In the current work, the effect of an annealing process on structural, morphological, and optical behaviors of cadmium sulfide thin films is presented. A chemical bath deposition method for cadmium sulfide deposition is based on the use of glycine as a complexing agent. Cadmium sulfide thin films were grown by chemical bath deposition and annealed in a nitrogen atmosphere at 0, 100, 150, 200, and 250 °C for 30 minutes. Crystallographic and morphological studies of CdS thin films were performed by scanning electron microscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and X-ray diffraction. The optical behavior of CdS thin films was evaluated by UV-Visible spectroscopy and band gap values were calculated by approximation using a Tauc plot. After annealing, densified CdS films formed by nanostructured crystallites exhibit suitable band gap values and present relatively high transmittance in the visible region, allowing their application as a window material for solar cells.
© 2014 Optical Society of America
The high demand for energy sources promotes the increasing interest in versatile and cheap materials and synthesis methods for thin film solar cell fabrication. Semiconductor materials properties and quality play an important role in the efficiency of solar cells based on thin film technologies. Among the several n-type semiconductors used as window materials in thin film solar cells, CdS is the most promising heterojunction partner material for PbS, CuIn(Ga)Se2 and CdTe, owing to its excellent properties . CdS is an n-type semiconductor with a typical direct band gap of 2.42 eV and appropriate optical absorption in the visible region. CdS thin films can be obtained by different techniques, such as: sputtering , thermal evaporation [3, 4], the SILAR method , spray pyrolysis [6, 7], and chemical bath deposition [8, 9]. Chemical bath deposition (CBD) is widely used for thin film deposition, because it is a simple, economic, large scale, and viable technique. In addition, by this technique can be produced thin films with acceptable quality for optoelectronic devices. CBD techniques have already been used for preparation of various semiconductor alloy films (Cd1−xPbxS, Cd1−xZnxS, and Cd1−xMnxS) and compounds (CdSe, CdTe, PbS, and CdS). The interest in the CBD technique to deposit semiconductor CdS thin films has increased because of the relatively high efficiencies achieved in thin film solar cells, specifically, CuIn(Ga)Se2 and CdTe based structures [1, 10–13]. In the case of CdTe thin film solar cells, it is possible to identify some attractive advantages, such as: the highest theoretical efficiency and the lowest inherent manufacturing costs. Also, they have a superior temperature coefficient and spectral response, resulting in a significant performance advantage compared to conventional crystalline silicon solar modules. Recently, CdTe commercial photovoltaic modules (First Solar®) have demonstrated superior performance advantages over conventional crystalline silicon solar modules, generating up to 8% more energy compared with the same power rating modules . The latest report of the world record efficiency is 19.4%  and their theoretical limit is near to 30%, bringing tremendous opportunities for their improvement through the enhancement of fabrication processes and semiconductor materials, such as: CdS [2, 10]. In that sense, the thermal treatment (annealing) after deposition and CdS nanostructured thin films can offer a way to improve their electrical and optical performance . In the present work, the effects of the annealing process on structural, morphological, and optical behaviors of CdS nanostructured thin films were investigated. Nanostructured thin films of crystalline CdS with 180 nm thickness were grown on glass substrate by CBD technique at 70 °C for 30 minutes. After that, CdS films were annealed in nitrogen atmosphere and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM/EDS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and UV-Vis spectroscopy.
2. Experimental details
CdS thin films were deposited on Corning® glass substrates by CBD technique. Prior to CdS deposition, glass substrates were cleaned in an ultrasonic bath using successive washing with acetone, isopropyl alcohol, and deionized water; with the purpose of removing impurities of the substrate surface. After that, the glass substrates were dried and placed in a beaker containing the chemical solution used for film deposition. Chemical solutions were prepared in a beaker of 80 mL of capacity by sequential additions of 31 mL of deionized water, 4 mL of 0.1 M cadmium nitrate tetrahydrate, 5 mL of 0.1 M glycine, 4 mL of borate (buffer pH 10), and 5 mL of 1 M thiourea. Then, deionized water was added to each solution until reaching 60 mL, as total volume. The deposition process of CdS thin films was carried out in a thermal bath at 70 °C for 30 minutes, without stirring. After that, films were annealed in a furnace under controlled nitrogen atmosphere at four different temperatures (100, 150, 200, and 250 °C) for 30 minutes. During thermal treatment of CdS films, ultra high purity nitrogen gas (UHP-N2) was fed to remove oxygen from the chamber and it was maintained slightly above atmospheric pressure with the purpose of avoiding phase transformation from CdS to CdO. Hence, as-deposited and annealed samples were prepared for their structural, morphological, and optical characterizations. Crystallographic study was accomplished by XRD in a Rigaku Ultima III instrument. Average diameter of crystal particles of the crystalline CdS samples was calculated using Debye-Scherrer’s formula , given by:
3. Results and discussion
3.1. Crystalline structure and morphological properties
Figure 1 shows representative XRD patterns obtained for the CdS thin films as-deposited and annealed at 250 °C.
Qualitative and quantitative analyses by XRD for both as-deposited and annealed films indicate that all reflections pertain only to the polymorphs of CdS phase, and they occur predominantly as cubic and hexagonal crystalline structures for as deposited and annealed films, respectively. For as deposited films, the predominant peak is located at 26.505° of 2θ and it corresponds to (111) crystallographic plane of the cubic phase of CdS (JCPDS No. 01-075-1546; cadmium sulfide (CdS) or Hawleyite; cubic, a=b=c=5.82 Å; and 26.50°, 43.96°, 52.07°, and 30.69°). On the other hand, XRD patterns corresponding to annealed deposits show that the main peak (100% of intensity) is placed at 26.66° of 2θ and it is associated with the (002) crystallographic plane of the hexagonal phase of CdS (JCPDS No. 00-041-1049; cadmium sulfide (CdS) or Greenockite, syn; hexagonal, a=b=4.14 Å, c=6.72 Å; and 28.18°, 26.51°, 47.840°, and 51.82°). Relevant parameters for the obtained CdS thin films with cubic (as-deposited) and hexagonal (annealed) crystalline structure are presented in Table 1.
Conversely to previous works by other authors [16, 18] the evidences found under the experimental conditions considered for this study are not enough to confirm the formation of the cubic phase of CdS in samples annealed at 250°C. Given that the main peaks at 26.50° and 30.69° of 2θ for the CdS cubic polymorph are not visible, the presence of CdS cubic polymorph was discarded for annealed samples at 250°C. This behavior can be explained by the following statements: (a) the amount of this phase is probably below the detection limits (less than 0.5%) of the XRD technique , and (b) it is possible to consider that the processing conditions (using glycine complexing agent) can promote the selective formation of CdS phases. In accordance with the previously cited works [16,18], it is clear that a phase transition from cubic to hexagonal exists during the annealing process. This change can be corroborated by the displacement of the mean peak at 26.50° of 2θ corresponding to the (111) plane of the cubic phase CdS for as deposited films to 26.66° of 2θ for annealed films. The last mentioned position in 2θ corresponds to the (002) crystallographic plane of the hexagonal phase of CdS. It has been reported [4, 19] that CdS can display a preferential orientation in that crystallographic plane when it is deposited by chemical bath and annealed. Additionally, it is clearly observed that the full-width at half-maximum (FWHM) of the main peak increases as a function of temperature, and is associated with the increase of the crystal size in CdS films. Measurement of the average particle sizes of the crystalline CdS thin films from XRD patterns by Debye-Scherrer’s formula indicate that CdS thin films as deposited and annealed are formed by nanosized crystallites of 13.87 and 17.49 nm, respectively. These results show that the augmentation of the crystal size is thermally activated and directly proportional to the annealing temperature. This behavior can be explained by the coarsening of the tiny crystals with high reactivity that is stimulated at relatively high temperatures (250 °C).
Qualitative and quantitative analyses for CdS nanostructured films (as deposited and annealed) by X-ray photoelectron spectroscopy (XPS) are validated and presented in Fig. 2 and Table 1, respectively. In addition, chemical oxidation states for Cd and S are included.
In Fig. 2, the representative XPS survey spectra (a) for the CdS thin films as deposited and annealed at different temperatures (100, 150, 200, and 250 °C) confirm the presence of Cd and S in the films. Also, Cd 3d binding energy spectra indicate that the binding energies of the Cd 3d5/2 and Cd 3d3/2 peaks are 405 and 411.7 eV, respectively. The S 2p binding energy spectra shows that the binding energy of S 2p is 162 eV. These values of binding energy are directly related with the CdS phase existence . The quantitative analysis by XPS corresponding to CdS films is presented in Table 2.
Qualitative chemical analyses by XPS of the CdS films as deposited and annealed show that annealing treatment has a noteworthy effect on the composition of CdS deposits, as follows: the CdS phase of films as deposited present the following chemical formula Cd0.6S0.4. Then, they change to Cd0.55S0.45, when the annealing temperature increases to 250 °C. These changes can be attributed to the formation of stoichiometric CdS in the deposited films and the removal of volatile phases present in the films and the remaining reagent used as raw materials for their synthesis.
Figure 3 corresponds to typical SEM photomicrographs of CdS nanostructured films as deposited and annealed at 100, 150, 200, and 250 °C.
According to Fig. 3, the surfaces of CdS films show variations in the structure and the average chemical composition (EDS quantitative analysis corresponds to the chemical composition of four zones of the films in the SEM photomicrographs for each condition, but does not represent the bulk composition of the surface of CdS film obtained by XPS) as a function of temperature. They present changes in morphology and grain size taking features from porous (a, b, and c) to compact deposits (d and e), when the temperature increases from 100 to 250°C. Microstructural evolution of CdS films can be described as follows: (a) samples as deposited correspond to granular deposits with relative surface chemical composition of Cd and S of 45.48 and 54.52 atomic percent (At. %), respectively; (b) samples annealed at 100 °C, correspond to a mix of granular and laminated deposits with 45.75 and 54.25 atomic percent of Cd and S, respectively; (c) samples annealed at 150 °C present particulate structures, high porosity and they contain 47.83 and 52.17 atomic percent of Cd and S, respectively. (d) samples annealed at 200 °C have compact and well defined grain structures and contain 47.75 and 52.25 atomic percent of Cd and S, respectively; and (e) samples annealed at 250 °C have compact and well defined grain structure with fewer bright spots than Fig. 3(d) and contain 47.67 and 52.33 atomic percent of Cd and S, respectively. The error associated to the average EDS surface chemical composition measurements corresponds to 4.84%.
In the next paragraph, a detailed analysis of the surface characteristics of CdS nanostructured films annealed at different temperatures using AFM images is presented. Figure 4 shows representative 3D images of the CdS nanostructured films obtained by AFM and WSxM .
AFM results indicate that the thermal treatment promotes an increase in the surface roughness degree (root mean squared, RMS) until 200 °C, after that, RMS roughness presents an abrupt decrease to 3.20 nm, when the temperature reaches 250 °C. The last mentioned event can be associated with the reorganization of the crystallites and the densification of CdS films. As a result, more compact films are obtained at higher temperatures (See Fig. 4).
3.2. Optical properties and band gap
CBD-CdS films are yellowish and homogeneous. Additionally, these films present suitable adhesion at the glass substrate and thickness values of 179.80, 180.00, 179.90, 180.00, and 180.10 nm for as deposited and annealed films at 100, 150, 200, and 250 °C, respectively. As expected, the thermal treatments have no remarkable effect on the film thicknesses, and their values remain around 180 nm for as-deposited and annealed films. On the other hand, the optical behavior is evidently affected by thermal treatment. In Fig. 5, optical transmittance (a) and absorbance (b) spectra of CBD-CdS nanostructured films as a function of the annealing temperature are shown.
Transmission curves for as deposited and annealed films exhibit a good response in the visible spectrum with values of 60% in the range of 500 to 800 nm, except in the sample thermally treated at 200 °C. For the annealed film at 200 °C, a fast increase in the transmittance from 46 to 77% in the wavelength range of 500 to 800 nm can be observed. Films with the same behavior have been suggested for solar cell application with relatively high efficiency, given that under this condition, more light can penetrate into the active region of the solar cells [22, 23]. In the same way, the absorbance spectra for CdS films reveals that the absorption values for all samples in the range of 500 to 800 nm is low. The optical behavior of annealed CdS films is in good agreement with other studies [24–26].
Additionally, the direct band gap (Eg) can be determined using the Tauc relation given by :23,28]. In Fig. 6, the plot of (αhν)2 as a function of photon energy hν is shown.
According to the optical characterization, films without annealing present an optical band gap of 2.52 eV, and after annealing treatment, the band gap of the films can be modified in the range of 2.51 to 2.46 eV with increasing annealing temperature from 100 to 250 °C, respectively. Hence, the change of band gap values is indirectly proportional to annealing temperature.
Lastly, structural and optical properties of annealed CdS films obtained by CBD, specifically their suitable band gap values and relatively high transmittance in the visible region, allow us to suggest the obtained films as efficient window materials for solar cells. The efficiency reported for solar cells based on CdS/PbS semiconductor materials is around 1.65% . Solar cells based on CdS semiconductor materials with transmittance of 45% in the wavelength range of 500 to 1100 nm have been reported with efficiencies of 1.35%  and semiconductors with 50% show a maximum efficiency of 1.68% . According to these results, it is estimated that solar cells based on CdS with transmittance of 77% in wavelength range of 300–800 nm could improve the efficiency up to the efficiencies reported in the literature [22, 23] for solar cells based on PbS/CdS. Also, these kinds of annealed thin films of nanostructured CdS can improve the efficiency of CuIn(Ga)Se2 and CdTe based structures
According to current research, annealing processes have a strong effect on the structural, morphological, and optical behaviors of CdS nanostructured films obtained by CBD using glycine as ligand. CBD-CdS nanostructured films are formed by nanosized crystallites of 13.8 nm. After the annealing process, CdS films become more compact and densified deposits, reaching crystallite sizes of 17.49 nm for the maximum annealing temperature (250 °C). Crystallite size increase is due to the activation of coarsening phenomena by the increase of temperature. In the densification process of the CdS films, glycine plays an important role and it acts as agglutinate agent and allows the densification of CdS films at relatively low temperature (250 °C). In addition, an increase of the crystallinity degree and RMS roughness is achieved. A decrease of optical band gap from 2.52 eV to 2.46 eV is generated for as-deposited and annealed films (250 °C), respectively. Annealed nanostructured CdS films based in glycine as complexing agent with the aforementioned characteristics can find potential applications as window materials for PbS, CuIn(Ga)Se2 and CdTe based structures for thin film solar cells.
The authors gratefully acknowledge the laboratory of XPS and the technician Roberto Mora for technical support. We also express thanks to Laboratorio de Nanomateriales-FIC of the Universidad de Colima in Coquimatlán, Colima for technical assistance and facilities.
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