Computational Optimization for CdS/CIGS/GaAs Layered Solar Cell Architecture

Abstract

Multi-junction solar cells are vital in developing reliable, green, sustainable solar cells. Consequently, the computational optimization of solar cell architecture has the potential to profoundly expedite the process of discovering high-efficiency solar cells. Copper indium gallium selenide (CIGS)-based solar cells exhibit substantial performance compared to those utilizing cadmium sulfide (CdS). Likewise, CIGS-based devices are more efficient according to their device performance, environmentally benign nature, and thus, reduced cost. Therefore, the paper introduces an optimization process of three-layered n-CdS/p-CIGS/p-GaAs (NPP)) solar cell architecture based on thickness and carrier charge density. An in-depth investigation of the numerical analysis for homojunction PPN-junction with the ’GaAs’ layer structure along with n-ZnO front contact was simulated using the Solar Cells Capacitance Simulator (SCAPS-1D) software. Subsequently, various computational optimization techniques for evaluating the effect of the thickness and the carrier density on the performance of the PPN layer on solar cell architecture were examined. The electronic characteristics by adding the GaAs layer on the top of the conventional (PN) junction further led to optimized values of the power conversion efficiency (PCE), open-circuit voltage (VOC), fill factor (FF), and short-circuit current density (JSC) of the solar cell. Lastly, the paper concludes by highlighting the most promising results of our study, showcasing the impact of adding the GaAs layer. Hence, using the optimized values from the analysis, thickness of 5 (μm) and carrier density of $1\times {10}^{20}$ (1/cm) resulted in the maximum PCE, VOC, FF, and JSC of 45.7%, 1.16 V, 89.52%, and 43.88 $\left({\mathrm{mA}/\mathrm{m}}^2\right)$, respectively, for the proposed solar cell architecture. The outcomes of the study aim to pave the path for highly efficient, optimized, and robust multi-junction solar cells.

1. Introduction

With the rapid increase in electricity demand, solar cells are playing a vital role in producing green, reliable, and efficient energy sources to meet the United Nations’ Sustainable Development Goal 7 [1,2,3]. One of the promising solar cell architectures involves the thin film CIGS (copper indium gallium selenide) and CdS (copper sulfide) solar cell, which is well known for yielding higher Power Conversion Efficiencies (PCEs) and generates efficient Levelized Cost of Electricity (LCOE), with minimal manufacturing costs as compared to other solar cell architectures [4,5]. This paper proposes simulation-based modelling of thin film layer CIGS/CdS solar cells followed by the optimization of the nanowire GaAs (gallium arsenide) layer, which was added to the top layer of the baseline multi-junction solar cell architecture [6,7]. Moreover, the simulations presented in this paper were performed using the Solar Cells Capacitance Simulator (SCAPS-1D), a one-dimensional solar simulation tool that provides an in-depth investigation of electronics and information systems [8,9,10]. Moreover, SCAPS-1D evaluates some of the vital electrical characteristics, such as $PCE$, Fill factor $\left(FF\right)$, Current Density curves $\left(J_{SC}\right)$, and the open circuit voltage $\left(V_{OC}\right)$ of the solar cells [11,12,13].

Over the last decade, there has been an exponential increase in interest in CIGS and CdS solar cells, owing to their attractive properties and applications [14,15]. For the CIGS semiconductor material, the wide direct bandgap range lies between 1.0 and 1.7 eV, whereas that of CdS lies from 2.2 to 2.4 eV and is an important parameter in terms of determining the range wavelengths of light that could be possibly absorbed by the solar cell and help in predicting the PCE of converting the sunlight into electricity [16,17,18,19]. These factors, specifically, make CIGS and CdS semiconductor materials highly favorable materials for photovoltaic energy conversion applications [20,21,22,23]. Furthermore, CIGS and CdS materials contribute towards high efficiency, low cost of manufacture, thin film fabrication techniques, environmental friendliness, varied spectral response, high radiation tolerance, and good absorption coefficient [24]. Therefore, research in these materials has led to the advanced development and fabrication of semiconductor devices, optoelectronics, and nanotechnology, from photovoltaics to biosensors, and they are widely used as catalysts in chemical reactions involving hydrogen production from water [25].

On the contrary, one limiting factor in developing more efficient solar cells lies in various temperature influences caused by intrinsic characteristics of the semiconductor materials [26]. Following this, CdS also led to a concern about toxicity and stability because of the availability of cadmium in the solar cells [27]. However, mechanical stacking of multi-junction semiconductor materials can help to mitigate these concerns whilst enhancing the overall solar cell efficiency [28]. In our paper, we run several simulations involving the traditional single-junction CIGS/CdS solar cell layers and then later add the GaAs layer to study the properties of the multi-junction solar cell. From the literature, several attempts have been made to optimize the overall efficiency of the solar cell [29,30,31]. The main idea behind this analysis is the improvement of the device efficiency using materials cheaper than conventional CIGS [32,33]. A new 5 μm layer of p-GaAs has been added for that purpose. Various thicknesses of the CIGS absorber layer ranging from 0.5 to 5 μm have been used [34].

Accordingly, the findings of our study showcase that an increase in the absorber layer thickness improves the performance and overall power conversion efficiency of the new CIGS solar cells. Initially, the study incorporated a window layer of ZnO, a buffer layer (CdS), an absorbent layer (CIGS), and a GaAs layer with varied values of thickness, including 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, and 5 μm. Also, the optimization of the GaAs material is performed with the help of the heatmap confusion matrix to analyze the most optimized thickness and carrier density of different layers of solar cells. In addition, a comparison of results including the performance and efficiency of the PN-junction (optimized CIGS and CdS) solar cell was carried out. The most promising results of the paper revealed that a thin top layer of p-GaAs on the conventional solar cell (p-CIGS and n-CdS) with an optimized thickness layer and high carrier density had a considerable influence on the performance of the solar cell architecture. Adding a p-GaAs layer as thin as 5 μm on the top of the PN-junction solar cell substantially improved the conversion efficiency of the solar cell from 20.07% (unoptimized PN) to 45.47 % (optimized NPP). The results showed that the new ultra-thin CIGS solar cell structure has performance parameters that are comparable to those of the conventional ones with reduced cost.

Therefore, our paper proposes a simple three-layered n-CdS/p-CIGS/p-GaAs (NPP) solar cell with a practical thickness and a high conversion efficiency. The PPN solar cell comprises a high-p-GaAs composition on top of p-CIGS and n-CdS multi-junction solar cells. In addition, computing a thin p-GaAs layer created a graded energy bandgap and the solar PN junction could only absorb photons having an energy equal to or greater than the energy bandgap [35,36]. Herein, it is worth mentioning that a graded energy bandgap has a wider energy bandgap to absorb more photons. Moreover, adding an extra thin layer increased the number of holes in the solar cells, ultimately increasing power conversion efficiency. The behavior and performance of the solar cells were evaluated by performing a comprehensive set of simulations under different configurations (i.e., thickness and carrier density of the layer) using the SCAPS-1D software (version 3.8).

1.1. Organization of the Paper

Our paper is divided into eight Sections. Section 2 of the paper discusses the significance, need, and limitations of the CdS/CIGS multi-junction solar cell architecture. Following this, Section 3 describes the process of setting up our simulation environment in the SCAPS-1D. Section 4 includes the efficiency optimization of the CdS/CIGS solar cell with the help of thickness and carrier density optimization using the heatmap confusion matrix. Section 5 presents the results of electrical parameters after adding a GaAs layer on top of the CdS/CIGS solar cell. Next, Section 6 discusses a critical comparison of IV, PV, and QE characteristics and the high-temperature effect. Lastly, Section 7 concludes the paper.

1.2. Contributions to the Research Paper

The key contributions to the research article are presented as follows:

• We discussed the significance, need, and limitations along with a thorough literature review of the CIGS and CdS multi-junction solar cell.
• We implemented simulations on the SCAPS-1D tool for designing the most optimized, high-efficiency, and robust solar cell architecture.
• We performed efficiency optimization for the CIGS/CdS multi-junction solar cell architecture by adjusting the thickness and carrier density using the heatmap confusion matrix.
• Whilst performing simulations, critical optimization techniques are applied to the CIGS/CdS solar cell as a first step, and then to CdS/CIGS/GaAs as a second step, with the help of the heatmap confusion matrix ranging from 0.5 μm to 5 μm for thickness and 10 (1.00 En) (1/${\mathrm{cm}}^3$) and 20 current density, respectively.
• In the second step, we introduced a novel multi-junction solar cell architecture by adding an n-GaAs layer on top of the pre-existing CIGS/CdS multi-junction solar cell and carried out the thickness and carrier density optimization to achieve the highest PCE value.
• Further, we investigated the electronic and electrical characteristics, such as PCE, fill factor, current density, and open circuit voltage, of the n-CdS/p-CIGS/p-GaAs multi-junction solar architecture.
• Lastly, a thorough comparative analysis is presented, showing the IV, PV, and QE characteristics graphs for the most optimized solar cell architectures, with their respective optimized thickness and current density values.

2. CIGS and CdS Multi-Junction Solar Cell

Initially, a multi-junction solar cell architecture using the CIGS and CdS semiconductor materials is designed. CIGS and CdS materials are widely used for manufacturing purposes, mainly because of key parameters such as low production cost and high solar energy yield. The literature suggests that the materials have achieved higher PCE values than their counterpart, silicon solar cells, installed on household rooftops. Moreover, the CIGS solar cells use a thin layer of CIGS as an absorbing layer and have high absorption coefficient values, which eventually results in high-efficiency solar cells. The flexible nature of the CIGS semiconductor materials broadens their application in various industries, research, and industry perspectives, such as photovoltaics, biosensors, portable electronic devices, off-grid power systems, utility-scale solar power plants, and for powering remote sensing and communication devices [37].

Subsequently, the semiconductor CdS is used as the window, also known as the buffer layer in the solar cells. The window layer is defined as a thin, transparent layer allowing the sunlight to pass through the absorber layer and also restricting the unwanted recombination of the holes and electrons generated by the p-CIGS (absorbed photons) layer. Moreover, the window layer is typically made of a transparent conducting oxide (TCO), such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), or zinc oxide (ZnO). These materials have high transparency to visible light and low electrical resistance, which allows them to efficiently collect and transport the electrons generated by the absorbed photons. Furthermore, the choice of window layer material depends on several factors, including the specific absorber material used in the solar cell, the desired efficiency, and the cost of the materials. Different TCOs have different properties that can affect the performance of the solar cell, such as their work function, surface roughness, and doping level.

Therefore, the window layer plays a critical role in the performance of a solar cell by controlling the flow of charge carriers and minimizing energy losses due to recombination, while also allowing sunlight to pass through to the absorber layer. Accordingly, in the multi-junction solar cell of n-CdS/p-CIGS, CdS is often used as the window layer in the respective solar cell architecture. Additionally, the transparent window layer sits on top of the absorbing layer and allows light to pass through to reach the CIGS layer. CdS has a high optical transmission, which allows it to efficiently transmit light to the CIGS layer, while also helping to protect the CIGS layer from degradation due to exposure to air and moisture. However, it is important to note that cadmium is a toxic substance, raising concerns about CdS solar cells’ environmental impact. Subsequently, efforts are being made to develop alternative materials for window layers, such as zinc oxide (ZnO), a non-toxic alternative to CdS.

2.1. CIGS/CdS: Significance and Need

Previous research studies indicate that the combination of CIGS and CdS multi-junction solar cells is known to provide promising electrical and electronic characteristics even in harsh environments along with achieving considerably higher efficiencies [38,39,40,41]. CIGS and CdS solar cell layers are used because they offer numerous advantages over the widely existing materials manufactured in the solar industry [42,43,44,45]. Not only does CIGS have high efficiency in converting sunlight into electricity, but it can also generate more power (Ws) per unit of surface area (A) in comparison with other materials. Accordingly, this makes CIGS and CdS materials ideal for applications in solar cells, where either the space is limited or reduced size is targeted, for example, rooftops or portable devices [46]. In addition, CIGS materials are known to be flexible, which allows them to be used in various applications, including curved or irregular surfaces [47]. Moreover, this property also makes the design of solar cells suitable for building-integrated photovoltaics (BIPV), where they are integrated into the design of buildings and other complex structures. Lastly, CdS materials proved to be an excellent window or buffer layer material due to their high optical transmission, which allows them to efficiently transmit light to the CIGS layer while also protecting from environmental factors that can degrade the solar cell [48]. Overall, the combination of CIGS and CdS offers a high-efficiency, flexible, and durable solution for solar cells that can be used in a wide range of applications [49].

2.2. CIGS/CdS: Limitations

While CIGS and CdS solar cells offer many advantages, there are also some challenges associated with their use. One challenge is the cost of producing CIGS solar cells [50]. The materials used to make CIGS solar cells are relatively expensive, making the manufacturing process more costly than other types of solar cells. This has limited the widespread adoption of CIGS solar cells, especially in utility-scale applications. Another challenge is the potential environmental impact of using CdS as a window layer material [51]. Cadmium is a toxic substance, and concerns have been raised about the possibility of environmental contamination from CdS solar cells during their production, use, and disposal. Efforts are being made to develop non-toxic alternatives to CdS for use in window layers, such as zinc oxide (ZnO), but these materials are not yet widely used in commercial applications. Finally, CIGS solar cells are susceptible to degradation over time due to exposure to moisture and air. This can reduce the efficiency of the solar cell and limit its lifespan. However, research is ongoing to develop new encapsulation methods and materials that can protect the CIGS layer from degradation and extend the lifespan of CIGS solar cells. Overall, while there are some challenges associated with using CIGS and CdS solar cells, ongoing research and development efforts are aimed at addressing these challenges and improving the efficiency and durability of these solar cell technologies [52].

The overall efficiency of CIGS and CdS solar cells has improved significantly over the past few decades, making them competitive with other types of solar cells in terms of efficiency. The current record for CIGS solar cell efficiency is around 23.35%, which is close to the efficiency of traditional silicon-based solar cells. The high efficiency of CIGS solar cells is due to their ability to absorb a broad range of wavelengths of light, including the blue and green parts of the spectrum, which traditional silicon solar cells cannot absorb efficiently [53]. The CdS window layer in CIGS solar cells also plays a critical role in their overall efficiency. CdS has high optical transmission, which allows it to efficiently transmit light to the CIGS layer, resulting in higher conversion efficiencies. However, as mentioned earlier, the use of CdS raises concerns about its potential environmental impact. Overall, the efficiency of CIGS and CdS solar cells is highly competitive with other types of solar cells, and ongoing research and development efforts are focused on improving their efficiency and reducing their cost [54].

The efficiency of CIGS and CdS solar cells can be evaluated in both simulation-based environments and real-world environments. In simulation-based environments, the efficiency of solar cells can be modeled using computer simulations that take into account various factors, such as material properties, cell design, and environmental conditions. These simulations can provide insights into the theoretical efficiency of solar cells and can help guide the design and optimization of solar cell materials and structures. In real-world environments, the efficiency of solar cells can be evaluated by measuring their performance under actual operating conditions. Factors such as temperature, humidity, and shading can affect the performance of solar cells in real-world environments, leading to variations in efficiency compared to simulation-based results. Generally, the efficiency of CIGS and CdS solar cells in real-world environments is lower than in simulation-based environments due to various factors such as partial shading, thermal losses, and other environmental factors. However, ongoing research and development efforts are focused on improving the real-world performance of CIGS and CdS solar cells, including developing new encapsulation techniques, optimizing the cell design, and developing new materials for use in the window layer. Overall, the efficiency of CIGS and CdS solar cells is evaluated in both simulation-based and real-world environments, and ongoing research is focused on improving their efficiency and performance in both environments.

3. Simulation Setup

For the analysis of the different solar cell architectures, a simulation environment was developed on the SCAPS-1D tool for examining the electronic and electrical parameters along with measuring the PCE, FF, open-circuit voltage (VOC), and JSC values. Initially, a solar cell was designed using the three-layered P-N-N semiconductor layers. With light incident from the right contact (front), followed by a layer of n-ZnO, n-CdS, and p-GaAs to the left contact (back), a multi-junction solar cell architecture was formed. The incident light plays a pivotal role in determining the efficiency of the solar cell, and thus, in our simulation setup, we introduced the incident light from the right contact (front) throughout the simulation settings. Furthermore, the initial working conditions of the simulations were set as follows: temperature—300 K, voltage—0 V, frequency—${10}^6$ Hz, and the number of points was 5.

In addition to this, for plotting the electrical characteristics curves, the settings for Pause at each stop were applied as V1 (0 V to −0.8 V), V2 (0.8 V to 0.8 V), frequency (f1: ${10}^2$ Hz to f2: ${10}^6$ Hz), and wavelength (WL1: 300 nm to WL2: 1200 nm). Moreover, at each step, the number of points was set as 41, 81, 21, and 61 with an increment of 0.02 V, 0.02 V, 5 points per decade, and 10 nm, respectively. It is worth mentioning that the author later in the study also introduced a layer of GaAs to achieve the highest efficiency, and accordingly, all the settings of the working point remain the same for different multi-junction solar cell architectures. Subsequently, Figure 1represents the direction of the incident light along with the n-ZnO/n-CdS/p-CIGS multi-junction solar cell architecture and

Table 1. The table below depicts the input parameters that were set for the simulation setup.

Input Electrical Parameters	Measurement Unit	Layer 4: p-GaAs	Layer 3: p-CIGS	Layer 2: n-CdS	Layer 1: n-ZnO
Bandgap	(eV)	$1.420$	$1.100$	$2.450$	$3.300$
Electron Affinity	(eV)	$4.070$	$4.500$	$4.400$	$4.600$
Dielectric Permittivity	(relative)	$12.900$	$13.600$	$10.000$	$9.000$
Conduction Band Effective Density of states	(1/${\mathrm{cm}}^3$)	$2\times {10}^{18}$	$2.2\times {10}^{18}$	$2.2\times {10}^{18}$	$2.2\times {10}^{18}$
Valence Band Effective Density of states	(1/${\mathrm{cm}}^3$)	$1\times {10}^{19}$	$1.8\times {10}^{19}$	$1.8\times {10}^{19}$	$1.8\times {10}^{19}$
Electron Thermal Velocity	(cm/s)	$1\times {10}^7$	$1\times {10}^7$	$1\times {10}^7$	$1\times {10}^7$
Hole Thermal Velocity	(cm/s)	$1\times {10}^7$	$1\times {10}^7$	$1\times {10}^7$	$1\times {10}^7$
Electron Mobility	(${\mathrm{cm}}^2$/Vs)	$1\times {10}^3$	$1\times {10}^2$	$1\times {10}^2$	$1\times {10}^2$
Hole Mobility	(${\mathrm{cm}}^2$/Vs)	$1\times {10}^2$	$1\times {10}^1$	$1\times {10}^1$	$2.5\times {10}^1$

depicts the input electrical parameters such as the bandgap, electron affinity (eV), dielectric permittivity (relative), conduction band effective density of states, valence band effective density of states, electron thermal velocity, hole thermal velocity, and electron and hole mobility of the respective four-layered solar cell architecture.

4. Efficiency Optimization

After incorporating the necessary input electrical values and setting up the simulation environment, in the next step, the authors conducted the efficiency optimization, consisting of three important steps. Accordingly, to meet the objective of this paper, a critical optimization of different solar cell architectures is performed. We first optimized thickness and then the doping [55,56]. The first step of optimization involves varying the thickness of the CIGS and CdS layers in the multi-junction solar cell of p-CIGS/n-CdS/n-ZnO. Followed this, the second step included the same solar cell architecture; however, the efficiency optimization is evaluated with the help of changing the doping values. Lastly, the third step introduces a GaAs layer to the existing solar cell architecture, and accordingly, both the thickness and doping optimization are performed simultaneously.

4.1. STEP 1: Optimization of Thickness for CIGS and CdS Layer

Initially, a baseline solar cell architecture is proposed consisting of n-ZnO/n-CdS/p-CIGS, and as a first step, the thickness optimization is performed to estimate the overall efficiency of the solar cell at different values of thickness, ranging from 0.5 to 5.0 μm, at an increment of 0.5 μm. Accordingly, the incrementing the thickness values is performed in such a manner that an increment is made for each 0.5 μm value for both the CIGS and CdS semiconductor materials of the solar cell independently. Therefore, to present a relation between the two parameters’ increments in thickness, the authors used the heatmap correlation matrix to provide a clear indication of the values of the overall performance of the solar cell at varied values of thickness of both materials.

The estimation of thickness is highly essential for designing the most optimized solar cell due to saving costs on materials, and subsequently, the solar cell manufacturers could use the materials optimally. The heat maps of Voc, Jsc, FF and $\eta$ are presented in Figure 2. The results from the heatmap confusion matrix showcase four parameters: efficiency, fill factor, current density, and open-circuit voltage. For thickness optimization of the materials n-CdS/p-CIGS, the heatmap confusion matrix indicates a thickness value of 0.5 μm for CdS and 5.0 μm for CIGS, giving a maximum efficiency of solar cell design as 20.07%. Following this, a thickness of 1 μm for CdS and 5.0 μm for CIGS yields a maximum fill factor of 80.88%, whereas the maximum values of the parameters current density and open-circuit voltage are 43.59 mA/cm² and 0.57 V, respectively, for thickness values of 4.5 μm for CdS and 5.0 μm for CIGS.

In addition, for the electrical performance of the proposed solar cell, PV, IV, and QE characteristics are plotted in Figure 3 measure the maximum power point tracking of the solar cell. Accordingly, the maximum value, as observed from the IV curve, is 45 mA/cm², and the maximum power, as analyzed from the PV curve, is 32 mA/cm². Subsequently, the maximum quantum efficiency of the baseline proposed solar cell architecture is 95%.

4.2. STEP 2: Optimization of Carrier Density for CIGS and CdS Layer

After optimizing the thickness values of the n-ZnO/p-CdS/p-CIGS multi-junction solar cell architecture, the authors conducted the optimization of the carrier density parameter of the solar cell using the same input values as set in the working simulation environment. However, the authors used the most optimized value of the thickness of the solar cell, as calculated in the above subsection, i.e., 0.5 μm for CdS semiconductor material and 5.0 μm for CIGS material. It is worth mentioning that the authors used this value of thickness for the remaining simulations of the study.

Furthermore, the optimization of the acceptor density of the CIGS $1.0\times {10}^n$ (1/cm³) and the donor density of the CdS $1.0\times {10}^n$ (1/cm³) is evaluated using the heatmap confusion matrix, which gives the values of the doping concentration for both the CdS and CIGS. Similar to the previous subsection, herein, the authors measured the electrical characteristics, such as efficiency (%), fill factor (%), current density (mA/cm²), and the open-circuit voltage (V) for the critical optimization of the doping concentration. Accordingly, Figure 4a,b,d indicate that the optimization of the doping concentration at $1\times {10}^{20}$ (1/cm³) for both CIGS and CdS materials yields a maximum value of efficiency, fill factor, and open-circuit voltage, at 32.12 %, 86.93 %, and 0.87 V, respectively. On the contrary, the optimized value of doping concentration for donor density of CdS lies in the range of $1\times {10}^{10}$ (1/cm³) to $1\times {10}^{20}$ (1/cm³) at an acceptor density of CIGS, as $1\times {10}^{10}$ (1/cm³) resulted in a maximum value of current density equivalent to 43.89 (mA/cm²).

In addition to this, the electrical characteristics are calculated from the SCAPS-1D simulation tool. Figure 5 showcases the IV, PV, and quantum efficiency for the most optimized values of the doping concentration values at $1\times {10}^{10}$ (1/cm³) for the donor density of CdS and $1\times {10}^{20}$ (1/cm³) for the CIGS semiconductor materials. Accordingly, the curves indicate that the maximum value of current density is 43.2 mA/cm² and the maximum power is 32.91 mA/cm². Subsequently, the maximum value of the quantum efficiency at the optimized inputs of doping concentration results in 96%.

5. Introducing GaAs Layer

One of the most common semiconductor materials used in the multi-junction solar cell is gallium arsenide (GaAs), which is primarily stacked on the top of either layer or at the top of all layers proposed in a solar cell architecture.

Each layer is designed to absorb a specific portion of the solar spectrum, allowing the cell to capture a broader range of sunlight and convert it into electricity more efficiently. One common material used in multi-junction solar cells is gallium arsenide (GaAs). GaAs is a semiconductor material with unique properties that make it well-suited for solar cell applications. Here is why GaAs is used as a semiconductor layer in multi-junction solar cells:

• High energy conversion efficiency: GaAs has a relatively high energy conversion efficiency compared to other semiconductor materials used in solar cells. It has a direct bandgap, which means it can efficiently convert sunlight into electricity without losing much energy as heat.
• Wide bandgap: GaAs has a wide bandgap, which allows it to absorb higher-energy photons from the solar spectrum. By incorporating GaAs in the solar cell stack, it can absorb photons from the blue and green regions of the spectrum, which are not efficiently absorbed by other materials such as silicon (commonly used in single-junction solar cells).
• Tandem cell configuration: In a multi-junction solar cell, the semiconductor layers are arranged in a tandem configuration, with each layer tuned to absorb a specific part of the solar spectrum. GaAs is often used as the top layer in the stack because it has a higher bandgap than other materials, making it suitable for capturing higher-energy photons. The layers beneath the GaAs layer can be designed to absorb lower-energy photons, ensuring efficient use of the entire solar spectrum.
• Temperature stability: GaAs has excellent temperature stability, allowing it to maintain its high performance even at elevated temperatures. This characteristic is crucial for solar cells, as they can heat up under intense sunlight.
• Mature technology: GaAs has been extensively researched and developed for various applications, including solar cells. It benefits from a well-established manufacturing process and has a proven track record in high-performance photovoltaic devices [57].
• High electron mobility: GaAs has a higher electron mobility compared to other common semiconductor materials like silicon. This property makes GaAs suitable for high-speed electronic devices, such as field-effect transistors (FETs) and integrated circuits, where fast switching and high-frequency operation are required.
• Low noise characteristics: GaAs exhibits low noise characteristics, making it ideal for applications in low-noise amplifiers and microwave devices. This property is particularly advantageous in high-frequency communication systems and radar technology.
• Wide frequency range: GaAs exhibits excellent performance across a wide frequency range, including microwave and millimeter-wave frequencies. It enables the development of devices and circuits for wireless communications, satellite communications, radar systems, and high-frequency electronics.
• High power handling capability: GaAs materials can handle high power levels without significant degradation in performance. This property makes GaAs suitable for power amplifiers and other high-power electronic devices, including those used in telecommunications and defense applications.
• Optoelectronic applications: GaAs is widely used in optoelectronic devices such as light-emitting diodes (LEDs), laser diodes, and photodetectors. GaAs-based LEDs and laser diodes have superior performance in terms of efficiency, brightness, and wavelength range, making them valuable for applications in lighting, optical communications, and optical sensing.
• Compatibility with complementary metal-oxide-semiconductor (CMOS) technology: GaAs can be integrated with CMOS technology, allowing for the development of hybrid circuits and systems that leverage the advantages of both GaAs and CMOS. This integration enables the fabrication of high-performance, mixed-signal devices and integrated circuits with diverse functionality.
• Radiation hardness: GaAs exhibits inherent radiation hardness, meaning it can withstand the effects of ionizing radiation without significant degradation in performance. This characteristic makes GaAs suitable for applications in space technology, nuclear power plants, and high-energy physics experiments.

By incorporating a GaAs semiconductor layer in a multi-junction solar cell, the overall efficiency of the cell can be significantly increased. GaAs helps capture a broader range of sunlight, including higher-energy photons, and converts them into electricity more effectively, leading to improved solar cell performance.

While GaAs materials offer numerous advantages, there are also some limitations associated with their use. One limitation is the higher cost of GaAs compared to other semiconductor materials, primarily due to the complex manufacturing processes involved. This cost factor restricts the widespread adoption of GaAs in certain applications where cost-effectiveness is a critical consideration. Additionally, GaAs is a brittle material, making it more prone to cracking and breakage during handling and fabrication. Moreover, GaAs-based devices may face challenges in scaling down to smaller dimensions due to material properties and technological constraints. Despite these limitations, ongoing research and advancements aim to address these issues and further enhance the capabilities and cost-effectiveness of GaAs materials for broader application domains.

STEP 3: Optimization of Thickness and Carrier Density for GaAs Layer

Furthermore, Figure 6 represents the proposed solar cell architecture, which includes another layer of the p-GaAs layer to the baseline multi-junction solar cell. Subsequently, the GaAs layer is added on top of the p-CIGS layer next to the left contact (back) of the proposed solar cell architecture for analyzing the electrical and electronic performance in the real-world environment with the help of the SCAPS-1D simulation tool. Moreover, the light in this scenario is incident from the right contact (front) to achieve the maximum possible efficiency of the proposed solar cell architecture. In addition to this, all working environments of the simulation setup were kept the same to avoid any discrepancy in the analysis of the output results and thus, the overall performance of the multi-junction solar cell.

Subsequently, the optimization of the GaAs semiconductor material was performed in terms of the thickness and doping concentration with the value ranging from 0.5 μm to 5.0 μm and from $1\times {10}^{11}$ (1/cm³) to $1\times {10}^{20}$ (1/cm³), respectively. Accordingly, Figure 7 showcases that the maximum efficiency and open-circuit voltage of the proposed solar cell architecture are achieved at 45.47 and 1.16 V, respectively, at a thickness of 5.0 μm and acceptor density of GaAs material of $1\times {10}^{20}$ (1/cm³). One of the most promising results in the heatmap confusion matrix indicates that the current density of the solar cell remains unaffected, i.e., 43.88 (mA/cm²), even by changing the thickness and doping concentration values of the solar cell. On the contrary, the percentage of the maximum fill factor is 89.52% at the thickness and doping concentration values of 5.0 μm and $1\times {10}^{16}$ (1/cm³), respectively.

Additionally, the current density (blue curve), as obtained from the IV curve, outputs a peak value of 44.8 (${\mathrm{mA}/\mathrm{cm}}^2$), as shown in Figure 8, and thus, the maximum power observed is equivalent to 46 W (the orange curve) at a thickness and doping concentration value of 5.0 μm and $1\times {10}^{20}$ (1/cm³), respectively. Following this, the maximum quantum efficiency achieved for the proposed solar cell with GaAs with the most optimized values of thickness and doping concentration is equal to 99.2 %, as shown in Figure 8. It is worth mentioning that the quantum efficiency curve consists of the quantum efficiency (%) vs. the wavelength curve (nm) with a range between 300 (nm) and 1130 (nm).

6. Discussions

In this section of the manuscript, we discuss the results of the comparison of various IV, PV, and QE characteristics of the proposed baseline solar cell, thickness optimization curve, and doping concentration curve, and a thorough comparison is made for the electrical properties of the solar cell after adding the p-GaAs layer on top of the proposed baseline solar cell architecture. The comparison gives a clear indication of the most optimized thickness and doping concentration values that need to be set whilst manufacturing the different solar cell architectures at a mass level.

6.1. Comparison of IV Characterisitcs

Figure 9 represents the IV characteristics in terms of the current density, JSC (${\mathrm{mA}/\mathrm{cm}}^2$) of the baseline—after thickness optimization (blue), the optimized curve for doping concentration (orange), and the optimization results after introducing the GaAs layer (green) on the proposed solar cell with varied values of the open-circuit voltage, i.e., 0.5 V, 0.83 V, and 1.17 V, respectively. The curve indicates that the optimization of the solar cell architecture along with the thickness and current density led to an increase in the open-circuit voltage of the solar cell architecture. In addition, an enhancement in a solar cell’s performance or a change in its operating circumstances is often indicated by an increase in the VOC of the solar cell on the IV curve. This improvement of VOC is attributable to circumstances like better material quality, less charge carrier recombination, lower series resistance, or different operating environments, like temperature. However, to appropriately evaluate the effectiveness and overall performance of the solar cell, it is crucial to take into account the entire IV curve, including elements like the short circuit current (Jsc), fill factor (FF), and power output, which is discussed in the subsequent subsection of the manuscript.

6.2. Comparison of PV Characterisitcs

Another comparison of power output is presented in Figure 10, showing a relation of the power density (${\mathrm{mW}/\mathrm{cm}}^2$) vs. voltage (V) characteristics, where like the previous step, the thickness optimization (blue), doping concentration (orange), and the p-GaAs layer optimization (green) are evaluated. The maximum of the power curve, represented as $P_{MP}$, is the point at which the solar cell should be operated to produce the most electricity. It occurs at a voltage of $V_{MP}$ and a current of $I_{MP}$ and is also referred to as $P_{MAX}$ or maximum power point (MPP). Figure 10 showcases that the power output for the thickness optimization of the proposed baseline solar cell architecture has a peak of 25 mW/cm², whereas that of the doping concentration is measured as 32 mW/cm². On the contrary, introducing the p-GaAs layer on the top of the proposed solar cell yields a peak value of power at 45.47 mW/cm².

Therefore, adding a p-GaAs layer highly affects the output power of the solar cell, and accordingly, it also results in an increased efficiency of the solar cell to 45.47%, which is thus far the maximum efficiency recorded for a multi-junction solar cell consisting of n-CdS/p-CIGS/p-GaAs semiconductor materials. Moreover, the solar cell shows an overall improvement in functionality and effectiveness. The increased power output is due to the solar cell’s improved material quality having improved, allowing for better light absorption and electrical conversion with optimized thickness and doping concentration values used on the SCAPS-1D simulation tool. Additionally, improved operating conditions, such as temperature and illumination levels, decreased recombination losses, and lower series resistance, are the other miscellaneous reasons that contribute to an increase in the overall power output.

6.3. Comparison of QE Characterisitcs

Subsequently, an improvement in the proposed solar cell’s overall capacity to convert photons of various wavelengths into electrical current is evaluated using the quantum efficiency vs. lambda (wavelength) curve. The solar cell is growing more effective at absorbing a larger variety of photons across the electromagnetic spectrum as QE rises. The increased light-trapping techniques, improved material characteristics, decreased recombination losses, and improved device topologies are some of the major contributing factors that led to a sudden rise in the overall quantum efficiency. Therefore, an increase in the quantum efficiency (%) in the quantum efficiency vs. lambda (wavelength in nm) curve depicts that the solar cell is performing efficiently in regards to capturing a wide range of wavelength spectrum, which further results in an exponential rise in the total power conversion efficiency of the proposed solar cell. Accordingly, the authors presented a comparative analysis of the quantum efficiency (%) for baseline thickness optimization (blue), doping concentration optimization (orange), and optimization after introducing the GaAs layer. As analyzed from Figure 11, the quantum efficiency for baseline thickness, doping concentration, and the GaAs layer is 82%, 94%, and 99.48% at the 1130 nm wavelength.

7. Conclusions

This paper proposes an efficient three-layered n-CdS/p-CIGS/p-GaAs (NPP), a unique solar cell architecture. Copper indium gallium selenide (CIGS)-based solar cells exhibit substantial performance compared to those utilizing cadmium sulfide (CdS). On the contrary, CIGS-based devices are more efficient, considering their device performance, environmentally benign nature, and reduced cost. Therefore, our paper proposes a numerical analysis of the homojunction PPN-junction GaAs solar cell structure along with n-ZnO front contact that was simulated using the Solar Cells Capacitance Simulator (SCAPS-1D) software. Moreover, we investigated optimization techniques for evaluating the effect of the thickness and the carrier density on the performance of the PPN layer on solar cell architecture. Subsequently, the paper discusses the electronic characteristics of adding GaAs material on the top of the conventional (PN) junction, further leading to improved values of the parameters, such as the PCE, VOC, FF, and short-circuit current density (JSC) of the solar cell. The most promising results of our study show that adding the GaAs layer using the optimized values of thickness as 5 (μm) and carrier density as $1\times {10}^{20}$ (1/cm³) will result in the maximum PCE, VOC, FF, and JSC of 45.7%, 1.16 V, 89.52%, and 43.88 $\left({\mathrm{mA}/\mathrm{m}}^2\right)$, respectively, for the proposed solar cell architecture.