Numerical design and optimisation of Cs2SnI6– <i>n</i> Br <i>n</i> -based lead-free perovskite solar cells

The demand for environmentally sustainable, high-efficiency photovoltaic technologies has accelerated the search for lead-free perovskite solar cells (PSCs). In this study, we numerically design and optimise a fully inorganic double-layer PSC using Cs2SnI6−nBrn as the light absorber, modelled through the one-dimensional solar cell capacitance simulator. A comprehensive parametric analysis was performed to evaluate the effects of absorber thickness, electron and hole transport layer thicknesses, interfacial defect densities, and back contact work functions on device performance. The optimised structure, FTO/graphene oxide (GO)/Cs2SnI4Br2/copper(I) oxide (Cu2O)/gold (Au), achieves a power conversion efficiency of 20.37%, with an open-circuit voltage (VOC) of 0.81 V, short-circuit current density (JSC) of 31.26 mA·cm−2, and fill factor of 78.72%. Moderate bromide incorporation (n ≈ 2) not only stabilises the Cs2SnI6 lattice but also tunes the bandgap to ≈1.30 eV, enhancing both light absorption and voltage output. The results highlight that high-work-function back contacts (≥5.1 eV) are essential for efficient hole extraction, while careful control of interfacial defect densities (≤1018 cm−³) is critical to suppress recombination losses. These findings establish clear design guidelines for the development of stable, high-efficiency, lead-free PSCs, and underline the potential of Cs2SnI6−nBrn absorbers for next-generation sustainable photovoltaics.