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Effects of boron doping in TiO2 nanotubes and the performance of dye-sensitized solar cells
Applied Surface Science 258 (2012) 6479–6484
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Effects of boron doping in TiO2 nanotubes and the performance of dye-sensitized solar cells Alagesan Subramanian, Hong-Wen Wang ∗ Department of Chemistry, Center for Nanotechnology at CYCU, Chung-Yuan Christian University, 200 Chung-Pei Road, Chung Li 320, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 23 February 2012 Received in revised form 9 March 2012 Accepted 12 March 2012 Available online 20 March 2012 Keywords: Boron doping TiO2 nanotube Dye-sensitized solar cell
a b s t r a c t Titanium nanotubes doped with boron used as the photoelectrode for dye-sensitized solar cells were investigated. The materials were characterized by SEM, XRD, and UV–vis spectroscopy and their photoconversion efficiencies were evaluated. The chemical compositions of TiO2 nanotubes (TNA) and boron doped TNA (B-TNA) were identified by the energy dispersive X-ray spectroscopy (EDS). XRD evidenced the presence of anatase as the main phase and presented the existence of boron elements at interstitial sites between the TiO2 lattices. The UV–vis spectra indicated the narrowing of band gap upon doping boron into titanium nanotubes. The photovoltaic properties were measured by a current–voltage meter under AM1.5 simulated light radiation. The boron-doped TiO2 nanotube arrays showed an enhanced performance with a photocurrent density of 7.85 ± 0.20 mA/cm2 and an overall conversion efficiency () of 3.44 ± 0.10%. The enhanced performance was attributed to the enhanced electron injection rate and retardation of the charge recombination, which could be due to perfect matching between the LUMO of dye molecules and the conduction band of TiO2 . Electrochemical impedance spectroscopy (EIS) measurement indicated the longer electron lifetime and reduced TiO2 /dye/electrolyte interface resistance for boron doped TiO2 nanotubes than that of undoped TiO2 nanotubes. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Dye-sensitized solar cells are currently attracting widespread academic and commercial interests for the conversion of sunlight into electricity with low cost and high efficiency [1–4]. Excellent photo-electron conversion efficiencies have been reported for regenerative photo electrochemical cells based on nanocrystalline titanium dioxide (TiO2 ) films, sensitized by ruthenium-dyes using iodide/triiodide (I− /I3 − ) redox couple in an organic solvent as the electrolyte [5,6]. TiO2 nanotubes (TNA) have attracted tremendous attention due to the combination of a wide band gap semiconductive nature with high surface area and high aspect ratio. TiO2 nanotube arrays have been used for a variety of applications including hydrogen production by water photoelectrolysis [7,8], gas sensing [9] and dye-sensitized solar cells [1,10,11]. Detailed studies have shown that the charge recombination in nanotube-arraybased DSSCs is much slower than that in nanoparticle-based DSSCs, which results in improved charge collection efficiency [12,13]. Compared with nanoparticles, TiO2 nanowires or nanotubes are suggested to be superior in chemical and photoelectrochemical
∗ Corresponding author. Tel.: +886 3 2653310; fax: +886 3 2653399. E-mail addresses: [email protected] (A. Subramanian), [email protected] (H.-W. Wang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.03.064
performance, due to their one-dimensional channel for carrier transportation [14,15]. The arrangement of the highly ordered titania nanotube array perpendicular to the surface permits facile charge transfer along the length of the nanotubes from the solution to the conductive substrate, thereby reducing the losses incurred by charge-hopping across the nanoparticle grain boundaries [16]. TiO2 has a band gap of 3.0 eV for the rutile phase and 3.2 eV for the anatase phase. These values require ultraviolet (UV) radiation to activate. Recently, many attempts have been made to promote TiO2 activities under visible light excitation to allow the utilization of the solar spectrum, by modifying titanium dioxide with nonmetals, such as N, C and F to efficiently extend the photoresponse of TiO2 from UV to visible light region [17–19]. The introduction of foreign atoms in these materials simultaneously alters the charge transport properties, optical properties, and crystallinity. Therefore, the DSSCs based on these nonmetal-doped TiO2 film deserved more studies to fully explore the doping effect on the TiO2 nanotubes for DSSCs. In recent years, doping nitrogen into TiO2 nanoparticles (TNP) had been studied intensively for photocatalytic activities as well as in the DSSCs [20–24]. However, a little attention and less detailed studies were given to boron doping into TiO2 nanoparticles for photocatalytic activities, especially, in DSSC study. There were very few reports on boron doped TiO2 nanotubes for the DSSC efficiency study to the best of our knowledge. Ruan et al. [25] were the first to report the boron doping of the titania nanotube arrays
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in the DSSC. However, there was no detailed study about the DSSC in their work. In this regard, TiO2 nanotube doped with boron was investigated and its performance on DSSC was studied in details in our study. 2. Experimental procedures 2.1. Preparation of TiO2 electrodes The growth of TiO2 nanotube arrays (TNA) was accomplished by direct anodization of Ti foil in a fluoride-containing ethylene glycol electrolyte [26–28]. At first, Ti foil was cut into a 2.5 cm × 1.5 cm area. Before electrochemical anodization, Ti foil was cleaned by ultrasonication in a mixture of acetone and isopropanol for 30 min, followed by subsequent rinsing in deionized water and drying. Electrochemical anodization of the Ti foils was performed at room temperature in an electrolyte containing 0.5 wt% NH4 F and 5 wt% H2 O in ethylene glycol solution by applying a 60 V dc potential for 5 h, in which a Cu sheet was used as the counter electrode. After the first anodization, the Ti foil was ultrasonicated in ethanol to remove the electrolytes, followed by annealing at 450 ◦ C for 30 min to crystallize it. A secondary anodization was carried out on the as-annealed film in the same electrolyte solution for 15 min. with an applied potential of 60 V. After rinsing in ethanol, the secondary anodized Ti foil was then immersed in 10% H2 O2 solution for 12 h to remove the underlayer amorphous TiO2 and separate the TNA from the Ti substrate. To prepare boron doped TNA (B-TNA), the same procedure was applied as explained above except that the electrolyte containing 0.5 wt% NH4 F and 5 wt% 0.5 M boric acid (H3 BO3 ) in ethylene glycol solution. Anatase TiO2 nanoparticles were also synthesized [28] using microwave hydrothermal method. Generally, 7.5 mL of titanium tetraisopropoxide (TTIP) was hydrolyzed in 45 mL of 0.1 M HNO3 solution under stirring. Then, the suspension was stirred at 80 ◦ C for 8 h, followed by microwave autoclaving at 170 ◦ C for 2 h. Then, the colloid was washed with de-ionized water. To prepare the paste, polyethylene glycol (molecular weight = 20,000 g/mol.) was added (30 wt% w.r.t. TiO2 ) to the colloid, followed by stirring for 12 h. Fluorine-doped tin oxide glasses (FTO glass) were cleaned in an acetone and isopropyl alcohol mixture (1:1, v/v) in an ultrasonic water bath. To prepare TiO2 electrodes, a very thin layer of the TiO2 nanoparticle paste was formed on FTO glass using a glass rod and then the as-synthesized TiO2 nanotube arrays (TNA and B-TNA) were transferred on the thin film. An adhesive tape was used to control the thickness of the thin layer to make it identical for both samples. Then, the electrodes were sintered again at 450 ◦ C for 30 min. Multiple samples (three samples were prepared for each case) were prepared for the reliability of results. 2.2. Fabrication of the DSSC devices An active area of 0.30 cm2 was selected from the TiO2 electrodes and immersed in a 5 × 10−3 M solution of the dye [RuL2 (NCS)2 ]TBA2 for overnight, where TBA is tetra-n-butylammonium (N719 dye, Everlight Chemical, Taiwan). The specimens were washed with ethanol after immersing in N719 dye solution. Pt sputtered FTO glass was used as the counter electrode. The iodide/triiodide (I− /I3 − ) electrolyte (Iodolyte R-150) was cast onto the dye absorbed TiO2 electrodes. The TiO2 electrodes and the Pt coated electrodes were clamped together in order to assemble the DSSC devices. 2.3. Characterization Surface morphology and thickness of the films were measured by field emission scanning electron microscope (FESEM, JEOL
Table 1 Parameters extracted from X-ray diffraction pattern of TNA and B-TNA. Sample name
Peak center (2(,◦ )
d-spacing (nm)
TNA B-TNA
25.30 25.10
3.507 3.547
S-4100). To determine the elements present in the as synthesized nanotubes, the field emission scanning electron microscope (FESEM, JEOL JSM 7600F) equipped with energy dispersive X-ray spectroscopy (EDS) was used (X-MAS, Oxford Instrumentals). The crystalline phase of obtained titania electrodes were analyzed using Rigaku D/MAX-3C X-ray diffractometer with a Cu target and Ni filter at a scanning rate of 2◦ /min from 2 = 20◦ to 80◦ . To obtain the information about band gap energy of the as-synthesized nanotubes, spectrum has been recorded using UV/vis spectrophotometer (UV2550, SHIMADZU). The photovoltaic characteristics of DSSC devices were measured from an illuminated area of 0.30 cm2 by an electrochemical analyzer (CHI 6173b, CH Instruments Co., U.S.A) under a standard AM 1.5 sunlight illumination (XES-151S, San-Ei, Japan) with 100 mW/cm2 light source. The electrical impedance spectra were also measured in the range of 0.01 Hz to 100 kHz using the same equipment and setup. The incident photon-to-current conversion efficiency (IPCE) was measured within the range of 350–800 nm using an IPCE system (Enlitech, Taiwan, ROC) that was specifically designed for DSSCs. 3. Results and discussion 3.1. Microstructures Fig. 1(a) and (b) shows the top-view and cross-sectional view micrograph of TNA formed in the mixture of NH4 F, ethylene glycol and water. The length and average diameter of nanotubes were 18 m and 145 nm, respectively. Nanotube arrays prepared in mixture of NH4 F, ethylene glycol and 0.1 M boric acid (B-TNA) are shown in Fig. 1(c) and (d). The length and average diameter of B-TNA were 15 m and 115 nm, respectively. A well ordered with tubular structure and no collapse was observed in the assynthesized TNAs (TNA and B-TNA). The wall thickness of B-TNA was slightly thicker (Fig. S1, Supporting information) than that of TNA, whereas the B-TNA tube length decreases by 3 m compared to that of TNA at the same anodizing time (5 h) and the same applied potential (60 V). 3.2. EDS analysis The EDS spectra and chemical compositions of TNA and B-TNA are shown in Fig. 2(a) and (b). Quantitative analysis of the EDS spectrum showed that the main elements present in the TNA were Ti and O and the atomic ratio of titanium to oxygen was closely equal to the stoichiometric of 1:2. For B-TNA, the main elements were Ti, O and boron and the atomic ratio of titanium to oxygen was also closely equal to 1/2. The boron content in the as-synthesized B-TNA was determined to be 4.21 wt% (9.6 at.%), which indicates that the nanotubes obtained were doped with boron. 3.3. X-ray diffraction study Fig. 3 shows the X-ray diffraction patterns of TNA and B-TNA. The d-spacing value of crystallites was calculated using Bragg’s law [29], and the d-spacing values are summarized in Table 1. XRD pattern evidenced that the presence of anatase as the main phase and no other second phases were found in the TNA and B-TNA. The FWHM decreases from 0.64 in TNA to about 0.30 after boron
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Fig. 1. SEM micrographs of the (a) top-view and (b) cross sectional view of TNA, and (c) top-view and (d) cross sectional view of B-TNA, respectively.
doping (B-TNA). The increase in intensities and sharpening of Bragg peaks of (1 0 1) planes of B-TNA are the characteristics of larger crystal sizes and improved crystallinity. Therefore, crystallinity is increased upon boron doping. Also, the anatase peak of (1 0 1) planes of B-TNA shifted to lower angle side (inset of Fig. 3) and increased d-spacing value was found from Bragg’s law calculation. This suggests that a certain amount of structural rearrangement has occurred upon intercalation of boron into TNA. It indicates that boron was introduced into TiO2 lattice interstitial sites thereby
Fig. 3. XRD patterns of TNA and B-TNA. Inset shows the enlarged portion of anatase (1 0 1) peaks of these two nanotube arrays.
distorting the crystal lattice and hence increased d-spacing was observed. It has been reported that the radius of B3+ (0.023 nm) is much smaller than that of Ti4+ (0.068 nm), and it would be difficult for B3+ to replace the Ti4+ site [29,30]. This implies the existence of boron elements at interstitial sites between the TiO2 lattices, which can balance the residual charge of the TiO2 nanoparticles, rather than the substitution of boron atoms on the TiO2 lattice [29]. 3.4. UV–vis spectroscopy analysis Fig. 4 shows the UV–vis spectra of TNAs and the B-TNAs. The optical absorption edges of the boron doped TNAs shifted to a lower energy in the visible-light region compared to that of undoped TNAs. This result indicates the narrowing of band gap upon doping boron into TNAs. The band gap energy of TNAs and B-TNAs can be estimated from the following equation [22,31]: Fig. 2. EDS spectra and their corresponding quantitative elemental analysis (inset) of (a) TNA and (b) B-TNA, respectively.
Eg =
1239.8
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Fig. 6. EIS analysis of the DSSCs under an illumination of 100 mW/cm2 . Bode phase plot for the TNA and B-TNA electrodes. Fig. 4. UV–vis absorption of spectra of the TNAs and B-TNAs. Table 2 Photovoltaic parameters of the un-doped and boron doped TiO2 nanotubes electrodes. Samples
Voc (V)
Jsc (mA/cm2 )
FF
Efficiency (%)
TNAs B-TNAs
0.70 ± 0.01 0.66 ± 0.01
6.81 ± 0.16 7.85 ± 0.20
0.64 ± 0.01 0.66 ± 0.01
3.02 ± 0.10 3.44 ± 0.10
where Eg is the band gap (eV) and (nm) is the wavelength of the absorption edge in the spectrum. The band gap for the absorption edge in the visible region was calculated to be 3.15 ± 0.01 eV and 3.07 ± 0.01 eV for TNAs and B-TNAs, respectively. This suggests that boron doping contributed to the conduction band shift i.e. red shift of the band gap. 3.5. Photovoltaic characteristics Fig. 5 shows the comparison of J–V characteristics of the TiO2 nanotubes electrodes and boron doped TiO2 nanotubes electrodes. The average photovoltaic parameters of TNAs and B-TNAs measured under illumination AM 1.5 simulated sunlight (100 mW/cm2 ) are summarized in Table 2. The B-TNA electrodes showed a better performance than those of TNA electrodes, which exhibited a short-circuit current density (Jsc ) of 6.81 ± 0.16 mA/cm2 and an overall conversion efficiency () of 3.02 ± 0.10%. The B-TNAs showed a short-circuit current density (Jsc ) of 7.85 ± 0.20 mA/cm2 and an overall conversion efficiency () of 3.44 ± 0.10%. The
Fig. 5. J–V curves for DSSCs constructed with TNA and B-TNA electrodes.
enhanced performance of the B-TNA could be attributed to the TiO2 conduction band shift due to boron doping. Photocurrents can be influenced by electrons injection rate, charge transfer and charge recombination process. The electron injection rate depends on the relative position of the LUMO of dye molecules and the conduction band of TiO2 [32]. It appears that a perfect match between LUMO of dye molecules and the conduction band of TiO2 is achieved by a downward shift, i.e., red shift, in the TiO2 energy level due to boron doping, which enhanced the electron injection rate and hence the overall DSSC efficiency. The influence of boron doping on the open-circuit potential (Voc ) was also found. It has been observed that the Voc value decreased in boron doped TNA electrodes. The Voc value is related to the conduction band edge [24,32] of TNA. It is evident that the conduction band edge was changed by the boron doping as observed in Section 3.4. 3.6. Electrochemical impedance spectroscopy (EIS) analysis 3.6.1. Bode plot Electrochemical impedance spectroscopy (EIS) is a powerful tool to clarify the electronic and ionic transport processes in DSSCs [33]. The EIS result of Bode plot is shown in Fig. 6. The responses in the frequency regions around 104 Hz (ω1 ), 103 Hz (ω2 ), 10 Hz (ω3 ) and 0.1–1 Hz (ω4 ), are assigned to charge transfer processes at the Pt/electrolyte interface, the TiO2 /TiO2 particles interface, the TiO2 /dye/electrolyte interface, and the Nernst diffusion within the electrolyte, respectively [32,34]. Thus, the lifetime of electrons in the TiO2 film ( eff ) can be estimated from the middle-frequency (fmax ) of the peak, which appears at around 10 Hz, as [35,36]: 1/2fmax . The characteristic frequency of TiO2 /electrolyte interface peak (∼10 Hz) for B-TNAs are shifted to a lower frequency compared with those of the TNA photoanodes. The average electron lifetimes in the TNA and B-TNA electrodes are summarized in Table 2. The calculated electron lifetimes of B-TNAs were much longer than that of the TNAs. The longer electrons lifetimes for the B-TNAs demonstrated that more effective retardation of the charge recombination reaction between photoelectrons from boron-doped TiO2 nanotubes and the electrolyte [35,37]. 3.6.2. Nyquist plot The internal resistances of the two kinds of photoelectrodes (TNAs and B-TNAs) were studied using Nyquist plot, which is shown in Fig. 7(a) in order to investigate the electron transfer at the TiO2 /dye/electrolyte interface. To estimate the interface resistance from the electrochemical impedance spectra, an equivalent
A. Subramanian, H.-W. Wang / Applied Surface Science 258 (2012) 6479–6484
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Fig. 8. Incident photon to current efficiency (IPCE) action spectra of the TNA and B-TNA solar cells.
Fig. 7. (a) EIS analysis of the DSSCs under an illumination of 100 mW/cm2 . Nyquist plot for the TNA and B-TNA electrodes. (b) An equivalent circuit model for the electrochemical impedance spectra. Table 3 Parameters obtained from the electrochemical impedance spectra analysis. Samples
Rs
R1
R2
R3
Electron life time (ms)
TNAs B-TNAs
14.1 ± 0.3 14.6 ± 0.1
7.7 ± 0.4 7.7 ± 0.5
20.2 ± 0.9 15.4 ± 0.7
8.0 ± 0.3 7.7 ± 0.4
26.2 ± 0.5 29.0 ± 0.2
circuit model has been adopted and is given in Fig. 7(b) [38]. According to the equivalent circuit model, the ohmic resistance Rs corresponds to FTO/electrolyte resistance. The first semicircle that appeared in the region 104 Hz (ω1 )–103 Hz (ω2 ) corresponds to charge transfer resistance (R1 ) at the Pt/electrolyte interface and the TiO2 /TiO2 particles interface. The second semicircle in the region 10 Hz (ω3 ) associated with the charge resistance (R2 ) at the TiO2 /dye/electrolyte interface. The third semi circle in the region 0.1–1 Hz (ω4 ) belongs to resistance R3 due to the Nernst diffusion within the electrolyte. [32,37]. The average values of the internal resistances for TNA and B-TNA electrodes were evaluated from the impedance spectra analysis and the characteristic parameters obtained from the electrochemical impedance spectra analysis are given in Table 3. The FTO/electrolyte interface resistance (Rs ) and the resistance R3 due to the Nernst diffusion within the electrolyte for TNAs and B-TNAs were very similar. The TiO2 /TiO2 particles interface resistance (R1 ) was almost the same (∼7.7 ) for both electrodes. The resistance R2 becomes smaller for the BTNA electrodes compared to those of TNA electrodes because of more electron injection rate due to better energetic matching of the LUMO of dye molecules and the conduction band of TiO2 as a consequence of boron doping and thereby reducing the corresponding resistance at the interface [32,38]. 3.7. Incident photon-to-current conversion efficiency (IPCE) analysis The incident photon to current efficiency (IPCE) action spectra of the TNA and B-TNA are shown in Fig. 8. The IPCE value of the B-TNA was remarkably increased compared to the TNA electrode. The maximum IPCE value was 34% at the wavelength of 500 nm for
B-TNA and 25% at the wavelength of 530 nm for TNA. The major enhancement in IPCE value was from the UV region and expended its spectral response to visible light region. Therefore, we conclude that the increases in the photocurrent and IPCE are primarily related to the enhanced electron injection and transfer ability of the boron-doped TNA. The enhancement in the IPCE values for B-TNA could be attributed to the substantial suppression of recombination and back electron transfer, which resulted in high harvesting efficiency and short-circuit photocurrent density observed in the solar cells. 4. Conclusions The effects of boron doping into TiO2 nanotubes were studied and their solar cell performances were compared with those of undoped TiO2 nanotubes. It was found that the boron was introduced into the interstitial sites of TiO2 lattice and contributed to the shift of conduction band. The boron-doped TiO2 nanotube arrays showed an enhanced performance compare to those of undoped TNA. The enhanced performance of the B-TNA could be attributed to longer electron lifetime in the TiO2 nanotubes and the enhanced electron-injection due to better matching of the LUMO of dye molecules and the conduction band of TiO2 nanotubes. Acknowledgments The financial support for this research provided by the NSC 99-2113-M-033-003-MY3 and CYCU Distinguish International Graduate Scholarship (DIGS) is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apsusc.2012.03.064. References [1] B.O. Regan, M. Gratzel, Nature 353 (1991) 737. [2] J.B. Asbury, R.J. Ellingson, H.N. Ghosh, S. Ferrere, A.J. Nozik, T. Lian, J. Phys. Chem. B 10 (1999) 3110. [3] R.J. Ellingson, J.B. Asbury, S. Ferrere, H.N. Ghosh, J.R. Sprague, T. Lian, A.J. Nozik, J. Phys. Chem. B 102 (1998) 6455. [4] V.D.J. van de Lagemaat, A.J. Frank, J. Phys. Chem. B 105 (2001) 11194. [5] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Muller, P. Liska, N. Vlachopoulos, M. Gratzel, J. Am. Chem. Soc. 115 (1993) 6382. [6] M.K. Nazeeruddin, P. Pechy, T. Renouard, S.M. Zakeeruddin, R. Humphry-Baker, P. Comte, P. Liska, C. Le, E. Costa, V. Shklover, L. Spiccia, G.B. Deacon, C.A. Bignozzi, M. Gratzel, J. Am. Chem. Soc. 123 (2001) 1613.
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Effects of boron doping in TiO2 nanotubes and the performance of dye-sensitized solar cells Alagesan Subramanian, Hong-Wen Wang ∗ Department of Chemistry, Center for Nanotechnology at CYCU, Chung-Yuan Christian University, 200 Chung-Pei Road, Chung Li 320, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 23 February 2012 Received in revised form 9 March 2012 Accepted 12 March 2012 Available online 20 March 2012 Keywords: Boron doping TiO2 nanotube Dye-sensitized solar cell
a b s t r a c t Titanium nanotubes doped with boron used as the photoelectrode for dye-sensitized solar cells were investigated. The materials were characterized by SEM, XRD, and UV–vis spectroscopy and their photoconversion efficiencies were evaluated. The chemical compositions of TiO2 nanotubes (TNA) and boron doped TNA (B-TNA) were identified by the energy dispersive X-ray spectroscopy (EDS). XRD evidenced the presence of anatase as the main phase and presented the existence of boron elements at interstitial sites between the TiO2 lattices. The UV–vis spectra indicated the narrowing of band gap upon doping boron into titanium nanotubes. The photovoltaic properties were measured by a current–voltage meter under AM1.5 simulated light radiation. The boron-doped TiO2 nanotube arrays showed an enhanced performance with a photocurrent density of 7.85 ± 0.20 mA/cm2 and an overall conversion efficiency () of 3.44 ± 0.10%. The enhanced performance was attributed to the enhanced electron injection rate and retardation of the charge recombination, which could be due to perfect matching between the LUMO of dye molecules and the conduction band of TiO2 . Electrochemical impedance spectroscopy (EIS) measurement indicated the longer electron lifetime and reduced TiO2 /dye/electrolyte interface resistance for boron doped TiO2 nanotubes than that of undoped TiO2 nanotubes. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Dye-sensitized solar cells are currently attracting widespread academic and commercial interests for the conversion of sunlight into electricity with low cost and high efficiency [1–4]. Excellent photo-electron conversion efficiencies have been reported for regenerative photo electrochemical cells based on nanocrystalline titanium dioxide (TiO2 ) films, sensitized by ruthenium-dyes using iodide/triiodide (I− /I3 − ) redox couple in an organic solvent as the electrolyte [5,6]. TiO2 nanotubes (TNA) have attracted tremendous attention due to the combination of a wide band gap semiconductive nature with high surface area and high aspect ratio. TiO2 nanotube arrays have been used for a variety of applications including hydrogen production by water photoelectrolysis [7,8], gas sensing [9] and dye-sensitized solar cells [1,10,11]. Detailed studies have shown that the charge recombination in nanotube-arraybased DSSCs is much slower than that in nanoparticle-based DSSCs, which results in improved charge collection efficiency [12,13]. Compared with nanoparticles, TiO2 nanowires or nanotubes are suggested to be superior in chemical and photoelectrochemical
∗ Corresponding author. Tel.: +886 3 2653310; fax: +886 3 2653399. E-mail addresses: [email protected] (A. Subramanian), [email protected] (H.-W. Wang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.03.064
performance, due to their one-dimensional channel for carrier transportation [14,15]. The arrangement of the highly ordered titania nanotube array perpendicular to the surface permits facile charge transfer along the length of the nanotubes from the solution to the conductive substrate, thereby reducing the losses incurred by charge-hopping across the nanoparticle grain boundaries [16]. TiO2 has a band gap of 3.0 eV for the rutile phase and 3.2 eV for the anatase phase. These values require ultraviolet (UV) radiation to activate. Recently, many attempts have been made to promote TiO2 activities under visible light excitation to allow the utilization of the solar spectrum, by modifying titanium dioxide with nonmetals, such as N, C and F to efficiently extend the photoresponse of TiO2 from UV to visible light region [17–19]. The introduction of foreign atoms in these materials simultaneously alters the charge transport properties, optical properties, and crystallinity. Therefore, the DSSCs based on these nonmetal-doped TiO2 film deserved more studies to fully explore the doping effect on the TiO2 nanotubes for DSSCs. In recent years, doping nitrogen into TiO2 nanoparticles (TNP) had been studied intensively for photocatalytic activities as well as in the DSSCs [20–24]. However, a little attention and less detailed studies were given to boron doping into TiO2 nanoparticles for photocatalytic activities, especially, in DSSC study. There were very few reports on boron doped TiO2 nanotubes for the DSSC efficiency study to the best of our knowledge. Ruan et al. [25] were the first to report the boron doping of the titania nanotube arrays
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in the DSSC. However, there was no detailed study about the DSSC in their work. In this regard, TiO2 nanotube doped with boron was investigated and its performance on DSSC was studied in details in our study. 2. Experimental procedures 2.1. Preparation of TiO2 electrodes The growth of TiO2 nanotube arrays (TNA) was accomplished by direct anodization of Ti foil in a fluoride-containing ethylene glycol electrolyte [26–28]. At first, Ti foil was cut into a 2.5 cm × 1.5 cm area. Before electrochemical anodization, Ti foil was cleaned by ultrasonication in a mixture of acetone and isopropanol for 30 min, followed by subsequent rinsing in deionized water and drying. Electrochemical anodization of the Ti foils was performed at room temperature in an electrolyte containing 0.5 wt% NH4 F and 5 wt% H2 O in ethylene glycol solution by applying a 60 V dc potential for 5 h, in which a Cu sheet was used as the counter electrode. After the first anodization, the Ti foil was ultrasonicated in ethanol to remove the electrolytes, followed by annealing at 450 ◦ C for 30 min to crystallize it. A secondary anodization was carried out on the as-annealed film in the same electrolyte solution for 15 min. with an applied potential of 60 V. After rinsing in ethanol, the secondary anodized Ti foil was then immersed in 10% H2 O2 solution for 12 h to remove the underlayer amorphous TiO2 and separate the TNA from the Ti substrate. To prepare boron doped TNA (B-TNA), the same procedure was applied as explained above except that the electrolyte containing 0.5 wt% NH4 F and 5 wt% 0.5 M boric acid (H3 BO3 ) in ethylene glycol solution. Anatase TiO2 nanoparticles were also synthesized [28] using microwave hydrothermal method. Generally, 7.5 mL of titanium tetraisopropoxide (TTIP) was hydrolyzed in 45 mL of 0.1 M HNO3 solution under stirring. Then, the suspension was stirred at 80 ◦ C for 8 h, followed by microwave autoclaving at 170 ◦ C for 2 h. Then, the colloid was washed with de-ionized water. To prepare the paste, polyethylene glycol (molecular weight = 20,000 g/mol.) was added (30 wt% w.r.t. TiO2 ) to the colloid, followed by stirring for 12 h. Fluorine-doped tin oxide glasses (FTO glass) were cleaned in an acetone and isopropyl alcohol mixture (1:1, v/v) in an ultrasonic water bath. To prepare TiO2 electrodes, a very thin layer of the TiO2 nanoparticle paste was formed on FTO glass using a glass rod and then the as-synthesized TiO2 nanotube arrays (TNA and B-TNA) were transferred on the thin film. An adhesive tape was used to control the thickness of the thin layer to make it identical for both samples. Then, the electrodes were sintered again at 450 ◦ C for 30 min. Multiple samples (three samples were prepared for each case) were prepared for the reliability of results. 2.2. Fabrication of the DSSC devices An active area of 0.30 cm2 was selected from the TiO2 electrodes and immersed in a 5 × 10−3 M solution of the dye [RuL2 (NCS)2 ]TBA2 for overnight, where TBA is tetra-n-butylammonium (N719 dye, Everlight Chemical, Taiwan). The specimens were washed with ethanol after immersing in N719 dye solution. Pt sputtered FTO glass was used as the counter electrode. The iodide/triiodide (I− /I3 − ) electrolyte (Iodolyte R-150) was cast onto the dye absorbed TiO2 electrodes. The TiO2 electrodes and the Pt coated electrodes were clamped together in order to assemble the DSSC devices. 2.3. Characterization Surface morphology and thickness of the films were measured by field emission scanning electron microscope (FESEM, JEOL
Table 1 Parameters extracted from X-ray diffraction pattern of TNA and B-TNA. Sample name
Peak center (2(,◦ )
d-spacing (nm)
TNA B-TNA
25.30 25.10
3.507 3.547
S-4100). To determine the elements present in the as synthesized nanotubes, the field emission scanning electron microscope (FESEM, JEOL JSM 7600F) equipped with energy dispersive X-ray spectroscopy (EDS) was used (X-MAS, Oxford Instrumentals). The crystalline phase of obtained titania electrodes were analyzed using Rigaku D/MAX-3C X-ray diffractometer with a Cu target and Ni filter at a scanning rate of 2◦ /min from 2 = 20◦ to 80◦ . To obtain the information about band gap energy of the as-synthesized nanotubes, spectrum has been recorded using UV/vis spectrophotometer (UV2550, SHIMADZU). The photovoltaic characteristics of DSSC devices were measured from an illuminated area of 0.30 cm2 by an electrochemical analyzer (CHI 6173b, CH Instruments Co., U.S.A) under a standard AM 1.5 sunlight illumination (XES-151S, San-Ei, Japan) with 100 mW/cm2 light source. The electrical impedance spectra were also measured in the range of 0.01 Hz to 100 kHz using the same equipment and setup. The incident photon-to-current conversion efficiency (IPCE) was measured within the range of 350–800 nm using an IPCE system (Enlitech, Taiwan, ROC) that was specifically designed for DSSCs. 3. Results and discussion 3.1. Microstructures Fig. 1(a) and (b) shows the top-view and cross-sectional view micrograph of TNA formed in the mixture of NH4 F, ethylene glycol and water. The length and average diameter of nanotubes were 18 m and 145 nm, respectively. Nanotube arrays prepared in mixture of NH4 F, ethylene glycol and 0.1 M boric acid (B-TNA) are shown in Fig. 1(c) and (d). The length and average diameter of B-TNA were 15 m and 115 nm, respectively. A well ordered with tubular structure and no collapse was observed in the assynthesized TNAs (TNA and B-TNA). The wall thickness of B-TNA was slightly thicker (Fig. S1, Supporting information) than that of TNA, whereas the B-TNA tube length decreases by 3 m compared to that of TNA at the same anodizing time (5 h) and the same applied potential (60 V). 3.2. EDS analysis The EDS spectra and chemical compositions of TNA and B-TNA are shown in Fig. 2(a) and (b). Quantitative analysis of the EDS spectrum showed that the main elements present in the TNA were Ti and O and the atomic ratio of titanium to oxygen was closely equal to the stoichiometric of 1:2. For B-TNA, the main elements were Ti, O and boron and the atomic ratio of titanium to oxygen was also closely equal to 1/2. The boron content in the as-synthesized B-TNA was determined to be 4.21 wt% (9.6 at.%), which indicates that the nanotubes obtained were doped with boron. 3.3. X-ray diffraction study Fig. 3 shows the X-ray diffraction patterns of TNA and B-TNA. The d-spacing value of crystallites was calculated using Bragg’s law [29], and the d-spacing values are summarized in Table 1. XRD pattern evidenced that the presence of anatase as the main phase and no other second phases were found in the TNA and B-TNA. The FWHM decreases from 0.64 in TNA to about 0.30 after boron
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Fig. 1. SEM micrographs of the (a) top-view and (b) cross sectional view of TNA, and (c) top-view and (d) cross sectional view of B-TNA, respectively.
doping (B-TNA). The increase in intensities and sharpening of Bragg peaks of (1 0 1) planes of B-TNA are the characteristics of larger crystal sizes and improved crystallinity. Therefore, crystallinity is increased upon boron doping. Also, the anatase peak of (1 0 1) planes of B-TNA shifted to lower angle side (inset of Fig. 3) and increased d-spacing value was found from Bragg’s law calculation. This suggests that a certain amount of structural rearrangement has occurred upon intercalation of boron into TNA. It indicates that boron was introduced into TiO2 lattice interstitial sites thereby
Fig. 3. XRD patterns of TNA and B-TNA. Inset shows the enlarged portion of anatase (1 0 1) peaks of these two nanotube arrays.
distorting the crystal lattice and hence increased d-spacing was observed. It has been reported that the radius of B3+ (0.023 nm) is much smaller than that of Ti4+ (0.068 nm), and it would be difficult for B3+ to replace the Ti4+ site [29,30]. This implies the existence of boron elements at interstitial sites between the TiO2 lattices, which can balance the residual charge of the TiO2 nanoparticles, rather than the substitution of boron atoms on the TiO2 lattice [29]. 3.4. UV–vis spectroscopy analysis Fig. 4 shows the UV–vis spectra of TNAs and the B-TNAs. The optical absorption edges of the boron doped TNAs shifted to a lower energy in the visible-light region compared to that of undoped TNAs. This result indicates the narrowing of band gap upon doping boron into TNAs. The band gap energy of TNAs and B-TNAs can be estimated from the following equation [22,31]: Fig. 2. EDS spectra and their corresponding quantitative elemental analysis (inset) of (a) TNA and (b) B-TNA, respectively.
Eg =
1239.8
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Fig. 6. EIS analysis of the DSSCs under an illumination of 100 mW/cm2 . Bode phase plot for the TNA and B-TNA electrodes. Fig. 4. UV–vis absorption of spectra of the TNAs and B-TNAs. Table 2 Photovoltaic parameters of the un-doped and boron doped TiO2 nanotubes electrodes. Samples
Voc (V)
Jsc (mA/cm2 )
FF
Efficiency (%)
TNAs B-TNAs
0.70 ± 0.01 0.66 ± 0.01
6.81 ± 0.16 7.85 ± 0.20
0.64 ± 0.01 0.66 ± 0.01
3.02 ± 0.10 3.44 ± 0.10
where Eg is the band gap (eV) and (nm) is the wavelength of the absorption edge in the spectrum. The band gap for the absorption edge in the visible region was calculated to be 3.15 ± 0.01 eV and 3.07 ± 0.01 eV for TNAs and B-TNAs, respectively. This suggests that boron doping contributed to the conduction band shift i.e. red shift of the band gap. 3.5. Photovoltaic characteristics Fig. 5 shows the comparison of J–V characteristics of the TiO2 nanotubes electrodes and boron doped TiO2 nanotubes electrodes. The average photovoltaic parameters of TNAs and B-TNAs measured under illumination AM 1.5 simulated sunlight (100 mW/cm2 ) are summarized in Table 2. The B-TNA electrodes showed a better performance than those of TNA electrodes, which exhibited a short-circuit current density (Jsc ) of 6.81 ± 0.16 mA/cm2 and an overall conversion efficiency () of 3.02 ± 0.10%. The B-TNAs showed a short-circuit current density (Jsc ) of 7.85 ± 0.20 mA/cm2 and an overall conversion efficiency () of 3.44 ± 0.10%. The
Fig. 5. J–V curves for DSSCs constructed with TNA and B-TNA electrodes.
enhanced performance of the B-TNA could be attributed to the TiO2 conduction band shift due to boron doping. Photocurrents can be influenced by electrons injection rate, charge transfer and charge recombination process. The electron injection rate depends on the relative position of the LUMO of dye molecules and the conduction band of TiO2 [32]. It appears that a perfect match between LUMO of dye molecules and the conduction band of TiO2 is achieved by a downward shift, i.e., red shift, in the TiO2 energy level due to boron doping, which enhanced the electron injection rate and hence the overall DSSC efficiency. The influence of boron doping on the open-circuit potential (Voc ) was also found. It has been observed that the Voc value decreased in boron doped TNA electrodes. The Voc value is related to the conduction band edge [24,32] of TNA. It is evident that the conduction band edge was changed by the boron doping as observed in Section 3.4. 3.6. Electrochemical impedance spectroscopy (EIS) analysis 3.6.1. Bode plot Electrochemical impedance spectroscopy (EIS) is a powerful tool to clarify the electronic and ionic transport processes in DSSCs [33]. The EIS result of Bode plot is shown in Fig. 6. The responses in the frequency regions around 104 Hz (ω1 ), 103 Hz (ω2 ), 10 Hz (ω3 ) and 0.1–1 Hz (ω4 ), are assigned to charge transfer processes at the Pt/electrolyte interface, the TiO2 /TiO2 particles interface, the TiO2 /dye/electrolyte interface, and the Nernst diffusion within the electrolyte, respectively [32,34]. Thus, the lifetime of electrons in the TiO2 film ( eff ) can be estimated from the middle-frequency (fmax ) of the peak, which appears at around 10 Hz, as [35,36]: 1/2fmax . The characteristic frequency of TiO2 /electrolyte interface peak (∼10 Hz) for B-TNAs are shifted to a lower frequency compared with those of the TNA photoanodes. The average electron lifetimes in the TNA and B-TNA electrodes are summarized in Table 2. The calculated electron lifetimes of B-TNAs were much longer than that of the TNAs. The longer electrons lifetimes for the B-TNAs demonstrated that more effective retardation of the charge recombination reaction between photoelectrons from boron-doped TiO2 nanotubes and the electrolyte [35,37]. 3.6.2. Nyquist plot The internal resistances of the two kinds of photoelectrodes (TNAs and B-TNAs) were studied using Nyquist plot, which is shown in Fig. 7(a) in order to investigate the electron transfer at the TiO2 /dye/electrolyte interface. To estimate the interface resistance from the electrochemical impedance spectra, an equivalent
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Fig. 8. Incident photon to current efficiency (IPCE) action spectra of the TNA and B-TNA solar cells.
Fig. 7. (a) EIS analysis of the DSSCs under an illumination of 100 mW/cm2 . Nyquist plot for the TNA and B-TNA electrodes. (b) An equivalent circuit model for the electrochemical impedance spectra. Table 3 Parameters obtained from the electrochemical impedance spectra analysis. Samples
Rs
R1
R2
R3
Electron life time (ms)
TNAs B-TNAs
14.1 ± 0.3 14.6 ± 0.1
7.7 ± 0.4 7.7 ± 0.5
20.2 ± 0.9 15.4 ± 0.7
8.0 ± 0.3 7.7 ± 0.4
26.2 ± 0.5 29.0 ± 0.2
circuit model has been adopted and is given in Fig. 7(b) [38]. According to the equivalent circuit model, the ohmic resistance Rs corresponds to FTO/electrolyte resistance. The first semicircle that appeared in the region 104 Hz (ω1 )–103 Hz (ω2 ) corresponds to charge transfer resistance (R1 ) at the Pt/electrolyte interface and the TiO2 /TiO2 particles interface. The second semicircle in the region 10 Hz (ω3 ) associated with the charge resistance (R2 ) at the TiO2 /dye/electrolyte interface. The third semi circle in the region 0.1–1 Hz (ω4 ) belongs to resistance R3 due to the Nernst diffusion within the electrolyte. [32,37]. The average values of the internal resistances for TNA and B-TNA electrodes were evaluated from the impedance spectra analysis and the characteristic parameters obtained from the electrochemical impedance spectra analysis are given in Table 3. The FTO/electrolyte interface resistance (Rs ) and the resistance R3 due to the Nernst diffusion within the electrolyte for TNAs and B-TNAs were very similar. The TiO2 /TiO2 particles interface resistance (R1 ) was almost the same (∼7.7 ) for both electrodes. The resistance R2 becomes smaller for the BTNA electrodes compared to those of TNA electrodes because of more electron injection rate due to better energetic matching of the LUMO of dye molecules and the conduction band of TiO2 as a consequence of boron doping and thereby reducing the corresponding resistance at the interface [32,38]. 3.7. Incident photon-to-current conversion efficiency (IPCE) analysis The incident photon to current efficiency (IPCE) action spectra of the TNA and B-TNA are shown in Fig. 8. The IPCE value of the B-TNA was remarkably increased compared to the TNA electrode. The maximum IPCE value was 34% at the wavelength of 500 nm for
B-TNA and 25% at the wavelength of 530 nm for TNA. The major enhancement in IPCE value was from the UV region and expended its spectral response to visible light region. Therefore, we conclude that the increases in the photocurrent and IPCE are primarily related to the enhanced electron injection and transfer ability of the boron-doped TNA. The enhancement in the IPCE values for B-TNA could be attributed to the substantial suppression of recombination and back electron transfer, which resulted in high harvesting efficiency and short-circuit photocurrent density observed in the solar cells. 4. Conclusions The effects of boron doping into TiO2 nanotubes were studied and their solar cell performances were compared with those of undoped TiO2 nanotubes. It was found that the boron was introduced into the interstitial sites of TiO2 lattice and contributed to the shift of conduction band. The boron-doped TiO2 nanotube arrays showed an enhanced performance compare to those of undoped TNA. The enhanced performance of the B-TNA could be attributed to longer electron lifetime in the TiO2 nanotubes and the enhanced electron-injection due to better matching of the LUMO of dye molecules and the conduction band of TiO2 nanotubes. Acknowledgments The financial support for this research provided by the NSC 99-2113-M-033-003-MY3 and CYCU Distinguish International Graduate Scholarship (DIGS) is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apsusc.2012.03.064. References [1] B.O. Regan, M. Gratzel, Nature 353 (1991) 737. [2] J.B. Asbury, R.J. Ellingson, H.N. Ghosh, S. Ferrere, A.J. Nozik, T. Lian, J. Phys. Chem. B 10 (1999) 3110. [3] R.J. Ellingson, J.B. Asbury, S. Ferrere, H.N. Ghosh, J.R. Sprague, T. Lian, A.J. Nozik, J. Phys. Chem. B 102 (1998) 6455. [4] V.D.J. van de Lagemaat, A.J. Frank, J. Phys. Chem. B 105 (2001) 11194. [5] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Muller, P. Liska, N. Vlachopoulos, M. Gratzel, J. Am. Chem. Soc. 115 (1993) 6382. [6] M.K. Nazeeruddin, P. Pechy, T. Renouard, S.M. Zakeeruddin, R. Humphry-Baker, P. Comte, P. Liska, C. Le, E. Costa, V. Shklover, L. Spiccia, G.B. Deacon, C.A. Bignozzi, M. Gratzel, J. Am. Chem. Soc. 123 (2001) 1613.
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