Journal of Materials Processing Technology 211 (2011) 382–387
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Growth of CdS thin films and nanowires for flexible photoelectrochemical cells Nurdan D. Sankir ∗ , Bahadir Dogan Nanotechnology and Membrane Science Laboratory, TOBB University of Economics and Technology, Ankara, Turkey
a r t i c l e
i n f o
Article history: Received 16 February 2010 Received in revised form 24 September 2010 Accepted 17 October 2010
Keywords: Cadmium sulfide Nanocrystal Thin film Nanowire dc-Electrochemical deposition Optical properties Photoelectrochemical cell
a b s t r a c t In this study, cadmium sulfide (CdS) nanocrystal thin films and nanowires have been deposited onto mechanically flexible substrates via dc-electrodeposition, which is a very suitable technique for large area manufacturing. For the first time with this study, flexible CdS nanocrystal thin films were integrated into photoelectrochemical (PEC) cells and their performances were compared with CdS nanowires. It has been demonstrated that PEC performance of both nanocrystal thin films and nanowires were a strong function of production conditions such as deposition time and voltage. The maximum power conversion efficiency of the CdS nanocrystal thin films obtained in this study was 0.3%. On the other hand, higher efficiencies (about 1.4%) were observed for the CdS nanowires. UV–vis analysis confirmed that both transmittance and band gap energies of the CdS nanowires were lower than that of CdS nanocrystal thin films. X-ray diffraction analysis revealed that both nanocrystal thin films and nanowires have a preferred orientation at 26◦ (2), which can be attributed to the CdS (0 0 2) structure. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Cadmium sulfide (CdS) is among the most promising candidates for photoelectrochemical (PEC) applications due to its proper band-edge positions for reduction/oxidation of water, high optical absorption and high electron affinity. There have been many CdS deposition techniques including vacuum evaporation (Grynko et al., 2009), RF sputtering (Akkad and Ashour, 2009), chemical vapor deposition (Jones et al., 2009) and molecular beam epitaxy (Ueta et al., 2000). All these methods require special tools, and therefore their manufacturing cost is high. Moreover, their areas are limited by the sample size. Conversely, solution based techniques provide large areas and low cost. Murali et al. (2007) has brush-deposited CdS thin films onto conducting glass and titanium substrates. They have demonstrated the effect of heat treatment on PEC performances and obtained an increase in conversion efficiency by increasing the annealing temperature. Although in that study CdS electrodes resulted in very good PEC performances, the high annealing temperatures (>450 ◦ C) limited the substrate type. Similarly, Jia et al. (2006) reported that the conversion efficiency of CdS thin films deposited onto glass substrates via the doctor blade technique depended strongly on annealing temperature. In this study,
∗ Corresponding author at: Nanotechnology and Membrane Science Laboratory, TOBB University of Economics and Technology, Sogutozu Cad. No. 43, Sogutozu, Ankara, Turkey. Tel.: +90 312 2924331; fax: +90 312 2924121. E-mail address:
[email protected] (N.D. Sankir). 0924-0136/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2010.10.010
we used a dc-electrodeposition technique to manufacture flexible CdS PEC cells. The major advantages of a dc-electrodeposition technique can be listed as follows: (i) large area manufacturing; (ii) compatibility with a variety of substrates including plastics; and (iii) very low manufacturing costs. According to our best knowledge, this is the first report on the PEC properties of nanostructured CdS thin films deposited on polyethylene terephthalate (PET). Morphological, structural and optical properties of nanostructred CdS samples have also been reported in this study. Additionally, it has been demonstrated here that the conversion efficiency of the CdS nanostructures grown by dc-electrodeposition was very high without any post-treatment. 2. Experimental method Flexible CdS thin films have been deposited onto indium tin oxide (ITO) coated PET samples, which were kindly donated by Sheldahl, USA. Samples were used after cleaning in 2-propanol. During the CdS thin film deposition, the ITO coated PET sample and a platinum counter electrode were immersed into a solution containing 0.055 M CdCl2 and 0.19 M elemental sulfur. The deposition voltage was kept constant at 3 V dc, while the temperature was varied from 90 to 110 ◦ C. During the deposition in order to prevent oxide formation, a constant nitrogen flow was maintained. After electrodeposition of the CdS, samples were washed with dimethyl sulfoxide to remove excess sulfur from the surface. CdS nanowires were deposited on aluminum coated anodized alumina membranes (AAM) using the same method. In this processes, the aluminum
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flex X-ray diffraction system equipped with Cu K␣ radiation has been used. The electrical response of the nanowires under illumination (35 mW/cm2 ) has been recorded using a Gamry G750 Potentiostat/Galvanostat/ZRA system. The electrolyte solution was prepared by using 0.1 M Na2 S, 0.1 M NaOH and 0.1 M S.
3. Results and discussion 3.1. CdS thin films
Fig. 1. Picture of CdS thin films deposited on PET.
coated AAM were immersed into a solution containing 0.55 M CdCl2 and 0.19 M elemental sulfur. In order to investigate the effects of process conditions on nanowire formation, deposition voltage, time and temperature were varied. For further characterizations, CdS nanowires have been transferred on commercial epoxy resin. During this process, first the gold-coated side of AAM was covered with a thin layer of epoxy. After curing the epoxy, AMM was dissolved in 1 M NaOH solution. Morphological characterization of the thin films has been performed using a QUANTA 400F Field Emission Scanning Electron Microscope. The absorbance spectra of the samples were measured by a Pharmicia LKB Ultraspec III UV–vis spectrophotometer over the wavelength range of 325–900 nm at room temperature. In order to confirm the crystal structure of the films, Rigaku Mini-
The CdS thin film deposited onto PET had good adherence and was a light yellowish color (Fig. 1). It is worth to mention here that the selected deposition technique is very suitable for large-area production. The average sample size produced in this study was 10 cm × 4 cm. Fig. 2(a)–(c) shows the SEM pictures of CdS thin films deposited for 20 min at various temperatures. As can be seen in Fig. 2, all CdS thin films were molecularly homogenous and pin-hole free. When the deposition temperature decreased from 110 to 90 ◦ C the films became smoother. The thickness of the CdS thin films was measured using a DekTak profilometer. It was observed that the average thickness increased from 300 to 500 nm by increasing the deposition temperature. This increase in average thickness due to the increase in deposition temperature may result in rougher surfaces. On the other hand, deposition time did not significantly affect the thickness and surface morphology of the CdS thin films. Energy dispersed X-ray (EDX) analysis was used to investigate the relationship between the process conditions and the Cd:S atomic ratio. It was observed that for all deposition temperatures Cd:S atomic ratios were very close to 1:1 (Fig. 2(d)–(f)). Fig. 3 shows the X-ray diffraction pattern of CdS thin films deposited onto ITO coated PET. As can be seen in this figure all CdS thin films have a large peak around 26◦ (2). This peak could be attributed to either PET or CdS. Although the crystal structure of PET changes depending on the process conditions, such as depo-
Fig. 2. SEM pictures and EDX analysis results of CdS thin films deposited onto ITO PET under 3 V bias for 20 min at (a and d) 110 ◦ C; (b and e) 100 ◦ C and (c and f) 90 ◦ C.
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N.D. Sankir, B. Dogan / Journal of Materials Processing Technology 211 (2011) 382–387 Table 1 Grain size and Eg values of the CdS thin films. Substrate
Deposition temperature (◦ C)
Glass
125 90
PET
100 110
Deposition time (min)
Grain size (nm)
Eg (eV)
10 20 60 20 30 20 10 5 20
35 44 45 6.4 6.6 6.5 6.3 6.3 6.7
2.68 2.60 2.53 2.75 2.69 2.72 2.77 2.78 2.66
minor peak around 55◦ (2) can be attributed to the CdS (1 0 0). The effect of deposition temperature on crystal structure has also been investigated. It has been observed that the intensity of the peaks increased very slightly by increasing the deposition time. Hence, one can say that the crystallinity of the CdS thin films increases very slightly with increasing deposition time. The grain size of the CdS thin films can be calculated using the Debye–Scherrer formula: d=
ˇ Cos
(1)
where d is the crystal size; is the X-ray wavelength used; ˇ is the angular line width of half maximum intensity; and is the Bragg’s angle. The grain size of CdS thin films deposited on glass substrates varied between 35 and 45 nm depending on the deposition time. On the other hand, the average grain size of the CdS thin films deposited on PET was about 6.5 nm. Therefore, regardless of the substrate material, it is possible to conclude that the CdS thin films produced in this study were nanocrystalline (Table 1). The band gap energy (Eg ) calculations of the CdS thin films will be discussed in Section 3.3. However, it is worth to mention here that the Eg of the CdS thin films deposited on both glass and PET decreased with increasing the grain size and deposition time. This decrease in Eg could be attributed to the grain size dependent grain boundary barrier height. Previously, Tyagi and Vedeshwar (2001) reported a similar behavior for the cadmium iodide thin films. 3.2. CdS nanowires
Fig. 3. X-ray diffraction patterns of CdS thin films deposited onto ITO coated PET substrate at 100 ◦ C for (a) 10 min, (b) 20 min, and (c) 30 min (inset figure: CdS deposited onto ITO coated glass).
sition and annealing temperature, it has been known that PET has characteristic XRD peaks between 20◦ and 30◦ (2). Tsai et al. (2009) reported that PET films have a very broad XRD peak around 20◦ (2) for low annealing temperatures (∼100 ◦ C). They also showed that when the annealing temperature increased three sharp absorption peaks were observed around 15, 23 and 26◦ (2). It is also well known that CdS thin films have two crystalline modifications: the hexagonal (wurtzite) phase and the cubic (zincblende) phase. Both phases have peaks around 26◦ (2) (Derin and Kantarlı, 2009; Natalia et al., 2009). Therefore, it is very difficult to differentiate the CdS peak from the XRD pattern of CdS thin films deposited onto ITO coated PET. Therefore, we have grown CdS thin films on an ITO coated glass substrate under the same experimental conditions to eliminate the PET peaks. In this case, as can be seen in the inset of Fig. 4(c), there is no peak around 26◦ (2) for the substrate material. As a result, it is possible to conclude that the major XRD peak at 26◦ (2) belongs to the CdS (0 0 2). Also the
As explained in our previous study, deposition conditions such as temperature, time and voltage affect the CdS nanowire formation (Sankir and Dogan, 2009). Typically, in an electrochemical nanowire formation charged reactive species in the solution, which are Cd+2 and S0 in our case, diffuse through the pores of the template and reach the electrode surface by means of an applied electric field. Then nucleation and growth process of nanowire occurs in the pores of the template. Although the pores of templates determine the shape of the nanowire, the length and the diameter can be controlled by the synthesis conditions. We reported that the optimum deposition temperature of the CdS nanowires varied between 100 and 125 ◦ C. Below this temperature no CdS nanowire formation was observed, most probably due to the poor solubility of the elemental sulfur. Above 125 ◦ C, the nanowire formation again failed because of the electrical contact loss. SEM studies revealed that the morphology of the CdS nanowires did not change significantly for the deposition temperatures ranging between 100 and 125 ◦ C (Fig. 4). However, there has been a dramatic change in morphology when the deposition time increased. Fig. 5(a) shows the SEM images of CdS nanowires deposited at 125 ◦ C under 30 V for 10 min. The average diameter and the length of the CdS nanowires deposited at 125 ◦ C under 30 V for 10 min were around 100 and 500 nm, respectively. On the other hand, when the deposition
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Fig. 4. SEM images of CdS nanowires deposited under 30 V for 30 min at (a) 125 ◦ C and (b) 100 ◦ C.
Fig. 5. SEM images of CdS nanowires deposited at 125 ◦ C (a) under 30 V for 10 min and (b) under 20 V for 30 min.
time was increased to 30 min, the diameter and the length of the nanowires reached to 200 nm and 3 m, respectively (Fig. 4(a)). Another factor effecting the CdS nanowires formation was the deposition voltage. As can be seen in Fig. 5(b), when the deposition voltage decreased to 20 V, both the length and the diameter of the nanowires decreased significantly. For lower deposition potentials no CdS nanowire formation were observed.
Fig. 6 shows the XRD patterns of the CdS nanowires. Similar to the CdS thin films, peaks around 26◦ (2) were observed. This result is consistent with previously reported studies. Yang et al. (2002) has reported that CdS nanowires grown in AAM had XRD peaks around 25, 27 and 29◦ (2) and indexed these peaks to hexagonal CdS. Similarly Mo et al. (2008) showed that CdS nanowires grown in ion-track templates had a preferred orientation along the (0 0 2) plane. Therefore, the XRD peaks around 26◦ and 38◦ (2) obtained in this study can be attributed to (0 0 2) and (1 0 2) CdS. Obviously, other minor peaks in XRD spectrum resulted from the gold-coated AAM template. Additionally, Fig. 6 indicates that XRD patterns of nanowires synthesized at 30 V is sharper than that at 20 V. This shows the increase in crystal structure with increasing voltage. Moreover, prolonged deposition time also resulted in sharper XRD patterns. 3.3. Optical and photoelectrochemical properties of the CdS nanocrystal thin films and nanowires
Fig. 6. X-ray diffraction pattern of CdS nanowires deposited at 125 ◦ C.
Fig. 7 shows that all CdS nanocrystal thin films had very high optical transmittance in the wavelength region greater than 500 nm. Also for the same wavelength region, an oscillatory behavior has been obtained due to the interference fringes produced by the reflected light waves from the interfacial region of thin film. The maximum transmittance of the nanocrystal thin films was about 80%. On the other hand, the transmittance of the CdS nanowires decreased by up to 30%. Although the particle size was very small for
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N.D. Sankir, B. Dogan / Journal of Materials Processing Technology 211 (2011) 382–387 Table 2 Eg values of the CdS nanowires. Deposition voltage (V)
20
30
Fig. 7. UV–vis spectra of CdS nanowires (Nw) and nanocrystal thin films (Nc).
the nanowires, the number of nanowires per unit area was very high (number density ≈108 nanowires/cm2 ). Most probably because of this high density the transmittance of the nanowires was lower than that for the thin films. Additionally, UV–vis spectroscopy of the CdS thin films and nanowires were different in terms of their band edge of the absorption bands. As can be seen in Fig. 8, a blue shift has been observed for both the CdS thin films and nanowires compared to the absorption band edge of the bulk CdS crystal (512 nm) (King, 1994). The band edge of the absorption band of the CdS nanowires was around 479 nm, which indicated a blue shift of 33 nm with respect to the value of the bulk crystal. Interestingly, a more pronounced blue shift of about 77 nm has been determined for the thin film samples. Generally, the reason for the blue shift of optical absorption spectra of CdS samples is quantum confinement effects. Aforementioned, the grain size of the CdS thin films on PET was around 7 nm. On the other hand, the average diameter of the CdS nanowires changed between 100 and 200 nm depending on the process conditions. Most probably the very small grain size of the CdS thin films resulted in more blue shift in UV–vis spectrum. The optical band gap energy (Eg ) was calculated from the following variation of the absorption coefficient (˛) with photon energy: (˛h) = A(h − Eg )
n
(2)
where A is a constant, and n is the power exponent, which takes a value of 1/2 for direct allowed transition and 2 for indirect allowed transition. It has been observed that the UV–vis data of both the nanocrystal and nanowire samples fitted very well to n = 1/2. This indicated that the optical transition in CdS nanocrystal thin films and nanowires were direct allowed transition. Table 1 summarizes the band gap energies of CdS thin films. As can be seen in this table,
Fig. 8. J–V plot of CdS nanowires (Nw) and nanocrystal thin films (Nc).
Deposition time (min)
Eg (eV)
10 20 30 10 20 30
2.51 2.48 2.48 2.42 2.40 2.39
Eg of the nanocrystal thin films depends on the deposition conditions. Eg values decreased with both increasing deposition time and temperature. This result is consistent with the XRD data where the crystal structure of the CdS nanocrystal thin films improved with increasing deposition time and temperature. Also, Eg for the CdS thin films deposited on glass substrates were lower than Eg for thin films deposited on PET. This could be attributed to the high deposition temperature used to deposit CdS on glass. On the other hand, Eg of the CdS nanowires were smaller when compared to that for the nanocrystal thin film samples (Table 2). Unlike the nanocrystal samples, Eg of the nanowires were not significantly affected by deposition time and temperature. However, when the deposition voltage increased from 20 to 30 V, Eg decreased from 2.51 to 2.42 eV. In this study, the PEC performances of the CdS nanocrystal thin films and nanowires have been compared. Two parameters have been used to determine the PEC performances. The first parameter is the Fill Factor (FF), which is equal to the ratio of theoretical power to the actual power of the cell. FF can be calculated from the following well-known equation: FF(%) =
Imax Vmax × 100 Isc Voc
(3)
where, Imax and Vmax are the maximum current and voltage respectively, Isc is the short circuit current, and Voc is the open circuit potential. The second parameter is the power conversion efficiency (), which is a measure of how much of the solar energy is converted to electrical energy and can be calculated from: (%) =
Imax Vmax × 100 Pi A
(4)
where, Pi is the power of incident light and A is the area of the electrode. Fig. 8 shows the current density (J) versus potential (V) graph. Here it is worth to mention that the active sample area we used was 1 cm2 for both the nanocrystal thin films and nanowires. Hence Jsc and Isc take the same values. As can be seen in Fig. 8, Voc of the all nanocrystal thin films were comparable and about 500 mV. However, the Isc values varied and increased dramatically for the 30 min deposition time and reached to 1.0 mA. This indicates the high quality film formation for 30 min deposition. A similar effect of the deposition time on PEC performance of electrochemically deposited CdTe thin films has been reported by Kokate et al. (2007). Table 3 summarizes the PEC performances of both CdS nanocrystal thin films and nanowires. The maximum for CdS thin films obtained in this study was 0.29%. There has been very limited information about the PEC performances of CdS nanowires. Among these studies, core–shell heterojunctions of CdS with oxide semiconductors have been investigated. Lee et al. (2009) deposited CdS layers on TiO2 nanowires by chemical vapor transport technique. After CdS deposition power conversion of TiO2 nanowire arrays improved approximately 7 times. Tak et al. (2009) reported ZnO/CdS core–shell heterojunctions prepared by a two-step chemical solution method. Dongre et al. (2009) compared the PEC performances of CdS nanoparticles and nanowires. They observed that the junction ideality factor of nanowire photoelectrodes is higher than that of nanoparticles. All these studies showed that nanowire photoelectrodes made it pos-
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Table 3 Photoelectrochemical parameters of CdS nanocrystal thin films and nanowires. Material
Nanocrystal
Nanowire
Deposition time (min)
Deposition voltage (V)
Jsc (mA/cm2 )
Voc (mV)
FF (%)
(%)
10 20 25 30 35 10 30 10 30
3 3 3 3 3 30 30 20 20
0.19 0.06 0.09 0.98 0.16 0.65 0.89 0.08 0.08
452 500 514 480 476 1590 402 418 562
22 24 23 22 26 46 22 16 23
0.05 0.03 0.02 0.29 0.06 1.36 0.22 0.02 0.03
sible to transport the electrons directly to the conducting substrate without going through the multiple steps of the tunneling process, which improves the overall efficiency of the solar cell. In this study, it has been observed that the efficiencies of CdS nanowires deposited at 20 V were comparable with the efficiencies of the CdS thin films. On the other hand, Isc of the CdS nanowires deposited under 30 V increased significantly. Among the nanowire samples, the highest Isc , about 0.9 mA, has been observed for 30 V and 30 min deposition. Conversely, nanowires deposited at 30 V for 10 min had a very high Voc , which is 1590 mV. Therefore, the average (1.36%) has been obtained for these samples. This value is one of the highest efficiencies reported for the CdS nanostructures. 4. Conclusions In this study, CdS nanocrystal thin films and nanowires have been electrochemically synthesized. It has been observed that the length and the diameter of the CdS nanowires depend on the deposition time and voltage. XRD analysis showed that both CdS nanocrystal thin films and nanowires have a major peak around 26◦ (2). UV–vis analysis showed that Eg of CdS nanocrystal thin films varied between 2.53 and 2.78 eV. Conversely, Eg ’s of the nanowires were lower than that of the nanocrystal thin films. The average Eg of CdS nanowires was about 2.44 eV. The photoelectrochemical performances of both nanocrystal thin films and nanowires have been investigated at room temperature and under 35 mW/cm2 illumination. As a general trend the PEC performance of the CdS nanowires was better than the performance of the CdS nanocrystal thin films. The maximum of the CdS nanowires obtained in this study was 1.36%. Acknowledgement This study was supported by The Scientific and Technological Research Council of Turkey under the research grant TBAG107T354.
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