This simulation-based study investigates the influence of carbon-based interlayers on the theoretically predicted performance of CsSnCl₃ perovskite solar cells by incorporating C₆₀ and single-wall carbon nanotubes (SWCNTs) between TiO₂ and the absorber. All results reported herein are obtained from numerical simulations using SCAPS- 1D and represent theoretical upper bounds under idealized, defect-minimized conditions. It is important to note that the assumed CsSnCl₃ bandgap of 1.52 eV is a hypothetical, strain-tuned value; experimentally synthesized CsSnCl₃ typically exhibits a bandgap of approximately 2.8 eV. Therefore, this study should be interpreted as a design exploration for future bandgap engineering efforts, not as a prediction of currently achievable experi- mental performance. Device simulations reveal that the TiO₂-only structure exhibits the highest charge transfer resistance and lowest built-in potential, resulting in moderate efficiency. Introducing C₆₀ improves interfacial band alignment and increases both the open circuit voltage and built-in potential, yielding a notable enhance- ment in power conversion efficiency. The most significant improvement is achieved with SWCNTs, which sub- stantially reduce the charge transfer resistance and facilitate more efficient electron extraction. This leads to a marked increment in the highest overall efficiency and short circuit current density among the studied archi- tectures. Nyquist analysis confirms the correlation between reduced impedance, improved interfacial charge transfer, and enhanced photovoltaic performance. These findings highlight the effectiveness of SWCNTs as an interfacial layer for optimizing charge transport and maximizing the theoretically predicted performance of lead- free perovskite solar cells under idealized conditions.
