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CdS structure
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Investigation of CdS and CdTe thin films and influence of CdCl2 on CdTe/CdS structure K.A.M.H. Siddiquee a,b , M.A.K. Pathan a,b , S. Alam a,b , O. Islam a,b,∗ , M.R. Qadir c a b c
Department of Physics, University of Dhaka, Dhaka 1000, Bangladesh Centre for Advanced Research in Sciences, University of Dhaka, Dhaka 1000, Bangladesh PP & PDC, BCSIR, Dhaka 1205, Bangladesh
a r t i c l e
i n f o
Article history: Received 16 August 2012 Accepted 7 January 2013 Keywords: Thin film Cadmium telluride Cadmium chloride Optical properties Structural properties
a b s t r a c t CdTe/CdS heterojunction solar cell structure has been fabricated using simple, easy and low-cost methods. To fabricate this structure, CdS and CdTe thin films are deposited onto FTO-coated conducting glass substrates by chemical bath deposition (CBD) and electrodeposition method, respectively. The optimized growth conditions are chosen for both CdS and CdTe films by investigating the optical, structural and morphological properties of both the as-deposited and annealed films. Optical measurement showed that CdS films have higher transmittance and lower absorbance, and CdTe films have lower transmittance and higher absorbance in the near infrared region. The band gap of CdS films is estimated to lie in the range 2.29–2.41 eV and that of CdTe films is in the range 1.53–1.55 eV. X-ray diffraction (XRD) study reveals that CdS and CdTe films are polycrystalline with preferential orientation of (1 1 1) plane. Scanning electron microscopy (SEM) study reveals that both films are smooth, voidfree and uniformly distributed over the surface of the substrate. Fabricated CdTe/CdS structure showed the anticipated rectifying behaviour, and the rectifying behaviour is observed to improve due to CdCl2 treatment. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction The semiconducting metal chalcogenides represent as interesting class of materials, which are attractive for large-scale applications because of the easy availability, and low cost of the starting materials. In recent years, increasing interest in the applications of solar cells has led to intensive research on the development of thin-film polycrystalline materials with acceptable conversion efficiency [1–3]. Among II–VI compounds CdTe, CdS, CdSe, etc. are prominent because of their properties like direct band gap, high absorption coefficients, etc. CdTe and CdS layers find their applications in photovoltaic devices. Several techniques, including chemical bath deposition, spray pyrolysis, electrodeposition, chemical vapor deposition, sputtering and vacuum evaporation, have been used for the deposition of CdS thin films [4–7]. Among them, chemical bath deposition (CBD) of CdS films from an aqueous solution is the most cost-effective and relatively easy process for solar cell fabrication [8]. Chalcogenide CdTe thin films have attracted considerable attention in recent years [9–11]. Cadmium telluride
∗ Corresponding author at: Department of Physics, University of Dhaka, Dhaka 1000, Bangladesh. E-mail addresses: [email protected], [email protected] (O. Islam).
is one of the most suitable materials for use in photovoltaic structures such as sensor, solar photovoltaic cells, nanodevices and other optoelectronic devices [12–14]. With its narrow and direct band gap, stability, and other optoelectronic and photoelectrochemical properties, it is the most ideal material for photovoltaic structures [15]. CdTe films have been deposited by various techniques such as electrodeposition, sputtering, close spaced sublimation (CSS) and vacuum evaporation, etc. [16–19]. However, electrodeposition is a particularly attractive method that has already been considered as a low-cost and large-area deposition technique for fabricating CdTe thin films [20]. A commonly used process, to enhance the grain size and density of the CdTe film, is the introduction of an annealing aid such as CdCl2 either during or after CdTe film growth [21]. Lot of works has been done for the improvement of CdS [8,22–24] and CdTe [25–27] thin films. But still there is lack of clear understanding of the growth mechanism and treatment of CdCl2 on CdTe and CdTe-based structure. This article reports the characterization of chemically deposited CdS and electrodeposited CdTe thin films by UV–vis spectrophotometer, XRD and SEM. Fabrication of CdTe/CdS solar cell structure, its characterization and annealing effect in presence of CdCl2 for CdTe/CdS structure are also investigated. Heat treatment on CdTe films without CdCl2 treatment is referred to as “air-annealed” and that on CdTe films with CdCl2 treatment is referred to as “CdCl2 -annealed” in the text.
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Please cite this article in press as: K.A.M.H. Siddiquee, et al., Investigation of CdS and CdTe thin films and influence of CdCl2 on CdTe/CdS structure, Optik - Int. J. Light Electron Opt. (2013), http://dx.doi.org/10.1016/j.ijleo.2013.01.099
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2. Experimental details 2.1. Chemical bath deposition (CBD) of CdS thin films Deposition of CdS films was carried out on FTO-coated conducting glass substrates by CBD method at different growth conditions using a bath contains 0.02 M cadmium chloride (CdCl2 ), 0.14 M thiourea ((NH2 )2 CS) and 0.083 M ammonium chloride (NH4 Cl) in distilled water. Ammonia solution was used to adjust the pH value of the bath. Before deposition, substrates were ultrasonically cleaned with acetone and rinsed with distilled water, and subsequently dried in air. The deposition was carried out at 80 ◦ C for 30 min. At the end of deposition process, the films were taken out from the bath and immediately cleaned in warm distilled water to remove the loosely adhered CdS particles on the film and finally dried in air. After getting photoelectrochemical, optical, structural and morphological measurements, the as-deposited CdS films were then annealed at 400 ◦ C for 60 min in air ambient to investigate the annealing effect.
2.2. Electrodeposition of CdTe thin films The deposition of CdTe films was carried out on FTO-coated conducting glass substrates by electrodeposition method using a BASI Epsilon electrochemical workstation. The electrodeposition cell employed a conventional three electrode geometry comprising a Pt counter electrode, a Ag/AgCl reference electrode and a FTOcoated conducting glass substrate as a working electrode. The bath for non-aqueous electrodeposition was prepared by dissolving 1 M cadmium chloride (CdCl2 ), 0.0015 M tellurium tetrachloride (TeCl4 ) and 0.05 M cadmium iodide (CdI2 ) in ethylene glycol. The temperature of the bath was maintained at 130 ◦ C during deposition and the deposition was carried out at different deposition time and at different potentials applied with respect to the reference electrode. Good quality films were observed to grow at −600 mV with respect to the reference electrode. Immediately after the deposition, the grown CdTe films were soaked in warm ethylene glycol and then rinsed in warm distilled water to remove the traces of the solvent and were subsequently dried in air. The deposited CdTe films were dipped into saturated CdCl2 –ethylene glycol solution for 5 min before heat treatment, and they were then annealed at 400 ◦ C for 60 min in air ambient to investigate the annealing effect in both absence and presence of CdCl2 .
2.3. Fabrication of solar cell structures After getting good quality films of CdS and CdTe, a CdTe/CdS structure was fabricated in our lab under optimum condition. To clean the surfaces from contaminants (like oxides), to remove damaged surface layers, to modify surface composition of the CdS films and to passivate their surface states, at first, chemically deposited and annealed CdS thin film samples were etched for 30 s in diluted NaOH and Na2 S2 O3 ·5H2 O solution. The deposition of CdTe onto CdS/FTO/glass was then performed at −600 mV with respect to the reference electrode for 20 min with keeping all other parameters constant as per Section 2.2. The fabricated CdTe/CdS structures were treated with saturated CdCl2 –ethylene glycol solution for 5 min, and then annealed at 400 ◦ C for 60 min in air ambient. The same heat treatment was also carried out on CdTe/CdS structures in air without CdCl2 treatment. After completing the deposition, the formation of metal (Al) contact was achieved by vacuum evaporation method and arrangement was made for I–V characteristics.
Fig. 1. Variation of transmittance (T, %) with wavelength (nm) of CdS films: (a) as-deposited and (b) annealed at 400 ◦ C.
2.4. Characterization techniques Photoelectrochemical cell (PEC) measurements were performed to determine the electrical conductivity type of the CdS and CdTe films. The films were studied using an UV–vis spectrophotometer for investigating the optical properties and for the estimation of band gap values. The X-ray diffraction (XRD) method was used to investigate the structural properties of deposited layer using a Bruker XRD system with Cu-K␣ radiation. Scanning Electron Microscopy (SEM) measurement was performed with JEOL JSM6490LA apparatus to carry out the SEM measurement of CdS and CdTe films. The CdTe/CdS and CdTe/CdS/CdCl2 structures were characterized by I–V measurements. 3. Results and discussion 3.1. Characterization of CdS thin films The photovoltage, created with white light illumination, was estimated by measuring the voltage under dark and illuminated conditions. A negative photovoltage was demonstrating the n-type character of the material means that the as-deposited CdS samples are n-type in electrical conduction [28]. Fig. 1 represents the variation of transmittance T (%) at wavelength range of 350–900 nm for different as-deposited and annealed CdS films prepared at different conditions. It is observed that as-deposited CdS films have lower transmittance in the UV–visible region; lower to moderate transmittance in the visible region and moderate to higher transmittance in the visible-infrared region as shown in Fig. 1a. The transmittance is slightly changed upon annealing the CdS films as shown in Fig. 1b. This higher transmittance of CdS films makes it suitable for use as a window material. Fig. 2 shows the variation of absorbance with wavelength range of 350–900 nm for different as-deposited and annealed CdS films. It is observed that the absorbance of the CdS films increases continually from the near-infrared towards the visible region, and become the highest absorbing at 350 nm. It is observed that as-deposited CdS films have higher absorbance in the UV–visible region; higher to moderate absorbance in the visible region and moderate to lower absorbance in the visible-infrared region as shown in Fig. 2a. A slight change is observed in absorbance spectra after annealing the CdS films as shown in Fig. 2b. The as-deposited film has the highest absorbance within the UV region of the absorbance spectrum and there is a slight lack of trend in the absorbance values displayed in the figure, caused by annealing the film.
Fig. 2. Variation of absorbance with wavelength (nm) of CdS films: (a) asdeposited and (b) annealed at 400 ◦ C.
Please cite this article in press as: K.A.M.H. Siddiquee, et al., Investigation of CdS and CdTe thin films and influence of CdCl2 on CdTe/CdS structure, Optik - Int. J. Light Electron Opt. (2013), http://dx.doi.org/10.1016/j.ijleo.2013.01.099
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Fig. 4. XRD spectra of CdS film: (a) as-deposited and (b) annealed at 400 ◦ C.
Fig. 3. (˛h)2 vs. h for CdS films.
The band gap energy (Eg ) for CdS films was determined by plotting (˛h)2 versus photon energy (h) for the corresponding wavelength graphs, where ˛ is the coefficient of optical absorption. The value of ˛ can be measured either using transmittance or absorbance. In this study we used absorbance for measuring the coefficient of optical absorption. From the optical data, the absorption coefficient (˛) was calculated using Lambert’s law [29]: 2.303A = ad
(1) n
˛=
Ao (h − Eg) ; h
n = 0.5 or 2
(2)
where h is the photon energy, Eg is the band gap energy and Ao is a constant which is related to the effective masses associated with the bands, A is the optical absorbance, and d is the film thickness. The film thickness was estimated to 0.1 m by using Fizeau Interferometer. The value of n is equal to 0.5 for direct band gap material and 2 for indirect band gap material [30]. Since CdS is a direct band gap material [31], the value of n = 0.5 is used in this experiment. Extrapolating the straight line portion of the curve in energy axis gives the values of band gap energy (Eg ) as shown in Fig. 3. The estimated values of the direct energy band gap lie in the range 2.29–2.41 eV, which is slightly lower than the value of 2.42 eV for single crystal [31]. Upon annealing the film, the band gap energy is observed to decrease due to the improvement of crystalline quality of the grown films. Fig. 4 shows the XRD pattern of as-deposited and annealed CdS thin film. Several well-defined peaks are observed in the XRD pattern. The XRD analysis reveals that the films are polycrystalline, and the sharp peaks are identified as (1 1 1), (2 2 0), and (3 1 1) planes of CdS. The strong peak at 26.54◦ corresponds to the reflection of cubic (1 1 1) plane, which agrees with the standard values [32]. The low intensity peaks observed in XRD pattern shows that the films are coarsely fine crystallites or nano-crystalline. The broad hump in the displayed pattern is due to the amorphous glass substrate and also possibly due to some amorphous phases present in the CdS thin films. The intensity of the peaks of as-deposited films became stronger after annealing the film indicates the improvement of crystalline quality due to annealing. Fig. 5 shows the SEM image of as-deposited CdS thin film on FTOcoated glass substrate. SEM image shows that the substrate is well covered with the deposited material without cracks and pinholes.
Fig. 5. SEM images of CdS film: (a) as-deposited and (b) annealed at 400 ◦ C.
The approximate average cluster sizes were estimated from different clusters within the film and are found to be about 150–300 nm SEM study for CdS films on the FTO-coated glass substrates reveals the uniform distribution of spherical grains over total coverage of the surface of the substrate. The grain size is observed to increase upon annealing the film in air ambient (as shown in Fig. 2b) indicates the improvement of crystalline quality of the CdS layer. 3.2. Characterization of CdTe thin films A positive photovoltage, measured by PEC, demonstrating the p-type character of the material means that the as-deposited CdTe samples are p-type in electrical conduction. Fig. 6 represents the variation of transmittance (T, %) with wavelength in the range 450–900 nm for as-deposited, air-annealed and CdCl2 -annealed CdTe films. The spectra show that the transmittance increases
Fig. 6. Variation of transmittance (T, %) with wavelength (, nm) of CdTe films: (a) as-deposited, (b) air-annealed at 400 ◦ C, and (c) CdCl2 -annealed at 400 ◦ C.
Please cite this article in press as: K.A.M.H. Siddiquee, et al., Investigation of CdS and CdTe thin films and influence of CdCl2 on CdTe/CdS structure, Optik - Int. J. Light Electron Opt. (2013), http://dx.doi.org/10.1016/j.ijleo.2013.01.099
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Fig. 7. Variation of absorbance with wavelength (nm) of CdTe films: (a) asdeposited, (b) air-annealed at 400 ◦ C, and (c) CdCl2 -annealed at 400 ◦ C.
with wavelength up to middle of the visible region as shown in Fig. 6a. Then the transmittance decreases with wavelength up to beginning of the infrared region. In the infrared region the transmittance becomes steady. The overall transmittance is significantly decreased upon air-annealing of the CdTe films than that of asdeposited films as shown in Fig. 6b. This behaviour may due to improved crystalline quality of CdTe films upon air-annealing [33]. In the case of CdCl2 -annealing of the CdTe films, overall transmittances is increased than that of air-annealed films, but lower than that of as-deposited films as shown in Fig. 6c, which may due to the presence of further Cd upon CdCl2 treatment. Fig. 7 shows the variation of absorbance with wavelength in the range 450–900 nm of as-deposited, air-annealed and CdCl2 annealed CdTe films. The spectra show that the absorbance decreases with wavelength up to middle of the visible region as shown in Fig. 7a, which agrees well with the standard value reported for bulk CdTe material [30]. Then the absorbance increases with wavelength up to beginning of the infrared region. The absorbance is significantly increased upon air-annealing of the CdTe films as shown in Fig. 7b. It is also observed that overall absorbance of CdCl2 -annealed CdTe films is increased than that of as-deposited CdTe films as shown in Fig. 7c. This behaviour may due to improved crystalline quality of CdTe films upon air-annealing as mentioned in the case of transmittance study [33]. Therefore, CdCl2 -treatment on CdTe improves the quality of CdTe films to use as an absorber material. The band gap energy (Eg ) was estimated by plotting (˛h)2 versus photon energy (h) graphs using Eqs. (1) and (2). The film thickness (d) was estimated to 1.0 m using Fizeau Interferometer. The value of n is equal to 0.5 for direct band gap material and 2 for indirect band gap material [30]. Since the transition from valence band to conduction band is known to be direct transition for CdTe [15], n = 0.5 is used in this experiment. Extrapolating the straight line portion of the curve in the energy axis gives the value of band gap energy (Eg ) as shown in Fig. 8. The estimated values of the direct energy band gap lie in the range 1.53–1.55 eV, which agree well with the standard value reported for bulk CdTe material [15]. Upon annealing the sample, the band gap energy is observed to decrease to 1.52 eV for the air-annealed and to 1.50 eV for the CdCl2 -annealed CdTe film deposited. The decrease of band gap upon annealing may due to the improvement of crystalline quality of the grown films. Fig. 9 shows the typical XRD pattern for as deposited, airannealed and CdCl2 -annealed CdTe films. The sharp diffraction peak observed at 27.50◦ correspond to (1 1 1) plane of the cubic CdTe structure [34]. XRD peaks at 2 equal to 27.50◦ , 40.38◦ , 45.91◦ and 49.67◦ correspond to the reflections from (1 1 1), (2 2 0), (3 1 1) and (3 3 1) planes, respectively. No diffraction peaks associated
Fig. 8. (˛h)2 vs. h for CdTe films: (a) as-deposited, (b) air-annealed at 400 ◦ C, and (c) CdCl2 -annealed at 400 ◦ C.
with metallic Cd, Te or other compounds were observed. This indicates that respective layered structures present a single phase with highly oriented CdTe crystallites with the (1 1 1) planes parallel to the substrate [21]. The [1 1 1] direction is the close-packing direction of the zinc-blend structure [35]. It is known that the grain orientation in the film is influenced by the nature of the substrate on which it is deposited [36]. The strong and sharp diffraction peaks indicate the formation of well crystalline sample. It can be seen that the major peak (1 1 1) is strongly dominating the other peaks as shown in Fig. 9a. The intensity of the peaks of as-deposited CdTe films became stronger after air-annealing as shown in Fig. 9b, which may due to the improvement of crystalline quality upon air-annealing. The peak intensity of (1 1 1) plane is significantly reduced after CdCl2 -annealing as shown in Fig. 9c. This result suggests that the CdTe films may lose their preferred orientation and become polycrystalline by heat treatment with CdCl2 [21]. The SEM images of air-annealed and CdCl2 -annealed CdTe thin films are shown in Fig. 10. SEM images show that the substrate is well covered with the deposited material without cracks and pinholes. In the case of air-annealed CdTe films, the grain size was small. The CdCl2 -annealing of the film appears to enlarge the grains of CdTe films by recrystallization and the CdTe film with the grain size of 0.8 m was obtained. The film after CdCl2 -annealing shows smooth and uniform crack free surface with larger granular-shaped identical grains with almost equal dimension spread all over the surface. It can be concluded that the increase of the grain size upon CdCl2 -annealing indicates the improvement of crystalline quality of the CdTe film, which is also confirmed from band gap measurement.
Fig. 9. XRD pattern of CdTe films: (a) as-deposited, (b) air-annealed at 400 ◦ C, and (c) CdCl2 -annealed at 400 ◦ C.
Please cite this article in press as: K.A.M.H. Siddiquee, et al., Investigation of CdS and CdTe thin films and influence of CdCl2 on CdTe/CdS structure, Optik - Int. J. Light Electron Opt. (2013), http://dx.doi.org/10.1016/j.ijleo.2013.01.099
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Fig. 10. SEM images of CdTe film: (left) air-annealed at 400 ◦ C and (right) CdCl2 annealed at 400 ◦ C.
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upon annealing due to improvement of crystalline quality. CdTe films are found to be polycrystalline with the (1 1 1) preferential orientation. From SEM study, the CdTe layers appeared dense and showed crack free surfaces with regular granular shaped grains. Annealing in the presence of CdCl2 is observed to have significant effect on the properties of CdTe films. After getting the good quality of CdS and CdTe layers, a fabrication of CdTe/CdS heterojunction solar cell structure has been performed. The dark current–voltage characteristics of the fabricated structure showed the anticipated rectifying behaviour. A thermal annealing of CdTe/CdS structure in presence of CdCl2 significantly improved the anticipated rectifying behaviour.
References
Fig. 11. Current vs. voltage behaviour of (a) CdTe/CdS and (b) CdTe/CdS/CdCl2 structure.
3.3. I–V measurements The current–voltage behaviour was recorded across FTO and Al contact. The dark current–voltage characteristics of the CdTe/CdS and CdTe/CdS/CdCl2 structures are shown in Fig. 11. In forward direction of the applied bias, the current is found to increase. The forward current exhibited a sluggish increase till 0.75 V beyond which a sharp threshold was observed. In reverse direction of the applied bias, at first the current is found to increase very slowly. After the applied voltage of −1.5 V, there exists a breakdown and the I–V behaviour of the CdTe/CdS structure showed the anticipated rectifying behaviour. There is a considerable series resistance effect. This behaviour is consistent with the large numbers of defects indicated by the carrier densities of CdTe found via Schottky barrier studies. I–V behaviour of the CdTe/CdS/CdCl2 structures showed significant improvement of the anticipated rectifying behaviour. After threshold voltage, forward current increases rapidly. After applying the bias voltage of −1.5 V, there exists a sharp breakdown. 4. Conclusions The optical, structural and morphological characterization of chemically deposited CdS and electrodeposited CdTe thin films has been performed for both as-deposited and annealed samples. Annealing effects of CdS films in air ambient and that of CdTe films with CdCl2 and without CdCl2 -treatment have also been investigated for choosing the quality of the films. Optical measurement showed that CdS layers exhibited band gap values vary from 2.29 to 2.41 eV and decreased upon annealing due to improvement of crystalline quality. XRD study reveals that CdS films are polycrystalline with preferential orientation of (1 1 1) plane with hexagonal phase. SEM study reveals that CdS layers are smooth, void free and uniformly distributed over the surface of the substrates. For absorber layers, optical measurement showed that CdTe layers exhibited band gap values vary from 1.53 to 1.55 eV for the films decreased
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Investigation of CdS and CdTe thin films and influence of CdCl2 on CdTe/CdS structure K.A.M.H. Siddiquee a,b , M.A.K. Pathan a,b , S. Alam a,b , O. Islam a,b,∗ , M.R. Qadir c a b c
Department of Physics, University of Dhaka, Dhaka 1000, Bangladesh Centre for Advanced Research in Sciences, University of Dhaka, Dhaka 1000, Bangladesh PP & PDC, BCSIR, Dhaka 1205, Bangladesh
a r t i c l e
i n f o
Article history: Received 16 August 2012 Accepted 7 January 2013 Keywords: Thin film Cadmium telluride Cadmium chloride Optical properties Structural properties
a b s t r a c t CdTe/CdS heterojunction solar cell structure has been fabricated using simple, easy and low-cost methods. To fabricate this structure, CdS and CdTe thin films are deposited onto FTO-coated conducting glass substrates by chemical bath deposition (CBD) and electrodeposition method, respectively. The optimized growth conditions are chosen for both CdS and CdTe films by investigating the optical, structural and morphological properties of both the as-deposited and annealed films. Optical measurement showed that CdS films have higher transmittance and lower absorbance, and CdTe films have lower transmittance and higher absorbance in the near infrared region. The band gap of CdS films is estimated to lie in the range 2.29–2.41 eV and that of CdTe films is in the range 1.53–1.55 eV. X-ray diffraction (XRD) study reveals that CdS and CdTe films are polycrystalline with preferential orientation of (1 1 1) plane. Scanning electron microscopy (SEM) study reveals that both films are smooth, voidfree and uniformly distributed over the surface of the substrate. Fabricated CdTe/CdS structure showed the anticipated rectifying behaviour, and the rectifying behaviour is observed to improve due to CdCl2 treatment. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction The semiconducting metal chalcogenides represent as interesting class of materials, which are attractive for large-scale applications because of the easy availability, and low cost of the starting materials. In recent years, increasing interest in the applications of solar cells has led to intensive research on the development of thin-film polycrystalline materials with acceptable conversion efficiency [1–3]. Among II–VI compounds CdTe, CdS, CdSe, etc. are prominent because of their properties like direct band gap, high absorption coefficients, etc. CdTe and CdS layers find their applications in photovoltaic devices. Several techniques, including chemical bath deposition, spray pyrolysis, electrodeposition, chemical vapor deposition, sputtering and vacuum evaporation, have been used for the deposition of CdS thin films [4–7]. Among them, chemical bath deposition (CBD) of CdS films from an aqueous solution is the most cost-effective and relatively easy process for solar cell fabrication [8]. Chalcogenide CdTe thin films have attracted considerable attention in recent years [9–11]. Cadmium telluride
∗ Corresponding author at: Department of Physics, University of Dhaka, Dhaka 1000, Bangladesh. E-mail addresses: [email protected], [email protected] (O. Islam).
is one of the most suitable materials for use in photovoltaic structures such as sensor, solar photovoltaic cells, nanodevices and other optoelectronic devices [12–14]. With its narrow and direct band gap, stability, and other optoelectronic and photoelectrochemical properties, it is the most ideal material for photovoltaic structures [15]. CdTe films have been deposited by various techniques such as electrodeposition, sputtering, close spaced sublimation (CSS) and vacuum evaporation, etc. [16–19]. However, electrodeposition is a particularly attractive method that has already been considered as a low-cost and large-area deposition technique for fabricating CdTe thin films [20]. A commonly used process, to enhance the grain size and density of the CdTe film, is the introduction of an annealing aid such as CdCl2 either during or after CdTe film growth [21]. Lot of works has been done for the improvement of CdS [8,22–24] and CdTe [25–27] thin films. But still there is lack of clear understanding of the growth mechanism and treatment of CdCl2 on CdTe and CdTe-based structure. This article reports the characterization of chemically deposited CdS and electrodeposited CdTe thin films by UV–vis spectrophotometer, XRD and SEM. Fabrication of CdTe/CdS solar cell structure, its characterization and annealing effect in presence of CdCl2 for CdTe/CdS structure are also investigated. Heat treatment on CdTe films without CdCl2 treatment is referred to as “air-annealed” and that on CdTe films with CdCl2 treatment is referred to as “CdCl2 -annealed” in the text.
0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.01.099
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2. Experimental details 2.1. Chemical bath deposition (CBD) of CdS thin films Deposition of CdS films was carried out on FTO-coated conducting glass substrates by CBD method at different growth conditions using a bath contains 0.02 M cadmium chloride (CdCl2 ), 0.14 M thiourea ((NH2 )2 CS) and 0.083 M ammonium chloride (NH4 Cl) in distilled water. Ammonia solution was used to adjust the pH value of the bath. Before deposition, substrates were ultrasonically cleaned with acetone and rinsed with distilled water, and subsequently dried in air. The deposition was carried out at 80 ◦ C for 30 min. At the end of deposition process, the films were taken out from the bath and immediately cleaned in warm distilled water to remove the loosely adhered CdS particles on the film and finally dried in air. After getting photoelectrochemical, optical, structural and morphological measurements, the as-deposited CdS films were then annealed at 400 ◦ C for 60 min in air ambient to investigate the annealing effect.
2.2. Electrodeposition of CdTe thin films The deposition of CdTe films was carried out on FTO-coated conducting glass substrates by electrodeposition method using a BASI Epsilon electrochemical workstation. The electrodeposition cell employed a conventional three electrode geometry comprising a Pt counter electrode, a Ag/AgCl reference electrode and a FTOcoated conducting glass substrate as a working electrode. The bath for non-aqueous electrodeposition was prepared by dissolving 1 M cadmium chloride (CdCl2 ), 0.0015 M tellurium tetrachloride (TeCl4 ) and 0.05 M cadmium iodide (CdI2 ) in ethylene glycol. The temperature of the bath was maintained at 130 ◦ C during deposition and the deposition was carried out at different deposition time and at different potentials applied with respect to the reference electrode. Good quality films were observed to grow at −600 mV with respect to the reference electrode. Immediately after the deposition, the grown CdTe films were soaked in warm ethylene glycol and then rinsed in warm distilled water to remove the traces of the solvent and were subsequently dried in air. The deposited CdTe films were dipped into saturated CdCl2 –ethylene glycol solution for 5 min before heat treatment, and they were then annealed at 400 ◦ C for 60 min in air ambient to investigate the annealing effect in both absence and presence of CdCl2 .
2.3. Fabrication of solar cell structures After getting good quality films of CdS and CdTe, a CdTe/CdS structure was fabricated in our lab under optimum condition. To clean the surfaces from contaminants (like oxides), to remove damaged surface layers, to modify surface composition of the CdS films and to passivate their surface states, at first, chemically deposited and annealed CdS thin film samples were etched for 30 s in diluted NaOH and Na2 S2 O3 ·5H2 O solution. The deposition of CdTe onto CdS/FTO/glass was then performed at −600 mV with respect to the reference electrode for 20 min with keeping all other parameters constant as per Section 2.2. The fabricated CdTe/CdS structures were treated with saturated CdCl2 –ethylene glycol solution for 5 min, and then annealed at 400 ◦ C for 60 min in air ambient. The same heat treatment was also carried out on CdTe/CdS structures in air without CdCl2 treatment. After completing the deposition, the formation of metal (Al) contact was achieved by vacuum evaporation method and arrangement was made for I–V characteristics.
Fig. 1. Variation of transmittance (T, %) with wavelength (nm) of CdS films: (a) as-deposited and (b) annealed at 400 ◦ C.
2.4. Characterization techniques Photoelectrochemical cell (PEC) measurements were performed to determine the electrical conductivity type of the CdS and CdTe films. The films were studied using an UV–vis spectrophotometer for investigating the optical properties and for the estimation of band gap values. The X-ray diffraction (XRD) method was used to investigate the structural properties of deposited layer using a Bruker XRD system with Cu-K␣ radiation. Scanning Electron Microscopy (SEM) measurement was performed with JEOL JSM6490LA apparatus to carry out the SEM measurement of CdS and CdTe films. The CdTe/CdS and CdTe/CdS/CdCl2 structures were characterized by I–V measurements. 3. Results and discussion 3.1. Characterization of CdS thin films The photovoltage, created with white light illumination, was estimated by measuring the voltage under dark and illuminated conditions. A negative photovoltage was demonstrating the n-type character of the material means that the as-deposited CdS samples are n-type in electrical conduction [28]. Fig. 1 represents the variation of transmittance T (%) at wavelength range of 350–900 nm for different as-deposited and annealed CdS films prepared at different conditions. It is observed that as-deposited CdS films have lower transmittance in the UV–visible region; lower to moderate transmittance in the visible region and moderate to higher transmittance in the visible-infrared region as shown in Fig. 1a. The transmittance is slightly changed upon annealing the CdS films as shown in Fig. 1b. This higher transmittance of CdS films makes it suitable for use as a window material. Fig. 2 shows the variation of absorbance with wavelength range of 350–900 nm for different as-deposited and annealed CdS films. It is observed that the absorbance of the CdS films increases continually from the near-infrared towards the visible region, and become the highest absorbing at 350 nm. It is observed that as-deposited CdS films have higher absorbance in the UV–visible region; higher to moderate absorbance in the visible region and moderate to lower absorbance in the visible-infrared region as shown in Fig. 2a. A slight change is observed in absorbance spectra after annealing the CdS films as shown in Fig. 2b. The as-deposited film has the highest absorbance within the UV region of the absorbance spectrum and there is a slight lack of trend in the absorbance values displayed in the figure, caused by annealing the film.
Fig. 2. Variation of absorbance with wavelength (nm) of CdS films: (a) asdeposited and (b) annealed at 400 ◦ C.
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Fig. 4. XRD spectra of CdS film: (a) as-deposited and (b) annealed at 400 ◦ C.
Fig. 3. (˛h)2 vs. h for CdS films.
The band gap energy (Eg ) for CdS films was determined by plotting (˛h)2 versus photon energy (h) for the corresponding wavelength graphs, where ˛ is the coefficient of optical absorption. The value of ˛ can be measured either using transmittance or absorbance. In this study we used absorbance for measuring the coefficient of optical absorption. From the optical data, the absorption coefficient (˛) was calculated using Lambert’s law [29]: 2.303A = ad
(1) n
˛=
Ao (h − Eg) ; h
n = 0.5 or 2
(2)
where h is the photon energy, Eg is the band gap energy and Ao is a constant which is related to the effective masses associated with the bands, A is the optical absorbance, and d is the film thickness. The film thickness was estimated to 0.1 m by using Fizeau Interferometer. The value of n is equal to 0.5 for direct band gap material and 2 for indirect band gap material [30]. Since CdS is a direct band gap material [31], the value of n = 0.5 is used in this experiment. Extrapolating the straight line portion of the curve in energy axis gives the values of band gap energy (Eg ) as shown in Fig. 3. The estimated values of the direct energy band gap lie in the range 2.29–2.41 eV, which is slightly lower than the value of 2.42 eV for single crystal [31]. Upon annealing the film, the band gap energy is observed to decrease due to the improvement of crystalline quality of the grown films. Fig. 4 shows the XRD pattern of as-deposited and annealed CdS thin film. Several well-defined peaks are observed in the XRD pattern. The XRD analysis reveals that the films are polycrystalline, and the sharp peaks are identified as (1 1 1), (2 2 0), and (3 1 1) planes of CdS. The strong peak at 26.54◦ corresponds to the reflection of cubic (1 1 1) plane, which agrees with the standard values [32]. The low intensity peaks observed in XRD pattern shows that the films are coarsely fine crystallites or nano-crystalline. The broad hump in the displayed pattern is due to the amorphous glass substrate and also possibly due to some amorphous phases present in the CdS thin films. The intensity of the peaks of as-deposited films became stronger after annealing the film indicates the improvement of crystalline quality due to annealing. Fig. 5 shows the SEM image of as-deposited CdS thin film on FTOcoated glass substrate. SEM image shows that the substrate is well covered with the deposited material without cracks and pinholes.
Fig. 5. SEM images of CdS film: (a) as-deposited and (b) annealed at 400 ◦ C.
The approximate average cluster sizes were estimated from different clusters within the film and are found to be about 150–300 nm SEM study for CdS films on the FTO-coated glass substrates reveals the uniform distribution of spherical grains over total coverage of the surface of the substrate. The grain size is observed to increase upon annealing the film in air ambient (as shown in Fig. 2b) indicates the improvement of crystalline quality of the CdS layer. 3.2. Characterization of CdTe thin films A positive photovoltage, measured by PEC, demonstrating the p-type character of the material means that the as-deposited CdTe samples are p-type in electrical conduction. Fig. 6 represents the variation of transmittance (T, %) with wavelength in the range 450–900 nm for as-deposited, air-annealed and CdCl2 -annealed CdTe films. The spectra show that the transmittance increases
Fig. 6. Variation of transmittance (T, %) with wavelength (, nm) of CdTe films: (a) as-deposited, (b) air-annealed at 400 ◦ C, and (c) CdCl2 -annealed at 400 ◦ C.
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Fig. 7. Variation of absorbance with wavelength (nm) of CdTe films: (a) asdeposited, (b) air-annealed at 400 ◦ C, and (c) CdCl2 -annealed at 400 ◦ C.
with wavelength up to middle of the visible region as shown in Fig. 6a. Then the transmittance decreases with wavelength up to beginning of the infrared region. In the infrared region the transmittance becomes steady. The overall transmittance is significantly decreased upon air-annealing of the CdTe films than that of asdeposited films as shown in Fig. 6b. This behaviour may due to improved crystalline quality of CdTe films upon air-annealing [33]. In the case of CdCl2 -annealing of the CdTe films, overall transmittances is increased than that of air-annealed films, but lower than that of as-deposited films as shown in Fig. 6c, which may due to the presence of further Cd upon CdCl2 treatment. Fig. 7 shows the variation of absorbance with wavelength in the range 450–900 nm of as-deposited, air-annealed and CdCl2 annealed CdTe films. The spectra show that the absorbance decreases with wavelength up to middle of the visible region as shown in Fig. 7a, which agrees well with the standard value reported for bulk CdTe material [30]. Then the absorbance increases with wavelength up to beginning of the infrared region. The absorbance is significantly increased upon air-annealing of the CdTe films as shown in Fig. 7b. It is also observed that overall absorbance of CdCl2 -annealed CdTe films is increased than that of as-deposited CdTe films as shown in Fig. 7c. This behaviour may due to improved crystalline quality of CdTe films upon air-annealing as mentioned in the case of transmittance study [33]. Therefore, CdCl2 -treatment on CdTe improves the quality of CdTe films to use as an absorber material. The band gap energy (Eg ) was estimated by plotting (˛h)2 versus photon energy (h) graphs using Eqs. (1) and (2). The film thickness (d) was estimated to 1.0 m using Fizeau Interferometer. The value of n is equal to 0.5 for direct band gap material and 2 for indirect band gap material [30]. Since the transition from valence band to conduction band is known to be direct transition for CdTe [15], n = 0.5 is used in this experiment. Extrapolating the straight line portion of the curve in the energy axis gives the value of band gap energy (Eg ) as shown in Fig. 8. The estimated values of the direct energy band gap lie in the range 1.53–1.55 eV, which agree well with the standard value reported for bulk CdTe material [15]. Upon annealing the sample, the band gap energy is observed to decrease to 1.52 eV for the air-annealed and to 1.50 eV for the CdCl2 -annealed CdTe film deposited. The decrease of band gap upon annealing may due to the improvement of crystalline quality of the grown films. Fig. 9 shows the typical XRD pattern for as deposited, airannealed and CdCl2 -annealed CdTe films. The sharp diffraction peak observed at 27.50◦ correspond to (1 1 1) plane of the cubic CdTe structure [34]. XRD peaks at 2 equal to 27.50◦ , 40.38◦ , 45.91◦ and 49.67◦ correspond to the reflections from (1 1 1), (2 2 0), (3 1 1) and (3 3 1) planes, respectively. No diffraction peaks associated
Fig. 8. (˛h)2 vs. h for CdTe films: (a) as-deposited, (b) air-annealed at 400 ◦ C, and (c) CdCl2 -annealed at 400 ◦ C.
with metallic Cd, Te or other compounds were observed. This indicates that respective layered structures present a single phase with highly oriented CdTe crystallites with the (1 1 1) planes parallel to the substrate [21]. The [1 1 1] direction is the close-packing direction of the zinc-blend structure [35]. It is known that the grain orientation in the film is influenced by the nature of the substrate on which it is deposited [36]. The strong and sharp diffraction peaks indicate the formation of well crystalline sample. It can be seen that the major peak (1 1 1) is strongly dominating the other peaks as shown in Fig. 9a. The intensity of the peaks of as-deposited CdTe films became stronger after air-annealing as shown in Fig. 9b, which may due to the improvement of crystalline quality upon air-annealing. The peak intensity of (1 1 1) plane is significantly reduced after CdCl2 -annealing as shown in Fig. 9c. This result suggests that the CdTe films may lose their preferred orientation and become polycrystalline by heat treatment with CdCl2 [21]. The SEM images of air-annealed and CdCl2 -annealed CdTe thin films are shown in Fig. 10. SEM images show that the substrate is well covered with the deposited material without cracks and pinholes. In the case of air-annealed CdTe films, the grain size was small. The CdCl2 -annealing of the film appears to enlarge the grains of CdTe films by recrystallization and the CdTe film with the grain size of 0.8 m was obtained. The film after CdCl2 -annealing shows smooth and uniform crack free surface with larger granular-shaped identical grains with almost equal dimension spread all over the surface. It can be concluded that the increase of the grain size upon CdCl2 -annealing indicates the improvement of crystalline quality of the CdTe film, which is also confirmed from band gap measurement.
Fig. 9. XRD pattern of CdTe films: (a) as-deposited, (b) air-annealed at 400 ◦ C, and (c) CdCl2 -annealed at 400 ◦ C.
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Fig. 10. SEM images of CdTe film: (left) air-annealed at 400 ◦ C and (right) CdCl2 annealed at 400 ◦ C.
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upon annealing due to improvement of crystalline quality. CdTe films are found to be polycrystalline with the (1 1 1) preferential orientation. From SEM study, the CdTe layers appeared dense and showed crack free surfaces with regular granular shaped grains. Annealing in the presence of CdCl2 is observed to have significant effect on the properties of CdTe films. After getting the good quality of CdS and CdTe layers, a fabrication of CdTe/CdS heterojunction solar cell structure has been performed. The dark current–voltage characteristics of the fabricated structure showed the anticipated rectifying behaviour. A thermal annealing of CdTe/CdS structure in presence of CdCl2 significantly improved the anticipated rectifying behaviour.
References
Fig. 11. Current vs. voltage behaviour of (a) CdTe/CdS and (b) CdTe/CdS/CdCl2 structure.
3.3. I–V measurements The current–voltage behaviour was recorded across FTO and Al contact. The dark current–voltage characteristics of the CdTe/CdS and CdTe/CdS/CdCl2 structures are shown in Fig. 11. In forward direction of the applied bias, the current is found to increase. The forward current exhibited a sluggish increase till 0.75 V beyond which a sharp threshold was observed. In reverse direction of the applied bias, at first the current is found to increase very slowly. After the applied voltage of −1.5 V, there exists a breakdown and the I–V behaviour of the CdTe/CdS structure showed the anticipated rectifying behaviour. There is a considerable series resistance effect. This behaviour is consistent with the large numbers of defects indicated by the carrier densities of CdTe found via Schottky barrier studies. I–V behaviour of the CdTe/CdS/CdCl2 structures showed significant improvement of the anticipated rectifying behaviour. After threshold voltage, forward current increases rapidly. After applying the bias voltage of −1.5 V, there exists a sharp breakdown. 4. Conclusions The optical, structural and morphological characterization of chemically deposited CdS and electrodeposited CdTe thin films has been performed for both as-deposited and annealed samples. Annealing effects of CdS films in air ambient and that of CdTe films with CdCl2 and without CdCl2 -treatment have also been investigated for choosing the quality of the films. Optical measurement showed that CdS layers exhibited band gap values vary from 2.29 to 2.41 eV and decreased upon annealing due to improvement of crystalline quality. XRD study reveals that CdS films are polycrystalline with preferential orientation of (1 1 1) plane with hexagonal phase. SEM study reveals that CdS layers are smooth, void free and uniformly distributed over the surface of the substrates. For absorber layers, optical measurement showed that CdTe layers exhibited band gap values vary from 1.53 to 1.55 eV for the films decreased
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Please cite this article in press as: K.A.M.H. Siddiquee, et al., Investigation of CdS and CdTe thin films and influence of CdCl2 on CdTe/CdS structure, Optik - Int. J. Light Electron Opt. (2013), http://dx.doi.org/10.1016/j.ijleo.2013.01.099