Ni2+ doped CdS quantum dots cosensitized solar cells: Enhanced power conversion efficiency and durability

Electrochimica Acta 173 (2015) 812–818

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Synthesis of PbS/Ni2+ doped CdS quantum dots cosensitized solar cells: Enhanced power conversion efficiency and durability Yanli Chen, Qiang Tao, Wuyou Fu, Haibin Yang * State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 5 February 2015 Received in revised form 9 April 2015 Accepted 3 May 2015 Available online xxx

A new photoanode by employing Ni2+ doping of CdS used to fabricate PbS and Ni2+ doped CdS cosensitized quantum dot-sensitized solar cell (QDSSCs). Under AM 1.5 G (100 mW/cm2) illumination, the cell device exhibit a power conversion efficiency (h) of 3.60 %, which is higher than the value of 2.65 % obtained with CdS without dopant. The improved photoelectric performance is due to the impurities from Ni2+ doping of CdS, which have an impact on the electronic and photophysical properties, and the dopant creates electronic states in the midgap region of CdS thus altering the charge separation and recombination dynamics. Furthermore, the cell device based on the Ni2+ doped CdS photoanode shows superior stability in the sulfide/polysulfide electrolyte, resulting in a highly reproducible performance, which is a serious challenge for the Ni2+ doped solar cell. This finding can provide an effective method for the fabrication of new photoanode, which can pave the way to further improve the power conversion efficiency of the future QDSSCs. ã2015 Elsevier Ltd. All rights reserved.

Keyword: Ni2+ doped CdS quantum dots PbS TiO2 nanorod arrays Solar cells Power conversion efficiency

1. Introduction Quantum dot sensitized solar cells (QDSSCs) can be regarded as a derivative of dye-sensitized solar cells (DSSCs), which have attracted worldwide scientific and technological interest since the breakthrough work done by O’Regan and Grätzel in 1991 for the lower cost compared to silicon-based solar cells [1–3]. The versatile properties of semiconductor quantum dots (QDs) such as tunability of the bandgap, high absorption coefficient, generation of multiple electron carriers under high energy excitation, and delivery of hot electrons make them attractive candidates for QDSSCs [4–9]. As sensitizer for sensitized solar cells, inorganic semiconductor QDs, such as CdS, CdSe or PbS has been reported in lots of works [10–20]. As a common sensitizer, CdS is more promising due to its reasonable band-gap of 2.4 eV matching the solar visible spectrum well and sufficiently negative flatband potential [21–23]. However, the low separation efficiency of photogenerated electron-hole pairs and the fact that it is easily corroded are not favorable for the wide applications of CdS in semiconductor solar cells [24–25]. Therefore, it is highly desirable to develop highly efficient and corrosion resistant sensitizer based on the conventional semiconductor materials. Recently, many research attempts to improve

* Corresponding author. Tel.: +86 431 85168763; fax: +86 431 85168763. E-mail addresses: [email protected] (W. Fu), [email protected] (H. Yang). http://dx.doi.org/10.1016/j.electacta.2015.05.013 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.

their photoelectric performance focus on structure design or surface modification [26–31]. Considering the approach of modifying intrinsic property of semiconductor nanocrystals is to introduce dopants [32–33]. By doping optically active transition metal ions, such as Mn2+, Co2+, Fe3+, Ni2+, etc., [34–38] is possible to modify the electronic and photophysical properties of QDs [39]. The dopant creates electronic states in the midgap region of the QDs thus altering the charge separation and recombination dynamics. In addition, it is also possible to tune the optical and electronic properties of semiconductor QDs by controlling the type and concentration of dopants. Especially Ni2+ doping, it is possible to modify the electronic and optical properties of QDs, this is the origin of our research impetus. Synthesis of Ni2+ doped CdS, CdSe, ZnS, and ZnSe QDs and their photophysical properties had been the subject of recent reports [40–43]. Ni2+ doped CdS QDs show near infrared emission due to the strong p–d hybridization of Ni2+ in doped semiconductor. Moreover, this hybridization can result in a very long lifetime [44]. Thus it should be advantageous to utilize long-lived charge carriers to boost the efficiency of solar cells using Ni2+ doped quantum dots. To our knowledge, there has been no research concerning to use of Ni2+ doped CdS (named NCdS) in QDSSCs. Herein, we report NCdS and PbS QDs applied to the QDSSCs via successive ionic layer adsorption and reaction (SILAR) method. The solar cell shows significantly enhanced light absorption. Under standard simulated AM 1.5 G (100 mW/cm2) illumination, the solar cell based on NCdS and PbS QDs co-sensitized TiO2 nanorod arrays photoelectrode can

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Fig. 1. (A): XRD patterns of (a) TiO2 nanorod arrays, (b) CdS/TiO2 electrode, and (c) NCdS/TiO2 electrode; (B): the magnified peaks of CdS and NCdS.

present a power conversion efficiency of 3.60 %. Most importantly, by exchanging CdS with the NCdS, the photostability of the cell has been improved significantly. 2. Experimental 2.1. Fabrication of PbS/NCdS photoanode The rutile TiO2 nanorod arrays photoanode was synthesized by a hydrothermal method, details of the fabrication for TiO2 nanorod arrays is similar to that described by Liu and Aydil [45]. Typically,

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15 ml deionized water was mixed with 15 ml concentrated hydrochloric acid, after it was stirred at ambient conditions for 10 min, 1 ml tetrabutyl titanate was added to the mixture and stirred for another 10 min. The mixture was transferred to a sealed teflon reactor (80 mL volume) after two pieces of cleaned FTO (NSG GROUP, TCO-17, 16 V/sq, with a thickness of 3.2 mm) substrates were placed within the reactor. The hydrothermal synthesis was conducted at 150  C in an electric oven for 14 h, then cooled down to room temperature slowly. A white TiO2 nanorod arrays was uniformly coated on the FTO glass substrate. The sample was thoroughly washed with DI water and air dried. Finally, the sample was annealed in air at 550  C for 3 hours to increase the crystallinity of TiO2 nanorod arrays and improve their contact to the substrate. The successive ionic layer adsorption and reaction (SILAR) was used to prepare NCdS and PbS semiconductor sensitizers. In the SILAR process, ethanol and absolute methanol were employed to dissolve Cd(NO3)2, Ni(CH3COO)2 and Pb(NO3)2 as the cation precursor solution. For NCdS sensitizer, the TiO2 photoanode was immersed in a solution containing 0.5 M Cd(NO3)2 in ethanol for 5 min. To incorporate doping of Ni2+, Ni(CH3COO)2 (0.075 M) was mixed with Cd(NO3)2. This allowed coadsorption of Cd2+ and Ni2+ ions, which in turn facilitated incorporation of Ni2+ in the CdS QDs. The photoanode was sonication-assisted rinsed with ethanol to remove the excess Cd2+ after immersed. The photoanode were dried in a gentle stream of N2 for 2 min. Subsequently the dried photoanode was dipped into 0.5 M Na2S mixed with methanol and deionized water (1:1, v/v) for 5 min. The photoanode was then sonication-assisted rinsed with methanol and dried again with N2. All these procedures were considered one SILAR cycle. The incorporated amount of sensitizer could be increased by repeating the assembly cycle. Preparing PbS sensitizer is similar to NCdS, just the concentrations of precursors and the assembly time are minor differences. In preparation of PbS, the photoanode was dipped into the 1 mM Pb(NO3)2 in absolute methanol for 2 min, and then sonication-assisted rinsed with absolute methanol to remove the excess Pb2+. The photoanode was then dried in a gentle stream of N2 for 2 min. Subsequently the dried photoanode was dipped into 1 mM Na2S in absolute methanol for 2 min. The photoanode was then sonication-assisted rinsed with methanol and dried again with N2. For ZnS capping, 0.5 M Zn(CH3COO)2 in deionized water and 0.5 M Na2S in deionized water were used for a SILAR process with a dipping time of 10 min each. In this work, all samples prepared were capped with three cycles of ZnS. In addition, ethanol is chosen rather than water here is because ethanol has better wetting and faster evaporation characteristics, which could lead to form better-defined particles through gaps during the TiO2 nanorod arrays [11].

Fig. 2. Typical FESEM images: (A) is cross section view of bare TiO2 nanorod arrays; (B) is cross section view of PbS/NCdS/TiO2 nanorod arrays. The samples prepared for FESEM did not cap ZnS for the sake of a better investigation on the growth of NCdS and PbS.

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Fig. 3. (A) TEM image of bare TiO2 nanorod arrays; (B) is HRTEM of bare TiO2 nanorod arrays; (C) is the TEM image of PbS/NCdS/TiO2 nanorod arrays; (D), (E) and (F) are HRTEM images of PbS/NCdS/TiO2 nanorod arrays. STEM images and corresponding STEM-EDX elemental mapping of PbS/NCdS/TiO2 nanorod arrays are shown, revealing the homogeneous distribution of Cd, Pb, S and Ni elements over the whole TiO2 nanorod arrays.

2.2. Material characterization The morphologies and structure properties of photoanodes were characterized by field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy analyses (STEMEDX), X-ray diffraction (XRD) and UV–vis diffused reflectance absorption spectra. FESEM was received on a JEOL JEM-6700F

microscope operated at 8 KV. High-resolution transmission electron microscopy (HRTEM) images, scanning TEM and energy dispersive X-ray spectroscopy analyses (STEM-EDX) were obtained on a JEM-2100F microscope with an accelerated voltage of 200 KV. XRD patterns were obtained by conducted on a Rigaku D/max2500 X-ray diffractometer with Cu Ka radiation (l=1.5418 Å). UV–vis diffuse reflectance absorption spectra were recorded on a UV-3150 double-beam spectrophotometer.

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Table 1 Parameters obtained from the photocurrent-voltage measurements of multiple semiconductors cosensitized TiO2 solar cells with 3 cycles of ZnS coating.

Fig. 4. Diffuse reflectance absorbance spectra of CdS/TiO2, NCdS/TiO2, PbS/CdS/TiO2 and PbS/NCdS/TiO2.

2.3. Photovoltaic characterization The photoanodes were sealed in sandwich structures with a 60 mm spacer by using Pt catalysts as counter electrode, which were prepared by spreading a drop of 5 mM H2PtCl6 in 2-propanol on fluorine-doped tin oxide (FTO) glass and heating it to 385  C for 15 min in ambient air. The liquid electrolyte composed of 1 M Na2S and 1 M S solution in Milli-Q ultrapure water was injected between two electrodes. A mask with a window of 0.25 cm2 was clipped on the TiO2 side to define the active area of the cell. Photocurrent– voltage measurement was performed with a Keithley model 2400 Source Meter and a 500 W Xe lamp (Spectra Physics) with a monochromator was simulated sunlight. A laser power meter (BG26M92C, Midwest Group) was used to revise the light intensity, as effective as AM 1.5 G (100 mW/cm2) illumination. The incident photon-to-current conversion efficiency (IPCE) was measured with an action spectrum measurement setup (PEC-S20, Peccell Ltd.). 3. Results and discussion 3.1. Phase structure The XRD patterns of TiO2, CdS/TiO2 and NCdS/TiO2 samples are shown in Fig. 1. After subtracting the diffraction peaks of the FTO substrate, five diffraction peaks were observed in every sample at 2

Fig. 5. Photocurrent-voltage curves of different working electrodes measured under AM 1.5 G (100 mW/cm2) illumination: CdS/TiO2, NCdS/TiO2, PbS/CdS/TiO2 and PbS/NCdS/TiO2. The working electrodes area was 0.25 cm2, Pt catalysts as counter electrode and aqueous 1 M Na2S/1 M S as electrolyte.

Photoelectrodes

Jsc (mA/cm2)

Voc (V)

FF (%)

h (%)

CdS/TiO2 NCdS/TiO2 PbS/CdS/TiO2 PbS/NCdS/TiO2

7.41 8.91 15.52 18.56

0.46 0.50 0.37 0.41

51.95 54.55 46.07 47.31

1.77 2.43 2.65 3.60

theta of 36.06 , 54.33 , 62.75 , 69.03 and 69.74 corresponding to the (101), (211), (002), (301) and (112) planes in TiO2. These sharp peaks are indexed to the characteristic peaks of tetragonal rutile TiO2 [JCPDS card No. 21-1276]. No diffraction peaks from other crystalline forms are detected, which indicates high purity and crystallinity of the TiO2. The XRD of CdS/TiO2 and NCdS/TiO2 were shown in Fig. 1 A (b) and (c), there have one main diffraction feature corresponding to (111) planes [JCPDS card No. 80-0019]. With the Ni2+ doping in the CdS QDs, the spectra remain the same, indicating that the cubic structure is not tailored by the addition of Ni2+ into the CdS matrix at least up to the detection level of XRD. The Fig. 1 B shows the magnified XRD patterns in the range of 2 u = 23–30 . There are no signals of NiS, but a slight shift toward higher 2 u values for NCdS indicates that Ni2+ (effective ionic radius 0.69 Å) substitutionally occupied Cd2+ sites (effective ionic radius 0.95 Å). 3.2. SEM images and HRTEM observation The FESEM images of the bare TiO2 nanorod arrays and the PbS/NCdS/TiO2 nanorod arrays are shown in Fig. 2. Fig. 2 A gives the cross section view of the as-prepared TiO2 nanorod arrays which obviously shows that the relatively smooth TiO2 nanorod arrays are uniformly formed with rod length of 2.73.0 mm and diameters 200 nm. After assembled with NCdS and PbS, the ordered TiO2 nanorod arrays structure is retained and the PbS/NCdS/TiO2 nanorod arrays with rougher surfaces are observed as shown in Fig. 2 B, which reveals that QDs have covered the entire surfaces of TiO2 nanorod arrays. The detailed microscopic characterization of the bare TiO2 nanorod arrays and the PbS/NCdS/TiO2 nanorod arrays were performed by using TEM and high-resolution TEM (HR-TEM). Fig. 3 A and B shows the typical TEM and the HRTEM image of the bare TiO2 nanorod arrays, respectively, of a bare TiO2 nanorod, confirming that the nanorod is single crystalline structure would be favorable for electron transport. Lattice fringes with interplanar

Fig. 6. Incident photon to current conversion efficiency (IPCE) spectra of PbS/CdS/ TiO2 and PbS/NCdS/TiO2.

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Fig. 7. Photocurrent stability of PbS/NCdS/TiO2 under continuous illumination of 100 mW/cm2 for 1 h.

spacing d(110) = 0.322 nm and d(001) = 0.294 nm are consistent with the tetragonal rutile phase [JCPDS card No. 21-1276]. The nanorods grow along the (110) crystal plane with a preferred (001) orientation. The typical TEM image of a TiO2 nanorod arrays deposited with NCdS and PbS QDs are shown in Fig. 3C where we can see that the whole surface area of TiO2 nanorod arrays have been covered with QDs. HR-TEM images of PbS/NCdS/TiO2 heterojunction region have indicated the high crystallinity of TiO2, NCdS and PbS. The measured lattice spacings in Fig. 3 D are consistent with the d-spacings of TiO2 and CdS. Lattice fringes with interplanar spacings d(002) = 0.144 nm in the left of Fig. 3 D and d (220) = 0.202 nm in the right of Fig. 3 D are consistent with the rutile phase of TiO2 [JCPDS card No. 21-1276] and the cubic phase of CdS [JCPDS card No. 80-0019] apparently. Thus we found the lattice spacing of NCdS is similar with the original CdS, its may be due to the doping amount is too small. While d(331) = 0.131 nm, d (400) = 0.144 nm and d(200) = 0.283 nm corresponds to the cubic phase of CdS [JCPDS card No. 80-0019]; d(222) = 0.169 nm, d (400) = 0.145 nm and d(440) = 0.103 nm correspond to the cubic phase of PbS [JCPDS card No. 65-0241]. STEM images and corresponding STEM-EDX elemental mapping of the sample (PbS/NCdS/TiO2 nanorod arrays) was also shown to reveal the distribution of Cd, Pb, S and Ni elements over the whole TiO2 nanorod arrays. These results confirm that NCdS and PbS QDs have been successfully deposited on the surface of the TiO2 nanorod arrays.

circuit voltage (Voc), fill factor (FF), and power conversion efficiency (h) of doped and undoped systems are summarized in Table 1. The detailed research on the cycles of sensitized CdS without dopants and PbS were reported in our previous work [46]. The Jsc and Voc of the photoelectrode with undoped CdS QDs sensitized are 7.41 mA/cm2 and 0.46 V, respectively, which followed a relatively low value of power conversion efficiency (1.77 %). The Jsc and Voc were increased, when Ni2+ doped CdS as sensitizer in QDSSCs. Similarly, the photoelectric performance of PbS/NCdS/TiO2 is superior to PbS/CdS/TiO2, and obtained fairly good power conversion efficiency (3.60 %). To further understand the reason of increasing the power conversion efficiency by NCdS QDs sensitized the QDSSCs, the incident photon to current conversion efficiency (IPCE) spectra were measured. As shown in Fig. 6, we found the IPCE value of both the PbS/CdS/TiO2 and PbS/NCdS/TiO2 were over 60 % in the wavelength range from 400 to 620 nm, and the end of the curve extended up to 900 nm or more. These results were consistent with the UV–vis absorption spectroscopy of PbS/CdS/TiO2 photoanodes. However, comparing with the IPCE of PbS/CdS/TiO2, the value of IPCE are bit higher in the PbS/NCdS/TiO2 at the range of 400–550 nm, and the wavelength of PbS/NCdS/TiO2 has slightly red shift at 600–750 nm, this result indicating that the PbS/NCdS/ TiO2 has a stronger photoresponse than the PbS/CdS/TiO2. The improved IPCE value of PbS/NCdS/TiO2 can be attributed to the Ni2+ doped can provided an electronic state in the band gap of CdS, which can capture the thermal relaxation and reverse photogenerated electron-hole pair recombination, ultimately prolong the life of electronic excited states. In order to show the stability of the electrodes, photocurrent density versus time curves were measured for 1 h. From the results in Fig. 7, during the 1 h of illumination, the steady photocurrent from PbS/NCdS/TiO2 shows no obvious degradation over 1 h. While for the PbS/CdS/TiO2, the photocurrent decreases quickly after 45 min irradiation. To understand the physical processes underlying the broad light absorption and effective carrier extraction, a type II heterojunction model is proposed to elucidate the possible charge transfer mechanism in the PbS/NCdS/TiO2 QDSSCs system based on the above experimental facts. As shown in Scheme 1, photons are captured by QDs under illumination, yielding electron–hole pairs that are rapidly separated into electrons and holes at the interface between the TiO2 and QDs. The electrons are injected into the TiO2 film, and the holes are simultaneously scavenged to the CE via the hole transporting redox couple of the polysulfide electrolyte [47].

3.3. UV–vis absorption spectroscopy The optical properties of the QDs sensitized TiO2 nanorod arrays with different sensitizers were measured by diffuse reflectance absorption spectra in the wavelength range from 400 to 800 nm (Fig. 4). The absorption spectra of CdS/TiO2 photoanode shows absorption edge around 520 nm, which corresponds to a bandgap of 2.4 eV. The Ni2+ doped CdS shows a red shift in the absorption with the edge around 540 nm. The absorption intensities of PbS/CdS/TiO2 and PbS/NCdS/TiO2 are greatly enhanced in the visible light region and the onset edges undergo a significant red shift, it's indicating that the effective bandgaps of PbS/CdS/TiO2 and PbS/NCdS/TiO2 become narrow. 3.4. Photoelectric performance of the electrodes The photocurrent-voltage curves of these four QDSSCs are presented in Fig. 5. The short circuit current density (Jsc), open

Scheme 1. Schematic diagram view of the possible charge transfer mechanism in PbS/NCdS/TiO2 QDSSCs.

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The Ni2+ dopant creates spatially indirect energy gap in the midgap region of CdS QDs thus altering the charge separation and recombination dynamics [48–50]. This unique feature enables the photogenerated electron–hole pairs to quickly separate. The midgap states created by Ni2+ doping cause electrons to get trapped and screen them from charge recombination with holes and oxidized polysulfide electrolyte [51]. Therefore, the spatial separation of electrons can be efficiently collected using an external circuit. 4. Conclusions In conclusion, we have fabricated PbS/NCdS/TiO2 by the facile SILAR method. We obtained a detailed comparison between PbS/ CdS/TiO2 and PbS/NCdS/TiO2 photoanodes, which revealed that the NCdS can be considered as an alternative photoanode to CdS in many applications, owing to its faster charge transport, more efficient charge separation, lower recombination resistance, and greater stability. The solar cells present the power conversion efficiency of 3.60 % under AM 1.5 G (100 mW/cm2) illumination. With these results, we demonstrated that Ni2+ doped CdS is a conceptually different approach to enhance the energy conversion efficiency for quantum dot sensitized solar cells. Acknowledgements This work was financially supported by Science and Technology Development Program of Jilin Province (20110417) and National Natural Science Foundation of China (No. 51272086). References [1] B. O’Regan, M. Gräetzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (1991) 737. [2] C.J. Barbé, F. Arendse, P. Comte, M. Jirousek, F. Lenzmann, V. Shklover, M. Grätzel, Nanocrystalline Titanium Oxide Electrodes for Photovoltaic Applications, J. Am. Ceram. Soc. 80 (1997) 3157. [3] A. Yella, H.W. Lee, H.N. Tsao, C.Y. Yi, A.K. Chandiran, M. Nazeeruddin, E.W.G. Diau, C.Y. Yeh, S.M. Zakeeruddin, M. Grätzel, Porphyrin-Sensitized Solar Cells with Cobalt (II/III)–Based Redox Electrolyte Exceed 12 Percent Efficiency, Science 334 (2011) 629. [4] C.B. Murray, D.J. Noms, M.G. Bawendi, Synthesis and Characterization of Nearly Monodisperse CdE (E = S Se, Te) Semiconductor Nanocrystallites, J. Am. Chem. Soc. 115 (1993) 8706. [5] D.D. Sarma, A. Nag, P.K. Santra, A. Kumar, S. Sapra, P. Mahadevan, Origin of the Enhanced Photoluminescence from Semiconductor CdSeS Nanocrystals, J. Phys. Chem. Lett. 1 (2010) 2149. [6] W.W. Yu, L.H. Qu, W.Z. Guo, X.G. Peng, Experimental Determination of the Extinction Coefficient of CdTe CdSe, and CdS Nanocrystals, Chem. Mater. 15 (2003) 2854. [7] M.C. Beard, Multiple Exciton Generation in Semiconductor Quantum Dots, J. Phys. Chem. Lett. 2 (2011) 1282. [8] A. Pandey, P.G. Sionnest, Hot Electron Extraction From Colloidal Quantum Dots, J. Phys. Chem. Lett. 1 (2010) 45. [9] W.A. Tisdale, K.J. Williams, B.A. Timp, D.J. Norris, E.S. Aydil, X.Y. Zhu, HotElectron Transfer from Semiconductor Nanocrystals, Science 328 (2010) 1543. [10] Y.L. Chen, Q. Tao, W.Y. Fu, H.B. Yang, X.M. Zhou, Y.Y. Zhang, S. Su, P. Wang, M.H. Li, Enhanced solar cell efficiency and stability using ZnS passivation layer for CdS quantum-dot sensitized actinomorphic hexagonal columnar ZnO, Electrochimica Acta 118 (2014) 176. [11] C.H. Chang, Y.L. Lee, Chemical bath deposition of CdS quantum dots onto mesoscopic TiO2 films for application in quantum-dot-sensitized solar cells, Appl. Phys. Lett. 91 (2007) 053503. [12] G. Zhu, L.K. Pan, T. Xu, Z. Sun, One-Step Synthesis of CdS Sensitized TiO2 Photoanodes for Quantum Dot-Sensitized Solar Cells by Microwave Assisted Chemical Bath Deposition Method, ACS Appl. Mater. Interfaces 3 (2011) 1472. [13] Y. Tak, S.J. Hong, J.S. Lee, K.J. Yong, Fabrication of ZnO/CdS core/shell nanowire arrays for efficient solar energy conversion, J. Mater. Chem. 19 (2009) 5945. [14] Y.L. Lee, B.M. Huang, H.T. Chien, Highly Efficient CdSe-Sensitized TiO2 Photoelectrode for Quantum-Dot-Sensitized Solar Cell Applications, Chem. Mater. 20 (2008) 6903. [15] J. Chen, D.W. Zhao, J.L. Song, X.W. Sun, W.Q. Deng, X.W. Liu, W. Lei, Directly assembled CdSe quantum dots on TiO2 in aqueous solution by adjusting pH value for quantum dot sensitized solar cells, Electrochemistry Communications 11 (2009) 2265.

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