Journal of Wuhan University of
Technology-Mater. Sci. Ed.
Feb.2013
17
DOI 10.1007/s11595-013-0632-6
CdS Quantum Dots-sensitized TiO2 Nanotube Arrays for Solar Cells
SUI Xiaotao, TAO Haizheng, LOU Xianchun, WANG Xuelai, FENG Jiamin, ZENG Tao, ZHAO Xiujian* (State Key Laboratory of Silicate Materials for Architectures (Wuhan University of Technology), Wuhan 430070, China)
Abstract: CdS quantum dots(QDs) sensitized TiO2 nanotube arrays photoelectrodes were investigated for their photovoltaic performance of quantum dots-sensitized solar cells. The highly ordered TiO2 nanotube arrays(TNAs) were synthesized on Ti foils by anodic oxidation method. Then CdS quantum dots were deposited onto the TiO2 nanotube arrays by successive ionic layer absorption and reaction(SILAR) method to serve as the sensitizers. Cd(NO3)2 and Na2S were used as the precursor materials of Cd+ and S2- ions, respectively. It is found that the CdS QDs sensitizer may significantly increase the light response of TiO2 nanotube arrays. With increasing CdS QDs deposition cycles, the visible light response increases. Maximum photocurrent was obtained for the QDs that have an absorption peak at about 500 nm. Under AM 1.5 G illuminations(100 mW cm-2), a 4.85 mA/cm2 short circuit current density was achieved, and the maximium energy conversion efficiency of the asprepared CdS QDs-sensitized TNAs solar cells was obtained as high as 0.81 % at five SILAR cycles. Key words: quantum dots sensitized solar cell; successive ionic layer adsorption and reaction; TiO2 nanotube arrays
1 Introduction Quantum dots-sensitized solar cell(QDSC) is a promising alternative to conventional Si-based solar cells due to low cost and easy manufacturing. Inorganic semiconductor quantum dots (QDs) are usually used as sensitizers in QDSCs. Compared with dye sensitizers, inorganic semiconductor QDs sensitizers exhibit some advantages in solar cell applications: First, energy gap of the semiconductor QDs can be tuned by controlling their sizes. Second, most of the QDs has a large extinction coefficients. Third, under visible light irradiation, some QDs may produce multiple excitons per photon. For these reasons, much attention has been recently concentrated on sensitizing wide bandgap semiconductors with narrow bandgap chalcogenides QDs, such as CdS[1],CdSe[2], CdTe[3], PbS[4],Ag2S[5],and ©Wuhan University of Technology and SpringerVerlag Berlin Heidelberg 2013 (Received: Dec. 29, 2011; Accepted: Sept. 26, 2012) SUI Xiaotao(隋小涛): Ph D; E-mail:
[email protected] *Corresponding author: ZHAO Xiujian(赵修建): Prof.; Ph D; E-mail:
[email protected] Funded by Major State Basic Research Development Program of China (973 Program) (No.2009CB939704) and Key Project of Chinese Ministry of Education (No.309021)
Sb 2 S 3 [6] .Recently, CdS and CdSe are the most common QDs sensitizers applied in QDSCs. Lee et al investigated the effects of CdS modification on the photoelectric properties of CdS sensitized porous nanocrystalline TiO 2 solar cells [7]. In the previous study, we investigated the photoelectrochemical properties of screen-printing TiO2 nanoparticles with CdS QDs[8]. Morphology of TiO2 electrodes plays an important role for enhancing efficiency of QDSCs. Highly ordered TiO2 nanotube arrays (TNAs)[9, 10] have been widely used as photoanodes in QDSCs due to their much advantageous photon-induced properties and the separation of the photo-excited charges. So the synthesis and photoelectrochemical properties of CdS QDs-sensitized TNAs were studied here. In this study, we synthesized CdS QDs-sensitized TNAs photoelectrodes using a successive ionic layer absorption and reaction (SILAR) method. The assynthesized samples were characterized by field emission scanning electron microscopy (FESEM), field emission scanning electron microscopy (FETEM), X-ray diffraction (XRD) and UV-vis diffuse reflectance absorption spectra (DRS).We also investigated the photoelectrochemical properties of the as-synthesized photoelectrodes under visible illumination.
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2 Experimental 2.1 Synthesis of TiO2 nanotube arrays (TNAs) Highly ordered self-organized TNAs electrodes were synthesized by electrochemical anodization of titanium foils (99.6 % purity, 0.25 mm thick), based on a previous work[11]. The titanium foil was immersed in formamide solution containing 5 wt % of deionized water and 0.6 wt % NH 4F (98%), and applied to a constant 30 V anodic potential for 8 h. After anodic oxidation, sonication of 1-5 s was performed to remove any surface deposits. The resulting amorphous TiO2 TNAs film was then transferred to anatase phase by annealing at 450 ℃ for 2 h with a heating rate of 3 ℃ /min. 2.2 Synthesis of CdS QDs-sensitized TNAs CdS QDs were deposited on TNAs electrodes by SILAR method, similar with the process reported by Lee et al[7]. The CdS sensitized TNAs electrodes were prepared by first dipping the as-prepared TNAs in 2 M Cd(NO3)2 ethanol solution for 5 min, rinsed with ethanol and then dipped in 2 M Na2S methanol/water solution for another 5 min followed by rinsing with methanol/water solution . The methanol/water solvent is composed of 50 vol% of methanol and 50 vol% of deionized water. This constituted one cycle and the process was repeated for 1, 2, 3, 4, 5 cycles. The CdS-sensitized TNAs electrodes were then combined with the Pt counter electrode, in the presence of water/methanol (7:3 by volume) solution polysulfide electrolyte (0.5 M Na2S and 0.125 M S and 0.2 M KCl) to assemble a typical QDSC. The active area of the cells was nearly 0.16 cm2. 2.3 Morphological and structural characterization and photoelectrochemical measurements XRD measurements were obtained on X’ pert PRO XRD (PANalytical,Netherlands) with a monochromatic CuKα radiation (λ=1.540 5 Å) and Ni filter. The morphologies were investigated using an S-4800 FESEM (Hitachi, Japan) and JEM2100F FETEM(JEOL, Japan). Energy dispersive spectroscopy (EDS) measurements were made with JSM-5610LV (JEOL, Japan). Diffuse reflectance spectra were recorded using a Shimadzu UV-3100PC spectrophotometer. The photocurrent-voltage (I-V) curve was measured under an illumination of a solar simulator (Newport, Oriel class B) at one sun (AM1.5, 100 mW cm-2). An Eco Chemie Autolab potentiostat/
galvanostat was used to record the current-voltage (I-V) characteristics.
3 Results and discussion Fig.1 shows the FESEM images of the asfabricated TNAs and CdS QDs-sensitized TNAs. The as-anodized TNAs with an outer diameter of about 100 nm and inner diameter of 80 nm were uniformly grown on Ti substrate (Fig.1(a)). Fig.1(b) shows that the as-anodized TNAs has an average length of about 13 μm. FESEM images (Figs.1(c) and (d)) show that the TNAs are covered with many small CdS QDs, and some large aggregates of the QDs are observed at some TiO2 nanotube entrances. These results indicate the effectiveness of the SILAR approach for the deposition of CdS QDs on TNAs and some aggregates of CdS QDs in deposition process. In addition, the wall of the CdS QDs-sensitized TNAs (Fig.1(c)) looks a little thicker than that of the pure TNAs (Fig.1(a)), which is attributed to the deposition of CdS QDs on the tube walls. Fig.1(e) and (f) show the outer wall of TNAs before and after CdS deposition. It is obviously observed that CdS QDs deposition make the outer wall of TNAs much rough. This result shows that the CdS QDs can deposited onto the outer wall of TNAs.
Journal of Wuhan University of
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to anatase TiO2. Therefore, the XRD results along with the TEM results demonstrate that crystalline CdS QDs are deposited on anatase TiO 2 tubes by the SILAR approach.
The microscopic structure of the CdS QDssensitized TNAs was further investigated by FETEM. Fig.2(a) shows that a large number of QDs are deposited inside TiO 2 tubes and on the outer wall surface. Fig.2(b) provides a cross-sectional image of a single TiO2 nanotube decorated with CdS QDs.The TEM image shows that QDs deposited inside TiO 2 tubes with the size about 5 nm. High-resolution TEM image (Fig.2(c)) suggests that the QDs are tightly adhered to the tube surfaces. The high-resolution lattice image confirms that the NTs are anatase TiO2 and the QDs are CdS. Fig.2(d) shows the energy dispersive X-ray spectroscopy (EDS) of the CdS QDs decorated TiO2 NTAs. The elemental composition of CdS/TiO2 TNAs film has been analyzed and the characteristic elements were identified using an EDX detection spectrometer. The area with CdS/TiO 2 NTAs film shows that strong Ka and Kb peaks from Ti element appear at 4.51 and 4.92 keV, while a moderate Ka peak of the element O appears at 0.52 keV. Besides the above peaks, elements S and Cd can also be found. Quantitative analysis of the EDS spectrum gives a Cd/ S atomic ratio of about 1:1, indicating that high-grade CdS QDs were formed on the TiO2 NTAs. The crystalline structure of the TNAs and the CdS QDs-sensitized TNAs were characterized using XRD and typical XRD patterns are shown in Fig.3. The XRD spectrum of the TNAs clearly confirms that the TNAs are anatase structure of TiO2 (JCPDS card No. 21-1272) and titanium (JCPDS card No. 44-1294) (Fig.3(a)). The XRD spectrum of the CdS QDs-sensitized TNAs displays many new peaks corresponding to CdS (JCPDS card No. 80-0019) as well as the peaks corresponding
Fig.4 shows the UV-vis DRS of the TNAs and the CdS QDs-sensitized TNAs prepared at different deposition cycles (1 to 5 cycles). The spectra reveal that TNAs absorb mainly UV light with a wavelength below 400 nm. However, the absorption spectra of the CdS QDs-sensitized TNAs shift toward the visible light region (400-500 nm). Consequently, these results indicate that deposition of CdS QDs can improve the visible light absorption property of TNAs. As the deposition cycles increase, the absorption edge of the spectra red-shift and the absorbance increase in both UV and visible light spectrum region. This red shift is due to the increased size of CdS QDs. Photovoltaic properties of as-prepared CdS QDs-sensitized TNA architectures were evaluated by constructing QDSCs and the energy conversion efficiency was evaluated using the equation η=FF•Jsc• Voc/Pin, where Jsc is the short circuit current density, Voc is the open circuit voltage, FF is the fill factor, and Pin
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Vol.28 No.1 SUI Xiaotao et al: CdS Quantum Dots-sensitized TiO2 Nanotube Arrays for...
is the incident light power density. The device requires backside illumination (through the Pt cathode) because the Ti foil is opaque. Fig.5 shows the photocurrent density vs voltage (J-V) characteristics of QDSCs based on the CdS QDs-sensitized TNA solar cells. The Voc, Jsc, FF and η of the cells at different deposition cycles are listed in Table 1. When the SILAR processs was introduced, both Jsc and η increased with increasing SILAR cycles, reaching a maximum value at five cycles. As a result, a maximum value as high as 0.81 % of the overall energy conversion efficiency was obtained at five SILAR cycles. On the early SILAR cycles, the coverage ratio of CdS on TNAs was gradually increased. After each SILAR cycle, CdS QDs deposition amount increases. Such increment of CdS loading leads to more excited electrons under the illumination of light, which is advantageous to photocurrent of a QDSC. However, if the QDs deposition amount is too much, it will be more difficult to inject an excited electron generated in the outer layer QDs into the TNAs. Meanwhile, the increase of SILAR cycles is caused to CdS/electrolyte contacting area decrease. The competition between these effects determines an optimal thickness of the CdS QDs deposition amount to be used as a sensitizer of a QDSC cell. In our previous studies[8,12], we studied both TiO2 nanoparticles (TiO2 -NP) and TiO2 nanorods arrays(TiO2 -NR) applied in CdS quantum dots sensitized solar cells. The CdS/TNAs electrode exhibited greater photocurrent than that of CdS/TiO2 -NP and CdS/TiO2
-NR electrode(CdS/TNAs: 4.85 mA cm-2, CdS/TiO2NP:2.2 mA cm -2 , CdS/TiO 2 -NR: 3.09 mA cm -2 ). Because the nanotubes have a higher surface area and aspect ratio to favorable CdS QDs adsorption,when the thickness of TiO2-NP, TiO2-NR and TNAs films is similar (TiO2-NP:12 μm, TiO2-NR:10 μm and TNAs:13 μm).The open circuit potential(Voc), corresponds to the difference between the apparent Fermi level of the CdS/TiO 2 composite and the redox electrolytes electrode. With polysulfide electrolytes, the Voc of CdS/ TNAs system(0.5 V) is similar to that of CdS/TiO2NR system(0.45 V)[12]. This maybe due to that the size of CdS QDs in TNAs is smaller than that in TiO 2NR electrode. It can be demonstrated that the UVVis absorbance onset of CdS/TNAs system(about 500 nm) is smaller than that of CdS/TiO2-NR system(about 520 nm). Such a blue shift reflects the changes in the bandgap of quantum dots, demonstrating the quantum confinement effects in the nanotubes architecture. This Voc stability confirms that the CdS/TiO2 interface and the apparent Fermi level are unaffected by support architecture, but can only be tuned by the modulation of CdS particle size. It is also noted that in CdS/TiO2-NP system with I-/I3- electrolytes, the Voc(about 0.7 V) is larger than in polysulfide electrolytes[8], but CdS is not stable in I-/I3- electrolytes. In spite to superior surface area ratio and QDs size confinement of nanotube architecture, the energy efficiency of the resulted CdS/ TNAs system is similar to that of TiO2-NR or TiO2NP system. This is mainly caused by a lower FF(about 0.4) of CdS/TNAs system while the FF of TiO2-NR or TiO2-NP system is almost 0.6. The TNAs are based on the opaque metallic Ti substrate, so it must be backside illuminated from the Pt counter electrode. Otherwise, a thin layer of amorphous TiO2 between TNAs and the underlying Ti support also influence the electrons collection at FTO glass substrate. These factors may result in FF of CdS/TNAs system being not ideal. Efforts are underway to improve the FF of CdS/TNAs to boost overall efficiency.
4 Conclusions CdS QDs-sensitized TNAs were prepared by a SILAR approach. TNAs were prepared by anodizing Ti foils at 30 V for 8 h, which resulted in highly ordered nanotube arrays with an average inner pore diameter of approximately 80 nm and wall thickness of approximately 20 nm. Sensitization of CdS QDs extended the absorption spectra of TNAs
Journal of Wuhan University of
Technology-Mater. Sci. Ed.
significantly to the visible light region. The CdS QDs-sensitized TNAs exhibited much enhanced photocurrent generation under visible irradiation. The photoelectrochemical properties of the QDssensitized TNAs solar cell were performed under AM 1.5 G illumination conditions. The maxmium energy conversion efficiency of the as-prepared CdS QDssensitized TNAs solar cells was obtained as high as 0.81 % at five SILAR cycles.
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