Effect of Surface Passivation on CdxNi1-xS Thin Films Embedded with Nickel Nanoparticles Текст научной статьи по специальности «Физика»

Effect of Surface Passivation on CdxNi1-xS Thin Films Embedded with Nickel Nanoparticles

Cliff Orori Mosiori 1

1 Technical University of Mombasa P. O. Box 90420-80100, Mombasa, Kenya

DOI: 10.22178/pos.25-4

LCC Subject Category: TP155-156, QC450-467, QD450-801

Received 12.06.2017 Accepted 10.08.2017 Published online 16.08.2017

Corresponding Author: corori@tum.ac.ke

© 2017 The Author. This article is licensed under a Creative Commons Attribution 4.0 License

Abstract. Certain treatments done to binary CdS, such as incorporating Ni onto CdS produces ternary thin films may cause major optical parameters that have a number of applications including for solar cell device fabrication. In this paper, we report on the effect of surface passivation on the band gap and other related optical properties of CdNiS thin films. Thin films for CdxNi1-xS were prepared on glass substrates by chemical solution method. Effects of surface passivation and variation of the volume of nickel ions on the optical properties CdS hence obtaining CdxNi1-xS thin films was investigated. It was observed that the thin films hard an average Transmittance above 68 %, with reflectance below 25 % across UV-VIS-NIR region. A plot of (ahv) 2 versus hv gave energy band gap between 2.553.49 eV for as-grown samples and 2.82-3.50 eV for annealed samples. The passivated samples had band gap energy values within the range 2.85-3.12 eV. It was concluded that an increase in concentration of Cd2+ and Ni2+ ions in the reaction led to an increase the band gap while optical conductivity ranged between 3.78 x 1011-2.40 x 1012 S-1.

Keywords: absorbance; annealing; optical conductivity; solar cell; spectrophotometer.

INTRODUCTION

Certain treatments done to binary CdS, such as incorporating Ni onto CdS produces ternary thin films. Such ternary thin film materials may cause major change in the optical parameters that may have a number of optoelectronic applications. It may have a direct influence on the electrical properties of the resulting material or the modification may result into some properties useful for single hetero-junction solar cells [8, 10] and especially nickel, Ni. The concept used in this report to explain observations made in CdxNii-xS is based on the ion-to-ion model [7, 12]. In this growth condensation of Cd+2 and S-2 that results in thin film formation. The growth of CdxNii-xS results from the incorporation of the Ni ions in the CdS precipitate [4, 28]. Thus a thin film photovoltaic cell is made by depositing one or more thin layers of photovoltaic material on a substrate. The substrates are immersed in an alkaline solution containing the chalcogenide source, metal ion and base. A chelating agent is

added to control the release of the metal ions. Here, we report on a CBD process that promotes large area deposition [24] for efficient photovoltaic cells, sensors and lasers applications [2, 6].

MATERIAL AND METHODS

Preparation of substrates. Ordinary glass slides were used as substrates with dimensions 76.2 mm x 254 mm x 10 mm. Prior to use; they were soaked in a bath of acetone and then rinsed in dei-onized water after which they were de-greased in dilute hydrochloric acid, rinsed in dei-onized water, dried and then stored in a desiccators ready to be used.

Growth of CdxNi1-xS thin films. The set up in Figure 1 was used. The reaction bath composed of 0.01 M CdCl2 solution, 1.0 M CH4N2S, some drops of Concentrated NH3 solution with varying volumes of 0.01 M NiCl2 solution as tabulated in Table 1.

Table 1 - Optimized depositional Parameters for CdxNhxS at 0.01 M NCL2 and 0.01 M CdCL2

RB CdCl2 NÍCI2 Value of x Value of (1-x)

Conc. (M) Vol. (ml) Conc.(M) Vol. (ml)

N1 0.01 10 0.01 0 1 0

N2 0.01 10 0.01 1.1 0.9 0.1

N3 0.01 10 0.01 2.5 0.8 0.2

N4 0.01 10 0.01 4.3 0.7 0.3

N5 0.01 10 0.01 6.7 0.6 0.4

N6 0.01 10 0.01 10 0.5 0.5

Figure 1 - Chemical bath deposition Schematic diagram

Deposition was done at 0.8 M for NiCh and CdCh in the order shown in Table 2 in which 10 ml CdCl2 solution was put in 100 ml beaker and few drops of concentrated NH3 solution added. Specified volumes of NiCh solution were then added

followed by accurately measured volumes of 1 M CH4N2S solution where the value of x varied between 1.0 and 0.5 according to the expression CdxNii-xS.

Table 2 - Optimized depositional Parame

ers for CdxNi1-xS at 0.8 M NiCl2 and 0.8 M CdCl2

RB CdCl2 NiCl2 Value of x Value of (1-x)

Conc. (M) Vol. (ml) Conc. (M) Vol. (ml)

N7 0.8 10 0.8 0 1 0

N8 0.8 10 0.8 1.1 0.9 0.1

N9 0.8 10 0.8 2.5 0.8 0.2

N10 0.8 10 0.8 4.3 0.7 0.3

N11 0.8 10 0.8 6.7 0.6 0.4

N12 0.8 10 0.8 10 0.5 0.5

The volume of NiCh solution was equally varied according to the formula (1):

x

1

Ni

■2 +

Ni2 + + Cd

2+

(1)

where Ni2+ was the value of varying volume of NiCh solution;

Cd2+ was the volume of CdCh solution.

The mixture was then topped to 100 °C by adding some distilled water, placed in a warm water bath at 45 °C for 1 hour. Six samples were prepared at same deposition time. NH3 solution was used as a complexion agent and a pH controller. Substrates were inserted at an inclined angle and after deposition; they were rinsed in distilled water; dried and kept for characterization.

Some of the thin films were annealed in a tube furnace in argon atmosphere at a temperature of 250 °C for 1 hour. The films were passivated by annealing them in a nitrogen atmosphere in a tube furnace in figure 3 for 1 hour at an average temperature of 250 °C.

GLASS BEAKER OIL

Figure 3 - Schematic diagram for a horizontal tube furnace

Optical Characterization. Transmittance measurements were measured using a computerized double beam Solid-Spec 3700 DUV Shimadzu Spectrophotometer in the spectral range from 300-1100 nm of wavelength. Scout 2.4 Software was used for simulation to obtain refractive indi-

ces, absorption coefficients, dielectric constants, real and imaginary parts and energy losses. EDX-800 HS Energy Dispersive X-rays spectrometer was used to ascertain presence of S2-, Ni2+ and Cd2+ ions and their abundances (Figure 4, 5).

5.00 10.00

Figure 4 - XRF spectrum for as-deposited CdS thin film (N1) from EDX 800HS

Figure 5 - XRF spectrum for as-deposited CdxNi1-xS thin film (N5) from EDX 800 HS

RESULTS AND DISCUSSION

Elemental Composition Analysis. Qualitative results in Table 3 (sample b) shows presence of S, Rh, Ni, Cd and K in sample N5. The sample comprised of Cd, Ni and S ions with Ni appearing in trace quantity. This implies that Ni acted as an impurity in CdxNii-xS thin films. Part of the results obtained are tabulated in Table 3 (sample a) in which S and Cd were found to be the major elements in sample N1, though this include, Si with a trace impurity which might be due to silicon internal fluorescence peak due to the photoelectric absorption of X-rays by the silicon dead layer in the detector resulting in the emission of Si K-X rays from this layer into the active volume of the detector.

Table 3 - The elemental composition analysis results

Presence of Rh element was as a result of the X-ray tube. Presence of Sb, Ca and Fe might have been caused by Spectral interference problems in EDX machine which occur at low energy (<3 keV) [25].

Optical Properties. Transmittance range was found to range between 37-44 %, 57-66 % and 8387% in UV, VIS and NIR region respectively for CdxNii-xS thin films deposited at 0.01 M concentration of Cd2+ and Ni2+ ions in Figure 6a while for annealed samples slightly dropped to 36-42 % in UV region, improved to 60-67 % in VIS region and remained high at 86-88 % in NIR region in Figure 6b. Highest transmittance was at 4.3 ml of nickel concentration at about 66 % and 67 % for as-grown and annealed respectively. Transmittance dropped to 12-52 %, 55-84 % and 7786 % for as-grown films and after annealing the films (Figure 6d) respectively. The passivated films had a transmittance in the range 38-45 %, 58-68 % and 82-88% in UV, VIS and NIR region respectively (Figure 6e). The increase in trans-mittance spectra after annealing was attributed to enhanced crystallity and packing orderliness. This was as a result of minimal number of free carriers coupling to the electric field hence reducing the reflection and causing the transmit-tance to be enhanced [3]. An increase in transmittance with the volume of Cd2+ ions in the reaction bath was attributed to decrease in diffuse and multiple reflections caused by increase in grain size and in light-scattering effect [17, 26].

Sample Analysed Qualitative Results Quantitative Results, %

(a) N1 -(CdxNii-xS) , x =1 S 55.223

Cd 44.642

Sb 0.029

Ca 0.101

Fe 0.005

(b)N5 - (CdxNii-xS), x = 0.6 S 21.643

Ni 4.006

Cd 74.345

K 0.006

Figure 6a - Transmittance for as-deposited CdxNi1-xS films at 0.01 M of Ni2+ ions, %

E-

S H

1 tA

i»*

t Ii''

— ami

100 90 80 70 60 50 40 30 20 10

300 400 500 600 700 800 900 1000 1100 Wavelength,^. (um)

Figure 6b - Transmittance for annealed CdxNi1-xS films at 0.01 M of Ni2+ ions, %

r

—■— AN1

—•— AN2

—A.— AN3

—T— AN4

—»— AN5

< AN6

Figure 6c - Transmittance for as-deposited CdxNi1-xS films at 0.8 M of Ni2+ ions, %

Figure 6d - Transmittance for annealed CdxNi1-xS films at 0.8 M of Ni2+ ions, %

In all the Figures 6, transmittance decreased as

n i n i

the concentration of Cd and N2 ions increased from 0.01 M to 0.8 M and this was attributed to increased free carriers coupling to the electric field hence increasing the reflection [20, 3, 16, 26]. Surface passivation had minimal influence on transmittance while an increase in transmi-tance at 4.3 ml of nickel ions was attributed to decrease in diffuse and multiple reflections caused by increase in grain size and in light -scattering effect [27].

Films deposited at 0.01 M concentration of ions (Figure 7a) had average reflectance range between 24-27 %, 21-30 % and 10-15 % in the UV, VIS and NIR region respectively. Annealed thin films at 0.01 M concentration (Figure 7b) had reflectance range between 21-25 %, 21-

24 % and 10-13 % in the UV, VIS and NIR region respectively. Thin films deposited at 0.8 M concentration (Figure 7d) had the reflectance range decrease to between 6-14 %, 7-19 % and 712 % in the UV, VIS and NIR region respectively. Annealing had minimal influence on the reflectance spectra (Figure 7e).

A decrease in the reflectance was attributed to increased transmittance. With low reflectance value in the VIS-NIR region, the material is best for photovoltaic devices like solar cells as a window layer as this reduces reflective loss on the cell surface [2, 6]. Passivated thin films deposited at 0.01 M concentration of Ni+ ions (Figure 7c) had average reflectance range between 19-28 %, 22-28 % and 10-16% in the UV, VIS and NIR region respectively. Surface passivation had mini-

mal influence on average reflectance spectra due to optical interference which a rises as a result of difference in refractive indices of the thin films

and the glass substrate used which results in multiple reflections [12, 23].

Figure 7a - Reflectance for as-deposited CdxNi1-xS thin films at 0.01 M of Ni2+ ions, %

35 -,

30-

25-

20'

m 15.

10

it « '. 4

v

■ PN1

• PN2

A PN3

T PN4

♦ PN5

4 PN6

i TT i«| 4a

t-

300 400 500 600 700 800 900 1000 1100 Wavelength,/, (nm)

Figure 7c - Reflectance for Passivated CdxNi1-xS thin films at 0.01 M of Ni2+ ions, %

22- A

20- • ▲

18-

£

Qi <D 16- • i

O C JH o JD 14- ♦ ■ Ï ♦

ai DC 12-

10

I A

■ AN7

• AN8

a AN9

▼ AN10

♦ AN11

4 AN12

▼ ♦.

4444,

444

44

Ui

300 400 500 600 700 800 900 1 000 1100 Wavelength, X (nm)

Figure 7e - Reflectance for annealed CcfxA//VxSthin films at 0.8 M of Ni2+ ions, %

Figure 7b - Reflectance for annealed CdxNi1-xS thin at 0.01 M of Ni2+ ions, %

Figure 7d - Reflectance for as-deposited CdxNi1-xS thin films at 0.8 M of Ni2+ ions, %

High value of reflectance observed at 500 nm was attributed to high value of refractive index (Figure 8a-8d) with 33-42 %, 10-17 % ; 0-2 % ; 41-74 %, 9-30 % ; 9-12 % in UV, VIS and NIR region respectively [2].

The increase in the absorbance spectra with increase in the concentration of Cd2+and Ni2+ ions in the reaction bath was attributed decrease in transmittance and reflectance as a result of increased photon absorption [17]. Highest value of absorption was noted at 2.5 ml doping of Ni2+ ions and the least value at 10 ml forming films fit for window layers in p-n junction solar cell [11, 25]. There was low absorption at photon energy less than 2.5 eV and 3.75 eV at 0.01 M and 0.8 M concentrations respectively. The range of the refractive index for films deposited at 0.01 M and

0.8 M ranged between 1-2 and 1-2.6 respectively. The maximum values of k and n occurred at same photon energy of 4.125 eV and these findings agree with the findings of Greenway and

Harbeke [2, 7] of average magnitude for J0 close to 1012 S-1 and this occurs at high photon energy.

Figure 8a - Absorbance for as-deposited CdxNi1-xS thin films at 0.01 M of Ni2+ ions

Figure 8c - Absorbance for as-deposited CdxNi1-xS thin films at 0.8 M of Ni2+ ions

The variation of a versus photon energy (Figure 8c) gave a straight line curve indicating presence of direct optical transitions [1] in which the real part indicates how the speed of light in the material can be slowed down while the imaginary part deals with the absorption of energy by a dielectric from the electric field due to dipole motion [10]. sr and st were obtained from

Scout 2.4 software by fitting the experimental transmittance data within the wavelength range 300-1100 nm. sr was averagely close to 2.0 from lowest to the highest photon energy values while si values were below 0.5 for lower energy values and showed upward trend for energy values above 3.5 eV. The values for sr can be attributed to the fact that for semiconductors k2<<n2.

Figure 8b - Absorbance for annealed CdxNi1-xS thin films at 0.01 M of Ni2+ ions

Figure 8d - Absorbance for annealed CdxNi1.xS thin films at 0.8 M of Ni2+ ions

Band gap variation. The band gap varied between 2.55-3.17 eV (Figure 9) for as deposited films while annealing the thin films narrowed the band gap as it increased with ion concentration. An increase in the band gap attributed to the Burstein-Moss effect [17].

The decrease in the band gap due to annealing was attributed to annealing which facilitates ordered packing of crystallites of molecules reducing the intermolecular defects within the material and these causes a reduction in the band gap value [19]. High band gap values ranging between 2.56 to 3.6 eV are in good agreement with results obtained by [18] and [6]. This also increases the chance that an ejected electron will meet up with a previously created hole in the material before reaching the p-n junction [9, 15].

Optical band gap at 0.01 m

■ As-deposited ■ Annealed ■ Passivated

N1 N2 N3 N4 N5 N6

SAMPLES

Optical band gap at 0.8 m

■ As-deposited ■ Annealed

N7 N8 N9 N10 N11 N12

SAMPLES

Figure 9 - Optical Band Gap for CdNhxS of Ni2+ and Cd2+ ions

Effect of Passivation. Figures 6 and Figure 7c show the transmittance and reflectance spectra of as-grown passivated CdxNii-xS thin film. Surface passivation had very little influence on transmittance and reflectance spectra since the absorb-ance spectra ranged from 0 % to 35 % in the VIS-NIR region for passivated thin films and 0 % to 32.5 % for as-grown and annealed thin films. Low absorption at photon energy less than 2.5 eV for passivated thin films was noted. Values obtained for these constants for as-grown, passivated and annealed thin films were within the same range. The band gap ranges were 2.85 eV-3.12 eV for passivated, 2.56 eV-3.42 eV for as-grown and 3.12 eV-3.48 eV for annealed thin films. The band gap was also least influenced by passivation.

CONCLUSION

CdxNii-xS thin films were grown using CBD technique. The films were found to have low reflectance value in the UV-VIS-NIR regions but low transmittance values at UV region while very high transmittance at VIS-NIR regions. The average band gap has been found to be above 2.80 eV

while surface passivation was found to be negligible effect on the optical properties and band gap and thus the films were well suitable for solar cell applications.

ACKNOWLEDGEMENTS

The author thank the Technical University of Mombasa, University of Nairobi, and Kenyatta University for allowing the authors to access of equipment and the technical staffs for accepting to engaged in this work. Special thanks to Mr. Muthoka and Mr. Mudimba of the University of Nairobi, Material Science laboratory and Dr. Bathsheba Kerubo Menge of the Technical University of Mombasa for the support and encouragement given to the authors during the laboratory processes.

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