC, 14.50 mA/cm2 of JSC, 0.53 of fill factor (FF), and 5.56% of efficiency. Therefore, dye arrangement of N749 top/N719 bottom was fixed in subsequent experiments. Above results confirmed that quite weakened incident light arrived at bottom cell. Therefore, bottom cell required very effective light harvesting for better performance. Accordingly, TiO2 thickness of bottom cell was controlled. 7 and 10 mm thick TiO2 layers were prepared by repetition of doctor blading. The increase of TiO2 thickness led to the enhancement on light confinement and absorption with increased dye amount [28]. Fig. 3 shows (a) photovoltaic properties and (b) IPCE of N749 top/N719 bottom tandem DSCs with different TiO2 thickness of bottom cell. There was no improvement with thicker TiO2 against our expectation. The performance of both tandem DSCs was almost same regardless of TiO2 thickness. It meant that all incident photons to bottom cell were fully harvested by 7 mm thick TiO2/dye layer. Accordingly, 10 mm thick TiO2/dye layer had longer path of electron transfer and some dyes were not photo-excited due to insufficient photons. Long electron path increased the recombination rate and dyes of ground state disturbed charge transfer in nano-porous TiO2 network. Their similar IPCE verified insufficient photon energy of bottom cell and the disturbance of charge transfer was verified by slightly decreased FF. Therefore, TiO2 thickness of bottom cell was fixed as 7 mm in subsequent experiments. After proposed tandem DSC was structurally optimized, its performance was compared with conventional tandem DSC. Conventional tandem DSC had N719 top/N749 bottom structure as other researches reported. Fig. 4 shows their (a) photovoltaic properties and (b) IPCE. The performance of proposed tandem DSC was above-defined and performance of conventional tandem DSC was defined by 0.72 V of VOC, 13.55 mA/cm2 of JSC, 0.68 of fill factor (FF), and 6.66% of efficiency. Structural alternation had slightly low VOC. As mentioned in Fig. 2, it came from original characteristics of dyes. However, JSC was much increased. Photo-generation in long wavelength range was much enhanced by structural alternation
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Fig. 3. (a) I-V characteristic curves and (b) Incident photon to current conversion efficiencies of tandem DSCs based on mesh type of counter electrodes according to TiO2 thickness of bottom cell.
although IPCE peak value of proposed tandem DSC was lower than that of conventional tandem DSC. Both top cells were fully photo-generated by 1 sun irradiation. Accordingly, the performance of bottom cells was dominant in overall performance of tandem DSCs. In proposed tandem DSC, the amount of incident photons to bottom cell was increased by the removal of two TCO layers and glass substrate. The increase of incident photons made
Fig. 4. The comparison of (a) I-V characteristics and (b) Incident photon to current conversion efficiencies of conventional and proposed tandem DSCs.
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Fig. 5. Cyclic voltammetry of conventional and proposed tandem DSCs.
more electrons photo-generated from bottom cell. It finally led to the increase of JSC. However, the increase of overall performance was not large due to the decrease of FF. CV analysis verified the decrease of FF. Fig. 5 shows CV of conventional and proposed tandem DSCs. In general CV of a DSC, there are two peaks in positive and negative domains. Positive domain is concerned in the regeneration of dye and its peak indicates the activity of redox reaction between dye and electrolyte. Negative domain is related to the catalytic reaction of counter electrode and its peak indicates the redox activity between electrolyte and counter electrode [31–34]. We focused on negative domain because counter electrode was mainly changed in this work. The mesh counter electrode of proposed tandem DSC had lower negative peak than conventional tandem DSC. In other words, proposed tandem DSC had relatively slow catalytic reaction on counter electrode. Slow electron supply from counter electrode to electrolyte caused slow dye recovery. Consequently, FF was decreased and the improvement on the photocurrent waslimited by slow electron cycle. This deterioration resulted from structural change because catalytic material of counter electrode was identical. Proposed tandem DSC had some factors of FF degradation. Amount of Pt catalysis was much decreased because two Pt thin films of 0.20 cm2 were substituted by Pt adsorbed on a mesh electrode. Mesh counter electrode was shared by two photo electrodes. Nevertheless, the decrement in FF was only 0.06 thanks to the increase of catalytic interface by structural change from flat to mesh counter electrodes. Therefore, the performance of proposed tandem DSC is expected to be more improved with structural optimization of mesh counter electrode and catalytic activation. At initial stage, a 25 mm thick sealant was inserted between photo and counter electrodes and its performance was rapidly decreased. Mesh counter electrode had an uneven structure that Ti strings were weaved while conventional counter electrode was flat. Therefore, thin sealant was easy to be destroyed by mesh electrode. It meant that proposed tandem DSC had a risk of volatile electrolyte leakage and contact with photo electrodes. They deteriorated the overall performance. Therefore, solid sealing was very important for the long-term stability. Thick sealants are effective for solid sealing but wide gap between photo and counter electrodes is not good for higher performance. The stability of proposed tandem DSC was examined according to the thickness of sealing walls. The thickness was controlled as 25, 50, and 75 mm. Fig. 6 shows the change of photovoltaic performance according to irradiation time. The photovoltaic performance was continuously deteriorated with 25 mm thick sealant since first measurement. It was not enough for sealing due to relatively thick Ti mesh. 50 mm thick sealant was solider than 25 mm thick sealant and tandem DSC did not contact with photo electrodes. Proposed tandem DSC with
Fig. 6. The performance change of conventional and proposed tandem DSCs according to irradiation time
50 mm thick sealant maintained 90% of initial performance during 500 h of irradiation. However, its performance was also deteriorated below 70% of initial performance after irradiation for 1000 h. On the other hand, 75 mm thick sealant was enough to seal proposed tandem DSC. Its performance was not deteriorated up to 1000 h of irradiation. It was relatively thick as compared with conventional tandem DSC using 25 mm thick sealants. More electrolyte was necessary and it was also one of reasons for low FF because path of charge transfer in redox electrolyte was lengthened. Thicker sealant was not applied any longer after long-term stability was secured by 75 mm thick sealant. 4. Conclusions In this work, we proposed structural alternation of tandem DSC based on Ti mesh counter electrode. It was different from conventional tandem DSC which was the assembly of two independent DSCs. The fabrication was simplified like single DSC and manufacturing cost was down with the reduction of TCO and Pt. Above all, the performance was expected to be enhanced by the increase of incident photons to bottom cell. Proposed tandem DSC was optimized by dye arrangement and TiO2 thickness variation of bottom cell. As a result, proposed tandem DSC had higher performance than conventional tandem DSC although FF was decreased by relatively low catalytic activity and long charge transfer path. Finally, the stability test according to the thickness of sealing wall was examined and tandem DSC based on mesh type of counter electrode had good long-term stability using 75 mm thick sealants. Higher performance is expected by subsequent research to improve catalytic activity of mesh counter electrode. Acknowledgment This work was supported by New Energy and Industrial Technology Development Organization (NEDO). References [1] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films, Nature 353 (1991) 737. [2] B. O’Regan, M. Grätzel, D. Fitzmaurice, Optical electrochemistry I: steady-state spectroscopy of conduction-band electrons in a metal oxide semiconductor electrode, Chem. Phys. Lett. 183 (1991) 89. [3] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells, J. Am. Chem. Soc. 131 (2009) 6050. [4] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites, Science 338 (2012) 643. [5] J. Burschka, N. Pellet, S. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin, and Michael Grätzel Sequential deposition as a route to high-performance perovskite-sensitized solar cells, Nature 499 (2013) 316.
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Please cite this article in press as: H. Seo, et al., Structural alternation of tandem dye-sensitized solar cells based on mesh-type of counter electrode, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.04.105