Elucidating the Inverse Relation Between Current Density and Electron Transport Layer Thickness in Perovskite‐Based Organic In‐Organic Heterojunction Solar Cells

Lead‐free perovskite solar cells (PSCs) are promising alternatives to Pb‐based devices because of their reduced toxicity, low‐cost fabrication, and environmental compatibility. In this work, a fully lead‐free ZnO/BiFeO ₃ /Sn‐based perovskite/Spiro‐OMeTAD heterojunction solar cell was numerically investigated using COMSOL Multiphysics. BiFeO ₃ (BFO) was employed as the primary ferroelectric absorber, while CsSnI ₃ and FASnI ₃ were introduced as secondary absorbers to improve photon harvesting and carrier generation. Increasing BFO thickness from 50 to 600 nm enhances the short‐circuit current density ( J _sc ) from 11.2 to 27.8 mA/cm ² and power conversion efficiency (PCE) from 10.5% to 26.3%, mainly due to stronger optical absorption. CsSnI ₃ thickness study reveals a trade‐off between current enhancement and voltage loss because of increased recombination. ZnO electron transport layer (ETL) exhibited inverse relation with current density, as J _sc decreases from 27.8 to 22.7 mA/cm ² when ETL thickness increases from 10 to 150 nm. Conversely, thicker Spiro‐OMeTAD improves hole extraction, raising PCE to 27.30%. The optimized BFO/FASnI ₃ configuration achieved 29.8% PCE, highlighting the potential of absorber and transport layer engineering for efficient lead‐free PSCs.