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STUDY EFFECTS OF AMMONIUM SOLUTION ON CHEMICAL BATH AS- DEPOSITED COPPER ALUMINUM DISELENIDE THIN FILMS

aS. C. Ezike and A .N. Ossai

aDepartment of Physics, Modibbo Adama University of Technology, Yola, Adamawa State, Nigeria.

bFirst Ghana Experimental School, Yola, Adamawa State, Nigeria

Chineduezike81@gmail.com; +2348037834194

Keywords: pH, Chemical bath deposition, thin films, Optical properties and Copper aluminum

diselenide.

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Abstract

Because of expensive nature of silicon – based solar cells, many scientists are looking for new materials that are cheap for the solar cell applications. Ternary chalcogenide materials were deposited onto microscope glass slide using simple chemical bath deposition technique. Here, we study the influence of ammonium solution (pH) on the preparation of thin films. The optical properties of deposited films have been studied using Uv-Vis spectrophotometer. The results
showed that the films have absorbance,

A ≈ 20%

, transmittance, T ≈ 64% , absorption
coefficient,

δ op

α ≈ 4.14 x105 m −1 , extinction coefficient, κ ≈ 15.72 x103 , optical conductivity,

ε r

≈ 2.55x1013 S −1 , real dielectric function,
≈ 5.33 and energy band gap,

E g ≈ 2.52 − 2.102eV

. The pH played important role during the deposition process.

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Introduction

The development of nanomaterials with special size and shape may lead to new chances to explore materials, physical and chemical properties. In recent years, semiconductor thin films have been attracting much attention due to their great potential in solar cells, optoelectronic devices, sensors, and laser materials [ Kassim et al, 2010]. Thin films photovoltaic devices have high worldwide demand to generate an efficient, renewable and clean solar energy as fossil fuel sources will be exhausted in future [Liang et al, 2008]. In recent years, considerable efforts have been made to find out low cost materials such as metal chalcogenide materials for solar energy conversion applications [kassim et al, 2010]. There are several methods that can be used to prepare thin films such as chemical bath deposition (CBD) [ Oladeji et al, 1999, Fajinmi et al.,
2009, Offiah et al., 2012, Kassim et al., 2010, Osuwa et al., 2011 and Maria et al.,2009], electrodeposition [Kassim et al., 2009 and Liang et al., 2008], low-pressure metalorganic chemical- vapor deposition [Chichibu et al., 1995 and Yoshiyuki et al., 2002], travelling – heat method [Alonso et al., 2000], mechanical alloying technique [Rafea et al., 2008], Vacuum Evaporation (Prabahar et al, 2010) and Sulphurization (Antony et al, 2003). The chemical bath deposition method is preferred for its simplicity, inexpensive and capability to achieve large surface area coatings [Kassim et al., 2010].
CBD has been used over the years to deposit chalcogenide material thin films: mainly ZnS [oladeji et al., 1999], ZnCdS [Maria et al., 2009], NiS2 [Osuwa et al., 2011 and Kassim et al.,
2010], CuS [Offiah et al., 2012], CdS [Fajinmi et al., 2009], Cu4 SnS4 [Kassim et al., 2010].
Generally, in a CBD process, ammonia (NH3 ) is used as a complexing agent to bind the metallic ions so as to minimize the precipitation of corresponding bulk compounds [Oladeji et al., 1999]. In this method, films are deposited on substrates whether metallic or non-metallic by dipping

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them into suitable solution bath containing metal salts. The metallic ions and non-metallic ions which are present in the deposited solution react with each other and become compound. The basic principle is that in order to precipitate a certain compound from solution, its ionic product (I.P) must exceed the solubility product (S.P) [Yoshida et al., 2009]. The Copper aluminum diselenide semiconductor is one of the wide-gap members belonging to the I – II – VI2 - type of ternary compounds that crystallize in the chalcopyrite structure [Chichibu et al, 1995]. Chalcopyrite Cu – III – VI2 compounds are promising for optoelectronic applications [Shirakata et al, 2000 and Alonso et al, 2000]. In this paper, we report study effects of ammonium solution as a pH adjuster on chemical bath deposited copper aluminum diselenide thin films on microscope glass substrates by varying the volume (1 – 9 ml). The optical properties of the films
were investigated.

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Experimental

The CuAlSe2 compound thin films were obtained from CuCl2 .2H2 O, Al2 (SO4 )3 .14H2 O), and Na2 SeSO3 as sources of Cu2+, Al3+ and Se2-. All the reagents used were analytical grade. EDTA was used as a complexing agent during the deposition and ammonia solution acted as source of
pH adjuster. A glass rod was used as a stirrer, mercury in glass thermometer was used to measure the bath temperature and Mac digital pH meter (MSW-552) model was used to measure the pH of the solution. The chemical baths were prepared by putting 5ml of 0.1 molar solution of copper chloride, 5mls of 0.1 molar solution of Al2 (SO4 ) 3 .14H2 O, 5ml of 0.1 molar solution of EDTA in a growth beaker were prepared and the solution was latter stirred before adding 5ml of sodium selenosulphate and 1 – 9 ml of ammonia solution respectively and deep bluish coloration was observed. When sodium selenosulphate was added, the solution did not dissolve until it was heated to temperature of 33oC. Each bath was filled up to 50ml mark with distilled water and stirred gently and long enough to ensure uniformity of the mixture. Pre-degreased microscope glass substrates cleaned with distilled water and dried in air were then inserted vertically in the reaction bath while synthetic foam which partly covered the top of the bath. Five different mixtures using 1.0 ml, 3.0 ml, 5.0 ml, 7.0 ml and 9.0 ml of ammonia solution were prepared at room temperature of 28 oC and left undisturbed for twenty four hours. All the samples were further washed in distilled water and air – dried for analysis.
The substrates introduced into the reaction bath gave reaction details as hereunder stated; CuCl2 .2H2 O + EDTA → [Cu-EDTA] 2+ + 2Cl- + 2H2 O
[Cu – EDTA]2+ → Cu2+ + EDTA
(Al2 (SO4 ) 3 .14H2 O) +EDTA → 2[Al-EDTA]3+ + 3SO4 2-+ 14H2 O

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[Al-EDTA]3+→ Al3+ + EDTA
Na2 SeSO3 +OH- → Na2 SO4 + HSe2-
2HSe2- + 2OH- →2H2 O + 2Se2-
Cu2+ + Al3+ + 2Se2- → CuAlSe2
The optical characterization recorded the optical absorbance carried out with the aid of Jenway
6405 UV-Vis spectrophotometer. The instrument gives the extent of absorbance, A, of the optical spectra for the range of 300nm - 700nm. The film-coated microscope glass substrates were placed across the sample radiation pathway while the uncoated substrate was put across the
reference path. The absorption coefficient,𝛼, Photon-energy, ℎ𝜈, and Refractive index, 𝑛 are
given by Okoli [2007] as

𝛼 = 𝐴

𝜆

(1)

ℎ𝜈 = 1.2𝑒𝑉

𝜆

(2)

𝑛 = 1+𝑅2

1− 𝑅2

(3)
Transmittance, 𝑇, reflectance R, real dielectric constant, ε r
and extinction coefficient, 𝑘, are
given by Rafea [2008] as
(1 − R) 2 e −αt

T =

(1 − R 2
)e −2αt
(4)

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(n − 1) 2 + k 2

R = (5) (n + 1) 2 + k 2
ε = n 2 − k 2
(6)

𝑘 = 𝜆𝛼

4𝜋

(7)
And films thickness, 𝑡 given by Fajinmi [2009] as

𝑡 = 𝐼𝑛 (1/𝑇)

𝛼

(8)
was determined by optical method. The energy band gaps were determined by extrapolation of
the straight portion of the graphs of α2 against photon energy (hν) at point α2 = 0 (Rafea et al,
2008).

Results And Discussion

The effect of ammonia quantity (in ml) on growth of the films was examined. The quantity of ammonia in the bath varied from 1 – 9 ml increased the pH of the bath from 8.35 – 10.1. As the pH of the bath increased, the film thickness increased till at certain volume of ammonia (7 ml) before it begins to decline as seen in fig.1. The films were thick, porous, non – reflecting and weakly adhered to the substrate. At the intermediate pH values (8.75 ≤ pH ≤ 9.2) , the films are uniform, smooth, non- porous and tightly adherent to the substrate support. There was low

deposition rate at pH interval of 8.35 to 8.75. This may result to non-precipitation of OH- ions in
the formation of complex, thereby lowering the concentration of metals ions required for thin films formation. When the OH- ions participate in the formation of the complex, they stabilize

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the complex leading to slow release of metal ions into the bath [Ahmed et al,2006] as in pH of interval 8.75 – 9.2. The variation of the film thickness with pH value in this region seems to be quasi – linear. We attribute the layer thickness in this region to the controlled rate of release of ions species. At higher pH values (9.2 ≤ pH ≤ 10.10) , layer thickness decreased considerably. A pH value of 9.2 was selected for good growth of the films.

In the case of absorbance of the films as in fig.2, it increases from the pH interval of 8.35 - 9.2
and dropped to a lower value. This also results from the explanation above on that film thickness versus pH. Sample A –D correspond to pH of 8.35 – 9.2. This is where the precipitation of OH- ions occurs in the formation of the complex. The average absorbance of the films is

approximately 20%. This shows that the films have poor absorbance. From the equation,𝛼 = 𝐴,

𝜆

it shows that absorption coefficient is directly proportional to absorbance of the films. This shows that the films have poor absorption coefficient as shown in fig. 3. The films have high average value of transmittance of 64%. The graph from fig. 4 shows that films with pH interval of 8.35 – 9.2 have high transmittance value and declined with pH value of 10.1. This x-rays those films with pH value range of 8.35 – 9.2 decrease with transmittance. Films obtained from bath having lower film thickness are most transparent and optical homogeneous. This is confirmed from the reflectance versus wavelength graph, as in fig.5, which indicates that the films have low reflectance value. Also it was said earlier that the films have low absorbance and all these contributed to high value transmittance of the films. In fig.6, the optical conductivity of the material decreases from wavelength of 380 – 800 nm/ increases from photon energy of
1.7732 – 3.2664 eV. It has low optical conductivity at near infrared. The optical conductivity of the films increases as the pH increases. The extinction coefficient of the films was examined in
fig. 7. It was noticed that all the samples have low value of extinction coefficient in visible

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region. This makes it a good candidate in solar cell application since its longevity in visible region is high. Increase in pH brought about increase in extinction coefficient of the films. In fig.
8, the refractive index of all the samples has nearly constant value in all the samples in the visible region. The higher the pH value the higher the refractive index. It has average refractive index of 2. The pH of the films increases alongside with the refractive index. The real dielectric function of the material as shown in fig. 9, shows that the increase in pH of the films brought about increase in the values of dielectric function. In fig. 10 – 13, they show plot of absorption
coefficient squared versus photon energy. The effect of pH on energy band gap was seen to
decrease it as the pH increase. The value range of energy band gap is
≈ 2.520 − 2.102
eV.
Sample D has higher energy band gap than sample E due to the extent of complex precipitation. Immediately after the peak pH (9.2), the rate of releasing complexes dropped and increasing the
inter-band energy.

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Conclusion

We have presented the study effects of ammonium solution on CuAlSe2 . The film growth is found to depend on the pH of the reaction bath. The results showed that the films have
absorbance,

A ≈ 20%

, transmittance, T ≈ 64% , absorption coefficient,

δ op

α ≈ 4.14 x105 m −1 ,

extinction coefficient, κ ≈ 15.72 x103 , optical conductivity,

≈ 2.55x1013 S −1 , real dielectric
function, ε r ≈ 5.33 and energy band gap,

E g ≈ 2.52 − 2.102eV . These show that CuAlSe2 is a

good candidate for photovoltaic applications, nonlinear optics, blue light emitting diodes.

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1.2

1.18

1.16

1.14

1.12

1.1

1.08

1.06

1.04

1.02

0.25

0.2

0.15

0.1

0.05

Sample A Sample B Sample C Sample D

380 420 460 500 540 580 620 660 700

Wavelength (nm)

0.98

8.35 8.75 8.86 9.2 10.1

Fig. 3. Variation of films absorption coefficient with wavelength

Fig. 1.Variation of average films thickness with pH for all samples

0.9

0.8

0.7

0.6

0.4

0.35

0.3

0.25

0.2

0.15

0.1

Sample A

Sample B

Sample C

Sample D

Sample E

0.5

0.4

0.3

0.2

0.1

Sample A Sample B Sample C Sample D Sample E

Wavelength (nm)

0.05

380 420 460 500 540 580 620 660 700

Wavelength (nm)

Fig.4. Variation of films transmittance with wavelength

0.25

Fig. 2. Variation of films absorbance with wavelength

0.2

0.15

0.1

0.05

Wavelength (nm)

Fig.5. Variation of films reflectance with wavelength

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6 8

5 6

4 5

3 3 Sample A

2 Sample B

2 Sample C

1 Sample D

1 0 Sample E

Photon Energy (eV)

Fig.6. Variation of Films optical conductivity with photo energy

Photon Energy (eV)

Fig.9. Variation of real dielectric function with photon energy

0.2

0 Sample E

.15

0.1

.05

Sample A

Photon Energy (eV)

Fig.7. Variation of extinction coefficient with photon energy

2.5

Photon Energy (eV)

Fig.10. Variation of absorption coefficient squared with photon energy for sample A

0.3

0.25

0.2

0.15

1.5

0.5

Sample A Sample B Sample C Sample D Sample E

0.1

0.05

Sample C

Photon Energy (eV)

Photon Energy

Fig.11. Variation of absorption coefficient squared with photon energy for sample C

Fig.8. Variation of refractive index with photon energy

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0.7

0.6

0.5

0.4

0.3

0.2

0.1

Sample D .2

Sample E

Photon Energy (eV)

Fig.12. Variation of absorption coefficient squared with photon energy for sample D

Photon Energy (eV)

Fig.13. Variation of absorption coefficient squared with photon energy for sample E

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