Halide-Dependent Optoelectronic Properties of CsPbX3 Nanocomposite-Based Perovskite Solar Cells

INTRODUCTION
There has been great interest in photovoltaic technologies, especially perovskite solar cells (PSCs), as promising candidates for next-generation solar cells compared to conventional silicon-based solar cells due to their high efficiency, low cost, and tunable bandgap [1-3]. Among them, all-inorganic cesium perovskite solar cells (CsPbX₃, X = I, Br, Cl) have been widely investigated owing to their high thermal stability and excellent long-term operational stability [4,5]. The optoelectronic properties of CsPbX₃ can be tuned by varying the halide composition, which strongly influences device performance [6-8].
Here, we develop a new device architecture using fullerene (C₆₀) as the electron transport layer (ETL) and single-walled carbon nanotubes (SWCNTs) as the hole transport layer (HTL), along with three different CsPbX₃ absorber compositions. The design aims at improving carrier transport, reducing recombination losses, and enhancing the power conversion efficiency (PCE). The photovoltaic characteristics of the designed cells were evaluated using SCAPS-1D simulations, including short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE).

MATERIALS AND METHODS
The inorganic perovskite solar cell was simulated using SCAPS-1D, following the structure:
C₆₀ / CsPbX₃ / SWCNT / Au.
C₆₀ (ETL): Ensures efficient electron extraction and hole blocking.
CsPbX₃ (Absorber, X = I, Br, Cl): Main light-absorbing layer with tunable bandgap.
SWCNT (HTL): Provides stable, conductive pathways for hole transport.
Au (Back Contact): Offers high work function and stable hole collection.
Simulations were conducted under AM1.5G (1000 W/m²) illumination at 300 K, evaluating Jsc, Voc, FF, and efficiency (η).
Figs. 1A and B show the device structure and corresponding energy band alignment.
The simulated solar cell is composed of four functional layers as follows: C60 used as the electron transport layer (ETL); CsPbX3, inorganic halide perovskite type, for the intrinsic layer absorbing the sunlight; SWCNT for hole transporting layer (HTL) and an Au back electrode. Two critical parameters, thickness and doping concentration determine each layer. A proper incorporation of the C SWCNT layer in the M3 structure ensures that excellent electron extraction and hole transport toward the rear contact are achieved. Photoactive “absorber” refers to a CsPbX3 perovskite layer that is intrinsic (i.e. undoped). The back contact coverage with gold (Au) guarantees excellent charge collection and a good electrical back contact. The specific parameters of each layer are shown in Table 1.

RESULTS AND DISCUSSION
The C60 layer, as the ETL, has a moderate bandgap (2 eV) and high donor concentration (ND = 1017 cm−3), which is beneficial for efficient electron extraction and transport. The SWCNT (which works as the hole transport layer (HTL)) has a small bandgap of 0.9 eV and a large acceptor doping level (NA = 1012−1016 cm−3), resulting in increased hole conduction [9,10].
Depending on the halide composition (I, Br direction: L, I) the absorber layer in CsPbX3 absorbers shows greater variation in bandgap size and therefore gives way for a wider range of spectral tunability. The lack of doping numbers for the absorber layers indicates that they are either intrinsic or slightly p-doped in order to minimize the recombination losses [11].
The simulation results indicate that the performance of the perovskite solar cell is strongly influenced by the thickness of the C60 electron transport layer. The highest power conversion efficiency (12.41%) is achieved at 10 nm, but ultra-thin layers may suffer from poor coverage and instability. Increasing the thickness beyond 50 nm leads to a decline in Voc, Jsc, FF, and efficiency due to higher series resistance and reduced charge extraction. At 200 nm, efficiency drops to 7.44%. Therefore, a thickness range of 20-50 nm is recommended to maintain a balance between efficiency and device stability [12].
The performance of the solar cell is affected by the thickness of the CsPbX3 perovskite layer. As thickness increases from 250 to 600 nm, both Jsc and FF improve due to better light absorption, while Voc gradually decreases because of enhanced recombination in thicker layers. The highest efficiency (11.77%) is recorded at 250 nm, then it declines beyond 450 nm despite the rise in Jsc and FF, indicating a tradeoff caused by Voc reduction.
Therefore, a thickness range of 250−400 nm is recommended to achieve balanced efficiency, light absorption, and device stability [13].
The simulation results demonstrate that the power conversion efficiency (PCE) of the solar cell improves with increasing the thickness of the SWCNT layer, reaching a maximum value of 12.40% at thicknesses of 300 nm,350 nm, and 400 nm. This enhancement in efficiency is attributed to the improved charge transport and hole collection capability provided by the thicker SWCNT layer, which reduces series resistance and enhances the short-circuit current density (JSC) without compromising the open-circuit voltage (VOC). Beyond a certain threshold, the efficiency appears to saturate, indicating that further thickness increase beyond 300 nm does not yield additional performance gains. Therefore, a thickness in the range of 300−400 nm is considered optimal for maximizing device performance [14].

Analysis of the Solar Cell Structure Using C60, SWCNT, and Perovskite Layers
In this solar cell configuration, the structure was designed to include fullerene (C60) as the electron transport layer (ETL), single-walled carbon nanotubes (SWCNT) as the hole transport layer (HTM), and a perovskite absorber layer. The optimal thickness for each layer was selected based on the highest achieved power conversion efficiency. (PCE), as determined through SCAPS-1D simulations.
The selected parameters and their corresponding efficiencies are summarized in the Table 5.
These thickness values were applied in a simulated solar cell structure in SCAPS, while other parameters were kept constant.

Effect of Temperature on Solar Cell Performance
The performance of the solar cell was analyzed using SCAPS-1D over a temperature range of 283− 343 K. The structure included a C60 electron transport layer, a perovskite absorber, and an SWCNT hole transport layer. The open-circuit voltage (Voc) dropped significantly from 3.25 V to 0.91 V with rising temperature due to increased recombination. In contrast, the short-circuit current density (Jsc) remained nearly constant, indicating minimal impact on light absorption and charge generation. The fill factor (FF) improved notably from 22.35% to 67.85%, suggesting better charge extraction and reduced internal losses. Despite the Voc drop, the enhanced FF helped maintain a relatively high-power conversion efficiency (η), which only slightly decreased from 12.25% to 10.57%.
This table demonstrates the impact of temperature on solar cell efficiency, which is consistent with previous findings reported in the literature [4].
The results show that increasing temperature negatively affects the solar cell performance by significantly reducing the open-circuit voltage (VOC) and efficiency (η), due to the rise in reverse saturation current. Meanwhile, the short-circuit current density (Jsc) and fill factor (FF) slightly increase as a result of improved charge mobility and reduced internal resistance. However, the drop in voltage dominates, leading to an overall decline in efficiency.

The Role of Band Gap in CsPbX3 Layer Performance
The band gap plays a key role in determining the efficiency of the CsPbX3 perovskite layer by affecting its ability to absorb light and generate charge carriers. Proper control of the band gap enhances solar spectrum absorption and improves compatibility with other cell components, leading to better performance and stability. Approximate band gap values based on halogen type
CsPbCl3 ≈ 3.0eV, CsPbBr3 ≈ 2.3eV, and CsPbI3 ≈ 1.73eV.
The results showed that increasing the energy band gap in CsPbI3 solar cells leads to a gradual rise in the open-circuit voltage (Voc) but a decrease in the short-circuit current density (Jsc). Nevertheless, the fill factor (FF) and efficiency (η) remained relatively stable. It can be concluded that an energy band gap between 1.7 and 1.75 eV offers an optimal balance for cell performance [15-17]. 
The results indicate that the energy band gap of the SWCNT layer has a minimal effect on the photovoltaic performance of the solar cell. As the band gap increases from 0.8 eV to 1.5 eV , the short-circuit current density (Jsc) remains constant at 15.81 mA/cm2, and both the fill factor (FF) and the efficiency (η) stay nearly unchanged at 34.9% and 11.32%, respectively. A slight increase in the open-circuit voltage (Voc)is observed at 0.9 eV , but it does not significantly influence the overall performance.
This stability may be attributed to the fact that the SWCNT layer acts as an efficient hole transport material within this range of band gaps, minimizing the influence of slight energy band gap variations on device performance. Additionally, charge transport in this layer is not highly dependent on the band gap width within this narrow range, which leads to relatively stable solar cell behavior [18].
As the energy band gap of the C60 layer increases from 1.5 eV to 2 eV , the overall power conversion efficiency (η) of the solar cell improves steadily from 9.50% to 12.56%. This enhancement is mainly attributed to the increase in open-circuit voltage (Voc) and short-circuit current density (Jsc) with higher band gaps. Although the fill factor (FF) slightly decreases, its impact is offset by the larger gains in Voc and Jsc, indicating that optimizing the band gap of the electron transport layer (ETL) is crucial for improving device performance [19].
The results showed that the power conversion efficiency (η) remained nearly constant at approximately 12.09% despite the increase in acceptor density (NA) in the SWCNT layer from 1012 to 1016 cm−3. This stability can be attributed to the fact that the doping level was sufficient to enhance the electrical conductivity of the SWCNTs without turning them into a fully conductive (metallic) material. Such moderate doping improved hole transport and led to slight enhancements in the fill factor (FF) and short-circuit current density (Jsc), but these improvements were not significant enough to noticeably affect the overall efficiency. Therefore, it can be concluded that an optimal doping level exists, which enhances the performance of the layer while preserving the semiconducting properties essential for efficient solar cell operation. [8, 20, 21].
The results indicate that increasing the donor density (ND) in the C60 transport layer from 1012 to 1017 cm−3 led to a noticeable improvement in solar cell performance. The short-circuit current density (Jsc) increased from 16.89 to 17.15 mA/cm2, the fill factor (FF) improved from 23.18% to 31.7%, and the power conversion efficiency (η) rose from 12.09% to 12.49%, while the open-circuit voltage (Voc) remained constant at 2.22 V . This enhancement is attributed to better electrical conductivity of the C60 layer with higher ND, which facilitated electron transport and reduced series resistance. However, the improvement remains moderate, suggesting that there is an optimal doping level that should not be exceeded to maintain the semiconducting properties of the layer.

CONCLUSION
The proposed inorganic perovskite solar cell structure incorporating C60 as the electron transport layer (ETL) and SWCNT as the hole transport material (HTL) achieved a power conversion efficiency (PCE) of 19.9%. The selected perovskite absorber exhibited a suitable energy bandgap, enabling efficient light absorption and effective carrier generation. Optimization of the layer thickness played a crucial role in maximizing device performance. Furthermore, reducing the defect density significantly suppressed recombination losses and improved charge transport. The use of a low work function back-contact metal enhanced charge extraction efficiency, while moderate doping levels in the transport layers improved charge separation and reduced recombination. Overall, the C60/SWCNT configuration demonstrates strong potential for developing high-efficiency and stable inorganic perovskite solar cells.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.