Chemical deposition of Bi2S3 thin films on glass substrates pretreated with organosilanes

ELSEVIER

Thin Solid Films 268 ( 1995) 49-56

Chemical deposition of Bi2S3 thin films on glass substrates pretreated with organosilanes Ling Huang a,*, P.K. Nair b, M.T.S. Nair b, Ralph A. Zingaro a, Edward A. Meyers a aDepartment of Chemistry, Texas A&M University, College Station, TX 77843, USA ’ Photovoltaic Systems Group, Loboratorio de Energia Solar, IIM, Universidad National Autonoma de Mexico, 62580 Tern&o. Morelos, Mexico Received 5 April 1994; accepted 29 June 1995

Abstract The chemical deposition of Bi2S3 thin films on glass substrates modified by treatment with solutions of 3-mercaptopropyltrimethoxysilane and 3-aminopropyltrimethoxysilane is described. Such treatment helps prevent the peeling of thin films, a problem which is otherwise encountered in the chemical deposition process. Uniform thin films having thicknesses up to 0.32 pm were obtained on the modified surfaces. X-ray photoelectron spectroscopy was employed to demonstrate that silanization takes place at the surfaces of the glass substrates. The relative atomic concentrations of nitrogen or sulfur on these surfaces increase with the time of immersion in the silanizing solutions. X-ray diffraction patterns of air-annealed B&S, thin films were obtained. Optical transmittance and photoconductivity were measured and compared with those of the thin films deposited on untreated glass substrates. It was found that the thin films deposited on the silanized substrates were stable at 200 “C and maintain their original physical characteristics. Keywords:

Deposition process; Glass; Organic substances; Bismuth; Sulfide

1. Introduction Chemically deposited thin films of bismuth sulfide have been studied with respect to their application in photoelectrochemical cells [ 11, photoconductivity characteristics [ 21, enhancement of crystallinity and conductivity on air annealing [ 3,4], application as a component in B&-&S solar control and solar absorber coatings [5,6], as well as their application in thin film photography [ 71. One major problem associated with the chemical deposition of bismuth sulfide thin films is the peeling of the films from glass substrates which has been observed following long periods of deposition [2]. The thickness at which peeling of the films begins depends on a number of factors: cleanliness of the substrates, composition of the chemical bath, duration of deposition, and the swiftness of drying the film following its removal from the deposition bath. A procedure described as “double-dip” deposition involves transference of the films from an initial coating bath to a second bath before the peeling begins. This helps to reduce the peeling problem [ 21. Another way to avoid the peeling of bismuth sulfide thin films is to provide an initial coating of a thin film of zinc sulfide on the glass substrate [ 81. X-ray photoelectron spectroscopy studies indi* Corresponding author. 0040-6090/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDIOO40-6090(95)06685-3

cate that the improvement of adhesion of the bismuth sulfide thin films in this case results from the diffusion of atoms across glass-ZnS and ZnS-bismuth sulfide interfaces [ 71. Herein is described a new procedure which helps prevent the peeling of thin films. It involves the treatment of glass substrates with organosilanes prior to chemical deposition. B&S3 thin films 0.32 pm in thickness were obtained without peeling. Optical and photoconductivity characteristics of these films are discussed and compared with those of the films deposited on untreated glass substrates.

2. Experimental 2.1. Materials Analytical-grade methanol and toluene were purchased from EM Scientific, and 3-aminopropyltrimethoxysilane and 3-mercaptopropyltrimethoxysilane from Hills America. Baker-Analyzed Reagent-grade bismuth nitrate pentahydrate [ Bi( N03) 3 *5H20], triethanolamine (TEA), and thioacetamide (TA) were used in these studies. Corning glass microscope slides of 25 mm X 75 mm X 1 mm were used as substrates. These slides were washed with detergent, rinsed

L. Huang et al. /Thin Solid Films 268 (1995) 49-56

50

with deionized water, then methanol and dried in air prior to the surface modification treatment. 2.2. Preparation of a NH, surface A solution was prepared by adding 1.5 ml of 3aminopropyhrimethoxysilane ( ( CH30) $iCH2CH2CH2NH2) to a mixture of 135 ml of methanol and 15 ml of deionized water. Five drops (about 0.2 ml) of glacial acetic acid were added as a catalyst. The solution was stirred at room temperature for 30 min before use. Four glass slides were immersed vertically in the solution. The slides were removed at intervals of 2 min, 15 min, 30 min, and 45 min following the immersion in the solution. Following their removal, the slides were rinsed thoroughly with methanol and then baked in an oven at 110 “C for 1 h. The surfaces modified with 3-aminopropyltrimethoxysilane will be referred to as NH, surfaces. 2.3. Preparation of a SH surface The same procedure as described in Section 2.2 was used except that 1.5 ml of 3-mercaptopropyltrimethoxysilane ( (CH30)sSiCH&H2CH2SH) was mixed with 150 ml of toluene and the slides were rinsed with toluene after the silanization. The surfaces modified with 3-mercaptopropyltrimethoxysilane will be referred to as SH surfaces.

2.4. Deposition of bismuth suljidefilms Thin films of bismuth sulfide were deposited on glass substrates and modified substrates using a procedure reported earlier [ 21. A solution of 0.5 M bismuth nitrate was prepared by dissolving 24.25 g of Bi(NOs), . 5Hz0 in 70 ml of triethanolamine/water solution ( 1:l by volume). It was taken to a volume of 100 ml with deionized water. The deposition bath was prepared in a 100 ml beaker by the sequential addition of 10 ml of the bismuth solution, 8 ml of 50% triethanolamine, 4 ml of 1 M thioacetamide, and 78 ml of deionized water. B&S3 will be formed in this bath according to the following reaction: 2[Bi(TEA)J3++3CH3CSNH2+60H--+ Bi,S3 J + 3CH,CONH, + 2nTEA + 3H20 where the hydroxide ions come from the reaction of triethanolamine with water. Eight untreated glass slides, and eight slides with NH, surfaces and SH surfaces ( 15 min treatment) were used in the deposition of bismuth sulfide thin films. These were placed vertically in groups of four in 100 ml beakers, each containing the freshly prepared deposition solution. During a “single-dip” deposition, the films were grown for 2-7 h. The slides were removed from the chemical bath at intervals of 1.5 h, then rinsed with distilled water and dried in air. In the double-dip deposition, the slides were removed from the first bath after 4 h deposition, rinsed with distilled water and

dried in air. Then they were transferred to a second bath and the deposition was allowed to proceed for an additional 2-8 h. The total period of deposition for the double-dip process was, therefore, between 6 and 12 h. Again the slides were rinsed with distilled water and allowed to dry in air. The films were annealed at 200 “C for 1 h in a Fisher Coal Analyzer model 490 oven/furnace unit. This treatment was performed to improve the crystallinity and the photo and dark conductivities of the bismuth sulfide thin films, following the studies reported earlier [ 3,4]. 2.5. Characterization The thicknesses of the films were measured using an Alpha Step 200 unit. A sharp and clear edge of the film was created by dissolving a portion of the film in concentrated nitric acid, followed by rinsing with deionized water and methanol. X-ray photoelectron spectroscopy (XPS) multiplex studies were carried out on a Perkin-Elmer PHI 5500 ESCA unit using Mg K, radiation. 2 min of 4 keV argon ion sputtering were used to confirm the presence of the Si 2s peak on untreated glass surfaces. Scans of 200 sweeps were used for nitrogen and sulfur and 10 sweeps were used to scan for other elements, including carbon, oxygen, and silicon. Untreated glass surfaces were cleaned with a detergent solution, rinsed with deionized water and then baked at 110 “C for 1 h before analysis to match the conditions for samples having chemically modified surfaces. Optical transmittance spectra of the films in the 3mSOO nm range were recorded on a Perkin-Elmer UV-VIS Lambda 3B spectrophotometer. The film on one side of the substrate was removed using cotton swabs moistened with a dilute solution of nitric acid. In the case of partially peeled films obtained on untreated substrates, only adhering portions of the films were used for the measurements. The spectra were recorded with the light beam incident to the bismuth sulfidecoated side of the substrate. No blank was used in the reference beam. Photo and dark current measurements were performed using a Keithley 236 source-measure unit with an applied bias of 10 V. A pair of silver print electrodes of 5 mm length at a separation of 5 mm were applied on the surface of the film to serve as electrical contacts. To record the dark current, the samples were allowed to remain in the dark until the current became steady. The photocurrents were recorded at the end of 1 min of illumination under an intensity of 600 W m -’ radiation from a tungsten-halogen lamp (General Electric halogen flood, 90 W, 120 V). In the given geometry of the electrodes and bias, the sheet resistance R,( Sz) = 10/Z, where Z is the numerical value of the dark- or photo-current (A) and the electrical conductivity CT (n-i cm-‘) = 1ld R. where d is the film thickness (cm). Powder X-ray diffraction patterns were obtained using the Rigaku RU200 automated diffractometer with monochromatized Cu K, radiation.

L. Huang et al. /Thin Solid Films 268 (1995) 49-56

O-

1

SiCH2CH2CH2X

HAc

OH -

51

room temp.

OH + (CH30),SiCH2CH2CH2X OH

i) 0- yCHzCH,CH,X + CH,OH

-6

4

(glass surface)

0 *&CH&H,CH,X

X = -NH2 and -SH Fig. 1. Schematic representation of the reaction between organosilanes and a glass surface in an ideal case.

3. Results and discussion 3.1. Silanization of the glass surf&e Reaction scheme

Organosilanes react with various oxygen-containing surfaces and result in the formation of covalent bonds between silicon and oxygen [ 9, IO]. The reaction between 3-aminopropyltrimethoxysilane and 3-mercaptopropyltrimethoxysilane with glass surfaces can be described as shown in Fig. 1. In the presence of water, condensation may also occur among organosilane molecules which gives rise to cross linkages. The thickness of the organosilane layer formed by selfassembled condensation may vary depending upon the extent of cross linking as reported in the case of other organosilanes [11,12]. XPS analysis

Table 1 gives the atomic percentages of Si, C, 0, and N or S on the untreated and treated glass surfaces obtained from the XPS measurements of the samples. The untreated glass slides did not show the presence of any nitrogen or sulfur. The slides must be handled with great care since any surface contaminant may affect the results. For instance, it was found Table 1 Chemical analyses of surfaces of modified and unmodified substrates by XPS Substrate

Glass NH2surface

SH-surface

Silanization time (min)

Atomic composition (46)’ N

S

Si

0

C

0 2

0.00 2.02

0.00

23.34 21.27

59.51 48.45

17.15 28.26

15 30 45 2 15 30 45

3.03 3.52 3.44

20.81 20.51 19.92 21.30 19.73 18.69 17.96

46.85 44.92 46.02 50.53 48.19 46.16 43.18

29.31 31.05 30.62 25.91 29.22 31.97 34.17

2.26 2.86 3.18 4.68

Traces of sodium, calcium, and magnesium on the glass are not considered. The estimated error of XPS analysis is f 10-158.

that traces of nitrogen (0.9 at.%) can be detected on a clean glass slide if the slide had remained exposed to the atmosphere for a long period of time (days) after it was cleaned and baked. This problem can be solved by removing the glass slide from the oven just prior to performance of the XPS measurement. The presence of adventitious carbon detected at surface of the untreated glass is a characteristic feature of surfaces studied by the XPS technique. The nitrogen 1s peak was observed in its expected position [ 13] of 402 eV in the case of the NH2 surfaces (Fig. 2(a) ) . The intensity of this peak increased with immersion time: the relative atomic concentration of nitrogen at the surface of the slides was found to increase from 2.02 at.% (2 min immersion) to 3.5 at.% (30 min immersion). The nitrogen content (3.44 at.%) at the NH, surface which was silanized for 45 min was somewhat lower. However, the deviation is within the range of error of f lO-15% for XPS analyses. In the case of the SH surface, three peaks were observed in the 155-180 eV region where the S 2p,,, peak [ 131 ( 165 eV) is normally expected (see Fig. 2(b) ) . One of them was attributed to the silicon 2s orbital ( 157 eV) according to published values. The assignment of the Si 2s peak was confirmed by the fact that this peak was also observed on the surface of clean glass and did not disappear even after 2 min of argon sputtering (see Fig. 2(c)). Peaks at 166.5 eV and 171.5 eV were only observed on the SH surface and can be attributed to S pJf2 with confidence. It was reasonable to believe that the peak at 166.5 eV arose from the thiol group since sulfur in this group had the oxidation state of - 2. Thiol can be oxidized by molecular oxygen [ 141, resulting in the formation of various sulfur-containing species such as RSSR, RSO- , RSO;, and RSO;. Thus, the peak at 171.5 eV might be attributed to oxidized sulfur species generated by the reaction of thiol with adsorbed oxygen on the surface while the spectrum was being recorded. Linderberg et al. [ 151, found that sodium p-nitrobenzenethiolate (S 2p,,, at 163.6 eV) was partially oxidized to thiosulfinate (S p312at 166.0 eV) when they were making a systematic study of sulfur-containing compounds with electron spectroscopy for chemical analysis (ESCA) . The atomic concentrations estimated from the XPS data indicate a doubling of the sulfur content at the surface when the immersion time increased from 2 to 45 min.

L. Huang et al. /Thin Solid Films 268 (1995) 49-56

52

8

8

8 y 14 2 0

175

l78

1%

l50

Fig. 2. XPS multiplex (relative counts per second (N(E) /E) versus binding energy) recorded using Mg Ka radiation ( 1253.6 eV) of: (a) a NH2surface baked at 110 “C for 1 h after soaking in a 1 vol.% solution of 3-aminopropyltrimethoxysilane for 15 min at room temperature; (b) a SH surface baked at 110 “C for 1 h after soaking in a 1 vol.% solution of 3-mercaptopropyltrimethoxysilane for 15 mm at room temperature; and (c) a clean glass slide baked at 110 “C for 1 h before the analyses.

3.2. Deposition of B&S3 thinfilms

sponding thickness was about 0.15 pm. The situation changed dramatically in the case of the films deposited on the modified surfaces. All films deposited on both types of modified surfaces remained intact and uniform following a period of 2-7 h deposition. In fact, films which remained in the chemical deposition bath for 2 days showed no signs of peeling. The thickness of the film grown in single-dip deposition was usually no more than 0.18 pm. To grow thicker films, double-dip depositions were employed. All slides were coated in the first chemical bath for 4 h. At this stage, all of the films, including those on untreated substrates, remained intact. The films were then removed to the second bath for an additional period of deposition. Again, the thin films deposited on untreated glass substrates for a total period of 6-10.5 h peeled to varying extents when rinsed with distilled water. (Regions in the films which survived the peeling were retained for measurement.) In contrast, no peeling occurred with any of the films deposited on SH surfaces. The thickness of films deposited for a period of 11.5 h (total) was about 0.32 pm. On NH2 surfaces the films grew as thick as 0.30 pm; instances of peeling were observed in the case of a total deposition time of 10.5 h or longer. The results are presented in Table 2. The mechanism of growth of a metal sulfide film on a glass substrate has been studied previously [ 2,16,17]. In general, it is believed that the metal ion first forms the hydroxide. Hydroxo groups on the glass surface function as seed nuclei on which the metal sulfide film grows. This mechanism is open to question, especially in those cases where the metal sulfide is less soluble than the hydroxide. The experimental results cited above show that, on the treated substrates, thicker films may be grown without peeling. In the following section we present the physical properties of the films which indicate that on the modified surfaces the films grow faster than on untreated surfaces and also preserve their original physical properties, even after being annealed.

Silanization for chemical deposition

3.3. Composition and physical properties of B&S3 thin

It is known that the covalent bonds formed between organosilanes and the surface of substrates may dissociate in alkaline media and result in the loss of the organosilane layer on the substrate [ 121. The initial pH of the chemical deposition bath for B&S3 was about 8.5. At this pH, it is necessary to optimize the silanization process so that an adequate thickness of the organosilane layer is retained in the chosen chemical deposition bath for the commencement of the thin film deposition. The surfaces which have undergone a silanization of 15 min or more were found to successfully control peeling during the deposition. At shorter durations of silanization, e.g. 2 min, the reproducibility of the control of peeling was not satisfactory.

films

Peeling control of chemically deposited B&S3 thin$lms

In single-dip deposition it was observed that the films that were grown on untreated surfaces for a period of 5 h, or longer, peeled to varying degrees when rinsed. The corre-

XRD spectra

In previous articles on chemically deposited bismuth sulfide thin films [ 1,3,4,18], it was reported that the as-prepared films present an amorphous-like X-ray diffraction (XRD) pattern, but that well-defined XRD peaks (matching BizSJ bismuthinite, JCPDS 17-320) appear for films annealed at temperatures of 150-300 “C. Fig. 3 shows the XRD patterns of the bismuth sulfide thin films deposited on an SH slide and on an NH2 slide and of the powder sample collected from the SH deposition bath. These samples were annealed at 200 “C for 1 h in order to enhance their crystallinity. A simulated diffraction pattern of B&S, (bismuthinite) is also given [ 191. A good match is evident among these patterns. This establishes that the silanization process does not alter the chemical composition of the thin films even after annealing.

53

L. Huang et al. /Thin Solid Films 268 (1995) 49-56

Table 2 Thickness of bismuth sulfide thin films deposited on the modified surfaces Substrate

Duration of deposition (h) Fit

NH,-surface

SH-surface

2.0 3.5 5.0 6.5 4.0 4.0 4.0 4.0 2.0 3.5 5.0 7.0 4.0 4.0 4.0 4.0

bath (h) (single dip)

Thickness (Km) Second bath (h)

Total for the double dip (h)

2.0 3.5 5.0 6.5

6.0 7.5 9.0 10.5

2.0 3.5 5.0 7.5

6.0 7.5 9.0 11.5

0.15 0.17 0.17 0.23 0.30 0.30” 0.05 0.08 0.11 0.16 0.14 0.22 0.29 0.32

‘Started to peel.

Optical transmittance spectra Fig. 4 shows the transmittance spectra of the films deposited for different durations. The NH1 surfaces and SH surfaces are both transparent in the 340-800 nm region, while the films of bismuth sulfide show continuous absorption. For the same time of deposition, 3.5 h, the films obtained on the chemically modified surfaces show lower transmittance (curves c and e) than that shown for the film deposited on the untreated glass surface (curve a) indicating higher film thicknesses for the former. Thus, the growth rate of bismuth sulfide thin films on the modified surfaces is greater than that on the untreated glass surface. This may be due to the formation of chemical bonds between bismuth and organosilane layers which eliminates the incubation period required for the formation of seed nuclei on which the growth of Bi& takes place on untreated glass substrates. Fig. 4 also shows the transmittance spectra of the samples annealed at 200 “C for 1 h. No major differences were observed between the optical properties of samples that were annealed or not annealed. This observation is consistent with a model in which both the bismuth sulfide thin films as well as the organosilane layers on the substrates are stable up to 200 “C. The reported values of the optical band gap of bismuth sulfide vary: 1.3 eV (955 nm) for bulk material is cited in Ref. [ 201; 1.47 eV (840 nm) is reported based on a simple absorbance-wavelength plot for as-prepared thin films ( < 0.1 km thick) deposited from a chemical bath containing thiourea as the source of sulfide ions [ 11; and 1.7 eV (730 nm) is given based on (crhv) “’ vs. hv (in standard notation for an indirect gap or amorphous material) plot of an asprepared thin film (about 0.2 p,m thick) deposited using a chemical bath similar to the one used in this work [ 181. The difference in the optical bandgap between the bulk value and

thin film values arises from the quantum confinement of charge carriers. Lack of crystallinity or very small grain size (nanometre size) could shift the bandgap to higher values (compared with the bulk crystalline value) [ 21,221. Analysis of the bandgap was not carried out in the present work, but it is apparent from Fig. 4 that such an analysis (using reflectance and transmittance spectra extending to the nearinfrared region) would result in values within the range reported for Bi2S3. Photoconductivity Previous studies have shown that bismuth sulfide films annealed at 200 “C display enhanced photo and dark conductivities [24]. In the present work, photocurrent measurements were made on the films deposited for durations of 211.5 h, with samples annealed at 200 “C. Results are presented in Figs. 5-7. The greater thicknesses achieved in films deposited on treated substrates leads to higher dark and photocurrent values. The reduction in the photocurrent-to-darkcurrent ratio with an increase in the deposition time (film thickness) seen in Figs. 5-7 is a feature which has been reported also in PbS thin films [ 231. For the experimental conditions used in the measurement, a current of = 1Ov4 A at 10 V recorded for the bismuth sulfide thin films deposited on treated glass slides (Fig. 6 and Fig. 7) represent sheet resistances of = ld R. The electrical conductivity of the films is E=0.3 a-’ cm-‘, when the thickness of the film is = 0.3 p,m. This is about two orders of magnitude higher than the conductivity value reported earlier for airannealed bismuth sulfide thin films (0.15 pm thickness) [ 31. The higher conductivity may be due to a larger crystalline grain size associated with increased film thickness since a larger grain size implies higher charge-carrier mobility and hence higher conductivity [ 241.

L. Huang et al. /Thin Solid Films 268 (1995) 49-56

Fig. 4. Optical transmittance (ST) spectra of as-prepared (top) and annealed (bottom) samples of bismuth sulfide thin films deposited for different durations: on glass substrates. curves a (3.5 h) and b (6 h) ; on NH* surfaces of 15 min treatment, curves c (3.5 h) and d (6Sh); and on SH surfaces of 15 min treatment, curves e (3.5 h) and f (5 h). 20

2 THETA Fig. 3. X-ray diffraction patterns of annealed (200 “C, 1 h) B&S,: thin films

deposited for (i) a total period of 13 h on an SH slide, (ii) for a total of period of 13 h on a NH2 slide, and of (iii) bismuth sulfide powder collected from the SH deposition bath. Also shown is the simulated pattern for B&S, (bismuthinite, JCPDS 17-320) calculated by means of program xpow, part of the program package SHELXTL-PLUS [191.

lo-’

,:e 4-

10”

Previous studies [ 1,3,25 ] have shown that the conductivity of a bismuth sulfide thin film/single crystal is of n-type. Thermoelectric measurements (hot probe) on the present films has confirmed this. It is also known that the conductivity of the films can be increased by three to five orders of magnitude (as compared with air annealing at 200 “C) if the films are annealed in vacuum at 300 “C [ 31. This results from the increased crystallinity obtained at higher annealing temperatures and the partial loss of sulfur from the film (creating a metal-rich n-type, by non-stoichiometry in the film). Conductivity of 100 R-’ cm-’ could be obtained in such films (single dip, 0.15 Frn thickness). Vacuum annealing of the film was not performed in the present work, but it is reasonable to expect conductivities of > lo3 n-l cm-’ in the films deposited by the double-dip method on the SH slides as well as the NH2 slides. Measurement of the photoconductivity illustrates the benefits of depositing films of higher thickness since thicker films lead to substantially higher electrical conductivities. Inter-

-

__.dJ”

1k1tbalk

10-I

--0’

*

10”

--*

._-J’ - - ~~ond bath

1o-10 1o-1’ 0

I 2

, 4

deposition

I 6

I 0

, 10

-I 12

time (hl

Fig. 5. Dark and photo currents, I (A), at 10 V bias as a function of the duration of deposition of annealed (200 “C, 1 h) bismuth sulfide thin films on glass substrates. The steady-state dark current values and photocurrent values recorded at the end of 1 min illumination (600 W m-’ tungstenhalogen) are shown.

actions between the thin films and the organosilane layers, which form the basis of the growth of the films to higher thickness, are being studied in detail, and evidence has been obtained for the initial formation of Bi-S bonds to the thiolsilane bonded at glass surfaces [ 261.

L. Huang et al. / Thin Solid Films 268 (199s) 49-56

. on NH,-slide.

dark

A on NH,-slide.

photo

deposition

time

Ih)

Fig. 6. Dark and photo currents, I (A), at 10 V bias as a function of the duration of deposition of annealed bismuth sulfide thin films on a NH, surface. The conditions of measurements are as given in Fig. 5.

0.001

$

10-4

advantageous in other reported applications: solar control coatings [4-6] and thin film photography [ 781. The peeling of the thin films from glass substrates at some stage during chemical deposition is a commonly observed feature, for example, in CuS [ 271 and Sb2Se3 [ 28 1. Mechanical stabilization of the films on glass substrates can greatly improve the prospects of chemically deposited semiconductor thin films for various applications. The advantage of the new method is that it is simple, relatively inexpensive, and easily controlled. It should be applicable to the deposition of other thin films by a proper choice of the functional group in an organosilane as well as the reaction time in the silanizing solution. Such variations exist because the pH of the chemical deposition bath and the mechanism of bonding of the constituent atoms of the film to the organosilane layer formed on the glass substrate may vary from one material to another.

Acknowledgements . on SH-did,.

dark

0 on Sii-slide,

photo

The authors are grateful to the Robert Welch Foundation (Houston, TX) and to the Center for Energy and Mineral Resources at Texas A&M University for the financial support. The work was also assisted by an NSF (USA, Grant No. INT9402007) -CONACyT (Mexico) collaboration program. Special thanks to Dr. Paul A. Lindahl for the use of his WVis spectrophotometer.

- - recond bath

1 o-s 1o-’

2

55

1o-7 1 o-1 1o-’ to-‘0

References

1 o-‘1 0

2

4

deposition

6

6

10

12

time (hl

Fig. 7. Dark and photo currents, I (A), at 10 V bias as a function of the duration of deposition of annealed bismuth sulfide thin films on a SH surface. The conditions of measurements are as given in Fig. 5.

4. Conclusions Glass substrates can be modified by treatment with organosilanes at room temperature. The modified surfaces with the functional groups -SH and-NH, are resistant to undesired peeling of bismuth sulfide thin films deposited by the chemical-bath technique. The thin films deposited on the organosilane layers are not only stable at 200 “C but also retain their original characteristics. The maximum thickness (0.32 pm) of B&S) thin films and best results (inhibition of peeling) were obtained on surfaces modified with 3-mercaptopropyltrimethoxysilane. Relatively high electrical conductivity (ntype, 0.1-1000 0-i cm-’ available with post-deposition heat treatments) associated with films of such thickness as well as the high optical absorption in these films (Fig. 3) makes them promising candidates_ for polycrystalline thin film solar-cell technology. The surface treatment with the silanizing agents helps to overcome the major difficulty in depositing these films, namely the peeling. This will also be

[ 11 R.N. Bhattacharyaand

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[ 171 P.K. Nair and M.T.S. Nair, Solar Cells, 22 (1987) 103.

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