Numerical investigation of lead-free Cs₂AgBiBr₆-based tandem solar cells with single-walled carbon nanotubes

This study employs SCAPS-1D numerical simulation to systematically establish and validate a design strategy for two-terminal (2 T) lead-free Cs₂AgBiBr₆/SWCNT tandem solar cells. Through global screening of the transport layers, co-optimization of absorber thickness and defect density, temperature-stability analysis, and current-matching optimization, the device is predicted to attain a power conversion efficiency (PCE) of 40.39% (open-circuit voltage V oc = 2.128 V; short-circuit current density J sc = 22.84 mA/cm²; fill factor FF = 83.11%), corresponding to 85.9% of the Shockley–Queisser tandem theoretical limit and exceeding the best reported Cs₂AgBiBr₆/Si and CZTS/SWCNT tandems by 4.13 and 5.59 %age points, respectively. SnS₂ and MoS₂ are identified as the optimal electron and hole transport layers, whose band alignment effectively suppresses non-radiative interfacial recombination; the optimal Cs₂AgBiBr₆ absorber thickness is 0.869 μm, which sustains efficient carrier transport. Compared with a silicon bottom cell, the superior bandgap matching and near-infrared spectral response of SWCNT enhance spectral complementarity and tandem current matching, further improving the theoretical efficiency utilization. The material parameters of all functional layers are taken from experimentally measured or first-principles values reported in the literature, while the absorber defect density is treated as a design variable and scanned over 10 ¹ ²–10 ¹ ⁷ cm⁻³ ; the adopted N t = 10 ¹ ⁴ cm⁻³ corresponds to the performance knee identified by this scan rather than a best-case selection, and the efficiency obtained at the presently reported experimental level (10 ¹⁶ cm⁻³) is reported alongside it, ensuring the physical traceability and transparency of the model. The resulting simulation framework and quantitative design guidelines therefore provide a reliable theoretical basis for material selection, device design, and experimental fabrication of Cs₂AgBiBr₆/SWCNT lead-free tandem solar cells.