Effect of Adding Cu₂O as a Back Surface Field Layer on the Performance of Copper Manganese Tin Sulfide Solar Cells

Abstract

One of the major limitations causing deadlock in solar cells with higher sulfur content in the photovoltaic absorber material is the unintended formation of an uncontrollable MoS₂ layer between the absorber material and Mo back contact, which can affect negatively the efficiency of solar cells. Researchers reported that it is very difficult to control the MoS₂ properties such as the conductivity type, thickness, band gap, and carrier concentration in experiments. Considering these challenges, an initial step involved a thorough examination utilizing the one-dimensional solar cell capacitance simulator (SCAPS-1D) to assess the impact of n-MoS₂ interlayer thickness and donor concentration on the performance of CMTS solar cells. Our investigation revealed the formation of a “cliff-like CBO” at the CMTS/n-MoS₂ interface, facilitating the transport of electrons from the p-CMTS absorber to the Mo back contact, resulting in a significantly higher recombination rate. Subsequently, herein a novel approach is proposed, using Cu₂O as a back surface field (BSF) layer due to its low cost, intrinsic p-type properties, and non-toxic nature. Simulation results of a novel heterostructure (Mo/Cu₂O/CMTS/CdS/i-ZnO/AZO/Al) of the CMTS-based solar cell are discussed in terms of recombination rate and conduction band alignment at the absorber/BSF interface. A desired “spike-like CBO” is formed between CMTS/Cu₂O, which hinders the transport of electrons to the back contact. By optimizing the physical parameters such as thickness and the doping density of the Cu₂O layer, an efficiency η of 21.78% is achieved, with an open circuit voltage (Voc) of 1.26 V, short-circuit current density (Jsc) of 24.45 mA/cm², and fill factor (FF) of 70.85%. Our simulation results offer a promising research direction to further develop highly efficient and low-cost CMTS solar cells.

1. Introduction

Over the past few years, thin film photovoltaic (PV) solar cell technology has been receiving attention from the scientific community [1,2,3,4,5,6], primarily due to CIGS and CdTe solar cells which exhibit an efficiency of over 20% [7]. Nevertheless, the main development limitations of thin film solar cells based on these materials as an absorber layer are the scarcity of constituent elements (In, Te, and Ga), and the toxicity of Cd. In order to produce low-cost and environmentally friendly solar cells, new materials have been proposed and studied. One of the favorable materials that have been evolving recently is copper zinc tin sulfide (CZTS) due to its low cost, environment-friendly, and earth-abundant properties [8]. Katagiri et al. (1996) reported the first CZTS solar cell with a PCE of 0.66% [9]. The highest CZTS solar cells produced to date have an efficiency of around 13% [10], which is still insufficient and far away from the Shockley–Queisser limit [11]. The reason for this difference is mainly due to the larger open-circuit voltage (V_OC) scarcity related to the formation of impurity phases and inappropriate band alignment [12]. On the other hand, the similarity of the ionic radii of copper (Cu) and zinc (Zn) is responsible for undesirable antisite defects (Zn_Cu and Cu_Zn) in CZTS [12,13].

To address these challenges, several recent efforts have focused on substituting Zn with neighboring transition elements like iron (Fe), cobalt (CO), manganese (Mn), and nickel (Ni) or group IIA elements such as barium (Ba), magnesium (Mg), beryllium (Be), and calcium (Ca) or Groupe IIB elements like cadmium (Cd) and mercury (Hg). Among these alternatives, manganese (Mn) emerges as an especially promising candidate.

Furthermore, copper manganese tin sulfide (CMTS) is expected to be a good absorber material for earth-abundant thin-film solar cells [14] due to its high optical coefficient (>10⁴ cm⁻¹) and direct optical bandgap ranges from 1.3 to 1.6 eV [13]. The photovoltaic effect of thin film based on CMTS solar cells was demonstrated by Prabhakar et al. (2015), and they reported the success of the first thin film solar cell based on CMTS (device area of 0.15 cm²) with the AZO/ZnO/CdS/CMTS/Mo/SLG structure [14]. Despite these advantages, the maximum power conversion efficiency (η) achieved for CMTS-based PV solar cells is around 0.83% which is still not sufficient in comparison with other PV solar cells [13,15]. However, this result is not very surprising considering that the development of this material is relatively recent.

The MoS₂ layer is an inorganic compound formed invariably during the sulfurization process by combining molybdenum and sulfur. It was first noticed by Johnson et al. in 2010 in CZTS-based thin films [16,17]. However, experimentalists have not conclusively reported the MoS₂ electrical behavior [18] and have difficulties controlling the structural and electrical properties of this layer which is created involuntarily.

The formation mechanism of MoS₂ at the CXTS/Mo interface is according to the following equations (X = Zn, Ba, etc.) [19]:

$$\begin{array}{c}2C{u}_{2}XSn{S}_4+Mo \to 2 C{u}_{2}S+2 XS+2 SnS+Mo{S}_2\end{array}$$

(1)

$$\begin{array}{c}Mo+S_2 \to Mo{S}_2\end{array}$$

(2)

Considering these challenges, several groups have inserted an interlayer between the molybdenum and the absorber layer to avoid MoS₂ formation [20,21,22], and they have found that the outputs of thin film solar cells can be enhanced utilizing the back surface field (BSF) layer among the absorber and the Mo contact. Wei Li et al. used ZnO as an intermediate layer to block sulfur diffusion to the molybdenum back contact in CZTS solar cells. An improvement of the optoelectronic characteristics of these cells has been obtained due to the reduction of the thickness of MoS₂, which went from 300 nm to 80 nm thanks to the introduction of ZnO as the BSF layer [20]. In 2021, Sohel et al. enhanced the efficiency of CZTS-based solar cells from 8.55% to 22.03% by introducing p-CZTSe material as a BSF layer sandwiched between the back contact (Mo) and the absorber [21]. In 2020, Ghobadi et al. showed the influence of the addition of diverse BSF layers in Cu₂BaSnSSe₃ PV solar cells that boost the configuration performance. Their results indicate an enhancement in device performance when using the SnS BSF layer, resulting in an increase in efficiency from 5.2% to 7.31% [22]. Many such materials have been utilized as a BSF layer in thin film solar cells and investigated either experimentally or theoretically. Among the used BSF layers, Cu₂O has attracted considerable attention from the scientific community due to its excellent photovoltaic properties, suitable bandgap of 2.2 eV, non-toxic nature, high absorption coefficient, and intrinsic p-type conductivity [23,24]. Moreover, it can be deposited using simple and low-cost techniques of fabrication [25,26].

To the best of our knowledge, there are only three simulation studies about the optimization of CMTS solar cells presented in references [5,27,28]. This paucity of research highlights the novelty and potential importance of our current investigation. Furthermore, it is important to note that research involving the incorporation of Cu₂O as a back surface field (BSF) layer in CMTS devices has not yet been documented in the existing literature.

In the current study, a numerical investigation is conducted on the effect of the thickness and donor concentration of the n-MoS₂ interfacial layer on the photovoltaic parameters, including power conversion efficiency (η), open circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF) of CMTS solar cells. This analysis is carried out using the one-dimensional solar cell capacitance simulator (SCAPS-1D) [29,30]. Considering all the difficulties in controlling n-MoS₂, the main objective of this study is to reveal the prospect of using a novel nontoxic p-type Cu₂O as a back surface field. This innovative approach aims to inhibit MoS₂ formation and enhance the CMTS solar cell’s PV performance. Furthermore, the effect of thickness and doping density of the Cu₂O BSF layer on electrical phenomena like recombination rate, external quantum efficiency (EQE), and band alignment is also studied and analyzed. Finally, comparisons are made with previously reported experimental and theoretical CMTS-based devices. Thus, the outcomes of our simulations point researchers to a potential approach for further developing CMTS-based solar cells that are both extremely efficient and inexpensive.

2. Device Structure and Simulation

The device and simulation methodology used are described in this section. A key element in improving the solar cell’s electrical conductivity is the ZnO:Al transparent conduction oxide layer, which enables effective charge carrier movement. On the other hand, the i-ZnO greater resistive layer is crucial in reducing recombination losses within the device, which boosts overall efficiency. A crucial interface between the absorber and front contact, the CdS buffer layer makes it possible to extract photo-generated carriers quickly and effectively. The remarkable light-absorbing qualities of the copper, manganese, tin, and sulfur-based CMTS absorber layer allow it to efficiently capture a variety of solar photons. The Cu₂O back surface field (BSF) layer also improves charge collection and lessens rear surface recombination, which eventually improves the performance of the cell. Aluminium (Al) and molybdenum (Mo) are used as the back and front metal contacts, respectively, which helps to achieve the correct band alignment and ensures compatibility with the semiconductor layers. Figure 1 shows a precise schematic illustration of this solar cell construction, giving the reader a visual understanding of its layered structure. The SCAPS-1D simulator, a potent numerical tool specially made for the accurate characterization of thin film solar cell structures, was used for the computational study and modeling of this ground-breaking design. The simulator successfully forecasts the performance characteristics of the device using basic semiconductor equations, such as continuity equations for electrons and holes and Poisson’s equation. The program itself can produce efficiency close to the experimental values if the appropriate defect values are entered. This program simulates the mechanisms of light absorption, the generation of excitons, the transfer of charge and assembly, and recombination. Equation (3) defines Poisson’s equation, which explains how the electrostatic potential and charge density distribution relate to one another [31].

$$\frac{d^2}{d{x}^2}\psi \left(x\right)=\frac{q}{{\epsilon }_0{\epsilon }_r}[p\left(x\right)-n\left(x\right)+N_D-N_A+{\rho }_p-{\rho }_n$$

(3)

where $\psi$ signifies electric potential, ${\epsilon }_0$ signifies permittivity of free space, ${\epsilon }_r$ signifies relative permittivity, $N_D$ signifies donor density, ${\rho }_p$ signifies hole charge density, $N_A$ signifies acceptor density, ${\rho }_n$ signifies electron charge density, and $q$ signifies electronic charge.

Equations (4) and (5) depict the continuity equations for electrons and holes accordingly.

$$\frac{dn}{dt}=\frac{1}{q}\frac{\partial J_n}{\partial x}+\left(G_n-R_n\right)$$

(4)

$$\frac{dp}{dt}=\frac{1}{q}\frac{\partial J_p}{\partial x}+\left(G_p-R_p\right)$$

(5)

where $J_n$ signifies current densities of electrons, $G_n$ signifies electron generation rate, $J_p$ signifies current densities for holes, $G_p$ signifies hole generation rate, $R_n$ signifies the recombination rate for electrons, and $R_p$ signifies the recombination rate for a hole.

The computation of both the electron current density and the hole current density involves the utilization of the charge carrier drift-diffusion equation, which is given in Equations (6) and (7).

$$J_n=q{\mu }_{n}n\in +q{D}_n{\partial }_n$$

(6)

$$J_p=q{\mu }_{p}p\in +q{D}_p{\partial }_p$$

(7)

where $D_n$ signifies the electron’s diffusion coefficient, ${\mu }_n$ signifies the mobility of electrons, $D_p$ signifies the hole’s diffusion coefficient, and ${\mu }_p$ signifies the mobility of the hole.

Table 1. Features set for simulation of CMTS-based solar cells [22,28,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51].

Features	p-Cu2O	n-MoS2	p-CMTS	n-CdS	i-ZnO	n-AZO
Thickness (µm)	0.1–0.6	0.1–0.6	1.5	0.1	0.05	0.2
Eg (eV)	2.2	1.3	1.6	2.42	3.3	3.6
εr	7.1	13.6	9	8.25	9	9
X (eV)	3.2	4.5	4.35	4.5	4.4	4.6
NC (cm−3)	2.5 × 1018	7.5 × 1017	2.2 × 1018	3.1 × 1018	2.2 × 1018	2.2 × 1018
NV (cm−3)	1.5 × 1019	1.8 × 1018	1.8 × 1019	1.8 × 1019	1.8 × 1019	1.8 × 1019
μn (cm2 V−1 s−1)	1.0 × 102	1.0 × 102	1.6 × 10−1	1.0 × 102	1.0 × 102	1.0 × 102
μh (cm2 V−1 s−1)	0.8 × 102	1.5 × 102	1.6 × 10−1	2.5 × 101	2.5 × 101	2.5 × 101
Electron thermal velocity (cm/s)	1.0 × 107	1.0 × 107	1.0 × 107	2.3 × 107	1.0 × 107	1.0 × 107
Hole thermal velocity (cm/s)	1.0 × 107	1.0 × 107	1.0 × 107	1.4 × 107	1.0 × 107	1.0 × 107
ND (cm−3)	0	1.0 × 1015–1.0 × 1019	0	1.0 × 1019	1.0 × 1018	1.0 × 1020
NA (cm−3)	1.0 × 1015–1.0 × 1019		1.0 × 1016	0	0	0
Nt (cm−3)	1.0 × 1015	1.0 × 1015	1.0 × 1013	1.0 × 1016	1.0 × 1014	1.0 × 1014

lists the precise simulation parameters used in this investigation for comprehensive reference. The solar cell was exposed to illumination from the front contact using the global Air Mass 1.5 spectrum at a dose of 1000 W/m². The uniformity and comparability of the evaluation of the cell’s effectiveness and performance across diverse research and applications is ensured by the standard testing procedure.

3. Results and Discussion

The schematic device structure is shown in Figure 2a, and the corresponding energy band diagram is shown in Figure 2b. The CMTS solar cell with the structure Mo/n-MoS₂/CMTS/CdS/i-ZnO/AZO/Al (Figure 2a) is first simulated and optimized to act as a reference device. The n-MoS₂ layer will be examined first to analyze its impact on the cell’s performance. This layer forms spontaneously during the deposition of the CMTS absorber layer on molybdenum. In fact, the sulfur present in the CMTS diffuses towards the molybdenum to create a semiconductor layer of MoS₂.

It is observed from the energy band diagram (Figure 2b) that the conduction band offset at the n-MoS₂/CMTS interface is negative (cliff-like CBO). This is considered an undesirable band alignment as it facilitates the flow of electrons from CMTS to the rear contact, which is responsible for hole collection, thereby leading to a loss of efficiency of the cell.

3.1. Investigation of the n-MoS₂ Layer in the CMTS Solar Cell

MoS₂ thin films have been reported to exhibit both n- or p-type conductivity. However, the experimental results reported in the literature mentioned that most of the MoS₂ layers formed in CZTS solar cells are n-type due to the presence of native defects of sulfur vacancies. These defects act as deep donors introducing excess electrons into the material, thus leading to n-type behavior [18,52,53,54]. In this simulation study, it was assumed that the MoS₂ has n-type conductivity. This phenomenon is important to understand, particularly in the context of thin-film solar cells, because the conductivity type and properties of the MoS₂ layers play a critical role in the overall performance of the device.

3.1.1. Effect of n-MoS₂ Interlayer Thickness on the Performance of the Studied Cell

It is well acknowledged that the thickness of the interfacial MoS₂ layer is considered a critical point for device optimization. Thus, the consequence of n-MoS₂ interfacial layer thickness on cell PV output was studied in the range of 0.1 to 0.6 µm.

As can be seen in Figure 3, the open circuit voltage (V_OC), the fill factor (FF), the short-circuit current (J_SC), and the efficiency (η) increase greatly from 0.663 V to 0.751 V, 75.63% to 77.78%, 23.22 mA/cm² to 23.32 mA/cm², and 11.64% to 13.62%, correspondingly, when the thickness of n-MoS₂ increases from 0.1 to 0.3 µm. However, when the thickness of n-MoS₂ is further increased, the recombination rate becomes more important, resulting in the decline of cell outputs (Figure 4). Generally, when the n-MoS₂ thickness increases, the photogenerated holes cannot cross easily from p-CMTS to the Mo contact to be collected, and hence the hole current is reduced. Thus, the best outputs of the CMTS-based PV solar cell are obtained while the thickness of n-MoS₂ is 0.3 µm.

3.1.2. Effect of n-MoS₂ Interlayer Donor Concentration on the Performance of the Studied Cell

To study the impact of the donor density (N_D) on the key parameters of the PV solar cell, N_D is changed from 10¹⁵ to 10¹⁹ cm⁻³, while keeping the n-MoS₂ thickness at 0.3 µm as it is the optimal value. As shown in Figure 5, all the key PV parameters like V_OC, J_SC, FF, and η decrease steadily as N_D increases. These parameters decrease from 0.751 V to 0.121 V, 23.32 mA/cm² to 14.85 mA/cm², 77.78% to 37.86%, and 13.62% to 0.68%, respectively, in response to higher N_D values. Higher N_D in MoS₂ creates undesirable electric fields at the CMTS/MoS₂ interface that restrict the movement of light-generated charge carriers and elevate the carrier recombination by increasing the interface resistance. A similar trend of decrease in PV parameters with an increasing donor concentration of MoS₂ is reported in ref. [42].

To justify these results, the recombination rate has been plotted for two different doping concentrations of the n-MoS₂ layer (Figure 6). In the heavily doped n-MoS₂, large carrier traps are generated, resulting in higher recombination. On another side, the CMTS conduction band energy is higher than that of MoS₂, and this forms a cliff-like band alignment in the n-MoS₂/p-CMTS interface (Figure 2b), which results in a significantly higher recombination rate. In other words, the easy transfer of electrons from p-CMTS to n-MoS₂ increases the electron-hole pair recombination at the n-MoS₂ layer. Figure 6 also shows a higher recombination rate in CMST near the n-MoS₂/p-CMTS interface, which validates the accumulation of light-generated charge carriers and their recombination within the vicinity of the interface.

The best efficiency η is 13.62% when the n-MoS₂ donor concentration is 10¹⁵ cm⁻³ and the thickness is equal to 0.3 µm.

Table 2. The beneficial effect of using the BSF layer in PV solar cells.

	Absorber	BSF	η without BSF	η with BSF	References
E	Si	Al	12.96	13.75	[55]
E	Si	ZnS	6.40	11.02	[56]
E	CIGS	MoSe2	9	14	[57]
S	CZTS	CZTSe	8.55	22.03	[21]
S	CIGS	BaSi2	19.71	26.24	[32]
S	CdTe	Cu2Te	14.87	19.06	[58]
S	CBT(S, Se3)	SnS	5.23	7.31	[22]

Note: E = Experimental, S = Simulated.

summarizes the advantageous consequence of utilizing the BSF layer in solar cell technology.

3.2. Impact of the Addition of a Cu₂O-Based Back Surface Field (BSF) on the Cell’s Performance

The presence of the BSF layer is crucial in preventing the movement of charge carriers in undesirable directions and preventing interfacial recombination losses. The BSF layer creates an appropriate electric field that acts as an electrical mirror of the charge carrier and pushes the carrier in a backward direction. This phenomenon significantly reduces the recombination rate and elevates the performance of the device. The device under consideration utilizes a Cu₂O-based BSF; for better understanding of carrier dynamics within the device, the energy band diagram is obtained and reported in Figure 7.

In order to prevent the spontaneous formation of the n-MoS₂ layer, we suggest the incorporation of a Cu₂O layer as a back surface field (BSF) on the molybdenum. Figure 7 shows the band alignment of the device by adding Cu₂O as a BSF layer to help understand how the incorporation of a Cu₂O BSF layer improves PV performance. It is clear from this diagram that by adding the Cu₂O layer to the cell structure, the obtained performance is higher due to the large conduction band offset (CBO) spike at the CMTS/Cu₂O interface. This spike allows the minority carriers (electrons) at p-CMTS to be backscattered towards the front contact of the cell to be collected, thus preventing them from recombining at the rear face of the device.

3.2.1. Effect of Cu₂O BSF Layer Thickness

It is also important to understand the effect of BSF layer thickness on the performance of the device. Therefore, in this section, the outcome of BSF thickness on cell PV output (V_OC, J_SC, FF, and η) has been considered in the range of 0.1 to 0.6 µm and presented in Figure 8. It can be noted that both V_OC and J_SC remain constant at 1.26 V and 24.41 mA/cm², respectively, while the FF and η decline steadily with increasing the Cu₂O BSF layer thickness.

The η goes from 19.53% to 19.17% and shows a clear similarity to the trend seen in the FF (63.67% to 62.49%) curve, so the main factor contributing to the reduction in efficiency is connected with a huge decline in FF. The decline in FF with the increase in Cu₂O thickness goes back to the introduction of a resistive component in the PV device. With a thicker BSF, the photogenerated holes must cross distances greater than their diffusion length; hence, it causes a higher recombination rate (Figure 9). In this context, the optimal thickness for the Cu₂O BSF was set at 0.1 µm. This specific thickness was chosen after a rigorous evaluation of various thicknesses to achieve the best performance for the solar cell.

3.2.2. Influence of Cu₂O BSF Layer Acceptor Concentration

Acceptor doping in the BSF is one of the crucial parameters that affect the appropriate function of the device as it effectively determines the overall resistive losses in the BSF and the strength of the electric field at the interface. Therefore, in order to analyze the influence of the acceptor concentration density (N_A) within the Cu₂O back surface field layer, simulations covering a wide range from 10¹⁵ to 10¹⁹ cm⁻³ were carried out. The findings from this analysis are visually depicted in Figure 10, showing the correlation between acceptor concentration density and the key performance parameters of the investigated solar cell.

From Figure 10, an increase in Jsc, FF, and η is observed from 24.41 mA/cm² to 24.45 mA/cm², 63.67% to 70.85%, and 19.53% to 21.78%, respectively. During the optimization, the V_OC remained unchanged, while N_A changed from 10¹⁵ to 10¹⁹ cm⁻³. A carrier concentration gradient in the Cu₂O/CMTS interface produces the built-in potential which blocks the minority carrier flow toward the interface. The observed increase in efficiency can be attributed to the enhancement of the electric field strength at the interface between the Cu₂O and CMTS layers as a consequence of increasing the doping concentration in the Cu₂O layer. This intensified electric field leads to improved carrier collection efficiency within the solar cell structure. As a result, a higher proportion of generated charge carriers is effectively harvested, contributing to the overall enhancement of the solar cell’s efficiency. This effect underscores the critical role that optimizing the doping concentration of the Cu₂O layer plays in augmenting the performance of the photovoltaic device. Finally, after an evaluation of various acceptor concentration densities, the decision was made to optimize the value of N_A at 10¹⁹ cm⁻³ This choice was based on the superior photovoltaic (PV) performance characteristics observed at this concentration level, signifying its potential to yield the most favorable outcomes for the solar cell.

External quantum efficiency (EQE) is defined as the ratio of the number of carriers generated in the solar cell by an incident photon at a given wavelength. The optical performance of the device is probed using EQE. Three devices, namely CMTS with n-MoS₂ (1 × 10¹⁹ cm⁻³), CMTS with n-MoS₂ (1 × 10¹⁵ cm⁻³), and CMTS with Cu₂O (BSF), are subjected to optical study by simulating the EQE data in the wavelength range until 900 nm. Figure 11 shows the comparison of the external quantum efficiency (EQE) curves for Al/AZO/ZnO/CdS/CMTS/n-MoS₂/Mo and Al/AZO/ZnO/CdS/CMTS/Cu₂O/Mo structures in the wavelength range from 10 nm to 900 nm.

By incorporating the Cu₂O as a BSF layer, good performance has been revealed due to the Cu₂O’s properties, which makes it a good candidate for photovoltaic applications [59,60,61,62]. A similar increase in performance parameters (J_SC, FF, η) with increasing the doping concentration of the BSF layer is reported in refs. [63,64].

From this analysis, it has been found that after adding Cu₂O as a BSF layer, the efficiency of CMTS-based solar cells increases up to 21.78%. A comparison of experimental and theoretical values (provided from the literature) is given in

Table 3. Comparison of the results with other reported works on CMTS-based solar cells.

Type	Device Structure	VOC (V)	JSC (mA/cm²)	FF (%)	η (%)	References
E	Mo/MoS2/CMTS/CdS/i-ZnO/AZO/Al	0.289	1.59	29.9	0.14	[65]
E	Mo/CMTS/CdS/i-ZnO+AZO	0.354	5.8	40	0.83	[15]
S	Mo/CMTS/CdS/i-ZnO/FTO	0.88	24.10	77.90	16.5	[28]
S	Mo/CMTS/CdS/i-ZnO/AZO/Al	1.11	26.26	61.08	17.81	[27]
S	Mo/Cu2O/CMTS/CdS/i-ZnO/AZO/Al	1.26	24.45	70.85	21.78	This work

Note: E = Experimental, S = Simulated.

. The results are found to be in good accord with the previous simulation outputs. It is clear from the data in this table that many challenges remain in the experimental development of CMTS solar cells. One of the main reasons for the observed lower experimental outputs of CMTS-based solar cells without a BSF layer is the formation of uncontrollable n-MoS₂ films, which have unfavorable effects on cell performance.

While our study provides valuable insights into the potential benefits of incorporating Cu₂O in CMTS solar cells, it is essential to acknowledge that our proposal involving Cu₂O as a BSF layer requires experimental validation to assess its feasibility.

4. Conclusions

This paper addresses a critical limitation in the development of CMTS solar cells, which is the formation of a MoS₂ intermediate layer. To illustrate the impact of variations in MoS₂ parameters, a numerical simulation of a reference device with the structure Mo/n-MoS₂/CMTS/CdS/i-ZnO/AZO/Al was performed. Given the uncontrollable parameters of MoS₂, our goal was not to achieve higher performance but to demonstrate how even slight variations in MoS₂ parameters can significantly affect the device’s performance. In order to avoid the adverse effect of the MoS₂ layer, the addition of Cu₂O as a BSF layer between the molybdenum and the CMTS absorber was proposed and studied. The optimal physical properties of this layer (thickness and acceptor concentration) have been identified. According to the optimization, the highest efficiency achieved with a Cu₂O BSF layer is 21.78% for a thickness of 0.1 µm and a doping density of 10¹⁹ cm⁻³, which is much higher than the results obtained previously in the literature. Furthermore, the presence of Cu₂O as a BSF layer provides a desirable spike-like CBO which prevents the easy transfer and the recombination of electrons from the absorber layer to the back contact. In conclusion, the results presented in this study offer significant and enlightening insights that hold great promise for advancing the development of photovoltaic (PV) solar cells based on CMTS materials, with the potential to substantially enhance their efficiency and overall performance. These findings underscore the importance of further research and optimization efforts in harnessing the full potential of CMTS-based PV technology, thus contributing to the ongoing evolution of sustainable and efficient energy generation.