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High-efficiency flexible dye-sensitized solar cells fabricated by a novel friction-transfer technique
Electrochemistry Communications 12 (2010) 1000–1003
Contents lists available at ScienceDirect
Electrochemistry Communications 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 / e l e c o m
High-efficiency flexible dye-sensitized solar cells fabricated by a novel friction-transfer technique Li Yang a, Liqiong Wu a, Mingxing Wu a, Gang Xin a, Hong Lin b, Tingli Ma a,⁎ a b
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China State Key Laboratory of New Ceramic and Fine Processing, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 15 May 2010 Accepted 20 May 2010 Available online 27 May 2010 Keywords: All-flexible Dye-sensitized solar cell Friction-transfer Tetrabutyl titanate
a b s t r a c t All-transparent and all-flexible dye-sensitized solar cells (DSCs) were fabricated using a novel, facile, and low-lost friction-transfer technique, which involved assembling TiO2 films on flexible substrates via hightemperature sintering. This friction-transfer technique led to a 25% enhancement in conversion efficiency compared with the compression method. The efficiency of the optimized all-flexible DSC reached 5.7%, which is approximately 73% higher than that of the all-flexible DSC prepared by the low-temperature sintering method (3.3%). © 2010 Elsevier B.V. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSCs) have been widely investigated in recent years due to their potential as environment-friendly and lowcost photovoltaic cells [1–3]. Currently, the highest conversion efficiency (η) of DSCs made on fluorine-doped tin oxide (FTO) glass was reported to be 11% [4,5]. In the pursuit to meet the requirements of portable applications, lightweight and bendable DSCs have attracted increasing attention. Generally, conductive polymer substrates, such as poly(ethylene naphthalate) coated with indiumdoped tin oxide (ITO/PEN), were used to fabricate flexible DSCs. However, these substrates cannot withstand high processing temperatures of up to 400–500 °C. Although several low-temperature sintering methods have been developed to prepare TiO2 films, the interparticle connection was poor, which directly affected the performance of the DSCs [6–9]. To solve this problem, Dürr et al. developed a lift-off method for preparing a hybrid DSC based on flexible photoanode and the photoanode with rigid FTO counter electrode (CE) [10]. Although the DSC provided a high conversion efficiency, the transfer procedure was complicated and unsuitable for practical applications. In order to fabricate flexible DSCs via hightemperature sintering, metallic sheets made of Ti and stainless steel have been employed [11–13]. These devices offered relatively high efficiencies. However, the applications of the DSCs were limited due to their opacity. To date, reports on all-transparent and all-flexible DSCs are limited. Furthermore, the efficiencies of the DSCs remain low.
Recently, Iwasaki et al. reported a DSC with a η of 2.6% — a result attributed to the poor interparticles connection under low-temperature sintering [14]. In this work, in order to obtain porous TiO2 films via hightemperature sintering, a friction-transfer technique is developed for the preparation of flexible photoanodes. The entire transfer process, in which no chemical reagents or noble metals were introduced, is convenient, fast, and efficient, as well as low-cost. We have also optimized the photoanodes by adjusting the proportion of TiO2 pastes, employing an industrially-applied spray-coating technique, and treating TiO2 films with tetrabutyl titanate (TBT) to solve the internal dislocation phenomenon. The photovoltaic properties and electronic processes of the DSCs containing these photoanodes are discussed. 2. Experimental section 2.1. Preparation of TiO2 pastes A mixture of 0.5 g commercial P25 powders (Degussa), TBT, and 10 mL anhydrous ethanol was allowed to stand for 3.5 h, before dispersing with an ultrasonicator. The resulting TiO2 pastes denoted as paste A were used for preparing the underlying TiO2 layer on ITO/PEN (110, Tobi, Japan). The TiO2 pastes used to fabricate the transferred TiO2 layer on the ceramic tile, also called paste B, were prepared according to our previous report [15]. 2.2. Preparation of nanocyrstalline TiO2 electrodes
⁎ Corresponding author. Tel./fax: + 86 411 39893820. E-mail address: [email protected] (T. Ma). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.05.026
Titania electrodes were fabricated by a low-temperature sintering method as below: Briefly, paste A was sprayed on ITO/PEN, which was
L. Yang et al. / Electrochemistry Communications 12 (2010) 1000–1003
1001
by sandwiching a redox (I−/I− 3 ) electrolyte solution, which was composed of 0.03 M I2, 0.06 M LiI, 0.6 M 1-butyl-3-methylimidazolium iodide, 0.1 M guanidinium thiocyanate, and 0.5 M 4-tertbutylpyridine in acetonitrile.
then sintered at 125 °C for 1 h to form the TiO2 film with a thickness of 28 μm. The same technique was also used for preparing the underlying layer of the friction-transfer process. Preparing titania electrodes via the friction-transfer technique is described as follows: paste B was coated on a ceramic tile by a doctor-blade method, which was then sintered at 500 °C for 30 min to form a 15 μm thick TiO2 film. Subsequently, the sintered TiO2 layer on the ceramic tile surface-tosurface contacted with the underlying TiO2 layer on the ITO/PEN, and was then easily transferred from the ceramic tile to the ITO/PEN by means of friction acting on the two layers. The friction was produced when the two surfaces moved across each other. These two TiO2 layers were tightly connected by compression with a pressure of 100 kgf cm− 2, finally forming the TiO2 film with an 8–10 μm thickness. The resulting titania electrode was immersed in a TBT/nbutanol mixture for 15 min at 90 °C. For comparison, the same pressure was applied to the TiO2 film prepared by the lowtemperature sintering method, ultimately obtaining a TiO2 film with 8–10 μm thickness. The area of the prepared TiO2 electrodes was 0.2 cm2.
Current–voltage curves of the DSCs were measured by a Keithley digital source meter (Keithley 2601, USA). The intensity of the incident light mimics AM 1.5 solar light through a 300 W solar simulator (Solar Light, USA). The incident light intensity was calibrated using a solar power meter (TES-1333R, Taiwan) and a silicon photocell (BS-520, Japan), and was set at 100 mW/cm2. The surface morphology of the nanocrystalline TiO2 films was characterized by scanning electron microscopy (SEM, Quanta 200F, FEI, Slovakia). The electrochemical impedance spectroscopy (EIS, Zenium Zahner, Germany) was actualized in the dark at a −0.80 V bias. The measured frequency ranged from 10 mHz to 1 MHz, and the AC amplitude was set at 10 mV.
2.3. Preparation of CEs
3. Results and discussion
An isopropanol solution containing 0.5% chloroplatinic acid was sprayed on ITO/PEN, which was then dried at 125 °C for 1 h. The ITO/ PEN sheet coated with chloroplatinic acid was immersed in water/ ethanol (4:1, v/v) containing 30 mM NaBH4 until the gas bubble vanished and was then dried at 125 °C for 1 h. Sputtered-Pt CEs on FTO glass were prepared according to our previous work [16].
3.1. Optimization of all-flexible DSCs
2.4. Fabrication of DSCs The obtained TiO2 electrodes were dipped into acetonitrile/4-tertbutyl alcohol (1:1, v/v) containing 5 × 10− 4 M cis-bis(isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II) bis-tetrabuylammonium (dye N719) for 12 h. The dye-sensitized TiO2 electrodes and platinized CEs were assembled to form DSCs (0.2 cm2-sized cells)
2.5. Characterization
Fig. 1(A) shows the process of preparing photoanodes of the allflexible DSCs. An industrially-applied spray-coating technique was used to fabricate flexible both photoanodes and CEs. This procedure is especially suitable for roll-to-roll manufacturing process. Furthermore, the preparation of pastes A and B developed by our group is simple compared to the sol–gel and hydrothermal methods. It has been reported that the compression method is used to prepare flexible photoanodes [3]. However, the compression method may generate TiO2 films with poor porosity, which can affect the properties of DSCs. Based on the compression method, therefore, a frictiontransfer technique was developed to achieve porous TiO2 films via high-temperature sintering. This friction-transfer was implemented
Fig. 1. (A) Schematic diagram of the preparation procedure of flexible DSCs. (B) SEM images of the composite TiO2 film on ITO/PEN treated with a TBT/n-butanol mixture. The composite TiO2 film was composed of the underlying and transferred TiO2 layers. Two types of magnification: 100,000× (a) and 10,000× (b).
1002
L. Yang et al. / Electrochemistry Communications 12 (2010) 1000–1003
Thermal decomposition and magnetron sputtering are widely used to deposit Pt catalyst on CEs. In this work, we prepared flexible CEs by chemical platinization using NaBH4 as a reducing agent. In this way, high-temperature heating needed for thermal decomposition, as well as the use of expensive instruments for magnetron sputtering, was avoided, making this technique suitable for the mass production of flexible CEs. 3.2. Photovoltaic performance of DSCs We assembled hybrid DSCs by combining ITO/PEN photoanodes and platinized FTO/glass CEs. Fig. 2A shows the photocurrent–voltage curves of the hybrid DSCs based on the ITO/PEN photoanodes prepared by the low-temperature sintering, compression, and friction-transfer techniques, respectively. The corresponding performance parameters of the DSCs are summarized in Table 1. For lowtemperature sintering, the hybrid DSC provides a η value of 3.8% (curve a). When the pressure of 100 kgf cm− 2 is applied to the TiO2 film via low-temperature sintering, the η value increases to 5.2% (curve b). On the basis of the compression method, the conversion efficiency of the DSC is further improved by introducing the frictiontransfer technique, reaching 6.5% (curve c).This result indicates that the high-temperature sintering of TiO2 film leads to a 25% enhancement in efficiency. In addition, the ITO/PEN photoanodes fabricated by the lowtemperature sintering and friction-transfer techniques were assembled into all-flexible DSCs with ITO/PEN CEs, providing η of 3.3% and 5.7%, respectively. According to our knowledge, this η value of 5.7% is the highest efficiency attained for all-flexible and all-transparent DSCs. Furthermore, the efficiency of the all-flexible DSC prepared by the friction-transfer technique is ca. 73% enhanced compared to that prepared via the low-temperature sintering method. 3.3. EIS study of DSCs
Fig. 2. (A) Photocurrent–voltage curves of the hybrid DSCs based on ITO/PEN photoanodes prepared by different techniques: (a) low-temperature sintering technique; (b) compression technique; and (c) friction-transfer technique. (B) Nyquist plots of hybrid DSCs based on ITO/PEN photoanodes with and without TBT treatment. To prepare photoanodes by the low-temperature sintering method, paste A with (a′) and without (a) TBT were used. For friction-transfer technique, the TiO2 composite composed of the underlying and transferred layers was treated with (b′) and without (b) a TBT/n-butanol mixture.
by the friction force which is created when the two surfaces of the underlying and transferred TiO2 layers move across each other. Therefore, this technique is facile and cost-effective. The transfer may bring forth a dislocation problem between the underlying and transferred TiO2 layers; thus, the two layers were treated with a TBT/n-butanol mixture. The SEM images in Fig. 1(B) illustrate that the TiO2 film on ITO/PEN fabricated using the friction-transfer technique shows a porous network structure.
The effects of TBT treatments on the electronic processes in the DSCs were investigated by EIS. Fig. 2B displays the Nyquist plots of the DSC samples with and without TBT treatment. The semi-circle in the intermediate-frequency region corresponds to the RC networks of the TiO2/dye/electrolyte interface, including the charge transfer resistance (Rct) and the constant phase element (CPE). The radius of the semi-circle for the DSC using paste A with TBT (curve a′) is much smaller than that without TBT (curve a), indicating that a smaller Rct exists in the solar cell when TBT is added to TiO2 pastes. This result demonstrates that TBT, as an inorganic binder, contributes to faster electron transport in TiO2 films. Moreover, when the composite TiO2 film composed of the underlying and transferred layers is treated with a TBT/n-butanol mixture (curve b′), the radius of the semi-circle for the DSC without the TBT treatment (curve b) sharply decreases. The electron transport in the composite TiO2 film is significantly improved due to the enhanced interconnection of TiO2 nanoparticles between the two layers. Thus, the
Table 1 Photovoltaic performance of DSCs. Photoanodes of hybrid and all-flexible DSCs were made on ITO/PEN, while platinized CEs of hybrid and all-flexible DSCs were fabricated on FTO/ glass and ITO/PEN, respectively. Type of DSC
Hybrid Hybrid Hybrid All-flexible All-flexible
Technique of preparing TiO2 film on ITO/PEN A
B
C
√ √ √ √ √
× √ √ × √
× × √ × √
Jsc/mA/cm2
Voc/V
FF
η (%)
ta/μm
6.9 10.6 12.3 6.4 13.9
0.77 0.80 0.80 0.78 0.78
0.72 0.62 0.66 0.66 0.52
3.8 5.2 6.5 3.3 5.7
29 8 8 28 10
A: TiO2 film on ITO/PEN was prepared by low-temperature sintering technique. B: A pressure of 100 kgf cm− 2 was applied to the TiO2 film by low-temperature sintering. C: A transferred TiO2 layer was prepared by friction-transfer technique combined with TBT treatment. a Film thickness.
L. Yang et al. / Electrochemistry Communications 12 (2010) 1000–1003
TBT treatment effectively solves the dislocation problem resulting from the friction-transfer technique. 4. Conclusion The all-transparent and all-flexible DSC fabricated by the frictiontransfer technique showed a high conversion efficiency of 5.7%, which increased by ca. 73% compared to that prepared by the low-temperature sintering. Based on the compression method, the 25% enhancement in the conversion efficiencies of the DSCs was obtained by a simple and low-cost friction-transfer technique, due to the advantage of high-temperature sintering. Furthermore, EIS results show that the addition of TBT to the TiO2 paste and the TBT treatment of the TiO2 composite films effectively improved the electron transport in TiO2 films, thereby enhancing the photovoltaic properties of the DSCs. Thus, the friction-transfer technique opens up a new way to prepare high-efficiency flexible DSCs. Acknowledgements This research is supported by the National Natural Science Foundation of China (No. 50773008) and the National High Technology Research and Development Program for Advanced Materials of China (No. 2009AA03Z220).
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References [1] B. O'Regan, M. Grätzel, Nature (London) 353 (1991) 737. [2] M. Ikegami, J. Suzuki, K. Teshima, M. Kawaraya, T. Miyasaka, Sol. Energy Mater. Sol. Cells 93 (2009) 836. [3] T. Yamaguchi, N. Tobe, D. Matsumoto, T. Nagai, H. Arakawa, Sol. Energy Mater. Sol. Cells 94 (2010) 812. [4] Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide, L. Han, Jpn. J. Appl. Phys. 45 (2006) L638. [5] R. Buscaino, C. Baiocchi, C. Barolo, C. Medana, M. Grätzel, M.K. Nazeeruddin, G. Viscardi, Inorg. Chim. Acta 361 (2008) 798. [6] T. Miyasaka, M. Ikegami, Y. Kijitori, J. Electrochem. Soc. 154 (2007) A455. [7] H.C. Weerasinghe, P.M. Sirimanne, G.P. Simon, Y.B. Cheng, J. Photochem. Photobiol. A: Chem. 206 (2009) 64. [8] A.D. Pasquier, M. Stewart, T. Spitler, M. Coleman, Sol. Energy Mater. Sol. Cells 93 (2009) 528. [9] X. Li, H. Lin, J. Li, N. Wang, C. Lin, L. Zhang, J. Photochem. Photobiol. A: Chem. 195 (2008) 247. [10] M. Dürr, A. Schmid, M. Obermaier, S. Rosselli, A. Yasuda, G. Nelles, Nat. Mater. 4 (2005) 607. [11] J.H. Park, Y. Jun, H. Yun, S. Lee, M.G. Kang, J. Electrochem. Soc. 155 (2008) F145. [12] M.G. Kang, N. Park, K.S. Ryu, S.H. Chang, K. Kim, Sol. Energy Mater. Sol. Cells 90 (2006) 574. [13] S. Ito, N.C. Ha, G. Rothenberger, P. Liska, P. Comte, S.M. Zakeeruddin, P. Péchy, M.K. Nazeeruddin, M. Grätzel, Chem. Commun. (2006) 4004. [14] M. Iwasaki, C. Lee, T. Kim, W. Park, J. Ceram. Soc. Jpn. 116 (2008) 153. [15] T. Ma, T. Kida, M. Akiyama, K. Inoue, S. Tsunematsu, K. Yao, H. Noma, E. Abe, Electrochem. Commun. 5 (2003) 369. [16] T. Ma, X. Fang, M. Akiyama, K. Inoue, H. Noma, E. Abe, J. Electroanal. Chem. 574 (2004) 77.
Contents lists available at ScienceDirect
Electrochemistry Communications 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 / e l e c o m
High-efficiency flexible dye-sensitized solar cells fabricated by a novel friction-transfer technique Li Yang a, Liqiong Wu a, Mingxing Wu a, Gang Xin a, Hong Lin b, Tingli Ma a,⁎ a b
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China State Key Laboratory of New Ceramic and Fine Processing, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 15 May 2010 Accepted 20 May 2010 Available online 27 May 2010 Keywords: All-flexible Dye-sensitized solar cell Friction-transfer Tetrabutyl titanate
a b s t r a c t All-transparent and all-flexible dye-sensitized solar cells (DSCs) were fabricated using a novel, facile, and low-lost friction-transfer technique, which involved assembling TiO2 films on flexible substrates via hightemperature sintering. This friction-transfer technique led to a 25% enhancement in conversion efficiency compared with the compression method. The efficiency of the optimized all-flexible DSC reached 5.7%, which is approximately 73% higher than that of the all-flexible DSC prepared by the low-temperature sintering method (3.3%). © 2010 Elsevier B.V. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSCs) have been widely investigated in recent years due to their potential as environment-friendly and lowcost photovoltaic cells [1–3]. Currently, the highest conversion efficiency (η) of DSCs made on fluorine-doped tin oxide (FTO) glass was reported to be 11% [4,5]. In the pursuit to meet the requirements of portable applications, lightweight and bendable DSCs have attracted increasing attention. Generally, conductive polymer substrates, such as poly(ethylene naphthalate) coated with indiumdoped tin oxide (ITO/PEN), were used to fabricate flexible DSCs. However, these substrates cannot withstand high processing temperatures of up to 400–500 °C. Although several low-temperature sintering methods have been developed to prepare TiO2 films, the interparticle connection was poor, which directly affected the performance of the DSCs [6–9]. To solve this problem, Dürr et al. developed a lift-off method for preparing a hybrid DSC based on flexible photoanode and the photoanode with rigid FTO counter electrode (CE) [10]. Although the DSC provided a high conversion efficiency, the transfer procedure was complicated and unsuitable for practical applications. In order to fabricate flexible DSCs via hightemperature sintering, metallic sheets made of Ti and stainless steel have been employed [11–13]. These devices offered relatively high efficiencies. However, the applications of the DSCs were limited due to their opacity. To date, reports on all-transparent and all-flexible DSCs are limited. Furthermore, the efficiencies of the DSCs remain low.
Recently, Iwasaki et al. reported a DSC with a η of 2.6% — a result attributed to the poor interparticles connection under low-temperature sintering [14]. In this work, in order to obtain porous TiO2 films via hightemperature sintering, a friction-transfer technique is developed for the preparation of flexible photoanodes. The entire transfer process, in which no chemical reagents or noble metals were introduced, is convenient, fast, and efficient, as well as low-cost. We have also optimized the photoanodes by adjusting the proportion of TiO2 pastes, employing an industrially-applied spray-coating technique, and treating TiO2 films with tetrabutyl titanate (TBT) to solve the internal dislocation phenomenon. The photovoltaic properties and electronic processes of the DSCs containing these photoanodes are discussed. 2. Experimental section 2.1. Preparation of TiO2 pastes A mixture of 0.5 g commercial P25 powders (Degussa), TBT, and 10 mL anhydrous ethanol was allowed to stand for 3.5 h, before dispersing with an ultrasonicator. The resulting TiO2 pastes denoted as paste A were used for preparing the underlying TiO2 layer on ITO/PEN (110, Tobi, Japan). The TiO2 pastes used to fabricate the transferred TiO2 layer on the ceramic tile, also called paste B, were prepared according to our previous report [15]. 2.2. Preparation of nanocyrstalline TiO2 electrodes
⁎ Corresponding author. Tel./fax: + 86 411 39893820. E-mail address: [email protected] (T. Ma). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.05.026
Titania electrodes were fabricated by a low-temperature sintering method as below: Briefly, paste A was sprayed on ITO/PEN, which was
L. Yang et al. / Electrochemistry Communications 12 (2010) 1000–1003
1001
by sandwiching a redox (I−/I− 3 ) electrolyte solution, which was composed of 0.03 M I2, 0.06 M LiI, 0.6 M 1-butyl-3-methylimidazolium iodide, 0.1 M guanidinium thiocyanate, and 0.5 M 4-tertbutylpyridine in acetonitrile.
then sintered at 125 °C for 1 h to form the TiO2 film with a thickness of 28 μm. The same technique was also used for preparing the underlying layer of the friction-transfer process. Preparing titania electrodes via the friction-transfer technique is described as follows: paste B was coated on a ceramic tile by a doctor-blade method, which was then sintered at 500 °C for 30 min to form a 15 μm thick TiO2 film. Subsequently, the sintered TiO2 layer on the ceramic tile surface-tosurface contacted with the underlying TiO2 layer on the ITO/PEN, and was then easily transferred from the ceramic tile to the ITO/PEN by means of friction acting on the two layers. The friction was produced when the two surfaces moved across each other. These two TiO2 layers were tightly connected by compression with a pressure of 100 kgf cm− 2, finally forming the TiO2 film with an 8–10 μm thickness. The resulting titania electrode was immersed in a TBT/nbutanol mixture for 15 min at 90 °C. For comparison, the same pressure was applied to the TiO2 film prepared by the lowtemperature sintering method, ultimately obtaining a TiO2 film with 8–10 μm thickness. The area of the prepared TiO2 electrodes was 0.2 cm2.
Current–voltage curves of the DSCs were measured by a Keithley digital source meter (Keithley 2601, USA). The intensity of the incident light mimics AM 1.5 solar light through a 300 W solar simulator (Solar Light, USA). The incident light intensity was calibrated using a solar power meter (TES-1333R, Taiwan) and a silicon photocell (BS-520, Japan), and was set at 100 mW/cm2. The surface morphology of the nanocrystalline TiO2 films was characterized by scanning electron microscopy (SEM, Quanta 200F, FEI, Slovakia). The electrochemical impedance spectroscopy (EIS, Zenium Zahner, Germany) was actualized in the dark at a −0.80 V bias. The measured frequency ranged from 10 mHz to 1 MHz, and the AC amplitude was set at 10 mV.
2.3. Preparation of CEs
3. Results and discussion
An isopropanol solution containing 0.5% chloroplatinic acid was sprayed on ITO/PEN, which was then dried at 125 °C for 1 h. The ITO/ PEN sheet coated with chloroplatinic acid was immersed in water/ ethanol (4:1, v/v) containing 30 mM NaBH4 until the gas bubble vanished and was then dried at 125 °C for 1 h. Sputtered-Pt CEs on FTO glass were prepared according to our previous work [16].
3.1. Optimization of all-flexible DSCs
2.4. Fabrication of DSCs The obtained TiO2 electrodes were dipped into acetonitrile/4-tertbutyl alcohol (1:1, v/v) containing 5 × 10− 4 M cis-bis(isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II) bis-tetrabuylammonium (dye N719) for 12 h. The dye-sensitized TiO2 electrodes and platinized CEs were assembled to form DSCs (0.2 cm2-sized cells)
2.5. Characterization
Fig. 1(A) shows the process of preparing photoanodes of the allflexible DSCs. An industrially-applied spray-coating technique was used to fabricate flexible both photoanodes and CEs. This procedure is especially suitable for roll-to-roll manufacturing process. Furthermore, the preparation of pastes A and B developed by our group is simple compared to the sol–gel and hydrothermal methods. It has been reported that the compression method is used to prepare flexible photoanodes [3]. However, the compression method may generate TiO2 films with poor porosity, which can affect the properties of DSCs. Based on the compression method, therefore, a frictiontransfer technique was developed to achieve porous TiO2 films via high-temperature sintering. This friction-transfer was implemented
Fig. 1. (A) Schematic diagram of the preparation procedure of flexible DSCs. (B) SEM images of the composite TiO2 film on ITO/PEN treated with a TBT/n-butanol mixture. The composite TiO2 film was composed of the underlying and transferred TiO2 layers. Two types of magnification: 100,000× (a) and 10,000× (b).
1002
L. Yang et al. / Electrochemistry Communications 12 (2010) 1000–1003
Thermal decomposition and magnetron sputtering are widely used to deposit Pt catalyst on CEs. In this work, we prepared flexible CEs by chemical platinization using NaBH4 as a reducing agent. In this way, high-temperature heating needed for thermal decomposition, as well as the use of expensive instruments for magnetron sputtering, was avoided, making this technique suitable for the mass production of flexible CEs. 3.2. Photovoltaic performance of DSCs We assembled hybrid DSCs by combining ITO/PEN photoanodes and platinized FTO/glass CEs. Fig. 2A shows the photocurrent–voltage curves of the hybrid DSCs based on the ITO/PEN photoanodes prepared by the low-temperature sintering, compression, and friction-transfer techniques, respectively. The corresponding performance parameters of the DSCs are summarized in Table 1. For lowtemperature sintering, the hybrid DSC provides a η value of 3.8% (curve a). When the pressure of 100 kgf cm− 2 is applied to the TiO2 film via low-temperature sintering, the η value increases to 5.2% (curve b). On the basis of the compression method, the conversion efficiency of the DSC is further improved by introducing the frictiontransfer technique, reaching 6.5% (curve c).This result indicates that the high-temperature sintering of TiO2 film leads to a 25% enhancement in efficiency. In addition, the ITO/PEN photoanodes fabricated by the lowtemperature sintering and friction-transfer techniques were assembled into all-flexible DSCs with ITO/PEN CEs, providing η of 3.3% and 5.7%, respectively. According to our knowledge, this η value of 5.7% is the highest efficiency attained for all-flexible and all-transparent DSCs. Furthermore, the efficiency of the all-flexible DSC prepared by the friction-transfer technique is ca. 73% enhanced compared to that prepared via the low-temperature sintering method. 3.3. EIS study of DSCs
Fig. 2. (A) Photocurrent–voltage curves of the hybrid DSCs based on ITO/PEN photoanodes prepared by different techniques: (a) low-temperature sintering technique; (b) compression technique; and (c) friction-transfer technique. (B) Nyquist plots of hybrid DSCs based on ITO/PEN photoanodes with and without TBT treatment. To prepare photoanodes by the low-temperature sintering method, paste A with (a′) and without (a) TBT were used. For friction-transfer technique, the TiO2 composite composed of the underlying and transferred layers was treated with (b′) and without (b) a TBT/n-butanol mixture.
by the friction force which is created when the two surfaces of the underlying and transferred TiO2 layers move across each other. Therefore, this technique is facile and cost-effective. The transfer may bring forth a dislocation problem between the underlying and transferred TiO2 layers; thus, the two layers were treated with a TBT/n-butanol mixture. The SEM images in Fig. 1(B) illustrate that the TiO2 film on ITO/PEN fabricated using the friction-transfer technique shows a porous network structure.
The effects of TBT treatments on the electronic processes in the DSCs were investigated by EIS. Fig. 2B displays the Nyquist plots of the DSC samples with and without TBT treatment. The semi-circle in the intermediate-frequency region corresponds to the RC networks of the TiO2/dye/electrolyte interface, including the charge transfer resistance (Rct) and the constant phase element (CPE). The radius of the semi-circle for the DSC using paste A with TBT (curve a′) is much smaller than that without TBT (curve a), indicating that a smaller Rct exists in the solar cell when TBT is added to TiO2 pastes. This result demonstrates that TBT, as an inorganic binder, contributes to faster electron transport in TiO2 films. Moreover, when the composite TiO2 film composed of the underlying and transferred layers is treated with a TBT/n-butanol mixture (curve b′), the radius of the semi-circle for the DSC without the TBT treatment (curve b) sharply decreases. The electron transport in the composite TiO2 film is significantly improved due to the enhanced interconnection of TiO2 nanoparticles between the two layers. Thus, the
Table 1 Photovoltaic performance of DSCs. Photoanodes of hybrid and all-flexible DSCs were made on ITO/PEN, while platinized CEs of hybrid and all-flexible DSCs were fabricated on FTO/ glass and ITO/PEN, respectively. Type of DSC
Hybrid Hybrid Hybrid All-flexible All-flexible
Technique of preparing TiO2 film on ITO/PEN A
B
C
√ √ √ √ √
× √ √ × √
× × √ × √
Jsc/mA/cm2
Voc/V
FF
η (%)
ta/μm
6.9 10.6 12.3 6.4 13.9
0.77 0.80 0.80 0.78 0.78
0.72 0.62 0.66 0.66 0.52
3.8 5.2 6.5 3.3 5.7
29 8 8 28 10
A: TiO2 film on ITO/PEN was prepared by low-temperature sintering technique. B: A pressure of 100 kgf cm− 2 was applied to the TiO2 film by low-temperature sintering. C: A transferred TiO2 layer was prepared by friction-transfer technique combined with TBT treatment. a Film thickness.
L. Yang et al. / Electrochemistry Communications 12 (2010) 1000–1003
TBT treatment effectively solves the dislocation problem resulting from the friction-transfer technique. 4. Conclusion The all-transparent and all-flexible DSC fabricated by the frictiontransfer technique showed a high conversion efficiency of 5.7%, which increased by ca. 73% compared to that prepared by the low-temperature sintering. Based on the compression method, the 25% enhancement in the conversion efficiencies of the DSCs was obtained by a simple and low-cost friction-transfer technique, due to the advantage of high-temperature sintering. Furthermore, EIS results show that the addition of TBT to the TiO2 paste and the TBT treatment of the TiO2 composite films effectively improved the electron transport in TiO2 films, thereby enhancing the photovoltaic properties of the DSCs. Thus, the friction-transfer technique opens up a new way to prepare high-efficiency flexible DSCs. Acknowledgements This research is supported by the National Natural Science Foundation of China (No. 50773008) and the National High Technology Research and Development Program for Advanced Materials of China (No. 2009AA03Z220).
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