Numerical design and optimisation of Cs₂SnI_6–nBr_n-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 Cs₂SnI_6−nBr_n 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 (V_OC) of 0.81 V, short-circuit current density (J_SC) 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 Cs₂SnI_6−nBr_n absorbers for next-generation sustainable photovoltaics.