Numerical Modelling Analysis for Carrier Concentration Level Optimization of CdTe Heterojunction Thin Film–Based Solar Cell with Different Non-Toxic Metal Chalcogenide Buffer Layers Replacements: Using SCAPS-1D Software

Cadmium telluride (CdTe), a metallic dichalcogenide material, has been utilized as an absorber layer for thin film-based solar cells with appropriate configurations, and the SCAPS-1D structures program has been used to evaluate the results. In both known and developing thin film photovoltaic systems, a CdS thin film buffer layer has been frequently employed as a traditional n-type heterojunction partner. In this study, numerical simulation was used to find a suitable non-toxic material for the buffer layer instead of CdS, among various types of buffer layers (ZnSe, ZnO, ZnS, and In2S3), and carrier concentrations for the absorber layer (NA) and buffer layer (ND) were varied to determine the optimal simulation parameters. carrier concentrations (NA from 2 x 10 cm to 2 x 10 cm and ND from 1 x 10 cm to 1 x 10 cm) have been differed. The results showed that the CdS as buffer layer based CdTe absorber layer solar cell has the highest efficiency (η%) of 17.43%. Furthermore, high conversion efficiencies of 17.42% and 16.27% have been found for ZnSe and ZnO based buffer layers, respectively. As a result, ZnO and ZnSe are potential candidates for replacing the CdS buffer layer in thin-film solar cells. Here, the absorber (CdTe) and buffer (ZnSe) layers were chosen to improve the efficiency by finding the optimal density of the carrier concentration (acceptor and donor). The simulation findings above provide helpful recommendations for fabricating high-efficiency metal oxide-based solar cells in the lab.


Introduction
The challenge of global warming has prompted further study of solar and other renewable energy sources. Solar cells are a fundamental component of solar energy. Different materials are used to make solar cells, with silicon being the most commercially feasible and prevalent. The majority of the alternative materials were developed with the goal of producing low-cost, high-efficiency, and long-lasting solar cells. Although the efficiency is still modest, Nanostructured Metal Oxide Solar Cells have moved a step farther in delivering clean, cheap, and sustainable solar cells [1]. Solar energy conversion to useable power using a solid-state pn-junction based photovoltaic (PV) device offers enormous promise in the effort to reduce our current reliance on fossil fuels and, as a result, reduce harmful greenhouse gas emissions [2,3].
Due to its unique properties, Cadmium Telluride (CdTe) thin film is widely employed in a variety of optical and electrical applications. CdTe thin film cells are gaining popularity because of their abundant, excellent efficiency, long-term stability, and low cost of manufacture [4], such as nanodevices, sensors, and solar cells [5]. CdTe is classified as an II-VI transition metallic dichalcogenide with a high absorption coefficient (> 10 ) that is greater than other known semiconductor materials with a narrow band gap ( ~1.5 ) [6,7]. This band gap value is suitable for the visible solar light spectrum [8][9][10].
CdTe, in the form of p-type semiconductors, is a potential absorbing material for thin-film PV technology [11]. Despite the widespread usage of CdTe thin films, its primary form has a low conversion efficiency in PEC procedures. When electrodeposited on Ni substrates, CdTe thin films have poor conversion efficiency, depending on the redox couplings and the type of conduction utilized [12]. PEC performance was also poor when CdTe thin films were formed on FTO or ITO substrates [13,14]. When CdTe films were deposited by spray pyrolysis [15], the conversion efficiency was 3.4 %, whereas chemical bath formed films by treatment with CdCl2 had a conversion efficiency of 2.5 % [16][17][18]. CdTe thin film has been reported to have a conversion efficiency of 17.5% or more under certain circumstances [19]. To increase low PEC performance, CdTe thin films are frequently combined with other systems, such as CdS films [13,20]. Cadmium sulfide (CdS) is a well-known II-VI compound semiconductor with excellent transparency, a straight band gap transition (Eg~2.4 eV), strong electron affinity (~ 4.2 eV), and n-type conductivity [21,22]. CdS also enhances the interface fit of lattice heterojunctions, increases the surplus carrier lifetime, and optimizes the band alignment of the devices in which it is utilized [23]. The optical, electrical, and structural properties of CdS films are useful in a wide variety of scientific, technical, and commercial applications involving optoelectronic devices, particularly solar cells [24]. Due to its properties of low surface recombination and little absorption loss, CdS is a promising option for use as a buffer layer in CdTe thin film-based solar cells.
Cadmium (Cd), on the other hand, is a metal that, due to its high toxicity, can be hazardous to the environment and human health. Different materials with a larger band gap, as well as non-toxic compounds like ZnS (O,OH) and ZnS, have been studied as suitable buffer layers for thin film solar cells [25][26][27].
However, because of the complex reaction mechanism and light soaking effects of these buffer layers, cell durability and repeatability may be compromised [28].
CdS/CdTe thin films produced on ITO substrates had a conversion efficiency of 3.5%, and when silver (Ag) was coated on the films, the conversion efficiency increased to 9.82% [29]. Multijunction CdTe/CdS combinations Conversion efficiencies of 13% have also been recorded [30].
Multi-junction CdS/CdTe/ZnTe/ZnTe:Cu cells have a high conversion efficiency of 13.38% [31,32]. The efficiency of the CdS/CdTe:Cu/CNT structure has been reported to be up to 14.1% [33].
The buffer layer connects the absorber and window layers, and it's important for a variety of reasons, including providing structural stability for the thin film and preventing static electricity in the absorber layer [34,35].
In heterojunction thin-film solar cells, the buffer layer generally serves as a focus point. The photons that reach the absorption layer through the reach-in layer travel via the buffer layer. As a result, the number of photons lost due to absorption in the buffer layer should be kept to a minimum. As a result, in the buffer layer, electrical resistance and minimal surface recombination are required. In order to provide the buffer layer between the absorber layer and the transparent window layer, it is necessary to provide thin-film solar cell stability. As a result, the buffer layer Green and less dangerous chemicals (such as ZnS, ZnSe, ZnO, Zn1-xMgxO, and In2S3) should be studied and assessed as a substitute for the traditional hazardous semiconductors used in heterojunction thin-film solar cells [36]. Numerical simulations may be used to investigate the influence of various materials on the final properties of solar cells. The results of such numerical research and analyses can be utilized to improve the device's performance [35,[37][38][39]. The optimum and best structure of thin film-based solar cells is determined by numerical modeling.
There is currently a scarcity of thin-film solar cell simulation research. As a result, we have narrowed the scope of our numerical simulation in this work by utilizing SCAPS-1D software to investigate the needed material for the buffer layer and substituting the CdS with another material.
A different buffer layer's effect on cell performance has been investigated. Different buffer layer materials (CdS, ZnO, ZnSe, In2S3, ZnS) have been shown to exhibit J-V characteristics (Voc, Jsc, FF%, and η%) under standard illumination AM1.5G, 100 mW/cm 2 , 300K. ( Table 1). The primary goal of this research is to replace CdS with a different buffer material. Furthermore, the concentration densities of carriers (acceptor and donor) have been considered in this study.

Numerical Modeling and Material Parameters
SCAPS -1D was created at ELIS, University of Ghent, and it may be used for free in photovoltaic research investigations [40,41]. The SCAPS-1D structures program is frequently used to model Where is electrostatic potential, is elementary charge, is relative permittivity and is vacuum permittivity, is hole concentration, is electron concentration, , are donor and acceptor charge concentrations respectively, and are holes and electrons distribution, respectively.
Also continuity equation (Eq. 2) [43]: Where and are hole and electron current densities respectively, and G are recombination rates respectively.
Carrier transport occurs by drift and diffusion according to Eq. (3) and Eq. (4), respectively: Where, is potential difference, and are electron and hole diffusion constant, respectively. and are electron and hole mobility, and are electron and hole carrier concentration.
2.2. The suggested thin-film solar cell device structure
Where q represents the fundamental electrical charge, ( ) represents photogenerated current, and ( ) represents photon flux. On the light spectrum, Fig. 4 depicts external quantum efficiency QE % for various buffer layers. The results reveal that when the buffer layer is CdS, the efficiency is at its peak [63].The impact of the different buffers on the light spectrum might be seen in the

Modelling and optimization of CdTe absorber layer doping level
The acceptor carrier concentration (NA) of the absorber (CdTe) has varied between 2 x 10 12 and 2 x 10 17 , as indicated in Table 5. The main objective of this study is to maximize the carrier concentration (NA) of the CdTe absorber layer while maintaining cell performance. The electrical parameter performance with acceptor (hole) carrier charge concentration (NA) at 2000 nm thickness of the CdTe absorber is shown in Fig. 5 (a to d). Fig. 5

(b) depicts a linear reduction in
short-circuit current density (JSC) with ( > 2 × 10 ), this can be ascribed to an increase in free carrier charge recombination inside the bulk [64]. On the other hand, fill factor (FF%) as shown in Fig. 5 (c), increases linearly with ( > 2 × 10 ). Fig. 5 (d) also demonstrates that a low hole doping level ( < 2 × 10 ) leads to a significant reduction in device conversion efficiency, with values of less than 3%. When the hole concentration of the absorber layer increases, however, minor cell efficiency changes are found, as shown by Eq. (6), Eq. (7), Eq. (8), and Eq. (9): Where q denotes elementary charge, ∅ denotes spectral power density, ( ) denotes optical transmission, and ∆ denotes the distance between two adjacent wavelength values.
= ln ( + 1) The improved efficiency (Fig. 5 (d)) in the simulated findings is explained by the combined impact of current density JSC saturation ( Fig. 5 (b)), as well as the rapid increase of VOC and FF% ( Fig. 5 (a) and Fig. 5 (c)) with acceptor carrier charge concentration (NA). As a result, (NA ~ 2 x10 15 cm -3 ) provides the best performance for the CdTe thin film. The effect of changes in CdTe acceptor charge carrier concentration (NA) on solar cell fundamental characteristics was thoroughly explored. The thin film's spectral response to the CdTe acceptor carrier charge concentration (NA) (Fig. 6). The simulated findings show that when the acceptor concentration increases from 2 x 10 12 cm -3 to 2 x 10 19 cm -3 ., the quantum efficiency (QE%) decreases. The enhanced gathering of photons at longer wavelengths can be ascribed to this. The production of additional pairs of electron holes in the thin-film solar cell has resulted from the absorption of longer wavelength photons, resulting in an increase in JSC at low acceptor charge carrier concentration (NA) (Fig. 7). The J-V curves show that the VOC increases as the acceptor charge carrier concentration (NA) of the CdTe layer increases ( > 2 × 10 ). This rise shows that the open-circuit voltage (VOC) of the CdTe layer is substantially influenced by the acceptor charge carrier concentration (NA). The generated electric field in the depletion region is altered when the acceptor (hole) carrier charge concentration (NA) of the CdTe layer is high [65].
As a result, the free charge carrier recombination decreased, increasing the VOC. While lowering the CdTe acceptor carrier charge concentration below 2 x10 15 cm -3 results in increased optical losses, which might be due to surface recombination at the back contact [66].

Modelling and optimization of ZnSe buffer layer doping level
The major goal of this section is to decrease the buffer layer's losses (both optical and electrical).
Following that, the carrier charge concentration level of the ZnSe layer was adjusted from 1 x 10 16 cm -3 to 1 x 10 22 cm -3 . The effect of the ZnSe buffer on thin film performance characteristics is shown in Table 6 and Fig. 8. With ( > 1 × 10 ), the simulated results show that the modification is a tiny bit in cell performance characteristics. The effectiveness of the thin film improves by 3% when the donor concentration is increased to ( = 1 × 10 ). It is better to have a high doping level in thin film to retain its exceptional overall performance [68]. The maximum conversion efficiency is 17.42% when the donor carrier charge concentration ( ~1 × 10 ) is used.   performance. This is owing to the apparent requirement for a minimum buffer layer thickness to compensate for the dislocation effect caused by the grid mismatch between the ZnSe and CdTe layers. Although the , % and η% parameters all rise ( Fig. 8 (b-d)), the drops ( Fig. 8 (a)).
The explanation for this may be ascribed to photon loss on a large buffer layer, as seen in Fig. 9.
As the concentration of the buffer layer (ND) decreases, more incident photons generated by the ZnSe layer are absorbed, reducing the number of photons that the absorber (CdTe) layer can absorb. As illustrated in Fig. 10, absorbed photons generate fewer electron-hole pairs, resulting in a lower QE%. As the donor carrier charge concentrations increase, so does the QE %. In the simulation, it is better to have a high buffer layer donor concentration ( > 1 × 10 ) for thin films.

Band diagram:
One of the most notable factors impacting thin film performance and current transmission across heterojunctions is band alignment. Fig. 11 illustrates our findings. There is a good band, as can be seen. CdTe is used as the absorber layer, with ZnSe as the buffer layer and SnO2 as the window layer. Fig. 11. shows the CdTe absorber layer from 0 to 2 μm, the ZnSe buffer layer from 2 μm to 2.025 μm, and the SnO2 window layer from 2.025 μm to 2.275 μm. When the absorber layer's conduction band is higher than the buffer layer's conduction band, the result is a "cliff" type band alignment [69]. As seen in Fig. 11, this is the situation with CdTe thin films. It can be shown that the absorber, buffer, and window layers have acceptable band alignment. Four recombination regions may be seen in the band diagram. Reasonable neutral interface defects for recombination were also included at the mid-gap to accommodate for recombination at the CdTe/CdS and ZnSe/SnO2 interfaces [70]. The reflectance of the rear and front contact surfaces was adjusted to 0.1 and 0.9, respectively ( Table 3). Photons that traverse the absorber are reflected by this high reflectivity upon return contact, which improves absorption in the absorber.

Current mode:
The  Figure 12 shows the output cell efficiency parameters. The carrier concentration of the absorber layer / buffer layer interface recombination or the absorber/back contact was measured using this advantage [71].

Quantum Efficiency ( %):
The optimal QE% for the thin film is shown in Fig. 13. The ratio of the number of captured electrons to the number of incident photons on the solar cell is known as the QE%. The QE% will be 100% when all of the carriers have been gathered and all of the photons have been absorbed by CdTe.
Photons (ℎ ≥ ). are absorbed by the absorber layer. Because the absorption layer cannot absorb low-energy photons, high-energy photons are able to contribute to the thermalization process, resulting in a variety of losses such as shading losses, spectral mismatch losses, shading losses, incomplete absorption, and collection losses, all of which reduce quantum efficiency [67]. SnO2 was employed as the TCO in our situation. SnO2 has a bandgap of 3.6 eV, which is sufficient to cover the whole visible wavelength range. SnO2 has a thickness of 250 nm and a donor concentration of 1 × 10 22 cm -3 , respectively. The window layer contributes a little to the production of electron-hole pairs. Table 7 shows that the proposed work outperforms the recent published studies in terms of open circuit voltage (V ), shot circuit current ( ), and conversion efficiency ( %) of the cell construction. The proposed cell structure Glass/Mo/CdTe/ZnSe/SnO2 outperforms others due to high and V , which result in higher conversion efficiency. The low FF value might be related to defect states in any of the device's layers. If the proposed cell structure can be effectively manufactured, this design method will become the superior option.

Conclusions
In this article, from a numerical simulation standpoint, it employs several buffer layers (CdS, ZnSe, ZnS, In2S3, ZnO), and the outcome indicates that CdS is the best buffer layer. Thus, it can be stated that ZnSe and ZnO are a good option as an alternate buffer layer to the CdS of CdTe solar cells, considering the findings from the simulation using SSAPS-1D. Also, the appropriate material for the CdS buffer layer must be changed. Furthermore, numerical simulation analysis has shown that the rise in NA and ND results in an increase in solar cell performance. The effect on cell performance was also studied via the ZnSe buffer layer. Our analysis also showed that it can be