Solar energy is a clean, environmentally beneficial, renewable source with an endless supply [1]. The primary photovoltaic conversion material for solar cells has evolved throughout the years, ranging from crystalline silicon (c-Si) [2], semiconductor thin films [3] to inexpensive organic/polymers and dye-sensitized materials [4,5]. Solar cells made of silicon are still good prospects for the switch from fossil fuels to low-carbon energy sources [6,7]. However, polished Si wafers' high surface reflection limits their use in high-efficiency solar cells [8]. Texturing is an efficient approach to increase light trapping in Si-based solar cells [9]. It is a helpful technique to improve light absorption from oblique angles and boost photogenerated excitons [10]. Due to its high surface-to-volume ratio and enhanced photon absorption coefficient, porous silicon (PSi) has been used as an intriguing material for efficient solar cells [11]. However, despite significant efforts to improve the efficiency of PSi-based devices, it isn't easy to maintain photovoltaic stability under various environmental factors [12]. The existence of metastable bonds, such as Si–H terminations, at the PSi surface is primarily responsible for the temperature dependency of PSi devices [13]. These inherently unstable terminations on the PSi surface lead to quenching in the photoluminescence (PL) spectra of PSi samples under extended laser illumination [14]. According to the literature, exposure to various ambient temperatures can alter PSi's optical characteristics [15,16]. Therefore, the initial metastable terminations can evolve at high temperatures, changing the PL spectra and causing the PL quenching induced by low-level laser illumination [17]. As a result, it is more challenging to develop PSi-based devices that can function in harsh environments like high temperatures and highly corrosive chemicals. Thermal oxidation or carbonization of PSi substrates has been proposed as a solution for developing durable devices based on PSi substrates [18]. Replacement of hydrogen species with more stable terminations is the common goal of all these techniques [19]. Although these methods effectively stabilize the optical characteristics of PSi samples, the formation of an opaque and thick layer on the substrates causes them to alter the physical characteristics of the porous surface [20].

Graphene can be considered a single mono-molecular layer of graphite consisting of carbon atoms that are closely packed together in a honeycomb hexagonal lattice to form a transparent single layer. It offers unique characteristics, including outstanding electron mobility, high surface-to-volume ratio, excellent chemical and mechanical stability, and exceptional thermal and electrical conductivity [21]. As a result, it is presumable to be a useful nanostructure with a range of photonic and electrical applications. Another distinctive two-dimensional material is graphene oxide (GO), consisting of several oxygen-functional species such as epoxy, carboxyl, and hydroxyl [22]. Therefore, it has a lower electrical conductivity than pure graphene despite dissolving significantly more readily in various solvents [23]. Consequently, it is possible to deposit GO using different liquid-based techniques [24,25]. One of the conventional chemical-based techniques for depositing large-sized GO flakes on a variety of materials while maintaining the morphology and structural characteristics of the substrate is electrophoretic deposition (EPD). An electric field is used in the EPD process to drive the suspended particles from the suspension media onto the substrate. In a two-electrode cell, which is often used for EPD, the electric field can operate in either a modulated or direct electric current mode. Any charged colloidal system with suspended particles less than 30 μm can be used in EPD process. The two phases of the EPD for graphene are electrophoresis and deposition. When an electric field is applied to a graphene suspension, charged graphene flakes are propelled toward an oppositely charged electrode by an electric force. This process is known as electrophoresis, and it ends with a deposition process on the electrode surface where the graphene flakes accumulate due to the electric force [26]. The applied voltage, deposition time, and electrode spacing are among the many available parameters with this approach to adjust the thickness and shape of the transferred GO layers [27]. After transferring homogeneous layers of GO, a chemical or thermal reduction process can be followed to form reduced graphene oxide (rGO) layers, which have optical and structural characteristics similar to pure graphene [28,29].

While silicon dioxide (SiO2) passivation layers are commonly used in silicon solar cells to improve their performance stability, there are some potential disadvantages associated with this passivation technique from an engineering point of view; the deposition of silicon dioxide passivation layers often involves high-temperature processes, which can be a concern in the manufacturing of PSi-based solar cells [30]. High-temperature processing may collapse the pore walls and coalesce pores, leading to a decrease in the surface area of PSi substrate. Moreover, it increases the overall manufacturing cost and energy consumption. Furthermore, silicon dioxide passivation layers may introduce defects and surface states. These defects can act as recombination centers for charge carriers, reducing the effectiveness of passivation [31]. Silicon dioxide is hygroscopic, meaning it can absorb moisture from the environment. Moisture absorption can lead to the formation of additional defects and impact the passivation quality. This sensitivity to moisture is a concern for the long-term stability of the passivation layer. In contrast, graphene has unique properties that make it an interesting material for various applications, including potential use in PSi-based solar cells [32]. It is known for its high electrical conductivity, which can be beneficial for facilitating efficient charge carrier transport and reducing resistive losses in the solar cell. Moreover, graphene is transparent, allowing it to serve as a passivation layer without significantly affecting the absorption of sunlight by the silicon [33]. It is chemically stable and less prone to oxidation compared to some other materials. Moreover, due to the feasibility of its deposition at room-temperature, graphene can been investigated as an alternative to traditional silicon oxide passivation layers for PSi-based solar cells.

This study presents the effect of the rGO capping layer in enhancing photovoltaic stability without reducing the performance of PSi-based solar cells. Silicon substrates were electrochemically etched in this experiment to produce homogenous and highly porous PSi substrates. Then, GO layers were deposited onto the porous substrates using the EPD process. Different solar cells with and without graphene layers were made, and their photovoltaic performances were examined at various temperatures to explore the graphene influence on the photovoltaic stability of porous substrates.