A modeling study on utilizing low temperature sprayed In2S3 as the buffer layer of CuBaSn(S, Se) solar cells

This study represents the investigation of In2S3 thin films as an electron transport layer in the CuBaSn(S, Se)-CBT(S, Se) solar cells, which have been deposited using the Chemical Spray Pyrolysis method. For studying the electrical properties of films such as conduction and valence band, carrier densities, Fermi level, flat band potential, and semiconductor type, the Mott–Schottky analysis has been used. UV–VIS, XRD, and FESEM have been applied to investigate the optical properties of the layers and the layer’s morphologies. The experimental CBT(S, Se) solar cell has been simulated and validated as the next step. After that, the In2S3 layer has been used as the electron transport layer. The results represent that the In2S3 layer is a suitable substitution for toxic CdS. Finally, the In2S3 properties are varied in reasonable ranges, which means different electron transport layers are screened.

www.nature.com/scientificreports/ that an electron transport layer with an optimal conduction band offset (CBO) reduces the V OC deficit, which directly affects the device performance [33][34][35] . Indium sulfide (In 2 S 3 ) stands out as a promising candidate among the other semiconducting materials to use as an electron transport layer in solar cell fabrication. This n-type material has attracted considerable attention among scientists due to its wide direct bandgap (2.0-2.8 eV), non-toxic components, chemical stability, high transparency in the visible region, and photoconductive nature [36][37][38][39] .
Among all the other simulation software, the solar cell capacitance simulator (SCAPS-1D) has been employed in this work to calculate photovoltaic parameters, such as open-circuit voltage (V OC ), short-circuit current density (J sc ), and efficiency (η) under standard illumination (AM 1.5 solar radiation/the power density of 100 mW/cm 2 is used as the source of illumination) [40][41][42][43] . The calculations of this software are based on the Poisson and the Continuity equations for electrons and holes. The numerical study is carried out to represent an accurate work with optimum model parameters and investigate the influence of physical parameters on the device's performance.
In this work, we used the Chemical Spray Pyrolysis method to deposit In 2 S 3 thin films as the electron transport layer and study their morphological, structural, optical, and electrochemical properties. This way not only enabled the low-temperature deposition of In 2 S 3 at 250 °C but also supplied the ability to use the non-toxic material of the In 2 S 3 at the Cu 2 BaSnS x Se 4−x (CBTSSe) solar cells. In order to get more insight on the influence of In 2 S 3 thin films on device performance, theoretical current density-voltage characteristics under irradiation are investigated. Device modeling and numerical simulations of CBTSSe/In 2 S 3 solar cells are achieved to examine the impacts of the valence band offset, conduction band offset, donor density, electron mobility, the thickness of buffer layer, effective density of the conduction band states, absorption coefficient constant, interface density affect a proposed structure's performance. For this purpose, numerical simulations of electrical responses of the solar cell device are made using the solar cell capacitance simulator (SCAPS-1D) program (version 3.3.05). The numerical simulation results propose an optimal geometrical structure for the CBTSSe/In 2 S 3 solar cells.

Material and methods
Material. The In 2 S 3 molecular-based precursor solution was prepared using a mixture of chloride metal salt and thiourea (TU) dissolved in water and ethanol (Merck 98%) with a ratio of 1 (ethanol):3 (water) to study the effect of low-temperature deposition on electron transport layer properties. Then, Indium (III) chloride (InCl 3 , STREM CHEMICALS, 99.99%), and thiourea (ACROS,99%) was used as indium and sulfur source. The molar ratio of In:S was 1:4. All chemicals were used without further purification. Experimental method. The investigation of the effect of the low-temperature electron transport layer by means of In 2 S 3 molecular-based precursor solution has been explained above. For more explanation, with 100% consideration, 0.1 M of thiourea concentration and 25 mM of indium concentration were kept constant for all experiments. The samples were located on a hot plate in the air with a surface temperature of 250 °C to deposit In 2 S 3 layers. It must be stressed that the surface temperature should be constant over the sample. The rate of deposition 4 ml/min, 17 cm height (distance between hot plate and nozzle), and 250 °C deposition temperature are the optimum conditions for In 2 S 3 deposition 44 . The carrier gas was the air with a constant rate of 3 L/min. The total volume of the solution sprayed and the minimum size of the samples were 2 ml and 1.1 in. × 1.1 in., respectively. As one can see, Fig. 1 represents the scheme of the spray pyrolysis set-up.

Device analysis
Philips XL30 system has been employed for FESEM investigation of the layer morphologies. Moreover, X-ray diffraction (X'Pert Pro MPD, PANalytical) was the method to analyze the crystal structural properties. The optical properties of the deposited layers were figured out by calculating the transmittance spectra using UV-Vis spectroscopy (Lamda 25, Perkin Elmer). The Mott-Schottky analysis was utilized in a three-electrode configuration, using a quartz cell and 0.5 M Na 2 SO 4 solution (pH 6.0) using an IRASOL EIS-26H system. The working reference and counter electrodes were In 2 S 3 /FTO, Ag/AgCl (3 mol/kg KCl), and Pt. The signal frequency was 1 kHz, and the bias voltage was scanned from − 0.8 to 0.3 V, with 50 mV/s speed (peak-to-peak) in the environmental condition. www.nature.com/scientificreports/

Results and discussion
Structural and optical properties. Figure 2 demonstrates the side view field emission scanning electron microscope (FESEM) of the In 2 S 3 layer. The In 2 S 3 layers prepared have a cauliflower, compact, homogenous, and crack-free structure. A compact In 2 S 3 electron transport layer can improve the interfacial contact with CBT (S, Se) layers and boost cell performance. The ideal connection between the In 2 S 3 and CBT (S, Se) is expected to provide a perfect pathway for photo-generated carriers 45 . The usual transparent conductive electrode with a compact blocking structure used as the front contact in CBT (S, Se) solar cells is the indium tin oxide (ITO). The primary responsibility of this thin layer is as a front electrode is reducing series resistance [46][47][48] . According to Fig. 3, the XRD spectra helps to investigate phase compounds and the crystalline structure of the layer. Figure 3a depicts the spectral resolution of the In 2 S 3 layer in comparison to the (JCPDS.NO. 25-0390) standard card. Peak diffraction (1 0 15) is characteristic of the formation of the tetragonal structure, which contrasts with the cubic structure. Notably, the peak diffraction (1 0 15) of the crystallization characteristic tetragonal sequentially because of the diffraction peaks of the two-crystal cubic and tetragonal structures overlapped 44 .
Additionally, none of the oxide compounds peaks were found in the XRD pattern, which means that these compounds are not present in the accuracy of XRD analysis detection. Some works reported that sulfur produces exhaustion of oxygen in the proximity of the film that prevents the formation of indium oxide 49 .
The UV-Vis study was carried out to characterize the total optical transmittance to define the optical bandgap of the In 2 S 3 layers. As can be seen, Fig. 4a illustrated the transition spectrum and indirect bandgap energy of the In 2 S 3 layer prepared at 250° C. The prepared In 2 S 3 layer revealed fine transparency in the spectral range of 300 1100 nm. For wavelengths smaller than the energy gap, the layer's transparency is about 90% in the visible and near-infrared region of the spectrum. The optical absorption coefficient (α), electron transfer, is related to the energy gap and is defined in terms of (α ≥ 10 4 cm −1 ) as (Eq. 1):  www.nature.com/scientificreports/ where A is a constant that depends on the transition probability, E g shows the energy gap, hv is the incident photon energy, and n is a parameter that describes the electron transfer between the conduction and valence bands. n = 2 for an indirect electron transfer. Thus, according to Fig. 4b, the E g value for indirect transition is determined by extrapolating the linear part of the plot ( αhv) 0.5 versus ( hv ) in the abscissa (axis x), which depicts an indirect optical transition. The E g value of the In 2 S 3 layer deposited was 2.12 eV (indirect transitions).
Electrochemical properties. All semiconducting properties of materials, such as the free-carrier concentration, flat-band potential, energy levels, and conductivity type, are critical components that lead to better device performance. These properties can be determined from a Mott-Schottky (M-S) plot consisting of the space-charge-layer capacitance (C SC ) inverse square versus the bias potential 50 . It is well defined that the slope of such plot is used to determine the semiconductor doping density N D, and extrapolation of intercept may be used to compare the flat band potential V fb , Fig. 5, using the well-known Mott-Schottky relation (Eq. 2): which N D , e, ε 0 , ε r , V fb, and V are carrier concentration, electron charge, vacuum permittivity, the dielectric constant of semiconductor, flat band potential, and applied potential, respectively. T is the temperature of the operation (300 K), K is the Boltzmann's constant (1.38 × 10 −23 J/K), and C SC is the space charge capacitance. In this work dielectric constant for In 2 S 3 films is equal to 8.5. As Mott-Schottky plot depicts (Fig. 5), the positive slope of the curves indicates that films all have an n-type conductivity 51 .
To understand precisely, the carrier density (N D ) has been calculated alongside the calculation of the In 2 S 3 carrier transport. From the Mott-Schottky (M-S) curve slope, the shallow donor density derived 2.11 × 10 17 cm −3 for In 2 S 3 . The flat band potential (V fb ) is the other parameter, which has been investigated. The redox potentials are conveniently expressed in the band edges by flat band potential (V fb ). Actually, V fb is the needed potential for applying to the semiconductor to reduce the band bending to zero. The flat band potential is obtained as − 0.74 V (relative to Ag/AgCl) for the In 2 S 3 layer. The different V fb values emerge as a result of conductivity differences among the other crystallographic planes. (2) -4.03 -4.14 -6.14 2.11×10 17 www.nature.com/scientificreports/

Numerical study
As mentioned above, SCAPS-1D software was the package, which has been used for the simulated part of the current work. The numerical model of this work has been validated with the experimental data to define instructions for better illustrating the cell PCE improvement. The validation results are available in the supporting information file.
Investigation of In 2 S 3 as the electron transport layer. The effect of substituting a uniform, thin, and continuous interfacial In 2 S 3 layer as the electron transport layer with toxic CdS has been investigated in the present study, which is based on examining the following parameters.

Examination of the valence band offset (VBO).
It should be stated that the CBT (S, Se) solar cell characteristics define in accordance with the buffer layer valence band offset (VBO) and conduction band offset (CBO).
According to Fig. 6, increment in VBO leads to PCE enhancement because hole particles at point C (E V -High) can easily flow from the buffer to the absorber without spending additional energy. An increment in opencircuit voltage (V OC ), short-circuit current density (J sc ), and Fill Factor (%), as well as the PCE, would be the outcome. On the other hand, the blue hole particle at point B (E V -Low) represents trapped in the well passing needs a considerable amount of energy, and it is impossible for a hole to gain this high amount of energy so, the consequence would be a reduction in parameters and PCE 52,53 . Functional parameters for two selected points are gathered in Table 1. The electron affinity of the electron transport layer should be more than the absorber's because, in this case, CBO becomes higher. As a result, the electron can easily cross the electron transport layer to enhance the opencircuit voltage (V OC ) and short-circuit current density (J sc ). Higher V OC and J SC lead to lower recombination of photo-generated carriers and thus enhances the PCE.

Examination of the donor density (N D ).
Based on the Fermi level (E fn ) definition, its position fixes the charge carrier concentration, and higher-level leads to higher short-circuit current density (J sc ) as well as the efficiency. As can be seen in Fig. 8, E fn -high (Red line) depicts a higher Fermi level, which gradually decreases by changing the position. According to the E fn -high, the cell records its highest J sc and PCE at N D = 10 12 cm −3 . It should be stated that from N D = 10 12 to 10 17 cm −3 , all the PV parameters show a constant value, but after N D = 10 17 cm −3 , the open-circuit voltage (V OC ) remains constant, and Fill Factor (FF) changes is not so effectual. The only drastic change is in short-circuit current density (J sc ), which directly affects the PCE.
Examination of the electron mobility (µ e ). Investigation of the electron mobility (µ e ) is the next step ( Fig. 9), which has been done in the range of 10 −6 -10 3 cm 2 /V s. As a critical point, more electrons respond to the electric field rather than the holes due to the electron mobility increment (µ e ). The increment of electron mobility (µe) helps the buffer layer catch more electrons from the conduction band. Therefore, the effect of the majority carrier denied, which increases the recombination at the back contact. Consequently, a better flow of electrons between the buffer/absorber interface causes an increment in the PCE. This could be sensible based on the following electric field graphs and integrations (electrostatic potentials); equation (Eqs. 5 and 6): Examination of the thickness (μm). As can be seen in Fig. 10, in the range of 10 −3 -10 −1 μm (optimum depletion width), the cell parameters remain constant, as well as the PCE. By increasing the buffer thickness (reducing the depletion width), photon aggregation happens, and this significantly increases the recombination rate, which is detrimental for the cell performance because the short-circuit current density (J sc ) goes to zero; subsequently, all the PV parameters become zero.
Examination of the effective density of the conduction band states (N C ). The CBT (S, Se) is a p-type semiconductor, and according to its definition, for a p-type semiconductor, holes are the major charge carriers. Moreover, for a p-type semiconductor, the Fermi level (E f ) position locates very close to the top of the valence band. According to Fig. 11, the effective density of the conduction band states (N C ) can influence the cell performance in the range of 10 12 -10 17 cm −3 , but it has a negligible effect on cell parameters after a certain value (10 17 cm −3 ).  008 1.017 1.026 1.035 1.044 1.053 1.062 1.071 1.080 1   www.nature.com/scientificreports/ Examination of the absorption coefficient constant (α). Figure 12 depicts that the recombination rate increases due to the absorption increment, and as can be seen, the increment of the absorption deteriorated the cell performance.

Examination of the interface density (N t ).
On the report of Fig. 13, higher N t-interface records higher recombination current, and it would be detrimental for the cell parameters and dramatically reduces the device performance.

Conclusion
This paper is a device modeling study to evaluate In 2 S 3 thin films as a possible electron transport layer in CBT(S, Se) solar cells. We deposited In 2 S 3 layers by low-temperature spray pyrolysis and characterized the films to obtain valid input values for modeling the device. The physical and electrical properties of In 2 S 3 layers have been investigated. There is a good correlation between the results obtained from different characterization techniques. In 2 S 3 thin films obtained revealed good transparency in the spectral range 300-1100 nm. The value of the indirect bandgap is measured as 2.11 eV. In 2 S 3 films show a tetragonal ordering structure of β-In 2 S 3 . The FESEM analysis confirms that films have a homogenous, crack-free dense microstructure that covers the entire substrates perfectly. In addition, all the films have an n-type conductivity. The modeling results show that In 2 S 3 is a promising material with electronic properties similar to CdS. In 2 S 3 is an attractive material, and we believe that this material has a high potential for being optimized as an n-type buffer layer for CBT (S, Se) solar cells, as well as other types of solar cells. In the proposed optimized structure, it can be found that good buffer layer candidates for CBT (S, Se) solar cells.   www.nature.com/scientificreports/