Fulfilling the demand for energy is one of the major issues for the research community that requires substantial focus [1]. So, to resolve problems of energy crisis, it is a major goal to develop and find renewable and non-conventional energy resources [2]. Renewable and non-conventional energy resources are pollution-free and environmentally friendly energy resources. Solar energy is a vast resource of renewable and non-conventional energy sources that might be a good option for producing clean energy [3,4]. Solar cells employ solar energy and change it into electrical energy without creating any pollution [5]. Currently, crystalline silicon photovoltaic devices are leading the photovoltaic market. Crystal silicon solar cells offer more than 26 % power conversion efficiency [6]. But the several issues related with c-Si are its low absorption coefficient, high manufacturing cost and difficulties in purification and processing steps [7,8]. To overcome these problems, researchers are going towards thin film materials such as CdTe, CZTS, CIGS, and perovskite solar cell technology [[9], [10], [11]].

Single junction solar cell’s utmost efficiency is restricted up to 33 %, and this theory was given by William Schockley and Hans Joachim Queisser in 1961, which is known as S-Q limit [12]. Lattice thermalization loss and non-absorbed photon loss are the main causes of the reduction in efficiency [13]. The photons with greater energy would be absorbed by small bandgap materials and resulted the thermalization loss however, photons with larger energies would be used to excite electrons in high bandgap materials and result in below bandgap loss [14]. Therefore, to overcome these losses and effectively use the maximum spectrum, large and narrow bandgap materials are taken to design upper and lower sub-cells in tandem solar cell structures [15]. In 1955, Jackson was first presented this idea [16]. The quest to exceed the PCE of a single-junction photovoltaic (PV) cell has prompted numerous attempts to design multi-junction (tandem) structures, serially connecting cells with different bandgaps of sub-cells. A tandem structure can be realized through mechanical stacking, monolithic integration, or a spectral splitting architecture. A monolithic series connection of dual junction photovoltaic cells is designed by Lamort et al., in 1980 [17]. Kim et al. reported CIGS/Si tandem cell and achieved the highest efficiency of 19.8 % [18]. Mousa et al. designed MaPbI3/CIGS tandem solar cell in 2021 and reported a efficiency of 30.52 % [14]. Another theoretical study on CIGS/perovskite tandem solar cell was done by Ahmed et al. and published an efficiency of 24.69 % [19]. M. Jost et al. designed a perovskite/CIGS tandem solar cell and attained more than 30% efficiency by optical simulation [20]. PCE of 22.73% is achieved for polymer/CIGS tandem solar cell by optimization of thickness and conduction band offset [21]. Perovskite/CIGS tandem solar cell was fabricated, and after a reduction in bulk and interface defects, it reported 28.4% efficiency [22].

CIGS material is a p-type semiconductor suitable for PV cell due to its favourable bandgap for photovoltaic applications and large absorption coefficient of up to 105 cm−1 [23,24]. Other advantages of the CIGS as an absorber material are low-cost mass production because of the easier fabrication process and the very thin layer required to absorb photons compared to silicon material [25]. CIGS is very compatible for designing of tandem solar cell due to a tuneable bandgap between 1.04 and 1.69 eV by varying the content of gallium (Ga) in CIGS material [26]. SWCNT has suitable optoelectronic properties for photovoltaic applications, which has drawn researchers' attention to SWCNT materials [27,28]. The conductivity can be tuned by varying the bandgap of SWCNT from 0 to 2 eV [29]. They are lightweight and flexible and have high photo absorption, greater carrier mobility, and low carrier transport scattering [30]. All these properties make it as a very good absorber material for solar cell. CNT has been synthesized by several methods such as chemical vapour deposition (CVD), spin coating, sol-gel method, arc discharge, injection floating catalyst chemical vapour deposition (FCCVD), laser ablation and other techniques which are reported by various researchers [29,31,32].

CdS is generally taken as a buffer material in conventional thin-film PV cell due to its favourable bandgap for the transmittance of photons. However, the major problem with CdS is the toxic nature of Cd elements. So, researchers focused on finding an alternative to CdS for the buffer layer. Recently, transition metal di-chalcogenides (TMDs) materials have been receiving attention due to suitable optoelectronic properties for the buffer layer [33]. Janus WSeTe monolayer belongs to TMDs family which is a direct bandgap semiconductor less than 1.79 eV and greater than 1.4 eV. The absorption coefficient of WSeTe layer is more than 104 cm−1. The reflectivity of WSeTe is less than 35 %. Therefore, it passes maximum photons to the absorber layer when it is used as a buffer layer. Zinconium disulfide (ZrS2) also a member of group IV TMDCs, which is n-type material and have a tuneable bandgap between 1.2 and 2.2 eV [34].

In the current work, we demonstrate the simulation of a tandem device with a CIGS absorber as an upper cell and a SWCNT absorber as a lower cell using SCAPS-1D software. First, CIGS and SWCNT absorber-based solar cell are simulated and calibrated to validate the reported performance. Then, we designed and analysed the standalone upper cell (CIGS absorber) and lower cell (SWCNT absorber) by using validated devices. After that, CIGS/SWCNT tandem solar cell was designed and optimized for performance. To ensure the identical current for both the upper and lower cell, the thickness and bandgap of the CIGS absorber of the top cell have been varied. To the best of the author’s knowledge, CIGS/SWCNT tandem solar cell configuration has not been reported before. The proposed tandem (CIGS/SWCNT) solar cell offers more than 38 % conversion efficiency. Tandem solar cell research adds value to the field of solar energy by pushing efficiency boundaries and exploring innovative materials and design strategies. These advancements contribute to the design of more efficient and cost-effective PV cells, helping to accelerate the transition to sustainable and renewable energy sources.