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 received significant attention in the photovoltaic field due to their low cost, environmental friendliness, and lead-free composition, which make them ideal candidates for next-generation solar cell applications. In this work, the photovoltaic performance of solar cells using Cs<sub>2</sub>AgBiI<sub>6</sub> as the light-absorbing layer is systematically investigated through simulations using Silvaco ATLAS software. Based on the previously reported single hole transport layer device architecture, namely ITO/ZnO/Cs<sub>2</sub>AgBiI<sub>6</sub>/HTL/Au, a new dual hole transport layer structure ITO/ZnO/Cs<sub>2</sub>AgBiI<sub>6</sub>/HTL1/HTL2/Au is proposed. Different dual hole transport layer combinations are explored, and their influence on the internal physical mechanism and the device performance are analyzed and optimized in detail. The simulation results show that the devices using Cu<sub>2</sub>O/NiO and NiO/Si respectively as dual hole transport layer significantly improve charge extraction and generate a negative electric field at the interface, thereby reducing recombination losse and accelerating the transport of hole carriers. These two configurations exhibit substantially higher efficiencies than those configurations with a single hole transport layer, confirming the advantages of the dual hole transport layer structure. Additionally, devices using Cu<sub>2</sub>O/CZTS and MoO<sub>3</sub>/CZTS as dual hole transport layer show better performance than the reference structure using Spiro-OMeTAD/CZTS, indicating the potential for further improvement by optimizing material selection and layer properties. Of the various dual hole transport layer combinations tested, the structure utilizing Cu<sub>2</sub>O/CZTS achieves the highest simulated power conversion efficiency (PCE) of 22.85%. By optimizing the thickness of each functional layer, the efficiency can be further increased to 25.62%, and the optimal layer thickness is determined to be 40 nm for ZnO, 850 nm for Cs<sub>2</sub>AgBiI<sub>6</sub>, 140 nm for Cu<sub>2</sub>O, 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 are investigated. This study lays a theoretical foundation for the design and enhancement of double perovskite solar cells. By demonstrating the potential that the dual hole transport layer structures can significantly improve device efficiency, their value in advancing environmentally friendly and lead-free photovoltaic technologies becomes very prominent. The insights gained from this research pave the way for developing high-performance double perovskite solar cells with optimized architectures and material properties.