High Efficiency of Cadmium Telluride Thin-Film Solar Cells Achieved Through Front and Back Surface Optimization

Abstract

The global transition to renewable energy has made photovoltaics a potential primary energy source, due to the abundance of solar radiation worldwide and the technological maturity of conversion systems. In this context, cadmium telluride (CdTe) remains one of the leading thin-film absorber materials due to its near-optimal bandgap (~1.5 eV) and high absorption coefficient; however, conventional CdTe-based devices face three factors that limit their efficiency: parasitic absorption in the standard cadmium sulfide (CdS) window layer, non-radiative recombination at the interfaces, and the economically costly requirement for thick absorber layers (3 to 5 µm) given the high cost of CdTe. This work proposes a dual-interface passivation strategy that simultaneously addresses loss mechanisms at both the front and back surfaces. At the front interface, the conventional CdS buffer layer is replaced by a Cd_(1−x)Zn_(x)S alloy, whose tunable bandgap (2.4–3.7 eV) suppresses parasitic absorption in the ultraviolet and blue regions and improves band alignment with CdTe, thereby increasing the short-circuit current and open-circuit voltage. At the rear interface, a CuInTe₂ (CIT) rear surface field layer is introduced between the CdTe absorber and the molybdenum (Mo) rear contact to counteract the Schottky barrier responsible for degrading hole collection. These two modifications, when implemented together, yield efficiency gains greater than those previously reported in the literature, while also enabling a substantial reduction in the thickness of the CdTe absorber to 1 µm. Using one-dimensional drift-diffusion simulations (wxAMPS) under standard AM 1.5G illumination, this study systematically evaluates the influence of the zinc content in the Cd_(1−x)Zn_(x)S buffer layer and the thickness of the CuInTe₂ layer on the key photovoltaic performance metrics (Jsc, Voc, FF, and efficiency), with the aim of identifying the optimal device configuration for high-performance and cost-effective CdTe thin-film solar cells.

1. Introduction

In the context of the global energy transition driven by climate change, photovoltaic energy has emerged as one of the most promising solutions [1,2]. This energy source is no longer considered only as a secondary solution for specific applications, such as off-grid sites [3] or charging systems [4], but as a potentially primary energy source capable of contributing to the stabilization of the electrical grid [5]. This choice is justified by the worldwide abundance of solar radiation [6] and the technological maturity of conversion systems [7]. In this context, research efforts have focused on improving efficiency, most notably through power maximization techniques such as Maximum Power Point Tracking [8], or the use of intelligent approaches for fault detection and performance optimization [9]. Among these research directions, optimizing photovoltaic cell performance during manufacturing constitutes a key approach [10].

Optimization of photovoltaic cell performance at the manufacturing stage depends on multiple factors such as the selection and quality of semiconductor materials, the arrangement of the absorber and buffer layers, and the electrical contacts to improve light absorption and lower recombination of lost carriers [11]. Using buffer layers or Back Surface Field (BSF) layers reduces interfacial recombination and more efficient charge harvesting. On top of that, using novel alloys or semiconductors for the same materials leads to bandgap extension, less parasitic absorption, and better thermal and chemical stability. Altogether, these methods raise the output power, decrease the use of expensive materials, make the photovoltaic system perform better, as well as be more cost-effective.

Cadmium telluride (CdTe) is considered among the thin-film technologies as an outstanding option for the production of high-performance solar cells [12]. The physical properties of CdTe are very favorable, such as a bandgap that almost matches the optimum for solar (~1.5 eV), extremely high optical absorption ability, and good electron mobility, which make it possible to achieve complete light absorption even with very thin absorber layers [12]. However, their efficiency is still constrained by several issues, e.g., parasitic absorption in the usual CdS window layer [13,14], non-radiative interfacial recombinations, and thicker absorbent layers which result in higher production costs [13].

In order to address these issues, numerous optimization methods have been investigated. One particularly interesting study examined the use of an MXene (Ti₂C₃) enriched buffer layer in SnO₂ as a front-side passivation, resulting in an improvement in the fill factor from 82.87% to 84.82% and in efficiency from 21.86% to 22.42%. Other researchers have chosen to optimize the cells by incorporating Mg into the ZnO layer to increase the bandgap to 3.75 eV, thereby improving efficiency [15]. Further optimization studies have focused on passivating the back side of the cell by adding an additional layer between the absorber and the rear contact, such as a BSF, to create a field that repels carriers toward the space charge region. Research on the insertion of a ZnTe- and ZnTe:Cu-based BSF [16], as well as the use of perovskite such as CH₃NH₃SnBr₃ [17], has resulted in significant optimization in terms of efficiency. The studies cited, as well as most previous studies, use standard CdTe thicknesses (3–5 µm), which represents a significant economic loss given the high cost of producing CdTe [18].

The approach of substituting Cadmium Sulphide (CdS) with Cadmium Zinc Sulphide Cd_(1−x)Zn_(x)S alloy allows bandgap expansion and reduces parasitic absorption, which leads to an increase in short-circuit current and Voc (open-circuit voltage). At the same time, the employment of a Back Surface Field (BSF) layer of Copper Indium Ditelluride (CuInTe₂) improves the carrier collection from the rear contact and reduces recombination losses. This combined approach improves electrical performance while reducing the thickness of the CdTe layer to just 1 µm, which both reduces material consumption and maintains very high solar cell conversion efficiencies. Indeed, the integrated optimization of both the buffer and BSF layers therefore represents a rigorous and economically viable pathway toward high-performance CdTe solar cells capable of meeting the demands of modern photovoltaic systems.

In this work, we aimed to:

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Perform front-side passivation by introducing Cd_(1−x)Zn_(x)S: The CdS is known as the conventional window layer, traditionally chosen for its good crystalline compatibility and ease of deposition with CdTe; however, it has a major limitation: its relatively narrow bandgap (~2.4 eV) absorbs a significant portion of blue and ultraviolet photons (λ < 500 nm) [13,14], which directly reduces the short-circuit current density (Jsc) compared to a theoretically transparent window layer [19]. When we replace the CdS with a Cd_(1−x)Zn_(x)S alloy in which the zinc fraction varies from 0 to 1, this substitution widens the bandgap, suppressing parasitic absorption in the 300–500 nm spectral range. Although the reduction in interfacial recombination is recognized as being beneficial for Voc in wide-bandgap buffer-layer heterojunctions [20], our simulations show that, for the structure studied here, which includes a CIT layer at the back contact, Voc remains limited by a mechanism distinct from that at the front interface such that the improvement resulting from the incorporation of Zn is primarily reflected in an increase in Jsc rather than in an increase in Voc [20].

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Perform back-side passivation by introducing a CuInTe₂ layer simultaneously with the front-side passivation: At the rear CdTe/Mo interface, the Schottky barrier arising from the electron affinity of CdTe (~4.3 eV) and the work function of molybdenum (~4.6 eV) impedes hole collection, reducing the fill factor by 5–10% and lowering the open-circuit voltage by 50–100 mV relative to theoretical predictions [21]. To overcome this limitation, a CuInTe₂ (CIT) Back Surface Field layer is inserted between the CdTe absorber and the Mo rear contact. As a chalcopyrite semiconductor with a bandgap of ~1.0 eV and a crystal structure compatible with CdTe, CuInTe₂ establishes a reverse electric field through the bandgap gradient at the rear interface, thereby reducing electron injection barriers, improving hole collection efficiency, and enhancing both the fill factor and the overall device voltage [22,23].

The combined integration of the Cd_(1−x)Zn_(x)S buffer layer and the CuInTe₂ BSF layer addresses two distinct and complementary loss pathways simultaneously: Cd_(1−x)Zn_(x)S primarily mitigates photocurrent losses attributable to parasitic absorption at the front interface, while the CuInTe₂ BSF layer targets rear-contact recombination, improving voltage and fill factor. The synergy between these two optimizations enables simultaneous enhancement of both photocurrent and voltage, yielding an overall cell efficiency that surpasses what either modification could achieve independently.

In addition to these electrical advantages, the proposed double-passivation approach allows the use of an ultra-thin CdTe absorber layer of just 1 µm, significantly thinner than the standard industrial thickness of 3 to 5 µm [18]. Much research has concluded that a CdTe thickness of around 2 µm is sufficient to absorb all visible light, given CdTe’s high absorption coefficient, ranging from 10⁴ to 10⁵ cm⁻¹, and that this thickness is optimal for both optical and electrical performance [12,24].

This significant reduction in the absorber thickness considerably reduces the consumption of raw materials, which provides a decisive economic advantage over conventional CdTe device architectures, given the high production costs of this material. It also reduces the use of cadmium, which is known in the literature as a toxic element [16,25].

In this article, we have investigated the influence of the zinc content in Cd_(1−x)Zn_(x)S and the thickness of the CuInTe₂ layer on the main photovoltaic parameters, short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF) and energy conversion efficiency (EFF), evaluated under standard illumination conditions (AM 1.5G, 1000 W/m², 300 K), using comprehensive physical simulations of the devices with the wxAMPS-1D 3.0 software to identify the optimal configuration combining performance and ultra-thin-film technology. The results obtained using this approach are very interesting, showing a conversion efficiency of around 25.73%, which far exceeds those of other optimization approaches (see Table 1). Table 1 provides a comparative overview of previous work focused on the optimization of CdTe solar cells compared the present work.

Table 1. Comparative performance table with other works.

Works	Optimization Approach	Performance Evaluation
		Jsc (mA/cm2)	Voc (V)	FF (%)	Eff (%)
[15]	Front passivation	22.73	1.05	85.73	20.57
[16]	Back passivation	21.78	0.98	76.43	17.11
[26]	Front passivation	26.20	1.10	88.10	25.43
[27]	Back passivation	26.15	0.92	87.30	21.04
[28]	Front passivation	23.25	0.99	67.03	15.45
[29]	Back passivation	25.45	1.09	66.22	20.56
[30]	Front passivation	30.12	0.96	87.26	25.24
[31]	Back passivation	20.90	0.80	88.45	15.00
Our work	Dual passivation	30.52	0.97	86.70	25.73

Following this theoretical part, we will present a section about simulation methods, in which we will briefly describe the software used (wxAMPS-1D 3.0), as well as the structure under study and its physical and optical parameters. The sections on results and discussion will follow in order to highlight the effect of each variation on the cell’s performance.

2. Simulation Method

wxAMPS-1D 3.0 (Widget-Provided Analysis of Microelectronic and Photonic Structures) is a powerful simulation software designed to model thin-film solar cells and photovoltaic devices. It is an enhanced version of the original AMPS-1D 1.0 (Analysis of Microelectronic and Photonic Structures) software developed by Stephen Fonash and his research group at Pennsylvania State University, USA, in 1997. Later, the software was improved when the University of Illinois at Urbana-Champaign, USA and Nankai University in China worked together to create wxAMPS-1D 3.0 [32,33]. The essence of wxAMPS-1D 3.0 is its implementation of the drift-diffusion model, which relates material properties to device response under various operating conditions, including light exposure, electrical bias, and temperature variations. This one-dimensional, steady-state simulation approach enables researchers to solve the three fundamental equations that govern semiconductor device behavior: Poisson’s and the continuity equations for holes and electrons.

$${\displaystyle \frac{\mathrm{d}}{\mathrm{dx}}}\left(-\mathsf{\epsilon }\left(\mathrm{x}\right){\displaystyle \frac{\mathrm{d}\mathsf{\psi }}{\mathrm{dx}}}\right)=\mathrm{q}\left[p_{\left(x\right)}-n_{\left(x\right)}+{{ND}^{+}}_{\left(x\right)}-{{NA}^{-}}_{\left(x\right)}+{Pt}_{\left(x\right)}-{nt}_{\left(x\right)}\right]$$

(1)

$${\displaystyle \frac{\mathrm{d}{P}_n}{\mathrm{dt}}}=G_p-{\displaystyle \frac{P_n-{P_n}_0}{{\mathsf{\tau }}_p}}-P_n{\mu }_p{\displaystyle \frac{\mathrm{d}\mathsf{\xi }}{\mathrm{dx}}}-{\mu }_p\mathsf{\xi }{\displaystyle \frac{{dP}_n}{d_x}}+\mathrm{Dp}{\displaystyle \frac{{\mathrm{d}}^2{P}_n}{{d_x}^2}}$$

(2)

$${\displaystyle \frac{\mathrm{d}{n}_p}{\mathrm{dt}}}=G_n-{\displaystyle \frac{n_p-{n_p}_0}{{\mathsf{\tau }}_n}}-n_p{\mu }_n{\displaystyle \frac{\mathrm{d}\mathsf{\xi }}{\mathrm{dx}}}-{\mu }_n\mathsf{\xi }{\displaystyle \frac{{dn}_p}{d_x}}+\mathrm{Dp}{\displaystyle \frac{{\mathrm{d}}^2{n}_p}{{d_x}^2}}$$

(3)

where ψ is the electrostatic potential, n and p are the free electron and hole, respectively; nt and pt are the trapped electron and hole, respectively; Nd+ and Na− are the ionized donor and acceptor-like doping concentrations; ξ is the electric field; G is the generation rate; D is the diffusion coefficient, and the electron and hole lifetimes; ε is the dielectric constant; and q is the charge of electron, and the electron and hole mobilities.

The software’s ability to handle complex heterojunction structures makes it invaluable for device analysis such as that proposed here. The drift-diffusion model coupled with tunneling mechanisms provides results that are reliable and comparable to experimental data. A distinguishing feature of wxAMPS-1D 3.0 is its flexibility in defining material parameters. It does not limit the number of layers that can be incorporated into a device structure. Unlike other simulation software, such as SCAPS-1D [14], rather than relying on default values, wxAMPS-1D 3.0 requires users to enter optical parameters, particularly absorption coefficients. This requirement guarantees that simulations are based on data derived from experimental works [34,35,36].

Figure 1 shows the configuration of the simulated structure. The structure consists of a substrate, a back contact, an absorber, a buffer layer, a window layer, and a front contact. First, a layer of molybdenum is sputtered onto glass to form the back contact. Next, a Copper Indium Ditelluride (CIT) P-type booster current layer is added, followed by a CdTe absorber and a Cd_(1−x)Zn_(x)S buffer layer. The latter has a direct bandgap and n-type conductivity. In this simulation, the bandgap and electron affinities of the buffer alloy Cd_(1−x)Zn_(x)S, as well as the thickness of the CuInTe2 layer, are considered variable parameters. The doping density in the buffer layer must exceed that of the absorber by a sufficient amount to confine the space charge region at the absorber, where the electric field helps separate charge carriers [37]. Next, a transparent conductive oxide (TCO) is deposited. Its high direct bandgap of 3.3 eV allows a large number of photons to reach the absorber layer. The TCO constitutes the front contact. A thin, highly resistive ZnO layer is typically positioned between the TCO and the buffer layer.

Figure 1. Schematic view of our CdTe solar cell structure.

Table 2 summarizes the parameters of the different layers used to model the solar cell, taken from literature values and experimental data [38,39,40].

Table 2. Input parameters used in numerical analysis [38,39,40].

Parameters	ZnO:N	ZnO:i	Cd(1−x)Zn(x)S	CdTe	CuInTe2
Thickness (µm)	0.15	0.1	0.02	1	0.02–0.3
Bandgap (eV)	3.3	3.3	2.4–3.7	1.5	1.1
Electron affinity (eV)	4.4	4.4	4.50–4.02	4.28	4.28
Dielectric constant	9	9	9.3	9.4	11
Donor concentration, ND (cm−3)	1 × 1018	1 × 1017	1.5 × 1017	0	0
Acceptor concentration, NA (cm−3)	0	0	0	1 × 1016	1 × 1020
Effective conduction band density, Nc (cm−3)	2.2 × 1018	2.2 × 1018	2.1 × 1018	7.5 × 1017	5.08 × 1016
Effective valence band density, Nv (cm−3)	1.8 × 1019	1.8 × 1019	1.7 × 1019	1.8 × 1019	5.65 × 1018
Electrons mobility µn (cm2·v−1·s−1)	10	15	100–60	320	300
Hole mobility µp (cm2·v−1·s−1)	5	5	25–10	40	20
Defect type	Gaussian	Gaussian	Gaussian	Gaussian	Gaussian
NDG/NAG (cm−3)	1 × 1017/-	1 × 1016/-	-/5 × 1016	1 × 1013/-	2 × 1014/-
EDONG/EACPG (eV)	Mid-gap	Mid-gap	Mid-gap	Mid-gap	Mid-gap
WDSDG/WDSAG (eV)	0.1/-	0.1/-	0.01	0.1	0.1
GSIG-ND/GSIG-NA (cm2)	1 × 10−12/-	1 × 10−12/-	1 × 10−15	1 × 10−12	1 × 10−12
GSIG-PD/GSIG-PA (cm2)	1 × 10−15/-	1 × 10−15/-	5 × 10−13	1 × 10−14	1 × 10−14

3. Results and Discussion

3.1. Effect of Cd_(1−x)Zn_(x)S Buffer Layer

One efficient method for enhancing the performance of CdTe-based solar cells is to incorporate zinc into the CdS buffer layer.

As illustrated in (Figure 2c), the short-circuit current first decreases for Zn concentrations below 0.2, and then significantly increases until Zn = 1. The unfavorable alignment of the conduction and valence bands at the Cd_(1−x)Zn_(x)S/CdTe interface can be used to explain the initial drop in current for low Zn content from CdS to Cd_0.8Zn_0.2S. Interface recombination is greatly increased by the energy barriers introduced by this unalignment. In this instance, a higher density of trap states is developed at the Cd_(1−x)Zn_(x)S/CdTe interface.

Figure 2. Simulated performance parameters of CdTe solar cell As a function of Zn content and thickness of CIT layer: (a) EFF, (b) FF, (c) Jsc and (d) Voc.

This higher density of trap states causes photogenerated carrier recombination and lowers the collected current. The progressive improvement in the current above 0.2 of zinc is due to the gradual adaptation of the band alignment. As the zinc concentration increases, several advantageous factors become effective. At first, better band alignment between Cd_(1−x)Zn_(x)S and a more suitable match with CdTe occur, which reduce energy barriers at the interface.

However, it also decreases the density of active trap states at the Cd_(1−x)Zn_(x)S/CdTe interface. Zinc addition can also improve electronic transport properties in the buffer layer, facilitating carrier collection. (Figure 2d) shows the evolution of the open-circuit voltage (Voc) as a function of the Zn concentration in the Cd_(1−x)Zn_(x)S layer. It stays practically unchanged, at approximately 0.97 V, for the whole Zn range from 0 to 1. Adding zinc (Zn) to cadmium zinc sulfide Cd_(1−x)Zn_(x)S mainly changes the properties of the buffer layer. It only has an effect on the cadmium telluride (CdTe) gap, which indicates that the variation in Zn in Cd_(1−x)Zn_(x)S does not significantly modify the volume recombination in CdTe. That corresponds to passivation of the interfaces and a low density of volume defects, which could otherwise affect Voc.

As shown in Figure 2b, the fill factor (FF) varies only slightly with Zn content at a fixed CIT thickness (e.g., from ~86.5% to ~86.7% at 300 nm), indicating that FF plays only a secondary role in the efficiency enhancement. This near-invariance suggests that junction quality series resistance, shunt resistance, and diode ideality is not substantially altered by moderate Zn incorporation. The slight local dip observed near Zn = 0.2 is consistent with the same trap-state formation and band-offset mismatch discussed above for Jsc, while the subsequent recovery reflects the improved Cd_(1−x)Zn_(x)S/CdTe band alignment beyond this composition. (Figure 2a) shows the conversion efficiency. Based on the previous interpretation, we can see a close relationship between JSC and EFF.

We can also observe that the efficiency decreases slightly in the Zn < 0.2 region. This is due to the simultaneous decrease in short-circuit current (Jsc) and form factor (FF), caused by increased recombination at the interfaces or band misalignment. Efficiency then improves markedly in the Zn = 0.2 to 0.8% range carrier collection and goes up due to the interface quality being greater, and CdTe absorbs in the ultraviolet and blue regions of the sun’s spectrum. After that, for the range Zn = 0.8 to 1%, efficiency reaches its peak and then remains relatively stable; this means that the cell has reached the best carrier collecting. But for ZnS, this could cause additional defects that make conversion even less efficient.

This confirms that the efficiency trend closely follows Jsc rather than FF, identifying Jsc driven by reduced parasitic absorption and improved band alignment as the dominant contributor to the Zn-related efficiency gain.

3.2. Effect of BSF (CuInTe₂) Layer

The CuInTe₂ layer goes between the CdTe absorber and the back contact. This is a common setup for a Back Surface Field (BSF), which is used to make the electrical properties of the solar cell’s back interface better. The P-doped CIT layer(CuInTe₂) lets holes through but keeps electrons out (which are the minority carriers in P-type CdTe). This forces electrons into the middle of the absorber and prevents their recombination at the back surface.

The recombination of minority carriers (electrons) at the back interface is a significant source of loss in CdTe solar cells. The CuInTe₂ layer generates a localized electric field through its high doping that repels electrons into the absorber, as seen in Figure 3, thereby reducing the probability of surface recombination. It should be noted that this mechanism does not result from a discontinuity in electronic affinity between CdTe and CIT, since the two layers have virtually identical electronic affinities (4.28 eV, Table 2). However, the CIT layer forms a p⁺/p junction with the CdTe absorber, due to its significantly higher acceptor doping concentration (NA = 10²⁰ cm⁻³) compared with 10¹⁶ cm⁻³ in CdTe. This difference in doping produces a narrow and highly localized band bending, confined to the immediate vicinity of the CIT/MO interface (Figure 4), which establishes an internal electrostatic field that repels minority electrons whilst facilitating the transport of majority carriers (holes) towards the back contact, which is consistent with the well-established BSF mechanisms reported for top-bottom junctions in thin-film photovoltaic devices [41,42,43].

Figure 3. Simulated field of best case X_Zn = 0.8/thickness CIT = 300 nm.

Figure 4. Simulated band diagram of the CdTe/CIT/Mo rear-contact region (zoom on the 1.4–1.55 µm interval) showing the localized band bending associated with the highly doped CIT layer.

This effect is enhanced by reducing the interface state density (passivation), which decreases the number of available recombination centers at the rear surface. The CuInTe₂ layer increases the lifetime of minority carriers by limiting recombination. This allows more carriers to reach the junction and be collected, thereby improving the photogenerated current and, consequently, the cell efficiency. The introduction of the CIT layer results in a significant increase in open-circuit voltage (Voc), fill factor (FF), and overall cell efficiency Figure 2.

For example, in the case of Zn = 0.8, simulations reveal that efficiency rises from 22.72% (with CIT 50 nm) to 25.73% (with CIT 300 nm), with a Voc of 0.97 V, FF of 86.70%, and short-circuit current density (Jsc) of 30.52 mA/cm². These gains are related to better carrier collection and lower recombination losses at the back interface. The CuInTe₂ layer functions as a passivation layer that reduces defect density at the back interface and improves energy band alignment. The CIT layer creates a potential gradient that favors hole transport to the back contact and electron repulsion in the absorber. This is crucial to avoiding the formation of energy barriers that would limit carrier transport and increase recombination. The simulations indicate that the thickness of the CIT layer is a key parameter, not because the amplitude of the electric field at the back interface increases indefinitely with layer thickness, but because this thickness determines whether the heavily doped BSF region is sufficiently extensive to establish and maintain the electrostatic potential distribution necessary for effective carrier selectivity. In a (p⁺/p) junction such as that formed at the CdTe/CIT interface, the thickness-dependent behavior observed in our simulations is interpreted as an electrostatic effect related to the thinness of the CIT layer, combined with the ability of the heavily doped BSF layer to maintain the potential distribution at the rear junction, Choe et al. [44] reported that the doping concentration of the BSF layer has a stronger influence on the solar cell’s efficiency than the thickness of this layer.

3.3. The CdTe Solar Cell Optimization

The incorporation of zinc into the CdS buffer layer of CdTe-based solar cells significantly enhances their performance. The bandgap of Cd_(1−x)Zn_(x)S ranges from 2.4 to 3.7 eV, depending upon the zinc content. The increased gap reduces parasitic absorption in the ultraviolet and blue regions of the spectrum, hence enhancing light transmission to the CdTe absorber layer and resulting in higher photocurrent efficiency (Figure 7b). The band alignment at the CdTe interface has been enhanced. However, with zinc concentrations below 0.2%, the short-circuit current falls, mostly due to band misalignment and higher recombination at the interface. Above this value, band alignment gradually improves, reducing energy barriers and the density of trap states, thus assisting charge carrier collection. Open-circuit voltage stays steady at roughly 0.97 V, minimally impacted by zinc change in the buffer layer. As for the fill factor, it peaks at a zinc concentration of roughly 0.8%, then declines slightly, which may be explained by a degradation in junction quality or higher recombination at the interface. Conversion efficiency consequently leads to a rise in the short-circuit current, as seen in Figure 5.

Figure 5. Simulated output parameters ((a) Jsc, (b) EFF, (c) FF and (d) Vco) of the studied structure as a function of Zn content for CIT layer of 300 nm.

The CBO (conduction band offset) refers to the conduction band minimum of the buffer layer Cd_(1−x)Zn_(x)S and the minimum energy gap of the absorber (CdTe) at their interface. This parameter is significant because it influences the extent to which photogenerated electrons can move freely from the buffer layer to the absorber, directly affecting the efficiency of the solar cell. There are three possible scenarios. First, if the CBO is too negative (often referred to as a negative spike or cliff), it creates a potential well that favors electron recombination at the interface, leading to a reduction in the open-circuit voltage (Voc) and the overall efficiency of the cell. Second, if the CBO is too positive (a positive spike), it forms a barrier that inhibits electrons from traveling through the interface, which limits the short-circuit current (Jsc) and consequently reduces efficiency.

Finally, the ideal configuration is characterized by a small positive CBO, which is typically measured in the range of 0.1 to 0.3 eV. This condition allows for optimal electron recombination while still facilitating efficient electron transport, resulting in the best performance of the solar cell, according to prior research [13,36].

In this work, we varied the concentration of Zn in Cd_(1−x)Zn_(x)S. This variation affects the CBO and tunes the lower conduction band of the buffer layer, thereby adjusting the CBO to CdTe. Figure 6 shows the standard case of CdS with a CBO of about 0.01 eV and the optimal case (Zn = 0.8) with 0.25 eV. The latter is considered a value that allows for the best compromise between reduced recombination and efficient carrier transport as we see in J (V) Characteristic curve on (Figure 7a).

Figure 6. Simulated Band Conduction Offset (CBO) of CdS case and the optimized cell with (X_Zn = 0.8).

Figure 7. (a) J (V) Characteristic curve and (b) quantum efficiency for the CdTe solar cell with CdS buffer layer and the optimized cell with (Cd_0.2Zn_0.8S) buffer layer.

4. Conclusions

Our work has demonstrated that a dual-interface passivation approach, which combines a front buffer layer of Cd_(1−x)Zn_(x)S with a rear surface field layer of CuInTe₂, is an effective method for producing high-efficiency, ultra-thin CdTe solar cells. Simulations using the wxAMPS-1D 3.0 software have shown that increasing the Zn content widens the bandgap of the buffer layer and improves band alignment at the buffer/absorber interface, which increases the short-circuit current (Jsc) and the fill factor (FF), and that the introduction of the CuInTe₂ layer at the back contact proved to be crucial: for a fixed Zn content of 0.8, increasing its thickness from 50 to 300 nm increased the efficiency from 22.72% to 25.73% by reducing recombination at the back contact and facilitating hole collection.

The optimal configuration identified (Zn = 0.8, CIT thickness = 300 nm) yields Jsc = 30.52 mA/cm², Voc = 0.97 V, FF = 86.70%, and a conversion efficiency of 25.73%, with a CdTe absorber only 1 µm thick. This efficiency is significant compared to previous front- or back-passivation approaches reported in the literature. Beyond electrical performance, reducing the CdTe thickness to 1 µm well below the standard industrial range of 3 to 5 µm lowers the consumption of tellurium and cadmium, directly addressing concerns regarding material costs and toxicity that currently limit the large-scale deployment of CdTe photovoltaic systems.

Several perspectives remain open for future work. First, replacing the uniform Cd_(1−x)Zn_(x)S composition with a graded Zn profile across the entire thickness of the buffer layer (higher near the front contact, lower near the CdTe) could smooth the bandgap transition and further reduce interfacial recombination. Alternatively, investigating other candidates for the BSF role, such as CuGaTe₂, CuInSe₂, or Cu(In,Ga)Te₂ alloys, could be compared to CuInTe₂ within the same simulation framework.