Low-Temperature Synthesis, Structural and Optical Characterizations of The Novel Bisbs3 Thin Films as Anew Absorber Layer for Solar Cells

In this research work, thin films of BiSbS 3 have been successfully synthesized onto well cleaned soda-lima glass substrates via the chemical bath deposition procedure at different thicknesses (t= 159, 243, 296 and 362 nm). The X-ray diffraction patterns of the chemically deposited BiSbS 3 films depicted that the synthesized films exposed polycrystalline nature and have an orthorhombic structure. The structural parameters of the chemically deposited BiSbS 3 films were evaluated by Debye-Scherer’s formulas. The surface morphologies of the BiSbS 3 films were fixed via the field-emission-scanning-electron microscope. The analyses of the linear optical parameters of the chemically deposited BiSbS 3 thin films refer to improving the values of the absorption coefficient, α and the linear refractive index, n via the increase in the film thickness. In addition, there is an observed reduction in the energy gap, E g values from 1.38 to 1.22 eV occurred by raising the film thickness. Furthermore, there is an enhancement in the nonlinear optical constants and the optoelectrical parameters occurred by raising the film thickness where the nonlinear refractive index, 𝑛 2 , the optical free carrier concentration, and the optical conductivity σ opt were enlarged with increasing the values of film thickness. Moreover, the hot probe procedure was applied to the BiSbS 3 thin films and this method demonstrated that the chemically deposited BiSbS 3 films are p-type semiconductors. =============================================================================================


Introduction
Recently, binary and ternary chalcogenide thin films attract vast attention in recent time owing to their high refractive index, absorption coefficient and suitable band gap energy (Akande et al. 2020;Jia et al. 2020;Yang et al. 2021a). So, the binary and ternary chalcogenide thin films are widely suitable for various applications like memories, light-emitting diode, solar cells, semiconductor devices, transistors and computer chips (Wang et al. 2020b;Xie et al. 2021;Zhang et al. 2020). The binary chalcogenide (B.C) thin films like CdS, Sb2S3, SnS, ZnS and CdTe are important semiconductor materials that exhibited better performance in thin film solar cells (Eensalu et al. 2022;Jrad et al. 2021;Rahman et al. 2021;Sharma et al. 2021). ZnS and CdS thin films are important window layers for thin film solar cells due to these films characterized by n-type conductivity, high optical transmittance and wide bandgap ranged from 3 eV to 3.8 eV (Kathalingam et al. 2021;Shakoury et al. 2020). Moreover, the Sb2S3, SnS and CdTe are important p-type semiconductors characterized by p-type conductivity, high absorption coefficient and narrow bandgap ranged from 1.2 eV to 1.5 eV So, these films are suitable for producing convenient absorber layer for thin film solar cells (Javed et al. 2020;Rahman et al. 2020;Yang et al. 2021b).
The ternary chalcogenide (T.C) thin films such as CuSbS2, Cu3SnS3, CuInS2, ZnAl2S4 and CdAl2S4 are impressive optical materials due to their high absorption coefficient, linear and nonlinear refractive indices (Nagamalleswari et al. 2021;Yang et al. 2020). CuSbS2, Cu3SnS3, CuInS2 ternary films were used as an absorber layer for thin film solar cells due to these films exhibited narrow bandgap ranged from 1.5 eV to 1.7 eV, p-type conductivity and high absorption coefficient (Sawant et al. 2021;Vinayakumar et al. 2017). On the other hand, ZnAl2S4 and CdGa2S4 thin films are important n-type semiconductors characterized by high optical transmittance, n-type conductivity and wide bandgap ranged from 3 eV to 3.8 eV. So, these films are suitable for producing a good window layer for thin film solar cells El Radaf et al. 2020b). The experimental procedures of synthesized the binary and ternary chalcogenide thin films were classified into two categories.
Among binary chalcogenides the antimony chalcogenides. The Sb-S and their ternary systems like Cu-Sb-S, Sn-Sb-S, In-Sb-S and Bi-Sb-S are important materials that received considerable attention in different articles owing to these materials are earth-abundant, inexpensive, non-toxic and have good electrical and optical properties (Bennaji et al. 2018;Maiti et al. 2019). Many articles focused on studying the physical properties of the CuSbS2, CuSbSe2 and SnSb2S4 thin films due to these films characterized by high values of absorption coefficient, ease of preparation, p-type conductivity and narrow bandgap energy. The BiSbS3 is a novel semiconductor material that has excellent electrochemical performance, good thermoelectric properties and high specific capacity (Patra et al. 2017;Wang et al. 2020a;Wen et al. 2019). The previous article focused on preparing the BiSbS3 in the nanorods form by the hydrothermal method (Wen et al. 2019). On the other side, no article presents the synthesis and characterization of the BiSbS3 thin films. So in the present work aims to synthesis the novel BiSbS3 thin films for the first time by the cost-effective chemical bath deposition technique. The authors have tried to fabricate the chemically deposited BiSbS3 film with good quality to study the structural and optical properties of BiSbS3 films. Then, the authors have discussed some optoelectrical parameters, the linear and nonlinear optical parameters of the chemically deposited BiSbS3 films.

Preparation of the BiSbS3 thin films
BiSbS3 films were synthesized at room temperature on precleaned soda-lima glass substrates by an inexpensive chemical bath deposition procedure. To obtain different film thicknesses, we prepare the BiSbS3 thin films at several deposition times 1, 3, 5, 7 hours. High purity chemical salts have been used to synthesize the BiSbS3 solution like antimony chloride (SbCl3) Sigma-Aldrich with purity degrees (99.99%), sodium thiosulfate (Na2S2O3) Sigma-Aldrich with purity degrees (99.99%) and bismuth nitrate (BiNO3) Sigma-Aldrich with purity degrees (99.99%). The BiSbS3 precursor solution was synthesized by the interaction between: (i) 1 M antimony chloride (SbCl3) was dissolved in 5 ml acetone. (ii) 1 M sodium thiosulfate (Na2S2O3) was dissolved in 25 ml deionized water. (iii) 1 M bismuth nitrate (BiNO3) was dissolved in 10 ml diluted Nitric acid. The BiSbS3 solution was stirred well for 30 minutes to produce a brown solution. Then the cleaned glass slides were placed in the BiSbS3 solution at different deposition times 1, 3, 5 and 7 hours to get thin films of different thicknesses. At the finish of the deposition time, the BiSbS3 thin films were extracted from the chemical bath and washed well with the deionized water and after this the BiSbS3 films were dried in air.

Characterization of the BiSbS3 thin films
The thickness of the BiSbS3 samples was determined by stylus profiler type (Dektak 150 surface profiler) and the values of the thickness for the BiSbS3 samples equal 159, 243, 296 and 362 nm. The surface morphological features and the compositional element percentages of the chemically deposited BiSbS3 samples deposited on the glass substrate were characterized via using the Quanta-FeG-250 USA field emission scanning electron microscope (FE-SEM). The X-ray diffraction (XRD) data of the chemically deposited BiSbS3 samples were examined using a Philips-X'Pert X-ray diffractometer, with CuK radiation. The optical properties of the BiSbS3 films were inspected via measuring the reflectance, R and transmittance, T of the films in the wavelength ranged from 400 nm to 2500 nm using UV-Visible spectrophotometer, JASCO (V-570.).

Structural analysis
The X-ray investigations of the BiSbS3 films with various thicknesses (t= 159, 243, 296 and 362 nm) were presented in Fig (1)

=
(2) Here represent the Bragg diffraction angle and β denotes the experimental full-width corresponding to the half maximum (FWHM).
On the other hand, the micro-strain values (ε) and the number of crystallites per unit surface area values ( of the chemically deposited BiSbS3 films was calculated by the formulas (Akl et al.

2021; Akl and Hassanien 2021):
Here t denotes the thickness of the chemically deposited BiSbS3 films.

Morphological and compositional studies
The FE SEM investigations of chemically deposited BiSbS3 films were presented in Fig. 3. We can see from this figure that the chemically deposited BiSbS3 film with thickness 159 nm has a homogenous surface consists of small grains as illustrated in Fig.3 (a). Moreover, the number and size of grains were increased by increasing the film thickness to 362 nm as displayed in Fig.3 (b). The EDAX result of the chemically deposited BiSbS3 films synthesized at various thicknesses (t= 159, 243, 296 and 362 nm) demonstrated that the BiSbS3 films exhibit a stoichiometric composition and the EDAX pattern display the existence of the Bi, Sb and S peaks at their energy positions.

Optical studies
The linear optical parameters of the BiSbS3 films synthesized at various thicknesses (t= 159, 243, 296 and 362 nm) have been computed from transmittance and the reflectance data in the wavelength ranged from 400 to 2500 nm. The absorption coefficient values (α) of the chemically prepared BiSbS3 films can be calculated according to the formula (Shaban et al. 2016;Sharma et al. 2017): Here t represents the film thickness. The energy gap values of the chemically prepared BiSbS3 films can be estimated according to Tauc's formula (Shi et al. 2018;Tauc et al. 1966): Here E denotes the photon energy which equals hν, Q represents a constant and p is the number that illustrates the optical transition kind and it is having the value 1/2 and 2 for the direct allowed and the indirect allowed optical transitions, respectively. In this work, the best fitting is obtained at p = 1/2 which indicated the allowed direct optical transition.   The variation of the ( 2 − 1) −1 versus (ℏ ) 2 for the chemically prepared BiSbS3 films is presented in Fig.7 (b). It is noted from this figure that the values of the Eo and Ed at different film thicknesses were estimated from the straight-lines slope and intercept that was fitted, as illustrated in The values of the f, no, and for the chemically prepared BiSbS3 films were recorded in Table 1 and it is observed that by enlarging the film thickness, the values of no, f and were enlarged.

Optoelectrical parameters
The optical properties of the semiconductor materials were expressed in terms of the complex dielectric function = 1 + 2 . The real, 1 , and imaginary, 2 parts of this description are both frequency-dependent quantities, which contain all the desired response information [53].
In this study, the values of 1 and 2 for the chemically prepared BiSbS3 films was deduced Here α is the absorption coefficient, c is the speed of light and n denotes the refractive index.
Here represents the electronic charge, represents the speed of light and is the electric permittivity of free space. Fig.9 (b) demonstrated the variation of 2 versus 2 for the chemically prepared BiSbS3 films. The magnitudes of the ( / * ) and were evaluated from this plot and recorded in Table.2. It is noted that the values of and / * were enlarged with growing the film thickness. This thickness enlargement is associated with the rise in the charge carrier concentration. This was related to the possibility of attaining a degree of ordering in the chemically prepared BiSbS3 films (Fouad et al. 2006).

Nonlinear optical parameters
The determining of the nonlinear optical constants of the chalcogenide films has a vital role in nonlinear optical devices like optical switching, optical signal processing and frequency doubling (Kasap and Capper 2017). Miller's relationships have been employed to deduce the first-order nonlinear susceptibility χ (1) , the magnitude of the nonlinear refractive index 2 and the third-order nonlinear optical susceptibility, (3) for the chemically prepared BiSbS3 films by (Darwish et al. 2017;Hassanien et al. 2021;Lougdali et al. 2021): Where 0 , B denotes the static refractive index of the chemically prepared BiSbS3 films and a constant value, equals 1.7 × 10 −10 esu respectively.
The magnitude of the (1) , (3) and 2 for chemically prepared BiSbS3 films were listed in table 3. It is obvious that by enlarging the film thickness, both of the (1) , (3) and n2 were enlarged.

Detecting of the semiconductor kind
To detect the semiconductor kind of the chemically prepared BiSbS3 thin films we apply the hot-probe procedure. In this experiment, the two probes were connected to a sensitive digital multimeter and we join the hot probe to the positive port and the cold probe was joined with the negative port as illustrated in Fig.11. This experiment demonstrated a negative voltage on the multimeter that detects a p-type semiconductor and in case of appearing positive voltage on the multimeter, it refers to the n-type semiconductor (Axelevitch and Golan 2013). The experiment shows negative voltage for all samples, which shows that the chemically prepared BiSbS3 films reveal the ptype conductivity.

Conclusions
In this study, BiSbS3 thin films were synthesized at room temperature on precleaned soda-lima glass substrates by an inexpensive chemical bath deposition procedure. The XRD of the chemically deposited BiSbS3 films indicated that the as-prepared BiSbS3 films exposed polycrystalline nature and have an orthorhombic structure. The structural parameters of the chemically deposited BiSbS3 films were evaluated by Debye-Scherer's formulas. The FE SEM investigation of chemically deposited BiSbS3 films displays that the investigated films have homogeneous surfaces and the EDAX pattern of the chemically deposited BiSbS3 films confirms a stoichiometric composition for the BiSbS3 films.
The optical results show that the refractive index, n, absorption coefficient, α and the extinction coefficient, k of the chemically prepared BiSbS3 films were enlarged with growing the film thickness.
On the other side, the optical bandgap of the BiSbS3 films was reduced by increasing the film thickness.
Moreover, the optoelectrical parameters of the chemically prepared BiSbS3 films like the optical conductivity (σopt) and the optical free carrier concentration to effective mass ( / * ) were enlarged with enlarging the film thickness. The non-linear optical constants (1) , (3) and 2 for the chemically prepared BiSbS3 films were enlarged by raising the film thickness. The hot-probe procedure shows that the chemically prepared BiSbS3 films reveal the p-type conductivity.

Conflicts of interest
The authors declare that they have no conflict of interest.