Numerical Investigation of MA₃Bi₂I₉-Based Lead-Free Perovskite Solar Cells: Impact of ETL Selection and Interface Band Alignment

Abstract

This study presents a comprehensive numerical investigation of leadfree perovskite solar cells (PSCs) employing methylammonium bismuth iodide (MA3Bi219) as the absorber material in an FTO/ETL/MA3Bi2I9/Cu2O/Ni device architecture. Using SCAPS-1D simulation software, the photovoltaic performance of three electron transport layers (ETLs), TiO2, SnO2, and ZnO was analysed under identical absorber thickness (1.4 µm) and defect density (1× 1012 cm-3). The resulting power conversion efficiencies (PCEs) were 16.16% for TiO2, 14.82% for SnO2, and 16.23% for ZnO, with all devices achieving high open-circuit voltages (Voc≈ 1.44 V). All three ETLs yield a CBO of+ 0.26 eV and a VBO of–0.95 eV, both within the literature-reported optimal thresholds (CBO< 0.30 eV and VBO< 0.2 eV) for achieving high efficiency. This alignment enables efficient hole extraction and creates an ideal spike-and-cliff band structure that reduces interfa-cial recombination while maintaining effective carrier flow. Therefore, the overall band alignment in all configurations supports favorable operation, and the remain-ing performance disparities stem from charge transport and interface quality. This work promotes MA3Bi2I9 as a stable, non-toxic absorber and validates Cu2O/Ni contacts as effective, low-cost alternatives to spiro-OMeTAD/Au.