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Synthesis and study of optical and structural properties of thin films based on new photovoltaic materials
ARTICLE IN PRESS
Microelectronics Journal 39 (2008) 1351–1353 www.elsevier.com/locate/mejo
Synthesis and study of optical and structural properties of thin films based on new photovoltaic materials G. Gordillo, M. Botero, J.S. Oyola Departamento de Fı´sica, Universidad Nacional de Colombia, Bogota´, Colombia Available online 19 March 2008
Abstract We have synthesized SnS:Bi thin films using a novel method based on sulphurization of the metallic precursors species. The films were characterized through spectral transmittance and X-ray diffraction (XRD) measurements to determine both, the optical constants and structural properties. The studies revealed that the SnS:Bi films tend to grow with a mixture of the SnS and Bi2 S3 phases. It was also found that the SnS:Bi films present an absorption coefficient greater than 104 cm1 and an energy band gap ranging from 1.37 to 1.47 eV, indicating that this compound has good properties to perform as absorber layer in thin film solar cells. r 2008 Elsevier Ltd. All rights reserved. PACS: 61.10.Nz; 78.20.Ci; 81.15.Ef Keywords: SnS thin films; Bi2 S3 thin films; Optical properties; Structural properties
1. Introduction The most extensively studied thin film solar cells are based on CuðIn; GaÞSe2 (CIGS) as absorbent material [1]. However, there is concern respect to the use of cadmium in this type of solar cells. Current efforts are being made to develop new photovoltaic materials. The factors that should be considered in developing these new materials include: the possibility of depositing them using low cost methods, abundance of the precursor elements and low environmental impacts. SnS is one of the promising materials for low cost thin film solar cells technology thanks to its optimum energy band gap Eg and a high fundamental absorption coefficient [2]. Elements Sn and S are abundant in nature and they do not contribute to pollution during the (deposition) process. 2. Experimental The SnS:Bi thin films were prepared by conversion of a Sn:Bi layer deposited by evaporation at room temperature Corresponding author.
E-mail address: [email protected] (G. Gordillo). 0026-2692/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.01.034
into the compound, by annealing it at temperatures around 400 C in the presence of elemental sulphur (sulphurization). The optical and structural properties of the SnS:Bi films were studied from transmittance and X-ray diffraction (XRD) measurements carried out with a VIS-IR Oriel spectrophotometer and a X-ray diffractometer Shimadzu 6000, respectively. The conductivity type was also determined from thermoelectric power measurements. 3. Results and discussion The synthesized SnS:Bi films were initially characterized through XRD measurements in order to get information regarding the phase, crystalline structure and lattice parameters. Fig. 1 shows typical XRD spectra of SnS:Bi films deposited varying the atomic content of Bi in the Sn:Bi alloy, according to the relation: x ¼ [mols of Bi/mols of (Bi+Sn)]. Theoretical simulations of the XRD spectra of Fig. 1 were also performed, in order to improve the confidence degree of the analysis on these spectra. It was found that the SnS:Bi films grow with orthorhombic structure independently of the Bi concentration; however, the phase in which these compounds grow is affected by the content of Bi in the Sn:Bi alloy (see Table 1).
ARTICLE IN PRESS G. Gordillo et al. / Microelectronics Journal 39 (2008) 1351–1353
1352
Bi2S3 (020) SnS (111) Bi2S3 (111) Bi2S3 Bi2S3 (221)(240)
Bi2S3 (120)
Bi2S3 Bi2S3 (251) (022) Bi2S3 (250) 100
Sn2S3 (020) Sn2S3 (120)
50
Bi2S3 (430)
%
5
Bi s
m
ut h
10
0 10
15
20
25
30 35 2θ (degrees)
40
45
50
55
Fig. 1. Typical XRD spectra of SnS:Bi thin films, with Bi concentrations varying between x ¼ 0 (SnS) and x ¼ 1 (BiS).
Table 1 List of phases identified in the studied SnS:Bi samples Bi conc. (%)
Phase
2.0 SnS
Lattice constant (A˚)
1.8
SnS:Bi (10%) SnS:Bi (50%)
a
b
c
1.6 SnS
4.101
11.205
3.861
p50%
Sn2 S3 SnS Bi2 S3
8.852 4.229 11.800
13.970 11.185 12.300
3.772 3.986 5.010
450%
SnS Bi2 S3
4.219 11.115
11.189 11.180
3.964 4.029
100%
Bi2 S3
11.115
11.180
4.029
Corresponding lattice constants are also listed.
In Fig. 2 are shown curves of ðahnÞ2 vs hn used to determine the optical gap Eg of the SnS:Bi films, from the intercept with the hn axis at the linear part of the graph. The absorption coefficient a was calculated using experimental data obtained from spectral transmittance measurements and a procedure described in Ref. [3]. In Table 2 are listed values of Eg, resistivity and conductivity type of the SnS:Bi films as functions of the Bi content. It is observed that the optical gap of the synthesized films varies between 1.37 and 1.47 eV which are considered adequate for solar cells fabrication. The results of Table 2 show that it is possible to get samples with p or n conductivity, controlling adequately the Bi content in the SnS:Bi films. This is an important result because it indicates that it is possible to fabricate p-SnS/n-SnS:Bi solar cells in situ, which facilitates the industrial production.
1.4
(αhν)2 (eV2cm-2)
0
Bi2S3
1.2
1.0
0.8
0.6
0.4
0.2
0.0 1.1
1.2
1.3
1.4
1.5
hν (eV) Fig. 2. Curves of ðahnÞ2 vs hn corresponding to SnS:Bi thin films prepared with Bi concentrations varying between x ¼ 0 and 1.
4. Conclusions Conditions were found for the synthesis of SnS:Bi films with orthorhombic structure and optical gap ranging from
ARTICLE IN PRESS G. Gordillo et al. / Microelectronics Journal 39 (2008) 1351–1353
1353
Table 2 Conductivity type and values of Eg and conductivity of SnS:Bi films as a function of their Bi-content
This is an important result because it indicates that it is possible to fabricate p-SnS/n-SnS:Bi solar cells in situ.
Bi conc. (%)
Conductivity type
r ðO cmÞ
Eg (eV)
Acknowledgement
0 (SnS) p50% 450% 100
p p n n
10:5 2 103 –6 105 1 103 –2:5 103 7.5
1.37 1.47 1.40 1.40
1.37 to 1.47 eV which are considered adequate for solar cells fabrication. It was possible to get samples with p or n conductivity controlling the Bi content in the SnS:Bi films.
This work was supported by Universidad Nacional de Colombia and Colciencias. References [1] B. von Roedem, K. Zweibel, H.S. Ulall, in: 31st IEEE Photovoltaic Specialist Conference and Exhibition, Lake Buena Vista, Florida, 2005, p. 183. [2] M.T.S. Nair, P.K. Nair, Semicond. Sci. Technol. 6 (1991) 132. [3] M. Gracia, F. Rojas, G. Gordillo, in: 20th European Photovoltaic Solar Energy Conference, Barcelona, Spain, 2005.
Microelectronics Journal 39 (2008) 1351–1353 www.elsevier.com/locate/mejo
Synthesis and study of optical and structural properties of thin films based on new photovoltaic materials G. Gordillo, M. Botero, J.S. Oyola Departamento de Fı´sica, Universidad Nacional de Colombia, Bogota´, Colombia Available online 19 March 2008
Abstract We have synthesized SnS:Bi thin films using a novel method based on sulphurization of the metallic precursors species. The films were characterized through spectral transmittance and X-ray diffraction (XRD) measurements to determine both, the optical constants and structural properties. The studies revealed that the SnS:Bi films tend to grow with a mixture of the SnS and Bi2 S3 phases. It was also found that the SnS:Bi films present an absorption coefficient greater than 104 cm1 and an energy band gap ranging from 1.37 to 1.47 eV, indicating that this compound has good properties to perform as absorber layer in thin film solar cells. r 2008 Elsevier Ltd. All rights reserved. PACS: 61.10.Nz; 78.20.Ci; 81.15.Ef Keywords: SnS thin films; Bi2 S3 thin films; Optical properties; Structural properties
1. Introduction The most extensively studied thin film solar cells are based on CuðIn; GaÞSe2 (CIGS) as absorbent material [1]. However, there is concern respect to the use of cadmium in this type of solar cells. Current efforts are being made to develop new photovoltaic materials. The factors that should be considered in developing these new materials include: the possibility of depositing them using low cost methods, abundance of the precursor elements and low environmental impacts. SnS is one of the promising materials for low cost thin film solar cells technology thanks to its optimum energy band gap Eg and a high fundamental absorption coefficient [2]. Elements Sn and S are abundant in nature and they do not contribute to pollution during the (deposition) process. 2. Experimental The SnS:Bi thin films were prepared by conversion of a Sn:Bi layer deposited by evaporation at room temperature Corresponding author.
E-mail address: [email protected] (G. Gordillo). 0026-2692/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.01.034
into the compound, by annealing it at temperatures around 400 C in the presence of elemental sulphur (sulphurization). The optical and structural properties of the SnS:Bi films were studied from transmittance and X-ray diffraction (XRD) measurements carried out with a VIS-IR Oriel spectrophotometer and a X-ray diffractometer Shimadzu 6000, respectively. The conductivity type was also determined from thermoelectric power measurements. 3. Results and discussion The synthesized SnS:Bi films were initially characterized through XRD measurements in order to get information regarding the phase, crystalline structure and lattice parameters. Fig. 1 shows typical XRD spectra of SnS:Bi films deposited varying the atomic content of Bi in the Sn:Bi alloy, according to the relation: x ¼ [mols of Bi/mols of (Bi+Sn)]. Theoretical simulations of the XRD spectra of Fig. 1 were also performed, in order to improve the confidence degree of the analysis on these spectra. It was found that the SnS:Bi films grow with orthorhombic structure independently of the Bi concentration; however, the phase in which these compounds grow is affected by the content of Bi in the Sn:Bi alloy (see Table 1).
ARTICLE IN PRESS G. Gordillo et al. / Microelectronics Journal 39 (2008) 1351–1353
1352
Bi2S3 (020) SnS (111) Bi2S3 (111) Bi2S3 Bi2S3 (221)(240)
Bi2S3 (120)
Bi2S3 Bi2S3 (251) (022) Bi2S3 (250) 100
Sn2S3 (020) Sn2S3 (120)
50
Bi2S3 (430)
%
5
Bi s
m
ut h
10
0 10
15
20
25
30 35 2θ (degrees)
40
45
50
55
Fig. 1. Typical XRD spectra of SnS:Bi thin films, with Bi concentrations varying between x ¼ 0 (SnS) and x ¼ 1 (BiS).
Table 1 List of phases identified in the studied SnS:Bi samples Bi conc. (%)
Phase
2.0 SnS
Lattice constant (A˚)
1.8
SnS:Bi (10%) SnS:Bi (50%)
a
b
c
1.6 SnS
4.101
11.205
3.861
p50%
Sn2 S3 SnS Bi2 S3
8.852 4.229 11.800
13.970 11.185 12.300
3.772 3.986 5.010
450%
SnS Bi2 S3
4.219 11.115
11.189 11.180
3.964 4.029
100%
Bi2 S3
11.115
11.180
4.029
Corresponding lattice constants are also listed.
In Fig. 2 are shown curves of ðahnÞ2 vs hn used to determine the optical gap Eg of the SnS:Bi films, from the intercept with the hn axis at the linear part of the graph. The absorption coefficient a was calculated using experimental data obtained from spectral transmittance measurements and a procedure described in Ref. [3]. In Table 2 are listed values of Eg, resistivity and conductivity type of the SnS:Bi films as functions of the Bi content. It is observed that the optical gap of the synthesized films varies between 1.37 and 1.47 eV which are considered adequate for solar cells fabrication. The results of Table 2 show that it is possible to get samples with p or n conductivity, controlling adequately the Bi content in the SnS:Bi films. This is an important result because it indicates that it is possible to fabricate p-SnS/n-SnS:Bi solar cells in situ, which facilitates the industrial production.
1.4
(αhν)2 (eV2cm-2)
0
Bi2S3
1.2
1.0
0.8
0.6
0.4
0.2
0.0 1.1
1.2
1.3
1.4
1.5
hν (eV) Fig. 2. Curves of ðahnÞ2 vs hn corresponding to SnS:Bi thin films prepared with Bi concentrations varying between x ¼ 0 and 1.
4. Conclusions Conditions were found for the synthesis of SnS:Bi films with orthorhombic structure and optical gap ranging from
ARTICLE IN PRESS G. Gordillo et al. / Microelectronics Journal 39 (2008) 1351–1353
1353
Table 2 Conductivity type and values of Eg and conductivity of SnS:Bi films as a function of their Bi-content
This is an important result because it indicates that it is possible to fabricate p-SnS/n-SnS:Bi solar cells in situ.
Bi conc. (%)
Conductivity type
r ðO cmÞ
Eg (eV)
Acknowledgement
0 (SnS) p50% 450% 100
p p n n
10:5 2 103 –6 105 1 103 –2:5 103 7.5
1.37 1.47 1.40 1.40
1.37 to 1.47 eV which are considered adequate for solar cells fabrication. It was possible to get samples with p or n conductivity controlling the Bi content in the SnS:Bi films.
This work was supported by Universidad Nacional de Colombia and Colciencias. References [1] B. von Roedem, K. Zweibel, H.S. Ulall, in: 31st IEEE Photovoltaic Specialist Conference and Exhibition, Lake Buena Vista, Florida, 2005, p. 183. [2] M.T.S. Nair, P.K. Nair, Semicond. Sci. Technol. 6 (1991) 132. [3] M. Gracia, F. Rojas, G. Gordillo, in: 20th European Photovoltaic Solar Energy Conference, Barcelona, Spain, 2005.