Electrochimica Acta 89 (2013) 510–515
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CdS quantum dots sensitized SnO2 photoelectrode for photoelectrochemical application Xiaoming Zhou, Wuyou Fu, Haibin Yang ∗ , Yixing Li, Yanli Chen, Meiling Sun, Jinwen Ma, Lihua Yang, Bo Zhao, Lecheng Tian State Key Laboratory of Superhard Materials, Jilin University, 2699 Qianjin Street, Changchun 130012, China
a r t i c l e
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Article history: Received 30 July 2012 Received in revised form 21 November 2012 Accepted 22 November 2012 Available online 29 November 2012 Keywords: CdS Quantum dot SnO2 Photoelectrochemical properties
a b s t r a c t In this study, the SnO2 spherical particles ﬁlm was ﬁrstly grown directly on FTO substrates by using one step hydrothermal method, and CdS QDs were deposited on the surface of the SnO2 spherical particles to act a light absorber by using the successive ionic-layer adsorption and reaction (SILAR) method. The photovoltaic performances of the sensitized-type solar cells based on SnO2 /CdS electrodes were investigated. A maximum 1.47 mA cm−2 short circuit current density and 0.22% conversion efﬁciency under one sun illumination has been achieved. These results demonstrate that the CdS QDs-sensitized SnO2 photoelectrode has a potential application in solar cells. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Semiconductor quantum dots (QDs) have attracted research interest for the development of solar cell devices due to their extraordinary optical and electrical properties [1,2]. These include size-dependent optical absorption spectra, large extinction coefﬁcients, extended photostability and impact ionization effects, make QDs materials are very promising candidates for efﬁcient light harvesting materials in sensitized solar cells [3,4]. As sensitizer for sensitized solar cells, inorganic semiconductor quantum dots (QDs), such as CdS [5–8], CdSe [9–11] or PbS [12–14] has been reported in lots of works. To construct QD-sensitized solar cells, a typical strategy is to use QDs as the sensitizer on metal oxide nanostructure ﬁlms. In most reports, TiO2 nanocrystalline is usually used as the photoanode, but further improvements in the photovoltaic performance have been limited due to high electron recombination rates related to the low electron mobility and transport properties of TiO2 . Compared to TiO2 , tin oxide (SnO2 ) is a promising wide band gap oxide material because of its higher electronic mobility and large band gap (3.6 eV). Furthermore, SnO2 has a low sensitivity to UV degradation due to its larger band gap and hence has better long term stability . Also, it is a better charge acceptor because
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(H. Yang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.11.080
of a more negative conduction band minimum (CBM) that should facilitate charge transfer from low band gap sensitizers such as near-infrared light absorbers, PbS , PbSe , and CuInSe2 , etc. Therefore, SnO2 is an interesting photoanode materials, which will show greatly promise in semiconductor sensitized solar cells (SSCs). But its application is conﬁned by the difﬁculty in actual synthesization on transparent conductive glass substrate. To the best of our knowledge, there are only a few reports on QDs sensitization of SnO2 nanostructure . QDs-sensitized SnO2 can extend the light absorbance to the visible light region and then improve the photoelectrochemical efﬁciency, so the investigation of this novel QDs-sensitized SnO2 nanostructure should be signiﬁcant . In this work, we prepared a CdS QDs-sensitized SnO2 spherical particles ﬁlm on FTO photoelectrodes by using the successive ioniclayer adsorption and reaction (SILAR) method and investigated their photoelectrochemical properties. In the as-prepared structure, the SnO2 spherical particles ﬁlm was ﬁrstly grown directly on FTO substrates by using one step hydrothermal method, and CdS QDs were deposited on the surface of the SnO2 nanocrystals to act a light absorber. CdS is a short bandgap semiconductor (Eg = 2.5 eV) with its conduction band (−0.8 V vs NHE) more negative than that of SnO2 (0.0 V vs NHE). The matching of band edges between CdS and SnO2 was important to form a type II heterojunction (Scheme 1). We will show here that efﬁcient charge separation has been observed in a photoelectrochemical cell and promising photovoltaic performance has been achieved. For three-electrode measurements do not give the parameters of the cell but rather the parameters of
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Scheme 1. Energy band of SnO2 –CdS coupled semiconductor and photoinduced charge separation and transport in the composite structures.
photoelectrode. The data measured from three-electrode measurements have been corrected. To the best of our knowledge, this is the ﬁrst detailed study of the CdS QDs-sensitized SnO2 nanostructure. 2. Experimental 2.1. Preparation of SnO2 spherical particles ﬁlm The SnO2 spherical particles ﬁlms were synthesized on ﬂuorinedoped tin oxide (FTO) conductive glass substrates by using hydrothermal method. Typically, SnCl4 ·5H2 O (1.05 g), HCl (0.6 mL), and PVP (0.945 g) were added in order to ethanol/distilled water (18 mL, 1/1, v/v) under intense ultrasonic treatment at 50 ◦ C. The resulting solution was then transferred to a Teﬂon-lined stainless autoclave (50 mL volume) after two pieces of cleaned FTO (NSG GROUP, TCO-17, 16 /sq, with a thickness of 2 mm) substrates were placed within the reactor. The hydrothermal synthesis was conducted at 210 ◦ C in an electric oven for 12 h. After synthesis, the Teﬂon reactor was cooled to room temperature and the FTO substrates were taken out and rinsed extensively with deionized water and ethanol. After preparation, SnO2 electrodes were immersed into 40 mM aqueous TiCl4 solution at 70 ◦ C for 40 min and then washed with water and ethanol followed by drying in an electric oven at 70 ◦ C. 2.2. Preparation of CdS QDs-sensitized SnO2 spherical particles ﬁlms CdS QDs were deposited on the SnO2 electrodes by the SILAR method. In a typical procedure, the SnO2 electrodes were immersed in a solution containing 0.5 M Cd(NO3 )2 in ethanol for 5 min, and then rinsed with ethanol to remove the excess Cd2+ . Electrodes were then dried in a gentle stream of N2 for 1 min. Subsequently the dried electrodes were dipped into a solution containing 0.5 M Na2 S in a mixture of methanol and deionized water (1:1, v/v) for 5 min. Electrodes were then rinsed in methanol and dried again with N2 , the electrode completes one cycle of deposition. The same procedure was repeated several times to obtain suitable CdS loading on SnO2 electrodes. 2.3. Characterization X-ray power diffraction (XRD) analysis was conducted on a Rigaku D/max-2500 X-ray diffractometer with Cu K␣ radiation ˚ A model JEOL JSM-6700F ﬁeld-emission scanning ( = 1.5418 A). electron microscopy ﬁtted with an energy dispersive X-ray spectrometer (EDX) was used to characterize the morphologies and elemental analysis of the samples. Transmission electron microscope (TEM) and high-resolution TEM (HR-TEM) images were taken by a JEM-2100F high-resolution transmission microscope
Fig. 1. XRD patterns of the FTO substrate and the as-synthesized SnO2 spherical particles ﬁlm on FTO.
operating at 200 kV with a point resolution of 0.23 nm. Optical characterization of the ﬁlms was performed using a UV-3150 double-beam spectrophotometer. 2.4. Photovoltaic measurements The photoelectrochemical properties were probed using the conventional three-electrode system which is made of quartz cell and linked with the electrochemical workstation (CH Instruments, model CHI601C). The as-prepared ﬁlm electrodes were used as the working electrode, while a platinum mesh as the counter electrode and a saturated Ag/AgCl as the reference electrode. A mixture of 0.25 M Na2 S and 0.35 M Na2 SO3 aqueous solution was used as electrolyte. The electrolyte had a PH of about 12 and the solution potential was about −0.43 V versus Ag/AgCl. The CHI electrochemical workstation was used to measure dark and illuminated current at a scan rate of 10 mV/s. Sunlight was simulated with a 500 W xenon lamp (Spectra Physics). The light intensity was calibrated at 100 mW/cm2 by a laser power meter (BG26M92C, Midwest Group). 3. Results and discussion 3.1. Characterization of SnO2 and CdS QDs-sensitized SnO2 electrodes The crystalline structure of the as-prepared ﬁlm was conﬁrmed by XRD. As shown in Fig. 1, all the diffraction peaks in the XRD pattern can be indexed to the rutile phase SnO2 (JCPDS card 411445), and no impurity is observed. The top view FESEM image of the bare SnO2 ﬁlm is shown in Fig. 2a reveals the surface of the FTO substrate is covered with a large-scale of high-purity SnO2 spherical particles. The high-magniﬁcation FESEM image in Fig. 2b, clearly shows the spherical structure actually are composed of number of nanorods. Each nanorod is about 100 nm in length and 30 nm in diameter. Fig. 2c is the cross section view of the as-prepared SnO2 ﬁlm, which obviously shows that the resulting ﬁlm is about 2.1 m in thickness. TEM and HRTEM were used to further characterize the detailed structure of the as-prepared SnO2 nanostructure. Fig. 3a is a typical TEM image of the SnO2 spheres, which shows that the sphere is composed of number of nanorods, in accordance with SEM results. Fig. 3b shows a HRTEM image and its corresponding
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Fig. 2. Typical top view FESEM images of the as-synthesized SnO2 ﬁlm at (a) low and (b) high magniﬁcations. (c) Cross sectional FESEM image of the as-synthesized SnO2 ﬁlm.
Fourier transform patterns (FFT) (inset) of one typical nanorod of the spherical structure, respectively, conﬁrming that the SnO2 nanorods are single-crystalline. Lattice fringes with interplanar spacing d110 = 0.334 nm is consistent with the tetragonal rutile phase (JCPDS card no. 41-1445). Fig. 3c is the typical HRTEM image of a SnO2 nanorod of the spherical structure deposited with CdS for 5 cycles, showing that a large amount of CdS QDs have been deposited on the SnO2 nanorod. The CdS QDs with a diameter smaller than 10 nm are visible as the dark sports on the surface of the nanorod. The observed lattice spacing of 0.334 nm corresponds to the (1 1 0) plane of rutile SnO2 . The observed 0.289 nm and 0.339 nm fringes of the QDs on the nanorod correspond to the (2 0 0) and (1 1 1) planes of the cubic phase of CdS, respectively (JCPDS card no. 80-0019). Energy dispersive spectroscopy (EDS) was applied to determine the composition of the nanostructure. In the spectrum (Fig. 3d), Sn and O peaks result from the SnO2 spherical particles. The atomic ratio of Si, Sn, O, Cd and S in the sample is 0.94%, 27.56%, 55.99%, 7.86% and 7.65%, respectively. The 1:1 molar ratio of Cd to S in the sample indicates that high-grade CdS are formed. These results prove that CdS QDs have been successfully deposited on the SnO2 microstructure.
3.2. UV–vis absorption spectroscopy Fig. 4 shows the UV-vis absorption spectra for the pure SnO2 ﬁlm and the ﬁlms sensitized by different cycles of CdS. The FTO/SnO2 electrode can absorb only ultraviolet light, because of its large energy gap (3.6 eV). After CdS QDs are deposited, the light absorbance extends to visible light region. With increasing the number of SILAR cycles, the color of the ﬁlm changed from white to orange, and the optical absorption in visible region is gradually enhanced due to the increased amounts of deposited CdS QDs. The enhanced ability to absorb visible-light of this type of SnO2 /CdS electrode makes it promising application in photovoltaic devices.
3.3. Photoelectrochemical properties of the electrodes Fig. 5 shows the current density versus potential (J–V) curves of FTO/SnO2 and FTO/SnO2 /CdS photoelectrodes with CdS QDs deposition for different cycles in a three-electrode conﬁguration, respectively. The measurement process follows a similar approach to that discuss elsewhere . Under light illumination, the FTO/SnO2 photoelectrode has a current density of 0.26 mA cm−2 at 0 V versus Ag/AgCl. With increasing the number of SILAR cycles, the current density increase obviously and approach a maximum value of 1.51 mA cm−2 at 0 V versus Ag/AgCl for 5 cycles of CdS QDs deposited, which is near 6 times higher than the FTO/SnO2 photoelectrode. Besides, the SnO2 photoelectrode for 5 cycles of CdS QDs deposited shows negligible current under dark condition. For FTO/SnO2 photoelectrode, the zero-current potential (ZCP) is only around 0.79 V versus Ag/AgCl. After 5 cycles of CdS QDs deposited, this value shifts to around 0.88 V, which indicates a shift in Fermi level to more negative potential as a result of the coupling between CdS and SnO2 in the composite system . Open circuit potential (Voc ) and conversion efﬁciency () are important parameters of the solar cell. Three-electrode measurements do not give the parameters of the cell but rather the parameters of photoelectrode . The misinterpretation of the data from three-electrode measurements that has resulted in claims of the parameters of the cell sometimes higher than were actually the case, if it is not corrected for the electrolyte potential. The cell voltage at any point on the J–V curve will be the difference between the photoelectrode potential and the counter electrode potential at that point. For three-electrode measurements the counter electrode potential will be the same as the solution potential . So in our experiments, the potential at zero current is not the real Voc , which is the difference between the potential at zero current and the electrolyte potential (Vel ). The counter electrode will maintain the electeolyte (0.25 M Na2 S and 0.35 M Na2 SO3 aqueous solution) potential of about −0.43 V versus
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Fig. 3. (a) Typical TEM image of the SnO2 spheres. (b) HRTEM image and its corresponding Fourier transform patterns (FFT) (inset) of one typical nanorod of the spherical structure. (c) HRTEM image of a SnO2 nanorod of the spherical structure deposited with CdS for 5 cycles. (d) Energy dispersive spectroscopy (EDS) spectrum of the FTO/SnO2 /CdS electrode (5 cycles).
Ag/AgCl. The vertical dashed line in Fig. 5 represents the electrolyte potential and is therefore the zero point of the cell voltage. Fig. 6 shows the corrected J–V curves of FTO/SnO2 and FTO/SnO2 /CdS photoelectrodes with CdS QDs deposition for
different cycles, respectively. The short circuit current density (Jsc ), open circuit potential (Voc ), ﬁll factor (FF) and conversion efﬁciency () of all the photoelectrodes are list in Table 1. The Jsc and Voc of the photoelectrode without CdS QDs sensitized are 0.20 mA cm−2 and 0.36 V, respectively, resulting in a very low value of conversion efﬁciency (0.017%). When increasing the number of SILAR cycles, more CdS QDs are deposited onto the SnO2 ﬁlms. The Jsc , Voc and increase obviously with increasing layers of CdS QDs and approach a maximum value of 1.47 mA cm−2 , 0.45 V and 0.22% at 5 cycles of CdS QDs. Then decrease with a further increase of CdS amount (6 cycles). This result is ascribed to the overloading of CdS which results in signiﬁcant pore blocking. Thus, the CdS/electrolyte contacting area will decrease, it is unfavorable the Table 1 Parameters obtained from the corrected photocurrent-voltage (J–V) measurements of the cells for different electrodes.
Fig. 4. Diffuse reﬂectance absorption spectra of FTO/SnO2 and FTO/SnO2 /CdS electrodes with CdS QDs deposition for different cycles by SILAR method.
Jsc (mA/cm2 )
0 cycle 1 cycles 2 cycles 3 cycles 4 cycles 5 cycles 6 cycles
0.20 0.50 0.73 0.96 1.27 1.47 1.15
0.36 0.41 0.43 0.44 0.45 0.45 0.45
0.24 0.35 0.27 0.33 0.30 0.34 0.31
0.017 0.074 0.085 0.14 0.17 0.22 0.16
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charge separation and transport properties. The synergistic effect of there factors contributes to the superior photovoltaic properties. 4. Conclusions For the ﬁrst time, CdS QDs-sensitized SnO2 spherical particles ﬁlm on FTO photoelectrodes were fabricated by a simple chemical method, and their photoelectrochemical properties were investigated. The data measured from three-electrode measurements have been corrected. A maximum 1.47 mA cm−2 short circuit current density and 0.22% conversion efﬁciency under one sun illumination has been achieved. As the ﬁrst report of CdS QDs-sensitized SnO2 photoelectrode with viable performance, we believe that further research of QDs sensitization of SnO2 nanostructure will be of great signiﬁcance. Acknowledgments This work was ﬁnancially supported by Science and Technology Development Program of Jilin Province (20110417) and National Natural Science Foundation of China (No. 51272086). Fig. 5. J–V curves of FTO/SnO2 and FTO/SnO2 /CdS electrodes with CdS QDs deposition for different cycles, respectively.
Fig. 6. Corrected J–V curves of FTO/SnO2 and FTO/SnO2 /CdS photoelectrodes with CdS QDs deposition for different cycles, respectively.
electron transportation at SnO2 /CdS/electrolyte interface [24,25]. Moreover excess CdS layer can act as potential barrier for charge transfer, which will increase the chance of the recombination of the photoelectrons and holes. The optimized thickness of CdS layer is obtained at 5 cycles, due to a well-covered CdS layer on the SnO2 surface. It can lead to more excited electrons under the illumination of light, which is advantageous to the photocurrent. A best of the CdS-sensitized SnO2 spherical particles photoelectrode is 0.22%, which is obviously higher than that of bare SnO2 spherical particles photoelectrode. The superior photovoltaic properties should be due to the follow reasons. Firstly, compared with bare SnO2 electrode, this CdS-sensitized SnO2 electrode has an intense absorption in the visible region, greatly raised the utilization rate of the solar energy. Secondly, SnO2 nanostructure forms a heterojunction with CdS QDs, which plays an important role in effective separation of light-induced electrons and holes. Thirdly, the SnO2 spherical structures are composed of number of nanorods. It not only have large speciﬁc surface areas, but also have light scattering abilities, while single crystalline nanorods have efﬁcient
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