Reflectance Minimization of GaAs Solar Cell with Single- and Double-Layer Anti-Reflection Coatings: A Simulation Study

Abstract

Renewable energy is in high demand, with significant contributions from the solar industry encouraging research into more efficient, cost-effective, and versatile solar cell technologies. Anti-reflection coating (ARC) is an important method for improving solar cell efficiency by minimizing light reflectance and maximizing photon absorption. This study investigates the electrical and optical behaviors of single- and double-layer ARCs for gallium arsenide (GaAs) solar cells, using PC1D simulation for single-layer SiO₂, and ZnSe, and double-layer SiO₂/ZnSe configurations. The findings indicate that the double-layer SiO₂/ZnSe ARC structure significantly reduces reflectance and enhances light absorption, leading to a higher current density (J_sc) and overall efficiency. With optimized layer thicknesses of 60 nm (ZnSe) and 100 nm (SiO₂), the efficiency increased from 20.628% to 30.904%, representing a 49.81% improvement. This enhancement is primarily attributed to the increased photon absorption and a higher electron–hole generation rate, confirming the superior performance of double-layer ARCs over single-layer configurations.

1. Introduction

As conventional fossil-based energy sources continue to decline and global warming intensifies, the demand for clean, renewable energy has become more critical than ever [1]. Among various sustainable energy sources, solar energy emerges as a viable alternative to address the pressing issues of energy scarcity and environmental challenges [2,3,4]. Substantial research and development initiatives are being conducted to maximize the efficiency, stability, and scalability of solar cells [5,6,7]. The effectiveness of solar cells is heavily affected by factors such as heterojunction design, precise band alignment, doping concentration, carrier transport efficiency, reduced recombination losses, and effective photon management [8,9,10,11]. One widely adopted approach for improving solar cell performance is minimizing light reflection from the surface [12]. By reducing optical reflection and maximizing light transmittance over a broad wavelength range, solar cells can achieve a higher energy conversion efficiency. To reduce the reflection losses on solar cells, superabsorbent nanostructures, such as core–shell nanowires and nanocones, can be applied as they enhance light absorption through guided resonance modes [13,14,15]. Additionally, anti-reflection coatings (ARCs) minimize reflection across a broad light spectrum by changing the in light, which is crucial for optimizing photovoltaic performance [16,17,18].

The optical and electrical behavior of the ARC layer depends on the thickness and refractive index (RI) of ARC films deposited on solar cells. Over the years, various ARC methods and materials have been employed to optimize solar cell performance. These include single-layer, double-layer, and triple-layer ARCs with both constant and graded refractive indices [19]. However, single-layer ARCs have restrictions, like a narrow anti-reflection band, which limits their applicability [20]. To address the single-layer limitations, multi-layer ARCs with graded refractive indices are employed, providing improved transmittance across a wider wavelength range [21]. High-refractive-index materials such as ZnS, TiO₂, CeO₂, and low-refractive-index materials like MgF₂ Al₂O, and SiO₂ are commonly used materials for ARCs [22]. Silicon dioxide (SiO₂), with its low refractive index and broad optical transparency, is an excellent choice as an intermediary layer to reduce reflection through destructive interference [23]. Meanwhile, zinc selenide (ZnSe), characterized by a higher refractive and superior transparency, is ideal for multi-layer coatings. Together, their complementary optical and physical properties make this combination highly effective for ARCs [24,25]. ZnSe and ZnSe-based compounds exhibit strong exciton–phonon coupling and minimal conduction-band offset [26], which play a crucial role in optimizing and matching the coating with the substrate, thereby effectively reducing reflection and enhancing light transmission.

Although single-layer ARCs are typically simpler in design, incorporating two or more ARC layers with a graded refractive index can result in improved transmittance across a broader spectral range. Selecting the appropriate ARC material has a crucial role in reducing the reflection losses at the substrate. For single-layer ARCs, to minimize reflection, a transparent material with a quarter-wavelength coating and a refractive index ‘η₁’ is used. For incident light with a free-space wavelength ‘λ₀’, the thickness ‘d₁’ is calculated by the following:

$${\mathrm{d}}_1={\displaystyle \frac{{\mathsf{\lambda }}_0}{4{\mathsf{\eta }}_1}}$$

(1)

and

$${\mathsf{\eta }}_1=\sqrt{{\mathsf{\eta }}_0{\mathsf{\eta }}_3}$$

(2)

where ${\mathsf{\eta }}_0$ and ${\mathsf{\eta }}_3$ are the refractive index of air and the emitter layer, respectively. For double-layer anti-reflection coatings, the refractive indices of the ARC layers can be calculated as follows:

$${\mathsf{\eta }}_2=\sqrt{{\mathsf{\eta }}_1{\mathsf{\eta }}_3}$$

(3)

Several studies have demonstrated the effectiveness of double-layer over single-layer ARCs. However, relatively few studies have explored the double-layer ARC in gallium arsenide (GaAs)-based thin-film solar cells. GaAs, a direct bandgap III–V material, possesses excellent optoelectronic properties, including efficient charge transport, photon trapping, and a near-optimal bandgap, positioning it as an excellent option for high-efficiency photovoltaic applications [27]. GaAs, with its high solar spectrum absorption, is particularly suitable for aerial systems and space technologies due to its low sensitivity to heat and resistance to radiation damage. Furthermore, GaAs facilitates the incorporation of multiple layers with varying compositions, enabling the precise optimization of electron generation and hole collection, making it a preferred material for advanced photovoltaic systems [28,29].

Previous studies provide insights into ARC optimization for GaAs solar cells. For instance, Ali Bahrami et al. identified the optimal thickness and refractive indices for single- and double-layer ARCs using simulations, achieving a maximum efficiency of 16.97%, a short-circuit current of 27.91 mA, and an open-circuit voltage of 0.944 V with a 5 nm TiO₂ layer in an Al₂O₃/TiO₂ double-layer coating [30]. Similarly, Shi Liu and Yong-Hang Zhang’s modeling results found that a MgF₂/ZnS double-layer ARC can diminish reflectance below 1.5% in ultra-thin GaAs single-junction solar cells [31]. Z.I. Alexieva et al. reported that an Al₂O₃/ZrO₂ double-layer ARC minimizes the average weighted reflectance to 2.17%, with optimal thicknesses of 49 nm and 45 nm for the bottom and top layers, respectively [32]. Furthermore, InGaP/GaAs dual-junction solar cells with Al₂O₃/TiO₂ and Al₂O₃/ITO double-layer ARCs achieved a maximum efficiency of 39.97% [33].

Although significant progress has been achieved in designing ARCs for solar cell technologies, relatively few studies have focused on optimizing double-layer ARCs for GaAs based thin-film solar cells, despite their superior optoelectronic properties. In addition, previous research has not explored the synergistic effects of SiO₂ and ZnSe layers in reducing the reflectance for GaAs. To address these gaps, this study employs PC1D simulations to analyze and optimize the electrical and optical performance of single- and double-layer ARCs for GaAs solar cells. Here, the coating techniques and performance of single and double-layered ARC films in a wide range of wavelength spectrum will be simulated. The impact of the refractive index and thickness of single- and double-layer ARCs on a GaAs single-junction solar cell, including their electrical and optical behavior in relation to variations in ARC thickness and key solar cell parameters, has been analyzed with the help of the PC1D simulation tool. Dielectric materials, such as SiO₂, in combination with the conductive crystalline ZnSe, are investigated to evaluate the structure and performance of single- and double-layer ARCs in GaAs solar cells. Overall, this study aims to highlight the appropriate ARC materials that can be applied to GaAs solar cells to minimize reflection losses and improve overall performance.

2. Materials and Methods

The proposed structure of the GaAs solar cell is illustrated in Figure 1, with the input parameters shown in Table 1. The solar cell is composed of p-type GaAs as the base layer, while the emitter layer is made of n-type GaAs. For the ARC materials, ZnSe and SiO₂ have been selected due to their appropriate refractive indices and thickness. The design of the solar cell structure is based on the bandgap energy and refractive index of the chosen materials. For the inner ARC material, ZnSe having a refractive index ${(\mathsf{\eta }}_2)$ of 2.6 was selected and for the outer ARC material, SiO₂ with a refractive index (${\mathsf{\eta }}_1$) of 1.48 was chosen.

To analyze the optoelectrical properties of the solar cell, simulations were conducted using the PC1D simulation tool. PC1D is a one-dimensional simulation model developed by the University of New South Wales, Australia [34]. Although primarily designed for c-Si, PC1D supports other materials, including Ge, GaAs, InP, a-Si, GaInN, and AlGaAs [35,36]. The software features an intuitive interface that allows users to define the essential parameters for device layers, such as thickness, doping concentration, energy bandgap, and optical properties [37]. It enables the detailed customization of input conditions, including device geometry, surface recombination velocities, and excitation settings like the AM 1.5G solar spectrum, one-sun illumination (0.1 W/cm²), and 300 K temperature, ensuring realistic simulations under standard test conditions. PC1D generates critical outputs, including current-voltage (I–V) characteristics, quantum efficiency (QE), and energy conversion efficiency (ECE), providing detailed insights into solar cell performance [38]. Its ability to systematically vary parameters makes it an important tool for optimizing the design and intrinsic properties of photovoltaic devices. The simulation results are easily exportable for further analysis. With a material library and flexible parameter adjustments, it is a reliable and precise tool for simulating and optimizing photovoltaic devices.

PC1D provides a one-dimensional model for simulating the key properties of photovoltaic devices made from semiconducting materials. The simulations were executed under AM 1.5 solar radiation, at a constant light intensity of 0.1 W/cm² (equivalent to one sun) and a temperature of 300 K.

Table 1. Material parameters used in simulation.

Parameters	n-GaAs	p-GaAs
Thickness	300 nm [28]	5 μm [39]
Energy bandgap (eV)	1.45 [40]	1.45 [40]
Electron affinity	4.28 [41]	4.28 [41]
Refractive index	3.0 [41]	3.0 [41]
Bulk recombination time	100 μs [41]	100 μs [41]
Doping concentration	1 × 1016 cm−3 [42]	1 × 1016 cm−3 [42]
Excitation mode	transient	transient
Light intensity	One sun	One sun
Dielectric constant	13.8 [43]	13.8 [43]
Temperature	300 K	300 K
Light intensity	0.1 W/cm2	0.1 W/cm2
Primary light source	AM 1.5 D spectrum	AM 1.5 D spectrum
Other parameters	Internal PC1D	Internal PC1D

Table 1. Material parameters used in simulation.

Parameters	n-GaAs	p-GaAs
Thickness	300 nm [28]	5 μm [39]
Energy bandgap (eV)	1.45 [40]	1.45 [40]
Electron affinity	4.28 [41]	4.28 [41]
Refractive index	3.0 [41]	3.0 [41]
Bulk recombination time	100 μs [41]	100 μs [41]
Doping concentration	1 × 1016 cm−3 [42]	1 × 1016 cm−3 [42]
Excitation mode	transient	transient
Light intensity	One sun	One sun
Dielectric constant	13.8 [43]	13.8 [43]
Temperature	300 K	300 K
Light intensity	0.1 W/cm2	0.1 W/cm2
Primary light source	AM 1.5 D spectrum	AM 1.5 D spectrum
Other parameters	Internal PC1D	Internal PC1D

3. Results and Discussion

To evaluate the optimum thickness of the ARC layers, the thickness was varied from 40 nm to 120 nm. Figure 2 illustrates the impact of the ZnSe and SiO₂ ARC layer thickness on the current density (J_sc), open-circuit voltage (V_oc), and efficiency of the GaAs solar cell. As shown in Figure 2a, the J_sc increased with the thickness of the ZnSe ARC layer, reaching its peak value of 29.28 mA/cm² at 60 nm. Beyond this thickness, J_sc slightly declined due to increased optical losses. For the SiO₂ ARC layer, J_sc reached its maximum value of 28.37 mA/cm² at 100 nm, after which it also started to decrease. This trend indicates that both materials exhibit the optimum anti-reflective behavior at a specific thickness, balancing light absorption and minimal reflection. Similarly, Figure 2b shows the variation in V_oc with ARC layer thickness, where the maximum V_oc of 1.097 V was observed for ZnSe at 60 nm and for SiO₂ at 100 nm. This behavior aligns with the improved light absorption and reduced recombination losses at these specific thicknesses, enhancing carrier generation and collection. Figure 2c highlights the critical role of ARC thickness in optimizing solar cell efficiency, as the trends closely follow those of J_sc and V_oc. The highest efficiency for ZnSe was achieved at 60 nm, while for SiO₂, the optimum efficiency was observed at 100 nm. These optimum thicknesses correspond to the maximum light trapping and effective suppression of reflection losses, leading to an improved overall cell performance. The drop in efficiency beyond the optimal thickness is attributable to an increase in reflection which diminishes the photocurrent.

The optimized thicknesses of the ZnSe (60 nm) and SiO₂ (100 nm) layers were calculated using Equation (1) and verified by PC1D simulations. At these thicknesses, the ARC layers satisfy the conditions for perfectly destructive interference, where the light waves reflected from the ARC and the incident light waves at the underlying GaAs surface cancel each other out. This occurs when the optical thickness (the product of physical thickness and refractive index) of the ARC is approximately a quarter of the incident light wavelength, as described by the quarter-wavelength criterion [28]. While the quarter-wavelength criterion theoretically minimizes reflectance, practical variations in the refractive index and the wavelength dependence of solar spectra require further investigation to identify the true optimum thickness. ARC thickness was varied between 40 nm and 120 nm to explore practical considerations, as this range aligns with the quarter-wavelength principle for visible and near-infrared wavelengths, where GaAs solar cells perform most effectively. The verification through PC1D simulations confirms that the optimized thicknesses balance theoretical design with practical performance, underscoring the importance of ARC engineering to improve efficiency [31].

The J-V characteristics describe the comprehensive performance of solar cells. Figure 3 compares the J-V characteristics of a bare GaAs solar cell, single-layer ZnSe and SiO₂ ARC, and a double-layer SiO₂/ZnSe ARC. The characteristics of J-V were studied corresponding to the optimized thickness of ARC. It was noticed that the double-layer SiO₂/ZnSe ARC shows a better performance in comparison with the single-layer SiO₂ and ZnSe ARC. The solar cell parameters of bare GaAs solar cell, single-layer ZnSe and SiO₂ ARC, and a double-layer SiO₂/ZnSe ARC are presented in

Table 2. Solar cell parameters of GaAs solar cells without an ARC, with SiO₂ and ZnSe single-layer ARCs, and with a SiO₂/ZnSe double-layer ARC.

ARC Layers	Current Density (mA/cm2)	Voc (V)	FF	Efficiency (%)
No ARC	21.284	1.089	0.890	20.628
SiO2	28.378	1.097	0.890	27.706
ZnSe	29.285	1.097	0.891	28.623
SiO2/ZnSe2	31.561	1.099	0.891	30.904

. With the double-layer SiO₂/ZnSe ARC, the efficiency of the bare GaAs increased from 20.628% to 30.904%, with an enhancement of 49.81%. The main factor for the increment in efficiency is the increment in the J_sc. The J_sc of the bare GaAs increased from 21.284 mA/cm² to 31.56 1 mA/cm² for the GaAs with a double-layer SiO₂/ZnSe ARC. In contrast, the roles of V_oc and FF are not that significant. The J_sc increment is attributed to the higher light intensity reaching the solar cell. Since the current density is proportional to the photon flux incident on the cell, J_sc is directly related to light intensity [44]. By applying an ARC, more light enters the device and increases the number of photons in the absorber layer which enhances the formation of electron–hole pairs and subsequently improves the J_sc. However, V_oc and FF are not directly affected by light intensity; instead, they depend on factors such as material quality, defect density, and charge carrier recombination.

As the increase in current density (J_sc) is caused by the high generation rate of electron–hole pairs, studying the cumulative photogeneration rate in solar cells is essential. Since light comprises multiple wavelengths, it is important to account for different generation rates at each wavelength when designing a solar cell, as generation rate varies with wavelength. The photogeneration rate is illustrated in Figure 4. The cumulative photogeneration rate refers to the generated electrons at various points within the solar cell due to photon absorption [45]. It provides critical insight into the efficiency of carrier generation as a function of the material’s optical and structural properties. The photogeneration rate $G$ at a given point can be calculated using the formula, $G=\alpha N_0{e}^{-\alpha x}$ [46,47] where $N_0$ = photon flux at the surface, $\alpha$ = absorption coefficient, and $x$ = distance into the material. The cumulative photogeneration rate of a GaAs solar cell with single-layer ZnSe, and SiO₂, and double-layer SiO₂/ZnSe ARCs have been analyzed. It starts at 0/s at the front of the GaAs solar cell and rises steadily with distance. The cumulative photogeneration rate with the SiO₂/ZnSe double-layer ARC reaches 1.97 × 10¹⁹/s, which is higher than the rates observed with the ZnSe (1.82 × 10¹⁹/s) and SiO₂ (1.77 × 10¹⁹/s) single-layer ARCs.

The J_sc in a solar cell is influenced by the photogeneration rate, which is directly affected by the reflectance. Minimizing reflectance is crucial for enhancing photogeneration and improving the J_sc. The simulated reflectance spectra for the bare GaAs, single-layer ARCs, and a double-layer ARC are presented in Figure 5. The analysis includes wavelengths from 300 nm to 1200 nm. The reflectance of the bare GaAs exceeds 30% across the 350 nm to 1200 nm spectrum range, mainly because of the substantial difference in refractive indices between GaAs and air, resulting in significant Fresnel reflections. This high reflectance reduces the amount of light absorbed, thereby lowering the photogeneration rate and short-circuit current. Introducing a single-layer ARC, such as SiO₂ or ZnSe, significantly reduces the reflectance. The average reflectance for a SiO₂ single-layer ARC is approximately 10%, while that of a ZnSe single-layer ARC is about 7%. The difference arises due to the variations in the refractive indices of SiO₂ (n ≈ 1.45) [48] and ZnSe (n ≈ 2.5) [49] relative to GaAs (n ≈ 3.5) [50]. However, single-layer ARCs are generally limited in their ability to minimize reflectance over a broad spectral range due to the inherent wavelength dependence of their performance [51]. A double-layer ARC of SiO₂/ZnSe further minimizes reflectance. Here, the SiO2 layer acts as a midway between air and ZnSe, creating a more gradual refractive index transition and allowing the layers to cancel out reflections effectively over a wider spectrum. The simulated results show that the double-layer SiO₂/ZnSe ARC achieves an average reflectance of as low as 2% between 400 nm and 1000 nm. This significant improvement confirms that double-layer ARCs are far more effective than single-layer coatings for broadband reflectance reduction.

Quantum efficiency is another important parameter of solar cells which links the electrical and optical parameters [52]. The reflection and photogeneration rate of a solar cell are closely linked to its external quantum efficiency (EQE). A higher EQE corresponds to reduced reflectance and enhanced photogeneration [53] as it reflects the fraction of incident photons converted into charge carriers. ARCs have a critical role in improving EQE by minimizing reflection losses and enhancing light absorption [54]. Figure 6 illustrates the impact of single-layer and double-layer ARCs on the EQE of solar cells. Notably, the SiO₂/ZnSe double-layer ARC achieves an EQE exceeding 95% in the wavelength range of 400–870 nm. This significant improvement is attributed to the optimized design of SiO₂/ZnSe.

Light absorption and charge carrier generation are fundamental to solar cell performance. However, the ability to effectively transport charge carriers to the p-n junction without significant recombination depends on the diffusion length. Diffusion length is the average distance an electron or a hole can travel before recombination. A longer diffusion length enables more carriers to be generated by light absorption to reach the p-n junction, increasing the photocurrent and efficiency. Conversely, shorter diffusion lengths are indicative of higher recombination rates. Diffusion length can be calculated by the equation $iffusion Length \left(L_D\right)=\sqrt{{\displaystyle \frac{K_{B}T}{q}}{\tau }_{bulk}\times \mu }$, where $K_B=$ Boltzmann coefficient, $T=$ temperature, $q=$ charge, $\mu =$ mobility, and ${\tau }_{bulk}=$ bulk lifetime [55]. Materials with longer diffusion lengths generally have longer carrier lifetimes, whereas shorter diffusion lengths result in higher recombination rates. Figure 7 shows that the diffusion length remains constant when the diffusion from the front is below 300 nm; however, a diffusion length beyond 300 nm begins to increase spontaneously. A higher diffusion length means that excitons generated near an interface are more likely to reach their corresponding electrode before they decay, thereby increasing the current density [17]. Our results demonstrate that the diffusion length remains nearly the same before applying an anti-ARC, after applying a single-layer ARC, and even with a double-layer ARC. This confirms that while ARC methods effectively reduce reflectance, they do not directly influence the diffusion length.

4. Conclusions

In this research, we simulated and analyzed the performance enhancement of GaAs solar cells through the application of single- and double-layer anti-reflection coatings (ARCs). The study demonstrates that a double-layer SiO₂/ZnSe ARC configuration significantly reduces reflectance across a broad wavelength range compared to single-layer coatings, achieving an average reflectance as low as 2%. This reduction in reflectance enhances light absorption and increases the short-circuit current, resulting in an overall efficiency improvement of 49.81%. While the ARC application highly affected current density (J_sc) and efficiency, it does not significantly affect the open-circuit voltage (V_oc) or fill factor (FF). Furthermore, the diffusion length analysis confirms that ARCs primarily optimize light management without influencing charge carrier transport. The findings of this work emphasize the importance of precise ARC design for improving the performance of GaAs solar cells, paving the way for more efficient solar cell devices. However, the study is limited to simulations and does not account for practical fabrication challenges, such as coating uniformity, material compatibility, and environmental stability. Despite these limitations, the scalability and industrial applicability of the proposed double-layer SiO₂/ZnSe ARC design are promising, owing to the widespread availability of these materials and the feasibility of employing established thin-film deposition techniques. Future research should focus on the experimental validation of the proposed ARC designs, the exploration of alternative materials to further reduce reflectance, and the optimization of ARCs for other high-performance solar cell devices. Given the significant efficiency improvement observed, the proposed ARC design is particularly relevant for high performance applications, such as satellite power systems.