J. Nig. Soc. Phys. Sci. 4 (2022) 123–129 Journal of the Nigerian Society of Physical Sciences Influence of Precursor Temperature on Bi Doped ZnSe Material via Electrochemical Deposition Technique for Photovoltaic Application Imosobomeh L. Ikhioyaa, Eli Danladib,∗, Okoli D. Nnanyerec, Abdulazeez O. Salawud aDepartment of Physics and Astronomy, Faculty of Physical Sciences, University of Nigeria, Nsukka, Enugu State, Nigeria bDepartment of Physics, Faculty of Science, Federal University of Health Sciences, Otukpo, Benue State, Nigeria cDepartment of Physics and Industrial Physics, Faculty of Physical Sciences, Nnamdi Azikiwe University, Awka, Anambra State, Nigeria dDepartment of Computer Science, Nile University of Nigeria Abstract 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 Keywords: ZnSe, Doping, Precursor temperature, Photovoltanic, Electrochemical deposition Article History : Received: 05 December 2021 Received in revised form: 03 February 2022 Accepted for publication: 15 February 2022 Published: 28 February 2022 c©2022 Journal of the Nigerian Society of Physical Sciences. All rights reserved. Communicated by: E. Etim 1. Introduction Among the various II–VI semiconductors, Zinc selenide dis- plays unique optical properties that makes it found application in light-emitting diodes [1,2], magneto-optical devices [3], ul- traviolet lasers [4,5], gas sensors [6,7], solar cells [8,9], photo- catalysis to mention but a few. The band gap tunability of ZnSe makes its optical properties outstanding and it can be used as ∗Corresponding author tel. no: +2348063307256 Email address: danladielibako@gmail.com (Eli Danladi ) transparent conductive oxide thin films. Many methods, both chemical and physical, including sputtering [10], thermal evap- oration [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 123 Ikhioya et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 123–129 124 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 con- trolled [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], cop- per (Cu) [16], Ag [17], Mn [18], etc. have been systematically studied. The semi-metal element bismuth (Bi) is used as a suit- able doping material for electronic applications because of its high anisotropic electrical properties and high electron mobil- ity [10,19], which is rarely reported in published works. Bi- doped 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 Spec- troscopy (EDS), ultraviolet-visible absorption spectroscopy and four point probe to explore the structural, optical and electrical properties of the as-deposited thin film. 2. Experimental Details 2.1. Material and Methods The chemicals used in this work were analytical grade and purchased from Sigma-Aldrich. They include: zinc tetraoxo- sulphate (VI) heptahydrate (ZnSO4.7H2O), Bismuth nitrate pen- tahydrate (Bi(NO2)3.5H2O), selenium metal powder (Se), and hydrogen chloride (HCl). In this work, electrochemical depo- sition technique (ECD) is used, which involves the deposition of any substance on electrodes due to electrolysis. The electro- chemical bath system consists of cation source (ie Bi(NO2)3.5H2O, ZnSO4.7H2O for Bi2+, Zn2+), anion source (i.e. selenium metal powder Se2−) and deionized water, all in a 100 ml beaker. The reaction bath was stirred using a magnetic stirrer, and a pow- der 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. 2.2. 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 so- lution. Final rinsing was done in distilled water and the sub- strates were dried using an oven. 2.3. 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 ZnSO4.7H2O as the cationic precursor while the anionic precursor was 0.1 mol solution of selenium metal powder dissolved in 5 ml of hy- drochloric acid (HCl) to ensure uniform deposition. The elec- trochemical bath system was composed of sequential variations of the zinc selenide molar concentrations. The varied precur- sor temperature of the solution are presented in Table 1. The deposited films were afterward annealed at 300 oC. 2.4. Characterization and Measurement The grown films were characterized for their morphologi- cal, structural, elemental, optical and electrical properties us- ing Zeiss scanning electron microscope (SEM), Bruker D8 Ad- vance X-ray diffractometer with Cu-Kα line (λ =1.5405 Å) in 2θ range from 10 ◦ - 90 ◦, energy dispersive X-ray spectro- scope (EDX), Uv-1800 visible spectrophotometer and a four- point probe device respectively. 3. Results and Discussion 3.1. 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 (26.000), (31.98o), (32.01o) 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 possi- bly 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 dif- ferent temperature room, 50oC, 55oC and 60oC which agrees to similar studies by other researchers [20-23]. d = nλ 2 sin θ (1) D = kλ B cos θ (2) 124 Ikhioya et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 123–129 125 Table 1: Varied molar concentration of growth materials Temperature (degree) Bi(NO2)3.5H2O (ml) ZnSO4.7H2O (ml) Se(ml) Temperature (degree) Time (s) Voltage (V) ZnSe 10 20 20 00 5 10 Bi/ZnSe 50 10 20 20 50 5 10 Bi/ZnSe 55 10 20 20 55 5 10 Bi/ZnSe 60 10 20 20 60 5 10 Table 2: Structural parameters of BiZnSe thin film Sample 2θ (degree) d (spac- ing) Å Lattice constant (Å) (β) FWHM (hkl) Grain Size(D) nm Dislocation density,σ lines/m2 ZnSe room 27.07 3.52076 7.1890 0.32717 220 4.54275 0.04845 38.98 2.87509 0.32717 221 4.60447 0.04716 51.01 2.87246 0.32717 300 4.60481 0.04716 Bi/ZnSe 500C 27.07 3.52076 7.1890 0.40959 220 3.62863 0.07594 38.98 2.87509 0.40959 221 3.67793 0.07392 51.01 2.87246 0.40959 300 3.67820 0.07391 Bi/ZnSe 550C 27.07 3.52076 7.1890 0.45864 220 3.24056 0.09522 38.98 2.87509 0.45864 221 3.28459 0.09269 51.01 2.87246 0.45864 300 3.28483 0.09267 Bi/ZnSe 600C 27.07 3.52076 7.1890 0.36444 220 4.07818 0.06012 38.98 2.87509 0.36444 221 4.13358 0.05852 51.01 2.87246 0.36444 300 4.13389 0.05851 Figure 2: XRD pattern of BiZnSe 3.2. Surface Morphology Analysis Using Scanning Electron Mi- croscopy (SEM) of BiZnSe Figure 3 shows the micrograph of ZnSe and BiZnSe. The materials were deposited at room, 500C, 550C, and 600C. The morphologies of the samples at different temperatures are smooth with bigger grains with decrease in temperature. When the de- position temperature increased to 600C the surface of the as de- posited 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 [20-23]. 3.3. 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 elemen- tal composition of the (FTO) substrate used for the deposition of the films. 3.4. Optical Analysis The optical absorbance-wavelength spectra of BiZnSe thin films is shown in Figure 5a. The films grown at different tem- peratures (say, room, 50oC, 55oC and 60oC) reveals that, the absorbance has an inverse relationship with its corresponding wavelength (in another word, as the wavelength of the inci- dent radiation increases the absorbance of BiZnSe thin films decreases). The material grown at 50oC, 55oC and 60oC ex- hibited 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 appli- cations [24-26]. The optical transmittance spectra of BiZnSe thin films in Figure 5b shows that the transmittance of the films grown at 125 Ikhioya et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 123–129 126 Figure 3: SEM Micrograph of ZnSe and BiZnSe Figure 4: EDX Spectra of ZnSe and BiZnSe different temperatures (room, 50oC, 55oC and 60oC) increases with increased in wavelength, which means that, the transmit- tance is directly related with the wavelength of BiZnSe thin films which agrees with similar studies [24-26]. 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, 50oC, 55oC and 60oC) as the wavelength of the incident radiation increase the reflectance of BiZnSe thin films decreases. The energy gap (Eg) was determined based on the wave- length of maximum absorption (λ) according to the formula Eg = 1240/λ. Here, the maximum absorption wavelength is ob- tained from the absorption wavelength data. The Eg was ob- tained in the formula describes as Tauc plot [27]. The Eg 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 deposi- tion temperature. The bandgap demonstrated an increase in the range 1.75-2.25 eV with increase in temperature (room, 50oC, 55oC and 60oC). Figure 7a shows the optical refractive index of BiZnSe thin films grown at different temperature (room, 50oC, 55oC and 126 Ikhioya et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 123–129 127 Figure 5: Absorbance (a), transmittance (b) and reflectance (c) versus wavelength Figure 6: Plot of absorption coefficient square versus photon energy 60oC). The result shows that, as the photon energy of the mate- rial 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 en- ergy 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 scat- tering 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 op- tical 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. 3.5. Electrical Properties of BiZnSe The material deposited at room, 50◦C, 55◦C and 60◦C re- veals an increase in thickness from 117.19 – 132.21 nm with in- crease in the resistivity of the deposited material from 5.3210× 103 −−6.2943 × 103(Ω.cm) which result to the decrease in the conductivity of the deposited material from 1.8793 × 1011 − 1.5887 × 1011(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 127 Ikhioya et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 123–129 128 Figure 7: Refractive index (a), extinction coefficient (b) and optical conductivity (c) versus photon energy Figure 8: Plot of real dielectric (a) imaginary dielectric constant (b) versus photon energy Table 3: Electrical Properties of BiZnSe Samples Thickness, t(nm) Resistivity, ρ (Ω.cm) Conductivity, σ (S/m) ZnSe room 117.19 5.3210x103 1.8793x1011 Bi/ZnSe 50◦C 122.15 6.7231x103 1.4874x1011 Bi/ZnSe 55◦C 128.15 6.9874x103 1.4311x1011 Bi/ZnSe 60◦C 132.21 6.2943x103 1.5887x1011 film resistivity is very suitable for use as a buffer layer in solar cells, photovoltaic panels and photovoltaic devices. 4. Conclusion Undoped ZnSe and Bismuth (Bi) doped ZnSe thin films have been prepared and deposited using electrochemical de- 128 Ikhioya et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 123–129 129 Figure 9: Resistivity and conductivity versus thickness position technique at room, 50, 55, 60 oC. 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 de- posited 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 tempera- ture (say room, 50oC, 55oC and 60oC), the absorbance and re- flectance of BiZnSe thin films decreases with increase in wave- length 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. Acknowledgements We acknowledge Nanosciences African Network (NANOAFNET), iThemba LABS National Research Foundation as well as the staff of Nano Research Group, University of Nigeria, Nsukka for their research support References [1] S. J. Pearton & F. Ren, “Advances in ZnO-based materials for light emit- ting diodes”, Current Opinion in Chemical Engineering 3 (2014) 51. [2] H. Dae-Kue, O. Min-Suk, L. Jae-Hong & P. Seong-Ju, “ZnOthinfilms and light-emitting diodes”, Journal of Physics D: Applied Physics 40 (2007) R387. [3] K. Ando, H. Saito, Z. Jin, T. Fukumura, M. Kawasaki, Y. Matsumoto & H. Koinuma, “Large magneto-optical effect in an oxide diluted magnetic semiconductor Zn1-xCoxO”, Applied Physics Letters 78, (2001) 2700. [4] M. H. Huang, S. Mao, H. Feick, H. Q. Yan, Y. Y. Wu, H. Kind, E. We- ber, R. Russo & P. D. Yang, “Room-temperature ultraviolet nanowire nanolasers”, Science 292 (2001) 1897. [5] K. Govender, D. S. Boyle, P. O’Brien, D. Binks, D. West & D. Coleman, “Room temperature lasing observed from ZnO nanocolumns grown by aqueous solution deposition”, Advanced Materials 14 (2002) 1221. [6] L. Wang, Y. Kang, X. Liu, S. Zhang, W. Huang & S. Wang, “ZnO nanorod gas sensor for ethanol detection”, Sensors and Actuators B: Chemical 162 (2012) 237. [7] P. H. Yeh, Z. Li & Z. L. Wang, “Schottky-gated probe-free ZnO nanowire biosensor”, Advanced Materials 21 (2009) 4975. [8] J. Huang, Z. Yin & Q. Zheng, “Applications of ZnO in organic and hybrid solar cells, Energy and Environmental Science 4 (2001) 3861. [9] B. Weintraub, Y. G. Wei & Z. L. Wang, “Optical fiber/nanowire hy- brid structures for efficient three-dimensional dye-sensitized solar cells”, Angewandte Chemie 48 (2009) 8981. [10] A. Kennedy, V. S. Kumar & K. P. Raj, “Influence of substrate temperature on structural, morphological, optical and electrical properties of Bi-doped MnInS4 thin films prepared by nebuliser spray pyrolysis technique”, Jour- nal of Physics and Chemistry of Solids 110 (2017) 100. [11] R. Inov & D. Desheva, “Preparation and characterization of amorphous SeTe/CdSe superlattices and their constituent thin layers”, Thin Solid Films 213 (1992) 230. [12] R. P. Raffaelle, H. Forsell, T. Potdevis, R. Fridefeld, J. G. Mortovani, S. G. Bailey, S.M. Hubbard, E.M. Gordon & A.F. Hepp, “Elecrodeposited CdS on CIS pn junction”, Solar Energy Materials and Solar Cells 57 (1999) 167. [13] K. Subbaramaiah & V. S. Raja, “Preparation and characterization of all spray-deposited p − Culn(S 0,5S e0,5)2/n − CdZnS : In thin film solar cells” Solal Energy Materials and Solar Cells 32 (1994) 1. [14] S. Tec-Yam, R. Potino & A. I. Oliva, “Chemical bath deposition of CdS films on different substrate orientations”, Current Applied Physics 11 (2011) 914. [15] N. B. Singh, C. H. Su, F. S. Choa, B. Arnold, P. Gill, C. Su, I. Emge & R. Sood, “Effect of Doping on the Electrical Characteristics of ZnSe”, Crystals 10 (2020) 551. [16] M. M. Rahman, C. Das, M. M. Rahaman, K. M. A. Hussain & S. Choud- hury, “Effect of thickness on structural, morphological and optical prop- erties of copper (Cu) doped zinc selenide (ZnSe) thin filmd by vacuum evaporation method”, Bangladesh Academy of Science 43 (2019) 159. [17] K. Yadav & N. Jaggi, “Effect of Ag doping on structural and optical prop- erties of ZnSe nanophosphors”, Materials Science in Semiconductor Pro- cessing 30 (2015) 376. [18] F. Qiao, R. Kang, Q. Liang, Y. Cai, J. Bian & X. Hou, “Tunability in the Optical and Electronic Properties of ZnSe Microspheres via Ag and Mn Doping”, ACS Omega 4 (2019) 12271. [19] M. Mrad, B. Chouchene & T. B. Chaabane “Effects of Zinc Precursor, Ba- sicity and Temperature on the Aqueous Synthesis of ZnO Nanocrystals” South African Journal of Chemistry 71 (2018) 103. [20] K. Yadav Y. Dwivedi & N. Jaggi, “Structural and optical properties of Ni doped ZnSe nanoparticles”, Journal of Luminnescence 158 (2015) 181. [21] Y. Wang, X. Liang, E. Liu, X. Hu & J. Fan, “Incorporation of lanthanide (Eu3+) ions in ZnS semiconductor quantum dots with a trapped-dopant model and their photoluminescence spectroscopy study”, Nanotechnol- ogy 26 (2015) 375601. [22] G. Colibaba, M. Caraman, I. Evtodiev, S. Evtodiev, E. Goncearenco, D. Nedeoglo & N. Nedeoglo, “Influence of annealing medium on photolu- minescence and optical properties of ZnSe:Cr crystals. Journal of Lumi- nescence 145 (2014) 237. [23] G. M. Lohar, S. K. Shinde, M. C. Rath & V. J. Fulari, “Structural, opti- cal, photoluminescence, electrochemical, and photoelectrochemical prop- erties of Fe doped ZnSe hexagonal nanorods”, Materials Science in Semi- conductor Processing 26 (2014) 548. [24] G. M. Lohar, S. T. Jadhav, M. V. Takale, R. A. Patil, Y. R. Mab, M. C. Rath & V. J. Fulari, “Photoelectrochemical cell studies of Fe2+ doped ZnSe nanorods using the potentiostatic mode of electrodeposition”, Journal of Colloid Interface Science 458 (2015) 136. [25] E. R. A. Moses, D. S. Mary, C. Sanjeeviraja & M. Jayachandran, “Growth of ZnSe thin layers on different substrates and their structural conse- quences with bath temperature”, Physica B 405 (2010) 485. [26] H. Metina, S. Durmus, S. Erat & M. Ari, “Characterization of chemi- cally deposited ZnSe/SnO2/glass films: influence of annealing in Ar at- mosphere on physical properties”, Applied Surface Science 257 (2011) 6474. [27] J. Tauc, Amorphous and liquid semiconductors, Plenum press, London & New York (1974). [28] N. F. Mott & E. A. Davis, Electronic Processes in Non-Crystalline Mate- rials, 2nd edition Clarendon, Oxford (1979). 129