- Email: [email protected]
CdS and CdSe quantum dots subsectionally sensitized solar cells using a novel double-layer ZnO nanorod arrays
Journal of Colloid and Interface Science 388 (2012) 118–122
Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
CdS and CdSe quantum dots subsectionally sensitized solar cells using a novel double-layer ZnO nanorod arrays Jianping Deng, Minqiang Wang ⇑, Xiaohui Song, Yanhua Shi, Xiangyu Zhang Electronic Materials Research Laboratory (EMRL), Key Laboratory of Education Ministry, International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 25 June 2012 Accepted 8 August 2012 Available online 16 August 2012 Keywords: Double-layer structure Quantum dots sensitized ZnO nanorods Solar cell
a b s t r a c t We report a novel approach for synthesizing CdS and CdSe quantum dots subsectionally sensitized double-layer ZnO nanorods for solar cells, which are comprised of CdS QDs-sensitized bottom-layer ZnO NRs and CdSe QDs-sensitized top-layer ZnO NRs. X-ray diffraction study and scanning electron microscopy analysis indicate that the solar cells of subsectionally sensitized double-layer ZnO NRs, which are the hexagonal wurtzite crystal structure, have been successfully achieved. The novel structure enlarged the range of absorbed light and enhanced the absorption intensity of light. The I–V characteristics show that the double-layer structure improved both the current density (Jsc) and fill factor (FF) by 50%, respectively, and power conversion efficiency (g) was increased to twice in comparison with the CdS QDs-sensitized structure. Crown Copyright Ó 2012 Published by Elsevier Inc. All rights reserved.
1. Introduction As a new class of photoelectrode materials for photovoltaic devices, the well aligned one-dimensional ZnO nanostructures, such as nanorods (NRs) and nanowires (NWs), have been widely investigated as photoelectrodes for dye-sensitized solar cells (DSSCs), due to the large surface area to adsorb more sensitizer and the direct electrical pathway to ensure the rapid collection of carriers generated by optical excitations [1–5]. However, nanostructured metal oxide photoelectrodes, such as ZnO and TiO2, are facing the major limitation of poor visible light absorption. Numerous efforts have been made to improve their visible light absorption, including dye or quantum dots (QDs) sensitization [6–8] and constructing hybrid structure [9–13]. Considering the technical challenge in the synthesis of the multi-junction nanostructures for photovoltaic devices and the instability of ZnO contacting with the common dyes, QDs-sensitized solar cells (QDSSCs) have attracted lots of attention, especially in view of the large absorption coefficient of QDs and the tunability of the absorption spectrum by quantum size confinement [14–16]. Various QDs, such as CdSe, CdS, PbS, and CdTe, have been developed for QDSSCs [6–8,17– 20]. CdS is an excellent choice, because it has a narrower band gap (2.4 eV) and a higher conduction band edge (CBE) than ZnO. CdS-sensitized ZnO NRs structure facilitates the injection of excited electrons from CdS to ZnO and reduces the recombination between electron–hole pairs. However, it is impossible for CdS to absorb the whole range of the visible light. Recently, some researchers have ⇑ Corresponding author. Fax: +86 029 82668794. E-mail address: [email protected] (M. Wang).
demonstrated that a better performance could be offered by coupling the photoanode materials with two kinds of semiconductor QDs with different exciton wavelength (co-sensitization) [11,12]. At present, there are three types of structure on QDs-sensitized ZnO nanorods solar cell, such as single-sensitized, core/shell cosensitized, and double-sided co-sensitized solar cell. The core/shell structure has some limitations: (1) the CBE and VBE of shell are higher than that of core, respectively, which promote the electron transfer from shell to core and the holes transfer from core to shell. So the selected QDs species are limited by the structure of energy level; (2) sometimes, the interface of core/shell increases the electron–hole recombination rate, especially for CdS and CdSe. They almost have the same conduction band position, and thus the electron cannot quickly transfer between them, which will eventually lead to the recombine of electron–hole pairs. Wang et al. [9] proposed the ‘‘double-sided CdS and CdSe quantum dot co-sensitized ZnO nanowire arrays.’’ They found that the efficiency of the structure was higher than that of core/shell, but the I–V measurement was performed in a three-electrode electrochemical cell. If the cell is encapsulated, the input of light will be difficult. In addition, the adsorption of QDs affected the cell efficiency. Increasing the length of nanorods can increase the surface areas where will be deposited more QDs, but the fusion will appear at bottom when the nanorods are longer. To solve the above problem, we try to prepare double-layer ZnO NR arrays sensitized subsectionally by CdS and CdSe. This could provide more potential than conventional mono-layer photoanode structure does. In the present work, we studied the fabrication and characterization of CdS and CdSe QDs subsectionally sensitized double-layer ZnO NRs-based solar cells. The double-layer ZnO photoanode
0021-9797/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.08.017
J. Deng et al. / Journal of Colloid and Interface Science 388 (2012) 118–122
composes of bottom-layer ZnO/CdS structure, top-layer ZnO/CdSe structure, and interlayer ZnO/CdS/CdSe (co-sensitization) structure. Three main strong points are as follows: (1) it removes the limitation of energy level on selecting QDs and the interface effect of core/shell on the electron separation and transfer; (2) it plays the role of bilateral sensitization and solves the encapsulating problem of cells; (3) the QDs adsorbing on the nanorods inhibit the lateral growth of the bottom-nanorods and eliminate the fusion, so the longer nanorods can be prepared. 2. Experimental section 2.1. Preparation of photoelectrods The detailed strategy of synthesizing double-layer ZnO NR photoelectrodes is illustrated in Fig. 1. ZnO seeds were deposited on the indium-tin-oxide (ITO) substrates by the conventional sol–gel spincoated method as shown in Fig. 1a [21,22]. In the typical synthesis, the concentration of zinc acetate dihydrate (Zn(CH3COO)22H2O) in the solution was assigned to 0.75 M, and 2-methoxyethanol and monoethanolamine (MEA) were used as a solvent and stabilizer, respectively. Zn(CH3COO)22H2O was firstly dissolved in a mixture of 2-methoxyethanol and MEA. The molar ratio of MEA to zinc acetate was kept at 1.0. Then, the resultant solution was stirred at 60 °C for 30 min to yield a clear and homogeneous solution, which was used as the coating solution. The precursor solution was spincoated onto the ITO substrates (4000 rpm, 30 s); then, the wet films were dried at 300 °C for 15 min to evaporate the solvent and remove organic residuals. The procedure from coating to drying was repeated twice. After the deposition, the films were annealed at 500 °C for 1 h in air; thus, ZnO seeds were obtained. Fig. 1b shows the growth of the bottom-layer ZnO NRs, which were synthesized by chemical bath deposition (CBD) method [23,24]. The ZnO seeds were immersed in a 100 ml mixed aqueous solution of 0.04 M Zn(NO3)2 and 0.8 M NaOH at 80 °C for 1 h, and then the samples were washed by deionized water and dried by argon. The deposition of CdS QDs on the down-layer ZnO NRs was achieved by the successive ion layer adsorption and reaction (SILAR) method [25] (Fig. 1c). The films were dipped into a 50 mM of Na2S methanol solution for 1 min, rinsed with methanol, then dipped for 1 min into a 50 mM of Cd(NO3)2 methanol solution, and rinsed with methanol. The two-step dipping procedure was repeated for five cycles. After depositing CdS QDs, the top of ZnO NRs was polished with a 2000 grit sand paper in order to better grow the top-layer ZnO NRs, which was the key of the novel double-layer ZnO NRs. As
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the same method, the growth of top-layer ZnO NRs was achieved (Fig. 1e). Fig. 1f shows the process of depositing CdSe QDs by SILAR method [26]. SeO2 (0.23 g) was dissolved in 40 mL of ethanol (50 mM) in the round-bottom flask and stirred for about 3 min with Ar gas to make the inside atmosphere with a low oxygen level. After that, 0.27 g of KBH4 (125 mM) was added into the solution. With stirring for about 1 h, the solution color changed from deep red to transparent; thus, the Se2 solution was obtained. Meanwhile, 0.5 g of Cd(NO3)2 was dissolved in 40 mL of ethanol (40 mM), which was served as Cd2+ solution. The double-layer CdS-modified electrode was dipped into the Se2 solution for 1 min, pure ethanol, Cd2+ solution for 1 min, and then pure ethanol successively for deposition. The process was repeated for seven cycles. 2.2. Fabrication and characterization of QDSSCs A 45-lm-thick hot-melt ionomer film (Surlyn) under heating (110 °C) was sandwiched between the subsectionally sensitized double-layer ZnO NRs film and a counter electrode of Pt-coated ITO. Polysulfide electrolyte was injected via the predrilled hole of Pt CE, and each hole was sealed. A polysulfide electrolyte was composed of 0.5 M Na2S, 2 M S, 0.2 M KCl in a methanol/water (7:3 by volume) [27]. The crystalline phase and orientation of ZnO NRs were identified by the X-ray diffraction (XRD) using a D/max2400 X-ray diffraction spectrometer (Rigaku) with Cu Ka radiation and operated at 40 kV and 100 mA from 20° to 70°, and the scanning speed was 15° min 1 at a step of 0.02°. The changes in the sample before and after QDs sensitized were studied using a digital camera. The morphologies of the samples were analyzed using a scanning electron microscope (SEM) (Quanta 400, Fei. Co.). The optical absorption spectra of the photoanodes were recorded at 300–750 nm with a Jasco V-570 UV–Vis–NIR photospectroscometer. The current–voltage characteristics of the cells were obtained under one sun illumination (AM 1.5 G, 100 mW cm 2) with a solar simulator (1000 W Xe source) and a Keithley 2400 source meter (active area is 0.36 cm2). The intensity of incident solar illumination was adjusted to one sun condition using a Si reference cell equipped with a KG-5 filter. 3. Results and discussion 3.1. XRD and SEM analysis Fig. 2a shows the XRD pattern with the standard JCPDS card (No. 36-1451), which confirms the formation of ZnO phase with
Fig. 1. The detailed strategy of the synthesis of the double-layer ZnO NRs photoelectrode.
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Fig. 2. (a) The XRD image of double-layer ZnO NRs and (b) the SEM image of double-layer ZnO NRs (inset of the top view).
hexagonal wurtzite crystal structure, and the double-layer structure exhibits higher oriented peak (0 0 2) than other, which presents a high single crystalline quality of ZnO NRs. Therefore, it can provide direct electrical pathways to collect excited electron. The SEM image of double-layer ZnO NRs is presented in Fig. 2b. It can be seen that there is a good growth (circled in the rectangle) but no apparent dislocation between the two layers. The result illustrates that the top-layer NRs started their growth from the top ends of the bottom-layer NRs and grew along the same orientation to the bottom-layer NRs. Because the top QDs of the bottomlayer were removed and the top ends of ZnO NRs were uncovered, the successive growth of ZnO NRs was achieved. The length of the bottom-layer ZnO NRs is about 2 lm, and the diameter 100 nm. The top-layer ZnO NRs with perfect verticality are length of 4 lm and diameter of 150 nm. The interesting phenomenon is that the growth rate of top-layer was twice higher than that of bottomlayer, and there was enough space to adsorb QDs between bottom-layer ZnO NRs. On one hand, the bottom-layer growth would take much time to realize the formation of the nucleate, and the second layer directly grew on the first layer, which explained the reason that the growth rate of the second layer was twice as large as the first layer; on the other hand, this space demonstrated that the CdS QDs deposited on the bottom-layer ZnO NRs inhibited radial growth of the NRs.
3.2. Mechanism analysis In order to address our process in detail, we designed the schematic diagram that explains the architecture and the corresponding energy of CdS and CdSe QDs subsectionally sensitized double-layer ZnO NRs. Meanwhile, we propose a possible chargetransfer mechanism, which is schematically illustrated in the right inset of Fig. 3a. From this modified energy-band diagram (Fig. 3b), it can be seen that for the top-layer and bottom-layer NRs, the CBEs of CdSe and CdS QDs are higher than that of ZnO NRs, so that the photogenerated electrons in the CBs of CdS and CdSe QDs can easily transfer to the CB of ZnO NRs, respectively. Moreover, the interlayer is co-sensitized by CdSe and CdS QDs, mentioned as ZnO/CdS/ CdSe structure. For the part between the top-layer and bottomlayer NRs, due to the Fermi-level alignment between CdSe and CdS QDs, a stepwise alignment in the band-edge levels is expected. Thus, the photogenerated electrons can transfer from the CB of CdSe QDs to the CB of CdS QDs and then to the CB of ZnO NRs. The light absorption of bottom-layer CdS and top-layer CdSe is uppermost, in which photogenerated electrons can quickly transfer to the same ZnO NR, respectively. Fig. 3c shows the photographs of NRs in the process. The color changes from white for A to yellow for B, to primrose yellow for C, and then to dark red for D. C became primrose yellow, due to
Fig. 3. Schematic diagrams illustrating (a) the architecture and (b) the corresponding energy diagram of double-layer CdSe/ZnO/CdS NR arrays photoanode. The dashed box highlights the CdSe/CdS/ZnO, CdS/ZnO, and CdSe/ZnO interfaces. (c) The surface change of photoanode during the process, A – bottom-layer ZnO NRs, B – CdS sensitized bottom-layer ZnO NRs, C – top-layer ZnO NRs, D – CdSe sensitized top-layer ZnO NRs illustrating.
J. Deng et al. / Journal of Colloid and Interface Science 388 (2012) 118–122
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Fig. 4. (a) UV–vis absorption spectra and (b) I–V characteristics of CdS10, CdSe10, and CdS5/CdSe7 cells.
the shelter of the top-layer ZnO NRs. Both yellow B and dark red D indicate the depositions of CdS and CdSe QDs. 3.3. Performance of QDSSCs In order to determine the effect of novel structure on the photovoltaic performance of the solar cells, the cells of CdS, and CdSe QDs single-sensitized ZnO NRs, which were synthesized with continual growth for twice under the same condition, were prepared by SILAR method for 10 cycles, respectively. The three cells were defined CdS10 (ten cycles), CdSe10 (ten cycles), and CdS5/CdSe7 (five cycles and seven cycles). Fig. 4a shows the UV–vis absorption spectroscopy of CdS10, CdSe10, and CdS5/CdSe7 photoelectrodes. It can be seen that the adsorption edges are about 520, 700, and 650 nm. Although the absorption edge of CdS5/CdSe7 has shifted to blue slightly in comparison with CdSe10, its absorbance is higher than that of CdSe10, which indicates the novel structure adsorbed QDs effectively and enlarged the range of absorbed light and enhanced the intensity of that either. The absorption edge of CdSe10 is not sharp, for the appearance of aggregations and different size of CdSe QDs with the increase in SILAR cycles. The I–V characteristics of samples (CdS10, CdSe10, and CdS5/ CdSe7) are shown in Fig. 4b and in Table 1 at simulated one sun. The CdSe10 cell gives a short-circuit current density (Jcs) of 1.15 mA cm 2, an open-circuit voltage (Voc) of 0.48 V, a fill factor (FF) of 26%, a power conversion efficiency (g) of 0.14%. The Jcs, Voc, FF, and g of CdS10 cell are 1.51 mA, 0.76 V, 22%, 0.25%, respectively. The parameters of CdS5/CdSe7 cell have more improvement, the Jcs and FF increased to 2.08 mA, 38%. Especially, its power conversion efficiency is trial and twice larger than that of CdSe10 and CdS10, respectively, owing to the increases in Jsc and FF. The high Jsc and FF could be attributed to two possible reasons. Firstly, there was a uniform coverage of CdS QDs on the bottom-layer NRs, which restrained the radial growth of ZnO NRs and left the certain space to be filled with the electrolyte, and it could increase the interaction between electrolyte and sensitizer. Secondly, in the novel structure, the range and intensity of light absorption were
Table 1 Photovoltaic parameters obtained from the I–V curves using various electrodes. Structure
Jsc (mA/cm2)
Voc (V)
FF
g (%)
ZnO/CdSe10 ZnO/CdS10 CdS5/ZnO/CdSe7
1.15 1.51 2.08
0.48 0.76 0.63
0.26 0.22 0.38
0.14 0.25 0.49
increased, subsequently the Jsc was increased. Importantly, the double-layer subsectionally sensitized sample, with CdS and CdSe QDs that directly contact ZnO NRs which facilitates the electron transfer quickly from QDs to the ‘‘highway’’ of ZnO NRs. That decreased the chance of electron–hole recombination. Although the ZnO/CdS/CdSe core/shell structure can increase light absorption and the electrons created in CdSe transfer to ZnO through the CdS layer, the presence of this intermediate layer (CdS) would increase the electron–hole recombination and limit electron collection efficiency [9]. Having compared the parameters of cells with those of others [28,29], the g is lower due to the lower Jcs. The adsorption of QDs influence basically on the Jcs. lots of efforts will be done to optimize the deposition of QDs. 4. Conclusions In conclusions, we report the fabrication and characterization of the novel double-layer ZnO NR arrays-based CdS and CdSe QDs subsectionally sensitized solar cell. The XRD pattern shows double-layer ZnO NRs with hexagonal wurtzite crystal structure. The SEM image of double-layer ZnO NRs electrode shows that there was a good growth of top-layer ZnO NRs but no apparent dislocation between the bottom-layer and the top-layer. The growth rate of the top-layer was about twice higher than that of the bottomlayer and the deposition of CdS QDs on the bottom-layer ZnO NRs inhibited radial growth of the NRs. We proposed three possible charge-transfer mechanisms of CdS ? ZnO, CdSe ? ZnO, and CdSe ? CdS ? ZnO. Among them, the CdS ? ZnO and CdSe ? ZnO are primary. The subsectionally sensitized structure enhanced light absorption from the UV–vis absorption spectroscopy. The I–V characteristics shows that the double-layer structure improved the Jsc and FF by 50%, respectively, and g was increased to twice in comparison with the CdS QDs-sensitized structure. Improving the structure is important to the g of cells in the future work. Acknowledgments The authors gratefully acknowledge financial support from Natural Science Foundation of China (Grant Nos. 61176056 and 91123019). This work has been financially supported by the Key Project of Basic Science Research of Shaanxi Province (2009JZ2015), the ‘‘13115’’ Innovation Engineering of Shaanxi Province (2010ZDKG58), the Fundamental Research Funds for the Central Universities (Grant No. xjj20100145), and the open projects from Institute of Photonics and Photo-Technology, Provincial Key Laboratory of Photoelectronic Technology, Northwest University, China.
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Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
CdS and CdSe quantum dots subsectionally sensitized solar cells using a novel double-layer ZnO nanorod arrays Jianping Deng, Minqiang Wang ⇑, Xiaohui Song, Yanhua Shi, Xiangyu Zhang Electronic Materials Research Laboratory (EMRL), Key Laboratory of Education Ministry, International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 25 June 2012 Accepted 8 August 2012 Available online 16 August 2012 Keywords: Double-layer structure Quantum dots sensitized ZnO nanorods Solar cell
a b s t r a c t We report a novel approach for synthesizing CdS and CdSe quantum dots subsectionally sensitized double-layer ZnO nanorods for solar cells, which are comprised of CdS QDs-sensitized bottom-layer ZnO NRs and CdSe QDs-sensitized top-layer ZnO NRs. X-ray diffraction study and scanning electron microscopy analysis indicate that the solar cells of subsectionally sensitized double-layer ZnO NRs, which are the hexagonal wurtzite crystal structure, have been successfully achieved. The novel structure enlarged the range of absorbed light and enhanced the absorption intensity of light. The I–V characteristics show that the double-layer structure improved both the current density (Jsc) and fill factor (FF) by 50%, respectively, and power conversion efficiency (g) was increased to twice in comparison with the CdS QDs-sensitized structure. Crown Copyright Ó 2012 Published by Elsevier Inc. All rights reserved.
1. Introduction As a new class of photoelectrode materials for photovoltaic devices, the well aligned one-dimensional ZnO nanostructures, such as nanorods (NRs) and nanowires (NWs), have been widely investigated as photoelectrodes for dye-sensitized solar cells (DSSCs), due to the large surface area to adsorb more sensitizer and the direct electrical pathway to ensure the rapid collection of carriers generated by optical excitations [1–5]. However, nanostructured metal oxide photoelectrodes, such as ZnO and TiO2, are facing the major limitation of poor visible light absorption. Numerous efforts have been made to improve their visible light absorption, including dye or quantum dots (QDs) sensitization [6–8] and constructing hybrid structure [9–13]. Considering the technical challenge in the synthesis of the multi-junction nanostructures for photovoltaic devices and the instability of ZnO contacting with the common dyes, QDs-sensitized solar cells (QDSSCs) have attracted lots of attention, especially in view of the large absorption coefficient of QDs and the tunability of the absorption spectrum by quantum size confinement [14–16]. Various QDs, such as CdSe, CdS, PbS, and CdTe, have been developed for QDSSCs [6–8,17– 20]. CdS is an excellent choice, because it has a narrower band gap (2.4 eV) and a higher conduction band edge (CBE) than ZnO. CdS-sensitized ZnO NRs structure facilitates the injection of excited electrons from CdS to ZnO and reduces the recombination between electron–hole pairs. However, it is impossible for CdS to absorb the whole range of the visible light. Recently, some researchers have ⇑ Corresponding author. Fax: +86 029 82668794. E-mail address: [email protected] (M. Wang).
demonstrated that a better performance could be offered by coupling the photoanode materials with two kinds of semiconductor QDs with different exciton wavelength (co-sensitization) [11,12]. At present, there are three types of structure on QDs-sensitized ZnO nanorods solar cell, such as single-sensitized, core/shell cosensitized, and double-sided co-sensitized solar cell. The core/shell structure has some limitations: (1) the CBE and VBE of shell are higher than that of core, respectively, which promote the electron transfer from shell to core and the holes transfer from core to shell. So the selected QDs species are limited by the structure of energy level; (2) sometimes, the interface of core/shell increases the electron–hole recombination rate, especially for CdS and CdSe. They almost have the same conduction band position, and thus the electron cannot quickly transfer between them, which will eventually lead to the recombine of electron–hole pairs. Wang et al. [9] proposed the ‘‘double-sided CdS and CdSe quantum dot co-sensitized ZnO nanowire arrays.’’ They found that the efficiency of the structure was higher than that of core/shell, but the I–V measurement was performed in a three-electrode electrochemical cell. If the cell is encapsulated, the input of light will be difficult. In addition, the adsorption of QDs affected the cell efficiency. Increasing the length of nanorods can increase the surface areas where will be deposited more QDs, but the fusion will appear at bottom when the nanorods are longer. To solve the above problem, we try to prepare double-layer ZnO NR arrays sensitized subsectionally by CdS and CdSe. This could provide more potential than conventional mono-layer photoanode structure does. In the present work, we studied the fabrication and characterization of CdS and CdSe QDs subsectionally sensitized double-layer ZnO NRs-based solar cells. The double-layer ZnO photoanode
0021-9797/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.08.017
J. Deng et al. / Journal of Colloid and Interface Science 388 (2012) 118–122
composes of bottom-layer ZnO/CdS structure, top-layer ZnO/CdSe structure, and interlayer ZnO/CdS/CdSe (co-sensitization) structure. Three main strong points are as follows: (1) it removes the limitation of energy level on selecting QDs and the interface effect of core/shell on the electron separation and transfer; (2) it plays the role of bilateral sensitization and solves the encapsulating problem of cells; (3) the QDs adsorbing on the nanorods inhibit the lateral growth of the bottom-nanorods and eliminate the fusion, so the longer nanorods can be prepared. 2. Experimental section 2.1. Preparation of photoelectrods The detailed strategy of synthesizing double-layer ZnO NR photoelectrodes is illustrated in Fig. 1. ZnO seeds were deposited on the indium-tin-oxide (ITO) substrates by the conventional sol–gel spincoated method as shown in Fig. 1a [21,22]. In the typical synthesis, the concentration of zinc acetate dihydrate (Zn(CH3COO)22H2O) in the solution was assigned to 0.75 M, and 2-methoxyethanol and monoethanolamine (MEA) were used as a solvent and stabilizer, respectively. Zn(CH3COO)22H2O was firstly dissolved in a mixture of 2-methoxyethanol and MEA. The molar ratio of MEA to zinc acetate was kept at 1.0. Then, the resultant solution was stirred at 60 °C for 30 min to yield a clear and homogeneous solution, which was used as the coating solution. The precursor solution was spincoated onto the ITO substrates (4000 rpm, 30 s); then, the wet films were dried at 300 °C for 15 min to evaporate the solvent and remove organic residuals. The procedure from coating to drying was repeated twice. After the deposition, the films were annealed at 500 °C for 1 h in air; thus, ZnO seeds were obtained. Fig. 1b shows the growth of the bottom-layer ZnO NRs, which were synthesized by chemical bath deposition (CBD) method [23,24]. The ZnO seeds were immersed in a 100 ml mixed aqueous solution of 0.04 M Zn(NO3)2 and 0.8 M NaOH at 80 °C for 1 h, and then the samples were washed by deionized water and dried by argon. The deposition of CdS QDs on the down-layer ZnO NRs was achieved by the successive ion layer adsorption and reaction (SILAR) method [25] (Fig. 1c). The films were dipped into a 50 mM of Na2S methanol solution for 1 min, rinsed with methanol, then dipped for 1 min into a 50 mM of Cd(NO3)2 methanol solution, and rinsed with methanol. The two-step dipping procedure was repeated for five cycles. After depositing CdS QDs, the top of ZnO NRs was polished with a 2000 grit sand paper in order to better grow the top-layer ZnO NRs, which was the key of the novel double-layer ZnO NRs. As
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the same method, the growth of top-layer ZnO NRs was achieved (Fig. 1e). Fig. 1f shows the process of depositing CdSe QDs by SILAR method [26]. SeO2 (0.23 g) was dissolved in 40 mL of ethanol (50 mM) in the round-bottom flask and stirred for about 3 min with Ar gas to make the inside atmosphere with a low oxygen level. After that, 0.27 g of KBH4 (125 mM) was added into the solution. With stirring for about 1 h, the solution color changed from deep red to transparent; thus, the Se2 solution was obtained. Meanwhile, 0.5 g of Cd(NO3)2 was dissolved in 40 mL of ethanol (40 mM), which was served as Cd2+ solution. The double-layer CdS-modified electrode was dipped into the Se2 solution for 1 min, pure ethanol, Cd2+ solution for 1 min, and then pure ethanol successively for deposition. The process was repeated for seven cycles. 2.2. Fabrication and characterization of QDSSCs A 45-lm-thick hot-melt ionomer film (Surlyn) under heating (110 °C) was sandwiched between the subsectionally sensitized double-layer ZnO NRs film and a counter electrode of Pt-coated ITO. Polysulfide electrolyte was injected via the predrilled hole of Pt CE, and each hole was sealed. A polysulfide electrolyte was composed of 0.5 M Na2S, 2 M S, 0.2 M KCl in a methanol/water (7:3 by volume) [27]. The crystalline phase and orientation of ZnO NRs were identified by the X-ray diffraction (XRD) using a D/max2400 X-ray diffraction spectrometer (Rigaku) with Cu Ka radiation and operated at 40 kV and 100 mA from 20° to 70°, and the scanning speed was 15° min 1 at a step of 0.02°. The changes in the sample before and after QDs sensitized were studied using a digital camera. The morphologies of the samples were analyzed using a scanning electron microscope (SEM) (Quanta 400, Fei. Co.). The optical absorption spectra of the photoanodes were recorded at 300–750 nm with a Jasco V-570 UV–Vis–NIR photospectroscometer. The current–voltage characteristics of the cells were obtained under one sun illumination (AM 1.5 G, 100 mW cm 2) with a solar simulator (1000 W Xe source) and a Keithley 2400 source meter (active area is 0.36 cm2). The intensity of incident solar illumination was adjusted to one sun condition using a Si reference cell equipped with a KG-5 filter. 3. Results and discussion 3.1. XRD and SEM analysis Fig. 2a shows the XRD pattern with the standard JCPDS card (No. 36-1451), which confirms the formation of ZnO phase with
Fig. 1. The detailed strategy of the synthesis of the double-layer ZnO NRs photoelectrode.
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Fig. 2. (a) The XRD image of double-layer ZnO NRs and (b) the SEM image of double-layer ZnO NRs (inset of the top view).
hexagonal wurtzite crystal structure, and the double-layer structure exhibits higher oriented peak (0 0 2) than other, which presents a high single crystalline quality of ZnO NRs. Therefore, it can provide direct electrical pathways to collect excited electron. The SEM image of double-layer ZnO NRs is presented in Fig. 2b. It can be seen that there is a good growth (circled in the rectangle) but no apparent dislocation between the two layers. The result illustrates that the top-layer NRs started their growth from the top ends of the bottom-layer NRs and grew along the same orientation to the bottom-layer NRs. Because the top QDs of the bottomlayer were removed and the top ends of ZnO NRs were uncovered, the successive growth of ZnO NRs was achieved. The length of the bottom-layer ZnO NRs is about 2 lm, and the diameter 100 nm. The top-layer ZnO NRs with perfect verticality are length of 4 lm and diameter of 150 nm. The interesting phenomenon is that the growth rate of top-layer was twice higher than that of bottomlayer, and there was enough space to adsorb QDs between bottom-layer ZnO NRs. On one hand, the bottom-layer growth would take much time to realize the formation of the nucleate, and the second layer directly grew on the first layer, which explained the reason that the growth rate of the second layer was twice as large as the first layer; on the other hand, this space demonstrated that the CdS QDs deposited on the bottom-layer ZnO NRs inhibited radial growth of the NRs.
3.2. Mechanism analysis In order to address our process in detail, we designed the schematic diagram that explains the architecture and the corresponding energy of CdS and CdSe QDs subsectionally sensitized double-layer ZnO NRs. Meanwhile, we propose a possible chargetransfer mechanism, which is schematically illustrated in the right inset of Fig. 3a. From this modified energy-band diagram (Fig. 3b), it can be seen that for the top-layer and bottom-layer NRs, the CBEs of CdSe and CdS QDs are higher than that of ZnO NRs, so that the photogenerated electrons in the CBs of CdS and CdSe QDs can easily transfer to the CB of ZnO NRs, respectively. Moreover, the interlayer is co-sensitized by CdSe and CdS QDs, mentioned as ZnO/CdS/ CdSe structure. For the part between the top-layer and bottomlayer NRs, due to the Fermi-level alignment between CdSe and CdS QDs, a stepwise alignment in the band-edge levels is expected. Thus, the photogenerated electrons can transfer from the CB of CdSe QDs to the CB of CdS QDs and then to the CB of ZnO NRs. The light absorption of bottom-layer CdS and top-layer CdSe is uppermost, in which photogenerated electrons can quickly transfer to the same ZnO NR, respectively. Fig. 3c shows the photographs of NRs in the process. The color changes from white for A to yellow for B, to primrose yellow for C, and then to dark red for D. C became primrose yellow, due to
Fig. 3. Schematic diagrams illustrating (a) the architecture and (b) the corresponding energy diagram of double-layer CdSe/ZnO/CdS NR arrays photoanode. The dashed box highlights the CdSe/CdS/ZnO, CdS/ZnO, and CdSe/ZnO interfaces. (c) The surface change of photoanode during the process, A – bottom-layer ZnO NRs, B – CdS sensitized bottom-layer ZnO NRs, C – top-layer ZnO NRs, D – CdSe sensitized top-layer ZnO NRs illustrating.
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Fig. 4. (a) UV–vis absorption spectra and (b) I–V characteristics of CdS10, CdSe10, and CdS5/CdSe7 cells.
the shelter of the top-layer ZnO NRs. Both yellow B and dark red D indicate the depositions of CdS and CdSe QDs. 3.3. Performance of QDSSCs In order to determine the effect of novel structure on the photovoltaic performance of the solar cells, the cells of CdS, and CdSe QDs single-sensitized ZnO NRs, which were synthesized with continual growth for twice under the same condition, were prepared by SILAR method for 10 cycles, respectively. The three cells were defined CdS10 (ten cycles), CdSe10 (ten cycles), and CdS5/CdSe7 (five cycles and seven cycles). Fig. 4a shows the UV–vis absorption spectroscopy of CdS10, CdSe10, and CdS5/CdSe7 photoelectrodes. It can be seen that the adsorption edges are about 520, 700, and 650 nm. Although the absorption edge of CdS5/CdSe7 has shifted to blue slightly in comparison with CdSe10, its absorbance is higher than that of CdSe10, which indicates the novel structure adsorbed QDs effectively and enlarged the range of absorbed light and enhanced the intensity of that either. The absorption edge of CdSe10 is not sharp, for the appearance of aggregations and different size of CdSe QDs with the increase in SILAR cycles. The I–V characteristics of samples (CdS10, CdSe10, and CdS5/ CdSe7) are shown in Fig. 4b and in Table 1 at simulated one sun. The CdSe10 cell gives a short-circuit current density (Jcs) of 1.15 mA cm 2, an open-circuit voltage (Voc) of 0.48 V, a fill factor (FF) of 26%, a power conversion efficiency (g) of 0.14%. The Jcs, Voc, FF, and g of CdS10 cell are 1.51 mA, 0.76 V, 22%, 0.25%, respectively. The parameters of CdS5/CdSe7 cell have more improvement, the Jcs and FF increased to 2.08 mA, 38%. Especially, its power conversion efficiency is trial and twice larger than that of CdSe10 and CdS10, respectively, owing to the increases in Jsc and FF. The high Jsc and FF could be attributed to two possible reasons. Firstly, there was a uniform coverage of CdS QDs on the bottom-layer NRs, which restrained the radial growth of ZnO NRs and left the certain space to be filled with the electrolyte, and it could increase the interaction between electrolyte and sensitizer. Secondly, in the novel structure, the range and intensity of light absorption were
Table 1 Photovoltaic parameters obtained from the I–V curves using various electrodes. Structure
Jsc (mA/cm2)
Voc (V)
FF
g (%)
ZnO/CdSe10 ZnO/CdS10 CdS5/ZnO/CdSe7
1.15 1.51 2.08
0.48 0.76 0.63
0.26 0.22 0.38
0.14 0.25 0.49
increased, subsequently the Jsc was increased. Importantly, the double-layer subsectionally sensitized sample, with CdS and CdSe QDs that directly contact ZnO NRs which facilitates the electron transfer quickly from QDs to the ‘‘highway’’ of ZnO NRs. That decreased the chance of electron–hole recombination. Although the ZnO/CdS/CdSe core/shell structure can increase light absorption and the electrons created in CdSe transfer to ZnO through the CdS layer, the presence of this intermediate layer (CdS) would increase the electron–hole recombination and limit electron collection efficiency [9]. Having compared the parameters of cells with those of others [28,29], the g is lower due to the lower Jcs. The adsorption of QDs influence basically on the Jcs. lots of efforts will be done to optimize the deposition of QDs. 4. Conclusions In conclusions, we report the fabrication and characterization of the novel double-layer ZnO NR arrays-based CdS and CdSe QDs subsectionally sensitized solar cell. The XRD pattern shows double-layer ZnO NRs with hexagonal wurtzite crystal structure. The SEM image of double-layer ZnO NRs electrode shows that there was a good growth of top-layer ZnO NRs but no apparent dislocation between the bottom-layer and the top-layer. The growth rate of the top-layer was about twice higher than that of the bottomlayer and the deposition of CdS QDs on the bottom-layer ZnO NRs inhibited radial growth of the NRs. We proposed three possible charge-transfer mechanisms of CdS ? ZnO, CdSe ? ZnO, and CdSe ? CdS ? ZnO. Among them, the CdS ? ZnO and CdSe ? ZnO are primary. The subsectionally sensitized structure enhanced light absorption from the UV–vis absorption spectroscopy. The I–V characteristics shows that the double-layer structure improved the Jsc and FF by 50%, respectively, and g was increased to twice in comparison with the CdS QDs-sensitized structure. Improving the structure is important to the g of cells in the future work. Acknowledgments The authors gratefully acknowledge financial support from Natural Science Foundation of China (Grant Nos. 61176056 and 91123019). This work has been financially supported by the Key Project of Basic Science Research of Shaanxi Province (2009JZ2015), the ‘‘13115’’ Innovation Engineering of Shaanxi Province (2010ZDKG58), the Fundamental Research Funds for the Central Universities (Grant No. xjj20100145), and the open projects from Institute of Photonics and Photo-Technology, Provincial Key Laboratory of Photoelectronic Technology, Northwest University, China.
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