Theoretical Analysis and Performance Optimization of Cs<sub>2</sub>AgBiI<sub>6</sub> Solar Cells with Dual Hole Transport Layers

Double perovskite materials have garnered significant attention in the photovoltaic field due to their low cost, environmental friendliness, and lead-free composition, making them ideal candidates for next-generation solar cell applications. In this work, the photovoltaic performance of solar cells using Cs₂AgBiI₆ as the light-absorbing layer was systematically investigated through simulations conducted with Silvaco ATLAS software. Building upon the previously reported single hole transport layer device architecture, ITO/ZnO/Cs₂AgBiI₆/HTL/Au, a new dual hole transport layer structure, ITO/ZnO/Cs₂AgBiI₆/HTL1/HTL2/Au, was proposed. Different dual hole transport layer combinations were explored, and their influence on the internal physical mechanisms and device performance was analyzed and optimized in detail. The simulation results demonstrated that devices employing Cu₂O/NiO and NiO/Si as dual hole transport layer, significantly improved charge extraction and generates a negative electric field at the interface, thereby reducing recombination losses and accelerating the transport of hole carriers. These configurations exhibited substantially higher efficiencies compared to those with a single hole transport layer, confirming the advantages of the dual hole transport layer structure. Additionally, devices using Cu₂O/CZTS and MoO₃/CZTS as dual hole transport layer showed better performance than the reference structure employing Spiro-OMeTAD/CZTS, indicating the potential for further improvement by optimizing material selection and layer properties. Among the various dual hole transport layer combinations tested, the structure utilizing Cu₂O/CZTS achieved the highest simulated power conversion efficiency (PCE) of 22.85%. By optimizing the thickness of each functional layer, the efficiency was further increased to 25.62%, with the optimal layer thicknesses determined to be 40 nm for ZnO, 850 nm for Cs₂AgBiI₆, 140 nm for Cu₂O, and 150 nm for CZTS. Furthermore, the effects of environmental and material parameters, such as temperature and hole transport layer doping concentration, on device performance were investigated. This study establishes a theoretical foundation for the design and enhancement of double perovskite solar cells. By demonstrating the potential of dual hole transport layer structures to significantly improve device efficiency, it underscores their value in advancing environmentally friendly and lead-free photovoltaic technologies. The insights gained from this research pave the way for developing high-performance double perovskite solar cells with optimized architectures and material properties.