Improvement in Optoelectronic Properties of Bismuth Sulphide Thin Films by Chromium Incorporation at the Orthorhombic Crystal Lattice for Photovoltaic Applications

By using the chemical bath deposition approach, binary bismuth sulphides (Bi2S3) and chromium-doped ternary bismuth sulphides (Bi2−xCrxS3) thin films were effectively produced, and their potential for photovoltaic applications was examined. Structural elucidation revealed that Bi2S3 deposited by this simple and cost-effective method retained its orthorhombic crystal lattice by doping up to 3 at.%. The morphological analysis confirmed the crack-free deposition, hence making them suitable for solar cell applications. Optical analysis showed that deposited thin films have a bandgap in the range of 1.30 to 1.17 eV, values of refractive index (n) from 2.9 to 1.3, and an extinction coefficient (k) from 1.03 to 0.3. From the Hall measurements, it followed that the dominant carriers in all doped and undoped samples are electrons, and the carrier density in doped samples is almost two orders of magnitude larger than in Bi2S3. Hence, this suggests that doping is an effective tool to improve the optoelectronic behavior of Bi2S3 thin films by engineering the compositional, structural, and morphological properties.


Introduction
To satisfy the need for renewable energy, new efforts are required to efficiently gather incident photons [1][2][3]. First-generation photovoltaic devices, such as single-crystal siliconbased devices, although having an efficiency of up to 15%, are expensive to manufacture and install. While second-generation devices, i.e., polycrystalline semiconductor thin filmbased solar cells, are cost-effective, their poor efficiency limits their applicability [4][5][6]. In the Figure 1 illustrates how an ellipsometeric method was used to gauge the films' thickness. Film formation often slows as time goes on as a result of reactant consumption in reactions that typically start off quickly [32]. It is possible that the precipitation process was altered based on the thickness of the films with various Cr concentrations for the same deposition duration. The selectivity of EDTA for one metal ion over another and the ensuing difference in the strength of one metal-EDTA complex over another, i.e., Bi and Cr, are attributed to variations in the precipitation process and, finally, the film thickness [36]. Figure 1 reveals that Cr addition slowed down the precipitation reaction by strong chelation, developed between the Cr-EDTA [37][38][39], which resulted in a slow precipitation process by slowly releasing the Cr ions for the doped samples for the same period of deposition time, i.e., six hours.

Results and Discussion
Molecules 2022, 27, x FOR PEER REVIEW 3 of 1 reactions that typically start off quickly [32]. It is possible that the precipitation proces was altered based on the thickness of the films with various Cr concentrations for the same deposition duration. The selectivity of EDTA for one metal ion over another and the en suing difference in the strength of one metal-EDTA complex over another, i.e., Bi and Cr are attributed to variations in the precipitation process and, finally, the film thickness [36] Figure 1 reveals that Cr addition slowed down the precipitation reaction by strong chela tion, developed between the Cr-EDTA [37][38][39], which resulted in a slow precipitation pro cess by slowly releasing the Cr ions for the doped samples for the same period of deposi tion time, i.e., six hours. The XRD patterns are shown in Figure 2. The polycrystalline structure of the depos ited thin films is evident by sharp and well-defined peaks. XRD analysis shows that both undoped and doped materials (Bi2S3 and Bi2−xCrxS3) fit the bismuthinite phase of bismuth sulphide, with an orthorhombic structure (ICSD No: 01-075-1306), as shown by black ver tical lines in Figure 2. The lack of additional peaks matching Cr or Cr-related, as well a Bi-related, peaks, suggests the formation of a single phase of Bi2S3 with high Cr homoge neity. For doped samples, a preferred orientation along the 021 plane is observed. Upon doping, the thin film growth process is influenced, resulting in the shifting of preferred planes [40].
Defects that are introduced as a result of dopant inclusion cause lattice deformation and shifts in the XRD peaks. The XRD peak locations move to either a higher or lowe angle as a function of the external entity, i.e., dopant [41]. Change in the preferred orien tation of thin films while transforming into the doped ones, as in the current case instead of the (221) plane to (021), is a common phenomenon [40]. Slide shifting of diffracted peak at 2θ~35.0° and 49.0° towards larger angles give a clear indication of the incorporation o Cr ions in the Bi2S3 lattice [42]. Grain sizes were found to be decreased with the addition of Cr ions, as with the addition of Cr, more nucleation centers and sites are created fo crystal growth. As both cations, i.e., Bi and Cr, act as seed nuclei, with the incorporation and increasing concentration of Cr ions, nucleation cites increased, resulting in a greate number of grains with a consequent reduction in size [43]. The XRD patterns are shown in Figure 2. The polycrystalline structure of the deposited thin films is evident by sharp and well-defined peaks. XRD analysis shows that both undoped and doped materials (Bi 2 S 3 and Bi 2−x Cr x S 3 ) fit the bismuthinite phase of bismuth sulphide, with an orthorhombic structure (ICSD No: 01-075-1306), as shown by black vertical lines in Figure 2. The lack of additional peaks matching Cr or Cr-related, as well as Bi-related, peaks, suggests the formation of a single phase of Bi 2 S 3 with high Cr homogeneity. For doped samples, a preferred orientation along the 021 plane is observed. Upon doping, the thin film growth process is influenced, resulting in the shifting of preferred planes [40].
Defects that are introduced as a result of dopant inclusion cause lattice deformation and shifts in the XRD peaks. The XRD peak locations move to either a higher or lower angle as a function of the external entity, i.e., dopant [41]. Change in the preferred orientation of thin films while transforming into the doped ones, as in the current case instead of the (221) plane to (021), is a common phenomenon [40]. Slide shifting of diffracted peaks at 2θ~35.0 • and 49.0 • towards larger angles give a clear indication of the incorporation of Cr ions in the Bi 2 S 3 lattice [42]. Grain sizes were found to be decreased with the addition of Cr ions, as with the addition of Cr, more nucleation centers and sites are created for crystal growth. As both cations, i.e., Bi and Cr, act as seed nuclei, with the incorporation and increasing concentration of Cr ions, nucleation cites increased, resulting in a greater number of grains with a consequent reduction in size [43]. Lattice factors "a, b, and c", unit cell volume "Vcell", Scherrer crystallite size "D" [44], X-ray density "ρx-ray" dislocation density "ẟ", and microstrain "Ɛ" were calculated using Equations (1)- (6).
where ꞵ is the full width at half maximum intensity, λ is the X-ray wavelength and is equal to 0.15406 nm, θ is Bragg's angle, k is the constant equal to 0.94, Z is the number of molecules per formula unit, and M is the molar mass. Vcell and NA have their usual meanings. Crystallographic parameters calculated for both Bi2S3 and Bi2−xCrxS3 thin films calculated from XRD data are tabulated in Table 1, which seem to be influenced by Cr addition. Transitions from binary to ternary, elemental to compound, and complex compounds often result in compositional and positional chaos [45]. Lattice factors "a, b, and c", unit cell volume "V cell ", Scherrer crystallite size "D" [44], X-ray density "ρ X-ray " dislocation density "δ", and microstrain "ε" were calculated using Equations (1)- (6).
where β is the full width at half maximum intensity, λ is the X-ray wavelength and is equal to 0.15406 nm, θ is Bragg's angle, k is the constant equal to 0.94, Z is the number of molecules per formula unit, and M is the molar mass. V cell and N A have their usual meanings. Crystallographic parameters calculated for both Bi 2 S 3 and Bi 2−x Cr x S 3 thin films calculated from XRD data are tabulated in Table 1, which seem to be influenced by Cr addition. Transitions from binary to ternary, elemental to compound, and complex compounds often result in compositional and positional chaos [45]. The surface morphologies of undoped Bi 2 S 3 and Cr-doped Bi 2 S 3 thin films are shown in Figure 3. A noticeable difference was observed between the morphological properties of films with the addition of Cr from 0-3 at.%. Figure 3a depicts the surface morphology of an undoped sample, which has compact, homogenous, and interconnected particles; Figure 3b depicts the surface morphology of a sample with 1% Cr, which has incredibly small particles that are comparable to those of pure Bi 2 S 3 , while the texture of the particles was preserved after Cr insertion. Upon further increase in the dopant, Figure 3c,d indicates irregular-shaped particles with a broad range of sizes. The particle size is in the range of 150 to 80 nm for all the deposited samples, which is in agreement with the XRD findings. Upon increasing the dopant concentration, the particle size decreased. As both cations, i.e., Bi and Cr, serve as seed nuclei for the growth of nanoparticles by Ostwald's ripening, particles were discovered to grow at the cost of previously deposited particles, resulting in agglomeration owing to the overgrowth of microscopic grains on previously deposited particles with uneven boundaries. Higher dopant concentrations resulted in loosely organized, smallerparticle-sized films on the substrate as evident by both SEM and AFM studies, hence validating the findings of XRD data. Atomic force microscopic studies (inset figures) showed that with an increasing doping concentration, the thickness increased from 51 to 57 nm by offering more surface area for photon interactions. The differences in the compositions of the deposited samples, which are determined by the Cr-to-Bi ratio, are connected to variations in their morphologies at various dopant concentrations.  The absorbance-versus-wavelength plot of chromium-doped bismuth sulphide thin film systems is shown in Figure 4. The strong absorbance region in this figure is between 400 and 800 nm, while in the infrared region, there is noteworthy absorbance. Furthermore, the absorption in the near-infrared region harvests more photons to invert into photocurrent [46]. The position of the absorption edge shifts red as the Cr content increases from 0 to 3 at. Percent. By modifying the ratio of Bi and S atoms, the addition of Cr to the system changes the average atomization energy, leading to this shift. The red shift in the absorption spectra will be helpful to enhance the ability of the synthesized materials to absorb a wider spectrum of light (more in the visible region). Additionally, Table 2 shows the compositions' optical absorption coefficients, which ranged from 10 5 to 10 6 cm −1 , and confirms their potential as effective absorber materials for solar applications.    Figure 5 illustrates the band gaps of the thin films, which were calculated using UV-Vis spectroscopy and the Tauc equation. These band gaps are in good agreement with known values and are appropriate for applications as visible light absorber materials [47]; the value of exponent n is 2, indicating a direct and allowed transition.   Figure 5 illustrates the band gaps of the thin films, which were calculated using UV-Vis spectroscopy and the Tauc equation. These band gaps are in good agreement with known values and are appropriate for applications as visible light absorber materials [47]; the value of exponent n is 2, indicating a direct and allowed transition. To calculate the band gap (Eg) of deposited films, the Tauc equation is used: "h" stands for Planck's constant (6.62 × 10 −34 Js), "υ" stands for light frequency, A is the constant, and "α" stands for the absorption coefficient calculated from this relationship.
Regarding the dependence of the composition of thin films on the band gap, a decrease in the band gap as shown in Figure 6 is credited to manifestation in the band structure by introducing discrete impurity levels [48]. Crystallinity, an important factor, might also speculate its role in the decrement in the band gap [49]. Transmittance (T) and absorbance (A) are inter-related through the following equation: The absorption coefficient (α) is calculated by the following equation: To calculate the band gap (E g ) of deposited films, the Tauc equation is used: "h" stands for Planck's constant (6.62 × 10 −34 Js), "υ" stands for light frequency, A is the constant, and "α" stands for the absorption coefficient calculated from this relationship.
Regarding the dependence of the composition of thin films on the band gap, a decrease in the band gap as shown in Figure 6 is credited to manifestation in the band structure by introducing discrete impurity levels [48]. Crystallinity, an important factor, might also speculate its role in the decrement in the band gap [49]. To calculate the band gap (Eg) of deposited films, the Tauc equation is used: "h" stands for Planck's constant (6.62 × 10 −34 Js), "υ" stands for light frequency, A is the constant, and "α" stands for the absorption coefficient calculated from this relationship. Regarding the dependence of the composition of thin films on the band gap, a decrease in the band gap as shown in Figure 6 is credited to manifestation in the band structure by introducing discrete impurity levels [48]. Crystallinity, an important factor, might also speculate its role in the decrement in the band gap [49]. Transmittance (T) and absorbance (A) are inter-related through the following equation: The absorption coefficient (α) is calculated by the following equation: The absorption coefficient (α) is calculated by the following equation: The value of the extinction coefficient is calculated using the absorption coefficient (α) and optical wavelength (λ o ).
Equation (11) demonstrates how to use the reflectance (R) and extinction coefficient (k) data to obtain the refractive index (n).
The refractive index (n) and extinction coefficient are related to the dielectric constant (k).
The dielectric constant is connected to certain substances, such as those used in capacitors, printed circuit board substrates, and cable insulation. It is a complex number, with the imaginary component corresponding to dielectric losses and the real part (ε r ) indicating the degree of the polarizability of a material. They were calculated by the relations: Electrical conductivity (σ e ) is estimated from the values of the wavelength (λ), refractive index (n), and speed of light (c = 2.8 × 10 8 m/s). Equation (15) may be used to determine it mathematically. σ e (Ωcm −1 ) = 2π/λnc (15) Thermal conductivity (σ t ) is assessed by Equation (16).
L is the Lorentz number, 2.45 × 10 −8 W Ω K −2 and T is the temperature. Table 2 shows absorption coefficient values that are suitable for use as an absorber layer in photovoltaic applications [50]. The real portion (ε r ) of the complex dielectric constant describes how much light is retarded in the material, while the imaginary part (ε i ) describes how much energy is absorbed from an electric field owing to the dipole signal. The real component of the dielectric constant is bigger than the imaginary part in this case, suggesting that the material's reaction to light is visible and distinct [51]. Hence, the dielectric properties (ε) of materials contribute to mainly dipolar or orientation polarization, which arises from molecules that change their dipolar orientation when an electric field is applied. Values of both real and imaginary dielectric constants lie in the visible region, and this behavior leads to increased electronic transfers through the material from the valence band to the conduction band [50]. The Urbach energy, another critical optical characteristic of the material, is related to the width of the band tail of the localized states in the bandgap (E u o ). The Urbach energy value is determined by the degree of defect in the chalcogenides [52]. The Urbach slope is calculated by plotting the logarithm of alpha versus photon energy and fitting a line after determining the linear zone. The width of tail states into the forbidden gap is quantified by E u o , which is the inverse of that slope. Table 2 shows the Eu values determined for undoped and doped thin films. The value of E u o is zero in a perfect semiconductor. The Urbach energy was found to increase from 0.26 to 0.35 eV when the bandgap region below the bandgap became wider and included more tail-absorbing states. E u o 's behavior is shown to be influenced by Cr content, since Cr incorporation resulted in an increase in the number of abnormalities and diseases. Figure 6 shows the considerable compositional dependence of doping and doping concentration on the refractive index and extinction coefficient. Values of the refractive index (n) decreased from 0.72 to 0.62, while an enhancement of the values of the extinction coefficient (k) was observed, i.e., from 0.001 to 0.012.
The incorporation of the Cr dopant strongly influenced the resistivity and conductivity of thin films, as shown in Figure 7a,b. The conductivity of binary thin films was enhanced up to 1.54 × 10 −2 ohm −1 cm −1 , and a consequent decrease in the resistivity of films from 299.9 to 59.0 Ω cm upon transforming the binary compound to a ternary compound is observed ( Table 3). The value of sheet carrier mobility (µ s ) is calculated and found to be 47.7 cm 2 /V s, which is considered to be higher than those published before in the literature (i.e., 28 cm 2 /V s) [53]. The value of carrier concentrations (N s ) decreases while µ s increases with an increasing concentration of Cr, as shown in Figure 7. Recombination of the stimulated carriers by the traps, which may be a shadow or deep, led to a decrease in mobility [54]. The behavior of thin films in the present study is n-type.   Figure 8 depicts the IV behavior of undoped and selected Cr-doped thin films. It is clear that with an increasing Cr content, the diode behavior of the film is enhanced, hence making the ternary material more suitable for photovoltaic applications. An improvement in the photocurrent signal of treated Bi2S3 compared to that of pure Bi2S3 under visiblelight irradiation has also been reported previously [55].   Figure 8 depicts the IV behavior of undoped and selected Cr-doped thin films. It is clear that with an increasing Cr content, the diode behavior of the film is enhanced, hence making the ternary material more suitable for photovoltaic applications. An improvement in the photocurrent signal of treated Bi 2 S 3 compared to that of pure Bi 2 S 3 under visible-light irradiation has also been reported previously [55].  Figure 8 depicts the IV behavior of undoped and selected Cr-doped thin films. It is clear that with an increasing Cr content, the diode behavior of the film is enhanced, hence making the ternary material more suitable for photovoltaic applications. An improvement in the photocurrent signal of treated Bi2S3 compared to that of pure Bi2S3 under visiblelight irradiation has also been reported previously [55].

Materials and Methods
Well-cleaned commercially available soda-lime microscopic glass slides were used as substrates. The chemicals used were Bi(NO3)3·5H2O, chromium nitrate pentahydrate (Cr(NO3)3·9H2O, Sigma, Schnelldorf, Germany, 98%), thioacetamide (CH3CSNH2, Aldrich, 99%), nitric acid (HNO3, Sigma), and ethylene-diamine-tetra-acetic-acid, (EDTA, Sigma, 99%). To deposit pure and chromium-doped samples in the range of 1-3 at%, four baths with varying concentrations of Cr(NO3)3·9H2O and Bi(NO3)3·5H2O were prepared to deposit undoped and doped films, labeled as 0 at.% Cr, 1.0 at.% Cr, 2.0 at.% Cr, and 3.0 at.% Cr. Equal-volume and equimolar (10 mL of 0.10 M) bismuth nitrate and EDTA solutions for the pure sample were mixed in a bath at pH 2. To synthesize the doped derivative samples, different concentrations of the chromium solution were added to the same bath. Thioacetamide (10 mL of 0.1 M) was added to the resultant mixture as the sulphur source. Pre-cleaned-glass films were placed vertically in the resultant mixture beaker for six hours at room temperature.
In order to assess how the planned material will behave, synthetic samples were exposed to various characterizations. Using a PANalytical Xpert' Pro (Holland) X-Ray Diffractometer, the phase composition of the deposited thin films was investigated using an X-ray diffraction study in the 20-700 range with Cu K irradiation (k = 0.15406 nm). Optical analysis was performed using the Perkin Elmer Lambda 25 spectrophotometer. Investigation of the morphology and content of samples was carried out using the JSM-6360A SEM and the 'Contact mode AFM' nasoscope digital equipment with a silicon nitride cantilever. Using a nano-chip dependability grade Hall effect device, the Hall experiments were examined. The optical properties of thin films were verified using the Systronics-117 spectrophotometer's ellipsometry method (sensor). Additionally, the Keithley-2635A source meter was used to assess IV behavior while in ohmic contact with an Ag electrode.

Materials and Methods
Well-cleaned commercially available soda-lime microscopic glass slides were used as substrates. The chemicals used were Bi(NO 3 ) 3 ·5H 2 O, chromium nitrate pentahydrate (Cr(NO 3 ) 3 ·9H 2 O, Sigma, Schnelldorf, Germany, 98%), thioacetamide (CH 3 CSNH 2, Aldrich, 99%), nitric acid (HNO 3 , Sigma), and ethylene-diamine-tetra-acetic-acid, (EDTA, Sigma, 99%). To deposit pure and chromium-doped samples in the range of 1-3 at%, four baths with varying concentrations of Cr(NO 3 ) 3 ·9H 2 O and Bi(NO 3 ) 3 ·5H 2 O were prepared to deposit undoped and doped films, labeled as 0 at.% Cr, 1.0 at.% Cr, 2.0 at.% Cr, and 3.0 at.% Cr. Equal-volume and equimolar (10 mL of 0.10 M) bismuth nitrate and EDTA solutions for the pure sample were mixed in a bath at pH 2. To synthesize the doped derivative samples, different concentrations of the chromium solution were added to the same bath. Thioacetamide (10 mL of 0.1 M) was added to the resultant mixture as the sulphur source. Pre-cleaned-glass films were placed vertically in the resultant mixture beaker for six hours at room temperature.
In order to assess how the planned material will behave, synthetic samples were exposed to various characterizations. Using a PANalytical Xpert' Pro (Holland) X-ray Diffractometer, the phase composition of the deposited thin films was investigated using an X-ray diffraction study in the 20-700 range with Cu K irradiation (k = 0.15406 nm). Optical analysis was performed using the Perkin Elmer Lambda 25 spectrophotometer. Investigation of the morphology and content of samples was carried out using the JSM-6360A SEM and the 'Contact mode AFM' nasoscope digital equipment with a silicon nitride cantilever. Using a nano-chip dependability grade Hall effect device, the Hall experiments were examined. The optical properties of thin films were verified using the Systronics-117 spectrophotometer's ellipsometry method (sensor). Additionally, the Keithley-2635A source meter was used to assess IV behavior while in ohmic contact with an Ag electrode.

Conclusions
Low bandgap energy, preferably in the visible range, high surface area, and conductivity are prerequisite properties for an efficient photocatalytic material. In the current study, chromium-doped bismuth sulphide thin films with good lateral homogeneity and an energy bandgap between 1.3 and 1.15 eV were successfully deposited in an acidic medium via the chemical bath deposition technique. The optical characteristics of the films were modified by dopant incorporation by modifying the lattice parameters and thickness of the films, according to a correlation between the optical band gap and lattice parameters of the films. According to the films' optical properties, almost all of them were found to be efficient absorbers in the targeted UV-Vis range. Top-view scans and AFM observations indicate that the surfaces of the films were affected by the Cr contributions. We determined that the Cr concentration in the ternary chromium-doped bismuth sulphide chalcogenide had an effect on all of the distinctive characteristics of the deposited films without disrupting the crystal lattice. It is necessary to relate the influence of the dopant concentration on the distinctive characteristics at the same thickness by altering the deposition duration, since all optoelectronic properties rely on the thickness of the film.  Data Availability Statement: The data will be available on request.