Assessment of Thickness-Dependent Recombination Mechanisms and Realistic Performance Limits in CsBi₃I₁₀ Lead-Free Perovskite Solar Cells

Lead-free perovskite solar cells (PSCs) offer a promising, environmentally sustainable alternative to their lead-based counterparts, yet their performance is often constrained by charge transport and recombination limitations. This study presents a comprehensive numerical investigation into the thickness-dependent performance of lead-free bismuth-based PSCs employing CsBi₃I₁₀ as the absorber layer. Through systematic variation of the absorber, electron transport layer (ETL), and hole transport layer (HTL) thicknesses within a planar FTO/TiO₂/CsBi₃I₁₀/Spiro-OMeTAD/Ag architecture modeled using SCAPS-1D, an optimal absorber thickness range of 0.5–1.0 µm is identified. Under idealized conditions, a peak power conversion efficiency (PCE) of 21.62% is predicted, primarily to illustrate thickness-dependent trends. However, the introduction of realistic non-idealities, including finite series/shunt resistances, transmission losses, and intrinsic recombination mechanisms, reveals a significant performance degradation, with the PCE dropping to approximately 8%. This numerical analysis identifies that under the selected numerical parameter space, radiative recombination exhibits a pronounced contribution to performance degradation; however, the relative dominance of recombination pathways remains strongly dependent on defect density and carrier lifetime assumptions. These findings are further corroborated by capacitance (Mott-Schottky) and impedance (Nyquist) analyses, which reveal improved recombination resistance near the optimal thickness. This work provides realistic performance bounds and critical design insights that can guide future defect passivation and interface engineering strategies to unlock the full potential of CsBi₃I₁₀-based PSCs. Furthermore, this work establishes a progressive non-ideality incorporation framework for SCAPS-based modeling, enabling realistic performance bounding of emerging lead-free perovskite materials under coupled optical, electrical, and thermal constraints.