Accepted Manuscript Morphological control and characterization of CdTe microstructure arrays synthesized by one-step electrodeposition Kailin Chi, Qian Li, Xianwei Meng, Li Liu, Dong Ding, Haibin Yang, Wuyou Fu PII: DOI: Reference:
S0167-577X(17)30204-5 http://dx.doi.org/10.1016/j.matlet.2017.02.023 MLBLUE 22125
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Received Date: Revised Date: Accepted Date:
27 November 2016 19 January 2017 9 February 2017
Please cite this article as: K. Chi, Q. Li, X. Meng, L. Liu, D. Ding, H. Yang, W. Fu, Morphological control and characterization of CdTe microstructure arrays synthesized by one-step electrodeposition, Materials Letters (2017), doi: http://dx.doi.org/10.1016/j.matlet.2017.02.023
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Morphological control and characterization of CdTe microstructure arrays synthesized by one-step electrodeposition Kailin Chi, a Qian Li, b Xianwei Meng, a Li Liu, a Dong Ding, a Haibin Yang, * a and Wuyou Fua a
State key Laboratory of Superhard Materials, Jilin University, Changchun, 130012, PR China.
*E-mail: [email protected]
, Fax: +86 431 85168763; Tel: +86 431 85168763. b
Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, PR China.
ABSTRACT Without templates and any addition of surfactants, cadmium telluride (CdTe) microstructures with various novel morphologies were fabricated by one-step electrodeposition in the acid system during the deposition potential range from -0.65V to -0.50V (vs.Ag/AgCl). The structure and properties of the as-synthesized films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis absorption spectroscopy and photoelectrochemical measurements. XRD pattern revealed that the high-purity CdTe films were cubic phase and exhibited the preferential orientation along (111) direction. The SEM indicated that by varying the deposition potential, we can obtain CdTe structure arrays with different morphologies evolution from microrod arrays to highly ordered flower-like structures. In addition, based on the better optical and photoelectrochemical performances, these CdTe microstructures would be potentially applied for substrate configuration photovoltaic device applications. KEYWORDS: CdTe, electrodeposion, deposition potential, microstructure 1. INTRODUCTION Due to the unique and intriguing performance arising from the size and high specific surface area, micro/nanostructure materials have tremendous potential to be applied in solar cells to enhance photoelectric conversion efficiency. Appropriate complicated structures are advantageous to the efficient and directional pathways of carriers and promote the separation and transmission efficiencies of photogenerated electrons and holes. Various functional inorganic materials have been fabricated possessing many micro/nanostructures through strategies such as electrodeposition [1-2], solvo/hydrothermal reaction [3-4], chemical oxidationreduction processing [5-8], ultrasonication and chemical activation , self-assembly method , and γ
irradiation routes , etc. As one of the most important II-VI optoelectronic compound semiconductor, CdTe has been attracting widespread interest in the photoelectric field and microelectronic devices. The performances of CdTe films rest with the level of preparation technology and growth. Synthesis of highaspect-ratio CdTe micro/nanostructure arrays is a potential factor to enhance performance in photovoltaic cells. In this research work, our strategy to synthesize the well-defined CdTe microstructures directly on Ni foils with a variety of morphologies without using any catalyst or template, using one-step electrodeposition and investigate the effects of deposited potential on the morphological, optical and photoelectrochemical properties. Such complicated and unique CdTe morphologies are reported rarely in the previous literature. 2. EXPERIMENTAL All chemicals used in the work were analytical pure. The electrolyte which contained distilled water, 0.006M Na2TeO3 and 0.02 M CdSO4•3/8H2O was added 0.10M sulfuric acid under the condition as magnetic stirring. The CdTe films depositions were fabricated respectively under a constant potential of -0.65V, -0.60V, -0.55V and -0.50V for 1 hour by a three electrodes system, in which a saturated silver/silver chloride electrode, the pretreated Ni substrates, and graphite flake served as the reference electrode counter electrode, the working electrode and the counter electrode respectively. After the depositions completed, the as-prepared films were cleaned and annealed at 400℃ in high purity argon gas for 0.5h. 3. RESULTS AND DISCUSSION 3.1. Structural characterization The structural characterization and phase purity of the as-deposited films deposited at different potentials were studied by X-ray diffraction (XRD), as shown in Figure 1. With the exception of Ni diffraction peaks, the rest peaks can be readily indexed to the (111), (220), (311), (400), (331) and (422) planes reflection of CdTe cubic phase respectively according to the CdTe standard JCPDS NO.75-2086. No characteristic peaks observed due to impurities in the patterns indicating that the synthesized material contain high-purity CdTe. The sharp and narrow diffraction peaks demonstrate the CdTe films are well-crystallized. When deposition potentials increase from -0.65V to -0.50V, the plane (111) in all patterns always shows the maximum intensity peak, which indicates the synthesized cubic CdTe films present a preferential growth oriented along the  direction . Figure 1. XRD patterns of the as-synthesized CdTe films deposited at different potentials on the Ni substrates: (a) -0.65V, (b) -0.60V, (c) -0.55V, (d) -0.50V.
3.2. Morphological characterization The influences of the deposition potentials on the representative morphologies of the CdTe films were investigated systemically via SEM. As the electrodeposition occurs at -0.65V, Figure 2a and b show that the well-aligned microrod arrays are formed, the microrods are vertically aligned and parallel to each other with lengths in the range of 2–3 µm, m, with with non-uniform diameters. When the deposition potential is -0.60V, highdensity randomly oriented arrays are observed which is dominated by flake-like petals with different dimensions, as shown in Figure 2c and d, a small quantity of larger petals are produced and the microstructure is irregular and defective. Whatever the size, the coarsely surfaces of the petals always have numerous folds, which has advantageous of large specific surface areas. In Figure 2e, the composite morphology deposited Figure 2. The SEM of the as-synthesized CdTe films deposited at different potentials: (a, b) -0.65V, (c, d) 0.60V, (e, f) -0.55V, (g, h) -0.50V. at -0.55V is composed of asparagus-like microrods with an inconsistency length in the range of 2 to 5µm.
Some microflowers are mixed in the asparagus-like structure. From the high-magnification image (Fig.2f) we can clearly confirm the structural characteristic that the upper of the asparagus-like microrods are cleft towards several directions, which perform a strong trend to form microflowers structures for further growth.
For a less negative potential (-0.50V), Figure 2g and h shows that as-prepared film is highly aligned flowerlike arrays structure with the length about 4-5µm m, few defects and straight. This upward growth structure will provide channels for the photo-induced carriers to transmit along the perpendicular orientation with high efficiency and minimum loss, combining with the higher surface-to-volume ratio, flower-like arrays structure should exhibits better optical and photoelectrochemical properties theoretically. 3.3. Photoelectrochemical measurements The UV-vis absorption spectra of the CdTe films are shown in Figure 3(a). It obviously shows that CdTe films with different microstructures have one strong absorption peak in the range of visible light region. With the deposited voltage decreasing, the visible absorption intensities increase gradually, making them potentially suitable absorption materials in solar cells. Due to the maximum specific surface area, the flowerlike arrays structure shows the relatively best performance of visible light absorption compared to the other specimens. Figure 3b displays the series of current density and voltage (J–V) curves of the as-synthesized CdTe films deposited at different potentials, which were measured by a conventional three-electrode photoelectrochemical system. The polysulfide (Sn2-) was the major active ingredient of the aqueous
Figure 3. UV-vis absorption spectrum (a) and J-V curves (b) of the as-synthesized CdTe films deposited at different potentials. 2
Isc (mA/cm )
Table 1. J–V data of the as-synthesized CdTe films. electrolyte. The key photovoltaic parameters which include open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF) and conversion efficiency (η) are given in Table 1. On decreasing the deposited voltage, the values of FF have a slight improvement from 0.466 to 0.555; Voc and Jsc present a significant 2
increasing from 0.468 to 0.543V and 2.138 to 4.102 mA/cm , respectively. The specimen with flower-like arrays structure deposited at -0.50V presents the best photovoltaic performance with the conversion efficiency 1.236%. All the performances of the fill factor and conversion efficiency are much better than that of the CdTe microstructure reported in the other literatures [1, 12-13]. These regular changes of the absorption intensities and photovoltaic parameters are caused by the interactions of crystallinity and increasingly complex morphology. The fine crystallinity of the CdTe film is benefit to diminish the internal defects and prolong the charge carrier lifetime. Meanwhile, as the deposited potential shifts to more positive, the more complex structures and higher specific surface areas amplify the effective area to contact the polysulphide electrolyte and chemical process take place on the CdTe microstructure surface, so that more light can be absorbed and photo-induced carriers are collect effectively. The excellent optical and electrical performances in visible region based on the higher specific surface areas make them a great potential application for hetero-microstructure substrate configuration solar energy conversion devices. On the basis of the research above, further experimental studies should be taken to investigate the morphology control and assemble complete substrate configuration modules for the conversion from solar energy to electricity with the same or better photoelectric conversion efficiency than those reported in previous literature (11.3%) , and even can be applied to commercial products. 4. CONCLUSION In this work, we had focused on the designing of CdTe films with novel morphologies by electrodeposition. The potentiostatic electrodeposition was conducted in the potential in the range from -0.65V to -0.50V. It was found that the deposited potential had dramatic effects on the CdTe morphologies such as rod-like, flake-like petals, asparagus-like and flower-like, which was conductive to enhance the specific surface area. At different potential, the synthesized CdTe films were cubic phase and showed an oriented growth along the  direction. In addition, UV-vis spectra and J-V curves verified that the deposited potential was the key parameter in determining the optical properties and photoelectric performances of the synthesized CdTe films.
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Highlights Various CdTe structures were prepared by electrodeposition at different deposition potential. Deposition potential in the range from -0.65V to -0.50V affected the CdTe morphologies obviously. Novel CdTe morphologies such as rod, flake-like petal, asparagus and flower were synthesized.