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Multi-step hydrothermally synthesized TiO2 nanoforests and its application to dye-sensitized solar cells
Materials Chemistry and Physics 135 (2012) 723e727
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Multi-step hydrothermally synthesized TiO2 nanoforests and its application to dye-sensitized solar cells Chih-Min Lin, Yun-Ching Chang, Jimmy Yao, Chao Wang, Claire Luo 1, Stuart (Shizhuo) Yin* Department of Electrical Engineering, The Pennsylvania State University, University Park, PA 16802, USA
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< Synthesize large surface area TiO2 nanoforests (TNF) by multi-step hydrothermal process. < TNF consists of trunks (nanorods) and branches (nanowires). < TNF offer both large surface area and high carrier transportation rate. < Enable a high efficiency dyesensitized solar cell (DSSC). < 3.93% conversion efficiency with a 5.1 micron high nanoforest.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 November 2011 Received in revised form 14 May 2012 Accepted 16 May 2012
Three-dimensional (3-D) titanium dioxide (TiO2) nanoforests with a large surface area and a high electron transport property have been successfully synthesized on transparent conducting oxide (TCO) substrate by using a unique multi-step hydrothermal synthesizing process. First, TiO2 nanorods with well-controlled density and length are grown on TCO glass substrate.TiO2 nanowires are then synthesized on TiO2 nanorods with another hydrothermal process. In comparison with previously reported one-dimensional (1-D) TiO2 nanorod arrays, TiO2 nanoforests offer a larger surface area for the same height, which is beneficial for achieving higher efficiency dye-sensitized solar cell (DSSC). The experimental results confirm that a conversion efficiency of 3.93% can be achieved with a 5.1 mm long TiO2 nanoforest based DSSC under AM1.5 illumination. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Three-dimensional (3-D) Titanium dioxide nanoforest Hydrothermal synthesizing process Dye-sensitized solar cell
1. Introduction Dye-sensitized solar cell (DSSC) is one of the most promising next generation solar cells because (1) it offers a relatively high photo-to-electric energy conversion efficiency 11.2%, (2) it has a lower manufacturing cost than that of silicon solar cell, (3) it can be fabricated on the flexible substrate, and (4) it is highly anticipated that it can meet the stability requirement (i.e., outdoor operation for at least 20 years) [1e4]. To further enhance the competitiveness of DSSC technology and expedite the * Corresponding author. E-mail address: [email protected] (S.(Shizhuo) Yin). 1 General Opto Solutions, LLC. 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.05.050
commercialization process, a higher conversion efficiency (e.g., w15% that closes the efficiency of commercial silicon solar cell) is desperately needed. Titanium dioxide (TiO2) is widely used for DSSCs because it has a wide band gap and a superior charge diffusion coefficient. In recent years, one-dimensional (1-D) nanostructured TiO2 thin films (including 1-D nanorod and 1-D nanotube) were proposed and investigated for the DSSC application [5e12] because they can provide a much longer electron diffusion length than that of the conventional TiO2 nanoparticle based structure. However, there is a fundamental tradeoff between the size of dye absorption area and electron diffusion length for the 1D nanostructured TiO2 thin film. For example, the larger diameter 1D nanorod has a lower recombination rate during the charge transport process but it also results in a smaller surface area [13].
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C.-M. Lin et al. / Materials Chemistry and Physics 135 (2012) 723e727
To overcome aforementioned limitation of existing 1D nanostructured TiO2 thin film, in this paper, we presents a new 3D morphology: TiO2 nanoforest, which is composed of nanorods with a diameter of several hundred nanometers as trunks to provide high charge transport capability and nanowires with a diameter of tens of nm as branches to offer a large surface area. It should be noted that although ZnO nanoforest has been demonstrated recently [14], the large surface area TiO2 nanoforest and its application to DSSC is reported in this paper. The higher power conversion efficiency of DSSCs based on TiO2 nanoforest thin film with less thickness than ZnO thin film is also demonstrated in the following discussion. 2. Experimental methods and procedures 2.1. Synthesis of TiO2 nanoforest A unique multi-step hydrothermal process was used to synthesize TiO2 nanoforest, as illustrated in Fig. 1. First, fluorine doped tin oxide (FTO) glass substrates (TEC8/TEC15) were well cleaned by ultrasonication in ethanol, isopropanol, and acetone for 10 min, respectively. Second, a two-step hydrothermal process was used to synthesize TiO2 nanoforest, as described in the following. 2.1.1. Growth of TiO2 nanorods (trunks) The first step was to synthesize TiO2 nanorod (i.e., trunk) on the well-cleaned FTO substrate. FTO glass substrates were put in a Teflon reactor which contained a mixture solution of DI water (30 ml), HCl aqueous solution (30 ml with a 37% concentration), and titanium butoxide (0.5 ml). The reactor was sealed in the metal bomb and baked at a temperature of 150 C for 6e20 h. 2.1.2. Growth of TiO2 nanowires (branches) The second step was to synthesize TiO2 nanowires (i.e., branches) on the previously synthesized TiO2 nanorods with following procedures. First, after rinsing the samples with deionized (DI) water and ethanol for several times and drying with gentle nitrogen flow, the TiO2 nanorod samples were transferred to another Teflon reactor that contained 10 mL toluene, 0.25 mLe2.0 mL titanium butoxide, 1 mL HCl aqueous solution (37%), and 1 mL TiCl4
solution(1 M in toluene). Second, the reactor was sealed in a metal bomb again and baked at a temperature of180 C for several hours. Last, the sample was removed from the reactor, and then rinsed with DI water and ethanol for several times and annealed at a temperature of 450 C for 1 h. 2.2. Fabrication and test of TiO2 nanoforest based DSSC The DSSC was fabricated after synthesizing TiO2 nanoforest with following procedures: First, TiO2 nanoforest samples were immersed with 0.3 mM N719 dye (made by Solaronix Inc., Switzerland) solution for 24 h. Second, the Redox electrolyte was synthesized by mixing a commercial electrolyte (MPN-100, Solaronix Inc., Switzerland) with acetonitrille that contained 0.1 M LiI at a ratio of 1:2. Third, a 25 mm hot-melt spacer SX-1170-25PF (Solaronix Inc., Switzerland) was loaded between nanoforest sample and counter electrode of platinum (Pt) coated FTO glass. Finally, DSSC was fabricated by filling Redox electrolyte into the sandwiched structure. The performance of TiO2 nanoforest based DSSC samples was quantitatively tested with a standard AM1.5 solar simulator made by Newport Corporation. 3. Results and discussion By using the procedures as described in Section 2, TiO2 nanorods with a length around 2 mm and a diameter of several hundred nanometers were synthesized in one single round hydrothermal process. The density of nanorods synthesized on FTO substrate was controlled by the concentration of titania precursor. Furthermore, to achieve a long nanorod without making them contact each other, a multi-step growing process was employed. As reported in Ref. 12, the maximum length of TiO2 nanorod was only about 2.8 microns if only one growing cycle was used because the growing solution reached the chemical equilibrium state after certain time (e.g., 20 h). In our experiment, eight growing cycles were used and growing time for each cycle was around 6 h. And, in each growing cycle, a fresh solution with same composition was employed. Thus, the nanorod could continue to grow without the maximum length limitation of the conventional one cycle growing method since the chemical equilibrium condition was not reached during each growing cycle.
Fig. 1. An illustration of growing process of TiO2 nanoforest and its application to DSSC.
C.-M. Lin et al. / Materials Chemistry and Physics 135 (2012) 723e727
To achieve a large surface area of nanoforest, longer nanorods and larger spacing between the adjacent nanorods are needed. The length and spacing of nanorod depends on the length of baking time and baking temperature of hydrothermal process. The longer baking time and higher baking temperature can result in longer nanorods. However, they can also reduce the spacing between the adjacent nanorods. To avoid contact of adjacent nanorods (that can result in a reduced surface area), the length of baking time for each cycle was shortened to 6 h rather than 20 hours as used in Ref.[12]. The observation of the growing process of nanorods showed that nothing appeared on the surface of FTO substrates when the growing time was less than 3 h. The nanorods began to grow on FTO substrates after 3 h, and the density of nanorods on FTO substrates increased as well. To restrain the increasing rate of the density of nanorods, a compulsive termination of hydrothermal process was taken by the end of each 6 h. After cooling down, the samples were transferred to a fresh solution immediately for the next step of process. The advantage of this process is that the existing nanorods would continue to grow for each new hydrothermal cycle, but new nanorods would only appear after 3 h, which reduces the generation rate of new nanorods so that longer nanorods with larger spacing among them can be achieved. The ambient condition for the hydrothermal process was set as 30 mL DI water þ 30 mL HCl aqueous solution (37% concentration) þ 0.5 ml titanium butoxide. The experimental result showed that there were no noticeable grown nanorods after the first and second growing cycles. However, one can started to observe the grown nanorods after the third growing cycle. As an example, Fig. 2(a) and (b) showed the scanning electron microscope (SEM) images of the top and cross-section views of the grown nanorods after the third growing cycle. One could clearly see the grown nanorods. The average length and
725
diameter of the nanorods were around 1.3 mm and 50 nm, respectively. With the increase of the number of the cycles of hydrothermal processes, the length, the diameter, and the amounts of the nanorods is increased. Fig. 2(c) and (d) shown the top and cross-section views of the TiO2 nanorods after eight growing cycles. It can be observed that the average length and diameter of nanorods of were around 4.1 mm and 300 nm, respectively. These experimental results clearly demonstrated that longer nanorods could indeed be grown by employing above described multiple step growing technique. Furthermore, since the growing happens in both the longitudinal and transversal directions (i.e., the nanorod becomes thicker after each growing cycle), to avoid the contact among adjacent nanorods, the separation distance for the initial growing cycle was controlled by adjusting the growing condition (e.g., the baking temperature and baking time). For the second step of hydrothermal process, non-polar solvent, toluene, was adopted as the main solvent to synthesize smaller diameter of TiO2 nanowire as branches. The nanowires were grown on the nanorod substrates synthesized during the first step of hydrothermal process. The growing rate, direction and dimension of the nanowire depend on the concentration of titanium precursor. The experimental results shown that nanowires had a tendency to grow in vertical directions as the concentration level of titanium precursor increased (e.g., >2.0 ml of titanium precursor added in 10 ml toluene). As an example, Fig. 3(a) is a conceptual drawing of the grown nanowires on nanorods under this condition. Fig. 3(b) and (c) shown the SEM images of the top and cross-section views of the grown nanowires on nanorods (with a length around 1.3 mm). It clearly shows a nanoforest structure. The total height of the nanoforest is around 2.4 mm. The length of the nanowires can be over 1 mm and the average diameter of the nanowire is less than 30 nm. It is noted that although nanowires can also grow on TCO
Fig. 2. (a) The top view of TiO2 nanorods after three growing cycle with an average length around 1.3 mm and a diameter around 50 nm. (b) The cross-section view of TiO2 nanorods after three growing cycle with an average length around 1.3 mm and a diameter around 50 nm. (c) The top view of TiO2 nanorods after eight growing cycle with an average length around 4.1 mm and a diameter around 300 nm. (d) The cross-section view of TiO2 nanorods after eight growing cycle after eight growing cycle with an average length around 4.1 mm and a diameter around 300 nm.
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C.-M. Lin et al. / Materials Chemistry and Physics 135 (2012) 723e727 Table 1 Characteristics of TiO2 nanoforest based DSSC for Samples 1, 2, 3, 4, and 5, respectively.
Sample Sample Sample Sample Sample
1 2 3 4 5
Thickness(mm)
Efficiency(%)
JSC(mA/cm2)
VOC(V)
Fill factor
1.5 2.0 2.3 3.7. 5.1.
0.04 0.58 1.25 2.50 3.93
0.28 1.15 2.57 4.90 7.30
0.504 0.797 0.774 0.756 0.787
0.30 0.63 0.63 0.67 0.68
the conversion efficiency, h, was also calculated based on following equation
h¼
JSC Voc FF ; Is
(1)
where JSC denotes the short-circuit photocurrent density, Voc represents the open-circuit photovoltage, FF stands for the fill factor, and Is is the intensity of the incident light. Table 1 and Fig. 4 summarize the testing results for Samples 1, 2, 3, 4, and 5, of which the thickness are 1.5, 2.0, 2.3, 3.7, and 5.1 microns, respectively. It can be clearly seen that the conversion efficiency increases as the height of the nanoforest increases due to the increased surface area. The conversion efficiency is up to 3.93% for a 5.1 mm height TiO2 nanoforest, which is much higher than the DSSC based on ZnO nanoforest. For instance, the conversion efficiency of ZnO nanoforest is only about 2.63% even when the thickness of ZnO nanoforest is13 microns, which is substantially lower than the conversion efficiency achieved with a 5.1 micron thick TiO2 nanoforest based DSSC. Thus, TiO2 nanoforest based DSSC can offer a much higher conversion efficiency than that of ZnO nanoforest based DSSC when the thickness of the nanoforest is the same.This is because TiO2 nanoforest has a higher electron injection efficiency than that of ZnO nanoforest [15]. It can be also noted that the reason of conversion efficiency increases with the increase of the height of the nanoforest is due to the increased surface area so that a larger amount of dyes can be attached on the surface of nanoforest, which results in a better light absorption. The conversion efficiency increases with increase in the height of nanoforest, however, it reaches a certain maximum level at certain height (e.g., 20 microns). After major of light (e.g., 99.9%) is absorbed by dye, the further increase in height can reduce the conversion efficiency due to the reduced transport efficiency.
Fig. 3. (a) A conceptual illustration of the grown nanowires on nanorod substrates. (b) A SEM image of top view of the grown nanowires on nanorod substrates, which shows a nanoforest configuration. (c) A SEM image of cross-section view of the grown nanowires on nanorod substrates, which shows a nanoforest configuration.
substrate, they can only grow a thin layer (<100 nm) because the further growing in upper direction is blocked by the nanowires grown on the nanorod. Finally, a set of TiO2 nanoforest based DSSCs with different height nanoforest from 1.5 mm to 5.1 mm were synthesized by using the procedures as described in Section 2.2. The performances of the DSSCs were also quantitatively tested with a standard AM1.5 solar simulator made by Newport Corporation. The important parameters of the solar cell, including the photocurrentevoltage curve, the short-circuit current density, the open-circuit voltage, and the fill factor were measured. Based on these measured data,
Fig. 4. IeV characteristics of TiO2 nanoforest based DSSCs for Sample 1, Sample 2, Sample 3, Sample 4, and Sample 5, respectively.
C.-M. Lin et al. / Materials Chemistry and Physics 135 (2012) 723e727
4. Conclusion A new type of DSSC based on TiO2 nanoforest was demonstrated in this paper. The TiO2 nanoforest was synthesized by using the unique multi-step hydrothermal process. The major advantages of TiO2 nanoforest based DSSC included following aspects: First, the nanoforest structure overcame a fundamental limitation of the conventional DSSC that was the tradeoff between the light harvesting efficiency and the electron collection efficiency. Second, unlike ZnO nanoforest that had a poor electron injection efficiency, TiO2 nanoforest also had a good electron injection efficiency. Thus, all the required characteristics for achieving the high conversion efficiency, including the high light harvesting efficiency, high electron collection efficiency, and high electron injection efficiency, could be achieved simultaneously, which enabled the realization of low cost and high conversion efficiency solar cells. The experimental results confirmed that a conversion efficiency of 3.93% could be achieved for a TiO2 nanoforest with a 5.1 micron long trunk length, which was higher than ZnO nanoforest based DSSC that had a much longer trunk length (13 microns). This experimental result clearly demonstrated the advantage of using TiO2 nanoforest to replace ZnO nanoforest for the application of DSSC. A even higher conversion efficiency is anticipated if a longer trunk (e.g., 13 micron) TiO2
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nanoforest is employed due to the increased surface area., which will be investigated in the future.
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[10] [11] [12] [13] [14]
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B. O’regan, M. Gratzel, Nature 353 (1991) 737. M. Gratzel, Inorg. Chem. 44 (2005) 6841. M. Gratzel, Acc. Chem. Res. 42 (2009) 1788. Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide, L. Han, Jpn. J. Appl. Phys. 45 (2006) L638. A.C. Fisher, L.M. Peter, E.A. Ponomarev, A.B. Walker, K.G. Wijayantha, J. Phys. Chem. B 104 (2000) 949. T. Oekermann, D. Zhang, T. Yoshida, H. Minoura, J. Phys. Chem. B 108 (2004) 2227. S. Nakade, M. Matsuda, S. Kambe, Y. Saito, T. Kitamura, T. Sakata, Y. Wada, H. Mori, S. Yanagida, J. Phys. Chem. B 106 (2002) 10004. G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Nano Lett. 5 (2005) 191. Karthik Shankar, Gopal K. Mor, Haripriya E. Prakasam, Sorachon Yoriya, Maggie Paulose, Oomman K. Varghese, Craig A. Grimes, Nanotechnology 18 (2007) 065707. Oomman K. Varghese, Maggie Paulose, Craig A. Grimes, Nature Nanotechnology 4 (2009) 592. X. Feng, K. Shankar, O.K. Varghese, M. Paulose, T.J. Latempa, C.A. Grimes, Nano Lett. 8 (2008) 3781. B. Liu, E.S. Aydil, J. Am. Chem. Soc. 131 (2009) 3985e3990. A. Soudi, P. Dhakal, Y. Gu, Appl. Phys. Lett. 96 (2010) 253115. Seung Hwan Ko, Daeho Lee, Hyun Wook Kang, Koo Hyun Nam, Joon Yeob Yeo, Suk Joon Hong, Costas P. Grigoropoulos, Hyung Jin Sung, Nano Lett. 11 (2011) 666. M. Quintana, T. Edvinsson, A. Hagfeldt, G. Boscholoo, J. Phys. Chem. 111 (2007) 1035.
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Multi-step hydrothermally synthesized TiO2 nanoforests and its application to dye-sensitized solar cells Chih-Min Lin, Yun-Ching Chang, Jimmy Yao, Chao Wang, Claire Luo 1, Stuart (Shizhuo) Yin* Department of Electrical Engineering, The Pennsylvania State University, University Park, PA 16802, USA
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< Synthesize large surface area TiO2 nanoforests (TNF) by multi-step hydrothermal process. < TNF consists of trunks (nanorods) and branches (nanowires). < TNF offer both large surface area and high carrier transportation rate. < Enable a high efficiency dyesensitized solar cell (DSSC). < 3.93% conversion efficiency with a 5.1 micron high nanoforest.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 November 2011 Received in revised form 14 May 2012 Accepted 16 May 2012
Three-dimensional (3-D) titanium dioxide (TiO2) nanoforests with a large surface area and a high electron transport property have been successfully synthesized on transparent conducting oxide (TCO) substrate by using a unique multi-step hydrothermal synthesizing process. First, TiO2 nanorods with well-controlled density and length are grown on TCO glass substrate.TiO2 nanowires are then synthesized on TiO2 nanorods with another hydrothermal process. In comparison with previously reported one-dimensional (1-D) TiO2 nanorod arrays, TiO2 nanoforests offer a larger surface area for the same height, which is beneficial for achieving higher efficiency dye-sensitized solar cell (DSSC). The experimental results confirm that a conversion efficiency of 3.93% can be achieved with a 5.1 mm long TiO2 nanoforest based DSSC under AM1.5 illumination. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Three-dimensional (3-D) Titanium dioxide nanoforest Hydrothermal synthesizing process Dye-sensitized solar cell
1. Introduction Dye-sensitized solar cell (DSSC) is one of the most promising next generation solar cells because (1) it offers a relatively high photo-to-electric energy conversion efficiency 11.2%, (2) it has a lower manufacturing cost than that of silicon solar cell, (3) it can be fabricated on the flexible substrate, and (4) it is highly anticipated that it can meet the stability requirement (i.e., outdoor operation for at least 20 years) [1e4]. To further enhance the competitiveness of DSSC technology and expedite the * Corresponding author. E-mail address: [email protected] (S.(Shizhuo) Yin). 1 General Opto Solutions, LLC. 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.05.050
commercialization process, a higher conversion efficiency (e.g., w15% that closes the efficiency of commercial silicon solar cell) is desperately needed. Titanium dioxide (TiO2) is widely used for DSSCs because it has a wide band gap and a superior charge diffusion coefficient. In recent years, one-dimensional (1-D) nanostructured TiO2 thin films (including 1-D nanorod and 1-D nanotube) were proposed and investigated for the DSSC application [5e12] because they can provide a much longer electron diffusion length than that of the conventional TiO2 nanoparticle based structure. However, there is a fundamental tradeoff between the size of dye absorption area and electron diffusion length for the 1D nanostructured TiO2 thin film. For example, the larger diameter 1D nanorod has a lower recombination rate during the charge transport process but it also results in a smaller surface area [13].
724
C.-M. Lin et al. / Materials Chemistry and Physics 135 (2012) 723e727
To overcome aforementioned limitation of existing 1D nanostructured TiO2 thin film, in this paper, we presents a new 3D morphology: TiO2 nanoforest, which is composed of nanorods with a diameter of several hundred nanometers as trunks to provide high charge transport capability and nanowires with a diameter of tens of nm as branches to offer a large surface area. It should be noted that although ZnO nanoforest has been demonstrated recently [14], the large surface area TiO2 nanoforest and its application to DSSC is reported in this paper. The higher power conversion efficiency of DSSCs based on TiO2 nanoforest thin film with less thickness than ZnO thin film is also demonstrated in the following discussion. 2. Experimental methods and procedures 2.1. Synthesis of TiO2 nanoforest A unique multi-step hydrothermal process was used to synthesize TiO2 nanoforest, as illustrated in Fig. 1. First, fluorine doped tin oxide (FTO) glass substrates (TEC8/TEC15) were well cleaned by ultrasonication in ethanol, isopropanol, and acetone for 10 min, respectively. Second, a two-step hydrothermal process was used to synthesize TiO2 nanoforest, as described in the following. 2.1.1. Growth of TiO2 nanorods (trunks) The first step was to synthesize TiO2 nanorod (i.e., trunk) on the well-cleaned FTO substrate. FTO glass substrates were put in a Teflon reactor which contained a mixture solution of DI water (30 ml), HCl aqueous solution (30 ml with a 37% concentration), and titanium butoxide (0.5 ml). The reactor was sealed in the metal bomb and baked at a temperature of 150 C for 6e20 h. 2.1.2. Growth of TiO2 nanowires (branches) The second step was to synthesize TiO2 nanowires (i.e., branches) on the previously synthesized TiO2 nanorods with following procedures. First, after rinsing the samples with deionized (DI) water and ethanol for several times and drying with gentle nitrogen flow, the TiO2 nanorod samples were transferred to another Teflon reactor that contained 10 mL toluene, 0.25 mLe2.0 mL titanium butoxide, 1 mL HCl aqueous solution (37%), and 1 mL TiCl4
solution(1 M in toluene). Second, the reactor was sealed in a metal bomb again and baked at a temperature of180 C for several hours. Last, the sample was removed from the reactor, and then rinsed with DI water and ethanol for several times and annealed at a temperature of 450 C for 1 h. 2.2. Fabrication and test of TiO2 nanoforest based DSSC The DSSC was fabricated after synthesizing TiO2 nanoforest with following procedures: First, TiO2 nanoforest samples were immersed with 0.3 mM N719 dye (made by Solaronix Inc., Switzerland) solution for 24 h. Second, the Redox electrolyte was synthesized by mixing a commercial electrolyte (MPN-100, Solaronix Inc., Switzerland) with acetonitrille that contained 0.1 M LiI at a ratio of 1:2. Third, a 25 mm hot-melt spacer SX-1170-25PF (Solaronix Inc., Switzerland) was loaded between nanoforest sample and counter electrode of platinum (Pt) coated FTO glass. Finally, DSSC was fabricated by filling Redox electrolyte into the sandwiched structure. The performance of TiO2 nanoforest based DSSC samples was quantitatively tested with a standard AM1.5 solar simulator made by Newport Corporation. 3. Results and discussion By using the procedures as described in Section 2, TiO2 nanorods with a length around 2 mm and a diameter of several hundred nanometers were synthesized in one single round hydrothermal process. The density of nanorods synthesized on FTO substrate was controlled by the concentration of titania precursor. Furthermore, to achieve a long nanorod without making them contact each other, a multi-step growing process was employed. As reported in Ref. 12, the maximum length of TiO2 nanorod was only about 2.8 microns if only one growing cycle was used because the growing solution reached the chemical equilibrium state after certain time (e.g., 20 h). In our experiment, eight growing cycles were used and growing time for each cycle was around 6 h. And, in each growing cycle, a fresh solution with same composition was employed. Thus, the nanorod could continue to grow without the maximum length limitation of the conventional one cycle growing method since the chemical equilibrium condition was not reached during each growing cycle.
Fig. 1. An illustration of growing process of TiO2 nanoforest and its application to DSSC.
C.-M. Lin et al. / Materials Chemistry and Physics 135 (2012) 723e727
To achieve a large surface area of nanoforest, longer nanorods and larger spacing between the adjacent nanorods are needed. The length and spacing of nanorod depends on the length of baking time and baking temperature of hydrothermal process. The longer baking time and higher baking temperature can result in longer nanorods. However, they can also reduce the spacing between the adjacent nanorods. To avoid contact of adjacent nanorods (that can result in a reduced surface area), the length of baking time for each cycle was shortened to 6 h rather than 20 hours as used in Ref.[12]. The observation of the growing process of nanorods showed that nothing appeared on the surface of FTO substrates when the growing time was less than 3 h. The nanorods began to grow on FTO substrates after 3 h, and the density of nanorods on FTO substrates increased as well. To restrain the increasing rate of the density of nanorods, a compulsive termination of hydrothermal process was taken by the end of each 6 h. After cooling down, the samples were transferred to a fresh solution immediately for the next step of process. The advantage of this process is that the existing nanorods would continue to grow for each new hydrothermal cycle, but new nanorods would only appear after 3 h, which reduces the generation rate of new nanorods so that longer nanorods with larger spacing among them can be achieved. The ambient condition for the hydrothermal process was set as 30 mL DI water þ 30 mL HCl aqueous solution (37% concentration) þ 0.5 ml titanium butoxide. The experimental result showed that there were no noticeable grown nanorods after the first and second growing cycles. However, one can started to observe the grown nanorods after the third growing cycle. As an example, Fig. 2(a) and (b) showed the scanning electron microscope (SEM) images of the top and cross-section views of the grown nanorods after the third growing cycle. One could clearly see the grown nanorods. The average length and
725
diameter of the nanorods were around 1.3 mm and 50 nm, respectively. With the increase of the number of the cycles of hydrothermal processes, the length, the diameter, and the amounts of the nanorods is increased. Fig. 2(c) and (d) shown the top and cross-section views of the TiO2 nanorods after eight growing cycles. It can be observed that the average length and diameter of nanorods of were around 4.1 mm and 300 nm, respectively. These experimental results clearly demonstrated that longer nanorods could indeed be grown by employing above described multiple step growing technique. Furthermore, since the growing happens in both the longitudinal and transversal directions (i.e., the nanorod becomes thicker after each growing cycle), to avoid the contact among adjacent nanorods, the separation distance for the initial growing cycle was controlled by adjusting the growing condition (e.g., the baking temperature and baking time). For the second step of hydrothermal process, non-polar solvent, toluene, was adopted as the main solvent to synthesize smaller diameter of TiO2 nanowire as branches. The nanowires were grown on the nanorod substrates synthesized during the first step of hydrothermal process. The growing rate, direction and dimension of the nanowire depend on the concentration of titanium precursor. The experimental results shown that nanowires had a tendency to grow in vertical directions as the concentration level of titanium precursor increased (e.g., >2.0 ml of titanium precursor added in 10 ml toluene). As an example, Fig. 3(a) is a conceptual drawing of the grown nanowires on nanorods under this condition. Fig. 3(b) and (c) shown the SEM images of the top and cross-section views of the grown nanowires on nanorods (with a length around 1.3 mm). It clearly shows a nanoforest structure. The total height of the nanoforest is around 2.4 mm. The length of the nanowires can be over 1 mm and the average diameter of the nanowire is less than 30 nm. It is noted that although nanowires can also grow on TCO
Fig. 2. (a) The top view of TiO2 nanorods after three growing cycle with an average length around 1.3 mm and a diameter around 50 nm. (b) The cross-section view of TiO2 nanorods after three growing cycle with an average length around 1.3 mm and a diameter around 50 nm. (c) The top view of TiO2 nanorods after eight growing cycle with an average length around 4.1 mm and a diameter around 300 nm. (d) The cross-section view of TiO2 nanorods after eight growing cycle after eight growing cycle with an average length around 4.1 mm and a diameter around 300 nm.
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C.-M. Lin et al. / Materials Chemistry and Physics 135 (2012) 723e727 Table 1 Characteristics of TiO2 nanoforest based DSSC for Samples 1, 2, 3, 4, and 5, respectively.
Sample Sample Sample Sample Sample
1 2 3 4 5
Thickness(mm)
Efficiency(%)
JSC(mA/cm2)
VOC(V)
Fill factor
1.5 2.0 2.3 3.7. 5.1.
0.04 0.58 1.25 2.50 3.93
0.28 1.15 2.57 4.90 7.30
0.504 0.797 0.774 0.756 0.787
0.30 0.63 0.63 0.67 0.68
the conversion efficiency, h, was also calculated based on following equation
h¼
JSC Voc FF ; Is
(1)
where JSC denotes the short-circuit photocurrent density, Voc represents the open-circuit photovoltage, FF stands for the fill factor, and Is is the intensity of the incident light. Table 1 and Fig. 4 summarize the testing results for Samples 1, 2, 3, 4, and 5, of which the thickness are 1.5, 2.0, 2.3, 3.7, and 5.1 microns, respectively. It can be clearly seen that the conversion efficiency increases as the height of the nanoforest increases due to the increased surface area. The conversion efficiency is up to 3.93% for a 5.1 mm height TiO2 nanoforest, which is much higher than the DSSC based on ZnO nanoforest. For instance, the conversion efficiency of ZnO nanoforest is only about 2.63% even when the thickness of ZnO nanoforest is13 microns, which is substantially lower than the conversion efficiency achieved with a 5.1 micron thick TiO2 nanoforest based DSSC. Thus, TiO2 nanoforest based DSSC can offer a much higher conversion efficiency than that of ZnO nanoforest based DSSC when the thickness of the nanoforest is the same.This is because TiO2 nanoforest has a higher electron injection efficiency than that of ZnO nanoforest [15]. It can be also noted that the reason of conversion efficiency increases with the increase of the height of the nanoforest is due to the increased surface area so that a larger amount of dyes can be attached on the surface of nanoforest, which results in a better light absorption. The conversion efficiency increases with increase in the height of nanoforest, however, it reaches a certain maximum level at certain height (e.g., 20 microns). After major of light (e.g., 99.9%) is absorbed by dye, the further increase in height can reduce the conversion efficiency due to the reduced transport efficiency.
Fig. 3. (a) A conceptual illustration of the grown nanowires on nanorod substrates. (b) A SEM image of top view of the grown nanowires on nanorod substrates, which shows a nanoforest configuration. (c) A SEM image of cross-section view of the grown nanowires on nanorod substrates, which shows a nanoforest configuration.
substrate, they can only grow a thin layer (<100 nm) because the further growing in upper direction is blocked by the nanowires grown on the nanorod. Finally, a set of TiO2 nanoforest based DSSCs with different height nanoforest from 1.5 mm to 5.1 mm were synthesized by using the procedures as described in Section 2.2. The performances of the DSSCs were also quantitatively tested with a standard AM1.5 solar simulator made by Newport Corporation. The important parameters of the solar cell, including the photocurrentevoltage curve, the short-circuit current density, the open-circuit voltage, and the fill factor were measured. Based on these measured data,
Fig. 4. IeV characteristics of TiO2 nanoforest based DSSCs for Sample 1, Sample 2, Sample 3, Sample 4, and Sample 5, respectively.
C.-M. Lin et al. / Materials Chemistry and Physics 135 (2012) 723e727
4. Conclusion A new type of DSSC based on TiO2 nanoforest was demonstrated in this paper. The TiO2 nanoforest was synthesized by using the unique multi-step hydrothermal process. The major advantages of TiO2 nanoforest based DSSC included following aspects: First, the nanoforest structure overcame a fundamental limitation of the conventional DSSC that was the tradeoff between the light harvesting efficiency and the electron collection efficiency. Second, unlike ZnO nanoforest that had a poor electron injection efficiency, TiO2 nanoforest also had a good electron injection efficiency. Thus, all the required characteristics for achieving the high conversion efficiency, including the high light harvesting efficiency, high electron collection efficiency, and high electron injection efficiency, could be achieved simultaneously, which enabled the realization of low cost and high conversion efficiency solar cells. The experimental results confirmed that a conversion efficiency of 3.93% could be achieved for a TiO2 nanoforest with a 5.1 micron long trunk length, which was higher than ZnO nanoforest based DSSC that had a much longer trunk length (13 microns). This experimental result clearly demonstrated the advantage of using TiO2 nanoforest to replace ZnO nanoforest for the application of DSSC. A even higher conversion efficiency is anticipated if a longer trunk (e.g., 13 micron) TiO2
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nanoforest is employed due to the increased surface area., which will be investigated in the future.
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