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A novel cheap, one-step and facile synthesis of hierarchical TiO2 nanotubes as fast electron transport channels for highly efficient dye-sensitized solar cells
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
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Original Research Paper
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A novel cheap, one-step and facile synthesis of hierarchical TiO2 nanotubes as fast electron transport channels for highly efficient dye-sensitized solar cells
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Hong Chen, Na Li, Yang-Hong Wu, Jia-Bin Shi, Bing-Xin Lei ⇑, Zhen-Fan Sun ⇑
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School of Chemistry and Chemical Engineering, Key Laboratory of Electrochemical Energy Storage and Energy Conversion of Hainan Province, Hainan Normal University, Haikou 571158, China
a r t i c l e
i n f o
Article history: Received 8 November 2019 Received in revised form 9 January 2020 Accepted 15 January 2020 Available online xxxx Keywords: Titania Hierarchical structure Nanotube Charge transport Dye-sensitized solar cell
a b s t r a c t The well-aligned hierarchical TiO2 nanotubes (HTNTs) have been synthesized by a one-step hydrothermal method employing potassium titanium oxalate, ethanol and H2O, which are strongly adhered onto transparent conducting oxide glass. The preparation is straightforward, cheap and applicative for mass manufacture. The thickness of membranes is changed from 12 to 22 lm by adjusting the reaction time. The HTNTs consist of one-dimensional (1D) long TiO2 nanotube trunks and numerous short TiO2 nanorod branches, which can balance surface area and charge transport. By employing optimized HTNTs for dye-sensitized solar cells, a remarkable power conversion efficiency of 9.89% is obtained. The result is superior to P25 (8.34%), because 1D trunk-1D branch structure of HTNTs offers the advantages of strong light-harvesting, directed electron transport, and efficient charge collection. The HTNTs may find an underlying application in the manufacture of photovoltaic devices. Ó 2020 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
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1. Introduction
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Dye-sensitized solar cell (DSSC) has attracted much attention as an alternative energy harvesting device [1–3], because this type of cell exhibits superior features, such as cheap raw materials, simplicity in process, and relatively high power conversion efficiency (g). The g of DSSC has so far reached a value of 14.3% at 100 mW cm2 [4] and 32% at 0.1012 mW cm2 [5]. One of the most crucial components in DSSC is the photoanode, which is responsible for light-scattering, dye loading, electron injection, and charge transport. Thus, the photoanode should be carefully designed and constructed. A conventional photoanode is built by using the randomly packed TiO2 nanoparticles, which can anchor sufficient dye molecules and result in a good current density. However, the disordered structure can significantly retard the transport dynamics and augment the recombination rate of photoelectrons with I-3, and thus lower the charge collection efficiency in DSSC [6]. In addition, the conventional TiO2 photoanode exhibits high transparency of visible light, and thus utilizes part of the visible light. The layered-
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⇑ Corresponding authors.
stacking TiO2 photoanode represents a considerable advancement composed of a light-scattering overlayer and a nano-sized transparent active underlayer. By employing large TiO2 particles (100–400 nm) as scattering centers, the transparency of visible light can be reduced and the g of DSSC displays a satisfying improvement [7–10]. Regrettably, electron transport goes through a zigzag diffusion pathway, causing charge recombination with electrolyte to some extent [11]. A competitive advantage of one-dimensional (1D) TiO2 nanostructure arrays, such as nanowire arrays, nanorod arrays, and nanotube arrays (NTAs), is a directional electron transport path. Among these nanostructures, TiO2 NTAs are extremely striking because the inner and outer surfaces can absorb dyes [12]. The well-aligned TiO2 NTAs are classically prepared on a nontransparent Ti substrate by direct anodization. When the as-anodized TiO2 NTAs on Ti substrate are used as photoanode, the incident light passes preferentially through the counter electrode and electrolyte, which can reduce the light capture efficiency [13]. To solve these limits, the well-aligned TiO2 NTAs are synthesized on a fluorinedoped tin oxide glass (FTO) substrate by combining magnetron sputtering with anodization or a membrane transfer technique [14–16]. Although light illumination is irradiated from the working electrode, the weak adhesion between TiO2 membrane and FTO
E-mail addresses: [email protected] (B.-X. Lei), [email protected] (Z.-F. Sun). https://doi.org/10.1016/j.apt.2020.01.020 0921-8831/Ó 2020 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
Please cite this article as: H. Chen, N. Li, Y. H. Wu et al., A novel cheap, one-step and facile synthesis of hierarchical TiO2 nanotubes as fast electron transport channels for highly efficient dye-sensitized solar cells, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2020.01.020
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substrate limits the length of TiO2 NTAs and electron transport. The annealing process inevitably introduces cracks by thermal expansion and the resulting stresses [17]. In addition, owing to oxygen-deficient stoichiometry (TiO2-x), impurities as well as grain boundaries, the deficiency of anodic TiO2 NTAs impeded the applications [18–20]. TiO2 NTAs on FTO substrate were also developed by using ZnO template agent [21,22]. However, the type of TiO2 NTAs exhibited weak adhesion with FTO substrate because ZnO seed layer was easily etched. Our group prepared hierarchical anatase TiO2 NTAs via in situ conversion from anatase TiO2 nanowire arrays, which is a complex multistep approach [23]. In spite of continuously improved efficiency of TiO2 nanotube based cells, frustratingly, the electron mobility in such nanotube photoanode is low; in some cases, even inferior to TiO2 nanoparticle photoanode [19,20]. Hierarchical TiO2 nanotubes (HTNTs) directly grown on FTO substrate were prepared by a one-step hydrothermal reaction applying potassium titanyl oxalate (PTO), diethylene glycol (DEG) and H2O [24,25]. Hierarchical anatase TiO2 NTAs is an ideal photoanode, balancing specific surface area, electron transport, and lightscattering effect. Kim’s group assembled solid-state DSSC with 19 lm long hierarchical TiO2 NTAs and exhibited an ideal g of 8.0% [24]. Mai’s group prepared 15 lm long hierarchical TiO2 NTAs, and the DSSC achieved a g of 7.15% [26]. At present, the length of hierarchical TiO2 NTAs is still less than 19 lm using PTO, DEG and H2O by a one-step hydrothermal method, and the efficiency of the cell based on such hierarchical TiO2 NTAs is still less than 8%. The main purpose of our research is to develop a simple, low-cost, and reliable solution-based method to prepare TiO2 NTAs to improve the electron transport property. In this work, the HTNTs on FTO glass have been prepared by a one-step hydrothermal method using three cheap commercially improvable constituents: PTO, ethanol and H2O. The low-cost and straightforward method is appropriate for large-scale manufacture by utilizing ethanol instead of DEG. The length of TiO2 NTAs can be controlled by adjusting the reaction time. The 1D trunk-1D branch structure ensures fast electron transport, slow electron recombination, pronounced light-scattering effect, providing a remarkably prominent charge collection efficiency of over 90%. By employing optimized HTNTs for DSSCs, an astonishing efficiency of 9.89% is achieved.
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2. Experimental
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2.1. Materials
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Potassium titanium oxalate (PTO), ethanol, N719 dye and FTO glass (14 X cm2) substrates were acquired from Shanghai Macklin Biochemical Co., Ltd., Xilong Scientific Co., Ltd., Everlight Chemical Industrial Corp., Taiwan, and Nippon Sheet Glass Co., Ltd., respectively. No further treatment for all chemicals.
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2.2. Synthesis of HTNTs
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Firstly, FTO glass substrates were treated by using the TiO2 collosol to from a seed-layer [27]. We applied a modified approach to synthesize HTNTs [28]. Typically, 1.77 g of PTO, 13.5 mL of ethanol and 9.0 mL of H2O were introduced to a 50 mL Teflon-liner and stirred for 1 h. The hydrothermal reaction was accomplished at 200 for different times (3, 6, 9, 12 and 15 h, named TiO2-3, TiO2-6, TiO2-9, TiO2-12 and TiO2-15, respectively). After the reaction was finished, the as-fabricated samples were rinsed with deionized water and calcined at 500 °C for 1 h.
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2.3. Fabrication of DSSCs
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According to previously reported procedure [29], the working electrode, the counter electrode and electrolyte solution were prepared. The working electrode and the counter electrode were assembled into a sandwich type cell. The active area of working electrode is 0.16 cm2.
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2.4. Characterizations
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The morphology of sample was characterized by the field emission scanning electron microscopy (FESEM, JEOL JSM-7100F) and transmission electron microscopy (TEM, JEOL JEM-2100PLUS). The phase of sample was recorded by X-ray diffraction (XRD, Rigaku Ultima IV) using Cu Ka radiation (k = 1.5418 Å). The diffuse reflectance spectra (DRS) were measured by Uv–vis spectrometry (UV-2600, SHIMADZU Excellence in Science Co., Ltd., Japan). The adsorbed dye amount was calculated UV–vis absorption spectra (UV-T10, Beijing Purkinje General Instrument Co., Ltd., China). Under simulated AM 1.5 G illumination (100 mV cm2) using a solar light simulator (Oriel, Model: 91192), the photocurrent density-voltage (J-V) and open-circuit photo-voltage decay (OCVD) were recorded using a Keithley model 2400 digital source meter. The light source was a 450 W lamp (Oriel), which was calibrated by a NREL-calibrated Si solar cell (Oriel, P/N91150V). Photo current spectra system of CIMPS (PP211, Zahner) with tunable light source (TLS03) was employed to test intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS). The incident photon-to-electron conversion efficiency (IPCE) measurement was performed under monochromatic light illumination from 300 W Xenon lamp through a monochromator (Model: 90074900) in the tunable light source system (TLS-300XU, Newport). The electrochemical impedance spectroscopy (EIS) was conducted on the Zennium electrochemical workstation with a frequency range from 10 mHz to 1 MHz and the alternative signal magnitude of 10 mV under a bias of the opencircuit voltage in the dark.
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3. Results and discussion
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Fig. 1a and b are plane FESEM images of the as-synthesized HTNTs on FTO glass (200 for 12 h). It is observed that the asprepared HTNTs display a flocky appearance, which seem like a head of dandelion built from spindly nanorods (Fig. 1a), The length of the nanorods is approximately 500–1000 nm (Fig. 1b). As shown in the inset of Fig. 1a, the as-prepared HTNTs fully cover the FTO glass without crack formation, indicating uniform anisotropic growth and large-scale preparation of HTNTs [24]. The crosssectional FESEM image (Fig. 1c) reveals that the as-prepared HTNTs are well-aligned against the FTO glass substrate with a thickness of 20 lm. Interestingly, a two-layer assembly of HTNTs with different morphologies is observed, each which is 10 lm thick. The top layer (Fig. 1d) is made up of sparse nanotubes, which are connected by nanorods to form a network, indicating the presence of the hierarchical structure. The bottom layer (Fig. 1e) consists of densely packed nanotubes with a diameter of 300 nm, which ensures a large amount of dye adsorption. It is worth noting from Fig. 1c that TiO2-12 sample have good adhesion to FTO glass, which is ascribed to this buffer layer. Compared with P25 film, TiO2-12 sample is very difficult to separate completely from FTO glass with a slide (Supplemental Video). The attachment between the HTNT membrane and FTO is firm, which can ensure the charge transfer between the interface of HTNTs and FTO. As shown in the TEM image of an individual HTNT (Fig. 1f), a lot of nanorod branches with 10 nm in diameter are grown on the nanotube trunk. The
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Fig. 1. FESEM (a-e) and TEM (f, g) images of TiO2-12.
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tubular structure is more advantageous than solid rods/wires because N719 can be adsorbed outside as well as inside the tubes. Fig. 1g shows the high magnification TEM image. The lattice fringes are 0.35 nm, corresponding to the (1 0 1) plane of anatase TiO2. The 1D trunk-1D branch structure has strong light-harvesting, high specific surface area, and directed electron transport, which is a promising photoanode to address the problems encountered by the photoanodes using 1D structure or nanoparticles alone. The phase composition and topography of HTNTs were investigated by XRD analysis, as shown in Fig. 2. The sharp peaks at 25.3, 37.8, 48.0, 55.1, 62.6, 68.7, 70.3 and 75.0° correspond to the (1 0 1), (0 0 4), (2 0 0), (2 1 1), (2 0 4), (1 1 6), (2 2 0) and (2 1 5) crystal
planes of anatase TiO2, respectively, which indicates that no other phases are observed in the as-prepared HTNTs. As a photoanode of DSSCs, the anatase phase is superior to the rutile phase, on account of quicker electron transport, lower carrier recombination rate, and higher specific surface area associate with the crystal plane (1 0 1) [30,31]. To enable comparison, the different length HTNTs by controlling the growth time between 3 and 15 h are showed clearly in Fig. 3. After 3 h of the reaction time (as shown in Fig. 3a–e), the top surface FESEM images (Fig. 3a-b) of the as-obtained product display two types of nanorods: smooth nanorods and rough nanorods. The length of nanorods is about 12 lm (Fig. 3c). At this stage,
Please cite this article as: H. Chen, N. Li, Y. H. Wu et al., A novel cheap, one-step and facile synthesis of hierarchical TiO2 nanotubes as fast electron transport channels for highly efficient dye-sensitized solar cells, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2020.01.020
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no tubular structure is discovered (Fig. 3d and e). Fig. 3f–j show FESEM images of HTNTs synthesized for 6 h. During this period, the branches become longer and more numerous (Fig. 3f and g). From the cross-sectional image (Fig. 3h), the thickness of the film is about 16 lm. Delightedly, the tubular structure has appeared at 6 h (Fig. 3i and j). When the reaction time increases from 9 to 15 h (Fig. 3k–t), the thickness of films increases from 18 to 22 lm. The branches become denser and slender. The tubular structure is more visible. In this way, the dandelion-like HTNTs with high adhesion characteristics consisting of long TiO2 nanotube trunks and short TiO2 nanorod branches on FTO are successfully achieved by one-step convenient hydrothermal reaction using ethanol as a capping reagent instead of DEG. Fig. 4a shows the typical J-V curves of DSSCs fabricated with TiO2-3, TiO2-6, TiO2-9, TiO2-12 and TiO2-15 photoanodes. The photovoltaic data are summarized in Table 1. The g augments gradually from 3.43% to 9.89% when the hydrothermal reaction time increases from 3 to 12 h, and then decreases to 8.36% as the reacFig. 2. XRD pattern of TiO2-12 powder scraped from FTO glass.
Fig. 3. FESEM images of TiO2-3 (a–e), TiO2-6 (f–j), TiO2-9 (k–o), and TiO2-15 (p–t).
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Fig. 4. (a) J-V curves of DSSCs based on different photoanodes; (b) DRS of the different films without adsorbed N719 dye.
Table 1 Dye loading, the film length, photovoltaic parameters of DSSCs based on different photoanodes, and impedance data simulated from EIS in Fig. 5.
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Cell
Dye loading (10-8 mol cm2)
Length (lm)
Voc (V)
Jsc (mA cm2)
FF (%)
g (%)
Rct2 (X)
s (ms)
TiO2-3 TiO2-6 TiO2-9 TiO2-12 TiO2-15
7.00 12.60 16.10 17.60 22.70
12 16 18 20 22
0.846 0.832 0.824 0.818 0.798
5.66 12.28 15.08 16.73 14.31
71.79 72.73 74.93 72.20 73.21
3.43 7.45 9.30 9.89 8.36
217.9 102.2 90.7 83.7 70.8
199.3 94.83 84.00 77.92 65.64
tion time prolonged to 15 h. It is seen that TiO2-12 based DSSC achieves a higher g of 9.89%, a short circuit current density (Jsc) of 16.73 mA cm2, an open-circuit voltage (Voc) of 0.818 V and a fill factor (FF) of 72.2%. The Jsc has a consistent trend of g, which increases from 5.66 to 16.73 mA cm2 significantly and then decreases to 14.31 mA cm2 when the hydrothermal reaction time varies from 3 to 15 h. The enhancement of Jsc is probably ascribed to the hereinafter facts: (1) both the length of trunk and the number of branches increase with the prolonged reaction time, compared with TiO2-3 sample, which could anchor more dye molecules (shown in Table 1); (2) numerous branches fill the intervals between neighboring trunks, which can enhance multi-angle light-scattering effects (shown in Fig. 4b). These merits improve the light-harvesting efficiency, leading to a higher Jsc. Despite the fact that more dye amounts and stronger light-scattering for TiO2-15, the Jsc experiences an appropriate reduction (from 16.73 to 14.31 mA cm2), which is likely ascribed to a great deal of surface defects and recombination centers due to large surface area [32]. The recombination resistance (Rct2) of the TiO2/dye/electrolyte interfaces was characterized by EIS analysis (Fig. 5). The fitted Rct2 and electron lifetime (s) are detailed in Table 1. As displayed in Table 1, the thicker photoanode has a smaller value of Rct2, implying a quicker recombination rate (or shorter s) of photoelectrons and holes at the TiO2/dye/electrolyte interfaces. Therefore, the quicker recombination rate (or shorter s) of TiO2-15 h based DSSC is possibly associated with the bigger specific surface area, which can provide more surface trapping and result to a smaller Voc [33]. Furthermore, for highlighting the advantages of the assynthesized TiO2-12 based DSSC, a series of comparisons with the photovoltaic performance of the conventional P25 photoanode with a similar film thickness (20 lm) are made. As shown in Fig. 6a, the cell based on P25 obtains Voc of 0.802 V, Jsc of 14.26 mA cm2, FF of 73.04% and g of 8.34%. It is apparent that
Fig. 5. EIS curves of DSSCs based on different photoanodes.
TiO2-12 cell yields a higher efficiency of 9.89%, including Voc of 0.818 V, Jsc of 16.73 mA cm2 and FF of 72.20%. Compared to DSSC based on P25, the higher efficiency for TiO2-12 can be awarded to enhance the values of Jsc and Voc. The enhanced Jsc can be attributed to the more dye amounts (Table 2.) and more excellent light scattering ability (Fig. 6b) for maximizing light-harvesting efficiency and the direct electron transfer pathway for enhancing charge collection efficiency. Moreover, IPCE spectra (Fig. 6c) indicate a comparable variation tendency to the corresponding Jsc of DSSCs. The prosperous Voc and longer s can be rooted in its unique 1D trunk-1D branch combined nanostructure with less grain boundaries and defects to decrease charge recombination. Eventually, the above factors are expected to result in an outstanding g of TiO2-12 based DSSC.
Please cite this article as: H. Chen, N. Li, Y. H. Wu et al., A novel cheap, one-step and facile synthesis of hierarchical TiO2 nanotubes as fast electron transport channels for highly efficient dye-sensitized solar cells, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2020.01.020
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Fig. 6. (a) J-V curves and (c) IPCE spectra of DSSCs based on the P25 and TiO2-12 photoanodes; (b) DRS based on P25 and TiO2-12 films without adsorbed N719 dye.
Fig. 7. (a) Dark current density-voltage, (b) OCVD and (c) EIS curves of DSSCs based on P25 and TiO2-12 photoanodes. The inset is electron lifetime.
Table 2 Dye loading, photovoltaic parameters of DSSCs based on TiO2-12 and P25 photoanodes, and impedance data simulated from EIS in Fig. 7c. Cell
Dye loading (10-8 mol cm2)
Voc (V)
Jsc (mA cm2)
FF (%)
g (%)
Rct2 (X)
s (ms)
P25 TiO2-12
15.00 17.60
0.802 0.818
14.26 16.73
73.04 72.20
8.34 9.89
72.1 83.7
68.06 77.92
Please cite this article as: H. Chen, N. Li, Y. H. Wu et al., A novel cheap, one-step and facile synthesis of hierarchical TiO2 nanotubes as fast electron transport channels for highly efficient dye-sensitized solar cells, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2020.01.020
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Fig. 8. (a) sd, (b) sr, (c) Dn, and (d) gcc of DSSCs based on P25 and TiO2-12 photoanodes. 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324
Fig. 7a displays the dark current curves of DSSCs based on TiO212 and P25. The dark current of TiO2-12 is smaller than that of P25 at the uniform forward bias voltage. The OCVD curves of DSSCs based on P25 and TiO2-12 are exhibited (Fig. 7b). The Voc decay of TiO2-12 is much slower than that of P25. The s is obtained by the according equation from OCVD [34]: s = (kT/q)(dVov/dt)-1, where k is the Boltzmann constant, T is the temperature, and q is the positive elementary charge. The inset of Fig. 7b shows s Voc curves of DSSCs fabricated with P25 and TiO2-12. The values of s for TiO2-12 cell are bigger than those of P25 cell. These observations indicate that the more recombination reactions take place in P25 cell because numerous TiO2 particles present a large number of grain boundaries and surface traps to offer more recombination site. For DSSC based on TiO2-12, the photoelectrons injected from the dye can survive for a longer time and vehemently transfer to the external circuit because TiO2-12 provides direct electronic transmission path and has a good connectivity with FTO glass (as shown in Fig. S1). Therefore, the high Voc is obtained in the TiO212 based DSSC as a matter of course. To further elucidate the charge recombination process, EIS measurement was conducted (Fig. 7c). The Rct2 and s are concluded in Table 2. Obviously, the Rct2 (83.7 X) of DSSC based on TiO2-12 is higher than that of DSSC based on P25 (72.1 X), suggesting that a lower recombination rate and faster charge transfer rate in TiO2-12 cell. The s (77.92 ms) of TiO2-12 is higher than that of P25 (68.06 ms), indicating that TiO2-12 provide less recombination sites and an unblocked electron channel. The electron transport time (sd), the electron recombination time (sr), the electron diffusion coefficient (Dn), and the charge col-
lection efficiency (gcc) were studied using IMPS/IMVS measurements [6,35]. The values of sd, sr, Dn and gcc are calculated by the following formulas (1–4):
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sd ¼ 1=ð2pf d Þ
ð1Þ
328 330
sr ¼ 1=ð2pf r Þ
ð2Þ
331 333
Dn ¼ d =ð2:35sd Þ 2
ð3Þ
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gcc ¼ 1 sd =sr
ð4Þ
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334
where fd and fr represent the minimum characteristic frequency of IMPS and IMVS, respectively. Fig. 8a and b exhibit sd and sr of cells based on TiO2-12 and P25. Obviously, the sd of DSSC based on TiO212 is shorter than that of DSSC based on P25, indicating that the electrons of TiO2-12 electrode could transport to the external circuit more easily. The sr of DSSC based on TiO2-12 is longer than that of DSSC made of P25, which reveals that the electrons of TiO2-12 electrode have slower electron recombination rate. This result is attributed to its unique 1D trunk-1D branch combined nanostructure, in which the electrons can transport through a well-organized orientation and grain boundaries would diminish momentously by comparison with P25 nanoparticles. However, for P25 photoanode, free carriers are transported through a porous nanoparticle network by means of a random walk and suffer more recombination reactions. The small sr correlates with the low Voc for P25 cell. As shown in Fig. 8c, the Dn of TiO2-12 is larger than that of P25, again authenticating faster electron transport capability of DSSC based on TiO212. As shown in Fig. 8d, the average of gcc for TiO2-12 is above 90%,
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P25 just only up to around 70%. The enhancement of gcc for TiO2-12 is the result of a combination of sd and sr, which is attributed to the directional electron transport path and high adhesion characteristic of TiO2-12 (as shown in Fig. S1, Supplemental Video). Compared to P25, the higher gcc of TiO2-12 also leads to the higher Jsc and g. As a consequence, the higher dye loading, superior light scattering effect, faster electron transport and slower electron recombination of TiO2-12 are together responsible for the higher Jsc and g.
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4. Conclusions
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In conclusion, we have successfully prepared the lengthcontrollable flocky HTNTs by a one-step facile hydrothermal method using by three inexpensive commercial reagents: PTO, ethanol and water. The method for preparing HTNTs is simple, low cost and suitable for large-scale manufacturing. The anatase HTNTs are composed of omnidirectional nanorod branches on the nanotube steam surfaces, which is an effective structure to balance the light-harvesting, the charge transport and specific surface area. The DSSC based on HTNTs yields a superior g of 9.89%, which is higher than that of P25 (8.34%). The improvement of g for HTNTs can be ascribed to its high light-utilizing efficiency, superior linear electron transport, and efficient charge collection.
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Acknowledgements
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This work is financially supported by the National Natural Science Foundation of China (No. 21561010 and 21965013), the Science and Research Key Project of Universities of Hainan Province (HNky2019ZD-16), the Natural Science Foundation of Hainan Province (2017CXTD007), the Key Science and Technology Program of Haikou City (2017042), and the Graduate Student Research and Innovation Program of Hainan Province (Hys2018-218).
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Appendix A. Supplementary material
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Supplementary data to this article can be found online at https://doi.org/10.1016/j.apt.2020.01.020.
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Sun, A novel hierarchical homogeneous nanoarchitecture of TiO2 nanosheets branched TiO2 nanosheet arrays for high efficiency dye-sensitized solar cells, Electrochim. Acta 149 (2014) 264–270. [30] T. Krishnamoorthy, V. Thavasi, G.M. Subodh, S. Ramakrishna, A first report on the fabrication of vertically aligned anatase TiO2 nanowires by electrospinning: Preferred architecture for nanostructured solar cells, Energy Environ. Sci. 4 (2011) 2807–2812. [31] N.G. Park, J. van de Lagemaat, A.J. Frank, Comparison of dye-sensitized rutileand anatase-based TiO2 solar cells, J. Phys. Chem. B 104 (2000) 8989–8994. [32] W.Q. Wu, B.X. Lei, H.S. Rao, Y.F. Xu, Y.F. Wang, C.Y. Su, D.B. Kuang, Hydrothermal fabrication of hierarchically anatase TiO2 nanowire arrays on FTO glass for dye-sensitized solar cells, Sci. Rep. 3 (2013) 1352. [33] Z.Q. Li, Y.P. Que, L.E. Mo, W.C. Chen, Y. Ding, Y.M. Ma, L. Jiang, L.H. Hu, S.Y. 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Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
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Original Research Paper
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A novel cheap, one-step and facile synthesis of hierarchical TiO2 nanotubes as fast electron transport channels for highly efficient dye-sensitized solar cells
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Hong Chen, Na Li, Yang-Hong Wu, Jia-Bin Shi, Bing-Xin Lei ⇑, Zhen-Fan Sun ⇑
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School of Chemistry and Chemical Engineering, Key Laboratory of Electrochemical Energy Storage and Energy Conversion of Hainan Province, Hainan Normal University, Haikou 571158, China
a r t i c l e
i n f o
Article history: Received 8 November 2019 Received in revised form 9 January 2020 Accepted 15 January 2020 Available online xxxx Keywords: Titania Hierarchical structure Nanotube Charge transport Dye-sensitized solar cell
a b s t r a c t The well-aligned hierarchical TiO2 nanotubes (HTNTs) have been synthesized by a one-step hydrothermal method employing potassium titanium oxalate, ethanol and H2O, which are strongly adhered onto transparent conducting oxide glass. The preparation is straightforward, cheap and applicative for mass manufacture. The thickness of membranes is changed from 12 to 22 lm by adjusting the reaction time. The HTNTs consist of one-dimensional (1D) long TiO2 nanotube trunks and numerous short TiO2 nanorod branches, which can balance surface area and charge transport. By employing optimized HTNTs for dye-sensitized solar cells, a remarkable power conversion efficiency of 9.89% is obtained. The result is superior to P25 (8.34%), because 1D trunk-1D branch structure of HTNTs offers the advantages of strong light-harvesting, directed electron transport, and efficient charge collection. The HTNTs may find an underlying application in the manufacture of photovoltaic devices. Ó 2020 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
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1. Introduction
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Dye-sensitized solar cell (DSSC) has attracted much attention as an alternative energy harvesting device [1–3], because this type of cell exhibits superior features, such as cheap raw materials, simplicity in process, and relatively high power conversion efficiency (g). The g of DSSC has so far reached a value of 14.3% at 100 mW cm2 [4] and 32% at 0.1012 mW cm2 [5]. One of the most crucial components in DSSC is the photoanode, which is responsible for light-scattering, dye loading, electron injection, and charge transport. Thus, the photoanode should be carefully designed and constructed. A conventional photoanode is built by using the randomly packed TiO2 nanoparticles, which can anchor sufficient dye molecules and result in a good current density. However, the disordered structure can significantly retard the transport dynamics and augment the recombination rate of photoelectrons with I-3, and thus lower the charge collection efficiency in DSSC [6]. In addition, the conventional TiO2 photoanode exhibits high transparency of visible light, and thus utilizes part of the visible light. The layered-
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⇑ Corresponding authors.
stacking TiO2 photoanode represents a considerable advancement composed of a light-scattering overlayer and a nano-sized transparent active underlayer. By employing large TiO2 particles (100–400 nm) as scattering centers, the transparency of visible light can be reduced and the g of DSSC displays a satisfying improvement [7–10]. Regrettably, electron transport goes through a zigzag diffusion pathway, causing charge recombination with electrolyte to some extent [11]. A competitive advantage of one-dimensional (1D) TiO2 nanostructure arrays, such as nanowire arrays, nanorod arrays, and nanotube arrays (NTAs), is a directional electron transport path. Among these nanostructures, TiO2 NTAs are extremely striking because the inner and outer surfaces can absorb dyes [12]. The well-aligned TiO2 NTAs are classically prepared on a nontransparent Ti substrate by direct anodization. When the as-anodized TiO2 NTAs on Ti substrate are used as photoanode, the incident light passes preferentially through the counter electrode and electrolyte, which can reduce the light capture efficiency [13]. To solve these limits, the well-aligned TiO2 NTAs are synthesized on a fluorinedoped tin oxide glass (FTO) substrate by combining magnetron sputtering with anodization or a membrane transfer technique [14–16]. Although light illumination is irradiated from the working electrode, the weak adhesion between TiO2 membrane and FTO
E-mail addresses: [email protected] (B.-X. Lei), [email protected] (Z.-F. Sun). https://doi.org/10.1016/j.apt.2020.01.020 0921-8831/Ó 2020 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
Please cite this article as: H. Chen, N. Li, Y. H. Wu et al., A novel cheap, one-step and facile synthesis of hierarchical TiO2 nanotubes as fast electron transport channels for highly efficient dye-sensitized solar cells, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2020.01.020
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substrate limits the length of TiO2 NTAs and electron transport. The annealing process inevitably introduces cracks by thermal expansion and the resulting stresses [17]. In addition, owing to oxygen-deficient stoichiometry (TiO2-x), impurities as well as grain boundaries, the deficiency of anodic TiO2 NTAs impeded the applications [18–20]. TiO2 NTAs on FTO substrate were also developed by using ZnO template agent [21,22]. However, the type of TiO2 NTAs exhibited weak adhesion with FTO substrate because ZnO seed layer was easily etched. Our group prepared hierarchical anatase TiO2 NTAs via in situ conversion from anatase TiO2 nanowire arrays, which is a complex multistep approach [23]. In spite of continuously improved efficiency of TiO2 nanotube based cells, frustratingly, the electron mobility in such nanotube photoanode is low; in some cases, even inferior to TiO2 nanoparticle photoanode [19,20]. Hierarchical TiO2 nanotubes (HTNTs) directly grown on FTO substrate were prepared by a one-step hydrothermal reaction applying potassium titanyl oxalate (PTO), diethylene glycol (DEG) and H2O [24,25]. Hierarchical anatase TiO2 NTAs is an ideal photoanode, balancing specific surface area, electron transport, and lightscattering effect. Kim’s group assembled solid-state DSSC with 19 lm long hierarchical TiO2 NTAs and exhibited an ideal g of 8.0% [24]. Mai’s group prepared 15 lm long hierarchical TiO2 NTAs, and the DSSC achieved a g of 7.15% [26]. At present, the length of hierarchical TiO2 NTAs is still less than 19 lm using PTO, DEG and H2O by a one-step hydrothermal method, and the efficiency of the cell based on such hierarchical TiO2 NTAs is still less than 8%. The main purpose of our research is to develop a simple, low-cost, and reliable solution-based method to prepare TiO2 NTAs to improve the electron transport property. In this work, the HTNTs on FTO glass have been prepared by a one-step hydrothermal method using three cheap commercially improvable constituents: PTO, ethanol and H2O. The low-cost and straightforward method is appropriate for large-scale manufacture by utilizing ethanol instead of DEG. The length of TiO2 NTAs can be controlled by adjusting the reaction time. The 1D trunk-1D branch structure ensures fast electron transport, slow electron recombination, pronounced light-scattering effect, providing a remarkably prominent charge collection efficiency of over 90%. By employing optimized HTNTs for DSSCs, an astonishing efficiency of 9.89% is achieved.
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2. Experimental
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2.1. Materials
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Potassium titanium oxalate (PTO), ethanol, N719 dye and FTO glass (14 X cm2) substrates were acquired from Shanghai Macklin Biochemical Co., Ltd., Xilong Scientific Co., Ltd., Everlight Chemical Industrial Corp., Taiwan, and Nippon Sheet Glass Co., Ltd., respectively. No further treatment for all chemicals.
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2.2. Synthesis of HTNTs
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Firstly, FTO glass substrates were treated by using the TiO2 collosol to from a seed-layer [27]. We applied a modified approach to synthesize HTNTs [28]. Typically, 1.77 g of PTO, 13.5 mL of ethanol and 9.0 mL of H2O were introduced to a 50 mL Teflon-liner and stirred for 1 h. The hydrothermal reaction was accomplished at 200 for different times (3, 6, 9, 12 and 15 h, named TiO2-3, TiO2-6, TiO2-9, TiO2-12 and TiO2-15, respectively). After the reaction was finished, the as-fabricated samples were rinsed with deionized water and calcined at 500 °C for 1 h.
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2.3. Fabrication of DSSCs
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According to previously reported procedure [29], the working electrode, the counter electrode and electrolyte solution were prepared. The working electrode and the counter electrode were assembled into a sandwich type cell. The active area of working electrode is 0.16 cm2.
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2.4. Characterizations
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The morphology of sample was characterized by the field emission scanning electron microscopy (FESEM, JEOL JSM-7100F) and transmission electron microscopy (TEM, JEOL JEM-2100PLUS). The phase of sample was recorded by X-ray diffraction (XRD, Rigaku Ultima IV) using Cu Ka radiation (k = 1.5418 Å). The diffuse reflectance spectra (DRS) were measured by Uv–vis spectrometry (UV-2600, SHIMADZU Excellence in Science Co., Ltd., Japan). The adsorbed dye amount was calculated UV–vis absorption spectra (UV-T10, Beijing Purkinje General Instrument Co., Ltd., China). Under simulated AM 1.5 G illumination (100 mV cm2) using a solar light simulator (Oriel, Model: 91192), the photocurrent density-voltage (J-V) and open-circuit photo-voltage decay (OCVD) were recorded using a Keithley model 2400 digital source meter. The light source was a 450 W lamp (Oriel), which was calibrated by a NREL-calibrated Si solar cell (Oriel, P/N91150V). Photo current spectra system of CIMPS (PP211, Zahner) with tunable light source (TLS03) was employed to test intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS). The incident photon-to-electron conversion efficiency (IPCE) measurement was performed under monochromatic light illumination from 300 W Xenon lamp through a monochromator (Model: 90074900) in the tunable light source system (TLS-300XU, Newport). The electrochemical impedance spectroscopy (EIS) was conducted on the Zennium electrochemical workstation with a frequency range from 10 mHz to 1 MHz and the alternative signal magnitude of 10 mV under a bias of the opencircuit voltage in the dark.
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3. Results and discussion
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Fig. 1a and b are plane FESEM images of the as-synthesized HTNTs on FTO glass (200 for 12 h). It is observed that the asprepared HTNTs display a flocky appearance, which seem like a head of dandelion built from spindly nanorods (Fig. 1a), The length of the nanorods is approximately 500–1000 nm (Fig. 1b). As shown in the inset of Fig. 1a, the as-prepared HTNTs fully cover the FTO glass without crack formation, indicating uniform anisotropic growth and large-scale preparation of HTNTs [24]. The crosssectional FESEM image (Fig. 1c) reveals that the as-prepared HTNTs are well-aligned against the FTO glass substrate with a thickness of 20 lm. Interestingly, a two-layer assembly of HTNTs with different morphologies is observed, each which is 10 lm thick. The top layer (Fig. 1d) is made up of sparse nanotubes, which are connected by nanorods to form a network, indicating the presence of the hierarchical structure. The bottom layer (Fig. 1e) consists of densely packed nanotubes with a diameter of 300 nm, which ensures a large amount of dye adsorption. It is worth noting from Fig. 1c that TiO2-12 sample have good adhesion to FTO glass, which is ascribed to this buffer layer. Compared with P25 film, TiO2-12 sample is very difficult to separate completely from FTO glass with a slide (Supplemental Video). The attachment between the HTNT membrane and FTO is firm, which can ensure the charge transfer between the interface of HTNTs and FTO. As shown in the TEM image of an individual HTNT (Fig. 1f), a lot of nanorod branches with 10 nm in diameter are grown on the nanotube trunk. The
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Please cite this article as: H. Chen, N. Li, Y. H. Wu et al., A novel cheap, one-step and facile synthesis of hierarchical TiO2 nanotubes as fast electron transport channels for highly efficient dye-sensitized solar cells, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2020.01.020
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Fig. 1. FESEM (a-e) and TEM (f, g) images of TiO2-12.
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tubular structure is more advantageous than solid rods/wires because N719 can be adsorbed outside as well as inside the tubes. Fig. 1g shows the high magnification TEM image. The lattice fringes are 0.35 nm, corresponding to the (1 0 1) plane of anatase TiO2. The 1D trunk-1D branch structure has strong light-harvesting, high specific surface area, and directed electron transport, which is a promising photoanode to address the problems encountered by the photoanodes using 1D structure or nanoparticles alone. The phase composition and topography of HTNTs were investigated by XRD analysis, as shown in Fig. 2. The sharp peaks at 25.3, 37.8, 48.0, 55.1, 62.6, 68.7, 70.3 and 75.0° correspond to the (1 0 1), (0 0 4), (2 0 0), (2 1 1), (2 0 4), (1 1 6), (2 2 0) and (2 1 5) crystal
planes of anatase TiO2, respectively, which indicates that no other phases are observed in the as-prepared HTNTs. As a photoanode of DSSCs, the anatase phase is superior to the rutile phase, on account of quicker electron transport, lower carrier recombination rate, and higher specific surface area associate with the crystal plane (1 0 1) [30,31]. To enable comparison, the different length HTNTs by controlling the growth time between 3 and 15 h are showed clearly in Fig. 3. After 3 h of the reaction time (as shown in Fig. 3a–e), the top surface FESEM images (Fig. 3a-b) of the as-obtained product display two types of nanorods: smooth nanorods and rough nanorods. The length of nanorods is about 12 lm (Fig. 3c). At this stage,
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no tubular structure is discovered (Fig. 3d and e). Fig. 3f–j show FESEM images of HTNTs synthesized for 6 h. During this period, the branches become longer and more numerous (Fig. 3f and g). From the cross-sectional image (Fig. 3h), the thickness of the film is about 16 lm. Delightedly, the tubular structure has appeared at 6 h (Fig. 3i and j). When the reaction time increases from 9 to 15 h (Fig. 3k–t), the thickness of films increases from 18 to 22 lm. The branches become denser and slender. The tubular structure is more visible. In this way, the dandelion-like HTNTs with high adhesion characteristics consisting of long TiO2 nanotube trunks and short TiO2 nanorod branches on FTO are successfully achieved by one-step convenient hydrothermal reaction using ethanol as a capping reagent instead of DEG. Fig. 4a shows the typical J-V curves of DSSCs fabricated with TiO2-3, TiO2-6, TiO2-9, TiO2-12 and TiO2-15 photoanodes. The photovoltaic data are summarized in Table 1. The g augments gradually from 3.43% to 9.89% when the hydrothermal reaction time increases from 3 to 12 h, and then decreases to 8.36% as the reacFig. 2. XRD pattern of TiO2-12 powder scraped from FTO glass.
Fig. 3. FESEM images of TiO2-3 (a–e), TiO2-6 (f–j), TiO2-9 (k–o), and TiO2-15 (p–t).
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Fig. 4. (a) J-V curves of DSSCs based on different photoanodes; (b) DRS of the different films without adsorbed N719 dye.
Table 1 Dye loading, the film length, photovoltaic parameters of DSSCs based on different photoanodes, and impedance data simulated from EIS in Fig. 5.
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Cell
Dye loading (10-8 mol cm2)
Length (lm)
Voc (V)
Jsc (mA cm2)
FF (%)
g (%)
Rct2 (X)
s (ms)
TiO2-3 TiO2-6 TiO2-9 TiO2-12 TiO2-15
7.00 12.60 16.10 17.60 22.70
12 16 18 20 22
0.846 0.832 0.824 0.818 0.798
5.66 12.28 15.08 16.73 14.31
71.79 72.73 74.93 72.20 73.21
3.43 7.45 9.30 9.89 8.36
217.9 102.2 90.7 83.7 70.8
199.3 94.83 84.00 77.92 65.64
tion time prolonged to 15 h. It is seen that TiO2-12 based DSSC achieves a higher g of 9.89%, a short circuit current density (Jsc) of 16.73 mA cm2, an open-circuit voltage (Voc) of 0.818 V and a fill factor (FF) of 72.2%. The Jsc has a consistent trend of g, which increases from 5.66 to 16.73 mA cm2 significantly and then decreases to 14.31 mA cm2 when the hydrothermal reaction time varies from 3 to 15 h. The enhancement of Jsc is probably ascribed to the hereinafter facts: (1) both the length of trunk and the number of branches increase with the prolonged reaction time, compared with TiO2-3 sample, which could anchor more dye molecules (shown in Table 1); (2) numerous branches fill the intervals between neighboring trunks, which can enhance multi-angle light-scattering effects (shown in Fig. 4b). These merits improve the light-harvesting efficiency, leading to a higher Jsc. Despite the fact that more dye amounts and stronger light-scattering for TiO2-15, the Jsc experiences an appropriate reduction (from 16.73 to 14.31 mA cm2), which is likely ascribed to a great deal of surface defects and recombination centers due to large surface area [32]. The recombination resistance (Rct2) of the TiO2/dye/electrolyte interfaces was characterized by EIS analysis (Fig. 5). The fitted Rct2 and electron lifetime (s) are detailed in Table 1. As displayed in Table 1, the thicker photoanode has a smaller value of Rct2, implying a quicker recombination rate (or shorter s) of photoelectrons and holes at the TiO2/dye/electrolyte interfaces. Therefore, the quicker recombination rate (or shorter s) of TiO2-15 h based DSSC is possibly associated with the bigger specific surface area, which can provide more surface trapping and result to a smaller Voc [33]. Furthermore, for highlighting the advantages of the assynthesized TiO2-12 based DSSC, a series of comparisons with the photovoltaic performance of the conventional P25 photoanode with a similar film thickness (20 lm) are made. As shown in Fig. 6a, the cell based on P25 obtains Voc of 0.802 V, Jsc of 14.26 mA cm2, FF of 73.04% and g of 8.34%. It is apparent that
Fig. 5. EIS curves of DSSCs based on different photoanodes.
TiO2-12 cell yields a higher efficiency of 9.89%, including Voc of 0.818 V, Jsc of 16.73 mA cm2 and FF of 72.20%. Compared to DSSC based on P25, the higher efficiency for TiO2-12 can be awarded to enhance the values of Jsc and Voc. The enhanced Jsc can be attributed to the more dye amounts (Table 2.) and more excellent light scattering ability (Fig. 6b) for maximizing light-harvesting efficiency and the direct electron transfer pathway for enhancing charge collection efficiency. Moreover, IPCE spectra (Fig. 6c) indicate a comparable variation tendency to the corresponding Jsc of DSSCs. The prosperous Voc and longer s can be rooted in its unique 1D trunk-1D branch combined nanostructure with less grain boundaries and defects to decrease charge recombination. Eventually, the above factors are expected to result in an outstanding g of TiO2-12 based DSSC.
Please cite this article as: H. Chen, N. Li, Y. H. Wu et al., A novel cheap, one-step and facile synthesis of hierarchical TiO2 nanotubes as fast electron transport channels for highly efficient dye-sensitized solar cells, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2020.01.020
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Fig. 6. (a) J-V curves and (c) IPCE spectra of DSSCs based on the P25 and TiO2-12 photoanodes; (b) DRS based on P25 and TiO2-12 films without adsorbed N719 dye.
Fig. 7. (a) Dark current density-voltage, (b) OCVD and (c) EIS curves of DSSCs based on P25 and TiO2-12 photoanodes. The inset is electron lifetime.
Table 2 Dye loading, photovoltaic parameters of DSSCs based on TiO2-12 and P25 photoanodes, and impedance data simulated from EIS in Fig. 7c. Cell
Dye loading (10-8 mol cm2)
Voc (V)
Jsc (mA cm2)
FF (%)
g (%)
Rct2 (X)
s (ms)
P25 TiO2-12
15.00 17.60
0.802 0.818
14.26 16.73
73.04 72.20
8.34 9.89
72.1 83.7
68.06 77.92
Please cite this article as: H. Chen, N. Li, Y. H. Wu et al., A novel cheap, one-step and facile synthesis of hierarchical TiO2 nanotubes as fast electron transport channels for highly efficient dye-sensitized solar cells, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2020.01.020
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Fig. 8. (a) sd, (b) sr, (c) Dn, and (d) gcc of DSSCs based on P25 and TiO2-12 photoanodes. 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324
Fig. 7a displays the dark current curves of DSSCs based on TiO212 and P25. The dark current of TiO2-12 is smaller than that of P25 at the uniform forward bias voltage. The OCVD curves of DSSCs based on P25 and TiO2-12 are exhibited (Fig. 7b). The Voc decay of TiO2-12 is much slower than that of P25. The s is obtained by the according equation from OCVD [34]: s = (kT/q)(dVov/dt)-1, where k is the Boltzmann constant, T is the temperature, and q is the positive elementary charge. The inset of Fig. 7b shows s Voc curves of DSSCs fabricated with P25 and TiO2-12. The values of s for TiO2-12 cell are bigger than those of P25 cell. These observations indicate that the more recombination reactions take place in P25 cell because numerous TiO2 particles present a large number of grain boundaries and surface traps to offer more recombination site. For DSSC based on TiO2-12, the photoelectrons injected from the dye can survive for a longer time and vehemently transfer to the external circuit because TiO2-12 provides direct electronic transmission path and has a good connectivity with FTO glass (as shown in Fig. S1). Therefore, the high Voc is obtained in the TiO212 based DSSC as a matter of course. To further elucidate the charge recombination process, EIS measurement was conducted (Fig. 7c). The Rct2 and s are concluded in Table 2. Obviously, the Rct2 (83.7 X) of DSSC based on TiO2-12 is higher than that of DSSC based on P25 (72.1 X), suggesting that a lower recombination rate and faster charge transfer rate in TiO2-12 cell. The s (77.92 ms) of TiO2-12 is higher than that of P25 (68.06 ms), indicating that TiO2-12 provide less recombination sites and an unblocked electron channel. The electron transport time (sd), the electron recombination time (sr), the electron diffusion coefficient (Dn), and the charge col-
lection efficiency (gcc) were studied using IMPS/IMVS measurements [6,35]. The values of sd, sr, Dn and gcc are calculated by the following formulas (1–4):
325 326 327
sd ¼ 1=ð2pf d Þ
ð1Þ
328 330
sr ¼ 1=ð2pf r Þ
ð2Þ
331 333
Dn ¼ d =ð2:35sd Þ 2
ð3Þ
336
gcc ¼ 1 sd =sr
ð4Þ
337 339
334
where fd and fr represent the minimum characteristic frequency of IMPS and IMVS, respectively. Fig. 8a and b exhibit sd and sr of cells based on TiO2-12 and P25. Obviously, the sd of DSSC based on TiO212 is shorter than that of DSSC based on P25, indicating that the electrons of TiO2-12 electrode could transport to the external circuit more easily. The sr of DSSC based on TiO2-12 is longer than that of DSSC made of P25, which reveals that the electrons of TiO2-12 electrode have slower electron recombination rate. This result is attributed to its unique 1D trunk-1D branch combined nanostructure, in which the electrons can transport through a well-organized orientation and grain boundaries would diminish momentously by comparison with P25 nanoparticles. However, for P25 photoanode, free carriers are transported through a porous nanoparticle network by means of a random walk and suffer more recombination reactions. The small sr correlates with the low Voc for P25 cell. As shown in Fig. 8c, the Dn of TiO2-12 is larger than that of P25, again authenticating faster electron transport capability of DSSC based on TiO212. As shown in Fig. 8d, the average of gcc for TiO2-12 is above 90%,
Please cite this article as: H. Chen, N. Li, Y. H. Wu et al., A novel cheap, one-step and facile synthesis of hierarchical TiO2 nanotubes as fast electron transport channels for highly efficient dye-sensitized solar cells, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2020.01.020
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P25 just only up to around 70%. The enhancement of gcc for TiO2-12 is the result of a combination of sd and sr, which is attributed to the directional electron transport path and high adhesion characteristic of TiO2-12 (as shown in Fig. S1, Supplemental Video). Compared to P25, the higher gcc of TiO2-12 also leads to the higher Jsc and g. As a consequence, the higher dye loading, superior light scattering effect, faster electron transport and slower electron recombination of TiO2-12 are together responsible for the higher Jsc and g.
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4. Conclusions
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In conclusion, we have successfully prepared the lengthcontrollable flocky HTNTs by a one-step facile hydrothermal method using by three inexpensive commercial reagents: PTO, ethanol and water. The method for preparing HTNTs is simple, low cost and suitable for large-scale manufacturing. The anatase HTNTs are composed of omnidirectional nanorod branches on the nanotube steam surfaces, which is an effective structure to balance the light-harvesting, the charge transport and specific surface area. The DSSC based on HTNTs yields a superior g of 9.89%, which is higher than that of P25 (8.34%). The improvement of g for HTNTs can be ascribed to its high light-utilizing efficiency, superior linear electron transport, and efficient charge collection.
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Acknowledgements
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This work is financially supported by the National Natural Science Foundation of China (No. 21561010 and 21965013), the Science and Research Key Project of Universities of Hainan Province (HNky2019ZD-16), the Natural Science Foundation of Hainan Province (2017CXTD007), the Key Science and Technology Program of Haikou City (2017042), and the Graduate Student Research and Innovation Program of Hainan Province (Hys2018-218).
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Appendix A. Supplementary material
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Supplementary data to this article can be found online at https://doi.org/10.1016/j.apt.2020.01.020.
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References
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