Influence of Precursor Temperature on Bi Doped ZnSe Material via Electrochemical Deposition Technique for Photovoltaic Application

In this study, Bismuth (Bi) doped ZnSe thin films were deposited on conducting glass substrates by electrochemical deposition technique and the influence of precursor temperature (room, 50, 55, 60 oC) on their optical and structural properties were systematically studied using the combined effect of X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM) and UV-VIS spectrophotometer. The XRD patterns show a face-centred cubic structure indexed with peaks at (220), (221) and (300). The grain size was in the range of 3.24056 to 4.60481 nm with a lattice constant of 7.189Å. The material deposited at room, 500C, 550C, and 600C reveals agglomeration of particle on the surface of the substrate indicating uniform deposition. The optical spectra show that at different temperature (say room, 50oC, 55oC and 60oC), the absorbance and reflectance of BiZnSe thin films decreases with increase in wavelength of the incident radiation while the transmittance shows direct proportionality with the increase in wavelength. The bandgap demonstrated an increase in the range 1.75-2.25 eV with increase in temperature. DOI:10.46481/jnsps.2022.502


Introduction
Among the various II-VI semiconductors, Zinc selenide displays unique optical properties that makes it found application in light-emitting diodes [1,2], magneto-optical devices [3], ultraviolet lasers [4,5], gas sensors [6,7], solar cells [8,9], photocatalysis to mention but a few. The band gap tunability of ZnSe makes its optical properties outstanding and it can be used as transparent conductive oxide thin films. Many methods, both chemical and physical, including sputtering [10], thermal evaporation [11], electro-deposition [12,20,21], spray pyrolysis [13] and chemical vapour deposition [14] have been used to produce doped and undoped semiconductor thin films. So far, some of the methods have drawbacks such as toxic reducing agents, and organic solvents, or may require special conditions such as high temperature or low pressure, and sometimes can require costly and time consuming procedure. Among them, the electrochemical deposition of thin films is a viable alternative to vacuum-based deposition process. Its Figure 1: Schematic diagram of electrochemical deposition technique major advantages are that processing can take place at room temperatures and pressures and thin film properties can be controlled [4,10]. Generally speaking, adding an appropriate amount of dopant can change the physical properties of a semiconductor film. The effects of sulfur (S), iron (Fe), chromium (Cr) [15], copper (Cu) [16], Ag [17], Mn [18], etc. have been systematically studied. The semi-metal element bismuth (Bi) is used as a suitable doping material for electronic applications because of its high anisotropic electrical properties and high electron mobility [10,19], which is rarely reported in published works. Bidoped ZnSe thin film has a transparent nature, and its band gap nature results to potential support for heterojunction layers in solar cells. This work aims to use X-ray diffraction (XRD), scanning electron microscopy (SEM), Energy Dispersive Spectroscopy (EDS), ultraviolet-visible absorption spectroscopy and four point probe to explore the structural, optical and electrical properties of the as-deposited thin film.

Material and Methods
The chemicals used in this work were analytical grade and purchased from Sigma-Aldrich. They include: zinc tetraoxosulphate (VI) heptahydrate (ZnSO 4 .7H 2 O), Bismuth nitrate pentahydrate (Bi(NO 2 ) 3 .5H 2 O), selenium metal powder (Se), and hydrogen chloride (HCl). In this work, electrochemical deposition technique (ECD) is used, which involves the deposition of any substance on electrodes due to electrolysis. The electrochemical bath system consists of cation source (ie Bi(NO 2 ) 3 .5H 2 O, ZnSO 4 .7H 2 O for Bi 2+ , Zn 2+ ), anion source (i.e. selenium metal powder Se 2− ) and deionized water, all in a 100 ml beaker. The reaction bath was stirred using a magnetic stirrer, and a powder supplied was used to supply an electric field (DC voltage), a conductive glass (FTO) with electrical resistance of 16.6 Ω for the cathode, and a carbon electrode for the anode. Finally, the uniform deposition of the thin film was achieved through electrochemical deposition technique.

Substrate Cleaning Procedure
The substrates were conducting glass materials. Acetone was used to cleaned the substrates. It was therefore rinsed with distilled water and ultrasonicated for 30 minutes in acetone solution. Final rinsing was done in distilled water and the substrates were dried using an oven.

Growth of ZnSe and Bi/ZnSe Films
The growth of ZnSe and Bi/ZnSe thin film materials was carried out using an aqueous solution of 0.1 mol of ZnSO 4 .7H 2 O as the cationic precursor while the anionic precursor was 0.1 mol solution of selenium metal powder dissolved in 5 ml of hydrochloric acid (HCl) to ensure uniform deposition. The electrochemical bath system was composed of sequential variations of the zinc selenide molar concentrations. The varied precursor temperature of the solution are presented in Table 1. The deposited films were afterward annealed at 300 o C.

XRD analysis
The XRD pattern of BiZnSe thin films deposited on FTO substrates at different temperatures is as shown in Figure 2. From the Figure, the diffraction peaks were at (220), (221), (300) which correspond to the following angles (26.00 0 ), (31.98 o ), (32.01 o ) respectively. From the pattern, BiZnSe was confirmed to be a face-centred cubic structure which agrees with the JCPDS card no. 01-088-2400. The un-indexed peaks could have possibly resulted from the FTO substrates used for deposition. The lattice constant a = 7.1890 Å was obtained using equation (1). From the pattern the higher peaks could be due to the fact that the thickness of the film increases with an increase in dopant concentration, thus creating larger surface areas for photovoltaic devices and solar cell activities. The average crystallite size of the film is determined using equation (1), and the value of the Scherrer constant K is estimated to be 0.94 of the crystallite size using equation (2). Table 2 shows the calculated crystallite or grain size and dislocation density of the films deposited at different temperature room, 50 o C, 55 o C and 60 o C which agrees to similar studies by other researchers [20][21][22][23].    Figure 3 shows the micrograph of ZnSe and BiZnSe. The materials were deposited at room, 50 0 C, 55 0 C, and 60 0 C. The morphologies of the samples at different temperatures are smooth with bigger grains with decrease in temperature. When the deposition temperature increased to 60 0 C the surface of the as deposited film resulted to formation of islands with reduced grains due to smaller crystallites formed. The large grains formed with reduced precursor temperature was due to agglomeration of the crystallites. It is evidenced that the crystallites get bigger as the deposition temperature reduces which agrees with similar studies [20][21][22][23].

Elemental Composition Analysis Using Energy Dispersive
X-ray (EDX) for BiZnSe Utilizing Energy Dispersive X-ray technique, the formation of ZnSe and BiZnSe are evident in Figure 4a&b and the other elements which include Si, Ca, and O resulted from the elemental composition of the (FTO) substrate used for the deposition of the films.

Optical Analysis
The optical absorbance-wavelength spectra of BiZnSe thin films is shown in Figure 5a. The films grown at different temperatures (say, room, 50 o C, 55 o C and 60 o C) reveals that, the absorbance has an inverse relationship with its corresponding wavelength (in another word, as the wavelength of the incident radiation increases the absorbance of BiZnSe thin films decreases). The material grown at 50 o C, 55 o C and 60 o C exhibited the same trend and reveals the highest behavior and the material grown at room reveals lowest which shows that ZnSe and BiZnSe will be a good material that will absorb energy from the Sun and can serve as a good device for photovoltaic applications [24][25][26].
The optical transmittance spectra of BiZnSe thin films in Figure 5b shows that the transmittance of the films grown at 125 with increased in wavelength, which means that, the transmittance is directly related with the wavelength of BiZnSe thin films which agrees with similar studies [24][25][26].
Similarly, the optical reflectance spectra of BiZnSe thin films in Figure 5c shows an inverse relationship with wavelength which means that, at different temperatures (room, 50 o C, 55 o C and 60 o C) as the wavelength of the incident radiation increase the reflectance of BiZnSe thin films decreases.
The energy gap (E g ) was determined based on the wavelength of maximum absorption (λ) according to the formula E g = 1240/λ. Here, the maximum absorption wavelength is obtained from the absorption wavelength data. The E g was obtained in the formula describes as Tauc plot [27]. The E g was obtained from extrapolation of the linear part (αhv) 2 versus hv.
The optical band gap energy of BiZnSe thin films in Figure  6 shows an increase in band gap energy with increase in deposition temperature. The bandgap demonstrated an increase in the range 1.75-2.25 eV with increase in temperature (room, 50 o C, 55 o C and 60 o C). Figure 7a shows the optical refractive index of BiZnSe thin films grown at different temperature (room, 50 o C, 55 o C and  The result shows that, as the photon energy of the material increases the refractive index also increases. The increase in the refractive index as a result of photon energy increase was due to larger crystallites as observed in the SEM micrograph while the refractive index increases with increasing photon energy as a result of the increase in grain size observed in the SEM analysis. The refractive index (n) is the range of frequencies in which films are weakly absorbing.
The optical extinction coefficient which is the measure of the fraction of light lost due to scattering and absorption per unit distance of the penetration medium is as shown in Figure  7b. The increase in the extinction coefficient indicates the scattering loss of light while travelling through the medium with high absorption. The optical conductivity of BiZnSe thin films is shown in Figure 7c. The optical conductivity has been found to increase with increase photon energy. This increase in optical conductivity attributed to increase in density of localized states in the energy band itself is due to the rise in new defect states [28]. The real and imaginary dielectric constant of BiZnSe thin films in figure 8a&b has a direct relationship with the photon energy. This shows that, as the photon energy of the material increases, the real and imaginary dielectric constants increases.

Electrical Properties of BiZnSe
The material deposited at room, 50 • C, 55 • C and 60 • C reveals an increase in thickness from 117.19 -132.21 nm with increase in the resistivity of the deposited material from 5.3210 × 10 3 − −6.2943 × 10 3 (Ω.cm) which result to the decrease in the conductivity of the deposited material from 1.8793 × 10 11 − 1.5887 × 10 11 (S/m) (See Figure 9).
The high resistance value allows the film to be used in solar cell applications to improve conversion efficiency, because it can reduce the inevitable defects in solar cell manufacturing during the actual production process. Therefore, BiZnSe thin  film resistivity is very suitable for use as a buffer layer in solar cells, photovoltaic panels and photovoltaic devices.

Conclusion
Undoped ZnSe and Bismuth (Bi) doped ZnSe thin films have been prepared and deposited using electrochemical de-128