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Formation of aligned ZnO nanotube arrays by chemical etching and coupling with CdSe for photovoltaic application
Thin Solid Films 518 (2010) 5146–5152
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Formation of aligned ZnO nanotube arrays by chemical etching and coupling with CdSe for photovoltaic application Lijuan Luo a, Gang Lv a, Bihui Li a, Xiaoyan Hu a, Lei Jin b, Jianbo Wang b, Yiwen Tang a,⁎ a b
Institute of Nano-science and Technology, Central China Normal University, Wuhan, 430079, China Department of Physics and Center for Electron Microscopy, Wuhan University, Wuhan 430072, China
a r t i c l e
i n f o
Article history: Received 27 September 2009 Received in revised form 8 February 2010 Accepted 9 March 2010 Available online 16 March 2010 Keywords: Zinc oxide Nanotubes Cadmium selenide Chemical etching Photovoltaic properties Solar cells
a b s t r a c t High density aligned ZnO nanotube (NT) arrays were synthesized using a facile chemical etching of electrochemically deposited ZnO nanorods (NRs). The influence of etching time and solution concentration on the ZnO NT formation was investigated. Moreover, cadmium selenide (CdSe) nanoparticles as sensitizers were assembled onto the ZnO NT and NR arrays for solar cell application. A conversion efficiency (η) of 0.44% was achieved for CdSe/ZnO NT-based solar cell under the white light illumination intensity of 85 mW/cm2. An 8% enhancement in η was observed between the CdSe/ZnO NT-based and NR-based solar cell due to the enhancement of the photocurrent density. ZnO NT arrays have been proved to have a superior ability as compared with ZnO NR arrays when employed as a semiconductor film. © 2010 Elsevier B.V. All rights reserved.
1. Introduction One dimensional (1D) ZnO nanostructures, such as nanowires, nanobelts and nanotubes, have been synthesized for wide application in gas sensors, field emission, nanolasers, solar cells and so on [1–3]. Among these nanostructures, arrays of ZnO nanotube (NT) have received much attention for their high porosity and large surface area [4]. Many methods have been developed to prepare ZnO NT arrays including chemical vapor deposition, solvothermal synthesis and electrochemical deposition [5–9]. Among these methods, electrodeposition presents a cost-effective method in terms of its low temperature and precise control of the reaction parameters. The electrodeposition method employed for ZnO NT synthesis can be divided into two sorts: the direct electrodeposition method and the two-step electrochemical method including the electrodeposition of ZnO nanorods (NRs) and the following etching process. Of particular interest are two reports on the direct electrodeposition of ZnO NTs [9,10]. In Yu's report, ZnO NTs were obtained by applying a high voltage up to 30 V between two electrodes, while our previous report on the synthesis of ZnO NTs employed a potentiostatic deposition method. But the length of the ZnO NTs was limited by the increased film resistance observed during electrodeposition. In the case of the two-step electrochemical method, the formation of ZnO NRs is similar,
⁎ Corresponding author. Tel.: +86 27 62075995; fax: +86 27 67861185. E-mail address: [email protected] (Y. Tang). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.03.014
although the etching process is different. Zhang et al. reported a defect selective etching method to obtain ZnO NTs under an applied potential of 1.1 V [11]. Zaera et al. reported the formation of ZnO NTs by etching ZnO NR in aqueous sodium chloride solution at neutral pH without applying a voltage [12]. Although a large amount of research has been performed on the two-step electrochemical method to synthesize ZnO NT arrays, a perfect array of ZnO NTs for application in solar cells still remains a challenge. As a wide band gap semiconductor (∼3.37 eV), 1D ZnO arrays are unable to absorb and utilize the visible region of the solar spectrum. In order to absorb visible light and generate electron–hole pairs in dyesensitized solar cells (DSSCs) or nanocrystal-sensitized solar cells, low band gap materials such as organic dye molecules and inorganic low band gap semiconductor should be coupled to the ZnO array as a photoanode. Recently, both ZnO NR and NT arrays have been applied as the semiconductor film for DSSCs because of their high surface areas and their vertically aligned electrical pathways, which are expected to increase the efficiencies of those photoelectrical devices [5,13,14]. Gao's group incorporated ZnO NRs into DSSC photoanode to obtain an energy conversion efficiency of 1.7% [13]. Hupp et al. on the other hand, introduced high surface area ZnO NT array photoanodes templated by anodic aluminum oxide into DSSCs [5]. The resulting power efficiency was found to be high up to 1.6% despite of being illuminated through the Pt/conductive glass rear electrode, which attenuated ∼20% of the light at the visible wavelength due mostly to Pt absorption. In the case of nanocrystal-sensitized solar cells, much research has been done to obtain ZnO NR array photoanodes
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sensitized by a low band gap semiconductor including CdSe, CdS, and CdTe [15,16]. Despite of the advantages known for using low band gap semiconductor sensitizers, the efficiencies of the ZnO NR array devices are limited by their low light-harvesting ability. Increasing the surface area of the ZnO film will operate but it is a significant technological problem to synthesize higher aspect ratio wires through mild chemical method. Obviously NTs have a larger surface area than NRs of similar length and diameter. However, no report has been found in the literatures where low band gap semiconductors are coupled with ZnO NT arrays as a photoanode. According to the results of a 1D TiO2 investigation, it was found that the higher lightharvesting efficiencies of the NT-based TiO2 DSSCs were owed to the stronger internal light scattering effects and more efficient change separation at the enormously enhanced junction areas [17–19]. We propose that ZnO NT arrays also have the good light scattering efficiency on account of their 1D tubular nature. In this paper, high density vertically aligned ZnO NT arrays were prepared on a F-doped SnO2 (TCO) glass substrate using a facile chemical etching of electrodeposited ZnO NRs [20]. Selective dissolution along the c-axis induced the 1D tubular structure formation [12,21,22]. The effects of etching time and solution composition were investigated. By controlling the preparation condition, highly oriented ZnO NT arrays were fast formed. Then, CdSe/ZnO NT photoelectrodes containing a three dimensional junction were synthesized using a low temperature chemical bath deposition method and its photovoltaic performance was investigated. The NT-based CdSe/ZnO photoelectrode exhibited an obviously better cell performance as compared with the corresponding NRbased CdSe/ZnO photoelectrodes. 2. Experimental details In the first step, ZnO NR arrays were grown on a TCO glass substrate (10–15 Ω/□, Asahi Company) through electrodeposition. Before the electrodeposition process, a thin ZnO compact seed layer was coated on TCO by a dip-coating method. For preparing the ZnO seed layer, the synthesis of the ZnO precursor sol has been described in detail (refer to our previous work [23]). The electrodeposition procedure was carried out using three electrode systems, including a Pt counter electrode, a saturated calomel electrode (SCE) as reference electrode and the ZnO seed coated TCO used as the working electrode. The films were electrodeposited from a bath containing 0.03 M Zn (NO3)2 and 0.03 M (CH2)6 N4 in deionized water. The bath temperature was controlled at 70 °C and the electrolyte was stirred continuously using a magnetic stirrer. The ZnO NR films were grown under a −1.0 V potentiostatic condition using a Princeton Applied Research Model 263A Potentiostat/Galvanostat. Subsequently, the TCO substrate covered with ZnO NRs was used as the working electrode for the second-step dissolution process. The etching solution consisted of 0.1 M ethylenediamine (EDA), and the reaction temperature was kept at 70 °C. The applied potential was 0.04 V versus SCE. The shape and size of the ZnO NT arrays could be controlled by adjusting the reaction parameters. The as-obtained ZnO NT array films were placed vertically into a beaker containing a solution of 80 mM CdSO4, 80 mM Na2SeSO3 and 120 mM N(CH2COOK)3 at 0 °C in the dark. The solution was continuously stirred to achieve a homogenous, clear distribution of the chemical components during the deposition. After deposition for 1 h, the films were rinsed with deionized water and dried at room temperature. The annealing processes of the composite films were performed in a tubular furnace at 400 °C under a N2 atmosphere for 30 min. The morphology of the as-prepared films was analyzed by scanning electron microscopy (SEM, JEOL 6700F). Further structural analysis of the ZnO NT and CdSe/ZnO NT nanostructures was carried out using transmission electron microscopy, selected area electron
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diffraction (TEM/SAED, JEOL JEM-2100FEF) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010). Photoluminescence (PL, F-7000, Hitachi) spectra were measured at room temperature on a fluorescence spectrophotometer. The excitation wavelength was 325 nm, the scanning speed was 1200 nm min−1, and the photomultiplier tube voltage was 700 V. The widths of the excitation slit and emission slit were both 5.0 nm. A Perkin Elmer Lambda35 spectrometer UV–Visible system was used to obtain the absorption spectra of the samples over a range of 350–700 nm. The CdSe/ZnO electrodes were incorporated into thin layer sandwich-type cells with a Pt sputtered TCO as the counter electrode and an electrolyte solution to measure the power conversion efficiency. The electrolyte used was 0.60 M 1-butyl-3-methylimidazolium iodide, 0.03 M I2, and 0.50 M tertbutylpyridine in acetonitrile/valeronitrile (85:15). The CdSe/ZnO electrodes of 0.2 cm2 area were illuminated by white light with an intensity of 85 mW/cm2 white light intensity. 3. Results and discussions 3.1. Fabrication and characterization of ZnO NT arrays Fig. 1a displays a typical surface SEM image of the ZnO NR film prepared by the first-step electrodeposition at a cathodic potential of −1.0 V for 60 min. The rods with hexagonal morphology are mainly 200 nm in diameter and about 3.5 μm in length. As expected, fairly well-aligned flat-top hexagonal ZnO NRs have been grown vertically onto the surface of the substrate and spaced apart sufficiently. It is anticipated that flat-top NRs would present a fascinating opportunity to form a tubular structure in the subsequent etching step. It is also anticipated that needle like ZnO NRs would induce the destruction of the ZnO array. Fig. 1b shows an SEM image of the ZnO NT array after selective dissolution in 0.1 M EDA under an applied potential of 0.04 V. The array consists of tubes having a smooth surface and a top diameter of 100–250 nm. It can be seen from Fig. 1c that the tubes are aligned perpendicularly to the substrate and the diameter of the bottom is larger than that of the top. The length of the NTs is about 3 μm, which is a little shorter than that of the previous NRs. Further structural characterization of single ZnO tube was performed by TEM (Fig. 1d). The resulting TEM image clearly indicates the hollow structure of the nanotubes, while the accompanying SAED pattern inset shows that the ZnO NTs are a single crystal with a hexagonal structure. Room temperature PL spectra of the vertically aligned ZnO NR and NT arrays were investigated following an excitation with a Xe light excitation source at 325 nm. From the PL spectra (Fig. 2), it can be seen that both the NRs and NTs show a sharp and strong peak at approximately 380 nm, which corresponds to the near band edge peak that is responsible for the recombination of the free excitons. As we can see, the emission intensity of the ZnO NTs (curve b) is weaker than that of the NRs (curve a), for the ZnO NTs are evolved from the NRs by etching. It should be noted that neither the PL spectra from ZnO NRs nor that of NTs show a broad deep-level (visible) emission which has been reported in literature [20,24]. According to these literature reports, the deep-level emissions are related to the various intrinsic defects produced during the growth process. No deep-level emission indicates that the as-prepared NRs and NTs have good crystal quality and a low concentration of defects [25]. To get a better understanding of the formation mechanism and optimization of the microstructure of ZnO NTs, the effects of etching time and EDA solution concentration on the morphology of ZnO NTs were studied by SEM. Fig. 3 shows images corresponding to the morphology evolution of the electrodeposited ZnO NRs after an etching process of 0 min, 10 min, 20 min and 30 min in 0.1 M EDA aqueous solution under an applied potential of 0.04 V. It can be clearly observed from Fig. 3a that the ZnO NRs obtained from the first electrodeposition have a flat-top hexagonal morphology. After 10 min
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Fig. 1. SEM images of the samples: (a) ZnO NR array, (b) ZnO NT array, and (c) cross-section of the ZnO NT array. (d) TEM image and the corresponding SAED of the NTs removed from the NT array.
of etching reaction, small pits can be observed in the top of the ZnO NRs, and a part of the NRs retain their rod shape (Fig. 3b). As illustrated in Fig. 3c, all tops of the ZnO NRs have been etched and obvious holes are formed after a 20 min reaction. The unbalanced distribution of defects in the ZnO nanorods leads to the preferential etching in the center part of the nanorods [11]. With a further increase in the reaction time to 30 min, perfect tube-like structures appeared. The further increase in etching time would induce the dissolution of the wall of ZnO NTs. According to the above experimental observation,
the fabrication procedure of the ZnO NT arrays could be proposed as follows. First, well-aligned arrays of ZnO NRs were electrodeposited onto the TCO substrate coated with ZnO compact layer. The wellknown electrochemical reaction is as follows [26]: −
−
−
2+
+ 2OH →ZnO + H2 O
NO3 + H2 O + 2e →NO2 + 2OH Zn
−
−
ð1Þ ð2Þ
Subsequently, the ZnO nanorods were soaked in EDA aqueous solution and etched at 70 °C. EDA is a type of molecule with a strong coordination ability to zinc [27]. In the EDA aqueous solution, OH− were formed by the hydrolysis of EDA (Eq. 3) and the OH− further reacted with ZnO to form a hydroxyl complex such as [Zn(OH)4]2− (Eq. 4). þ − ðCH2 NH2 Þ2 + 2H2 O → CH2 NH3 + 2OH 2
−
ZnO + 2OH + H2 O→ZnðOHÞ4
Fig. 2. Room temperature PL spectra of the electrodeposited ZnO NRs (a) and ZnO NTs after the etching process (b).
2
ð3Þ ð4Þ
When the c-axis preferentially grown ZnO NRs prepared by the firststep electrodeposition were soaked in aqueous EDA solution, the preferential adsorption of EDA molecules occurred on the lateral facets rather than on the (0001) facets and OH− were formed by the hydrolysis of EDA. Subsequently, the OH− anions reacted with ZnO to form a soluble hydroxyl complex such as [Zn(OH)4]2− along the caxis. The selective dissolution of ZnO along the c-axis caused the formation of the tubes. In addition, the applied potential was also crucial to perfecting the tubes. A lower positive bias would accelerate the dissolution process of the ZnO NRs [20]. When the ZnO NR film
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Fig. 3. SEM images of the products etched after different deposition times in 0.1 M EDA solution: (a) 0, (b) 10, (c) 20, and (d) 30 min.
was put into the EDA solution without applying a positive bias, the tubes with porous wall would be obtained after a long time soaking. Previous research suggests that the adsorption of OH− plays a major role during the etching of ZnO NRs to form NTs, and OH− are formed by the hydrolysis of EDA. To gain further insight, the effects of solution concentration of EDA on the morphologies were studied. ZnO NR arrays were held for 30 min at 70 °C in a solution with 0 M, 0.01 M, 0.2 M and 0.4 M EDA. It was found that no ZnO NT could be obtained in the absence of EDA or at a too low concentration, and the ZnO kept a perfect rod-like structure. However, a partial dissolution of the NRs core occurred when the concentration of EDA increased. When the concentration of EDA was in the range from 0.05–0.1 M, perfect NT structures as shown in Fig. 1d were obtained. But when the EDA concentration was further increased, not only the cores of the ZnO NRs but also the walls were dissolved. Fig. 4a shows the SEM image of the sample which etched in a 0.2 M EDA solution under 0.04 V (vs SCE) at 70 °C. The image shows that further increasing the concentration of the EDA concentration induces the completed dissolution of the tubes as shown in Fig. 4b. From this figure, we could observe the TCO substrate and some fragmentary tubes of ZnO. The variation of the morphologies of ZnO with the concentration inferred from the above experiments suggests that OH− play a major role on the dissolution of the ZnO NR core. It is well known that hexagonal wurtzite ZnO possesses two oppositely charged polar facets (0001) and (0001−) which are perpendicular to the [0001] direction. The two facets are metastable while the other six side nonpolar (101−0) facets are mostly stable and chemically inert. The stability of the top (0001) facet requires that it become less positive [28]. When ZnO NRs are soaked in the EDA solution, the OH− of EDA will preferentially adsorb on the (0001) facet because of the electrostatic adherence. The EDA molecules, on the other hand, preferentially adsorb onto the lateral facets of the ZnO NRs thereby providing electrons to the Zn atoms, which will change
Fig. 4. SEM images of samples etched in 0.2 M EDA (a) and 0.4 M EDA (b) solution for 30 min.
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the distribution of Zn and O. This process will lead to an enhancement of the Zn–O bond and protect the edge of the (0001) while slowing the dissolution rate of the edge. The formation of the tubular structure should be contributed to the elective dissolution of ZnO along the caxis. Moreover, OH− will compete with EDA to absorb on the lateral facets of the ZnO NRs and dissolve the tube wall when the concentration of OH− is high enough. So in our experiments, control of the concentration of EDA is critical for the formation of the perfect tubular structure. NTs can be obtained in a narrow range of EDA concentration.
3.2. Coupling ZnO NTs with CdSe and their application in photovoltaic cells The hollow nature of the ZnO tubes makes both inner and outer surface areas accessible for the modification with CdSe. Obviously, ZnO NT arrays have a larger surface area than that of ZnO NR arrays with the same length and diameter because NTs comprise both an outer and inner surface. When the CdSe/ZnO NT composite material was applied in a photoelectrode, the light from the illumination were perfectly trapped in the tube arrays and reduplicatively absorbed by the CdSe particles both inside and outside of the tubes. Then, the photoelectrons generated in the CdSe pass through the ZnO arrays and rapidly reached the back contact smoothly because of the 1D nature of the ZnO NTs. A simple low temperature chemical bath deposition method was employed to deposit CdSe onto the ZnO tubes. As can be seen in Fig. 5a, both the inner and outer walls of the ZnO NTs are covered by CdSe nanoparticles causing the surface of the tubes to become
Fig. 5. SEM images of the as-prepared CdSe/ZnO NT array (a) and its cross-sections (b).
rougher. The diameters of the CdSe particles range from 17 to 25 nm. The super thin layer of CdSe ensures the photoinduced electron the immediate transfer to ZnO. The size and thickness of CdSe strongly depend on the chemical bath deposition parameters such as temperature and deposition time. It would affect the photovoltaic performance of the electrode greatly. Fig. 5b shows the cross-sections of the as-prepared CdSe/ZnO NT arrays. From this image, we can see that the diameter of the top of the NR is smaller than that of the bottom. The length of the NTs can be tuned by the electrodeposition process of the NRs. In order to confirm the composite nanostructure of CdSe on ZnO NT, we further performed TEM investigation. The inset in Fig. 6a shows the TEM image of an individual CdSe/ZnO NT. It can be seen that the CdSe particles are uniformly attached to the surface of the NT. HRTEM image shown in Fig. 6a proves that CdSe crystallizes as polycrystals with its grain sizes in the range of 3–3.5 nm. The corresponding SAED pattern of the ZnO/CdSe nanostructure in Fig. 6b shows a set of sharp diffraction spots and other spots belonging to several discrete diffraction rings. It indicates that the ZnO NT is a single crystal with a hexagonal structure, and all other spots in the diffraction rings could be indexed to CdSe nanocrystals with cubic symmetry. In order to elucidate which part of the composite structure contributed to the diffraction pattern, dark-field images were captured with the hexagonal ZnO diffraction spots and the cubic CdSe diffraction spots circled with markers C, and D (Fig. 6c and d). From the dark-field image of Fig. 6c, we could infer that the core is ZnO. From Fig. 6d, several CdSe bright dots could be seen on the surface of the ZnO tube. It confirms that the nanostructure composites are crystalline ZnO NT and CdSe QD composed nanoparticles. As we all know, the UV–vis spectra of the CdSe/ZnO electrodes are well dependent on the CdSe size and load amount [29,30]. Fig. 7 depicts the UV–vis absorption spectra of pure ZnO NT, CdSe/ZnO NR and CdSe/ZnO NT arrays. The thickness of these films was almost the same (∼3.5 μm). The samples were subsequently used to evaluate the photovoltaic performance. It has been observed from curve (a) that pure ZnO absorbed in the UV region with a band edge ∼400 nm. On the other hand, the spectra of CdSe/ZnO absorb (curve b, c) in both UV and visible region, and the absorption range increases up to 550 nm after the coupling process. The optical absorption properties of the CdSe were preserved when they were attached to the ZnO NT array. By comparing this spectrum to the sizing curves reported by Kamat et al. [29], the diameter of CdSe we obtained can be evaluated as ∼3.0 nm, which matched well with the TEM shown in Fig. 6a. Furthermore, there is an obvious difference in the absorption intensity between CdSe/ZnO NT (curve c) and the NR films (curve b). The enhancement depends on the absorption of CdSe onto both inner wall and out wall of NT for the same length and diameter of NT and NR. These results indicate that the CdSe will play the role of generating electron in the visible region and the NT-based electrode can absorb light more efficiently in the visible region. Fig. 8 compares the I–V characteristics of CdSe sensitized 1D ZnO NT-based and NR-based solar cells incorporated with 3.5 μm length of the array films. The corresponding values are summarized in Table 1, which demonstrates the current density at short circuit (Jsc), the voltage at open circuit (Vop), the fill factor (FF), and the efficiency of power conversion (η) in different samples. From the results in Table 1, the CdSe/ZnO NT-based solar cell exhibits a higher η of 0.44% with Jsc of 2.09 mA/cm2, open circuit Vop of 0.44 V, and the fill factor FF of 40.6%. Compared with the CdSe/ZnO NR-based solar cell, the η increases from 0.4 to 0.44 % although the Vop and FF slightly decrease. The enhancement of η (8%) is attributed to the strong effects of the increases of Jsc from 1.54 to 2.09 mA/cm2. It demonstrates that the increased surface area provided by the inner walls of ZnO tubes enhance the total amount of CdSe in the array films, thus the NT arrays have advantages over NR arrays in light-harvesting efficiencies. The slight decline of Vop and FF would not depress η of the CdSe/ZnO NT-
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Fig. 6. (a) HRTEM and TEM (inset) images and (b) SAED pattern of an individual CdSe/ZnO NT. (c, d) Dark-field images captured from the diffraction spots circled with markers C and D respectively.
based solar cell greatly as the strong enhancement of Jsc. The decline of Vop and FF could be attributed to the slight destruction of the compact ZnO seed layer which is expected to reduce the recombination between the electrolyte and the TCO substrate and improve the Vop and FF of DSSCs [31]. As shown in the image of excessively etched film (Fig. 4), bulk TCO substrate could be observed implying that the etching process could affect the compact layer. Proper etching condition and shorter reaction duration would not lead to dissolve
the compact layer completely but slightly destroy it. Furthermore, it can be seen in Fig. 8 that ZnO NT films without CdSe sensitization show no significant response under light illumination. The performance comparing with bare ZnO NT-based solar cell infers CdSe is necessary in solar cells. These results confirm that the ZnO NT-based photoelectrode exhibits a better performance compared to the ZnO NR-based photoelectrode. It is remarkable that the η obtained from the CdSe/ZnO NT system is up to 0.44%, which is also higher than that
Fig. 7. Light absorbance spectra of a bare ZnO NT array (a), CdSe/ZnO NR array (b), and CdSe/ZnO NT array (c).
Fig. 8. Current–voltage characteristics of ZnO NT (a), CdSe/ZnO NT (b) and CdSe/ZnO NR (c) array-based solar cells which were illuminated by a white light intensity of 85 mW/cm2.
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Table 1 Photovoltaic parameters of the ZnO NT, CdSe/ZnO NR and CdSe/ZnO NT-based solar cells. Film type
Jsc (mA/cm2)
Vop (V)
FF
η (%)
ZnO NT CdSe/ZnO NR CdSe/ZnO NT
0.11 1.54 2.09
O.27 0.47 0.44
0.36 0.47 0.41
0.01 0.40 0.44
of the CdSe/ZnO NT system (0.40%) reported in the literature [32]. By the way, the length of the ZnO array in the literature is about 9 μm.
4. Conclusion In summary, vertically aligned ZnO NT array films were simply synthesized by etching the electrodeposited ZnO NRs in a solution of EDA. The formation of the 1D tubular structure was contributed to the elective dissolution of ZnO along the c-axis. The reaction time and concentration of EDA were crucial to form perfect NT arrays. Then, CdSe/ZnO NT photoelectrodes were obtained by using a low temperature chemical bath deposition method. This architecture leads to better solar light harvesting in the visible region. An 8% enhancement in energy conversion efficiency was observed between the CdSe sensitized ZnO NT-based and ZnO NR-based solar cell. We believe that the improvement results from the larger surface area for CdSe deposition. This approach to design photovoltaic electrode would give a direction in the field of multi-junction solar cell materials.
Acknowledgments Financially supported by self-determined research funds of CCNU from the colleges' basic research and operation of MOE of China (CCNU09A02011) and by the Key Project of Ministry of Education of China (Grant 108097).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
J. Lu, S. An, W. Park, G. Yi, Adv. Mater. 16 (2004) 1661. T. Sun, J. Qiu, C. Liang, J. Phys. Chem. C 112 (2008) 715. S. Kar, S. Santra, J. Phys. Chem. C 112 (2008) 8144. A.B.F. Martinson, J.W. Elam, J.T. Hupp, M.J. Pellin, Nano Lett. 8 (2007) 2183. X.L. Yuan, B. Dierre, J.B. Wang, B.P. Zhang, T. Sekiguchi, J. Nanosci. Nanotechnol. 7 (2007) 3323. Y. Sun, G.M. Fuge, N.A. Fox, D.J. Riley, M.N.R. Ashfold, Adv. Mater. 17 (2005) 2477. N.G. Ndifor-Angwafor, D.J. Riley, Phys. Status. Sol. 205 (2008) 2351. H.Q. Wang, G.Z. Wang, L.C. Jia, C.J. Tang, G.H. Li, J. Phys. D: Appl. Phys. 40 (2007) 6549. Y.W. Tang, L.J. Luo, Z.G. Chen, B.H. Li, Z.Y. Jia, L. Xu, Electrochem. Commun. 9 (2007) 289. L.G. Yu, G.M. Zhang, S.Q. Li, Z.H. Xi, D.Z. Guo, J. Cryst. Growth 299 (2007) 184. G.W. She, X.H. Zhang, W.S. She, X. Fan, J.C. Chang, Electrochem. Commun. 9 (2007) 2784. J. Elias, R.T. Zaera, G.Y. Wang, Chem. Mater. 20 (2008) 6633. Y.F. Gao, M. Nagai, T.C. Chang, J.J. Shyue, Cryst. Growth. & Des. 7 (2007) 2467. E. Galoppini, J. Rochford, H.H. Chen, G. Saraf, Y.C. Lu, A. Hagfeldt, G. Boschloo, J. Phys. Chem. C 110 (2006) 16159. W. Lee, S.K. Min, V. Dhas, S.B. Ogale, S.H. Han, Electrochem. Commun. 11 (2009) 103. X.B. Cao, P. Chen, Y. Guo, J. Phys. Chem. C 112 (2008) 20560. K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Nano Lett. 7 (2007) 69. C.C. Chen, H.W. Chung, C.H. Chen, H.P. Lu, E.G. Diau, J. Phys. Chem. C 112 (2008) 19151. K. Zhou, T.B. Vinzant, N.R. Neale, A.J. Frank, Nano Lett. 7 (2007) 3739. L.F. Xu, Q. Liao, D.S. Xu, J. Phys. Chem. C 111 (2007) 4549. C. Yan, D. Xue, Electrochem. Commun. 9 (2007) 1247. G. She, X. Zhang, W. Shi, X. Fan, J.C. Chang, C. Lee, S. Lee, C. Liu, Appl. Phys. Lett. 92 (2008) 053111. Y.W. Tang, X.Y. Hu, M.J. Chen, L.J. Luo, B.H. Li, L.Z. Zhang, Electrochim. Acta 54 (2009) 2742. D.C. Look, G.C. Farlow, P. Reunchan, S. Limpijumnong, S.B. Zhang, K. Nordlund, Phys. Rev. Lett. 95 (2005) 225502. A. Umar, Y.B. Hahn, Nanotechnology 17 (2006) 2174. M. Izaki, T. Omi, J. Electrochem. Soc. 144 (2004) 2575. L.F. Xu, Y. Guo, Q. Liao, D.S. Xu, J. Phys. Chem. B 109 (2005) 13519. F. Li, Y. Ding, P. Gao, Z.L. Wang, Angew. Chem. Int. Ed. 43 (2004) 5238. I. Robel, V. Subramanian, M. Kuno, P.V. Kamat, J. Am. Chem. Soc. 128 (2006) 2385. W.W. Yu, L.H. Qu, W.Z. Guo, X.G. Peng, Chem. Mater. 15 (2003) 2854. A. Burke, S. Ito, H. Snaith, U. Bach, J. Kwiatkowski, M. Grätzel, Nano Lett. 8 (2008) 977. K.S. Leschkies, R. Divakar, J. Basu, E.E. Pommer, J.E. Boercker, C.B. Carter, U.R. Kortshagen, D.J. Norris, E.S. Aydil, Nano Lett. 7 (2007) 1793.
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Formation of aligned ZnO nanotube arrays by chemical etching and coupling with CdSe for photovoltaic application Lijuan Luo a, Gang Lv a, Bihui Li a, Xiaoyan Hu a, Lei Jin b, Jianbo Wang b, Yiwen Tang a,⁎ a b
Institute of Nano-science and Technology, Central China Normal University, Wuhan, 430079, China Department of Physics and Center for Electron Microscopy, Wuhan University, Wuhan 430072, China
a r t i c l e
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Article history: Received 27 September 2009 Received in revised form 8 February 2010 Accepted 9 March 2010 Available online 16 March 2010 Keywords: Zinc oxide Nanotubes Cadmium selenide Chemical etching Photovoltaic properties Solar cells
a b s t r a c t High density aligned ZnO nanotube (NT) arrays were synthesized using a facile chemical etching of electrochemically deposited ZnO nanorods (NRs). The influence of etching time and solution concentration on the ZnO NT formation was investigated. Moreover, cadmium selenide (CdSe) nanoparticles as sensitizers were assembled onto the ZnO NT and NR arrays for solar cell application. A conversion efficiency (η) of 0.44% was achieved for CdSe/ZnO NT-based solar cell under the white light illumination intensity of 85 mW/cm2. An 8% enhancement in η was observed between the CdSe/ZnO NT-based and NR-based solar cell due to the enhancement of the photocurrent density. ZnO NT arrays have been proved to have a superior ability as compared with ZnO NR arrays when employed as a semiconductor film. © 2010 Elsevier B.V. All rights reserved.
1. Introduction One dimensional (1D) ZnO nanostructures, such as nanowires, nanobelts and nanotubes, have been synthesized for wide application in gas sensors, field emission, nanolasers, solar cells and so on [1–3]. Among these nanostructures, arrays of ZnO nanotube (NT) have received much attention for their high porosity and large surface area [4]. Many methods have been developed to prepare ZnO NT arrays including chemical vapor deposition, solvothermal synthesis and electrochemical deposition [5–9]. Among these methods, electrodeposition presents a cost-effective method in terms of its low temperature and precise control of the reaction parameters. The electrodeposition method employed for ZnO NT synthesis can be divided into two sorts: the direct electrodeposition method and the two-step electrochemical method including the electrodeposition of ZnO nanorods (NRs) and the following etching process. Of particular interest are two reports on the direct electrodeposition of ZnO NTs [9,10]. In Yu's report, ZnO NTs were obtained by applying a high voltage up to 30 V between two electrodes, while our previous report on the synthesis of ZnO NTs employed a potentiostatic deposition method. But the length of the ZnO NTs was limited by the increased film resistance observed during electrodeposition. In the case of the two-step electrochemical method, the formation of ZnO NRs is similar,
⁎ Corresponding author. Tel.: +86 27 62075995; fax: +86 27 67861185. E-mail address: [email protected] (Y. Tang). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.03.014
although the etching process is different. Zhang et al. reported a defect selective etching method to obtain ZnO NTs under an applied potential of 1.1 V [11]. Zaera et al. reported the formation of ZnO NTs by etching ZnO NR in aqueous sodium chloride solution at neutral pH without applying a voltage [12]. Although a large amount of research has been performed on the two-step electrochemical method to synthesize ZnO NT arrays, a perfect array of ZnO NTs for application in solar cells still remains a challenge. As a wide band gap semiconductor (∼3.37 eV), 1D ZnO arrays are unable to absorb and utilize the visible region of the solar spectrum. In order to absorb visible light and generate electron–hole pairs in dyesensitized solar cells (DSSCs) or nanocrystal-sensitized solar cells, low band gap materials such as organic dye molecules and inorganic low band gap semiconductor should be coupled to the ZnO array as a photoanode. Recently, both ZnO NR and NT arrays have been applied as the semiconductor film for DSSCs because of their high surface areas and their vertically aligned electrical pathways, which are expected to increase the efficiencies of those photoelectrical devices [5,13,14]. Gao's group incorporated ZnO NRs into DSSC photoanode to obtain an energy conversion efficiency of 1.7% [13]. Hupp et al. on the other hand, introduced high surface area ZnO NT array photoanodes templated by anodic aluminum oxide into DSSCs [5]. The resulting power efficiency was found to be high up to 1.6% despite of being illuminated through the Pt/conductive glass rear electrode, which attenuated ∼20% of the light at the visible wavelength due mostly to Pt absorption. In the case of nanocrystal-sensitized solar cells, much research has been done to obtain ZnO NR array photoanodes
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sensitized by a low band gap semiconductor including CdSe, CdS, and CdTe [15,16]. Despite of the advantages known for using low band gap semiconductor sensitizers, the efficiencies of the ZnO NR array devices are limited by their low light-harvesting ability. Increasing the surface area of the ZnO film will operate but it is a significant technological problem to synthesize higher aspect ratio wires through mild chemical method. Obviously NTs have a larger surface area than NRs of similar length and diameter. However, no report has been found in the literatures where low band gap semiconductors are coupled with ZnO NT arrays as a photoanode. According to the results of a 1D TiO2 investigation, it was found that the higher lightharvesting efficiencies of the NT-based TiO2 DSSCs were owed to the stronger internal light scattering effects and more efficient change separation at the enormously enhanced junction areas [17–19]. We propose that ZnO NT arrays also have the good light scattering efficiency on account of their 1D tubular nature. In this paper, high density vertically aligned ZnO NT arrays were prepared on a F-doped SnO2 (TCO) glass substrate using a facile chemical etching of electrodeposited ZnO NRs [20]. Selective dissolution along the c-axis induced the 1D tubular structure formation [12,21,22]. The effects of etching time and solution composition were investigated. By controlling the preparation condition, highly oriented ZnO NT arrays were fast formed. Then, CdSe/ZnO NT photoelectrodes containing a three dimensional junction were synthesized using a low temperature chemical bath deposition method and its photovoltaic performance was investigated. The NT-based CdSe/ZnO photoelectrode exhibited an obviously better cell performance as compared with the corresponding NRbased CdSe/ZnO photoelectrodes. 2. Experimental details In the first step, ZnO NR arrays were grown on a TCO glass substrate (10–15 Ω/□, Asahi Company) through electrodeposition. Before the electrodeposition process, a thin ZnO compact seed layer was coated on TCO by a dip-coating method. For preparing the ZnO seed layer, the synthesis of the ZnO precursor sol has been described in detail (refer to our previous work [23]). The electrodeposition procedure was carried out using three electrode systems, including a Pt counter electrode, a saturated calomel electrode (SCE) as reference electrode and the ZnO seed coated TCO used as the working electrode. The films were electrodeposited from a bath containing 0.03 M Zn (NO3)2 and 0.03 M (CH2)6 N4 in deionized water. The bath temperature was controlled at 70 °C and the electrolyte was stirred continuously using a magnetic stirrer. The ZnO NR films were grown under a −1.0 V potentiostatic condition using a Princeton Applied Research Model 263A Potentiostat/Galvanostat. Subsequently, the TCO substrate covered with ZnO NRs was used as the working electrode for the second-step dissolution process. The etching solution consisted of 0.1 M ethylenediamine (EDA), and the reaction temperature was kept at 70 °C. The applied potential was 0.04 V versus SCE. The shape and size of the ZnO NT arrays could be controlled by adjusting the reaction parameters. The as-obtained ZnO NT array films were placed vertically into a beaker containing a solution of 80 mM CdSO4, 80 mM Na2SeSO3 and 120 mM N(CH2COOK)3 at 0 °C in the dark. The solution was continuously stirred to achieve a homogenous, clear distribution of the chemical components during the deposition. After deposition for 1 h, the films were rinsed with deionized water and dried at room temperature. The annealing processes of the composite films were performed in a tubular furnace at 400 °C under a N2 atmosphere for 30 min. The morphology of the as-prepared films was analyzed by scanning electron microscopy (SEM, JEOL 6700F). Further structural analysis of the ZnO NT and CdSe/ZnO NT nanostructures was carried out using transmission electron microscopy, selected area electron
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diffraction (TEM/SAED, JEOL JEM-2100FEF) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010). Photoluminescence (PL, F-7000, Hitachi) spectra were measured at room temperature on a fluorescence spectrophotometer. The excitation wavelength was 325 nm, the scanning speed was 1200 nm min−1, and the photomultiplier tube voltage was 700 V. The widths of the excitation slit and emission slit were both 5.0 nm. A Perkin Elmer Lambda35 spectrometer UV–Visible system was used to obtain the absorption spectra of the samples over a range of 350–700 nm. The CdSe/ZnO electrodes were incorporated into thin layer sandwich-type cells with a Pt sputtered TCO as the counter electrode and an electrolyte solution to measure the power conversion efficiency. The electrolyte used was 0.60 M 1-butyl-3-methylimidazolium iodide, 0.03 M I2, and 0.50 M tertbutylpyridine in acetonitrile/valeronitrile (85:15). The CdSe/ZnO electrodes of 0.2 cm2 area were illuminated by white light with an intensity of 85 mW/cm2 white light intensity. 3. Results and discussions 3.1. Fabrication and characterization of ZnO NT arrays Fig. 1a displays a typical surface SEM image of the ZnO NR film prepared by the first-step electrodeposition at a cathodic potential of −1.0 V for 60 min. The rods with hexagonal morphology are mainly 200 nm in diameter and about 3.5 μm in length. As expected, fairly well-aligned flat-top hexagonal ZnO NRs have been grown vertically onto the surface of the substrate and spaced apart sufficiently. It is anticipated that flat-top NRs would present a fascinating opportunity to form a tubular structure in the subsequent etching step. It is also anticipated that needle like ZnO NRs would induce the destruction of the ZnO array. Fig. 1b shows an SEM image of the ZnO NT array after selective dissolution in 0.1 M EDA under an applied potential of 0.04 V. The array consists of tubes having a smooth surface and a top diameter of 100–250 nm. It can be seen from Fig. 1c that the tubes are aligned perpendicularly to the substrate and the diameter of the bottom is larger than that of the top. The length of the NTs is about 3 μm, which is a little shorter than that of the previous NRs. Further structural characterization of single ZnO tube was performed by TEM (Fig. 1d). The resulting TEM image clearly indicates the hollow structure of the nanotubes, while the accompanying SAED pattern inset shows that the ZnO NTs are a single crystal with a hexagonal structure. Room temperature PL spectra of the vertically aligned ZnO NR and NT arrays were investigated following an excitation with a Xe light excitation source at 325 nm. From the PL spectra (Fig. 2), it can be seen that both the NRs and NTs show a sharp and strong peak at approximately 380 nm, which corresponds to the near band edge peak that is responsible for the recombination of the free excitons. As we can see, the emission intensity of the ZnO NTs (curve b) is weaker than that of the NRs (curve a), for the ZnO NTs are evolved from the NRs by etching. It should be noted that neither the PL spectra from ZnO NRs nor that of NTs show a broad deep-level (visible) emission which has been reported in literature [20,24]. According to these literature reports, the deep-level emissions are related to the various intrinsic defects produced during the growth process. No deep-level emission indicates that the as-prepared NRs and NTs have good crystal quality and a low concentration of defects [25]. To get a better understanding of the formation mechanism and optimization of the microstructure of ZnO NTs, the effects of etching time and EDA solution concentration on the morphology of ZnO NTs were studied by SEM. Fig. 3 shows images corresponding to the morphology evolution of the electrodeposited ZnO NRs after an etching process of 0 min, 10 min, 20 min and 30 min in 0.1 M EDA aqueous solution under an applied potential of 0.04 V. It can be clearly observed from Fig. 3a that the ZnO NRs obtained from the first electrodeposition have a flat-top hexagonal morphology. After 10 min
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Fig. 1. SEM images of the samples: (a) ZnO NR array, (b) ZnO NT array, and (c) cross-section of the ZnO NT array. (d) TEM image and the corresponding SAED of the NTs removed from the NT array.
of etching reaction, small pits can be observed in the top of the ZnO NRs, and a part of the NRs retain their rod shape (Fig. 3b). As illustrated in Fig. 3c, all tops of the ZnO NRs have been etched and obvious holes are formed after a 20 min reaction. The unbalanced distribution of defects in the ZnO nanorods leads to the preferential etching in the center part of the nanorods [11]. With a further increase in the reaction time to 30 min, perfect tube-like structures appeared. The further increase in etching time would induce the dissolution of the wall of ZnO NTs. According to the above experimental observation,
the fabrication procedure of the ZnO NT arrays could be proposed as follows. First, well-aligned arrays of ZnO NRs were electrodeposited onto the TCO substrate coated with ZnO compact layer. The wellknown electrochemical reaction is as follows [26]: −
−
−
2+
+ 2OH →ZnO + H2 O
NO3 + H2 O + 2e →NO2 + 2OH Zn
−
−
ð1Þ ð2Þ
Subsequently, the ZnO nanorods were soaked in EDA aqueous solution and etched at 70 °C. EDA is a type of molecule with a strong coordination ability to zinc [27]. In the EDA aqueous solution, OH− were formed by the hydrolysis of EDA (Eq. 3) and the OH− further reacted with ZnO to form a hydroxyl complex such as [Zn(OH)4]2− (Eq. 4). þ − ðCH2 NH2 Þ2 + 2H2 O → CH2 NH3 + 2OH 2
−
ZnO + 2OH + H2 O→ZnðOHÞ4
Fig. 2. Room temperature PL spectra of the electrodeposited ZnO NRs (a) and ZnO NTs after the etching process (b).
2
ð3Þ ð4Þ
When the c-axis preferentially grown ZnO NRs prepared by the firststep electrodeposition were soaked in aqueous EDA solution, the preferential adsorption of EDA molecules occurred on the lateral facets rather than on the (0001) facets and OH− were formed by the hydrolysis of EDA. Subsequently, the OH− anions reacted with ZnO to form a soluble hydroxyl complex such as [Zn(OH)4]2− along the caxis. The selective dissolution of ZnO along the c-axis caused the formation of the tubes. In addition, the applied potential was also crucial to perfecting the tubes. A lower positive bias would accelerate the dissolution process of the ZnO NRs [20]. When the ZnO NR film
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Fig. 3. SEM images of the products etched after different deposition times in 0.1 M EDA solution: (a) 0, (b) 10, (c) 20, and (d) 30 min.
was put into the EDA solution without applying a positive bias, the tubes with porous wall would be obtained after a long time soaking. Previous research suggests that the adsorption of OH− plays a major role during the etching of ZnO NRs to form NTs, and OH− are formed by the hydrolysis of EDA. To gain further insight, the effects of solution concentration of EDA on the morphologies were studied. ZnO NR arrays were held for 30 min at 70 °C in a solution with 0 M, 0.01 M, 0.2 M and 0.4 M EDA. It was found that no ZnO NT could be obtained in the absence of EDA or at a too low concentration, and the ZnO kept a perfect rod-like structure. However, a partial dissolution of the NRs core occurred when the concentration of EDA increased. When the concentration of EDA was in the range from 0.05–0.1 M, perfect NT structures as shown in Fig. 1d were obtained. But when the EDA concentration was further increased, not only the cores of the ZnO NRs but also the walls were dissolved. Fig. 4a shows the SEM image of the sample which etched in a 0.2 M EDA solution under 0.04 V (vs SCE) at 70 °C. The image shows that further increasing the concentration of the EDA concentration induces the completed dissolution of the tubes as shown in Fig. 4b. From this figure, we could observe the TCO substrate and some fragmentary tubes of ZnO. The variation of the morphologies of ZnO with the concentration inferred from the above experiments suggests that OH− play a major role on the dissolution of the ZnO NR core. It is well known that hexagonal wurtzite ZnO possesses two oppositely charged polar facets (0001) and (0001−) which are perpendicular to the [0001] direction. The two facets are metastable while the other six side nonpolar (101−0) facets are mostly stable and chemically inert. The stability of the top (0001) facet requires that it become less positive [28]. When ZnO NRs are soaked in the EDA solution, the OH− of EDA will preferentially adsorb on the (0001) facet because of the electrostatic adherence. The EDA molecules, on the other hand, preferentially adsorb onto the lateral facets of the ZnO NRs thereby providing electrons to the Zn atoms, which will change
Fig. 4. SEM images of samples etched in 0.2 M EDA (a) and 0.4 M EDA (b) solution for 30 min.
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the distribution of Zn and O. This process will lead to an enhancement of the Zn–O bond and protect the edge of the (0001) while slowing the dissolution rate of the edge. The formation of the tubular structure should be contributed to the elective dissolution of ZnO along the caxis. Moreover, OH− will compete with EDA to absorb on the lateral facets of the ZnO NRs and dissolve the tube wall when the concentration of OH− is high enough. So in our experiments, control of the concentration of EDA is critical for the formation of the perfect tubular structure. NTs can be obtained in a narrow range of EDA concentration.
3.2. Coupling ZnO NTs with CdSe and their application in photovoltaic cells The hollow nature of the ZnO tubes makes both inner and outer surface areas accessible for the modification with CdSe. Obviously, ZnO NT arrays have a larger surface area than that of ZnO NR arrays with the same length and diameter because NTs comprise both an outer and inner surface. When the CdSe/ZnO NT composite material was applied in a photoelectrode, the light from the illumination were perfectly trapped in the tube arrays and reduplicatively absorbed by the CdSe particles both inside and outside of the tubes. Then, the photoelectrons generated in the CdSe pass through the ZnO arrays and rapidly reached the back contact smoothly because of the 1D nature of the ZnO NTs. A simple low temperature chemical bath deposition method was employed to deposit CdSe onto the ZnO tubes. As can be seen in Fig. 5a, both the inner and outer walls of the ZnO NTs are covered by CdSe nanoparticles causing the surface of the tubes to become
Fig. 5. SEM images of the as-prepared CdSe/ZnO NT array (a) and its cross-sections (b).
rougher. The diameters of the CdSe particles range from 17 to 25 nm. The super thin layer of CdSe ensures the photoinduced electron the immediate transfer to ZnO. The size and thickness of CdSe strongly depend on the chemical bath deposition parameters such as temperature and deposition time. It would affect the photovoltaic performance of the electrode greatly. Fig. 5b shows the cross-sections of the as-prepared CdSe/ZnO NT arrays. From this image, we can see that the diameter of the top of the NR is smaller than that of the bottom. The length of the NTs can be tuned by the electrodeposition process of the NRs. In order to confirm the composite nanostructure of CdSe on ZnO NT, we further performed TEM investigation. The inset in Fig. 6a shows the TEM image of an individual CdSe/ZnO NT. It can be seen that the CdSe particles are uniformly attached to the surface of the NT. HRTEM image shown in Fig. 6a proves that CdSe crystallizes as polycrystals with its grain sizes in the range of 3–3.5 nm. The corresponding SAED pattern of the ZnO/CdSe nanostructure in Fig. 6b shows a set of sharp diffraction spots and other spots belonging to several discrete diffraction rings. It indicates that the ZnO NT is a single crystal with a hexagonal structure, and all other spots in the diffraction rings could be indexed to CdSe nanocrystals with cubic symmetry. In order to elucidate which part of the composite structure contributed to the diffraction pattern, dark-field images were captured with the hexagonal ZnO diffraction spots and the cubic CdSe diffraction spots circled with markers C, and D (Fig. 6c and d). From the dark-field image of Fig. 6c, we could infer that the core is ZnO. From Fig. 6d, several CdSe bright dots could be seen on the surface of the ZnO tube. It confirms that the nanostructure composites are crystalline ZnO NT and CdSe QD composed nanoparticles. As we all know, the UV–vis spectra of the CdSe/ZnO electrodes are well dependent on the CdSe size and load amount [29,30]. Fig. 7 depicts the UV–vis absorption spectra of pure ZnO NT, CdSe/ZnO NR and CdSe/ZnO NT arrays. The thickness of these films was almost the same (∼3.5 μm). The samples were subsequently used to evaluate the photovoltaic performance. It has been observed from curve (a) that pure ZnO absorbed in the UV region with a band edge ∼400 nm. On the other hand, the spectra of CdSe/ZnO absorb (curve b, c) in both UV and visible region, and the absorption range increases up to 550 nm after the coupling process. The optical absorption properties of the CdSe were preserved when they were attached to the ZnO NT array. By comparing this spectrum to the sizing curves reported by Kamat et al. [29], the diameter of CdSe we obtained can be evaluated as ∼3.0 nm, which matched well with the TEM shown in Fig. 6a. Furthermore, there is an obvious difference in the absorption intensity between CdSe/ZnO NT (curve c) and the NR films (curve b). The enhancement depends on the absorption of CdSe onto both inner wall and out wall of NT for the same length and diameter of NT and NR. These results indicate that the CdSe will play the role of generating electron in the visible region and the NT-based electrode can absorb light more efficiently in the visible region. Fig. 8 compares the I–V characteristics of CdSe sensitized 1D ZnO NT-based and NR-based solar cells incorporated with 3.5 μm length of the array films. The corresponding values are summarized in Table 1, which demonstrates the current density at short circuit (Jsc), the voltage at open circuit (Vop), the fill factor (FF), and the efficiency of power conversion (η) in different samples. From the results in Table 1, the CdSe/ZnO NT-based solar cell exhibits a higher η of 0.44% with Jsc of 2.09 mA/cm2, open circuit Vop of 0.44 V, and the fill factor FF of 40.6%. Compared with the CdSe/ZnO NR-based solar cell, the η increases from 0.4 to 0.44 % although the Vop and FF slightly decrease. The enhancement of η (8%) is attributed to the strong effects of the increases of Jsc from 1.54 to 2.09 mA/cm2. It demonstrates that the increased surface area provided by the inner walls of ZnO tubes enhance the total amount of CdSe in the array films, thus the NT arrays have advantages over NR arrays in light-harvesting efficiencies. The slight decline of Vop and FF would not depress η of the CdSe/ZnO NT-
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Fig. 6. (a) HRTEM and TEM (inset) images and (b) SAED pattern of an individual CdSe/ZnO NT. (c, d) Dark-field images captured from the diffraction spots circled with markers C and D respectively.
based solar cell greatly as the strong enhancement of Jsc. The decline of Vop and FF could be attributed to the slight destruction of the compact ZnO seed layer which is expected to reduce the recombination between the electrolyte and the TCO substrate and improve the Vop and FF of DSSCs [31]. As shown in the image of excessively etched film (Fig. 4), bulk TCO substrate could be observed implying that the etching process could affect the compact layer. Proper etching condition and shorter reaction duration would not lead to dissolve
the compact layer completely but slightly destroy it. Furthermore, it can be seen in Fig. 8 that ZnO NT films without CdSe sensitization show no significant response under light illumination. The performance comparing with bare ZnO NT-based solar cell infers CdSe is necessary in solar cells. These results confirm that the ZnO NT-based photoelectrode exhibits a better performance compared to the ZnO NR-based photoelectrode. It is remarkable that the η obtained from the CdSe/ZnO NT system is up to 0.44%, which is also higher than that
Fig. 7. Light absorbance spectra of a bare ZnO NT array (a), CdSe/ZnO NR array (b), and CdSe/ZnO NT array (c).
Fig. 8. Current–voltage characteristics of ZnO NT (a), CdSe/ZnO NT (b) and CdSe/ZnO NR (c) array-based solar cells which were illuminated by a white light intensity of 85 mW/cm2.
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Table 1 Photovoltaic parameters of the ZnO NT, CdSe/ZnO NR and CdSe/ZnO NT-based solar cells. Film type
Jsc (mA/cm2)
Vop (V)
FF
η (%)
ZnO NT CdSe/ZnO NR CdSe/ZnO NT
0.11 1.54 2.09
O.27 0.47 0.44
0.36 0.47 0.41
0.01 0.40 0.44
of the CdSe/ZnO NT system (0.40%) reported in the literature [32]. By the way, the length of the ZnO array in the literature is about 9 μm.
4. Conclusion In summary, vertically aligned ZnO NT array films were simply synthesized by etching the electrodeposited ZnO NRs in a solution of EDA. The formation of the 1D tubular structure was contributed to the elective dissolution of ZnO along the c-axis. The reaction time and concentration of EDA were crucial to form perfect NT arrays. Then, CdSe/ZnO NT photoelectrodes were obtained by using a low temperature chemical bath deposition method. This architecture leads to better solar light harvesting in the visible region. An 8% enhancement in energy conversion efficiency was observed between the CdSe sensitized ZnO NT-based and ZnO NR-based solar cell. We believe that the improvement results from the larger surface area for CdSe deposition. This approach to design photovoltaic electrode would give a direction in the field of multi-junction solar cell materials.
Acknowledgments Financially supported by self-determined research funds of CCNU from the colleges' basic research and operation of MOE of China (CCNU09A02011) and by the Key Project of Ministry of Education of China (Grant 108097).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
J. Lu, S. An, W. Park, G. Yi, Adv. Mater. 16 (2004) 1661. T. Sun, J. Qiu, C. Liang, J. Phys. Chem. C 112 (2008) 715. S. Kar, S. Santra, J. Phys. Chem. C 112 (2008) 8144. A.B.F. Martinson, J.W. Elam, J.T. Hupp, M.J. Pellin, Nano Lett. 8 (2007) 2183. X.L. Yuan, B. Dierre, J.B. Wang, B.P. Zhang, T. Sekiguchi, J. Nanosci. Nanotechnol. 7 (2007) 3323. Y. Sun, G.M. Fuge, N.A. Fox, D.J. Riley, M.N.R. Ashfold, Adv. Mater. 17 (2005) 2477. N.G. Ndifor-Angwafor, D.J. Riley, Phys. Status. Sol. 205 (2008) 2351. H.Q. Wang, G.Z. Wang, L.C. Jia, C.J. Tang, G.H. Li, J. Phys. D: Appl. Phys. 40 (2007) 6549. Y.W. Tang, L.J. Luo, Z.G. Chen, B.H. Li, Z.Y. Jia, L. Xu, Electrochem. Commun. 9 (2007) 289. L.G. Yu, G.M. Zhang, S.Q. Li, Z.H. Xi, D.Z. Guo, J. Cryst. Growth 299 (2007) 184. G.W. She, X.H. Zhang, W.S. She, X. Fan, J.C. Chang, Electrochem. Commun. 9 (2007) 2784. J. Elias, R.T. Zaera, G.Y. Wang, Chem. Mater. 20 (2008) 6633. Y.F. Gao, M. Nagai, T.C. Chang, J.J. Shyue, Cryst. Growth. & Des. 7 (2007) 2467. E. Galoppini, J. Rochford, H.H. Chen, G. Saraf, Y.C. Lu, A. Hagfeldt, G. Boschloo, J. Phys. Chem. C 110 (2006) 16159. W. Lee, S.K. Min, V. Dhas, S.B. Ogale, S.H. Han, Electrochem. Commun. 11 (2009) 103. X.B. Cao, P. Chen, Y. Guo, J. Phys. Chem. C 112 (2008) 20560. K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Nano Lett. 7 (2007) 69. C.C. Chen, H.W. Chung, C.H. Chen, H.P. Lu, E.G. Diau, J. Phys. Chem. C 112 (2008) 19151. K. Zhou, T.B. Vinzant, N.R. Neale, A.J. Frank, Nano Lett. 7 (2007) 3739. L.F. Xu, Q. Liao, D.S. Xu, J. Phys. Chem. C 111 (2007) 4549. C. Yan, D. Xue, Electrochem. Commun. 9 (2007) 1247. G. She, X. Zhang, W. Shi, X. Fan, J.C. Chang, C. Lee, S. Lee, C. Liu, Appl. Phys. Lett. 92 (2008) 053111. Y.W. Tang, X.Y. Hu, M.J. Chen, L.J. Luo, B.H. Li, L.Z. Zhang, Electrochim. Acta 54 (2009) 2742. D.C. Look, G.C. Farlow, P. Reunchan, S. Limpijumnong, S.B. Zhang, K. Nordlund, Phys. Rev. Lett. 95 (2005) 225502. A. Umar, Y.B. Hahn, Nanotechnology 17 (2006) 2174. M. Izaki, T. Omi, J. Electrochem. Soc. 144 (2004) 2575. L.F. Xu, Y. Guo, Q. Liao, D.S. Xu, J. Phys. Chem. B 109 (2005) 13519. F. Li, Y. Ding, P. Gao, Z.L. Wang, Angew. Chem. Int. Ed. 43 (2004) 5238. I. Robel, V. Subramanian, M. Kuno, P.V. Kamat, J. Am. Chem. Soc. 128 (2006) 2385. W.W. Yu, L.H. Qu, W.Z. Guo, X.G. Peng, Chem. Mater. 15 (2003) 2854. A. Burke, S. Ito, H. Snaith, U. Bach, J. Kwiatkowski, M. Grätzel, Nano Lett. 8 (2008) 977. K.S. Leschkies, R. Divakar, J. Basu, E.E. Pommer, J.E. Boercker, C.B. Carter, U.R. Kortshagen, D.J. Norris, E.S. Aydil, Nano Lett. 7 (2007) 1793.