ELSEVIER Solar Energy Materials and Solar Cells 35 (1994) 179-184

Solar Energy Materials and Solar Ce~s

Preparation conditions of CdS thin films by flowed liquid film method

Kazuo Ito *, Kanehiro Shiraishi

Osaka Prefectural College of Technology, Department of Applied Chemistry 26-12, Saiwai, Neyagawa, Osaka 572, Japan

Abstract

The flowed liquid film method was applied for the CdS thin film preparation. This method is a kind of solution growth, but it can prepare the films continuously. The CdS films prepared by this method were polycrystalline films with hexagonal phase only and were sufficiently coherent, uniform, and transparent. The preparation of CdS thin films by this method was affected considerably by the reaction pH. The CdS thin films were prepared in the range of pH 7 to 9. The electrical resistivity of the films varied appreciably with the reaction pH, and had a minimum value of 101 l~ cm at pH 8. The size of the particles in the CdS films increased with an increase of the reaction pH. The reaction temperature also affected the resistivity of the CdS films.

I. Introduction

CdS thin films have been studied in order to make solar cells. CdS thin films are prepared by many methods, such as hot wall vacuum evaporation [1], spray pyrolysis [2], sputtering [3], electrodeposition [4], and screen-printing-sintering [5,6]. C d S / C u I n S e z solar cells and C d S / C d T e solar cells increase their solar conversion efficiency remarkably, when the CdS layers are prepared by solution growth [7-9]. We have been studying a flowed liquid film method (LF method) for the thin films [10,11]. The LF method is a kind of solution growth, and is characterized by a simple apparatus and as a relatively low temperature process. The LF method, different from the chemical bath deposition, can prepare the films continuously, and provides an easily scalable and low-cost technique. We

* Corresponding author.

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180 K. Ito, K. Shiraishi / Solar Energy Materials and Solar Cells 35 (1994) 179-184

.... CdS Thin Film

T / L . . _ _ Substrate In let Outlet

Fig. 1. Scheme of apparatus for the flowed liquid film method. Cd 2~ aqueous solution and thioac- etamide solution are flowed into reaction room (space of 0.5 mm) of apparatus.

reported that CdS thin films could be prepared by the LF method [12]. This method was once used for the ferrite plating [13,14]. This paper presents the preparation conditions for CdS thin films by the LF method.

2. Experimental

Our method is based on a simple chemical reaction for CdS precipitation. Cd 2+ aqueous solution (CdC12) and thioacetamide solution were flowed together into the reaction room (space of 0.5 mm) of the apparatus and reacted to form a polycrystalline CdS film on a glass substrate. Fig. 1 shows the apparatus for the LF method. During the reaction for the film preparation, the temperature (= 40 ~ 80°C), the flow rate (= 0.2 m l / m i n ) and the pH ( = 7.5 ~ 9.0) were kept constant. Cd(NO3)2, CdSO 4 and Cd(CH3COO) 2 were also used for Cd 2+ aqueous solution.

The morphology was analyzed by a Jeol scanning electron microscope (15 kV). The optical transmission measurements were performed with an ultraviolet-visible Shimazu spectrophotometer (UV-1200). X-ray diffraction spectra were obtained by Rigaku RINT-2100 X-ray diffractometer with CuK~ radiation. The electrical resistivity of the films was measured using the four-probe method.

3. Results and discussion

3.1. Preparation at uarious pH

CdS thin films were prepared by the LF method at a reaction pH of 7.5 ~ 9.0. These films were deposited on glass substrates at 60°C. X-ray diffraction analysis reveals that these films are polycrystalline CdS with hexagonal phase only and oriented with the c-axis perpendicular to substrates.

Fig. 2a shows the relationship between the electrical resistivity of the CdS films and the reaction pH. This reveals that the resistivity of the films varies appreciably

IE Ito, BL Shiraishi / Solar Energy Materials and Solar Cells 35 (1994) 179-184 181

c ,O

(1)

o. I 0.

0,5

0.4

0.3

0,2

0,1

0 I I I I 7,5 8,0 8,5 9,0

pH

0.7

0.6 % ~0 .5

~t~

N0,4 tED

m0.

~0, O.

1 I I I 7.5 8.0 8,5 9,0

pH Fig. 2. (a) Relationship between electrical resistivity of CdS thin films and reaction pH. (b) Relationship between particle size (mean diameter) of CdS thin films and reaction pH. Reaction temperature is 60°C.

with the reaction pH. The film prepared at around pH 8 has a minimum resistivity. Fig. 2b shows the relationship between the particle size (mean diameter) in the CdS films and the reaction pH. The mean particle size was estimated from the scanning electron micrograph. The particle here refers to the single particle or the aggregate of the fine particles. Fig. 2b reveals that the particle size increases with the increase of the reaction pH. As can be seen from Fig. 2a and 2b, the resistivity of the films shows a minimum value ranging in particle size from 0.3 to 0.4 ixm. The resistivity rises steeply below 0.3 Ixm, or beyond 0.4 p,m. The rise of the resistivity can be explained that the grain boundaries increase under 0.3 ~m and the large aggregates increase over 0.4 ~m.

Fig. 3 shows the optical transmission spectra of the CdS thin films with a thickness of about 0.05 p,m prepared at various pH. There is not clear difference among these spectra at various pH. The spectra have a high transmission region

182 K. Ito, K. Shiraishi / Solar Energy Materials and Solar Cells 35 (1994) 179-184

IO0

o~ 801

o 60

~= 40

h

,- 20 '/ a,pH7,5

b'pHS, 0 c~H9, O

0 |

O0 500 700 900 Wavelength / nm

Fig. 3. Optical transmission spectra of CdS thin films prepared at various pH. Reaction temperature is 60°C. Thickness of the films is about 0.05 ~m.

(over 90%) with a absorption edge at about 500 ~m. The absorption edge gives a direct optical bandgap of 2.4 eV, which agrees closely with the generally accepted value of 2.42 eV. Fig. 4 shows the scanning electron micrograph of the CdS film prepared at pH 8. This CdS film is a uniform and polycrystalline film and no crack

Fig. 4. Scanning electron micrograph of CdS thin film prepared at pH 8.0. Reaction temperature is

60°C.

I~ Ito, I~ Shiraishi / Solar Energy Materials and Solar Cells 35 (1994) 179-184 183

E t..)

c o

O~ t ' r "

1041

10 3

10 ~

10

o ' o / °

I I I I

40 50 60 70 80 Temperature /

Fig. 5. Relationship between electrical resistivi~ of CdS thin films and reaction temperature. Reaction pH is 8.0.

or peeling is observed. Since the LF method can provide the fresh reaction solution continuously for the glass substrate, the concentration of the reacting species are kept constant during the CdS deposition, which may be the reason this LF method contributes to prepare a pure and uniform CdS film.

3.2. Preparation at various temperature and with various starting reagents.

The reaction temperature affected the preparation of the CdS thin films by the LF method. The appropriate CdS films were prepared at a temperature between 40°C and 80°C. Under 40°C the film consisted of amorphous CdS and over 80°C the flatness of the film was poor because of the evolution of bubbles.

Fig. 5 shows the relationship between the resistivity of the CdS films and the reaction temperature. As the temperature increased, the resistivity of the films decreased from 104 to 101 D cm, saturated at about 65°C, and then increased from 101 to 4 x 10 2 D cm. The decrease of the resistivity probably is due to the increase of the particle size and the crystallinity. The CdS thin films were prepared with various starting reagents by the LF method. The resistivity of the films prepared with CdCI 2 Cd(NO3)2, CdSO 4 and C d ( C H 3 C O O ) 2 was 2 × 101, 0.5 X 101, 1 × 101 and 0.4 X 101 l~ cm, respectively. The reaction pH is kept to 8.0 with the buffer solution contained chloride ion. Chlorine is one of dopant for CdS, so all of the CdS films prepared with various starting reagents may be chlorine doped CdS films. From the results mentioned above, these starting reagents seem to have the same effects on the resistivity of the CdS films, though they have different complexing anions.

184 K. Ito, K. Shiraishi / Solar Energy Materials" and Solar Cells 35 (1994) 179-184

4. Conclusions

The f lowed l iquid film m e t h o d was used to p r e p a r e CdS thin films f rom aqueous so lu t ion at 60°C. T h e CdS thin films were polycrys ta l l ine films with hexagona l phase only and a d h e r e well the subst ra tes . The fi lms p r e p a r e d by so lu t ion growth usual ly conta in a small amoun t of cubic phase CdS bes ides hexagona l phase CdS. The e lec t r ica l resist ivity and the par t ic le size of the films were a f fec ted c lear ly by the reac t ion pH. The reac t ion t e m p e r a t u r e a f fec ted app rec i ab ly the resist ivity of the films, but the s tar t ing reagen t s a f fec ted a lit t le. The opt ica l t ransmiss ion of the CdS thin films have a high t ransmiss ion region (over 90%) with a abso rp t ion edge which c o r r e s p o n d s to an energy b a n d g a p of 2.4 eV. The resist ivity of the fi lms was in the r ange 101 to 10 4 D cm. This resistivity is lower than the resist ivity of the CdS fi lms p r e p a r e d by o the r methods . These CdS fi lms may be accep tab le for the app l i ca t ion to CuInSe 2 solar cells as a he te ro junc- t ion pa r tne r .

Acknowledgements

The au thors a re gra tefu l to Dr. T. W a d a and Dr. M. Ikeda of Matsush i t a E lec t r ic Indus t r i a l Co., and Prof. Y. T a m a u r a and Dr. T. I to of Tokyo Ins t i tu te of Techno logy for the i r useful discussion. T h e au thors also t hank Prof. S. Yanag ida , Dr. Y. W a d a and Dr. K. M u r a k o s h i for suppor t in e l ec t ron microscopy.

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