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Sprayed CuInS2 thin films for solar cells: The effect of solution composition and post-deposition treatments
Solar Energy Materials & Solar Cells 69 (2001) 93}98
Sprayed CuInS thin "lms for solar cells: The e!ect of solution composition and post-deposition treatments M. Krunks *, O. Bijakina , V. Mikli, H. Rebane , T. Varema , M. Altosaar , E. Mellikov Institute of Materials Technology, Tallinn Technical University, Ehitajate 5, 19086 Tallinn, Estonia Centre for Materials Research, Tallinn Technical University, Ehitajate 5, 19086 Tallinn, Estonia Received 16 August 2000
Abstract CuInS thin "lms were prepared by spray pyrolysis from solutions with di!erent composi tions. Etching in KCN solution and thermal treatments in vacuum and hydrogen were applied to as-deposited "lms. KCN etching removes conductive copper sul"de from the surface of Cu-rich "lms but has no e!ect on matrix composition. Vacuum annealing at 5003C and hydrogen treatment at 400}5003C puri"es the "lms, prepared from the solutions with the Cu/In"1, from secondary phases, reduces chlorine content and improves crystallinity. Vacuum annealing results in n-type "lms due to the formation of In O phase. Treatment in hydrogen reduces oxygen-containing residues and results in p-type CuInS "lms with resistivity close to 10 cm. 2001 Elsevier Science B.V. All rights reserved. Keywords: Copper indium disul"de; Thin "lms; Spray pyrolysis; Hydrogen treatment; Vacuum annealing
1. Introduction The high production cost of the conventional solar cells requires a search in order to look for cheaper methods and materials for solar energy conversion. The simple and cheap spray pyrolysis method is a widely used process to produce large-area
* Corresponding author. Tel.: #372-620-33-62; fax: #372-620-27-98. E-mail address: malle@sta!.ttu.ee (M. Krunks). 0927-0248/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 3 8 1 - 0
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metal oxide "lms and it seems attractive to deposit low-cost layers for thin-"lm photovoltaic solar cells also [1}5]. The composition of the starting solution and the growth temperature act on the chemical composition, the structure and the electrical properties of the as-sprayed CuInSe -type "lms [1}7]. Highly textured in the (1 1 2) direction, CuInSe -type "lms were grown using Cu-rich solutions at growth temperatures of 300}4003C. The Cu-rich solutions support the formation of chalcopyrite structure, low resistivity and large crystallites [1}3,5,6]. Although the XRD shows single phase composition of these "lms, the SEM and EDS studies indicate di!erences in bulk and surface compositions as well as nonuniformity in crystallite size [6,7]. The In- and S-rich solutions result in the "lms with poor crystallinity, but with smooth surface and uniform crystallite size [1}3,5}7]. In the present work, CuInS "lms were prepared by spray pyrolysis using solutions of di!erent compositions. The post-deposition chemical etching in KCN solution was applied to Cu-rich "lms. As-sprayed "lms with poor crystallinity and secondary phases were thermally treated in hydrogen and vacuum. In our knowledge, the e!ect of post-deposition thermal treatments onto the properties of sprayed CuInS has not been formerly studied.
2. Experimental Aqueous solutions of CuCl , InCl and SC(NH ) were used to deposit CuInS "lms. The solution (1 mmol/l) was sprayed onto heated glass substrates at temperatures 320}3803C using nitrogen as carrier gas. The Cu/In molar ratio in the solution was varied in the range of 0.8}1.2 at the S/Cu"3 and 6. Chemical etching in 10% KCN solution was made at room temperature. The thermal treatment in dynamic vacuum at 10\ Torr and annealing in #owing hydrogen atmosphere were made in the temperature range 400}5003C. The XRD patterns of the "lms were recorded using the Bruker AXS D5005 di!ractometer with monochromatic Cu K radiation. The optical transmittance spectra were measured in the range of 400}2500 nm on a double beam Beckman Ratio Recording spectrometer. The surface morphology was examined by SEM on the Jeol JSM-840A. The chemical composition was studied by EDS on the Link Analytical AN 10 000 spectrometer. The carrier type was examined by a hot probe method and electrical resistivity was measured by four-point method.
3. Results and discussion 3.1. As-sprayed xlms and chemical etching CuInS "lms with chalcopyrite structure could be deposited from Cu-rich solutions in the temperature range 300}3803C and from the solution with the Cu/In"1, S/Cu"3 (named stoichiometric) at growth temperatures close to 3803C according to
M. Krunks et al. / Solar Energy Materials & Solar Cells 69 (2001) 93}98
95
Fig. 1. The electrical resistivity of as-sprayed "lms in dependence of the growth temperature at di!erent solution compositions (Cu/In"1.0}1.2, S/Cu"3).
XRD [2,3,6]. These CuInS "lms show optical band gap of 1.45 eV and p-type conductivity. The electrical resistivity of sprayed "lms (Cu/In"1.0; 1.1) decreases, if growth temperature increases (Fig. 1). The observed phenomenon is obviously caused by increase in grain size due to the recrystallization processes [2,6]. In the case of Cu/In"1.2 in the solution, the as-deposited "lms show resistivity close to 1 cm independent of growth temperature. After etching in KCN solution the resistivity increases up to 100}200 cm. The "lms deposited from solutions with Cu/In*1 show the Cu/In'1 in the "lm. The Cu/In"1.2 in solution leads to the "lms with the Cu/In"1.3 in the matrix and Cu/In<1 in the region of large crystals formed in Cu-rich "lms. KCN etching has small e!ect on the matrix composition but Cu-rich phase was removed from the area of large crystals resulting in the Cu/In 1 in these "lm regions. It con"rms that high conductivity of as-sprayed "lms produced from Cu-rich solutions should be attributed to the presence of Cu S phase. \V The "lms grown from the stoichiometric solutions at 3203C and deposited from Inor S-rich solutions show, independent of the growth temperature, poor crystallinity and the presence of secondary phases according to XRD (Figs. 2 and 3). The "lms deposited from In-rich solutions (Cu/In"0.8) have In excess (Table 1) and high resistivity. 3.2. Post-deposition thermal treatments in hydrogen and vacuum The hydrogen annealing signi"cantly improves the structure of the "lms deposited from stoichiometric solutions at 3203C according to XRD (Fig. 2). The treatment at 4003C for 30 min is not su$cient to remove the re#ections at 2"26.53, which could belong to In S [3] or organic residues [7]. The annealing time 3 h at 4003C or 30 min at 5003C leads to CuInS "lms textured in the (1 1 2) direction (Fig. 2). The sharp rise in the transmittance spectra in the region of fundamental absorption edge con"rms
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M. Krunks et al. / Solar Energy Materials & Solar Cells 69 (2001) 93}98
Fig. 2. The XRD patterns of the "lms deposited from stoichiometric solution (Cu:In:S"1:1:3) at 3203C and annealed in vacuum and hydrogen. The indexed di!raction peaks belong to CuInS (PDF 27-0159) and unidenti"ed re#ections are marked by `xa.
Fig. 3. The XRD patterns of the "lms deposited from In-rich solution (Cu:In:S"0.8:1:3) at 3203C and S-rich solution (Cu:In:S"1:1:6) at 3803C and annealed in vacuum and hydrogen.
the improvement in crystallinity. The SEM micrographs show smooth surface with well-shaped crystallites of 50}100 nm. The puri"cation of the "lms from chlorine containing residues by annealing at 400}5003C is con"rmed by EDS (Table 1). The "lms show the Cu/In 1 and chlorine content of 0.1 mass% or less. Hydrogen treated "lms have p-type conductivity with resistivity "1}10 cm. The "lms produced from S-rich solutions require higher treatment temperatures (Fig. 3) to obtain puri"cation e!ects similar to stoichiometric solution "lms. The reason could be in di!erent residue amounts in these two types of the "lms and in smaller crystallite size in the case of S-rich solutions [7].
M. Krunks et al. / Solar Energy Materials & Solar Cells 69 (2001) 93}98
97
Table 1 Composition and electrical properties of as-sprayed and thermally treated "lms Cu/In/S
¹ (3C) As-deposited 1
in solution
1/1/3 1/1/6 0.8/1/3
320 380 320
H 4503C, 3 h
Vacuum 5003C, 1 h
Cu/In
Cl Cu/In (mass%)
Cl Cond. (mass%) type
1.19 1.08 0.79
1.8 0.9 1.2
(0.1 0.1 0.2
0.97 0.96 0.73
p p High
( cm) Cu/In
Cl Cond. (mass%) type
6 15 Resist.
(0.1 0.2 0.2
0.76 0.69 0.61
n n n
(cm) 0.1 0.2 0.5
The annealing of the "lms deposited from stoichiometric and S-rich solutions at 5003C in vacuum results in In-rich composition and also reduces chlorine content in the "lms according to EDS (Table 1). Vacuum annealing leads to textured CuInS "lms containing additionally In O according to XRD (Figs. 2 and 3). The SEM study shows rough surface with pin holes. The "lms have n-type conductivity (0.1}0.5 cm) independent of the solution composition. The observed change in conductivity type is obviously connected with the formation of conductive In O phase and supported by the changes in stoichiometry. The decomposition of metal sulfates and oxichlorides could be the origin of oxide formation according to the thermoanalytical study of precursors formed in spraying solution [8]. The "lms deposited from In-rich solutions did not show any change in the structure in both thermal treatment processes according to XRD (Fig. 3).
4. Conlusions CuInS "lms prepared by spray pyrolysis contain impurity phases. It is shown in the present study that post-deposition treatments signi"cantly improve the purity and crystallinity of the "lms. The etching in KCN solution removes conductive copper sul"de phase from the surface of the "lms deposited from Cu-rich solutions. The annealing in dynamic vacuum at temperatures close to 5003C results in n-type CuInS "lms containing In O . The hydrogen treatment at temperatures 4003C and higher puri"es the "lms, produced from stoichiometric and sulfur-rich solutions, from chlorine and oxygen-containing residues and results in highly textured CuInS "lms with chlorine content of 0.1 mass% or less. The "lms are p-type with the resistivity close to 10 cm. The puri"cation of the "lms prepared from In-rich solutions is a more complicated process and did not give satisfactory results.
Acknowledgements This study was partly "nancially supported by the Estonian Science Foundation under the contracts No. 2841 and 4270.
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M. Krunks et al. / Solar Energy Materials & Solar Cells 69 (2001) 93}98
References [1] [2] [3] [4] [5]
S. Shirakata, T. Murakami, T. Kariya, S. Isomura, Jpn. J. Appl. Phys. 35 (1996) 191. M. Krunks, O. Bijakina, T. Varema, V. Mikli, E. Mellikov, Thin Solid Films 338 (1999) 125. M. Ortega-Lopez, A. Morales-Acavedo, Thin Solid Films 330 (1998) 96. K.T. Ramakrishna Reddy, R.B.V. Chalapathy, Solar Energy Mater. Solar Cells 50 (1998) 19. H. Bihri, C. Messaoudi, D. Sayah, A. Boyer, A. Mzerd, A. Abd-Lefdil, Phys. Stat. Sol. (A) 129 (1992) 193. [6] M. Krunks, V. Mikli, O. Bijakina, E. Mellikov, Appl. Surf. Sci. 142 (1999) 356. [7] M. Krunks, V. Mikli, O. Bijakina, H. Rebane, A. Mere, T. Varema, E. Mellikov, Thin Solid Films 361}362 (2000) 61. [8] M. Krunks, T. LeskelaK , R. Mannonen, L. NiinistoK , J. Therm. Anal. 53 (1998) 355.
Sprayed CuInS thin "lms for solar cells: The e!ect of solution composition and post-deposition treatments M. Krunks *, O. Bijakina , V. Mikli, H. Rebane , T. Varema , M. Altosaar , E. Mellikov Institute of Materials Technology, Tallinn Technical University, Ehitajate 5, 19086 Tallinn, Estonia Centre for Materials Research, Tallinn Technical University, Ehitajate 5, 19086 Tallinn, Estonia Received 16 August 2000
Abstract CuInS thin "lms were prepared by spray pyrolysis from solutions with di!erent composi tions. Etching in KCN solution and thermal treatments in vacuum and hydrogen were applied to as-deposited "lms. KCN etching removes conductive copper sul"de from the surface of Cu-rich "lms but has no e!ect on matrix composition. Vacuum annealing at 5003C and hydrogen treatment at 400}5003C puri"es the "lms, prepared from the solutions with the Cu/In"1, from secondary phases, reduces chlorine content and improves crystallinity. Vacuum annealing results in n-type "lms due to the formation of In O phase. Treatment in hydrogen reduces oxygen-containing residues and results in p-type CuInS "lms with resistivity close to 10 cm. 2001 Elsevier Science B.V. All rights reserved. Keywords: Copper indium disul"de; Thin "lms; Spray pyrolysis; Hydrogen treatment; Vacuum annealing
1. Introduction The high production cost of the conventional solar cells requires a search in order to look for cheaper methods and materials for solar energy conversion. The simple and cheap spray pyrolysis method is a widely used process to produce large-area
* Corresponding author. Tel.: #372-620-33-62; fax: #372-620-27-98. E-mail address: malle@sta!.ttu.ee (M. Krunks). 0927-0248/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 3 8 1 - 0
94
M. Krunks et al. / Solar Energy Materials & Solar Cells 69 (2001) 93}98
metal oxide "lms and it seems attractive to deposit low-cost layers for thin-"lm photovoltaic solar cells also [1}5]. The composition of the starting solution and the growth temperature act on the chemical composition, the structure and the electrical properties of the as-sprayed CuInSe -type "lms [1}7]. Highly textured in the (1 1 2) direction, CuInSe -type "lms were grown using Cu-rich solutions at growth temperatures of 300}4003C. The Cu-rich solutions support the formation of chalcopyrite structure, low resistivity and large crystallites [1}3,5,6]. Although the XRD shows single phase composition of these "lms, the SEM and EDS studies indicate di!erences in bulk and surface compositions as well as nonuniformity in crystallite size [6,7]. The In- and S-rich solutions result in the "lms with poor crystallinity, but with smooth surface and uniform crystallite size [1}3,5}7]. In the present work, CuInS "lms were prepared by spray pyrolysis using solutions of di!erent compositions. The post-deposition chemical etching in KCN solution was applied to Cu-rich "lms. As-sprayed "lms with poor crystallinity and secondary phases were thermally treated in hydrogen and vacuum. In our knowledge, the e!ect of post-deposition thermal treatments onto the properties of sprayed CuInS has not been formerly studied.
2. Experimental Aqueous solutions of CuCl , InCl and SC(NH ) were used to deposit CuInS "lms. The solution (1 mmol/l) was sprayed onto heated glass substrates at temperatures 320}3803C using nitrogen as carrier gas. The Cu/In molar ratio in the solution was varied in the range of 0.8}1.2 at the S/Cu"3 and 6. Chemical etching in 10% KCN solution was made at room temperature. The thermal treatment in dynamic vacuum at 10\ Torr and annealing in #owing hydrogen atmosphere were made in the temperature range 400}5003C. The XRD patterns of the "lms were recorded using the Bruker AXS D5005 di!ractometer with monochromatic Cu K radiation. The optical transmittance spectra were measured in the range of 400}2500 nm on a double beam Beckman Ratio Recording spectrometer. The surface morphology was examined by SEM on the Jeol JSM-840A. The chemical composition was studied by EDS on the Link Analytical AN 10 000 spectrometer. The carrier type was examined by a hot probe method and electrical resistivity was measured by four-point method.
3. Results and discussion 3.1. As-sprayed xlms and chemical etching CuInS "lms with chalcopyrite structure could be deposited from Cu-rich solutions in the temperature range 300}3803C and from the solution with the Cu/In"1, S/Cu"3 (named stoichiometric) at growth temperatures close to 3803C according to
M. Krunks et al. / Solar Energy Materials & Solar Cells 69 (2001) 93}98
95
Fig. 1. The electrical resistivity of as-sprayed "lms in dependence of the growth temperature at di!erent solution compositions (Cu/In"1.0}1.2, S/Cu"3).
XRD [2,3,6]. These CuInS "lms show optical band gap of 1.45 eV and p-type conductivity. The electrical resistivity of sprayed "lms (Cu/In"1.0; 1.1) decreases, if growth temperature increases (Fig. 1). The observed phenomenon is obviously caused by increase in grain size due to the recrystallization processes [2,6]. In the case of Cu/In"1.2 in the solution, the as-deposited "lms show resistivity close to 1 cm independent of growth temperature. After etching in KCN solution the resistivity increases up to 100}200 cm. The "lms deposited from solutions with Cu/In*1 show the Cu/In'1 in the "lm. The Cu/In"1.2 in solution leads to the "lms with the Cu/In"1.3 in the matrix and Cu/In<1 in the region of large crystals formed in Cu-rich "lms. KCN etching has small e!ect on the matrix composition but Cu-rich phase was removed from the area of large crystals resulting in the Cu/In 1 in these "lm regions. It con"rms that high conductivity of as-sprayed "lms produced from Cu-rich solutions should be attributed to the presence of Cu S phase. \V The "lms grown from the stoichiometric solutions at 3203C and deposited from Inor S-rich solutions show, independent of the growth temperature, poor crystallinity and the presence of secondary phases according to XRD (Figs. 2 and 3). The "lms deposited from In-rich solutions (Cu/In"0.8) have In excess (Table 1) and high resistivity. 3.2. Post-deposition thermal treatments in hydrogen and vacuum The hydrogen annealing signi"cantly improves the structure of the "lms deposited from stoichiometric solutions at 3203C according to XRD (Fig. 2). The treatment at 4003C for 30 min is not su$cient to remove the re#ections at 2"26.53, which could belong to In S [3] or organic residues [7]. The annealing time 3 h at 4003C or 30 min at 5003C leads to CuInS "lms textured in the (1 1 2) direction (Fig. 2). The sharp rise in the transmittance spectra in the region of fundamental absorption edge con"rms
96
M. Krunks et al. / Solar Energy Materials & Solar Cells 69 (2001) 93}98
Fig. 2. The XRD patterns of the "lms deposited from stoichiometric solution (Cu:In:S"1:1:3) at 3203C and annealed in vacuum and hydrogen. The indexed di!raction peaks belong to CuInS (PDF 27-0159) and unidenti"ed re#ections are marked by `xa.
Fig. 3. The XRD patterns of the "lms deposited from In-rich solution (Cu:In:S"0.8:1:3) at 3203C and S-rich solution (Cu:In:S"1:1:6) at 3803C and annealed in vacuum and hydrogen.
the improvement in crystallinity. The SEM micrographs show smooth surface with well-shaped crystallites of 50}100 nm. The puri"cation of the "lms from chlorine containing residues by annealing at 400}5003C is con"rmed by EDS (Table 1). The "lms show the Cu/In 1 and chlorine content of 0.1 mass% or less. Hydrogen treated "lms have p-type conductivity with resistivity "1}10 cm. The "lms produced from S-rich solutions require higher treatment temperatures (Fig. 3) to obtain puri"cation e!ects similar to stoichiometric solution "lms. The reason could be in di!erent residue amounts in these two types of the "lms and in smaller crystallite size in the case of S-rich solutions [7].
M. Krunks et al. / Solar Energy Materials & Solar Cells 69 (2001) 93}98
97
Table 1 Composition and electrical properties of as-sprayed and thermally treated "lms Cu/In/S
¹ (3C) As-deposited 1
in solution
1/1/3 1/1/6 0.8/1/3
320 380 320
H 4503C, 3 h
Vacuum 5003C, 1 h
Cu/In
Cl Cu/In (mass%)
Cl Cond. (mass%) type
1.19 1.08 0.79
1.8 0.9 1.2
(0.1 0.1 0.2
0.97 0.96 0.73
p p High
( cm) Cu/In
Cl Cond. (mass%) type
6 15 Resist.
(0.1 0.2 0.2
0.76 0.69 0.61
n n n
(cm) 0.1 0.2 0.5
The annealing of the "lms deposited from stoichiometric and S-rich solutions at 5003C in vacuum results in In-rich composition and also reduces chlorine content in the "lms according to EDS (Table 1). Vacuum annealing leads to textured CuInS "lms containing additionally In O according to XRD (Figs. 2 and 3). The SEM study shows rough surface with pin holes. The "lms have n-type conductivity (0.1}0.5 cm) independent of the solution composition. The observed change in conductivity type is obviously connected with the formation of conductive In O phase and supported by the changes in stoichiometry. The decomposition of metal sulfates and oxichlorides could be the origin of oxide formation according to the thermoanalytical study of precursors formed in spraying solution [8]. The "lms deposited from In-rich solutions did not show any change in the structure in both thermal treatment processes according to XRD (Fig. 3).
4. Conlusions CuInS "lms prepared by spray pyrolysis contain impurity phases. It is shown in the present study that post-deposition treatments signi"cantly improve the purity and crystallinity of the "lms. The etching in KCN solution removes conductive copper sul"de phase from the surface of the "lms deposited from Cu-rich solutions. The annealing in dynamic vacuum at temperatures close to 5003C results in n-type CuInS "lms containing In O . The hydrogen treatment at temperatures 4003C and higher puri"es the "lms, produced from stoichiometric and sulfur-rich solutions, from chlorine and oxygen-containing residues and results in highly textured CuInS "lms with chlorine content of 0.1 mass% or less. The "lms are p-type with the resistivity close to 10 cm. The puri"cation of the "lms prepared from In-rich solutions is a more complicated process and did not give satisfactory results.
Acknowledgements This study was partly "nancially supported by the Estonian Science Foundation under the contracts No. 2841 and 4270.
98
M. Krunks et al. / Solar Energy Materials & Solar Cells 69 (2001) 93}98
References [1] [2] [3] [4] [5]
S. Shirakata, T. Murakami, T. Kariya, S. Isomura, Jpn. J. Appl. Phys. 35 (1996) 191. M. Krunks, O. Bijakina, T. Varema, V. Mikli, E. Mellikov, Thin Solid Films 338 (1999) 125. M. Ortega-Lopez, A. Morales-Acavedo, Thin Solid Films 330 (1998) 96. K.T. Ramakrishna Reddy, R.B.V. Chalapathy, Solar Energy Mater. Solar Cells 50 (1998) 19. H. Bihri, C. Messaoudi, D. Sayah, A. Boyer, A. Mzerd, A. Abd-Lefdil, Phys. Stat. Sol. (A) 129 (1992) 193. [6] M. Krunks, V. Mikli, O. Bijakina, E. Mellikov, Appl. Surf. Sci. 142 (1999) 356. [7] M. Krunks, V. Mikli, O. Bijakina, H. Rebane, A. Mere, T. Varema, E. Mellikov, Thin Solid Films 361}362 (2000) 61. [8] M. Krunks, T. LeskelaK , R. Mannonen, L. NiinistoK , J. Therm. Anal. 53 (1998) 355.