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Influence of mole concentration on nano crystalline Bi-doped CuInS2 thin films with the temperature by chemical spray method
Optik 126 (2015) 4237–4242
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Influence of mole concentration on nano crystalline Bi-doped CuInS2 thin films with the temperature by chemical spray method C. Mahendran a,∗ , N. Suriyanarayanan b a b
Department of Physics, Sri Ramakrishna Mission Vidyalaya College of Arts & Science, Coimbatore 641020, Tamil Nadu, India Department of Physics, Government College of Technology, Coimbatore 641013, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 16 August 2014 Accepted 24 August 2015 Keywords: Chalcopyrite thin films Optical and electrical property Surface morphology Spray pyrolysis
a b s t r a c t Copper Indium Disulphide (CuInS2 ) is an efficient absorber material for photovoltaic and solar cell applications. In this paper Bi (0.02 and 0.03 M) doped CuInS2 thin films are (Cu/In = 1.25) deposited on to glass substrates in the temperature range 300–400 ◦ C. The optical bandgap energy (Eg ) increases with the increase in temperatures for both 0.02 and 0.03 M samples. SEM photographs reveal the formation of a large number of flower like small crystals of size ranging from 70 to 200 nm. The EDAX results confirm the presence of Cu, In, S and Bi in the films. All the films present low resistivity () values and exhibit semiconducting nature. Hence, Bi species can be used as a donor and acceptor impurity in CuInS2 thin films to fabricate efficient solar cells, photovoltaic devices and good IR Transmitters. © 2015 Elsevier GmbH. All rights reserved.
1. Introduction Chalcopyrite semiconductors CuInS2 and CuInSe have drawn considerable attention in recent years for the fabrication of solar cells [1] and LED devices [2]. These applications are based on P–N junctions. Controlled doping of impurities on these materials seems to be desirable, where the required free carrier concentrations are in the range of 1016 –1018 cm−3 . But it is very difficult to intrinsically dope chalcopyrite semiconductors due to the following reasons. (a) 12 types of possible intrinsic defects can compensate extrinsic donors (or) acceptors [3]. (b) Depending on site occupation, acceptors and donors can be formed by the same dopant. (c) Low dissociation temperature of chalcopyrite crystals hampers diffusion doping. (d) Secondary phases containing host and doping elements can form. The above said possible effects may limit the number of elements for extrinsic doping of chalcopyrites. Among the various compound semiconductors CuInS2 has high absorption coefficient, direct bandgap of 1.5 eV, emits no toxic components and belongs to semiconducting ternary compounds of I-III-VI2 with chalcopyrite’s structure. Among the various methods available [4–10] for the deposition of thin films, the spray pyrolysis method enables the deposition of thin films of a larger area with good uniformity [11–15]. For temperature controlling a conduction type and obtaining low resistivity, several impurities doped
∗ Corresponding author. Tel.: +91 9965615395. E-mail address: [email protected] (C. Mahendran). http://dx.doi.org/10.1016/j.ijleo.2015.08.133 0030-4026/© 2015 Elsevier GmbH. All rights reserved.
CuInS2 thin films [16]. The semimetal Bismuth (Bi) is an attractive material for electronics because of its highly anisotropic electronic behaviour, low conduction band effective mass and high electron mobility [17]. But considerable work is not available on Bi-doped CuInS2 thin films by spray pyrolysis. This compositional analysis presents the influence of mole concentration on Bi-doped CuInS2 thin films with the temperature by chemical spray method.
2. Experimental Bi-doped CuInS2 thin films are deposited by spray pyrolysis on to glass substrates from aqueous solutions of CuCl2 , InCl3 , SC(NH2 )2 and BiCl2 using compressed air as carrier gas. First, aqueous solutions (0.02 and 0.03 M) of the salts are prepared, and then they are mixed with appropriate portions in order to have copper to indium molar ratio (Cu/In = 1.25) and (Cu + In)/S fixed to 1 in the solution. The copper (II) chloride and indium (III) chloride are mixed and then thiourea solution is added. The resulting solution is doped with Bismuth chloride (BiCl2 ) of (0.01 and 0.02 M). The solutions are prepared by dissolving in de-ionized water. Then the resulting solution is sprayed using is compressed air as carrier gas with spray rate of 2 ml/min in air on to glass substrates (2.5 × 2.5 cm2 ) heated at different temperatures from 300 to 400 ◦ C. The X-ray diffraction (XRD) patterns of sprayed films are recorded using the XPERT-PRO Gonio scan diffractometer with CuK␣ radiation. The optical transmittance spectra are recorded in the wavelength range of 300–1100 nm using double beam Beckman Ratio Recording spectrophotometer. The surface morphology of the film is investigated
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Fig. 3.1. XRD of sprayed Bi-doped (0.02 M) CuInS2 film prepared at different substrate temperatures.
Fig. 3.3. EDAX spectra of sprayed Bi-doped (0.02 M) CuInS2 films prepared at different substrate temperatures: (a) 300 ◦ C and (b) 400 ◦ C.
using a Jeol, JSM-6390, JM-Spot size 35. The compositional analysis is carried out using energy dispersive X-ray spectroscopy (EDAX). Photoluminescence (PL) spectra of the film are recorded using a Cary Eclipse instrument in fluorescence emission scan mode. The electrical resistivity and conductivity study is carried out by the four probe method. 3. Result and discussions 3.1. Structural analysis Figs. 3.1 and 3.2 show the X-ray diffraction (XRD) patterns of (0.02 and 0.03 M) Bi-doped CuInS2 polycrystalline thin films associated with amorphous phase grown in the temperature range from 300 to 400 ◦ C. It is found that the dopant concentration has great effects on the formation of polycrystalline CuInS2 . Figs. 3.1 and 3.2 show the characteristic peaks along (0 0 3), (2 2 0), (1 1 6) and (3 1 2) with little preferential orientation along (1 1 2) at 23.9◦ . It is observed that the intensity of the (1 1 2) peak decreases with increasing temperature [13]. Bi, Cu, CuIn5 S8 , Bi2 S3 , Cu2 S, In2 O3 or Bi/Cu/In/S alloys are found in the secondary phases with the lower intensity and these additional phases are responsible for other reflections [19]. From the reflections of XRD patterns, it is concluded that the polycrystalline Bi-doped CuInS2 films are incorporated with amorphous phase. The Bi doped CuInS2 thin films become Cu-rich below 350 ◦ C and In-rich above 300 ◦ C. One of the reasons may be that the atoms or binary compound resolved from CuInS2 compound could be stacked [20]. The Bi-doped CuInS2 thin films are composed of compactly packed nano-sized crystals of grain size in the range 3–8 nm.
Fig. 3.4. EDAX spectra of sprayed Bi-doped (0.03 M) CuInS2 films prepared at different substrate temperatures: (a) 300 ◦ C and (b) 400 ◦ C.
The grain size calculated from the Debye–Scherrer relation 5.2. At 350–375 ◦ C (Figs. 3.1d and 3.2c,d) almost all the height of reflections are suppressed [21]. The XRD results of undoped samples show chalcopyrite structure and polycrystalline nature of the films. At higher temperatures, 375–400 ◦ C, oxidation leads to the growth of In2 O3 and it is transparent at higher wavelength. The average grain size is found to be in the range of 26–125 nm. 3.2. EDAX analysis EDAX analysis is carried out to (Figs. 3.3 and 3.4) determine the chemical composition of the films (0.02 and 0.03 M). The presence of all elements Cu, In, S and Bi is confirmed. Similar to 0.01 M doped samples, 0.02 and 0.03 M samples contain oxygen at all deposited temperatures. Stoichiometry of the film is achieved when oxygen is taken into relative compositions. The concentration of Cu, In, S and Bi from the film decreases as the temperature increases from 300 to 400 ◦ C. Due to chemisorption, oxygen containing phase has been formed in all the films. But the oxygen content increases as the temperature of the film increases from 300 to 400 ◦ C and maximum of 25.6% is measured at 400 ◦ C. This oxygen surrounds the grain boundaries without entering into crystallites hence the properties of the film are not altered by oxygen content but decrease the crystallographic defects and improves the stoichiometry of the films. 3.3. Optical properties
Fig. 3.2. XRD of sprayed Bi-doped (0.03 M) CuInS2 film prepared at different substrate temperatures.
The optical transmission and absorption spectra were recorded (0.02 and 0.03 M) in the spectral wavelength range of 300–1100 nm (Figs. 3.5 and 3.6). Intrinsic inter-band absorption occurs for 0.02 M doped samples in the UV–vis region when the substrate
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Fig. 3.5. The transmittance spectra of Bi-doped (0.02 M) CuInS2 film sprayed at different substrate temperatures: (a) 300 ◦ C, (b) 325 ◦ C, (c) 350 ◦ C, (d) 375 ◦ C and (e) 400 ◦ C.
Fig. 3.6. The transmittance spectra of Bi-doped (0.03 M) CuInS2 film sprayed at different substrate temperatures: (a) 300 ◦ C, (b) 325 ◦ C, (c) 350 ◦ C, (d) 375 ◦ C and (e) 400 ◦ C.
temperature is at 300 ◦ C (Fig. 3.5) and the absorption decreases and transmission increases from 325 to 400 ◦ C. About 70% light transmission is noticed when the substrate temperature is at 400 ◦ C. But, no intrinsic inter-band absorption is noticed in the case of 0.03 M doped samples (Fig. 3.6). Transmission increases gradually as the temperature increases from 300 to 400 ◦ C and maximum of 55% light transmission is noticed at 400 ◦ C. The decrease in transmission for 0.03 M doped samples might be due to the increase in crystallite size [22,23]. However, improved optical property is observed in the case of 0.02 and 0.03 M doped CuInS2 thin films than the films prepared by thermal and vacuum evaporation methods [20,24]. Bi doped CuInS2 thin films recognized to the Bi atoms may occupy volume as well as near the surface of the films [24] and decrease in film thickness or change in structure may also be the reason for improved transmittance (Table 3.1). The bandgap energy (Eg ) and absorption co-efficient (˛) for the sprayed CuInS2 films are determined from the optical transmission data. The absorption co-efficient (˛) can be calculated by Eq. (4.1). ˛=
2.303 1 log t T
(4.1)
where t is the thickness of the film and T is the transmittance. The absorption coefficient (˛) is calculated for different wavelengths ().
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Fig. 3.7. The absorption spectra of Bi-doped (0.02 M) CuInS2 film sprayed at different substrate temperatures: (a) 300 ◦ C, (b) 325 ◦ C, (c) 350 ◦ C, (d) 375 ◦ C and (e) 400 ◦ C.
Fig. 3.8. The absorption spectra of Bi-doped (0.03 M) CuInS2 film sprayed at different substrate temperatures: (a) 300 ◦ C, (b) 325 ◦ C, (c) 350 ◦ C, (d) 375 ◦ C and (e) 400 ◦ C.
For the direct band gap semiconductors, ˛ can be related to through the equation: 2
(˛h) = A(h − Eg )
(4.2)
where A is a constant, Eg is the Band gap energy and h is the photon energy. The absorption co-efficient (˛) versus the photon energy (h) of the Bi-doped CuInS2 thin film in the temperature range 300–400 ◦ C is shown in Figs. 3.7 and 3.8. The films have high absorption co-efficient in the range 0.5 × 105 –7.6 × 105 cm−1 for 0.02 M and 1 × 105 cm−1 –5 × 105 cm−1 for 0.03 M doped samples in the UV–vis spectral region. Comparing 0.02 and 0.03 M, the 0.02 M doped samples show higher ˛ value (7.6 × 105 cm−1 ). However, both 0.02 and 0.03 M doped CuInS2 thin films have absorption coefficient in the order of 105 /cm between the temperature ranges 300 and 400 ◦ C. Hence, both the samples can be used as an efficient solar cell absorber and other photovoltaic applications in the UV–vis region. But 0.02 M doped samples are more effective than 0.03 M doped samples. For undoped sample ˛ value is quite higher which may affect the solar conversion efficiency of solar cells. The absorption co-efficient (˛), the relation shown by Eq. (4.1) is used. A plot of (˛h)2 versus h for the Bi-doped CuInS2 films (0.02 and 0.03 M) deposited at various temperatures is presented in Figs. 3.9 and 3.10. The bandgap energy increases from 2.10 to 2.92 eV for 0.02 M and 1.60 to 1.82 eV for 0.03 M doped samples. The increase in bandgap energy in the case of 0.02 M doped sample is attributed to crystallo-chemical rearrangement of principal
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Table 3.1 Variation of optical bandgap energies (Eg ) and thickness of Bi doped (0.02 and 0.03 M) CuInS2 film with temperatures. Temperature (◦ C)
300 325 350 375 400
CuInS2 /0.02 M Bi-doped
CuInS2 /0.03 M Bi-doped
Eg (eV)
Thickness (nm)
Eg (eV)
Thickness (nm)
2.10 2.15 2.49 2.58 2.92
677 663 651 647 641
1.60 1.67 1.70 1.72 1.82
732 723 719 711 709
Fig. 3.9. Variation of optical bandgap energy values (Eg ) of sprayed Bi-doped (0.02 M) CuInS2 films prepared at different substrate temperatures: (a) 300 ◦ C, (b) 325 ◦ C, (c) 350 ◦ C, (d) 375 ◦ C and (e) 400 ◦ C.
Fig. 3.10. Variation of optical bandgap energy values (Eg ) of sprayed Bi-doped (0.03 M) CuInS2 films prepared at different substrate temperatures: (a) 300 ◦ C, (b) 325 ◦ C, (c) 350 ◦ C, (d) 375 ◦ C and (e) 400 ◦ C.
structural structures which increase the distance between the top of the valance band formed by P-chalcogen states and bottom of conduction band formed by s–d cationic states [25]. But in the case of 0.03 M doped sample the bandgap energy decreases compared to 0.02 M sample. This may be due to the filling up of S, Cu, Indium vacancies by the Bi atoms or Cu in an In, S sites or In may be in S, Cu sites. The bandgap energy can be improved by passing hydrogen at 400 ◦ C annealing temperature. But, even at 375 ◦ C and beyond, the film (0.02 and 0.03 M) composed of polycrystalline with amorphous phase leading to further deterioration of crystalline properties, which gives rise to defect states and smearing of absorption edge. From Table 3.1 it is noticed that the energy gap (Eg ) values and thickness of the film decreases as the substrate temperature increases from 300 to 400 ◦ C [13].
Fig. 3.11. SEM micrographs of sprayed Bi-doped (0.02 M) CuInS2 films prepared at: 300 ◦ C.
Fig. 3.12. SEM micrographs of sprayed Bi-doped (0.02 M) CuInS2 films prepared at: 300 ◦ C.
3.4. Surface morphology The SEM micrographs of Bi doped (0.02 M) CuInS2 thin films deposited in the temperature range 300–400 ◦ C are shown in Figs. 3.11–3.13. A large number of small flower like crystals of size ranging from 77 to 134 nm are formed at 300 ◦ C (3.12 and 3.13). These crystals grow in size and combine to form large crystals as the temperature increases from 325 to 400 ◦ C. It is quite evident that even at 400 ◦ C, it is polycrystalline associated with amorphous phase as depicted by XRD studies. But the undoped, Sb doped, Zn doped CuInS2 thin films completely formed into an amorphous structure beyond 375 ◦ C [26,27]. In the case of 0.03 M doped samples the crystals size ranging from 106 to 160 nm are formed at 300 ◦ C (Fig. 3.14). Due to the formation of some mediate products in the molten phase accelerated growth of crystals are formed when the substrate temperature is 300–400 ◦ C (Fig. 3.15). These accelerated growths of crystals in
C. Mahendran, N. Suriyanarayanan / Optik 126 (2015) 4237–4242
Fig. 3.13. SEM micrographs of sprayed Bi-doped (0.02 M) CuInS2 films prepared at: 400 ◦ C.
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Fig. 3.16. Variations of the resistivity () of Bi-doped CuInS2 thin films with different substrate temperatures: (a) 0.02 M and (b) 0.03 M.
P-type conductivity. The lowest value obtained is 5 × 10−3 -cm (at 350 ◦ C) which is higher than 0.01 M doped samples. The observed P-type conductivity can be explained by the fact that the introduced Bi acceptor may compensate with donors of sulphur vacancy defects [29]. On further increasing the temperature to 350–400 ◦ C an increase in is observed due to scattering of thermal generated defects, compensation of Cu vacancy defects and interstitials by Bi atoms (or) attributed to localisation of impurity states. For 0.03 M doped samples the resistance further decreases due to the compensation and saturation of valances by Bi atoms. The lowest value obtained at 350 ◦ C for 0.03 M doped sample is 1 × 10−3 -cm and further increase in temperature leads to N-type conductivity which requires further investigations 4. Conclusion Fig. 3.14. SEM micrographs of sprayed Bi-doped (0.03 M) CuInS2 films prepared at: 300 ◦ C.
Fig. 3.15. SEM micrographs of sprayed Bi-doped (0.03 M) CuInS2 films prepared at: 400 ◦ C.
certain regions on the surface are attributed to insufficient wetting of the surface of growing films by molten phase [28]. Some of the crystals of size ranging from 90 to 100 nm are shown in Fig. 3.15. 3.5. Electrical properties Fig. 3.16a and b depicts the variation of resistivity () of Bi (0.02 and 0.03 M) doped CuInS2 thin films. For 0.02 M doped samples the value decreases as the temperature increases with considerable
Bi doped (0.02 and 0.03 M) CuInS2 thin films are deposited in the temperature range 300–400 ◦ C. The growth of CuInS2 along the (1 1 2) preferred plane and other characteristic planes are suppressed by Bi doping. Bi concentration (0.02 and 0.03 M) affects the structure of the film as the temperature increases. Bi doped polycrystalline CuInS2 films are associated with amorphous phase. With the increase in temperature, 375 ◦ C and above, the polycrystalline growth disappears and becomes almost amorphous. EDAX results confirm the presence of Cu, In, S and Bi in the films. Optical properties show that there is an intrinsic inter-band absorption in the 0.02 M Bi doped samples in the UV–vis region when the substrate temperature is at 300 ◦ C. The absorption coefficient (˛ = 7.5 × 105 cm−1 ) for 0.02 M Bi doped sample is found to be higher than 0.03 M doped samples. Hence, 0.02 M Bi doped CuInS2 thin films at 300 ◦ C can be used as an efficient solar cell absorber. The optical bandgap energy (Eg ) increases with the increase in temperatures for both 0.02 and 0.03 M samples. SEM photographs reveal the formation of a large number of flower like small crystals of size ranging from 70 to 200 nm. Photoluminescence properties are completely suppressed by 0.02 and 0.03 M doped samples. No emissions are noticed. The electrical study reveals the semiconducting nature of Bi doped CuInS2 thin films. References [1] H.W. Schock, A. Shah, Proceedings of the European Conference on Photovoltaic Solar Energy Conversion, Stephens & Associates, Barcelona, Spain, 1997, p. 2000. [2] S. Chichibu, S. Shirakata, A. Isomura, H. Nakanishi, Jpn. J. Appl. Phys. 36 (1997) 1703. [3] H.Y. Ueng, H.L. Wang, Mater. Sci. Eng.: B (1992) 261. [4] L.L. Kazmerski, G.A. Sanborn, J. Appl. Phys. 48 (1977) 3178.
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[5] R. Scheer, T. Walter, H.W. Schock, M.L. Fearheily, H.J. Lewerenz, Appl. Phys. Lett. 63 (1993) 3294. [6] T. Walter, R. Menner, Ch. Koble, H.W. Schock, Proceedings of the 12th European Photovoltaic Solar Energy Conference, Amsterdam, 1994, p. 684. [7] T. Watanabe, M. Matsui, Jpn. J. Appl. Phys. 35 (1996) 1681. [8] Y. Ogawa, A. Jager-Waldau, Y. Hashimoto, K. Ito, Jpn. J. Appl. Phys. 33 (1994) L1775. [9] Y. Ogawa, S. Uenishi, K. Tohoyama, K. Ito, Sol. Energy Mater. Sol. Cells 35 (1994) 157. [10] C. Dzionk, H. Metzner, S. Hessler, H.-E. Mahnke, Cryst. Res. Technol. 31 (1996) 773. [11] M. Ortega-Lopez, A. Morales-veco, Proceedings of the 25th IEEEPVSC, Washington, May13–17, 1996, p. 13. [12] H. Onnagawa, K. Miayashita, Jpn. J. Appl. Phys. 23 (1984) 965. [13] H. Bihri, C. Messaoudi, D. Sayah, A. Boyer, A. Mzerd, M. Abd-Lefdil, Phys. Status Solidi 129 (1992) 193. [14] R.P.V. Lakshmi, R. Venugopal, D.R. Reddy, B.K. Reddy, Solid State Commun. 82 (1992) 997. [15] C. Messaoudi, H. Bihri, D. Sayah, M. Cedene, M. Abd-Lefdil, J. Mater. Sci. Lett. 11 (1992) 1234.
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Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Influence of mole concentration on nano crystalline Bi-doped CuInS2 thin films with the temperature by chemical spray method C. Mahendran a,∗ , N. Suriyanarayanan b a b
Department of Physics, Sri Ramakrishna Mission Vidyalaya College of Arts & Science, Coimbatore 641020, Tamil Nadu, India Department of Physics, Government College of Technology, Coimbatore 641013, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 16 August 2014 Accepted 24 August 2015 Keywords: Chalcopyrite thin films Optical and electrical property Surface morphology Spray pyrolysis
a b s t r a c t Copper Indium Disulphide (CuInS2 ) is an efficient absorber material for photovoltaic and solar cell applications. In this paper Bi (0.02 and 0.03 M) doped CuInS2 thin films are (Cu/In = 1.25) deposited on to glass substrates in the temperature range 300–400 ◦ C. The optical bandgap energy (Eg ) increases with the increase in temperatures for both 0.02 and 0.03 M samples. SEM photographs reveal the formation of a large number of flower like small crystals of size ranging from 70 to 200 nm. The EDAX results confirm the presence of Cu, In, S and Bi in the films. All the films present low resistivity () values and exhibit semiconducting nature. Hence, Bi species can be used as a donor and acceptor impurity in CuInS2 thin films to fabricate efficient solar cells, photovoltaic devices and good IR Transmitters. © 2015 Elsevier GmbH. All rights reserved.
1. Introduction Chalcopyrite semiconductors CuInS2 and CuInSe have drawn considerable attention in recent years for the fabrication of solar cells [1] and LED devices [2]. These applications are based on P–N junctions. Controlled doping of impurities on these materials seems to be desirable, where the required free carrier concentrations are in the range of 1016 –1018 cm−3 . But it is very difficult to intrinsically dope chalcopyrite semiconductors due to the following reasons. (a) 12 types of possible intrinsic defects can compensate extrinsic donors (or) acceptors [3]. (b) Depending on site occupation, acceptors and donors can be formed by the same dopant. (c) Low dissociation temperature of chalcopyrite crystals hampers diffusion doping. (d) Secondary phases containing host and doping elements can form. The above said possible effects may limit the number of elements for extrinsic doping of chalcopyrites. Among the various compound semiconductors CuInS2 has high absorption coefficient, direct bandgap of 1.5 eV, emits no toxic components and belongs to semiconducting ternary compounds of I-III-VI2 with chalcopyrite’s structure. Among the various methods available [4–10] for the deposition of thin films, the spray pyrolysis method enables the deposition of thin films of a larger area with good uniformity [11–15]. For temperature controlling a conduction type and obtaining low resistivity, several impurities doped
∗ Corresponding author. Tel.: +91 9965615395. E-mail address: [email protected] (C. Mahendran). http://dx.doi.org/10.1016/j.ijleo.2015.08.133 0030-4026/© 2015 Elsevier GmbH. All rights reserved.
CuInS2 thin films [16]. The semimetal Bismuth (Bi) is an attractive material for electronics because of its highly anisotropic electronic behaviour, low conduction band effective mass and high electron mobility [17]. But considerable work is not available on Bi-doped CuInS2 thin films by spray pyrolysis. This compositional analysis presents the influence of mole concentration on Bi-doped CuInS2 thin films with the temperature by chemical spray method.
2. Experimental Bi-doped CuInS2 thin films are deposited by spray pyrolysis on to glass substrates from aqueous solutions of CuCl2 , InCl3 , SC(NH2 )2 and BiCl2 using compressed air as carrier gas. First, aqueous solutions (0.02 and 0.03 M) of the salts are prepared, and then they are mixed with appropriate portions in order to have copper to indium molar ratio (Cu/In = 1.25) and (Cu + In)/S fixed to 1 in the solution. The copper (II) chloride and indium (III) chloride are mixed and then thiourea solution is added. The resulting solution is doped with Bismuth chloride (BiCl2 ) of (0.01 and 0.02 M). The solutions are prepared by dissolving in de-ionized water. Then the resulting solution is sprayed using is compressed air as carrier gas with spray rate of 2 ml/min in air on to glass substrates (2.5 × 2.5 cm2 ) heated at different temperatures from 300 to 400 ◦ C. The X-ray diffraction (XRD) patterns of sprayed films are recorded using the XPERT-PRO Gonio scan diffractometer with CuK␣ radiation. The optical transmittance spectra are recorded in the wavelength range of 300–1100 nm using double beam Beckman Ratio Recording spectrophotometer. The surface morphology of the film is investigated
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Fig. 3.1. XRD of sprayed Bi-doped (0.02 M) CuInS2 film prepared at different substrate temperatures.
Fig. 3.3. EDAX spectra of sprayed Bi-doped (0.02 M) CuInS2 films prepared at different substrate temperatures: (a) 300 ◦ C and (b) 400 ◦ C.
using a Jeol, JSM-6390, JM-Spot size 35. The compositional analysis is carried out using energy dispersive X-ray spectroscopy (EDAX). Photoluminescence (PL) spectra of the film are recorded using a Cary Eclipse instrument in fluorescence emission scan mode. The electrical resistivity and conductivity study is carried out by the four probe method. 3. Result and discussions 3.1. Structural analysis Figs. 3.1 and 3.2 show the X-ray diffraction (XRD) patterns of (0.02 and 0.03 M) Bi-doped CuInS2 polycrystalline thin films associated with amorphous phase grown in the temperature range from 300 to 400 ◦ C. It is found that the dopant concentration has great effects on the formation of polycrystalline CuInS2 . Figs. 3.1 and 3.2 show the characteristic peaks along (0 0 3), (2 2 0), (1 1 6) and (3 1 2) with little preferential orientation along (1 1 2) at 23.9◦ . It is observed that the intensity of the (1 1 2) peak decreases with increasing temperature [13]. Bi, Cu, CuIn5 S8 , Bi2 S3 , Cu2 S, In2 O3 or Bi/Cu/In/S alloys are found in the secondary phases with the lower intensity and these additional phases are responsible for other reflections [19]. From the reflections of XRD patterns, it is concluded that the polycrystalline Bi-doped CuInS2 films are incorporated with amorphous phase. The Bi doped CuInS2 thin films become Cu-rich below 350 ◦ C and In-rich above 300 ◦ C. One of the reasons may be that the atoms or binary compound resolved from CuInS2 compound could be stacked [20]. The Bi-doped CuInS2 thin films are composed of compactly packed nano-sized crystals of grain size in the range 3–8 nm.
Fig. 3.4. EDAX spectra of sprayed Bi-doped (0.03 M) CuInS2 films prepared at different substrate temperatures: (a) 300 ◦ C and (b) 400 ◦ C.
The grain size calculated from the Debye–Scherrer relation 5.2. At 350–375 ◦ C (Figs. 3.1d and 3.2c,d) almost all the height of reflections are suppressed [21]. The XRD results of undoped samples show chalcopyrite structure and polycrystalline nature of the films. At higher temperatures, 375–400 ◦ C, oxidation leads to the growth of In2 O3 and it is transparent at higher wavelength. The average grain size is found to be in the range of 26–125 nm. 3.2. EDAX analysis EDAX analysis is carried out to (Figs. 3.3 and 3.4) determine the chemical composition of the films (0.02 and 0.03 M). The presence of all elements Cu, In, S and Bi is confirmed. Similar to 0.01 M doped samples, 0.02 and 0.03 M samples contain oxygen at all deposited temperatures. Stoichiometry of the film is achieved when oxygen is taken into relative compositions. The concentration of Cu, In, S and Bi from the film decreases as the temperature increases from 300 to 400 ◦ C. Due to chemisorption, oxygen containing phase has been formed in all the films. But the oxygen content increases as the temperature of the film increases from 300 to 400 ◦ C and maximum of 25.6% is measured at 400 ◦ C. This oxygen surrounds the grain boundaries without entering into crystallites hence the properties of the film are not altered by oxygen content but decrease the crystallographic defects and improves the stoichiometry of the films. 3.3. Optical properties
Fig. 3.2. XRD of sprayed Bi-doped (0.03 M) CuInS2 film prepared at different substrate temperatures.
The optical transmission and absorption spectra were recorded (0.02 and 0.03 M) in the spectral wavelength range of 300–1100 nm (Figs. 3.5 and 3.6). Intrinsic inter-band absorption occurs for 0.02 M doped samples in the UV–vis region when the substrate
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Fig. 3.5. The transmittance spectra of Bi-doped (0.02 M) CuInS2 film sprayed at different substrate temperatures: (a) 300 ◦ C, (b) 325 ◦ C, (c) 350 ◦ C, (d) 375 ◦ C and (e) 400 ◦ C.
Fig. 3.6. The transmittance spectra of Bi-doped (0.03 M) CuInS2 film sprayed at different substrate temperatures: (a) 300 ◦ C, (b) 325 ◦ C, (c) 350 ◦ C, (d) 375 ◦ C and (e) 400 ◦ C.
temperature is at 300 ◦ C (Fig. 3.5) and the absorption decreases and transmission increases from 325 to 400 ◦ C. About 70% light transmission is noticed when the substrate temperature is at 400 ◦ C. But, no intrinsic inter-band absorption is noticed in the case of 0.03 M doped samples (Fig. 3.6). Transmission increases gradually as the temperature increases from 300 to 400 ◦ C and maximum of 55% light transmission is noticed at 400 ◦ C. The decrease in transmission for 0.03 M doped samples might be due to the increase in crystallite size [22,23]. However, improved optical property is observed in the case of 0.02 and 0.03 M doped CuInS2 thin films than the films prepared by thermal and vacuum evaporation methods [20,24]. Bi doped CuInS2 thin films recognized to the Bi atoms may occupy volume as well as near the surface of the films [24] and decrease in film thickness or change in structure may also be the reason for improved transmittance (Table 3.1). The bandgap energy (Eg ) and absorption co-efficient (˛) for the sprayed CuInS2 films are determined from the optical transmission data. The absorption co-efficient (˛) can be calculated by Eq. (4.1). ˛=
2.303 1 log t T
(4.1)
where t is the thickness of the film and T is the transmittance. The absorption coefficient (˛) is calculated for different wavelengths ().
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Fig. 3.7. The absorption spectra of Bi-doped (0.02 M) CuInS2 film sprayed at different substrate temperatures: (a) 300 ◦ C, (b) 325 ◦ C, (c) 350 ◦ C, (d) 375 ◦ C and (e) 400 ◦ C.
Fig. 3.8. The absorption spectra of Bi-doped (0.03 M) CuInS2 film sprayed at different substrate temperatures: (a) 300 ◦ C, (b) 325 ◦ C, (c) 350 ◦ C, (d) 375 ◦ C and (e) 400 ◦ C.
For the direct band gap semiconductors, ˛ can be related to through the equation: 2
(˛h) = A(h − Eg )
(4.2)
where A is a constant, Eg is the Band gap energy and h is the photon energy. The absorption co-efficient (˛) versus the photon energy (h) of the Bi-doped CuInS2 thin film in the temperature range 300–400 ◦ C is shown in Figs. 3.7 and 3.8. The films have high absorption co-efficient in the range 0.5 × 105 –7.6 × 105 cm−1 for 0.02 M and 1 × 105 cm−1 –5 × 105 cm−1 for 0.03 M doped samples in the UV–vis spectral region. Comparing 0.02 and 0.03 M, the 0.02 M doped samples show higher ˛ value (7.6 × 105 cm−1 ). However, both 0.02 and 0.03 M doped CuInS2 thin films have absorption coefficient in the order of 105 /cm between the temperature ranges 300 and 400 ◦ C. Hence, both the samples can be used as an efficient solar cell absorber and other photovoltaic applications in the UV–vis region. But 0.02 M doped samples are more effective than 0.03 M doped samples. For undoped sample ˛ value is quite higher which may affect the solar conversion efficiency of solar cells. The absorption co-efficient (˛), the relation shown by Eq. (4.1) is used. A plot of (˛h)2 versus h for the Bi-doped CuInS2 films (0.02 and 0.03 M) deposited at various temperatures is presented in Figs. 3.9 and 3.10. The bandgap energy increases from 2.10 to 2.92 eV for 0.02 M and 1.60 to 1.82 eV for 0.03 M doped samples. The increase in bandgap energy in the case of 0.02 M doped sample is attributed to crystallo-chemical rearrangement of principal
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Table 3.1 Variation of optical bandgap energies (Eg ) and thickness of Bi doped (0.02 and 0.03 M) CuInS2 film with temperatures. Temperature (◦ C)
300 325 350 375 400
CuInS2 /0.02 M Bi-doped
CuInS2 /0.03 M Bi-doped
Eg (eV)
Thickness (nm)
Eg (eV)
Thickness (nm)
2.10 2.15 2.49 2.58 2.92
677 663 651 647 641
1.60 1.67 1.70 1.72 1.82
732 723 719 711 709
Fig. 3.9. Variation of optical bandgap energy values (Eg ) of sprayed Bi-doped (0.02 M) CuInS2 films prepared at different substrate temperatures: (a) 300 ◦ C, (b) 325 ◦ C, (c) 350 ◦ C, (d) 375 ◦ C and (e) 400 ◦ C.
Fig. 3.10. Variation of optical bandgap energy values (Eg ) of sprayed Bi-doped (0.03 M) CuInS2 films prepared at different substrate temperatures: (a) 300 ◦ C, (b) 325 ◦ C, (c) 350 ◦ C, (d) 375 ◦ C and (e) 400 ◦ C.
structural structures which increase the distance between the top of the valance band formed by P-chalcogen states and bottom of conduction band formed by s–d cationic states [25]. But in the case of 0.03 M doped sample the bandgap energy decreases compared to 0.02 M sample. This may be due to the filling up of S, Cu, Indium vacancies by the Bi atoms or Cu in an In, S sites or In may be in S, Cu sites. The bandgap energy can be improved by passing hydrogen at 400 ◦ C annealing temperature. But, even at 375 ◦ C and beyond, the film (0.02 and 0.03 M) composed of polycrystalline with amorphous phase leading to further deterioration of crystalline properties, which gives rise to defect states and smearing of absorption edge. From Table 3.1 it is noticed that the energy gap (Eg ) values and thickness of the film decreases as the substrate temperature increases from 300 to 400 ◦ C [13].
Fig. 3.11. SEM micrographs of sprayed Bi-doped (0.02 M) CuInS2 films prepared at: 300 ◦ C.
Fig. 3.12. SEM micrographs of sprayed Bi-doped (0.02 M) CuInS2 films prepared at: 300 ◦ C.
3.4. Surface morphology The SEM micrographs of Bi doped (0.02 M) CuInS2 thin films deposited in the temperature range 300–400 ◦ C are shown in Figs. 3.11–3.13. A large number of small flower like crystals of size ranging from 77 to 134 nm are formed at 300 ◦ C (3.12 and 3.13). These crystals grow in size and combine to form large crystals as the temperature increases from 325 to 400 ◦ C. It is quite evident that even at 400 ◦ C, it is polycrystalline associated with amorphous phase as depicted by XRD studies. But the undoped, Sb doped, Zn doped CuInS2 thin films completely formed into an amorphous structure beyond 375 ◦ C [26,27]. In the case of 0.03 M doped samples the crystals size ranging from 106 to 160 nm are formed at 300 ◦ C (Fig. 3.14). Due to the formation of some mediate products in the molten phase accelerated growth of crystals are formed when the substrate temperature is 300–400 ◦ C (Fig. 3.15). These accelerated growths of crystals in
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Fig. 3.13. SEM micrographs of sprayed Bi-doped (0.02 M) CuInS2 films prepared at: 400 ◦ C.
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Fig. 3.16. Variations of the resistivity () of Bi-doped CuInS2 thin films with different substrate temperatures: (a) 0.02 M and (b) 0.03 M.
P-type conductivity. The lowest value obtained is 5 × 10−3 -cm (at 350 ◦ C) which is higher than 0.01 M doped samples. The observed P-type conductivity can be explained by the fact that the introduced Bi acceptor may compensate with donors of sulphur vacancy defects [29]. On further increasing the temperature to 350–400 ◦ C an increase in is observed due to scattering of thermal generated defects, compensation of Cu vacancy defects and interstitials by Bi atoms (or) attributed to localisation of impurity states. For 0.03 M doped samples the resistance further decreases due to the compensation and saturation of valances by Bi atoms. The lowest value obtained at 350 ◦ C for 0.03 M doped sample is 1 × 10−3 -cm and further increase in temperature leads to N-type conductivity which requires further investigations 4. Conclusion Fig. 3.14. SEM micrographs of sprayed Bi-doped (0.03 M) CuInS2 films prepared at: 300 ◦ C.
Fig. 3.15. SEM micrographs of sprayed Bi-doped (0.03 M) CuInS2 films prepared at: 400 ◦ C.
certain regions on the surface are attributed to insufficient wetting of the surface of growing films by molten phase [28]. Some of the crystals of size ranging from 90 to 100 nm are shown in Fig. 3.15. 3.5. Electrical properties Fig. 3.16a and b depicts the variation of resistivity () of Bi (0.02 and 0.03 M) doped CuInS2 thin films. For 0.02 M doped samples the value decreases as the temperature increases with considerable
Bi doped (0.02 and 0.03 M) CuInS2 thin films are deposited in the temperature range 300–400 ◦ C. The growth of CuInS2 along the (1 1 2) preferred plane and other characteristic planes are suppressed by Bi doping. Bi concentration (0.02 and 0.03 M) affects the structure of the film as the temperature increases. Bi doped polycrystalline CuInS2 films are associated with amorphous phase. With the increase in temperature, 375 ◦ C and above, the polycrystalline growth disappears and becomes almost amorphous. EDAX results confirm the presence of Cu, In, S and Bi in the films. Optical properties show that there is an intrinsic inter-band absorption in the 0.02 M Bi doped samples in the UV–vis region when the substrate temperature is at 300 ◦ C. The absorption coefficient (˛ = 7.5 × 105 cm−1 ) for 0.02 M Bi doped sample is found to be higher than 0.03 M doped samples. Hence, 0.02 M Bi doped CuInS2 thin films at 300 ◦ C can be used as an efficient solar cell absorber. The optical bandgap energy (Eg ) increases with the increase in temperatures for both 0.02 and 0.03 M samples. SEM photographs reveal the formation of a large number of flower like small crystals of size ranging from 70 to 200 nm. Photoluminescence properties are completely suppressed by 0.02 and 0.03 M doped samples. No emissions are noticed. The electrical study reveals the semiconducting nature of Bi doped CuInS2 thin films. References [1] H.W. Schock, A. Shah, Proceedings of the European Conference on Photovoltaic Solar Energy Conversion, Stephens & Associates, Barcelona, Spain, 1997, p. 2000. [2] S. Chichibu, S. Shirakata, A. Isomura, H. Nakanishi, Jpn. J. Appl. Phys. 36 (1997) 1703. [3] H.Y. Ueng, H.L. Wang, Mater. Sci. Eng.: B (1992) 261. [4] L.L. Kazmerski, G.A. Sanborn, J. Appl. Phys. 48 (1977) 3178.
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