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Preparation and characterization of chemically deposited CuInSe2 films
Thin Solid Films, I67 (1988) L19-L22
L19
Letter
Preparation aod characterization of chemically deposited CuIn!Se, films K. R. MURAL1
Central Electrochemical Research Institute, Karaikudi623006 (India) (Received July 12,198s;
accepted September 26,1988)
The structural and electrical properties of thin films of copper indium selenide (CuInSe,) prepared by the chemical bath deposition technique are described. The composition of the polycrystalline films deviate from ideal composition as observed from EPMA analysis. X-ray diffraction analyses showed that the films were single phase CuInSe, with chalcopyrite structure. The films were p-type and the resistivity is in the range 50-5OOohmcm, depending on the deposition parameters. Preliminary data on CdS/CuInSe, heterojunctions are also presented. I. Introduction
In recent years the ternary chalcopyrite semiconductor CuInSe, has received considerable attention since it shows promise for practical use as an alternative absorber generator material in solar cells’. The films have been prepared by a number of methods2-‘. However, the chemical bath deposition technique which is well suited for commercial large-scale production has not been thoroughly investigated. Although some preliminary results on the growth of CuInSe2 films by chemical bath deposition were reported by Bhattacharya”, a systematic study was not carried out. In this work, CuInSe, thin films were grown by the chemical bath deposition method and the effect of deposition parameters on structural and electrical properties of these films are observed. 2. Experimental methods The deposition mixture consisted of 5 ml CuCl(O.05 M), 6 ml InCl, (0.05 M), 0.7 ml ammonia, 0.65 ml triethanolamine and 10 ml sodium selenosulphate (0.3 M). Solutions of CuCl, InCl, and triethanolamine were taken in a beaker and stirred for a period of about 6O-80min. All the chemicals were of analytical grade and the solutions were prepared using deionized water. Mixtures of sodium selenosulphate and ammonia were added from a burette. CuInSe, thin films were deposited on glass slides. The glass slides were mounted vertically in the deposition mixture kept at 85 “C, which was vigorously stirred during deposition of the films. While films obtained at temperatures lower than 85 “C were less adherent and powdery, those obtained at 85 “C were extremely adherent to the substrate and were homogeneous. Several layers of the films were 0040-60!40/88/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
L20
LETTERS
deposited by repeating the above process. In most of the cases the deposition time was around 2 h. An increase in the deposition time beyond 2 h increases the concentration of the chalcogen, and precipitation becomes more significant; hence the increase in film thickness becomes negligibly small. Various thin films were grown under different deposition conditions such as (i) varying the quantity of triethanolamine added to the CuCl plus InCl, mixture (from 0.2 to 0.8 ml), (ii) varying the quantity of ammonia added to sodium selenosulphate solution (from 0.2 to 0.9 ml) and (iii) by varying the quantity of CuCl(3.0-6.0 ml). Before deposition of the films, the glass substrates were first boiled in chromic acid and then cleaned with acetone. Just before use they were finally cleaned several times with boiling water and acetone. Deposition took place at a pH value greater than 9.,The triethanolamine complex of copper and indium dissociates to give a controlled number of copper and indium ions which combine with the selenium ions to form CuInSe,. Se2- ions are obtained from the sodium selenosulphate solution according to the following chemical reaction: Na,SeSO,+20H-+
Na2S04+H20+Se2-
(1)
The film thicknesses measured by weighing were around OSpm. Two- and three-layer depositions yielded thicknesses of the or&r of 0.9 pm. The films were analysed by X-ray diffraction usmg CuKa radiation. Compositional data were obtained by electron probe microanalysis. The electrical resistivity of CuInSe, fihns was measured using the Van der Pauw method. Evaporated silver was used as an ohmic contact. 3. Results and discussion The X-ray diffraction pattern of a film deposited at 85 “C by using 5 ml CuCl, 5ml InCI, (both 0.05 M), 0.65ml triethanolamine, 018ml ammonia and lOm1 sodium selenosulphate (0.3 M) is shown in Fig. 1. The diffraction pattern was compared with the ASTM powder diffraction data and it was observed that the films exhibited only the prominent lines assignable to single-phase chalcopyrite CuInSel. Diffraction lines corresponding to copper selenide or indium selenide were absent.
Fig. 1.
1s (110 X-ray diffraction pattern of a chemically deposited CuInSe, 6lm.
L21
LETTERS
Several films were grown by using the above mixture and the film composition was found to be reproducible. Scanning electron microscopy studies carried out on these films have indicated a grain size around 0.2 um. The compositional analysis of the films is indicated in Table I. Films deposited from the mixture cited above were nearly stoichiometric. By changing the quantity of triethanolamine (from 0.2 to 0.8 ml) it was observed that adherent coatings were obtained only for 0.62-0.65ml triethanolamine. The quantity of ammonia was changed from 0.2 to 0.9 ml in order to establish the optimum quantity necessary for a good and adherent coating. As the quantity of CuCl was changed from 3 to 6 ml, it was observed that for 3 and 4 ml CuCl(O.05 M) the films were slightly deficient in copper and a slight excess of copper was observed for 6 ml CuCl(O.05 M). TABLE I EFFECTS OF
&cl
CONCENTRATION
ON TTE COMPOSI~ON
Quantity of Cdl (0.05 M) (ml)
Film composition
3.0 4.0 5.0 5.5 6.0
Cu0.&.&%.1 Cu&.&,.0*%.0, cu 1.021nb0bSez.0~ cu I.O&bO$ez.OS CU1.0&k0sSe,.07
Composition of deposition mixture: 6ml InCl,(O.OS M), lOm1 sodium selenosulphate (0.3 M), 0.65ml triethanolamine and 0.8 ml ammonia. Temperature: 85 “C.
The resistivity of the films deposited on glass substrates was measured by the Van der Pauw method and hot probe measurements indicated the films to be p type. The resistivity was found to depend on the Cu:In ratio. Lower Cu: In ratios resulted in high resistivity, and higher Cu:In ratios resulted in low resistivity films. The results are presented in Table II. With a view to testing the use of these layers in solar cells, a CuInSe, layer was chemically deposited on conducting glass and a layer of CdS was vacuum evaporated on the CuInSe, layer kept at 200 “C. Finger-type contacts were made by evaporating indium on CdS. The heterojunction exhibited an open-circuit voltage of 150mV and a short-circuit current density of 4mA cme2 under air mass 1 illumination. Further work on the deposition of large-area CuInSe, films is in progress. TABLE II PROPERTIES
OF CHEMICALLY
DEPOSITED
FILMS
Composition
Type
Resistiuity(S2 cm)
Cu0.91nl.02Se2.1
P P P P P
500 400 325 150 50
CuO.&l.O&,.O, cu 1.02In I.04 S co5 ~l.oJn~.o~Se~.o~ cu 1.0sIn1.0s%.07
L22
LBTTBRS
1 J. Shay and J. Wemick, Ternary Chalcopyrite Semiconductors: Growth, Elecironic Properties and Applications, Pergamon, oxford, 1975. 2 E. Elliot, R. D. Tomlinson, J. Parkes and M. J. Hampshire, Thin SolidFilms, 20 (1975) S25. 3 L. Y. Sun, L. L. Kazmerski, A. H. Clark, P. J. Ireland and D. N. Morton, J. Vat. Sci. Technol., 15 (1978) 265. 4 R. A. Mickelson, W. S. Chen, Y. R. Hsiao and V. E. Lowe, Proc. IEEE Photouoltaic Specialists, Co& IEEE, New York, 1985, p. 542. 5 J. Piekoszewski, J. J. Lofenki, R. Beaulieu, J. Beall, B. Roessler and J. Shewchun, Sol. Energy Mater., 2 (1980) 363. 6 F. R. White, A. H. Clark, M. C. Graf and L. L. Kazmerski, J. Appl. Phys., 50 (1979) 544. 7 B. R. Pamplin, Prog. Crysl. Growth Charact., I (1979) 395. 8 K. R. Murali, unpublished results. 9 R. P. Singh, S. L. Singh and S. Chandra, J. Phys. D, 19 (1986) 1299. 10 R. N. Bhattacharya, J. Electrochem. SOL, 130 (1983) 2040.
L19
Letter
Preparation aod characterization of chemically deposited CuIn!Se, films K. R. MURAL1
Central Electrochemical Research Institute, Karaikudi623006 (India) (Received July 12,198s;
accepted September 26,1988)
The structural and electrical properties of thin films of copper indium selenide (CuInSe,) prepared by the chemical bath deposition technique are described. The composition of the polycrystalline films deviate from ideal composition as observed from EPMA analysis. X-ray diffraction analyses showed that the films were single phase CuInSe, with chalcopyrite structure. The films were p-type and the resistivity is in the range 50-5OOohmcm, depending on the deposition parameters. Preliminary data on CdS/CuInSe, heterojunctions are also presented. I. Introduction
In recent years the ternary chalcopyrite semiconductor CuInSe, has received considerable attention since it shows promise for practical use as an alternative absorber generator material in solar cells’. The films have been prepared by a number of methods2-‘. However, the chemical bath deposition technique which is well suited for commercial large-scale production has not been thoroughly investigated. Although some preliminary results on the growth of CuInSe2 films by chemical bath deposition were reported by Bhattacharya”, a systematic study was not carried out. In this work, CuInSe, thin films were grown by the chemical bath deposition method and the effect of deposition parameters on structural and electrical properties of these films are observed. 2. Experimental methods The deposition mixture consisted of 5 ml CuCl(O.05 M), 6 ml InCl, (0.05 M), 0.7 ml ammonia, 0.65 ml triethanolamine and 10 ml sodium selenosulphate (0.3 M). Solutions of CuCl, InCl, and triethanolamine were taken in a beaker and stirred for a period of about 6O-80min. All the chemicals were of analytical grade and the solutions were prepared using deionized water. Mixtures of sodium selenosulphate and ammonia were added from a burette. CuInSe, thin films were deposited on glass slides. The glass slides were mounted vertically in the deposition mixture kept at 85 “C, which was vigorously stirred during deposition of the films. While films obtained at temperatures lower than 85 “C were less adherent and powdery, those obtained at 85 “C were extremely adherent to the substrate and were homogeneous. Several layers of the films were 0040-60!40/88/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
L20
LETTERS
deposited by repeating the above process. In most of the cases the deposition time was around 2 h. An increase in the deposition time beyond 2 h increases the concentration of the chalcogen, and precipitation becomes more significant; hence the increase in film thickness becomes negligibly small. Various thin films were grown under different deposition conditions such as (i) varying the quantity of triethanolamine added to the CuCl plus InCl, mixture (from 0.2 to 0.8 ml), (ii) varying the quantity of ammonia added to sodium selenosulphate solution (from 0.2 to 0.9 ml) and (iii) by varying the quantity of CuCl(3.0-6.0 ml). Before deposition of the films, the glass substrates were first boiled in chromic acid and then cleaned with acetone. Just before use they were finally cleaned several times with boiling water and acetone. Deposition took place at a pH value greater than 9.,The triethanolamine complex of copper and indium dissociates to give a controlled number of copper and indium ions which combine with the selenium ions to form CuInSe,. Se2- ions are obtained from the sodium selenosulphate solution according to the following chemical reaction: Na,SeSO,+20H-+
Na2S04+H20+Se2-
(1)
The film thicknesses measured by weighing were around OSpm. Two- and three-layer depositions yielded thicknesses of the or&r of 0.9 pm. The films were analysed by X-ray diffraction usmg CuKa radiation. Compositional data were obtained by electron probe microanalysis. The electrical resistivity of CuInSe, fihns was measured using the Van der Pauw method. Evaporated silver was used as an ohmic contact. 3. Results and discussion The X-ray diffraction pattern of a film deposited at 85 “C by using 5 ml CuCl, 5ml InCI, (both 0.05 M), 0.65ml triethanolamine, 018ml ammonia and lOm1 sodium selenosulphate (0.3 M) is shown in Fig. 1. The diffraction pattern was compared with the ASTM powder diffraction data and it was observed that the films exhibited only the prominent lines assignable to single-phase chalcopyrite CuInSel. Diffraction lines corresponding to copper selenide or indium selenide were absent.
Fig. 1.
1s (110 X-ray diffraction pattern of a chemically deposited CuInSe, 6lm.
L21
LETTERS
Several films were grown by using the above mixture and the film composition was found to be reproducible. Scanning electron microscopy studies carried out on these films have indicated a grain size around 0.2 um. The compositional analysis of the films is indicated in Table I. Films deposited from the mixture cited above were nearly stoichiometric. By changing the quantity of triethanolamine (from 0.2 to 0.8 ml) it was observed that adherent coatings were obtained only for 0.62-0.65ml triethanolamine. The quantity of ammonia was changed from 0.2 to 0.9 ml in order to establish the optimum quantity necessary for a good and adherent coating. As the quantity of CuCl was changed from 3 to 6 ml, it was observed that for 3 and 4 ml CuCl(O.05 M) the films were slightly deficient in copper and a slight excess of copper was observed for 6 ml CuCl(O.05 M). TABLE I EFFECTS OF
&cl
CONCENTRATION
ON TTE COMPOSI~ON
Quantity of Cdl (0.05 M) (ml)
Film composition
3.0 4.0 5.0 5.5 6.0
Cu0.&.&%.1 Cu&.&,.0*%.0, cu 1.021nb0bSez.0~ cu I.O&bO$ez.OS CU1.0&k0sSe,.07
Composition of deposition mixture: 6ml InCl,(O.OS M), lOm1 sodium selenosulphate (0.3 M), 0.65ml triethanolamine and 0.8 ml ammonia. Temperature: 85 “C.
The resistivity of the films deposited on glass substrates was measured by the Van der Pauw method and hot probe measurements indicated the films to be p type. The resistivity was found to depend on the Cu:In ratio. Lower Cu: In ratios resulted in high resistivity, and higher Cu:In ratios resulted in low resistivity films. The results are presented in Table II. With a view to testing the use of these layers in solar cells, a CuInSe, layer was chemically deposited on conducting glass and a layer of CdS was vacuum evaporated on the CuInSe, layer kept at 200 “C. Finger-type contacts were made by evaporating indium on CdS. The heterojunction exhibited an open-circuit voltage of 150mV and a short-circuit current density of 4mA cme2 under air mass 1 illumination. Further work on the deposition of large-area CuInSe, films is in progress. TABLE II PROPERTIES
OF CHEMICALLY
DEPOSITED
FILMS
Composition
Type
Resistiuity(S2 cm)
Cu0.91nl.02Se2.1
P P P P P
500 400 325 150 50
CuO.&l.O&,.O, cu 1.02In I.04 S co5 ~l.oJn~.o~Se~.o~ cu 1.0sIn1.0s%.07
L22
LBTTBRS
1 J. Shay and J. Wemick, Ternary Chalcopyrite Semiconductors: Growth, Elecironic Properties and Applications, Pergamon, oxford, 1975. 2 E. Elliot, R. D. Tomlinson, J. Parkes and M. J. Hampshire, Thin SolidFilms, 20 (1975) S25. 3 L. Y. Sun, L. L. Kazmerski, A. H. Clark, P. J. Ireland and D. N. Morton, J. Vat. Sci. Technol., 15 (1978) 265. 4 R. A. Mickelson, W. S. Chen, Y. R. Hsiao and V. E. Lowe, Proc. IEEE Photouoltaic Specialists, Co& IEEE, New York, 1985, p. 542. 5 J. Piekoszewski, J. J. Lofenki, R. Beaulieu, J. Beall, B. Roessler and J. Shewchun, Sol. Energy Mater., 2 (1980) 363. 6 F. R. White, A. H. Clark, M. C. Graf and L. L. Kazmerski, J. Appl. Phys., 50 (1979) 544. 7 B. R. Pamplin, Prog. Crysl. Growth Charact., I (1979) 395. 8 K. R. Murali, unpublished results. 9 R. P. Singh, S. L. Singh and S. Chandra, J. Phys. D, 19 (1986) 1299. 10 R. N. Bhattacharya, J. Electrochem. SOL, 130 (1983) 2040.