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Investigation of solution pH effect on electrochemical, microstructural, optical and photoelectrochemical properties of CdSe thin films
Solid State Sciences 15 (2013) 142e151
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Investigation of solution pH effect on electrochemical, microstructural, optical and photoelectrochemical properties of CdSe thin films Sethuramachandran Thanikaikarasan a, *, Chinnapyan Vedhi b, Xavier Sahaya Shajan a, Thaiyan Mahalingam c a b c
Centre for Scientific and Applied Research, School of Basic Engineering and Sciences, PSN College of Engineering and Technology, Tirunelveli 627 152, Tamil Nadu, India Department of Chemistry, VOC College, Tuticorin 628 008, Tamil Nadu, India Department of Physics, School of Science and Humanities, Karunya University, Coimbatore 641 114, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 November 2011 Received in revised form 3 April 2012 Accepted 2 September 2012 Available online 10 September 2012
Thin films of Cadmium Selenide have been prepared on indium doped tin oxide coated conducting glass substrates using cathodic electrodeposition technique. The deposition mechanism has been analyzed using linear sweep voltammetry. X-ray diffraction analysis revealed that the deposited films have hexagonal structure with most prominent reflection along (002) plane. X-ray line profile analysis technique has been used to evaluate the microstructural parameters. Surface morphology and film composition have been analyzed using scanning electron microscopy and energy dispersive analysis by X-rays. Optical absorption analysis has been carried out to find out the parameters such as band gap, refractive index, extinction coefficient, real and imaginary dielectric constants. Photoelectrochemical solar cell analysis has been showed that the films obtained at solution pH value around 2.5 has better cell parameters as compared to the films obtained at other solution pH values. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Thin films Texture coefficient RMS Microstrain Stacking fault probability
1. Introduction IIeVI group binary semiconductors are considered important technological materials due to its potential applications in solar cells, semiconducting, photovoltaic and optoelectronic devices [1e3]. Among them, Cadmium Selenide (CdSe) is found to be an excellent material with a band gap value around 1.7 eV which make them fairly interesting for the fabrication of solar cells through photoelectrochemical route [1,4,5]. Thin films of CdSe are normally crystallized in hexagonal structure (JCPDS-ICDD 2003, PDF 080459) with lattice constants (a ¼ 4.299 A; c ¼ 7.010 A) and in cubic structure (JCPDS-ICDD 2003, PDF 19-0191) with lattice constant (a ¼ 6.077 A). There are number of methodologies have been reported earlier for the preparation CdSe thin films such as sputtering [6], vacuum evaporation [7], thermal evaporation [8], pulsed laser deposition [9], chemical bath deposition [10]. Among the deposition techniques mentioned above, electrodeposition appears to be attractive due to its low cost of synthesis, low temperature growth and its possibility to control film thickness and morphology by readily adjusting the electrical parameters as well as composition of the electrolytic solution [1,11,12]. One obvious
* Corresponding authors. Tel.: þ91 4634 279067; fax: þ91 4634 279681. E-mail address: [email protected] (S. Thanikaikarasan). 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2012.09.001
requirement is that the substrate must be conductive. This is not a problem for thin film solar cell, since they are deposited on conducting ITO substrate [12]. Preparation of CdSe thin films on stainless steel, FTO coated glass substrates with minor variation in solution pH value and its effect on electrochemical, structural, morphological, compositional, optical and photoelectrochemical properties have been investigated by Pawar et al. [13]. Gudage and Rampal Sharma [14] have obtained (111) plane oriented cubic CdSe thin films on FTO coated conducting glass substrates by electrochemical deposition technique and investigated the effect of solution pH on electrochemical, structural, compositional, optical and photoelectrochemical properties. Both of them, have analyzed the film properties with minor variation in solution pH values. To the best of our knowledge, there is no such detailed investigation is available regarding the properties of hexagonal CdSe thin films with larger variation in solution pH values. In the present investigation, thin films of CdSe have been prepared on indium doped tin oxide coated conducting glass (ITO) substrates at various solution pH values. The mechanism of formation of CdSe has been analyzed using linear sweep voltammetry. The deposited films have been subjected to X-ray diffraction, scanning electron microscopy, atomic force microscopy, energy dispersive analysis by X-rays, optical and photoelectrochemical techniques, respectively. The effect of solution pH on electrochemical, microstructural, morphological, compositional, optical and photo
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electrochemical properties of the deposited films are studied and the results are reported. 2. Experimental details All the chemicals used in the present work were of Analar grade reagents. Thin films of CdSe were deposited on ITO substrates from an aqueous acidic bath containing 0.25 M CdSO4, 0.0025 M SeO2. The first working solution of CdSO4 was obtained by dissolving 48.095 g CdSO4 in 250 cc deionized water. The second working solution of SeO2 was obtained by dissolving 0.0694 g SeO2 in 250 cc deionized water. Each 20 cc of the two solutions forms the reaction mixture and this mixture was used as an electrolytic bath for all depositions. The electrochemical experiments were carried out using PAR scanning potentiostat/galvanostat unit (EG & G, Model 362, Princeton Applied Research, USA) employing three electrode configuration with ITO substrate, Platinum electrode and Saturated Calomel Electrode (SCE) as working, counter and reference electrode, respectively. The electrolytic processes were monitored by linear sweep voltammetry and governed potentiostatically. The deposition potential was fixed as 700 mV versus SCE using linear sweep voltammetry. The bath temperature and deposition time were fixed as 70 C and 40 min, respectively. The films were deposited at various solution pH values in the range between 1.5 and 3.0 0.1. Initially, the pH value of the electrolytic bath was 2.5 0.1. By adding an adjustable amount of dilute H2SO4 the pH value was adjusted to 2.0 and 1.5 0.1, respectively. If the pH value is kept below 2.0 0.1, there is rapid growth of film followed by its peeling out from the substrate. The pH value was increased to 3.0 0.1 by adding an adjustable amount of NaOH to the electrolytic bath. At higher pH value such as above 2.5 0.1, precipitation of electrolytic bath occurs which in turn yields poor quality films. Hence, the solution pH value was fixed as 2.5 0.1 to obtain films with better quality. Linear sweep voltammetry was carried out using BAS 200A electrochemical analyzer. Thickness of the deposited films was measured using stylus profilometer (Mitutoyo SJ 301, Japan). X-ray diffraction data of the deposited films was analyzed using an X-ray diffractometer (XPERT PRO PANalytical, Netherland) with CuKa radiation with wavelength (l ¼ 1.5418 A). The parameters such as crystallite size, rms microstrain, dislocation density and stacking fault probability were evaluated from X-ray diffraction data using X-line profile analysis. Surface morphology of the deposited films was analyzed using scanning electron microscope (Philips Model XL30) and atomic force microscope (PSIA XE100). Film composition composition was analyzed using an energy dispersive X-ray analysis set up attached with scanning electron microscope. Optical parameters such as band gap, refractive index, extinction coefficient, real and imaginary dielectric constants were evaluated from optical absorption, transmission and reflection data which is obtained using an UVeViseNIR spectrophotometer (HR-2000, M/S Ocean Optics, USA). Photoelectrochemical solar cells were constructed using CdSe thin film as photocathode, platinum electrode as anode and SCE as reference electrode, respectively. 3. Results and discussion 3.1. Linear sweep voltammetric studies Electrochemical growth of CdSe thin films on ITO substrates has been carried out potentiostatically from an aqueous electrolytic bath containing CdSO4 and SeO2. The electrochemical equations involved in the co-deposition of CdSe are described as follows [Eq. (1)] [14]
143
0 3CdSO4 8H2 O/3Cd2þ þ SO2 þ 8H2 O ECd ¼ 240 mV 4 3
# " a2þ RT Cd þ þ ln aCd 2F
(1)
ECd ¼
0 ECd
(2)
The formation of selenous acid occurs which may be due to the following Eq. (3)
H2 O þ SeO2 /H2 SeO3
(3)
Reduction of H2SeO3 to Se occurs which may be due to the following Eq. (4)
H2 SeO3 þ 4Hþ þ 4e /Se þ 3H2 O 0 ESe ¼ ESe þ
0 ESe ¼ 740 mV
4 aSeO2 RT RT þ ln þ ln aþ þ H 2F 4F aSe
(4) (5)
Reaction of formation of CdSe on ITO substrate may be due to the following Eq. (6)
Cd þ Se/CdSe
DG0 ¼ 100 kJ mole
(6)
Here ECd and ESe are the equilibrium electrode potential of cadmium and selenium, R is universal gas constant and T is absolute and aSeO2 are the activities temperature of the electrolytic bath. a2þ Cd of Cd and Se ions present in the electrolytic bath, while aCd and aSe are the activities of respective atoms present in the deposited films. The deposit is formed by the solid state reaction of plated Cd and Se atoms rather than the precipitation of H2Se and Cd. Hence, the reaction of formation of CdSe is described by Eq. (6) with free energy formation DG0 ¼ 100 kJ mol [15]. Linear sweep voltammetric studies have been carried out in a standard three compartment cell comprising of ITO substrate as cathode, platinum electrode as anode and saturated calomel electrode as reference electrode, respectively. The electrolytic bath consists of 0.25 M CdSO4 and 0.0025 M SeO2. The pH value is found to be vary in the range between 1.5 and 3.0 0.1 which may be due to the addition of H2SO4 and NaOH in the electrolytic bath. Linear sweep voltammetric curves recorded for ITO glass electrode in an aqueous electrolytic bath maintained at various solution pH values is shown in Fig. 1. The pH value is maintained at 1.5, 2.0 0.1, there is evolution of hydrogen gas in the electrolytic bath. This process leads to development of poor quality films with irregular surface and discontinuously distributed grains which reduces the Faradaic formation of CdSe thin films [Fig. 1(a,b)]. The pH value is increased to 2.5 0.1, cathodic current increases sharply and two reduction waves obtained at 715 and 810 mV versus SCE thus leads to formation of CdSe on ITO substrate (Fig. 1c) according to Eq. (6). It is observed from Fig. 1d, that two reduction waves obtained at 760 and 850 mV versus SCE leads to deposition of individual metallic selenium on ITO substrate according to Eq. (4) and white precipitation of cadmium ion source occurs which is observed at the bottom of the flask. Hence, the deposition potential is fixed as 700 mV versus SCE to prepare CdSe thin films with well-defined crystallinity and stoichiometry which is confirmed using X-ray diffraction and EDX analyses. 3.2. Growth kinetics Thickness is one of the important film parameters which depend upon the pH value, concentration of the electrolytic bath, bath temperature and deposition time [14]. Growth of CdSe thin
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2.0 0.1, the electrolytic bath containing more Hþ ions thus leads to give higher conductivity to the electrolytic bath. During deposition hydrogen evolution takes place thus leads to give films with irregular surface and lower thickness which is shown in Fig. 2(a,b). If the solution pH value is increased to 2.5 0.1, the solution becomes transparent and high conductivity. This higher conductivity increases the deposition current passing through the electrolytic bath thus enhances the overall rate of deposition which in turn increases thickness of the film prepared at solution pH value 2.5 0.1. Further increasing the solution pH value above 2.5 0.1, the electrolytic bath becomes slightly turbid and milky due to the formation of Cd(OH)2 ions [14]. Therefore, there is formation of porous, foggy and less adherent films which results lower value of film thickness as shown in Fig. 2d. Hence, an optimum solution pH value of 2.5 0.1 may be selected to get better quality films with higher thickness value around 1480 nm. 3.3. Structural studies
Fig. 1. Linear sweep voltammogram of CdSe thin films obtained at different solution pH values: (a) 1.5 (b) 2.0 (c) 2.5 (d) 3.0 0.1.
films on ITO substrates has been carried out at various solution pH values with different intervals of time. The reduction potential maintained between the electrodes and bath temperature are fixed as 700 mV versus SCE and 80 C, respectively. Fig. 2 shows the variation of film thickness with deposition time for CdSe thin films prepared at various solution pH values in the range between 1.5 and 3.0 0.1. It is found that the film thickness build up slowly and increases linearly at the beginning stage, thereafter it tend to attain its saturated value (640e1480 nm) after 30 min for films prepared at all solution pH values. If the deposition time is increased above 40 min thickness of the deposited films remains constant upto 60 min, thereafter the film thickness is found to decrease slightly which is not shown in Fig. 2. The decrease in value of film thickness after 60 min may be attributed to higher rate of dissolution than the rate of deposition of CdSe thin films [15,16]. The films deposited at lower pH value such as 1.5 and
Fig. 2. Variation of film thickness with deposition time for CdSe thin films electrodeposited at various solution pH values: (a) 1.5 (b) 2.0 (c) 2.5 (d) 3.0 0.1.
The pH value of an electrolytic bath play an important key role in determine the crystal structure and stoichiometry of the deposited films. X-ray diffraction pattern recorded for CdSe thin films prepared at (potential: 700 mV versus SCE; bath temperature: 80 C) various solution pH values in the range between 1.5 and 3.0 0.1 is shown in Fig. 3. XRD pattern showed that the deposited films are found to exhibit hexagonal structure with lattice constants (a ¼ 4.299 A; c ¼ 7.010 A). The observed diffraction peaks of hexagonal CdSe are found at 2q values of angles 23.88, 25.87, 27.10, 35.08, 42.07, 45.73, 48.83, 50.71, 55.83, 63.92, 66.25 and 76.78 corresponding to the lattice planes (100), (002), (101), (102), (110), (103), (200), (201), (202), (203),(210) and (310), respectively. The different peaks in the diffractogram are indexed and the corresponding values of interplanar spacing ‘d’ are calculated and compared with standard JCPDS-ICDD file for hexagonal CdSe [17]. It is observed from the diffractogram that all the identified peaks are from CdSe and hence no additional peaks corresponding to Cd and Se are present. Electrochemical growth of CdSe thin films are carried out at various solution pH values in the range between 1.5 and 3.0 0.1. At lower pH value such as below 1.5 0.1, the
Fig. 3. X-ray diffraction pattern of CdSe thin films electrodeposited at various solution pH values: (a) 1.5 (b) 2.0 (c) 2.5 (d) 3.0 0.1.
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formation of film may be hindered due to the process of hydrogen evolution reaction (HER). This process leads to give films with poor crystallinity. At higher pH value such as above 3.0 0.1, precipitation of electrolytic bath occurs which in turn yields films with lower crystallinity. For detailed investigation of solution pH effect on structural properties, the value of solution pH is fixed in the range between 1.5 and 3.0 0.1. It is observed from Fig. 3a, that the films prepared at solution pH value around 1.5 0.1 are found to exhibit diffraction peaks of hexagonal CdSe with most prominent reflection along (100) plane. If the pH value is increased to 2.0 0.1, there is slight decrement in peak height of (100) plane and slight increment in peak height of (002) plane is observed which is shown in Fig. 3(a,b). The films prepared at solution pH value around 2.5 0.1 are found to exhibit well defined crystallites with most prominent reflection along (002) plane which is indicated in Fig. 3c. Further increasing the solution pH value above 2.5 0.1, the intensity of all the observed peaks are found to decrease which is shown in Fig. 3d. It is also observed that there is no change in crystal structure, but there is change in preferential orientation from (100) to (002) plane is observed for films prepared at various solution pH values in the range between 1.5 and 3.0 0.1. Similar trend is observed for CdCr2S4 thin films on ITO substrates has been reported earlier [18]. The value of lattice constants (‘a’ and ‘c’) of the deposited films are calculated using the following Eq. (7) [12].
2 1 4 h2 þ hk þ k2 l þ 2 ¼ 3 a2 c d2hkl
(7)
Fig. 4(a,b) shows the variation of lattice constants (‘a’ and ‘c’) with solution pH value for CdSe thin films obtained at different solution pH values. It is observed that the value of ‘a’ and ‘c’ are found to be 0.04% different from ‘a’ and ‘c’ values which is given in JCPDS-ICDD file for hexagonal CdSe [17]. It is also observed that the value of a, c is found to decrease while increasing the solution pH value from 1.5 to 2.5 0.1, thereafter there is slight increment is observed which is indicated in Fig. 4a. Increase in value of lattice constants while increasing the solution pH value above 2.5 0.1, may be due to shift of diffraction peak to larger angles [19]. Also, the variation of ‘c/a’ ratio with solution pH value for CdSe thin films obtained at various solution pH values is shown in Fig. 4b. NelsoneRiley plot is drawn for CdSe thin films obtained at various solution pH values using the expression which is given in Eq. (8) [20]. The effect of orientation of polycrystalline CdSe thin films on deposition condition such as solution pH value, potential, electrolyte concentration are determined by evaluating the texture coefficient of the (hkl) plane using the expression which is given in Eq. (9) [1,21]:
Dd d
¼
1 cos2 q cos q þ q 2 sin q
IðhklÞ=I0 ðhklÞ # Tc ðhklÞ ¼ " P 1 IðhklÞ=I0 ðhklÞ N N
(8)
(9)
where Tc(hkl) is the texture coefficient of the (hkl) plane, I(hkl) is the measured intensity and I0(hkl) is the JCPDS-ICDD standard intensity. NelsoneRiley plot drawn for CdSe thin films obtained at various solution pH values is shown in Fig. 5a. Variation of texture coefficient with solution pH value for CdSe thin films obtained at various solution pH values is shown in Fig. 5b. The diffraction peak corresponding to (002) plane is used for the calculation of crystallite size of the deposited films. It is observed from Fig. 5b that the value of texture coefficient for (002) plane is found to increase
Fig. 4. a. Variation of lattice constants with solution pH value for CdSe thin films electrodeposited at various solution pH values. b. Variation of c/a ratio with solution pH value for CdSe thin films electrodeposited at various solution pH values.
while increasing the solution pH value from 1.5 to 2.5 0.1, thereafter it is found to decrease slightly. It is also observed that the texture coefficient value of (100) plane is found to decrease while increasing the solution pH value from 1.5 to 3.0 0.1. The films prepared at solution pH value around 2.5 0.1 have higher texture coefficient value indicating that the deposited films have better crystallinity. The films deposited at solution pH value around 2.5 0.1, have better crystallinity and well adherent to the substrates. The average crystallite size of the deposited films is calculated using FWHM data and Debye-Scherrer formula [1,12,21]:
P ¼
0:9l bcos q
(10)
where b is full width at half maximum of the peak position in A), q is Braggs radian, l is the wavelength of CuKa target (l ¼ 1.540 diffraction angle at peak position in degrees. The crystallite size of the deposited film is found to be in the range between 19 and 47 nm (Fig. 6a). 3.4. Microstructure and surface morphology Line profiles in observed XRD patterns are generally due to convolution of various factors such as crystallite size, rms microstrain and stacking faults. The line profiles are subjected to variance
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Fig. 5. a. NelsoneRiley plot for CdSe thin films electrodeposited at solution pH value 2.5 0.1. b. Variation of texture coefficient along (100), (110), (002) and (200) planes for CdSe thin films electrodeposited at various solution pH values.
analysis in order to calculate the value of crystallite size and rms microstrain [21,22]. An aggregate of distorted crystallites as a measure of crystallite size and strain could affect the variance of X-ray diffraction line profiles. Since, this method is sensitive to variation near the tails of the peak, a careful adjustment of the background has been carried out following the method given by Mitra and Misra [23]. For instrumental broadening the line profiles has been corrected by substracting the variance of the corresponding profiles of well-annealed CdSe sample because of the additive effect of the variance. Assuming the broadening of diffracted line is due to crystallite size and strain, the variance can be written as
W2q ¼
ls
2p2 Pcos q
þ 4tan2 q3e2 _1=2
Fig. 6. a. Variation of crystallite size and rms microstrain with solution pH value for CdSe thin films electrodeposited at various solution pH values. b. Variation of dislocation density and stacking fault probability with solution pH value for CdSe thin films electrodeposited at various solution pH values.
varying inversely as a square of the distance from the mean, the variance will be the linear function of total range s and can be written as W ¼ K s þ C, where K and C are constants which depend upon the physical conditions of the sample and geometrical factors. Strain is defined as the restoring force acts on the surface of the film to restrict the formation of crystallites on its surface [24]. Dislocation density is defined as the number of dislocation lines per unit volume of the crystal [1,11,12]. Dislocation density is calculated using the following Eq. (12) which is given by Williamson and Smallman [25]:
r¼
ð3nK=FÞ1=2 3e2 _1=2 bP
(12)
(11)
where l is the wavelength of X-ray used, s the angular range over which the intensity distribution is appreciable, P is the crystallite size, q is Bragg diffraction angle,1/2 mean square strain. However, the variance is a range sensitive parameter and consequently depends on the choice of the background level which has a marked influence on the range to be selected for integration. In fact, it is found that the diffraction profile approaches zero, rather asymptotically following an inverse square law. For such a function
where n be the number of dislocations per unit volume of the crystal, K the constant depend on strain distribution,1/2 is rms strain, F is an interaction parameter, b is burgers vector and P the crystallite size. For Cauchy strain profile, the value of K is 25 whereas for Gaussian strain profile the value of K is nearly 4. In the absence of extensive polygonization, the dislocation density is calculated from Eq. (12) by assuming n ¼ F, b ¼ d, the interplanar spacing and K ¼ 4 now the above Eq. (12) reduces which in turn gives the value of dislocation density [Eq. (13)]:
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r¼
pffiffiffiffiffiffi 123e2 _1=2 dP
147
(13)
Stacking fault probability (a) is the fraction of layers undergoing stacking sequence faults in a given crystal and hence one fault is expected to be found in 1/a layers [21]. The presence of stacking sequence faults gives rise to shift in peak position of different reflections with respect to ideal position of a fault free wellannealed sample. A well-annealed powder sample reference is used to compare the shift in peak position of different reflections and hence to evaluate the microstructural parameters. The relation connecting stacking fault probability (a) with peak shift D(2q) is given by Eq. (14) [21]:
a ¼
Dð2qÞ 2p2 pffiffiffi 45 3 tan qhkl
(14)
Using above Eq. (14) stacking fault probability is calculated by measuring the peak shift with the well annealed sample. X-ray diffraction pattern of CdSe thin films prepared at various solution pH values in the range between 1.5 and 3.0 0.1 are recorded. Using FWHM data and DebyeeScherrer formula the crystallite size of the deposited films is calculated. Variation of crystallite size and rms microstrain with solution pH value for CdSe thin film is shown in Fig. 6a. Variation of dislocation density and stacking fault probability with solution pH value is also shown in Fig. 6b. It is observed from Fig. 6a that the value of crystallite size is found to increase while increasing the solution pH value from 1.5 to 2.5 0.1, thereafter it is found to decrease slightly. For thinner films, the microstrain and dislocation density are found to be larger. Due to building up of film thickness with increase in solution pH value, the dislocation density and microstrain are reduced due to the release of stresses in the films. Also, with increase in film thickness, crystallite size increases gradually and shows a maximum value at pH value around 2.5 0.1. The cumulative effect of decrease in rms microstrain and dislocation density may be responsible for gradual reduction of stacking fault of the layers with increase in solution pH values. Similar functional dependency of microstructural parameters with solution pH value and bath temperature for FeSe thin films which has been reported earlier [21]. The effect of solution pH on surface morphology of CdSe thin films has been analyzed using SEM and AFM. Scanning electron microscope image of CdSe thin films deposited at different solution pH values is shown in Fig. 7(a,b). It is observed from Fig. 7a that the films prepared at solution pH value around 2.0 0.1 are found to exhibit island like structure. If the solution pH value is increased to 2.5 0.1, there is improvement in crystallinity is observed which is shown in Fig. 7b. The pH value of the solution reaches 2.5 0.1, the surface morphology is completely changed. The surface is covered with uniform spherically shaped grains. The grains visible in the SEM picture thus represent an aggregate of very many small crystallites. The sizes of the grains for films prepared at solution pH value 2.5 0.1 are found to be in the range between 0.28 and 0.71 mm. The average size of the grains is found be 0.52 mm. AFM (2D & 3D) image of CdSe thin films prepared at solution pH value 2.5 0.1 is shown in Fig. 8(a,b). It is observed from Fig. 8a that the films prepared at solution pH value 2.5 0.1 have cone shaped grains in addition with some tiny spherically shaped grains. The grains are distributed uniformly over the entire surface of the films. Few holes are seen at few places on the surface of the film. The value of Ra is found to be 11.97 nm.
Fig. 7. (a,b). SEM picture of CdSe thin films electrodeposited at different solution pH values: (a) 2.0 (b) 2.5 0.1.
3.5. Compositional and optical absorption analyses The film composition has been analyzed using an energy dispersive analysis by X-rays set up attached with SEM. Typical EDX spectrum of CdSe thin films deposited at solution pH value around 2.5 0.1 is shown in Fig. 9a. It is observed that the emission lines of Cd and Se are present in the investigated energy range indicates the formation of CdSe thin films. Variation of Cd and Se content with solution pH value for films obtained at various solution pH values is shown in Fig. 9b. It is found that the content of Cd decreases and the content of Se increases while increasing the solution pH value from 1.5 to 2.5 0.1, thereafter both of them changes vice-versa. The average atomic ratio (Cd:Se) for CdSe thin films prepared at a solution pH value around 2.5 0.1 is found to be 49.78:50.22 and is nearly 0.99:1 indicating well defined stoichiometric nature of the deposited films. This result is also consistent with X-ray diffraction analysis of the sample with phase corresponding to CdSe. The stoichiometric ratio observed in the present work is found to be slightly higher than the value which has been reported earlier for cubic CdSe thin films by electrodeposition technique [14]. Optical parameters such as absorption coefficient, band gap are determined from optical absorption and transmittance measurements. The value of absorption coefficient for strong absorption region of thin film is calculated using the following Eq. (15) [1,21]
ahw ¼ A hw Eg
n
(15)
where a is the absorption coefficient in cm1, hn is the photon energy, A is an energy dependent constant and n is an integer. The value of ‘n’ determines the type of transition present in the material. In this case n ¼ 1/2 indicates that the transition present in the material is direct allowed. A plot of hn versus (ahn)2 is drawn for
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Fig. 8. (a,b). AFM (2D and 3D) image of CdSe thin films electrodeposited at solution pH value 2.5 0.1.
CdSe thin films obtained at various solution pH values is shown in Fig. 10. Extrapolation of linear portion of the graph to energy (hn) axis gives the band gap value of the material. The band gap value of the material obtained in the present work is found to be in the range between 1.69 and 1.74 eV for films obtained at various solution pH values. Refractive index (n) and extinction coefficient (k) are determined using the following Eq. (16) and Eq. (17) [11]. Fig. 11 shows the variation of refractive index (n) with wavelength (l) for films obtained at various solution pH values. It is observed that there is sudden decrease of dispersion (n) at 730 nm shows the presence of characteristic absorption at that wavelength. On the other hand, the extinction coefficient (k) is found to decrease with wavelength (l) and attained almost constant value after 730 nm (Fig. 12). The value of (n) is found to be almost constant at wavelength greater than 730 nm. The very strong absorption exhibited for photon energies above Eg value is very likely due to high density of cadmium ion within the lattice. Real and imaginary dielectric constants are determined by using the following Eqs. (18) and (19) [11]. Fig. 13 shows the variation of real dielectric constant (3 1) with wavelength
(l) for films obtained at various solution pH values. It is observed that the value of (3 1) indicated a similar variation as that of (n). The value of (3 2) decreases with increasing wavelength just like (k) which is shown in Fig. 14. It is observed from above Figures that all of them such as n, k, 31, 3 2 shows decreasing dependence with wavelength (l). This may be due to inter band transition for photon energy smaller than the smallest band gap. The value of n, k, 31, 3 2 are found to be 3.3, 0.64, 10.60, 4.32 correspond to the band gap value of the material.
n ¼
k ¼
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1þR 4R þ k2 1R ð1 RÞ2
al
4p
(16)
(17)
31
¼ n2 k2
(18)
32
¼ 2nk
(19)
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Fig. 11. Variation of refractive index (n) with wavelength (l) for CdSe thin films prepared at various solution pH values: (a) 1.5 (b) 2.0 (c) 2.5 0.1.
Fig. 9. a. Typical EDX spectrum of CdSe thin films obtained at solution pH value 2.5 0.1. b. Variation of Cd and Se content with solution pH value for CdSe thin films electrodeposited at various solution pH values.
3.6. Photoelectrochemical solar cell studies From the point of view of photoelectrochemical solar cells n-type semiconducting materials endowed with acceptable stability and compatibility are required so that the enhanced photoresponsiveness to illumination may become possible. The quality
Fig. 10. Plot of (hy) versus (ahy)2 for CdSe thin films electrodeposited at various solution pH values: (a) 1.5 (b) 2.0 (c) 2.5 0.1.
of electrodeposited semiconducting photoelectrode needed for photovoltaic applications depends to a large extent on the applied deposition potential, solution pH and composition of electrolytic bath. Photoelectrochemical solar cell studies are carried out in a standard three compartment cell comprising of n-(CdSe) as photocathode, a platinum electrode as anode and SCE as reference electrode, respectively. The electrolyte consists of 1 M each of Na2S, S and NaOH. A collimated narrow beam of light from tungsten filament (150 W) lamp is used as light source and spectral response measurements are carried out in the wavelength range between 360 and 900 nm using Keithley Multimeter (Model 2000, USA). Spectral response measurements have shown that the photocurrent density is found to increase with wavelength and attained its maximum value around (720e740) nm for films obtained at various solution pH value in the range between 2.0 and 3.0 0.1. If the wavelength is increased above 740 nm the value of photocurrent
Fig. 12. Variation of extinction coefficient (k) with wavelength (l) for CdSe thin films prepared at various solution pH values: (a) 1.5 (b) 2.0 (c) 2.5 0.1.
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Fig. 13. Variation of real dielectric constant (3 1) with wavelength (l) for CdSe thin films prepared at various solution pH values: (a) 1.5 (b) 2.0 (c) 2.5 0.1.
density is found to decrease thus indicates that value of photocurrent density exhibit a peak near the absorption edge of CdSe. Decrease in wavelength on longer wavelength side may be due to recombination of photogenerated carriers caused by surface states [1,26]. Similarly, decrease in value of photocurrent density on shorter wavelength side may be due to transition between defect levels [1,26]. Variation of photopotential with photocurrent density for CdSe thin films obtained at various solution pH values in the range between 2.0 and 3.0 is shown in Fig. 15. The cell parameters such as open circuit voltage, short circuit current, fill factor and efficiency are determined from power output characteristics. The value of fill factor and efficiency for films obtained at solution pH value 2.5 0.1 is found to be and 0.29 and 0.93%, respectively. The films obtained at solution pH value around 2.0 and 3.0 0.1 has lower value of fill factor and efficiency when compared to the film obtained at solution pH value 2.5 0.1. The lower value of efficiency may be due to the presence of defects in the deposited films. The process of annealing at higher temperatures which remove the defects and sizes of the grains of fabricated photoelectrodes and are
Fig. 15. Power output (IeV) characteristics of CdSe thin film photoelectrodes prepared at various solution pH values: (a) 2.0 (b) 2.5 (c) 3.0 0.1.
found to be increase through the diffusion of small crystallites. Increase in value of crystallite size resulted increase in value of diffusion length of charge carriers which enhances the fill factor and efficiency of the deposited films which could be reported by Hosun Moon et al. [26]. Hence, the process of annealing has been carried out for films obtained at solution pH value around 2.5 0.1 to increase the efficiency of the deposited films is in progress. 4. Conclusions Semiconducting CdSe thin films have been prepared by potentiostatic electrodeposition on ITO substrates. X-ray diffraction analysis showed that the deposited films possess polycrystalline nature with hexagonal structure with most prominent reflection along (002) plane. There is change in preferential orientation of the plane from (100) to (002) is observed for films prepared at various solution pH values in the range between 1.5 and 3.0 0.1. Microstructural parameters exhibit monotonic variation with solution pH values. Morphological and compositional analyses showed that films with spherical shaped grains and well-defined stoichiometry has been obtained at solution pH value around 2.5 0.1. Optical absorption analysis showed that the deposited film has a direct band gap value of 1.72 eV. The dispersion of n and k with frequency is similar to the behavior reported for semiconducting chalcogenides. Photoelectrochemical solar cell studies showed that the films prepared at solution pH value around 2.5 0.1 has better cell parameters than the films obtained at other solution pH values. Acknowledgement One of the authors (S. Thanikaikarasan) gratefully acknowledge the Council of Scientific and Industrial Research (CSIR), New Delhi, India for the award of Senior Research Fellowship (SRF) with File No:9/688(0010) 2008 to carry out this research work. References
Fig. 14. Variation of imaginary dielectric constant (3 2) with wavelength (l) for CdSe thin films prepared at various solution pH values: (a) 1.5 (b) 2.0 (c) 2.5 0.1.
[1] S. Thanikaikarasan, K. Sundaram, T. Mahalingam, S. Velumani, Jin-Koo Rhee, Mater. Sci. Eng. B 174 (2010) 242e248. [2] R. Bhargava (Ed.), Properties of Wide Band Gap Semiconductors, INSPEC Publications, London, 1997. [3] T. Mahalingam, A. Kathalingam, S. Velumani, Soonil Lee, M.H. Sun, K.Y. Deak, J. Mater. Sci. 41 (2006) 3553e3559.
S. Thanikaikarasan et al. / Solid State Sciences 15 (2013) 142e151 [4] Kehar Singh, Sameer S.D. Mishra, Solar Energ. Mater. Solar Cells 72 (2002) 115e129. [5] Dong Ham, Kamal K. Mishra, Krishnan Rajeshwar, J. Electrochem. Soc. 138 (1991) 100e108. [6] C.M. Rouleau, D.H. Lowndes, Appl. Surf. Sci. 127-129 (1998) 418e424. [7] U. Pal, D. Samanta, S. Ghorai, B.K. Samantaray, A.K. Chaudhuri, J. Phys. D. Appl. Phys. 25 (1992) 1455e1494. [8] Cristian Baban, G.I. Rusu, Appl. Surf. Sci. 211 (2003) 6e12. [9] G. Perna, V. Capozzi, M. Ambrico, V. Augelli, T. Ligonzo, A. Minafra, L. Schiavulli, M. Pallara, Appl. Surf. Sci. 233 (2004) 366e372. mec, D. Mikes, T. Rohovec, E. Uhlîrová, F. Trojánek, P. Malỳ, Mater. Sci. [10] R. Ne Eng. B 70 (2000) 500e504. [11] S. Thanikaikarasan, T. Mahalingam, J. Alloys Compd. 511 (2012) 115e122. [12] S. Thanikaikarasan, T. Mahalingam, K. Sundaram, A. Kathalingam, Yong Deak Kim, Taekyu Kim, Vacuum 83 (2009) 1066e1072. [13] S.M. Pawar, A.V. Moholkar, C.H. Bhosale, Mater. Lett. 61 (2007) 1034e 1038. [14] Y.G. Gudage, Ramphal Sharma, Curr. Appl. Phys. 10 (2010) 1062e1070.
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[15] E.B. Chubenko, A.A. Klyshko, V.A. Petrovich, V.P. Bandarenko, Thin Solid Films 517 (2009) 5981e5987. [16] S.J. Lade, M.D. Uplane, M.M. Uplane, C.D. Lokhande, J. Mater. Sci. Mater. Electron. 9 (1998) 477e482. [17] JCPDSICDD, PDF No.08-0459, 2003. [18] T. Mahalingam, S. Thanikaikarasan, V. Dhanasekaran, R. Mariappan, P. Jayamurugan, S. Velumani, Jin-Koo Rhee, Mater. Sci. Eng. B 174 (2010) 249e252. [19] A. Kariper, E. Güneri, F. Göde, C. Gümüs, Chalcogen. Lett. 9 (2012) 27e40. [20] T. Mahalingam, V.S. John, S. Rajendran, G. Ravi, P.J. Sebastian, Surf. Coat. Tech. 155 (2002) 245e249. [21] S. Thanikaikarasan, T. Mahalingam, M. Raja, Taekyu Kim, Yong Deak Kim, J. Mater. Sci. Mater. Electron. 20 (2009) 727e734. [22] G.B. Mitra, Acta Crystallogr. 17 (1964) 765e767. [23] G.B. Mitra, N.K. Misra, Br. J. Appl. Phys. 17 (1966) 1319e1328. [24] B.E. Warren, E.P. Warekois, Acta Metall. 3 (1955) 473e479. [25] G.K. Williamson, R.E. Smallman, Philos. Magn. 1 (1956) 34e46. [26] Hosun Moon, A. Kathalingam, Thaiyan Mahalingam, J.P. Chu, Yong Deak Kim, J. Mater. Sci. Mater. Electron. 18 (2007) 1013e1019.
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Investigation of solution pH effect on electrochemical, microstructural, optical and photoelectrochemical properties of CdSe thin films Sethuramachandran Thanikaikarasan a, *, Chinnapyan Vedhi b, Xavier Sahaya Shajan a, Thaiyan Mahalingam c a b c
Centre for Scientific and Applied Research, School of Basic Engineering and Sciences, PSN College of Engineering and Technology, Tirunelveli 627 152, Tamil Nadu, India Department of Chemistry, VOC College, Tuticorin 628 008, Tamil Nadu, India Department of Physics, School of Science and Humanities, Karunya University, Coimbatore 641 114, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 November 2011 Received in revised form 3 April 2012 Accepted 2 September 2012 Available online 10 September 2012
Thin films of Cadmium Selenide have been prepared on indium doped tin oxide coated conducting glass substrates using cathodic electrodeposition technique. The deposition mechanism has been analyzed using linear sweep voltammetry. X-ray diffraction analysis revealed that the deposited films have hexagonal structure with most prominent reflection along (002) plane. X-ray line profile analysis technique has been used to evaluate the microstructural parameters. Surface morphology and film composition have been analyzed using scanning electron microscopy and energy dispersive analysis by X-rays. Optical absorption analysis has been carried out to find out the parameters such as band gap, refractive index, extinction coefficient, real and imaginary dielectric constants. Photoelectrochemical solar cell analysis has been showed that the films obtained at solution pH value around 2.5 has better cell parameters as compared to the films obtained at other solution pH values. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Thin films Texture coefficient RMS Microstrain Stacking fault probability
1. Introduction IIeVI group binary semiconductors are considered important technological materials due to its potential applications in solar cells, semiconducting, photovoltaic and optoelectronic devices [1e3]. Among them, Cadmium Selenide (CdSe) is found to be an excellent material with a band gap value around 1.7 eV which make them fairly interesting for the fabrication of solar cells through photoelectrochemical route [1,4,5]. Thin films of CdSe are normally crystallized in hexagonal structure (JCPDS-ICDD 2003, PDF 080459) with lattice constants (a ¼ 4.299 A; c ¼ 7.010 A) and in cubic structure (JCPDS-ICDD 2003, PDF 19-0191) with lattice constant (a ¼ 6.077 A). There are number of methodologies have been reported earlier for the preparation CdSe thin films such as sputtering [6], vacuum evaporation [7], thermal evaporation [8], pulsed laser deposition [9], chemical bath deposition [10]. Among the deposition techniques mentioned above, electrodeposition appears to be attractive due to its low cost of synthesis, low temperature growth and its possibility to control film thickness and morphology by readily adjusting the electrical parameters as well as composition of the electrolytic solution [1,11,12]. One obvious
* Corresponding authors. Tel.: þ91 4634 279067; fax: þ91 4634 279681. E-mail address: [email protected] (S. Thanikaikarasan). 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2012.09.001
requirement is that the substrate must be conductive. This is not a problem for thin film solar cell, since they are deposited on conducting ITO substrate [12]. Preparation of CdSe thin films on stainless steel, FTO coated glass substrates with minor variation in solution pH value and its effect on electrochemical, structural, morphological, compositional, optical and photoelectrochemical properties have been investigated by Pawar et al. [13]. Gudage and Rampal Sharma [14] have obtained (111) plane oriented cubic CdSe thin films on FTO coated conducting glass substrates by electrochemical deposition technique and investigated the effect of solution pH on electrochemical, structural, compositional, optical and photoelectrochemical properties. Both of them, have analyzed the film properties with minor variation in solution pH values. To the best of our knowledge, there is no such detailed investigation is available regarding the properties of hexagonal CdSe thin films with larger variation in solution pH values. In the present investigation, thin films of CdSe have been prepared on indium doped tin oxide coated conducting glass (ITO) substrates at various solution pH values. The mechanism of formation of CdSe has been analyzed using linear sweep voltammetry. The deposited films have been subjected to X-ray diffraction, scanning electron microscopy, atomic force microscopy, energy dispersive analysis by X-rays, optical and photoelectrochemical techniques, respectively. The effect of solution pH on electrochemical, microstructural, morphological, compositional, optical and photo
S. Thanikaikarasan et al. / Solid State Sciences 15 (2013) 142e151
electrochemical properties of the deposited films are studied and the results are reported. 2. Experimental details All the chemicals used in the present work were of Analar grade reagents. Thin films of CdSe were deposited on ITO substrates from an aqueous acidic bath containing 0.25 M CdSO4, 0.0025 M SeO2. The first working solution of CdSO4 was obtained by dissolving 48.095 g CdSO4 in 250 cc deionized water. The second working solution of SeO2 was obtained by dissolving 0.0694 g SeO2 in 250 cc deionized water. Each 20 cc of the two solutions forms the reaction mixture and this mixture was used as an electrolytic bath for all depositions. The electrochemical experiments were carried out using PAR scanning potentiostat/galvanostat unit (EG & G, Model 362, Princeton Applied Research, USA) employing three electrode configuration with ITO substrate, Platinum electrode and Saturated Calomel Electrode (SCE) as working, counter and reference electrode, respectively. The electrolytic processes were monitored by linear sweep voltammetry and governed potentiostatically. The deposition potential was fixed as 700 mV versus SCE using linear sweep voltammetry. The bath temperature and deposition time were fixed as 70 C and 40 min, respectively. The films were deposited at various solution pH values in the range between 1.5 and 3.0 0.1. Initially, the pH value of the electrolytic bath was 2.5 0.1. By adding an adjustable amount of dilute H2SO4 the pH value was adjusted to 2.0 and 1.5 0.1, respectively. If the pH value is kept below 2.0 0.1, there is rapid growth of film followed by its peeling out from the substrate. The pH value was increased to 3.0 0.1 by adding an adjustable amount of NaOH to the electrolytic bath. At higher pH value such as above 2.5 0.1, precipitation of electrolytic bath occurs which in turn yields poor quality films. Hence, the solution pH value was fixed as 2.5 0.1 to obtain films with better quality. Linear sweep voltammetry was carried out using BAS 200A electrochemical analyzer. Thickness of the deposited films was measured using stylus profilometer (Mitutoyo SJ 301, Japan). X-ray diffraction data of the deposited films was analyzed using an X-ray diffractometer (XPERT PRO PANalytical, Netherland) with CuKa radiation with wavelength (l ¼ 1.5418 A). The parameters such as crystallite size, rms microstrain, dislocation density and stacking fault probability were evaluated from X-ray diffraction data using X-line profile analysis. Surface morphology of the deposited films was analyzed using scanning electron microscope (Philips Model XL30) and atomic force microscope (PSIA XE100). Film composition composition was analyzed using an energy dispersive X-ray analysis set up attached with scanning electron microscope. Optical parameters such as band gap, refractive index, extinction coefficient, real and imaginary dielectric constants were evaluated from optical absorption, transmission and reflection data which is obtained using an UVeViseNIR spectrophotometer (HR-2000, M/S Ocean Optics, USA). Photoelectrochemical solar cells were constructed using CdSe thin film as photocathode, platinum electrode as anode and SCE as reference electrode, respectively. 3. Results and discussion 3.1. Linear sweep voltammetric studies Electrochemical growth of CdSe thin films on ITO substrates has been carried out potentiostatically from an aqueous electrolytic bath containing CdSO4 and SeO2. The electrochemical equations involved in the co-deposition of CdSe are described as follows [Eq. (1)] [14]
143
0 3CdSO4 8H2 O/3Cd2þ þ SO2 þ 8H2 O ECd ¼ 240 mV 4 3
# " a2þ RT Cd þ þ ln aCd 2F
(1)
ECd ¼
0 ECd
(2)
The formation of selenous acid occurs which may be due to the following Eq. (3)
H2 O þ SeO2 /H2 SeO3
(3)
Reduction of H2SeO3 to Se occurs which may be due to the following Eq. (4)
H2 SeO3 þ 4Hþ þ 4e /Se þ 3H2 O 0 ESe ¼ ESe þ
0 ESe ¼ 740 mV
4 aSeO2 RT RT þ ln þ ln aþ þ H 2F 4F aSe
(4) (5)
Reaction of formation of CdSe on ITO substrate may be due to the following Eq. (6)
Cd þ Se/CdSe
DG0 ¼ 100 kJ mole
(6)
Here ECd and ESe are the equilibrium electrode potential of cadmium and selenium, R is universal gas constant and T is absolute and aSeO2 are the activities temperature of the electrolytic bath. a2þ Cd of Cd and Se ions present in the electrolytic bath, while aCd and aSe are the activities of respective atoms present in the deposited films. The deposit is formed by the solid state reaction of plated Cd and Se atoms rather than the precipitation of H2Se and Cd. Hence, the reaction of formation of CdSe is described by Eq. (6) with free energy formation DG0 ¼ 100 kJ mol [15]. Linear sweep voltammetric studies have been carried out in a standard three compartment cell comprising of ITO substrate as cathode, platinum electrode as anode and saturated calomel electrode as reference electrode, respectively. The electrolytic bath consists of 0.25 M CdSO4 and 0.0025 M SeO2. The pH value is found to be vary in the range between 1.5 and 3.0 0.1 which may be due to the addition of H2SO4 and NaOH in the electrolytic bath. Linear sweep voltammetric curves recorded for ITO glass electrode in an aqueous electrolytic bath maintained at various solution pH values is shown in Fig. 1. The pH value is maintained at 1.5, 2.0 0.1, there is evolution of hydrogen gas in the electrolytic bath. This process leads to development of poor quality films with irregular surface and discontinuously distributed grains which reduces the Faradaic formation of CdSe thin films [Fig. 1(a,b)]. The pH value is increased to 2.5 0.1, cathodic current increases sharply and two reduction waves obtained at 715 and 810 mV versus SCE thus leads to formation of CdSe on ITO substrate (Fig. 1c) according to Eq. (6). It is observed from Fig. 1d, that two reduction waves obtained at 760 and 850 mV versus SCE leads to deposition of individual metallic selenium on ITO substrate according to Eq. (4) and white precipitation of cadmium ion source occurs which is observed at the bottom of the flask. Hence, the deposition potential is fixed as 700 mV versus SCE to prepare CdSe thin films with well-defined crystallinity and stoichiometry which is confirmed using X-ray diffraction and EDX analyses. 3.2. Growth kinetics Thickness is one of the important film parameters which depend upon the pH value, concentration of the electrolytic bath, bath temperature and deposition time [14]. Growth of CdSe thin
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2.0 0.1, the electrolytic bath containing more Hþ ions thus leads to give higher conductivity to the electrolytic bath. During deposition hydrogen evolution takes place thus leads to give films with irregular surface and lower thickness which is shown in Fig. 2(a,b). If the solution pH value is increased to 2.5 0.1, the solution becomes transparent and high conductivity. This higher conductivity increases the deposition current passing through the electrolytic bath thus enhances the overall rate of deposition which in turn increases thickness of the film prepared at solution pH value 2.5 0.1. Further increasing the solution pH value above 2.5 0.1, the electrolytic bath becomes slightly turbid and milky due to the formation of Cd(OH)2 ions [14]. Therefore, there is formation of porous, foggy and less adherent films which results lower value of film thickness as shown in Fig. 2d. Hence, an optimum solution pH value of 2.5 0.1 may be selected to get better quality films with higher thickness value around 1480 nm. 3.3. Structural studies
Fig. 1. Linear sweep voltammogram of CdSe thin films obtained at different solution pH values: (a) 1.5 (b) 2.0 (c) 2.5 (d) 3.0 0.1.
films on ITO substrates has been carried out at various solution pH values with different intervals of time. The reduction potential maintained between the electrodes and bath temperature are fixed as 700 mV versus SCE and 80 C, respectively. Fig. 2 shows the variation of film thickness with deposition time for CdSe thin films prepared at various solution pH values in the range between 1.5 and 3.0 0.1. It is found that the film thickness build up slowly and increases linearly at the beginning stage, thereafter it tend to attain its saturated value (640e1480 nm) after 30 min for films prepared at all solution pH values. If the deposition time is increased above 40 min thickness of the deposited films remains constant upto 60 min, thereafter the film thickness is found to decrease slightly which is not shown in Fig. 2. The decrease in value of film thickness after 60 min may be attributed to higher rate of dissolution than the rate of deposition of CdSe thin films [15,16]. The films deposited at lower pH value such as 1.5 and
Fig. 2. Variation of film thickness with deposition time for CdSe thin films electrodeposited at various solution pH values: (a) 1.5 (b) 2.0 (c) 2.5 (d) 3.0 0.1.
The pH value of an electrolytic bath play an important key role in determine the crystal structure and stoichiometry of the deposited films. X-ray diffraction pattern recorded for CdSe thin films prepared at (potential: 700 mV versus SCE; bath temperature: 80 C) various solution pH values in the range between 1.5 and 3.0 0.1 is shown in Fig. 3. XRD pattern showed that the deposited films are found to exhibit hexagonal structure with lattice constants (a ¼ 4.299 A; c ¼ 7.010 A). The observed diffraction peaks of hexagonal CdSe are found at 2q values of angles 23.88, 25.87, 27.10, 35.08, 42.07, 45.73, 48.83, 50.71, 55.83, 63.92, 66.25 and 76.78 corresponding to the lattice planes (100), (002), (101), (102), (110), (103), (200), (201), (202), (203),(210) and (310), respectively. The different peaks in the diffractogram are indexed and the corresponding values of interplanar spacing ‘d’ are calculated and compared with standard JCPDS-ICDD file for hexagonal CdSe [17]. It is observed from the diffractogram that all the identified peaks are from CdSe and hence no additional peaks corresponding to Cd and Se are present. Electrochemical growth of CdSe thin films are carried out at various solution pH values in the range between 1.5 and 3.0 0.1. At lower pH value such as below 1.5 0.1, the
Fig. 3. X-ray diffraction pattern of CdSe thin films electrodeposited at various solution pH values: (a) 1.5 (b) 2.0 (c) 2.5 (d) 3.0 0.1.
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145
formation of film may be hindered due to the process of hydrogen evolution reaction (HER). This process leads to give films with poor crystallinity. At higher pH value such as above 3.0 0.1, precipitation of electrolytic bath occurs which in turn yields films with lower crystallinity. For detailed investigation of solution pH effect on structural properties, the value of solution pH is fixed in the range between 1.5 and 3.0 0.1. It is observed from Fig. 3a, that the films prepared at solution pH value around 1.5 0.1 are found to exhibit diffraction peaks of hexagonal CdSe with most prominent reflection along (100) plane. If the pH value is increased to 2.0 0.1, there is slight decrement in peak height of (100) plane and slight increment in peak height of (002) plane is observed which is shown in Fig. 3(a,b). The films prepared at solution pH value around 2.5 0.1 are found to exhibit well defined crystallites with most prominent reflection along (002) plane which is indicated in Fig. 3c. Further increasing the solution pH value above 2.5 0.1, the intensity of all the observed peaks are found to decrease which is shown in Fig. 3d. It is also observed that there is no change in crystal structure, but there is change in preferential orientation from (100) to (002) plane is observed for films prepared at various solution pH values in the range between 1.5 and 3.0 0.1. Similar trend is observed for CdCr2S4 thin films on ITO substrates has been reported earlier [18]. The value of lattice constants (‘a’ and ‘c’) of the deposited films are calculated using the following Eq. (7) [12].
2 1 4 h2 þ hk þ k2 l þ 2 ¼ 3 a2 c d2hkl
(7)
Fig. 4(a,b) shows the variation of lattice constants (‘a’ and ‘c’) with solution pH value for CdSe thin films obtained at different solution pH values. It is observed that the value of ‘a’ and ‘c’ are found to be 0.04% different from ‘a’ and ‘c’ values which is given in JCPDS-ICDD file for hexagonal CdSe [17]. It is also observed that the value of a, c is found to decrease while increasing the solution pH value from 1.5 to 2.5 0.1, thereafter there is slight increment is observed which is indicated in Fig. 4a. Increase in value of lattice constants while increasing the solution pH value above 2.5 0.1, may be due to shift of diffraction peak to larger angles [19]. Also, the variation of ‘c/a’ ratio with solution pH value for CdSe thin films obtained at various solution pH values is shown in Fig. 4b. NelsoneRiley plot is drawn for CdSe thin films obtained at various solution pH values using the expression which is given in Eq. (8) [20]. The effect of orientation of polycrystalline CdSe thin films on deposition condition such as solution pH value, potential, electrolyte concentration are determined by evaluating the texture coefficient of the (hkl) plane using the expression which is given in Eq. (9) [1,21]:
Dd d
¼
1 cos2 q cos q þ q 2 sin q
IðhklÞ=I0 ðhklÞ # Tc ðhklÞ ¼ " P 1 IðhklÞ=I0 ðhklÞ N N
(8)
(9)
where Tc(hkl) is the texture coefficient of the (hkl) plane, I(hkl) is the measured intensity and I0(hkl) is the JCPDS-ICDD standard intensity. NelsoneRiley plot drawn for CdSe thin films obtained at various solution pH values is shown in Fig. 5a. Variation of texture coefficient with solution pH value for CdSe thin films obtained at various solution pH values is shown in Fig. 5b. The diffraction peak corresponding to (002) plane is used for the calculation of crystallite size of the deposited films. It is observed from Fig. 5b that the value of texture coefficient for (002) plane is found to increase
Fig. 4. a. Variation of lattice constants with solution pH value for CdSe thin films electrodeposited at various solution pH values. b. Variation of c/a ratio with solution pH value for CdSe thin films electrodeposited at various solution pH values.
while increasing the solution pH value from 1.5 to 2.5 0.1, thereafter it is found to decrease slightly. It is also observed that the texture coefficient value of (100) plane is found to decrease while increasing the solution pH value from 1.5 to 3.0 0.1. The films prepared at solution pH value around 2.5 0.1 have higher texture coefficient value indicating that the deposited films have better crystallinity. The films deposited at solution pH value around 2.5 0.1, have better crystallinity and well adherent to the substrates. The average crystallite size of the deposited films is calculated using FWHM data and Debye-Scherrer formula [1,12,21]:
P ¼
0:9l bcos q
(10)
where b is full width at half maximum of the peak position in A), q is Braggs radian, l is the wavelength of CuKa target (l ¼ 1.540 diffraction angle at peak position in degrees. The crystallite size of the deposited film is found to be in the range between 19 and 47 nm (Fig. 6a). 3.4. Microstructure and surface morphology Line profiles in observed XRD patterns are generally due to convolution of various factors such as crystallite size, rms microstrain and stacking faults. The line profiles are subjected to variance
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Fig. 5. a. NelsoneRiley plot for CdSe thin films electrodeposited at solution pH value 2.5 0.1. b. Variation of texture coefficient along (100), (110), (002) and (200) planes for CdSe thin films electrodeposited at various solution pH values.
analysis in order to calculate the value of crystallite size and rms microstrain [21,22]. An aggregate of distorted crystallites as a measure of crystallite size and strain could affect the variance of X-ray diffraction line profiles. Since, this method is sensitive to variation near the tails of the peak, a careful adjustment of the background has been carried out following the method given by Mitra and Misra [23]. For instrumental broadening the line profiles has been corrected by substracting the variance of the corresponding profiles of well-annealed CdSe sample because of the additive effect of the variance. Assuming the broadening of diffracted line is due to crystallite size and strain, the variance can be written as
W2q ¼
ls
2p2 Pcos q
þ 4tan2 q3e2 _1=2
Fig. 6. a. Variation of crystallite size and rms microstrain with solution pH value for CdSe thin films electrodeposited at various solution pH values. b. Variation of dislocation density and stacking fault probability with solution pH value for CdSe thin films electrodeposited at various solution pH values.
varying inversely as a square of the distance from the mean, the variance will be the linear function of total range s and can be written as W ¼ K s þ C, where K and C are constants which depend upon the physical conditions of the sample and geometrical factors. Strain is defined as the restoring force acts on the surface of the film to restrict the formation of crystallites on its surface [24]. Dislocation density is defined as the number of dislocation lines per unit volume of the crystal [1,11,12]. Dislocation density is calculated using the following Eq. (12) which is given by Williamson and Smallman [25]:
r¼
ð3nK=FÞ1=2 3e2 _1=2 bP
(12)
(11)
where l is the wavelength of X-ray used, s the angular range over which the intensity distribution is appreciable, P is the crystallite size, q is Bragg diffraction angle,
where n be the number of dislocations per unit volume of the crystal, K the constant depend on strain distribution,
S. Thanikaikarasan et al. / Solid State Sciences 15 (2013) 142e151
r¼
pffiffiffiffiffiffi 123e2 _1=2 dP
147
(13)
Stacking fault probability (a) is the fraction of layers undergoing stacking sequence faults in a given crystal and hence one fault is expected to be found in 1/a layers [21]. The presence of stacking sequence faults gives rise to shift in peak position of different reflections with respect to ideal position of a fault free wellannealed sample. A well-annealed powder sample reference is used to compare the shift in peak position of different reflections and hence to evaluate the microstructural parameters. The relation connecting stacking fault probability (a) with peak shift D(2q) is given by Eq. (14) [21]:
a ¼
Dð2qÞ 2p2 pffiffiffi 45 3 tan qhkl
(14)
Using above Eq. (14) stacking fault probability is calculated by measuring the peak shift with the well annealed sample. X-ray diffraction pattern of CdSe thin films prepared at various solution pH values in the range between 1.5 and 3.0 0.1 are recorded. Using FWHM data and DebyeeScherrer formula the crystallite size of the deposited films is calculated. Variation of crystallite size and rms microstrain with solution pH value for CdSe thin film is shown in Fig. 6a. Variation of dislocation density and stacking fault probability with solution pH value is also shown in Fig. 6b. It is observed from Fig. 6a that the value of crystallite size is found to increase while increasing the solution pH value from 1.5 to 2.5 0.1, thereafter it is found to decrease slightly. For thinner films, the microstrain and dislocation density are found to be larger. Due to building up of film thickness with increase in solution pH value, the dislocation density and microstrain are reduced due to the release of stresses in the films. Also, with increase in film thickness, crystallite size increases gradually and shows a maximum value at pH value around 2.5 0.1. The cumulative effect of decrease in rms microstrain and dislocation density may be responsible for gradual reduction of stacking fault of the layers with increase in solution pH values. Similar functional dependency of microstructural parameters with solution pH value and bath temperature for FeSe thin films which has been reported earlier [21]. The effect of solution pH on surface morphology of CdSe thin films has been analyzed using SEM and AFM. Scanning electron microscope image of CdSe thin films deposited at different solution pH values is shown in Fig. 7(a,b). It is observed from Fig. 7a that the films prepared at solution pH value around 2.0 0.1 are found to exhibit island like structure. If the solution pH value is increased to 2.5 0.1, there is improvement in crystallinity is observed which is shown in Fig. 7b. The pH value of the solution reaches 2.5 0.1, the surface morphology is completely changed. The surface is covered with uniform spherically shaped grains. The grains visible in the SEM picture thus represent an aggregate of very many small crystallites. The sizes of the grains for films prepared at solution pH value 2.5 0.1 are found to be in the range between 0.28 and 0.71 mm. The average size of the grains is found be 0.52 mm. AFM (2D & 3D) image of CdSe thin films prepared at solution pH value 2.5 0.1 is shown in Fig. 8(a,b). It is observed from Fig. 8a that the films prepared at solution pH value 2.5 0.1 have cone shaped grains in addition with some tiny spherically shaped grains. The grains are distributed uniformly over the entire surface of the films. Few holes are seen at few places on the surface of the film. The value of Ra is found to be 11.97 nm.
Fig. 7. (a,b). SEM picture of CdSe thin films electrodeposited at different solution pH values: (a) 2.0 (b) 2.5 0.1.
3.5. Compositional and optical absorption analyses The film composition has been analyzed using an energy dispersive analysis by X-rays set up attached with SEM. Typical EDX spectrum of CdSe thin films deposited at solution pH value around 2.5 0.1 is shown in Fig. 9a. It is observed that the emission lines of Cd and Se are present in the investigated energy range indicates the formation of CdSe thin films. Variation of Cd and Se content with solution pH value for films obtained at various solution pH values is shown in Fig. 9b. It is found that the content of Cd decreases and the content of Se increases while increasing the solution pH value from 1.5 to 2.5 0.1, thereafter both of them changes vice-versa. The average atomic ratio (Cd:Se) for CdSe thin films prepared at a solution pH value around 2.5 0.1 is found to be 49.78:50.22 and is nearly 0.99:1 indicating well defined stoichiometric nature of the deposited films. This result is also consistent with X-ray diffraction analysis of the sample with phase corresponding to CdSe. The stoichiometric ratio observed in the present work is found to be slightly higher than the value which has been reported earlier for cubic CdSe thin films by electrodeposition technique [14]. Optical parameters such as absorption coefficient, band gap are determined from optical absorption and transmittance measurements. The value of absorption coefficient for strong absorption region of thin film is calculated using the following Eq. (15) [1,21]
ahw ¼ A hw Eg
n
(15)
where a is the absorption coefficient in cm1, hn is the photon energy, A is an energy dependent constant and n is an integer. The value of ‘n’ determines the type of transition present in the material. In this case n ¼ 1/2 indicates that the transition present in the material is direct allowed. A plot of hn versus (ahn)2 is drawn for
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Fig. 8. (a,b). AFM (2D and 3D) image of CdSe thin films electrodeposited at solution pH value 2.5 0.1.
CdSe thin films obtained at various solution pH values is shown in Fig. 10. Extrapolation of linear portion of the graph to energy (hn) axis gives the band gap value of the material. The band gap value of the material obtained in the present work is found to be in the range between 1.69 and 1.74 eV for films obtained at various solution pH values. Refractive index (n) and extinction coefficient (k) are determined using the following Eq. (16) and Eq. (17) [11]. Fig. 11 shows the variation of refractive index (n) with wavelength (l) for films obtained at various solution pH values. It is observed that there is sudden decrease of dispersion (n) at 730 nm shows the presence of characteristic absorption at that wavelength. On the other hand, the extinction coefficient (k) is found to decrease with wavelength (l) and attained almost constant value after 730 nm (Fig. 12). The value of (n) is found to be almost constant at wavelength greater than 730 nm. The very strong absorption exhibited for photon energies above Eg value is very likely due to high density of cadmium ion within the lattice. Real and imaginary dielectric constants are determined by using the following Eqs. (18) and (19) [11]. Fig. 13 shows the variation of real dielectric constant (3 1) with wavelength
(l) for films obtained at various solution pH values. It is observed that the value of (3 1) indicated a similar variation as that of (n). The value of (3 2) decreases with increasing wavelength just like (k) which is shown in Fig. 14. It is observed from above Figures that all of them such as n, k, 31, 3 2 shows decreasing dependence with wavelength (l). This may be due to inter band transition for photon energy smaller than the smallest band gap. The value of n, k, 31, 3 2 are found to be 3.3, 0.64, 10.60, 4.32 correspond to the band gap value of the material.
n ¼
k ¼
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1þR 4R þ k2 1R ð1 RÞ2
al
4p
(16)
(17)
31
¼ n2 k2
(18)
32
¼ 2nk
(19)
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Fig. 11. Variation of refractive index (n) with wavelength (l) for CdSe thin films prepared at various solution pH values: (a) 1.5 (b) 2.0 (c) 2.5 0.1.
Fig. 9. a. Typical EDX spectrum of CdSe thin films obtained at solution pH value 2.5 0.1. b. Variation of Cd and Se content with solution pH value for CdSe thin films electrodeposited at various solution pH values.
3.6. Photoelectrochemical solar cell studies From the point of view of photoelectrochemical solar cells n-type semiconducting materials endowed with acceptable stability and compatibility are required so that the enhanced photoresponsiveness to illumination may become possible. The quality
Fig. 10. Plot of (hy) versus (ahy)2 for CdSe thin films electrodeposited at various solution pH values: (a) 1.5 (b) 2.0 (c) 2.5 0.1.
of electrodeposited semiconducting photoelectrode needed for photovoltaic applications depends to a large extent on the applied deposition potential, solution pH and composition of electrolytic bath. Photoelectrochemical solar cell studies are carried out in a standard three compartment cell comprising of n-(CdSe) as photocathode, a platinum electrode as anode and SCE as reference electrode, respectively. The electrolyte consists of 1 M each of Na2S, S and NaOH. A collimated narrow beam of light from tungsten filament (150 W) lamp is used as light source and spectral response measurements are carried out in the wavelength range between 360 and 900 nm using Keithley Multimeter (Model 2000, USA). Spectral response measurements have shown that the photocurrent density is found to increase with wavelength and attained its maximum value around (720e740) nm for films obtained at various solution pH value in the range between 2.0 and 3.0 0.1. If the wavelength is increased above 740 nm the value of photocurrent
Fig. 12. Variation of extinction coefficient (k) with wavelength (l) for CdSe thin films prepared at various solution pH values: (a) 1.5 (b) 2.0 (c) 2.5 0.1.
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Fig. 13. Variation of real dielectric constant (3 1) with wavelength (l) for CdSe thin films prepared at various solution pH values: (a) 1.5 (b) 2.0 (c) 2.5 0.1.
density is found to decrease thus indicates that value of photocurrent density exhibit a peak near the absorption edge of CdSe. Decrease in wavelength on longer wavelength side may be due to recombination of photogenerated carriers caused by surface states [1,26]. Similarly, decrease in value of photocurrent density on shorter wavelength side may be due to transition between defect levels [1,26]. Variation of photopotential with photocurrent density for CdSe thin films obtained at various solution pH values in the range between 2.0 and 3.0 is shown in Fig. 15. The cell parameters such as open circuit voltage, short circuit current, fill factor and efficiency are determined from power output characteristics. The value of fill factor and efficiency for films obtained at solution pH value 2.5 0.1 is found to be and 0.29 and 0.93%, respectively. The films obtained at solution pH value around 2.0 and 3.0 0.1 has lower value of fill factor and efficiency when compared to the film obtained at solution pH value 2.5 0.1. The lower value of efficiency may be due to the presence of defects in the deposited films. The process of annealing at higher temperatures which remove the defects and sizes of the grains of fabricated photoelectrodes and are
Fig. 15. Power output (IeV) characteristics of CdSe thin film photoelectrodes prepared at various solution pH values: (a) 2.0 (b) 2.5 (c) 3.0 0.1.
found to be increase through the diffusion of small crystallites. Increase in value of crystallite size resulted increase in value of diffusion length of charge carriers which enhances the fill factor and efficiency of the deposited films which could be reported by Hosun Moon et al. [26]. Hence, the process of annealing has been carried out for films obtained at solution pH value around 2.5 0.1 to increase the efficiency of the deposited films is in progress. 4. Conclusions Semiconducting CdSe thin films have been prepared by potentiostatic electrodeposition on ITO substrates. X-ray diffraction analysis showed that the deposited films possess polycrystalline nature with hexagonal structure with most prominent reflection along (002) plane. There is change in preferential orientation of the plane from (100) to (002) is observed for films prepared at various solution pH values in the range between 1.5 and 3.0 0.1. Microstructural parameters exhibit monotonic variation with solution pH values. Morphological and compositional analyses showed that films with spherical shaped grains and well-defined stoichiometry has been obtained at solution pH value around 2.5 0.1. Optical absorption analysis showed that the deposited film has a direct band gap value of 1.72 eV. The dispersion of n and k with frequency is similar to the behavior reported for semiconducting chalcogenides. Photoelectrochemical solar cell studies showed that the films prepared at solution pH value around 2.5 0.1 has better cell parameters than the films obtained at other solution pH values. Acknowledgement One of the authors (S. Thanikaikarasan) gratefully acknowledge the Council of Scientific and Industrial Research (CSIR), New Delhi, India for the award of Senior Research Fellowship (SRF) with File No:9/688(0010) 2008 to carry out this research work. References
Fig. 14. Variation of imaginary dielectric constant (3 2) with wavelength (l) for CdSe thin films prepared at various solution pH values: (a) 1.5 (b) 2.0 (c) 2.5 0.1.
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