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Some electrical and optical studies on bismuth trisulphide thin films
Solar Energy Materials and Solar Cells 28 (1992) 249-254 North-Holland
Solar Energy Materials and Solar Cells
Some electrical and optical studies on bismuth trisulphide thin films L.P. Deshmukh, K.V. Zipre, A.B. Palwe, B.P. Rane, P.P. H a n k a r e and A.H. Manikshete Department of Physics and Chemistry, Shit:aji University, Centre for Post-Graduate Studies, Solapur 413 003, M.S,, India Received 5 April 1991; in revised form 18 December 1991 An electroless chemical method for the deposition of bismuth trisulphide thin films is presented. For the deposition, the triethanolamine complex of bismuth nitrate was allowed to react with aqueous thiourea solution. The substrates used are glass microslides. Good-quality samples are obtained at 950C deposition temperature. The deposition time was 30 min and solution pH was about 9.5. The substrates are kept rotating at a speed of 70 rpm. Layers of Bi2S3, 0.8 to 1.2 p,m thick are obtained by this process at the above deposition conditions. The films have n-type conduction with high electrical resistance (105 to 10 6 l'~ cm) compared to single crystals. The activation energies of electrical conduction in low and high temperature regions are 0.16 eV and 0.80 eV, respectively. The films are polycrystalline in nature. Optical absorption studies revealed a high absorption coefficient (104 cm-1), with a direct type of transition. The observed bandgap is 1.60 eV.
1. Introduction
Direct bandgap semiconductors with bandgaps in the range 1.2 to 1.7 eV are well suited to convert light into electricity. In this respect, bismuth trisulphide, an n-type direct bandgap semiconductor whose experimentally observed bandgap varies from 1.3 to 1.7 eV, seems to be a promising candidate. Since photovoltaic properties are directly related to material properties, the choice lies in both the preparation method and the characterization techniques. Only a few methods have been used to prepare and characterize the bismuth trisulphide material, both in single-crystal and polycrystalline forms, and different aspects of electrochemical behavior have been reported [1-9]. The optical to electrical conversion efficiency so far reported is quite below expectation and is generally supposed to be due to its poor conductivity. Taking intoaccount these experimental observations, we have proposed to deposit the thin films of bismuth trisulphide by an electroless chemical deposition method for end use in photoelectrochemical cells. Correspondence to: L.P. Deshmuth, Department of Physics and Chemistry, Shivaji University, Centre for Post-graduate Studies, Solapur 413 003, M.S., India. 0927-0248/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
250
L.P. Deshmuth et al. / Bismuth trisulphide thin films
This paper briefly describes the procedure for the deposition of bismuth trisulphide thin films and some measurements on such material properties as conductivity, thermoelectric power, and optical absorption.
2. Experimental details 2.1. Preparation o f the samples
Bismuth trisulphide thin films have been prepared by a modified chemical deposition process [10-12] by allowing the triethanolamine complex of Bi 3÷ to react with S 2- ions, which are released slowly by the dissociation of thiourea [3,4,9]. The detailed procedure is: 20 ml of 0.01 M freshly prepared solution of Bi 3÷, obtained by triturating G.R. grade bismuth nitrate with triethanolamine, was placed in a glass beaker 250 ml in capacity. 30 ml 1 M thiourea solution and 5 ml (14 N) aqueous ammonia were added to the beaker and the total volume of the reaction mixture was made up to 200 ml with distilled water. The pH of the solution was adjusted to around 9.5. The glass substrates, of dimensions 72 × 8 x 2 mm, were thoroughly cleaned and kept immersed in the reaction mixture by means of a specially designed substrate holder. The substrate holder was attached to a constant-speed gear motor. The whole assembly was fitted to an oil trough whose temperature was controlled to 95 + 0.5°C. The substrates were rotated at a speed of 70 rpm. in the reaction mixture for about 30 min. After 30 min the samples were taken out, washed with distilled water, dried in air, and left in a dark desiccator. 2.Z Characterization o f the samples The samples were tested for their electrical conductivity in the temperature range 300 K to 600 K. A DC power supply was used to pass the current through the sample. The current was measured by a HIL-2665, 4½ digit current meter. The thermoelectric studies were performed in the 300 K to 500 K temperature range. The temperature gradient, and thermovoltage were recorded with 3½ digit (Vasavi make) and 4½ digit (HIL-2665) microvoltmeters, respectively. A Cr-AI thermocouple was used to record the temperature in both cases. The structural and optical studies were examined with XRD and optical absorption techniques. The X-ray diffractometer (PW-1710) was operated on 30 kV-20 mA using Cu Ket radiation. The scanning rate was 2°/min for angles from 10° to 100°. A Leitz Laborlux optical microscope was used to observe the microstructure ( M = 630). The films were scanned for further optical studies in the wavelength 500 nm to 900 nm with a Hitachi photospectrometer (Japan). The scan rate was 100 nm/min. 3. Results and discussion
An electroless method of chemical deposition of thin films is a relatively inexpensive, simple, and convenient technique for the preparation of large area
L.P. Deshmuth et al. / Bismuth trisulphide thin films
251
II-VI and IV-VI compounds. The method has several overriding advantages over the other conventional techniques [9-12]. These advantages are (1) the method requires negligible power consumption, (2) it is ideally suited for large-area deposition, (3) the substrate material is not an important criterion, (4) it avoids oxidation and corrosion of metallic surfaces, (5) it results in reproducible deposits both in structural and electrical properties, (6) the stoichiometry can easily be maintained, and (7) doping can be easily incorporated and allows slow growth of the film' with better orientation of crystallites. Using this technique, thin films of bismuth trisulphide have been deposited on glass substrates by allowing them to remain in an aqueous solution of the reactants for a period equal to deposition time. The basic overall reaction is [4]
B i [ N ( C H 2 C H 2 O H ) 3 ] 3+ +
S=C
/ NH 2 \ NH 2
~ Bi2S 3 +
O=C
/ NH 2 \
+
n A,
NH2
where A is the complexing agent, N(CH2CH2OH) 3. For a freshly prepared complex solution and at a deposition temperature around 95°C, good-quality (very uniform, reproducible, tightly adherent, specularly reflecting, and crack free) deposits are obtained for pH around 9.5. The color of the deposit is dark brown. If the solution is not freshly prepared (prepared 5-7 h before the deposition) and other conditions are held constant, the color of the deposit appears to be light brown. In both cases, when the reaction mixture is subjected to 95 + 0.5°C, the solution starts taking its respective color after 5 to 7 minutes. The thickness of the layer, measured by weight difference-density considerations, lies in the 0.8 to 1.2 Ixm range. Considerable change in thickness is observed for the samples deposited from a non-fresh solution. These films are found to be thick, spotty, and less reflecting. The possibility of thick and spotty samples in the case of bismuth solution kept for a few hours before deposition may be due to incorporation of some of the precipitated bismuth hydroxide clusters into the sample during deposition. These observations are in close agreement with the results of Bhattacharya and Paramanik [4]. The DC electrical conductivity of the samples was measured in the temperature range 300 K to 600 K for the both heating and cooling cycles. The change in conductivity was noted for every 5°C rise in temperataure. It is understood that both for heating and cooling cycles, the log or versus 1/T variation (fig. 1) follows almost the same track. This observation partly confirms the uniformity of the samples [12,13]. The conductivity increases with increasing temperature. Two distinct regions are clearly seen corresponding to two activation energies. The activation energies of electrical conduction have been determined in both low and high temperature regions and found to be 0.16 eV and 0.80 eV, respectively. A considerable difference is observed in the values of activation energies in our case and the values so far reported [3,4,6]. This can be partly attributed to the comparatively lower resistivity of our samples than the samples prepared by other conventional techniques, viz. solution gas interface, dip and dry, sulfurization, and
L.P. Deshrnuth et al. / Bismuth trisulphMe thin films
252
oN
°\ oN
~z
°\ ° \
o\ °\ o \ o
1°6
i
i
o ~ o a
2°0
2,Z,
2,8
--
o - - o
--
1000/T,(K-1 ) Fig. 1. T e m p e r a t u r e dependence of the conductivity for Bi2S 3 thin films.
spray pyrolysis. The lower resistivity observed in our case can be due to the relatively higher thickness, and by the improved degree of crystallinity provided by the preferred orientation while depositing the samples. We believe that this flexibility can thus be accounted for in the use of substrate holder design. This effect is also reflected in the lower magnitudes of activation energies [4]. In general, resistance of bismuth sulphide thin films is very high compared to the single-crystal resistance, and this difference is mainly because of the grain boundary discontinuities and thickness of the films. Moreover, the grain boundary and discontinuity of the samples are dependent on the deposition conditions [4]. The thermoelectric power generated by the sample was measured from 300 K to 500 K, and the polarity of the thermovoltage is negative towards the hot end of the sample. This indicates that the samples are of n-type conductivity. The observed thrmoelectric power is of the order of IxV/°C, and it increases with increasing temperature. The temperature dependence of thermoelectric power is shown in fig. 2. The material was further characterized by optical and structural techniques. The samples are of polycrystalline nature. This is further supported by microscopic observations. These observations are in close agreement with the results reported by Pawar et al. [6,8] and Bhattacharya and Pramanik [4]. The absorption spectra have been studied at 300 K in the 500 nm to 900 nm wavelength range. The absorption coefficient is of the order of 104 cm- 1, indicating that the material is of the direct-bandgap type and that transitions are allowed [3,4,6,8]. The energy-absorbance spectrum is shown in fig. 3. The absorption edge is much broader than that expected for a direct-bandgap-type material. This can be ascribed to the grain-boundary discontinuity effect in the structure and lack of stoichiometry generally observed in polycrystalline materials [4,6,12,13]. The optical bandgap has been determined from the absorbance spectra by plotting (t~hv) z versus hr. The
L.P. Deshmuth et al. / Bismuth trisulphide thin films
/°
O
) gs0
253
/
/
g
o~°
E o .......o -+~°/
loo
300
200
Temperature (°C)
Fig. 2. Variation of thermoelectric power with temperature.
/ / 0 0 ~0
o~ ,~
/°
.
/ /o
2
°/ , ~
O / O f ~
0
/
,
1,8
1.2
2,/;
3,0
hv (ev) Fig. 3. Variation of optical absorption coefficient(a)with photon energy (hv).
A
/
v
"o r4
/
./
~4 t~ A
/ 0
..,
lo2
_?~o
.....,..,o~
loL, hv
J
t,6 (eV)
Io8
Fig. 4. Variation of ( a h v ) 2 versus h r .
2,0
254
L.P. Deshmuth et al. / Bismuth trisulphide thin films
extrapolation to the energy axis gives a value of b a n d g a p equal to 1.6 eV (fig. 4). This agrees well with the values r e p o r t e d by others [3,6,8]. T h e magnitude of b a n d g a p seems to be higher than that of the observations on single-crystal studies. The difference can be accounted for by interference by diffuse reflectance within the material itself [14] and may be due to the anisotropy of the film-formation process [12]. Detailed studies on parameterization of the material with respect to its deposition conditions and preparative p a r a m e t e r s are in progress.
4. Conclusions Good-quality deposition o f n-Bi2S 3 thin films, reproducible both in electrical and optical properties, is easily possible with less c o n s u m p t i o n o f both energy and active materials. T h e technique offers a way to avoid the clumsy and time-consuming processes involved in the deposition of this material by o t h e r techniques. T h e deposition temperature, time, and p H of the solution play very important roles in producing high quality samples. T h e deposits are polycrystalline, with resistivities in the 10 5 to 10 6 ~ cm range. T h e material is o f the direct-bandgap type, with a high absorption coefficient, 10 4 c m - 1 . T h e estimated b a n d g a p is 1.60 eV.
Acknowledgements O n e o f the authors ( L P D ) thanks Professor R.N. Patil and Professor S.H. Pawar, University D e p a r t m e n t of Physics, for their constant e n c o u r a g e m e n t and moral support.
References [1] S.M. Polyakov, E.N. Loverko, I.S. Lisker, A.L. Pukashanskii and V.M. Yagodkin, Sov. Phys. Solid State 17 (1975) 386. [2] B. Miller and A. Heller, Nature 262 (1976) 680. [3] P. Pramanik and R.N. Bhanacharya, J. Electrochem. Soc. 127 (1980) 2087. [4l R.N. Bhattacharya and P. Pramanik, J. Electrochem. Soc. 129 (1982) 332. [5] P.K. Mahapatra and B. Roy, Sol. Cells 7 (1983) 225. [6] S.H. Pawar, P.N. Bhosale and M.D. Uplane, Thin Solid Films 110 (1983) 165. [7] P.A.K. Moorthy and G.K. Shivakumar, Mater Lett. 2 (1984) 223; Thin Solid Films 12 (1984) 151. [8] S.H. Pawar, S.P. Tamhankar and C.D. Lokhande, Mater. Chem. Phys. 11 (1984) 401. [9] C.V. Suryanarayana, A.S. Lakshmanan, V. Subramanian, and R.K. Kumar, Bull. Electrochem. 2 (1986) 57. [10] L.P. Deshmukh, A.B. Palwe and V.S. Sawant, Sol. Cells 28 (1990) 1. [11] L.P. Deshmukh, A.B. Palwe and V.S. Sawant, Sol. Energy Mater. 20 (1990) 341. [12] L.P. Deshmukh, K.V. Zipre, D.S. Hulle, P.P. Hankare and A.H. Manikshete, in: Proc. Nat. Conference on Oxide Ceramics and Technology, 21-23 February 1991, Shivaji University, Kolhapur, M.S. (India) p. 63. [13] P.N. Bhosale, Ph.D. Thesis, Shivaji University, Kolhapur, M.S. (India) (1985) p. 86. [14] D.E. Aspnes, E. Kinsborn and D.D. Bacon, Phys. Rev. B 21 (1980) 329.
Solar Energy Materials and Solar Cells
Some electrical and optical studies on bismuth trisulphide thin films L.P. Deshmukh, K.V. Zipre, A.B. Palwe, B.P. Rane, P.P. H a n k a r e and A.H. Manikshete Department of Physics and Chemistry, Shit:aji University, Centre for Post-Graduate Studies, Solapur 413 003, M.S,, India Received 5 April 1991; in revised form 18 December 1991 An electroless chemical method for the deposition of bismuth trisulphide thin films is presented. For the deposition, the triethanolamine complex of bismuth nitrate was allowed to react with aqueous thiourea solution. The substrates used are glass microslides. Good-quality samples are obtained at 950C deposition temperature. The deposition time was 30 min and solution pH was about 9.5. The substrates are kept rotating at a speed of 70 rpm. Layers of Bi2S3, 0.8 to 1.2 p,m thick are obtained by this process at the above deposition conditions. The films have n-type conduction with high electrical resistance (105 to 10 6 l'~ cm) compared to single crystals. The activation energies of electrical conduction in low and high temperature regions are 0.16 eV and 0.80 eV, respectively. The films are polycrystalline in nature. Optical absorption studies revealed a high absorption coefficient (104 cm-1), with a direct type of transition. The observed bandgap is 1.60 eV.
1. Introduction
Direct bandgap semiconductors with bandgaps in the range 1.2 to 1.7 eV are well suited to convert light into electricity. In this respect, bismuth trisulphide, an n-type direct bandgap semiconductor whose experimentally observed bandgap varies from 1.3 to 1.7 eV, seems to be a promising candidate. Since photovoltaic properties are directly related to material properties, the choice lies in both the preparation method and the characterization techniques. Only a few methods have been used to prepare and characterize the bismuth trisulphide material, both in single-crystal and polycrystalline forms, and different aspects of electrochemical behavior have been reported [1-9]. The optical to electrical conversion efficiency so far reported is quite below expectation and is generally supposed to be due to its poor conductivity. Taking intoaccount these experimental observations, we have proposed to deposit the thin films of bismuth trisulphide by an electroless chemical deposition method for end use in photoelectrochemical cells. Correspondence to: L.P. Deshmuth, Department of Physics and Chemistry, Shivaji University, Centre for Post-graduate Studies, Solapur 413 003, M.S., India. 0927-0248/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
250
L.P. Deshmuth et al. / Bismuth trisulphide thin films
This paper briefly describes the procedure for the deposition of bismuth trisulphide thin films and some measurements on such material properties as conductivity, thermoelectric power, and optical absorption.
2. Experimental details 2.1. Preparation o f the samples
Bismuth trisulphide thin films have been prepared by a modified chemical deposition process [10-12] by allowing the triethanolamine complex of Bi 3÷ to react with S 2- ions, which are released slowly by the dissociation of thiourea [3,4,9]. The detailed procedure is: 20 ml of 0.01 M freshly prepared solution of Bi 3÷, obtained by triturating G.R. grade bismuth nitrate with triethanolamine, was placed in a glass beaker 250 ml in capacity. 30 ml 1 M thiourea solution and 5 ml (14 N) aqueous ammonia were added to the beaker and the total volume of the reaction mixture was made up to 200 ml with distilled water. The pH of the solution was adjusted to around 9.5. The glass substrates, of dimensions 72 × 8 x 2 mm, were thoroughly cleaned and kept immersed in the reaction mixture by means of a specially designed substrate holder. The substrate holder was attached to a constant-speed gear motor. The whole assembly was fitted to an oil trough whose temperature was controlled to 95 + 0.5°C. The substrates were rotated at a speed of 70 rpm. in the reaction mixture for about 30 min. After 30 min the samples were taken out, washed with distilled water, dried in air, and left in a dark desiccator. 2.Z Characterization o f the samples The samples were tested for their electrical conductivity in the temperature range 300 K to 600 K. A DC power supply was used to pass the current through the sample. The current was measured by a HIL-2665, 4½ digit current meter. The thermoelectric studies were performed in the 300 K to 500 K temperature range. The temperature gradient, and thermovoltage were recorded with 3½ digit (Vasavi make) and 4½ digit (HIL-2665) microvoltmeters, respectively. A Cr-AI thermocouple was used to record the temperature in both cases. The structural and optical studies were examined with XRD and optical absorption techniques. The X-ray diffractometer (PW-1710) was operated on 30 kV-20 mA using Cu Ket radiation. The scanning rate was 2°/min for angles from 10° to 100°. A Leitz Laborlux optical microscope was used to observe the microstructure ( M = 630). The films were scanned for further optical studies in the wavelength 500 nm to 900 nm with a Hitachi photospectrometer (Japan). The scan rate was 100 nm/min. 3. Results and discussion
An electroless method of chemical deposition of thin films is a relatively inexpensive, simple, and convenient technique for the preparation of large area
L.P. Deshmuth et al. / Bismuth trisulphide thin films
251
II-VI and IV-VI compounds. The method has several overriding advantages over the other conventional techniques [9-12]. These advantages are (1) the method requires negligible power consumption, (2) it is ideally suited for large-area deposition, (3) the substrate material is not an important criterion, (4) it avoids oxidation and corrosion of metallic surfaces, (5) it results in reproducible deposits both in structural and electrical properties, (6) the stoichiometry can easily be maintained, and (7) doping can be easily incorporated and allows slow growth of the film' with better orientation of crystallites. Using this technique, thin films of bismuth trisulphide have been deposited on glass substrates by allowing them to remain in an aqueous solution of the reactants for a period equal to deposition time. The basic overall reaction is [4]
B i [ N ( C H 2 C H 2 O H ) 3 ] 3+ +
S=C
/ NH 2 \ NH 2
~ Bi2S 3 +
O=C
/ NH 2 \
+
n A,
NH2
where A is the complexing agent, N(CH2CH2OH) 3. For a freshly prepared complex solution and at a deposition temperature around 95°C, good-quality (very uniform, reproducible, tightly adherent, specularly reflecting, and crack free) deposits are obtained for pH around 9.5. The color of the deposit is dark brown. If the solution is not freshly prepared (prepared 5-7 h before the deposition) and other conditions are held constant, the color of the deposit appears to be light brown. In both cases, when the reaction mixture is subjected to 95 + 0.5°C, the solution starts taking its respective color after 5 to 7 minutes. The thickness of the layer, measured by weight difference-density considerations, lies in the 0.8 to 1.2 Ixm range. Considerable change in thickness is observed for the samples deposited from a non-fresh solution. These films are found to be thick, spotty, and less reflecting. The possibility of thick and spotty samples in the case of bismuth solution kept for a few hours before deposition may be due to incorporation of some of the precipitated bismuth hydroxide clusters into the sample during deposition. These observations are in close agreement with the results of Bhattacharya and Paramanik [4]. The DC electrical conductivity of the samples was measured in the temperature range 300 K to 600 K for the both heating and cooling cycles. The change in conductivity was noted for every 5°C rise in temperataure. It is understood that both for heating and cooling cycles, the log or versus 1/T variation (fig. 1) follows almost the same track. This observation partly confirms the uniformity of the samples [12,13]. The conductivity increases with increasing temperature. Two distinct regions are clearly seen corresponding to two activation energies. The activation energies of electrical conduction have been determined in both low and high temperature regions and found to be 0.16 eV and 0.80 eV, respectively. A considerable difference is observed in the values of activation energies in our case and the values so far reported [3,4,6]. This can be partly attributed to the comparatively lower resistivity of our samples than the samples prepared by other conventional techniques, viz. solution gas interface, dip and dry, sulfurization, and
L.P. Deshrnuth et al. / Bismuth trisulphMe thin films
252
oN
°\ oN
~z
°\ ° \
o\ °\ o \ o
1°6
i
i
o ~ o a
2°0
2,Z,
2,8
--
o - - o
--
1000/T,(K-1 ) Fig. 1. T e m p e r a t u r e dependence of the conductivity for Bi2S 3 thin films.
spray pyrolysis. The lower resistivity observed in our case can be due to the relatively higher thickness, and by the improved degree of crystallinity provided by the preferred orientation while depositing the samples. We believe that this flexibility can thus be accounted for in the use of substrate holder design. This effect is also reflected in the lower magnitudes of activation energies [4]. In general, resistance of bismuth sulphide thin films is very high compared to the single-crystal resistance, and this difference is mainly because of the grain boundary discontinuities and thickness of the films. Moreover, the grain boundary and discontinuity of the samples are dependent on the deposition conditions [4]. The thermoelectric power generated by the sample was measured from 300 K to 500 K, and the polarity of the thermovoltage is negative towards the hot end of the sample. This indicates that the samples are of n-type conductivity. The observed thrmoelectric power is of the order of IxV/°C, and it increases with increasing temperature. The temperature dependence of thermoelectric power is shown in fig. 2. The material was further characterized by optical and structural techniques. The samples are of polycrystalline nature. This is further supported by microscopic observations. These observations are in close agreement with the results reported by Pawar et al. [6,8] and Bhattacharya and Pramanik [4]. The absorption spectra have been studied at 300 K in the 500 nm to 900 nm wavelength range. The absorption coefficient is of the order of 104 cm- 1, indicating that the material is of the direct-bandgap type and that transitions are allowed [3,4,6,8]. The energy-absorbance spectrum is shown in fig. 3. The absorption edge is much broader than that expected for a direct-bandgap-type material. This can be ascribed to the grain-boundary discontinuity effect in the structure and lack of stoichiometry generally observed in polycrystalline materials [4,6,12,13]. The optical bandgap has been determined from the absorbance spectra by plotting (t~hv) z versus hr. The
L.P. Deshmuth et al. / Bismuth trisulphide thin films
/°
O
) gs0
253
/
/
g
o~°
E o .......o -+~°/
loo
300
200
Temperature (°C)
Fig. 2. Variation of thermoelectric power with temperature.
/ / 0 0 ~0
o~ ,~
/°
.
/ /o
2
°/ , ~
O / O f ~
0
/
,
1,8
1.2
2,/;
3,0
hv (ev) Fig. 3. Variation of optical absorption coefficient(a)with photon energy (hv).
A
/
v
"o r4
/
./
~4 t~ A
/ 0
..,
lo2
_?~o
.....,..,o~
loL, hv
J
t,6 (eV)
Io8
Fig. 4. Variation of ( a h v ) 2 versus h r .
2,0
254
L.P. Deshmuth et al. / Bismuth trisulphide thin films
extrapolation to the energy axis gives a value of b a n d g a p equal to 1.6 eV (fig. 4). This agrees well with the values r e p o r t e d by others [3,6,8]. T h e magnitude of b a n d g a p seems to be higher than that of the observations on single-crystal studies. The difference can be accounted for by interference by diffuse reflectance within the material itself [14] and may be due to the anisotropy of the film-formation process [12]. Detailed studies on parameterization of the material with respect to its deposition conditions and preparative p a r a m e t e r s are in progress.
4. Conclusions Good-quality deposition o f n-Bi2S 3 thin films, reproducible both in electrical and optical properties, is easily possible with less c o n s u m p t i o n o f both energy and active materials. T h e technique offers a way to avoid the clumsy and time-consuming processes involved in the deposition of this material by o t h e r techniques. T h e deposition temperature, time, and p H of the solution play very important roles in producing high quality samples. T h e deposits are polycrystalline, with resistivities in the 10 5 to 10 6 ~ cm range. T h e material is o f the direct-bandgap type, with a high absorption coefficient, 10 4 c m - 1 . T h e estimated b a n d g a p is 1.60 eV.
Acknowledgements O n e o f the authors ( L P D ) thanks Professor R.N. Patil and Professor S.H. Pawar, University D e p a r t m e n t of Physics, for their constant e n c o u r a g e m e n t and moral support.
References [1] S.M. Polyakov, E.N. Loverko, I.S. Lisker, A.L. Pukashanskii and V.M. Yagodkin, Sov. Phys. Solid State 17 (1975) 386. [2] B. Miller and A. Heller, Nature 262 (1976) 680. [3] P. Pramanik and R.N. Bhanacharya, J. Electrochem. Soc. 127 (1980) 2087. [4l R.N. Bhattacharya and P. Pramanik, J. Electrochem. Soc. 129 (1982) 332. [5] P.K. Mahapatra and B. Roy, Sol. Cells 7 (1983) 225. [6] S.H. Pawar, P.N. Bhosale and M.D. Uplane, Thin Solid Films 110 (1983) 165. [7] P.A.K. Moorthy and G.K. Shivakumar, Mater Lett. 2 (1984) 223; Thin Solid Films 12 (1984) 151. [8] S.H. Pawar, S.P. Tamhankar and C.D. Lokhande, Mater. Chem. Phys. 11 (1984) 401. [9] C.V. Suryanarayana, A.S. Lakshmanan, V. Subramanian, and R.K. Kumar, Bull. Electrochem. 2 (1986) 57. [10] L.P. Deshmukh, A.B. Palwe and V.S. Sawant, Sol. Cells 28 (1990) 1. [11] L.P. Deshmukh, A.B. Palwe and V.S. Sawant, Sol. Energy Mater. 20 (1990) 341. [12] L.P. Deshmukh, K.V. Zipre, D.S. Hulle, P.P. Hankare and A.H. Manikshete, in: Proc. Nat. Conference on Oxide Ceramics and Technology, 21-23 February 1991, Shivaji University, Kolhapur, M.S. (India) p. 63. [13] P.N. Bhosale, Ph.D. Thesis, Shivaji University, Kolhapur, M.S. (India) (1985) p. 86. [14] D.E. Aspnes, E. Kinsborn and D.D. Bacon, Phys. Rev. B 21 (1980) 329.