Article pubs.acs.org/JPCC
Influence of Morphology on the Optical Properties of Self-Grown Nanowire Arrays Liqiang Li,† Zhufeng Liu,† Ming Li,† Lan Hong,† Hui Shen,† Chaolun Liang,‡ Hong Huang,‡ Dan Jiang,‡ and Shan Ren*,† †
State Key Laboratory of Optoelectronic Materials and Technologies, The Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong Province, The Center for Nanotechnology Research, School of Physics Science and Engineering and ‡ Instrumental Analysis & Research Center, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, P. R. China S Supporting Information *
ABSTRACT: Nanostructured materials such as nanowire arrays often absorb light more strongly than thin films, so they can be used to improve the performance of solar cells. In this study, a series of self-grown Cu2S nanowire arrays with different diameters, lengths, and morphologies is prepared by solid− gas reaction between Cu foil and a mixture of H2S and O2. The structure of the arrays is characterized by XRD, TEM, XPS, and Raman. Their light absorption performance is investigated systematically by diffuse reflectance and photoluminescence spectroscopy. The nanowire arrays are single-crystal Cu2S semiconductors, and their band gap can be adjusted by changing the morphology, diameter, and length of nanowires in the arrays. The lightabsorption ability is enhanced from 70% for a planar Cu2S film to 93.5% for a Cu2S nanowire arrays and is less sensitive to both the wavelength and incident angle of light because of the morphology and distribution of the nanowires. The light absorption is high (about 92−95%) over a wide range of wavelengths (240−670 nm) and only decreases by 3% as the incident angle increases from 10 to 40°. This research shows the potential of Cu2S nanowire arrays for use in solar energy applications.
1. INTRODUCTION
Increasing light absorption is an important way to improve the performance and reduce the cost of photovoltaic cells. Diedenhofen’s group20 investigated the light absorption of semiconductor nanowire arrays with three different geometries and found that layers of conical nanowires absorbed the most light. In contrast, the absorption relative to the volume fraction of semiconductor was the highest for layers of cylindrical nanowire arrays. Broadband, omnidirectional enhanced light absorption was observed for base-tapered nanowire arrays, with absorption exceeding 97.5% in the wavelength range of 400− 850 nm for a combined system of nanowires and substrate. The absorption in the nanowire layer alone is in the order of 80%, which is increased by almost 22% compared with the corresponding film. This geometry can also reduce the amount of semiconductor material required in a device. Fan et al.21 investigated changes in the optical properties of highly ordered Ge nanopillar arrays upon changing their shape and geometry. Ge nanopillar arrays with a diameter of 130 nm absorbed ∼85% of the incident light with minimal wavelength dependence for λ = 300−900 nm. Ge nanopillars with minimal reflectance and large diameter base exhibited 95−100% absorption for λ = 300−900 nm, an increase of 18%. Most of the above
To improve the conversion efficiency of solar cells, various nanostructures such as nanofilms,1−4 nanoparticles,5 nanowire arrays,6−12 nanotubes,13,14 and nanorod arrays14 have been explored extensively in recent years. One-dimensional nanostructures have received considerable attention due to their distinctive structure and properties that include enhanced light trapping, reduced light reflection, improved band gap tuning, facile strain relaxation, and increased structure defect tolerance compared with those of bulk materials.15 One-dimensional nanostructures also use previously disregarded low-cost materials and processing options and can be applied in photoconductors16 and field-emission devices.8 Various 1D nanomaterials have been synthesized for use in solar cells. Yang’s group17 prepared dye-sensitized solar cells containing ZnO nanowires, which increased the efficiency of the cell at higher dye loadings because of their large surface area. Tsakalakos et al.18 fabricated Si nanowire-based solar cells on metal substrates that exhibited enhanced optical properties compared with those of planar thin-film devices. Tang and coworkers19 prepared photovoltaic cells containing CdS−Cu2S core−shell nanowires that showed excellent open circuit voltage, fill factor, and response to low-light levels. However, it is still difficult to produce large-area nanowire arrays that might further increase the photoelectric efficiency of solar cells. © 2013 American Chemical Society
Received: November 2, 2012 Revised: January 21, 2013 Published: January 30, 2013 4253
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Figure 1. SEM side-view images of Cu2S nanowire arrays and film. (S1) Cu2S film formed at 18 °C in 2 h and Cu2S nanowire arrays prepared at (S2) 30, (S3) 28, (S4) 25, (S5) 20, and (S6) 18 °C for 15 h. (The scale bar is 2 μm.).
2. EXPERIMENTAL DETAILS Copper foil (99.9% purity) was carefully polished with number 0−6 abrasive paper and then washed with deionized water. The copper foil was cleaned sequentially in an ultrasonic bath containing 3% sulfuric acid, then absolute ethanol, and finally deionized water three times. The solid−gas reaction was performed in an airtight ceramic pipe. Pure O2 with a flow of 800 sccm was introduced into the pipe for 5 min and then switched to the mixed gas of H2S (3000 sccm) and O2 (240 sccm) for ∼3 min. The pipe was sealed and kept at a fixed temperature of 18, 20, 25, 28, or 30 °C for 15 h. The area of each sample was about 15 × 25 mm. Scanning electron microscopy (SEM) was carried out using a scanning electron microscope (Quanta 400F, FEI, France), and transmission electron microscopy (TEM) was performed on an electron microscope (JEOL JEM-2010HR) equipped with an energy-dispersive X-ray spectrometer (EDS, Oxford, U.K.). Xray diffraction (XRD) was performed on a diffractometer (Rigaku D/maxIIIA) with Cu−Kα radiation (λ = 0.154056 nm). X-ray photoelectron spectroscopy (XPS) was performed on an X-ray photoelectron spectrometer (Escalab 250). Diffuse reflectance spectra (DRS) were measured with a UV−vis-NIR spectrophotometer (UV-3150). Photoluminescence (PL) spectra were measured using a combined fluorescence lifetime and steady-state spectrometer (FLS920). A Raman spectrum was measured on a micro-Raman spectrometer (Renishaw inVia) using an excitation wavelength of 514.5 nm from an Ar ion laser.
investigations are of nanowire arrays with very regular arrangement. The vapor−liquid−solid (VLS) method is commonly used to prepare nanowire arrays to investigate their light absorption ability or application in solar cells. For example, InP nanowires have been prepared by metal−organic vapor phase epitaxy,20 and Ge nanopillars have been prepared by a combination of the VLS method and electroplating.21 However, the VLS method usually requires a metal catalyst, such as Au nanoparticles, which can contaminate the nanowires and increase leakage currents in devices. The nanowire arrays prepared by these methods are highly ordered. Cu2S is a p-type semiconductor with a bulk band gap of 1.21 eV and was one of the first semiconductors used to make solar cells. Cu2S nanowire arrays can be converted into copper indium gallium selenide (CIGS) or copper zinc tin sulfide (CZTS) nanowire arrays by further processing by electrodeposition or dipping in solution containing In, Ga, and Se ions and subsequent annealing. This is because the single-crystal structure of Cu2S nanowire arrays benefits from being converted to the perfect crystallinity of CIGS. Cu2S nanowire arrays have been synthesized by many methods.9−11,22−27 Among them, the solid−gas reaction is a cost-effective, easy scaled-up method to prepare large-area Cu2S nanowire arrays on Cu foil or films.9,10,28 This approach does not use any catalysts or templates but simply uses mild reaction conditions and gives high yields. In the solid−gas reaction, nanowire arrays are self-grown from a Cu substrate or Cu film, and the strong bonding between nanowires and the substrate ensures excellent electrical contact between them. The Cu foil or film used as a precursor promotes the growth of nanowire arrays and can also be used as an electrode and heat sink when the nanowire arrays are used in solar cells. We previously found that self-grown Cu2S nanowire arrays enhanced light absorption29 but did not determine the full details of this relationship. We systematically control the morphology and distribution of Cu2S nanowire arrays to analyze their influence on light absorption.
3. RESULTS AND DISCUSSION The SEM images in Figure 1 show the structures of the Cu2S nanowire arrays. The morphology of each Cu2S nanowire array depends on the temperature used in the solid−gas reaction. The average diameter of each nanowire increases with temperature, whereas the average length decreases. For example, the nanowire arrays prepared at 18 °C possessed an 4254
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average diameter of 114 nm and an average length of 8 μm, whereas those prepared at 30 °C have an average diameter of 847 nm and an average length of 1.1 μm. The SEM images also show that the nanowires are more or less deviated from the normal direction of the substrate. We used the inclination angle, which is the angle between the nanowires and the normal direction of the substrate, to describe this deviation, as shown in Figure 1 (S1) and (S6), where the marked inclination angle is 10 and 40°, respectively. The relationships between the preparation conditions and the diameter, length, and spacing density of the Cu2S nanowire arrays are presented in Table 1. Table 1. Relationship between Preparation Conditions and the Diameter, Length, and Density of Cu2S Nanowire Arrays sample number
synthesis temperature (°C)
reaction time (h)
average diameter (nm)
average length (μm)
spacing density (p/μm2)
S1 S2 S3 S4 S5 S6
18 30 28 25 20 18
2 15 15 15 15 15
847 425 264 184 114
1.1 1.45 1.5 5 8
0.1 0.4 5 9 13
Figure 3. (a) TEM image of Cu2S nanowire and (b) HRTEM image of the region marked in panel a. The inset shows the SAED pattern for this region.
the nanowires is covered with a thin layer of CuxO (∼4 nm thick) to form a core/shell nanowire structure. The lattice spacing perpendicular to the radial direction of a nanowire obtained from a HRTEM image (Figure 3b) of the core of the nanowire is ∼0.67 nm and is 1.345 nm along the radial direction, corresponding to the interplanar distances of the (1̅ 0 2) and (100) crystal planes of monoclinic Cu 2 S, respectively. The lattice spacing of the shell part is ∼0.302 nm, corresponding to the interplanar distance of the (110) crystal plane of cubic Cu2O, but most of the Cu2O on the nanowire surface has low crystallinity. The selected-area electron diffraction (SAED) pattern of this region of the nanowire (inset in Figure 3b) confirms the presence of Cu2O and Cu2S. This finding is also consistent with a Raman spectrum (Figure 4) and O 1 s and Cu 2p XPS (Figure 5) of the Cu2S nanowire arrays.
The color of samples changed from black to gray as the nanowire diameter increased, and the Cu2S film was light gray, as shown in the photographs in Figure S3 in the Supporting Information. An XRD spectrum of the nanowire arrays is presented in Figure 2. The diffraction peaks can be indexed as monoclinic
Figure 2. XRD patterns of Cu2S nanowire arrays.
Cu2S (JCPDS no. 33-0490). The relative intensity of the (2̅04) diffraction peak of the nanowire arrays increases the most compared with the intensity ratio in the standard PDF card of Cu2S, demonstrating that the {1̅02} crystal planes of the Cu2S nanowire arrays preferentially align parallel to the surface of the substrate. EDS indicates that the nanowires are composed of Cu, S, and O. The atomic ratio of Cu to S is greater than 2 because of the additional copper signal coming from the Cu substrate (Figure S4 in the Supporting Information). TEM and HRTEM observations (Figure 3) indicate that the growth direction of Cu2S nanowire is [1̅02], consistent with XRD results. The images also clearly show that the surface of
Figure 4. Raman spectrum of Cu2S nanowire arrays prepared at 18 °C for 15 h (S6).
Raman spectrum and XPS also confirm that the shell of the nanowire contains both CuO and Cu2O. Raman peaks at 219, 298, 407, and 625 cm−1 are attributed to Cu2O and CuO,30−32 and a peak at 472 cm−1 corresponds to Cu2S in the core of the nanowire.28,33,34 Comparing Figure 5a,b reveals that the relative amounts of both CuxO and Cu2S increase as the Cu2S nanowire grows (Table S1 in the Supporting Information). PL spectra of the Cu2S nanowire arrays (Figure 6) show that their band gap varies between 1.251 and 1.272 eV. As the 4255
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Figure 5. XPS of Cu2S nanowire arrays. (a) O 1s XPS and Cu 2p XPS of Cu2S nanowire arrays prepared at 18 °C for 15 h (S6) and (b) O 1s XPS, Cu 2p XPS, and S 2p XPS of the Cu2S film grown at 18 °C for 2 h (S1).
reasons for this phenomenon: one is that the diameter of the nanowires influences both the band gap and the thickness of CuxO film, and the other one is that the thickness of CuxO film influences the band gap of the Cu2S nanowires. However, the detailed relationship between the thickness of CuxO film, the band gap, and the diameter of Cu2S is still remained investigation. The above results also indicate that the band gap of Cu2S nanowire arrays can be adjusted by changing the morphology and size of the nanowires. This is another advantage for Cu2S nanowire arrays in photovoltaic or heating harvesting applications. We systematically measured the light-absorption properties of Cu2S nanowire arrays. DRS of the nanowire arrays with different diameters and morphologies are presented in Figure 7a1. When the incident light is along the normal direction of the sample surface, all Cu2S nanowire arrays show good light absorption over a wide range of wavelengths from 240 to 800 nm, including visible, UV, and infrared regions. The nanowire arrays absorb light more strongly than the Cu2S thin film. The average light absorption increases by ∼34% as the nanowire diameter decreases, from 70% for the Cu2S film (S1 curve in Figure 7a1) to 93.5% for the Cu2S nanowires with a diameter of 114 nm (S6 curve in Figure 7a1), at light wavelength of 433 nm, the light absorption values even increases by 43%, and the enhanced light absorption abilities remain almost constant (about 92−95%) over a wide range of wavelengths from 240 to 670 nm, presenting a very strong enhanced light absorption. The light absorption properties of ordered nanowire arrays of different sizes and shapes have been investigated extensively, for example, Chen’s work on silicon nanowire arrays with diameters between 50 and 80 nm and lengths of 1.16−4.66 μm35 and Fan’s study of Ge dual-diameter nanopillars with tips of small diameter (D1 = 60 nm), base of large diameter (D2 = 130 nm), and length of 2 μm.21 In these reports, nanowire
Figure 6. PL spectra of the as-prepared Cu2S nanowire arrays. The inset shows the relationships of the diameter of the Cu2S nanowires versus band gap and fwhm of PL.
diameter of the nanowires increases from 114 to 847 nm, the band gap first decreases from 1.266 to 1.251 eV and then increases to 1.272 eV. In contrast, the full width at halfmaximum (fwhm) of the PL peaks reflects the amount of CuxO on the surface of the nanowires. The inset in Figure 6 shows that the fwhm first decreases from 0.14 to 0.026 eV when the nanowire diameter increased from 114 to 425 nm. It then increased to 0.151 eV when the diameter increased to 847 nm, indicating that the amount of CuxO on the surface of the nanowires first decreases and then increases as the nanowire diameter increases. The PL experiments show that the band gap and amount of CuxO change in the same manner with respect to nanowire diameter, first decreasing and then increasing as the nanowire diameter increases. There are two possible 4256
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Figure 7. (a1) DRS of Cu2S nanowires with different diameters. (a2) and (a3) show DRS with different light incidence angles for S2 and S6 Cu2S nanowire arrays, respectively. (b1) and (b2) show the relationships between the diameter, length and average absorbance of samples. (b3) Dependence of absorbance on material filling ratio, which is defined as the occupation of the cross section of a Cu2S nanowire array per 1 μm2 area.
arrays were very regular vertical arrays with more even size and spacing distribution than the nanoarrays produced here. The light absorption of silicon nanowire arrays is sensitive to wavelength: higher in the high-frequency regime and lower in
the low-frequency regime. In addition, the larger the diameter of the nanowires, the higher its absorption for wires of the same length. In the high-frequency regime, nanowires absorb more light than optically dense thin films. In the low-frequency 4257
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photon regime, films absorb more efficiently than nanowire arrays, which can be improved by increasing the length of the nanowire because of the small extinction coefficient of silicon. This means that the tip portion of the Ge dual-diameter nanopillars with small diameter has low reflectance, leading to efficient photon trapping and transmission down to the base. Meanwhile, the base layer with large diameter has a high material filling ratio and thus can efficiently absorb light; such arrays exhibit 95−100% absorption for λ = 300−900 nm. In our work, light absorption can reach 91−95% over the whole wavelength range from 240 to 800 nm for the nanowires of smaller diameter, even though our Cu2S nanowires have a slightly uneven distribution and much larger diameters than those discussed above. This may be related to their larger extinction coefficient36,37 and the distinct distribution and geometry of these nanowire arrays. Figure 7a2,a3 shows DRS measured at different light incident angles (0, 10, 20, and 40°) for Cu2S nanowire arrays with different morphologies and diameters over the wavelength range of 200−850 nm. The nanowires with bigger diameters (847 nm) are relatively evenly distributed (Figure 1 (S2)), and have small inclination angles (
