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CdS thin film solar cells
Optik 127 (2016) 7986–7992
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Effect of annealing on the structural properties of thermal evaporated CdIn2 Te4 /CdS thin film solar cells I˙ brahim Kırbas¸, Rasim Karabacak ∗ Pamukkale University, Engineering Faculty, Mechanical Engineering Department, Kınıklı Campus, 20070 Denizli, Turkey
a r t i c l e
i n f o
Article history: Received 11 April 2016 Accepted 1 June 2016 Keywords: Thin films solar cells II–VI Semiconductors Thermal evaporation CdIn2 Te4
a b s t r a c t In this study, a thermal evaporation technique and the effect of annealing on the structural properties of CdIn2 Te4 /CdS thin film solar cells were investigated. Thin film solar cells were deposited onto an indium tin oxide (ITO)-coated glass substrate using a thermal evaporation technique. The nitrogen atmosphere was 400 ◦ C for 1 h of annealing. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray (EDAX) analysis were performed on the solar cells. XRD analysis revealed two peaks (2 = 27.2◦ and 33.6◦ ). We observed increased peak severity but identical peak position in the annealed films. The Xray diffraction patterns of the annealed and as-deposited solar cells’ preferred orientations in nature have been detected as (200) and (202), respectively. Crystallite size (D), interplanar distance (d), and lattice constant (a) values were calculated for the thin film solar cells using the XRD data. When examining the EDAX analysis and element placement, we detected only CdIn2 Te4 in the absorber layer and only CdS atoms in the window layer, but no impurity atoms in the structure. We also observed an increase in surface roughness of the annealed films in SEM images. The I–V characteristics show that the current is increased for annealed thin films solar cells. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction Thin film solar cells have attracted attention in recent years due to their potential to lower costs and increase energy conversion efficiency. Thin film solar cells have been found to be lower in efficiency than silicon solar cells, but these films have been chosen for their low production costs [1]. According to studies reported in the literature, the efficiency rates of thin film solar cells; cadmium tellurium (CdTe), copper indium gallium selenide (CIGS), and amorphous silicon (a-Si) are 18.3%, 20%, and 12.3%, respectively [2]. Researchers have extensively studied II–VI compounds for applications in the optoelectronic and photovoltaic industry [3–6]. CdTe, one of the most studied compounds in this family, is suitable for use in the production of solar cells because it has a solar energy conversion bandwidth of 1.5 eV [7–12]. These compounds generally crystallize in the form of cubic or hexagonal systems, and they have a defective chalcopyrite structure [13–15]. The most appropriate structure for CdTe thin film solar cells is produced by matching n-type CdS and p-type CdTe [16]. II-III2 -VI4 compounds, such as CdIn2 Te4 compounds and II–VI compounds, have received great attention in recent years [17,18]. In the last decade, the emphasis has been on the development of low-cost deposition techniques [19]. The thermal evaporation technique for
∗ Corresponding author. E-mail address: [email protected] (R. Karabacak). http://dx.doi.org/10.1016/j.ijleo.2016.06.014 0030-4026/© 2016 Elsevier GmbH. All rights reserved.
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Fig. 1. Schematic diagram of CdIn2 Te4 /CdS solar cells on ITO-coated glass substrate [16,32].
depositing CdInTe [18] and CdS thin films is one of the most widely used because it is very simple due to having relatively few control parameters [20]. CdInTe and CdTe films are very sensitive, so various preparation techniques are used to prepare them, including thermal evaporation [21,22], electro deposition [23], the molecular beam epitaxial technique [24], closed-space sublimation [25–27], closed-space vapor transport [28,29], magnetron sputtering [30], and chemical deposition [31]. The properties of CdInTe films have been investigated in many studies [11,15,17,18,29], but more information is needed. In this study, for the first time, solar cells were manufactured by depositing CdIn2 Te4 and CdS on an ITO-coated glass substrate using the thermal evaporation technique. The effects of annealing on the structural characteristics of these solar cells were examined. The results are presented in graphs and tables. 2. Experimental details 2.1. Materials and methods 2.1.1. Depositing CdIn2 Te4 CdIn2 Te4 polycrystalline was deposited on an ITO-coated substrate at a pressure of 5 × 10−5 Torr with a thermal evaporation system. During the deposition process, the substrate was rotated at a constant speed so that the deposition parameters ´˚ were consistent and homogenous. The evaporation rate of the material was maintained at approximately 15–25 A/s. When ´ ˚ of closed cutting thickness was reached, the storage process was terminated. 1 m (10 kA) 2.1.2. Annealing process CdIn2 Te4 deposited substrates [16] were annealed for 1 h in a PROTHERM brand horizontal furnace, which was heated up to 400 ◦ C and maintained in a nitrogen atmosphere. 2.1.3. Depositing CdS After the glass/ITO/CdIn2 Te4 structure was formed, annealed and as-deposited samples were placed in the thermal evaporation system holder to store polycrystalline CdS, which has a window layer. After the CdS was placed into the tungsten crucible in the form of a powder of 99.999% purity, the system was closed. The process of rotating the substrate holder and vacuuming the system began, and the vacuum process continued until the inner pressure of the vacuum circle reached ´˚ The evaporation rate 5 × 10−5 Torr. The deposition process then continued until the cutting thickness reached 1 m (10 kA). ´ ˚ of the material was maintained at approximately 10–15 A/s during the process. 2.1.4. Contact process After the glass/ITO/CdIn2 Te4 structure was formed, annealed and as-deposited samples were placed in the thermal evaporation system holder for the contacting process. Solid In was placed into the tungsten crucible, and the contacting process started when the inner pressure of the vacuum circle reached 5 × 10−5 Torr. The deposition process was stopped by closing ´˚ coating thickness value from the monitor. The schematic structure of the prepared the cutter after reading a 0.5 m (5 kA) solar cells is given in Fig. 1. 2.2. Physical characterization and electrochemical measurements 18 mm × 18 mm × 2 mm ITO-coated glass was used as the substrate material. 12 mm × 12 mm CdIn2 Te4 and 8 mm × 8 mm CdS were obtained as active areas in the solar cells. The surface morphology and composition of the films was obtained through scanning electron microscopy (SEM) and energy dispersive X-ray (EDAX) analysis with a QUANTA (FEG-250) model.
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Fig. 2. The X-ray diffraction patterns of as-deposited (A0) and annealed (A400) thin film solar cells. Table 1 Inter-planar distance (d), the lattice constant (a), and crystallite size (D) of A0 and A400 samples. Samples A0
Samples A400 ´˚ d (A) 3.271 2.657
◦
hkl (200) (202)
2 27.23 33.69
´˚ a (A) 6.542 7.515
D (nm) 25.87 18.06
◦
2 27.26 33.62
´˚ d (A) 3.267 2.662
´˚ a (A) 6.535 7.530
D (nm) 27.54 28.46
X-ray diffraction measurements were taken using a BRUKER XRD system (D8 Advance). The current–voltage characteristic in dark and under light was measured by Keithley 2400 source meter. 3. Results and discussion 3.1. Structural analysis A CdIn2 Te4 /CdS (A0) thin film solar cell and CdIn2 Te4 /CdS (A400) thin film solar cell which was annealed for 1 h at 400 ◦ C were deposited onto the ITO substrate by thermal evaporation. Fig. 2 shows the XRD diffractions of A0 and A400 thin film solar cells. The peak positions of the A0 and A400 samples were observed to be approximately 2 = 27.2◦ . The positions of the observed peak were the same for both sample types, and for both sample types the positions of the observed peak increased depending on the duration of annealing time. In the same samples, a sharp peak emerged at the 2 = 33.6◦ position. Annealing caused no change in the peak position, but there was an increase in the intensity of the peak due to the annealing. This may be due to the thin films passing from an amorphous structure to a polycrystalline structure, and it may be an indication of decreasing structural deficiency. Diffraction lines generated by the 2 = 27.2◦ and 2 = 33.6◦ peaks corresponded to structure (200) and (202), respectively [33]. The crystallite sizes of thin films were calculated with XRD data using the Scherrer formula [34]: D=
k ˇCos
(1)
where D is the crystal size, is the wavelength of the X-ray source,  is half the maximum width of the diffraction peak in radians, is the Bragg angle XRD diffraction peak, and k is a constant related to the film whose particle size was calculated [34]. Using the XRD profile, the inter-planar distance (d) and lattice constant (a) can be calculated according to Bragg ‘s law [19]: d=
2sin
(2)
where is the wavelength of the X-ray, (d) is the lattice spacing, and is the Bragg’s angle. The plane-spacing equation for cubic crystal is given by Formula-3: 1 = d2
h2 + k2 + l2 a2
(3)
where (hkl) are the Miller indices of the planes [35]. The crystallite size of the thin film solar cells was calculated with the help of XRD using the Scherrer formula. The interplanar distance (d) and lattice constant (a) were calculated using Bragg’s law. A0 and A400 values calculated for thin film solar cells are given in Table 1. Increased crystallite size was determined due to annealing, which means that films passed from an amorphous to a polycrystalline state. The detected crystallite size is consistent with those of other studies in the literature [18,36]. The inter-planar distance (d) calculations are similar to those obtained by Jain et al. [17], and the lattice constant (a) calculations are similar to those obtained by Dongol et al. [19].
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Fig. 3. (a) The SEM image of A0 and (b) the SEM image of A400.
Fig. 4. Energy dispersive X-ray (EDAX) spectrums taken for the as-deposited thin film (a) CdIn2 Te4 layer and (b) CdS layer.
Fig. 5. Energy dispersive X-ray (EDAX) spectrums taken for the annealed thin film (a) CdIn2 Te4 layer and (b) CdS layer. Table 2 EDS results for CdIn2 Te4 thin film. Elements
Average weight (%)
Atomic weight (%)
Cd In Te
11.55 29.13 59.33
12.51 30.38 56.61
3.2. Surface morphology analysis The SEM images used to determine the morphology of the surface material for annealed and as deposited solar cells are shown in Fig. 3. Fig. 3 displays the increased surface roughness of the annealed film. Similar behavior has been reported by Pandey et al. [36]. 3.3. Composition analysis The energy dispersive X-ray (EDAX) spectrums of the annealed and as-deposited samples are shown in Figs. 4 and 5. EDS measurements were taken in order to determine the nature of the compound formed by sintering at 1150 ◦ C, the Cd:In:Te ratio of the compound, and whether the compound contained impurity atoms. As seen in Table 2, the EDS analysis shows that there are no impurity atoms apart from Cd, In, and Te atoms. In addition, elemental analysis revealed that a triple
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7990 Table 3 EDS results of the absorber layer (CdIn2 Te4 ).
A0
A400
Elements
Average weight (%)
Atomic weight (%)
Cd In Te Cd In Te
3.93 23.52 72.55 5.40 23.31 71.28
4.33 25.34 70.34 5.93 25.07 69.00
Elements
Average weight (%)
Atomic weight (%)
Cd S Cd S
74.36 25.64 75.40 24.60
54.73 45.27 53.35 46.65
Table 4 EDS results of the window layer (CdS).
A0 A400
Fig. 6. Element placement.
Fig. 7. Element profile.
CdIn2 Te4 compound was formed with a 1:2:4 ratio which correspond to stoichiometry within 5% experimental error limit. These values [37] seem to be compatible with the results of previous studies. CdIn2 Te4 was used as the absorber layer material, and its EDS analysis is given in Table 2. CdS (in powder form, 99.999% pure) was used as the window layer material. Materials were deposited on an ITO-coated glass substrate by thermal evaporation. The deposited thin film solar cell EDS analysis results are shown in Tables 3 and 4. An increase in the Cd ratio and a decrease in In and Te ratio was observed in the deposited absorber layer obtained by the thermal evaporation technique after annealing. The CdS values for the window layer material are consistent with the results of the study by Rmili et al. [38]. Figs. 6 and 7 show the absence of impurity atoms in the structure. The absorber layer consists of Cd, In, and Te atoms, and the window layer consists of Cd and S atoms. 3.4. Electrical analysis The transverse current–voltage characteristics of as-deposited and annealed CdIn2 Te4 thin films solar cells are presented in Fig. 8. It is clearly visible in Fig. 8 that the variation in current with voltage for as-deposited and annealed CdIn2 Te4 thin films solar cells is found to be linear. It is observed that the current of the solar cells is found to be increased for annealed solar cells. The results are well agreed with earlier reported work [4].
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Fig. 8. Dark I–V characteristics of as-deposited and annealed CdIn2 Te4 /CdS thin films solar cells.
4. Conclusion The thermal evaporation technique was used to form p-CdIn2 Te4 and n-CdS structures on an ITO-coated glass substrate. Thin film solar cells were annealed at 400 ◦ C, and as-deposited thin film solar cells were investigated to determine their structural properties. Through XRD analysis, both annealed and as-deposited solar cells were found in the direction of (200) and (202). This indicates the transition from an amorphous to a polycrystalline structure. Particle size was also calculated; annealed films were found to have increased particle size, whereas a polycrystalline structure to become clearer. SEM and EDAX analysis showed an increase in surface roughness of the annealed solar cells and no impurity atom in structure. The I–V characteristics show that the current is increased for annealed thin films solar cells. When solar cells produced by the thermal evaporation technique are compared with those produced through other methods, the results are similar. Therefore, the thermal evaporation technique is preferred for producing CdIn2 Te4 /CdS solar cells due to its lower cost and higher degree of simplicity. Acknowledgement This work was financially supported by the Scientific Research Projects Center (BAP) of Pamukkale University (project number: 2014FBE007). References [1] H.A. Mohamed, Optimized conditions for the improvement of thin film CdS/CdTe solar cells, Thin Solid Films 589 (2015) 72–78. [2] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar cell efficiency tables (version 41), Prog. Photovol.: Res. Appl. 21 (2013) 1–11. [3] S. Lalitha, S.Zh. Karazhanov, P. Ravindran, S. Senthilarasu, R. Sathyamoorthy, J. Janabergenov, Electronic structure, structural and optical properties of thermally evaporated CdTe thin films, Physica B 387 (2007) 227–238. [4] S. Singh, R. Kumar, K.N. Sood, Structural and electrical studies of thermally evaporated nanostructured CdTe thin films, Thin Solid Films 519 (2010) 1078–1081. [5] S. Shanmugan, D. Mutharasu, Effect of Ar+ ion irradiation on structural and optical properties of e-beam evaporated cadmium telluride thin films, Mater. Sci. Semicond. Process. 13 (2010) 298–302. [6] S. Lalitha, R. Sathyamoorthy, S. Senthilarasu, A. Subbarayan, K. Natarajan, Characterization of CdTe thin film—dependence of structural and optical properties on temperature and thickness, Sol. Energy Mater. Sol. Cells 82 (2004) 187–199. [7] J. Fritsche, D. Kraft, A. Thißen, T. Mayer, A. Klein, W. Jaegermann, Band energy diagram of CdTe thin film solar cells, Thin Solid Films 403–404 (2002) 252–257. [8] H. Bayhan, C¸. Erc¸elebi, Effects of post deposition treatments on vacuum evaporated CdTe thin films and CdS/CdTe heterojunction devices, Turk. J. Phys. 22 (1998) 441–451. [9] T. Okamoto, A. Yamada, M. Konagai, Optical and electrical characterizations of highly efficient CdTe thin film solar cells, Thin Solid Films 387 (2001) 6–10. [10] N.G. Dhere, R.G. Dhere, Thin-film photovoltaics, J. Vac. Sci. Technol. A Vac. Surf. Films 23 (2005) 1208–1214. [11] A.Y. Shenouda, M.M. Rashad, L. Chow, Synthesis, characterization and performance of Cd1 − x Inx Te compound for solar cell applications, J. Alloys Compd. 563 (2013) 39–43. [12] E.R. Shaaban, N. Afify, A. El-Taher, Effect of film thickness on microstructure parameters and optical constants of CdTe thin films, J. Alloys Compd. 482 (2009) 400–404. [13] V.B. Patil, P.D. More, D.S. Sutrave, G.S. Shahane, R.N. Mulik, L.P. Deshmukh, A new process for deposition of the CdTe thin films, Mater. Chem. Phys. 65 (2000) 282–287. [14] U.P. Khairnar, D.S. Bhavsar, R.U. Vaidya, G.P. Bhavsar, Optical properties of thermally evaporated cadmium telluride thin films, Mater. Chem. Phys. 80 (2003) 421–427. [15] S.H. You, K.J. Hong, T.S. Jeong, C.J. Youn, J.S. Park, D.C. Shin, J.D. Moon, Point defects in p-type CdIn2 Te4 Bridgman grown crystals, J. Cryst. Growth 256 (2003) 116–122. [16] B.V. Rajendra, D. Kekuda, Flexible cadmium telluride/cadmium sulphide thin film solar cells on mica substrate, J. Mater. Sci.: Mater. Electron. 23 (2012) 1805–1808. [17] K. Jain, R.K. Sharma, S. Kohli, K.N. Sood, A.C. Rastogi, Electrochemical deposition and characterization of cadmium indium telluride thin films for photovoltaic application, Curr. Appl. Phys. 3 (2003) 251–256. [18] K. Yılmaz, D. Gölcür, Investigations on structural, electrical and optical properties of polycrystalline CdInTe thin films grown by thermal evaporation, SDU J. Sci. (E-Journal) 9 (2014) 150–162.
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Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Effect of annealing on the structural properties of thermal evaporated CdIn2 Te4 /CdS thin film solar cells I˙ brahim Kırbas¸, Rasim Karabacak ∗ Pamukkale University, Engineering Faculty, Mechanical Engineering Department, Kınıklı Campus, 20070 Denizli, Turkey
a r t i c l e
i n f o
Article history: Received 11 April 2016 Accepted 1 June 2016 Keywords: Thin films solar cells II–VI Semiconductors Thermal evaporation CdIn2 Te4
a b s t r a c t In this study, a thermal evaporation technique and the effect of annealing on the structural properties of CdIn2 Te4 /CdS thin film solar cells were investigated. Thin film solar cells were deposited onto an indium tin oxide (ITO)-coated glass substrate using a thermal evaporation technique. The nitrogen atmosphere was 400 ◦ C for 1 h of annealing. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray (EDAX) analysis were performed on the solar cells. XRD analysis revealed two peaks (2 = 27.2◦ and 33.6◦ ). We observed increased peak severity but identical peak position in the annealed films. The Xray diffraction patterns of the annealed and as-deposited solar cells’ preferred orientations in nature have been detected as (200) and (202), respectively. Crystallite size (D), interplanar distance (d), and lattice constant (a) values were calculated for the thin film solar cells using the XRD data. When examining the EDAX analysis and element placement, we detected only CdIn2 Te4 in the absorber layer and only CdS atoms in the window layer, but no impurity atoms in the structure. We also observed an increase in surface roughness of the annealed films in SEM images. The I–V characteristics show that the current is increased for annealed thin films solar cells. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction Thin film solar cells have attracted attention in recent years due to their potential to lower costs and increase energy conversion efficiency. Thin film solar cells have been found to be lower in efficiency than silicon solar cells, but these films have been chosen for their low production costs [1]. According to studies reported in the literature, the efficiency rates of thin film solar cells; cadmium tellurium (CdTe), copper indium gallium selenide (CIGS), and amorphous silicon (a-Si) are 18.3%, 20%, and 12.3%, respectively [2]. Researchers have extensively studied II–VI compounds for applications in the optoelectronic and photovoltaic industry [3–6]. CdTe, one of the most studied compounds in this family, is suitable for use in the production of solar cells because it has a solar energy conversion bandwidth of 1.5 eV [7–12]. These compounds generally crystallize in the form of cubic or hexagonal systems, and they have a defective chalcopyrite structure [13–15]. The most appropriate structure for CdTe thin film solar cells is produced by matching n-type CdS and p-type CdTe [16]. II-III2 -VI4 compounds, such as CdIn2 Te4 compounds and II–VI compounds, have received great attention in recent years [17,18]. In the last decade, the emphasis has been on the development of low-cost deposition techniques [19]. The thermal evaporation technique for
∗ Corresponding author. E-mail address: [email protected] (R. Karabacak). http://dx.doi.org/10.1016/j.ijleo.2016.06.014 0030-4026/© 2016 Elsevier GmbH. All rights reserved.
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Fig. 1. Schematic diagram of CdIn2 Te4 /CdS solar cells on ITO-coated glass substrate [16,32].
depositing CdInTe [18] and CdS thin films is one of the most widely used because it is very simple due to having relatively few control parameters [20]. CdInTe and CdTe films are very sensitive, so various preparation techniques are used to prepare them, including thermal evaporation [21,22], electro deposition [23], the molecular beam epitaxial technique [24], closed-space sublimation [25–27], closed-space vapor transport [28,29], magnetron sputtering [30], and chemical deposition [31]. The properties of CdInTe films have been investigated in many studies [11,15,17,18,29], but more information is needed. In this study, for the first time, solar cells were manufactured by depositing CdIn2 Te4 and CdS on an ITO-coated glass substrate using the thermal evaporation technique. The effects of annealing on the structural characteristics of these solar cells were examined. The results are presented in graphs and tables. 2. Experimental details 2.1. Materials and methods 2.1.1. Depositing CdIn2 Te4 CdIn2 Te4 polycrystalline was deposited on an ITO-coated substrate at a pressure of 5 × 10−5 Torr with a thermal evaporation system. During the deposition process, the substrate was rotated at a constant speed so that the deposition parameters ´˚ were consistent and homogenous. The evaporation rate of the material was maintained at approximately 15–25 A/s. When ´ ˚ of closed cutting thickness was reached, the storage process was terminated. 1 m (10 kA) 2.1.2. Annealing process CdIn2 Te4 deposited substrates [16] were annealed for 1 h in a PROTHERM brand horizontal furnace, which was heated up to 400 ◦ C and maintained in a nitrogen atmosphere. 2.1.3. Depositing CdS After the glass/ITO/CdIn2 Te4 structure was formed, annealed and as-deposited samples were placed in the thermal evaporation system holder to store polycrystalline CdS, which has a window layer. After the CdS was placed into the tungsten crucible in the form of a powder of 99.999% purity, the system was closed. The process of rotating the substrate holder and vacuuming the system began, and the vacuum process continued until the inner pressure of the vacuum circle reached ´˚ The evaporation rate 5 × 10−5 Torr. The deposition process then continued until the cutting thickness reached 1 m (10 kA). ´ ˚ of the material was maintained at approximately 10–15 A/s during the process. 2.1.4. Contact process After the glass/ITO/CdIn2 Te4 structure was formed, annealed and as-deposited samples were placed in the thermal evaporation system holder for the contacting process. Solid In was placed into the tungsten crucible, and the contacting process started when the inner pressure of the vacuum circle reached 5 × 10−5 Torr. The deposition process was stopped by closing ´˚ coating thickness value from the monitor. The schematic structure of the prepared the cutter after reading a 0.5 m (5 kA) solar cells is given in Fig. 1. 2.2. Physical characterization and electrochemical measurements 18 mm × 18 mm × 2 mm ITO-coated glass was used as the substrate material. 12 mm × 12 mm CdIn2 Te4 and 8 mm × 8 mm CdS were obtained as active areas in the solar cells. The surface morphology and composition of the films was obtained through scanning electron microscopy (SEM) and energy dispersive X-ray (EDAX) analysis with a QUANTA (FEG-250) model.
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Fig. 2. The X-ray diffraction patterns of as-deposited (A0) and annealed (A400) thin film solar cells. Table 1 Inter-planar distance (d), the lattice constant (a), and crystallite size (D) of A0 and A400 samples. Samples A0
Samples A400 ´˚ d (A) 3.271 2.657
◦
hkl (200) (202)
2 27.23 33.69
´˚ a (A) 6.542 7.515
D (nm) 25.87 18.06
◦
2 27.26 33.62
´˚ d (A) 3.267 2.662
´˚ a (A) 6.535 7.530
D (nm) 27.54 28.46
X-ray diffraction measurements were taken using a BRUKER XRD system (D8 Advance). The current–voltage characteristic in dark and under light was measured by Keithley 2400 source meter. 3. Results and discussion 3.1. Structural analysis A CdIn2 Te4 /CdS (A0) thin film solar cell and CdIn2 Te4 /CdS (A400) thin film solar cell which was annealed for 1 h at 400 ◦ C were deposited onto the ITO substrate by thermal evaporation. Fig. 2 shows the XRD diffractions of A0 and A400 thin film solar cells. The peak positions of the A0 and A400 samples were observed to be approximately 2 = 27.2◦ . The positions of the observed peak were the same for both sample types, and for both sample types the positions of the observed peak increased depending on the duration of annealing time. In the same samples, a sharp peak emerged at the 2 = 33.6◦ position. Annealing caused no change in the peak position, but there was an increase in the intensity of the peak due to the annealing. This may be due to the thin films passing from an amorphous structure to a polycrystalline structure, and it may be an indication of decreasing structural deficiency. Diffraction lines generated by the 2 = 27.2◦ and 2 = 33.6◦ peaks corresponded to structure (200) and (202), respectively [33]. The crystallite sizes of thin films were calculated with XRD data using the Scherrer formula [34]: D=
k ˇCos
(1)
where D is the crystal size, is the wavelength of the X-ray source,  is half the maximum width of the diffraction peak in radians, is the Bragg angle XRD diffraction peak, and k is a constant related to the film whose particle size was calculated [34]. Using the XRD profile, the inter-planar distance (d) and lattice constant (a) can be calculated according to Bragg ‘s law [19]: d=
2sin
(2)
where is the wavelength of the X-ray, (d) is the lattice spacing, and is the Bragg’s angle. The plane-spacing equation for cubic crystal is given by Formula-3: 1 = d2
h2 + k2 + l2 a2
(3)
where (hkl) are the Miller indices of the planes [35]. The crystallite size of the thin film solar cells was calculated with the help of XRD using the Scherrer formula. The interplanar distance (d) and lattice constant (a) were calculated using Bragg’s law. A0 and A400 values calculated for thin film solar cells are given in Table 1. Increased crystallite size was determined due to annealing, which means that films passed from an amorphous to a polycrystalline state. The detected crystallite size is consistent with those of other studies in the literature [18,36]. The inter-planar distance (d) calculations are similar to those obtained by Jain et al. [17], and the lattice constant (a) calculations are similar to those obtained by Dongol et al. [19].
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Fig. 3. (a) The SEM image of A0 and (b) the SEM image of A400.
Fig. 4. Energy dispersive X-ray (EDAX) spectrums taken for the as-deposited thin film (a) CdIn2 Te4 layer and (b) CdS layer.
Fig. 5. Energy dispersive X-ray (EDAX) spectrums taken for the annealed thin film (a) CdIn2 Te4 layer and (b) CdS layer. Table 2 EDS results for CdIn2 Te4 thin film. Elements
Average weight (%)
Atomic weight (%)
Cd In Te
11.55 29.13 59.33
12.51 30.38 56.61
3.2. Surface morphology analysis The SEM images used to determine the morphology of the surface material for annealed and as deposited solar cells are shown in Fig. 3. Fig. 3 displays the increased surface roughness of the annealed film. Similar behavior has been reported by Pandey et al. [36]. 3.3. Composition analysis The energy dispersive X-ray (EDAX) spectrums of the annealed and as-deposited samples are shown in Figs. 4 and 5. EDS measurements were taken in order to determine the nature of the compound formed by sintering at 1150 ◦ C, the Cd:In:Te ratio of the compound, and whether the compound contained impurity atoms. As seen in Table 2, the EDS analysis shows that there are no impurity atoms apart from Cd, In, and Te atoms. In addition, elemental analysis revealed that a triple
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A0
A400
Elements
Average weight (%)
Atomic weight (%)
Cd In Te Cd In Te
3.93 23.52 72.55 5.40 23.31 71.28
4.33 25.34 70.34 5.93 25.07 69.00
Elements
Average weight (%)
Atomic weight (%)
Cd S Cd S
74.36 25.64 75.40 24.60
54.73 45.27 53.35 46.65
Table 4 EDS results of the window layer (CdS).
A0 A400
Fig. 6. Element placement.
Fig. 7. Element profile.
CdIn2 Te4 compound was formed with a 1:2:4 ratio which correspond to stoichiometry within 5% experimental error limit. These values [37] seem to be compatible with the results of previous studies. CdIn2 Te4 was used as the absorber layer material, and its EDS analysis is given in Table 2. CdS (in powder form, 99.999% pure) was used as the window layer material. Materials were deposited on an ITO-coated glass substrate by thermal evaporation. The deposited thin film solar cell EDS analysis results are shown in Tables 3 and 4. An increase in the Cd ratio and a decrease in In and Te ratio was observed in the deposited absorber layer obtained by the thermal evaporation technique after annealing. The CdS values for the window layer material are consistent with the results of the study by Rmili et al. [38]. Figs. 6 and 7 show the absence of impurity atoms in the structure. The absorber layer consists of Cd, In, and Te atoms, and the window layer consists of Cd and S atoms. 3.4. Electrical analysis The transverse current–voltage characteristics of as-deposited and annealed CdIn2 Te4 thin films solar cells are presented in Fig. 8. It is clearly visible in Fig. 8 that the variation in current with voltage for as-deposited and annealed CdIn2 Te4 thin films solar cells is found to be linear. It is observed that the current of the solar cells is found to be increased for annealed solar cells. The results are well agreed with earlier reported work [4].
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Fig. 8. Dark I–V characteristics of as-deposited and annealed CdIn2 Te4 /CdS thin films solar cells.
4. Conclusion The thermal evaporation technique was used to form p-CdIn2 Te4 and n-CdS structures on an ITO-coated glass substrate. Thin film solar cells were annealed at 400 ◦ C, and as-deposited thin film solar cells were investigated to determine their structural properties. Through XRD analysis, both annealed and as-deposited solar cells were found in the direction of (200) and (202). This indicates the transition from an amorphous to a polycrystalline structure. Particle size was also calculated; annealed films were found to have increased particle size, whereas a polycrystalline structure to become clearer. SEM and EDAX analysis showed an increase in surface roughness of the annealed solar cells and no impurity atom in structure. The I–V characteristics show that the current is increased for annealed thin films solar cells. When solar cells produced by the thermal evaporation technique are compared with those produced through other methods, the results are similar. Therefore, the thermal evaporation technique is preferred for producing CdIn2 Te4 /CdS solar cells due to its lower cost and higher degree of simplicity. Acknowledgement This work was financially supported by the Scientific Research Projects Center (BAP) of Pamukkale University (project number: 2014FBE007). References [1] H.A. Mohamed, Optimized conditions for the improvement of thin film CdS/CdTe solar cells, Thin Solid Films 589 (2015) 72–78. [2] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar cell efficiency tables (version 41), Prog. Photovol.: Res. Appl. 21 (2013) 1–11. [3] S. Lalitha, S.Zh. Karazhanov, P. Ravindran, S. Senthilarasu, R. Sathyamoorthy, J. Janabergenov, Electronic structure, structural and optical properties of thermally evaporated CdTe thin films, Physica B 387 (2007) 227–238. [4] S. Singh, R. Kumar, K.N. Sood, Structural and electrical studies of thermally evaporated nanostructured CdTe thin films, Thin Solid Films 519 (2010) 1078–1081. [5] S. Shanmugan, D. Mutharasu, Effect of Ar+ ion irradiation on structural and optical properties of e-beam evaporated cadmium telluride thin films, Mater. Sci. Semicond. Process. 13 (2010) 298–302. [6] S. Lalitha, R. Sathyamoorthy, S. Senthilarasu, A. Subbarayan, K. Natarajan, Characterization of CdTe thin film—dependence of structural and optical properties on temperature and thickness, Sol. Energy Mater. Sol. Cells 82 (2004) 187–199. [7] J. Fritsche, D. Kraft, A. Thißen, T. Mayer, A. Klein, W. Jaegermann, Band energy diagram of CdTe thin film solar cells, Thin Solid Films 403–404 (2002) 252–257. [8] H. Bayhan, C¸. Erc¸elebi, Effects of post deposition treatments on vacuum evaporated CdTe thin films and CdS/CdTe heterojunction devices, Turk. J. Phys. 22 (1998) 441–451. [9] T. Okamoto, A. Yamada, M. Konagai, Optical and electrical characterizations of highly efficient CdTe thin film solar cells, Thin Solid Films 387 (2001) 6–10. [10] N.G. Dhere, R.G. Dhere, Thin-film photovoltaics, J. Vac. Sci. Technol. A Vac. Surf. Films 23 (2005) 1208–1214. [11] A.Y. Shenouda, M.M. Rashad, L. Chow, Synthesis, characterization and performance of Cd1 − x Inx Te compound for solar cell applications, J. Alloys Compd. 563 (2013) 39–43. [12] E.R. Shaaban, N. Afify, A. El-Taher, Effect of film thickness on microstructure parameters and optical constants of CdTe thin films, J. Alloys Compd. 482 (2009) 400–404. [13] V.B. Patil, P.D. More, D.S. Sutrave, G.S. Shahane, R.N. Mulik, L.P. Deshmukh, A new process for deposition of the CdTe thin films, Mater. Chem. Phys. 65 (2000) 282–287. [14] U.P. Khairnar, D.S. Bhavsar, R.U. Vaidya, G.P. Bhavsar, Optical properties of thermally evaporated cadmium telluride thin films, Mater. Chem. Phys. 80 (2003) 421–427. [15] S.H. You, K.J. Hong, T.S. Jeong, C.J. Youn, J.S. Park, D.C. Shin, J.D. Moon, Point defects in p-type CdIn2 Te4 Bridgman grown crystals, J. Cryst. Growth 256 (2003) 116–122. [16] B.V. Rajendra, D. Kekuda, Flexible cadmium telluride/cadmium sulphide thin film solar cells on mica substrate, J. Mater. Sci.: Mater. Electron. 23 (2012) 1805–1808. [17] K. Jain, R.K. Sharma, S. Kohli, K.N. Sood, A.C. Rastogi, Electrochemical deposition and characterization of cadmium indium telluride thin films for photovoltaic application, Curr. Appl. Phys. 3 (2003) 251–256. [18] K. Yılmaz, D. Gölcür, Investigations on structural, electrical and optical properties of polycrystalline CdInTe thin films grown by thermal evaporation, SDU J. Sci. (E-Journal) 9 (2014) 150–162.
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