Investigation of Structure, Optical, and Electrical Properties of CuS Thin Films by CBD Technique

Copper Sulfide (CuS) thin films were deposited onto a glass substrate using the Chemical Bath Deposition (CBD) technique. The chemical bath Precursors were made up of CuSO4, SC(NH2)2, and C4H6O6. Different parameters have been considered to specify the optimum conditions for fabricating CuS thin films, such as solution temperature, deposition time, pH level, and different precursor concentrations. It has been found that the optimum deposition time is 20 min at temperature 80 °C and pH = 11. The optimum precursor concentrations were 0.15 M, 0.2 M, and 0.1 M of CuSO4, SC(NH2)2, and C4H6O6, respectively. The structural properties of the thin film were studied using X-ray diffraction (XRD), and a single peak was observed for the thin film made at optimum conditions, while all other cases were amorphous. It is obvious from the optical characterization that the transmission spectra show a red-shift for the cases of increasing deposition time, bath temperature, C4H6O6 concentration, and pH. For the case of increasing CuSO4, blue shifts in the transmission spectra were observed. The energy band gap, resistivity, and activation energy of CuS thin films under optimum conditions are 2.35 eV, 0.7 Ω·cm, and 0.0152 eV, respectively.


Introduction
In recent years, transition metal chalcogenides have been increasingly studied due to their appropriate physical and chemical properties and potential for use in many industries. There are various techniques for the thin films to be prepared, such as pulsed laser deposition [1], electrodeposition [2], vacuum evaporation [3], chemical bath deposition [4], spray pyrolysis [5], successive ionic layer adsorption and reaction (SILAR) [6], and electron beam evaporation methods [7].
Among different metal chalcogenides, copper sulfide has been extensively studied and has attracted much interest in the recent research due to its special properties and potential applications such as in Solar cells [8], biomedical applications [9,10], photocatalysis [10], sensors [11], and as a cathode material in lithium chargeable batteries [12], etc. It is a binary inorganic chemical compound that occurs in nature as a dark indigo blue mineral, which has a general formula Cu x S y . It is present in both synthetic materials and in minerals in the form of CuS (covellite) and Cu 2 S (chalcocite). Characterization and synthesis of CuS nanostructures have become an interesting area of research. It has been observed that CuS provides different absorbance edges both in the UV and visible regions [13].
The CBD is also called nucleation growth and is widely used in the laboratory for the deposition of chalcogenide semiconductors. It is a simple deposition technique requiring only a substrate to be placed in a vessel containing a supersaturated solution of dilute aqueous precursors such as metal salts, complexing agents, and pH buffers. It has many advantages, such as low-temperature operation, atmospheric pressure, low cost, and the ability to deposit large areas [14]. The deposition of CuS films  The thickness of the thin films was measured using the optical interferometer technique, where thickness (d) is calculated by the below formula.
where λ is the wavelength of the He-Ne laser (632.8 nm), Δ is the distance between two fringes, and is the fringe width [17]. Structure properties were studied using the X-ray diffraction system X-Pert Pro PANalytical with a scanning range of 2Theta set between 20° and 70°, with a wavelength of 1.5406 Å from CuKα operating at 40 kV, 30 mA.
The optical transmittance spectra of the CuS thin films were recorded by the UV-VIS Spectrophotometer in the range of 200-1100 nm. The thickness of the thin films was measured using the optical interferometer technique, where thickness (d) is calculated by the below formula. where λ is the wavelength of the He-Ne laser (632.8 nm), ∆x is the distance between two fringes, and x is the fringe width [17]. Structure properties were studied using the X-ray diffraction system X-Pert Pro PANalytical with a scanning range of 2Theta set between 20 • and 70 • , with a wavelength of 1.5406 Å from CuK α operating at 40 kV, 30 mA.
The optical transmittance spectra of the CuS thin films were recorded by the UV-VIS Spectrophotometer in the range of 200-1100 nm.
The direct energy gap of the thin films is calculated from the Tauc's equation as shown below: where α is the absorption coefficient that is given by (α = 2.303 log (T/d)), A is the absorption, hυ is the incident photon energy, E g is the bandgap energy, T is the transmittance, and d is the film thickness [18]. The electrical properties of the CuS thin films were measured from the I-V characteristic curve using (KEITHLEY 2450) and the electrical resistivity of the deposited films was determined by the equation: where R is the sheet resistance, ρ is the resistivity of the thin film, L is the length between two probes, and A is the cross-sectional area of the probes [19]. The activation energy of the thin films was calculated by the Arrhenius Equation [20,21].
where σ is conductivity, A is the pre-exponential factor, E a represents activation energy, K B is Boltzman constant, and T is absolute temperature [22].

Thickness and Growth Rate
It can be seen that at certain temperatures CuS could be precipitated either in the bath solution with the formation of colloids or at the surface of the immersed substrate in the solution. The formation of CuS layers occurs when the ionic product of Cu 2+ and S 2− exceeds the solubility product of CuS (K sp = 5 × 10 −36 ) [23]. Figure 2a,b shows the thickness of CuS thin films as a function of deposition temperature and deposition time, respectively. The curves in Figure 2a show a linear increase of film thickness with the deposition temperature at specific deposition time, with no indication of saturation. Figure 2b shows the thin film thickness as a function of deposition time. From the figure, one can notice that at 50 • C it reached saturation at 15 min of deposition time, while at higher deposition temperature it is not reached even at 25 min of deposition time [23,24]. Figure 3a,b shows the growth rate of CuS deposition thin films. Figure 3a indicates the growth rate of CuS thin film as a function of deposition temperature at constant deposition time. From the figure, it can be seen that the deposition growth rate gradually increases with deposition temperature and this is due to more generation of colloidal ions. The growth rate is decreasing with the increasing deposition time at a constant deposition temperature due to precursor consumption over time, as shown in Figure 3b [4]. shows the thin film thickness as a function of deposition time. From the figure, one can notice that at 50 °C it reached saturation at 15 min of deposition time, while at higher deposition temperature it is not reached even at 25 min of deposition time [23,24].  Figure 3 (a, b) shows the growth rate of CuS deposition thin films. Figure 3a indicates the growth rate of CuS thin film as a function of deposition temperature at constant deposition time. From the figure, it can be seen that the deposition growth rate gradually increases with deposition temperature and this is due to more generation of colloidal ions. The growth rate is decreasing with the increasing deposition time at a constant deposition temperature due to precursor consumption over time, as shown in Figure 3b [4].  Figure 4 (a, b) shows the effect of different CuSO4 precursor concentration on the thickness and growth rate of CuS thin films, respectively. Figure 4a shows that the thickness of CuS films decreases from 148 to 136 nm as the CuSO4 concentration increases from 0.1 M to 0.15 M [25]. The deposition on a substrate mainly depends on the formation of nucleation sites and subsequent growth of the thin films from the center. Hence, irregularly shaped grains with different sizes are observed on the surface of the film. The film thickness almost increases exponentially with the increase from 136 nm to 152 nm in the concentration of copper ions from 0.15 M to 0.5 M. Depositions with concentrations above 0.1 M result in the formation of outer porous layer stress which tends to cause delamination,   Figure 3 (a, b) shows the growth rate of CuS deposition thin films. Figure 3a indicates the growth rate of CuS thin film as a function of deposition temperature at constant deposition time. From the figure, it can be seen that the deposition growth rate gradually increases with deposition temperature and this is due to more generation of colloidal ions. The growth rate is decreasing with the increasing deposition time at a constant deposition temperature due to precursor consumption over time, as shown in Figure 3b [4].  Figure 4 (a, b) shows the effect of different CuSO4 precursor concentration on the thickness and growth rate of CuS thin films, respectively. Figure 4a shows that the thickness of CuS films decreases from 148 to 136 nm as the CuSO4 concentration increases from 0.1 M to 0.15 M [25]. The deposition on a substrate mainly depends on the formation of nucleation sites and subsequent growth of the thin films from the center. Hence, irregularly shaped grains with different sizes are observed on the surface of the film. The film thickness almost increases exponentially with the increase from 136 nm to 152 nm in the concentration of copper ions from 0.15 M to 0.5 M. Depositions with concentrations above 0.1 M result in the formation of outer porous layer stress which tends to cause delamination,   Figure 4a shows that the thickness of CuS films decreases from 148 to 136 nm as the CuSO 4 concentration increases from 0.1 M to 0.15 M [25]. The deposition on a substrate mainly depends on the formation of nucleation sites and subsequent growth of the thin films from the center. Hence, irregularly shaped grains with different sizes are observed on the surface of the film. The film thickness almost increases exponentially with the increase from 136 nm to 152 nm in the concentration of copper ions from 0.15 M to 0.5 M. Depositions with concentrations above 0.1 M result in the formation of outer porous layer stress which tends to cause delamination, resulting in peeling off the film [26]. Furthermore, the competition of heterogeneous nucleation on the substrate and homogeneous nucleation in the solution [25] would alter the growth of the thin film, and here the thickness increased again. The surface morphology of the film deposited at 0.15 M is quite uniform and well covered on the substrate when compared with the other concentrations using the optical microscope [24].
An increase in growth rate with CuSO 4 concentration is observed from Figure 4b. The induction time also becomes shorter at a higher CuSO 4 concentration. In order to determine the apparent reaction orders, the growth rate is assumed to be dependent on the variation of initial reagent concentration during the initial linear growth region [24].
is quite uniform and well covered on the substrate when compared with the other concentrations using the optical microscope [24].
An increase in growth rate with CuSO4 concentration is observed from Figure 4b. The induction time also becomes shorter at a higher CuSO4 concentration. In order to determine the apparent reaction orders, the growth rate is assumed to be dependent on the variation of initial reagent concentration during the initial linear growth region [24].  Figure 5 shows the growth rate of CuS thin films as a function of C4H6O6 concentration. The thicknesses of the films depend strongly on the molarity of the C4H6O6, where the thickness increases as the concentration is increased. Copper ions (Cu 2+), introduced into the solution as copper salts, can form different complex species with complexing agents such as tartaric acid and ammonia. Appropriate complexing agents are present to produce a stable complex of Cu 2+ ions in the solution in which Cu 2+ ions are slowly released on dissociation, resulting in a controllable reaction rate. For a metal M and complexing agent, A, the existence of free metal ions in the solution can be expressed by the equilibrium reaction [24].
It has been seen that a saturation region occurs at higher concentrations of C4H6O6, which is clearly defined in Figure 5.   Figure 5 shows the growth rate of CuS thin films as a function of C 4 H 6 O 6 concentration. The thicknesses of the films depend strongly on the molarity of the C 4 H 6 O 6, where the thickness increases as the concentration is increased. Copper ions (Cu 2+), introduced into the solution as copper salts, can form different complex species with complexing agents such as tartaric acid and ammonia. Appropriate complexing agents are present to produce a stable complex of Cu 2+ ions in the solution in which Cu 2+ ions are slowly released on dissociation, resulting in a controllable reaction rate. For a metal M and complexing agent, A, the existence of free metal ions in the solution can be expressed by the equilibrium reaction [24].
It has been seen that a saturation region occurs at higher concentrations of C 4 H 6 O 6 , which is clearly defined in Figure 5.
film, and here the thickness increased again. The surface morphology of the film deposited at 0.15 M is quite uniform and well covered on the substrate when compared with the other concentrations using the optical microscope [24].
An increase in growth rate with CuSO4 concentration is observed from Figure 4b. The induction time also becomes shorter at a higher CuSO4 concentration. In order to determine the apparent reaction orders, the growth rate is assumed to be dependent on the variation of initial reagent concentration during the initial linear growth region [24].  Figure 5 shows the growth rate of CuS thin films as a function of C4H6O6 concentration. The thicknesses of the films depend strongly on the molarity of the C4H6O6, where the thickness increases as the concentration is increased. Copper ions (Cu 2+), introduced into the solution as copper salts, can form different complex species with complexing agents such as tartaric acid and ammonia. Appropriate complexing agents are present to produce a stable complex of Cu 2+ ions in the solution in which Cu 2+ ions are slowly released on dissociation, resulting in a controllable reaction rate. For a metal M and complexing agent, A, the existence of free metal ions in the solution can be expressed by the equilibrium reaction [24].
It has been seen that a saturation region occurs at higher concentrations of C4H6O6, which is clearly defined in Figure 5.  Figure 6a,b shows the influence of the thiourea concentration on the film thickness and the growth rate of CuS thin films. An increase of the initial growth rate with the SC(NH 2 ) 2 concentrations is observed; such an increment can be attributed to the increase in the number of S 2 in the thiourea volume, which accelerates the reaction forming CuS [27] and has good agreement with the work of Reference [24]. a temperature of 80 °C, deposition time of 20 min, and pH = 11. Figure 6a, b shows the influence of the thiourea concentration on the film thickness and the growth rate of CuS thin films. An increase of the initial growth rate with the SC(NH2)2 concentrations is observed; such an increment can be attributed to the increase in the number of S 2 in the thiourea volume, which accelerates the reaction forming CuS [27] and has good agreement with the work of Reference [24].  Figure 7 shows a significant change of the thin film thickness with respect to different pH rates; the thickness of the thin film increases as the pH rate increase [28], and an increase in pH value leads to a higher film growth rate. A higher concentration of OH − ions in the solution will push the reaction of thiourea hydrolysis forward, and hence a higher generation of sulfide ions has occurred. In Reference [29], the optimum pH was found to be 11, and in Figure 7 we see that at pH = 12 the thickness was higher but the thin film morphology was not uniform and little cracks were seen on the thin film surface when using an optical microscope [15].   Figure 7 shows a significant change of the thin film thickness with respect to different pH rates; the thickness of the thin film increases as the pH rate increase [28], and an increase in pH value leads to a higher film growth rate. A higher concentration of OH − ions in the solution will push the reaction of thiourea hydrolysis forward, and hence a higher generation of sulfide ions has occurred. In Reference [29], the optimum pH was found to be 11, and in Figure 7 we see that at pH = 12 the thickness was higher but the thin film morphology was not uniform and little cracks were seen on the thin film surface when using an optical microscope [15]. a temperature of 80 °C, deposition time of 20 min, and pH = 11. Figure 6a, b shows the influence of the thiourea concentration on the film thickness and the growth rate of CuS thin films. An increase of the initial growth rate with the SC(NH2)2 concentrations is observed; such an increment can be attributed to the increase in the number of S 2 in the thiourea volume, which accelerates the reaction forming CuS [27] and has good agreement with the work of Reference [24].  Figure 7 shows a significant change of the thin film thickness with respect to different pH rates; the thickness of the thin film increases as the pH rate increase [28], and an increase in pH value leads to a higher film growth rate. A higher concentration of OH − ions in the solution will push the reaction of thiourea hydrolysis forward, and hence a higher generation of sulfide ions has occurred. In Reference [29], the optimum pH was found to be 11, and in Figure 7 we see that at pH = 12 the thickness was higher but the thin film morphology was not uniform and little cracks were seen on the thin film surface when using an optical microscope [15].    Figure 8 shows that the film fabricated at the 20 min deposition time has more homogeneity over the entire film area without any cracks. The film prepared at 15 min of deposition time has voids and is not smooth. The film prepared at 25 min deposition time has many cracks and is not a continuous film. Based on the above, 20 min of deposition time has been considered as the optimum deposition time.  Figure 8 shows that the film fabricated at the 20 min deposition time has more homogeneity over the entire film area without any cracks. The film prepared at 15 min of deposition time has voids and is not smooth. The film prepared at 25 min deposition time has many cracks and is not a continuous film. Based on the above, 20 min of deposition time has been considered as the optimum deposition time.       Figure 8 shows that the film fabricated at the 20 min deposition time has more homogeneity over the entire film area without any cracks. The film prepared at 15 min of deposition time has voids and is not smooth. The film prepared at 25 min deposition time has many cracks and is not a continuous film. Based on the above, 20 min of deposition time has been considered as the optimum deposition time.        Figure 11 represents the XRD patterns of CuS thin films deposited at different pH (8, 9, 10, 11, and 12), a deposition time of 20 min, a deposition temperature of 80 °C, and precursor concentrations of 0.1M CuSO4, 0.1 M C4H6O6, and 0.1 M SC(NH2)2. It gives the same result, which is an amorphous structure for all samples prepared with the above parameters.

Optical Properties of CuS Thin Films
The transmittance of CuS thin films is studied for thin films prepared at different deposition times and temperatures at pH 10, and precursor concentrations of 0.1M CuSO4, 0.1M SC(NH2)2, and 0.1 M C4H6O6.

Optical Properties of CuS Thin Films
The transmittance of CuS thin films is studied for thin films prepared at different deposition times and temperatures at pH 10, and precursor concentrations of 0.1 M CuSO 4 , 0.1 M SC(NH 2 ) 2 , and 0.1 M C 4 H 6 O 6 . Figure 16 shows the transmittance of CuS thin films as a function of wavelength prepared at different deposition times at constant deposition temperatures. Strong regions of the absorption edge can be notices in the range of 300 to 340 nm for all samples.
From Figure 16d, the transmittance decreases with increasing deposition time, and this is coinciding with thin film thickness, as shown in Figure 2. One can see that the maximum transmittance is (96%) and decreases to (83%) in the visible region. In Figure 16e, we can see that at 25 min of deposition time, transmittance is higher than at 20 min. This is due to the peeling of the film from the surface.
The energy bandgap (Eg) of CuS is direct [32] Figure 17 represents the relation between (αhν) 2 and photon energy hν for different deposition times. From the extrapolation of the above relation in Figure 17, energy band gaps are determined.
can be notices in the range of 300 to 340 nm for all samples.
From Figure 16d, the transmittance decreases with increasing deposition time, and this is coinciding with thin film thickness, as shown in Figure 2. One can see that the maximum transmittance is (96%) and decreases to (83%) in the visible region. In Figure 16e, we can see that at 25 min of deposition time, transmittance is higher than at 20 min. This is due to the peeling of the film from the surface. The energy bandgap (Eg) of CuS is direct [32] Figure 17 represents the relation between (αhν) 2 and photon energy hν for different deposition times. From the extrapolation of the above relation in Figure 17, energy band gaps are determined.  Figure 18 shows the relation between the energy bandgap the maximum energy band gap is 2.399 eV at a deposition time of 5 min, and the minimum energy bandgap is 2.363 eV at a deposition time of 25 min, which indicates that the energy band gap decreased with the increase of the grain size of the film.      Figure 18. The relationship between the energy band gap and deposition time at a constant deposition temperature. Figure 19 indicates the optical behavior of the case at different deposition temperatures. In Figure 19d, a significant difference in the behavior of the transmission line is noticed, and for temperatures of 50, 60, and 70 °C the transmission is higher at the IR region and slowly comes down until reaching the 300 nm wavelength and sharply becomes zero at this point. However, in the higher temperature of the solution, the behavior changes, and for 90 °C the low range of the transmission is shown in the IR region and slowly becomes higher until it reaches the peak at 650 nm and then sharply reduces to zero at 300 nm. Although the influence of the increase of the number of free  Figure 19 indicates the optical behavior of the case at different deposition temperatures. In Figure 19d, a significant difference in the behavior of the transmission line is noticed, and for temperatures of 50, 60, and 70 • C the transmission is higher at the IR region and slowly comes down until reaching the 300 nm wavelength and sharply becomes zero at this point. However, in the higher temperature of the solution, the behavior changes, and for 90 • C the low range of the transmission is shown in the IR region and slowly becomes higher until it reaches the peak at 650 nm and then sharply reduces to zero at 300 nm. Although the influence of the increase of the number of free carriers cannot be ruled out, it is more likely that this effect results from the increase in the film roughness with thickness [27]. This is a good indicator that the films are strong absorbers in the visible region [33].  Figure 19 indicates the optical behavior of the case at different deposition temperatures. In Figure 19d, a significant difference in the behavior of the transmission line is noticed, and for temperatures of 50, 60, and 70 °C the transmission is higher at the IR region and slowly comes down until reaching the 300 nm wavelength and sharply becomes zero at this point. However, in the higher temperature of the solution, the behavior changes, and for 90 °C the low range of the transmission is shown in the IR region and slowly becomes higher until it reaches the peak at 650 nm and then sharply reduces to zero at 300 nm. Although the influence of the increase of the number of free carriers cannot be ruled out, it is more likely that this effect results from the increase in the film roughness with thickness [27]. This is a good indicator that the films are strong absorbers in the visible region [33].    Figures 20 and 21 represent the relationship between photon energy hν and (αhν) 2 for different bath temperatures at a constant deposition time and the relationship between the energy gap and temperature, respectively. A reverse relationship can be seen between both the deposition temperature and energy gap, and for a constant deposition time of 5 min a sharper decrease is shown in the energy gap for the bath temperatures of 50, 60, 70, 80, and 90 • C, found to be 2.41, 2.39, 2.37, 2.36, and 2.35 eV, respectively. The bandgaps were decreased due to the fact that the transmission is strongly dependent on the thickness and surface conditions [33,34]. Figures 20 and 21 represent the relationship between photon energy hν and (αhν) 2 for different bath temperatures at a constant deposition time and the relationship between the energy gap and temperature, respectively. A reverse relationship can be seen between both the deposition temperature and energy gap, and for a constant deposition time of 5 min a sharper decrease is shown in the energy gap for the bath temperatures of 50, 60, 70, 80, and 90 °C, found to be 2.41, 2.39, 2.37, 2.36, and 2.35 eV, respectively. The bandgaps were decreased due to the fact that the transmission is strongly dependent on the thickness and surface conditions [33,34].  The relationship between energy bandgap with respect to the time of deposition is shown in Figure 18. At 5 min of deposition time, a larger decrease of the energy gap can be noticed because the initial growth of the film at 5 min of deposition time was faster according to Figure 2a [31,34].  The relationship between energy bandgap with respect to the time of deposition is shown in Figure 18. At 5 min of deposition time, a larger decrease of the energy gap can be noticed because the initial growth of the film at 5 min of deposition time was faster according to Figure 2a [31,34].  The relationship between energy bandgap with respect to the time of deposition is shown in Figure 18. At 5 min of deposition time, a larger decrease of the energy gap can be noticed because the initial growth of the film at 5 min of deposition time was faster according to Figure 2a [31,34]. Optical transmittance using different pH levels is shown in Figure 22. The transmission line varies from 60% to 87% from the wavelength in the range 300 to 600 nm and shows a gradual decrease from 600 nm to 1100 nm, and it has the highest peak at the visible range of 600 nm for pH=9. pH=8 has a sharper transmittance increase from 40% to 84% in the range of 300 to 600 nm [28], and pH 12 and 11 are almost the same across the transmission line. The transmission line shows a blue shift with increasing pH. Figure 18 represents the effect of the pH rate on the optical energy gaps. A strong linear decrease of the energy gap with respect to pH rate is observed as in Figure 18 from 2.61 eV to 2.47 eV. This is related to the growth conditions; films grown at higher pH are thicker than the films made at low pH, and hence the band gap decreases. These results are confirmed by Reference [35]. Optical transmittance using different pH levels is shown in Figure 22. The transmission line varies from 60% to 87% from the wavelength in the range 300 to 600 nm and shows a gradual decrease from 600 nm to 1100 nm, and it has the highest peak at the visible range of 600 nm for pH = 9. pH = 8 has a sharper transmittance increase from 40% to 84% in the range of 300 to 600 nm [28], and pH 12 and 11 are almost the same across the transmission line. The transmission line shows a blue shift with increasing pH.  Clear regular change in the behavior of transmittance is observed in Figure 24a for different tartaric acid (C4H6O6) concentrations. It can be seen that at higher concentrations, the transmittance is lower throughout the whole range of wavelength because thickness increases with the increasing tartaric acid concentration. Figure 24b,c shows the Tauc's relation of the energy gap with respect to the tartaric acid concentration and the effect of the concentration on the energy gap. The energy gap decreases with   Figure 18 represents the effect of the pH rate on the optical energy gaps. A strong linear decrease of the energy gap with respect to pH rate is observed as in Figure 18 from 2.61 eV to 2.47 eV. This is related to the growth conditions; films grown at higher pH are thicker than the films made at low pH, and hence the band gap decreases. These results are confirmed by Reference [35].
Optical transmittance using different copper concentrations is shown in Figure 23a. It can be seen that at the higher the concentration, the transmittance is higher [25]. While there is a slightly noticeable change in the IR region, these high values may be attributed to low band gaps due to the increased thickness of the films since thicker films have more atoms present so more states will be available for the photons to be absorbed [36,37], or due to quantum confinement effects [25,38] Crystals 2020, 10 Optical transmittance using different copper concentrations is shown in Figure 23a. It can be seen that at the higher the concentration, the transmittance is higher [25]. While there is a slightly noticeable change in the IR region, these high values may be attributed to low band gaps due to the increased thickness of the films since thicker films have more atoms present so more states will be available for the photons to be absorbed [36,37], or due to quantum confinement effects [25,38] At the visible region, all the concentrations showed the same behavior, and less than 5% transmittance is noticed. Figure 23b,c shows the Tauc plot of the energy gap and the effect of copper concentration on the optical bandgap energy, respectively. A clear dependence of the energy gap on copper concentration is seen; as the concentration of CuSO4 increases, the band gap decreases [25].  At the visible region, all the concentrations showed the same behavior, and less than 5% transmittance is noticed. Figure 23b,c shows the Tauc plot of the energy gap and the effect of copper concentration on the optical bandgap energy, respectively. A clear dependence of the energy gap on copper concentration is seen; as the concentration of CuSO 4 increases, the band gap decreases [25].  [25,27,39].
Clear regular change in the behavior of transmittance is observed in Figure 24a for different tartaric acid (C 4 H 6 O 6 ) concentrations. It can be seen that at higher concentrations, the transmittance is lower throughout the whole range of wavelength because thickness increases with the increasing tartaric acid concentration.
Clear regular change in the behavior of transmittance is observed in Figure 24a for different tartaric acid (C4H6O6) concentrations. It can be seen that at higher concentrations, the transmittance is lower throughout the whole range of wavelength because thickness increases with the increasing tartaric acid concentration. Figure 24b,c shows the Tauc's relation of the energy gap with respect to the tartaric acid concentration and the effect of the concentration on the energy gap. The energy gap decreases with the increasing concentration of the tartaric acid.         Figure 25 shows the influence of the SC(NH 2 ) 2 concentration on optical transmittance. No significant change is seen at higher concentrations of SC(NH 2 ) 2 , but at 0.1 M the transmission showed the highest peak of 94%. Another point that can be noticed, which is that no shift in the transmission line is seen with the increasing SC(NH 2 ) 2 concentration.  Figure 25 shows the influence of the SC(NH2)2 concentration on optical transmittance. No significant change is seen at higher concentrations of SC(NH2)2, but at 0.1 M the transmission showed the highest peak of 94%. Another point that can be noticed, which is that no shift in the transmission line is seen with the increasing SC(NH2)2 concentration.      Figure 26 represents the current-voltage characteristics for CuS thin films fabricated at different deposition parameters. The electrical contacts are made from Aluminum (Al) [16].  Figure 26 represents the current-voltage characteristics for CuS thin films fabricated at different deposition parameters. The electrical contacts are made from Aluminum (Al) [16].  Figure 26 represents the current-voltage characteristics for CuS thin films fabricated at different deposition parameters. The electrical contacts are made from Aluminum (Al) [16].

Electrical Properties of CuS Thin Films
From the figures, it can be seen that the relationship of the current-voltage is linear. The forward and reverse bias linear relationship proves that there is an ohmic contact between Al-CuS. Tables 2 and 3 represent the effect of different deposition parameters on the electrical resistivity of the thin films. The resistivity in CuS films decreases as the deposition time increases. This decrease is due to thickness dependence [27,31,40] An increase in resistivity is obtained with increasing deposition temperatures, and that could be due to an increase in grain boundary barrier height [27,41]. For different pH levels of the solution, resistivity increases, as shown in Table 3. This is may be attributed to a deviation from stoichiometry due to the enhancement of the concentration of copper ions [27].
A large decrease in the resistivity is shown when increasing the concentration thiourea. As shown in Table 3, this large thiourea volume is induced by the creation of excess copper vacancies which give rise to a large number of free holes. Actually, as thiourea is the source for Sulphur, one can expect a larger concentration of copper vacancies [27].
Both the cases of increasing concentration of CuSO4 and tartaric acid lead to larger electrical resistivity from 0.7 to 2.41 Ω·cm for 0.15 and 0.2 M of CuSO4 concentration, and 0.78 to 1795.1 Ω·cm for 0.1 and 0.5 M of tartaric acid, respectively.    Tables 4 and 5 represent the values of the electrical activation energy of different bath parameters. At a low time of deposition, the activation decreases with time, and the activation energy decreased from 0.012 eV to 0.009 eV. These results are in agreement with Reference [27]. For different pH levels of the solution, the activation energy decreases with increasing pH, as is shown in Table 5 [27]. Deposition temperature has a reverse relationship with the activation energy, and it has been decreased from 0.009 eV to 0.003 eV for deposition temperatures of 80 °C and 90 °C, respectively. The activation energy was in the range mentioned in References [41,42].
The activation energy decreases with the increase of both concentrations of CuSO4 and SC(NH2)2 as shown in Table 5, but activation energy increases with increasing tartaric acid concentration mentioned in Table 5.  From the figures, it can be seen that the relationship of the current-voltage is linear. The forward and reverse bias linear relationship proves that there is an ohmic contact between Al-CuS. Tables 2 and 3 represent the effect of different deposition parameters on the electrical resistivity of the thin films. The resistivity in CuS films decreases as the deposition time increases. This decrease is due to thickness dependence [27,31,40]  An increase in resistivity is obtained with increasing deposition temperatures, and that could be due to an increase in grain boundary barrier height [27,41]. For different pH levels of the solution, resistivity increases, as shown in Table 3. This is may be attributed to a deviation from stoichiometry due to the enhancement of the concentration of copper ions [27].
A large decrease in the resistivity is shown when increasing the concentration thiourea. As shown in Table 3, this large thiourea volume is induced by the creation of excess copper vacancies which give rise to a large number of free holes. Actually, as thiourea is the source for Sulphur, one can expect a larger concentration of copper vacancies [27].
Both the cases of increasing concentration of CuSO 4 and tartaric acid lead to larger electrical resistivity from 0.7 to 2.41 Ω·cm for 0.15 and 0.2 M of CuSO 4 concentration, and 0.78 to 1795.1 Ω·cm for 0.1 and 0.5 M of tartaric acid, respectively. Tables 4 and 5 represent the values of the electrical activation energy of different bath parameters. At a low time of deposition, the activation decreases with time, and the activation energy decreased from 0.012 eV to 0.009 eV. These results are in agreement with Reference [27]. For different pH levels of the solution, the activation energy decreases with increasing pH, as is shown in Table 5 [27]. Deposition temperature has a reverse relationship with the activation energy, and it has been decreased from 0.009 eV to 0.003 eV for deposition temperatures of 80 • C and 90 • C, respectively. The activation energy was in the range mentioned in References [41,42]. The activation energy decreases with the increase of both concentrations of CuSO 4 and SC(NH 2 ) 2 as shown in Table 5, but activation energy increases with increasing tartaric acid concentration mentioned in Table 5.

Conclusions
CuS thin films were successfully deposited onto commercial glass according to various deposition parameters using chemical bath deposition. CBD is an easy, available, and cheap technology for controlling the structure, optical, and electrical properties of CuS thin films. It has been concluded that the thickness of the Cus thin film is affected by the concentration of the precursors. The use of 0.15 M of CuSO 4 and thiourea gives a low thickness, and 0.1 M of tartaric acid gives a low thickness.
The structure of the obtained CuS thin films indicated that all the films show an amorphous structure except the case of the optimum conditions.
The transmittance of the CuS thin films tell us that the films absorb heavily in the UV and partly in the VIS regions but moderately in the near-IR regions.
The deposition time, deposition temperature, pH, and CuSO 4 concentration have less of an effect on electrical resistivity. In contrast, the concentrations of C 4 H 6 O 6 and SC(NH 2 ) 2 have a large influence on the resistivity. Activation energy decreases with increasing deposition temperature, deposition time, solution pH, CuSO 4 , and thiourea concentration, while it has a direct relationship with the increase of C 4 H 6 O 6 .
The optical characterization within the UV-VIS region shows that the growth CuS thin film has potential applications as absorbers for photovoltaic conversion in solar cells.  Acknowledgments: The acknowledgment is directed to Soran University and Koya University for their technical support.

Conflicts of Interest:
First the authors declare no conflict of interest. Second the funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.