Silvaco TCAD modeling, optical simulation, and optimization for high-current perovskite and u-CIGS tandem solar cells with efficiencies above 30%

Tandem perovskite solar cells (TPSCs) have attracted considerable attention due to their potential for achieving high efficiency, low production cost, and excellent scalability. In this study, a two-terminal monolithic tandem solar cell combining a lead-free Methylammonium Bismuth Iodide ((CH₃NH₃)₃Bi₂I₉, abbreviated as MBI) perovskite top sub-cell (Eg = 1.9 eV, absorber thickness 320 nm) and an thin CIGS bottom sub-cell (Eg = 1.68 eV, absorber thickness 500 nm) was designed and comprehensively optimized using Silvaco Atlas TCAD. To eliminate the use of scarce and expensive indium, fluorine-doped tin oxide (FTO) was deliberately selected as the front transparent conductive oxide (TCO) instead of the conventionally used indium tin oxide (ITO). The superior thermal stability of FTO (stable up to 600 °C versus 350 °C for ITO), its higher tolerance to physical abrasion, and its direct deposition capability on glass without an intermediate passivation layer make it a more robust and cost-effective choice for large-scale manufacturing and for subsequent high-temperature processing steps required in CIGS deposition. The standalone optimized MBI-perovskite single-junction cell using FTO achieved a power conversion efficiency of 15.13%. After individual calibration and optimization of both sub-cells, the fully coupled two-terminal monolithic tandem device delivered a realistic and reproducible efficiency of 35.67% (Voc = 4.53 V, Jsc = 29.23 mA/cm², FF = 77.88%) under standard AM1.5G illumination. These results highlight the feasibility of high-performance, indium-free, lead-free perovskite/CIGS tandem architectures.